Low-temperature positron-lifetime studies of proton-irradiated silicon

The positron-lifetime technique has been used to identify defects created in high-purity single-crystal silicon by irradiation with 12-MeV protons at 15 K, and the evolution of the defects has been studied by subsequent annealings between 20 and 650 K. Two clear annealing steps were seen in the samples, the first starting at 100 K and the other at 400 K. The first is suggested to be a result of the migration of free, negatively charged monovacancies, and the second is connected to the annealing of some vacancy-impurity complexes, probably negatively charged vacancy-oxygen pairs. The specific trapping rate of positrons to both of these negatively charged monovacancy-type defects has been found to have a clear ${\mathit{T}}^{\mathrm{-}0.5}$ dependence. The positron lifetime in perfect Si is measured to be 217±1 ps, and the monovacancy lifetime is found to be 275±5 ps. Also the negatively charged vacancy-oxygen complexes were found, both experimentally and theoretically, to give rise to a positron lifetime of about 275 ps.