New Optical Transitions in Ge-Si Atomic Layer Superlattices Grown on (001) Ge

Reflectance spectra taken at 300K and 20K show the presence of new optical transitions in Ge-Si Superlattices with 10 atomic layer periodicity. A prominent new optical transition is clearly resolved 70 meV below the lowest direct optical transition in Ge. Both the amplitude and narrow linewidth of this superlattice-induced feature suggest behavior characteristic of an allowed direct optical transition. Our results can be interpreted in terms of the folding of the strain split-off Si (001) conduction band minimum to the zone center by the superlattice potential. The measured transition energy is in good agreement with calculations based on the envelope function approximation.