Phonon modes in spontaneously orderedGaInP2studied by micro-Raman scattering measurements

We have performed micro-Raman-scattering experiments on ordered ${\mathrm{GaInP}}_2$ alloy samples in three different geometries where the phonon wave vector is either parallel or perpendicular to the ordering axis of the crystal. By comparing results from the (1¯11) backscattering and right-angle scattering measurements with the $C_{3v}$ symmetry of the crystal, we found that the recently discovered peaks at 205 and 354 ${\mathrm{cm}}^{\mathrm{-}1}$ in the Raman spectra of ordered alloys are due to longitudinal-phonon modes with $A_1$ symmetry in these geometries. In the (110) backscattering geometry, selection-rule forbidden longitudinal-phonon modes appear in the Raman spectra measured in parallel polarizations. Possible mechanisms for this selection-rule violation are discussed.