New optical transitions in strained Si-Ge superlattices

First-principles total-energy and electronic-structure calculations for SiₙGeₙ superlattices grown epitaxially on an (001) Si substrate reveal a nearly direct band gap despite the pronounced indirectness of its constituents. Whereas the lowest conduction-band wave function extends over both Si and Ge sublattices, the (higher energy) lowest direct state shows strong confinement to Si states. We predict that a substrate with a larger lattice constant than Si will produce a nearly direct band-gap superlattice (indirect only by 0.01 eV).