Doubly resonant LO-phonon Raman scattering observed with GaAs-AlxGa1-xAs quantum wells.

Abstract

Very sharp line emission has been seen in the n=1 heavy-hole exciton ${E}_{1h}$ luminescence of certain GaAs-${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Al}}_{\mathrm{x}}$As quantum-well structures under optical excitation at ${E}_{1h}$+n\ensuremath{\Elzxh}${\ensuremath{\omega}}_{\mathrm{LO}}$ (n=1,2,3,4) where \ensuremath{\Elzxh}${\ensuremath{\omega}}_{\mathrm{LO}}$=36.7 meV is the LO-phonon energy of GaAs. The samples showing this effect all have a resonance between ${E}_{1h}$+\ensuremath{\Elzxh}${\ensuremath{\omega}}_{\mathrm{LO}}$ and some higher excitonic transition, in most cases the nominally forbidden transition involving the n=1 electron and n=3 heavy hole, ${E}_{13h}$. The effect, which we call doubly resonant Raman scattering (DRRS), has been seen for excitation with ${E}_{13h}$ and ${E}_{2h}$ in square-well structures and resonant with ${E}_{21h}$ in one half-parabolic-well sample. The DRRS line has nearly three times the circular polarization of the broad ordinary luminescence and a large linear polarization (optical alignment) not usually seen in luminescence from quantum wells. The polarization properties of DRRS at 6 K for one sample have been completely determined, as well as the magnitude of the scattering relative to the ordinary luminescence. A theoretical discussion is given for the first-order DRRS characterizing the principal mechanisms, the strength, the LO-phonon involved, the effects of symmetry, and polarization relaxation. It is concluded that the Fr\ohlich interaction can account for the strength but there could also be significant impurity-assisted Fr\ohlich scattering, the deformation potential interaction is negligible, and the polarization relaxation suggests as much as 55% impurity-assisted scattering.