Resonant Raman scattering in short-period (Si)n/(Ge)m superlattices.

We have measured the resonant Raman efficiencies of different modes of (Si)„/(Ge) shortperiod superlattices as well as those of a Sio.46e0.6 random alloy in the range 1.8 eV «hmL «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. The possibility of growing high-quality strained-layer superlattices made of ultrathin Si/Ge layers on Si, Ge, or Sii — „Ge„substrates has been demonstrated by several authors. ' ' Recent modulated reflectivity measurements have shown a richness of optical transitions which are attributed to folded and confined electronic states. Band calculations using diverse theoretical methods reproduce these results with varying degrees of success. These calculations also give the spatial extent of the electronic wave functions, thus allowing discrimination between states that are localized in either of the two types of layers, and those that are extended throughout the whole superlattice.