Changes in Structural and Optical Properties of CdS Thin Films During the Process of Chemical Bath Deposition
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Changes in Structural and Optical Properties of CdS Thin Films During the Process of Chemical Bath Deposition Li Wei, Feng Liang-huan, Cai Ya-ping, Zhang Jing-quan, Zheng Jia-gui, Cai Wei, Wu Li-li, Li Bing, Shao Ye, Cai Dao-lin and Len Wen-jian Department of Materials Science, Sichuan University, Chengdu 610064, P. R. China ABSTRACT Chemical bath deposition (CBD) techniques for the growth of CdS polycrystalline thin films are now well developed. However, there usually appear different structures, grain sizes and energy gaps in CdS polycrystalline films by CBD, which implies that the influence of growth process on the structures and properties of CBD CdS thin films must be considered. In this paper, the optical, compositional and structural properties of CBD CdS thin films in the reaction process have been studied by XRD, AFM, XPS and optical transmission spectra measurements. The results can be explained using atomic mechanism and lattice transition in the CdS thin films. INTRODUCTION CBD is a very attractive method to fabricate CdS thin films for photovoltaic applications such as the ease of coating large-area devices, low-cost of the process and low energy consumption [1-7]. For optimizing film properties, much of the work has been done on the CBD mechanism, such as effects of the solvent concentration, temperature, and PH values. However, its structural and optical characteristics are not yet fully investigated. As to variations in the structural, chemical composition and optical characteristics of CdS thin films with time, there have hardly been reported on this aspect. In this paper, we have studied the changes in structural and optical properties for CBD CdS thin films during the formation reaction. And we report our own interpretation of the relationship between the deposition time and the structural, compositional and optical changes. EXPERIMENT Cadmium sulfide thin films have been prepared by using the following overall reaction for the process -
Cd(NH3)42+ + S=C(NH2)2 + OH
CdS + 4NH + 2 H O + CN H 3
2
2
2
(1)
The experimental apparatus to deposition is the same as the described by Kaur et.al. [1]. CdS thin films were grown by CBD technique using CdCl2 0.0014 M, thiourea 0.005 M, ammonia 0.02 M and were deposited onto ultrasonically cleaned glass slides. Solutions were prepared using analytical-grade reagents. The glass slides were placed Vertically in a closed beaker, where the solution of cadmium salt and NH4Cl were first added to 2000 ml deionized water, and the ammonia solution was then added to reaction beaker, finally the thiourea solution was then continuously added to reaction beaker only when an appropriate temperature was reached and maintained at 80- 830C by means of a thermostat water bath. The solution was
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vigorously stirred throughout the process and the deposition lasted for about 1 h. During the formation reaction, the coated substrates were removed at 30 min, 35 min, 40 min, and 50 min respectively, rinsed in deionized water, and blown dry in a N2 stream. The fi
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