Effects of hydrostatic pressure on the optic and transverse-acoustic vibrations of a (012) GaAs/AlAs superlattice

We report on hydrostatic-pressure Raman measurements (to 2 GPa) of a superlattice composed of pure GaAs and pure AlAs grown in the (012) direction. We observe Raman and resonance-Raman scattering, where the resonance is achieved by pressure tuning the direct superlattice transition to match the excitation photon energy. The GaAs confined longitudinal and transverse optic modes are seen to resonate via the Fröhlich electron-phonon interaction. This is possible for the transverse mode due to mixing with the GaAs longitudinal-optic vibrations. Growth in this direction permits the observation of folded quasitransverse-acoustic phonons. The observation of these modes in first-order Raman scattering is supported by the weak pressure softening at a relative rate of -0.025±0.005 ${\mathrm{GPa}}^{\mathrm{-}1}$, implying a mode-Grüneisen parameter of -1.9±0.4 for the transverse-acoustic phonon.