Raman line asymmetry in alloys and in ion-implanted polar crystals

We compare the effect of ion bombardment with that of alloying on the Raman-scattering line shape and its phonon frequency. In the case of ion implantation (no annealing), the modifications are attributed to the confinement dictated by the spatial correlation of the phonons and the formation of disorder therein, which partially relaxes the selection rules causing a clear dependence on the dispersion relations. We present results taken on GaAs, InP, and ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$, for both TO and LO lines. Our results agree with the main assumptions of the spatial-correlation model and add information about the phonon-dispersion relations. As opposed to that, the asymmetry of the Raman line shape of ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ alloys is explained here in a totally different way. It is assumed that the random cation distribution (with no clustering) gives rise to a collection of various phonon modes having the same symmetry, which, therefore, interact with each other. This interaction transfers charge, such that in the case of LO modes the more ionic LO mode is favored, while in the case of TO modes the mode favored is the one of the stronger bond. This implies that in both cases the mode exhibiting the higher frequency strengthens, causing asymmetry. The alloy disorder-activated modes under resonance conditions are also discussed.