Electronic and optical properties of ultrathin Si/Ge (001) superlattices

We report the electronic and optical properties of very-small-period (10–30-Å) Si/${\mathrm{Si}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Ge}}_{\mathrm{x}}$ superlattices. We present the first conduction dispersion relations (band structures) and the oscillator strengths for these systems. The magnitude of the oscillator strength associated with the transition across the fundamental superlattice gap is explained in terms of the relative importance of the strain and microscopic potential. In particular, it is found that the oscillator strength strongly reflects the relationship between the degree of momentum mixing and the corresponding wave-function overlap in real space concerning the states involved in the transition. A detailed description is given of the symmetry properties of the optical spectra in question and their relationship to the splitting of the degeneracy at the top of the valence band.