Magic number vacancy aggregates in GaAs: Structure and positron lifetime studies

To investigate the stability of various atomic arrangements of vacancy aggr egates in GaAs, we make use of a self-consistent-charge density-functional- based tight-binding method. In contrast to silicon, where the corresponding number is 6, we find the first such stable-aggregate to be made up of 12 v acancies. This is explained by the added possibility of energy-lowering: Ga -Ga and As-As dimer formation. Since these results are consistent with meas ured and calculated defect-related positron lifetimes, they give powerful e vidence for the existence of such magic number vacancy aggregates in GaAs.