Resonant-Raman-scattering study on short-period Si/Ge superlattices

Resonant Raman scattering is performed between 1.6 and 3.0 eV on a series of strain-symmetrized (Si${)}_{\mathit{n}}$(Ge${)}_{\mathit{n}}$ superlattices with n=4–12 and a (Si${)}_4$(Ge${)}_{12}$ superlattice pseudomorphic to the Ge substrate. The dependence of the resonance energies and the degree of localization of states on single-layer thickness and strain are investigated. The results are compared with band-structure calculations and electroreflectance results on similar samples. We observe strong transitions confined within the Ge layers below 2.5 eV and transitions that are attributed to the intermixed interface regions, since they appear independently of layer thickness for all phonon modes around 2.6 eV. The shift of the dominant Ge-like transitions with layer thickness is much weaker than calculated for the localized ${\mathit{E}}_0$ superlattice transitions. These resonance maxima cannot be explained by bulk Ge-like ${\mathit{E}}_1$ transitions, since their energy is below ${\mathit{E}}_1$ of strained bulk Ge. Folded acoustic modes show a pronounced upward shift of the energy of the resonance maximum with increasing mode index. Forbidden confined ${\mathrm{LO}}_{\mathit{m}}$ modes with even index m were observed under resonance conditions in nonpolar superlattices.