Defects in ZnS and ZnSe investigated by positron annihilation spectroscopy

Abstract

Positron lifetime measurements have been performed on as-grown and electron-irradiated single crystals of the wide-gap compound semiconductors ZnS and ZnSe. The temperature dependence of the positron lifetime in as-grown samples reveals the presence of certain grown-in defects which act as shallow positron traps. No evidence for positron trapping at room temperature is found in as-grown crystals. The values of 230+or-3 ps and 240+or-5 ps are proposed for the bulk positron lifetime in ZnS and ZnSe, respectively. The positron lifetime in samples electron irradiated at 320, 77 and 20 K is interpreted in terms of positron trapping at Zn vacancy-related defects and negatively charged Zn antisites. The low-temperature inhibition of the positron trapping at open-volume defects is attributed to the presence of negatively charged Zn antisites. The positron lifetime in the samples irradiated at 20 K reveals the formation of voids in ZnS and ZnSe after annealing above approximately 600 K and approximately 500 K, respectively. These voids in ZnSe appear to anneal for temperatures above 600 K.