Stability and band offsets of heterovalent superlattices: Si/GaP, Ge/GaAs, and Si/GaAs

First-principles pseudopotential theory is used to calculate the formation enthalphies ΔH and valence-band offsets for the heterovalent (${\mathrm{Si}}_2$ ${)}_{\mathit{n}}$/(GaP${)}_{\mathit{n}}$, (${\mathrm{Ge}}_2$ ${)}_{\mathit{n}}$/(GaAs${)}_{\mathit{n}}$, and (${\mathrm{Si}}_2$ ${)}_{\mathit{n}}$/(GaAs${)}_{\mathit{n}}$ superlattices with repeat periods n≤4 and growth directions G?[001], [110], and [111]. All of these superlattices are found to be unstable with respect to phase separation; the [111] system is the least unstable. The unreconstructed [001] and [111] polar superlattices have large internal electric fields, causing the [001] superlattices to reconstruct for n≥2; [111] are predicted to reconstruct for n≥6. Different types of reconstruction lead to different band offsets. A simple model for ΔH which includes the short-range nonoctet-bond energies and the long-range electro-static interactions between donor and acceptor bonds is shown to capture the chemical trends of all calculated ΔH’s.