1. technology and computing

Optical and thermal investigations of CdS/Si(100)
nanostructures
Y. Al-Douri1,2, ∗, U. Hashim1, M. Ameri3, A. Bouhemadou4
1
Institute of Nano Electronic Engineering, University Malaysia Perlis, 01000 Kangar,
Perlis, Malaysia
2
Physics Department, Faculty of Science, University of Sidi-Bel-Abbes, 22000-Algeria
3
Laboratory physico-chemistry of advanced materials, University of Djillali Liabes, BP 89, SidiBel- Abbes, 22000, Algeria.
4
Laboratory for Developing New Materials and their Characterization, Department of Physics,
Faculty of Science, University of Setif, 19000 Setif, Algeria.
Abstract
CdS nanostructures have grown on p-type silicon (Si) (100) substrates using sol-gel method. The
crystalline quality, surface morphology, optical and electrical properties of the deposited CdS
nanostructures have been characterized and analyzed using atomic force microscopy (AFM),
scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric analysis
(TGA), differential thermal analysis (DTA), UV-vis spectroscopy and electrical characterization,
respectively. The effect of annealing temperature in the range 200-600 oC on the structural,
morphological, optical and electrical properties has been elaborated. The XRD analysis shows
that the crystalline quality can be improved by increasing the temperature to 400 oC, but further
increase to 600 oC leads to degradation of crystalline quality. The bulk modulus is calculated and
showed good agreement with experimental and theoretical results. The optical properties of
absorption, reflection, energy band gap and extinction coefficient are obtained by UV-vis
spectroscopy. The calculated refractive index and optical dielectric constant have shown good
agreement with other results. The electrical and thermal properties are studied for antireflection
coating applications.
Keywords: II-VI; Nanostructure; Characterization; Annealing temperature.
PACS: 78.30.Fs; 79.60.Jv; 68.49.Df; 61.72.Cc.
∗
) For correspondence; Tel.: + (60) 4 9775021, Fax: + (60) 4 9798578, E-mail: [email protected].
1
1. Introduction
Cadmium sulfide (CdS) is an important semiconductor with direct band gap, 2.43 eV at room
temperature. Extensive research has been done in the last decades due to its applications in
electronic devices like field-effect transistors, solar cells, photoconductors, optical thin films
filter, nonlinear integrated optical devices, light emitting diodes (LEDs) and laser
heterostructures for emission in the visible spectral range [1-6]. For these purposes, CdS
nanostructures have been deposited by different techniques of spray pyrolysis (SP), close space
vapor transport (CSVT), vacuum evaporation [7], chemical bath deposition (CBD), solution
growth technique (SGT) [8], sputtering, chemical vapor deposition (CVD), pulsed laser
deposition (PLD) and spin coating technique. It is proven that the spin coating technique has
many features of creating highly energetic growth precursor, leading to growth conditions, so
that high quality films can be obtained at fairly low substrate temperature. Up to now, the spin
coating deposition technique was currently used in the laboratory research to obtain CdS
nanostructures [9,10].
Merdes et al. [11] have reported total area efficiencies reaching up to 12.9% for
CdS/Cu(In,Ga)S 2 solar cells. They have confirmed the highest externally efficiencies for such
cell, and mentioned the absorbers were prepared from sputtered metals subsequently sulfurized
using rapid thermal processing in sulfur vapor. Also, they have presented the structural,
compositional and electrical properties of these cells and discussed the correlation between the
Ga distribution profile and solar cell properties. Otherside, Kim et al. [12] have fabricated
CdTe/CdS heterostructure on fluorine tin oxide glass to produce thin-film photovoltaic devices.
CdCl 2 layer was deposited onto CdTe absorber layer and the subsequent annealing of the stack
2
was performed in He atmosphere. They have investigated the influence of CdCl 2 -activation step
on the interfaces by monitoring the phase transition of CdCl 2 -heat-treated CdTe specimens
during temperature ramp annealing via high-temperature X-ray diffraction. While, the feasibility
of measuring contact wetting angles to characterize processing induced changes to thin film
semiconductors of CdTe/CdS solar cells is evaluated by Angelo et al. [13]. They have elaborated
the changes in surface energies resulting from processing that are correlated to changes in
surface chemistry and structure as detected by glancing incidence X-ray diffraction (GIXRD), Xray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). They have exhibited
indium tin oxide (ITO) and CdS films increased polar surface energy corresponding to enhanced
crystallization of surfaces resulting from processing and increasing CdS growth temperature.
Theoretically, Al-Douri et al. [14] have employed density functional theory (DFT) of the full
potential-linearized augmented plane wave (FP-LAPW) method as implemented in WIEN2K
code for energy band calculations of the indirect energy gap. The EngeleVosko generalized
gradient approximation (EV-GGA) formalism is used to optimize the corresponding potential for
energetic transition and optical properties calculations of CdS and CdTe as a function of
quantum dot diameter that is used to test the validity of specific model of quantum dot potential.
In the present work, we have used spin coating technique to prepare CdS nanostructures. The
effect of annealing temperature on structural, morphological, optical, electrical and thermal
properties of CdS nanostructures deposited on p-Si substrates was investigated. The novelty of
this work is converting thermal into electrical energy using lunar material.
3
2. Experimental
All chemicals were used as received from Sigma-Aldrich Company CdS nanostructures are
grown by sol-gel spin coating technique at room temperature. Polyethylene glycol PEG200 was
prepared by mixing 0.5 ml of PEG, 10 ml of ethanol and 0.5 ml of acetic acid under stirring for 1
h. 0.1 mol/L of thiourea and 0.1 mol/L of cadmium nitrate as a source of S and Cd, respectively
and 15 ml of ethanol accompanying at 60 oC. Prepared solution was slowly added to PEG200
with vigorous stirring for 6 hr until homogeneous solution was obtained. As the reaction was
started, the reaction system is gradually changed from transparent to light yellow. The prepared
solution was stored at room temperature for at least 24 hr.
CdS nanostructures have been grown on 12x12 mm2 commercial p-Si wafers with (100)
orientation as substrates. The wafers have single side polished. The substrates were cleaned by
acetone and rinsing with distilled water. After that the prepared solution was spin coated on p-Si
substrates at spinning speed 3000 rpm for 30 sec. The precipitate collected from centrifugation
was dried on hot plate at 120 ºC and annealed using muffle furnace at 200, 400, 500 and 600 oC
for 1 hr. Finally, the evaporated silver (Ag) on the substrate surface as electrode; and the
substrates were annealed by nitrogen gas flow at 450 oC for 30 minutes. The dried and annealing
temperature for CdS nanostructures are used to be analyzed and characterized by X-ray
diffraction pattern (XRD) (JEOL-JSM-6460 LA analytical), (Philips PW 1710 X-ray
diffractometer), Atomic Force Microscopy (AFM), (SII Sciko Instrument INC, SPI 3800N Probe
station scan area 2000 nm scan speed 2 Hz), Scanning Electronic Microscopy (SEM), (JEOLJSM-6460 LA). Thermogravimetric analysis (TGA) and Differential thermal analysis (DTA),
(PerkinElmer Pyris Diamond Instrument SII TG/DTA) with heating rate of 10 oC/min in air
4
atmosphere from 10 to 900 oC, using sample weight of about 51.104 mg, Ultra Violet (UV-vis)
spectroscopy, (Perkin Elmer Lambda 950) and I-V characterization, (Kiethly 2400 source meter).
For the electrical properties, Ag electrode was deposited on p-Si using shadow mask and thermal
evaporator (Auto 306) with pressure 5.25 x 105 Torr.
For solar cells applications, Schottky contact was deposited onto CdS nanostructures and it
consists of four fingers at each electrode to record the I-V relationships at different bias voltages
from 5 to -5 V. The tested fixture was placed with wires connected from the probes to Keithley
device for measuring the current-voltage (I-V) characteristics.
3. Results and discussion
3.1 Analysis and characterization
The nanostructured CdS deposited on p-Si at different annealing temperatures; 200, 400, 500 and
600 oC have been investigated by XRD as shown in Fig. 1. XRD pattern has provided
information about the crystalline phase of nanoparticles as well as the crystallite size. Table 1
shows the structural properties formed at different annealing temperatures. The CdS
nanostructure exhibits at 200 oC ten obvious peaks (Fig. 1a). These peaks are located at 2 theta =
21.6o, 23.2o, 26.60o, 29.52o, 31.60o, 36.18o, 39.51o, 43.26o, 47.63o and 48.67o were attributed to
CdS (114), CdS (112), CdS (202), CdS (203), Si (200), CdS (200), CdS (114), CdS (300), CdS
(212) and CdS (202), respectively. While CdS nanostructure deposited at 400 oC exhibits eleven
peaks that are located at 2 theta = 15.22o, 21.46o, 22,92o, 26.56o, 29.37o, 33.12o, 36.03o, 39.37o,
43.32o, 47.49o and 48.53o were attributed to CdS (004), CdS (112), CdS (202), CdS (203), Si
(200), Si (200), CdS (114), CdS (300), CdS (212), CdS (202) and Si (200). Otherside, CdS
5
nanostructure deposited at 500 oC exhibits four peaks that are located at 2 theta = 26.67o, 29.58o,
33.33o and 56.65o were attributed to CdS (202), CdS (203) Si (200) and CdS (202), and CdS
nanostructure deposited at 600 oC exhibits eleven peaks that are located at 2 theta = 21.37o,
23.03o, 26.57o, 29.48o, 31.36o, 36.15o, 39.48o, 43.23o, 47.60o, 48.64o, 57.59o were attributed to
CdS (004), CdS (112), CdS (202), CdS (203), Si (200), Si (200), CdS (114), CdS (300), CdS
(212), CdS (202) and Si (200).
All the mentioned peaks are exactly matched with the hexagonal (wurtzite) structure
corresponding to standard (JCPDS Data Card no. 02-0563) with lattice constants a = b = 4.142
Å, c = 6.724 Å. It is resulted that there is a hexagonal phase. The lattice constants a and c of CdS
wurtzite structure can be calculated [15]:
(1)
c=
(2)
The calculated values for a and c of (203) plane at 200, 400, 500 and 600 oC are given in Table
1. The CdS nanostructures consist of crystallites with mixed c-axis orientations, parallel and
perpendicular to Si surface.
Crystallite size (D) was calculated using Scherrer’s formula [15]:
D=
(3)
where k is a constant equals 0.91, λ is X-ray wavelength of Cu-kα (λ = 1.54 Å), θ is the angle
between the incident beam and the reflection lattice planes and β is the full width at half maxima
(FWHM) of the diffraction peak in radian. It is noticed that particle size increases as temperature
increases due to agglomerating particles to become large. The operating is at voltage = 40.0 kV,
6
current = 30.0 mA, scan range =10.000 to 50.000, scan speed = 4.000 deg/min and present time
= 0.30 sec. The dislocation and strain for (203) plane is given in Table 1 using [15]:
δ=
(4)
ε=
(5)
The interplaner distance (d) is calculated using Bragg’s formula [15]:
d=
(6)
where n is a constant equals 1. The number of crystallites per unit area (N) is calculated by the
following relation [15]:
N=
(7)
where t is the thickness as indicated in Table 1.
The bulk modulus is a reflectance of the materials stiffness that it is important in different
industries. Many authors [16-21] have made various efforts to explore thermodynamic properties
of solids. In these studies, authors have examined the thermodynamic properties such as the
inter-atomic separation and the bulk modulus of solids with different approximations and best-fit
relations [18-21]. It has become possible to compute with great accuracy an important number of
structural and electronic properties of solids. The ab initio calculations are complex and require
significant effort. Therefore, more empirical approaches have been developed [22-23] to
compute properties of materials. In many cases, the empirical methods offer the advantage of
applicability to a broad class of materials and to illustrate trends. In many applications, these
empirical approaches do not give highly accurate results for each specific material, but are still
very useful.
7
Cohen [23] has established an empirical formula to calculate the bulk modulus (B 0 ); based on the
nearest-neighbor distance. His result is in agreement with experimental values. Lam et al. [24]
have derived an analytical expression for the bulk modulus from the total energy. This
expression is different in structure from the empirical formula but gives similar numerical
results. Also, they have obtained an analytical expression for the pressure derivative B 0 of the
bulk modulus. Our group [25] has used a concept based on the lattice constant to establish an
empirical formula for the calculation of the bulk modulus. The calculated results are in
agreement with experimental data and other calculations. Consideration of hypothetical structure
and simulation of experimental conditions are required to make practical use of this formula. The
aim is to see how a qualitative concept, such as the bulk modulus, can be related to the lattice
constant. It was argued that the dominant effect is the degree of covalence characterized by
Phillips’ homopolar gap E h [22], and one reason for presenting these data in this work is that the
validity of our calculations that is not restricted in computed space. We thus believe that the data
will prove valuable for future work in this field. An important reason for studying B o is the
observation of clear differences between the lattice constants of different CdS nanostructures.
While the basis of our model is the lattice constant as seen in Table 1. Fitting of these data gives
the following empirical formula [25]:
(8)
where a is the lattice constant (in Å) and λ is an empirical parameter equals 2. In Table 1, the
calculated bulk modulus value is compared with experimental and theoretical [23-25] results. We
may result that the calculated bulk moduli are in accordance with other results [23-25] and
exhibit the same trends as those found for the values derived from the experimental [24] value as
seen in Table 1.
8
To characterize the surface topography of CdS nanostructures, Fig. 2 shows the AFM images at
various annealing temperatures 200, 400, 500 and 600 oC. The 2-D and 3-D images show the
CdS nanostructures. The surface roughness is related to the substrates type, annealing
temperatures, spin coating speed, addition atoms mobility and diffusion. It is obvious that the
surface roughness of CdS nanostructures increases as the temperature increases. The thickness
was measured and observed that the thickness values is found to be 15, 15, 40 and 80 nm for
200, 400, 500 and 600 oC, respectively as given in Table 1. It is indicated that the thickness
increases as the temperature increases. A similar trend was observed with the obtained results of
AFM images.
The surface morphology is very helpful to study and investigate the CdS nanostructures surface
using SEM under temperature effect. The SEM images are shown in Fig. 3. Formerly, particles
of structures are agglomerated together to become large, all CdS nanostructures are dense and
have strong adherence to the substrates which improved due to increasing temperature.
3.2 Optical properties
The optical properties of absorption and reflection measurements are performed at room
temperature using UV-vis spectroscopy in the range of 200-800 nm to get information at
different annealing temperatures 200, 400, 500 and 600 oC. The absorption and reflectance
spectra of CdS nanostructures are shown in Fig. 4. It is observed that the wavelength ranges are
found at 280-380 and 320-380 nm for absorption and reflectance, respectively. The optical
absorption is the highest at 0.13% at 400 oC and decreased to 0.110%, 0.109% and 0.104% at
200, 500 and 600 oC, respectively. While, the reflectance is the lowest values at 5.363% at 400
9
o
C and increased to 5.48, 5.67 and 8.24% at 600, 500 and 200 oC, respectively. All of these are
attributed to quantum confinement effect. Among CdS nanostructures, we can see the best
crystallinity, the highest absorption and lowest reflectance are found for CdS nanostructures
deposited on p-Si at 400 oC. The energy band gap has been calculated using absorption spectra as
shown in Fig. 4. To measure the absorption coefficient from absorption spectra [26],
α = 2.3026 (A/t),
(9)
where A is the absorption spectra value and t is the thickness of the Tauc relation [26,27];
αhv = A (hv-E g )n
(10)
where hv is the photon energy, α is the absorption coefficient, E g is the energy band gap and A is
a constant equals 1 and n = 1/2 for direct band gap. To measure the energy band gap from
absorption spectra, (αhv)2 versus hv is plotted (Fig. 5). The extrapolation of straight line to (αhv)2
= 0 gives the energy band gap value. The measured energy band gaps of CdS nanostructures are
given in Table 2. Additionally, Fig. 6 shows the extinction coefficient (K) obtained for CdS
nanostructures as given in Table 2 using the following [26,28]:
K = α λ /4 π
(11)
The refractive index n is an important physical parameter related to microscopic atomic
interactions. Theoretically, the two different approaches in viewing this subject are the refractive
index related to density, and the local polarizability of these entities [29]. On the other hand, the
crystalline structure represented by a delocalized picture, n will be closely related to the energy
band structure of the material, complicated quantum mechanical analysis requirements and the
obtained results. Many attempts have been made to relate the refractive index n and the energy
gap E g through simple relationships [30-35]. Here, the various relationships between n and E g
10
will be reviewed. Ravindra et al. [35] had suggested different relationships between the band gap
and the high frequency refractive index and presented a linear form of n as a function of E g [35]:
n = α + β Eg,
(12)
where α = 4.048 and β = 0.62 eV-1.
To be inspired by simple physics of light refraction and dispersion, Herve and Vandamme [36]
have proposed an empirical relation as [36]:
2
n
=


A

1+ 
 E g +B 


(13)
where A = 13.6 eV and B = 3.4 eV. Ghosh et al. [37] have took a different approach to the
problem by considering the band structural and quantum-dielectric formulations of Penn [38] and
Van Vechten [39]. Introducing A as the contribution from the valence electrons and B as a
constant additive to the lowest band gap E g , the expression for the high-frequency refractive
index are written as [37]:
n2 – 1 = A / (E g + B)2 ,
(14)
where A = 25E g + 212, B = 0.21E g + 4.25 and (E g + B) refers to an appropriate average energy
gap. Thus, these three models of variation n with energy gap have been calculated. Also, the
calculated values of the optical dielectric constant (ε ∞ ) were obtained using the relation ε ∞ = n2
[40]. Our calculated refractive index values are in good agreement with experimental value as
given in Table 2. This is giving an appropriate model of Ghosh et al. for solar cells applications.
11
3.3 Electrical and thermal properties
To fabricate Schottky barrier junction of CdS nanostructures with different annealing
temperatures and silver (Ag) metal, the desired metal was separately thermally evaporated over
the CdS nanostructures which were already deposited on conducting p-Si substrate. I-V
characteristics of the junctions are formed under forward and reverse bias as shown in Fig. 7 for
dark, ambient and illumination conditions at 200, 400, 500 and 600 oC. Figure 7 shows the p-n
junction I-V characteristics grown on p-Si. The barrier height, Ф b is calculated from [41,42]:
kT  AA**T 2 
φb =
ln 

q  I0 
(15)
where q is electron charge equals -1.602176565(35) x 10-19 eV, T is the temperature and A is the
contact area, A** is the effective Richardson’s constant equals 22.8 A.cm2.K2 using [42]:
(16)
where k is Boltzmann constant equals 8.62 x 10-5 eV/K. At room temperature kT=25.9 x 10-3 eV.
The saturation current can be written as [42]:
Is
=A
A**
T2
exp
(17)
where A is the diode area (1mm2). For n-CdS, the barrier height was found to be 0.76 eV for CdS
nanostructures deposited on p-Si at 400 oC in ambient condition. To find saturation current
density, forward current density, ln I F versus forward voltage, V F , the barrier heights can be
obtained using Eq. (15) as given in Table 3. The ideality factor (m) is calculated via [42]:
(18)
12
It was observed that the significant improvement of ideality factor (m) which is found to be 3.05
corresponds to barrier height equals 0.76 at 400 oC at ambient condition for CdS nanostructures
deposited on p-Si substrates.
The thermo gravimetric analysis (TGA) was carried out to understand the thermal stability of the
CdS nanostructures as shown in Fig. 8a, the small weight loss of 8 wt% is between 110 and 300
o
C. This indicates that CdS nanostructures are very stable. Figure 4b shows the differential
thermal analysis (DTA) to be found very strong endothermic peak near 100 oC corresponds to
the first period. The second period occurs between 200 and 380 oC, which attributes to the
burning of citrate acid and decomposition of metal nitric. Thereafter, the weight remains
constant, which indicates that the decomposition and combustion of materials components in the
precursor have completed below 500 oC [43]. The DTA (Fig. 8b) from 230 to 500 oC could be
interpreted into two physical meanings: (1) decreasing of DTA curve from 230 oC corresponding
to initial decomposition of the precursor and the formation of CdS nanostructures. (2)
Exothermic peak at 370 oC corresponding to crystallization of cadmium nitrate to thiourea could
be obtained over 400 oC, which is confirmed with XRD results (Fig. 1b).
4. Conclusion
CdS nanostructures have grown on p-Si (100) substrates by sol-gel method. The XRD suggests
that the CdS nanostructures have wurtzite structure. The lattice constants for (203) plane are a =
1.0477 Å and c = 3.1252 Å. The surface morphology proves that the surface roughness increases
as temperature increases that reflect increasing of stiffness. In addition, the particles become
larger and denser as temperature increases. The calculated bulk modulus gave good agreement
13
with experimental and theoretical values. The preferred crystallinity, highest absorption and
lowest reflectance of CdS nanostructures are found at 400 oC. The measured E g , calculated
refractive index and optical dielectric constant are in good agreement with other results. The
interface junction of CdS nanostructures is characterized to be well at 400 oC, and confirmed
with high barrier height and low ideality factor at 400 oC also. The thermal analysis has verified
the stability and crystallization of CdS nanostructures.
Acknowledgements
Y. A. would like to acknowlegde University Malaysia Perlis for grant No. 9007-00111 and
TWAS-Italy for the full support of his visit to JUST-Jordan under TWAS-UNESCO
Associateship.
14
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18
Table 1 Nanostructured CdS particle size, full width half maxima, miller indices, interplaner
distance, lattice constants, strain, dislocation density, bulk modulus, number of crystallites and
thickness deposited on p-Si substrates at different annealing temperatures.
T
(oC)
2θ
Particle
size*
(D)
(nm)
Full
Width
Half
Maxima*
Miller
indices
* (hkl)
interplaner
distance*
(d) (Å)
Lattice
constants
* (a & c)
(Å)
Strain*
(ε)
(10-3)
Bulk
Modulus$
(B o )
(GPa)
0.2005
Dislocati
on
density*
(δ) (1014
lines/m2)
0.3276
200
29.52
1.7471
0.9218
203
1.5626
400
29.44
1.9606
0.8208
203
1.5661
a=1.0417
a=4.1a
a=4.1b
c=3.1252
c=6.7a
c=6.7b
a=1.044
c=3.1323
500
29.56
1.2899
1.249
203
1.5607
600
29.46
1.6104
0.9994
203
1.5655
Number of
crystallites*
particles/are
a
(N) (×1015)
2.8127
Thickness*
(nm)
0.1786
0.2601
51.47
1.9903
15
a=1.0404
c=3.1214
0.2716
0.6010
52.11
18.6376
40
a=1.0437
c=3.1311
0.2175
0.3855
51.54
19.1552
80
51.89
69c
62d
66.6e
15
*Measured value, $Calculated value, aRef. [15] Exp., bRef. [44] Exp.; cRef. [23] Theo., dRef. [24] Exp., eRef. [25]
Theo.
19
Table 2 Energy gap (E g ), refractive index (n), optical dielectric constant (ε ∞ ) and extinction
coefficient (K) of CdS nanostructures using Ravindra et al. [35], Herve and Vandamme [36] and
Ghosh et al. [37] models.
T
(oC)
E g * (eV)
Reflective index (n)
200
3.10
2.48a
2.348b
2.43c
2.359d
3.90e
5.97f
2.3189g
2.3499h
2.52i
400
500
Optical dielectric
constant (ε ∞ )
35.6409f
5.7073g
5.5520h
3.11
5.9762f
2.3160g
2.3478h
35.7149f
5.3638g
5.5121h
3.16
6.0072f
2.3017g
2.3375h
36.0864f
5.2978g
5.4639h
Extinction
coefficient (K)
0.0174
0.2686
4.8987 x10-4
33.0257f
5.7468f
g
2.74
5.9085g
3.7094 x10-4
600
2.4302
h
h
5.9005
2.4291
*Measured value, aRef. [1] Exp.; bRef. [15] Exp.; cRef. [27] Exp.; dRef. [14] Theo.; eRef. [28] Exp.; fRef. [35].; gRef.
[36].; hRef.; [37]; iRef. [44] Exp.
20
Table 3 Saturation current (I o ), ideality factor (m) and barrier heights (φb ) of CdS nanostructures
deposited on p-Si substrates at different annealing temperatures.
200 (oC)
Conditions\Temperatures
Is*
m*
400 (oC)
φb*
x10-7
Dark
9.1189
Is*
m*
500 (oC)
φb*
Is*
x10-7
4.8
0.73
0.44433
m*
600 (oC)
φb*
x10-7
5.39
0.69
I s * x10-
m*
φb*
7
0.49787
4.7
0.69
9.6444
6.0
0.73
a
0.73
Ambient
6.7522
4.8
0.74
3.6602
3.05
0.76
0.5928
4.8
0.68
0.54564
5.6
0.69
Illumination
4.6235
4.2
0.75
0.33836
6.07
0.70
0.80241
7.7
0.73
0.28050
6.3
0.70
*Measured value, aRef. [41] Exp.
21
Figure captions
Fig. 1 XRD patterns of CdS nanostructures deposited on p-Si substrates at a) 200, b) 400, c) 500
and d) 600 oC
Fig. 2 AFM images of CdS nanostructures deposited on p-Si substrates at 200, 400, 500 and d)
600 oC
Fig. 3 SEM images of CdS nanostructures deposited on p-Si substrates at different annealing
temperatures 200, 400, 500 and d) 600 oC
Fig. 4 a) Absorption (%) and b) Reflection (%) spectra of CdS nanostructures deposited on p-Si
substrates at different annealing temperatures 200, 400, 500 and 600 oC
Fig. 5 (αhv)2 versus hv dependency of CdS nanostructures deposited on p-Si substrates at a) 200,
b) 400, c) 500 and d) 600 oC
Fig. 6 Extinction coefficient spectra of CdS nanostructures deposited on p-Si substrates at a) 200,
b) 400, c) 500 and d) 600 oC
Fig. 7 I-V characteristics of CdS nanostructures deposited on p-Si substrates annealed at 200,
400, 500 and 600 oC on a) dark, b) ambient and c) illumination conditions.
Fig. 8 Thermo gravimetric analysis TGA (a) and differential thermal analysis DTA (b) for CdS
nanostructures
22
3500
(200 oC)
3000
(CdS)
(203)
Intensity (a. u.)
2500
2000
1500
1000
(CdS)
(202)
(CdS) (CdS)
(114) (112)
500
0
10
(CdS)
(212)
(CdS)
(202)
15
20
(Si)
(200)
25
30
(CdS) (CdS) (CdS)
(114) (300)
(200)
35
40
45
50
55
60
2 theta (degrees)
a)
3500
(400 oC)
3000
(CdS)
(203)
Intensity (a. u.)
2500
2000
1500
1000
(CdS)
(202)
(CdS)
500
0
10
(CdS)
(004) (112)
15
20
25
(Si)
(200)
30
(CdS)
(212) (CdS)
(CdS) (CdS)
(202)
(Si)
(114) (300)
(200)
35
40
45
50
(Si)
(200)
55
60
2 theta (degrees)
b)
23
3500
(500 oC)
3000
Intensity (a. u.)
2500
2000
1500
1000
(CdS)
(203)
500
(Si)
(200)
(CdS)
(202)
(CdS)
(202)
0
10
15
20
25
30
35
40
45
50
55
60
2 theta (degrees)
c)
3500
(600 oC)
3000
(CdS)
(203)
Intensity (a. u.)
2500
2000
1500
1000
(CdS)
(202)
(CdS)
500
0
10
(CdS)
(004) (112)
15
20
25
(Si)
(200)
30
(CdS)
(212) (CdS)
(CdS) (CdS)
(202)
(Si)
(114) (300)
(200)
35
40
45
50
(Si)
(200)
55
60
2 theta (degrees)
d)
Fig. 1
24
200 oC
a)
a) 400 OC
b)
400 oC
25
c) 500 oC
d) 600 oC
Fig. 2
26
a) 200oC
b) 400 oC
b) 500 oC
d) 600 oC
Fig. 3
27
0.55
0.50
0.45
0.40
0.35
(200 oC)
(400 oC)
(500 oC)
(600 oC)
A%
0.30
0.25
0.20
0.15
0.10
0.05
0.00
200
300
400
500
600
700
800
Wavelength (nm)
a)
14
13
(200 oC)
(400 oC)
(500 oC)
(600 oC)
12
11
Reflection (%)
10
9
8
7
6
5
4
3
2
1
0
200
300
400
500
600
700
800
Wavelength (nm)
b)
Fig. 4
28
0.018
(200 oC)
0.016
0.014
0.014
0.012
0.010
0.010
(αhv)2
(αhv)2
0.012
0.008
0.006
0.008
0.006
0.004
0.004
0.002
0.000
(400 oC)
0.016
0.002
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
0.000
hv (V)
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
hv (V)
a)
b)
0.0006
(600 oC)
o
0.016
(500 C)
0.0005
0.012
0.0004
0.010
0.0003
(αhv)2
(αhv)2
0.014
0.008
0.0002
0.006
0.0001
0.004
0.0000
0.002
0.000
1.5
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
2.0
2.5
3.0
3.5
4.0
4.5
5.0
hv (v)
hv (V)
c)
d)
Fig. 5
29
5.5
6.0
6.5
0.08
6.0
0.07
(400 oC)
5.5
o
(200 C)
5.0
0.06
4.5
4.0
0.05
(K)
(K)
3.5
0.04
3.0
2.5
0.03
2.0
0.02
1.5
1.0
0.01
0.5
0.0
200 250 300 350 400 450 500 550 600 650 700 750 800
0.00
200 250 300 350 400 450 500 550 600 650 700 750 800
Wavelength (nm)
Wavelength (nm)
a)
b)
0.011
0.005
0.010
(500 oC)
0.009
0.008
(600 oC)
0.004
0.007
0.003
(K)
(K)
0.006
0.005
0.004
0.002
0.003
0.002
0.001
0.001
0.000
200
300
400
500
600
700
800
Wavelength (nm)
0.000
200
300
400
500
600
700
Wavelength (nm)
c)
Fig. 6
30
800
-6
-5
-4
-3
Current (A)
(200 oC)
(dark)
(ambient)
(illumination)
-2
30.0µ
20.0µ
10.0µ
0.0
-1
0
-10.0µ
1
2
3
4
5
6
Voltage (V)
-20.0µ
-30.0µ
-40.0µ
-50.0µ
-60.0µ
-70.0µ
-80.0µ
a)
(400 oC)
(dark)
(ambient)
(illumination)
250.0µ
Current (A)
200.0µ
150.0µ
100.0µ
50.0µ
-6
-5
-4
-3
-2
-1
0.0
0
-50.0µ
1
2
3
4
5
6
Voltage (V)
b)
31
600.0µ
Current (A)
(500 oC)
(dark)
(ambient)
(illumination)
400.0µ
200.0µ
0.0
-6
-5
-4
-3
-2
-1
0
1
2
3
Voltage (V)
4
5
6
-200.0µ
-400.0µ
-600.0µ
-800.0µ
c)
Current (A)
(600 oC)
(dark)
(ambient)
(illumination)
250.0µ
200.0µ
150.0µ
100.0µ
50.0µ
0.0
-6
-5
-4
-3
-2
-1
0
1
-50.0µ
d)
2
3
4
5
6
Voltage (V)
Fig. 7
32
100
(TGA)
90
80
Weight loss (%)
70
60
50
40
30
20
10
0
100
200
300
400
500
600
700
600
700
800
o
Temperature C
DT (mW)
a)
40.0
20.0
0.0
-20.0
-40.0
-60.0
-80.0
-100.0
-120.0
-140.0
-160.0
-180.0
-200.0
-220.0
-240.0
-260.0
-280.0
-300.0
(DTA)
(Endo)
100
200
300
400
500
800
o
Temperature C)
b)
Fig. 8
33