Effect of Deposition Time on ZnS Thin Films Properties by Chemical Bath Deposition (CBD) Techinique

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1 World Applied Sciences Journal 19 (8): , 01 ISSN IDOSI Publications, 01 DOI: /idosi.wasj Effect of Deposition Time on ZnS Thin Films Properties by Chemical Bath Deposition (CBD) Techinique Huda Abdullah, Norhabibi Saadah and Sahbuddin Shaari Department of Electric, Electronic and System Eng., Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, Malaysia Abstract: Zinc sulfide thin films were deposited on optical glass substrates by using chemical bath deposition (CBD) technique. The chemical bath contains the solutions of thiourea, zinc acetate, ammonia and sodium citrate. The deposition time of the samples were varied from 18 hours to 39 hours. SEM, XRD and UV-Vis-NIR were used to characterize the samples. SEM shows the films are thicker and have bigger grains size at 39 hours of deposition time compared to the film at 18 hours of deposition time in the range of 37.0 to 56.0 nm as the deposition time increases. X-ray diffraction (XRD) pattern prove that ZnS thin films are in disordered since it does not revealing any peaks and the surface of ZnS thin films are amorphous. UV-Vis spectra showed that the deposited ZnS thin films have more than 100% transmittance in the visible region and direct band gap of deposited films are in range of.45 ev to 3.53 ev. Time increasing of deposition will slightly decrease the transmittance of the film. Key words: Zinc Sulfide Chemical Bath Deposition (CBD) Transmittance Band Gap INTRODUCTION In CBD, deposition of metal chalcogenide semiconducting thin films is done by maintaining the substrate in contact Zinc sulfide (ZnS) is a wide gap semiconductor. with the solution of chemical bath containing the metal Consequently, it is a potentially important material to be and chalcogen ions. The film formation on substrate takes used as an antireflection coating for heterojunction solar place when ionic product exceeds solubility product. The cells. It is an important device material for the detection, precipitate formation in the bulk of solution is an inherent emission and modulation of visible and near ultra violet problem in CBD technique. This normally results light. With a large band gap, Zinc sulfide (ZnS) becomes unnecessary precipitation and loss of materials during film a highly efficient luminescent material, when doped deposition on glass substrate. In order to avoid such with manganese, copper or other ions. Owing to wide problem, proper optimization of chemical constituents of band gap value of 3.7 ev, it can be used for fabrication bath deposition is necessary. The purpose of this work of optoelectronic devices such as blue light-emitting was to study the effects of the growth conditions, such as diodes, electroluminescent devices, electro optic deposition time on optical and structural properties of the modulator, optical coating, n-window layers for thin film chemical bath ZnS thin films. In this paper, we focus on heterojunction solar cells, photoconductor and especially the preparation of nanocrystalline zinc sulfide by a simple photovoltaic devices [1],[], [3]. ZnS films can be prepared CBD technique. by different techniques such as molecular beam epitaxy sputtering, metal-organic chemical vapour deposition, MATERIALS AND METHODS electrodeposition or chemical bath deposition (CBD). Chemical bath deposition is a very attractive method for All experiments were carried out initially at room producing ZnS thin films due to its specific advantages temperature without further temperature control. For this [4]. This method has been successfully used in the experiment,.5 ml of 1M Zn(CH3COO), 0.5 ml of 3.75M obtaining of other metal chalcogenide thin films [5]. triethanolamine (TEA),.5 ml ammonia (NH ), 0.8 ml of 3 Corresponding Author: Huda Abdullah, Department of Electric, Electronic and System Eng., Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, Malaysia. Tel: , Fax:

2 World Appl. Sci. J., 19 (8): , M trisodium citrate (C6H5Na3O 7) and.5 ml of 1M thiourea were employed as starting materials. First of all, solution of zinc acetate and trisodium citrate were mixed and stirred for about 30 minutes. After the clear solution formed, thiourea was added slowly along with ammonia as buffer solution. TEA was then added into the solution and stirred about another 30 minutes until stable solution formed. The substrates will be immersed in the solution and put into the microwave oven at temperature of 80 C- 86 C for 18 hours to 39 hours to ensure the deposition time was varied. After deposition, the ZnS thin films were washed by distilled water and dried in air. The deposition take place on glass substrates where it was cleaned with detergent, dried and degreased with ethanol in an ultrasonic cleaner and kept immersed in distilled water prior to the deposition. The glass substrates were placed vertically in the reaction solution to deposit the film. The coated substrates were removed at the end of the chosen duration of deposition, washed in running tap water, rinsed in deionized water and dried in air. The structures of ZnS thin films were studied by X-Ray diffractometry. The optical transmittance of the films was measured by optical spectroscopy using UV-Vis-NIR spectrometer. Intensity (a.u) (1 1 1) Theta (degree) Fig. 1: X-ray diffraction pattern of ZnS produced by deposition time of (a) 18 hours, (b) 4 hours, (c) 36 hours, (d) 39 hours. RESULTS AND DISCUSSIONS The films obtained by this technique were white in colour, continuous and adhere very well. Fig. 1 shows X- Ray spectra made for ZnS thin films with different deposition times such as 18 hours, 4 hours, 36 hours and 39 hours. From Fig. 1, ZnS appears almost in amorphous Fig. : Optical absorption spectra of ZnS films on glass form. Similar pattern of XRD peak have been reported by substrates with deposition time of (a) 18 hours, Liu [4] for as-deposited CBD ZnS deposited at (b) 4 hours, (c) 36 hours, (d) 39 hours. temperature of 80 C. The XRD pattern of ZnS thin films also have been reported by Johnston [6], which X-ray diffraction studies of the film produced did not show any discernable peaks. In this study, there was only one peak that occurred in XRD pattern at Theta scale of 5.1 at (1 1 1) [7]. As the deposition time increases, the diffraction of samples lower due to the thickness of the films coated. The optical properties of the films deposited on glass substrates were determined from the transmission and absorption measurements in the range of nm. Fig. shows the absorption spectra of deposited ZnS thin films with different deposition time. It indicates that the ZnS thin films grown at deposition time of 39 hours have highest average percentage for absorbance of 9.81 %. This goes to show that the absorbance of the films with Fig. 3: Optical transmission spectra of ZnS films on glass respect to incident radiation increases as the time of substrates with deposition time of (a) 18 hours, deposition is increased. (b) 4 hours, (c) 36 hours, (d) 39 hours. 1088

3 World Appl. Sci. J., 19 (8): , Fig. 4: Plot of ( hv) against hv for ZnS thin films on glass substrate with deposition time of (a) 18 hours, (b) 4 hours, (c) 36 hours, (d) 39 hours. Table 1: Effect of deposition time on optical band gap of the ZnS thin films deposited on glass substrates. Deposition time (hours) Optical band gap (ev) Band Gap (ev) Deposition time (h) Fig. 5: Plot of band gap (ev) against deposition time (hours) for ZnS thin films deposited on glass substrates. Fig. 3 shows the transmittance (%) as a function of wavelength (nm) for ZnS thin films. It shows that sample deposited at 4 hours has higher average transmittance over 100% in this region than that of sample deposited at 18, 36 and 39 hours. This further proves that as the deposition time increases, the deposited films transmittance decreases and it is optimum at deposition time of 4 hours. (a) (b) (c) (d) Fig. 6: SEM image of ZnS thin films grown at deposition time of (a) 18 hours, (b) 4 hours, (c) 36 hours, (d) 39 hours. 1089

4 The values of absorption coefficient are calculated by using equation: I t = I oexp (- t) (1) World Appl. Sci. J., 19 (8): , 01 deposition time but does not improve the surface roughness. The thickness of the film was about nm, nm, nm, nm for each deposition time. CONCLUSIONS where is the optical absorption coefficient, t is the thickness of the film, I t and I o are the intensity of Zinc sulfide thin films were prepared on glass transmitted light and initial light respectively. The substrates by the CBD technique. The films are in good absorption coefficient ( ) is related to the incident photon quality, adherent and uniform. There is a good agreement energy as [8]: between XRD, microstructures and optical results. These studies show that the deposition time contributes n/ = K (hv-eg) / hv () noticeably to the growth and to the structure of deposited films. ZnS appears almost in amorphous form and as the where K is constant, Eg is the energy gap and n is deposition time increases, the diffraction of samples constant equal to 1 for direct gap compound. become lower. Optical study proves that as the deposition time increases, the deposited films transmittance Based on the obtained optical transmittance and decreases and it is optimum at deposition time of 4 hours absorption, the square of optical absorption coefficient, which is over 100%. The absorption figure shows that the ( hv ) is plotted as a function of photon energy (hv) in absorbance of the films with respect to incident radiation Fig. 4. It can be seen that the films have a steep optical increases as the time of deposition increased with the absorption feature indicating good homogeneity in the highest percentage is at 9.81 % for deposition time of 39 shape and size of the grains and lower defects density hours. The morphology of films also has been studied by near band edge. As can be seen, hv varies almost SEM analysis showing the thickness of ZnS thin films is linearly with hv above band gap energy (E g). Accordingly, in the range of 37.0 to 56.0 nm as the deposition time the following equation for a direct inter-band transition increases. can be applied. ACKNOWLEDGEMENTS hv = K (hv-e g) (3) The author wants to thanks to The Ministry of where K is a constant. The band gap is obtained by Science, Technology and Innovation of Malaysia is extrapolating the straight portion of the curve to zero gratefully acknowledged for the grant of EScience Fund absorption coefficients. The determined band gap values vote: SF0385 (Development of Nanostructured are listed in Table 1. It is seen that the band gap was ZnS/CIS Films for Advanced Solar Cell Application). Also increased from.45 to 3.53 ev as deposition time was thanks to Photonic Technology Laboratory, Institute of increased from 18 hours to 39 hours which is lower than Microengineering and Nanoelectronics (IMEN), Universiti the typical value of the bulk ZnS (~3.6 ev). Fig. 5 shows Kebangsaan Malaysia and Physics Laboratory, Faculty that band gap (ev) plotted against deposition time for ZnS Science and Technology, Universiti Kebangsaan thin films deposited on glass substrates. It is clearly Malaysia. shows that increasing time of deposition will increase the band gap as the thickness of the films also increases REFERENCES against deposition time [9]. SEM image has been obtained for ZnS thin film 1. Ndukwe, I.C., Characterization and Applications deposited on glass substrate in order to study thin film of Zinc Sulphide Thin Films. Solar Energy Material, surface. SEM brings microscopic information of the 40: surface structure and roughness. It appears to be a. Varitimos, T.E. and R.W. Tustison, Ion Beam helpful technique to specify the growth mode via the Sputtering of ZnS Thin Films. Thin Solid Films, study of a surface roughness and to determine the effect 151: of the different annealing temperature on the film 3. Yamaga, S., A. Yoshikawa and H. Kasai, morphology [10]. Fig. 6 shows the surface of ZnS layers Growth and Properties of Zn1-xCdxS Films on GaAs obtained at four different deposition time. A slight by Low-Pressure MOVPE. Journal of Crystal increase of the grain size follows the increase of the Growth, 99:

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