Evaporation mechanism of CdS single crystals. II. Diffusion controlled evaporation of cadmium- and sulfur-doped CdS

Abstract

Excesses of cadmium and sulfur over that of stoichiometry are found to lower the evaporation rate of CdS single-crystal c faces in the temperature range 700°-740°C. Initial rates for highly doped crystals are an order of magnitude lower than the steady-state evaporation rate. An initial induction period (0.1-6 h) is followed by a rapid upturn and subsequent attainment of the steady-state rate. This transient behavior is accompanied by drastic changes in crystal conductivity. Experiments indicate that excesses of cadmium or sulfur in the bulk, diffusing to the surface, cause the effect. A theory for evaporation controlled by the out diffusion of excess cadmium or sulfur is derived which, under proper conditions, yields a very convenient determination of the diffusion constant. There is evidence of the presence of precipitated Cd in the heavily doped crystals. The actual diffusing species in these experiments is presumed to be sulfur vacancies.