The DX center which was related to the Ge impurity was investigated by means of  ab initio  pseudopotential calculations within the local density approximation. The results indicated that the behavior of the GeGa defect was qualitatively different to the broken-bond model that was usually associated with SiGa; even if the electronic structure behaved in a very similar way. In the case of the Ge impurity, the present calculations showed that - for breathing-mode relaxations of the Ge neighbors in Td symmetry - a negative-U behavior was exhibited and many details of the experimental data could be explained.

T.M.Schmidt, A.Fazzio, M.J.Caldas: Physical Review B, 1996, 53[3], 1315-21