Phonon modes in spontaneously ordered GaInP2 studied by micro-Raman scattering measurements

We have performed micro-Raman-scattering experiments on ordered GaInP2 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) over bar 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 cm(-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.