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. KalyanasundaramPh.D. Scholar (part time)Department of Physics,Karpagam University,Coimbatore-641 021Tamil Nadu88838 44640sksmsc06@gmail.com

2. Dr. V. Senthil KumarAssociate Professor,Department of Physics,Karpagam University,Coimbatore-641 021Tamil Nadu94439 12108vsenkumar@yahoo.com

3. Dr. K. PanneerselvamAssociate Professor,Department of Physics,Karpagam University,Coimbatore-641 021Tamil Nadu98429 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

baths

Thin film Temperature

( C)⁰

pH Deposition Time

(minutes)

Bath 1 A 80 8.5 30

Bath 2 B 80 8.5 45

Bath 3 C 80 8.5 60

Figure 1: XRD pattern of thin film A

Figure 2: XRD pattern of Thin film B

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 Hexagonal Measured

angle (2θ)

Plane (h k l) Angle (2θ) Plane (h k l) Angle (2θ)

A

B

C

(1 1 1)

(2 2 0)

(1 1 1)

(2 2 0)

(1 1 1)

(2 2 0)

28.550

47.489

28.550

47.489

28.550

47.489

(0 0 8)

(1 0 1 1)

(0 0 8)

(1 0 1 1)

(0 0 8)

(1 0 1 1)

28.587

48.618

28.587

48.618

28.587

48.618

28.496

47.504

28.535

47.510

28.577

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

Wavelength (nm)

Transm

ittance

cba

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 X-

ray 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.

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.

Figure 5: Band gap of A, B, C

3 40.00E+000

5.00E+014

1.00E+015

1.50E+015

2.00E+015

2.50E+015

3.00E+015

3.50E+015

(ahv

)2

hv (eV)

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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