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 T-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

Structure of dislocations in Al and Fe as studied by positron-annihilation spectroscopy

Positron-lifetime-spectra measurements have been performed on single crystals of Al and polycrystals of Fe deformed under the tensile mode at room temperature and 77 K, respectively. It is shown that the positron-trapping component in Al depends on the number of slip systems activated: The positron lifetime is 215, 220, and 240 ps when single-, double-, and multiple-slip planes are activated, respectively. This dependence is well understood if positrons annihilate at dislocation-associated defects (jogs) with different positron-annihilation characteristics revealing different jog structures. In iron, the positron-lifetime spectra associated with dislocations were found to be independent of the annealing temperature between 110 and 360 K. In this temperature range, screw dislocations should transform into nonscrew dislocations, suggesting that the positrons are not sensitive to the different types of dislocations. These results are interpreted on the basis that positrons annihilate at defects associated with the dislocations rather than at the dislocation lines. © 1992 The American Physical Society.Peer Reviewe