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Proceedings of the 4th International Conference on Nanostructures (ICNS4)
12-14 March, 2012, Kish Island, I.R. Iran
Study of Te-enriched Cadmium Telluride Nano-Structured Thin Films at Cryogenic and Room
Temperature
Waqar A. Adil Syed1 Nazar Abbas Shah2, and M. Kamran2
1
Department of Physics, International Islamic University, Islamabad, Pakistan
2
Thin Films Technology Research Laboratory, Department of Physics COMSATS Institute of Information Technology,
Islamabad 46000, Pakistan
* Corresponding author Tel.: +92519049295; Fax: +92514442805. Cell: +923215105363
E-mail addresses: [email protected], [email protected]
Abstract: Te-enriched Cadmium Telluride (CdTe) polycrystalline thin films were grown on corning glass substrate.
The structural investigations performed by means of X-ray diffraction (XRD) technique showed that the deposited
samples exhibit a polycrystalline structure with <111> as preferred orientation. Compositional study was taken by
energy dispersive X-ray spectroscopy (EDX). Electrical properties of these samples were measured by Hall Effect. For
the Te-enriched samples, the electrical resistivity dropped several orders of magnitude at room temperature. The
deposited films also showed decreasing resistivity with increasing temperature, manifesting the semi conducting
behaviour. Hall measurements at room temperature as well as at cryogenic temperature are discussed
Keywords: Crystal structure; Optical microscopy; Thin films; Semiconductor; Vacuum coating
Introduction
Recent advances in the research of Cadmium
2CdTe ( s ) ⇔ 2Cd ( g ) + Te 2 ( g )
Telluride (CdTe) based solar cells [1-6], raised the
The source CdTe dissociates into its elements (Cd
interests in this highly efficient material for the use in
and Te) at high temperature, and they recombine on
thin film terrestrial photovoltaic applications [7-9].
the substrate surface, which is kept at a lower
CdTe has a direct band gap of 1.5 eV at room
temperature. The CSS system used in our experiment
temperature, which is optimum for single junction
is shown in Fig. 1.
solar cell efficiency [6]. Wu et al [10] have reported
CdTe / CdS solar cell with an efficiency of 16.5%.
Because of its low sublimation temperature, CdTe
polycrystalline films can be prepared by several
techniques, such as thermal evaporation [11], close
spaced
sublimation
(CSS). The
closed
space
sublimation (CSS) technique is one of the various
techniques that have produced encouraging results.
Experimental
The deposition of CdTe films is based on the
reversible process at high temperature.
864
Thin films of cadmium telluride powder (99.99%
pure) by Aldrich chemical company were placed on
microscope slides of water-white glass substrates.
Source material was put in the graphite boat as
shown in Fig 1 (b), heated by a halogen lamp of
Proceedings of the 4th International Conference on Nanostructures (ICNS4)
12-14 March, 2012, Kish Island, I.R. Iran
1000W, connected to the main power through
the transmittance curves of as deposited film along
temperature controller with K-type thermocouple.
with Te-enriched samples for different periods of
The substrate was fixed at a distance of about 5 mm
from the source material. It was heated by a halogen
lamp of 500 Watts, while the thermocouple was
placed over the substrate to measure its temperature.
Source and substrate temperatures were kept at
500oC and 400oC respectively. The chamber was
evacuated at ∼ 10-4 mbar with the help of rotary
pump and diffusion pump.
s
times.
The resistivity of
In the next step, Te was deposited on the top of CdTe
thin films by successively varying Te mass
evaporation in the graphite boat keeping the source
o
and substrate temperature at 200 and 100 C
respectively. The vacuum of the system was ∼ 10-4
mbar. All the thin films were heat treated at 400oC
sample 7 using Vander Pauw
geometry was ~ 156 Ω-cm at room temperature,
while the mobility and carrier concentration were
about 129.3 cm2 V-1
s
-1
and 3.09*1014 cm
–3
respectively at room temperature using Hall Effect.
for 30 minutes under the same vacuum. The silver
It has been observed by Hall measurements that p-
paste was used for making Ohmic contacts to
type doping is achieved in Te-doped CdTe films
perform Hall measurements. The samples denoted by
1-5 with successively varying Te content achieved
prepared by CSS technique, which ensures the excess
after tellurium depositing as shown in Table.1 by the
of Te forms a part of the lattice, might create Cd
above technique and the characterization of these
vacancies, which increases the p-type doping.
samples is described below.
Results and Discussion
8
6x10
In the XRD patterns, the main reflections of samples
according to fcc CdTe lattice. The data obtained for
the lattice constant ‘a’ of the investigated samples
series is 6.48-6.50Å, which agreed with the standard
values of lattice constant (ASTM Cards 15-0770).
During Te depositing, the in coming Te atoms collide
8
Resistivity at Room Temperature
5x10
Resistivity (Ωcm)
are found similar to each other and can be indexed
8
4x10
8
3x10
8
2x10
8
1x10
0
1
with the cadmium telluride, replacing the Cd atom in
2
3
Sample Number
4
5
the lattice, resulting an increase in the lattice
constant. Also the grains of the film became larger
with the increase of Te content in the samples, which
indicates that the crystallinity of the films can be
improved by appropriate Te doping. Fig. 2. Shows
Conclusion
In this work, Te-doped CdTe films have been
fabricated by the CSS technique. It is concluded on
865
Proceedings of the 4th International Conference on Nanostructures (ICNS4)
12-14 March, 2012, Kish Island, I.R. Iran
the basis of the results of XRD, SEM and Hall
measurements that the post deposition treatment
[4] G. Fulop, M. Doty, P. Meyers, J. Betz and C. H. Liu,
Appl. Phys. Lett.40 (1982) 327.
[5] K. Yamaguchi, H. Matsumoto, N. Nakayama and S.
strongly affects the morphology as well as electrical
properties. The samples had large grains with a size
∼ 2 µm and the crystallite size can grow larger with
appropriate Te doping. The refractive index was
increased with the increase of Te doping. The
transmission
spectra
showed
a
decrease
of
transmittance with an increasing level of Te doping.
The band gap values are also affected by Te doping.
The sheet resistance, dc electrical resistivity and
activation
energy
of
Te-doped
samples
have
decreased, on the other hand the dark conductivity,
mobility and carrier concentration have increased
significantly compared to pure CdTe samples at
cryogenic temperature.
Acknowledgment
The authors would like to thank Higher Education
Commission (HEC), Pakistan, for financial support
through the project 20-1187/R&D/09.
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