A self-consistent charge-density functional-based tight-binding method was used to investigate the stability of various atomic arrangements of vacancy aggregates. In contrast to Si, where the corresponding number was 6, it was found that the first such stable aggregate was made up of 12 vacancies. This was explained in terms of the additional possibility of energy-lowering Ga-Ga and As-As dimer formation. These results were consistent with measured and calculated defect-related positron lifetimes, and provided strong evidence for the existence of magic-number vacancy aggregates in GaAs.

Magic Number Vacancy Aggregates in GaAs - Structure and Positron Lifetime Studies T.E.M.Staab, M.Haugk, T.Frauenheim, H.S.Leipner: Physical Review Letters, 1999, 83[26], 5519-22