I. Raman scattering and x-ray diffraction study of annealing effects on confined LO and folded LA phonons in GaAs-AlAs superlattices ; II. Picosecond light scattering studies of relaxation and tunneling in GaAs/AlxGa1-xAs mulitple quatum well structures

We have used light scattering techniques to probe the vibrational properties ofGaAs/AlAs superlattices and the dynamics of electrons in GaAs/AlxGal-xAs multiple quantum well structures. In our study of the effects of annealing on a short-period GaAs/ AlAs superlattice we have used Raman scattering in conjunction with x-ray diffraction measurements to determine the effects of interface broadening on folded LA and confined LO phonons. During the annealing process we followed three folded LA phonon doublets and up to the ninth order of confined LO phonons in the Raman spectrum. The frequencies of folded LA phonons in annealed samples did not shift, but the intensities changed with annealing. Dependence of folded LA phonon Raman intensities on annealing time was similar to that of x-ray diffraction satellite peaks for the two lowest doublets, as predicted by the photoelastic model for acoustic phonons in superlattices. We saw downward frequency shifts of confined LO phonons with increasing annealing time. The different annealing effects on frequencies of optical phonons and intensities of acoustic phonons in superlattices are a reflection of the different properties of the phonons: confined for the former and propagating for the latter. The downward frequency shifts of confined LO phonons produced by annealing are caused by an effective narrowing of GaAs phonon quantum wells due to diffusion of Al and can be successfully explained using an effective mass model for the phonons. We have used a picosecond time-resolved Raman scattering technique to characterize the relaxation of electrons photoexcited onto the third subband of a 210A wide GaAs quantum well. Although the lifetime of these carriers is shorter than the temporal resolution of our technique, we have been able to gain qualitative information on their relaxation by monitoring the electron populations on the first two subbands. The relatively large density of electrons on the second subband at early times indicates that carriers on the third subband preferentially scatter into the second subband as they relax. This indicates that symmetry-dependent selection rules brought about by confinement of LO phonons play a significant role in the electron-phonon interaction in these structures. Finally, we have used a picosecond time-resolved absorption technique to measure tunneling rates between quantum wells in a series of asymmetric coupled quantum well structures. We have studied the tunneling rates for inelastic tunneling between GaAs quantum wells as functions of barrier width and photoexcited carrier density. As expected from simple quantum considerations the tunneling rate decreases exponentially with barrier width. The tunneling rate is fastest at the lowest carrier density and decreases more than exponentially with increasing density. On the basis of this behavior we argue that interface roughness or impurity scattering are the primary mechanisms responsible for tunneling in these structures.U of I OnlyThesi