Field effect and the role of excitons in the photoconductivity of CdS

Abstract

For many crystals of cadmium sulfide, the photoconductivity spectrum contains a great deal of structure due to excitons. In particular, two types of spectra have been found: type I, wherein photoconductivity maxima correspond to absorption maxima, and type II, wherein photoconductivity minima correspond to absorption maxima. Various methods have been used to change one type of response into the other, among them, heat treatment, mechanical surface treatment, electron bombardment, and ultraviolet irradiation. The evidence presented in this paper supports the view that the common denominator in all these treatments is the change in the electric field near the surface. The surface field is a sensitive function of defect concentration, adsorbed oxygen, and free carrier concentration.