Raman study of interface phonons in GaAs/AlAs quantum wells: Resonance with the e2-h2 exciton

We have performed light-scattering experiments from two GaAs/AlAs quantum-well structures. In one sample, the second conduction confinement subband e2 is near degenerate with the AlAs X-band minimum. Raman spectra excited resonantly with the e2-h2 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 cm-1 whereas in nonresonant Raman experiments the observed shift is 405 cm-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 e2 state is below the X-band minimum, contain only phonon modes associated with the GaAs layers.