Positron beam studies of argon irradiated CdS thin films

Abstract

Depth profiling studies of defects in CdS thin films using a low energy positron beam are reported. CdS films of 1 μm thickness on a glass substrate have been grown using the chemical bath deposition method and irradiated with 140 keV Ar ions to doses of 5×1014 and 5×1016 cm−2, respectively. The Doppler broadening line shape S parameter in as-grown and irradiated films has been monitored as a function of positron beam energy Ep. S vs Ep curves have been analyzed, using a positron diffusion model in terms of annihilations at surface, epithermal and bulk states. The S parameter at the surface (Ss) is found to be significantly higher than that in the bulk. This is understood as due to positron trapping at a large concentration of Cd vacancies in the surface region, which act as acceptors. The observed reduction of Ss with increasing Ar ion dose seems to indicate the formation of antisite defects in the surface region, brought about by irradiation. At sample depths corresponding to the peak damage layers, the observed behavior of the S parameter indicates that no large vacancy clusters are formed. Isochronal annealing studies have been carried out to investigate the thermal stability of defects in irradiated and as-grown samples.