Optical-phonon emission in GaAs/AlAs multiple-quantum-well structures determined by hot-electron luminescence

We have performed hot-electron photoluminescence experiments on a number of different Be-doped GaAs/AlAs multiple-quantum-well structures (MQW’s), with fixed well width of 40 Å and barrier thicknesses between 5 and 80 Å, 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/AlAs 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-Å GaAs and 80-Å AlAs, the energy relaxation is dominated by the AlAs phonons. However, for samples with smaller barrier widths, i.e., 40-Å GaAs and either 5- or 10-Å AlAs, 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 AlAs MQW optical-phonon energies.