Cadmium zinc sulfide films and heterojunctions

Abstract

Cadmium sulfide (CdS) and zinc sulfide (ZnS), direct gap semiconductors with room temperature band-gap energy of 2.42 and 3.66 eV, respectively, form a continuous series of solid solutions (Cd1−xZnxS). The band-gap energy of Cd1−xZnxS can be tailored in the range of the binary band gaps. In this work, polycrystalline films of Cd1−xZnxS have been deposited on glass, SnO2:F/glass, and ZnO:F/glass substrates by the reaction of dimethylcadmium (DMCd), diethlyzinc (DEZn), and propyl mercaptan (PM) in a hydrogen atmosphere. The deposition rate and properties of Cd1−xZnxS films depend on the substrate temperature and the composition and flow rate of the reaction mixture. The deposition rate of Cd1−xZnxS films has been measured at 375 and 425 °C as a function of the DMCd/DEZn molar ratio in the reaction mixture. Without intentional doping, the deposited films are of high lateral resistivity, and the resistivity increases with increasing ZnS concentration. The electrical resistivity of the deposited films can be reduced by using octyl chloride or trimethylaluminum as a dopant. The effects of DMCd/DEZn and (DMCd+DEZn)/PM molar ratios on the optical and electrical properties of Cd1−xZnxTe films have been investigated. Thin film heterojunctions have been prepared by the successive in situ metal organic chemical vapor deposition of Cd0.7Zn0.3S (Eg∼2.8 eV), an absorber, and the ohmic contact on a ZnO:F/glass substrate, and their electrical and photovoltaic properties characterized.