International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org
Volume 3, Issue 9, September 2014
ISSN 2319 – 4847
EFFECT OF GAMMA – RAY ON
STRUCTURAL AND OPTICAL PROPERTIES
OF CADIMUM SULFIDE (CdS) THIN FILMS
FAISAL HAMAD ANTAR
Physics Department Education College for Pure Sciences, Anbar University ,Iraq
ABSTRACT
In this work, CdS thin films have been deposited onto the glass substrates by thermal evaporation technique in vacuum of the
order of 10–5 mbar. at different thickness. The effect of gamma radiation from Co-60 by with ray dose 20 Mrad on the structural
and optical properties of the CdS thin films was investigated. The crystal structure and orientation of the CdS thin films were
investigated by X-ray diffraction (XRD) patterns. The optical transmittance measurements were recorded by using a double beam
spectrophotometer. The XRD spectra indicate that the films are of polycrystalline structure before and after irradiation with a
preferential growth of crystallites in the (002) plane. The grain size of crystallites was found to be in the range of 9-32 nm. After
gamma irradiation, the result indicates that the intensity of XRD spectra and grain size of crystallites increases, while the optical
energy gap decreases.
Keywords: Thin films, Gamma radiation, Cadmium sulphide.
1. INTRODUCTION
In recent years, the synthesis of chalcogenides has attracted significant interest and still is the subject of intense
investigation owing to their important non-linear properties, luminescent properties and other important physical and
chemical properties [1]. Among the wide band gap II-VI semiconductors cadmium sulphide (CdS) with its direct band
gap of 2.42 eV at room temperature is a promising material and is applied in wide variety of fields such as solar cells
[2,3]. Ionizing radiations such as X-rays, gamma-rays, beta particles, alpha particles, fission fragments, etc. are present in
several fields that include industry, medicine, military, particle accelerator based research, nuclear power plants, etc. It is
very much essential to ensure that the radiation levels in the environment surrounding these fields are within the
permissible limit which can be determined by proper dosimeters [4]. CdS is one of the most extensively investigated
semiconductors in thin film form and a large variety of deposition techniques have been utilized to obtain solar cell
quality layers of CdS. These preparation techniques include dry process such as sputtering, physical vapor deposition,
chemical vapor deposition, and molecular beam epitaxy(MBE)[5]-[8], and wet process such as electro-deposition, spray
pyrolysis, successive ionic layer adsorption and reaction, and chemical bath deposition [9]-[12]. The aim of this work is to
investigate the effect of Gamma irradiation on structural and optical properties of CdS thin films.
2. EXPERMENTAL
The cleanness of the substrate surface plays a decisive influence on film growth and adhesion. The commercial glass
substrates were washed with a detergent solution, acid and base solutions, after which they were ultrasonically cleaned in
distilled water and finally dried. The CdS thin films were deposited on glass substrates (using 99.99 % pure CdS powder)
by thermal evaporation technique in vacuum (CVD) at room temperature (300K) at a pressure of the order of 10-5 torr.
Molybdenum boat was used to hold powdered sample. The distance from source to substrate is kept at 20 cm and the rate
of deposition was 7 Å/sec. The samples thickness were 100 nm , 200 nm and 300 nm. was determined using thickness
monitor ( Edward 306 ). Then the as deposited films exposed to gamma radiation dose. Structure of deposited CdS films
was analyzed by X-ray diffractometer using Cukα radiation at a slow scanning rate in the 2θ range 20-60°, while the
optical transmittance measurements were recorded by using a double beam spectrophotometer.
3. RESULTS AND DISCUSION
3.1 Structural properties
Figures 1and 2 shows the X-ray diffraction pattern of the CdS thin films before and after irradiation by gamma radiation.
X-ray diffraction studies showed that these films are well oriented with a preferential growth of crystallites in the (002)
plane. XRD studies revealed that the crystallanity of these films was improved on irradiation with a gamma radiation.
Volume 3, Issue 9, September 2014
Page 89
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org
Volume 3, Issue 9, September 2014
ISSN 2319 – 4847
There is also observed several dominant peaks represent to (100), (101), (110) and (112) plane are present with low
intensities as compared to that of (002) plane.
Figure 1: X-ray diffraction pattern of CdS thin films before irradiation prepared at
different thickness (a) 100 nm, (b) 200 nm and (c) 300 nm.
Figure 2: X-ray diffraction pattern of CdS thin films after irradiation prepared at
different thickness (a) 100 nm, (b) 200 nm and (c) 300 nm.
The average grain size of the prepared CdS thin films can be determined from the full width at half maximum (FWHM)
of the prominent peak using Debye- Scherer formula [2]:
(1)
D hkl  K /( cos )
where λ = 1.54Ǻ is the wavelength of the X-ray radiation, β is the FWHM in radians of the XRD peak and is the angle of
diffraction. The crystallite sizes are summarized in the table 1 are found to be within the range 9- 32 nm.
Table 1: XRD results for CdS thin films before and after irradiation.
Sample
Thickness
2θ
Pla
FWHM
Gran
ne
Size
Before
1
100
1.036
9
irradiati
26.
00
2
200
0.423
21
on
5
2
3
300
0. 342
27
After
irradiati
on
1
2
3
Volume 3, Issue 9, September 2014
100
200
300
26.
5
00
2
0.559
0.345
0.333
16
26
32
Page 90
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org
Volume 3, Issue 9, September 2014
ISSN 2319 – 4847
3.2 Optical properties
Absorbance measurements are performed at room temperature in the range of 300 – 1100 nm to obtain information on the
optical properties of the CdS thin films obtained at different thickness values. Figures 3 and 4 shows the transmittance as
a function of wavelength of the CdS thin films before and after irradiation. The CdS films have high transmittance of
about 70 to 80% in the UV-Vis-NIR regions. From the figures we can see that as the thickness increase the transmittance
decrease. Also the transmittance decreases after irradiation. The small decreases in transmittance after irradiation may be
due to increase in the grain size of these films.
Figure 3: transmittance as a function of wavelength of the CdS thin films (A) before and, (B) after irradiation
From optical absorption spectrum of CdS the band gap was calculated using the Tauc relation [13]. We plotted (αhw)2
versus hν as shown in figures 4 and 5. Where α is the absorption coefficient and hν is the photon energy. Tauc relation
connecting the α, hν and optical band gap Eg takes the form:
h  A( h  E )
g
m
…….(2)
The direct band gap values were determined by extrapolating the linear portion of these plots to the energy axis. Where
m=1/2 for a direct band gap material, 2 for an indirect band gap material and 3/2 for a forbidden –direct energy gap. As
can be seen in Figuer 5, after irradiation the band gap energy is decreasing. The band gap energies of CdS thin films are
given in table 2.
Table 2: The bandgap energies for CdS thin films before and after irradiation.
Eg (eV)
Eg (eV)
Sample
Thickness
Before
After
(nm)
irradiation
irradiation
1
100
2.42
2.42
2
200
2.38
2.37
3
300
2.36
2.33
Figure 4: Optical energy gap of CdS thin films before irradiation prepared at
nm and (c) 300nm.
Volume 3, Issue 9, September 2014
different thickness (a) 100 nm, (b) 200
Page 91
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org
Volume 3, Issue 9, September 2014
Figure 5: Optical energy gap of CdS thin films after irradiation prepared at different thickness
and (c) 300 nm.
ISSN 2319 – 4847
(a) 100 nm, (b) 200 nm
4. CONCLUSION
CdS are synthesized through thermal evaporation technique in vacuum at different thickness. The x-ray diffraction
pattern shows that CdS thin film is polycrystalline with a hexagonal structure before and after irradiation by gamma
radiation and the intensity increase after irradiation. The grain size of crystallites was found to be in the range of 9-32
nm. The transmittance and optical band gap energies of the prepared CdS thin films decreases after irradiation.
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