We present measurements and calculations of the resonant Raman line shape due to optic phonons in GaAs/AlAs multiple quantum wells (MQW's). Under resonant photoexcitation conditions we observe broad features between the bulk LO and TO frequencies in both the GaAs and AlAs optic phonon regions, due to modes propagating in the layer plane (in-plane modes). These are much stronger for the outgoing than the incoming resonance condition due to relaxation of the photoexcited exciton to states of the finite in-plane center-of-mass wave vector. The broad feature in the GaAs region displays a number of dips that can be assigned to the anticrossing of the interface dispersion with odd-order confined modes. We present a macroscopic model for calculating the resonant Raman line shape that incorporates the coupling to the in-plane modes, described realistically as combinations of interfacelike and confinedlike parts. Calculated line shapes reproduce closely the spectra measured for several MQW's of difFering layer widths. In particular, good agreement is found for the dependence of both the GaAs and AlAs optic phonon regions on the AlAs thickness, which provides convincing proof of the role of interface modes in the spectra.
We have performed hot-electron photoluminescence experiments on a number of different Bedoped GaAs/AlAs multiple-quantum-well structures (MQW's), with fixed well width of 40 A and barrier thicknesses between 5 and 80 A, in order to determine the energy of the optical phonons emitted by the hot electrons before recombination with the acceptor levels of the GaAs quantum wells. A continuum theory of optical phonons in GaAs/A1As multiple quantum wells was used to estimate the effective energy of the optical phonons emitted during the hot-electron energy relaxation. The theoretical calculations are compared with the energy separation of the measured hot photoluminescence peaks and a detailed analysis of the different modes contributing to the energy relaxation is performed. For MQW's with large barriers, i.e. , 40-A GaAs and 80 A A1As, the energy relaxation is dominated by the AlAs phonons. However, for samples with smaller barrier widths, i.e. , 40-A GaAs and either 5or 10-A A1As, relaxation via the emission of GaAs modes is more important. Nevertheless, relaxation by AlAs phonons is still significant producing an effective energy separation of the hot photoluminescence peaks between that of the pure GaAs and the A1As MQW optical-phonon energies.
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