POSITRON LIFETIME AND 2D-ACAR STUDIES OF DIVACANCIES IN SI

We have measured positron lifetime and Two Dimensional Angular Correla tion of Annihilation Radiation (2D-ACAR) distributions of Floating-Zon e grown (FZ) Si specimens containing divacancies (V-2) with the defini te charge states, V-2(0), V-2(-1) or V-2(-2) from room temperature to about 10 K. These charge states are accomplished by an appropriate com bination of dopant species, their concentration and irradiation doses of 15 MeV electrons with reference to the currently accepted ionizatio n level of divacancies. The positron lifetime of the negatively charge d divacancy increases with temperature, while that of the neutral diva cancy shows little change with temperature. The positron trapping rate , obtained from lifetime and 2D-ACAR measurements, increases markedly with decreasing temperature. This is found not only for the negative d ivacancies but also for the neutral divacancy. We need a model which e xplains this temperature dependence. The 2D-ACAR distribution from pos itrons trapped at divacancies shows nearly the same distribution for t he different charge states, which differs considerably from the case o f As vacancies in GaAs studied by Ambigapathy et al. We have observed a small but definite anisotropy in the distribution of trapped positro ns in V-2(-) using a specimen containing oriented divacancies.