The structural properties of large vacancy agglomerates were investigated by using a self-consistent charge density functional based tight-binding method. Calculations were also made of defect-related positron lifetimes. Clear evidence was found for the existence, in GaAs, of vacancy aggregates having unusual magic numbers. Unlike the case of Si, the first stable agglomerate consisted of 12 vacancies instead of 6. This prediction was in agreement with experimental observations of deformed and irradiated samples.

Magic Number Vacancy Aggregates in Si and GaAs - Structure and Positron Lifetime Studies T.E.M.Staab, M.Haugk, A.Sieck, T.Frauenheim, H.S.Leipner: Physica B, 1999, 273-274, 501-4