We have measured the resonant Raman efficiencies of different modes of (Si${)}_{\mathrm{n}}$/(Ge${)}_{\mathrm{m}}$ short-period superlattices as well as those of a ${\mathrm{Si}}_{0.4}$${\mathrm{Ge}}_{0.6}$ random alloy in the range 1.8 eV≤ħ${\omega }_L$≤3.0 eV. The two peaks observed in the curves of Raman cross sections versus photon energy originate in extended (confined within the Ge layers) electronic states for the higher (lower) energy peak. These two types of optical transitions seem to merge as the layer thickness decreases. We attempt to explain these results on the basis of recent calculations of the electronic structure of these materials. As a byproduct we have obtained the absorption coefficients of our superlattices in the region below 3.1 eV.

• Received 17 April 1989

DOI:https://doi.org/10.1103/PhysRevB.40.1361