The mechanism and kinetics of chemisorption of oxygen on thin evaporated films of CdS have been investigated through changes in electrical conductivity caused by chemisorbed oxygen. Electrical conductivity of CdS films, when exposed to oxygen atmosphere falls exponentially with time, with a characteristic time determined by several factors such as temperature, electric field applied to the sample, illumination, etc. Electric field applied in appropriate direction is shown to enhance oxygen chemisorption. Complete theoretical treatment for these changes has been provided and the agreement between theory and experiment is shown to be excellent.

Photoelectric properties of pure and Al doped vacuum deposited CdS films have been studied to explore the possibility of their application in photoactivated liquid crystal light valves. The effect of heat treatment in oxygen atmosphere, rate of deposition and the extent of Al doping, etc. on the resistivity, photo-response spectral characteristics, rise and decay time of photo-current, etc. have been investigated. It has been found that in contrast to pure CdS films, the properties of Al-doped films significantly depend on both the rate of deposition and the extent of Al doping. The dark resistivity in all cases was found to increase with heat treatment in oxygen. Unlike pure CdS films, Al doped films show photo-conductivity which is enhanced by heat treatment. Al doped films deposited at higher rates show better photo response even at lower light levels. At various light levels the rise and decay time of Al-doped films were found to be fairly constant and lower than that for pure CdS films. All these properties have been explained in terms of the presence of trapping levels due to doping. These trapping levels are also indicated by TSC, optical absorption and EPR studies.