Electronic properties of the (100) (Si)/(Ge) strained-layer superlattices

By performing local-density pseudopotential calculations, we examine the electronic structure of the (Si${)}_{\mathrm{n}}$/(Ge${)}_{\mathrm{m}}$ (n,m∼3–7) strained-layer superlattices and show how strain and the layer thicknesses interplay in determining the nature of the electron states. A group-theoretical discussion of the symmetry properties of the electron states as well as an analysis of the selection rules for dipole-allowed optical transitions are given. For the (Si${)}_4$/(s-Ge${)}_4$ (where s denotes strained) superlattice on which experiments have been reported, we find that the lowest-energy transition observed at 0.76 eV is not a direct transition. The lowest direct transition at ∼1.1 eV is not optically allowed from symmetry considerations, with the lowest allowed direct transition occurring at ∼1.2 eV. Strain in the superlattice layers is shown to have an important effect on the nature of electron states in the gap region. We show that thin superlattices grown on Ge or Si-Ge alloy substrates are promising candidates for direct band-gap materials.