Resonance Raman scattering in GaAs/AlAs thin-layer superlattices under high pressure

We report observations of resonant Raman scattering by confined-optical and interface-vibrational overtones and their combinations in thin-layer GaAs/AlAs superlattices. The superlattice electronic-band energies are brought into resonance with the excitation photon energy via pressure tuning of the band gap. Resonance is determined to be with $E_{1h}$, the lowest-energy Γ-like valence-to-conduction quantum-well subband (n=1 heavy hole), for outgoing scattering via the Fröhlich electron-phonon interaction. This technique permits measurement of the $E_{1h}$ energy well above the pressure at which the superlattices become indirect-band-gap materials. Experimental results are compared with theoretical predictions for the phonon and energy-band symmetries for resonance due to Fröhlich electron–optic-phonon and electron-interface vibration scattering. The two-phonon resonance is seen to diminish in strength with decreasing superlattice period, despite the fact that the density of internal surfaces is increasing. This is attributed to weak broadening of the superlattice $E_{1h}$ band gap.