By using a density functional based tight-binding method, an investigation was made of the stability of various vacancy clusters; up to a size of 17 vacancies. In addition, positron lifetimes were computed for the most stable structures in order to compare them with experimental data. A simple bond-counting model was extended to take into account the formation of new bonds. This yields a very good agreement with the explicitly calculated formation energies of the relaxed structures for V6 to V14. The structures, where the vacancies form closed rings, such as V6 and V10, were especially stable against dissociation. For these structures, the calculated dissociation energies were in agreement with experimentally determined annealing temperatures and the calculated positron lifetimes were consistent with measurements.

Stability of Large Vacancy Clusters in Silicon. T.E.M.Staab, A.Sieck, M.Haugk, M.J.Puska, T.Frauenheim, H.S.Leipner: Physical Review B, 2002, 65[11], 115210 (11pp)