Resonant interference effects in the phonon Raman spectra of (311) GaAs/AlAs superlattices

We study the dependence on laser frequency (${\mathrm{\omega }}_{\mathit{L}}$) of the Raman spectra of GaAs/AlAs superlattices (SL’s) grown along the [311] direction. The observed phonon modes follow the polarization selection rules predicted by the SL crystal symmetry and, at the highest ${\mathrm{\omega }}_{\mathit{L}}$, the intensities predicted by the bulk Raman tensor. For lower ${\mathrm{\omega }}_{\mathit{L}}$ we observe a complex variation in the intensities of the confined LO and TO modes which deviates sharply from these predictions. We interpret these deviations as due to interference between a background associated with the ${\mathit{E}}_1$ and ${\mathit{E}}_1$+${\mathrm{\Delta }}_1$ gaps and a resonant contribution arising from the hh2→e2 transition of the SL, both induced by the deformation potential interaction. For the latter, the electron-phonon coupling with the hole bands is diagonal, in contrast to the off-diagonal (hh→lh) coupling of conventional (100) SL’s. Evidence for a Fröhlich induced resonance at the ${\mathit{E}}_0$+${\mathrm{\Delta }}_0$ gap of the SL’s is also obtained.