Electronic structure of [110] Si-Ge thin-layer superlattices

First-principles electronic structure calculations for SinGen superlattices ( for n=4, 6, and 8) grown epitaxially on a (110) Si substrate reveal a nearly direct band gap (to within ≊0.04 eV for n=4) despite the pronounced indirectness of its constituents. This is unlike superlattices grown in the [001] direction which are indirect when grown on Si and quasi-direct only on substrates with larger lattice constants, e.g., Ge. Transition dipole matrix elements for the lowest energy direct transition vanish for all repeat periods n but are finite for several other new low-energy transitions.