THEORY OF ZONE-FOLDED OPTICAL-TRANSITIONS IN SEMICONDUCTOR SUPERLATTICES

A one-dimensional two-band tight-binding model is used to develop simp le 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 op tical transitions calculated for the III-V superlattices as being due to the similarities of the constituents and predicts strong optical tr ansitions for large band discontinuities and type I alignment, particu larly in Gap/Si2. The optical strength in the Si2/Ge2 superlattice is found to depend strongly on the growth direction and on strain. The os cillator strength calculated by the simple model is checked in two cas es against a more elaborate calculation using the sp3d5 empirical tigh t-binding band structure and atomic wave functions.