Optical transitions in semiconductor superlattices with small band offset in one band and large in the other

Abstract

We show that optical transitions in superlattices (SLs) with small band offset in one band and large in the other have unique characteristics that are not found in `standard' SLs (i.e. those with significant offsets in both bands). We demonstrate this using a simple analytical model, which allows us to express wave function overlaps between various conduction and valence subband states in a way that keeps the physics clearly in evidence. It is found that, because of the very different confinement properties of the conduction and the valence band states, optical transitions in such large-small offset SLs can actually take place between any pair of conduction and valence subband states both at the SL Brillouin zone centre and at the zone edges, as long as the states have the same parity. We also obtain analytical expression for the dependence of the wave function overlap on the SL geometry, and we identify the geometrical condition required to maximize the intensity of specific transitions. The above results, demonstrated analytically, are found to be in very good agreement with numerical calculations obtained via a rigorous multiband formalism, thus providing verification for the analytical model and for its predictions.