THEORY OF SI DONOR-ACCEPTOR COMPLEXES IN GAAS

Local density functional cluster calculations are carried out on Si-Ga-As-X complexes where X = Li, Be, Cu, Zn and a vacancy. We find that that the donor and acceptor move away from each other contrary to a simple argument based on a Coulomb attraction between them. This result explains the insensitivity of the Si-related local vibrational modes to the identity of the impurity and their similarity to the case when X is a vacancy.