GERMANIUM NEGATIVE-U CENTER IN GAAS

The DX center related to the Ge impurity in GaAs is investigated by ab initio pseudopotential calculations within the local-density aproxima tion. Our results indicate that the behavior of the Ge-Ga defect is qu alitatively different from the broken-bond model usually associated to Si-Ga, even if the electronic structure behaves in a very similar way . Indeed, for the Ge impurity our calculations show that already for b reathing-mode relaxations of the Ge neighbors, in T-d symmetry, a nega tive-U behavior is found, and many details of the experimental data ca n be explained.