Raman scattering by acoustic phonons in semiconductor superlattices
We study raman scattering by acoustic phonons in semiconductor superlattices and multiple quantum wells and show that it allows novel access to growth-related and intrinsic material properties. In the ideal superlattice the acoustic-phonon dispersion is backfolded and gaps appear at edge and center of the mini-Brillouin zone. Raman spectra show doublets of folded acoustic phonons with a fixed transfer of crystal momentum. In real superlattices interface roughness and layer thickness fluctuations cause Raman scattering with phonons for which crystal-momentum conservation is partially relaxed. Under resonant excitation we observe continuous emission which reflects features of the folded-phonon density of states such as internal and zone-edge dispersion gaps. From these features we determine the lateral extent of interface growth islands. The resonance behavior of folded-phonon scattering and continuous emission depends on homogeneous and inhomogeneous broadenings of the electronic structure which we determine separately. From the temperature dependence of these effects we obtain information on electron-phonon coupling constants in two-dimensional systems.
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