International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org
Volume 3, Issue 5, May 2014
ISSN 2319 - 4847
Structural and Optical Properties of
Nanocrystalline CdS Thin Film Obtained by
Multi- Deposition Processes using CBD
Technique.
Jamal. F. Mohammad, Hamid. S. Al-jumaili and Shireen M. Abed
University of Anbar, College of education for pure Sciences, Physics Department
ABSTRACT
Nanocrystalline Cadmium sulfide (CdS) thin films have been prepared by chemical bath deposition technique on commercial
glass substrates at 70 ºC. Using cadmium chloride (CdCl2 ), thiourea (CS(NH 2)2) and ammonia solution (NH4 OH) were added to
maintain the pH value of the solution at 10. The characterization of thin films was carried out for the structural and optical
properties using X-ray diffraction (XRD) and UV-VIS spectrophotometer. XRD studies show that the preferential orientation
(111). A UV-VIS optical spectroscopy study was carried out to determine the band gap of the nanocrystalline CdS thin films and it
found to be higher as comparing with respect to the bulk value (2.4 eV).
Keyword: Nanocrystalline thin films, chemical bath deposition, cadmium sulfide.
1. INTRODUCTION
Current research on the study of nanometer-scale materials generates intense interest because it exhibits new or enhanced
size-dependent properties compared with the larger size particles of the same material [1]. The II-VI binary
semiconducting compounds, belonging to the cadmium chalcogenide family (CdS, CdSe, CdTe) are considered to be very
important due to their potential use in photoconductive devices and solar cells [2-4]. CdS has an energy band gap of 2.42
eV at room temperature [5,6]. The value of energy gap (Eg) defines the fundamental light absorption edge and thus the
range of the visible light spectrum which can be used to be converted in electricity by a solar cell [7]. As CdS has wide
band gap, it is used as window material for hetero junction solar cells to avoid the recombination of photogenerated
carriers which improves the solar cells efficiency. It has also application in light emitting diodes, photo detectors,
Sensors, address decoders, and electrically driven lasers [8]. The deposition of CdS film has been explored by various
techniques, such as molecular beam epitaxy (MBE) [9], spray pyrolysis [10], screen printing [11], spin coating [12],
electrodeposition [13], chemical bath deposition (CBD), etc[14-18]. Among these various techniques, the CBD is the
most successful method used in the production of uniform, adherent, and reproducible large-area thin films for solarrelated application[19,20]. The main goal of this work is to study the effects of multi- deposition on the structural and
optical properties of nanocrystalline CdS thin film prepared by CBD technique.
2. EXPERMENTAL DETAILS
2.1. Synthesis
The commercially available glass, Microscope Slides with the size of (1×25×75 mm) were used as substrates. Before
deposition, the substrates were degreased with de-ionized water, chromic acid solution, methanol, ultrasonically washed
with de-ionized water respectively and finally dried in air. aqueous solutions of 20 ml each of 0.1 M solution of CdCl2 as
a source of cadmium, CS(NH2)2 as a source of sulfur were used. First, 20 ml of CdCl2 solution placed in a 60 ml beaker.
The complexing agent to form a cadmium complex, which slowly released Cd2+ ions for subsequent reaction with S2−
ions, was ammonia solution. NH4OH was added drop by drop in CdCl2 solution under vigorous stirring to maintain the
pH value of the solution at 10.5 and a colorless solution was obtained. After stirring for several minutes, the solution
became clear and homogenous. Then under continuous stirring, 20 ml of CS(NH2)2 solution was introduced. Once, the
clean glass slide were vertically immersed in the solution. The beaker contain mixture solution was placed in chemical
bath whose temperature were kept at 70±2 oC during the growth. The reaction solution was no stirred during the
deposition process. Deposition time was 60 minute, during which the solution color changed to yellow and finally to
orange and precipitation occurs. After deposition, the sample were removed from the bath and washed in distilled water
and finally dried in air. Thinner film, prepared by mono-deposition, were not thick enough. Films of different thickness
were obtained by repeating the deposition procedure. The CdS films obtained were homogeneous, hard, and had very
good adhesion to the glass substrate. The variation reactions involved and their equilibrium constants at room
temperature are as follows [5].
Volume 3, Issue 5, May 2014
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International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org
Volume 3, Issue 5, May 2014
NH 3 + H 2 O → NH 4 + + OH −
ISSN 2319 - 4847
………… (1)
2+
The source of Cd :
CdCl2 ↔ Cd2+ + Cl22−
Cd2+ + 2OH− → Cd(OH)2
…………(2)
………… (3)
Cd2+ and NH3 form a complex species of cadmium (Cd(NH3)4)2+ (to slowly release Cd2+):
Cd2+ + 4NH3 → Cd(NH3)42+
(NH2)2CS + 2OH− → S2− + 2H2O + C(NH)2
Cd2+ + S2− → CdS
………… (4)
………… (5)
………… (6)
In the presence of sufficient NH3, the Cd salt exists predominantly in the form of Cd(NH3)42+. The room temperature
equilibrium constant of reaction 5 is very small. When the concentration product of Cd2+ and S2− in solution exceeds the
solubility product of CdS, 1.4 × 10−29 at 25ºC, CdS precipitates.
2.2 Characterization techniques.
The films were structurally characterized by X-ray diffraction (XRD) using a SHMADZU XRD-7000 (X-ray
diffractometer) in the range 10° to 60° with CuKα radiation λ =1.54059 Å, operated at 40 kV and 30 mA. Optical
properties of Cadmium sulfide films were measured at room temperature with UV-VIS spectrophotometer (Jenway 6800)
to measure the absorbance and transmittance of the films in the range of wavelengths 300-1100 nm.
3. RESULTS AND DISCUSSION
3.1 Structural characteristics of the films:
CdS films, which were deposited on glass, can have either a hexagonal or a cubic structure or, a mixed structure of the
two, depending on the preparation conditions [21]. Figure (1) shows the X-ray diffraction patterns of the three CdS thin
films deposited on glass substrates. XRD analysis reveals that films were polycrystalline in nature. XRD observation show
that a prominent broad peak appear at an angle 2θ = 26.5º which correspond to either the (111) cubic (JCPDS- 89-0440)
or the (002) hexagonal (JCPDS- 89-2944) planes. It is quite difficult to differentiate the cubic from the hexagonal
structure because the cubic (111) and the hexagonal (002) lines coincide with in 1% [1]. These peaks are quite broad,
which is indicative of nanosize particle [22]. Also there are two peaks appear at an angles 44º and 52º corresponding to
(220) and (311) plane of the cubic phase. In the hexagonal phase, these two peaks corresponding to the (110) and (112).
When we repeating the deposition procedure for the resulting film in the first stage, it can be seen from figure 1 (a and b)
of XRD, the intensity increase due to increase in the film thickness, this means the degree of crystallinity of the film
increases. We can interpreting this result as when the thickness of the CdS film increases, and full width at half maxima
decrease, the grain size increases and decrease in micro strain (εs = (β cos θ)/4). A comparison between observed and
standard d-values for the CdS thin films there is a good agreement between the d-values as shown in table 1. From the
XRD patterns it is possible to evaluate the average grain size of the samples by using the Well-known Deby-scherrer
formula [23]:
Dhkl= K λ /(β cosθ)
………… (7)
where k =0.94, λ is the wavelength of X-ray, β is the full width at half the peak maximum in radians and θ is Bragg’s
angle.
Figure 1: X-ray diffraction patterns of the three CdS thin films deposited on glass
substrate: (a) one deposition (b) double deposition (c) triple deposition.
Volume 3, Issue 5, May 2014
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International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org
Volume 3, Issue 5, May 2014
ISSN 2319 - 4847
Table 1: XRD data of nanocrystalline CdS thin films.
Samples
Single deposition
Double deposition
Tribal deposition
2θ
(deg.)
26.5
26.54
26.57
FWHM
(deg.)
3.3610
1.546
1.141
d-Spacing
Observed (Å)
3.3597
3.3548
3.3514
hkl
(111)
(111)
(111)
Grain size
(nm)
3
6
8
Micro strain (εs)
×10-4
142.74
65.65
48.45
3.2 Optical properties of CdS thin films
Figure (2) shows the transmittance curves as a function of wavelength for the nanocrystalline CdS thin film deposited
onto glass substrates for single, double and tribal depositions. The spectra show an average transmission of above 70 %
was obtained in the middle of visible range. The decreases in the transmittance in the case of double and tribal
depositions, may be due to the increase in film thickness as a result of multi-dip.
Figure 2:. Transmittance of the films for ; (a) Single deposition (b) double deposition and
(c) after triple deposition.
In semiconductors, the relation connecting the absorption coefficient α, the incident photon energy (hv) and optical band
gap Eg takes the form [10];
(αhv) = k( hv - Eg )m
………… (8)
Where ν is the frequency of the incident photon, h is Planck's constant, k is a constant which is different for different
transitions and it is related to the effective masses (of electron and hole) associated with the bands and m is the number
which characterizes the optical processes, m =1/2 for a direct allowed transition, 2 for the indirect allowed transition, 3/2
for a forbidden direct transition and 3 for a forbidden in direct transition. To determine whether the CdS films deposited
using CBD technique have direct or indirect band gap, The band gap of the films was determined by plotting a graph
between (αhv)2 and (hv). The band gap energy (E g) was estimated by a linear interpolation of each curve to energy axis
[24]. Figure 3 shows optical energy gaps of the films within the range
3.57 – 2.96 eV as a result of varying of
thickness of these films.
Figure 3: Optical energy gaps of the films; (a) Single deposition (b) Double deposition and
(c) Triple deposition.
Volume 3, Issue 5, May 2014
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International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org
Volume 3, Issue 5, May 2014
ISSN 2319 - 4847
4. CONCLUSION
The chemical bath method was successfully used to deposit CdS thin films. These films are smooth, uniform and good
adherent to substrate surface. The prepared films were found to be nanocrystalline thin films. XRD analysis reveals that
CdS thin films are polycrystalline having a hexagonal or cubic structure with a preferential orientation of (002) or (111)
plane. Film thickness can be increase by repeating the deposition procedure several times. When the thickness of the
films increase the grain size increase and the energy gap value was found to be thickness dependence. The film with triple
depositions can use in making solar cells.
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