Optical absorption spectroscopy of Si-Ge alloys and superlattices

We report calculations and measurements of the optical absorption coefficient of SinGem superlattices and alloys where n approximate to m. Our results can be used to determine the fundamental band gap, the dependence of the band gap on superlattice period, and the absorption coefficient near the band-gap energy. Both theory and experiment show that all short-period (n+m<20) superlattices display a similar linear dependence of absorption coefficient on photon energy. This behavior is different from alloys of the same average composition where we measure that the absorption coefficient increases with the 3/2 power of photon energy.