Preparation of Polycrystalline CdS Thin Films by Chemical Bath Deposition
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ABSTRACT We prepared CdS thin films from a solution containing cadmium acetate, thiourea, ammonia, and ammonium acetate. We varied fabrication conditions such as the concentrations of reactants, reaction temperature, and heat treatment, to investigate the changes in structural and optical properties of the film. Effects of substrate on the properties were also investigated. INTRODUCTION CdS has been recognized as a promising n-type window material for CdTe/CdS and CuInSe 2/CdS heterojunction thin film solar cells. Since quality of window materials has been critical for fabrication of high efficiency solar cells, there have been many researchers focused on manufacturing high quality CdS thin films that are transparent and electrically conductive. Deposition techniques of evaporation, electrodeposition, close spaced sublimation, chemical bath deposition, spray pyrolysis, and rf sputtering have been employed for the preparation of CdS thin films. Among these techniques, chemical bath deposition has been one of the most commonly accepted processes due to advantages of low cost, convenience, low temperature process, and large size fabrication. Various types of reactants, deposition parameters, and methods have been investigated for preparation of high quality thin film CdS. Kaur et al. [1] have studied the growth kinetics of CdS films during chemical bath deposition with respect to temperature and relative concentrations of the various reactants in the solution. It has been proposed that the films grow either by ion-by-ion condensation of Cd 2' and S2- ions at the surface (heterogeneous reaction) or by adsorption of colloidal particles of CdS (homogeneous reaction) formed in the solution., depending on the deposition parameters and the method of preparation. The films obtained from the heterogeneous reaction have been thin, hard, adherent and specularly reflecting. T. Nakanishi et aL [2] compared the properties of a CdS thin film deposited from different reaction baths containing cadmium halide (CdX 2), ammonium halide (NI- 4X), and thiourea. The film prepared from iodides and chlorides showed optical band gap of 2.62 and 2.45eV, respectively and dark resistivity of 108-10 and 10 6-c •na ec, respectively. T.L. Chu et al. [3] characterized the structural, electrical, and optical properties of CdS films deposited on glass and Sn0 2:F/glass substrate. From the investigation of boron addition effects on the electrical properties, it was found that the addition of boron into the solution reduces the dark resistivity. Maximum effects were reported when the B0 3 /Cd 2+ ratio was 0.001. In this study, we prepared CdS thin films from chemical baths containing cadmium acetate, thiourea, ammonia, and ammonium acetate. We investigated the changes in structural and optical properties of CdS film with respect to various fabrication conditions such as the concentration of reactants, reaction temperature, and heat treatment. Effects of substrate on the properties were also investigated. 277
Mat. Res. Soc. Symp. Proc. Vol. 410 0 1996 M
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