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 H and valence-band offsets for the heterovalent (Si2)n/(GaP)n, (Ge2)n/(GaAs)n, and (Si2)n/(GaAs)n superlattices with repeat periods n4 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 n2; [111] are predicted to reconstruct for n6. Different types of reconstruction lead to different band offsets. A simple model for H 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 Hs. © 1990 The American Physical Society.