Raman study of interface phonons in GaAs/AlAs quantum wells: Resonance with the ${\mathit{e}}_2$-${\mathit{h}}_2$ exciton

We have performed light-scattering experiments from two GaAs/AlAs quantum-well structures. In one sample, the second conduction confinement subband ${\mathit{e}}_2$ is near degenerate with the AlAs X-band minimum. Raman spectra excited resonantly with the ${\mathit{e}}_2$-${\mathit{h}}_2$ transition contain phonon modes associated with both the GaAs and the AlAs layers. The resonantly excited AlAs phonon modes have an average frequency of 385 ${\mathrm{cm}}^{\mathrm{-}1}$ whereas in nonresonant Raman experiments the observed shift is 405 ${\mathrm{cm}}^{\mathrm{-}1}$. This indicates that the AlAs phonon spectra observed under resonant conditions are dominated by interface modes. Resonant Raman spectra from the second sample, in which the ${\mathit{e}}_2$ state is below the X-band minimum, contain only phonon modes associated with the GaAs layers.