Stability and electronic structure of ultrathin [001] (GaAs)m(AlAs)msuperlattices

The general issues of stability towards disproportionation or disordering of (AC${)}_m$(BC${)}_m$ superlattices are addressed by simple model calculations based on detailed first-principles results for the (GaAs${)}_1$(AlAs${)}_1$ [001]-orientation alternate-monolayer superlattice. Valence-force-field (VFF) calculations permit isolation of strain-related contributions to the superlattice formation energy and a simple electrostatic energy model highlights the importance of charge transfer in stabilizing or destabilizing such ordered phases. We predict that bulk (GaAs${)}_1$(AlAs${)}_1$ is in fact unstable with respect to disproportionation into zinc-blende constituents because of insufficient Ga-Al charge transfer. Epitaxial growth on GaAs or AlAs simply makes this structure less unstable. A simple semiquantitative model for [001] (AC${)}_1$(BC${)}_1$ extracted from detailed self-consistent calculations makes clear the competition between (destabilizing) strain effects and (potentially stabilizing) charge transfer effects. We extract trends for thicker superlattices with the aid of VFF calculations and generalizations of the electrostatic model. We find that unstable thin superlattices become (per bond) less unstable as the repeat period increases, while stable ones become less stable per bond. Kinetic factors or surface effects must be invoked to explain the spontaneous occurrence of (GaAs${)}_{\mathrm{m}}$(AlAs${)}_{\mathrm{m}}$ structures. The electronic structure of (GaAs${)}_1$(AlAs${)}_1$ is analyzed in detail and interpreted in terms of simple distortions and band folding of the virtual-crystal-approximation band structure.