Positron lifetime and 2-dimensional angular correlation of annihilation radiation distributions in float-zone material were determined between room temperature and about 10K. The specimens contained di-vacancies with charge states of V20, V2-1 or V2-2. These charge states were obtained by using a suitable combination of dopant type, dopant concentration, and 15MeV electron irradiation dose. It was found that the positron lifetime of the negatively charged di-vacancy increased with temperature, while that of the neutral di-vacancy exhibited very little change with temperature. The positron trapping rate increased markedly with decreasing temperature. This was found to be true not only of negative di-vacancies but also of the neutral di-vacancy. The 2-dimensional angular correlation of annihilation radiation data on positrons which were trapped at di-vacancies exhibited almost the same distribution for the various charge states. By using a specimen which contained oriented di-vacancies, a small but definite anisotropy was detected in the distribution of positrons trapped at V2-.

M.Hasegawa, A.Kawasuso, T.Chiba, T.Akahane, M.Suezawa: Applied Physics A, 1995, 61[1], 65-70