Vacancies in SiGe: Jahn-Teller distortion and spin effects

The electronic structure of a vacancy in silicon-germanium is studied using ab initio total-energy minimization methods. The calculations are based on density-functional theory in the local-spin-density approximation. We repo rt ionic relaxations, defect formation energies and ionization levels of Si and Ge vacancies in a zinc blende model structure (SiGe). The Ge vacancy i n SiGe is characterized by symmetry-lowering Jahn-Teller (JT) distortions a nd a negative-effective-U effect, in those respects resembling the vacancy in elemental silicon. For Si vacancy, the exchange-coupling energy is found to overcome the JT energy, and symmetric high-spin ground states are predi cted. (C) 2000 American Institute of Physics. [S0003-6951(00)03028-X].