Vacancy aggregates in silicon

We have calculated the lowest-energy configurations, stability, and electronic structures of vacancy aggregates (V_n) containing up to n=7 vacancies. The calculations were done using first-principles tight-binding molecular-dynamics simulations (in periodic supercells) and at (and near) the ab-initio Hartree-Fock level (in molecular clusters). The results show that the reaction V_n-1+ V₁ → V_n is exothermic at least up to n=7. The most stable aggregate, by far, is the ring-hexavacancy (V₆) which has remarkable properties. In contrast to the other V_n's, it is not vacancy-like: It has no deep level in the gap and its dipole moment is extremely small. It is sufficiently stable to survive high-temperature annealing. V₆ is a small trigonal void which is likely to be a gettering center and is a plausible nucleus for a range of extended defects.