Vacancy aggregates in silicon

The properties of vacancy aggregates in crystalline silicon are studied using density-functional-based molecular-dynamics simulations in large periodic supercells as well as approximate nb initio and ab initio Hartree Fock in molecular clusters. The stability and properties of aggregates of up to seven vacancies are discussed. The central results deal with the remarkable properties of the ring hexavacancy. Theory predicts it to be very stable, trigonal, planar, electrically inactive, and virtually invisible by photoluminescence and infrared-absorption spectroscopy. However, it could be Raman active. This defect is most likely a gettering center and the nucleus or precursor of a range of extended defects. Further, it suggests that the history of the sample may play an important role. [S0163-1829(97)02740-9].