Declaration
I hereby declare that the work entitled, “Study on physical properties of ZnS thin films
prepared by chemical bath deposition” is the original work done by me under the
supervision of
Dr. V. Senthil Kumar, Associate Professor, Department of Physics,
Karpagam University, Coimbatore as a part of my Ph.D. work during the period from 2009 to
2013. I hereby declare that this manuscript or its contents have not formed a part of any other
paper or thesis and also has not been submitted elsewhere for publication.
Your’s truly,
S. Kalyanasundaram
Author Details
1.
S. Kalyanasundaram
Ph.D. Scholar (part time)
Department of Physics,
Karpagam University,
Coimbatore-641 021
Tamil Nadu
88838 44640
sksmsc06@gmail.com
2.
Dr. V. Senthil Kumar
Associate Professor,
Department of Physics,
Karpagam University,
Coimbatore-641 021
Tamil Nadu
94439 12108
vsenkumar@yahoo.com
3.
Dr. K. Panneerselvam
Associate Professor,
Department of Physics,
Karpagam University,
Coimbatore-641 021
Tamil Nadu
98429 21367
Study on physical properties of ZnS thin films prepared by chemical bath deposition
S. Kalyanasundaram*, Dr. K. Panneerselvam, Dr. V. Senthil Kumar
Department of Physics, Karpagam University, Coimbatore, Tamilnadu.
Abstract
ZnS thin films have been deposited by chemical bath deposition method using
ammonia as a complexing agent with various deposition times. The morphological, optical,
structural and compositional analysis of films have been investigated by SEM, UV
spectroscopy; XRD and EDAX. The XRD analysis of as deposited films shows that the films
were cubic structure, the grain size of the film increases with increase in the deposition time.
UV spectroscopy studies show good transmission characteristic with an average
transmittance of 70-85%. The band gap energy values of ZnS films increased from 3.84 to
3.90 eV with increasing deposition time.
Key words: Zinc compounds, CBD, XRD, SEM, EDAX, UV spectroscopy.
1. Introduction
Traditionally CdS buffer layers deposited by chemical bath deposition have been used
in thinfilm solar cells (TFSC) to obtain high conversion efficiency. But CdS thin films when
utilized in large scale solar cell production could raise environmental problems due to high
amount of Cd compounds waste (Sartale 2005, Jhonston 2002). The search for alternative to
Cd-free buffer layers is the subject of interest and many efforts to overcome these difficulties
are still ongoing (Asenjo 2008, Puspitasari 2008). Recently the nano and polycrystalline ZnS
thin films have attracted researchers as they play crucial role in photovoltaic technology and
opto-electronics devices (So Ra Kang 2010). Zinc sulphide thin films are the promising
materials for its use in various application devices. In opto electronics it can be used as light
emitting diode in the blue to ultraviolet spectral region due to its wide band gap of 3.7 eV at
room temperature (Yamaga 1998). In the area of optics, ZnS can be used as a reflector and
dielectric filter because of its high refractive index (2.35) and its high transmittance in the
visible range respectively (Ruffner 1989, Ledger 1979). Zinc sulphide is less toxic, efficient,
cost effective material and has good transparency.
ZnS thin films are synthesized by different methods such as thermal evaporation
(Dimitrova 2000), spray pyrolysis (Mustafa 2007), sputtering (Shao 2003), chemical vapor
deposition [Icimura 1999], successive ionic layer adsorption and reaction (SILAR) (Nomura
1995), and the metal organic vapour phase epitaxy (MOVPE) (Roy 2006, Dona 1995).
Among these, a very attractive method for producing ZnS thin films due to the possibility of
large area deposition at low cost is the so called chemical bath deposition (CBD) method.
Ammonia and hydrazine are popular choices as the complexing agent in the CBD of ZnS thin
films. In addition tri sodium citrate can significantly improve the quality of the ZnS films
obtained (Gumus 2005).
In this paper we report some studies of chemical bath deposition of ZnS thin films
from NH3/SC (NH2)2/Zn (CH3Coo)3 solutions. The CBD ZnS thin films characterisation
including structural, compositional and optical are shown.
2. Experimental work
Using CBD technique Zinc sulphide thin films are prepared on glass substrates by
various deposition times. Aqueous solution of 10 ml, 0.2 M of Zinc acetate Zn(CH3COO)2,
10 ml of 0.6 M thiourea SC(NH2)2, 10 ml tri sodium citrate and 5 ml ammonia were used to
prepare ZnS thin films. The solution is continuously stirred for several minutes and it
becomes clear and homogeneous. Deionised water was added to make the solution up to 50
ml. The pH of the solution is maintained as 8.5. The glass substrates were first cleaned in
pure water, isopropyl alcohol, and double distilled water. The cleaned substrates (25 mm x 75
mm) were immersed vertically in the bath. The temperature of the bath was set at 80o C for
all the deposited ZnS thin films.
A Bruker AXS D8 advance x-ray diffratometer with vertical goniometer fitted with
copper radiation (=5406A) was used for the structural analysis of thin films coated with
different deposition times. The surface morphology of ZnS thin films was studied using a
scanning electron microscope (JEOL Model JSM - 6390LV), Energy dispersive spectrometer
(JEOL Model JED – 2300) confirmed the composition of the constituents.
3. Results and discussions
The X-ray diffraction (XRD) profiles of ZnS thin films deposited at various
deposition times (Table 1) are shown in fig1,2 and 3. Table 1: Preparatory conditions of the
thin films of chemical baths.
Chemical
Thin film
baths
Temperature pH
Deposition Time
(⁰C)
(minutes)
Bath 1
A
80
8.5
30
Bath 2
B
80
8.5
45
Bath 3
C
80
8.5
60
Cubic - (111)
Cubic - (220)
100
80
60
47.341
40
48.797
120
Hexa gonal - (220)
A- 30 minutes
28.577
140
20
0
-20
20
25
30
35
40
45
50
55
60
Angle (2-Theta °)
Figure 1: XRD pattern of thin film A
100
B- 45 minutes
47.341
Hexa gonal - (220)
49.354
28.496
40
ASASXDAD
Cubic - (111)
60
Cubic - (220)
80
20
0
20
25
30
35
40
45
50
55
Angle (2-Theta °)
Figure 2: XRD pattern of Thin film B
60
C- 60 minitues
100
28.42
80
60
Cubic - (220)
Hexa gonal - (220)
120
ASASXDAD
Cubic - (111)
29.339
Hexa gonal - (111)
140
47.899
49.065
40
20
0
20
25
30
35
40
45
50
55
60
Angle (2-Theta)
Figure 3: XRD pattern of Thin film C
Generally ZnS crystals exist in two forms, cubic (Zinc blende) and hexagonal (wurtzite). The
cubic form is stable at room temperature while wurtzite the less dense hexagonal form is
stable at high temperature (Berger 1997). ZnS films exhibited peaks corresponding to the
(111), (220) plane of the cubic ZnS phase and exactly matching with JCPDS PDF 65-1691
with lattice parameter of 5.41 A° and cell volume of 158.4 A° . The matching of standard and
observed 2θ values with cubic and hexagonal faces are depicted in table 2 from which is it
concluded that deposited ZnS thin films are of cubic structure.
Table 2: The comparison of the different planes with 2θ values with cubic and hexagonal
structures for CBD-ZnS thin films deposited with different deposition time. The peaks can be
assigned to both cubic and hexagonal ZnS phases of the planes using [JCPDS data file No.:
65-1691 (ZnS/cubic)]. [JCPDS data file No.: 72-0163 (ZnS/Hexagonal)]
Thin films
Cubic
Measured
Hexagonal
angle (2θ)
A
B
C
Plane (h k l)
Angle (2θ)
Plane (h k l)
Angle (2θ)
(1 1 1)
28.550
(0 0 8)
28.587
28.496
(2 2 0)
47.489
(1 0 1 1)
48.618
47.504
(1 1 1)
28.550
(0 0 8)
28.587
28.535
(2 2 0)
47.489
(1 0 1 1)
48.618
47.510
(1 1 1)
28.550
(0 0 8)
28.587
28.577
(2 2 0)
47.489
(1 0 1 1)
48.618
47.509
The film prepared with deposition time 30 min are amorphous as well as highly transparent.
When the deposition time increases the intensity of the ZnS (111) peak increases indicating a
better crystallinity. This behaviour is explained as follows:
When the deposition time
increases the thickness of the ZnS film increases and the crystallinity is improved. The
diffraction peaks become slightly sharper and their intensity is relatively enhanced with
increasing deposition time while their location did not changed significantly. The high noise
content of XRD graph did not allow identification of other phases. The average grain size of
the ZnS films can be estimated from scherrer formula : D = 0.9λ / β cosθ, where λ is the Xray wavelength of 0.5406 nm, β is the full width at half maximum, and θ is the Bragg
diffraction angel (Ladar 2007). The calculated average grain sizes were 12.07, 14.92, and
19.01 nm for the films coated at deposition time 30, 45 and 60 minutes respectively.
Figure 4 shows the optical transmittance spectra of ZnS thin films under different
deposition time intervals.
Transmittan
ce
a
b
c
Wavelength (nm)
Figure 4: Transmittance spectra of thinfilms A,B,C
It is significant to note that these ZnS films were highly transparent in the visible region and
their transmittance was found to be over 70%. The thickness of ZnS films was dependent on
both the deposition time (30 to 60 minutes) and concentration of thiourea (upto 0.6 M) which
in turn results in decreasing trend for the transmittance of ZnS films from 85% to 70% in the
wavelength range of 600-800 nm. For a deposition time of 30 minutes, the transmittance
attained a maximum of 85% at a wavelength of 700nm. The transmittance of ZnS films was
inversely proportional to the deposition time which means at a short deposition time, higher
transmittance was observed that is highly desirable for enhancing the short circuit current.
The absorption were analyzed using the well known relation for near edge optical
absorption of semi conductors:
Αhυ = k (hυ - Eg)n/2,
Where k' is a constant, n is a constant equal to 1 for direct band gap semiconductors
and 4 for indirect band gap semiconductor materials. The variation of α2 versus h𝜐 in Figure
5 is linear at the absorption edge, which confirms that ZnS is a direct band gap
semiconductor.
3.50E+015
3.00E+015
(ahv)
2
2.50E+015
2.00E+015
1.50E+015
1.00E+015
5.00E+014
0.00E+000
3
4
hv (eV)
Figure 5: Band gap of A, B, C
Extrapolating the straight line portion of the α2 versus hυ plot to zero absorption coefficient
value gives the band gap energy Eg. (Lokhande 2005), the estimated band gap values are
listed in the table 3.
Table 3: Estimated band gap values
Thin films
Deposition Time (minutes)
Band gap (eV)
A
30
3.84
B
45
3.88
C
60
3.90
The band gap values of the deposited thin films increases than that of the value of the
bulk ZnS (3.6 ev), which could be attributed to quantum confinement effects due to the small
grain sizes of the polycrystalline films.
The scanning electron micrographs of the deposited ZnS thin films are shown in
Figures 6, 7 & 8 of magnification x 5000.
Figure 6: SEM image of Thin film A
Figure 7: SEM image of thin film B
Figure 8: SEM image of thin film C
The surface of the thin film with deposition time 30 minutes seems to have a coarser surface
when compared to the surface of the thin films with deposition time 45 and 60 minutes. The
surface of the thin film with deposition time 30 minutes containing particles of size 0.5 to 1
micron. The smoothness of the film increases with the increase in the deposition time. The
particles with different contrast on the top of the film are due to the free particles which are
sitting on the films and they are not attached with the films, also there is reduce in the free
particles with increase in the deposition time is found. This might be the reason for the
smoothness of the films. The films with coarser surface (deposition time 30 and 45 minutes)
have discontinuities.
The Figure 9 shows the EDAX result of ZnS thin film with deposition time 30 minutes.
EDAX analysis confirms the presence of zinc and sulphur in ZnS thinfilm with composition
Zn1.05, S0.95.
Figure 9: EDAX analysis for thin film A
The proportion of the constituent elements measured was Zn = 52.68% and S = 47.32%. The
composition was near stoichiometric films can be obtained from the single and economic
CBD technique. EDAX spectrum also shows that prepared films are free from impurities.
The presence of silicon (Si) and oxygen (O) are due to glass substrates (Johnson 2002, Afifi
1995).
4. Conclusion
ZnS thin films have been successfully deposited by chemical bath deposition
technique with different deposition time. Our experiment, XRD studies show formation of
pure ZnS films cubic structure. The crystallinity of the films and the average grain size are
increased with increasing deposition time from 30 minutes to 60 minutes. The film shows
better optical transmission (70-85%) in the visible region. The optical transmission of the
film decreased with increase in deposition time. The calculated band gap value are in the
range of 3.8 - 3.9 eV, which will be very useful for solar cell. The morphology of the
deposited films has been found as smooth and the films with coarser surface have
discontinuities. The chemical constituents and their compositions of the films have been
estimated by the energy dispersive X-ray analysis. The deposited films are identified as Zn
1.05
S 0.95.
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