Theory of zone-folded optical transitions in semiconductor superlattices

A one-dimensional two-band tight-binding model is used to develop simple criteria governing the optical transition strength of a direct gap formed by zone folding in a superlattice of alternating layers of two indirect band-gap semiconductors. The model study explains the weak optical transitions calculated for the III-V superlattices as being due to the similarities of the constituents and predicts strong optical transitions for large band discontinuities and type I alignment, particularly in GaP/Si2. The optical strength in the Si2/Ge2 superlattice is found to depend strongly on the growth direction and on strain. The oscillator strength calculated by the simple model is checked in two cases against a more elaborate calculation using the sp3d5 empirical tight-binding band structure and atomic wave functions.