Thin [001] and [110] GaAs/AlAs superlattices: Distinction between direct and indirect semiconductors

We have calculated oscillator strengths of across-gap transitions in n×{}n GaAs/AlAs superlattices (n=1,2) grown along the [001] and [110] directions using the augmented spherical wave ab initio band-structure method. We find that the rate for radiative emission across the direct gap in the very thin [001] superlattices is at least 2 orders of magnitude smaller than the corresponding quantity in GaAs. The radiative transition rates across the direct and indirect gaps turn out to be comparable in these superlattices. The 2×{}2 [110] superlattice is direct and has a direct-gap oscillator strength which is 78% of that of GaAs.