Positron annihilation studies, which combined lifetime and Doppler broadening measurements, were made of 10MeV electron-irradiated float-zone material. Following irradiation, a lifetime of 305ps was observed at 300K. This decreased to 290ps at 30K, and the positron trapping rate decreased markedly with increasing temperature. The Doppler measurements, when coupled with the lifetime data, yielded a defect S-value that was 6.7% larger than that for the bulk, and was nearly twice as high as the value which had previously been reported for di-vacancies. Isochronal annealing of the 1.8 infra-red absorption band was associated with an appreciable change in the defect S-value, until it was 3.8% larger than the bulk value. On the other hand, the trapping rate at 50K decreased by only 20% during the annealing out of the 1.8 infra-red absorption, while the positron lifetime remained essentially constant. It was suggested that loose vacancy complexes, a so-called sponge defect that consisted of resolvable mono-vacancies, formed during annealing as an intermediate step in vacancy clustering.

V.Avalos, S.Dannefaer: Physical Review B, 1996, 54[3], 1724-8