Theory of optical transitions in Si/Ge(001) strained-layer superlattices

Abstract

Superlattices alternating ultrathin (two, four, and six atoms) Si and Ge layers form artificial compound semiconductors. Trends and accurate transition energies for these materials are calculated based on the local-density-functional and quasiparticle self-energy approaches. The lowest transition for the Si substrate material is found to be indirect, but direct transitions to a zone-folded final state occur at slightly higher energy. The strain can be transferred to the Si layers with opposite sign by growing the superlattice on a Ge substrate. This reverses the order of the indirect and direct zone-folded transitions, which is predicted to yield an approximately direct-gap material. However, the allowed dipole matrix elements are small for these new transitions. The description of the near-band-edge states in these materials in terms of quantum wells in an effective-mass approach is found to be reasonable. In particular, both strain and confinement in the ultrathin quantum wells forming the superlattice are important. The quasiparticle energies for the particular case of the 4×4 structure have been calculated using the self-energy approach. This allows a direct comparison of the calculated transitions to recent experimental spectra for this material. In particular, the detailed features in those spectra are explained.