Magic number vacancy aggregates in Si and GaAs - structure and positron lifetime studies

We investigate structural properties of large vacancy agglomerates in Si an d GaAs using a self-consistent-charge density-functional based tight-bindin g method. We also calculated the defect-related positron lifetimes. Strong evidence is found for the existence of vacancy aggregates with unusual magi c numbers in GaAs. In contrast to Si - the first stable agglomerate consist s 12 vacancies instead of 6. This findings fit into experimental observatio ns on deformed and irradiated samples. (C) 1999 Elsevier Science B.V. All r ights reserved.