STRAIN AND DIPOLE EFFECTS IN COVALENT-POLAR SEMICONDUCTOR SUPERLATTICES

The energetics and electronic structure of lattice-matched (Ge)4/(GaAs)2 and strained, pseudomorphic (Si)4/(GaAs)2 (001) semiconductor superlattices have been studied with use of a self-consistent-field pseudopotential method. The interfaces are assumed to be uniform, but the interlayer distances of the pseudomorphic lattice are optimized to achieve a minimum-total-energy configuration. The calculated enthalpy of formation is in the 100-meV/atom range for these two superlattices, which is almost an order of magnitude larger than the strain component in (Si)4/(GaAs)2. The superlattice dipole induces a metal-insulator transition by periodically tilting the potential. The electrostatic energy derived from this dipole field is the main cause of the instability relative to disproportionation.