Raman study of the effects of annealing on folded LA and confined LO phonons in GaAs-AlAs superlattices

Abstract

We have observed annealing effects by measuring x-ray diffraction and Raman scattering spectra from folded LA and confined LO phonons on a GaAs-AlAs superlattice grown by molecular-beam epitaxy. 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. There were seven pairs of x-ray-diffraction satellite peaks to be seen for unannealed samples. 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. The third-order Raman doublet was relatively more intense, reflecting the stronger coupling of the 5145-Å laser light to the LA phonons in the AlAs-rich regions. 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 annealing effects measured by Raman scattering have allowed us to separate and assign clearly an interface phonon mode in the confined-LO-phonon frequency region. 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.