Stable fourfold configurations for small vacancy clusters in silicon from ab initio calculations

Using density-functional-theory calculations, we have identified new stable configurations for tri-, tetra-, and pentavacancies in silicon. These new configurations consist of combinations of a ring hexavacancy with three, two, or one interstitial atoms, respectively, such that all atoms remain fourfold. As a result, their formation energies are lower by 0.6, 1.0, and 0.6 eV, respectively, than the "part of a hexagonal ring" configurations, believed until now to be the lowest-energy states.