Electrical and optical properties of ion-activated CdS thin films

Abstract

This dissertation considers the fabrication and resulting electrical and optical properties of Bi ion-implanted CdS thin films. An optimum doping density, anneal temperature and time for maximum sensitization has been determined. With an implantation dose of 10(exponent_15) ions cm(exponent_-2) the light-to-dark current ratio is 50 times that of an unimplanted sample for an illumination level of 75 foot-candles. Damage which resulted due to implantation is studied with Scanning Electron Microscopy. The observed dark spots are interpreted as small interstitial clusters or unresolvable interstitial dislocation loops produced by displacement cascades from the bombarding ions. Observed superlinearity of photo current with light intensity is supported with a theoretical calculation based on a two-state model. Theoretical calculations are made from both closed form and numerical analytical approaches. Spectral response curves are interpreted in terms of volume and surface excitation of photo-carriers. In addition it is noted that the threshold for the absorption edge can be varied and the longer wavelength spectral response can be broadened with implantation dose. Variation of time constants with doping densities and light intensity is experimentally determined and theoretically analyzed. Moreover, their wavelength dependence is noted. A linear trap distribution profile affecting the response time characteristics is observed.