International Journal of Engineering and Science Invention (IJESI)
EFFECT OF DEPOSITION PERIOD AND pH
1. EFFECT OF DEPOSITION PERIOD AND pH ON
ELECTRODEPOSITION TECHNIQUE OF ZINC SELENIDE
THIN FILMS
I. L. Ikhioya and A. J. Ekpunobi
Department of physics Industrial Physics, Nnamdi AzikiweUniversity,
Awka, Anambra State, Nigeria. Email: ikhioyalucky@gmail.com, Mobile no:
+23408038684908
Department of Physics and Industrial Physics, Nnamdi AzikiweUniversity, Awka,
Anambra State, Nigeria. Email: ajekpunobi@yahoo.com, Mobile no:
+23408038763056
Keywords: Thin Film, ITO, Electrodeposition, ZnSe, Seo2, characterization,
application.
Abstract
Zinc selenide (ZnSe) thin films semiconductor were investigated at room
temperature by varying three different pH. X-ray diffraction analysis revealed
that the crystallininty of ZnSe films prepared at pH 2.1 slightly increased. XRD
pattern of ZnSe showed cubic structure. The optical properties of the films
were investigated in the wavelength range of 300nm-900nm. Optical absorption
investigated show that pH has a slight effect on the energy band gap of the
grown ZnSe films, the band gap decreases (2.7-1.9eV) as pH decreases from
2.1-1.8
INTRODUCTION
Zinc selenide (ZnSe) is a wide band gap II-VI semi-conductor and has attracted
considerable attention for their wide range of applications in various
optoelectronic devices and in solar cells. It is a direct band gap semiconductor
and is transparent over wide range of visible spectrum. Polycrystalline ZnSe
thin films have been identified as suitable material for electroluminescent
display and window layers in solar cells. In recent years efforts have been
devoted to develop blue green laser diodes based on ZnSe and its alloys. So far
these have not been realized, mainly because of the difficulty in controlling
electrical conductor. Preparation of conductor layer is essential for its use as
light emitting devices. As in other large band gap semiconductor systems,
2. progress in ZnSe was hindered by lack of shallow dopant source, essentially
acceptors (M.G.M. Choudhury et al., 2004, T.Niina, 1982 and T.Yao, 1985).
ZnSe is an important semiconductor material with a large band gap (2.7eV),
which has a vast potential use in thin films devices and as n-type window layer
for thin film herterojunction solar cells (Umesh Khairnar et al., 2011).
The direct conversion of solar energy into electricity by photovoltaic (PV) solar
cells was studied for the past 30years (M.A. Green, 1982). We are still far from
making these sources cost effective. Photovoltaic solar energy conversion offers
one of the few ways of producing electricity in urban areas which is free of
various emissions and noise. Functional considerations, and in particular
efficiency, dominated research and development, when terrestrial PV
applications were developed, the same considerations of functionality and
efficiency continued to be given priority among them ZnSe, CdSe and doped
CdSe thin film are queues (M.A. Green, 2001).
The major technical importance of semiconductor ZnSe thin films are
exhibiting simultaneously high electrical conductivity and optical transparency
for example in solar cells flat panel displays and electronic devices. The
nanocystalline II-VI films are prospective for development of resistive sensors,
operating at room temperature. (A. Ayeshamariam et al., 2014).
Various methods have been used for the deposition of ZnSe thin films. Most
frequently used methods are molecular beam epitaxy (MBE), thermal
evaporation, organo-metallic chemical vapour deposition (OMCVD), closed
spaced vapour transport (CSVT), liquid phase epitaxial (LPE), gas transport
(GT), pulse laser deposition, chemical bath deposition. However, all of these
methods required special device and usually included toxic metal organic
reagents as raw material (P. David Cozzoli et al., 2005) It is believed that the
most straightforward way to synthesize ZnSe is direct combination of element
zinc and selenium at high temperature. (Yadongli et al., 1999) has reported a
successful fabrication of ZnSe by hydrothermal method in specific solvents
such as pyridine which is virulent, so it must be synthesized at glove-box.
(Hua Gong et al., 2006), once reported analogy method at low-temperature and
the products presented hollow microsphere and powder In hydrothermal
syntheses, reaction time, temperature, and mole ratios of the precursors and
solvent were very important to the formation of nanocrystals. Low temperature,
high concentration, and short reaction time minimized the crystal size.
3. MATERIALS AND METHODS
ZnSe thin films were deposited by electrodeposition technique using 20cm3 of
0.063m of selenium IV oxide (Seo2) mixed 20cm3 of 0.069m of Zinc
tetraoxosulphate VI Heptehydrate (ZnSo4.7H2O) and 5cm3 of potassium
tetraoxosulphate VI (K2SO4) solution. Then, 5cm3 of 0.4m of tetraoxosulphate
VI acid (H2SO4). Which used to acidify the solution, it was added into the
mixture and stirred well. The indium doped tin oxide (ITO) glass was used as
substrate. The ultrasonically cleaned glass substrate was immersed vertically
into the solution for electrodeposition process. The films growth was carried
out at 300k. The films were deposited in various deposition periods of (60-180s)
and pH (2.1-1.8) in order to determine the optimum condition for the
deposition of ZnSe thin films. During deposition process, the deposited films
were tested for adhesion by subjecting it to a steady stream of distilled water.
X- Ray diffractometer (XRD) analysis was carried out using DM-10
diffractometer for the 2θ ranging from (15- 530) with cukal (λ = 1.540598Ǻ)
radiation. Optical absorbance study was carried out using M501 UV-Visible
spectrophotometer. The films coated indium thin oxide glass was placed across
the sample radiation pathway while the uncoated the reference path. The
absorption data were manipulated for the determination of band gap energy
Results and discussion
Optical properties of ZnSe films
The optical properties of ZnSe films were studied using a M501 UV-Visible
spectrophotometer in a wavelength range of 300-900nm. All the films prepared
at pH 2.1-1.8 absorb moderately in the visible region.
The transmission spectra of the ZnSe thin films deposited at different pH
values are shown in Fig. 2. It is evident that transmission is slightly improved
with increasing pH value. All deposited films exhibited transmittance between
46% and 84% in the visible region. The transmittance spectra in Fig. 2 show
very high transmittance in the VIS-NIR regions of the electromagnetic
spectrum.
In figure 2, it is observed that the transmittance of the films is as high as 46-
84% in visible & infrared regions. Both of them have peak transmittance in
infrared region but the transmittance of slide B film is not as high as that of
slide C. The wide transmission range (0.4nm – 0.8nm) revealed in the figure
makes the materials useful in manufacturing optical components, windows,
mirrors, lenses etc for high power infra red laser .The transmittance of (Slide B)
increases from UV to the peak value (70%) in infrared region and can be as
high as 43% in UV region. Absorbance of both films is high UV region and in
4. visible and IR regions as shown in figure 2. The absorbance of (slide C) is very low as
low as 0.052 in all the regions of the spectrum. Absorbance of ZnSe decrease from
ultraviolet region to infrared region.
Fig. 1 Absorbance spectra of Znse thin films
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0 1 2 3 4 5
Absorbance(arbitraryunit)
Photon Energy, eV
SLIDE B
SLIDE C
SLIDE D
5. Fig. 2 Spectral transmittance against wavelength of Znse films
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 200 400 600 800 1000
Transmittance(%)
Wavelength (nm)
SLIDE B
SLIDE C
SLIDE D
6. Fig. 3 Spectral reflectance against wavelength of Znse thin films
The band gap energy and transition types were derived from mathematical
processing of the data obtained from the optical absorbance versus wavelength
with the following relationships for near edge absorption:
α = (hυ -εg) n/2,
Where υ is the frequency, h is the Planck’s constant, while n carries the value
of either 1 or 4. The band gap could be obtained from a straight line plot of α²
as a function of hυ; an extrapolation of the value of α² to zero will give band
gap. If a straight line graph is obtained from n=1, it indicates a direct transition
between the states of the semiconductor, whereas the transition is indirect if a
straight line graph is obtained from n = 4 as shown in Fig. 4. The pH has no
significance effect on the band gap energy of fig.6 hence, the band gaps remain
the same (2.7 eV) as the pH value was decreased from 2.1- 1.8
0
0.05
0.1
0.15
0.2
0.25
0 200 400 600 800 1000
Reflectance(%)
Wavelength (nm)
SLIDE B
SLIDE C
SLIDE D
7. Fig.4 plot 0f α2 against photon energy
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0 1 2 3 4 5
AbsorptionCoefficientSquared(m-2)
Photon Energy (eV)
8. Fig.5 plot 0f α2 against photon energy
0
0.1
0.2
0.3
0.4
0.5
0.6
0 1 2 3 4 5
AbsorptionCoefficientSquared(m-2)
Photon Energy (eV)
9. Fig.6 plot 0f α2 against photon energy
Structural properties of Zinc selenide films
The change in structure and identification of phases is studied with the help of
an X-ray diffractometer using CuKal radiation (λ = 1.540598 Ǻ). The X-ray
diffraction patterns at pH 2.1 and 1.8 of ZnSe thin films are shown in Fig.7-8.
The X-ray diffraction patterns at different pH reveal that the samples show a
cubic structure which correspond to (111-222) planes. These result compare
well with the report values by (choudhury et al., 2004) who reported the XRD
preferred orientation of (111- 311) planes and (Andalip Islam et al., 2014)
reported the preferred orientation in the (111-311) and (Ivashchenko et al.,
2011) obtained a film orientation in the (111-222) planes. The diffraction angle
2ϴ value is 15.87˚ with d = 5.582Å ˚ at lower pH 1.8. The preferred orientation
lies along the (111) plane. The lattice constant as was given in the X-ray
diffraction analysis is found to be a = 5.6667Ǻ
0
0.05
0.1
0.15
0.2
0.25
0 1 2 3 4 5
AbsorptionCoefficientSquared(m-2)
Photon Energy (eV)
12. Conclusion
ZnSe thin films have been prepared by electrodeposition technique. The films
have peak transmittance in infrared region of the electromagnetic spectrum
and high rate of absorption in the UV and NIR regions. The reflectance is low in
all the regions of the electromagnetic spectrum. These make the films excellent
glazing material for solar control in warm climatic regions.
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