Electronic and acoustic-phonon inter-Landau-level Raman scattering in GaAs/AlxGa1-xAs multiple quantum wells.

Abstract

We present an experimental study of inter-Landau-level excitations in undoped GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As multiple quantum wells in high magnetic fields by means of Raman scattering. The experiments were performed in Faraday backscattering geometry with the field along the growth axis, using circularly polarized light for resonant excitation of low-index magneto-optical transitions between Landau levels. We observe two types of peaks. One of them, present in both Stokes and anti-Stokes regions at a constant Raman shift, corresponds to the electron cyclotron energy. We attribute it to electronic Raman scattering from a quasistationary population of photoexcited carriers. To account for the observation of these lines, which are forbidden in backscattering geometry, we present a theory of electronic Raman scattering that includes the effect of interface roughness. Such roughness allows Landau-number nonconserving steps and makes cyclotron energy scattering of electrons possible. Another type of structure occurs at the energy of magneto-optical interband transitions close to the exciting laser and is due to coherent acoustic-phonon emission processes. The breakdown of crystal momentum conservation along the growth axis due to layer thickness fluctuations leads to scattering by phonons from the whole Brillouin zone. We discuss possible acoustic-phonon scattering mechanisms and show that two-phonon emission causes triple resonances, which are responsible for the luminescencelike peaks observed at interband magneto-optical transition energies. The observation of cyclotron scattering in resonant Raman experiments leads to a way of determining electronic masses in undoped quantum well structures.