Raman scattering due to interface optical phonons in GaAs/AlAs multiple quantum wells

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.