Origin of the optical transitions in ordered SiGe(001) superlattices.

We report new experimental and theoretical results for SiGe (001) strained layer superlattices. The optical transitions are accounted for on the basis of first principles calculation of the quasiparticle energies and a quantum well analysis of the energy levels. In particular, the lowest transition observed near 0.8 eV is indirect, consistent with the shape of the photocurrent. However the amplitude of the corresponding electroreflectance feature is considerably larger than expected. Effects due to confinement in the narrow quantum wells that form the superlattice are apparent.