Theoretical calculations of heterojunction discontinuities in the Si/Ge system

We present a theoretical study of the structural and electronic properties of pseudomorphic Si/Ge interfaces, in which the layers are strained such that the lattice spacing parallel to the interface is equal on both sides. The self-consistent calculations, based on the local density functional and ab initio pseudopotentials, determine the atomic structures and strains of minimum energy, and the lineup of the Si and Ge band structures. The presence of the strains causes significant shifts and splittings of the bulk bands. We derive values for the band discontinuities for (001), (111), and (110) interfaces under different strain conditions, and discuss the validity of the density-functional methods for the analysis of the interface problem. Spin-orbit splitting effects in the valence bands are included a posteriori. We express our results in terms of discontinuities in the valence bands, and deformation potentials for the bulk bands, and compare them with recent experiments on Si/${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ge}}_{\mathrm{x}}$ heterostructures.