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Novel Uses of Titanium Dioxide for
Silicon Solar Cells
A thesis submitted as partial fulﬁllment
of the requirement for the Degree of
Doctor of Philosophy
by
Bryce Sydney Richards
at the
Centre for Photovoltaic Engineering
and the
School of Electrical Engineering
University of New South Wales
Sydney 2052
New South Wales
Australia
April 2002
CENTRE FOR
PHOTOVOLTAIC
ENGINEERING
UNSW
Certiﬁcate of Originality
I hereby declare that this submission is my own work and that, to the best of my knowledge
and belief, it contains no material previously published or written by any other person
nor material which to a substantial extent has been accepted for the award of any other
degree or diploma of a university or other institute of higher learning, except where due
acknowledgment is made in the text.
I also declare that the intellectual content of this thesis is the product of my own work,
even though I may have received assistance from others on style, presentation and language
expression.
Bryce Richards
Richards, Bryce Sydney
Novel Uses of Titanium Dioxide for Silicon Solar Cells
PhD Thesis
Centre for Photovoltaic Engineering
The University of New South Wales Sydney, NSW 2052, Australia
c 2002 all rights reserved
Copyright ISBN 0 7334 1971 2
Abstract
Titanium dioxide (TiO2 ) thin ﬁlms have a long history in silicon photovoltaics (PV) as
antireﬂection (AR) coatings due to their excellent optical properties and low deposition
cost. This work explores several novel areas where TiO2 thin ﬁlms could be use to enhance
silicon (Si) solar cell performance while reducing device fabrication costs.
Amorphous, anatase and rutile TiO2 thin ﬁlms are deposited using ultrasonic spraydeposition (USD) and chemical vapour deposition (CVD) systems, both designed and constructed by the author. Initial experiments conﬁrmed that no degradation in the bulk
minority carrier lifetime (τbulk ) occurred during high-temperature processing, although the
stability of the USD-deposited TiO2 ﬁlms was dependent on the furnace ambient.
A major disadvantage of TiO2 AR coatings is that they aﬀord little surface passivation. In
this work, a novel method of achieving excellent surface passivation on TiO2 -coated silicon
wafers is presented. This involved growing a 6 nm-thick SiO2 layer at the TiO2 :Si interface by
oxidising the wafer after TiO2 ﬁlm deposition. The increase in surface passivation aﬀorded by
the interfacial SiO2 layer results in a decrease in the emitter dark saturation current density
(J0e ) by nearly two orders of magnitude to 4.7 − 7.7 × 10−14 A/cm2 . This demonstrates the
compatibility of the TiO2 /SiO2 stack with high-eﬃciency solar cells designs.
By varying the ﬁlm deposition and annealing conditions, TiO2 refractive indices in the
range of 1.726 − 2.633 (at λ = 600 nm) could be achieved. Subsequently, a double-layer
antireﬂection (DLAR) coating was designed comprised of low and high TiO2 refractive index
material. The best experimental weighted average reﬂectance (Rw ) achieved was 6.5% on a
planar silicon wafer in air. TiO2 DLAR coatings are ideally suited to multicrystalline silicon
(mc-Si) wafers, which do not respond well to chemical texturing.
Modelling performed for a glass and ethyl vinyl acetate (EVA) encapsulated buried-contact
solar cell indicated that a TiO2 DLAR coating aﬀorded a 7% increase in the short circuit
current density, when compared to a standard, commercially-deposited TiO2 single-layer AR
coating.
Finally, it is demonstrated that chemical reactions with phosphorus prevent TiO2 from acting
as a successful phosphorus diﬀusion barrier or dopant source. The applicability of TiO2 thin
ﬁlms to various silicon solar cell structures is discussed.
Acknowledgements
Many people contributed to the success of this work and my survival throughout.
First, and foremost, I need to thank (yes, thank!) my partner, Andrea Sch¨afer, for leading
me along the path to the PhD. Somehow witnessing all the good and the bad moments
during her PhD, ended up creating a positive image for me! Andrea also provided invaluable
guidance and tips along the way, and created many shortcuts through the bureaucracy for
me. I will never forget your assistance Andrea, and am deeply indebted to you. Vielen
Dank, and may our love only grow stronger.
Another big Danke, goes out to our daughter, Moana Sch¨afer, who witnessed just over half
of my eﬀorts. Thanks for keeping my feet ﬁrmly planted on the ground and not letting me
drift too far oﬀ into “PhD land”! Thank you for all our fun times, and my apologies for the
times when my patience wasn’t suﬃcient to see your needs.
Naturally, I would like to thank the input from my supervisors of the years: to Stuart
Wenham (UNSW) for his enthusiasm and encouragement; to Christiana Honsberg (UNSW
and Georgia Institute of Technology, U.S.A.) for her moral and monetary support; to
Francesca Ferrazza (Eurosolare S.p.A.) for the opportunities to see the “real” side of
photovoltaics and for being a true friend; and to Jeﬀ Cotter for his valuable advice during
the latter stage of the thesis.
Several members brought their own special personalities to the Centre and made it a fun
and challenging work place. These people include Keith McIntosh, Hamid Mehrvarz, Holger
Neuhaus, Alex Slade, Bernhard Vogl, Rob Bardos, Matt Boreland, Martin Bruahart and
Tom Puzzer. Thanks for all the ethical, moral and technical conversations. Thanks too for
the great computer support, Laurie!
I would like to thank other people who assisted with TiO2 thin ﬁlm characterisation:
Dr. Tom Puzzer (UNSW) for SEM/AFM training; Prof. Robert Lamb (UNSW) and
Dr. Matt Boreland (Toyota Technical Institute, Japan) for XPS analysis; Prof. David
Jamieson (Univ. of Melbourne) for RBS analysis; Sally Rowlands and Prof. Trevor
Redgrave (both Univ. of Western Australia) for training and access to the variable-angle
spectroscopic ellipsometer (VASE); Dr. Alistair Sproul (UNSW), author of the forthcoming
book “Ellipsometry for Dummies”; and to my father, Dr. Ray Richards (Lower Hutt, New
Zealand), for his assistance in bringing me up to speed on thermochemistry analysis.
2
I am grateful for the guidance in my career provided by Dr. Andrea Sch¨afer, Prof. Mark Wainwright, Prof. Stuart Wenham and Prof. Martin Green. The ﬁnancial support provided by the
Faculty of Engineering, the School of Electrical Engineering and the Centre for Photovoltaic
Engineering was greatly appreciated.
Publications Resulting from this Thesis (to date)
B.S. Richards (2004) Comparison of Dielectric Coatings for Buried-Contact Solar Cells: A
Review, Progress in Photovoltaics 12 (in press).
B.S. Richards, S.R. Richards, M.B. Boreland, D.N. Jamieson (2004) High Temperature
Processing of TiO2 Thin Films for Application in Silicon Solar Cells, Journal of Vacuum
Science and Technology A, 22(2): 339-348.
B.S. Richards, S.F. Rowlands, A. Ueranatasun, J.E. Cotter, C.B. Honsberg (2004) Reducing
the Production Costs of Buried-Contact Solar Cells using Titanium Dioxide Thin Films,
Solar Energy, 76(1-3): 269-276.
B.S. Richards (2003) Single-Material TiO2 Double-Layer Antireﬂection Coatings, Solar
Energy Materials and Solar Cells, 79(3), 369-390.
B.S. Richards, S.F. Rowlands, C.B. Honsberg, J.E. Cotter (2003) TiO2 DLAR Coatings for
Planar Silicon Solar Cells, Progress in Photovoltaics, 11(1), 27-32.
B.S. Richards, J.E. Cotter and C.B. Honsberg (2002) Enhancing the surface passivation of
TiO2 coated silicon wafers, Appl. Phys. Letters, 80(7), 1123-1125.
B.S. Richards, S.F. Rowlands, A. Ueranatasun, J.E. Cotter, and C.B. Honsberg (2001)
Reducing the production costs of buried-contact solar cells using titanium dioxide thin ﬁlms,
Intl. Solar Energy Society Solar World Congress, 26-30 November, Adelaide.
B.S. Richards, J.E. Cotter, C.B. Honsberg and S.R. Wenham (2000) Novel Uses of TiO2
Films in Crystalline Silicon Solar Cells, 28th IEEE Photovoltaic Specialists Conference,
Alaska, 375-378.
C.B. Honsberg, J.E. Cotter, K.R. McIntosh, S. Pritchard, B.S. Richards and S.R. Wenham,
(1999), Design strategies for commercial solar cells using the buried contact technology,
IEEE Trans. Electron Devices, 46(10), 1984-92.
B.S. Richards, J.E. Cotter, F. Ferrazza, C.B. Honsberg and S.R. Wenham (1998) Lowering
the cost of commercial silicon solar cells, Proc. of the Environmental Engineering Research
Event 1998, Avoca Beach, New South Wales, 303-308.
J.E. Cotter, B.S. Richards, F. Ferrazza, C.B. Honsberg, T.W. Leong, H.R. Mehrvarz,
G.A. Naik and S.R. Wenham (1998) Design of a simpliﬁed emitter structure for buried contact
solar cells, 2nd World Conference Photovoltaic Energy Conversion, Vienna, 1511-1514.
Contents
1 Introduction
9
1.1
Motivation for this Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
1.2
Australia’s Solar Energy Resource . . . . . . . . . . . . . . . . . . . . . . . .
11
1.3
Brief Theory of Solar Cell Operation . . . . . . . . . . . . . . . . . . . . . .
12
1.4
Commercially Produced Silicon Solar Cells . . . . . . . . . . . . . . . . . . .
14
1.4.1
Screen-Printed Solar Cells . . . . . . . . . . . . . . . . . . . . . . . .
14
1.4.2
Buried-Contact Solar Cells . . . . . . . . . . . . . . . . . . . . . . . .
15
1.4.3
Buried-Contact Solar Cell Fabrication Sequence . . . . . . . . . . . .
16
1.4.4
Simpliﬁed Buried-Contact Solar Cell . . . . . . . . . . . . . . . . . .
16
Multicrystalline Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18
1.5.1
Issues with Multicrystalline Silicon . . . . . . . . . . . . . . . . . . .
19
Why use Titanium Dioxide? . . . . . . . . . . . . . . . . . . . . . . . . . . .
20
TiO2 Thin Films in Photovoltaics . . . . . . . . . . . . . . . . . . . .
21
Thesis Overview and Goal . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
1.5
1.6
1.6.1
1.7
2 Common Properties of TiO2 Thin Films
29
2.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
29
2.2
Physical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
30
The Amorphous − Anatase − Rutile Phase
Transformations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
30
2.2.1
2.2.2
The Eﬀect of Impurities on the Anatase − Rutile Phase Transformation 32
2.2.3
Substrate Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
33
4
CONTENTS
2.3
2.2.4
Film Defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
34
2.2.5
Film Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
34
2.2.6
Non-Stoichiometric TiO2−x Thin Films . . . . . . . . . . . . . . . . .
36
Optical Properties
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
37
Refractive Index, Extinction Coeﬃcient and
Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
37
2.3.2
TiO2 Thin Film vs. Single Crystal . . . . . . . . . . . . . . . . . . .
39
2.3.3
Variation with Deposition and Annealing Temperature . . . . . . . .
40
2.3.4
Variation with Deposition and Annealing Ambient . . . . . . . . . . .
45
2.3.5
Variation with Other Deposition Conditions . . . . . . . . . . . . . .
48
2.3.6
Optical Properties of Highly Porous TiO2 Films . . . . . . . . . . . .
48
Electrical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
50
2.4.1
TiO2 : Insulator or Conductor? . . . . . . . . . . . . . . . . . . . . . .
50
2.4.2
Non-Stoichiometric TiO2−x Thin Films . . . . . . . . . . . . . . . . .
50
2.4.3
Variation with Deposition or Annealing Ambient . . . . . . . . . . .
51
2.4.4
Doped TiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
51
Chemical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
52
2.5.1
Chemicals used in Making Solar Cells . . . . . . . . . . . . . . . . . .
52
2.5.2
Hydroﬂuoric Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
54
2.5.3
Other Acids and Bases . . . . . . . . . . . . . . . . . . . . . . . . . .
56
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
57
2.3.1
2.4
2.5
2.6
3 TiO2 Thin Film Deposition Equipment
59
3.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
59
3.2
Overview of TiO2 Thin Film Deposition Methods . . . . . . . . . . . . . . .
62
3.3
Ultrasonic Spray Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . .
65
3.4
Theory of Ultrasonic Spray Deposition . . . . . . . . . . . . . . . . . . . . .
67
3.5
TPT: The TiO2 Precursor . . . . . . . . . . . . . . . . . . . . . . . . . . . .
70
3.5.1
70
Why TPT? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CONTENTS
3.5.2
3.6
3.7
3.8
5
The TPT→TiO2 Reaction . . . . . . . . . . . . . . . . . . . . . . . .
72
Design of Ultrasonic Spray Deposition System . . . . . . . . . . . . . . . . .
74
3.6.1
Selection of Ultrasonic Nozzle . . . . . . . . . . . . . . . . . . . . . .
74
3.6.2
Ultrasonic Nozzle Performance . . . . . . . . . . . . . . . . . . . . . .
76
3.6.3
Liquid Delivery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
77
3.6.4
Substrate Heater . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
78
3.6.5
Motorized Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
81
3.6.6
Spray Shaping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
82
3.6.7
Miscellaneous Equipment . . . . . . . . . . . . . . . . . . . . . . . . .
82
3.6.8
Operation of the TiO2 Spray System . . . . . . . . . . . . . . . . . .
85
Design of CVD System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
87
3.7.1
Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
87
3.7.2
TPT Bubbler and Temperature Control . . . . . . . . . . . . . . . .
87
3.7.3
Water Vapour Bubbler . . . . . . . . . . . . . . . . . . . . . . . . . .
88
3.7.4
Operation of the CVD System . . . . . . . . . . . . . . . . . . . . . .
88
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
89
4 Characterisation of TiO2 Thin Films
91
4.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
91
4.2
FTIR Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
92
4.3
Raman Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
95
4.4
X-ray Photoelectron Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . .
98
4.5
Rutherford Back-Scattering Spectroscopy . . . . . . . . . . . . . . . . . . . .
99
4.6
Ellipsometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
4.6.1
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
4.6.2
Ellipsometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
4.6.3
Lorentz Oscillator Model . . . . . . . . . . . . . . . . . . . . . . . . . 106
4.6.4
Surface Roughness Model . . . . . . . . . . . . . . . . . . . . . . . . 107
4.6.5
Ellipsometric measurements of Spray Deposited TiO2 Thin Films . . 107
6
CONTENTS
4.6.6
SE measurements of CVD TiO2 Thin Films . . . . . . . . . . . . . . 109
4.7
Reﬂectance Spectrophotometry . . . . . . . . . . . . . . . . . . . . . . . . . 109
4.8
Electron Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
4.9
4.8.1
USD-Deposited TiO2 Thin Films . . . . . . . . . . . . . . . . . . . . 110
4.8.2
APCVD-Deposited TiO2 Thin Films . . . . . . . . . . . . . . . . . . 110
4.8.3
CVD-Deposited TiO2 Thin Films . . . . . . . . . . . . . . . . . . . . 110
Atomic Force Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
4.10 Chemical Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
4.11 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
5 Enhancing the Passivation of TiO2 -coated Wafers
121
5.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
5.2
Stability of TiO2 at High-Temperatures . . . . . . . . . . . . . . . . . . . . . 123
5.3
5.4
5.2.1
Titanium Contamination of Silicon . . . . . . . . . . . . . . . . . . . 123
5.2.2
Reduction of TiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
Methods of Achieving Surface Passivation with TiO2 Thin Films . . . . . . . 130
5.3.1
Growth of SiO2 at the TiO2 :Si Interface . . . . . . . . . . . . . . . . 133
5.3.2
TiO2 on PSG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
6 Performance of TiO2 Thin Films as a Phosphorus Diﬀusion Barrier and
Dopant Source
141
6.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
6.2
TiO2 as a Phosphorus Diﬀusion Barrier . . . . . . . . . . . . . . . . . . . . . 142
6.3
6.2.1
Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
6.2.2
Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
6.2.3
Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
TiO2 as a Phosphorus Dopant Source . . . . . . . . . . . . . . . . . . . . . . 149
6.3.1
Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
6.3.2
Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 151
CONTENTS
6.4
7
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
7 TiO2 Antireﬂection Coatings
153
7.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
7.2
Previous Developments in AR Coatings . . . . . . . . . . . . . . . . . . . . . 155
7.3
7.4
7.2.1
Theory and Design of AR Coatings . . . . . . . . . . . . . . . . . . . 155
7.2.2
TiO2 AR Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
7.2.3
Silicon Nitride AR Coatings . . . . . . . . . . . . . . . . . . . . . . . 165
Varying the Optical Properties of TiO2 . . . . . . . . . . . . . . . . . . . . . 167
7.3.1
Deposition Temperature . . . . . . . . . . . . . . . . . . . . . . . . . 168
7.3.2
Annealing Temperature
7.3.3
Deposition Ambient
7.3.4
Annealing Ambient . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
. . . . . . . . . . . . . . . . . . . . . . . . . 174
. . . . . . . . . . . . . . . . . . . . . . . . . . . 178
Development of Novel TiO2 AR Coatings . . . . . . . . . . . . . . . . . . . . 183
7.4.1
Single-layer TiO2 AR Coatings . . . . . . . . . . . . . . . . . . . . . 183
7.4.2
Double-layer TiO2 AR Coatings . . . . . . . . . . . . . . . . . . . . . 186
7.5
Performance of TiO2 DLAR-Coated Solar Cells . . . . . . . . . . . . . . . . 193
7.6
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
8 Conclusions
199
8.1
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
8.2
Applicability to Various Solar Cell Processes . . . . . . . . . . . . . . . . . . 203
8.3
Suggestions for Further Work . . . . . . . . . . . . . . . . . . . . . . . . . . 205
A TiO2 AR Coating Modelling Parameters
207
A.1 Variation of n and k with Deposition Temperature . . . . . . . . . . . . . . . 208
A.2 Variation of n and k with Annealing Temperature . . . . . . . . . . . . . . . 212
A.3 Variation of n and k with Deposition Ambient . . . . . . . . . . . . . . . . . 218
A.4 Variation of n and k with Annealing Ambient . . . . . . . . . . . . . . . . . 220
A.5 TiO2 DLAR Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225
8
CONTENTS
Chapter 1
Introduction
Photovoltaics (PV) will play an important role in the world’s future energy trends, however
the major hurdles faced in widespread implementation of renewable energy are of a social,
not technical, nature. This chapter brieﬂy discusses Australia’s potential to become a major
player in the future solar energy industry. The diﬀerences between the two dominant commercially produced silicon solar cells, screen-print (SP) and buried-contact (BC), are brieﬂy
examined. The BC technology was introduced in the mid-1980’s and designed for single
crystal silicon (c-Si) wafers. A trend over recent years has seen lower-cost, multicrystalline
silicon (mc-Si) wafers now dominate the marketplace. The BC fabrication sequence has
several lengthy high-temperature processing steps fabrication. The processing costs currently
outweigh the performance enhancement oﬀered by the buried-contact technology when using
mc-Si wafers and, today, only SP solar cells are produced on mc-Si substrates.
The most crucial high-temperature processing step in the buried-contact fabrication sequence
is the growth of a thick, thermal oxide layer. If this layer could be replaced by a thin dielectric ﬁlm, deposited at low temperature and such a ﬁlm was able to withstand processing
in phosphorus-containing ambients, a buried-contact solar cell could be fabricated with about
one hour of high-temperature processing steps. This proposition seemed attractive for application of the BC technology to multicrystalline silicon wafers in a cost-eﬀective manner.
One logical choice for this ﬁlm is titanium dioxide (TiO2 ), due to its prevalence in the PV
industry, high optical performance and low cost. A review of roles that TiO2 ﬁlms have played
in the photovoltaics industry is presented, before discussing the overview of the remainder of
the thesis.
1.1
Motivation for this Work
The amount of solar energy that strikes earth in a period of a few days is greater than the
amount of fuel burnt overt the course of the whole human history,1 which encourages one to
9
10
1. Introduction
think about ways in which this energy could be eﬀectively harnessed in order to satisfy our
ever-increasing demand for energy. While this may sound like a gross oversimpliﬁcation of
the impending energy crisis facing humankind, it brings into perspective the amount of power
that is constantly being radiated by the sun. The primary focus of most developed nations
needs at this stage should be looking at ways of radically reducing the current consumption
of energy and raw materials. In fact, we are all so used to consuming that our language now
deﬁnes our role in society as “consumers”. A 1995 study stated that for each American, 20
tonnes of new materials have to be provided every year, including energy equivalent to 7.6
tonnes of oil (or 12 tonnes of coal).2 If the world’s projected population in the year 2070
consumed energy at this rate, the world energy production would have to be fourteen times
greater than its current capacity and all potentially recoverable energy resources would be
depleted in about eighteen years2 !
Therefore, if close to 11 billion people are to live on this planet in 70-years time we need to
become “conservers” rather than consumers. The earth does not contain enough resources
for each person in a developed nation to sustain our resource-intensive lifestyle. In the same
manner, the economies of Western nations simply cannot continue to grow at targeted rate
of 4% p.a., no matter what the politicians say. The Australian environmental thinker and
activist, Ted Trainer, claims that the only type of economic growth that can be said to be
truly ecologically sustainable is one that has a 0% growth rate per annum.2 Trainer also
argues that “technological ﬁxes” will not get us there either, as the main problems faced
are social.2 So while the pursuit of renewable energy is a worthy cause, a truly sustainable
solution can only be reached if we were to consume a small fraction of what we use in our
aﬄuent lifestyle today.
If photovoltaics (PV), the direct conversion of sunlight into electricity via solar cells, is to
have a major and timely impact upon our current global predicament, the costs of production
need to be signiﬁcantly reduced before PV can compete head to head with fossil fuels.
The cost of generating electricity directly from solar cells is slowly, but steadily, reducing.
Figure 1.1 shows that the cost of purchasing PV modules is expected to reduce from today’s
US$4 /watt-peak (Wp ) to <US$2 Wp by about 2010.1 Many forecasts have been published
announcing that silicon or other thin-ﬁlm based technologies will dominate the marketplace
in ﬁve years time, however silicon-wafer-based technologies still comprise more than 85% of
all PV module sales.3 Additionally, since 1998 more multicrystalline silicon (mc-Si) solar
cells have been produced than crystalline silicon (c-Si) cells.3 This is due to the cheaper
fabrication costs of mc-Si and only a slight reduction in performance with current dominant
solar cell technologies.
Average selling price (1998 US$/Watt)
1.2 Australia’s Solar Energy Resource
1 0 0 .0 0
11
1978
1980
1985
1990
1995
1 0 .0 0
1998
2000
1 .0 0
1
10
100
1000
10000
Accumulated shipments (megawatts)
Figure 1.1: Historical cost of purchasing PV modules from 1978 to 2000,
and an extrapolation of the linear trend to increased shipments in the
future (from Green 1 ).
1.2
Australia’s Solar Energy Resource
Australia has recently implemented a PV rebate scheme has at both State and Federal
level in an attempt to help Australia meet its target of generating an additional 9500 GWh
(about 1%) of electricity from renewable energy sources by the year 2010. There is deﬁnite
potential for a much greater expansion in PV in Australia, considering that many areas
of Australia receive more than twice the amount of solar radiation than countries such as
Germany (as shown in Figure 1.2), which already has a large installed PV capacity. It is also
apparent from Figure 1.2(a) that the majority of Australians do not live in the sunny central
and northern regions of the country. However, the cities of Perth, Adelaide, and all of the
densely populated East Coast (Sydney and further north) still receive four or more sunshine
hours a day of full sunshine (1 kW/m2 ). This is an excellent (and free) resource, which
remains relatively untapped. Additionally, PV panels can easily be mounted on existing
north-facing roofs and included into fa¸cades of oﬃce buildings. This means that power can
be generated from within densely populated areas, obviating the requirement of setting aside
large amounts of land for such purposes. Australia’s excellent research record in the ﬁeld
of solar energy along with its abundant resources make it uniquely positioned to become a
major player in the future solar energy industry. One only needs to look to Denmark in
the case of wind energy to see how a strong domestic market can lead to a small country
dominating the world market.
12
1. Introduction
Figure 1.2: Maps indicating the level of solar radiation received on a
horizontal plane in (a) Australia and (b) Europe (in units of kWh/m2 ).4
1.3
Brief Theory of Solar Cell Operation
A typical silicon solar cell is shown in Figure 1.3(a). The standard n + -p silicon solar cell
has a shallow junction formed near the front surface, a front ohmic contact in the form of
ﬁngers and a busbar, and a full metal rear ohmic contact. Light that is absorbed by the solar
cell generates an electron-hole (e-h) pair that is able to contribute to current ﬂow from the
device. For small photon energies, the majority of the current is generated in the base (about
300 µm thick), while photons of energy greater than 2.5 eV generate current from within the
ﬁrst 1 µm of silicon. This is the reason why the recombination velocity at the front surface
can have such a profound eﬀect on this high-energy photocurrent. For high-eﬃciency solar
cells the eﬀect of rear surface recombination also becomes an important design parameter.
The area-normalised current density J (mA/cm2 ), of the solar cell is given by
V + JR
V +JRs
s
,
J = JL − J0 exp( nkT /q ) −1 +
Rsh
(1.1)
where J0 is the dark saturation current density (mA/cm2 ), JL is the light generated current
density (mA/cm2 ), q is the electronic charge (1.602 × 10−19 C), V is the operating voltage,
k is Boltzmann’s constant (1.380 × 10−23 J/K), Rs is the series resistance, Rsh is the shunt
resistance, n is the diode ideality factor, and T is the operating temperature (K). The special
case, where the voltage is zero and J = JL is deﬁned to be the short-circuit current density,
Jsc . Figure 1.3(a) shows the equivalent circuit of a solar cell, including series and shunt
resistances.
The Jsc is limited by optical losses such as reﬂectance from the front surface, shading of the
front surface due to the metal contacts, and transmission of lower energy light out the back
of the cell. An additional source of current loss is due to minority carriers recombining at
the surfaces before they are collected by the junction.
1.3 Brief Theory of Solar Cell Operation
13
J
Pmpp, Vmpp
Rs
V
J0(exp q(V-JRs )/nkT-1)
JL
Rsh
Current Density, Power
–
Jsc
Jmpp, Vmpp
Voltage
+
(a)
Voc
(b)
Figure 1.3: (a) Equivalent circuit of the basic p-n junction solar cell, and
(b) a typical I − V (solid) and P − V (dashed) curve of an illuminated
solar cell.
The operating point where J = 0 mA/cm2 in Equation 1.1 deﬁnes the open-circuit voltage
Voc of the solar cell, as shown in Figure 1.3(b). The minimization of both bulk and surface
recombination are important in order to maximise the voltage at maximum power point.
Excellent surface passivation can be achieved by using a thermally grown silicon dioxide
(SiO2 ) layer, as discussed in Section 5.3.1. Bulk recombination will always be higher in
mc-Si wafers compared to high quality c-Si FZ wafers, due to the grain boundaries between
the crystals, although lifetime enhancement measures such as gettering and hydrogenation
may improve the situation somewhat. In practice, the highest Voc ’s of mc-Si and c-Si solar
cells have been limited to 657 mV5 and 710 mV.6
The most eﬃcient operating point of the solar cell is at the maximum-power-point voltage
Vmpp and maximum-power-point current Impp , as shown in Figure 1.3(b). At this point, solar
energy is converted into electrical energy with an eﬃciency of
η=
Impp Vmpp
,
Pin
(1.2)
where Pin is the incident solar power. Another parameter commonly used is the ﬁll factor
(F F ), which is a measure of the “squareness” of the I − V curve and is deﬁned as
FF =
Impp Vmpp
.
Isc Voc
(1.3)
Recombination in the depletion region will lower a solar cells ﬁll factor, as will a small shunt
resistance or a large series resistance.
The interested reader is referred to the books by Green7, 8 and van Overstraeten and Mertens9
for a more detailed discussion on device physics and operation.
14
1.4
1.4.1
1. Introduction
Commercially Produced Silicon Solar Cells
Screen-Printed Solar Cells
The ﬁrst reference to a commercial screen-printed (SP) solar cell production line can be
found in 1975.10 Since this time the process has remained relatively unchanged. Figure
1.4(a) shows the structure of a screen-printed (SP) solar cell. As the name implies, the
metal contacts in Figure 1.4(a) are formed by screen-printing a metallic paste through a
mask. However, the contacts are the main limitation of the device, which has an eﬃciency
of typically 12 − 13%. This is because of the paste’s poor conductivity, its poor contact
resistance to silicon, the poor aspect ratios achieved, and the inability to reliably produce
thin lines in production. The nett result is that the metal lines are much wider than desirable
(150−200 µm). To still allow a signiﬁcant fraction of the light (92%) to strike the front surface
of the silicon, the ﬁngers are spaced widely (about 3 mm) apart. The electrons are then
required to travel a large lateral distance through the thin emitter region before reaching the
metal contact. For this reason the emitter is heavily doped with n-type (negative) carriers,
aﬀording better lateral conductivity. This, however, gives the cell a very poor response to
high-energy (ultraviolet-blue) light which is absorbed very close to the front surface. An
aluminium (Al) or silver/aluminium (Ag/Al) paste is screen-printed onto the rear of the cell
and ﬁred in a furnace to form a p-type ohmic contact. The aluminium also creates a backsurface-ﬁeld (BSF), which repels electrons that travel towards the rear of the cell instead
of towards the junction. A titanium dioxide (TiO2 ) antireﬂection (AR) coating is deposited
near the end of the process, increasing the amount of light absorbed by the silicon. The total
amount of high-temperature (> 750◦ C) processing involved in fabricating a SP solar cell is
typically about 30 − 45 min. Modules fabricated today with SP cells typically cost about
US$3.50 /Wp .
Figure 1.4: Schematic diagrams of a (a) screen-printed solar cell and a
(b) buried-contact solar cell (adapted from Green 8 ).
1.4 Commercially Produced Silicon Solar Cells
1.4.2
15
Buried-Contact Solar Cells
The performance of commercial silicon solar cells was enhanced greatly through the development of the buried-contact (BC) solar cell at the University of New South Wales in the
mid-1980’s.11, 12 The BC solar cell, shown in Figure 1.4(b), is currently commercially produced in large volumes under license by BP Solar. Conversion eﬃciencies of greater than
20% have been achieved on laboratory scale cells at UNSW,11 and an independently measured production BC solar cell had an eﬃciency of 16.7%.13 The ﬁngers are only 20 µm
wide but are 50 µm deep, the grooves being made either with a laser or a mechanical dicing
saw. The cell is less shaded due to the reduced metal area (about 3%) on the front surface,
and therefore the ﬁngers can be placed closer together, permitting the use of a lightly-doped
emitter and giving the cell an excellent response to blue light. Figure 1.5 provides a visual
comparison of the metal contact areas of a SP and BC solar cell. It can be easily seen that
the ﬁngers of the BC solar cell are much ﬁner and occupy a much smaller fraction of the
solar cell front surface.
Figure 1.5: Scanned images of (a) a Solarex screen-printed solar cell on
mc-Si, and (b) a BP Solar buried-contact solar cell on textured c-Si. The
AR coatings used are titanium dioxide (TiO2 ) and silicon nitride for the
Solarex and BP Solar cell, respectively (not to scale).
The drawback of the BC process is that although there are substantial materials costs savings
relative to SP cells, the processing costs are higher, over 30% of which can be attributed to
the high-temperature processing steps14 (see Section 1.4.3). In making the BC technology
commercially-viable, BP Solar have removed the lengthy high-temperature oxidation step
and have replaced it with an alternative dielectric, namely silicon nitride.15, 16 The silicon
nitride is deposited using a low-pressure chemical vapour deposition (LPCVD) system.16
LPCVD silicon nitride is typically deposited at a temperature of about 700◦ C,17 and is
16
1. Introduction
capable of acting as a phosphorus diﬀusion barrier and metallisation mask as well as an AR
coating.16 While the LPCVD silicon nitride ﬁlm acts in many ways as a drop-in replacement
for the thermally-grown silicon dioxide (SiO2 ) layer, the high deposition temperature means
that the surface passivation beneﬁts normally associated with silicon nitride are not realised
in that process.16, 18
1.4.3
Buried-Contact Solar Cell Fabrication Sequence
Figure 1.6 describes the fabrication sequence for a single sided BC solar cell on a high
quality ﬂoat zone (FZ) c-Si wafer. Four high-temperature (> 750◦ C) steps of varying length
are involved in fabricating a BC solar cell:
i) a deep, high-quality emitter is formed by performing a light (low dose) n-type (phosphorus, P) diﬀusion on the wafers. This creates the collecting junction in the p-type
wafers. While this process is relatively short (15 min), the length of the following
processes are in the order of hours each.
ii) a thick SiO2 layer is grown a) as a diﬀusion barrier to protect the lightly doped emitter
from the heavy groove diﬀusion, b) to bond with atoms of the disrupted silicon lattice
at the surface, thereby improving the surface passivation, c) to facilitate electroless
metal plating of the front contacts, and d) to act as an AR coating, reducing reﬂection
losses from the cell’s front surface.
iii) a heavy phosphorus diﬀusion (close to the solid solubility limit) in the grooves permits
good electrical contact between the silicon and the metal.
iv) an evaporated Al ﬁlm is sintered for a few hours to form an ohmic contact and create
a BSF at the rear.
1.4.4
Simpliﬁed Buried-Contact Solar Cell
The simpliﬁed buried-contact (SBC) solar cell was developed in an attempt to reduce the
number of high-temperature processing steps in the standard BC fabrication sequence.19, 20
In the SBC solar cell a single emitter and groove diﬀusion is performed, as opposed to
the two separate diﬀusions in the standard BC process. This reduces the number of hightemperature steps by one. More importantly, by performing the homogeneous diﬀusion
before the deposition of the AR coating, the limitations on the choice of front-surface dielectric layer are considerably relaxed. The dielectric ﬁlm now only has to act as a metallisation
mask and a good AR coating, and does not need to act as a phosphorus diﬀusion barrier.
This opens the door for low-temperature deposited ﬁlms such as plasma-enhanced chemical
1.4 Commercially Produced Silicon Solar Cells
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wafer.
17
18
1. Introduction
vapour deposited (PECVD) hydrogenated silicon nitride (a-SiN:H) and APCVD-deposited
TiO2 .
Cotter et al. performed PC1D21 modelling and determined that eﬃciencies greater than
16.5% could be achieved for a single emitter and groove diﬀusion with a sheet resistance
greater than 40 Ω/2 as long as the front-surface recombination velocity (SRVf ) was kept
below 20000 cm/s.22 This condition is relatively easy to achieve with SiO2 passivation which
typically results in SRVf ≈ 1000 cm/s.23
A necessary modiﬁcation to the BC process has been the optimisation of the nickel sintering
step. The electrolessly-plated nickel (Ni) ﬁlm is normally sintered at 350◦ C in order to form
a nickel silicide (Ni2 Si) ohmic contact. Additionally, the (Ni2 Si) layer may act as a diﬀusion
barrier to the subsequently deposited copper layer at normal module operating temperatures.
It was found that performing a 350◦ C Ni sintering step with a 45 Ω/2 groove diﬀusion
resulted in low ﬁll factors (< 75%) and a open-circuit voltage (Voc ) that is degraded by up
to 40 mV.24 It has been postulated that the degradation is caused by small-area Schottky
contacts between the metal and the p-type base.24 This is attributable to either the nickel
punching-through the n-doped grooves during sintering or the lack of a diﬀused region in
some areas of the grooves. By reducing the Ni sintering temperature to 250◦ C this problem
has been circumvented.24 This has resulted in the fabrication of 9 cm2 solar cells with a
conversion eﬃciency of 16.9 − 17.1%, with a sheet resistances of 39 − 50 Ω/2 on 1 Ω cm
boron-doped untextured CZ wafers.24 The use of a PECVD a-SiN:H ﬁlm as an AR coating
enabled a respectable short-circuit current density (Jsc ) of up to 34.8 mA/cm2 to be achieved.
These cells did not require any SiO2 passivation layer.25 The disadvantage of the current SBC
process is that it is not nearly as robust as the standard BC fabrication sequence, and requires
further reﬁning before it is able to withstand the rigours of a production environment.
1.5
Multicrystalline Silicon
The cost of the silicon wafers alone represent the largest fraction in the cost breakdown of a
solar cell. The cost of the growing the silicon ingot and cutting it into wafers contributes 46%
towards the ﬁnal PV module cost.26 This is not because silicon is a rare material, it is in fact
the most abundant element on earth, however the puriﬁcation and ingot growth processes
are extremely energy intensive. Additionally, the PV industry is somewhat reliant on silicon
scrap and oﬀ-cuts from the semiconductor industry for their feedstock. The reﬁning processes
to obtain the necessary purity for good electrical performance and cutting the ingots into
350 µm thick wafers costs about US$1.50 /Wp . The price is not strongly linked to economies
of scale, as the PV industry in the past has been able to rely on the much larger semiconductor
industry for technological advances in crystal growth. Additionally. as the semiconductor
industry has moved towards larger and larger wafer sizes - 300 mm is the current standard -
1.5 Multicrystalline Silicon
19
the PV industry has been able to purchase the outdated equipment relatively cheaply.
Only recently, with the strong growth of the PV industry, has dedicated equipment for PV
technologies been developed in order to reduce the cost of silicon wafers. Firstly, there is an
industry trend towards fabricating solar cells on thinner wafers. There are several handling
issues in production to be overcome before high yields can be obtained on 150 µm-thick
wafers. However, the French PV manufacturer, Photowatt, has demonstrated a high yield
with 200 µm thick wafers on an automated production line.27 Secondly, diﬀerent methods of
casting silicon into multicrystalline silicon ingots have been developed.
1.5.1
Issues with Multicrystalline Silicon
Multicrystalline silicon (mc-Si) ingots are now used by many of the world’s leading solar
cell manufacturers, including BP Solar (incorporating Solarex), Kyocera, Eurosolare, and
Photowatt, to name a few. Mc-Si ingots of up to 65 × 65 cm and weighing 230 kg are now
being grown.27 There are several advantages to such an ingot compared to the traditional
100−150 mm round Czochralski (CZ) wafers. Firstly, the cost of mc-Si wafers is on the order
of 15% less than c-Si wafers grown by the CZ process. Secondly, the overall geometrical yield
from a 150 mm diameter c-Si ingot is only about 66%.27 This is because the ingot must be
trimmed into pseudo-square wafers to permit a higher packing density of the solar cells once
they are laminated into modules. If this is not performed then module costs will increase
due to the extra amounts of glass required to fabricate a module with the same power rating.
In comparison, the wafer yield from a 65 × 65 cm mc-Si ingot is about 84%.
There are several disadvantages to using mc-Si wafers. Firstly, electrical properties of the
material quality are slightly poorer. This is due to increased numbers of minority carriers
recombining at the grain boundaries between the crystallites, before they can be collected
by the junction. Secondly, due to the random orientation of the crystallites in mc-Si, the
beneﬁts achieved from the standard alkaline chemical texturing are minimal and a good
AR coating is necessary to prevent large reﬂection losses. Thirdly, all mc-Si material is
diﬀerent, varying from manufacturer to manufacturer and from batch to batch. The material
behaves diﬀerently under high processing temperatures (> 950◦ C) and while the particular
parameters used for lifetime enhancement processes, like hydrogenation and gettering, may
work well for one material they may actually degrade the quality of another material. This
indicates that a robust technology is required that can tolerate these kinds of variations in
a production environment.
For screen-printed solar cells these disadvantages represent only a small loss in performance.
The mc-Si wafers are still chemically textured, even though the beneﬁts are minimal. Screenprinted solar cells fabricated on CZ c-Si substrates maintain a slight performance advantage
over mc-Si wafers, by about 0.5 − 1.0 % absolute eﬃciency. The device eﬃciency is limited,
20
1. Introduction
not by the substrate, but by the screen-print process itself, and therefore can be more costeﬀective on mc-Si wafers. This fact, along with the reduced cost of mc-Si wafers has seen
mc-Si screen-printed solar cells capture a major share of the PV market. Additionally, large
amounts of money are being invested in new facilities that will fabricate tens of megawatts
more of this product. Although investment in renewable energy is applauded, an opportunity
is being passed up by many companies to invest in more eﬃcient technologies that make
better use of the silicon substrate. The upper eﬃciency limit for a mc-Si solar cell has been
demonstrated at UNSW. A laboratory scale (1×1 cm) passivated-emitter rear-locally-diﬀused
(PERL) structure fabricated on Eurosil P48 material (Eurosolare S.p.A., Italy) resulted in a
conversion eﬃciency of 19.8%.28 This result indicates that a signiﬁcant improvement margin
exists for the 12 − 13% eﬃcient commercially-produced solar cells if the right technology can
be found. This work investigated the applicability of the BC process to mc-Si wafers, using
low-temperature deposited titanium dioxide thin ﬁlms as the replacement dielectric coating.
1.6
Why use Titanium Dioxide?
There are several motivations for investigating titanium dioxide (TiO2 ) thin ﬁlms in this
work. TiO2 thin ﬁlms are used currently as the PV industry standard AR coating on the
vast majority of screen-printed solar cells. The important implications of this are, ﬁrstly, that
the industry is familiar with the technology and will not be reluctant to adapt to fabrication
processes based around TiO2 and, secondly, that the necessary deposition equipment is
operating today on the factory ﬂoor. Thus, the development of a new silicon solar cell
technology that included TiO2 processing steps could be readily adopted by the PV industry
without the typical long lead-in time for a new technology.
TiO2 exists in nature as the minerals rutile, anatase, and brookite. Titanium dioxide of the
rutile form is a relatively abundant mineral,29 however anatase and brookite are extremely
rare in nature.30 TiO2 thin ﬁlms are generally amorphous for deposition temperatures ≤
350◦ C, above which anatase is formed. The most stable crystalline phase, rutile, is formed at
temperatures greater than about 800◦ C. The brookite phase is rarely observed in deposited
thin ﬁlms. The functional properties of TiO2 ﬁlms, powders and ceramics are strongly
dependent on the phase of the material. For many applications, the size of crystals that are
present also alter the behaviour of the ﬁlm. Typical properties of TiO2 include:
• Electrical: high electrical resistance - resistivities of 1014 Ω cm29 and dielectric constants
of up to 180 are possible for rutile crystals.31
• Mechanical: high durability32 and high hardness.33
• Optical: very high refractive index - up to 2.70 − 2.71 (at a wavelength of 600nm) for
rutile thin ﬁlms33, 34 - and excellent transmittance in the visible region.
1.6 Why use Titanium Dioxide?
21
• Chemical: good chemical resistance and high chemical stability.35, 36
TiO2 powders and thin ﬁlms are used in an extremely wide range of commercial applications
and research areas, including:
• Powders: as a white pigment in paint, plastic, inks, paper, and cosmetics; in washing
powder, toothpaste, sunscreen, foodstuﬀs, pharmaceuticals, photographic plates, for
creating synthetic gemstones; and as a catalyst.
• Thin ﬁlms: for ultra-thin capacitors and MOSFETs due to its extremely high dielectric constant; as humidity and oxygen sensor due to the dependence of its electrical
conductance on the gases present; as an optical coating and a material for waveguides
due to its high refractive index; as a protective coating and corrosion resistant barrier;
and as a photoanode in solar cells due its photoelectric activity.
1.6.1
TiO2 Thin Films in Photovoltaics
The use of TiO2 ﬁlms has already been explored to a certain degree in the ﬁeld of PV.
Antireﬂection Coating
Lord Rayleigh ﬁrst observed the antireﬂection (AR) eﬀect in 1887, and Bauer presented the
ﬁrst theoretical treatment based on interference eﬀects in 1934.37 Since this time the theory
and application of optical coatings have been well developed. The use of an AR coating for
solar cells is a more recent application, beginning in the 1960’s with the advent of PV as a
remote power source in space. For a good theoretical treatment of AR coatings, mainly on
glass substrates, the reader is referred to Heavens,38 and some theory is also presented in
Chapter 7.
Many references to TiO2 thin ﬁlms being experimented with as a solar cell AR coating
appeared in the early 1970’s.39–44 These early experiments were aimed at increasing the
eﬃciency of space cells, which commonly employed silicon monoxide (SiO) AR coatings. At
a wavelength of 600 nm, the refractive index of silicon is 3.94, glass about 1.52, SiO about
1.9, and TiOx in the range 1.9−2.4 (it is diﬃcult to know the exact stoichiometry of titanium
dioxide thin ﬁlms that are deposited via evaporation or sputtering, and therefore these ﬁlms
are denoted as TiOx ). As discussed in Section 7.2.1, an AR coating with a refractive index of
about 2.45 is optimal for achieving minimal reﬂection losses for a glass-encapsulated silicon
solar cell. Thus, the use of TiOx ﬁlms became widespread in the PV industry for providing
better optical coupling of light into the silicon. Although TiOx ﬁlms are more absorbing to
short wavelength light, this is of little importance for SP solar cells, which already exhibit
a poor blue response due to the phosphorus ”dead-layer” at the front surface.8 It should be
22
1. Introduction
noted that the ethyl-vinyl-acetate (EVA) ﬁlms used to encapsulate that silicon solar cells
and the glass cover plates also exhibit signiﬁcant absorption of short-wavelength light.45
Since the early 1970s, TiOx has been the main AR coating employed by the PV industry.
Nearly all SP solar cell production lines use an APCVD- or spray-deposited TiO2 AR coating.16, 19 Kern and Tracy provide an extensive review of early AR coatings for silicon solar
cells, focussing on TiO2 .46 Several improvements have been made to the process, including optimisation for the ﬁring-through of screen-printed contacts47, 48 and for deposition on
textured surfaces,49 and since 1994 TiO2 has been investigated for application in BC solar
cells.16, 19, 20, 22, 24, 35, 50 TiO2 thin ﬁlms are also employed as AR coatings on glass, transmitting
visible light while reﬂecting heat-producing IR radiation.51
Surface Passivation
Surface passivation is an extremely important design consideration for high-eﬃciency silicon
solar cells, especially at the front surface where the majority of the light is collected. The
predominant recombination losses in c-Si are via defect levels within the bandgap and the
large number of non-saturated Si bonds at the surfaces dominate those defect levels. In order
to reduce these recombination losses and achieve high conversion eﬃciency the surfaces must
be electronically passivated, and, in the case of solar cells, the passivation scheme should be
stable under ultraviolet (UV) illumination for at least 20 years.52 Two common and wellcharacterized methods for silicon surface passivation are thermal oxidation at temperatures
of about 1000◦ C to grow SiO2 and plasma-enhanced chemical vapour deposition (PECVD)
of silicon nitride. Methods of achieving surface passivation with TiO2 thin ﬁlms will be
discussed in detail in Chapter 5.
Metallisation Mask
TiO2 (anatase) ﬁlms have been used as a dielectric mask for preventing electroless nickel and
copper plating from occurring on the front surface of solar cells. TiO2 thin ﬁlms of about
70 nm in thickness have successfully replaced the 350 nm thick SiO2 employed in the BC solar
cell fabrication sequence for use as a metallisation mask.20 The necessary ﬁlm properties for
this application include chemical resistance and a dense, continuous ﬁlm with no pinholes.
TiO2 thin ﬁlms deposited by spray-deposition at temperatures above 400◦ C have satisﬁed
these criteria.20 Research performed at the University of Konstanz (Germany) has shown
that the use of PECVD deposited a-SiN:H thin ﬁlms as a metallisation mask is problematic.53
As mentioned previously, BP Solar employ an LPCVD silicon nitride coating on their BC
solar cell production line. Films deposited by LPCVD are typically very dense and are well
suited to acting as a metallisation mask. The primary diﬀerence between silicon nitride
ﬁlms deposited by PECVD and LPCVD is that the former can contain as much as 30 at. %
1.6 Why use Titanium Dioxide?
23
hydrogen. This, along with the signiﬁcantly lower deposition temperature of PECVD ﬁlms,
results in the PECVD ﬁlm having a signiﬁcantly lower density.17
Ohmic Contacts
Thin ﬁlms of TiO2 have been used to create ohmic contacts to p-type mc-Si solar cells.54
The TiO2 layers were deposited in between layers of aluminium screen-print paste and ptype silicon. The screen-print paste was then ﬁred (850◦ C for 5 − 30 min), which allowed the
aluminium to interdiﬀuse with the silicon, creating a titanium silicide (Tix Siy ) ﬁlm in the
process. With the above ﬁring conditions, the Tix Siy ﬁlms yielded a low contact resistivity
of 1 − 13 × 10−5 Ω cm2 .
MIS Solar Cells
Metal-insulator-semiconductor (MIS) solar cells rely on quantum mechanical tunnelling
through a very thin oxide layer, less than 2 nm thick, for carrier transport.7 This is made
possible by the extreme work functions of the metal. The top contact can be a thin metallic
layer (< 10 nm), which is essentially transparent to light. As this layer will have a high
resistivity a thicker contact grid is required to transport the current. Alternatively, very
ﬁne (5 − 10 µm) and closely spaced (50 − 100 µm) metallic ﬁngers can be used. In this case,
carriers that are generated between the ﬁngers can be collected by a nearby grating line
before recombining.
The latter MIS grid structure was investigated using a 2 nm thick SiO2 layer, followed by a
100 nm thick TiOx layer.55 The TiOx was intended to act as an inversion layer, inducing a
layer of minority carrier near the front surface. Although this apparently improved eﬃciencies, it was later found that the TiOx was acting as an accumulation layer on p-type silicon.56
This resulted in further research being performed with tantalum pentoxide, although TiOx
would be suitable for n-type solar cells.
Transparent Conducting Oxide Layers
In the PV industry, transparent conducting oxide (TCO) layers are most commonly used
in thin-ﬁlm solar cells where low current densities are present. The most common TCO is
indium-tin-oxide (ITO). However, more recently TCO layers have been employed for silicon
wafer based heterojunction solar cells from Sanyo.57 These cells have a p- and n-type amorphous silicon ﬁlms deposited onto the front and rear of the n-type silicon substrate. A TCO
layer is employed to reduce the front contact area required. There is always an electrical
vs. optical trade-oﬀ with TCO layers, and even though a relatively high short circuit current density is achieved (Jsc = 36.7 mA/cm2 ) the spectral response curve exhibits signiﬁcant
24
1. Introduction
absorption at wavelengths less than 700 nm.57
The TCO layers typically have a sheet resistance of about 10 − 50 Ω/2. Niobium-doped
TiO2 ﬁlms with this sheet resistance and a refractive index of 2.2 − 2.5 have been deposited
as TCO layers.58 However, a ﬁlm thickness of greater than 1.5 µm was required to achieve
the 50 Ω/2 sheet resistance. With thinner ﬁlms a logarithmic dependence of the resistivity
on the ﬁlm thickness was observed. Therefore, these ﬁlms are too thick to act as an eﬀective
AR coating. As discussed in Sections 2.4.2 and 2.4.4, oxygen deﬁcient TiO2−x thin ﬁlms can
have resistivities down to 102 − 10−3 Ω/2, indicating their potential as a TCO.
TiO2 /c-Si Heterojunction Solar Cells
Investigations have been performed on the addition of indium (In) to TiO2 to form p-TiO2 /nSi heterojunction solar cells.59 The TiO2 ﬁlms were deposited by a ”spray-CVD” process.60
The In-doped ﬁlms lead to an eﬃciency increase of about 40% over pure TiO2 ﬁlms, resulting
in an eﬃciency of 14.1% under 100 mW/cm2 (AM1) illumination. The open circuit voltage
approached 650 mV and a high ﬁll factor of 0.82 was achieved.
Gordon deposited doped TiO2 ﬁlm as an electrode underneath a ﬂuorine-doped tin oxide
(F:SnO2 ) layer to form a F:SnO2 /TiO2 /p-Si heterojunction solar cell.61 The patent deals
mainly with niobium-doped TiO2 , however other possible dopants that are also discussed
include tantalum, tungsten, phosphorus, arsenic, antimony and vanadium. The function
of the TiO2 layer is to, ﬁrstly, overcome the “interfacial resistance” observed in SnO2 /p-Si
heterojunction cells, and, secondly, to act as an intermediate AR coating between the SnO2
and Si.61 The SnO2 has a refractive index of about 1.85 at 600 nm.62 It is noted that the
TiO2 does not exhibit suﬃciently low resistance to act as a TCO. In both solar cells described
here the TiO2 layer was about 100 nm thick.
The dye-sensitized solar cells described below are also heterojunction solar cells, however as
these involve a liquid p-type electrolyte they will discussed in a separate section.
Dye-sensitized TiO2 Solar Cells
In 1991 a very diﬀerent solar cell concept was presented based on dye-sensitized nanocrystalline TiO2 thin ﬁlms and an iodine/iodide electrolyte.63 The sample structure is shown
in Figure 1.7. The TiO2 is n-type while the dye is p-type. The cell works by conversion of
photons to electrons by the dye and the subsequent transfer of electrons to the glass electrode
by the TiO2 layer. The device had an eﬃciency of 7.1% under full sunlight, which increased
to 12% under diﬀuse lighting. This solar cell has a large potential market due to drastically reduced fabrication costs and conversion eﬃciencies that are comparable to amorphous
silicon solar cells.64 The dye, in this case ruthenium based, is used to photosensitize the
1.7 Thesis Overview and Goal
25
TiO2 ﬁlm. A highly porous TiO2 layer with a large surface area to volume ratio is used to
increase the amount of adsorbed dye. This increases the absorption properties of the device
in the visible spectral region. Spray-deposition techniques have been used for depositing
the nanocrystalline TiO2 ﬁlms65 and the highly porous, CVD-deposited ﬁlms presented in
Chapter 7 could potentially be used for this type of solar cell.
Figure 1.7: Sample structure of a photoelectrochemical cell using
nanocrystalline TiO2 . The arrows indicate the direction of light (from
Li et al.64 ).
1.7
Thesis Overview and Goal
The aim of many silicon-wafer based PV research groups worldwide is to develop a new,
commercially-viable, fabrication technology suitable for mc-Si wafers, in order to bridge the
gap between the “ﬁve-year plan” for thin ﬁlm dominance of the PV marketplace and the
stock-standard product of the last 20 years, screen-printed solar cells.
The objective of this thesis project is to develop, understand and evaluate novel applications of TiO2 thin ﬁlms to silicon solar cells. TiO2 is identiﬁed as an unique material with
signiﬁcant potential owing to its excellent optical and electrical properties. TiO2 thin ﬁlms
also appeared to be an attractive option due to the possibility of depositing them at a lowcost. While amorphous TiO2 thin ﬁlms have a long history of being used as an AR coating
on screen-printed solar cells, very few examples of the application of polycrystalline TiO2
(especially anatase) thin ﬁlms to photovoltaics can be found in the literature.
Anatase TiO2 thin ﬁlms exhibit a high refractive index and low absorption coeﬃcient.
This, along with their insulating properties and excellent chemical resistance, suggested
26
1. Introduction
that anatase thin ﬁlms could be used as a direct replacement for the thermally-grown SiO2
layer in the standard BC technology. The replacement of the SiO2 layer with TiO2 promised
much lower thermal budgets, simpliﬁed fabrication sequences and reduced processing costs.
The lack of literature regarding the behaviour and stability of TiO2 thin ﬁlms under hightemperatures and diﬀerent gas ambients meant that a signiﬁcant amount of time was spent
increasing this knowledge base.
In order for TiO2 to successfully replace SiO2 in the standard BC process, several key parameters have to be explored. Firstly, it needs to be demonstrated that TiO2 thin ﬁlms do
not reduce the minority carrier lifetime of silicon wafers when processed at temperatures up
to 1000◦ C. Secondly, TiO2 is known to be a poor option for passivating the surfaces of a
silicon wafer, so therefore a successful method for enhancing the surface passivation of TiO2
coated silicon wafers needs to be developed in order to achieve high eﬃciencies. Fourthly,
one crucial role of the SiO2 layer in the BC process is to act as a phosphorus diﬀusion barrier.
The performance of a TiO2 thin ﬁlm in this role needs to be evaluated. Fifthly, an additional
reduction of thermal budget is envisaged by combining the emitter diﬀusion and AR coating
steps. In this manner, a TiO2 ﬁlm doped with phosphorus atoms would be deposited onto
the p-type wafer and, during a subsequent ﬁring process, the phosphorus atoms diﬀuse out
of the TiO2 and form an n-type emitter. Finally, as well as acting as a chemically resistant
layer and an electroless metal plating mask, the optical performance of TiO2 AR coatings
need to be optimised.
The primary goal is to develop a 16− 17% eﬃcient BC solar cell on planar mc-Si wafers. The
application of the BC technology to textured c-Si wafers has been demonstrated by industry,
however the current BC technology cannot be economically applied to mc-Si wafers due to
the high-processing costs. Therefore, a simpliﬁed process, centred around using TiO2 as the
thin dielectric ﬁlm is sought. If TiO2 ﬁlms prove to be successful then one expected outcome
would be the evolution of a new solar cell technology that is readily applicable to today’s
PV industry.
Following this introduction, Chapter 2 presents an extensive literature review, necessary
to understand the physical, optical, electrical and chemical properties of TiO2 . A review
of thin ﬁlm deposition techniques and a description of two deposition systems designed
and constructed by the author are described in detail in Chapter 3. Due to the novel
ways that TiO2 thin ﬁlms were being implemented into solar cell processing sequences,
extensive ﬁlm characterisation was necessary to determine the variation of ﬁlm properties
with process conditions (Chapter 4). A novel method of overcoming the limited surface
passivation achievable with TiO2 coated silicon surfaces is discussed in Chapter 5. Issues
such as ﬁlm stability at high temperatures and contamination are also addressed in this
chapter. Novel PV applications of TiO2 , such as its ability to act as a phosphorus diﬀusion
barrier and phosphorus dopant source, are investigated in Chapter 6. A high-performance,
commercially viable, double-layer antireﬂection (DLAR) coating, based on two or more TiO2
1.7 Thesis Overview and Goal
27
ﬁlms with diﬀering refractive indices, is demonstrated in Chapter 7. Modelling results of the
performance of this DLAR coating on a planar BC solar cell will also be presented. Finally,
Chapter 8 summarises the work, and presents opportunities for further research in the area.
28
1. Introduction
Chapter 2
Common Properties of TiO2 Thin
Films
The physical, optical, electrical and chemical properties of titanium dioxide (TiO2 ) depend
greatly on the amorphous or crystalline phase of the material. TiO2 is a complex material
with three crystalline phases, two of which are commonly observed in thin ﬁlms - anatase
and rutile. Anatase is commonly observed at ﬁlm deposition temperatures of 350 − 700◦ C,
while higher temperatures promote the growth of rutile. Deposition temperatures lower than
300◦ C generally result in the formation of amorphous TiO2 .
Amorphous TiO2 is a highest bandgap material (about 3.5 eV), and exhibits a low refractive
index (about 1.9 − 2.0 for 600 nm wavelength light) and extinction coeﬃcient. The chemical
resistance of amorphous TiO2 ﬁlms is poor in many acidic and basic solutions. Polycrystalline anatase thin ﬁlms, with an optical bandgap of about 3.2 eV, exhibit a much higher refractive index (as high as 2.532 at 600 nm for single crystal material) and a slightly increased
absorption coeﬃcient. With the crystalline structure comes increased chemical resistance,
and dense anatase ﬁlms are insoluble in many acids and bases. Rutile thin ﬁlms (3.05 eV
bandgap) have extremely high refractive indices (up to 2.70 at 600 nm for single crystal rutile)
and below the bandgap absorption is still low. The chemical resistance of rutile is excellent,
and after annealing at temperatures above 1000◦ C it is insoluble in nearly all acids and bases.
2.1
Introduction
The aim of this work was to evaluate the performance of titanium dioxide (TiO2 ) as a drop-in
replacement for the thick, thermally grown silicon dioxide (SiO2 ) layer in the buried-contact
solar cell. The use of TiO2 immediately obviates one of the high-temperature processing
steps, required to grow the SiO2 layer. TiO2 was chosen due to the ability of depositing ﬁlms
at low temperatures and at atmospheric pressure; the non-toxicity of the liquid precursor;
29
30
2. Common Properties of TiO2 Thin Films
the familiarity of solar cell manufacturers with this ﬁlm; and, their existing ownership of
the necessary deposition equipment. It was anticipated that these factors would facilitate
an easy transfer of a successful laboratory device into a commercial environment. However,
TiO2 is a relatively complex material, and three crystalline phases as well as the amorphous
form of TiO2 exist. Since each of the these materials has diﬀerent optical, electrical, and
chemical properties it was necessary to perform an extensive literature review in order to
predict how the TiO2 ﬁlms would behave in diﬀerent processing conditions.
This chapter will describe the properties of the crystalline phases most commonly observed
in thin ﬁlms, that of rutile and anatase, and amorphous TiO2 . The third crystalline phase,
brookite, is a less stable and common form of TiO2 is rarely observed in deposited thin ﬁlms
and will not be discussed here. There are many diﬀerent parameters that aﬀect the phase
of a deposited TiO2 thin ﬁlm. Some of these parameters are deposition method, deposition
temperature, annealing temperature, deposition rate, deposition pressure, precursor type,
reaction atmosphere, impurities present, and substrate type. The resulting phase or mixture
of phases, plays a large role in determining the physical, optical, chemical, and electrical
properties of the ﬁlm.
This work relies greatly on the excellent optical properties of TiO2 thin ﬁlms, as well as its
chemical resistance and insulating properties. A summary of the physical, optical, electrical
and chemical properties reported in the literature, with an emphasis on those relevant to
solar cell fabrication, is presented in the following sections of this chapter.
2.2
2.2.1
Physical Properties
The Amorphous − Anatase − Rutile Phase
Transformations
Amorphous TiO2 thin ﬁlms can be deposited at temperatures as low as 100 − 150◦ C.66, 67
Amorphous TiO2 does not have a strict crystallographic structure, often incorporates voids
within the material, and has a relatively low density. For TiO2 thin ﬁlms formed by chemical
reaction, the lowest temperature crystalline phase of TiO2 that can be obtained is anatase.
To obtain polycrystalline anatase, the ﬁlm can be either deposited as amorphous TiO2 and
then crystallised by annealing at a higher temperature, or deposited as polycrystalline material directly. Nearly all published results indicate that the transition from an amorphous
to anatase ﬁlm occurs at about 300 − 365◦ C, regardless of whether this is the deposition or
annealing temperature. Rutile ﬁlms are initially observed on silicon substrates at deposition
temperatures above 700◦ C, and more typically from 900 − 1100◦ C. It should be noted that
anatase is a metastable phase of TiO2 , and the conversion to rutile involves a collapse of the
anatase structure, which is irreversible.68, 69 Figure 2.1 indicates the structure of an anatase
2.2 Physical Properties
31
and rutile crystal. Although rutile and anatase are both of tetragonal crystallographic structure, rutile is more densely packed and thus possesses a greater density.
Figure 2.1: Models showing the tetragonal structure of both anatase and
rutile, and the denser structure of the rutile phase of the latter (adapted
from Du Pont, Inc.70 ).
The TiO2 thin ﬁlms deposited in this work are formed by chemical reaction, using chemical
vapour deposition (CVD), and spray pyrolysis and hydrolysis systems. In this scenario, the
substrate temperature is the primary means of controlling the deposited phase of the material. In contrast, physical vapour deposition (PVD) systems, such as evaporation, sputtering,
and ion-beam deposition, the resulting phase and ﬁlm structure is determined primarily by
the kinetic energy of the impinging atoms. Therefore, the progression through the amorphous, anatase, and rutile phases may not necessarily be expected. This is conﬁrmed by the
occurrence of rutile ﬁlms at low deposition temperatures (< 450◦ C) by carefully optimised
deposition methods,34, 71 ion-assisted deposition,72 and reactive evaporation.73 However, the
bulk of the discussion here pertains to TiO2 ﬁlms formed by a chemical reaction, and where
the substrate temperature dominates ﬁlm growth characteristics.
Several researchers73–75 observed that the processing temperatures required to convert an
anatase ﬁlm into a rutile one are much higher than temperature required to deposit a rutile
ﬁlm directly. Agreement with this observation can be found in the literature, where the
rutile phase is only present at low temperatures when it is deposited directly at that temperature.34, 71, 73, 74, 76–78 The formation of rutile at lower temperatures is facilitated by the
kinetic energy possessed by the TiO2 molecules during the deposition process, enabling the
lowest energy state to be reached on the substrate. In contrast to the above observation,
Fitzgibbons had previously claimed that the variation in physical and chemical properties
of the ﬁlms is determined solely by the maximum processing temperature, whether this be
the deposition temperature or a subsequent annealing temperature.67 Amores et al. have
published an excellent diagram indicating how the high-temperature sintering process and
transformation of anatase to rutile crystals proceeds, shown here in Figure 2.2.79 The pro-
32
2. Common Properties of TiO2 Thin Films
posed mechanism for the sintering and transformation of anatase into rutile involves several
steps. Initially, the smallest particles (a) coalesce, forming bigger particles (b). The fraction of particles that are already large have been shown not to undergo sintering. The heat
evolved from the exothermic sintering process causes the local nucleation of the rutile phase
(c). Finally, as the conversion to rutile is also an exothermic process, this results in the
transformation of the whole particle to rutile (d).
Figure 2.2: Proposed mechanism for the sintering and transformation of
anatase into rutile. The smallest particles (a) coalesce, forming bigger
particles (b). The fraction of particles that are already large have been
shown not to undergo sintering. Heat evolved from the exothermic sintering process causes the local nucleation of rutile (c). The conversion to
rutile is also an exothermic process, leading to the transformation of the
whole particle to rutile (d) (adapted from Amores et al.79 ).
2.2.2
The Eﬀect of Impurities on the Anatase − Rutile Phase
Transformation
Many researchers have observed that the inclusion of a certain amount of impurities into
TiO2 can drastically alter the physical properties of the ﬁlm. It has been shown that silicon
and phosphorus inhibit the transformation from anatase to rutile, with 100% anatase phase
being retained at temperatures as high as 870◦ C for up to 3 hr for thin ﬁlms80 and 1500 K for
bulk samples.81 The retardation of the anatase-rutile transformation can be achieved with
2−
3+ 80
the following impurities: PO3−
AlPO4 ;68 SiO2 ;68, 81 Co3 O4 and MoO3 ;79
4 , SO4 and Al ;
Ce and Nb;82 K+ ;83 WO3 ;84, 85 Na2 O;85 and P2 O5 .81, 86 Conversely, it is well known that
other impurities enhance the formation of rutile at lower temperatures. These impurities
include CuO2 ;79, 85 V2 O5 ;79, 87 and NiO, CoO, MnO2 , Fe2 O3 .85
Most researchers agree that oxygen vacancies are responsible for the overall transformation
mechanism.68, 69, 88 Thus, the oxides and ﬂuorides (such as Li+1 , Co+2 and Mn+4 ) that assist
2.2 Physical Properties
33
the transformation can substitute for Ti+4 in the anatase lattice, resulting in the creation of
−2
oxygen vacancies. On the other hand, the inhibiting eﬀect of other impurities (PO−3
4 , SO4 ,
Nb2 O5 ) has been explained by the reduction of oxygen vacancies due to the substitution of
Nb+5 and S+6 into the anatase lattice. Oxygen vacancies are also known to be created in
hydrogen ambients, thereby favouring the transformation to rutile.69, 85, 88
It should be noted that during the growth of TiO2 thin ﬁlms, contamination from the various
chemical precursors can result. Titanium alkoxides are common TiO2 precursors, with the
most frequently used being titanium isopropoxide (also called tetraisopropyl titanate, TPT).
The residue of the organic binders results in carbon contamination of typically a few at.%,
but as high as 13 at.%, being observed.64, 89–99 It is likely that carbon incorporation could be
higher at low growth temperatures, as when higher temperatures were used the carbonate
species decomposed, resulting in the removal of hydrocarbon fragments.64, 94, 99, 100 Titanium
tetrachloride (TiCl4 ) is another common TiO2 precursor, and this results in chlorine contamination of the deposited ﬁlm.91, 93, 101
2.2.3
Substrate Type
The eﬀect of substrate type upon the deposited TiO2 ﬁlm will be discussed only brieﬂy, as
nearly all experiments performed in this work employ silicon substrates. Several researchers
have studied the properties of TiO2 ﬁlms deposited onto various substrate types, including
silicon, silica, quartz, alumina, titanium, copper, gallium arsenide, stainless steel, as well
as several types of glass. The diﬀerent substrate types inﬂuence the physical properties of
the deposited ﬁlm, including the phase, texture or surface roughness. Optical and chemical
properties also change, however these are primarily dependent on the phase and density of
the polycrystalline TiO2 ﬁlm. Possible reasons for the dependence of the TiO2 phase and
properties on the substrate include the substrate’s surface conditions aﬀecting the orientation
and packing density of the molecules, and, with glass, the diﬀusion of metal ions into the
ﬁlm.102
Battiston et al. observed that anatase was the only phase present with ﬁlms deposited
onto titanium and stainless steel substrates and annealed at 750◦ C.103 In the same work, an
anatase-rutile mixed-phase was observed on barium ﬂuosilicate glass substrates at 1100◦ C
and alumina substrates at 900◦ C. At 1100◦ C a single rutile phase was detected on the alumina. This is in agreement with other results, where either single-phase rutile or epitaxiallygrown rutile ﬁlms have been achieved upon alumina substrates.94, 104–107
Film depositions on glass substrates are typically limited to anatase, or anatase-rutile mixed
phases due to the low melting point (typically ≤ 650◦ C) of most glasses. However, results
of depositions on quartz indicate a preference for the formation of anatase, even at temperatures greater than 1000◦ C.67, 108, 109 This is also true for glasses containing sodium, where
34
2. Common Properties of TiO2 Thin Films
it is possible for the Na+ ion to out-diﬀuse from the substrate and retard the formation
of rutile.85, 102, 107, 110 In contrast, aluminosilicate glasses such as Corning 7059 are known
to favour the formation of the rutile phase due to Al2 O3 impurity.107, 110 Out-diﬀusion of
substrate elements has also been observed by Yuan and Tsujikawa, where up to 30% copper
concentration was found in a TiO2 ﬁlm deposited onto copper sheet and ﬁred at 800◦ C.111
2.2.4
Film Defects
It important that the deposited ﬁlms are relatively uniform in thickness, and do not exhibit
pinholes. Several works have reported pinholes or similar defects in TiO2 ﬁlms. Using
scanning electron microscopy (SEM), Fitzgibbons observed an occasional pinhole at 20000×
magniﬁcation in ﬁlms deposited by CVD at 150◦ C, however no pinholes were observed after
annealing (300 − 1000◦ C).67 Additionally, it was found that tensile stress caused the ﬁlms to
crack when the ﬁlm thickness reached 400 − 500 nm. Nishide and Mizukami experimented
with spin-coating of TiO2 ﬁlms and found that some ﬁlms exhibited 1 µm diameter pores
when ﬁred at temperatures below 500◦ C, however these defects disappeared when the ﬁring
temperature was increased to 550◦ C.112 This is in accordance with other results indicating
that TiO2 ﬁlms deposited at lower temperatures are generally more heavily defected.113
The existence of craters was noticed by Szlufcik et al. when screen-printing a titanium
organo-metallic based ink, however the problem was alleviated with the addition on butanol
to the ink.114 Kern and Tracy commented that the existence of micro-pinholes and TiO2
particulates observed in pneumatically sprayed TiO2 antireﬂection (AR) coating did not impair solar cell performance.46 More recently, Golego et al. observed that some spray droplets
can react on their way to the substrate, form a particulate and then become incorporated
into the ﬁlm.115 Spray depositions performed at very low temperatures (90◦ C) resulted in
the liquid layer cracking upon drying. Again, as the deposition temperature was increased
slightly (to 120◦ C) these defects disappeared. Kurtz and Gordon noted that by maintaining
a large temperature diﬀerential between the substrate and the deposition equipment, avoids
TiO2 particulates from forming on the substrate.116
2.2.5
Film Density
While both rutile and anatase possess a tetragonal crystallographic structure, rutile is
more densely packed and thus possesses a greater density (4.26 g/cm3 ) than anatase (ρ =
3.84 g/cm3 ).70 For TiO2 thin ﬁlms the highest observed density published to date is the
range 4.09 − 4.10 g/cm3 for a rutile ﬁlm.33, 117 Amorphous TiO2 ﬁlms exhibit a wide range
of densities, from 2.4 g/cm3 for porous ﬁlms118 to more typical values of 3.2 − 3.65 g/cm3 ,119
while ﬁlms deposited with a high kinetic energy have achieved densities in the range
3.6 − 3.8 g/cm3 .33 It has been noted that the TiO2 ﬁlms with a lower density can favour
2.2 Physical Properties
35
impurity diﬀusion.120
It is widely accepted that there is a linear relationship between density and refractive index
of a TiO2 thin ﬁlm.33, 67, 121–125 The linear variation of the refractive index (measured at
550 nm) with density is shown in Figure 2.3 for ﬁlms deposited by ﬁve diﬀerent techniques.33
The equation of the line in Figure 2.3 is
nf = 0.42751 ρ + 0.91933 .
(2.1)
This can be more usefully expressed for this work as
ρ=
nf − 0.91933
,
0.42751
(2.2)
where ρ and nf are the TiO2 ﬁlm density (in g/cm3 ) and refractive index, respectively.
Figure 2.3: Experimental data from several researchers indicating that
a linear correlation between TiO2 ﬁlm density and refractive index is
observed over for a wide range of values. Previously published data from
Ottermann and Bange,121 Fitzgibbons et al.,67 Bendavid et al.,33 Hass 34
and Ribarsky 126 was used. (adapted from Bendavid et al.33 ).
Additionally, the porosity of the ﬁlm can be determined using the following relation127
Porosity = 1 −
n2f − 1
,
n2b − 1
(2.3)
where nb is the refractive index of the bulk single crystal material. It should be emphasised
that this value is an approximation due to the fact that, ﬁrstly, both anatase and rutile
crystals exhibit strong birefringence and, secondly, that mixed anatase/rutile phases can
exist. The values of mean refractive indices used in this thesis are 2.70 for rutile126 and
36
2. Common Properties of TiO2 Thin Films
2.532 for anatase,34 both measured at λ = 600 nm. Several researchers have noted that TiO2
ﬁlms (anatase) derived from sol-gels tend to be highly porous in nature, sometimes up to
49%.128, 129
When TiO2 ﬁlms are annealed at a temperature higher than their deposition temperature,
particle sintering and crystallisation contribute to an increase in ﬁlm density and refractive
index, and, accordingly, a reduction in ﬁlm thickness (see Wong et al. for example130 ).
Generally, the type of ﬁlm structure can be estimated before the deposition takes place,
based on the Movchan-Demchishin structure zone model (SZM).72 Guenther expanded the
SZM (see Figure 2.4) to include high density vitreous phases that can be achieved using
PVD techniques.72 However, for ﬁlms deposited via CVD or spray deposition, the structure
of the ﬁlm is predicted by dividing the substrate temperature (Tsub ) during TiO2 deposition
by its melting point, which for TiO2 is Tmelt = 1832◦ C.131
columnar
porous
dense
0.3
dense
polycryst.
0.4
vitreous
amorphous
1.0
Normalised substrate temperature (Tsub / Tmelt)
Figure 2.4: Structure zone model, expanded to include vitreous phases
observed with TiO2 (adapted from Guenther 72 ).
2.2.6
Non-Stoichiometric TiO2−x Thin Films
Titanium dioxide thin ﬁlms exhibit a bluish, purplish or greyish hue once they are reduced
(become poor in oxygen) to TiO2−x or Tiy Ox .29, 73, 88, 132–136 Oxygen deﬁciency can occur
as a result of the deposition conditions. This is a common problem with evaporated thin
ﬁlms, where the choice of source material (e.g., Ti, TiO, TiO2 or Ti3 O5 ) and oxygen partial
pressure of the system are critical. The formation of TiOx can also result from annealing
TiO2 in a vacuum or hydrogen (reducing) ambient. In this case, oxygen is lost from the
lattice to the furnace ambient. The optical and electrical properties of TiO2−x thin ﬁlms will
be presented in Sections 2.3.4 and 2.4.2, respectively. Non-stoichiometric thin ﬁlms observed
in this thesis will be discussed in Section 5.2.2.
2.3 Optical Properties
2.3
2.3.1
37
Optical Properties
Refractive Index, Extinction Coeﬃcient and
Scattering
The refractive index n of a material is primarily determined by the polarizability of the
valence electrons.132, 137 Increased shielding of the positive nucleus results from elements with
higher atomic weight, and this increases the polarizability of the electrons and consequently
the refractive index. Silicon and germanium are good examples with refractive indices in
the infrared spectrum of 3.4 and 4.0. respectively. In compounds, the type of bonding
also aﬀects the refractive index, with highly covalently bonded compounds yielding higher
refractive indices than predominantly ionically bonded compounds.137
The extinction coeﬃcient k plays an attenuating role in the material. When the attenuation
is solely due to absorption, it is termed the absorption coeﬃcient
α=
4πk
.
λ
(2.4)
Figure 2.5 displays a theoretical transmission spectrum for an optical thin ﬁlm. It can be
seen that a region of high transmittance (region II) is located between the region of shortwavelength fundamental absorption (region I) and the long wavelength limit (region III). The
region of fundamental absorption is dependent on the electronic structure of the material,
while absorption in the long-wavelength region is due to lattice vibrations and/or free carrier
absorption. The transmission of region II is strongly linked to the stoichiometry and purity
of the thin ﬁlm.
The extinction coeﬃcient of a ﬁlm can also be increased by the scattering of light by surface
and volume imperfections, such as surface roughness, porous microstructure, and density
ﬂuctuations due to crystallinity, and is thus dependent on the deposition method.132, 137 The
term optical loss L is deﬁned as being
L=A+S
(2.5)
1=R+T +L ,
(2.6)
with
where A is absorptance, S is the scattered component, R is reﬂectance and T is transmittance. Wang and Chao noted that an increase in the extinction coeﬃcient of amorphous
TiO2 thin ﬁlms deposited by sputtering upon annealing.138 The drastic increase observed in
k was attributed, ﬁrstly, to the formation of the anatase phase, and, secondly, to an increase
in the surface roughness due to the polycrystalline nature of the annealed ﬁlm. The scattering loss of a rough surface is related to the root-mean-squared (RMS) surface roughness σ
38
2. Common Properties of TiO2 Thin Films
band-band
absorption
transparent
region
impurity
absorption
I
II
lattice vibration
absorption
drop caused by
free carrier
absorption
III
Figure 2.5: Schematic of a theoretical transmission curve for an optical
thin ﬁlm. Three regions of absorption are shown, and each region has a
diﬀerent mechanism for optical absorption, as explained in the text (from
Pulker 137 ).
by139
TIS =
4πσ 2
,
λ2
(2.7)
where TIS is the total integrated scattering. Wang and Chao proposed that the extinction
coeﬃcient k could be divided into two components138
k = αa + αs ,
(2.8)
where αa is the absorption coeﬃcient (as deﬁned in Equation 2.4) and αs is termed a scattering coeﬃcient. Figure 2.6 shows how the two contributions to the extinction coeﬃcient, αa
and αs , vary as a function of annealing temperature in an oxygen ambient.138 It is observed
that αa decreases with temperature, due to the reduction of oxygen vacancies in the ﬁlm,
while αs increases as surface roughness of the ﬁlm becomes greater with the formation polycrystalline anatase. If this evaluation was performed, say, at λ = 400 nm (near the bandgap)
αa would increase at about 350◦ C due to lower bandgap of anatase (about 3.4 eV). It should
be emphasised that Figure 2.6 was derived empirically for sputtered TiO2 ﬁlms, and that
wrinkling, cracking, or peeling of the ﬁlm at temperatures above 300◦ C may not occur for
alternative deposition methods. For stoichiometric ﬁlms deposited by spray deposition or
CVD, αa would not decrease at low annealing temperatures as there are no oxygen vacancies.
At a temperature of about 800◦ C, αa would increase again due to the increase rutile fraction
in the ﬁlm (the bandgap of rutile is about 0.2 eV lower than that of anatase). The scattering
coeﬃcient αs would most likely level oﬀ with the increased rutile fraction, as the grain size
2.3 Optical Properties
39
for anatase thin ﬁlms is typically an order of magnitude greater than that of rutile ﬁlms
(e.g., 20 nm versus 200 nm, respectively).119
Figure 2.6: Qualitative illustration indicating the behaviour of the extinction coeﬃcient k, and its absorption component αa and its scattering
component αs with increased annealing temperature in an oxygen ambient
(from Wang and Chao 138 ).
2.3.2
TiO2 Thin Film vs. Single Crystal
The refractive index of a TiO2 thin ﬁlm is typically much less than that of an anatase or
rutile crystal, while the extinction coeﬃcient of the deposited material will generally be
greater than that of the bulk material. Figure 2.7 displays the refractive index for anatase
single crystals. Due to the limited studies performed on anatase single crystals the data
used for the anatase curves is shown in Table 2.1. Data for rutile is shown in Figure 2.7
for comparison.140 As both anatase and rutile are birefringent crystals it is necessary to
calculate the mean refractive index nmean for a randomly oriented polycrystalline thin ﬁlm.
This is done using Equation 2.9 below
2n⊥ + n
,
(2.9)
3
where n⊥ and n are for oscillations perpendicular and parallel to the optical axis, respectively. The mean refractive indices of 2.70 for rutile126 and 2.532 for anatase34 (both at
nmean =
40
2. Common Properties of TiO2 Thin Films
λ = 600 nm) were selected as a reference bulk value to represent dense polycrystalline material.
Figure 2.7: Published values for the refractive index of single crystal
anatase, taken from Meyer and Pietsch,141 Hass,34 Fitzgibbons et al.,67
Kingery et al.,142 Washburn,143 and Kim.144 The dispersive curve for
single crystal rutile from Kim is also given.144
Very little anatase and rutile single crystal absorption data has been published in the visible
as these materials are essentially transparent. The optical bandgap of anatase and rutile
is about 3.2 eV145, 146 and 3.05 eV,145 respectively. The optical bandgap of amorphous TiO2
is commonly reported as being around 3.5 eV.147 Hence, optical absorption will increase
with the successive transformations from amorphous to anatase to rutile material. It can
be seen in Figure 2.8 that anatase has an absorption edge with a lower steepness, which
is attributed to the presence of excitons and more imperfections and disorder in anatase
crystals.120 Figure 2.9 shows the exponential dependence of the absorption coeﬃcient at
10 K for diﬀerent polarisations when illuminated with UV light.120
2.3.3
Variation with Deposition and Annealing Temperature
As previously discussed, many researchers have observed a direct linear relationship between
the refractive index and the density of the ﬁlm.33, 67, 121–125 With TiO2 it is possible to deposit
increasingly dense ﬁlms, approaching the density of the bulk material, due to the mixture
of amorphous, anatase, and rutile phases that can exist. Furthermore, while high refractive
indices can be achieved for an amorphous ﬁlm, a diﬀerent set of deposition conditions can
yield an anatase or rutile ﬁlm with a higher refractive index.33 Thus, the optical properties
2.3 Optical Properties
41
Table 2.1: Refractive index data for anatase single crystal. The data for
the anatase and rutile curves in Figure 2.7 are not presented in the table
and can be found in Kim.144 The asterisked n values (∗ ) are for rutile.
Wavelength
λ (nm)
435.83
546.07
589.30
690.70
n
n⊥
2.7688 2.6576
2.5948 2.5161
2.5612 2.488
2.5097 2.4447
nmean
Reference
2.732
2.569
2.537
2.488
Meyer and Pietsch141
2.785 Washburn143
2.69467
2.59893
2.5431
2.5198
2.50333
2.49417
2.46727
2.46233
405
436
492
546
578
589
623
691
706
2.8760
2.7688
2.6586
2.5955
2.5694
2.534
2.5407
2.5106
2.5052
2.7395
2.6576
2.5691
2.5169
2.4950
2.488
2.4709
2.4456
2.4409
450
500
550
600
700
−
−
−
−
−
−
−
−
−
−
2.703
2.615
2.565
2.532
2.485
Hass34
550
−
−
2.57
Fitzgibbons et al.67
600
−
−
2.52
Kingery et al.142
600
2.60∗
2.90∗
2.70∗
Ribarsky126
of a TiO2 ﬁlm can be eﬀectively tuned by adjusting the deposition temperature, bearing in
mind that, in general, the extinction coeﬃcient of a polycrystalline TiO2 thin ﬁlm will be
higher than that of an amorphous thin ﬁlm.
Several works that have included extensive optical characterisation of TiO2 ﬁlms will be reviewed here. Hovel has published two excellent graphs demonstrating the trend of increasing
refractive index with increasing deposition temperature, as shown below in Figures 2.10(a)
and (b).148 The TiO2 ﬁlms were deposited by thermal spraying and the refractive index in
Figure 2.10(a) was measured using ellipsometry at 633 nm.
Absorption values are reported in a variety of ways, including calculations of α or the TiO2
bandgap energy, and plots of optical transmittance or absorptance versus wavelength. As
previously discussed, contributions towards optical losses in ﬁlm arise from the fundamental
42
2. Common Properties of TiO2 Thin Films
Figure 2.8: Fundamental absorption edge of anatase and rutile single
crystals, measured at a temperature of 10 K (adapted from Tang et al.120 ).
Figure 2.9: The exponential dependence of the absorption coeﬃcient of
single crystal anatase, measured at 10 K with light polarized in Ec and
E⊥c directions (adapted from Tang et al.120 ).
absorption edge of the TiO2 ﬁlm as well as surface and volume imperfections. Figure 2.11
indicates the general trend of increasing absorption coeﬃcient with annealing temperature.
This is in agreement with the formation of the anatase phase, which possesses a lower
bandgap than amorphous TiO2 . DeLoach et al. found that the absorption edge for sputtered
TiO2 ﬁlms decreased from 3.41 eV for ﬁlms with a small rutile fraction (< 0.17) to 3.22 eV
for ﬁlms with a large rutile fraction (> 0.7).149
2.3 Optical Properties
43
Figure 2.10: (a)Variation of TiO2 refractive index with annealing temperature, and (b) corresponding dispersive refractive index relations (adapted
from Hovel 148 ).
Figure 2.11: Absorption coeﬃcients of TiO2 ﬁlms deposited at various
temperatures (from Hovel 148 ).
Kamataki et al. performed optical characterisation of TiO2 thin ﬁlms deposited by atmospheric pressure chemical vapour deposition (APCVD) at 300◦ C in the wavelength range
250 − 850 nm.150 Measurements were also performed on samples that were annealed for 1 hr
in both oxygen (O2 ) and nitrogen (N2 ) ambients at temperatures of 500◦ C, 700◦ C and 900◦ C.
Figure 2.12(a) shows a maximum in n at about 310 nm, while Figure 2.12(b) indicates that
there is negligible absorption in the ﬁlms at wavelengths greater than 350 nm. Both n and
k exhibit a trend of increasing with higher annealing temperatures.
The optical constants of electron-beam (e-beam) evaporated TiO2 thin ﬁlms were measured
44
2. Common Properties of TiO2 Thin Films
Figure 2.12: (a) Refractive index and (b) extinction coeﬃcient of
APCVD-deposited and annealed TiO2 thin ﬁlms (adapted from Kamataki
et al.150 ).
by Kim using spectroscopic ellipsometry (SE) in the spectral region 1.5 − 5.5 eV.140, 144 A
double oscillator model for amorphous materials151 is used to ﬁt n and k values. Kim also
successfully modelled the ﬁlm as being polycrystalline anatase with a 16% void content.140
Figure 2.13 displays n and k for the TiO2 evaporated ﬁlm along with the “void-free” equivalent ﬁlm. A three-layer model with varying void fraction was used to model the 96.4 nm-thick
(total) ﬁlm. The surface roughness was successfully modelled by 13.1 nm-thick top layer with
a 34% void incorporation.
Mardare and Hones deposited TiO2 thin ﬁlms at 100◦ C (sample B) and 250◦ C (sample
A) onto glass substrates using RF sputtering and determined n and k using SE.82 Figure
2.14(a) indicates that high refractive indices were achieved even at these low deposition
temperatures, while Figure 2.14(b) demonstrates that the extinction coeﬃcient remained
below 0.02 for the sample deposited at 250◦ C for wavelengths > 400 nm. The dispersive
relations for Samples C and D in Figures 2.14(a) and (b) are for doped TiO2 thin ﬁlms and
will not be discussed here.
Szlufcik et al. investigated screen-printed TiO2 AR coatings and found that the refractive
index increased linearly with annealing temperature up to a maximum of 2.30 at 800◦ C.114
2.3 Optical Properties
45
Figure 2.13: Dispersive n and k values calculated from SE data for an
e-beam evaporated TiO2 ﬁlm (solid lines)and the equivalent “void-free”
ﬁlm (open circles). The refractive indices of polycrystalline anatase and
rutile are also shown for comparison (from Kim 144 ).
Figure 2.14: (a) Refractive index and (b) extinction coeﬃcient of RF
sputtered TiO2 thin ﬁlms. (from Mardare and Hones 82 ).
2.3.4
Variation with Deposition and Annealing Ambient
Photovoltaic researchers have noted that the optical absorption of non-stoichiometric TiO2−x
thin ﬁlms increased in the short wavelength region, precisely where higher eﬃciency solar cells
46
2. Common Properties of TiO2 Thin Films
exhibited their improved response.43 As mentioned previously, non-stoichiometric TiO2−x
ﬁlms can be achieved under a variety of deposition and annealing conditions. The eﬀect of
depositing TiO2 thin ﬁlms in an oxygen poor environment is demonstrated in Figure 2.15.152
Films were evaporated at two diﬀerent base pressures, 5 × 10−5 Torr and 1 × 10−4 Torr,
and at one oxygen partial pressure. As shown in Figure 2.15, ﬁlms evaporated without the
presence of oxygen exhibited a much lower transmittance over the visible spectrum than the
ﬁlm deposited with an oxygen partial pressure of 5 × 10−5 Torr. The exact stoichiometry of
the ﬁlms is unknown, however a direct relationship between base pressure and transmittance
can be observed.
Figure 2.15: Variation of transmittance of evaporated TiO2 ﬁlms with
base an oxygen partial pressure (from Jiao and Anderson 152 ).
Zakrzewska et al. correlated the optical and physical properties of sputtered TiO2−x thin
ﬁlms deposited with diﬀerent stoichiometries.153 The measure of stoichiometry was determined as the fraction I/I0 , which is the ratio of the titanium plasma line intensity during
deposition to the 100% metallic titanium plasma line intensity. Higher I/I0 values indicate
a greater departure from stoichiometry. The main graph in Figure 2.16 shows the increase in
the ﬁlm absorption coeﬃcient with increasing I/I0 . The inset graph in Figure 2.16 shows that
the refractive index, measured at λ = 800 nm, increases linearly with decreasing oxidation
state of the TiO2−x ﬁlm.
For TiO2 thin ﬁlms deposited by ion-beam sputtering, it was found that the refractive
index exhibited a maximum (2.52 at λ = 633 nm) with an oxygen concentration during
the deposition of [O2 ]=30%.154 The level of absorption in the ﬁlm was found to decrease
dramatically for [O2 ]< 30%, especially at the shorter wavelengths of 500 nm and 633 nm.
A diﬀerence in the optical properties of APCVD-deposited TiO2 ﬁlms annealed in O2 and N2
ambients was noted by Kamataki et al., as shown in Figure 2.17. This was attributed to the
growth of SiO2 at the TiO2 :Si interface of O2 annealed samples. The analysis of Kamataki
et al. determined that after 1 hr at 500◦ C an 11.6 nm-thick SiO2 layer had grown at the
2.3 Optical Properties
47
Figure 2.16: Variation of refractive index (inset) and absorption coeﬃcient (main graph) with oxidation state of the TiO2−x thin ﬁlms (from
Zakrzewska et al.153 ).
interface. This SiO2 growth rate is much greater than other researchers have observed at
the same temperature.17
Figure 2.17: Eﬀect of annealing ambient on the (a) refractive index and
(b) extinction coeﬃcient of APCVD-deposited TiO2 thin ﬁlms annealed
at 500◦ C (adapted from Kamataki et al.150 ).
Fuyuki and Matsunami noted that the presence of a small amount of water vapour may
cause scattered values of the dielectric constant in CVD-deposited TiO2 ﬁlms.147 Subse-
48
2. Common Properties of TiO2 Thin Films
quent research demonstrated that the presence of water vapour altered the refractive index
from about 2.0 (no H2 O) to about 2.15 (300 ppm H2 O).155 Ardakani observed that as the
hydrogen pressure in the laser ablation chamber increased, the measured reﬂectance of the
ﬁlms decreased due to the higher concentration of oxygen-deﬁcient phases of TiO2−x .132
Golego et al. determined that annealing spray-deposited TiO2 ﬁlms in hydrogen at 500◦ C
did not aﬀect the absorption spectra, but an absorption peak at 400 − 600 nm appeared
after the annealing temperature was increased to 900◦ C.115 This indicates that absorption
in the visible spectrum occurs after the ﬁlm is reduced to TiO(2−x) . Ottermann et al. determined that there was a clear relationship between the hydrogen content of TiO2 thin ﬁlms,
deposited using diﬀerent production processes, and their refractive index.156 An increase
in refractive index was observed for decreasing hydrogen content, regardless of whether the
ﬁlms were amorphous or anatase. Films formed by chemical reaction methods, such as the
sol-gel technique and dip-coating, exhibited high hydrogen contents. Thus, it is assumed
that the organic molecules from the liquid precursor have not been fully decomposed by
high-temperature processing.
Yoldas and O’Keeﬀe noted that spin-on TiO2 samples annealed in air had a refractive index
of 2.1 (at λ = 546.1 nm), whereas the same coating annealed in vacuum had a refractive index
of 2.4.157 This increase in refractive index was also observed for ﬁlms annealed in argon or
nitrogen, and was attributed to the increased densiﬁcation resulting from the thermochemical
reactions occurring with the residual terminal bonds in the absence of oxygen.102
2.3.5
Variation with Other Deposition Conditions
Depending on the type of deposition system used, there are many diﬀerent parameters that
may be tuned to adjust the optical properties of the TiO2 thin ﬁlm. The results of Bendavid
et al. are included here due to the extremely high refractive indices achieved - 2.56, 2.62
and 2.72 (at λ = 550 nm) for amorphous, anatase and rutile ﬁlms, respectively. The ﬁlms
were deposited by ﬁltered arc deposition (FAD) and a range of bias voltages were applied to
the substrate. Figure 2.18 shows the refractive index and extinction coeﬃcient of two TiO2
ﬁlms deposited with diﬀerent bias voltages, 0 V and −400 V, which were determined to be
anatase and rutile, respectively.
2.3.6
Optical Properties of Highly Porous TiO2 Films
A TiO2 ﬁlm with a high porosity, or low packing density, will have a reduced refractive
index. The optical properties of porous ﬁlms can be determined using a Bruggeman eﬀective
medium approximation (EMA), which can be expressed as158, 159
(1 − fv )
1 − ε
ε
m−ε
+ fv
=0,
ε
ε
1 + 2
ε
m + 2
(2.10)
2.3 Optical Properties
49
Figure 2.18: Refractive indices and extinction coeﬃcients of FADdeposited TiO2 thin ﬁlms. The ﬁlm deposited with 0 V substrate bias
is anatase and the −400 V ﬁlm is rutile (from Bendavid et al.33 ).
where fv is the volume fraction of void in the ﬁlm, and ε and εm are the dielectric functions
of the unknown ﬁlm and the main constituent material, respectively. The dielectric function
of the void is taken to be unity, while the complex dielectric constant of a material is related
to its refractive index and extinction coeﬃcient by
ε = (n + ik)2 .
(2.11)
TiO2 thin ﬁlms deposited using the sol-gel method have exhibited void fractions in the
ranging from 25% at the exposed surface to 38% at the TiO2 :substrate interface.128 This
variation in void content translates directly into a variation in refractive index of the ﬁlm,
which, in this case, was 2.15 (at λ = 500 nm) at the outer surface and 2.05 at the inner
surface.128, 160 The higher refractive index at the outer surface was attributed to densiﬁcation
and crystallisation processes that begin at the outer surface and gradually progress through
the depth of the ﬁlm during annealing.128 The void fraction observed in evaporated ﬁlms
was slightly lower at 19.6 − 24.5%.161 Kim determined that TiO2 thin ﬁlms deposited by
e-beam evaporation had a void content of 16% when compared to dense, polycrystalline
anatase.140, 144 TiO2 thin ﬁlms deposited by RF sputtering exhibited a much lower void
content (< 5%),82 which can be attributed to much greater kinetic energy possessed by the
atoms as the impinge on the surface.
Another issue related to porous ﬁlms is the sorption (either adsorption or absorption) of
water vapour. When the optical properties of a thin porous ﬁlm are measured in a vacuum
the results may be very diﬀerent to those achieved in air. This is because on exposure to
the atmosphere, the sorption of water vapour, which has a high refractive index than air
nH2 O = 1.33, results in a marked increase in the refractive index of the ﬁlm.162 Borgogno et
al. measured the refractive index of a TiO2 thin ﬁlm in vacuum and in air, and attributed
the 0.1 higher n in air to the sorption of water vapour,163 while Leprince-Wang et al. and
Nguyen Van et al. noted a 3% increase in n after exposure to air.161, 164 Ben Amor et al.
50
2. Common Properties of TiO2 Thin Films
observed that it was possible to detect the adsorbed water vapour using Fourier transform
infrared (FTIR) spectroscopy.165
2.4
2.4.1
Electrical Properties
TiO2 : Insulator or Conductor?
TiO2 is an n-type semiconductor with a relatively wide bandgap of 3.05 − 3.5 eV. Single
crystal titanium dioxide TiO2 has a resistivity of about 1013 Ω cm at room temperature,
and about 107 Ω cm at 250◦ C.29, 116, 155 These values are similar to conductivities reported
for single crystal rutile:141 at 30◦ C the conductivity was 5 × 10−14 Ω−1 cm−1 while at 260◦ C
it had decreased to 3.3 × 10−9 Ω−1 cm−1 . Therefore, TiO2 is generally considered to be an
insulator at temperatures less than 200◦ C.141, 166
There are a wide number of applications for highly insulating TiO2 ﬁlms, including its use
as a super-thin gate dielectric in MOSFET devices.167 However, the electrical properties of
the TiO2 ﬁlm can be altered to become highly conductive for various applications, such as
humidity and gas sensors,83, 168 or as an corrosion-resistant electrode.132, 169 The properties
of TiO2 thin ﬁlms have been likened to those of zinc oxide (ZnO), with typically a large
bandgap, small grain size, low carrier density, n-type conductivity, long photoconductivity
relaxation, and low mobility.115 This section will brieﬂy discuss the relationship between the
electrical properties of TiO2 and the deposition parameters, annealing ambients, and dopant
atoms.
2.4.2
Non-Stoichiometric TiO2−x Thin Films
The semiconducting properties of non-stoichiometric titanium dioxide are very dependent on
the extent of the oxygen deﬁciency in the ﬁlm.132, 170 The conductivity mechanism in undoped
TiO2 relies on a deﬁciency of oxygen atoms in the material. These oxygen deﬁciencies behave
like n-type defects, with a typical density of 1018 −1019 cm−3 .171 As the stoichiometry departs
from the ideal Ti:O ratio of 1:2, several changes in the ﬁlm result. Firstly, as previously
mentioned in Section 2.2.6, the TiO2−x ﬁlms exhibit a blue, purple or black colour depending
on the ﬁlm stoichiometry. Secondly, due to the increased number of oxygen vacancies, the
optical absorption in the visible is dramatically increased for these ﬁlms (see Section 2.3.4).
Additionally, these sub-oxides, especially TiO and Ti2 O3 , are known to exhibit metallic
conduction or semiconducting properties.58, 172, 173 The conductivity is observed to increase
dramatically with slight departures from stoichiometric TiO2 , with < 10−10 Ω−1 cm−1 for
TiO2.00 ; 10−1 Ω−1 cm−1 for TiO1.9995 ; 0.8 Ω−1 cm−1 for TiO1.995 ; and 102 Ω−1 cm−1 for
TiO1.75 .141 Rao et al. determined that the resistivity of Ti2 O3 is about 94 × 10−3 Ω−1 cm−1
2.4 Electrical Properties
51
at room temperature.174 Feuersanger noted that the deposition of TiO2 thin ﬁlms by direct
evaporation of a TiO2 source results in the loss of O2 , and the ﬁlms were semiconducting
rather than insulating.66 Tsuda et al. showed that Ti2 O3 exhibits a sharp transition in
conductivity at 227◦ C, exhibiting metallic conductivity above that temperature and semiconducting below it.173
Thus, TiO2−x thin ﬁlms would seem to be incompatible with high-eﬃciency solar cells, as the
increased optical absorption is undesirable in an AR coating the buried-contact fabrication
sequence requires an insulating ﬁlm to act as a metallisation mask. All ﬁlms used in this
work were intentionally stoichiometric.
2.4.3
Variation with Deposition or Annealing Ambient
Much work has been performed determining the electrical properties of as-deposited TiO2
ﬁlms, and the eﬀect of the deposition or annealing ambient. It is commonly reported that
thin ﬁlms deposited in a low oxygen partial pressure result in the formation of sub-oxides such
as TiO and Ti2 O3 .113, 135 This results in increased electrical conductivity in the deposited
ﬁlms.175
When stoichiometric TiO2 ﬁlms are annealed in a vacuum, hydrogen, or low oxygen pressure
ambient, loss of oxygen from the ﬁlm results.116, 132, 169, 176 However, at room temperature
it has been demonstrated that the ﬁlm conductance does not increase when placed in a
vacuum chamber and dry hydrogen gas is introduced.177 Yeung and Lam found that very low
conductivity (10−13 Ω−1 cm−1 ) ﬁlms could be achieved by performing an extended oxidation
of up to 30 hours.178 Stoichiometric, as-deposited TiO2 thin ﬁlms possess a negative ﬁxed
charge density, in the order of −5 × 10−8 C cm−2 .117, 178, 179 Erkov et al. observed that after
annealing in N2 O this ﬁxed charge density became zero, while annealing in vacuum created a
ﬁlm with a positive ﬁxed charge density.117 In comparison, SiO2 has a positive ﬁxed charge
density.179, 180
Takahashi et al. demonstrated that the resistivity of TiO2 thin ﬁlms could vary by more
than an order of magnitude depending on the measurement ambient.170 The lowest resistivity
was observed in a hydrogen (reducing) ambient, consistent with the properties of an n-type
semiconductor. Additionally, the ﬁlm conductivity was observed to increase by 104 when
illuminated with a ﬂuorescent lamp.
2.4.4
Doped TiO2
Doped TiO2 ﬁlms have been used as transparent conducting oxides (TCO),58 for creating
heterojunction solar cells,59, 61 for suppressing leakage currents in capacitors,166, 181 increasing
the sensitivity of gas detectors115, 168 and as electrodes for photoelectrolysis devices.63–65
52
2. Common Properties of TiO2 Thin Films
Dopant atoms have also been introduced into TiO2 thin ﬁlms to create a diﬀusion source for
an underlying silicon layer. These atoms then diﬀuse into the silicon during a subsequent
high-temperature anneal. This will be discussed in more detail in Chapter 6.
The majority of dopants enhance the n-type semiconducting properties of TiO2 . These
dopants include niobium (Nb),116, 168, 182 tantalum (Ta),116 vanadium (V),116 ﬂuorine (F),116
and hydrogen (H).116, 172 Dopants that change the ﬁlm to being p-type include aluminium
(Al),116 iron (Fe),116, 182, 183 and indium (In).59 There are instability issues with both
hydrogen-doped (n-type) and all p-type TiO2 thin ﬁlms. In the former case, the hydrogen is very mobile and is likely to result in electrically unstable devices.116 In the latter case,
the electrical behaviour of the Al- or Fe-doped ﬁlm depends on the oxygen concentration,
and it is possible for the ﬁlm to revert to n-type.116, 182 Additionally, dopants such as aluminium, iron,170 chromium and cadmium,59 are known to extend the photoactive response
from the TiO2 ﬁlm under visible light. Table 2.2 includes some references to published works
on both doped and undoped TiO2 crystals and ﬁlms, and their corresponding resistivity.
2.5
2.5.1
Chemical Properties
Chemicals used in Making Solar Cells
In general, the resistance of thin ﬁlms to chemical attack is a highly desirable property for
most applications.137 Schr¨oder stated the requirements for a ﬁlm to be considered chemically
resistant. A chemically resistant ﬁlm must36
i) be insoluble to the attacking medium
ii) be impervious to the attacking medium, and
iii) form a solid bond to the substrate, preventing chemical attack at edges or through
defects in the coating.
A large variety of chemicals are used to fabricate solar cells, but this is dependent on the
speciﬁc type of solar cell and whether it is produced in a laboratory or industrial environment.
A summary of the typical chemicals used at diﬀerent stages in fabricating buried-contact
solar cells include:
• Sodium hydroxide (NaOH), potassium hydroxide (KOH), or a mixture of hydroﬂuoric
(HF) and nitric (HNO3 ) acids: for texturing (basic solutions only) or etching silicon
• Ammonium hydroxide (NH4 OH), hydrochloric acid (HCl) or sulphuric acid (H2 SO4 ):
for removing organic and metallic contaminants from the surfaces of the wafers − each
2.5 Chemical Properties
53
Table 2.2: Measured resistivity of anatase and rutile crystals and thin
ﬁlms after annealing in various ambients and/or being doped with various
impurities (adapted from Kurtz and Gordon 116 ).
Resist.
(Ω cm)
1014
1
3
102 − 10−3
10−1 / 101
3 × 106
2 × 104
3 × 103
1.5
0.1
0.3
10
4 × 10
1010 − 109
107 /
5 × 10−2
106 − 104
107 − 102
103
107 − 103
104 − 103
4 × 102
Sample Preparation
Single Crystals - Undoped
Stoichiometric rutile
Rutile reduced at 1000◦ C with an O2 partial
pressure of 10−11 Pa.
Rutile reduced at 700◦ C for 2 hr in H2 , resulting in a carrier conc. of 4 × 1018 cm−3
Rutile reduced in H2 or vacuum at 750◦ C for
up to 2 hr
Anatase: as-grown/after repeated O2 annealing
Reference
Clark29
Gautron et al.184
Butler185
Ginley
and
186
Knotek
Forro et al.187
Single Crystals - Doped
TiO2 doped with 1 mol % boron
Johnson188
TiO2 doped with 1 mol % phosphorus
Johnson188
TiO2 doped with 1 mol % niobium (Nb)
Johnson188
TiO2−x Fx (x=0.002)
Subbarao et al.189
TiO2 doped with 1020 Nb atoms cm−3
Bogomolov et al.190
TiO2 doped with 1% Nb and reduced at Gautron et al.184
1000◦ C with an O2 partial pressure of
10−11 Pa
Thin Films - Undoped
Evaporated ﬁlms annealed in O2 for up to Yokota et al.191
14 hr
MOCVD at 200◦ C as-deposited
Fuyuki and Matsunami147
Reactively sputtered anatase as-deposited / Tang192
reduced at 450◦ C in vacuum
Evaporation of Ti under 10−3 − 10−6 Torr O2 Chen et al.135
1.6 − 17.2 µm thick ﬁlms (higher deposition Takahashi et al.71
temperature leads to higher resistivity ﬁlm)
Films deposited via spray-CVD method at Badawy and ElTaher193
400◦ C
Thin Films - Doped
CVD of Al-doped TiO2 ﬁlms, deposited at Takahashi et al.170
435 − 480◦ C
CVD of Fe-doped TiO2 ﬁlms, deposited at Takahashi et al.170
415 − 470◦ C
CVD of Cr-doped TiO2 ﬁlms, deposited at Takahashi et al.170
435◦ C
54
2. Common Properties of TiO2 Thin Films
of these chemicals is mixed with hydrogen peroxide (H2 O2 ) and de-ionised (DI) water
at a ratio of 1:1:5. With NH4 OH:H2 O2 :H2 O this is referred to as an ”RCA1” clean,
while HCl:H2 O2 :H2 O is called an ”RCA2” clean.194
• HF: for removing silicon dioxide (SiO2 ) and doped SiO2 layers − diluted to about 5%
in DI water
• Nickelex − commercial product from Transene Inc. (MA, U.S.A.) that contains nickel
chloride. It is used for forming a thin electrolessly plated nickel layer in the grooved
contacts in buried-contact solar cells.
• Enplate Cu-704 − another commercial product (Enthone, Melbourne) that has three
components (A, B, M), containing copper sulphate (CuSO4 ), formaldehyde (HCHO),
NaOH, potassium cyanate (KCN), as well as other organic wetting agents. It forms
the main bulk of the conductor in the grooves.
• Silver plating − contains (AgCN) silver cyanate. It is used as a ﬂash coating to improve
solderability
During the silicon etching procedures, the TiO2 ﬁlm would not normally be present on
the wafer, however the TiO2 would ideally be resistant to all of the chemicals used in the
subsequent cleaning and metal plating processes. As the majority of results for the chemical
resistance of TiO2 ﬁlms are against HF this acid will be dealt with in a separate section.
2.5.2
Hydroﬂuoric Acid
A critical step in the buried-contact solar cell fabrication sequence is the removal of the
P2 O5 :SiO2 from the grooves before performing electroless metal plating, using either dilute
hydroﬂuoric acid (HF) or buﬀered HF (1:15, HF:NH4 F). This allows nickel to plate to the
heavily doped silicon, however if even a thin oxide is present the metal will not plate. In
general, the crystalline phases of TiO2 , anatase and rutile, are much more chemically resistant
than amorphous TiO2 . Kurtz and Gordon noted that TiO2 ﬁlms deposited via atmospheric
pressure chemical vapour deposition at 400 − 600◦ C were very chemically inert and that
surpassed the chemical resistance of glass to attack by common solvents and acids.116 These
ﬁlms could not be removed mechanically or chemically, and etching away the glass substrate
in HF left an intact TiO2 ﬁlm.
Feuersanger reported that ﬁlms deposited at 150◦ C were easily etched in 10% HF.66 Fitzgibbons observed that ﬁlms deposited by CVD at 150◦ C etched at 50 − 75 ˚
A/s in 0.5% HF and
very rapidly in 48% HF.67 Upon subsequent annealing at 350◦ C, dilute HF undercut the
TiO2 layer by dissolving a thin interfacial SiO2 layer, while concentrated HF etched the ﬁlm
slowly and unevenly. Annealing at 700 − 1000◦ C made the ﬁlms resistant to dilute HF, and
2.5 Chemical Properties
55
concentrated HF resulted in undercutting. Spray deposited ﬁlms annealed for 30 s at 450◦ C
were removed in 2 − 5 min when subjected to 1 − 5% aqueous solutions of HF,46 however the
aim of this work was to ensure the TiO2 ﬁlms were amorphous. Yokozawa found that TiO2
(anatase) ﬁlms deposited by CVD at temperatures of 530 − 700◦ C in N2 hardly dissolved
(< 0.03 ˚
A/s), even in concentrated HF.118 On the other hand, ﬁlms deposited at the same
temperatures, but in an N2 and O2 ambient, were less crystalline and etched at 4 − 60 ˚
A/s in
diluted HF. Fuyuki et al. deposited TiO2 ﬁlm deposited by metal-organic CVD (MOCVD)
with water vapour present. The etch rates of these ﬁlms in 10% HF were about 5 nm/min
at 400◦ C and 200 − 1000 nm/min at 200◦ C.155 Brown and Grannemann used buﬀered HF to
remove TiO2 ﬁlms annealed in O2 at 1000◦ C, by undercutting grown SiO2 and leaving the
TiO2 ﬁlm intact.31 Rausch and Burte observed that ﬁlms deposited at 450◦ C by low-pressure
CVD (LPCVD) etched in 50% buﬀered HF.195 Frenck et al. determined that parameters
other than in situ and ex situ temperature have only a minor inﬂuence on etch rate in
buﬀered HF.124 If no post-deposition annealing step is performed, the PECVD ﬁlms are only
chemically resistant to buﬀered HF with deposition temperatures greater than 270◦ C. Lee
noted that the ﬁlm etch rates in buﬀered HF decreased from 2.9 nm/s to 0.14 nm/s as the
anatase fraction of the ﬁlm increased from 0.35 up to 0.7.113
Thus, there is a trend of increased chemical resistance of TiO2 thin ﬁlms deposited or annealed at higher deposition temperatures. The majority of ﬁlms treated at temperatures
greater than 300◦ C either could not be etched, or only etched slowly, in HF. This temperature coincides with the amorphous−anatase transformation.
There are a number of published works that contrast the above trend of increased chemical
resistance with increased annealing or deposition temperatures. Rausch and Burte claim
that the etch rate of TiO2 ﬁlms annealed at 900◦ C was four times greater than the asdeposited layer, which etched at a rate of 10 nm/min in 50% buﬀered HF.195 However later
in the paper, it is stated that the annealed TiO2 ﬁlm lifted oﬀ, exhibiting therefore a greater
chemical resistance. Secondly, Harbison and Taylor reported an etch rate of 700 ˚
A/min in
◦
179
48% HF for material grown by hydrolysis at 800 − 1000 C. This anomalous result does
not agree with the above trend, especially considering the high growth temperature. Other
results of TiO2 ﬁlms deposited at relatively high temperatures (550 − 650◦ C) that etched,
include the ﬁlms prepared by Balog et al. using MOCVD, which etched in a solution of 5%
HF.196 Burns also found that ﬁlms formed by rapid thermal annealing (30 s at 550 − 650◦ C)
of titanium metal at these temperatures could be etched in HF.197 It is claimed that the ﬁlms
are polycrystalline rutile, which is in agreement with the high dielectric constants reported,
but not the poor chemical resistance. It has been noted that the chemical resistance to HF
is highly dependent on the ﬁlm deposition technique and water-content of the TiO2 ﬁlms.141
The inclusion of a small fraction of SiO2 (6.25 at. %) into the TiO2 ﬁlm results in rapid
etching (70 nm/min) in buﬀered HF, even after annealing at 1260◦ C for 10 min.198
56
2.5.3
2. Common Properties of TiO2 Thin Films
Other Acids and Bases
Kurtz and Gordon found that TiO2 ﬁlms were chemically resistant to attack by common
solvents and acids, and surpassed the chemical resistance of glass.116 The chemical resistance
of TiO2 to sulphuric acid (H2 SO4 ) is very dependent on the ﬁlm preparation technique,141
but water-free TiO2 is insoluble in all other acids and bases. This is in agreement with
Barksdale, who observed that TiO2 is known to be slightly soluble in H2 SO4 , HF, and a few
strong alkalis, however after annealing at 1000◦ C it is almost completely chemically inert.199
TiO2 ﬁlms prepared by Balog using MOCVD at 550 − 650◦ C were able to be etched in a
solution of 70% H2 SO4 .196 Fitzgibbons found that the chemical resistance increased with
temperature, with ﬁlms deposited at 150◦ C etching at 25 − 40 ˚
A/s.67 However, even ﬁlms
annealed at 1000◦ C still etched slowly (1000 ˚
A/hr in boiling pure H2 SO4 . At the same annealing temperature, TiO2 ﬁlms were observed to etch very slowly in 85% H3 PO4 . Yoldas
and O’Keeﬀe found that for 1 wt. % concentrations of H2 SO4 , H3 PO4 , and HNO3 no observable deterioration was observed after 75 days.157 That research also showed an increase in
chemical resistance to acids for TiO2 ﬁlms annealed at higher temperatures (up to 400◦ C).
TiO2 ﬁlms deposited by the sol-gel technique and baked at 120◦ C were observed to etch in
boiling 0.3 mol HNO3 (pH=0.5).200
Schr¨oder found that an observable colour change with TiO2 ﬁlms baked at 200◦ C occurred
after about 4 hours immersion in a 10% HCl solution, whereas after baking at 550◦ C the
same colour change occurred after more than 200 hours.36 Szlufcik et al. determined that a
1 wt. % HCl solution completely deteriorated TiO2 ﬁlms ﬁred at 300◦ C after 26 days, however
no observable deterioration was achieved after ﬁring at temperatures greater than 600◦ C.114
For spray-deposited ﬁlms (450◦ C) no observable deterioration was found after placing the
ﬁlms in a 1% solution of boiling HCl for 1 hour.46
The resistance of TiO2 thin ﬁlms to various bases has also been reported in the literature.
Schr¨oder performed experiments with TiO2 thin ﬁlms in a 10% NaOH solution, and found
that for ﬁlms annealed at 200◦ C an observable colour change occurred after 5 hr.36 Increasing the annealing temperature to 500◦ C delayed the same colour change observation to more
than 200 hours. TiO2 ﬁlms baked at 120◦ C were badly corroded after placing them in boiling 0.5 mol NaOH solution (pH=13.5) for 30 min.200 The addition of 1 at. % boron oxide
(B2 O3 ) signiﬁcantly increased the chemical resistance of the ﬁlm. Kern and Tracy found
no observable deterioration for spray-deposited TiO2 ﬁlms annealed at 450◦ C in a 1% solution of NH4 OH.46 For screen-printed TiO2 ﬁlms the chemical resistance to 1 wt. % NaOH
and NH4 OH solutions was poor for annealing temperatures of 600◦ C.114 No observable deterioration was detected after increasing the annealing temperature to 800◦ C. Honsberg et
al. found that cleaning 60 nm-thick, spray-deposited TiO2 ﬁlms on silicon wafers in RCA
solutions resulted in thickness reduction in the ﬁlm of 6 nm.20 This is most likely due to the
NH4 OH in the RCA1 clean.194 Yoldas and O’Keeﬀe found that the chemical resistance of
2.6 Conclusions
57
TiO2 to 1 wt. % NH4 OH ﬁlms ﬁred in a vacuum at 500◦ C) was greater than that of ﬁlms ﬁred
in air.157 For the air-baked ﬁlms, the deterioration was obvious after 7 days, and complete
after 10 − 20 days. NH4 OH based etches can also be used for removing unreacted titanium
in ﬁlms.113
Changes in the reﬂectance spectra of TiO2 ﬁlms containing small fractions of SiO2 that were
ﬁred at 400◦ C were noted for 1% HCl solutions after 24 hours.201 No change was noted for a
1% NH4 OH solution after 192 hours, but the 1% NaOH solution resulted in the swelling of
the ﬁlm and a reduction in refractive index. The hierarchy of chemical attack for these ﬁlms
was given as NaOH > HCl > NH4 OH. This is in agreement with the results of Schr¨oder,36 but
in contrast with the NH4 OH results from other research.157 However, Yoldas and O’Keeﬀe
postulated that the increased chemical resistance to sol-gel ﬁlms baked at only 80◦ could be
due to the retention of organic groups.157 The results of Schr¨oder indicate that the chemical
resistance of TiO2 containing SiO2 is signiﬁcantly poorer than pure SiO2 ﬁlms.36
TiO2 ﬁlms deposited at above 400◦ C have also been shown to be resistant to the nickel and
copper electroless metal plating solutions used in the buried-contact solar cell fabrication
sequence.20 The chemical resistance to the copper plating solution is signiﬁcant as it is
strongly basic (pH=11) and contains NaOH.
Thus, increased deposition and annealing temperature result in greater chemical resistance
for the majority of other acids and bases - a similar trend to that observed with hydroﬂuoric
acid.
2.6
Conclusions
The properties of TiO2 depend greatly on whether the sample is bulk material or a thin ﬁlm
and the phase of the material. With CVD techniques the phase is usually directly related to
the substrate temperature, with amorphous TiO2 forming at temperatures less than 300◦ C,
the metastable crystalline phase of anatase in the temperature range 300 − 700◦ C, and the
stable crystalline phase of rutile at temperatures greater than 700◦ C. Various impurities and
substrate types are known to partially or totally impair or enhance the transformation of
a TiO2 thin ﬁlm to rutile. A useful linear relationship can be found between the refractive
index and TiO2 ﬁlm density.
While optical constant data for rutile is fairly easy to ﬁnd, it was necessary to collect small
sets of data for anatase from over many decades in order to construct a dispersive refractive
index model. Anatase has a fundamental absorption edge with a lower steepness than rutile,
due to the increased disorder observed in anatase crystals. A trend of increasing refractive
and extinction coeﬃcient with increasing deposition temperature is commonly observed.
Thus, accurate control of the temperature results in accurate tuning of the TiO2 thin ﬁlm’s
58
2. Common Properties of TiO2 Thin Films
refractive index and, to a certain extent, extinction coeﬃcient.
The electrical properties are brieﬂy discussed. Although TiO2 is a wide bandgap n-type
semiconductor, at room temperature it behaves like an insulator. The ﬁlm resistivity is
extremely sensitive to the deposition method and on the availability of oxygen to the system.
Films deposited in oxygen poor ambients exhibit greatly increased electrical conductivity and
optical absorption, however subsequent furnace oxidation processes can reduce these oxygen
vacancies. Doping of TiO2 thin ﬁlms in order to increase the electrical conductivity or
photoconductivity of the ﬁlm has been experimented with in many diﬀerent applications.
The chemical resistance of TiO2 thin ﬁlms change markedly during their amorphouspolycrystalline transition. Amorphous TiO2 ﬁlm are highly soluble in hydroﬂuoric acid,
while dense, polycrystalline ﬁlms can be insoluble. TiO2 ﬁlms seem to be most susceptible
to etching in strong basic solutions such as sodium hydroxide and ammonium hydroxide.
The chemical resistance to sulphuric acid is dependent on the ﬁlm preparation technique.
There is a deﬁnite trend of increased chemical resistance to all chemicals with increased ﬁlm
deposition or annealing temperature.
Chapter 3
TiO2 Thin Film Deposition
Equipment
After evaluating the desired ﬁlm properties and performing a literature survey on the possible deposition methods, the author designed and constructed two TiO2 thin ﬁlm deposition
systems. The ﬁrst system used an ultrasonic atomisation spray nozzle in order to create an
aerosol of the TiO2 precursor. The reasons for choosing ultrasonic spray deposition (USD)
and the TiO2 precursor, tetraisopropyl titanate (TPT) are discussed. A diagram of the system is presented and the necessary components described. With this system, very dense
TiO2 ﬁlms could be deposited at a temperatures of 450◦ C and the very shallow deposition
angles successfully prevented TiO2 ﬁlm deposition in grooves scribed in the front surface of
the wafer. Thickness uniformity and chemical resistance problems with the electroless metal
plating solutions, used for contact formation in the buried-contact solar cell fabrication sequence, arose due to the frequent inclusion of TiO2 particulates (1 − 30 µm in diameter) in
the 70 nm thick ﬁlms.
For this reason the TPT was placed in a stainless steel bubbler, resulting in the development
of a simple atmospheric pressure chemical vapour deposition (CVD) system. TiO2 ﬁlms
deposited using the CVD system exhibited much greater thickness uniformity and a lack of
particulates. Additionally, it was also possible to deposit ﬁlms anywhere in the temperature
range 150 − 450◦ C, enabling the refractive index to be tuned. The tradeoﬀ with the new
system was that the ﬁlm density decreased signiﬁcantly.
3.1
Introduction
The previous chapter described the physical, optical, electrical and chemical properties of
TiO2 thin ﬁlms, primarily as a function of deposition or annealing temperature. Based on
previous experience with a TiO2 deposition system at UNSW, TiO2 thin ﬁlms with the
59
60
3. TiO2 Thin Film Deposition Equipment
following properties were desirable for this work:
• Physical: Dense, defect-free ﬁlms with a thickness of about 70 nm. The thickness
uniformity should be within ±10%. Films that are dense and lack defects (such as
large, amorphous TiO2 particulates) will be impervious to chemical attack. Denser
ﬁlms will typically perform better as a diﬀusion barrier as well.120
• Optical: Dense TiO2 ﬁlms will also enable a refractive index of about 2.4 for light
of 600 nm wavelength to be achieved at low deposition temperatures (< 450◦ C). This
value is the optimum refractive index for an antireﬂection (AR) coating for a silicon
solar cell encapsulated under glass. An excellent AR coating is necessary to reduce the
amount of reﬂected light from the planar multicrystalline silicon (mc-Si) wafers.
• Electrical: The TiO2 ﬁlm must be insulating in order to act as a metallisation mask
to the electroless metal plating solutions. This requires that the ﬁlms are stoichiometric, and do not exhibit the oxygen vacancies that form the conduction mechanism in
reduced TiO2−x thin ﬁlms.
• Chemical: The TiO2 ﬁlms should be polycrystalline anatase or rutile in order to withstand the necessary wet chemical processing during solar cell fabrication. The ﬁlms
need to withstand RCA cleaning, dilute hydroﬂuoric acid, and the electroless metal
plating solutions.
A simple TiO2 pressurized spray system had already been in operation for some years at
the UNSW, using tetraisopropyl titanate (TPT) as the precursor.202 This system used a
spray-painting gun pressurized with nitrogen, and the TPT ﬂow was controlled via a needle
valve. A TPT aerosol was formed and transported to the wafer by the nitrogen ﬂow. The
wafer was held by vacuum onto a stainless steel block that was placed on top of a 2 kW stove
element. The whole deposition process was carried out inside a fumecupboard. This system
was used for several years, and it demonstrated that geometry could be used to direct the
emerging aerosol, so that when spraying at shallow angles the TiO2 could be kept out of the
grooves.202, 203
There were a number of problems with the TiO2 ﬁlms and the deposition system. Firstly,
it took over an hour to deposit a 70 nm thick TiO2 ﬁlm onto a 2”-diameter wafer. This
was not due to the low TPT ﬂow rates, although the needle valve was being operated at its
limits to avoid a too high a ﬂow of TPT from emerging from the nozzle, but because of the
cooling eﬀect of the aerosol on the stainless steel block. After several seconds spraying, the
temperature of the block dropped by over 30◦ C, and it was necessary to wait a few minutes
before continuing spraying. This highlighted the second problem, which was that the heater
was very ineﬃcient at transferring heat to the stainless steel block, and this excess heat made
it uncomfortable for the user. Thirdly, there was no way of controlling the relative humidity,
3.1 Introduction
61
and the relative humidity on rainy or overcast days (up to 65%) made spraying impossible
due to the formation of white TiO2 particulates in the ﬁlm.
A replacement TiO2 system was to be designed and constructed by the author. In order for
the TiO2 thin ﬁlms to exhibit the desired properties, listed above, the requirements for the
new system were determined as follows:
• Temperature: Signiﬁcant ﬂuctuations in the temperature at about 400◦ C could result
in the TiO2 ﬁlm having a mixed amorphous-anatase phase. Naturally, this outcome is
undesirable, so a target of achieving a maximum variation of substrate temperature of
±10◦ C was set.
• Relative Humidity: Excess humidity will result in large TiO2 particulates sticking
to deposited ﬁlm.202, 204 Therefore, a necessary feature of the system was to have
adjustable and repeatable humidity control. It was anticipated that mixtures of dry
nitrogen (N2 ) and wet nitrogen (N2 +H2 O) could be fed into the system in order to
control the relative humidity. A meter should display the current relative humidity in
the system.
• Deposition Time: To achieve accurate control of the ﬁlm thickness, a deposition time
of about 5 − 10 min per wafer was decided upon. The ﬁlm thickness would be judged
visibly.
• Deposition Area: As this project had commercial relevance, it was desirable that a 4”square wafer could be TiO2 -coated, although the research would typically be performed
on 2”-round and 5 × 5 cm-square wafers.
• Geometry Control: To prevent TiO2 from being deposited in the laser-scribed grooves,
ﬁlm deposition was to occur on the very top surface only. Any TiO2 that entered
the grooves would inhibit electroless metal plating. The pressurized spray-system at
UNSW had successfully fulﬁlled this goal by spraying a TPT aerosol onto the wafers
at an angle of 10 − 20◦ .
• Substrate Heater: A dedicated substrate heater is required to eﬃciently transfer heat
into the substrate. The temperature should be accurately controlled by dedicated
electronics.
• Automation: The previous pressurized spray system required the user to manually
operate a spray-gun. In the new system it was deemed necessary that the user should
only need to load and unload the wafer from the system, the user’s hands remaining
free during ﬁlm deposition.
• Safety: The system should be enclosed so that it can safely operate on a standard
laboratory bench. It also needs to have exhaust facilities. It was desirable to continue
62
3. TiO2 Thin Film Deposition Equipment
using TPT as the precursor as it is a safe, non-toxic liquid, and obviates the need for
expensive gas handling systems.
• Cost: There was a budget of A$10,000 for the new TiO2 deposition system.
Initially, a new system was designed using an ultrasonic atomisation nozzle. Figure 3.1 shows
the USD system in its ﬁnal form, including the ultrasonic nozzle and generator, syringe
pump, cartridge heaters embedded in the stainless steel block, motorized sample stage, and
temperature controller. Some components have been omitted for clarity, including a nitrogen
heater, air-knife heater, motor speed controller, both the oxygen and humidity sensors, as
well as regulators and ﬂow meters to accurately control gas ﬂows.
The following section will describe the literature review performed to evaluate simple, ﬂexible
and cost-eﬀective techniques for depositing TiO2 ﬁlms. Subsequently, the theory of ultrasonic
spraying will be discussed, and a thorough description of the necessary system components
will be presented.
3.2
Overview of TiO2 Thin Film Deposition Methods
Titanium dioxide has been deposited by many diﬀerent techniques, including
• hydrolysis and pyrolysis,49, 60, 65–67, 71, 74, 78, 115, 148, 168, 177, 179, 193, 205–207
• pneumatic spraying,46, 208
• ultrasonic spraying,65, 106, 206, 209–212
• dip coating100, 109, 111, 156, 157, 200, 207, 213
• plasma enhanced chemical vapour deposition (PECVD),93, 97, 113, 121, 124, 134, 214–216
• atmospheric pressure chemical vapour deposition (APCVD),116, 130, 150, 204, 217–221
• metal organic chemical vapour deposition
(MOCVD),58, 71, 77, 92, 94, 96, 103, 105, 106, 108, 147, 155, 167, 170, 195, 196, 211, 220–227
• ultra-high vacuum chemical vapour deposition (UHV-CVD),228
• low pressure chemical vapour deposition (LPCVD),90, 91
• evaporation,31, 34, 73, 76, 119, 121, 137, 156, 161, 213, 229–231
• spin-on methods,89, 100, 112, 121, 157, 198, 200, 232, 233
• sputtering,119, 120, 145, 149, 154, 165, 234–236
air-knife
syringe pump
0.02 ml/min
hot N2
vacuum
rings
motor
3-way
valve
TPT
cartridge
heaters
ultrasonic
nozzle
syringe
Figure 3.1: TiO2 ultrasonic spray deposition system.
1.8 W
445qC
450qC
12 VDC
vacuum
line
thermo
-couple
switch (x2)
st. steel
block
relay
housing
TPT storage
bottle
ultrasonic
generator
PID
temperature
contoller
3.2 Overview of TiO2 Thin Film Deposition Methods
63
64
3. TiO2 Thin Film Deposition Equipment
• ion assisted deposition,72, 107, 140, 237, 238
• plasma anodisation,113
• reactive ion plating,121, 156, 213, 231
• laser ablation,132, 239
• ﬁltered arc deposition,33
• atomic layer epitaxy,240 and
• screen-printing.114, 241
A primary consideration is that the growth morphology, crystalline structure and stoichiometry of TiO2 thin ﬁlms are very sensitive to the deposition conditions.73, 110, 122, 237, 242 This
a disadvantage for many physical vapour deposition methods, such as evaporation, where a
large variation in the observed optical properties arise from only a small changes in the deposition conditions.107, 161, 164 Therefore, the need for stoichiometric TiO2 ﬁlms with minimal
absorption suggested that a deposition method where the ﬁlm stoichiometry is controlled by
a chemical reaction would enable more consistent results.
The chemical reaction of a TiO2 -precursor to form TiO2 can either proceed by hydrolysis
or pyrolysis. With hydrolysis systems, separate gas lines of nitrogen or argon are bubbled
through heated baths of a liquid TiO2 precursor and water. The two delivery lines are then
brought together close to the substrate where the reaction takes place. Pyrolysis systems
are similar, except that a water bath is not required as the TiO2 precursor decomposes
upon reaching the heated substrate. Both of these systems have the advantage of simplicity,
although they may be relatively inﬂexible, as tubing diameters have to be designed around
predicted ﬂow rates.
To keep the deposition system as simple as possible, maximise throughput, and keep costs at
a minimum, systems with a vacuum chamber were not considered to be a viable option. This
excluded evaporation, sputtering, and the majority of CVD systems. However some experiments were performed with an APCVD system,204 owned by Eurosolare S.p.A. (Nettuno,
Italy). The APCVD system is designed around a belt furnace, and, as the name implies, the
depositions are performed at atmospheric pressure, so no vacuum is required. The system
is capable of depositing TiO2 onto 580 solar cells per hour,217 which corresponds to about a
5 MW yearly throughput for screen-printed solar cells.
Spin-on methods were not seriously considered, as the throughput of any such system would
be limited in a production environment. Screen-printing is commonly used in the PV industry for depositing metallic contacts, about 30 − 50 µm thick, to solar cells. Szlufcik et
al. demonstrated that screen-printing could also be used for depositing TiO2 thin ﬁlms.114
As the thickness of the metallic contacts are about 500 times thicker than the TiO2 thin
3.3 Ultrasonic Spray Deposition
65
ﬁlms it is not known how the thinner ﬁlms behaved with regard to reproducibility, squeegee
wear, and thickness uniformity. Following the screen-printing of the organometallic ink, the
samples were ﬁred in a three-step process of 30 min duration. The ﬁring is a relatively slow
process as ﬁrst the thick ﬁlm needs to settle for 15 min to obtain a uniform ﬁlm, with subsequent drying performed at 125◦ C for 5 min. The ﬁnal crystallisation was performed in a belt
furnace at temperatures between 500◦ C and 900◦ C. Lengthy drying procedures are required
to remove the substantial amount of organic solvents added to the TiO2 precursor. This
was also necessary in the pneumatic spraying technique used by Kern and Tracy.46 Kern
and Tracy developed a system for production was developed that was capable of coating
4500 cells per hour (around 30 MW per year), based on batch processing. Each batch took
30 s to receive a coating, followed by three separate heating steps to remove organic groups
in the ﬁlm. It has been noted by other researchers that the temperature required to crystallise an amorphous ﬁlm is signiﬁcantly greater than the temperature needed to grow such
a crystalline ﬁlm.74 Therefore, to lower thermal the budget and processing costs it would
be desirable to deposit a polycrystalline TiO2 thin ﬁlm in one step without subsequent heat
treatment steps.
3.3
Ultrasonic Spray Deposition
Several researchers have used USD for depositing TiO2 35, 65, 106, 206, 210, 212 and other thin
ﬁlms.209, 211, 243 Blandenet et al. describes a method for depositing many diﬀerent metallic oxides based on the pyrolysis of an aerosol.210 If an ultrasonic beam is focussed on the
surface of a liquid the vibrations result in the formation of on aerosol. The chemical to
be sprayed is contained in a glass container attached to a high frequency generator that
vibrates at 800 − 1000 kHz. The chemical precursor for TiO2 ﬁlms was butyl orthotitanate
diluted in acetyl-acetone and butanol. Air or nitrogen is passed through the glass container,
transporting the aerosol close to the heated substrate, which is subsequently decomposed by
pyrolysis. Other researchers have also used commercially available ultrasonic nebulizers,65, 211
while more recently ultrasonic atomizing nozzles have been used.35, 58, 106, 212, 243 Versteeg et
al. used an ultrasonic nozzle to inject small quantities of a TiO2 precursor into a vacuum
chamber, maintained at a pressure of 0.1 − 1 Torr.106 It is stated that the low pressure chamber facilitates ﬁlm uniformity over large areas. Liang implemented an ultrasonic nozzle as a
modiﬁed injector in an APCVD system.58 DeSisto and Henry deposited magnesium oxide
thin ﬁlms by USD.243
The primary advantages of USD are that, ﬁrstly, there is a very narrow size distribution of the
droplets in the aerosol. Secondly, by altering this droplet size, the droplet→solid reaction
mechanism can also be changed. This is also dependent on gas ﬂows and the nozzle to
substrate distance. For example, smaller droplets will have already decomposed by pyrolysis
and will strike the wafer as a solid, while larger droplets will not have had time to vaporise
66
3. TiO2 Thin Film Deposition Equipment
completely. Alternately, the spraying conditions can be adjusted so that the majority of the
aerosol is a gas when it contacts the wafer, resulting in a process similar to CVD.209, 210, 244
Additional advantages of USD that were potentially relevant for this work, included:
i) The potential ﬂexibility of the system and excellent parameter control. This included
low ﬂow rates, variation of spray angle, variation of power to atomize liquids of diﬀerent
viscosities and at diﬀerent ﬂow rates, and control over the shape of the spray plume
using air-jets.245, 246
ii) The possibility of spraying at a shallow angle to keep the TiO2 out of the grooves,
scribed on the front side of a buried-contact solar cell. In this manner, the simple
geometry of the system prevents the spray from entering the grooves.20, 202, 203 With
other deposition systems a lot more care would need be taken to keep the grooves
free of TiO2 . This is especially true for the majority CVD systems which provide a
conformal coating of the surface.
iii) Since the distribution of the droplet sizes can be varied by choosing the excitation
frequency (see Section 3.4), it was postulated that larger droplet sizes (greater than
the groove width) would not enter the grooves, but would still coat the front surface
of the wafer.247
iv) It is possible to pulse-feed the liquid into the nozzle, enabling very low ﬂow rates to
be achieved.106, 245
v) It is possible to shape the spray plume by directing jets of gas across it. This can be
used for focussing the spray plume or to spread it out.245
vi) As decomposition by hydrolysis or pyrolysis is a chemical process, this would ensure that stoichiometric TiO2 was deposited. The deposition of TiO2 by physical
vapour deposition methods, evaporation and sputtering for example, can result nonstoichiometric TiOx ﬁlms. TiOx ﬁlms exhibit very diﬀerent properties including increased absorption and a metallic or semiconducting behaviour.
vii) No vacuum chamber would be required.
viii) The nozzles do not clog or wear out.245
ix) The system would be easy to clean by ﬂushing with a solvent,248 such as isopropyl
alcohol.
x) Since overspraying is avoided, material consumption can be reduced by up to 80%.245
As well as avoiding waste, it was anticipated that this would reduce the build-up of
TiO2 powder in the system.
3.4 Theory of Ultrasonic Spray Deposition
67
xi) Spraying may be suitable for production if accompanied by a suitable pump, with a
continuous ﬂow, such as a gear pump or pressurized feed, and either multiple nozzles
or a single traversing nozzle.46
xii) An USD system can be used to deposit a variety of ﬁlms, including AR coatings and
SiO2 passivation layers. Additionally, it is possible to add other dopant liquids to the
main precursor.65
xiii) TiO2 ﬁlms deposited by spray pyrolysis at 450◦ C are known to produce nearly dense,
optically transparent, anatase ﬁlms.177, 206
The use of an ultrasonic spray nozzle appeared to oﬀer the best ﬂexibility and potential
for the new system. Out of the ﬁve surveyed models available on the US and Australian
markets, only one (Sono-Tek Corp., U.S.A.) had been previously used in published scientiﬁc
literature. This brand also had the advantage of being able to operate nozzles with a diﬀerent
atomisation frequency from the one ultrasonic generator.
3.4
Theory of Ultrasonic Spray Deposition
Blandenet et al. provide a good description of USD-deposited ﬁlms produced by an
aerosol.210 In the case of most metal oxide depositions, the aerosol is a colloidal dispersion of a organometallic liquid in a carrier gas. The liquid is atomized in a glass vessel with
a transducer operating in the kHz to MHz frequency range. The carrier gas transports the
aerosol close to the heated substrate, where it is decomposed by pyrolysis. Alternately, the
aerosol can react with water vapour to complete the reaction by hydrolysis. In such systems,
unlike pneumatic spraying, the gas ﬂow rate is independent of the aerosol ﬂow rate.
In an ultrasonic nozzle, two piezoelectric transducers are contained within the nozzle, as
shown in the cut-away view in Figure 3.2. This sets up a standing wave and both ends
of the nozzle become anti-nodes. The junction between the two transducers is a node, a
point of zero amplitude, because the transducers have their polarities opposed. This causes
the transducers to either expand or contract against each other. The standing waves and
anti-nodes are illustrated in Figure 3.3.248
As the TPT passes through the nozzle, the ultrasonic vibrations (48 − 120 kHz) form an
aerosol. The mean diameter of the droplets produced depend upon the excitation frequency
f , the surface tension σ, and the density of the liquid ρ being atomized210, 245
8πσ
3
d =
k
ρf 2
8πσ
3
∼
,
(3.1)
0.34
=
ρf 2
68
3. TiO2 Thin Film Deposition Equipment
Figure 3.2: Cut-away view of Sono-Tek ultrasonic nozzle.248
Figure 3.3: Standing waves inside ultrasonic nozzle.248
where k is a constant, and its value of 0.34 determined by Lang.249 This relationship is
plotted for a range of frequencies in Figure 3.4 for water (ρ = 1 g/cm3 and σ = 0.073 N/m)
and isopropyl alcohol (ρ = 0.80 g/cm3 and σ = 0.0217 N/m) at temperatures of 20 − 25◦ C.245
The intercepts on the graph indicate the median droplet diameters for water at the excitation
frequencies used in this work, 3.1µm at 48kHz and 1.7 µm at 120 kHz. Although the density
of TPT is similar to that of water at 0.955 g/cm3 ,250 no data on the surface tension could be
found. Therefore, the size of the TPT aerosol droplets could not be accurately estimated.
Although small droplet sizes are achievable with pneumatic spraying, the main advantage
of USD is the narrow size distribution of the droplets.210 Figure 3.5 shows the distribution
of water droplet sizes for the range of excitation frequencies 25 − 120 kHz.245 The number
median diameter deﬁnes the 50% value of drop size, meaning that one half of the drops
Median Droplet Diameter, d (µm)
3.4 Theory of Ultrasonic Spray Deposition
10
69
3
Water
Isopropyl Alcohol
o
10
2
10
1
10
0
(at T=20-25 C)
1
10
100
Excitation Frequency, f (kHz)
1000
Figure 3.4: Median droplet diameter as a function of excitation frequency
for water and isopropyl alcohol.
have diameters larger than this, while the remaining half are smaller. The number mean is
obtained by summing the diameters of each drop together and then dividing by the number of
drops in the sample. The weight mean diameter is calculated by taking the adding together
the volume of each drop in a spray sample, taking the cube root of this sum, and dividing by
the number of drops. The Sauter mean diameter measures the eﬀective ratio of drop volume
to surface area and is primarily used for combustion applications.
Other factors aﬀecting the operation of ultrasonic spraying are liquid viscosity, solids content, and the miscibility of components.245 Although there are no hard-and-fast rules for
determining the suitability of a liquid for ultrasonic spraying, there are several guidelines.
Generally, the higher the viscosity or solids-content of a liquid, the lower the maximum ﬂow
rate. Liquids with a viscosity of up to 500 mPa s (or 50 centipoise) can be readily atomized,248 where water and TPT have a viscosity of 1 mPa s and 3.5 mPa s,251 respectively, at
20◦ C. Liquids containing long polymer chains can interfere with the atomization process
and may inhibit the formation of discrete droplets. It has been found that liquids with a
solids content of up to 40% can be successfully atomized, however the particle size should
be less than one-tenth of the droplet diameter.245 The maximum ﬂow rate Fmax has been
empirically determined to be
Fmax = k
A
f 2/3
(3.2)
where the constant k has the value k ≈ 28500 l Hz2/3 /(s m2 ), and A is the atomizing surface
area. Figure 3.6 plots Equation 3.2 for the range of ultrasonic nozzles produced by Sono-Tek.
The speciﬁc ﬂow rate r is the maximum ﬂow rate Fmax divided by the atomizing area A.
The speciﬁc ﬂow rate for the 120 kHz nozzle, not shown in Figure 3.6, is 11 l s−1 m2 . More
importantly for research applications though, is the minimum ﬂow rate. The minimum ﬂow
70
3. TiO2 Thin Film Deposition Equipment
Figure 3.5: Drop size distribution for ultrasonic nozzles operating at various frequencies (from Berger 245 ).
rate is typically about 20% of the maximum ﬂow rate, as below this rate the liquid emerges
from the nozzle in a non-uniform manner and the spray plume becomes distorted.245
3.5
3.5.1
TPT: The TiO2 Precursor
Why TPT?
Titanium isopropoxide, also known as tetraisopropyl titanate (TPT), was chosen as the
TiO2 precursor. Apart from being the most commonly used precursor in the literature,
this chemical is also used on solar cell production lines. The use of any precursor can
result in contamination of the TiO2 ﬁlm with by-products of the precursor. Metal-organic
precursors, such as TPT, often result in carbon contamination due to the residue of the
organic binders.89–93, 95, 96, 98 This is typically in the order of a 1 − 2 at. % for ﬁlms deposited
at low temperatures. However, several researchers have observed that at higher deposition
or annealing temperatures (400 − 600◦ C) the carbonate species can decompose, resulting in
the removal of hydrocarbon fragments.64, 99, 252 Chen et al. noted that the decomposition
of TPT to TiO2 is a very clean process in which carbon is not signiﬁcantly trapped either
3.5 TPT: The TiO2 Precursor
71
Figure 3.6: Speciﬁc ﬂow rates as a function of excitation frequency (from
Berger.245 )
within the crystalline ﬁlm or at the grain boundaries.135 Titanium tetrachloride (TiCl4 ) is
another common TiO2 precursor, which results in chlorine contamination.91, 93, 101 In addition
corrosive by-products (HCl) are produced in the reaction.66, 91 In one instance the chlorine
contamination was so high that it prevented crystallisation of the ﬁlm and poor ﬁlm adhesion
onto the substrate resulted.101 In any case, the level of contamination observed with TPT
is much smaller than with TiCl4 .97
Further advantages of TPT are that:
i) It is non-corrosive124 and non-toxic, listed as being a mild skin and eye irritant.250
ii) It can be highly puriﬁed and has an almost indeﬁnite shelf-life.124
iii) As a liquid, it is relatively easy to handle,97 although it should not be exposed to a
naked ﬂame.250 The fact that it is not dangerous makes the addition of TPT to a
CVD system a relatively easy and safe task, as no special gas handling equipment is
required.97
iv) It is very volatile at low temperatures (50◦ C), which means that it will be readily
decomposed.97
v) It can be ultrasonically sprayed directly without dilution.49
vi) It has been observed that there is enough oxygen in the TPT molecule that the reaction
to form TiO2 can proceed without additional oxygen in the ambient.94, 99, 101, 124, 221
72
3. TiO2 Thin Film Deposition Equipment
3.5.2
The TPT→TiO2 Reaction
The mechanism of the reaction of TPT aerosol to form TiO2 depends on the droplet size.
If the majority of the aerosol is a gas when it contacts the substrate then the deposition
conditions are similar to a CVD process.210 Aerosols with larger droplet sizes will not have
had time to vaporise completely, while smaller droplets will be decomposed by pyrolysis before striking the substrate. Since the droplet size distribution is small in ultrasonic spraying,
the same decomposition conditions will apply to nearly all of the aerosol. The deposition
conditions vary also with temperature, as the diagram for pyrolysis in Figure 3.7 shows,
however other factors such as ﬂow rates and geometry also play a role. In process A, the
decomposition rate at very low temperatures (< 100◦ C) will be slower than the deposition
rate and a liquid ﬁlm will form on the surface. This layer will slowly dry, however it will still
contain many organics and probably cracks.115 In process B, the droplets evaporate before
reaching the surface and a precipitate strikes the substrate where decomposition occurs. In
process C, the solid precipitate melts and vaporises (or sublimes) and the vapour diﬀuses
to the substrate and undergoes a reaction there. This corresponds to true CVD. At higher
temperatures (process D), the vapour undergoes a chemical reaction before impinging upon
the substrate. The droplets in the aerosol have formed solid particles that stick to the surface
of the substrate. The product information sheet on DuPont’s “TYZOR” TPT notes that
TPT pyrolyses at temperatures greater than 350◦ C, and that ﬁlms deposited in this method
at 500 − 600◦ C are considerably harder than ﬁlms produced by hydrolysis and contain no
organic residue.252
VXEVWUDWH
ILQHO\GLYLGHG
VROLGSURGXFW
YDSRXU
SUHFLSLWDWH
{
{
{
{
$
%
&
'
/RZWHPSHUDWXUH
GURSOHWV
+LJKWHPSHUDWXUH
Figure 3.7: Pyrolysis decomposition as a function of temperature (adapted
from Vigui`e and Spitz 244 ).
The decomposition of TPT (by pyrolysis) to form TiO2 proceeds as follows:253
Ti(OC3 H7 )4 −→ TiO2 + 2C3 H7 (OH) + oleﬁns ,
(3.3)
3.5 TPT: The TiO2 Precursor
73
For the reaction of TPT to form TiO2 by hydrolysis, the reaction product will be strongly
dependent on the amount of water vapour present in the system, as well as the substrate
temperature. Wong et al. found that for APCVD-deposited TiO2 , there is not enough
oxygen to form TiO2 and the TPT remains unreacted on the wafer when less than 30%
relative humidity exists.204 The reason why the TPT did not react by pyrolysis in this
case (Tdep = 250◦ C) remains unclear. When spraying in an environment with greater than
45% relative humidity, the TPT reacts before long before reaching the wafer and a powdery
white deposit results.46, 202, 203 Wong et al. mentioned that laboratory scale CVD experiments
showed it was possible to deposit ﬁlms with 15% thickness uniformity at humidities above
45%.204 In general, the recommended range in relative humidity is from 30% to 45%, which
results in a transparent homogeneous ﬁlm being deposited.202, 203 In this case, the reaction
rate is limited by the amount of TPT reaching the wafer,205 which decomposes by hydrolysis
onto the heated wafer, according to the two-step hydrolysis/degradation reaction in Equation
3.4.210, 253 The “Tyzor” TPT product information sheet (Du Pont, Inc.) contains a complete
explanation of all the steps involved in the hydrolysis reaction.252 In the information it is
also noted that whether hydrous titanium dioxide (TiO2 ·H2 O) or TiO2 itself is formed as
the ﬁnal product is dependent on the temperature and the rate at which water is added to
the system.
Ti(OC3 H7 )4 + 2H2 O −→ Ti(OH)4 + 4C3 H7 (OH)
Ti(OH)4 + 4C3 H7 (OH) −→ TiO2 + 4CH3 CH(OH)CH3
(3.4)
In Equation 3.4, one mole of TPT reacts with two moles of water vapour to form one mole
of TiO2 and 4 moles of by-product (mostly 2-propanol). Practically however, the amount of
water vapour will depend on the geometry of the reaction zone, and gas ﬂow and exhaust
rates.217 TiO2 ﬁlms deposited in an APCVD system required more than four times this
amount of water vapour for the above reaction to occur.204 As the substrate temperature is
increased, the deposition chamber, or environment, will heat up and the relative humidity
will decrease. However this is not strictly of great concern, as the number of moles of water
vapour going into the system will remain constant, and this is what is required.
The amount of TPT and water vapour required for the reaction to occur, as given in Equation
3.4, can be calculated. The molar mass mm of TiO2 is 79.9 g/mol. The number of moles M
of TiO2 to cover a 25 cm2 surface area with a 70 nm thick ﬁlm (volume, ν = 175 × 10−6 cm3 )
is given by the following equation, where the density ρ of rutile is 4.26 g/cm3
ρν
mm
= 9.3 × 10−6 moles TiO2
M =
(3.5)
This means we require 9.3 × 10−6 mol of TPT and 18.6 × 10−6 mol of H2 O for the reaction in Equation 3.4 to occur. The molar mass of TPT is 284.26 g/mol and its density is
74
3. TiO2 Thin Film Deposition Equipment
0.955 g/cm3 .250 Rearranging Equation 3.5 we obtain
M mm
ρ
= 2.8 µl of TPT to cover the 25 cm2 area with a 70 nm thick ﬁlm.
ν =
(3.6)
The amount of water vapour required has been estimated at being four times greater again
due to unreacted water vapour being extracted out the exhaust.204 However, in the system
constructed for this work, many times this amount (approximately 0.5 − 1 ml) of TPT is
needed to be sprayed as the ﬁlm density is lower than that of single crystal rutile and not all
the TPT sprayed is deposited within this area. This is especially true for shallow deposition
angles, where the majority of the TPT passes across the surface of the wafer. Based on the
requirement of 1 ml of TPT being required for each 5 cm × 5 cm wafer we can expect that at
least 4 ml of water vapour would be required.
3.6
3.6.1
Design of Ultrasonic Spray Deposition System
Selection of Ultrasonic Nozzle
Two ultrasonic atomizing nozzles (UAN) were purchased from Sono-Tek Corporation (Milton, N.Y., U.S.A.). There were several reasons for selecting an ultrasonic nozzle, rather than
an ultrasonic nebulizer. Firstly, with a nozzle the spray could be directed at diﬀerent angles.
Secondly, it was believed that a spray system where the reaction mechanism could be altered
with the drop size could be tuned to behave similarly to a belt-furnace APCVD system204
as used in the PV industry. Thirdly, the potential of a solely spray-based processing system
for fabricating solar cells was attractive. As well AR coating deposition, spray systems have
been used for depositing passivation layers for solar cells.208
The two nozzles operated at frequencies of 48 kHz and 120 kHz, respectively. One motivating
factor for nozzles manufactured by Sono-Tek was that only one broadband ultrasonic generator (BUG) was required to operate any of their nozzles. In contrast, other manufacturers
require that a separate BUG be purchased for each nozzle. The 48 kHz nozzle was selected in
order to evaluate the system performance at high ﬂow rates, and also to determine whether
large droplets would be prevented from entering the 20 − 30 µm wide grooves on the front
surface of the wafer. The 120 kHz nozzle was selected because it was able to tolerate the
lowest ﬂow rates. Later on in the project the 48 kHz nozzle was exchanged for a newly
released nozzle called the MicroSpray. This nozzle used a 120 kHz atomizing frequency as
well, however it was ﬁtted with a micro-bore tube which signiﬁcantly reduced the inside
diameter of the nozzle. This enabled extremely low ﬂow rates to be tolerated, while still
achieving acceptable spray plumes. Figure 3.8 illustrates the 48 kHz standard nozzle and
the 120 kHz MicroSpray nozzle purchased from Sono-Tek. The 120 kHz standard nozzle is
3.6 Design of Ultrasonic Spray Deposition System
75
similar in appearance to the MicroSpray nozzle, except that it possessed a conical tip (see
Figure 3.8).
Figure 3.8: The 48 kHz (left) and 120 kHz (right) ultrasonic nozzles purchased from Sono-Tek Corp (adapted from Sono-Tek Corp.246 ).
The front and rear horns of the nozzles are manufactured from titanium alloy, Ti-6Al-4V,
while the nozzle housing and liquid inlet are made from 316 stainless steel. The titanium alloy
is chosen for is high mechanical strength, good acoustical properties and excellent chemical
resistance.245 Most chemicals, except hydroﬂuoric and sulphuric acid and strong oxidizing
agents, are compatible with these spray nozzles. Table 3.1 provides technical details on the
Sono-Tek MicroSpray ultrasonic atomizing nozzle as well as the standard 120 kHz and 48 kHz
nozzles. The dimensions in Table 3.1 correspond to the schematic of the nozzle shown in
Figure 3.9.
Figure 3.9: Dimensions of the Sono-Tek ultrasonic atomizing nozzles
that correspond to the values given in Table 3.1 (adapted from Sono-Tek
Corp.246 ).
76
3. TiO2 Thin Film Deposition Equipment
Table 3.1: Sono-Tek ultrasonic atomization nozzle data (from Sono-Tek
Corp.246 ). Note that dimensions are not exact as they are converted
from inches. So few experiments were performed with the 48 kHz nozzle
that some parameters are unknown and are labelled in the table as not
applicable (n/a).
Nozzle Type
Microspray
UAN
UAN
UAN
Atomisation frequency (kHz)
Oriﬁce diameter (mm)
Maximum ﬂow rate (ml/min)
Recommended minimum ﬂow rate (ml/min)
Actual minimum ﬂow rate (ml/min)
Atomization power at minimum ﬂow rate (W)
Median drop diameter (µm)
Weight (g)
Dimensions: A1 (mm)
B1 (mm)
A2 (mm)
B3 (mm)
C (mm)
D (mm)
E (mm)
F (mm)
120
0.38
2.4
0.48
0.02
1.1 − 2.0
18
120
1.32
21
4.2
0.35
1.1 − 2.0
18
196
5.8
11.2
−
−
25.4
36.6
12.7
8.6
48
2.18
72
14.4
n/a
n/a
38
309
11.7
26.9
−
−
37.3
38.1
42.9
−
Parameter
3.6.2
−
−
2.5
11.5
29.3
36.6
12.7
10
Ultrasonic Nozzle Performance
It was quickly determined that the 48 kHz nozzle would be unsuitable for our application.
This was due to the extremely high ﬂow rates, which caused a much thicker ﬁlm than desired
to be deposited in less than one second and was diﬃcult to control. The 120 kHz proved
to be better with spraying times in the order of 1 min for a 70 nm thick ﬁlm at the lowest
ﬂow rates. Both of these nozzles possessed a conical tip, as shown in the left hand side of
Figure 3.9, which is designed to spread the spray plume out over several inches. This tip was
initially favoured as it was believed that this coverage would be suﬃcient to coat a 2” wafer.
However, to achieve longer deposition times, the lowest ﬂow rates had to be used. Although a
ﬁne mist often emerged from the nozzle the low volumes being pumped were not suﬃcient to
create a plume of several inches in diameter. Occasionally, the nozzle would also “stall” and
spit larger droplets of liquid onto the substrate. This resulted in 1 − 30 µm diameter TiO2
particulates being incorporated into the ﬁlm. These defects reduced the chemical resistance
of the ﬁlms.
To improve the ﬁlm uniformity, reduce particulate incorporation, and to achieve lower ﬂow
3.6 Design of Ultrasonic Spray Deposition System
77
and ﬁlm deposition rates the 48 kHz nozzle was exchanged for a 120 kHz MicroSpray nozzle.
This nozzle had a much narrower bore enabling very low ﬂow rates could be used (down
to 0.02 ml/min). Additionally, the tapered tip (see tip on right-hand side of Figure 3.9)
resulted in a much ﬁner line of spray. Achieving a consistent spray was aided by the use of
the small-bore 1 ml gas-tight syringes (see Section 3.6.3). This setup resulted in a deposition
time of 1 − 2 min and the best performance of all conﬁgurations tried for the spray system.
The power required from the broadband ultrasonic generator (BUG) to atomize the TPT
with the 120 kHz nozzles was in the order of 1.5 W.
3.6.3
Liquid Delivery
Pump
Selecting the correct pump is a crucial step in designing the spray deposition system. This
is because the ultrasonic nozzle will atomize any liquid that reaches the atomizing surface.
A syringe pump (Yale YA-12, Kent Scientiﬁc, U.S.A.) was chosen primarily because it can
pump continuously in the µl/min range and pulsed volumes of nanolitres can be achieved.245
The upper limit on the pumping rate and the dose is limited by the maximum syringe size,
in this case 60 ml. The pumping action is very smooth, which is important for achieving
good spraying. The pump is programmable and can be controlled via TTL signals or an
RS-232 port. In this work it was controlled via the front panel only. This enabled a ﬂow
rate, typically 0.02 − 1.00 ml, to be set and the pumped volume to be monitored on the
display. It is also possible to inject or withdraw with the pump. The ability to withdraw
liquid was used in conjunction with a three-way valve in order to reﬁll the smallest (1 ml)
syringes (refer Figure 3.11).
Figure 3.10: The Yale YA-12 syringe pump.
78
3. TiO2 Thin Film Deposition Equipment
Syringes, Tubing, Valves and Bottles
Initially experiments were performed with disposable 5 ml and 10 ml plastic syringes with
rubber-tipped plungers. By observing the spray plume it was noticed that the plunger was
periodically sticking as it travelled down the syringe barrel. It was discovered that the
ability of a syringe pump to dose at consistently low ﬂow rates is inﬂuenced by the diameter
of the syringe. Therefore, several high-quality 1 ml and 2.5 ml gas-tight syringes equipped
with Teﬂon plungers were purchased from a chromatography supplier (SGE, Melbourne,
Australia). These performed excellently and could be ﬂushed with dilute HF to remove any
build-up of TiO2 .
All the syringes used were of the Luer-lock variety to ensure an air-tight and leak-proof
connection. A variety of chemically resistant plastic (HDPE) ﬁttings were purchased (Chromalytic Technology, Victoria) to adapt from Luer-lock to barbed ﬁttings for 1/8” outer
diameter (1/16” inner diameter) tubing. The 1/8” plastic tubing (LDPE) then connected
directly to the 1/8” Swagelok liquid inlet ﬁtting on the Sono-Tek nozzle.
As mentioned previously, a Teﬂon-lined gas-tight three-way valve (SGE, Australia) was later
installed to permit withdrawal of new TPT precursor from a 100 ml screw-top chemical
bottle. This was convenient for the 1 ml syringes since coating a 2”diameter wafer with a
70 nm thick TiO2 typically required 0.5 ml of TPT. The gas-tight syringes were able to screw
directly into the valve, however it was necessary to purchase Kalrez ﬁttings (SGE, Australia)
to connect the 1/8” tubing.
Figure 3.11 indicates how the Schott chemical bottles were modiﬁed to contain TPT. Initially
a hole was drilled in plastic lid and a thread tapped into the hole. A female Luer adapter
was screwed into the lid, sealed with a Viton o-ring, and the 1/8” tubing extended down into
the bottle. A Teﬂon solvent ﬁlter (SGE, Australia) was pressed onto the end of the tubing,
and this rested about 5 mm above the bottom of the bottle. This was to remove as many
particulates as possible from the TPT. Particulates can slowly form due to the bottle being
opened and closed, and also if it is not totally air-tight.
3.6.4
Substrate Heater
The previous substrate heater for the pneumatic spray system at UNSW consisted of a stainless steel block that was placed on a 2 kW stove element.203 This was less than ideal as the
majority of heat generated did not pass into the block. Therefore, a new block was designed.
Stainless steel was still used as it was anticipated that the block would need to withstand
temperatures of 500◦ C or more. Other materials considered included copper, graphite, nickel,
molybdenum and titanium. Copper is an undesirable material for semiconductor devices due
to the speed that which it can diﬀuse through silicon. The remainder of the materials were
3.6 Design of Ultrasonic Spray Deposition System
(QG
FDS
79
2'
/'3(WXELQJ
0DOH/XHUWR
EDUEDGDSWHU
)HPDOH/XHUILWWLQJ
VFUHZHGLQWROLGZLWK
9LWRQIOXRULQDWHG
UXEEHURULQJ
2'
/'3(WXELQJ
6ROYHQW
)LOWHU
Figure 3.11: The creation of a ﬁltered TPT reservoir from a chemical
bottle.
ruled out because of expense. Therefore, a 150 mm×150 mm×20 mm stainless steel block
was milled out for use as a substrate heater. Figure 3.12 shows the workshop drawings of
the block, including precisely milled holes for the six heater cartridges, thermocouple and
vacuum line.
Heater cartridges were selected as they would result in the best power to
heat conversion. The heater cartridges chosen were a split design (Dalton Electric Co., MA,
U.S.A.) that expand upon heating to ensure good thermal contact to the block (see Figure
3.13). This avoided the use of a copper thermally-conducting paste. The power rating of
each cartridge was 250 W and could be run oﬀ mains power (240 VAC . The cartridges were
custom designed so that a 20 mm length near the leads was not embedded into the block.
This cool-zone was important for keeping the shielding around the leads from burning or
melting and possibly shorting out.
Double layer ﬁbre-glass/ceramic sheets (6 mm thick each) were placed on ﬁve sides of the
block, and were contained by a stainless steel box with 1 mm thick walls. The insulation
was designed to prevent too much heat from being transmitted downwards, as this could
warp the rails of the driving mechanism below. A 3 mm diameter K-type thermocouple was
inserted into the block. The tip of the thermocouple was located underneath the centre of
the wafer. The output from the thermocouple was fed into a BTC-9090 PID temperature
controller. The output of the PID controller to the six heater cartridges was via a 240 VAC
10 A relay. As the heater cartridges could draw a maximum of 1.5 kW there was no risk of
80
3. TiO2 Thin Film Deposition Equipment
3
BACK
VIEW
Hole for thermocouple 3mm
diameter, 70mm deep
150
5
20
75
3
25
Vacuum
channels:
1mm wide,
1mm deep
35
45
15
3mm diam. hole
for thermocouple
75
150
TOP
VIEW
5
3mm diam.
vacuum line
10
1/8” NPT
thread
25
50
FRONT
VIEW
50
25
50
3
6x holes for heater
cartridge = 0.377”
10
0.377”
Figure 3.12: Schematic diagram of the stainless steel heater block.
20
3.6 Design of Ultrasonic Spray Deposition System
81
Figure 3.13: Six of these 6” long 250 W Dalton Watt-Flex heater cartridges were embedded in the stainless steel block to ensure maximum
heat transfer.254
blowing the fuse on the output side of PID controller. The block took about 15 min to reach
450◦ C from room temperature, and typically temperature ﬂuctuations while spraying were
not more than 5◦ C.
3.6.5
Motorized Stage
A motorized translation stage was designed so that the substrate could pass back-and-forth
beneath the spraying zone. The 12 VDC motor (RS Components, Sydney) had a continuous
torque rating of 300 mNm and a maximum torque rating of 600 mNm. At 12 VDC the motor
speed was 220 rpm. The motor was directly connected to a double-start threaded spindle
with a 2 mm thread pitch. This spindle passed through a Teﬂon nut that was mounted on a
plate connected to the underside of the heater block. The signiﬁcant mass of the heater block
(about 6 kg) was supported by two 12 mm diameter stainless steel rails. These 50 cm long
rails were machined precisely to accommodate four linear bearings. Once correctly aligned,
the block slid eﬀortlessly along the rails. Thus, by applying either a positive or negative
voltage to the motor the spindle turned and moved the block forwards or backwards. At
220 rpm the block travelled at a speed of 7 mm/s.
As it was necessary to keep the door to the spray chamber shut during depositions (see
Section 3.6.7), a simple circuit using a double-pole change-over (DPCO) 12 VDC relay was
implemented in order to reverse the direction of the stage. The relay has two pairs of
inputs. The ﬁrst pair consisted of +12 VDC and -12 VDC , while the second had the polarity
switched. Two sealed and chemically resistant momentary push-button micro-switches (RS
Components, Sydney) were used to trigger the state of the relay. Pressing one switch caused
the relay to “change-over” from its ﬁrst input to its second input pair, while pressing the
second switch caused the relay to go back to its initial state. Thus, the stage started moving
in one direction (the direction it was last travelling) until reaching the end of the rails
upon which the microswitch was depressed, and the stage changed direction. A fuse was
also included in this simple circuit to prevent the motor from burning out. A motor speed
controller was also added to the system to enable slower translation of the stage, however
this was rarely used. A 2 A 12 VDC power supply was suﬃcient to power all the electrics.
Finally, in order to keep the rails free from TiO2 dust, concertina-style rubber boots were
ﬁtted over the rails. These boots could be compressed to about 25% of their standard length.
82
3.6.6
3. TiO2 Thin Film Deposition Equipment
Spray Shaping
Spray shaping and directional control was necessary for several reasons. In early experiments
it was found that when the nozzle was placed directly above the substrate heater the ﬁne,
low-velocity (about 10 cm/s) mist did not fall on the wafer. This was because of hot air
rising from the 450◦ C block and carrying the spray droplets away with it. Experiments were
performed with the two “air shrouds” that were supplied with the nozzles (Sono-Tek Corp.).
This enabled the spray to reach the heated substrate, however there were many particulates
in the ﬁlm and the thickness uniformity was very poor.
After discussing our requirements with Sono-Tek Corp., a loan of a “vertical spray assembly”
was arranged. The vertical spray assembly, depicted in Figure 3.14, is designed to produce
wide spray patterns. The two streams of gas are slightly oﬀ-centre, resulting in the shearing
of spray plume.245 Wide spray patterns were able to be generated with the vertical spray
assembly, however these were far from uniform. Additionally, whatever nozzle-substrate
distance was used resulted in the incorporation of many particulates into the ﬁlm. It is
believed that the TiO2 particulates arise from droplets reacting with remnant humidity in
the air, entrained into the nitrogen gas stream. Since the nozzle is directly above the wafer
any particulates formed will fall onto the wafer and be incorporated into the ﬁlm.
As a solution, an “air-knife”, sometimes also called an air-guide, was purchased (Exair Corp.,
U.S.A.). The air-knife was 6” long and consists of two halves of an aluminium casing bolted
together. There is an 1/4” NPT ﬁtting at one end for the nitrogen inlet. At the front there
is a small slit running almost the whole length of the air-knife, which creates a high-velocity
sheet or curtain of nitrogen. The slit height is set by a plastic shim inside the air-knife. It
was found that by intersecting the emerging spray plume from the nozzle with a sheet of
nitrogen from the air-knife enabled the deposition of a visually acceptable ﬁlm on the wafer.
As the 6” wide curtain cooled the substrate signiﬁcantly a new shim was inserted, which
had created a slit with 4 mm width and 0.02” height. This reduced the amount of cooling
of the block signiﬁcantly.
3.6.7
Miscellaneous Equipment
Relative Humidity Sensor
As discussed previously in Section 3.5.2, the reaction of TPT to form TiO2 is extremely
sensitive to the relative humidity. Relative humidity RH is deﬁned as the partial pressure p
of water vapour in air divided by the vapour pressure of water ps at the same temperature.
To be able to monitor the relative humidity in the spray system, a sensor and digital meter
were purchased (Elan Technical Corp., CT, U.S.A.).
3.6 Design of Ultrasonic Spray Deposition System
83
Figure 3.14: Sono-Tek’s vertical spray assembly, designed for generating
wide spray patterns (from Sono-Tek Corp.246 ).
Oxygen Concentration Sensor
An oxygen sensor was added to the system because it was anticipated that spray depositions
with TPT diluted in a solvent, such as isopropanol, would be performed. Although the
volumes of solvents sprayed would be less than 1 ml, ensuring that the oxygen concentration
was low would inhibit the combustion of the solvent. Therefore an oxygen sensor (Electrovac
GmbH, Austria) was added to the system. As shown in Figure 3.15(a), when a voltage is
applied across the zirconia electrolyte cell, oxygen is pumped through the cell because oxygen
ions carry the current through the cell. By attaching a cap with a pinhole on the cathode
side of the cell and increasing the voltage, the current becomes saturated due to limited
transfer of oxygen ions to the cathode. This current is proportional to the ambient oxygen
concentration.255 The advantages of this sensor included having a linear output signal, no
cross-sensitivities to other gases, long life, and a very low temperature dependence of the
84
3. TiO2 Thin Film Deposition Equipment
signal. Due to a heater inside the package the sensor required the application of 2 V while
warming up (30 s) and then 4 V for constant operation. Figure 3.15(b) shows an image of
the oxygen sensor. Designing this power supply and installation of the sensor and a suitable
analogue meter was completed by a German practicum student, Manfred Fahr.
Figure 3.15: (a) Schematic indicating oxygen sensor operation, and (b)
Image of the oxygen sensor (adapted from Electrovac GmbH 255 ).
Spray Chamber
The spray chamber was fabricated from steel “speed-frame” and clear perspex sheeting,
with internal dimensions of 70 ×70 ×70 cm. The entire front side of the chamber was hinged
along one edge and could be opened to provide full access to the chamber. A series of 25 mm
diameter holes were drilled in the door and covered with a sliding plate. This was in order
to allow the entry of ambient air after spraying was completed. The top of the chamber was
connected to a 6” diameter exhaust line. The exhaust ﬂow could be controlled via a large
butterﬂy valve.
Nitrogen Heater
Due to the relatively high gas ﬂow rates used (see Section 3.6.8) signiﬁcant cooling of the
block occurred. With the block temperature set at 450◦ C the top surface of the wafer
was measured to be about 330◦ C while spraying. In order to reduce the amount of cooling
occurring it was decided to control the temperature of the nitrogen gas stream. The nitrogen
heater used a heater cartridge that was inserted inside a length of 3/8” stainless steel tubing.
It was designed to operate upright so that if the nitrogen gas ﬂow stopped the hot air for
around the heater would rise up to the top of the tube where a thermocouple was placed.
The system was designed to operate at temperatures up to 400◦ C, being controlled by a
BTC-2020 PID controller (ECE Fast, Melbourne).
3.6 Design of Ultrasonic Spray Deposition System
85
Air-Knife Heater
Although the nitrogen heater worked well, a with heated lines the temperature of the emerging gas had dropped only 30◦ C after passing through a 1 m length of tubing (with the heater
set to 200◦ C). However, when connected to the aluminium air knife, which acted as a very
eﬃcient heat sink, the emerging gas temperature was only a few degrees above ambient
temperature. Therefore a 240 VAC 100 W heater pad was obtained (RS Components, Australia) that was both ﬂexible and had a high-temperature adhesive applied to one side. This
was carefully folded around the air-knife and ﬁrmly held in place. This permitted heating
(not-controllable) of the air-knife up to temperatures of 190◦ C, which enabled the heated
nitrogen gas to reach the substrate.
3.6.8
Operation of the TiO2 Spray System
In its ﬁnal form, the spray system incorporated all of the above componentry, as well as
regulators and ﬂow meters to accurately control gas ﬂows (as shown in Figure 3.1). All the
ﬁlms deposited using this deposition system were formed by spray pyrolysis, due to the very
low relative humidity. The diagram in Figure 3.16 indicates the necessary steps to set-up
and perform depositions with the TiO2 spray system.
Table 3.2 in Section 3.7 lists the various deposition parameters of the system. The gas ﬂow
rates listed were optimised in order to extend the ﬁlm coverage in the forward direction,
while trying to reduce the amount of cooling of the substrate heater. At lower ﬂow rates, the
ﬁlm was not deposited onto the wafer due to heat arising from the 450◦ C block, while little
beneﬁt was achieved by using higher ﬂow rates. A typical atomisation power of 1.5 − 2.0 W
was required from the BUG for ﬂow rates between 0.02 − 0.10 ml/min. Spray depositions
were only performed at the maximum system operating temperature of 450◦ C as at lower
temperatures the number of particulates in the ﬁlm was unacceptable. It should be noted
that the deposition temperatures quoted throughout this work may be slightly higher than
the actual deposition temperature due to the cooling of the wafer by the gas ﬂow.
The deposition time and the eﬃciency at which the TPT is converted to TiO2 is greatly
inﬂuenced by the alignment of the system. The height of the nozzle and whether the tip sits
directly in the nitrogen ﬂow from the air-knife or just above it is important. The best results
were achieved by adjusting the height of the nozzle with the nitrogen ﬂowing, and when it
could be heard that the tip just entered the gas stream the nozzle was ﬁxed in that position.
The optimum angle for spraying was determined to be 5◦ below the horizontal. At this angle
the nitrogen emerging from the air-knife travels up towards the block and then adheres to
the top surface of the block as it passes across it. The adherence of a gas to a solid surface
is called the Coanda eﬀect. In general, spraying at angles closer to the horizontal reduced
the number of particulates observed in the ﬁlm, and also enabled only the top surface of
3. TiO2 Thin Film Deposition Equipment
Check that tip of spray nozzle is free TiO2 powder
Install a new 0.5 m length of LDPE 1/8" OD tubing between
three-way valve and nozzle
Withdraw 1 ml of TPT into gas-tight syringe
Syringe pump: set syringe diameter and desired flow rate
Purge chamber with N2 until RH ≈10%
Wait for substrate heater to reach 450°C
Inject TPT to fill "dead-space" in the tubing, valves and nozzle
Turn on motor to move stage away from centre and place
wafer on substrate heater
Using three-way valve, withdraw TPT from bottle into syringe
To begin spraying: adjust power on BUG to 1.2 W, start TPT
flow, and turn on air-knife N2
Leave motor off momentarily as there are often a few "spits"
at the start of spraying
Turn on motor and monitor colour of TiO2 film on wafer - stop
when it appears dark blue
To spray more wafers
86
Turn off syringe pump, motor and BUG, and remove wafer.
Open exhaust valve and purge system for 1 min.
To finish spraying, turn off heater block, remove nozzle and
syringe and rinse with IPA, discard tubing in waste container
Figure 3.16: Diagram indicating the necessary steps to set-up and perform depositions with the TiO2 spray system.
3.7 Design of CVD System
87
grooved wafers to be coated.
The order in which the various components are turned on and oﬀ are important. The tubing,
valve and nozzle should be ﬁlled with TPT before starting. Any excess TPT can be wiped
oﬀ the tip of the nozzle using a tissue and some isopropyl alcohol (IPA). The stage should
be positioned so that the initial spray plume will not land on the wafer. This is because
of a tendency for there to be more particulates in the initial spray plume. Secondly, the
air-knife ﬂow should be started and the BUG should be turned on. After a few seconds of
spraying the motor can be switched on and coating of the wafer will begin. To ﬁnish, it is
best to turn oﬀ the BUG ﬁrst, to stop he atomisation process as quickly as possible, then
the syringe pump, and ﬁnally the motor and air-knife ﬂow.
For the deposition of doped TiO2 ﬁlms (as in Section 6.3 of this work) the dopant liquid was
mixed together with the TPT in a small chemical bottle in a fumecupboard before spraying.
If many doped ﬁlms, especially of diﬀerent doping concentrations, were required it would
be recommended to replace the three-way valve with a four-way valve and to have separate
pure bottles of TPT and the dopant liquid. The syringe should be shaken to ensure good
mixing of the two liquids.
3.7
3.7.1
Design of CVD System
Motivation
Although the spray pyrolysis system produced quite dense anatase ﬁlms, the thickness uniformity on a macroscopic level was poor. On a scale of nanometres to micrometers the ﬁlms
were quite uniform, however, on a scale of millimetres to centimetres, thickness variations
of a similar magnitude to the average ﬁlm thickness were observed. This meant that, for a
70 nm thick ﬁlm, there were many points across the wafer that remained virtually uncoated.
This, along with the incorporation of particulates, typically 1 − 30 µm in diameter, had serious implications for chemical resistance and the electroless metal plating process. Other
limitations included only being able to operate the spray deposition system at the maximum
temperature of 450◦ C, in order to limit the number of particulates in the ﬁlm.
3.7.2
TPT Bubbler and Temperature Control
Therefore a simple CVD system was designed to replace the spray system. Attachai Ueranatusan (MEngSc, UNSW) had performed initial investigations into a CVD nozzle that
was fed with a vapour from a glass bubbler. This idea was adapted by the author into the
existing structure of the spray system. A one-litre stainless steel bubbler was purchased
(Meriter, U.S.A.) for safety as it could handle pressures up to 42 psi, much greater than the
88
3. TiO2 Thin Film Deposition Equipment
quartz bubbler. The use of electropolished stainless steel bubblers have been successfully
demonstrated in the semiconductor industry.256 It was found that there was essentially no
assay degradation when comparing the performance of quartz and stainless steel bubblers
(J.C. Schumacher, CA, U.S.A.) were the same and as long as moisture was prevented from
entering the bubbler. This is achieved by using metal gaskets instead of Teﬂon. A pressure
relief valve with a threshold of 35 psi was placed in parallel with the bubbler to prevent too
high a pressure being applied to the bubbler. The outlet of the pressure relief valve went
directly into the exhaust system.
A Schumacher temperature controller and temperature control unit were used to maintain
the bubbler temperature at 50◦ C. At this temperature, the TPT has a vapour pressure
of 1 mbar. The Teﬂon tubing leading from the bubbler to the nozzle was heated using a
1 m length of 240 VAC heater tape with a power of 90 W. This was insulated with glassﬁbre insulation tape, held in place with high-temperature heat-shrink tubing. This was to
prevent condensation of the TPT onto any cool surface so as to avoid the build up of TiO2
particulates in the tubing and nozzle. It was not necessary to hear the nozzle itself as its
close proximity to the heater block ensured that it maintained a temperature well above
50◦ C.
3.7.3
Water Vapour Bubbler
After initial success with TiO2 ﬁlms deposited using the CVD system, a second bubbler
containing de-ionised (DI) water was added to the system. This was designed to permit
reactions by hydrolysis. This was a standard quartz bubbler typically used for performing
wet oxidations in a tube furnace. A separate nitrogen regulator was used to limit the
pressure that could be applied to the bubbler to 5 psi. The bubbler had a matching base with
an integrated heater. The temperature was monitored with a thermometer and manually
maintained at approximately 100◦ C. A 3-way valve was attached to the outlet of the water
bubbler so that the water vapour could be ”switched-oﬀ” by redirecting the ﬂow to an empty
ﬂask. Figure 3.17 shows a diagram of the ﬁnal CVD system.
3.7.4
Operation of the CVD System
Operation of the TiO2 CVD system was quite similar to the spray system. About three
hours before depositions were performed it was necessary to switch on the temperature
controllers for the TPT and water bubblers. A timer was implemented for doing this so that
CVD depositions could begin early in the morning. Depositions could now be performed at
substrate temperatures of 150 − 450◦ C. Below 150◦ C the ﬁlm coverage became very nonuniform, probably due to the TPT striking the substrate as a liquid. As with the spray
system the air-knife angle and nozzle position were critical. It was found that the optimum
3.8 Conclusions
89
angle for the air-knife was about 2◦ above the horizontal, and the height on the nozzle was
adjusted in the same manner as previously described in Section 3.6.8. Again, the gas ﬂows
listed in Table 3.2 are the minimum ﬂows for successful TiO2 deposition to occur, while
trying to reduce the amount of cooling of the block occurring.
Table 3.2: Deposition conditions for the USD and CVD TiO2 ﬁlms
Process parameters
USD
CVD
3
Liquid TPT ﬂow rate (cm )
0.02 − 0.10
n/a
TPT bubbler ﬂow rate (scm3 )
n/a
1700
Air-knife N2 ﬂow rate (slpm)
5
13
Atomisation power (W)
1.5 − 2.0
n/a
Chamber pressure (kPa)
≈ 101
≈ 101
◦
TPT temperature ( C)
25
50
Substrate temperature (◦ C)
450
150 − 450
Nozzle-substrate distance (cm)
5−7
3−5
Relative humidity (%)
< 10
< 10
◦
Deposition angle ( )
−5
≈2
Deposition time (min)
1−2
10 − 20
3.8
Conclusions
A good understanding of basic TiO2 material properties along with knowledge of a previous
TiO2 spray deposition system at UNSW, formed the requirements for the TiO2 thin ﬁlms
in this application. Films were required to be dense and defect-free, exhibit good thickness
uniformity, possess a high refractive index and low optical absorption, and be insulating.
Examination of deposition techniques described in the literature lead to the author designing and constructing an ultrasonic spray deposition (USD) system. USD oﬀered several
many potential advantages, including shallow-angle depositions, low deposition rates, and
the ability to deposit for a wide range of thin ﬁlms via this method using diﬀerent liquid
precursors. Many of the desired TiO2 ﬁlm properties were obtained from ﬁlms deposited
using the USD system. However, the limitations of the USD system were, ﬁrstly, that the
thickness uniformity of these ﬁlms was poor, secondly, that large particulates were commonly
embedded into the thin ﬁlms (these ﬁlms will be characterised in detail in Chapter 4). Spray
depositions were performed at 450◦ C as a dramatic increase in the number of particulates
was observed at lower temperatures. Thus, although the ﬁlms exhibited a high refractive
index, the ability to tune the refractive index by varying the substrate temperature could not
be realised. This lead to the design and construction of a simple chemical vapour deposition
(CVD) system that could operate at atmospheric pressure. TiO2 ﬁlms deposited with this
simple CVD system are used throughout the majority of this work.
air-knife
quartz H2O
bubbler and
heater
N2
N2
N2 +
H2O
motor
1/8" st. vacuum
steel
rings
nozzles
N2 +
TPT
cartridge
heaters
heated
lines
TPT
heater
relay
housing
12 VDC
vacuum
line
thermo
-couple
switch (x2)
st. steel
block
st. steel
bubbler
N2
445qC
450qC
PID
temperature
contoller
TPT bubbler
temperature
contoller
49.9qC
50.0qC
90
3. TiO2 Thin Film Deposition Equipment
Figure 3.17: TiO2 CVD deposition system. The humidity sensor, exhaust
valve on TPT bubbler, and the 3-way valve on the water bubbler have been
omitted for clarity.
Chapter 4
Characterisation of TiO2 Thin Films
Extensive characterisation of TiO2 ﬁlms deposited using ultrasonic spray deposition (USD)
and chemical vapour deposition (CVD) was performed in order to determine the physical,
optical, electrical and chemical properties of the ﬁlms. All ﬁlms deposited at 450◦ C were of
the anatase phase. A surprising result was that USD anatase ﬁlms did not convert to rutile
after lengthy annealing at 950◦ C. USD TiO2 ﬁlms were found to be dense (3.64 g/cm3 ) and
exhibited a high refractive index (2.45 at 600 nm), ideal for acting as an antireﬂection (AR)
coating on a glass encapsulated silicon wafer. Although the USD ﬁlms appeared continuous
over a microscopic level, macroscopically the ﬁlms exhibited large variations in thickness
and the occasional pinhole. TiO2 ﬁlms deposited using CVD were found to exhibit much
lower surface roughness and better thickness uniformity, although the density was signiﬁcantly
lower than that of the USD-deposited ﬁlms. Shallow-angle depositions were successful in
maintaining the grooves free of TiO2 . However, shallow-angle depositions were not successful
on textured crystalline silicon wafers, with large tree-like structures growing at the tips of the
pyramids. The chemical resistance of all ﬁlms was excellent against acids, but relatively poor
against alkaline solutions, although the etch resistance did improve upon annealing of the
TiO2 ﬁlm.
4.1
Introduction
The literature review in Chapter 2 described the many diﬀerent material phases of TiO2 ,
and the variation of the material properties with those phases. The front-surface dielectric
ﬁlm in the buried-contact (BC) solar cell fabrication sequence is subjected to a number of
high-temperature processing steps in oxygen, nitrogen, and phosphorus-containing furnace
ambients. Therefore, before replacing the silicon dioxide (SiO2 ) layer in the original BC solar
cell with a TiO2 ﬁlm, it was necessary to perform extensive characterisation of the TiO2 thin
ﬁlms to, ﬁrstly, optimise the deposition parameters to obtain a TiO2 ﬁlm exhibiting the
91
92
4. Characterisation of TiO2 Thin Films
desired qualities and, secondly, to determine the behaviour of the ﬁlms under typical BC
fabrication sequence conditions. Diﬀerent characterisation techniques were utilised in order
to determine the physical, optical, chemical and electrical properties of the as-deposited and
annealed TiO2 thin ﬁlms, deposited using ultrasonic spray deposition and chemical vapour
deposition (CVD). Characterisation techniques can be roughly divided into the following
four categories:
• Physical Properties: The crystalline phase of the ﬁlm and the existence of oxygen
vacancies were determined using Fourier-transform infrared (FTIR) and Raman spectroscopy. The surface roughness of the ﬁlms was measured using atomic force microscopy (AFM). The presence of particulates and the density of the ﬁlms was observed using scanning electron microscopy (SEM). SEM work also provided a rough
indication of the grain size of the material. Elemental analysis of ﬁlm was performed
using both X-ray photoelectron spectroscopy (XPS) and Rutherford back-scattering
(RBS) spectroscopy.
• Optical Properties: The refractive index and extinction coeﬃcient of the ﬁlms were
measured using spectroscopic ellipsometry (350 − 1150 nm), ellipsometry (633 nm),
and reﬂectance measurements (300 − 1200 nm).
• Electrical Properties: Conductivity observed in some TiO2 ﬁlms that had undergone a
reaction was determined using a four-point probe (FPP). The degree of surface passivation aﬀorded by TiO2 ﬁlms and TiO2 /SiO2 stacks was determined using the transient
photoconductance decay (transient-PCD) technique (discussed further in Chapter 5).
• Chemical Properties: The chemical resistance of TiO2 thin ﬁlms were determined by
placing ﬁlms in various solutions commonly used in solar cell processing.
A brief introduction to each of these characterisation techniques will be provided here and
the results presented throughout this section.
4.2
FTIR Spectroscopy
FTIR spectra provide information regarding the bonding between atoms in a sample, and
are primarily used in the semiconductor industry to determine the existence of dopant or
impurity atoms. The nature of the technique is very quantitative in identifying the impurity
type, but very qualitative in determining the concentration of the impurity. FTIR spectra
are also demonstrated in this work to be useful for understanding changes occurring in TiO2
ﬁlms during high-temperature processing. For a discussion of the theory of FTIR refer to
works by Nakamoto257 and Griﬃths and de Haseth.258
4.2 FTIR Spectroscopy
93
FTIR measurements in this work were performed with a Nicolet 520 spectrometer in the
range 250 − 5000 cm−1 . A 4 cm−1 resolution was used and 256 scans were collected per
spectrum. Any oxygen and carbon in the ﬂoat-zone (FZ) wafers was accounted for by ﬁrst
performing a reference spectrum with only the FZ wafer. This reference spectrum was then
subtracted from subsequent sample spectra to yield information about the TiO2 ﬁlm.
Erkov et al. published some excellent FTIR spectra of 110 nm thick, LPCVD deposited TiO2
thin ﬁlms, as shown in Figure 4.1(a) and (b).117 The rutile ﬁlms are deposited on Si wafers
at 630◦ C using a TiCl4 precursor with N2 O and H2 ambient gases. The spectra shown in
Figure 4.1(a) are the as-deposited TiO2 ﬁlm (curve 1), samples annealed in a vacuum (curve
2), structures annealed in N2 O ambient (curve 3), and samples that had 10 nm of SiO2
grown prior to TiO2 deposition (curve 4). In Figure 4.1(b) the IR spectra of a bare Si wafer
with 0.4 − 0.6 nm natural oxide (curve 5) and of sample 3 after etching oﬀ the TiO2 in a hot
H2 SO4 etch (curve 6). The strong absorption peak at 608 cm−1 is inherent to the rutile phase,
however this is somewhat obscured by a weaker silicon absorption peak at 608− 610 cm−1 , as
seen in curve 5. The overlapping peaks at 423 cm−1 and 460 cm−1 are also characteristic of
the rutile modiﬁcation of TiO2 . An additional absorption peak is observed at 470− 480 cm−1
for samples annealed in N2 O (curve 3) and for structures with a 10 nm interfacial SiO2 layer
(curve 4). In the former case the peak at 480 cm−1 is attributed to the formation of Ti2 O3 .
This will be discussed in more detail in Section 5.2.2. The small peak at 515 cm−1 has been
observed for TiO2 samples annealed in vacuum only (curve 2). The existence of a peak
at 1108 cm−1 is indicative of the formation of SiO2 at the Si:TiO2 interface (curves 2, 3, 4
and 6). This peak is even observed in the vacuum (10−2 Torr) annealed samples where the
availability of oxygen would be extremely limited.
Figure 4.2 shows the FTIR spectra of two USD-deposited TiO2 ﬁlms from this work (TO-11
and TO-12), over the range where the spectra of coated wafers diﬀer markedly from the
bare silicon reference wafer. It can be seen that at wavenumbers greater than 570 cm−1 the
features observed in all spectra are similar and originate from the silicon substrate. A broad
absorption peak at 1083 cm−1 is the only diﬀerence between the TiO2 coated wafers and
the bare substrate. This peak is likely to be due to the formation of SiO2 at the TiO2 :Si
interface, and is normally observed at 1108 cm−1 .117
When comparing these thin ﬁlm FTIR spectra to reference spectra for bulk anatase and
rutile TiO2 obtained from the literature259, 260 it is immediately apparent that there is little
agreement between the absorption peaks. Only a local transmittance peak at about 380 cm−1
can be observed in all samples. The spectra of the bulk TiO2 samples may diﬀer greatly due
to other incorporated impurities or the measurements may not have been obtained at room
temperature. TiO2 samples TO-11 and TO-12 were loaded into the furnace under diﬀerent
conditions, and the eﬀect of the ambient furnace gas will be discussed in detail in Section
5.2.2. The spectrum of TiO2 ﬁlm TO-11 has been successfully modelled by the author using
the polarisation-dependent model recently applied to single crystal anatase.261, 262 The model
94
4. Characterisation of TiO2 Thin Films
Figure 4.1: (a) and (b) FTIR spectra of TiO2 thin ﬁlms on silicon wafers.
The spectra are of 1) an as-deposited TiO2 ﬁlm, 2) TiO2 ﬁlms annealed
in a vacuum, 3) TiO2 ﬁlms annealed in N2 O, 4) TiO2 ﬁlms deposited on
10 nm of SiO2 , 5) a bare Si wafer with 0.4 − 0.6 nm natural oxide, and
6) the spectra of sample 3 after etching oﬀ the TiO2 .117
presented in Equation 4.1 is based on the factorised form of the complex dielectric function
ε(ν).261
ε(ν) = ε1 (ν) − ıε2 (ν) = ε∞
ε2
n
LOn
ε2T On
− ε2 + ıγLOn ε
− ε2 + ıγT On ε
(4.1)
The longitudinal optical (LO) and transverse optical (TO) phonon oscillator frequency ν
(cm−1 ) and damping γ (cm−1 ) values are given in Table 4.1. The complex dielectric constant
is related to the complex refractive index by ε = n
2 (refer to Section 2.3.6 for more detail).
A similar model has been published in the past for rutile.263 The sample structure used
in the anatase model was a 300 µm thick polished silicon wafer with a 74 nm thick TiO2
ﬁlm deposited onto both surfaces. The dielectric constant of the TiO2 ﬁlm was determined
using a Bruggemann eﬀective medium approximation158 of 55.7% Ec-axis and 44.3% E⊥caxis. The modelling was performed using the WVASE32 software package.62 The best ﬁt
4.3 Raman Spectroscopy
95
obtained with the model is plotted in Figure 4.2 using the anatase values from Table 4.1.
It can be seen that the location of the absorption peaks are in excellent agreement with the
experimental results. This result is somewhat surprising as the as-deposited ﬁlms had been
subjected to 90 min of annealing at 950◦ C, which should be more than suﬃcient to observe
a transformation from anatase to rutile. The only similar result that can be found in the
literature (for undoped TiO2 ) is for anatase ﬁlms, deposited at 330◦ C, remained anatase after
annealing at 850◦ C.75 In order to conﬁrm that only the anatase phase was present Raman
spectroscopy measurements were performed on the ﬁlms.
Figure 4.2: Comparison of the FTIR spectra of spray deposited TiO2 thin
ﬁlms and the modelled anatase result obtained using Equation 4.1.
4.3
Raman Spectroscopy
Raman scattering occurs when light interacts with the optical phonons in a material. If the
photon gives up part of its energy to the lattice, in the form of a lattice vibration or phonon,
the photon emerges with a lower energy. It is also possible for a photon to absorb a phonon
and emerge at a higher energy, although these events much weaker. The theory of Raman
spectroscopy is discussed in detail in the works of Long264 and Nakamoto.257
Since the intensity of Raman scattered light is very weak, the intense monochromatic beam
from a laser is required. The collected signal is usually passed through a double monochromator and detected by a photodetector. The Renishaw Model 2000 machine located in the
School of Materials Science at UNSW is equipped with three lasers, with wavelengths of
514.5 nm, 632.8 nm and 780 nm. The system uses a prism to disperse the signal onto a CCD
96
4. Characterisation of TiO2 Thin Films
Table 4.1: LO and TO phonon frequencies for anatase and rutile to ﬁt dielectric function in Equation 4.1 (adapted from Gonzalez,261 and Gervais
and Piriou 263 ).
Anatase
Mode
E c-axis
(A2u )
Frequency
ν (cm−1 )
Rutile
Damping Frequency
γ (cm−1 ) ν (cm−1 )
Damping
γ (cm−1 )
TO
LO
367
755
68
79
172
796
76
38
E⊥ c-axis TO
(Eu )
LO
TO
LO
262
366
435
876
36
4.1
32
33
189
367
381.5
443.5
27
10
16.5
21.5
ε∞ (Ec-axis) = 5.41
ε∞ (E⊥c-axis) = 5.8
ε∞ (Ec-axis) = 7.8
ε∞ (E⊥c-axis) = 6.0
array. While this makes measurements very fast it also limits the accuracy of the instrument to about 1.6 cm−1 . An additional complication with the Renishaw machine is that
the absorption ﬁlter designed to remove the laser line at 0 cm−1 begins absorbing at about
200 cm−1 . This interferes with the dominant peak of anatase which lies at 143 − 144 cm−1 ,
and rutile also has a smaller peak that lies at the same frequency. Figure 4.3 illustrates the
Raman spectra for the three crystalline phases of TiO2 as well as the amorphous phase.265
Figure 4.3: Raman spectra of the various phases of TiO2 , obtained from
powder samples.265
4.3 Raman Spectroscopy
97
It can be seen that the peaks from each TiO2 phase are clearly separated in frequency and
therefore easily distinguishable. Table 4.2 presents the Raman peak assignments for single
crystal anatase and rutile, and also silicon. Silicon is included as the absorption depth of even
the most absorbing TiO2 ﬁlms deposited in this work is just over 1 µm at λ = 514.5 nm. Since
the ﬁlms are typically 70 nm thick, photons interact with the silicon, which has a similar
absorption depth to TiO2 at this wavelength, creating Raman scattering events. This is
conﬁrmed in the literature where the Raman peaks of silicon were still observed at 300 cm−1
and 520 cm−1 when performing measurements on 700 nm thick TiO2 ﬁlms deposited onto
silicon wafers using a laser of 532 nm.266 Thus, it is not possible to observe the A1g and B1g
modes of anatase for a thin ﬁlm deposited onto silicon.
Table 4.2: Raman active phonon frequencies for anatase and rutile
(adapted from Ohsaka et al.267 and Gonzalez 261 ).
Anatase
Mode Frequency
ν (cm−1 )
Eg
Eg
B1g
A1g
B1g
Eg
144
197
399
514
514
639
Rutile
Mode
Frequency
ν (cm−1 )
B1g
Eg
A1g
B2g
143
447
612
826
Figure 4.4 shows the behaviour of the Raman spectra during annealing to temperatures for
a ﬁxed time of 40 min.265 At about 250◦ C, the 141 cm−1 anatase peak appears and by 425◦ C
the conversion to anatase is complete.265 The anatase to rutile transformation is clearly
observable at 800◦ C, and at 1000◦ C the conversion to rutile is complete.
Raman measurements from this work are given in Figure 4.5. Note that the data for the
520 cm−1 silicon peak has been omitted for clarity. Samples 1 and 2 were deposited by
APCVD at Eurosolare S.p.A., Italy. Sample 1, deposited at 320◦ C, exhibits no anatase
peaks. Silicon peaks at 300 cm−1 and were observed, and the peaks at 622 cm−1 and 834 cm−1
could possibly be attributed to the A1g and B2g modes of rutile. The A1g peak is typically
very strong, while the B2g mode is very weak. Therefore it is concluded that the sample is
predominantly amorphous with a very small fraction of rutile. With increased temperature
(450◦ C) anatase peaks emerge at 143 cm−1 , 396 cm−1 and 637 cm−1 . The location of these
peaks is in excellent agreement with the bulk values given in Table 4.2. Samples 3 and
4 were both deposited at 450◦ C using ultrasonic spray pyrolysis, and sample 4 received a
subsequent 90 min anneal at 950◦ C. These results conﬁrm the earlier result that only the
anatase phase is present after a lengthy high-temperature anneal. The broad peak at about
98
4. Characterisation of TiO2 Thin Films
Figure 4.4: Raman spectra measured after annealing TiO2 powder samples at various temperatures.265
950 cm−1 is attributed to silicon-oxygen-titanium (Si-O-Ti) bond formation.268 Note that a
peak at 960 cm−1 is observed in silicon, however this is a multiple phonon event and it is
very weak.8 The Si-O-Ti peak indicates an interaction between the TiO2 layer and either
a thin or native SiO2 layer.266 This result is consistent with the FTIR ﬁnding that a SiO2
layer is formed at the TiO2 :Si interface. Fitzgibbons et al. noted that TiO2 depositions on
quartz substrates remained anatase after annealing at 1000◦ C for 20 hr.67 Therefore, it is
postulated that the presence of an interfacial SiO2 layer has resulted in the TiO2 maintaining
the anatase preferentially.
4.4
X-ray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy (XPS) is primarily used to identify chemical species at
the surface of a sample. When high-energy photons (X-rays) interact with atoms of the
sample via the photoelectric eﬀect, electrons are ejected from the core levels. However only
photoelectrons ejected from atoms in the top 5 − 50 ˚
A can escape and therefore be detected.
The primary strength of XPS is that it allows chemical and not only elemental identiﬁcation.
Depth proﬁling is also possible by performing ion-beam sputtering of the sample. Further
information on XPS theory and measurement techniques can be found in Briggs and Seah.269
The spectrometers used in this work were, ﬁrstly, a VG Scientiﬁc ESCALAB 220i-XL, using
monochromated Al Kα (1486.6 eV) radiation, with an accuracy of 0.2 at. %. Figure 4.6
shows the XPS spectrum resulting from a wide energy scan of a spray deposited TiO2 ﬁlm
on silicon. This was a surface scan performed with no etching of the ﬁlm. Apart from
the expected titanium and oxygen peaks, a signiﬁcant carbon peak is observed. This peak
is primarily due to adsorbed carbon from the atmosphere, as the sample was stored in
4.5 Rutherford Back-Scattering Spectroscopy
99
Figure 4.5: Raman spectra of TiO2 thin ﬁlms deposited by APCVD (samples 1 and 2) and ultrasonic spray pyrolysis (samples 3 and 4). Anatase
(A) and silicon (Si) Raman peak assignments are shown. Samples 1 and
2 were deposited at 320◦ C and 450◦ C, respectively, while samples 3 and
4 were both deposited at 450◦ C. Sample 4 received a subsequent 90 min
anneal at 950◦ C.
air. This adsorbed carbon had an atomic concentration of 13% at the top surface. As
the ﬁlm was etched this reduced quickly to below 1%, indicating that 1% was the level
of contamination resulting from the organic precursor (see Section 5.3.1). The location of
the carbon peak at 285 eV serves the useful purpose of providing a calibration point for
the spectra, accounting for any charging eﬀects.94 Windows around the titanium, oxygen,
carbon and silicon peaks were drawn and examined in greater resolution. The O1s peak at
531 eV exhibits a slight shoulder at exhibited a shoulder at about 533 eV, and this has been
attributed to the adsorption of water vapour onto the top surface.94 The results of further
XPS experiments will be discussed in Section 5.3.1.
4.5
Rutherford Back-Scattering Spectroscopy
Rutherford back-scattering spectroscopy (RBS) is based on bombarding a sample with highenergy helium ions and measuring the energy of these back-scattered ions. With this method,
the masses of elements and their depth distribution in the sample can be determined. The
depth resolution of RBS is typically 100 ˚
A, and atomic concentrations down to 10 − 100 ppm
can be detected. Figures 4.7(a) and (b) indicates how the measured RBS spectrum corresponds to the elements in the sample structure.270 Since Au, Ag and N are only on the
surface in Figure 4.7(a), the RBS signals have a narrow spectral distribution. It also demon-
100
4. Characterisation of TiO2 Thin Films
Figure 4.6: XPS surface scan over a wide range of binding energies of a
TiO2 ﬁlm spray deposited onto a silicon wafer.
strates that, ﬁrstly, the RBS yield increases with atomic number. Secondly, the RBS signal
of elements lighter than the substrate (nitrogen in our example) will appear superimposed
on the substrate signal, while heavier elements will be displayed as separate peaks.270 Figure
4.7(b) demonstrates how the RBS signal for gold broadens as ions back-scattered from deeper
within the gold ﬁlm have lower energies due to additional energy loss mechanisms within the
ﬁlm. More detailed theory on RBS can be obtained from Schroder270 or Breese et al.271
The RBS results in this work were obtained with a collimated beam of 4 He2+ ions at an
energy of 2 MeV. The beam was scanned over an area of 1 × 1 mm2 . Backscattered ions
were detected with a surface barrier detector with a solid angle of 28 msr at a scattering angle
of 145◦ . The beam was provided by the University of Melbourne 5U pelletron accelerator.
Figure 4.8 shows the RBS spectrum obtained from a TiO2 ﬁlm deposited by ultrasonic spray
pyrolysis onto a silicon wafer and a RUMP simulation272 of a 100 nm thick TiO2 ﬁlm.
As discussed in Section 2.2.5, the density of the ﬁlm can also be determined from the RBS
spectrum. The areal densities of the titanium and oxygen in the ﬁlm in Figure 4.8 were
ρareal = 1.51 ± 0.01 × 1017 atoms/cm2 ρareal = 3.1 ± 0.2 × 1017 atoms/cm2 , respectively. The
resulting stoichiometry is determined to be 2.02 ± 0.13, being limited by the noise in the
oxygen statistics.273 Further results from RBS will discussed in Chapter 6.
4.6 Ellipsometry
101
Figure 4.7: (a) Calculated RBS spectrum for a silicon sample with the
elements gold (Au), silver (Au) and nitrogen (N) on the top surface.
Note the narrow peaks. (b) Au depth proﬁle information corresponds to
an increased Au peak width.270
Figure 4.8: RBS spectrum of a TiO2 ﬁlm on a silicon wafer (solid line)
and a RUMP simulation 272 of a 100 nm thick TiO2 ﬁlm.
4.6
4.6.1
Ellipsometry
Overview
Ellipsometry is primarily used in the semiconductor industry for accurately measuring the
thickness of thin dielectric ﬁlms. However, ellipsometry is also a very powerful tool for
determining the optical constants of a ﬁlm. The following brief introduction to ellipsometry
has been adapted from J.A. Woollam.62
102
4. Characterisation of TiO2 Thin Films
Ellipsometry measures the change in the state of polarisation of light that is reﬂected from
the front surface of a sample. The measured ellipsometric parameters Ψ and ∆ are related
p and R
s for p- and s-polarised light, respectively.
to the ratio of the Fresnel coeﬃcients R
Figure 4.9 shows an incoming linearly polarised beam, with the p-direction lying in the plane
of incidence (the plane that contains the incident and reﬂected beams) and the s-direction
(from Senkrecht, German for perpendicular) lies perpendicular to the p-direction.
Figure 4.9: An ellipsometry experiment, showing the p- and s-directions
(from J.A. Woollam 62 ).
and the electric ﬁeld E
Figure 4.10 compares linearly, circularly and elliptically polarised light. By looking at the
in a plane perpendicular to the direction of propagation the polarisation
electric ﬁeld vector E
lies in one line at all times
of the light can be determined. For linearly polarised light (a), E
and there is no phase diﬀerence. In the case of circularly polarised light (b) the Ex and Ey
are of equal magnitude but 90◦ out of phase. Circularly polarised light
components of E
may precess either clockwise or counter-clockwise around the circle. In general, the Ex and
Ey -ﬁelds do not have to be of equal magnitude and could possess any phase relationship. In
traces out an ellipse as a function of time.
this case (c) the tip of the electric ﬁeld vector E
p and R
s is deﬁned as the complex reﬂection ratio
The ratio of the reﬂection coeﬃcients R
ρ, which is related to the measured ellipsometric parameters Ψ and ∆ by
ρ=
p
R
= tan(Ψ) expı∆
s
R
(4.2)
The schematic diagram in Figure 4.11 shows the typical componentry of a simple ellipsometer. A collimated beam of unpolarised light becomes linearly polarised after passing through
the polariser (P). The compensator (C), or retarder, changes this linearly polarised light
the elliptically polarised light. The compensator contains a fast and slow optical axis perpendicular to the direction of transmission. Therefore one component will become retarded
in phase relative to the other component. After being reﬂected oﬀ the front surface of the
sample the linearly polarised light enters the analyser (A) (similar to the polariser). With
4.6 Ellipsometry
103
Figure 4.10: Diagram looking into the propagating beam, showing (a)
linearly, (b) circularly and (c) elliptically polarised light (adapted from
J.A. Woollam 62 ).
null ellipsometry the signal is extinguished by the analyser and a zero output is observed at
the detector. Then by ﬁxing certain angles the number of solutions is reduced down to one
pair of Ψ and ∆. There are other ellipsometric conﬁgurations (e.g., rotating analyser like
the VASE instrument) and the reader is referred to J.A. Woollam62 for further information.
In the example shown in Figure 4.11 light is reﬂected at an air-substrate interface. In this
Figure 4.11: Schematic diagram of a typical ellipsometry setup (adapted
from 270 ).
simple case Fresnel’s equations can be used to show that the complex refractive index n
can
be determined from the measured Ψ and ∆ values:270
n
= n1 − ık1
= n0 tan(φ) 1 −
4ρ
sin2 (φ) .
(1 + ρ)2
(4.3)
104
4. Characterisation of TiO2 Thin Films
If the ellipsometric ratio ρ from Equation 4.2 is measured at the incident angle φ and n0
is known (unity for air) then the refractive index n and extinction coeﬃcient k can be
calculated.
In Figure 4.12(a) and (b) the behaviour of the p- and s-components of the reﬂectance and
the ellipsometric parameters Ψ and ∆ are plotted for a bare crystalline silicon substrate for
light of λ = 633 nm. It can be seen that as the angle of incidence increases Ψ reaches a
minimum at 75.5◦ . At this angle ∆ changes from very close to 180◦ to 0◦ . This angle is
known as the Brewster angle and is determined by
1
−1 n
,
φ
B = tan
n0
(4.4)
0 is the ambient
where φ
B is the Brewster angle (complex for an absorbing substrate), n
refractive index (air in our example) and n1 is the complex refractive index of the substrate
(silicon). The absorption in the silicon substrate prevents the p-polarised component and Ψ
from going to zero.
In summary, two important advantages of ellipsometry are that, ﬁrstly, because the ratio
of two numbers is measured it is highly accurate and reproducible and, secondly, the phase
information ∆ makes the measurement very sensitive.
4.6.2
Ellipsometers
Three diﬀerent ellipsometers were used in this work. A single wavelength G¨artner L116A
ellipsometer equipped with a He-Ne laser (632.8 nm) was used for quick determination of
ﬁlm thickness and refractive index immediately after ﬁlm deposition or high-temperature
processing. It was attempted to measure all ﬁlms at both 50◦ and 70◦ as a consistency
check. However, this often was not possible due to the ﬁlms acting as an AR coating a
reducing the signal signiﬁcantly. Instead, three to ﬁve measurements were made at 70◦ and
these values averaged.
The second ellipsometer equipped with three lasers (Paciﬁc Solar Pty Ltd, Sydney), with
wavelengths of 632.8 nm, 831.7 nm and 1299 nm. This enabled a few points on the dispersion
curve of n and k to be measured. Additionally, at each wavelength the sample was measured
at incident angles from 45◦ to 80◦ in 5◦ steps providing a high degree of conﬁdence in the
extracted thickness and optical constants.
Variable angle spectroscopic ellipsometry (SE) measurements were performed in the Department of Physiology at the University of Western Australia with a VASE machine (J.A.
Woollam Co., Inc.). Traditionally, a monochromatic light source such as a He-Ne laser is
used as a source, however in the case of SE a xenon lamp and a monochromator are implemented instead. The ability to measure data a wide range of wavelengths (250 − 1700 nm)
4.6 Ellipsometry
105
Figure 4.12: Behaviour of (a) p- and s-polarisation components, and (b)
Ψ and ∆ for a bare silicon substrate (adapted from J.A. Woollam 62 ).
and at multiple angles means that SE is a very powerful technique for determining the optical constants of single or multilayer samples. For the TiO2 on silicon samples measured in
this work the wavelength range was set to 350 − 1150 nm due to a noisy signal outside this
range. The noise in the UV was due to the limited signal from the lamp, while the noise
above 1150 nm arose from the type of ﬁbre optic cable implemented. The wavelength range
350 − 1150 nm is excellent for silicon solar cells, with little energy from the sun at less than
350 nm and silicon has an extremely low absorption coeﬃcient above 1150 nm. Data were
collected for each sample at angles of 65 − 80◦ in 5◦ steps.
106
4.6.3
4. Characterisation of TiO2 Thin Films
Lorentz Oscillator Model
All modelling of optical constants was performed using the WVASE32 software package.62
The Lorentz oscillator model was chosen for ﬁtting the dispersive optical constants to the
ellipsometric data. The Lorentz oscillator model is based on the assumption that the response
of electrons in a material to the light beam is similar to the response of a harmonically
driven mass on a spring subject to a force (friction).62 In this analogy, the mass represents
the electron, the spring corresponds to the electrostatic forces on the electron (due to all the
other electrons and nuclei in the material), and the friction represents the electron energy
loss due to the emission of a photon. The imaginary part of the dielectric function ε2 is
proportional to the power per unit volume absorbed from a monochromatic light source of
a given wavelength.62 To apply this to the mass on the spring analogy for the electron, the
power absorbed by the mass from the driving force is then calculated using electromagnetic
units. After determining ε2 the Kramers-Kronig transformation can be applied to obtain
the real part of the dielectric function ε1 . The relationships between the dielectric constants
ε1 and ε2 and the refractive index n and extinction coeﬃcient k are
ε1 = n 2 − k 2
(4.5)
ε2 = −2nk
(4.6)
The Lorentz oscillator model is Kramers-Kronig consistent, and is usually expressed in terms
of the dielectric constant, where
ε(E) = ε1 − ıε2
= ε1 (∞) +
N
2
Eni
i=1
Ai Bi Eni
.
− E 2 − ıBn E
(4.7)
In Equation 4.7, the incident photon energy E has the units of eV, ε1 (∞) is the value of
the real part of the dielectric function at very large photon energies (dimensionless), Ai is
the amplitude of the ith oscillator (dimensionless), Bi is the broadening of the ith oscillator
(eV), and Eni is the centre energy of the ith oscillator (eV). Initial modelling was performed
with a single Lorentz oscillator, however it was subsequently found that a double Lorentz
oscillator was able to model the experimental behaviour of some TiO2 ﬁlms much better.
These situations included when the measurements approached the bandgap of anatase or
rutile, or when there was an anatase/rutile phase mixture present in the sample. Other
researchers have made use of similar single and double oscillator models to describe the
dielectric function of a material.45, 140, 144, 151
WVASE32 uses the mean-squared error (MSE), based on the chi-square (χ2 ) test, to determine the quality of match between the data calculated from current model (ψ mod , ∆mod ) and
4.6 Ellipsometry
107
the experimental data (ψ exp , ∆exp ). The MSE is deﬁned in Equation 4.862

2 2 
N
exp
exp
mod
mod
∆i − ∆i
− ψi
1
 ψi exp

M SE =
+
exp
2N − M i=1
σψ,i
σ∆,i
=
(4.8)
1
χ2 ,
2N − M
where N is the number of (ψ, ∆) pairs, M is the number of variables in the model, and σ are
the standard deviations on the experimental data points. The MSE should reach an absolute
minimum for the best ﬁt, and this minimum should be fairly sharp. However, insensitive
parameters and correlation between parameters can hinder the determination of an absolute
minimum MSE. It is therefore necessary to spend a signiﬁcant amount of time developing
the initial model for the deposited material and substrate. Once established, it is then easier
to apply this model to similar ﬁlms with greater conﬁdence.
4.6.4
Surface Roughness Model
The most common method of modelling surface roughness of TiO2 thin ﬁlms is to use
a Bruggeman eﬀective medium approximation (EMA), comprised of 50% void and 50%
TiO2 .82, 128, 161, 274 The expression for the Bruggeman EMA was given in Equation 2.10 in
Section 2.3.6. The TiO2 fraction of the EMA was coupled to the underlying dense TiO2 layer,
linking the optical constants of the surface roughness layer to the denser TiO2 layer below.
The only parameter permitted to vary in the EMA layer was its thickness. This approximation for surface roughness has been used successfully by other researchers for modelling
TiO2 ﬁlms.82, 128, 275 Mardare and co-workers82, 275 noted that surface roughness signiﬁcantly
changed the values of the optical constants, and suggested independently conﬁrming the surface layer roughness using atomic force microscopy (refer Section 4.9). Furthermore, it was
stated that for large surface roughness a graded layer model becomes necessary. The author
evaluated this option however it was found that the increased number of ﬁtting parameters
easily lead to unphysical results. The results using either a single or double Lorentz oscillator
coupled with the 50% void/50% TiO2 EMA layer were found to be entirely satisfactory.
4.6.5
Ellipsometric measurements of Spray Deposited TiO2 Thin
Films
The optical constants of a TiO2 ﬁlm spray deposited at 450◦ C are plotted in Figure 4.13. A
second TiO2 ﬁlm deposited at 450◦ C but annealed at 950◦ C was also measured. The optical
constants were determined using the three-wavelength ellipsometer. As the optical constants
in the region 632.8 − 1299 nm are fairly non-dispersive the reﬂectance of the same samples
was also measured using a ultraviolet-visible-near infrared (UV-vis-NIR) spectrophotometer
108
4. Characterisation of TiO2 Thin Films
(see Section 4.7). A Lorentz single-oscillator model was used for both ellipsometric and
reﬂectance data. It can be seen from Figure 4.13 that the agreement in the refractive indices
is very good, while there is some discrepancy in the extinction coeﬃcient data (note that
k is on a log scale). The surface roughness of these ﬁlms is large (refer to Section 4.9)
and, as discussed previously in Section 2.3.1, surface roughness can aﬀect measurements
of the extinction coeﬃcient signiﬁcantly. Additionally, it is known that ellipsometry is very
sensitive to non-uniformities in both thickness and refractive index.276 For these reasons, the
reﬂectance data is believed to provide the best estimate of the behaviour of the extinction
coeﬃcient of the spray deposited TiO2 ﬁlms.
Figure 4.13: Comparison between the optical constants of as-deposited
and annealed TiO2 ﬁlms using three-wavelength ellipsometry (symbols)
and reﬂectance (curves) measurements. The hollow symbols (2,◦) are
refractive index values while the solid symbols (,•) represent extinction
coeﬃcient values.
The refractive indices of the spray deposited ﬁlms before and after annealing (at λ = 600 nm)
are 2.449 and 2.458, respectively. The high refractive indices observed here are possibly linked
to the high deposition rate of the spray system.277 Additionally, the high refractive index of
the as-deposited ﬁlm indicates that they are very dense. Equation 2.2 from Section 2.2.5 can
be used to estimate the densities of the two ﬁlms - the refractive indices of 2.470 and 2.476 (at
λ = 550 nm) correlate to densities of 3.63 g/cm3 and 3.64 g/cm3 , respectively. This compares
to a density of 3.84 g/cm3 for single crystal anatase. These results can be compared to other
TiO2 ﬁlm densities reported in the literature. Ottermann et al. deposited stoichiometric
anatase ﬁlms using e-beam evaporation at 320◦ C.278 These ﬁlms had a refractive index
of 2.345 at 550 nm and a density of 3.3 g/cm3 . Bendavid et al. measured the density of
ﬁltered-arc deposited anatase thin ﬁlms to be 3.82 g/cm3 with a refractive index of 2.62 at
4.7 Reﬂectance Spectrophotometry
109
550 nm.33 This result is interesting in that from Equation 2.2 a refractive index of 2.62
should correspond to a density of 3.98 g/cm3 , which is greater than the bulk anatase value.
Obtaining a density greater than the bulk phase would require Ti and O atoms to occur
interstitially within the anatase crystal. This would most likely result in a fraction of the
ﬁlm converting to the more stable phase of rutile. In the X-ray diﬀraction plot indicating
the anatase phase a small peak for rutile(110) is visible. This small rutile fraction could
explain the high ﬁlm density.
Furthermore, based on the results from Sections 4.2 and 4.3, which indicated that the ﬁlms
are 100% anatase, the porosity of the ﬁlms can then also be determined using Equation 2.3,
with the bulk anatase value nb being 2.532 at 600 nm. This calculation results in a ﬁlm
porosity of 7.6% for the as-deposited sample and 6.8% for the annealed sample.
4.6.6
SE measurements of CVD TiO2 Thin Films
The thin ﬁlms deposited via CVD had much smoother surfaces (refer to Section 4.9). Additionally, the restriction on the range of deposition temperatures was relaxed, and depositions
could be performed anywhere from 150−450◦ C. This enabled the refractive index of the TiO2
ﬁlms to be varied. The presence of water vapour during depositions was also investigated.
The results of these experiments are presented and discussed in detail in Chapter 7.
4.7
Reﬂectance Spectrophotometry
Reﬂectance spectrophotometry is typically employed for measuring the thicknesses and optical constants of deposited thin ﬁlms. The theory of the method is well developed and excellent explanations can be found in the literature.38, 279, 280 The reﬂectance spectra of TiO2
ﬁlms were measured using a Varian Cary 5G ultraviolet-visible-near infrared (UV-vis-NIR)
spectrophotometer equipped with a Labsphere DRA-50 integrating sphere. This enabled reﬂectance measurements in the wavelength range 250 − 2500 nm to be performed. Reﬂectance
measurements were performed for determining the optical properties of spray deposited TiO2
thin ﬁlms (see Section 4.6.5) where ellipsometric determination at short wavelengths was not
possible. Additionally, the optical constants of multilayer TiO2 ﬁlms (see Chapter 7) were
determined using both spectroscopic ellipsometry and reﬂectance spectrophotometry.
4.8
Electron Microscopy
Field emission scanning electron microscopy (FESEM) digital images of TiO2 ﬁlms were
recorded using a Hitachi S-900 instrument with an accelerating voltage of 10 − 12 kV. Even
110
4. Characterisation of TiO2 Thin Films
though the ﬁlms were insulating, it was not necessary to coat the ﬁlms with a thin layer of
chromium. Images of USD-, APCVD- and CVD-deposited TiO2 thin ﬁlms will be shown in
the following sections.
4.8.1
USD-Deposited TiO2 Thin Films
Cross-sectional SEM images of USD-deposited TiO2 ﬁlms on silicon substrates are shown in
Figure 4.14(a), (b) and (c). Figure 4.14(a) shows an as-deposited TiO2 ﬁlm. Figure 4.14(b)
indicates the possible existence of voids underneath the TiO2 ﬁlm. The uniform, thin ﬁlm on
top of the silicon substrate is SiO2 . The formation of this ﬁlm will be discussed in Chapter
5. The image in Figure 4.14(c) is the same ﬁlm as in (b), however the magniﬁcation is three
times greater at 600,000 times. In all the Figures it can be seen that the USD-deposited ﬁlms
appear to be continuous and dense. This is in agreement with the high refractive indices
measured in Section 4.6.5. Figure 4.15 shows a plan-view of an as-deposited (450◦ C) TiO2
ﬁlm. Again, the crystals appear to be densely packed, and the possibility of some sintering
having occurred is not ruled out.
4.8.2
APCVD-Deposited TiO2 Thin Films
The SEM image in Figure 4.16 indicates the presence of particulates, both incorporated into
and on top of the TiO2 ﬁlm, as well as cracks in the ﬁlm. Cracks were only observed in ﬁlms
deposited using a commercial APCVD system, and the limited and infrequent access to the
deposition system meant that it was not possible to determine their origin. Particulates
were observed in both USD- and APCVD-deposited ﬁlms. The particulates shown in Figure
4.16 are representative of smaller particulates, with larger sizes (up to 30µm) being observed
in USD-deposited samples. The inability to deposit TiO2 ﬁlm without the presence of
particulates was a major motivating factor for developing the CVD-based system.
4.8.3
CVD-Deposited TiO2 Thin Films
Figure 4.17 contains four plan-view SEM images of TiO2 thin ﬁlms, all deposited via CVD
at 450◦ C. In (a), angular crystallites can be seen, many of which are separated by large
voids. It is immediately apparent that the ﬁlms deposited via CVD are less dense than
the USD-deposited ones. The lower-density of the as-deposited CVD ﬁlms was reﬂected
in the refractive index measurements in Chapter 7. After annealing for one hour (b), the
agglomeration of grains are observed and the edges of the crystallites have become rounded.
Annealing at a slightly higher temperature for a period of 6 hr resulted in the majority
of grains sintering together. The largest voids in between the grains remain “unbridged”.
Annealing for a further 16 hr at 1000◦ C results in an almost continuous ﬁlm, with small voids
4.8 Electron Microscopy
111
Figure 4.14: USD-deposited TiO2 ﬁlms on Si substrates: (a) one sample
as-deposited, (b) a second sample, indicating the possible existence of
voids underneath sections of the ﬁlm, and (c) a higher magniﬁcation
image of (b) showing the dense and continuous nature of the ﬁlm.
(typically 40 nm in diameter) remaining. Figure 4.17(a)−(d) demonstrate that a sintering
process has occurred, similar to that illustrated in Section 2.2.1. The densiﬁcation of the
TiO2 layer is accompanied by a reduction in ﬁlm thickness, from 79 nm in (a) to 64 nm after
step (d). The correlation between density and refractive index is discussed in Section 2.2.5.
Due to the horizontal angle of the impinging vapour it is not possible to coat textured wafers
with this TiO2 deposition method. The result of performing a 1 hr deposition at 450◦ C is
dramatically shown in Figure 4.18. Large crystal structures over 1 µm in size have grown on
the tips of the pyramids, while the growth of smaller structures also appeared on the upper
regions of the pyramid edges. The structures observed here are very similar to the “grasses”
and “plants” observed by Goossens et al.,101 obtained using a TiCl4 /TPT precursor mixture.
It was noted that these fractal structures are good for solar harvesting in Gr¨atzel-type solar
cells, due to their large surface area. It was not attempted to further adjust the system due
to the requirement of the horizontally incident vapour in order to maintain the grooves free
112
4. Characterisation of TiO2 Thin Films
Figure 4.15: As-deposited TiO2 ﬁlm USD-deposited onto a silicon substrates. The crystals seem to be relatively densely packed. The darker
regions are most likely crystals of a diﬀerent orientation.
Figure 4.16: SEM image of an APCVD-deposited TiO2 ﬁlm exhibiting
particulates and cracks.
from TiO2 .
It was anticipated that the horizontally impinging stream of TPT vapour would result in
TiO2 ﬁlm deposition on the top surface of a wafer but not in the grooves. To test this
4.8 Electron Microscopy
113
Figure 4.17: SEM images of TiO2 ﬁlms deposited by CVD at 450◦ C: (a)
as-deposited; (b) 1 hr anneal (load 800◦ C and ramp to 950◦ C); (c) 6 hr
anneal at 1000◦ C; and (d) 22 hr anneal at 1000◦ C.
hypothesis the grooves and busbar on the wafer were oriented at 45◦ to the incoming vapour,
as illustrated in Figure 4.19(a). A new busbar design was implemented in order to keep the
grooves free of TiO2 . This involved increasing the step the laser took in between scribing
adjacent busbar grooves, as shown in Figure 4.19(b) and (c).
Figure 4.20(a), (b) and (c) are SEM images of a silicon wafer after performing CVD deposition at an incident angle of 45◦ . The V-shape groove in Figure 4.20(a) arises from the
114
4. Characterisation of TiO2 Thin Films
Figure 4.18: SEM images of CVD TiO2 ﬁlms deposited with a horizontally impinging TPT vapour onto textured silicon wafers at 450◦ C.
E
F
D
737
Figure 4.19: Diagram depicting: (a) the 45◦ orientation of the busbar
and ﬁngers to the impinging TPT vapour; (b) the cross-section of the new
busbar, designed to keep the grooves free of TiO2 ; and (c) a traditional
BC solar cell busbar.
NaOH based anisotropic groove etch used. Large overhanging TiO2 deposits can be seen on
the top-edges of both sides of the groove in Figure 4.20(b) and (c). These deposits are up to
3 µm in length. It is believed that this will not inhibit electroless metal plating and, in fact,
a previous work may indicate that the overhanging dielectric may help promote plating at
the depths of the groove rather than near the surface.281
An energy dispersive X-ray spectrometry (EDS) attachment to a SEM enables detection and
4.9 Atomic Force Microscopy
115
identiﬁcation of X-rays produced by the impact of an electron beam on the sample. The
detection limit of EDS is approximately 0.1 − 1.0 at.%. The EDS spectra of a TiO2 -coated
BC solar cell are given in Figure 4.21. The spectra from the top surface exhibits a large
peak at 1.75 keV (Si), a small Si sum-peak at 3.5 keV, and a large Ti peak at 7.5 keV. The
Si sum-peak results from two Si atoms hitting the detector simultaneously, and therefore an
energy of 3.5 keV is measured. The Si sum-peak is typically 1000 times smaller than the
main Si peak at 1.75 keV. In the groove spectrum, the height of the Ti peak is reduced to
roughly four times the height of the Si sum-peak. Although EDS is essentially a qualitative
measurement, the relative heights of the Si sum-peak and the Ti peak can provide some
indication as to the abundance of Ti in the grooves. Therefore, some TiO2 was observed in
the grooves, however it is believed that this amounts to a fraction of a percent. The peaks
at 4.5 keV and 4.95 keV belong to nickel (Ni). Thus, as well as plating in the heavily-doped
grooves, Ni is also plating either on top of the TiO2 or is contacting the doped emitter
through the TiO2 .
Figure 4.20: (a), (b) and (c): SEM images of large TiO2 deposits at the
edges of laser-scribed grooves.
4.9
Atomic Force Microscopy
Atomic force microscopy (AFM) was performed in order to determine the surface roughness of the TiO2 ﬁlms. The surface roughness can be quantiﬁed by the root-mean-squared
116
4. Characterisation of TiO2 Thin Films
Figure 4.21: EDS spectra from the top surface and the grooves of a TiO2
coated BC solar cell after electroless nickel plating.
roughness (RRM S ), which is deﬁned as the standard deviation of the AFM data,
RRM S =
N
− z¯)2
N −1
n=1 (zn
(4.9)
where zn is the height of the nth data, z¯ is the mean height and N is the number of data.
The measurements were performed with a Digital Instruments Nanoscope III microscope
(Electron Microscope Unit, UNSW) in contact mode.
Figure 4.22 depicts the TiO2 surface morphology of an as-deposited CVD ﬁlm (a), an annealed CVD ﬁlm (b), and a USD-deposited ﬁlm (c) over a 1.5 µm × 1.5 µm area. The sample
in (a), deposited at 450◦ C, is relatively smooth for a polycrystalline ﬁlm and has an surface
roughness of RRM S = 4.9 nm. The surface roughness is observed to increase upon annealing
(1 hr at 950◦ C) to RRM S = 7.9 nm due to growth in the grain sizes. The samples in Figure 4.22 (a) and (b) are the same samples depicted in Figure 4.17(a) and (b). The ﬂatter
regions of the annealed sample observed with SEM (see Figure 4.17(b)) are not visible in
the AFM image. Figure 4.22 (c) depicts a TiO2 ﬁlm USD-deposited at 450◦ C. The surface
roughness for this ﬁlm is RRM S = 10.6 nm, and a large variation in ﬁlm thickness can be
seen. Over a larger area (85 µm × 85 µm) the surface roughness increased dramatically to
RRM S = 32.9 nm and the occasional deep pinhole could be seen.
4.10 Chemical Resistance
117
Figure 4.22: AFM images of (a) as-deposited CVD TiO2 ﬁlms, (b) CVDdeposited TiO2 ﬁlm after 1 hr anneal (load 800◦ C and ramp to 950◦ C),
and (c) a USD-deposited TiO2 ﬁlm.
4.10
Chemical Resistance
Both USD- and CVD-deposited TiO2 ﬁlms were placed in various chemical solutions. The
chemical resistance of the ﬁlms were ascertained by observing any colour change and etching
that occurred before and after etching. Table 4.3 summarises the results for both USD- and
CVD-deposited TiO2 ﬁlms and the time spent in solution. In Table 4.3, a tick (✓) indicates
that no change in colour or the appearance of pinholes could be detected, while a cross (✗)
means that the TiO2 ﬁlms were signiﬁcantly etched. Additional explanations are included in
footnotes below Table 4.3. The exact composition of the solutions can be found in Section
2.5. CVD ﬁlms deposited at 450◦ C were not as chemically resistant as TiO2 ﬁlms deposited
by USD at the same temperature. All non-annealed TiO2 ﬁlms were susceptible to etching
from highly alkaline solutions. This included chemicals like NaOH, NH4 OH, and the Cu
plating solution (Enplate 704, Melbourne, pH=11). The resistance against acidic solutions
was very good except for the HF/HNO3 mixture used in the CP etch, which removed ﬁlms
and began etching the silicon substrate after a few seconds. The green Nickelex solution
(Transene Inc., MA, USA) used had a pH of 4.
118
4. Characterisation of TiO2 Thin Films
Table 4.3: Chemical resistance of both as-deposited annealed TiO2 ﬁlms.
Chemical
Solution
Saw-damage etch
Groove etch
CP etch
Dilute HF
BHF
RCA1
RCA2
H2 SO4 clean
Nickelex
Copper
Time in USD
Solution
20 min
20 min
2 min
1 min
1 min
5 min
5 min
5 min
2 min
> 3 hr
✗
✓
✗
✓
✓
✗‡
✓
✓
✓
✗∗
CVD
(450◦ C)
CVD
(anneal)
✗
✗
✗
✓
✓†
✗‡
✓
✓
✓
✗
✗
✓
✗
✓
✓†
✗‡
✓
✓
✓
✓
Some etching of the TiO2 ﬁlm occurred
Rapid etching of TiO2 ﬁlm occurred when wet samples were placed in BHF solution
‡
Slight etching of all TiO2 ﬁlms occurred during RCA1 cleaning, therefore an H2 SO4
based cleaning step was used instead
∗
Chemical resistance to basic copper plating solution only achieved after annealing
†
4.11
Conclusions
Extensive characterisation of the USD- and CVD-deposited TiO2 thin ﬁlms provided very
useful information about the material properties and the behaviour of these properties under
high temperature processing. A surprising result was that the Raman spectra of both asdeposited and annealed (950◦ C for 90 min) samples indicated that the USD-deposited TiO2
ﬁlms were anatase. This is the highest temperature reported where anatase ﬁlms have withstood the transformation to rutile. The exact reason for the retardation of the anatase-rutile
phase transformation is not understood, however it is postulated that the thin interfacial
SiO2 layer, grown at the start of the annealing step, could be inhibiting the phase transformation. The IR spectra of TiO2 thin ﬁlms were able to be modelled well using a factorised
form of the dielectric function. As well as conﬁrming the phase of the USD-deposited TiO2
thin ﬁlms as that of anatase, the FTIR spectra were able to detect non-stoichiometry in TiO2
ﬁlms annealed in nitrogen (discussed in detail in Section 5.2.2). Both XPS and RBS spectra
rendered excellent results, which will be discussed fully in Chapters 5 and 6, respectively.
Spectroscopic ellipsometry was found to be a very accurate method for determining the
optical constants of the TiO2 ﬁlms. The optical properties could be well described by a single
or double Lorentz oscillator model, implemented in the software package WVASE32.62 It was
necessary to use a 50% void/50% TiO2 EMA as a surface roughness model. The refractive
indices of as-deposited (450◦ C) and annealed (950◦ C) USD-deposited TiO2 thin ﬁlms were
2.449 and 2.458 at 600 nm, respectively. The high refractive indices are indicative of a
4.11 Conclusions
119
dense ﬁlm (3.64 g/cm3 ). The extinction coeﬃcient of the ﬁlms remained low at 0.1 or less
for wavelengths greater than 350 nm. The optical properties of CVD-deposited ﬁlms are
presented in Chapter 7.
SEM images conﬁrmed the dense appearance of the USD-deposited ﬁlms, although it is
possible that voids exist underneath sections of the ﬁlm. TiO2 ﬁlms deposited in an industrial APCVD system exhibited cracks and large numbers of particulates. With the simple
CVD system, as-deposited TiO2 ﬁlms had grains of about 30 nm diameter and the density
was noticeably less than USD-deposited ﬁlms. The density was observed to increase with
annealing time, due to a sintering process occurring in the furnace. SEM images and EDS
analysis have shown that the horizontally impinging vapour has prevented TiO2 deposition
in the grooves. Large overhanging TiO2 depositions were observed on both top-edges of the
groove. The surface roughness of USD-deposited ﬁlms was large at 32.9 nm, however CVDdeposited ﬁlms were much smoother at 4.9 nm and 7.9 nm, as-deposited and after annealing,
respectively.
The chemical resistance of both USD- and CVD-deposited ﬁlms was excellent against all
acids, except the aggressive CP etch. The chemical resistance of the ﬁlms versus basic
solutions was not as good, with slight etching occurring during RCA1 cleaning. The etch
resistance of the ﬁlms to alkaline solutions improved somewhat after annealing.
120
4. Characterisation of TiO2 Thin Films
Chapter 5
Enhancing the Passivation of
TiO2-coated Wafers
Novel applications for titanium dioxide (TiO2 ) thin ﬁlms have the potential to reduce production costs of high-eﬃciency commercial silicon solar cells, especially for structures like the
buried-contact solar cell. This chapter demonstrates, ﬁrstly, that a TiO2 ﬁlm deposited via a
cheap and industrially compatible process and does not contaminate the silicon wafer or furnace after lengthy high-temperature processing. It has been determined that spray-deposited
TiO2 ﬁlms are, however, sensitive to the furnace ambient. Thermochemistry and FTIR analysis conﬁrmed that reduction of the TiO2 ﬁlm occurred when samples were loaded into a pure
N2 environment and titanium sesquioxide (Ti2 O3 ) was formed.
It is demonstrated that good surface passivation of lightly diﬀused n-type solar cell emitters using TiO2 thin ﬁlms can be achieved when treated with a furnace oxidation process.
Transient-PCD, XPS and SEM measurements indicate that the silicon dioxide layer formed
at the TiO2 :Si interface provides excellent surface passivation. Emitter dark saturation current densities of 4.7 × 10−14 A/cm2 are achieved by this method, demonstrating for the ﬁrst
time that TiO2 ﬁlms are compatible with high-eﬃciency solar cell structures.
5.1
Introduction
Surface passivation is an extremely important design consideration for high-eﬃciency solar
cells, especially at the front surface where the majority of the light is absorbed. Common methods for Si surface passivation include thermal oxidation at temperatures of about
1000◦ C to grow silicon dioxide (SiO2 ) and plasma-enhanced chemical vapour deposition
(PECVD) of hydrogenated amorphous silicon nitride (a-SiN:H). Although TiO2 thin ﬁlms
are the prevalent antireﬂection (AR) coating in the PV industry, one shortcoming of is that
these ﬁlms aﬀord very little surface passivation to silicon surfaces. Therefore, one aim of
121
122
5. Enhancing the Passivation of TiO2 -coated Wafers
this work was to develop a method for enhancing the level of surface passivation achievable
using commercially-viable TiO2 deposition techniques. The standard buried-contact (BC)
solar cell uses a thermally grown SiO2 layer as its primary dielectric ﬁlm. The SiO2 ﬁlm can
easily withstand subsequent high-temperature processes, such as diﬀusion, oxidation and
aluminium alloying at temperatures up to and beyond 1100◦ C. Therefore, several experiments were performed to investigate the suitability of TiO2 thin ﬁlms to high-temperature
processing. These experiments examined the stability of the TiO2 ﬁlms, and whether hightemperature processing of TiO2 thin ﬁlms deposited using the 97% pure tetraisopropyl titanate (TPT) precursor would result in contamination of the silicon wafer or furnace.
In this work, transient photoconductance decay (transient-PCD) is used extensively in order
to determine the eﬀect that TiO2 thin ﬁlms, deposited onto the surfaces of silicon wafers,
have on the surface and bulk recombination properties of the sample. This technique has
become a very popular method for determining the bulk minority carrier lifetime (τbulk ) and
emitter dark saturation current density (J0e ) since its conception in 1985.282 In the transientPCD technique, minority carriers are injected into the sample using a ﬂash lamp, and the
conductivity of the sample is monitored using an inductively coupling bridge. Thus, the
method does not require any contacts to the sample. For measurements made with the bulk
region in high injection, the emitter recombination has a quadratic dependence on the excess
carrier density, while recombination in the base exhibits a linear dependence. This enables
the emitter dark saturation current density J0e to be separated from the bulk recombination
or at the surfaces. Thus, for a symmetrical n-type sample with the same n-type diﬀusion
and dielectric on both surfaces and assuming the minority carrier diﬀusion length is much
greater than the wafer thickness, the eﬀective minority carrier lifetime τef f extracted from
transient-PCD measurements is related to J0e and τbulk by
1
τef f
=
1
τbulk
+
1
2 J0e ∆p
+
2
q ni W
τAuger
(5.1)
where W is the wafer thickness, ∆p is excess minority carrier (hole) concentration, ni is the
intrinsic carrier concentration and q the electronic charge. After the Auger recombination
term has been subtracted, the J0e can be calculated from the constant slope regions of a plot
of 1/τef f versus ∆p.282
Lightly-doped (1000 Ω cm) n- and p-type ﬂoat zone (FZ) wafers were used in this work for J0e
and τbulk measurements to minimise bulk recombination. A lightly-doped emitter was used on
both surfaces, ﬁrstly, as the J0e will be extremely sensitive to any change in recombination
at the surfaces,22 which makes it a good indicator as to the level of surface passivation
achieved. Secondly, similar lightly diﬀused emitters are commonly used in the fabrication of
high-eﬃciency solar cells,23, 283 will be the most responsive to enhanced surface passivation
schemes. Surface recombination was reduced using either thermally grown SiO2 or P2 O5 :SiO2
layers for passivation. All experiments were performed in high injection (∆p ≈ 4×1015 cm−3 ),
where the dopant concentration for the n-type and p-type wafers were 4.4 × 1012 cm−3 and
5.2 Stability of TiO2 at High-Temperatures
123
1.3 × 1013 cm−3 , respectively.
5.2
5.2.1
Stability of TiO2 at High-Temperatures
Titanium Contamination of Silicon
As silicon solar cells are minority carrier devices, maintaining a high eﬀective minority carrier
lifetime is extremely important in order to achieve high conversion eﬃciency. As can be seen
from Equation 5.1, this can be achieved by minimizing recombination at the surfaces, represented by the emitter dark saturation current density J0e , and maximising the bulk minority
carrier lifetime τbulk . Ensuring that there are no metal ions present during high-temperature
processing steps, for example, is vital to maintain a high τbulk . Even low concentrations of
metal ions can diﬀuse into the silicon wafer and create regions of high recombination that
can drastically reduce τbulk .
Therefore, there was initially some concern as to whether the TPT precursor or the TiO2
ﬁlm would introduce contamination into the furnaces and reduce τef f of the silicon wafers.
Of all the 3d transition metals, titanium exhibits the lowest known solubility and diﬀusivity
in silicon,284 however its diﬀusivity is still several orders of magnitude greater than that of
dopant atoms like phosphorus and boron. Due to its strong aﬃnity for oxygen and nitrogen,
the diﬀusion of titanium is hindered by even small amounts of residual air in the furnace
ambient.284
Studies have been published measuring the performance degradation of solar cells that were
fabricated from silicon ingots that had controlled additions of titanium.285–287 An onset of
performance reduction was measured with titanium concentrations of 3 × 1011 atoms/cm3 ,
and at a concentration of 2×1014 atoms/cm3 , the eﬃciency of a p-type solar cell was reduced
to 37% of its uncontaminated value.287 Additionally, more than an order of magnitude
reduction in the bulk minority carrier lifetime resulted. However, the performance of an
n-type solar cell was only reduced to 80% of its initial value at a Ti concentration of 2 ×
1014 atoms/cm3 (4 parts-per-billion).285 The solid solubility of titanium in silicon is about
1014 atoms/cm3 at 1200◦ C and 1012 atoms/cm3 at 950◦ C.284 In the worst case scenario,
the performance reduction can be calculated by noting that the solid solubility of Ti in Si
is 1.5 × 1012 atoms/cm3 at a typical maximum processing temperature used in a solar cell
fabrication sequence (1000◦ C). This would result in a reduction in eﬃciency of about 5% for
p-type substrates, but no observable change in eﬃciency for n-type solar cells. The greater
impact on p-type material can be attributed to the high majority-carrier capture crosssections of titanium donors and acceptors, which have a marked inﬂuence on the carrier
lifetime in p-type silicon.287 One paper has also been published, indicating that titanium
124
5. Enhancing the Passivation of TiO2 -coated Wafers
may act as an n-type dopant atom in silicon 1 .
Importantly, it is known that TiO2 layers deposited onto SiO2 layers are very stable. Keddie
et al. determined that there was no intermixing of TiO2 and SiO2 layers after 10 hr at
450◦ C, despite signiﬁcant densiﬁcation of both ﬁlms and crystallisation of the TiO2 ﬁlm.233
The interface width was measured to be 0.8 nm. Guenther fabricated multi-layer SiO2 /TiO2
coatings using the ion-plating deposition technique and observed that the interfaces were
sharp when depositing SiO2 on TiO2 , but not in the other direction.72, 231 It is assumed that
the titanium ions possess enough energy during deposition to reduce the previously deposited
SiO2 layer. These diminishing interfaces resulted in a lower than expected transmittance
from the multilayer stacks. This phenomenon has not been observed with other deposition
methods. Additionally, it has been noted that crystalline TiO2 (either anatase or rutile)
and SiO2 do not form a mixed compound, although there is some solubility between the
amorphous phases of TiO2 and SiO2 .288
Experiment
Two MiniBrute quartz tube furnaces were available for annealing the TiO2 ﬁlms, while a
third, larger furnace was equipped with a phosphorus trioxychloride (POCl3 ) liquid dopant
source (Schumacher, CA, U.S.A.). A range of quartz boats enabled high-temperature processing of 4” and 2” diameter wafers as well as quarters of 4” round wafers and 5 × 5 cm2
multicrystalline silicon (mc-Si) wafers. All furnaces were equipped with nitrogen, oxygen,
forming gas (4% hydrogen in argon) and trichloroethane (TCA) for cleaning. The furnaces
were proﬁled to have a ﬂat-zone of at least 25 cm long with a temperature variation of ±0.5◦ C
across the ﬂat-zone.
The diagram in Figure 5.1 depicts the processing steps included for the “contamination”,
“reduction” and “surface passivation” experiments performed by the author. The substrates
for all experiments were either lightly-doped (1000 Ω cm) n- or p-type ﬂoat zone (FZ), (100)oriented wafers to minimise the amount of recombination in the base. For ellipsometry
measurements, polished 0.1 − 10 Ω cm n-type FZ wafers were used.
For the ”contamination” experiment in this section, several batches of wafers received a
heavier 10 Ω/2 POCl3 diﬀusion. The samples were heavily diﬀused in order to be less
sensitive to the poor level of surface passivation aﬀorded to the silicon wafer by the TiO2
layer. These wafers were loaded into the furnace 800◦ C in O2 before ramping the furnace
temperature up to 950◦ C in an N2 ambient. The total time spent in the furnace was 2 hr.
1
Brown and Grannemann reported that titanium diﬀused to a depth of 150 nm during a 15 min process
at 1000◦ C from a reduced titanium dioxide (TiO2−x ) ﬁlm.31
5.2 Stability of TiO2 at High-Temperatures
“Contamination”
125
“Reduction”
“Surface Passivation”
NaOH etch (30%, 85°C, 20 min)
RCA2, RCA1, RCA2, HF dip, DI rinse
POCl3 Emitter
Diffusion, 10 Ω/V
POCl3 Emitter
Diffusion, 175 Ω/V
Dilute HF dip to remove PSG
TiO2 Deposition (both sides, ~70 nm thick, Tdep=450°C)
H2SO4:H2O2:DI (1:1:5), RCA2, DI rinse
Furnace Step:
oxidation (O2) and
anneal (N2)
Furnace Step:
anneal (N2)
Furnace Step:
oxidation (O2) and
anneal (N2)
Figure 5.1: Diagram showing the processes steps used for the “contamination”, “reduction” and “surface passivation” experiments described in
this chapter.
Results and Discussion
Transient-PCD measurements were performed in order to determine whether any degradation in bulk minority carrier lifetime τbulk had occurred due to titanium contamination. It
can be seen from the results in Table 5.1 that τbulk remains above 2 ms in all cases. The
J0e is relatively insensitive to the change in surface conditions due to the heavy phosphorus
diﬀusion, and varies from 5.6 − 6.9 × 10−13 A/cm2 . FZ wafers with only an emitter diﬀusion
and no TiO2 coating placed in the furnace adjacent to the TiO2 coated samples also exhibited similarly high τbulk . This indicates that TiO2 thin ﬁlms, deposited using the same TPT
precursor as used in the PV industry, are compatible with high-temperature processing and
do result in contamination of the silicon wafer.
5.2.2
Reduction of TiO2
While performing the contamination experiments, it was noted that the gas ambient that
the TiO2 -coated wafer was loaded into was critical. This was investigated further to fully
126
5. Enhancing the Passivation of TiO2 -coated Wafers
Table 5.1: Transient-PCD analysis demonstrating that no bulk contamination has resulted from the TiO2 ﬁlm after processing at 950◦ C for 2
hours.
Transient-PCD
performed after:
10 Ω/2 POCl3 diﬀusion
TiO2 deposition
950◦ C anneal
τbulk
(ms)
J0e
(A/cm2 )
2.4
2.2
2.1
6.2 × 10−13
6.9 × 10−13
5.6 × 10−13
understand the reactions that were taking place. No changes in the appearance of the TiO2
ﬁlm were observed if oxygen (O2 ) was present in the furnace when loading the samples (at
800◦ C). However, if the wafers were loaded in pure N2 , a reaction between the TiO2 ﬁlm and
the silicon substrate occurred. This could be visually observed as regions with a purplish-blue
colour. It was postulated that in the absence of oxygen, the silicon reduces the TiO2 to form
a non-stoichiometric sub-oxide TiOx . Other researchers have noted that non-stoichiometric
Tiy Ox ﬁlms have a blue, purple, grey-blue or blue-black colour,29, 73, 132–136 depending on the
ﬁlm stoichiometry. Shannon and Pask demonstrated that the grey colour appeared in rutile
(the high temperature crystalline phase of TiO2 ) ﬁlms once the O:Ti ratio (x) decreased
from 2 to 1.991.88 Due to the increased number of oxygen vacancies, the optical absorption
in the visible is dramatically increased for TiOx ﬁlms.43, 58 It is imperative to minimise the
amount of absorption originating from the ﬁlm for it to act as an eﬃcient antireﬂection (AR)
coating for a solar cell.
Thermochemistry Analysis
Reactions that take place between thin ﬁlms and the substrate may be better understood
by considering the thermochemistry of the deposited ﬁlm in its ambient condition. The
standard free energy change of a reaction ∆G◦ is given by
∆G◦ =
∆f G◦P roducts −
∆f G◦Reactants ,
(5.2)
where ∆f G◦ is the standard free energy of formation for 1 mol of a compound from its
constituent elements in standard states and is given by the Gibbs-Helmholtz equation (at a
constant temperature)
∆f G◦ = ∆f H ◦ − T ∆S ◦ .
(5.3)
In Equation 5.3, ∆f H ◦ and ∆S ◦ are the change in enthalpy (kJ mol−1 ) and entropy (J mol−1
K−1 ), respectively, from O K to the reaction temperature T (K). Once ∆f G◦ is calculated for
each product and reactant in the system, the ∆G◦ for the whole reaction can be determined
from Equation 5.2. For a negative ∆G◦ value, energy is evolved and the reaction proceeds
spontaneously, however when ∆G◦ is positive, energy is absorbed in the process and the
reaction will not proceed spontaneously.
5.2 Stability of TiO2 at High-Temperatures
127
Table 5.2: Enthalpy ∆H ◦ , entropy S ◦ and calculated standard free energy
of formation ∆f G◦ values for various compounds. TiO2 (a) = anatase
and TiO2 (r) = rutile.131, 289–292
∆H ◦ at 298 K
Compound
(kJ mol−1 )
O2
0.0
Si
0.0
SiO2
-910.7
TiO
-519.7
TiO2 (r)
-944.7
TiO2 (a)
-938.7
Ti2 O3
-1520.9
Ti3 O5
-2459.4
TiSi2
-133.9
S ◦ at 298 K ∆f G◦ at 1223 K
(J mol−1 K−1 )
(kJ mol−1 )
205.2
0.0
18.8
0.0
41.5
-961.5
50.0
-580.9
50.3
-1006.2
49.9
-999.7
77.3
-1615.4
129.3
-2617.5
61.1
-208.6
The entropy and enthalpy data of several relevant compounds are given in Table 5.2. Also
included in Table 5.2 are the standard free energies of formation at 950◦ C (1223 K), calculated
using Equation 5.3. The stability of a TiO2 ﬁlm in an oxygen ambient is demonstrated by
chemical reactions in Table 5.3. In this environment, TiO2 will not react with oxygen to form
either Ti2 O3 or Ti3 O5 . The only reaction with a negative standard free energy of formation
is the reaction of silicon and oxygen to form SiO2 , which proceeds spontaneously at 1223 K.
Table 5.3: The stability of TiO2 in an oxygen ambient is demonstrated
by the standard free energy change ∆G◦ at 1223 K for each reaction. The
rutile phase of TiO2 was used in the reactions.
∆G◦
(kJ)
Possible TiO2 /Si Reactions
2 TiO2 + O2 −→ Ti2 O3 + 12 O2
+395.0
3 TiO2 + O2 −→ Ti3 O5 +
+398.1
3
O
2 2
TiO2 + O2 + Si −→ TiO2 + SiO2
-961.5
In a nitrogen ambient, the situation is somewhat diﬀerent. Several possible furnace reactions
are given in Table 5.4, along with the associated standard free energy change ∆G◦ at 1223 K
for each reaction. The reactions presented in Tables 5.3 and 5.4 assume that the TiO2 ﬁlm
is rutile. Although all reduction reactions are able to proceed spontaneously, the formation
of Ti2 O3 with a ∆G◦ = −165.9 kJ has the most negative free energy change. The strong
aﬃnity of the titanium atom for oxygen is evidenced by the large positive standard free
energy change in the case of TiSi2 .
128
5. Enhancing the Passivation of TiO2 -coated Wafers
Table 5.4: Possible reactions between TiO2 and Si in the absence of O2 ,
and the associated standard free energy change ∆G◦ at 1223 K for each
reaction. Nitrogen does not play an active role in the reaction and is
therefore not shown.
Possible TiO2 /Si Reactions
2 TiO2 + Si
4 TiO2 + Si
6 TiO2 + Si
TiO2 + 2 Si
−→
−→
−→
−→
2 TiO + SiO2
2 Ti2 O3 + SiO2
2 Ti3 O5 + SiO2
TiSi2 + O2
∆G◦
(kJ)
-110.9
-165.9
-156.9
+798.0
Experiment
The centre column of Figure 5.1 shows the processing steps for this “reduction” experiment.
Again, n-type 1000 Ω cm FZ (100)-oriented silicon wafers were used. To observe the reduction
of the spray-deposited TiO2 ﬁlm the samples were loaded at 800◦ C in an N2 ambient, and
the furnace ramped up to 950◦ C. After 2 hr the wafers were unloaded from the furnace. The
reduction of TiO2 was subsequently avoided by supplying oxygen (O2 ) to the furnace for the
ﬁrst 10 min. The eﬀect of adding oxygen to the gas ambient will be described in detail in
Section 5.3.1.
Results and Discussion
Figure 5.2 compares SEM images of (a) a typical spray-deposited TiO2 ﬁlm with the familiar
texture arising from columnar grain growth, and (b) the region of the N2 -annealed TiO2 ﬁlm
that exhibited a bluish-purple hue. The white features in Figure 5.2(b) are precipitates on
the surface, while grain boundaries can be seen in top and bottom left-hand corners. The
dramatic increase in grains size and the change in surface morphology from Figure 5.2(a)
to Figure 5.2(b) would seem to indicate that a sintering process has occurred, resulting in a
smoother ﬁlm with much larger grain sizes (see Figure 5.2(b)). The J0e of the samples with
a bluish-purple ﬁlm was typically very high, of the order of 2 − 4 × 10−11 A/cm2 . This is
approximately an order of magnitude higher than that achievable with either a bare silicon
wafer (J0e = 1 × 10−12 A/cm2 ) or a TiO2 -coated wafer (J0e = 4 × 10−12 A/cm2 ). Therefore,
the high J0e observed in this experiment could be due to a metallic compound at the TiOx :Si
interface, which would explain the high surface recombination velocity. Sub-oxides of TiO2 ,
including TiO and Ti2 O3 , are known to exhibit metallic conduction properties.58, 172, 173 The
dependence of the conductivity on stoichiometry was observed to change from less than
10−10 Ω−1 cm−1 for TiO2.00 to 102 Ω−1 cm−1 for TiO1.75 .141
FTIR Analysis
5.2 Stability of TiO2 at High-Temperatures
129
Figure 5.2: SEM image of (a) a typical spray-deposited TiO2 ﬁlm, and (b)
the region of the N2 annealed TiO2 ﬁlm that exhibited a bluish-purple hue.
The white features are precipitates on the surface, while grain boundaries
can be seen in top and bottom left-hand corners.
In order to determine which sub-oxide has formed and to understand the nature of the
chemical bonding, FTIR spectroscopic measurements were performed on samples loaded in
O2 and N2 . Figures 5.3(a) and (b) compares the spectra obtained from annealing samples
in an O2 and N2 ambient, and also a bare silicon wafer reference. In Figure 5.3(a), the
absorption peaks at about 260 cm−1 , 360 cm−1 , and 430 cm−1 can be assigned to the anatase
phase of TiO2 .261 However, Figure 5.3(a) shows that the sample annealed in N2 has a
distinctive absorption peak at 493 cm−1 , which cannot be assigned to either the anatase or
rutile phase of TiO2 . The literature discusses two possible origins of an absorption peak
for TiO2 samples in this wavelength range. Erkov et al. noted that, ﬁrstly, an absorption
peak at 470 − 480 cm−1 can be attributed to samples with a thin SiO2 layer at the TiO2 :Si
interface.117 Secondly, an absorption peak at 480 cm−1 has also been attributed to the
presence of Ti2 O3 .117, 293
Figure 5.3(a) shows that the sample loaded in O2 does not exhibit a strong, deﬁned absorption peak at 470 − 480 cm. Additionally, the TiO2 samples placed in the furnace in an O2
ambient are known to possess a thin interfacial SiO2 layer (see Section 5.3.1). Therefore,
there is a strong indication that the anomalous absorption peak of the N2 annealed sample is
due to the formation of Ti2 O3 during high-temperature processing in an N2 ambient. Figure
5.3(b) provides further conﬁrmation of this result, as the weak absorption peaks at 3750 cm−1
and 3840 cm−1 can also be attributed to the presence of Ti2 O3 on TiO2 substructure.294
In summary, spray-deposited TiO2 thin ﬁlms have been demonstrated to exhibit the following
properties when subjected to high-temperature processing.
130
5. Enhancing the Passivation of TiO2 -coated Wafers
Figure 5.3: (a) and (b): FTIR spectra of spray-deposited TiO2 ﬁlms after
high-temperature processing in an O2 and N2 ambient.
i) TiO2 ﬁlms do not result in contamination of the silicon wafer or quartz tube furnace.
This was determined by monitoring the bulk minority carrier lifetime of the wafer
before and after TiO2 ﬁlm deposition and high-temperature processing. Signiﬁcantly,
a standard commercial 97% pure TiO2 precursor was used for all experiments.
ii) TiO2 ﬁlms placed in the furnace in an oxygen ambient are stable. After 10 min of O2
the gas ﬂow can be changed to N2 with no adverse eﬀects.
iii) TiO2 ﬁlms loaded into the furnace in an nitrogen ambient are unstable and the TiO2
is reduced to sub-oxide, most likely titanium sesquioxide Ti2 O3 . Ti2 O3 exhibits very
diﬀerent optical and electrical properties than TiO2 and is not a suitable AR coating
for a solar cell (see Chapter 2).
5.3
Methods of Achieving Surface Passivation with
TiO2 Thin Films
Surface passivation is an extremely important design consideration for high-eﬃciency solar
cells, especially at the front surface where the majority of the light is absorbed. In the
previous section, it was demonstrated that spray-deposited TiO2 ﬁlms are stable under
high-temperature processing, as long as oxygen is present at the start of processing. It
is widely recognized that a stoichiometric TiO2 thin ﬁlm, such as those deposited using
spray deposition and chemical vapour deposition, on bare silicon aﬀords very little surface
5.3 Methods of Achieving Surface Passivation with TiO2 Thin Films
131
passivation.20, 239, 295, 296 Therefore, this section will investigate the options for capitalising
on the required oxidation step in order to enhance the level of surface passivation possessed
by TiO2 -coated silicon wafers.
A literature review will be presented, and the methods for achieving good surface passivation
with TiO2 ﬁlms will be discussed. The most common method is to grow a thin thermal SiO2
passivation layer on the silicon wafer and subsequently deposit the TiO2 ﬁlm, however, more
recently, limited success has been achieved using non-stoichiometric TiOx ﬁlms without an
SiO2 layer. Following this, experimental results that show that the novel method of growing
SiO2 layers at the TiO2 :Si interface after TiO2 ﬁlm deposition and method of depositing TiO2
on pre-existing phosphorosilicate glass (PSG) layers provides excellent surface passivation.
Deposition of TiO2 on SiO2
The most common method of achieving good surface passivation in conjunction with TiO2
AR coatings has been to initially grow a thin (5 − 30 nm) thermal SiO2 passivation layer and
subsequently deposit a TiO2 layer.16, 20, 22, 283, 295, 297–301 Thin SiO2 passivation layers have also
been used in conjunction with magnesium ﬂuoride (MgF2 )/TiO2 double-layer AR coatings.150
In that work, no reduction in optical performance was observed for SiO2 thicknesses up to
10 nm. Zhao et al. determined the minimum SiO2 thicknesses for high-eﬃciency passivated
emitter and rear cells (PERC).283 This was performed by monitoring the Voc and Isc of a
textured and planar solar cell as the oxide layer was etched back. The results showed that
for a textured surface the Voc started decreasing for a SiO2 thickness less than 25 nm, and at
10 nm for a planar surface. For both textured and planar surfaces the Isc started decreasing
for SiO2 thicknesses less than 10 nm.
The performance of BC solar cells fabricated on FZ silicon is limited by the relatively high
rear surface recombination velocity exhibited by the alloyed aluminium high-low p-type
junction.23 For BC solar cells fabricated on Czochralski (CZ) grade c-Si or multicrystalline
silicon (mc-Si) wafers the lower bulk lifetime, typically 20 µs, places additional performance
limitations on the device.23 The result is that a poorer front surface recombination velocity
caused by using 5 nm-thick SiO2 passivation layers, for example, may not result in a noticeable decrease in performance. Additionally, these thinner passivation layers increase the
optical transmittance of the TiO2 /SiO2 stack and improve device performance. Thin SiO2
layers (5 − 6 nm) have also been demonstrated to provide good surface passivation in some
high-eﬃciency solar cell designs.8 The sensitivity of the surfaces will be somewhat dependent
on the doping proﬁle of the emitter. Honsberg et al. published J0e results of TiO2 -coated
emitters with a thin SiO2 layer.20 An excellent result of 4 × 10−14 A/cm2 was achieved. The
TiO2 /SiO2 stack has been demonstrated to be compatible with high-eﬃciency solar cells,
with Voc ’s as high as 679 mV being achieved in practice.298
132
5. Enhancing the Passivation of TiO2 -coated Wafers
Growth of SiO2 at the TiO2 :Si Interface
Researchers have previously observed that oxygen is capable of diﬀusing through TiO2 thin
ﬁlms to form a SiO2 layer at the TiO2 :Si interface,31, 75, 89, 91, 114, 167, 179, 195, 207, 224, 226, 230, 302, 303
however none have fully explored the potential until now.35, 50 Previously, the formation
of SiO2 has been observed during the fabrication of super-thin capacitors and MOSFETs,
which take advantage of the high dielectric constant of TiO2 .31, 75, 89, 167, 224, 226, 302 Brief oxidations are performed on such samples in order to reduce the leakage current. However, if
the oxidation temperature is too high, the thin SiO2 layer that forms at the interface can
be detrimental to the device performance, drastically reducing the eﬀective dielectric constant.31, 197, 230 In the ﬁeld of PV, three research groups have noted that SiO2 can form at
the TiO2 :Si interface upon heat treatments.114, 217 In the work of Wong and Waugh,217 the
TiO2 ﬁlms were being applied to SP solar cells, which are not able to beneﬁt from improved
surface passivation due to the phosphorus “dead-layer” at the top surface. However, it was
noted that the ﬁlms were porous and oxygen could diﬀuse through the TiO2 ﬁlm. Szlufcik et
al. noted only that the reﬂectance of the TiO2 AR coating increased after heat treatments at
900◦ C, which was attributed to the formation of SiO2 at the interface. Murozono et al. observed SiO2 growth underneath TiO2 AR coatings (using Auger electron spectroscopy), when
the samples were ﬁred at temperatures up to 1000◦ C, but the concept of surface passivation
was not investigated.89
The composition of such thin interfacial oxides has also been described in the literature. In
the majority of publications related to MOS devices, the interfacial layer is simply assumed
to be SiO2 because lower than expected dielectric constants were observed.91, 179, 195, 230 Auger
electron spectroscopy (AES) has been used successfully to identify the interfacial layer as
being SiO2 .31, 89 Using RBS, Gartner et al. found that a 9 nm thick SiO2 had formed at the
interface after a 1 -hr oxidation at only 300◦ C.207 The research performed by Campbell et
al. has shown that the dielectric constant of the interfacial oxide is not as low as for pure
SiO2 .75, 167, 224, 226, 302, 303 Therefore, it was postulated that the observed 2 − 3 nm thick layer
is an amorphous mixed Ti-O-Si oxide. This appears to be likely when considering results
that indicate that the very ﬁrst stages of TiO2 ﬁlm deposition on a silicon wafer result in
an amorphous TiO2 ﬁlm.90, 96 Thus, it can be imagined that the initial SiO2 growth at
the TiO2 :Si interface may mix with the amorphous TiO2 region. Lee observed that nonstoichiometric TiOx oxides at the interface lead to a large leakage current in DRAM cells,
and found that it was necessary to include a thin (1 nm) thick SiO2 layer to improve device
performance.113
Non-stoichiometric TiOx
Non-stoichiometric TiOx ﬁlms are typically deposited using physical vapour deposition methods, such as evaporation or sputtering. In this scenario, ﬁlm growth does not occur due to
5.3 Methods of Achieving Surface Passivation with TiO2 Thin Films
133
a chemical reaction within the system and the oxygen concentration in the ambient plays a
critical role in determining the stoichiometry of the ﬁlm. Three works have demonstrated
that some degree of surface passivation can be achieved with non-stoichiometric TiOx thin
ﬁlms deposited directly onto a silicon wafer, however the application of non-stoichiometric
ﬁlms is not pursued in this work.
Wohlgemuth et al. postulated that the higher voltages observed for solar cells with a spray
deposited TiOx AR coating was due to reduced surface recombination velocity at the n + Si
surface.49 Crotty et al. demonstrated that solar cells with a TiOx AR coating had a similar
open-circuit voltage (Voc ) and eﬃciency (646 mV and 17.5%) to cells that possessed an 8−10
nm thick SiO2 passivation layer at the TiOx :Si interface.297 This method was dependent on
the samples undergoing a 400◦ C anneal for 5 min in a hydrogen ambient, otherwise the opencircuit voltage and eﬃciency remained low at about 626 mV and 13%, respectively. More
recently, Doeswijk et al. deduced an improvement in surface passivation after depositing
TiOx ﬁlms by laser ablation, based on modulated free carrier absorption measurements.239
The improved passivation achieved with this ﬁlm was attributed to ﬁxed positive charges
at the TiOx :Si interface, resulting from the oxygen deﬁciency in the ﬁlms. The ﬁxed charge
reduces the recombination by forming an electric ﬁeld, which bends the energy bands near
the surface of the wafer.
5.3.1
Growth of SiO2 at the TiO2 :Si Interface
By initially depositing TiO2 onto the silicon wafer and subsequently performing a brief
oxidation, a thin SiO2 layer can be formed at the TiO2 :Si interface. The possibility of
achieving this result was noted from thermochemistry analysis performed earlier, which
showed that the presence of TiO2 and silicon in an oxygen ambient at 950◦ C strongly favoured
the formation of SiO2 (refer to Table 5.3). The formation of a TiO2 /SiO2 passivation stack
in this manner, oﬀers several potential advantages over the previously described scheme of
depositing TiO2 onto a thermally grown SiO2 layer. These include:
i) The ease of chemical processing, due to the excellent chemical resistance of polycrystalline TiO2 .35, 36
ii) The TiO2 ﬁlm may act as a diﬀusion barrier to other elements during the high temperature processing stage, preventing contaminants from reaching the silicon wafer.
iii) The stoichiometry of the ﬁlm is ensured as the oxygen (O2 ) diﬀuses through the TiO2
layer, removing any oxygen deﬁciencies.34, 99 This is beneﬁcial for reducing the level of
optical absorption observed in non-stoichiometric TiO2 ﬁlms deposited by evaporation
or sputtering techniques.138
134
5. Enhancing the Passivation of TiO2 -coated Wafers
iv) Carbon contamination, resulting from the organo-metallic precursor, is reduced after
high temperature processing due to the decomposition of carbonate species.64, 99
v) The refractive index of the ﬁlm can be tuned by adjusting the deposition and annealing
temperatures.148
vi) The method of surface passivation presented here is applicable to all TiO2 ﬁlms that
are able to undergo a brief high-temperature oxidation. To the authors knowledge,
this may only exclude highly stressed ﬁlms deposited by techniques such as PECVD.
An additional advantage of the TiO2 /SiO2 stack is that its passivation properties do not
degrade under concentrated sunlight, unlike an SiO2 ﬁlm.300 This is attributed to the absorption of ultraviolet photons by the TiO2 layer.
Experiment
The aim of this “surface passivation” experiment (see Figure 5.1) was to determine whether
good quality surface passivation could be achieved by, ﬁrst, depositing TiO2 onto silicon
wafers, and then subsequently subjecting the wafers to a brief, high-temperature oxidation
process.
The wafers used for this experiment were n-type, high-resistivity 1000 Ω cm FZ Si(100). All
seven wafers were etched in a sodium hydroxide solution, cleaned in RCA1 and RCA2,194
dilute hydroﬂuoric acid (HF), rinsed in deionised (DI) water, and blown dry with nitrogen
(N2 ) before receiving a 175 Ω/2 phosphorus diﬀusion at 800◦ C. A lightly-doped emitter was
used, ﬁrstly, as the J0e will be extremely sensitive to any change in surface passivation.22
Secondly, similar lightly diﬀused emitters are used in the fabrication of high-eﬃciency solar
cells.23, 283 Then, the phosphorosilicate glass (PSG) was removed and the wafers well rinsed
in DI water and blown dry. Samples 1 − 3 received a spray-deposited polycrystalline TiO2
coating (≈ 70 nm thick) on both sides, with the wafers sitting on a heater block maintained
at 450◦ C. The processing steps for these samples are shown in Figure 5.1.
Subsequently, the wafers were RCA cleaned again and then loaded into a quartz tube furnace
for the post-TiO2 oxidation. The loading temperature was 800◦ C and the ambient O2 :N2
(1:1). After loading, the temperature was ramped to 950◦ C at a rate of 10◦ C/min, and the
O2 was switched oﬀ after 10 min, leaving a N2 ambient to anneal the samples. The wafers
were removed from the furnace at 950◦ C after a further 80 min.
As a control, samples 4−7 received an initial oxidation (10 min at 800◦ C) after removal of the
PSG. These wafers then underwent TiO2 deposition and oxidation, as per samples 1−3. The
thickness of the PSG, and initial and post-TiO2 oxide layers were measured using a G¨artner
L116A ellipsometer (λ = 633 nm) and were found to be 30 nm, 5 nm and 7.5 nm, respectively.
Transient-PCD measurements were performed in high injection (∆p ≈ 4 × 1015 cm−3 ) after
5.3 Methods of Achieving Surface Passivation with TiO2 Thin Films
135
each process step in order to determine the emitter saturation current density and to monitor
the minority carrier bulk lifetime.282
Results and Discussion
The cross-sectional SEM image in Figure 5.4(a) depicts the as-deposited TiO2 on the silicon
substrate, while the growth of a new layer at the TiO2 :Si interface can be seen in the
SEM image shown in Figure 5.4(b). The TiO2 , which has peeled away from the interfacial
SiO2 layer and substrate during cleaving, seems to be a dense and continuous ﬁlm, and the
interfaces appear to be abrupt. From Figure 5.4(b), the thickness of the TiO2 ﬁlm and the
newly formed interfacial layer are about 67 nm and 6 nm, respectively.
Figure 5.4: SEM images (10◦ tilt) showing (a) the TiO2 :Si interface before
oxidation, and (b) the 6 nm interfacial layer grown during the post-TiO2
oxidation (sample 2).
Results of XPS analysis performed at UNSW are shown in Figure 5.5. The top surface of
the TiO2 ﬁlm is at t=0 s, where the Ti:O ratio is about 1:2. A broad SiO2 peak can be
seen, centred at t=1600 s. Artifacts of the XPS technique, namely preferential sputtering
and islanding,304 make the SiO2 ﬁlm appear to be much thicker than is indicated in the SEM
image. There is a small amount of carbon adsorbed onto the surface of the TiO2 ﬁlm, due
to the samples being stored in air. The use of an organic precursor may have also resulted
in some carbon being retained in the ﬁlm,89 however the high temperature oxidations are
known to reduce these levels signiﬁcantly.99
The J0e of the wafers after phosphorus diﬀusion, measured with the PSG on both surfaces,
was 2.6 × 10−13 A/cm2 . The results of transient-PCD measurements after TiO2 deposition
and the subsequent oxidation are displayed in Table 5.5. The J0e of all samples exhibits a
marked increase after TiO2 deposition due to the poor surface passivation properties of this
136
5. Enhancing the Passivation of TiO2 -coated Wafers
Figure 5.5: XPS analysis chemically identifying the interfacial layer as
SiO2 .
oxide. However, the most signiﬁcant result is that the J0e of all TiO2 coated wafers is shown
to decrease to 4.5 − 7.7 × 10−14 A/cm2 after the post-TiO2 oxidation. In some cases, the
increase in surface passivation has decreased the J0e by nearly two orders of magnitude. The
poorer passivation (J0e = 2.4 × 10−12 A/cm2 ) of samples 4 − 7 before post-TiO2 oxidation can
be attributed to a poor-quality, non-annealed SiO2 layer grown during the initial oxidation
step. Notably, this did not limit the ﬁnal J0e values. The J0e values achieved here with
60 nm thick SiO2 layers compare favourably with the results achieved for high-eﬃciency,
buried-contact solar cells (7.4 × 10−14 A/cm2 ), with 158 Ω/2 emitters and thermal oxide
passivation.22
Table 5.5: Emitter saturation current density (J0e ) results measured using
transient-PCD. Samples 4 − 7 received an initial oxidation prior to TiO2
deposition, while samples 1 − 3 did not.
J0e (A/cm2 )
Sample After TiO2 After Post-TiO2
Number Deposition
Oxidation
1
2
3
1.8 × 10−12
1.9 × 10−12
1.9 × 10−12
5.1 × 10−14
4.7 × 10−14
7.7 × 10−14
4
5
6
7
3.9 × 10−12
4.0 × 10−12
4.1 × 10−12
4.0 × 10−12
6.6 × 10−14
4.5 × 10−14
7.6 × 10−14
5.4 × 10−14
The phosphorus pre-deposition step results in a relatively heavily doped and shallow emitter with a surface concentration of 2.0 × 1020 atoms/cm3 and a junction depth of 0.01 µm.
5.3 Methods of Achieving Surface Passivation with TiO2 Thin Films
137
After high-temperature processing the phosphorus surface concentration is reduced to
5.6 × 1018 atoms/cm3 and the junction depth increased to of 0.32 µm. Although some redistribution of the dopant atoms has occurred during oxidation, the shift in proﬁle cannot
fully account for the larger reduction in J0e . Additionally, as the TiO2 ﬁlms deposited in
this work are stoichiometric, the improvement in surface passivation cannot be ascribed to
ﬁxed charges within the ﬁlms. Thus, the large reduction in J0e observed after the hightemperature oxidation is attributed to the improved surface passivation aﬀorded by the thin
interfacial SiO2 layer.
5.3.2
TiO2 on PSG
A simple experiment was performed in order to determine the level of passivation achievable
by depositing TiO2 ﬁlms directly onto the thin phosphorosilicate glass (PSG, P2 O5 :SiO2 )
layer remaining after the emitter diﬀusion step. The use of an existing oxide layer for
passivation purposes is of interest for commercially produced solar cells. If successful, it
would obviate several steps in a fabrication process, including wet-chemical etching of the
PSG layer and the high-temperature growth of an additional passivation layer.
Experiment
Four p-type 1000 Ω cm FZ wafers received a phosphorus emitter diﬀusion (825◦ C for 10 min)
from a phosphorus oxychloride (POCl3 ) source. The POCl3 and O2 concentrations were both
1.5%. This pre-deposition step was followed by a drive-in at 950◦ C for 90 min in the same
furnace tube. This resulted in a relatively deep emitter with a sheet resistance of 100 Ω/2
and a PSG thickness of 5 nm. TiO2 ﬁlms (≈ 70 nm thick) were subsequently spray-deposited
on both surfaces of wafers 1 and 2 only. These wafers were loaded into an oxidation furnace
at 800◦ C. The ambient gases were a mixture of both 02 and N2 (3.2 slpm each). The furnace
was ramped to 950◦ C with the O2 being left on for the ﬁrst 10 min. The wafers remained in a
N2 ambient for another 80 min before being unloaded at 950◦ C. Samples 3 and 4 received the
same emitter diﬀusion, however the PSG layer was subsequently removed and a thin (50 nm,
sample 3) and thick (1000 nm, sample 4) SiO2 layer was thermally grown. The thin SiO2
layer for sample 3 was grown using the same conditions as the post-TiO2 oxidation above,
while the thick SiO2 layer for sample 4 was grown by loading the sample in dry O2 at 800◦ C
and ramping the furnace up to 950◦ C for a total of 90 min. Therefore, all samples received
exactly the same high-temperature process, with only the gas ambient being altered.
138
5. Enhancing the Passivation of TiO2 -coated Wafers
Results
Table 5.6 shows the J0e (in A/cm2 ) measured using transient-PCD analysis. It can be seen
that the J0e increases slightly after TiO2 deposition. The sensitivity of the measurement is
due to the minimal thickness of the PSG and the relatively lightly-doped emitter. Finally, it
should be noted that after a subsequent oxidation step and anneal, the J0e reduces further
by a factor of 2 − 3. There was no evidence of any reaction between the TiO2 ﬁlm and the
PSG layer, before or after high-temperature processing. Table 5.6 also shows the measured
J0e of SiO2 passivated wafers with the same emitter. The passivation achieved with a thin,
5 nm-thick SiO2 layer (sample 3) is very similar to that of the same thickness PSG-layer
with a TiO2 coating (sample 2). Improved surface passivation is aﬀorded by the 100 nmthick SiO2 layer with a dark saturation current density of 4.1 × 10−14 A/cm2 being achieved.
Table 5.6: Emitter saturation current density (J0e ) results measured using transient-PCD. The 5 nm-thick PSG that resulted from the 100 Ω/2
emitter diﬀusion step was left on the wafer. Only Samples 1 and 2 had
TiO2 deposited on them. Samples 3 and 4 had the PSG layer removed
and a thin (5 nm, sample 3) and thick (100 nm, sample 4) SiO2 layer was
thermally grown.
Sample After POCl3
Number
Diﬀusion
1
2
3
4
1.4 × 10−13
1.5 × 10−13
1.3 × 10−13
1.3 × 10−13
J0e (A/cm2 )
After TiO2 After Post-TiO2
Deposition
Oxidation
1.8 × 10−13
2.2 × 10−13
−
−
9.7 × 10−14
7.7 × 10−14
7.5 × 10−14
4.1 × 10−14
Discussion
From Table 5.6 it can be seen that a J0e of 1.8×10−13 can be achieved for TiO2 on P2 O5 :SiO2
layers, with thicknesses of 70 nm and 5 nm, respectively. This J0e result would limit the Voc
of a solar cell with such a 100 Ω/2 emitter to 671 mV (for an assumed Jsc of 33 mA/cm2 ),
which is much greater than a commercially produced BC solar cell (e.g., the best BC solar cell
fabricated by BP Solar cell exhibited a Voc of 636 mV15 ). Hence, such a combined emitter and
passivation scheme would not be the limiting factor in the solar cell. The J0e values of wafers
passivated with SiO2 only indicate, ﬁrstly, that very similar passivation can be achieved with
either PSG or SiO2 layers. Secondly, although thicker passivation layers result in a lower
J0e , these ﬁlms would exhibit greater optical (reﬂectance) losses than their electrical gains.
5.4 Conclusions
139
One disadvantage of such a passivation scheme is the tight control that is required on the
PSG thickness. For the method to be commercially feasible the PSG layer thickness would
have to be maintained between 5 − 10 nm. As discussed in Section 5.3, ﬁlms thinner than
5 − 6 nm have not been successfully implemented in a high-eﬃciency solar cell design, while
thicker ﬁlms will increase optical losses due to reﬂection. A thickness of 5 − 10 nm of SiO2 is
relatively easy to control by varying the oxidation time and temperature, however the growth
rate of P2 O5 :SiO2 is much higher. A second potential disadvantage of having a PSG layer
adjacent to a TiO2 layer is that a reaction between the two ﬁlms may occur slowly over time.
The mechanism for such a reaction is discussed in Chapter 6. The reaction results in a new
compound being formed which has both poor optical and passivation properties. No visual
signs of a reaction could be observed in these ﬁlms, however at lower P2 O5 concentrations
the reaction may still occur over a longer period of time.
5.4
Conclusions
The experiments performed in this chapter have demonstrated that TiO2 is compatible
with high-temperature processing without contaminating the wafers or furnaces. The bulk
minority carrier lifetimes of samples placed in a furnace for 2 hr at 950◦ C were maintained
at greater than 2 ms. It was found that the spray-deposited TiO2 ﬁlms are sensitive to the
initial gas ambient in the furnace. Samples loaded in oxygen were stable, however TiO2
ﬁlms loaded directly into a nitrogen ambient were reduced to a sub-oxide, most likely Ti2 O3 .
The formation sub-oxide was predicted using thermochemistry analysis and conﬁrmed using
FTIR spectroscopy.
Other researchers have demonstrated that some level of surface passivation can be achieved
using non-stoichiometric TiOx thin ﬁlms. However, many unanswered questions remain
regarding the use of TiOx ﬁlms, including, ﬁrstly, the limits of such a passivation scheme.
Secondly, greater knowledge regarding the range of stoichiometries (x values) that aﬀord
good surface passivation is required. Thirdly, the exact origin of the surface passivation
mechanism needs to be examined, and the role of hydrogen and oxygen vacancies studied.
Finally, considering the strong aﬃnity that the titanium atom has for oxygen, the long-term
stability of the TiOx passivation scheme needs to be carefully examined.
Importantly, the results of experiments performed in this work have indicated the presence
of a thin SiO2 layer, formed by oxidizing the wafer after TiO2 ﬁlm deposition. This was
conﬁrmed using SEM images and XPS analysis. The increase in surface passivation aﬀorded
by the interfacial SiO2 layer results in a decrease in J0e by nearly two orders of magnitude
from ∼ 2 × 10−12 A/cm2 after TiO2 deposition to 4.7 − 7.7 × 10−14 A/cm2 . This demonstrates
the ability of the TiO2 /SiO2 AR coating to provide excellent surface passivation. The low
J0e and high τbulk values demonstrated here are compatible with high-eﬃciency solar cells
140
5. Enhancing the Passivation of TiO2 -coated Wafers
with an open circuit voltage of the order of 700 mV. Any slight reduction in AR coating
performance due to the inclusion of the low refractive index SiO2 layer is far outweighed by
the passivation beneﬁts.22
Finally, it is also demonstrated in this work that a J0e of 1.8 × 10−13 A/cm2 can be achieved
by depositing a TiO2 ﬁlm directly onto a 5 nm-thick PSG layer that remains after a 100 Ω/2
phosphorus emitter diﬀusion. For an assumed Jsc of 33 mA/cm2 , a J0e of this magnitude
would limit the Voc to 671 mV, which is more than suﬃcient for commercially produced
silicon solar cells. The long term stability of this passivation scheme remains unknown, and
possibility of reactions between the PSG and TiO2 layers exist.
Chapter 6
Performance of TiO2 Thin Films as a
Phosphorus Diﬀusion Barrier and
Dopant Source
The ability of a dielectric ﬁlm to act either as a phosphorus dopant source or a phosphorus
diﬀusion barrier would enable a solar cell fabrication sequence to be simpliﬁed. In the former case, the phosphorus is incorporated into the antireﬂection coating and ﬁred at a later
time, allowing the P-atoms to diﬀuse into the silicon. The diﬀusion barrier is particularly
relevant for the buried-contact (BC) solar cell fabrication sequence, where the dielectric ﬁlm
is required to prevent phosphorus from reaching the lightly-doped emitter during the heavy
groove diﬀusion step.
The performance of TiO2 thin ﬁlms in both of these roles were investigated. It was discovered that TiO2 and P2 O5 are prone to react chemically at high enough temperatures and
concentrations, and that the P-dopant atom signiﬁcantly alters the properties of the TiO2
ﬁlm. Thermochemistry analysis and Rutherford backscattering spectroscopy are used to determine what chemical reactions are occurring, and the likelihood of success of both of these
simpliﬁcation schemes in the buried-contact solar cell process.
6.1
Introduction
This chapter investigates the performance of TiO2 thin ﬁlms as a barrier to phosphorus diffusion as well as a phosphorus dopant source. If TiO2 could successfully act as a phosphorus
diﬀusion barrier it could most likely be used a direct replacement for the thermally grown
silicon dioxide (SiO2 ) layer implemented in the original buried-contact (BC) solar cell. As
well as reducing the number of high-temperature processing steps, this would signiﬁcant
reduce the processing time and cost.
141
142
6. Performance of TiO2 Thin Films as a Phosphorus Diﬀusion Barrier and Dopant Source
The ﬁrst two steps in a typical commercial silicon solar cell fabrication sequence are, ﬁrstly,
emitter formation and, secondly, AR coating deposition. The possibility of combining these
two deposition steps by using a phosphorus-doped titanium dioxide (P:TiO2 ) thin ﬁlm is
also investigated in this chapter. If successful, this would enable the AR coating and emitter
dopant source to be deposited in a single low-temperature step, the phosphorus dopants
diﬀusing out from the ﬁlm and forming an emitter during a subsequent high-temperature
process. This could greatly simplify either a solar cell fabrication sequence.
6.2
TiO2 as a Phosphorus Diﬀusion Barrier
There are several requirements for a stable thin-ﬁlm diﬀusion barrier. Firstly, to minimise
chemical interdiﬀusion, the barrier should ideally be a large-grain polycrystalline thin ﬁlm to
reduce grain boundary diﬀusion.305 Secondly, to prevent chemical interaction the chosen material must have a large standard free energy of formation ∆G◦f .305, 306 This thermodynamic
criterion minimises the chance of a reaction proceeding as described in Equations 5.2 and
5.3. Thus, it is recommended to choose a material such as oxides, nitrides, and transition
metal nitrides, borides and silicides.306 Also, the presence of excess nitrogen, carbon, boron,
or oxygen in the ﬁlm can increase its eﬀectiveness as a diﬀusion barrier.307 It should be
noted that diﬀusion barriers do not eliminate the driving force for diﬀusion caused by the
concentration gradient, but merely reduce rate of interdiﬀusion.
The majority of the work performed on diﬀusion barriers has focussed on preventing interactions between aluminium and silicon, thus allowing the microelectronics industry to
continue to use aluminium contacts under increasingly demanding processing conditions.305
There has been very little work in the literature reporting the use of TiO2 as a barrier to
phosphorus diﬀusion. The only reference to TiO2 acting as a phosphorus diﬀusion barrier
was made in passing by Drynan et al.308 In that work, it was observed that when titanium
metal ﬁlms were deposited onto silicon, the titanium reacted with the silicon native oxide
to form non-stoichiometric TiOx , and this TiOx layer appeared to limit the diﬀusion of
phosphorus. TiO2 thin ﬁlms have been demonstrated to act as a diﬀusion barrier to various other compounds, such as lead titanate,309 and also between copper and bismuth,310
and between platinum-based metal contacts and silicon.311–313 Diﬀering experimental results
have been achieved with TiO2 acting as a diﬀusion barrier to hydrogen (H). Spiegel et al.
reported that APCVD-deposited TiO2 ﬁlms acted as a out-diﬀusion barrier to hydrogen.314
In contrast, Liang et al. found that the sheet resistance of H-doped TiO2 thin ﬁlms increased
from 30 Ω/2 to 500 Ω/2 after two days.58 The increased sheet resistance was attributed to
the out-diﬀusion of hydrogen from the ﬁlm. Tang et al. postulated that since anatase has a
less dense and more defected structure than rutile, this could favour impurity diﬀusion into
the material.120
6.2 TiO2 as a Phosphorus Diﬀusion Barrier
143
Thermochemistry Analysis
Thermochemistry analysis is used to predict any reactions that may take place between the
TiO2 ﬁlm and the phosphorus containing furnace ambient. The analysis performed here
assumes that the POCl3 and O2 have fully reacted upon entering the neck of the furnace
together, forming phosphorus pentoxide (P2 O5 ) vapour, which is then deposited as a solid
onto the surfaces of the wafer.315 Therefore, the chemical reactions postulated here are
between the compounds P2 O5 and TiO2 . The entropy and enthalpy data of the compounds
discussed in this chapter are given in Table 6.1, along with the standard free energies of
formation at 950◦ C (1223 K), calculated using Equation 5.3. The thermodynamic data is
taken from the literature.131, 289–291
Table 6.1: Enthalpy ∆H ◦ , entropy S ◦ and calculated standard free energy
of formation ∆f G◦ values for various compounds. TiO2 (a) = anatase,
TiO2 (r) = rutile, (g) = gas, (s) = solid.131, 289–291
Compound
∆H ◦ at 298 K
(kJ mol−1 )
O2
Si
SiO2
TiO2 (r)
TiO2 (a)
(P2 O5 )2 (g)
(P2 O5 )2 (s)
TiPO4
TiP2 O7
TiP3 O9
0.0
0.0
-910.7
-944.7
-938.7
-3904.1
-3009.9
-1671.5
-2539.7
-3255.2
S ◦ at 298 K ∆f G◦ at 1223 K
(J mol−1 K−1 )
(kJ mol−1 )
205.2
18.8
41.5
50.3
49.9
404.0
228.8
96.7
164.8
210.9
0.0
0.0
-961.5
-1006.2
-999.7
-4398.2
-3289.7
-1789.8
-2741.3
-3513.1
The six most likely reactions to occur at 950◦ C and the standard free energy change ∆G◦
of each reaction, are shown in Table 6.2. Care must be taken when calculating the heats
of formation in thin ﬁlms as some compound phases present in the bulk phase are not
observed with thin ﬁlms.316 Additionally, there is also the consideration of the ﬂux of
elements to the reacting surface, as not all elements in the system will be available in the
same concentration at the same time to the reacting surface. From Table 6.2 it is noted
that TiO2 will react spontaneously with P2 O5 to form titanium diphosphate (TiP2 O7 ) or
titanium triphosphate (TiP3 O9 ), but the reaction to form titanium phosphate (TiPO4 ) does
not proceed spontaneously at 950◦ C. However, if silicon is available to the reaction, noting
that silicon and P2 O5 are initially present on opposite sides of the TiO2 ﬁlms, any of the
previously mentioned titanium phosphate compounds (TiPx Oy ) may be formed.
144
6. Performance of TiO2 Thin Films as a Phosphorus Diﬀusion Barrier and Dopant Source
Table 6.2: Proposed chemical reactions between TiO2 and P2 O5 and their
standard free energy change ∆G◦ at 1223 K.
∆G◦ (kJ)
Proposed Chemical Reaction
TiO2 + P2 O5 −→ TiP2 O7
2 TiO2 + P2 O5 −→ 2 TiPO4 +
2 TiO2 + 3 P2 O5 −→ 2 TiP3 O9 +
O2
1
2
+78.5
O2
-78.5
4 TiO2 + Si + 2 P2 O5 −→ 4 TiPO4 + SiO2
-804.6
TiO2 + Si + P2 O5 + O2 −→ TiP2 O7 + SiO2
-1051.3
2 TiO2 + Si + 3 P2 O5 +
6.2.1
-89.8
1
2
1
2
O2 −→ 2 TiP3 O9 + SiO2
-1040.0
Experiment
Several batches of experiments were performed to conﬁrm the results predicted from thermochemistry analysis - that some reaction between TiO2 and P2 O5 would occur, resulting
in the formation of a TiPx Oy compound. The substrates used for both diﬀusion barrier
and dopant source experiments were p-type 1000 Ω cm ﬂoat zone (FZ) (100)-oriented silicon
wafers. The diagram in Figure 6.1 depicts the processing steps foe the “diﬀusion barrier” as
well as the “dopant source” experiments. The latter experiment is described in Section 6.3.
Five batches of experiments with eight wafers each were undertaken to determine the eﬀectiveness of spray-deposited TiO2 as a phosphorus diﬀusion barrier. The wafers were loaded
into a POCl3 furnace at 800◦ C and oxidised for 10 min, before the temperature was ramped
up to 925◦ C in N2 . The POCl3 and O2 concentrations were 1.5% and these gases were left
on for 10 min. A drive-in was performed at 950◦ C for 90 min in N2 before unloading the
wafers. This recipe is typically used to create deep emitters with a sheet resistance of about
45 − 50 Ω/2. Heavier phosphorus diﬀusions (5 Ω/2) were performed by leaving the POCl3
and O2 ﬂows on for 90 min at 950◦ C.
6.2.2
Results
Upon removing the diﬀusion barrier samples from the POCl3 furnace, it was noted that
there were yellow ﬂecks on the surface of the dark blue TiO2 ﬁlm surface. The experiments
were repeated several times with phosphorus diﬀusions as heavy as 5 Ω/2. For the heavier
diﬀusions it was evident that a reaction between the TiO2 ﬁlm and the phosphorus had
occurred in the furnace, as there were silvery-white, opaque areas on the surface of the ﬁlm,
a signiﬁcant fraction of the ﬁlm had a yellow colour to it, while a only a small area remained
dark blue. An image of the latter sample is shown in Figure 6.2. In order to determine the
nature of the resulting compound, transient-PCD, microwave-PCD, conductivity, and RBS
6.2 TiO2 as a Phosphorus Diﬀusion Barrier
“Diffusion Barrier”
145
“Dopant Source”
NaOH etch (30%, 85°C, 20 min)
RCA2, RCA1, RCA2, HF dip, DI rinse
TiO2 Deposition
(both sides,
~70nm thick,
Tdep=450°C)
P:TiO2 Deposition
(both sides,
~70nm thick,
Tdep=450°C)
H2SO4:H2O2:DI (1:1:5), RCA2, DI rinse
Furnace Step:
5–50 Ω/
POCl3 Diffusion
V
Furnace Step:
oxidation (O2)
and anneal (N2)
Figure 6.1: Diagram showing the processes steps used for TiO2 as a
phosphorus diﬀusion barrier and TiO2 as a phosphorus dopant source
experiments.
measurements were performed.
Opaque region
Dark blue
region
Yellow
region
Figure 6.2: Image of a TiO2 -coated wafer after a heavy (5 Ω/2) phosphorus diﬀusion. A small area of dark blue coloured ﬁlm remains, while
the majority of the ﬁlm has a either a yellow or an opaque, silvery white
colour.
146
6. Performance of TiO2 Thin Films as a Phosphorus Diﬀusion Barrier and Dopant Source
PCD and Conductivity Measurements
Transient-PCD analysis was performed in an attempt to determine the eﬀect of the reaction
on the surface quality (J0e ) and minority carrier bulk lifetime (τbulk ). The J0e had increased
considerably to 1 − 5 × 10−12 A/cm2 , primarily due to the poor surface passivation. Mapping
of τef f was performed with a microwave-PCD system, however there did not appear to be
a relationship to the value of τef f and the white, yellow, or blue regions of the sample.
The conductivity of the ﬁlms was measured using the four-point probe technique. It was
found that conductivity increased from the blue region (no measurable conductivity) to
the yellow region (able pass a small current through the ﬁlm) to the white region (about
700 Ω/2). The samples were placed undiluted 48% HF for 24 hours, which slowly etched
away the white regions, however the blue regions remained. The remaining blue TiO2 ﬁlm
was removed instantly by subsequently placing the wafer in undiluted NH4 OH. The measured
conductivity of the diﬀused layer on the silicon wafer varied from 6000 Ω/2 underneath the
white region to > 200000 Ω/2 under the blue region. A hot probe test indicated that an
n-type diﬀusion was present on the surfaces of the p-type wafer. Therefore, it is assumed
that a small amount of phosphorus was able to diﬀuse through the TiO2 ﬁlm and into the
silicon wafer.
RBS Measurements
Rutherford backscattering spectroscopy (RBS) allows the determination of the masses of
the elements within a sample, their depth distribution within a 10 nm resolution, and their
crystalline structure. Figure 6.3 compares the diﬀerent RBS spectra obtained from (a) a
yellow region, and (b) a silvery-white region of the sample. Curve ﬁtting using the software
package RUMP272 suggests that a signiﬁcant amount of phosphorus has been incorporated
into the ﬁlm. There are two notable diﬀerences between the spectra. Firstly, a large amount
of silicon has been incorporated into the ﬁlm in the white region, resulting in the formation
of a TiPx Siy Oz (see Figure 6.3(b)) compound instead of the TiPx Oy compound observed
in the yellow region. Secondly, the thickness has dramatically increased from about 70 nm
before being placed in the furnace, to 100 nm in yellow regions and 300 nm in the opaque,
white regions. A similar phosphorus concentration is observed in both spectra, and the
stoichiometry at the surface is similar in both regions.273
6.2.3
Discussion
There is no exact correlation between a likely product from the thermochemistry analysis
and the compounds detected using RBS analysis. Whether SiO2 is formed as a reaction
by-product or not remains unknown, however it is suspected that the thin SiO2 passivation
layer, grown before the POCl3 ﬂow was turned on, has been consumed in the reaction. This,
6.2 TiO2 as a Phosphorus Diﬀusion Barrier
147
Figure 6.3: Comparison of RBS analysis performed on (a) a yellow region
and (b) a silvery-white region of a phosphorus diﬀusion barrier sample.
The solid lines represent experimental data, while the dotted lines are a
ﬁt to the experimental data using the simulation package RUMP.272
along with the high J0e values, would seem to indicate that it was not merely a surface
reaction that occurred in the furnace and that the entire thickness of the TiO2 ﬁlm has
undergone a reaction, altering the electrical and optical properties of the ﬁlm.
A search of the relevant literature yields several pieces of information about TiPx Oy compounds. Ekambaram and Sevov noted that amorphous titanium diphosphate is a white
colour, and that it crystallises in the TiP2 O7 phase above 600◦ C.317 Glaum and Gruehn
observed that it was possible for titanium phosphate compounds to react with silica (SiO2 )
to form a new titanium silico-phosphate (Ti4 P6 Si2 O25 ) compound.290 The ratio between
the elements in this new compound are 1 : 1.25 : 0.5 : 6.25, which is remarkably similar to
that determined from the RBS data in Figure 6.3(b) for the silvery-white compound (c.f.
1 : 1.25 : 1.05 : 6). The diﬀering fraction of silicon in the compounds formed here could be
due to the non-equal availability of reactants to the system. For example, signiﬁcantly more
silicon is available to the reaction from the wafer than the TiO2 or P2 O5 constituents. Additionally, the Si:Ti ratio is too high (1 : 1.05) for the source of the silicon to be solely the
148
6. Performance of TiO2 Thin Films as a Phosphorus Diﬀusion Barrier and Dopant Source
SiO2 layer, as the ratio of ﬁlm thicknesses gives about dT iO2 /dSiO2 = 10 : 1. One possible
explanation is that the reaction has proceeded throughout the whole ﬁlm thicknesses and
consumed part of the silicon wafer.
The ability of TiO2 to act as a phosphorus diﬀusion barrier may be better understood by
comparing it to SiO2 . In comparison to SiO2 , the archetypical glass-former, it was noted
that the diﬀerences between the two compounds included covalent versus ionic bonding,
tetrahedral versus octahedral structural units, and that TiO2 is among the worst glass formers.262 Additionally, Zachariasen classiﬁed TiO2 as an intermediate or network-modifying
oxide, indicating that it does not appear to form a glass by itself.318 The most common
network formers in SiO2 are boron and phosphorus, as they are capable of forming glasses
by themselves.17 This allows a P2 O5 layer to form a mixed glass when it comes into contact
with a SiO2 layer, as occurs in the standard buried-contact process. SiO2 is a more eﬀective
diﬀusion barrier for boron than phosphorous because of the lower temperatures associated
with the B2 O3 :SiO2 glass composition. Examination of the TiO2 :P2 O5 phase diagram,319
shown in Figure 6.4, also reveals low temperature glass compositions. However, closer inspection reveals the formation of the compound TiP2 O7 at a wide range of compositions and
temperatures. Thus, TiO2 and P2 O5 are not miscible, and the two oxides readily react upon
contact.
Figure 6.4: TiO2 :P2 O5 phase diagram, indicating the prevalence for the
compound TiP2 O7 to form at both a wide range of compositions and at
low temperatures.319
6.3 TiO2 as a Phosphorus Dopant Source
149
The prerequisites for the diﬀusion barrier layer in the BC solar cell fabrication sequence
are demanding, requiring that the ﬁlm remain optically transparent to visible light and
maintain excellent surface passivation, in addition to preventing phosphorus from diﬀusing
through to the lightly doped emitter. Although TiO2 prevents all but a small fraction of
phosphorus from diﬀusing into the underlying silicon, the optical, insulating, and passivating
properties of the TiO2 :SiO2 stack are irreparably damaged in the process, and TiO2 must be
regarded as a sacriﬁcial phosphorus diﬀusion barrier. The author performed one experiment
demonstrating that a 200 nm-thick spin-on SiO2 ﬁlm is suﬃcient to protect the TiO2 during
a heavy groove diﬀusion, however it was not within the scope of this work to investigate the
full potential of suck “workarounds”.
6.3
TiO2 as a Phosphorus Dopant Source
There are several motivations for introducing dopant atoms into thin ﬁlms. For TiO2 , common reasons include altering the electrical conductivity, photoelectric response, chemical resistance, optical properties, melting temperature, and the resulting TiO2 crystalline phase,
as well as using the ﬁlm as a dopant source. There are quite a number of references to
P:TiO2 thin ﬁlms in the literature. In some papers, the phosphorus was introduced to increase the conductivity of a visibly transparent ﬁlm.61, 169, 188 In one instance, doping a TiO2
ﬁlm with 1 mol. % of P2 O5 increased conductivity of the ﬁlm by 1000 times.188 The use of
P:TiO2 in a heterojunction solar cell as an intermediary layer between tin oxide and silicon
was proposed, due to its increased electrical conductivity and optimal refractive index.61
Moriyama described several methods for depositing P-doped TiOx ﬁlms.181 These included
e-beam evaporation and sputtering of a phosphorus and titanium source followed by oxidation; deposition of a titanium ﬁlm followed by oxidation of the ﬁlm in POCl3 atmosphere;
deposition of a P-based ﬁlm onto a TiOx ﬁlm followed by a high temperature diﬀusion step;
and ﬁnally, ion-implantation of phosphorus into a TiOx ﬁlm. The aim of that work was to
suppress the leakage current of a TiOx capacitor.
Another frequent observation is that P:TiO2 enhances the formation of the TiO2 anatase
phase, and inhibits the formation of the rutile phase.80, 81, 86 Akhtar et al. deposited doped
TiO2 ﬁlms formed with TiCl4 and 15% POCl3 , and observed an increase in the lattice
constant. This indicates that the phosphorus is incorporated into the lattice. Additionally,
the smaller phosphorus atoms were observed to diﬀuse interstitially, which had the eﬀect of
inhibiting the phase change to rutile. Rao et al. observed that after 3 hr annealing at 870◦ C a
TiO2 ﬁlm containing 5 at. % phosphorus remained 100% anatase.80 The enhancement of the
anatase phase was also observed when analysing P-doped glass using Raman spectroscopy.86
In the remainder of the works examined here, the phosphorus was intended to be used as
a dopant source. Saﬁr initially described use of doped SiO2 as a diﬀusion source, however
150
6. Performance of TiO2 Thin Films as a Phosphorus Diﬀusion Barrier and Dopant Source
the potential of including dopant atoms, such as phosphorus, into titanium monoxide (TiO),
TiO2 , and other thin ﬁlms was noted. Toyokura and Taguchi demonstrated that after annealing a phosphorus-doped tantalum pentoxide (P:Ta2 O5 ) ﬁlm on a p-type wafer for 30 min at
1000◦ C a 0.3 µm deep junction is formed with a surface concentration of ND = 1019 cm−3 .320
It was mentioned that similar results could be achieved with P:TiO2 however no experimental
evidence was given in the paper.
Sharp Corporation (Japan) hold several patents regarding the diﬀusion of phosphorus from
a TiOx antireﬂection coating into p-type silicon to form a p-n junction.225, 321, 322 In the work
by Yokozawa et al., the titanium and phosphorus precursors, TPT and triethoxy phosphorus,
respectively, are introduced into a CVD chamber using an inert gas.225 It is claimed that a
subsequent thermal process enables the phosphorus to diﬀuse from the TiOx ﬁlm. In subsequent research, the technique was reﬁned so that highly-doped P:TiOx is deposited where the
evaporated contacts will be located.321 Then, a more lightly-doped P:TiOx layer is deposited
over the whole front surface of the p-type wafer. The subsequent high-temperature step then
results in the formation of a selective emitter and reduction of the contact resistance. The
most recent work by Ui et al., details the variation of the refractive index and resulting sheet
resistance with phosphorus concentration of CVD deposited P:TiOx ﬁlms.322 By increasing
the P:Ti atomic ratio from 0.1 to 1.0 the sheet resistance decreased from 100 Ω/2 to about
30 Ω/2. Two researchers have noted that it is much easier to perform p-type diﬀusions from
boron-doped TiO2 ﬁlms than n-type diﬀusions from P:TiO2 , however no further explanations were given.323, 324 Despite this, Yoldas and Yoldas reported that n-type diﬀusions could
be achieved from TiO2 ﬁlms containing, ﬁrstly, 15% P2 O5 and, secondly, 10% triethyl phosphate. The resulting sheet resistance after high-temperature processing at 1000 − 1050◦ C in
N2 for one hour was not stated.323
Investigations have also as been performed in the use of titanium silicide (TiSi2 ) as a phosphorus dopant source. Privitera et al. achieved shallow junctions (< 100 nm deep) with
P:TiSi2 using thermal treatments in the range 950 − 1150◦ C. However, this resulted in a
high density of silicon phosphide (SiP) precipitates at the interface. La Via et al. noted that
titanium-phosphorus compounds were formed in the P:TiSi2 , however phosphorus diﬀusion
was still observed due to the similar lattice spacings of the two compounds.325 Another work
noted that there was negligible diﬀusion of phosphorus into the silicon after annealing for
30 min at 800◦ C.326 Indium tin oxide has also been doped with phosphorus in order to form
an emitter in p-type silicon.327 However, no phosphorus diﬀused into the silicon, and it is
postulated that indium phosphide formed and was evaporated during the subsequent 900◦ C
thermal treatment.
In this section, initial investigations on the use of spray-deposited TiO2 as a phosphorus
dopant source for the formation of emitters in silicon solar cells are reported. The aims were
to determine, ﬁrstly, whether an emitter dopant could be contained within the dielectric
coating, and, secondly, if the dopant atoms could diﬀuse from the dielectric ﬁlm into the
6.3 TiO2 as a Phosphorus Dopant Source
151
silicon wafer during a subsequent high-temperature processing. If successful, this would
eliminate the need for one high-temperature step, namely the emitter diﬀusion.
6.3.1
Experiment
In order to realize phosphorus-doped TiO2 ﬁlms, small volumes (1 wt. % and 5 wt. %) of
triethyl phosphate (TEPO, C2 H5 O3 PO, Aldrich, 99%) were added to the TPT, and the
solution sprayed onto p-type wafers at 450◦ C. The samples were then cleaned in boiling
RCA2 solution for 5 min and rinsed in DI water. This resulted in the complete etching
of the 5% P:TiO2 ﬁlms, however the 1% P:TiO2 ﬁlms remained intact. After rinsing and
drying, the samples were separated into three batches. All batches were loaded at 900◦ C
in O2 , and a N2 ambient replaced the O2 after the ﬁrst 5, 10, and 20 min for each of the
batches, respectively, while ramping up to 950◦ C. In all cases the total processing time was
kept constant at 90 min.
6.3.2
Results and Discussion
The sheet resistance of TiO2 ﬁlms doped with 1 wt. % phosphorus were measured using
the four-point probe (FPP) technique. Results indicated that the ﬁlm itself had a sheet
resistance of 300 Ω/2. Transient-PCD analysis indicated that the τbulk of the samples were
high at 1.4 ms, however the J0e was also high at 9 − 10 × 10−13 A/cm2 . After etching oﬀ
the P:TiO2 ﬁlm in pure (48%) HF, a hot-probe test conﬁrmed that an n-type diﬀusion
was present, but the conductivity was too low to be measured with the FPP technique.
One published work has indicated that titanium atoms are able to diﬀuse out of a reduced
TiOx ﬁlm, resulting in an n-type diﬀusion in a p-type silicon wafer.31 The possibility of
this occurring in the spray-deposited samples cannot be ruled out, however the TiO2 ﬁlms
employed here are very close to stoichiometric (2.02 ± 0.13 from RBS measurements) and
the strong aﬃnity of titanium for oxygen would seem to rule out any diﬀusion of titanium
into the silicon.
Ferdjani et al. noted that phosphorus can be incorporated into an oxide as oxygenated anions Px Oy (n-) rather that atomic phosphorus.328 Additionally, within TiO2 , the formation
of titanium phosphide (TiP) is common as this compound is very stable.328 XPS experiments were performed on the spray-deposited, phosphorus-doped TiO2 ﬁlms at the Toyota
Technical Institute (Nagoya, Japan). Two 1 wt. % P:TiO2 samples had a total phosphorus
concentration at the surface of 3.2 at. % and 2.6 at. %, respectively. The phosphorus 2p peak
appears at 135.6 eV while the 2s peak is at 192.5 eV. The height of the phosphorus peaks
did not vary signiﬁcantly with the depth of the ﬁlm, indicating that the phosphorus dopant
atoms are quite uniformly incorporated into the TiO2 lattice.87
152
6. Performance of TiO2 Thin Films as a Phosphorus Diﬀusion Barrier and Dopant Source
It would seem that for P:TiO2 ﬁlms to act as an eﬀective dopant source, phosphorus dopant
concentrations of about 15% would be required along with higher processing temperatures,
as reported by Yoldas.323 However, if a 1 mol. % phosphorus-doped TiO2 ﬁlm increased the
conductivity by 1000 times,188 a 15% concentration would most likely make the ﬁlms very
conductive.
6.4
Conclusions
The results from this work indicate that a 70 nm thick TiO2 ﬁlm can function either as a
phosphorus diﬀusion barrier or a phosphorus dopant source. Although the TiO2 diﬀusion
barrier resulted in only very light phosphorus diﬀusions underneath the ﬁlm, a reaction with
P2 O5 substantially alters the optical and electrical properties and limiting its usefulness.
Therefore, TiO2 must be described as a sacriﬁcial diﬀusion barrier to phosphorus or, alternatively, the TiO2 must be protected by another ﬁlm (such as SiO2 ) during the diﬀusion
process.
The ability of TiO2 to act as a dopant source for phosphorus atoms is also somewhat limited.
The increased conductivity of the TiO2 ﬁlm due to the phosphorus incorporation is unlikely
to be compatible with the insulating requirements of the BC solar cell electroless metal
plating step. Further investigations would need to be performed to ascertain the beneﬁt of
P:TiO2 ﬁlms in other solar cell structures, such as those using screen-printed and evaporated
metal contacts. However, it is anticipated that the increased optical absorption in the ﬁlms,
along with the lengthy diﬀusion times and temperatures involved (> 1 hr at > 1000◦ C) will
limit the industrial uptake of this technology, even for screen-printed solar cells.
Chapter 7
TiO2 Antireﬂection Coatings
7.1
Introduction
Commercial solar cells fabricated on multicrystalline silicon (mc-Si) wafers exhibit poorer
short current densities (Jsc ) than their monocrystalline counterparts, primarily due to the
inability to adequately texture mc-Si wafers. This is due to the randomly oriented grains in
mc-Si material, and after texturing in a basic (NaOH or KOH) solution, only 20 − 30% of
the grains result in the familiar {111} pyramid faces. The pyramids enable the majority of
light striking the surface to have two chances at being transmitted into the silicon, instead
of being reﬂected. Research has been performed on alternate texturing methods, such as
reactive ion etching,329–331 laser- and photolithographically-deﬁned texturing,28, 332 mechanical texturing333, 334 and acid-based chemical etching to form either textured335 or porous
silicon.336, 337 While some of these methods enable low reﬂectance to be achieved on mc-Si
wafers, none of these techniques would appear to be particularly attractive in a commercial
environment, due to either the high costs (photolithography, maintaining a vacuum) or large
quantities of dangerous chemicals involved.
An alternate method of reducing the front surface reﬂection is with antireﬂection (AR)
coatings. This chapter will brieﬂy review the theory of AR coatings, before establishing the
performance of existing silicon nitride and TiO2 AR coatings. Subsequently, a novel and
simpliﬁed approach for the formation of TiO2 single- and multi-layer antireﬂection AR will
be presented and the performance of these coatings established.
Figure 7.1 shows the radiation intensity (φ) of the AM1.5 global spectrum (in mW/cm2 ) as
a function of wavelength. The data is taken from Green and is normalised to 100 mW/cm2 .8
Also shown in Figure 7.1 is the available short-circuit current density (Jsc ) at each wavelength. An AR coating should exhibit a minimum reﬂectance at about 600 nm in order
to take full advantage of the peak in the spectrum at 550 − 750 nm. A maximum Jsc of
45.97 mA/cm2 can be obtained in the 300 − 1200 nm wavelength region.8
153
154
7. TiO2 Antireﬂection Coatings
Figure 7.1: Global AM1.5 spectrum and short-circuit current density for
wavelengths 300 − 1200 nm. The data was taken from Green.8
There are two common standards for measuring the performance of a texturing scheme or
AR coating. The ﬁrst is the weighted average reﬂection Rw , deﬁned as:
λmax
Rw =
R(λ)Nph (λ)d(λ)
λmin
λmax
λmin
.
(7.1)
Nph (λ)d(λ)
In Equation 7.1, the values used for λmin and λmax are typically in the range 300 − 400 nm
and 1000 − 1200 nm, respectively. The amount of solar ﬂux under the AM 1.5 spectrum is
represented by Nph (λ), while Rw is the reﬂectance of the solar cell. A Rw that is evaluated
over a narrower wavelength range is likely to be lower than that evaluated over a wider
wavelength range. This is especially true if the short wavelength regions (< 400 nm) are
excluded, where the reﬂectance of silicon is very high, or if the long wavelengths (> 1000 nm)
are excluded, where reﬂectance from the back surface of the silicon wafer is observed. The
advantage of using Rw as a ﬁgure-of-merit is that it is purely optical and does not depend
on the electrical performance of the solar cell.
The second measure of performance is the short-circuit current density Jsc , as shown in
Equation 7.2,
λmax
Jsc = q
(1 − R(λ)) Nph (λ)IQE(λ)d(λ) ,
(7.2)
λmin
where the electrical response of the solar cell as a function of wavelength is contained in the
internal quantum eﬃciency (IQE) term, Nph is the photon ﬂux of the solar spectrum, and q
is the electronic charge with the value 1.602×10−19 C. For comparing texturing methods and
7.2 Previous Developments in AR Coatings
155
the majority of AR coatings, the quantity Rw will be used as many of these methods could
be applied to either screen-printed or high-eﬃciency solar cell designs. In later sections, the
Jsc will be used to determine the performance of single- and multi-layer TiO2 AR coatings
for buried-contact (BC) solar cells. The advantage of using Jsc is that this is one of the
important ﬁnal output parameters of a solar cell.
7.2
Previous Developments in AR Coatings
This section will brieﬂy introduce the theory behind single- and double-layer AR coatings,
before reviewing the two most common AR coatings in the PV industry, TiO2 and silicon
nitride (SiNx ). For really high-eﬃciency, laboratory-scale solar cells, the evaporation of
materials such as zinc sulphide (ZnS) and magnesium ﬂuoride (MgF2 ) is a standard process,
however this technology has no relevance in a production setting due to the high-cost and
low-throughput nature of the evaporation method.
7.2.1
Theory and Design of AR Coatings
Single Layer AR Coatings
A single layer antireﬂection (SLAR) coating is the minimum requirement for any silicon solar
cell produced today. Light is well absorbed by semiconducting materials such as silicon,
however these materials exhibit high refractive indices. For example, silicon has a refractive
index of nsi = 3.939 at 600 nm.8 This refractive index is much greater than air, which has
a constant refractive index of n0 = 1.0, and glass (n0 = 1.52 at 600 nm). The reﬂectance of
normally incident light at such an interface is given by
2
nSi − n0
,
(7.3)
R=
nSi + n0
which means that 35.4% or 19.6% of the light is reﬂected oﬀ an air:silicon or glass:silicon
interface in the ﬁrst bounce, respectively. If an optimum-thickness AR coating is inserted
between the silicon and ambient medium, the minimum reﬂectance is given by
2
2
nAR − n0 nSi
,
(7.4)
R=
n2AR + n0 nSi
where nAR is the refractive index of the coating. To achieve zero reﬂectance at one wavelength, the value of nAR should be
√
nAR = n0 nSi
(7.5)
and the ﬁlm thickness (dAR ) must meet the quarterwave optical thickness requirement
dAR =
λ0
,
4 nAR
(7.6)
156
7. TiO2 Antireﬂection Coatings
where λ0 is the wavelength of zero or minimum reﬂectivity.
This indicates that an AR coating for a silicon solar cell in air should have a refractive index
of 1.985 and a thickness of 75.6 nm, while a glass encapsulated cell requires an AR coating
with nAR = 2.447 and dAR = 61.3 nm. Figure 7.2 shows the calculated reﬂectance for four
diﬀerent SLAR coatings. The lines in Figure 7.2 represent theoretical AR coatings with
ﬁxed (non-dispersive) refractive indices of 1.985 and 2.447 (as determined above), while the
data points are two TiO2 ﬁlms deposited by the author, indicating that an excellent match
between the theoretical and practical coatings can be achieved. The reﬂectance minimum of
the encapsulated silicon is broader due to the higher refractive index of the glass, however
the ﬁrst reﬂectance bounce oﬀ the air:glass interface means that only a minimum of 4.3%
can be achieved at the design wavelength of λ0 = 600 nm. The weighted average reﬂectances
(350 − 1150 nm) of the ﬁxed AR coatings are Rw = 8.75% and Rw = 8.41% for air and glass
encapsulation, respectively. In comparison, the minimum weighted average reﬂectances that
can be achieved using experimental TiO2 coatings are Rw = 9.45% and Rw = 8.98% for air
and glass encapsulation, respectively. The TiO2 AR coatings deposited in this work will be
discussed in detail in Section 7.4.
Figure 7.2: Modelled reﬂectance of the ﬁxed refractive index coatings, and
two TiO2 AR coatings deposited in this work. The performance of the
TiO2 coatings is extremely close to that of the theoretical coatings.
Double Layer AR Coatings
Whereas a SLAR coating can be designed to achieve zero reﬂectance at one wavelength, a
double layer antireﬂection (DLAR) coating oﬀers a further reduction in reﬂectance. In this
7.2 Previous Developments in AR Coatings
157
scenario, the refractive indices are stacked as follows nSi > nAR2 > nAR1 > n0 , where nAR2
and nAR1 represent AR coatings with high and low refractive indices, respectively. For layers
with equal optical thickness, such that nAR2 dAR2 = nAR1 dAR1 = λ0 /4, the reﬂectance at λ0
becomes
2
2
nAR1 nSi − n2AR2 n0
R=
.
(7.7)
n2AR1 nSi + n2AR2 n0
The reﬂectance R will be either a maximum or minimum at λ0 depending on the relationship
between the refractive indices. From Equation 7.7, it can be seen that if n2AR1 nSi = n2AR2 n0 ,
zero reﬂectance at λ0 will be achieved. This is a much broader minimum than is achievable
with a SLAR coating. If nAR1 nAR2 = n0 nSi a maximum of R will occur at λ0 and double zero
reﬂectance will be achieved at two wavelengths either side of λ0 . The latter conﬁguration
is commonly used to achieve minimum reﬂection across the whole solar spectrum (300 −
1200 nm).
For two quarter-wavelength coatings, the optimal refractive indices of each layer in a DLAR
stack can be determined by279
n3AR1 = n20 nSi and n3AR2 = n0 n2Si .
(7.8)
The mathematics for determining the reﬂectance oﬀ double- or multi-layer AR coatings is
not diﬃcult, however it is much more laborious than the single coating case above, especially
for ﬁlms that exhibit absorption. The interested reader is referred to texts that discuss this
optical theory such as Heavens38 and Macleod.279
Absorbing AR Coatings
In many instances, the amount of absorption in the AR coating is negligible338 and Equation
7.1 (see Section 7.1) can be safely applied in order to determine Rw . However, for AR
coatings possessing high refractive indices, absorption is commonly observed in the short
wavelength spectrum35, 339, 340 and the use of Equation 7.1 will lead to an erroneous answer.
This problem can be solved by considering the weighted average transmission Tw through
the AR coating, deﬁned as
λmax
T (λ)Nph (λ)d(λ)
Tw = λmin
.
(7.9)
λmax
N
(λ)d(λ)
ph
λmin
This performance parameter is somewhat more complicated to determine experimentally,
compared to Rw . In order to determine Rw , the reﬂectance of the sample is simply measured using a spectrophotometer with an integrating sphere, and a numerical integration is
performed using a mathematical software package, such as Mathematica, giving a value for
Rw . In order to determine Tw , there are several options. Firstly, careful optical modelling
158
7. TiO2 Antireﬂection Coatings
of the reﬂectance data can yield information regarding the extinction coeﬃcient of the ﬁlm.
A certain amount of prior knowledge is required to ensure that a “physically real” answer
is reached. Secondly, this analysis can be greatly assisted by depositing the AR coating
in question onto a transparent substrate and measuring the transmittance. This provides
valuable data to assist in the determination of n and k. The only drawback to this approach
is that ﬁlms such as TiO2 may have a diﬀerent preferential phase when deposited onto glass
or quartz compared to silicon (see Section 2.2.3), and silicon nitride ﬁlms deposited using
PECVD may possess diﬀerent optical properties due to the insulating nature of the glass
substrate. Thirdly, an alternate method for obtaining the optical constants can be used,
such as ellipsometry. Once n and k are known the transmittance or reﬂectance curve can be
generated using software and Tw or Rw calculated.
The remaining quantity, average weighted absorptance Aw , then becomes another useful
ﬁgure of merit for solar cell design. The Aw is deﬁned as
λmax
A(λ)Nph (λ)d(λ)
.
(7.10)
Aw = λmin
λmax
N
(λ)d(λ)
ph
λmin
The inclusion of absorption into the optical model also means that the equation for determining the Jsc of a solar cell, previously deﬁned in Equation 7.2, must be modiﬁed and now
appears as
λmax
(1 − R(λ) − A(λ)) Nph (λ)IQE(λ)d(λ)
Jsc = q
λmin
λmax
= q
T (λ)Nph (λ)IQE(λ)d(λ) .
(7.11)
λmin
The wavelength limits used in this work are λmin = 350 nm and λmax = 1150 nm. These
limits were imposed by the spectroscopic ellipsometer, for which the data outside the 350 −
1150 nm range is typically quite noisy. The solar spectrum for silicon solar cells is usually
300 − 1200 nm, however only a small fraction of light is emitted from the sun at λ < 350 nm
(≈ 0.35 mA/cm2 ) and is absorbed by silicon at λ > 1150 nm.
The electrical response of a silicon solar cell varies with wavelength due to the varying
absorption coeﬃcient of silicon, as well as the level of front and rear surface passivation and
the quality of the silicon substrate. Silicon wafers with a lower bulk minority carrier lifetime
τbulk typically exhibit a reduced response to long wavelength light due to defects within the
crystal structure. This includes c-Si wafers grown with the Czochralski (CZ) method, which
have a low τbulk due to a high oxygen and carbon concentration, and mc-Si wafers, where
the grain boundaries between the crystallites reduces τbulk .
In Equation 7.11, the IQE(λ) term indicates the electrical response of the solar cell over the
solar spectrum, called internal quantum eﬃciency (IQE). The IQE of the solar cell is strongly
inﬂuenced by the quality of surface passivation, primarily at the front and to some degree
7.2 Previous Developments in AR Coatings
159
at the rear. The use of thin SiO2 layers at the AR coating:silicon interface was discussed in
Section 5.3. The IQE of a simulated BC solar cell, used to predict the performance of solar
cells with TiO2 AR coatings, is plotted in Figure 7.26 in Section 7.5.
Materials for AR Coatings
Common materials used for AR coatings are summarised in Table 7.1. The refractive indices
may vary somewhat depending upon the deposition method, deposition rate, and any postdeposition annealing steps. Transparent conducting oxides such as indium tin oxide and zinc
oxide, commonly used in thin ﬁlm devices, are not shown as their electrical conductivity
severely compromises their transparency.
Table 7.1: Refractive indices of materials commonly used in antireﬂection
coatings (adapted from Green 7 ).
Material
Magnesium ﬂuoride MgF2
Silicon dioxide
SiO2
Aluminium dioxide Al2 O3
Silicon monoxide
SiO
Cerium dioxide
CeO2
Tantalum pentoxide Ta2 O5
Silicon nitride
SiNx
Titanium dioxide
TiO2
Zinc sulphide
ZnS
n (λ = 600 nm)
1.38
1.46
1.8
1.8 − 1.9
2.2
2.1 − 2.3
1.9 − 2.1
1.9 − 2.4
2.3 − 2.4
Revesz made several important observations about the desirable properties of an AR coating material.43 Firstly, the use of a non-crystalline material is important in AR coating
design because the grain boundaries in polycrystalline ﬁlm cause scattering and decrease
transparency. Non-crystalline materials have also proven to be most suitable for fabricating
capacitors. However, it is desirable for the noncrystalline material to exhibit a reasonable
degree of short range order in order to avoid increased absorption due to unsaturated bonds.
Thus, a vitreous material is deemed to be more suitable than an amorphous material. A
good example of a vitreous material is thermally grown SiO2 , and this would be expected
to exhibit lower absorption than sputtered amorphous SiOx , for example. TiO2 ﬁlms grown
in a chemical process at low temperatures, such as spray-deposition or CVD, could also be
expected to exhibit a high degree of short-range order.
Roger and Colardelle noted that as a common rule for pre-determining the optical properties
of a thin ﬁlm: a slow deposition rate leads to low absorption and low refractive index, whereas
a fast deposition rate is conducive to a high refractive index, but also high absorption.277
160
7. TiO2 Antireﬂection Coatings
Thus, when designing multilayer AR coatings with absorbing materials, there may be a
design trade-oﬀ with perfectly matching the refractive indices or avoiding high absorption.
Optical Modelling Software
Two pieces of optical modelling software were used extensively throughout this thesis.
Firstly, WVASE,62 already mentioned in Section 4.6, was used for analysing measured ellipsometry and spectrophotometry data. This software was chosen for its speed, robustness,
and a large number of integrated models. A double Lorentz oscillator model was successfully used for ﬁtting the Ψ and ∆ data and extracting the dispersive refractive indices n
and extinction coeﬃcients k in the range 350 − 1150 nm. The n and k values were exported
from WVASE into TFCalc341 for optimisation of the AR coating layer thickness. TFCalc
enables the user to set either distinct or a range of wavelengths over which to minimise the
reﬂectance. A simplex downhill algorithm was normally used for optimising the layer thicknesses, and a passivating oxide layer could also be included at the interface. The reﬂectance,
transmittance, and absorptance data was then exported to a Mathematica program where
the average weighted reﬂectance, transmittance, and absorptance values (see Section 7.2.1)
were calculated, along with the maximum short circuit current density, Jsc .
7.2.2
TiO2 AR Coatings
The most commonly used material in the PV industry for AR coatings is TiO2 .16, 19 These
are SLAR coatings, primarily deposited onto screen-printed solar cells. This is primarily
due to the excellent optical properties of TiO2 , its ease of deposition and low cost. Doublelayer antireﬂection (DLAR) coatings are used in many high-eﬃciency solar cells, in order to
further reduce front surface reﬂectance losses. These DLAR coatings are normally utilised in
laboratory-scale devices, and the coatings are optimised for measurements in air. However,
some and, secondly, under glass encapsulation. Ultimately, however, any successful solar cell
design will require encapsulation to protect the device from the environment, and a review
of DLAR coatings under glass will be provided.
SLAR Coatings
The review paper by Kern and Tracy provides an excellent review of early AR coating
methods and development, focussing particularly on TiO2 .46 In that work, screen-printed
solar cells exhibited a 41% increase in eﬃciency after receiving an optimised TiO2 AR coating.
Many other researchers have observed a 42 − 50% increase in eﬃciency after depositing a
TiO2 AR coating,89, 157 the higher numbers coming from glass-encapsulated coatings with
nAR = 2.4.89
7.2 Previous Developments in AR Coatings
161
Brinker and Harrington optimised the ratio of SiO2 to TiO2 in similar SLAR coatings, and
found that a 90% TiO2 /10% SiO2 mixture achieved the lowest reﬂectance.201 The bandwidth,
or range of wavelengths, at which the reﬂectance was ≤ 5% was also determined for various
AR coatings, with the 90% TiO2 /10% SiO2 SLAR achieving 196 nm. This value was 10%
higher than the SLAR coating of Yoldas and O’Keeﬀe above, and it was concluded that this
was due to a slight gradient in the refractive index.
DLAR Coatings
Figure 7.3 shows a plot of the maximum Jsc achievable for a non-encapsulated silicon solar
with optimised single-, double-, and triple-layer AR coatings.45 The Jsc increases from
25.7 mA/cm2 with no AR coating, to 36.4 mA/cm2 with a SLAR coating. A DLAR coating
improves the Jsc further to 38.7 mA/cm2 , while a triple layer antireﬂection (TLAR) coating
can theoretically achieve a Jsc of 39.2 mA/cm2 . Thus, a theoretical increase in current of
6 − 8% provides the motivation for researchers to develop multi-layer antireﬂection (MLAR)
coatings for the photovoltaics industry. In reality, a TLAR coating provides little beneﬁt
over a DLAR coating due to the limited range of transparent materials that are available.
Currently, DLAR coatings are only used in industry for small-scale production of higheﬃciency solar cells.301 This is primarily due to diﬀerent materials and diﬀerent deposition
systems being required for each of the DLAR coating layers. Zhao and Green noted that
encapsulated DLAR coating designs using high refractive indices (nAR2 > 2.5) do not improve
much above the SLAR coating due to absorption in the lower coating.338 An additional
advantage of using a DLAR coating is the reduced sensitivity to layer thicknesses,342, 343
while a disadvantage is that the performance of a DLAR coating decreases rapidly once a
SiO2 passivation layer is included at the silicon interface.344
Non-Encapsulated Solar Cells
A number of researchers that high refractive index TiO2 ﬁlms are suitable for being paired up
with low refractive index SiO2 ﬁlms to obtain excellent performance from non-encapsulated
DLAR coatings.44, 97, 150, 152, 342, 344–348 In a study of many possible AR coating materials, Jellison and Wood determined that a SiO2 /TiO2 DLAR coating had the highest performance.344
Martinet et al. also concluded that SiO2 /TiO2 DLAR coatings closely approached the ideal,
with optimal refractive indices of nAR1 = 1.58 and nAR2 = 2.50 for a silicon in air system at
λ0 = 600 nm.97 Swartz et al. used SiO2 /TiO2 DLAR coatings for silicon concentrator solar
cells, operating up to 350 suns.345 Yoldas developed a DLAR coating comprised of a top
layer of 10% TiO2 /90% SiO2 (nAR1 = 1.4) and a bottom layer of TiO2 (nAR2 = 2.4), which
increased the performance of uncoated silicon solar cells by 49%.346
Bouhafs et al. demonstrated that a Rw of 2.65% (300 − 1100 nm) could be achieved for
a SiO2 /TiO2 DLAR coating, slightly less than the modelled result of 2.39%.347 Modelling
results from Wang et al. indicated that a SiO2 /TiO2 DLAR coating in air should be able
162
7. TiO2 Antireﬂection Coatings
Figure 7.3: Maximum Jsc achievable for a non-encapsulated silicon solar
cell with optimised single-, double-, and triple-layer AR coatings. The
ﬁlms used are hypothetical and absorption is assumed to be zero (adapted
from Nagel et al.45 ).
to achieve a Jsc of 42.2 mA/cm2 under AM0 illumination.44 Jiao and Anderson deposited
SiO2 /TiO2 DLAR coatings on metal/insulator/n-Si/p-Si (MINP) solar cells.152 The Jsc and
eﬃciency of the solar cells under AM 0 illumination improved by up to 46% after receiving
the DLAR coating, with a maximum Jsc of 44.3 mA/cm2 being observed. No degradation
in Voc or ﬁll factor (FF) was observed after evaporating the SiO2 and TiO2 coatings, as has
been observed by other researchers.348
Pettit et al. used spin-on SiO2 and TiO2 ﬁlms as a DLAR coating.342 A weighted average
reﬂectance, including reﬂectance from the rear silicon:air interface, of 5% was achieved for
experimental ﬁlms, while the minimum achievable Rw with these ﬁlms was modelled as
being 3.8% over the wavelength range 320 − 1120 nm. Figure 7.4 shows the Rw contours for
a wide range of SiO2 and TiO2 layer thicknesses. It can be seen that a wide range of layer
thicknesses, ±15% for SiO2 and ±10% for TiO2 , can still achieve a Rw of 4.3%, just 0.5%
above the minimum. It was also noted that these results were better than the Rw of 7%
achieved by Johnson for a SiNx quadruple-layer AR coating349 (see Section 7.2.3).
Kamataki et al. performed modelling to determine the optimum SiO2 and TiO2 layer thicknesses in a MgF2 /TiO2 DLAR coating with a SiO2 passivation layer.150 Figure 7.5(a) shows
the eﬀect of increasing the SiO2 passivation layer thickness and the optimum TiO2 ﬁlm
thickness. The MgF2 thickness was ﬁxed at 100 nm. A theoretical Jsc of 42.6 mA/cm2 was
achieved on a planar surface for SiO2 and TiO2 thicknesses of 6 nm and 60 nm, respectively.
For a V-grooved surface, the potential Jsc increased to 44.1 mA/cm2 . Modelling also demonstrated that the optical performance of the DLAR coating did not change signiﬁcantly for
SiO2 thicknesses less than 10 nm, as shown in Figure 7.5(b).
7.2 Previous Developments in AR Coatings
163
Figure 7.4: Optimised layer thicknesses of a SiO2 /TiO2 DLAR coating
with refractive indices nAR1 = 1.414 and nAR2 = 2.243 (at λ = 632.8 nm),
respectively. The ’X’ marks the experimental DLAR coating with Rw =
5% (including back reﬂectance), while the minimum Rw for this coating
is shown at •, corresponding to 95 nm SiO2 and 62 nm TiO2 .342, 343
Modelling performed by Nagel et al. indicated that a PECVD-deposited SiNx ﬁlm could
be used a surface passivation layer under a SiO2 /TiO2 DLAR coating.45 The SiNx has a
signiﬁcantly higher refractive index than SiO2 (2.2 vs. 1.46), reducing the reﬂectance losses
due to the inclusion of a passivation layer. A Jsc of 37.7 mA/cm2 was achieved with this
DLAR coating, however the use of several diﬀerent materials in one DLAR coating is likely
to make this coating too costly in a production environment.
Encapsulated Solar Cells
Research performed at Solarex found that hot-sprayed DLAR Al2 O3 /TiO2 DLAR coatings
exhibited a 10% performance advantage over TiO2 SLAR coatings, and further enhancement
was achieved once a cover slide was in place.49 The hot-sprayed coatings exhibited high
refractive indices, ideal for DLAR coatings under glass. In a subsequent study, it was found
that incorporating the Al2 O3 /TiO2 DLAR coating in a screen-printed solar cell process
would result in an 0.4% (absolute) increase in eﬃciency, reducing the cost of the module by
US$0.03/Wp (1990 dollars).350
Wang et al. performed experiments with many SLAR and modelled DLAR coatings in air
and under ﬂuorinated ethylene propylene (FEP) plastic sheets under AM0 illumination.44
The best results of Jsc = 44.0 mA/cm2 were achieved using a SiO (63 nm)/TiO2 (45 nm)
DLAR coating under the FEP (n0 = 1.34) cover.
Herpin equivalent layers can be used to create a layer with a desired refractive index, when a
single material with that refractive index is not available. Aiken used Herpin equivalent layers
to fabricate a quadruple-layer antireﬂection (QLAR) coating using TiO2 and Al2 O3 stacked in
164
7. TiO2 Antireﬂection Coatings
Figure 7.5: (a) Variation of calculated Jsc as a function of TiO2 ﬁlm
thickness and SiO2 passivation layer thickness for V-grooved and planar
silicon surfaces, and (b) optical dependence of Jsc on SiO2 passivation
layer thickness. Note that the change in electrical passivation with SiO2
is not included in the purely optical model (adapted from Kamataki et
al.150 ).
a high-low-high-low (HLHL) pattern.351 Modelled results have shown that a QLAR coating
can have a solar weighted reﬂectance as low as 7.0% (wavelength range not speciﬁed), nearly
matching the optical performance of a TLAR step-down (i.e., nAR1 < nAR2 < nAR3 ) coating
(6.7%), with only using two-materials. The beneﬁt of moving from a DLAR coating to either
a TLAR or QLAR coating for a silicon solar cell is minimal, and the 3- or 4-layer structures
were designed primarily for multijunction InGaP/GaAs cells, which exhibit a much greater
IR response.
Cudzinovic et al. deposited Al2 O3 /TiO2 and SiO/TiO2 stacks by e-beam and thermal evaporation, respectively, as DLAR coatings for high-eﬃciency solar cells.301 Severe degradation
of the surface passivation quality was reported for e-beam evaporation, while the damage
caused by thermal evaporation was also signiﬁcant. This damage was most apparent for
textured samples, and only a fraction of the surface passivation could be recovered by performing a forming gas anneal (FGA).
7.2 Previous Developments in AR Coatings
165
Textured AR Coatings
Gee et al. and Liang developed several textured SLAR coatings for application in mc-Si solar
cells.58, 218, 352 While experiments were performed with APCVD-deposited ZnO and TiO2 , the
best results were obtained using CVD-deposited diamond ﬁlms (nAR1 ≈ 2.4). The weighted
average reﬂectance of the encapsulated sample was 7.3% over the wavelength range 300 −
1000 nm. The TiO2 ﬁlms exhibited poor adherence and the research was discontinued.218
The best results were achieved using an evaporated TiO2 layer covered with a textured ZnO
layer. Even with the lower refractive indices of the ZnO ﬁlm (nAR1 ≈ 2.0), the excellent
texture reduced the Rw to 6% (300 − 1000 nm).218
7.2.3
Silicon Nitride AR Coatings
SLAR Coatings
Silicon nitride ﬁlms deposited by plasma enhanced chemical vapour deposition (PECVD)
have recently become popular due to their large hydrogen fraction (about 25%), which can
provide some level of surface passivation and assists in increasing the bulk minority carrier
lifetime in mc-Si wafers. The disadvantage to these coatings is the high cost of depositions
due to maintaining a vacuum.
Nagel et al. compared SiNx and TiO2 SLAR coatings for encapsulated and non-encapsulated
silicon solar cells.45 Their results show that while excellent performance can be achieved using
SiNx ﬁlms in air, the glass-encapsulated results show that the performance of the SiNx SLAR
(Jsc = 34.1 mA/cm2 ) and TiOx /20 nm-SiO2 (Jsc = 34.0 mA/cm2 ) coatings are similar once
surface passivation is taken into account. As discussed in Section 5.3.1, 10 nm of SiO2 is
usually suﬃcient to act as a good surface passivation layer, and if included in the modelling
this would increase the Jsc of the TiO2 coatings slightly, due to decreased reﬂectance losses
incurred by the low refractive index SiO2 layer.
Doshi et al. performed an extensive study into AR coating optimisation using PECVD
silicon nitride ﬁlms.339 Initially, ten ﬁlms with diﬀerent refractive indices were deposited,
ranging from 2.03 to 2.55 (at λ = 632.8 nm. The photocurrent lost due to light reﬂected oﬀ
the front surface or absorbed in the AR coating was deﬁned as
λmax
R(λ)Nph (λ)IQE(λ)d(λ) +
Jpcl = q
λmin
λmax
A(λ)Nph (λ)IQE(λ)d(λ)
q
(7.12)
λmin
= Jpcl(ref l) + Jpcl(abs) .
(7.13)
Minimizing the Jpcl is identical to maximising the Jsc . A 100% IQE in the range 400−1100 nm
166
7. TiO2 Antireﬂection Coatings
was assumed with 0% IQE elsewhere, for a maximum Jsc of 41.5 ,mA/cm2 .339 The highest
performance SiNx SLAR coating under glass was achieved with nAR1 = 2.23 (λ = 632.8 nm),
with Jpcl = 2.24 mA/cm2 , Rw = 4.37%, Aw = 1.03% and Tw = 94.60%. This value is
signiﬁcantly lower than the ideal value of nAR1 = 2.45 calculated previously (see Section
7.2.1). Doshi et al. determined that while the reﬂected losses were lower for a SiNx ﬁlm with
nAR1 = 2.42, the increased absorption losses (Aw = 2.66%) reduced the overall performance
of that SLAR coating. Performing a similar optimisation for a SiNx SLAR coating in air
showed that a ﬁlm with the lowest refractive index nAR1 = 2.03 resulted in the lowest losses,
with Jpcl = 3.32 mA/cm2 , Rw = 7.98%, Aw = 0.02% and Tw = 92.00%.
BP Solar have replaced rudimentary silicon dioxide (SiO2 ) AR coating, originally implemented in the BC solar cell fabrication sequence, with a low-pressure chemical vapour deposition (LPCVD) silicon nitride (SiNx ) layer.15, 16 LPCVD SiNx ﬁlms typically have a
refractive index of about 2.3 at 600 nm.17 These ﬁlms have achieved a Jsc of 38.3 mA/cm2
(unconﬁrmed) on the best BC solar cell fabricated on a textured crystalline silicon wafer.15
DLAR Coatings
Two solutions to minimizing the number of materials in a DLAR coating have been reported,
both using SiNx ﬁlms.339, 340 Winderbaum et al. used SiNx ﬁlms with refractive indices of
nAR1 = 1.95 and nAR2 = 2.5 as a DLAR coating for passivated emitter solar cells (PESC)
and SP solar cells.340 Figure 7.6 compares the optical properties of the spray-deposited TiO2
from Solarex and the bottom layer SiNx ﬁlm from the DLAR coating. The performance of the
TiO2 is superior, exhibiting a much lower absorption while maintaining a higher refractive
index. When applied to a PESC cell, a 49% improvement in the Jsc was observed after the
DLAR coating deposition. The eﬃciency of the cells remained low (12 − 14%) however due
to a 5% (absolute) reduction in FF and a Voc that remained < 600 mV. Several batches of SP
solar cells were produced with both coatings, and the SiNx DLAR coated samples typically
exhibited an eﬃciency advantage of 0.5% absolute. This increased eﬃciency is not purely
due to improved optics, as there are also the electrical beneﬁts of bulk passivation from the
hydrogen in the SiNx .
Doshi et al. designed a DLAR coating using two SiNx layers, nAR1 = 2.03 and nAR2 = 2.42,
for use under glass. The DLAR coating performed slightly better than an optimised SiNx
SLAR coating, with a photocurrent loss of Jpcl = 1.95 mA/cm2 (compared to 2.24 mA/cm2
for the SLAR coating). Although reﬂectance losses were low Rw = 2.96%, absorption losses
arising from the bottom layer (Aw = 1.74%) were signiﬁcant. Doshi et al. claim that even
the 0.3 mA/cm2 gain resulting from the DLAR coating is cost-eﬀective, as the wafers can
remain in the PECVD chamber for both depositions.339 An additional beneﬁt of such a
coating may be improved surface and bulk passivation resulting from the high hydrogen
concentration in the bottom layer.
7.3 Varying the Optical Properties of TiO2
167
Figure 7.6: Refractive index and extinction coeﬃcient comparison between the TiO2 SLAR coating used by Solarex and the bottom layer of a
SiNx DLAR coating (adapted from Winderbaum et al.).340
Graded Index AR Coatings
Johnson et al. postulated that a continually varying refractive index, from 1.5 at the air/ﬁlm
interface to 2.6 at the ﬁlm/silicon interface, should produce a coating with an average reﬂectance of about 3% over most of the visible and infrared wavelengths.349 As a ﬁrst approximation, four SiNx layers were deposited with the following refractive indices and thicknesses:
nAR1 = 1.83, dAR1 = 23 nm, nAR2 = 1.92, dAR2 = 31 nm, nAR3 = 2.20, dAR3 = 24 nm, and
nAR4 = 3.10, dAR1 = 19 nm. This SiNx MLAR coating was used in the fabrication of a 4 cm2
solar cell with a Jsc of 34.8 mA/cm2 .
7.3
Varying the Optical Properties of TiO2
The optical constants of TiO2 ﬁlms deposited in this work were determined using spectroscopic ellipsometry (SE), an optical characterisation technique introduced in Section 4.6. A
distinct advantage of SE is that the n and k of single- or multi-layer ﬁlms can be determined
relatively easily over a wide range of wavelengths, in our case 350 − 1150 nm. A number of
other researchers have used SE to determine the optical properties of TiO2 thin ﬁlms, as well
as other information, such as the void content of the TiO2 thin ﬁlms and the distribution
of the void content across the ﬁlm.82, 128, 140, 144, 150, 161, 353 Results from these works were discussed in Sections 2.3.3 (optical properties), and Section 2.3.6 (void content). Results from
this work will be presented, indicating the variation in optical properties of TiO2 thin ﬁlms
achieved by altering the deposition and annealing conditions (temperature and ambient).
168
7.3.1
7. TiO2 Antireﬂection Coatings
Deposition Temperature
Experiment
Before designing TiO2 AR coatings, it was necessary to determine the range of n and k
values that could be achieved with the CVD system. The aim of this experiment was to
deposit a range of TiO2 ﬁlms from the minimum (150◦ C) to maximum (450◦ C) deposition
temperature. The substrates used in this work were polished 0.1 − 10 Ω cm n-type FZ
silicon wafers. TiO2 depositions were performed using a CVD system, and the deposition
time and relative humidity (RH) for each sample ranged from 8 − 14 min and 9 − 15.4%,
respectively. The higher RH values were for the three samples deposited at ≤ 250◦ C. Two
additional samples were deposited at 250◦ C and 450◦ C at a later time in order to determine
the repeatability of results. During these depositions the RH was lower at 7 − 8% and ﬁlm
depositions took 21 − 23 min.
SE measurements were performed on each sample in the wavelength range 350 − 1150 nm
and at 65 − 80◦ in 10 nm and 5◦ intervals, respectively. The beam size on the sample was
about 3 mm in diameter. Sample scans took about 2.5 hr each. The ellipsometric data Ψ
and ∆ was modelled using a Lorentz double-oscillator (see Equation 4.7 in Section 4.6.3)
using version 3.361 of the software package WVASE32.62
Results and Discussion
Figure 7.7 shows the sample structure used to model the TiO2 ﬁlms on silicon. The optical
constants for the silicon substrate were taken from Green.8 An eﬀective medium approximation (EMA) layer, discussed in Section 4.6.4, comprised of 50% void and 50% TiO2 was used
to model the surface roughness in the ﬁlms. The optical properties of the TiO2 fraction of
the EMA layer were coupled to the optical constants of the dense TiO2 layer below it. The
thickness of the silicon wafer was ﬁxed at 300 µm.
The ﬁtting parameters used to model the dielectric constant function of this ﬁlm are shown
in Table A.1 (see Appendix A). The modelling parameters for all TiO2 ﬁlms discussed in
this section can also be found in Appendix A. Figures 7.8(a) and (b) demonstrate the excellent agreement reached between the measured and modelled values of the ellipsometric
parameters Ψ and ∆. The quality of the ﬁt indicates that it is not necessary to model an
inhomogeneity (gradient) in refractive index of the TiO2 layer, as has been required by other
researchers.128, 160, 353 Notably, this sample (TO-2-2) exhibited the poorest mean-squarederror (MSE) out of all the samples in this experiment, however the ﬁt to the experimental
data is still excellent. In the regions where cos(∆) ≈ 1 it is known that the extinction coefﬁcient k has a value close to zero.128 This agrees with the results of many other researchers
that TiO2 is essentially transparent at wavelengths greater than 400 nm (see Section 2.3.3).
7.3 Varying the Optical Properties of TiO2
169
air
surface roughness layer
(EMA: 50% void / 50%TiO2)
dsurf
TiO2
dTiO2
silicon
dSi
air
Figure 7.7: Schematic diagram of the ﬁlm structure used for analysing
the SE data using WVASE32.62
The variation of the TiO2 refractive index (n at λ = 600 nm) for a range of deposition temperatures is shown in Figure 7.9(a). As expected, n increases with Tdep due to densiﬁcation
and crystallisation processes. The exact values for n, as well as the other parameters plotted
in Figure 7.9(b), extinction coeﬃcient (k at λ = 400 nm) and the ratio of the surface layer
d
thickness to TiO2 layer thickness ( dTsurf
), are included in Table A.2 in Appendix A. Since the
iO2
optical constants of both the surface roughness and dense TiO2 layers are coupled, trends
observed in n and k for the TiO2 layer are applicable to both layers. However, it should be
noted that the absolute n and k values for the surface roughness layer will greatly reduced
due to the 50% void incorporation. There is no clear trend in the extinction coeﬃcient (see
Figure 7.9(b)) with k values spanning an order of magnitude, and sample TO-2-2 exhibiting
a much higher k than other ﬁlms.
d
The quantity dTsurf
, a result of the optical modelling, is used as a parameter to tie together the
iO2
physical roughness and the optical properties of the modelled layers. Although, SE analysis
provides a unique solution for the optical properties and layer thicknesses, the chosen EMA
void fraction will also inﬂuence the reulting layer thicknesses. Therefore, while the the
modelled thicknesses may not exactly match the measured layer thickness, the normalised
d
parameter dTsurf
serves as a useful indicator as to the behaviour of the surface roughness
iO2
layer.
The dramatic increase in dsurf for T ≥ 300◦ C corresponds with the transformation of amorphous TiO2 to the polycrystalline phase of anatase. Polycrystalline thin ﬁlms are known
to exhibit much greater surface roughness than amorphous ﬁlms.82, 138 The reason for the
reduction in surface roughness from the sample deposited at 200◦ C to that of the ﬁlm de-
170
7. TiO2 Antireﬂection Coatings
Figure 7.8: Measured (symbols) and modelled (lines) values for (a)
cos(∆) and (b) tan(Ψ) for sample TO-2-2.
posited at 300◦ C is not clear. It is possibly related to a more complete reaction of the TPT
vapour, as it was observed that the ﬁlms deposited at Tdep ≤ 250◦ C were soft and could be
easily scratched with plastic tweezers.
The dispersive relations for the refractive index and extinction coeﬃcient of the dense TiO2
layer are illustrated in Figures 7.10(a) and (b). The trends in n and k for the surface
roughness layer are identical as the optical constants are coupled to the dense TiO2 layer
(these are included for each TiO2 ﬁlm in Appendix A). The refractive index curves in Figures
7.10(a) are nearly ﬂat at longer wavelengths, and n rapidly increases at wavelengths less that
7.3 Varying the Optical Properties of TiO2
171
Figure 7.9: Measured parameters for the samples deposited at 150 −
450◦ C: (a) Refractive indices and (b) extinction coeﬃcients of the surface
roughness layer and the dense TiO2 layer, and the ratio of these layer
thicknesses. The lines between symbols are provided as a guide for the
eye only.
500 nm due to the approaching bandgap. Only sample TO-2-2 exhibits a dispersive curve
with a lower steepness than other samples. The reason for this is not known, but may be due
to variations in the amorphous phase of TiO2 . As mentioned earlier, there is no clear trend
for the extinction coeﬃcient values plotted in Figure 7.10(b) and Figure A.2(b). Sample
TO-2-2 deposited at 250◦ C exhibits the lowest k, while sample TO-2-4 (300◦ C) possesses
the highest k. Other researchers have also observed that there is not a direct relationship
172
7. TiO2 Antireﬂection Coatings
between k and Tdep ,148 as presented in Section 2.3.3.
Figure 7.10: Dispersive relations for the (a) refractive index and (b)
extinction coeﬃcient of the TiO2 layer of samples deposited using CVD
at a range of deposition temperatures.
It is possible that the samples deposited at lower temperatures have retained some organic
(OH) groups from the organometallic precursor. It was noted during the depositions that the
relative humidity was signiﬁcantly higher for these samples. It is known that temperatures
of 450◦ C are required to drive oﬀ all OH-groups from within the TiO2 ﬁlm.46 However, it
is not known exactly how these incorporated OH-groups would explain the variations in k
observed in the TiO2 ﬁlms.
7.3 Varying the Optical Properties of TiO2
173
Two additional samples were deposited at later date in order to check the repeatability of n
and k values from the TiO2 ﬁlms. The dispersive refractive indices and extinction coeﬃcients
for the TiO2 layer are plotted in Figure 7.11. Large diﬀerences in the optical properties can
be seen between the older and newer samples. At a wavelength of 600 nm, the refractive
index is 0.059 (or 3.1%) higher at 250◦ C and 0.128 (or 5.7%) greater at 450◦ C.
Figure 7.11: Dispersive relations for the refractive index (main graph)
and extinction coeﬃcient (inset graph) of the TiO2 layer of samples deposited using CVD at 250◦ C and 450◦ C.
The most striking variation in the extinction coeﬃcient is that the samples deposited at
450◦ C possess the lowest (TO-2-8) and highest (TO-3-1a) k values. It is believed that the
much slower deposition rate of samples TO-3-8a and TO-3-1a is responsible for the higher
n values observed. The longer time spent at the deposition temperature will enable the
TiO2 grains to sinter and densify somewhat, increasing the refractive index. This hypothesis
does not agree, however, with the rule-of-thumb that a slowly deposited ﬁlm leads to high
transparency and low n, whereas high deposition rates result in a high n but also high k.277
The fractionally lower relative humidity present during the later depositions is not likely to
have had such an impact on the refractive indices of the ﬁlms. Another possibility is that the
TPT vapour was impinging at a slightly shallower angle in the second round of depositions.
From past experience, it has been noted that a slight variation in “deposition angle” can
drastically alter the deposition eﬃciency (the fraction of TPT vapour that is incident upon
the substrate), and therefore the deposition rate. It can be imagined that a vapour-stream
174
7. TiO2 Antireﬂection Coatings
impinging on the substrate at a shallower angle could result in less cooling occurring, thus
bringing about an eﬀective rise in the deposition temperature. The latter explanation is the
most likely mechanism responsible for the large increase in n observed.
7.3.2
Annealing Temperature
Experiment
For this experiment, all TiO2 samples were deposited at 450◦ C. The relative humidity remained between 7 − 11% for all samples, while the deposition times ranged from 14 − 17 min.
The wafers were each cut into three pieces using a laser. One piece of each sample was
retained, while the other pieces were annealed in a quartz tube furnace at 450 − 1050◦ C in
a nitrogen (N2 ) ambient for a period of either 1 hr or 6 hr.
Results and Discussion
The variation of the TiO2 refractive index (n at λ = 600 nm) for a range of annealing
temperatures is shown in Figure 7.12(a). The refractive index is roughly constant up until
700◦ C for the ﬁlms annealed at both 1 hr and 6 hr. At Tann ≥ 800◦ C, there is a clear trend
of increasing refractive index with increasing annealing temperature. This increase in n at
higher temperatures is caused by an increasing rutile content in the TiO2 ﬁlms. The ﬁlms
annealed at 450−700◦ C exhibit very similar n values, and it is anticipated that these samples
are anatase only. The phase transformation between anatase and rutile can be seen at 800◦ C.
This sample (TO-4-4c) is thus a mixture of anatase and rutile phases. The samples annealed
at 900 − 1050◦ C possess signiﬁcantly higher refractive indices, due to a greater rutile content
in these ﬁlms. The samples annealed at 800 − 950◦ C for 6 hr exhibit the largest increases in
n, 0.135, 0.066 and 0.069 greater than the equivalent samples annealed for 1 hr, respectively.
Therefore, it can be concluded that at these temperatures, the crystallisation process is
limited by time. The fact that the refractive indices of these samples did not reach the 2.6
margin of the samples annealed at Tann ≥ 1000◦ C suggest that the phase transformation is
not complete and that these ﬁlms remain an anatase/rutile mixture.
The refractive indices of the surface roughness layers, plotted in Figure 7.12, are in the
range 1.501 − 1.747. These refractive indices are similar to the range represented by SiO2
and Al2 O3 , both of which are often used as the top layer in a DLAR coating. This suggests
the possibility that a single TiO2 deposition step could be used to form a “pseudo”-DLAR
coating. The term pseudo-DLAR coating is used as it is not possible to independently
optimise the thickness or refractive index of the surface roughness layer - its properties are
inherently linked to the denser TiO2 layer. However, the performance of a pseudo-DLAR
coating may oﬀer a performance enhancement over a SLAR coating while avoiding the more
7.3 Varying the Optical Properties of TiO2
175
Figure 7.12: Measured parameters for samples annealed at 450 − 1050◦ C
for a period of 1 hr and 6 hr: (a) refractive indices, and (b) extinction
coeﬃcients (only 1 hr data shown) of the surface roughness layer and the
dense TiO2 layer, and the ratio of these layer thicknesses. The lines
between symbols are provided as a guide for the eye only.
complex processing requirements of a regular DLAR coating. This idea will be explored in
greater depth in Section 7.4.
There is a general trend of the extinction coeﬃcients of the ﬁlms annealed for 1 hr increasing
at annealing temperatures greater than 500◦ C, as displayed in Figure 7.12(b). The exact
explanation for the samples annealed at 900◦ C and 950◦ C exhibiting the highest k values
remains unclear. This trend was observed with samples annealed for 6 hr as well (see Ap-
176
7. TiO2 Antireﬂection Coatings
pendix A). Therefore, as the samples annealed at the same temperature are pieces of the
same deposited ﬁlm the variation in k is attributed to slight ﬂuctuations in the deposition
conditions, which results in the ﬁlms sintering and crystallising slightly diﬀerently. It is not
expected to be due a result of the SE analysis, due to the use of a Kramers-Kronig consistent
optical model and the excellent MSE values achieved, as shown in Table A.5. Hovel also
found that a general trend of increasing k with increasing deposition temperatures existed in
spray-deposited ﬁlms, however the occasional ﬁlm did not ﬁt this trend (see Figure 2.11).148
It is interesting to note that while ﬁlms annealed at Tdep < 700◦ C have very similar n values,
their extinction coeﬃcients vary signiﬁcantly. It is possible that this disparity between n
and k is due to the existence of a small rutile fraction in the ﬁlms or increased disorder in
the ﬁlms with the impending phase transformation.
d
◦
The thickness ratio dTsurf
remains high for Tann ≤ 800 C, as shown in Figure 7.12(b), and is
iO2
similar to the values observed for non-annealed ﬁlms deposited at 450◦ (refer Figure 7.9(a)).
At annealing temperatures greater than 900◦ C, both the surface layer thickness dsurf and
dsurf
decrease rapidly. The reduced surface roughness at higher annealing temperatures
dT iO2
is attributed to sintering of the TiO2 crystallites. The samples annealed for 6 hr exhibit
d
slightly reduced dTsurf
values, and reach a minimum value at Tann ≥ 1000◦ C. SEM images
iO2
(refer Figure 7.13, later in this section provide independent conﬁrmation of decreased surface
roughness for annealed ﬁlms.
d
is expected due to
An increase in n and k, and the reduction in the thickness ratio dTsurf
iO2
two mechanisms, crystallisation and sintering. The eﬀect of sintering is demonstrated in the
scanning electron microscopy images in Figure 4.17. These are higher resolution images of
the same samples discussed in Section 4.8.3. After annealing the as-deposited (a) ﬁlms for
1 hr (b), the crystallites have become rounded and the grains are observed to agglomerate.
Annealing for 6 hr results in the majority of grains sintering together, although the largest
voids in between the grains remain “unbridged”. After a total of 22 hr annealing at 1000◦ C
an almost continuous ﬁlm is formed, with only small voids, typically 40 nm in diameter,
remaining. The sintering results in signiﬁcant densiﬁcation of the TiO2 layer, with the
ﬁlm thickness decreasing from 79 nm in (a) to 64 nm after step (d). The increase in ﬁlm
density, brought about by the reduced ﬁlm thickness, is responsible for a small increase in
the TiO2 refractive index. The grain sizes observed in the ﬁlms deposited at 450◦ C are
roughly 30 nm across, in agreement with nanocrystalline anatase thin ﬁlms deposited by
other researchers.354, 355
An additional mechanism taking place during annealing is crystallisation. Crystallisation is
primarily responsible for the large increase in n and k presented in Table A.6. At annealing
temperatures less than 800◦ C, no signiﬁcant increase in n is observed. Annealing at 450 −
700◦ C will transform any amorphous TiO2 fraction into the anatase phase. However, the
ﬁlms are already anatase as-deposited, and the stable n values indicate that no signiﬁcant
amorphous fractions were present prior to annealing. For Tann ≥ 800◦ C, a signiﬁcant increase
7.3 Varying the Optical Properties of TiO2
177
Figure 7.13: High resolution SEM images of TiO2 ﬁlms deposited by
CVD at 450◦ C: (a) as-deposited; (b) 1 hr anneal (load 800◦ C and ramp
to 950◦ C); (c) 6 hr anneal at 1000◦ C; and (d) 22 hr anneal at 1000◦ C.
in n is noted, reaching a maximum of about 2.6 at a wavelength of 600 nm. This increase
is due to the transformation of the anatase phase to the rutile phase. This transformation
temperature is in agreement with other works, reviewed in Section 2.2.1. A trend in the k
values, determined at λ = 400 nm, is not so clear from Table A.6.
Keddie et al. noted that the higher TiO2 ﬁlm densities were achieved with increased heating
rate.356 The higher ﬁlm density was attributed to delaying the crystallisation, which allowed
178
7. TiO2 Antireﬂection Coatings
greater ﬁlm densiﬁcation to occur prior to crystallisation. It was also emphasised that premature crystallisation can lead to ﬁlms that are only partially sintered, because diﬀusive
sintering mechanisms are much slower than viscous sintering mechanisms.356 This implies
that the densest ﬁlms may possibly be achieved by depositing amorphous TiO2 ﬁlms, followed
by annealing (sintering) the ﬁlms at a temperature less than 300◦ C to avoid crystallisation
into the anatase phase, and ﬁnally annealing (crystallising) the samples at a higher temperature. Fitzgibbons et al. commented that the crucial parameter determining the ﬁnal
TiO2 ﬁlm structure was simply the maximum processing temperature (either deposition or
annealing temperature).67 It was also noted that rutile could be achieved directly by performing the deposition at moderate temperatures. This is in agreement with the observation
of other researchers that the processing temperatures required to convert an anatase ﬁlm
into a rutile ﬁlm are signiﬁcantly higher than the temperature required to deposit a rutile
ﬁlm directly.73–75
7.3.3
Deposition Ambient
Experiment
The aim of this experiment was to determine whether the presence of water vapour (H2 O)
would alter the optical properties of the TiO2 ﬁlms. For this experiment, TiO2 samples were
deposited at 250◦ C and 450◦ C, both with and without H2 O. The water vapour was supplied
by heating DI water, contained in a quartz bubbler, to 100◦ C. The relative humidity during
depositions was about 30%. Certain pieces of the samples were annealed in N2 at either
450◦ C or 1000◦ C for a period of 1 − 6 hr. One sample was loaded in the annealing furnace in
oxygen (O2 ) for the ﬁrst 5 min, before the gas ﬂow was changed to N2 for the next 55 min.
The optical properties of all of the ﬁlms in this section were measured using SE, and the
results are given below. The ﬁtting parameters used to model the samples are provided in
Table A.7 in Appendix A.
Results and Discussion
The refractive indices of the ﬁlms deposited in the presence of water vapour were typically
low at n = 1.726 − 2.194. The samples deposited at 250◦ C exhibit the lowest refractive
indices, regardless of annealing time and temperature. In fact, the ﬁlm annealed at 1000◦ C
for 6 hr has a very similar refractive index to the ﬁlm annealed 450◦ C for 1 hr. However, Table
7.2 indicates that the former ﬁlm has an extinction coeﬃcient many orders of magnitude
greater than the latter ﬁlm. This increase in k can partly be attributed to the transformation
of the ﬁlm from anatase to rutile. It is also interesting that the sample TO-4-1c, deposited
at 450◦ C, exhibits a signiﬁcantly higher n and k than the ﬁlm annealed at 450◦ C. This
may indicate that the as-deposited amorphous phase is more porous than the as-deposited
7.3 Varying the Optical Properties of TiO2
179
anatase phase, and that the porosity can not be reduced during subsequent anneals. This
observation is in agreement with the results of other researchers. Wong et al. also reported
that TiO2 ﬁlms deposited at 265◦ C were porous and hence the rearrangement of the atoms by
a sintering process is limited.130 This phenomenon is also supported by the marked increase
in n observed for the ﬁlms from this work that were deposited at 450◦ C and subsequently
annealed, as shown in Figure A.7(a).
Table 7.2: Thickness, refractive index (at λ = 600 nm) and extinction coeﬃcient (at λ = 400 nm) of surface roughness and TiO2 layers deposited
using CVD at 250◦ C and 450◦ C with H2 O vapour. Some samples were
annealed at either 450◦ C or 1000◦ C in N2 for a period of 1 hr or 6 hr.
Surface Layer
dsurf
n
k
(nm)
TiO2 Layer
dT iO2 n
k
(nm)
63.7
73.2
1.395 0.00037
1.501 0.00639
52.1
32.0
1.829
2.004
0.00082 1.223
0.00733 2.288
1 hr Anneal in N2
TO-3-6b 250
450
TO-3-5a 450 1000
89.6
43.2
1.347 0.00022
1.562 0.03425
68.5
74.0
1.726
2.194
0.00047 1.308
0.07557 0.584
6 hr Anneal in N2
TO-3-7c 250 1000
TO-3-5b 450 1000†
68.7
49.7
1.350 0.01951
1.524 0.06151
60.7
80.0
1.731
2.111
0.04231 1.132
0.13568 0.621
Sample
Name
Tdep
(◦ C)
No Anneal
TO-3-6a 250
TO-4-1c 450
Tann
(◦ C)
−
−
dsurf
dT iO2
†
Sample TO-3-5b was annealed for 5 min in O2 and subsequently 5 hr 55 min in N2
in the same high-temperature step. This resulted in the growth of 11.1 nm of SiO2 at
the TiO2 :Si interface.
One surprising result is that the sample deposited at 250◦ C (TO-3-6a) had a refractive index
of 1.829, however after annealing at 450◦ C (TO-3-6b) this decreased to n = 1.726.
The samples deposited at 250◦ C (TO-3-6a, TO-3-6b an TO-3-7c) exhibited very low refractive indices. This reduction in n is attributed to a large volume of water vapour being
contained within the ﬁlm during deposition. The water vapour is then subsequently drivenoﬀ by heating of the sample. The porosity of sample To-3-6b, determined using Equation 2.3
and using nb = 2.532 (dense anatase), is 63.4%. As mentioned in Sections 2.3.6 and 2.3.4,
highly porous ﬁlms can absorb water vapour and this is known to increase the refractive
index of porous samples.155, 161, 163, 164 The voids in the ﬁlm become essentially “ﬁlled” with
water (nH2 O = 1.33) and the measured refractive index can be signiﬁcantly higher than if
the ﬁlm was measured in a vacuum. The possibility of this eﬀect could not be precluded in
the SE measurements, and the samples were measured in air.
The surface roughness layer of the samples deposited in the presence of water vapour was
180
7. TiO2 Antireﬂection Coatings
often thicker than the dense TiO2 layer, as shown in Table 7.2. The ﬁlms with the highest n
values are both pieces of the same sample (TO-3-5), and these ﬁlms also exhibit the lowest
dsurf
values. This is in agreement with the ﬁlms possessing a higher density than the others.
dT iO2
In general, the large dsurf values and the low refractive indices of the TiO2 layer indicate
that the ﬁlms are quite rough and porous.
Figure 7.14(a) is an SEM image of sample TO-3-6a, deposited with water vapour at 250◦ C.
Larger clusters of about 60 − 70 nm across are comprised of smaller grains with a diameter
of roughly 10 nm. The large voids in between the clusters are also immediately apparent.
Lottiaux et al. reported similar observations, noting that larger “blocks” of about 40 nm
in size were comprised of smaller blocks that were about ten times smaller in size.242 The
formation of the smaller blocks was found to be in agreement with nucleation theory and
the growth of amorphous thin ﬁlms. Additionally, SEM images showed that these blocks do
not change with deposition conditions, and that deep cracks separate the large blocks.
Figure 7.14: SEM images of (a) TiO2 ﬁlms deposited at 250◦ C (TO-3-6a)
and (b) subsequently annealed at 1000◦ C 6 hr (TO-3-7c).
Figure 7.14(b) shows the same ﬁlm after annealing for 6 hr at 1000◦ C. The clusters and
grains have undergone signiﬁcant densiﬁcation, however the large voids are still present. This
indicates that sintering has only occurred when there was TiO2 material directly adjoining
the grains. The refractive index of the sample annealed for 6 hr at 1000◦ C (TO-3-7c) was
very similar to the sample annealed for 1 hr at 450◦ C (TO-3-6b). Assuming that after a 6 hr
anneal the ﬁlm is converted to 100% rutile, this demonstrates that ﬁlms can be deposited that
exhibit certain properties of rutile (such as excellent chemical resistance) while maintaining
a low refractive index.
7.3 Varying the Optical Properties of TiO2
7.3.4
181
Annealing Ambient
Experiment
The aim of this experiment was to determine whether the gas ambient during annealing
would later the optical properties of the TiO2 ﬁlms. Other researchers have reported that
the optical properties of TiO2 can vary depending on the whether the gas ﬂowing through
the furnace is O2 or N2 .150 Therefore, samples were deposited at 450◦ C and annealed in a
quartz tube furnace at 1000◦ C for a total time of 1 hr or 6 hr. The only variable was whether
the samples received 5 min O2 at the start of the furnace step or not. The relative humidity
remained between 7 − 11% for all samples. In all cases the total time the sample spent in
the furnace is either 1 hr or 6 hr.
It was demonstrated in Section 5.3.1 that a thin SiO2 could be grown at the TiO2 :Si interface by performing a brief oxidation (dry O2 ). Therefore, the ﬁnal section of work was to
determine how the optical properties of the TiO2 ﬁlm changed with increase oxidation times
and temperatures. Five samples were deposited at 450◦ C (15 − 23 min deposition time and
RH ≈ 7%) and loaded individually into a quartz tube furnace at 800◦ C in an O2 (3.2 slpm)
ambient. Once loaded, the furnace was set to ramp to 1000◦ C, and the wafers were removed
after 1 min, 2 min, 4 min, 8 min and 16 min.
The optical properties of all of the ﬁlms in this section were measured using SE, and the
results are given below. The ﬁtting parameters used to model the samples in this section
are provided in Tables A.8 and A.9 in Appendix A.
Results and Discussion
To compare the eﬀect on the annealing ambient, pairs of samples were annealed either in
pure N2 for 1 hr or 6 hr, or initially 5 min O2 followed by N2 for a total processing time
of 1 hr or 6 hr. Table 7.3 details the thickness and optical properties of the various layers.
It was necessary to include an SiO2 layer at the TiO2 :Si interface in the model to achieve
the best ﬁt to the experimental data. The thickness of this layer dSiO2 varies somewhat,
from 6.2 − 10.9 nm. Slight variations in the SiO2 thickness may be caused by TiO2 ﬁlm
inhomogeneities. From Table 7.3 it can be seen that one pair of samples (TO-3-3b and
TO-3-3c) possess virtually identical refractive indices at 600 nm. However, for the remaining
pairs (TO-3-5a and TO-3-5b, and TO-4-7c and TO-3-3a), the ﬁlms annealed only in N2
exhibited higher refractive indices. The similarities between the n values of the former pair
and the diﬀerence in the latter pair is clearly illustrated in Figure A.8(a).
Figure A.8(b) indicates that the extinction coeﬃcient values for all samples are fairly similar,
except the sample TO-3-3a, which is signiﬁcantly higher than the others. Kamataki et al.
observed slightly lower n and k values for ﬁlms annealed in O2 .150 This research group
182
7. TiO2 Antireﬂection Coatings
Table 7.3: Thickness, refractive index (at λ = 600 nm) and extinction coeﬃcient (at λ = 400 nm) of surface roughness and TiO2 layers deposited
using CVD at 450◦ C. Some samples received a 5 min oxidation prior to
all samples being annealed at 1000◦ C in N2 for a total period of 1 hr or
6 hr.
Sample
Name
5 min dSiO2
O2 ? (nm)
Surface Layer
dsurf
n
k
(nm)
TiO2 Layer
dT iO2
n
k
(nm)
dsurf
dT iO2
1 hr Anneal
TO-4-7c ✗
TO-3-3a ✓
TO-3-5a† ✗
TO-3-5b† ✓
−
6.2
−
10.9
14.2
30.5
43.2
49.9
1.722
1.697
1.562
1.524
0.04047
0.05312
0.03425
0.06130
55.0
49.7
74.0
80.0
2.547
2.491
2.194
2.111
6 hr Anneal
TO-3-3b ✗
TO-3-3c ✓
−
10.7
10.5
22.0
1.761 0.02513
1.761 0.04309
64.0
46.9
2.633 0.05565 0.164
2.632 0.09540 0.469
†
0.08962
0.11766
0.07557
0.13517
0.258
0.614
0.584
0.624
Samples TO-3-5a and TO-3-5b were deposited in the presence of H2 O vapour.
attributed this to the growth of SiO2 at the TiO2 :Si interface of O2 annealed samples. The
analysis of Kamataki et al. determined that after 1 hr at 500◦ C an 11.6 nm-thick SiO2 layer
had grown at the interface. This SiO2 thickness is similar to the samples in this thesis,
however the annealing temperature is halved. It would seem unlikely that a similar dSiO2
would grow at a temperature of 500◦ C. It may be possible that a signiﬁcant native oxide
existed prior to TiO2 ﬁlm deposition instead. Examination of the results of Kamataki et al.
in Figure 2.17 indicate that an increased refractive index is primarily observed at wavelengths
less than 500 nm. In conjunction with the increased k at shorter wavelengths, it would seem
to suggest that the ﬁlm annealed in N2 may possess a small rutile fraction. The results from
this thesis also exhibit increased n values for ﬁlms annealed for 1 hr. After 6 hr annealing,
refractive indices were identical. Therefore, it postulated that oxygen may play a role in
slightly retarding the transformation from anatase to rutile.
The following results are for the samples deposited at 450◦ C and loaded into an oxidation
furnace at 800◦ C. The furnace temperature was set to ramp to 1000◦ C. The samples received
separate oxidations of varying length - 1, 2, 4, 8 and 16 min - and were unloaded without
ramping the furnace temperature down. Table A.10 (in Appendix A) and Figure 7.15 summarise the measured parameters, including refractive index, extinction coeﬃcient, SiO2 layer
d
thickness and dTsurf
.
iO
2
The refractive indices of the samples are observed to increase with increased oxidation time.
From the n values, it is believe that the transformation from anatase to rutile begins with
the sample oxidised for 8 min. There is also a large increase in k for samples for times
7.4 Development of Novel TiO2 AR Coatings
183
longer then 8 min, as shown in Figure A.9(b) (refer Appendix A). It remains unclear as to
d
why the thickness ratio dTsurf
steadily increases with increased oxidation time (and therefore
iO2
temperature). It is postulated that crystallisation is occurring ﬁrst and that densiﬁcation
has not had a chance to proceed yet. This is in agreement with the observations of Keddie
et al., who noted the rate of crystallisation is much greater than the rate of densiﬁcation,
and that a high density ﬁlm (corresponding to a thin surface roughness layer) is achieved by
delaying crystallisation.356
7.4
Development of Novel TiO2 AR Coatings
The achievement of a wide range of refractive indices by altering the deposition and annealing
conditions suggested the possibility of creating several novel types of AR coatings using only
TiO2 . This idea was supported by the fact that a single deposition resulted in a denser
TiO2 layer (n = 1.726 − 2.633 at λ = 600 nm) underneath a surface roughness layer (with
n = 1.347 − 1.761 at λ = 600 nm). Although the thickness of the surface roughness layer
could not be independently, the behaviour of dsurf as a function of deposition temperature
was known. This section presents new designs of SLAR and DLAR coatings based on these
CVD-deposited TiO2 thin ﬁlms.
7.4.1
Single-layer TiO2 AR Coatings
Measurements on the above TiO2 samples were performed in order to determine whether the
surface roughness layer would act as a “pseudo-DLAR coating”. A pseudo-DLAR coating is
deﬁned as an optical coating that can reducing the amount of reﬂected light by more than
is possible with an SLAR coating, but is still the result of a single ﬁlm deposition. It could
also be regarded as the ﬁrst step from a SLAR coating towards a graded refractive index.
Experiment
The deposition of the samples used in these measurements was previously described in the
‘Experiment’ section of Sections 7.3.1, 7.3.2, 7.3.3 and 7.3.4. Reﬂectance measurements were
performed with a Varian Cary 5G spectrophotometer in the wavelength range 210−1200 nm.
A deuterium lamp was used as the light source for wavelengths less than 350 nm, while a
tungsten-halogen lamp was used at longer wavelengths.
The data was modelled using the Lorentz oscillator model presented in Section 4.6.3. The
pseudo-DLAR coating model included a surface roughness layer and a dense TiO2 layer,
while the single layer model consisted of one dense TiO2 layer.
184
7. TiO2 Antireﬂection Coatings
Refractive Index, n (at λ =600 nm)
2.4
(a)
2.2
Surface roughness layer
Dense TiO2 layer
2.0
1.7
1.6
1.5
0
2
4
6
8
10
12
14
16
Oxidation Time, tox (min)
0.56
0.50
6
0.48
4
0.46
2
0.44
2
2
Thickness Ratio, dsurf / dTiO
8
0.52
SiO2 Layer Thickness, dSiO (nm)
10
(b)
0.54
0
0
2
4
6
O id ti
8
Ti
10
12
14
16
t ( i )
Figure 7.15: Measured parameters for samples oxidised for 1 − 16 min:
(a) refractive indices, and (b) thickness ratio of the surface roughness
layer and dense TiO2 layer and the SiO2 layer thickness grown. The
lines between symbols are provided as a guide for the eye only.
7.4 Development of Novel TiO2 AR Coatings
185
Results and Discussion
The reﬂectance spectra of a typical TiO2 ﬁlm, sample TO-3-1b, is shown in Figure 7.16. The
eﬀect of the surface roughness layer can be seen, broadening the reﬂectance minimum and
signiﬁcantly reducing the short wavelength reﬂectance. The weighted average reﬂectance
for the pseudo-DLAR coating is Rw = 8.95% over the wavelength range 350 − 1150 nm,
compared to Rw = 10.54% for the best single-layer ﬁt. Thus, the coating is behaving like
a DLAR coating with the refractive indices satisfying the relationship n2AR1 nSi = n2AR2 n0 .
The reduction in Rw can be attributed solely to the presence of the surface roughness layer
with an intermediate refractive index, since the extinction coeﬃcient of both the dense TiO2
layers was virtually identical.
Figure 7.16: Reﬂectance of sample TO-3-1b in air, modelled with a single
layer (dotted line), and EMA/dense layer (solid line).
It should be noted that, even though sample TO-3-1b exhibited one of the lowest reﬂectances,
its refractive index was greater than desirable (nAR1 = 2.08) for a AR coating in air. However,
from Table A.2 it can be seen that the thickness of the surface roughness layer decreases
signiﬁcantly for deposition temperatures lower than 450◦ C. This highlights the limitation
of the pseudo-DLAR coating - that the thickness of the surface roughness layer cannot be
independently controlled.
Figure 7.17 models the performance of sample TO-4-5c under 2 mm of B270 Crown glass
and 1 mm ethyl-vinyl-acetate (EVA) encapsulant. The optical models for the B270 Crown
glass and the EVA layer were taken from Nagel et al.45 Although, the extinction coeﬃcient
of the EVA very low (k = 7.7 × 10−6 at 400 nm), the large thickness results is the majority
of the light at wavelengths less than 400 nm being absorbed in the layer. Again, the optical
performance is seen to be enhanced slightly by the presence of the TiO2 surface roughness
layer, primarily at short wavelengths. This reduces the weighted average reﬂectance from
Rw = 8.95% to Rw = 8.42%. In addition, the inclusion of a TiO2 ﬁlm with a refractive index
186
7. TiO2 Antireﬂection Coatings
of about 2.4 results in a 0.5% (absolute) reduction in Rw compared to the case in air.
Figure 7.17: Reﬂectance of sample TO-4-5c under glass, modelled with a
single layer (dotted line), and EMA/dense layer (solid line).
7.4.2
Double-layer TiO2 AR Coatings
This experiment was aimed at demonstrating that a DLAR coating, comprising two TiO2
layers, could be achieved using the CVD-deposition technique. The existence of a DLAR
coating can be conﬁrmed by the observance of two minima in the reﬂectance spectrum of
the coating.
The refractive indices of air, glass and silicon are 1.0, 1.52 and 3.939 at a wavelength of
600 nm. Using Equations 7.8 and 7.6, the optimum refractive indices and thicknesses for a
DLAR coating can be determined for a silicon solar cell in air and under glass at a design
wavelength of λ0 = 600 nm. In air, the optimum refractive indices (and thicknesses in
parentheses) of the two layers are nAR1 = 1.58 (94.9 nm) and nAR2 = 2.49 (60.2 nm), while
under glass these values increase to nAR1 = 2.07 (72.5 nm) and nAR2 = 2.86 (52.4 nm).
These refractive indices satisfy the requirements of a DLAR coating in order to achieve zero
reﬂectance at two wavelengths either side of λ0 , nAR1 nAR2 = n0 nSi .
As can be seen from Table A.6, sample TO-4-5b exhibited a refractive index of 2.489, very
close to that required for the bottom layer of the DLAR coating in air. As this layer was
annealed and had undergone densiﬁcation and crystallisation processes, it was necessary to
estimate the degree of thickness reduction. An ellipsometer (λ = 632.8 nm) in the laboratory was used to perform quick measurements during sample processing. Sample TO-4-5b
possessed a thickness and refractive index of 95.1 nm and 1.991, respectively, after deposition. After annealing for 6 hr at 900◦ C, these values changed to 57.3 nm and 2.509 (at
λ = 632.8 nm). To achieve a minimum at λ0 = 600 nm with this ﬁlm (nAR = 1.991), a thickness of 75.2 nm is required. The thickness of TO-4-5b after annealing is very close to what
7.4 Development of Novel TiO2 AR Coatings
187
is required as a ﬁnal thickness for the annealed bottom layer. Therefore, when depositing
the bottom layer of the DLAR coating, this ﬁlm will appear thicker than typically desired
dark blue, possibly exhibiting a light blue or yellowish colour.
The lowest refractive indices for the dense TiO2 ﬁlms observed in this work (n ≈ 1.73) were
achieved by performing depositions at 250◦ C in the presence of water vapour, followed by
a subsequent annealing step of 450 − 1000◦ C (see samples TO-3-6b and TO-3-7c in Table
7.2). The refractive indices (and thicknesses) of these ﬁlms were 1.479 (137.6 nm) and 1.551
(157.6 nm), respectively, when measured with the λ = 632.8 nm ellipsometer. This piece of
ellipsometer is only able to model a single ﬁlm only, and cannot model surface roughness
layers. Therefore, these deposition settings for the upper layer of the DLAR coating. The
deposition of the upper layer would be stopped when a dark blue colour was observed in the
TiO2 /TiO2 stack.
Experiment
The wafers used for optical experiments were three 0.1 − 10 Ω cm n-type polished ﬂoat
zone (FZ) Si(100) wafers and one chemically-polished, mc-Si wafer (Eurosil, F43, p-type,
1.5 Ω cm). All wafers were cleaned in RCA1/RCA2/HF before being thoroughly rinsed in
DI water and dried. The bottom TiO2 layer was deposited at 450◦ C, with deposition times
and relative humidities ranging from 12 − 23 min and 7 − 11%, respectively. Film colours
ranged from dark blue to yellow. All samples were then cleaned in H2 SO4 :H2 O2 :H2 O (1:1:5)
and RCA2 before being rinsed and dried.
Two of the polished FZ wafers (TO-5-5 and TO-5-6) and the mc-Si wafer (TO-5-3) were
then placed in a quartz tube furnace for annealing at 900◦ C for 6 hr in an N2 ambient. The
remaining sample (TO-5-8) received the same amount of annealing time and at the same
temperature, however the ﬁrst 10 min were in O2 before switching the furnace gas to N2 .
Subsequently, the upper TiO2 layer was deposited at 250◦ C with H2 O vapour present. The
deposition times ranged from 5 − 20 min and the relative humidity was relatively constant
at 33 − 34%. As the layers deposited at low temperatures were quite soft, the samples were
then annealed for 2 hr at 700◦ C.
The reﬂectance of all samples was measured in the wavelength range 300 − 1200, while
spectroscopic ellipsometry measurements were performed in the wavelength range 350−1150
nm. SE data were collected for each sample at angles of 65 − 80◦ in 5◦ steps. The ﬁtting
parameters used to model the samples in this section are provided in Table A.11 in Appendix
A.
188
7. TiO2 Antireﬂection Coatings
Results and Discussion
The excellent agreement between the measured (symbols) and modelled (lines) ellipsometric
parameters for sample TO-5-5 is shown in Figure 7.18. As noted previously, in regions where
cos(∆) ≈ 1 it is known that the extinction coeﬃcient k is very close to zero.128
Figure 7.18: Measured (symbols) and modelled (lines) values for (a)
cos(∆) and (b) tan(Ψ) for sample TO-5-5.
The refractive indices, extinction coeﬃcients and thicknesses of the dense TiO2 and surface
roughness layers in the DLAR coatings are plotted in Figure 7.19(a), (b) and (c). When displayed graphically, the approximation of the four modelled TiO2 layers to a graded refractive
index coating can be seen. It was attempted to model the layer as a graded refractive index
7.4 Development of Novel TiO2 AR Coatings
189
coating. The void content of each of the four layers was varied, and the optical properties
where linked to a bottom layer that determined the optical properties of the ﬁlm. This model
was not successful, and it is postulated that the upper and lower TiO2 ﬁlms are composed
of diﬀerent phases of TiO2 , anatase and rutile, respectively, and possess diﬀerent optical
properties.
The reﬂectance of three of the TiO2 DLAR coatings is shown in Figure 7.20. The reﬂectance
spectra of sample TO-5-6d is not plotted as it is virtually identical to sample TO-5-5d. All
three samples in Figure 7.20 exhibit the characteristic double minimum of a DLAR coating.
The noisy data in the range 800 − 1000 nm is due to poor sensitivity in the spectrophotometer’s IR detector, however the modelled data clearly provides an excellent ﬁt. The lowest
weighted average reﬂectances achieved for a TiO2 DLAR coating in air in this work is 6.54%
and 8.04% for planar c-Si and mc-Si substrates, respectively (see Figure 7.20). While signiﬁcantly better than a TiO2 SLAR coating, the performance of this coating is less than
optimal due to the high refractive index of the lower TiO2 layer (nAR2 > 2.6 instead of 2.49
as determined at the start of this section).
The most interesting application for a TiO2 DLAR coating would be for planar mc-Si solar cells encapsulated under glass. Modelling was performed, using the software package
TFCalc,341 with the above coatings to determine the optimum performance that could be
expected for this case. The optical models for the 2 mm-thick B270 Crown glass and 1 mmthick ethyl-vinyl-acetate (EVA) layers were taken from Nagel et al.45 Although, the extinction coeﬃcient of the EVA very low (k = 7.7 × 10−6 at 400 nm), the large thickness results
is the majority of the light at wavelengths less than 400 nm being absorbed in the layer.
As previously discussed, the ideal refractive index for the lower DLAR coating layer is
nAR2 = 2.86. As this is not achievable in practice, the highest refractive index obtained
in this study was used, nAR2 = 2.63 (this can be achieved by annealing the TiO2 layer at
1000◦ C for 6 hr). With the reduced refractive index of the lower DLAR coating, the refractive
index of the upper DLAR coating also reduced from 2.07 to nAR1 = 1.95. Figure 7.21 plots
the reﬂectance, transmittance, and the absorptance of this DLAR coating. The weighted
averages of these parameters are Rw = 6.98% (including back reﬂectance), Tw = 91.65% and
Aw = 2.25%. The reﬂectance spectra is extremely ﬂat and lies between 4.7% and 7.7% for
all wavelengths in the range 410 − 1040 nm. This is excellent, considering that about 4.3%
(absolute) is lost in the ﬁrst bounce oﬀ the front surface of the glass. With the inclusion of
a 10 nm-thick SiO2 layer at the interface, the average weighted reﬂectance and absorptance
increase to Rw = 7.57% and Aw = 2.10%, respectively, while Tw = 90.33%.
A known advantage of DLAR coatings is that they are less sensitive to variations in their
layer thicknesses than SLAR coatings.342, 343, 349 Figure 7.22 plots the reﬂectance spectra of
the TiO2 DLAR coating with ideal layer thicknesses (the same as appears in Figure 7.21),
and with both layers 20% thicker and thinner than ideal. The lower TiO2 layer thickness
190
7. TiO2 Antireﬂection Coatings
Figure 7.19: Optical constants for four various layers of TiO2 DLAR
coatings: (a) sample TO-5-5d, (b) sample TO-5-6d and (c) sample TO5-8d (N.B. TO-5-8d has a thin SiO2 layer at the TiO2 :Si interface.
7.4 Development of Novel TiO2 AR Coatings
Figure 7.20: Measured reﬂectance of three TiO2 DLAR coatings: (a)
sample TO-5-5d, (b) sample TO-5-8d and (c) sample TO-5-3d (mc-Si).
Note that sample TO-5-8d has a thin SiO2 layer at the TiO2 :Si interface.
191
192
7. TiO2 Antireﬂection Coatings
Figure 7.21: Reﬂectance, transmittance and absorptance spectra for the
ideal TiO2 /TiO2 DLAR coating. Experimental data was used for the
dispersive refractive indices of the TiO2 layers.
is 78.0 ± 15.6, while the upper TiO2 layer has a thickness of 49.4 ± 9.9. For the scenario
where both layers are too thin , the weighted average reﬂectance increases from 6.98% to
only 7.10%, while thicker layers lead to a slightly higher value of Rw = 7.46%.
Figure 7.22: Variation of reﬂectance spectra with a 20% increase or reduction in both TiO2 layer thicknesses.
One disadvantage of DLAR coatings is that they more sensitive to thickness of the SiO2
passivation layer thickness than SLAR coatings.344 The results of modelling performed in
this work for SLAR and DLAR TiO2 coatings in air is shown in Figure 7.23. This graph
shows that the point where a DLAR coating oﬀers no beneﬁt over an SLAR coating is for
SiO2 layers with a thickness of 15 nm or more. As previously discussed, a 10 nm-thick SiO2
passivation layer has been found to achieve excellent results. Therefore, the eﬃciency of
solar cells incorporating a thin SiO2 layer can be expected to increase upon the deposition
7.5 Performance of TiO2 DLAR-Coated Solar Cells
193
of a TiO2 DLAR coating.
Figure 7.23: Dependence of weighted average reﬂectance on the SiO2
passivation layer thickness for TiO2 SLAR and DLAR coatings in air.
7.5
Performance of TiO2 DLAR-Coated Solar Cells
To predict the performance of solar cells with these DLAR coatings, the weighted average
reﬂectance curves were used as an input to a PC1D simulation. The intensity of the AM1.5
global spectrum was modiﬁed to account for absorption in the glass, EVA and TiO2 layers.
The short-wavelength absorptance in the glass, EVA and TiO2 layers is shown in Figure
7.24. This resulted in an intensity of 98.35 mW/cm2 , slightly lower than the AM1.5G value
of 100 mW/cm2 .
The source of the optical losses in the encapsulated solar cell is broken down in individual
layers in Figure 7.25 at two diﬀerent wavelengths, 350 nm and 400 nm. The main source of
absorptance is the 1 mm thick EVA layer, with the lower TiO2 absorbing a much smaller
fraction of the light. Minimal absorption is observed in the upper TiO2 thin ﬁlm, while the
level of absorption in the glass is close to zero at these wavelengths.
The parameters for the standard buried-contact solar cell, displayed in Table 7.4, were taken
from Honsberg et al.23 The IQE of this solar cell is plotted in Figure 7.26.
On 1 Ω cm p-type material, this modelled device resulted in a short-circuit current density of 37.5 mA/cm2 without an SiO2 passivation layer present, reducing slightly to Jsc =
37.2 mA/cm2 with the inclusion of 10 nm of SiO2 at the TiO2 :Si interface. A short-circuit
current density of greater than 37 mA/cm2 is excellent value for a planar encapsulated solar
cell. This compares favourably to the best buried-contact solar cell manufactured by BP
Solar, which achieved a Jsc of 38.3 mA/cm2 (measured in air) using a silicon nitride SLAR
coating on textured crystalline silicon.15 The emitter dark saturation current density was
194
7. TiO2 Antireﬂection Coatings
Figure 7.24: Intensity of the AM1.5 global spectrum striking the front
surface of the glass and the silicon wafer, indicating absorption in the
glass, EVA and TiO2 layers.
Figure 7.25: Percentage of optical losses attributable to various layers at
wavelengths of 350 nm and 400 nm.
kept constant for TiO2 DLAR coatings both with and without an SiO2 passivation layer
present. While it is not realistic to expect a J0e of 1 × 1013 A/cm2 with no SiO2 layer, this
value was used to indicate the maximum optical performance of the TiO2 DLAR coating.
For the device with a surface passivation layer, am open-circuit voltage of 640.6 mV was
achieved, resulting in a solar cell eﬃciency of η = 18.6%. The ﬁll-factor of this solar cell was
78.1%, which is believed to be achievable for a BC solar cell fabricated on mc-Si substrates.
The high bulk minority carrier lifetime of 100 µs assumes that gettering and/or hydrogenation
of the mc-Si wafers has occurred during processing. Gettering is known to occur during
the heavy phosphorus groove diﬀusion,357 while a forming gas anneal performed after the
aluminium alloying process has resulted in bulk minority carrier lifetimes in excess of 80 µs
on ungettered 1.5 Ω cm mc-Si wafers.35 For mc-Si material that has received no lifetime
7.5 Performance of TiO2 DLAR-Coated Solar Cells
195
Figure 7.26: IQE of a simulated BC solar cells on τbulk = 100 µs material.
Table 7.4: PC1D modelling parameters used to simulate a single-sided
buried-contact solar cell (adopted from Honsberg et al.23 ).
Parameter
Surface texturing
p-type wafer resistivity
Wafer thickness, dSi
Bulk minority carrier lifetime, τbulk
Peak emitter doping, N0
Junction depth, xj
Emitter sheet resistance
Emitter dark saturation current density, J0e
Rear surface recombination velocity, SRVrear
Series resistance, Rs
Shunt resistance, Rsh
PC1D Value
None
1 Ω cm
350 µm
100 µs
1 × 1019 cm−3
1 µm
150 Ω/2
1 × 10−13 mA/cm2
1000 cm/s2
1 Ω cm2
100000 Ω cm2
enhancement processes, a bulk minority carrier lifetime of 20 µs is more typical. On this
material, a BC solar cell with a TiO2 DLAR coating could be expected to achieve an eﬃciency
of 17.4% (Jsc = 35.8 mA/cm2 and Voc = 625.0 mV).
The DLAR coating results represent a 7% improvement over a typical commercial TiO2
SLAR coating, deposited at 320◦ C and annealed brieﬂy at 850◦ C to simulate ﬁring of the
front screen-printed contacts. When modelled together with the electrical parameters for
a BC solar cell (with τbulk = 100 µs), this SLAR coating resulted in a Jsc of 34.7 mA/cm2
(Voc = 621.3 mV and η = 16.8%). The refractive index and extinction coeﬃcient of this
commercial SLAR coating were n = 2.275 and k = 0, at wavelengths of 600 nm and 400 nm,
respectively.
196
7.6
7. TiO2 Antireﬂection Coatings
Conclusions
There are two main candidates for AR coatings in a PV production environment, TiO2 and
SiNx . TiO2 has excellent optical properties and can be deposited at a low-cost. SiNx ﬁlms
are deposited at a higher cost using vacuum-based deposition equipment and exhibit slightly
poorer optical properties (higher absorption for a similar refractive index). However, there
can also be electrical beneﬁts in using SiNx ﬁlms deposited using PECVD, due to the large
hydrogen content of these ﬁlms.
It is commonly noted that DLAR coatings have reduced sensitivity to variations in ﬁlm
thickness.342, 343 Johnson et al. noted that a 10% variation of the layer thicknesses in a
multilayer coating resulted in only a 1% variation in the reﬂectance.349 Jellison and Wood
noted that the position of the reﬂectance minimum in the UV spectrum depends primarily
on the thickness of the inner layer of the DLAR coating.344 The same layer thickness also
aﬀects the IR reﬂectivity to a much greater extent than the outer layer thickness. The same
authors also showed that the outer layer thickness only inﬂuences the absolute value of the
UV reﬂectance minimum. DLAR coatings are much more sensitive to the SiO2 passivation
layer thickness, and therefore it is important that the passivation layer be made as thin as
possible.344
Doshi et al. explored the idea of using a DLAR coating made from the same materials,
in this case SiNx , for use with encapsulated solar cells.339 While the absorption due to
the bottom, high refractive index SiNx layer was high(Aw = 1.74), the reduced reﬂectance
(Rw = 2.96%) lead to an overall increase in optical performance. It is claimed that even
a small photocurrent gain of 0.3/,mA/cm2 resulting from using the DLAR coating rather
than an optimized SLAR coating is cost-eﬀective, as the wafers can remain in the PECVD
chamber for both depositions.339 An additional beneﬁt of such a coating may be improved
surface and bulk passivation resulting from the high hydrogen concentration in the bottom
layer. When a 10 nm-thick SiO2 passivation layer was included underneath a MgF2 /SiNx
DLAR coating, it was noted that the bottom layer is reduced in thickness by roughly the
same amount, while the optimum refractive index for the same layer increases slightly.
All researchers that have investigated multilayer coatings based on SiNx found that the
increased extinction coeﬃcient, especially for ﬁlms with n ≥ 2.3, has resulted in poor optical
performance of the coating.45, 339, 340, 349 Therefore, to achieve a practical DLAR coating to
improve the performance of planar mc-Si solar cells an alternative dielectric needs to be
found.
The optical properties of TiO2 ﬁlms could be varied greatly, by altering the deposition and
annealing conditions (temperature and ambient). TiO2 ﬁlms with a range of refractive indices
from 1.879 − 2.097 (λ = 600 nm) were achieved by varying the deposition temperature from
150 − 450◦ C. Such a trend is not evident in the extinction coeﬃcient values, however all ﬁlms
7.6 Conclusions
197
exhibit low absorption with k ≤ 1 × 10−2 at a wavelength of 400 nm. The reproducibility
between depositions was within ±10% of refractive index. This value is high, and the large
variation is attributed to the shallower deposition angles causing less cooling of the substrate,
thereby resulting in a higher eﬀective substrate temperature.
By placing the samples in a furnace at 450 − 1050◦ C for a period of 1 hr or 6 hr, refractive
indices in the range 2.061 − 2.602 (λ = 600 nm) could be obtained. Again, the extinction
coeﬃcients of these ﬁlms did not increase with the same linear trend and, in fact, the
highest k values (at 400 nm) were exhibited by ﬁlms annealed at 900 − 950◦ C. However,
as the ﬁlms were deposited individually, there are slight variations in the initial n and
k values, and these variations still remain after annealing. The thickness of the surface
roughness layer decreased dramatically for annealing temperatures ≥ 900◦ C due to sintering
processes. TiO2 ﬁlms annealed for the ﬁrst 5 min in oxygen exhibited a 6.2 − 10.9 nm-thick
SiO2 layer at the TiO2 :Si interface. The refractive indices of samples annealed for 6 hr were
the highest observed in this work, at n = 2.633. TiO2 samples that underwent a furnace
step in an oxygen ambient showed an increasing SiO2 layer thickness for longer processing
times. For ﬁlms deposited in the presence of water vapour, the refractive indices were low
at n = 1.726 − 2.194 (λ = 600 nm), even after annealing at 1000◦ C. An anatase ﬁlm with a
refractive index of 1.726 is calculated to be extremely (63.4%) porous.
A DLAR coating was designed and fabricated, with both layers being comprised of TiO2 . A
denser bottom TiO2 layer was obtained by annealing a TiO2 ﬁlm deposited at 450◦ C, while a
highly porous upper TiO2 layer was achieved by performing the deposition in the presence of
water vapour. A distinctive double minima, characteristic of a DLAR coating, was observed
in the reﬂectance spectrum. The best weighted average reﬂectance measured was 6.54% for
a planar crystalline silicon wafers and 8.04% for a planar multicrystalline silicon wafer (both
in air). The layer thicknesses and refractive indices were both greater than ideal, indicating
that further improvement is possible in the experimental coatings.
Modelling determined that a TiO2 DLAR coating under glass could achieve a minimum
weighted average reﬂectance of 6.1%, while the inclusion of a 10 nm-thick SiO2 passivation
layer at the interface increased the Rw slightly to 6.9%. The performance of the DLAR
coating was signiﬁcantly higher than a SLAR, which exhibited an Rw of 8.3% and 8.1%,
with and without the SiO2 layer, respectively.
Optical modelling determined that the majority of the absorptance in the short wavelength
region occurred in the EVA layer, rather than in the glass or TiO2 layers. A reduced AM1.5G
spectrum and intensity was calculated These values, along with parameters describing the
electrical performance of a single-sided buried-contact (BC) solar cell, were used as inputs to
a PC1D simulation. The maximum Jsc achievable with a planar BC solar cell implementing
a TiO2 DLAR coating under glass was 37.5 mA/cm2 . When a 10 nm-thick SiO2 passivation
layer was included at the TiO2 :Si interface, this value decreased slightly to 37.2 mA/cm2 . The
198
7. TiO2 Antireﬂection Coatings
modelled eﬃciency for the latter solar cell was 18.6% (Voc = 640.6 mV), which is excellent
considering the planar nature of the device. The ﬁll-factor for this solar cell is F F = 78.1%,
which is achievable for a BC solar cell fabricated on mc-Si substrates. Use of the DLAR
coating results in a 7% improvement in Jsc over a typical commercial TiO2 SLAR coating
on the same device.
Chapter 8
Conclusions
8.1
Summary
Overview and Motivation
This thesis represents one of the most thorough investigations into the application of titanium
dioxide (TiO2 ) thin ﬁlms to silicon photovoltaics (PV). TiO2 thin ﬁlms have a long history
of usage in the PV industry, primarily as an antireﬂection (AR) coating on screen-printed
(SP) solar cells. This is due, ﬁrstly, to the excellent optical properties of TiO2 and, secondly,
to its low deposition cost.
In order to reduce the cost of fabricating buried-contact (BC) solar cells, an alternative
dielectric thin ﬁlm to silicon dioxide (SiO2 ) has to be found. The SiO2 growth step is lengthy
and has to be performed at high-temperatures, increasing the complexity and cost of solar
cell fabrication. Additionally, other research has demonstrated that aluminium alloying can
occur in a phosphorus ambient,358 enabling two high-temperature steps to be combined into
one. While silicon nitride has been demonstrated to be economic on a commercial scale for
textured crystalline silicon (c-Si) wafers, TiO2 thin ﬁlms appeared to be more suitable for
solar cells fabricated on lower cost, multicrystalline silicon (mc-Si) wafers. This provided
the motivation to investigate individual processes within the solar cell fabrication sequence
and identify areas where TiO2 thin ﬁlms could be used to either enhance performance and
reduce fabrication costs.
Thin Film Requirements
Depending on the fabrication sequence of the BC solar cell, a single dielectric ﬁlm, such as
TiO2 , could serve multiple purposes:
• an AR coating,
• a dopant source for emitter diﬀusions,
199
200
8. Conclusions
• a ﬁlm compatible with oxide passivation,
• a diﬀusion barrier to phosphorus diﬀusion,
• a mask for electroless metal plating.
These desired functions place relatively strict requirements on the dielectric ﬁlm, demanding:
• a high refractive index and low extinction coeﬃcient for excellent optical performance.
This is especially important for mc-Si, for which no eﬀective texturing method has
been demonstrated on an industrial scale yet.
• a high chemical resistance to wet chemicals used in PV manufacturing. This requires
a dense ﬁlm with no pinholes to prevent chemicals and metal plating solutions from
reaching to the front surface of the silicon wafer. Additionally, the ﬁlms have to
be insulating to prevent the electroless metal plating solution from adhering to the
dielectric ﬁlm.
• excellent stability during high-temperature processing in a variety of gas ambients,
including oxygen (O2 ), nitrogen (N2 ) and the dopant source, phosphorus oxychloride
(POCl3 ).
• and, to be attractive in a commercial environment, such a ﬁlm needs to be deposited
at a cost of less than US$0.05 per 5” square wafer.
While the BC technology was the main driver behind this work, much of the literature review
is relevant to any work involving TiO2 , while several of the experiments may be applicable
to other solar cell designs such as those with evaporated or SP contacts.
Titanium Dioxide
TiO2 is a complex material with two of the crystalline phases – the metastable lowtemperature phase of anatase as well as the stable high-temperature phase of rutile – as
well as the amorphous phase being commonly observed in thin ﬁlms. The transformation
from amorphous TiO2 to anatase occurs at 300 − 350◦ C, while anatase can be transformed
into rutile by heating at temperatures greater than about 800◦ C. TiO2 thin ﬁlms used in the
majority of PV applications are as deposited as amorphous ﬁlms, due to the requirement of
ﬁring SP pastes through the layer in order to contact the solar cell emitter. The refractive
index of these ﬁlms is low (n ≈ 2.0 − 2.1 at λ = 600 nm) and the chemical resistance to the
majority of acids and bases is poor. Anatase ﬁlms exhibit an increased refractive index of
(n ≈ 2.3−2.45 at λ = 600 nm) and greatly enhanced chemical resistance. Rutile ﬁlms exhibit
refractive indices of n > 2.45 at λ = 600 nm and also a signiﬁcantly increased extinction
coeﬃcient due to an optical bandgap at about 3.05 eV. Therefore, anatase thin ﬁlms seemed
to aﬀord the best optical properties and chemical resistance.
8.1 Summary
201
TiO2 Deposition Systems
Two systems were designed and fabricated by the author for the deposition of anatase thin
ﬁlms. Films deposited using a spray-deposition system exhibited ideal optical properties
(high refractive index n and low extinction coeﬃcient k) for a single-layer antireﬂection
(SLAR) coating on a glass encapsulated solar cell. However, due to large variations in the
ﬁlm thickness, this system was converted into a simple chemical vapour deposition (CVD)
system operating at atmospheric pressure. CVD-deposited TiO2 ﬁlms exhibited much more
uniformly thick layers, although the refractive index of these ﬁlms was nearly 15% lower at
the same deposition temperature of 450◦ C.
No Contamination During High-Temperature Processing
Experimental results demonstrated that TiO2 is compatible with high-temperature processing without contaminating the wafers or furnaces. The bulk minority carrier lifetimes (τbulk )
of TiO2 -coated samples placed in a furnace for 2 hr at 950◦ C were maintained at greater
than 2 ms. It was found that the spray-deposited TiO2 ﬁlms are sensitive to the initial gas
ambient in the furnace. Samples loaded in oxygen were stable, however TiO2 ﬁlms loaded
directly into a nitrogen ambient were reduced to a sub-oxide, most likely Ti2 O3 . The formation sub-oxide was predicted using thermochemistry analysis and conﬁrmed using FTIR
spectroscopy.
Overcoming the Surface Passivation Limitation
One major disadvantage in using TiO2 thin ﬁlms in silicon solar cells is that they aﬀord very
little surface passivation to the silicon wafer. A novel method was developed by the author to
overcome this limitation of achieving surface passivation on TiO2 -coated silicon wafers. This
involved growing a thin SiO2 layer at the TiO2 :Si interface by oxidizing the wafer after TiO2
ﬁlm deposition. The presence of the 6 nm-thick SiO2 layer was conﬁrmed using scanning
electron microscopy (SEM) images and X-ray photoelectron spectroscopy (XPS) analysis.
The increase in surface passivation aﬀorded by the interfacial SiO2 layer results in a decrease
in the emitter dark saturation current density (J0e ) by nearly two orders of magnitude from
∼ 2 × 10−12 A/cm2 after TiO2 deposition to 4.7 − 7.7 × 10−14 A/cm2 . This demonstrates the
ability of the TiO2 /SiO2 AR coating to provide excellent surface passivation. The low J0e
and high τbulk values demonstrated here are compatible with high-eﬃciency solar cells with
an open circuit voltage (Voc ) of the order of 700 mV.
TiO2 as a Phosphorus Diﬀusion Barrier
The results from this work indicate that a 70 nm thick TiO2 ﬁlm can function as a phosphorus diﬀusion barrier, thereby protecting the lightly-doped emitter (∼ 100 Ω/2) from the
heavy phosphorus groove diﬀusion (< 5 Ω/2). Although the TiO2 diﬀusion barrier resulted
in only very light phosphorus diﬀusions underneath the ﬁlm, a reaction in the POCl3 furnace substantially alters the optical and electrical properties and limits its usefulness. The
202
8. Conclusions
reaction between the TiO2 and phosphorus pentoxide (P2 O5 ) led to the formation of new
compounds on the wafer, determined to be either TiPx Siy Oz or TiPx Oy from Rutherford
Backscattering (RBS) and thermochemistry analysis. Therefore, TiO2 must be described as
a sacriﬁcial diﬀusion barrier to phosphorus. This limitation means that in order for TiO2
to directly replace SiO2 in the BC process, a protective layer (such as SiO2 ) must ﬁrst be
applied on top of the TiO2 ﬁlm.
TiO2 as a Phosphorus Dopant Source
By doping the TiO2 ﬁlm with a small concentration of phosphorus, it was anticipated that
these dopant atoms would diﬀuse out of the ﬁlm and into the silicon during subsequent hightemperature processing. In this manner an n-type emitter could be created in the p-type
silicon wafer, obviating the need for a separate emitter diﬀusion step. Results from this
work have determined that the ability of TiO2 to act as a phosphorus dopant source is also
somewhat limited. The increased conductivity of the TiO2 ﬁlm due to the phosphorus incorporation is not compatible with the insulating requirements of the BC solar cell electroless
metal plating step. Also, it is anticipated that the increased optical absorption in the ﬁlms,
along with the lengthy diﬀusion times and temperatures involved (> 1 hr at > 1000◦ C) will
limit the industrial uptake of this technology.
TiO2 Double-Layer Antireﬂection (DLAR) Coatings
The optical properties of TiO2 ﬁlms have been varied greatly by altering the deposition and
annealing conditions (temperature and ambient). The optical properties of the ﬁlms were
determined with high accuracy using spectroscopic ellipsometry over the wavelength range
350 − 1150 nm, at angles of 65 − 80◦ . Optical modelling was performed using two main
software packages, WVASE (J.A. Woollam) and TFCalc (Software Spectra). TiO2 ﬁlms
with refractive indices ranging from 1.726 to 2.633 at a wavelength of 600 nm have been
produced. The low refractive index ﬁlms were deposited in the presence of large amounts
of water vapour, while the high refractive index ﬁlms were achieved by annealing the asdeposited samples for a period of up to 6 hr at 1000◦ C.
This variation in optical properties suggested that a DLAR coating design could be fabricated, with both layers being comprised of TiO2 . A DLAR coating was designed for operation
in air, with ideal refractive indices of nAR1 = 1.58 and nAR2 = 2.49, for the upper and lower
layers respectively. The denser bottom TiO2 layer was obtained by annealing a TiO2 ﬁlm
deposited at 450◦ C, while a highly porous upper TiO2 layer was achieved by performing
the deposition in the presence of water vapour. A distinctive double minima, characteristic
of a DLAR coating, was observed in the reﬂectance spectrum. The best weighted average
reﬂectance achieved was 6.54% for a planar crystalline silicon wafers and 8.04% for a planar
multicrystalline silicon wafer (both in air). Both the individual layer thicknesses and the
refractive indices were greater than ideal, indicating that further improvement is possible.
The real opportunity for a TiO2 DLAR coating comes, not for silicon solar cells in air, but for
8.2 Applicability to Various Solar Cell Processes
203
those encapsulated under ethyl vinyl acetate (EVA) and glass. The ideal refractive indices
of nAR1 = 2.07 and nAR2 = 2.86 (at λ = 600 nm) are not achievable with any transparent
materials, however modelling by the author has shown that a TiO2 DLAR coating with
nAR1 = 1.95 and nAR2 = 2.60 could achieve a minimum weighted average reﬂectance (Rw )
of 6.1%. With the inclusion of a 10 nm-thick SiO2 passivation layer at the interface, the Rw
increased slightly to 6.9%. The performance of the DLAR coating was signiﬁcantly higher
than a SLAR, which exhibited an Rw of 8.3% and 8.1%, with and without the SiO2 layer,
respectively. This indicates the opportunities for implementing such a DLAR coating as a
cost-eﬀective means for reducing the reﬂectance of mc-Si wafers.
Optical modelling determined that the majority of the absorptance in the short wavelength
region occurred in the EVA layer, rather than in the glass or TiO2 layers. A PC1D simulation,
which included absorption in all layers, of an encapsulated, planar BC solar cell with a TiO2
DLAR coating fabricated on low lifetime (τbulk = 100 µs) silicon resulted in a maximum shortcircuit current density (Jsc ) of 37.5 mA/cm2 . When a 10 nm-thick SiO2 passivation layer was
included at the TiO2 :Si interface, this value decreased slightly to 37.2 mA/cm2 . The modelled
eﬃciency for the latter solar cell was 18.6% (Voc = 640.6 mV), which is excellent considering
the planar nature of the device. The ﬁll-factor for this solar cell is F F = 78.1%, which
is achievable for a BC solar cell fabricated on mc-Si substrates. Use of the DLAR coating
results in a 7% improvement in Jsc over a typical commercial TiO2 SLAR coating on the
same device. For mc-Si wafers that have not received any form of gettering or hydrogenation,
a bulk minority carrier lifetime of 20 µs is more typical. On this material, the simulated BC
solar cell with TiO2 DLAR coating achieved an eﬃciency of 17.4% (Jsc = 35.8 mA/cm2 and
Voc = 625.0 mV).
8.2
Applicability to Various Solar Cell Processes
The application of TiO2 ﬁlms to BC solar cells in order to reduce the fabrication costs and
make was the BC technology more applicable to mc-Si wafers was the primary motivation
of this research. However, the results of the experiments determining the plausible roles of
TiO2 ﬁlms are relevant to other types of silicon solar cells. This section will highlight the
opportunities for TiO2 thin ﬁlms in BC, screen-printed (SP) and evaporated-contact solar
cells.
BC Solar Cells
TiO2 is potentially a low-cost replacement for the thermally grown SiO2 layer as implemented
in the original BC solar cell sequence. TiO2 can only be used as a direct replacement for
SiO2 layer in the standard BC processing sequence (refer to Section 1.4.3) if it is protected
during the heavy phosphorus groove diﬀusion. Thus, the TiO2 ﬁlm would function as a ﬁlm
compatible with surface passivation, an AR coating and an electroless metal plating mask.
204
8. Conclusions
The author demonstrated that a 200 nm-thick spin-on SiO2 layer is suﬃcient to protect the
TiO2 ﬁlm during a phosphorus diﬀusion. This layer could remain on the solar cell until after
electroless metal plating and subsequently be removed in hydroﬂuoric acid, which would not
attack the TiO2 . The cost using an additional protective barrier would be small, estimated
at US$0.01 per 5” wafer, and along with the cost of depositing the TiO2 ﬁlm (∼US$0.04
per 5” wafer) it is believed that this option is still cheaper than growing either SiO2 in an
oxidation furnace or silicon nitride in an low-pressure chemical vapour deposition reactor.
A second option in the BC solar cell process is to use a single diﬀusion for both the emitter
and grooves (typically 40−50 Ω/2). In this manner, the TiO2 is deposited prior to electroless
metal plating, thereby avoiding exposing the ﬁlm to phosphorus containing gas ambients.
The use of phosphorus doped titanium dioxide (TiO2 :P) as a phosphorus dopant source is
not applicable to the BC process, as the inclusion of P-dopant atoms makes the TiO2 ﬁlm
conductive and renders it useless as a electroless metal plating mask.
SP Solar Cells
The traditional role of TiO2 ﬁlms in the PV industry is as an AR coating for SP solar cells.
An additional requirement of the TiO2 ﬁlm in SP solar cells is that it “soft” enough for the
SP paste to be able to be ﬁred-through the layer and to contact the emitter. Deposition
temperatures are typically about 200◦ C, and the ﬁlms retain many organics and exhibit a
very low refractive index. However, during the brief ﬁring step (∼ 800◦ C for 1 min 30 s) the
organics are baked oﬀ and the refractive index increases to about 2.25.
Thus, any role of TiO2 will only be successful if it is able to remain as an amorphous ﬁlm
up until the front contact ﬁring step. Surface passivation is not usually a requirement for
SP solar cells due to the heavily-doped emitter, which exhibits a phosphorus “dead-layer”
and is insensitive to the front surface preparation conditions. A possible role of TiO2 in the
SP process would be the use of TiO2 :P as an emitter dopant source. However, it may be the
case that the times required for achieving phosphorus diﬀusion are not compatible with the
metallisation paste ﬁring times.
A TiO2 DLAR coating could be implemented if both of the layers were deposited as amorphous and the front contact ﬁring step was optimised to achieve high refractive indices. This
may require the use of diﬀerent pastes.
Evaporated-Contact Solar Cells
All of the novel roles of TiO2 may be applicable to a solar cell that uses evaporated contacts.
There is greater ﬂexibility in the design of such a solar cell due to the large number of steps
(including photolithographic) involved. The most likely beneﬁcial step would be the use of a
TiO2 DLAR coating, combined with the growth of SiO2 at the front surface for passivation
purposes. The beneﬁts of a DLAR coating for such a solar cell will be small, as these cells are
normally fabricated on c-Si wafers, which are able to be chemically textured. However, the
8.3 Suggestions for Further Work
205
cost of a TiO2 DLAR coating will be substantially less than coatings comprised of evaporated
ﬁlms such as zinc sulphide and magnesium ﬂuoride.
8.3
Suggestions for Further Work
There are several areas of research involving TiO2 that would appear to be promising. It
should be noted that some of these “ideas” are merely the opinion of the author and there
may be no scientiﬁc literature available to support these claims.
TiO2 would seem to be more compatible with boron than phosphorus. This has several
implications, mainly for solar cells fabricated on n-type wafers. Firstly, shorter times and
lower temperatures may be required in order to achieve a p-type emitter from boron-doped
TiO2 . Secondly, it is possible that boron diﬀusion glass (B2 O3 ) will not react with or increase
the conductivity of the TiO2 ﬁlm. If demonstrated to be true, this would make an n-type
BC solar cell process very attractive. From the literature, it would also seem that tantalum
pentoxide (Ta2 O5 ) would be more compatible than TiO2 as a phosphorus dopant source.
Another interesting possibility is if aluminium oxide (Al2 O3 ) could act as a phosphorus
diﬀusion barrier. If so, then an Al2 O3 /TiO2 DLAR coating could be used in the BC process.
The use of highly porous TiO2 ﬁlms demonstrated in this work may be useful in other types
of solar cells, such as the Gr¨atzel cell. In this cell, a thick (∼ 5 µm) TiO2 ﬁlm is used a
matrix, and ﬂooded with electrolyte dye. A ﬁlm with a high surface area is required so that
the dye can contact as many points as possible.
A study of the optical and electrical properties of both doped TiO2 (say, with niobium) and
reduced TiO2−x would be interesting. It may be possible to fabricate a transparent conducting oxide layer from either of these ﬁlms. TiO2 -based ﬁlms have the advantage that they can
be deposited at high-speed, at atmospheric pressure and at low-cost. Therefore, it is possible
that conducting layers for thin ﬁlm modules could be deposited using atmospheric pressure
chemical vapour deposition (APCVD), rather than more expensive sputtering techniques.
206
8. Conclusions
Appendix A
TiO2 AR Coating Modelling
Parameters
The ﬁtting parameters used to model the dielectric constant function of the TiO2 ﬁlms from
Sections 7.3 and 7.4 are included in this appendix. The ﬁtting parameters, ε1 (∞), Ai , Bi and
Eni are described in Section 4.6.3, along with Equation 4.8, which deﬁnes the mean-squared
error (MSE).
In addition, the tabulated results of thickness and optical properties and their dispersive
n and k spectra of some of the TiO2 thin ﬁlms from Section 7.3 are also included in this
appendix.
207
208
A. TiO2 AR Coating Modelling Parameters
A.1
Variation of n and k with Deposition Temperature
Table A.1: Fitting parameters used to model the dielectric constant
(Lorentz double-oscillator model) of TiO2 ﬁlms deposited using CVD with
substrate temperatures 150 − 450◦ C. The ﬁlms are listed from lowest to
highest deposition temperature with 50◦ C intervals.
Sample
Name
ε1 (∞)
A1
B1
(eV)
En1
(eV)
A2
B2
(eV)
En2
(eV)
MSE
TO-2-1
TO-2-3
TO-2-2
TO-2-4
TO-2-5
TO-2-6
TO-2-8
2.9145
3.1056
2.9594
3.1135
3.2556
3.5291
3.7805
0.27128
0.19189
0.33326
0.27315
0.26939
0.30021
0.56632
0.19185
0.12474
0.34003
0.18417
0.17283
0.12739
0.18329
3.5666
3.5337
3.5342
3.5506
3.5563
3.5513
3.5637
406.16
523.59
471.25
329.25
429.78
464.72
196.30
0.0044184
0.0023403
0.0036487
0.0061043
0.0054490
0.0047787
0.0083855
4.0648
3.9785
3.9302
4.0383
4.0787
4.0308
3.9464
1.977
1.480
2.900
1.801
1.328
1.154
1.084
Table A.2: Thickness, refractive index (at λ = 600 nm) and extinction coeﬃcient (at λ = 400 nm) of surface roughness and TiO2 layers deposited
by CVD with substrate temperatures 150 − 450◦ C.
Sample
Name
Surface Layer
Tdep dsurf
n
k
◦
( C) (nm)
TO-2-1
TO-2-3
TO-2-2
TO-2-4
TO-2-5
TO-2-6
TO-2-8
150
200
250
300
350
400
450
23.1
26.7
21.7
14.8
14.9
29.6
36.2
1.419
1.419
1.428
1.451
1.479
1.506
1.517
TiO2 Layer
dT iO2
n
k
(nm)
0.00149 87.0 1.879
0.00056 76.7 1.881
0.00721 102.0 1.901
0.00157 83.6 1.952
0.00133 76.2 2.012
0.00091 65.4 2.072
0.00204 63.0 2.097
0.00326
0.00121
0.01144
0.00343
0.00291
0.00199
0.00583
dsurf
dT iO2
0.266
0.348
0.213
0.177
0.196
0.453
0.575
A.1 Variation of n and k with Deposition Temperature
Figure A.1: Dispersive relations for the (a) refractive index and (b) extinction coeﬃcient of the TiO2 layer of samples deposited using CVD at
a range of deposition temperatures.
209
210
A. TiO2 AR Coating Modelling Parameters
Figure A.2: Dispersive relations for the (a) refractive index and (b) extinction coeﬃcient of the surface roughness layer of samples deposited
using CVD at a range of deposition temperatures.
A.1 Variation of n and k with Deposition Temperature
211
Table A.3: Fitting parameters used to model the dielectric constant
(Lorentz double-oscillator model) of additional TiO2 ﬁlms deposited using CVD with substrate temperatures 250◦ C (TO-3-8a) and 450◦ C (TO3-1a).
Sample
Name
ε1 (∞)
A1
B1
(eV)
TO-3-8a 1.6877 73.911 0.03728
TO-3-1a 2.3540 7.5575 0.10686
En1
(eV)
A2
B2
(eV)
En2
(eV)
MSE
4.0662
3.6863
175.45
203.98
0.33451
0.07535
47.446
7.3323
1.841
1.468
Table A.4: Thickness, refractive index (at λ = 600 nm) and extinction coeﬃcient (at λ = 400 nm) of surface roughness and TiO2 layers deposited
by CVD with substrate temperatures of 250◦ C and 450◦ C.
Sample
Name
Surface Layer
Tdep dsurf
n
k
◦
( C) (nm)
TiO2 Layer
dT iO2
n
k
(nm)
TO-3-8a
TO-2-2
250
250
14.1
21.7
1.455 0.00295 96.9 1.960 0.00975 0.146
1.428 0.00721 102.0 1.901 0.01144 0.213
TO-3-1a
TO-2-8
450
450
34.8
36.2
1.576 0.00719
1.517 0.00204
56.0
63.0
2.225
2.097
dsurf
dT iO2
0.01586 0.621
0.00583 0.575
En1
(eV)
A2
151.30
173.79
212.71
395.75
469.93
498.96
1937.6
362.42
383.08
1.7879
1.6131
1.2638
0.59047
0.48433
0.44677
0.06670
0.41197
0.40191
10.840
25.776
34.738
20.244
14.808
13.109
16.847
33.735
19.637
50.160
50.014
42.444
21.082
16.367
15.076
36.479
10.527
10.219
B1
(eV)
6 hr Anneal in N2
TO-3-2b 450 -1.6873
TO-4-1b 500 -1.8589
TO-4-2b 600 -2.6751
TO-4-3b 700 -7.3910
TO-4-4b 800 -10.000
TO-4-5b 900 -10.000
TO-4-6b 950 1.0791
TO-4-7b 1000 -9.2070
TO-4-8b 1050 -10.000
A1
3.5800 0.57569
50.092 28.747
50.000 41.470
25.650 24.400
16.352 9.2176
16.308 9.2967
12.512 11.014
9.7629 20.518
7.9520 25.480
ε1 (∞)
1 hr Anneal in N2
TO-3-1b 450 3.8177 7.5739 0.10541
TO-4-1c 500 -1.8225 159.18 1.6749
TO-4-2c 600 -2.0668 183.72 1.5122
TO-4-3c 700 -5.8686 330.64 0.71979
TO-4-4c 800 -9.9736 468.91 0.48890
TO-4-5c 900 -10.000 521.69 0.45637
TO-4-6c 950 -4.1817 710.61 0.15483
TO-4-7c 1000 -5.2537 826.69 0.11261
TO-4-8c 1050 -1.1817 118.49 0.39351
Tann
(◦ C)
3.4834
3.9780
4.0728
3.9666
3.7119
3.6038
3.6601
3.8497
3.8535
En2
(eV)
0.15430 3.8313
0.055087 3.8267
0.066355 4.0231
0.0091141 3.8748
0.24770 4.0054
0.21158 3.6372
0.28769 3.7526
0.096759 3.7903
0.11758 3.7332
0.73656
0.074611
0.066487
0.099716
0.13795
0.26935
0.28993
0.23544
0.17741
B2
(eV)
1.978
2.187
1.312
1.167
1.108
1.484
1.824
1.384
1.805
3.434
2.033
1.660
2.042
0.847
1.049
1.819
1.477
1.728
MSE
A.2
Sample
Name
212
A. TiO2 AR Coating Modelling Parameters
Variation of n and k with Annealing Temperature
Table A.5: Fitting parameters used to model the dielectric constant
(Lorentz double-oscillator model) of TiO2 ﬁlms deposited using CVD at
450◦ C and annealed at 450 − 1050◦ C in N2 for a period of 1 hr or 6 hr.
A.2 Variation of n and k with Annealing Temperature
213
Table A.6: Thickness, refractive index (at λ = 600 nm) and extinction
coeﬃcient (at λ = 400 nm) of surface roughness and TiO2 layers deposited by CVD at Tdep = 450◦ C and annealed at 450 − 1050◦ C in N2 for
1 hr or 6 hr.
Surface Layer
dsurf
n
k
(nm)
dT iO2
(nm)
1 hr Anneal in N2
TO-3-1b
450
TO-4-1c
500
TO-4-2c
600
TO-4-3c
700
TO-4-4c
800
TO-4-5c
900
TO-4-6c
950
TO-4-7c
1000
TO-4-8c
1050
44.9
34.1
34.5
32.9
35.4
36.8
29.7
14.2
6.6
1.511
1.501
1.519
1.509
1.558
1.666
1.690
1.722
1.745
0.03861
0.00639
0.00582
0.01122
0.01905
0.05753
0.05737
0.04047
0.03775
80.4
65.4
61.0
68.4
64.2
63.2
60.6
55.0
63.5
2.082
2.061
2.100
2.077
2.184
2.423
2.477
2.547
2.598
0.08495
0.01404
0.01282
0.02470
0.04202
0.12740
0.12706
0.08962
0.08362
0.558
0.521
0.566
0.481
0.551
0.582
0.490
0.258
0.104
6 hr Anneal in N2
TO-3-2b
450
TO-4-1b
500
TO-4-2b
600
TO-4-3b
700
TO-4-4b
800
TO-4-5b
900
TO-4-6b
950
TO-4-7b 1000
TO-4-8b 1050
41.3
39.4
35.7
33.2
33.0
30.1
22.6
4.7
5.2
1.511
1.505
1.524
1.525
1.619
1.696
1.722
1.747
1.732
0.01326
0.00495
0.00583
0.01155
0.03084
0.04645
0.06018
0.02717
0.02966
70.5
67.5
60.0
66.0
59.4
63.7
63.7
61.2
66.5
2.081
2.069
2.110
2.113
2.319
2.489
2.546
2.602
2.569
0.02918
0.01087
0.01284
0.02544
0.06822
0.10287
0.13329
0.06017
0.06569
0.586
0.584
0.595
0.503
0.556
0.473
0.355
0.077
0.078
Sample
Name
Tann
(◦ C)
TiO2 Layer
n
k
dsurf
dT iO2
214
A. TiO2 AR Coating Modelling Parameters
Figure A.3: Dispersive relations for the (a) refractive index and (b) extinction coeﬃcient of the TiO2 layer of samples deposited at 450◦ C and
annealed for 1 hr in N2 .
A.2 Variation of n and k with Annealing Temperature
Figure A.4: Dispersive relations for the (a) refractive index and (b) extinction coeﬃcient of the TiO2 layer of samples deposited at 450◦ C and
annealed for 6 hr in N2 .
215
216
A. TiO2 AR Coating Modelling Parameters
Figure A.5: Dispersive relations for the (a) refractive index and (b) extinction coeﬃcient of the surface roughness layer of samples deposited at
450◦ C and annealed for 1 hr in N2 .
A.2 Variation of n and k with Annealing Temperature
Figure A.6: Dispersive relations for the (a) refractive index and (b) extinction coeﬃcient of the surface roughness layer of samples deposited at
450◦ C and annealed for 6 hr in N2 .
217
218
A.3
A. TiO2 AR Coating Modelling Parameters
Variation of n and k with Deposition Ambient
Figure A.7: Dispersive relations for the (a) refractive index and (b) extinction coeﬃcient of the TiO2 layer of samples deposited at 250◦ C and
450◦ C in the presence of H2 O vapour. Some samples were annealed at
either 450◦ C or 1000◦ C in N2 for a period of 1 hr or 6 hr.
Tdep
(◦ C)
B2
(eV)
En2
(eV)
0.18753
0.95360
MSE
3.7506 3.112
57.808 6.552
Sample TO-3-5b was annealed for 5 min in O2 and subequently 5 hr 55 min in N2 in the same hightemperature step. This resulted in the growth of 11.1 nm of SiO2 at the TiO2 :Si interface.
†
A2
86.830 232.99 0.006443 4.3773 2.456
9.1427 594.07 0.029195 6.6227 1.754
En1
(eV)
3.7608 100.00 246.31 0.003356 3.9991 4.900
0.29274 3.7236 323.72 0.37816 24.545 2.002
4.9956
1.9594
B1
(eV)
10.537 6.3942 25.798 3.6709
6.0971 0.39140 3.6764 172.98
0.10645
0.65607
13.863
1.1237
A1
(hr)
6 hr Anneal in N2
TO-3-7c 250 1000
TO-3-5b 450 1000†
2.1066
0.86073
ε1 (∞)
(◦ C)
1.9741 19.200
-0.97474 6.7456
−
−
Tann
(◦ C)
1 hr Anneal in N2
TO-3-6b 250
450
TO-3-5a 450 1000
No Anneal
TO-3-6a 250
TO-3-4a 450
Sample
Name
A.3 Variation of n and k with Deposition Ambient
219
Table A.7: Fitting parameters used to model the dielectric constant
(Lorentz double-oscillator model) of TiO2 ﬁlms deposited using CVD at
250◦ C and 450◦ C with H2 O vapour. Some samples were annealed at either 450◦ C or 1000◦ C in N2 for a period of 1 hr or 6 hr.
A1
B1
(eV)
En1
(eV)
A2
B2
(eV)
En2
(eV)
MSE
-3.6533
-1.1366
30.090
40.871
0.12581
0.18648
3.8632
3.8823
301.74 0.21910
241.44 0.30138
7.7274
13.965
1.083
1.896
-5.2537 826.69 0.11261 9.7629 20.518 0.23544 3.8497 1.477
-5.4320 64.836 0.12730 3.9827 34.292 5.0079 19.722 1.950
-0.97474 6.7456 0.29274 3.7236 323.72 0.37816 24.545 2.002
0.65607 6.0971 0.39140 3.6764 172.98 0.95360 57.808 6.552
ε1 (∞)
Samples TO-3-5a and TO-3-5b were deposited in the presence of H2 O vapour.
✗
✓
6 hr Anneal
TO-3-3b 450
TO-3-3c 450
†
✗
✓
✗
✓
1 hr Anneal
TO-4-7c 450
TO-3-3a 450
TO-3-5a 450†
TO-3-5b 450†
Tdep 5 min
(◦ C) O2 ?
A.4
Sample
Name
220
A. TiO2 AR Coating Modelling Parameters
Variation of n and k with Annealing Ambient
Table A.8: Fitting parameters used to model the dielectric constant
(Lorentz double-oscillator model) of TiO2 ﬁlms deposited using CVD at
450◦ C. Some samples received a 5 min oxidation prior to all samples being
annealed at 1000◦ C in N2 for a total period of 1 hr or 6 hr.
A.4 Variation of n and k with Annealing Ambient
Figure A.8: Dispersive relations for the (a) refractive index and (b) extinction coeﬃcient of the TiO2 layer of samples deposited at 450◦ C. Some
samples received a 5 min oxidation prior to all samples being annealed at
1000◦ C in N2 for a total period of 1 hr or 6 hr.
221
tann
(min)
1
2
4
8
16
Sample
Name
TO-4-1a
TO-4-2a
TO-4-3a
TO-4-4a
TO-4-5a
-2.0959
-1.8936
-1.9082
-3.4864
0.737663
ε1 (∞)
201.43
215.81
190.35
208.43
13.996
A1
(eV)
1.3653
1.3091
1.3604
1.1598
0.27947
B1
(eV)
50.705
52.835
49.804
35.499
3.6323
En1
23.324
24.777
28.435
24.532
209.27
A2
(eV)
0.10798
0.11224
0.12884
0.22635
0.87688
B2
(eV)
3.9790
4.0005
4.0914
4.0323
50.000
En2
1.760
1.779
1.524
1.536
2.393
MSE
222
A. TiO2 AR Coating Modelling Parameters
Table A.9: Fitting parameters used to model the dielectric constant
(Lorentz double-oscillator model) of TiO2 ﬁlms deposited using CVD at
450◦ C and oxidised for a period of 1 − 16 min at a loading temperature
of 800◦ C with the furnace set to ramp to 1000◦ C.
A.4 Variation of n and k with Annealing Ambient
223
Table A.10: Thickness, refractive index (at λ = 600 nm) and extinction
coeﬃcient (at λ = 400 nm) of surface roughness and TiO2 layers deposited using CVD at 450◦ C and oxidised for a periof of 1 − 16 min at a
loading temperature of 800◦ C with the furnace set to ramp to 1000◦ C
Sample
Name
TO-4-1a
TO-4-2a
TO-4-3a
TO-4-4a
TO-4-5a
tann dSiO2
(min) (nm)
1
2
4
8
16
0
0
0.4
3.7
8.7
Surface Layer
dsurf
n
k
(nm)
TiO2 Layer
dT iO2
n
k
(nm)
32.4
29.3
32.0
28.2
33.7
72.9
61.7
65.6
57.7
60.8
1.497
1.519
1.529
1.602
1.678
0.00974
0.01017
0.01232
0.03239
0.07174
2.050
2.100
2.122
2.283
2.449
0.02142
0.02240
0.02716
0.07163
0.15890
dsurf
dT iO2
0.444
0.475
0.488
0.489
0.554
224
A. TiO2 AR Coating Modelling Parameters
Figure A.9: Dispersive relations for the (a) refractive index and (b) extinction coeﬃcient of the TiO2 layer of samples deposited at 450◦ C and
oxidised for a periof of 1 − 16 min at a loading temperature of 800◦ C with
the furnace set to ramp to 1000◦ C.
Low
High
Low
High
Low
High
TO-5-5d
TO-5-8d
TO-5-6d
High/Low
Index
1.0261
3.6191
1.2686
4.6997
0.30975
5.6057
ε1 (∞)
B1
(eV)
En1
(eV)
A2
17.388
5.0788
82.390
−
3969.7 0.0022149 4.2949 3.7625
19.988
5.0002
74.719
−
2907.7 0.0015748 3.8822 4.8528
37.791
5.0586
84.695
−
2370.6 0.0015067 3.7147 4.6486
A1
−
0.26300
−
0.28336
−
0.19178
B2
(eV)
MSE
−
3.643
3.4587
−
3.945
3.4819
−
3.384
3.4987
En2
(eV)
A.5
Sample
Name
A.5 TiO2 DLAR Coatings
225
TiO2 DLAR Coatings
Table A.11: Fitting parameters used to model the dielectric constant
(Lorentz double-oscillator model) of TiO2 DLAR coatings measured using
spectroscopic ellipsometry. A dash (−) indicates that the second oscillator was not required in the model and that the amplitude was zero.
226
A. TiO2 AR Coating Modelling Parameters
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