Electrical properties of cadmium sulfide thin films in the thickness direction

Abstract

High-resistivity photoconductive thin films of CdS have been obtained by dc sputtering and by dual source evaporation, without the use of post-deposition heat treatments. Light-to-dark conductivity ratios up to 105 have been observed in the films. Spectral response and sensitivity are similar for photoconductivity in the thickness direction and in the plane of the films, but appreciable space-charge effects and current saturation were observed for current flow in the thickness direction. In darkness, the films exhibit space-charge-limited transient current in the presence of a high trap density, while under photoexcitation current flow is linear up to a definite saturation field, and constant at higher fields. The current saturation, attributed to piezoelectric effects in the CdS, occurs at (2–4) × 104 V/cm. Transient currents in excess of the saturation level are observed for short periods of time. Although the dc current-voltage characteristic is linear below saturation, small-signal ac conductivity is proportional to dc current. A model is presented that explains the ac and dc characteristics in terms of a distribution of localized centers in the vicinity of the dark Fermi level of the material. The density of these states is sufficient to suppress space-charge effects in the steady-state photocurrent and dark current by trapping nearly all injected charge in a distance short compared to the film thickness.