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
Effect of Cadmium Sources on Structural and
Optical Properties of Nanocrystalline CdS Thin
Films Prepared by CBD Technique
Jamal F. Mohammad, Hamid S. Al-jumaili and Faisal H. Antar
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, cadmium sulfate and cadmium acetate as a cadmium source. While thiourea
(CS(NH 2)2) used as a source of sulfur and ammonia solution (NH4 OH) were added to maintain the pH value of the solution at 10.
Crystalline structure and optical properties of the as-deposited thin films were analyzed by x-ray diffraction and UV-VIS
spectrophotometer technique, respectively. X-ray diffraction study indicates that the grown Cadmium sulfide films have cubic
structure. The optical characterizations show that the values of band are 3.59, 3.7 and 3.77 eV depending on cadmium source. The
films show high transmittance above 65% in the VIS – NIR range of the solar spectrum.
Keyword: Nanocrystalline thin films, Cadmium sulfide, chemical bath deposition.
1. INTRODUCTION
The synthesis of binary metal chalcogenides of group II–VI semiconductors in a nanocrystalline form has been a rapidly
growing area of research due to their important non-linear optical properties, luminescent properties, quantum size effect
and other important physical and chemical properties[1]. Nanocrystalline thin films are also polycrystalline in nature but
with sizes of crystallites of the order of a few nanometers. Many properties of nanocrystalline materials are found to
deviate from those of coarse grained polycrystalline materials with the same average chemical composition. These
deviations result from dimensionality of nanometer sized grains and numerous interfaces between adjacent crystallites[2].
CdS nanocrystalline thin films belonging to the cadmium chalcogenide family and used as window material for
CdS/CdTe solar cells continues as a subject of intense research due to its potential application in solar cells [3]. There are
several methods for depositing CdS thin films, such as molecular beam epitaxy (MBE) [4], spray pyrolysis [5], screen
printing [6], spin coating [7], electrodeposition [8], chemical bath deposition (CBD), etc. [9-13]. Among of all methods,
the CBD technique has many advantages such as simplicity, no requirement for sophisticated instruments, minimum
material wastage, economical way of large area deposition, and no need of handling poisonous gases. The CBD method is
a slow process, which facilitates the better orientation of the crystallites with improved grain structure [14]. The
deposition of CdS films by means of the CBD technique is done by immersing a substrate in an alkaline aqueous solution
containing Cd2+ and S2- ions resulting from chemical reactions in the solution [15]. I n this work. the effect of cadmium
sources on structural and optical properties of nanocrystalline CdS thin films prepared by CBD technique were
investigated.
2. EXPERMENTAL DETAILS
Before deposition, the commercially available glass substrates were degreased with chromic acid solution, washed with deionized water and finally dried in air. For preparation of Nanocrystalline CdS thin films, aqueous solutions of 0.1M of
cadmium chloride, sulfate and acetate were used as a source of cadmium ions, thiourea as a source of sulfur ions. The
complexing agent to form a cadmium complex, which slowly released Cd2+ ions for subsequent reaction with S2− ions, was
ammonia. Three beakers in the same bath were used, each beaker contained 20 ml of cadmium chloride, sulfate and
acetate respectively, ammonia solution was added drop by drop each beaker under vigorous stirring to maintain the pH
value of the solution at 10. After stirring for several minutes, the solution became clear and homogenous. Then under
continuous stirring, 20 ml thiourea solution was added in each beaker. The clean glass substrates were immersed vertically
into the beakers contain mixture solution were placed in chemical bath whose temperature were kept at 70±2 oC during the
growth. The reaction solutions was no stirred during the deposition process. Deposition time was 60 minute, during which
the solution color changed to yellow and finally to orange. The reaction of formation CdS of the film can be written as
follows [16];
NH3 + H2O → NH4+ + OH−
Cd2+ + 4NH3 → Cd(NH3)42+
Volume 3, Issue 5, May 2014
…………..(1)
… ……… (2)
Page 196
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
(NH2)2CS + 2OH− → S2− + 2H2O + H2CN2 …….……..(3)
HS−+ OH− → S2− + H2O
…. ………..(4)
Cd(NH3)42+ + S2− → CdS + 4NH3
…..……….(5)
In the presence of sufficient NH3, the Cd salt exists predominantly in the form of Cd(NH3)42+. When the concentration
product of Cd2+ and S2− in solution exceeds the solubility product of CdS, 1.4 × 10−29 at 25ºC, CdS precipitates. After
deposition, the samples were removed from the bath and washed in distilled water to remove the loosely adhered CdS
particles on the films and finally dried in air.
3. RESULTS AND DISCUSSION
3.1 Structural Properties
Figure 1 shows X-ray diffraction (XRD) patterns of Nanocrystalline CdS thin films fabricated from various cadmium
sources at bath conditions (70 °C for 60 min. and pH=10). All the CdS films whic h prepared three different cadmium
sources had the cubic crystalline structure. These patterns display an intense diffraction peak at approximately 2θ=26.6º,
which coincides with the (111) diffraction line of the CdS cubic crystalline phase. From the diffraction pattern, it can be
seen that there are two weak diffraction peaks exist at approximately 2θ=44º and 52º which can be ascribed due to (220)
and (311) reflection planes of the cubic CdS structure, respectively. Therefore it has been concluded that the deposited
CdS thin films are polycrystalline in nature. The intensity of the (111) diffraction peak of CdSO4 based film is stronger
than that of other peaks of Cd(CH3COO)2 and CdCl2 based films because of the smaller thickness of these two films. The
peak broadening in the XRD patterns clearly indicates the formation of CdS nanocrystal of very small size.
Figure 1: X-ray diffraction patterns of the three CdS thin films deposited on glass
substrate from three different cadmium sources: (a) CdCl2 (b) Cd(CH3COO)2 (c) CdSO4.
From the full-width at half-maximum of the (111) diffraction peaks, the grain size of the nanocrystalline CdS films is
estimated using the Debye-Scherrer formula[17];
Dhkl= k λ /(β cosθ)
……………..(6)
where k is a constant taken to be 0.94, β is the broadening of diffraction line measured at full width of half maximum
intensity (rad.) and λ= 1.5406 Å is the wavelength of CuKα radiation. The X-ray characterization of the prepared films are
summarized in table (1). The difference in grain size between the films may be attributed to two different deposition
processes in CBD technique, the first cluster by cluster deposition processes which leads to highest grain size due to faster
growth rate and the second ion by ion which leads to smaller grain size as in the case of CdCl 2-based film.
Table 1: XRD data of nanocrystallites CdS thin films deposited from three different cadmium source
2θ
d
Grain size (from
XRD) (nm)
Cadmium source
(degree)
(nm)
(hkl)
FWHM
CdCl2
Cd(CH3COO)2
CdSO4
Volume 3, Issue 5, May 2014
26.68
26.58
26.68
0.3350
0.3354
0.3340
(111)
(111)
(111)
3.268
2.379
1.878
3
4
5
Page 197
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
3.2 Optical Properties
Figure 2 shows the optical transmission spectra of CdS thin films as a function of wavelength for the films prepared from
three different cadmium sources (cadmium chloride, sulfate and acetate). The difference in the transmission values as a
result of different thickness of the prepared films. The difference in the thickness of these films are attributed to different
in the stability constants of cadmium chloride, sulfate and acetate, the high stability constant the least thickness. The
maximum transmission obtained was in the case of CdCl2 (least thickness), while the highest thickness obtained was in
the case of CdSO4 which leads to minimum transmission, Whereas the intermediate transmission in the case of
Cd(CH3COO)2. The film thickness dependence on the Cd source used in the deposition process and stability constants of
used Cd complexes are summarized in table (2). The spectra of these films show an average transmission of above 65 %
were obtained above the middle of visible range. Also optical transmission curves are observed to be slightly shifted
towards the longer wavelengths (red shift) with using CdCl2, Cd(CH3COO)2 and CdSO4 respectively. The sharp rise in
transmission near 500 nm is an identification of good crystallinity of films [18].
Table 2: film thickness and stability constants with different Cd source.
Cadmium source
CdCl2
Cd(CH3COO)2
CdSO4
Film thickness
(nm)
105
118
Stability constant Cd complexes
(log scale) [19].
2.93
2.19
127
<2
Figure 2: Transmittance as a function of wavelength of CdS thin films with three
different cadmium source: (a) CdCl2 (b) Cd(CH3COO)2 (c) CdSO4.
The optical band gap energy (Eg) was determined from the absorption spectra of the films by using the following relation
[14]:
(αhν) = k(hν – Eg)m ………….. (7)
where k is a constant, α is absorption coefficient, hν is the photon energy and m is a constant, equal to 1/2 for direct band
gap semiconductor. The value of the energy where α = 0 is known as energy band gap, Eg. From figure 3, the calculated
values of the direct energy band gap are 3.77, 3.7 and 3.59 eV for CdCl2, Cd(CH3COO)2 and CdSO4 as a ion cadmium
source respectively.
Figure 3: Optical energy gap as a function of wavelength of CdS thin films with three
different cadmium source: (a) CdCl2 (b) Cd(CH3COO)2 (c) CdSO4.
Volume 3, Issue 5, May 2014
Page 198
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
Nanocrystalline CdS thin films have been successfully deposited on glass substrate by chemical bath deposition technique
at 70 ºC using CdCl2, CdSO4 and Cd(CH3COO)2 as a cadmium source. The results obtained in structural characterization
indicate that the reflection peak at 2θ = 26.68○, 26.58○ and 26.68○ (111) for CdCl2, Cd(CH3COO)2 and CdSO4 as a
cadmium source , with two small another intensity peaks at 2θ = 44○ (220) and 52○ (311). The optical characterizations
show that the values of band are 3.59, 3.7 and 3.77 eV depending on cadmium source. The optical study shows that the
transmittance of CdS films above 65% in the VIS – NIR range of the solar spectrum.
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