The dependence on the scattering wave vector of a nonequilibrium LO phonon distribution generated by hot-electron relaxation is measured in thick GaAs and ${\mathrm{Al}}_{0.11}$${\mathrm{Ga}}_{0.89}$As layers, and a 500-Å GaAs layer using picosecond Raman scattering in back and forward scattering geometries. The absence of $q=0\mathrm{}$ nonequilibrium phonons in the thick samples, and their presence in the 500-Å sample, demonstrates that Raman-active LO phonons in ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ have well-defined wave vectors and are not localized by alloy disorder.

• Received 22 June 1988

DOI:https://doi.org/10.1103/PhysRevB.38.5776