Samples were studied in the as-grown state, or after 2MeV electron irradiation at low (4K) temperatures. A positron bulk lifetime of 235ps was measured in non-irradiated samples; thus indicating that the concentration of neutral and negatively charged vacancies was lower than 5 x 1015/cm3. The positron bulk lifetime was calculated theoretically and was found to be in good agreement with the experimental value. The defect-related lifetimes for mono-, di-, and trivacancies were also theoretically determined. An increased average positron lifetime after electron irradiation indicated the appearance of open-volume defects; most probably ones of divacancy type. The disappearance of this defect was observed during annealing at temperatures below 250K. Other defects were formed, thus leading to a divacancy signal which was stable up to at least 600K. It was concluded that defect complexes which were undetectable by positrons existed at temperatures ranging from 250 to 450K. These were probably complexes without vacancies, or positively charged defect complexes which contained vacancies.

A.Polity, R.Krause-Rehberg, T.E.M.Staab, M.J.Puska, J.Klais, H.J.Möller, B.K.Meyer: Journal of Applied Physics, 1998, 83[1], 71-8