structural and optical properties of nanocrystalline cds films … · 2017. 10. 16. · cds films...
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WSN 87 (2017) 175-190 EISSN 2392-2192
Structural and optical properties of nanocrystalline CdS films prepared by spray pyrolysis
Radhyah M. S. Aljarrah, Adnan H. Aljobory*
Department of Physics, Faculty of Science, University of Kufa, Box 21 Kufa, Najaf, Iraq
*E-mail address: [email protected]
ABSTRACT
Cadmium Sulphide (CdS) thin films are produced using spray pyrolysis deposition technique.
Films are annealed in air at 400, 500, and 600 K of 1 h. It characterized by X-Ray Diffraction (XRD),
Atomic Force Microscope (AFM) and optical properties of CdS. XRD shows that these films are
polycrystalline in nature with cubic and hexagonal crystalline structure. The crystallite size,
microstrain, and dislocation density were measured. AFM shows that the total substrate surface is
finely covered with uniformly distributed spherical shaped grains. Optical transmittance was shown
that direct transition with band gap energy was decreased between 2.44 to 2.27 eV with annealing.
Keywords: Cadmium Sulphide, CdS films, spray pyrolysis
1. INTRODUCTION
In recent years, semiconductor materials have been growing interest in II-VI for
potential applications in optoelectronic and photovoltaic industries. This compound is a strong
candidate as a window layer in a solar cell because it has a wide direct gap. CdS is a
promising material widely employed in the technology of optoelectronic devices (optical
filters, photodetectors, gas sensors and above all for solar cells) [1-5]
. The basic requirements of
CdS in applications are high optical transparency, wide direct band gap between 2.28 eV and
2.45 eV, high transmittance, stability and low cost, low dark electrical resistivity, high
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photoconductivity and better crystallinity [6-11]
. CdS is a chalcogenide n-type semiconductor.
It is known as CdS thin films may exist in a cubic or in hexagonal phase or as a mixture of
both phases depending on factors including deposition technique [9,12]
. CdS are fabricated
using various techniques (sputtering, chemical vapor deposition, spray pyrolysis, RF
sputtering, pulsed-laser deposition, and chemical bath deposition) [13-17]
. Spray pyrolysis was
suitable for manufacturing CdS is known as the simplest and economical for the large area
productions in obtaining thin films [6-45]
.
2. MATERIALS AND METHODS
Spray pyrolysis method is nanostructured thin-film preparation method with excellent
features such as no need sophisticated equipment and quality targets or substrates. Film
thickness and stoichiometry are easy to control and the resulting films are well compacted.
CdS produced using spraying the aqueous solution of 0.1 M of cadmium acetate (CH3COO)2· Cd·H2O ≡ 266.52 g ml
-1 onto the microscope slide (1×25×25 mm
3) at 350 °C. 50 ml thiourea
was used. Prior to deposition, the substrates were cleaned using cleaner solution, distilled
water, and alcohol using an ultrasonic bath. The spray rate was adjusted to one sprinkling in a
minute, the sprinkling time about of 11 s. The normalized distance between the spray nozzle
and the substrate is 30 cm. The temperature of the substrate was controlled by an Iron-
Constantan thermocouple.
The thickness of the films (t) was determined using weighing-method.
t =
where = Mass difference of slide, A = aria = 2.5×2.5 cm2
and ρ = CdS density = 4.824 g
cm-3
. X-ray with Cu Kα radiation λ = 0.15406 nm was used to investigate the film structure,
the grain size used to CdS was determined by average grain size in the c-axis orientation
estimated using the Debye-Scherrer relation [18]
:
D = 0.9λ / B cos θ
where D = Mean particle size, θ = Bragg diffraction angle and B = Full width at half
maximum (FWHM) of the diffraction peak. The optical absorption test was recorded by using
a Shimadzu UV1650 PC spectrophotometer. The absorbance and the reflectance measured for
the scanning of the electromagnetic spectrum (300-1100 nm). to The energy gap and the
optical constants such as refractive index (n), extinction coefficient (k), real, and imaginary
parts of dielectric constant (ε1and ε2) were measured. The optical energy gap (Eg) was
determined using Tauc Eq: [19,20]
(αhυ) =B (hυ-Eg )r
where B = Tauc constant and hυ = Photon energy, α = Absorption coefficient. R = 1/2,
indicate the direct transition. The optical energy gap of the film obtained by plotting (αhν)2 ~
hν and determination the straight line, and make it extend to meet the energy axis. The optical
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properties of a material are utilized to determine its optical. The extinction coefficient
describes the energy absorption of the electromagnetic wave during the process of
transmission of a wave through a material. The intensity of light (I) after crossing thickness of
material x in an isotropic medium can be estimated by [21,22]
.
I = I0 exp (–αx)
where I0 = Initial intensity. The extinction coefficient k is equal to [21,22]
:
k = αλ/4π
where λ = wavelength. The normal-incidence reflectivity R can also be given by [21,22]
:
Then the refraction index value can be calculated from the formula [21,22]
:
The real and imaginary part of dielectric constant determined:
εr = n2 − k2, εi = 2nk [23]
3. RESULTS AND DISCUSSION
Fig. 1 shows X-ray diffraction patterns of CdS. It was seen that as-deposited CdS has
the mixtures of cubic and hexagonal structures [24]
. It is difficult to distinguish between cubic
(1 1 1) and HCP (0 0 2), cubic (2 2 0) and HCP (1 1 0). After the high-temperature process,
new peaks of hexagonal structure appealed at XRD patterns. This phenomenon was thought to
be the phase change of CdS using heat treatment. It was believed that the mixtures of
hexagonal and cubic phase were changed to hexagonal phase by the heat treatment.
The hexagonal phase of CdS is thermodynamically stable than the cubic phase of CdS [25,26]
. The XRD patterns revealed that highest peaks correspond to the H (0 0 2) C (1 1 1),
(2 2 0), and (3 1 1) phase. It shows a small hexagonal (1 0 0), H (1 1 0), and C (2 2 0) peaks.
The different peaks in the diffractogram were indexed and the corresponding values of
interplanar spacing “d” were calculated and compared with standard values of JCPDS data [27]
.
The annealing resulted in good quality films with improved crystallinity as evidenced by
intense diffraction peaks indicated (Fig. 1). The position of HCP (0 0 2) peak was changed
from 27.21 to 26.81. It may be due to the relaxation of tensile stress. The tensile stress of the
as-deposited CdS developed by lattice mismatch of the hexagonal structure of CdS. Lattice
parameters of hexagonal structure of CdS reported being a = 4.1450 Å and c = 6.721 Å.
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The FWHMs of HCP (2 0 0) peaks and crystalline size (D). It was calculated by
Scherrer’s formula (Table 1). The FWHM and crystalline size (D) of as-deposited CdS are
0.4278 Å and 91.3 nm, respectively. After the high-temperature process, the FWHM and
crystalline size (D) increase, which indicates the grain growth of CdS. These results are
agreement with Mariappan et al, 2012 [28-45]
.
Fig 1. XRD for CdS at different annealing temperature (a-R. T, b-400K, c-500K,
and d-600K).
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Table 1. The peaks observed in all films and the standard peaks from JSPDS [27].
Ta (K) 2θ
(Deg.)
FWHM
(Deg.)
dhkl
Exp.(Å)
D
(nm) Phase hkl
dhkl
Std.(Å) Card No.
R.T
30.6731 0.4278 2.9124 19.3 Cub.
CdS (200) 2.9090
96-900-
8840
43.9820 0.5703 2.0571 15.0
Hex.
CdS (110) 2.0674
96-900-
8863
Cub.
CdS (220) 2.0570
96-900-
8840
400
24.4563 0.5348 3.6368 15.2 Hex.
CdS (100) 3.5808
96-900-
8863
26.7500 0.3922 3.3300 20.8
Hex.
CdS (002) 3.3745
96-900-
8863
Cub.
CdS (111) 3.3590
96-900-
8840
30.5000 0.4520 2.9285 18.2 Cub.
CdS (200) 2.9090
96-900-
8840
44.0420 0.4991 2.0544 17.2
Hex.
CdS (110) 2.0674
96-900-
8863
Cub.
CdS (220) 2.0570
96-900-
8840
48.1000 0.6300 1.8902 13.8 Hex.
CdS (103) 1.9049
96-900-
8863
52.4300 0.4278 1.7438 20.7 Cub.
CdS (311) 1.7542
96-900-
8840
500
24.8841 0.5348 3.5753 15.2 Hex.
CdS (100) 3.5808
96-900-
8863
26.5241 0.5348 3.3578 15.3
Hex.
CdS (002) 3.3745
96-900-
8863
Cub.
CdS (111) 3.3590
96-900-
8840
28.1996 0.5348 3.1620 15.3 Hex.
CdS (101) 3.1632
96-900-
8863
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30.4210 0.3922 2.9360 21.0 Cub.
CdS (200) 2.9090
96-900-
8840
44.0210 0.3921 2.0554 21.9
Hex.
CdS (110) 2.0674
96-900-
8863
Cub.
CdS (220) 2.0570
96-900-
8840
47.8000 0.6540 1.9013 13.3 Hex.
CdS (103) 1.9049
96-900-
8863
52.2100 0.4278 1.7506 20.7 Cub.
CdS (311) 1.7542
96-900-
8840
600
24.6702 0.3921 3.6058 20.7 Hex.
CdS (100) 3.5808
96-900-
8863
26.5600 0.3600 3.3534 22.7
Hex.
CdS (002) 3.3745
96-900-
8863
Cub.
CdS (111) 3.3590
96-900-
8840
28.2709 0.4950 3.1542 16.6 Hex.
CdS (101) 3.1632
96-900-
8863
30.5320 0.4230 2.9256 19.5 Cub.
CdS (200) 2.9090
96-900-
8840
43.9200 0.3460 2.0599 24.8
Hex.
CdS (110) 2.0674
96-900-
8863
Cub.
CdS (220) 2.0570
96-900-
8840
48.0214 0.4500 1.8931 19.3 Hex.
CdS (103) 1.9049
96-900-
8863
52.5300 0.4500 1.7407 19.7 Cub.
CdS (311) 1.7542
96-900-
8840
The Average Surface Roughness (RMS) values and the average surface grain size are
shown in Table 2. These results are agreement with Mazón-Montijo, et al, 2010 [29]
. The grain
size of the film was determined using the AFM, the average grain size increases as increasing
Ta because the increasing Ta can cause recrystallization in grains, leading to a reorientation of
the film and a significant increase in average grain size. Consequently, the surface roughness
increases as increase the grain size.
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Figs. (2a, b, c, and d) shows typical surface using AFM of CdS at 350oC and annealed at
temperatures (400, 500 and 600) K of 1h. All the images show a homogeneous distribution
with columnar structure.
a.. b
c.. d
Fig. 2 AFM images: (a) deposited CdS (b) annealed at 400K, (c) 500 K and (d) 600K.
The Average Surface Roughness (RMS) values and the average surface grain size are
shown in Table 2. These results are agreement with Mazón-Montijo, et al, 2010 [29]
. The grain
size of the film was determined using the AFM, the average grain size increases as increasing
Ta because the increasing Ta can cause recrystallization in grains, leading to a reorientation of
the film and a significant increase in average grain size. Consequently, the surface roughness
increases as increase the grain size.
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Table 2. Crystallite size, roughness average, and Ta of CdS.
Ta (K) Grain size average (nm) Root mean
square (nm)
Roughness
average (nm) Peak-peak (nm)
300 32.5 1.51 1.30 11.5
400 46.7 1.91 1.93 13.4
500 61.1 2.42 2.32 15.3
600 86.2 3.11 2.71 18.6
The optical transmittance spectra of as-deposited and annealed CdS was recorded as a
function of wavelength as shown in Fig 3.
.
Fig. 3. Transmittance variation with the wavelength
The transmittance spectra of CdS exhibited a sharp fall at the fundamental absorption
edge of approx. 500 nm. Fig. 4
Gradual enhancement in absorbance was noted with an increase in annealing
temperature. The annealed samples show a slight shift in the absorbance toward higher
wavelength with the increase of annealing temperature. The band gaps of the films were
calculated using Tauc equation by plotting the relations of (αhυ)1/2
versus incident photon
energy (hυ) as shown in Fig. 5.
0
20
40
60
80
100
120
200 300 400 500 600 700 800 900
a: RT
b: Ta=400 K
c: Ta=500 K
d: Ta=600 K
abcd
λ nm
T%
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Fig. 4. Absorbance with the wavelength
Fig. 5. (hυ)
2 vers. hυ
The gap energy was obtained by extrapolating the straight line portion of the graph to
zero absorption coefficients. The intercept on the energy axis gives the value of gap energy.
The plot indicates the transition is an allowed direct type. The as-deposited sample showed
direct band gap of 2.44 eV, which was found to reduce to 2.27 eV with annealing.
0
20
40
60
80
100
200 400 600 800 1000λ nm
A%
dc
b a
a: RT
b: Ta=400 K
c: Ta=500 K
d: Ta=600 K
0.0E+00
2.0E+09
4.0E+09
6.0E+09
8.0E+09
1.0E+10
1.2E+10
1.5 2 2.5 3
d
a
b
c
a: RT
b: Ta=400 K
c: Ta=500 K
d: Ta=600 K
hυ (eV)
(αhυ
)^2 (
eV
.Cm
)^-2
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The obtained values were found to be comparable with the previous reports [25–27]
. The
sample annealed at 600 °C showed minimum band gap (2.27 eV). This deviation of band gap
from 2.44 eV to ...2 eV can be attributed to the change in crystallite size as a result of
controlled annealing [28]
. The observed improvement in the crystallites of samples can be
correlated with the optical study, reflecting a red shift in the optical band gap of the material
which is justified by increased crystallite size and crystalline quality. The Different values of
the refractive index against incident photon energy (hυ) for all samples which prepared at R.T
and annealed is shown in Fig. 6, the refractive index for all samples is about 2.18.
Fig. 6. n vers. hυ
The demeanor of the extinction coefficient k of CdS for different annealing
temperatures is shown in Fig. 7. It makes us realize easily that the extinction coefficient, in
general, increases with increasing of annealing temperature (Ta).
The variation of the εr and εi versus photon energy (hυ) for CdS films for different
annealing temperature shown in Figs. (8, 9). The behavior of εr is similar to that of the (n)
because of the smaller value of k. compared with n
. , while εi is mainly depended on the k
values.
The variation of the energy gap and the optical parameter with annealing temperature
are given in Table 3. These results are agreement with Mariappan, et.al, 2012 [28]
.
0
0.5
1
1.5
2
2.5
1 1.5 2 2.5 3 3.5 4
hυ (eV)
a: RT
b: Ta=400 K
c: Ta=500 K
d: Ta=600 K
n
dcba
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Fig. 7. k vers. hυ
Fig. 8. ɛr vers. hυ
0
0.05
0.1
0.15
0.2
0.25
1.5 1.7 1.9 2.1 2.3 2.5 2.7 2.9
hυ (eV)
kd c b aa: RT
b: Ta=400 K
c: Ta=500 K
d: Ta=600 K
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
1 1.5 2 2.5 3 3.5 4
dc
ba
hυ (eV)
ɛ r
a: RT
b: Ta=400 K
c: Ta=500 K
d: Ta=600 K
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Fig. 9. ɛi vers. hυ
Table 3. The dependence of the band gap, extinction coefficient, refractive
Index, and the real and imaginary parts of dielectric constant
Ta (K) Eg (eV) n k εr εi
300 2.44 2.18 0.17 449 0.38
400 2.35 2.18 0.15 4.48 0. 41
500 2.32 2.18 0.11 4.49 0.65
600 2.27 2.18 0.08 4.50 0.71
4. CONCLUSIONS
CdS were prepared using spray pyrolysis and deposited on the slide at 350 °C. The
corresponding characteristics of the CdS as a function of annealing temperature were
reported. X-ray results show that the structure of CdS is polycrystalline with a hexagonal
quartzite structure. The refractive index is about 2.18. The grain size increased with
increasing annealing temperature. The linear dependence of (αhν)2 to hν indicates that CdS at
all different annealing temperature are direct transition type semiconductors. It concluded that
the decreases in the optical band gap of the films with increasing annealing temperature.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
1 1.5 2 2.5 3 3.5 4hυ (eV)
ɛi
a: RT
b: Ta=400 K
c: Ta=500 K
d: Ta=600 K
dcba
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Acknowledgement
The authors acknowledge the financial support of the Kufa and Baghdad Universities, Iraq. The authors are
grateful to Dr. Basim A. Almayahi, Department of Environment, College of Science, University of Kufa
([email protected]) for assisting us throughout conducting the present research.
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( Received 28 September 2017; accepted 16 October 2017 )