Atomic structure and properties of polar Ge-GaAs(100) interfaces

We present self-consistent calculations of the total energy, charge density, and electronic states of "compensated" (100) interfaces of Ge and GaAs. In this paper we test the methods using the simplest compensated interfaces—an abrupt single mixed layer, either (1/2)(Ge + Ga) or (1/2)(Ge + As), using the "average atom" approximation for the interface plane. The (1/2)(Ge + As) interface is found to be the more stable (however, this may be an artifact of the averaging). We find a large variation in the interface dipole with stoichiometry. This is not observed experimentally, suggesting that the stoichiometry is, in fact, varying only slightly in experimental Ga-rich and As-rich interfaces. No interface states are found in the fundamental gap although there are localized states at $J$ and $K$ in the two-dimensional Brillouin zone. Several points are discussed improving the efficiency of the self-consistent calculations. Finally, variation of the dipole with relaxation is used to determine interface effective charges.