The effect of group-IV impurities (C, Si, Ge) was investigated by using a first-principles pseudopotential method within the local density approximation. It was shown that a broken-bond configuration of group-IV impurities in cation sites occurred for the negative charge state, but was always metastable. The lowest barrier to the metastable transition was found for the SiGa defect. The DX center of Si-doped material could involve a large lattice relaxation whereas, in the case of Ge impurity, breathing-mode relaxation of the Ge neighbors in Td symmetry produced a negative-U behavior which was typical of DX centers. The C levels were low in the gap, even for the cation-substitutional site. Off-center displacements were not favored, and DX behavior was not predicted for this impurity. The behavior of the impurities could be understood as involving a competition between mechanical and quantum effects. These included differences in the covalent radii of impurity and host atoms, and other factors: bonding for anion-substitutional and anti-bonding for cation-substitutional.

T.M.Schmidt, A.Fazzio, M.J.Caldas: Materials Science Forum, 1995, 196-201, 273-8