Superlattice effects induced by atomic ordering onGaxIn1−xP Raman modes

A polarized Raman scattering study is undertaken in order to investigate the atomic ordering in (001)-oriented ${\mathrm{Ga}}_{0.51}$ ${\mathrm{In}}_{0.49}$P layers lattice matched to GaAs and preliminarily characterized by photoluminescence. The superlattice structure of trigonal symmetry which results from ordering manifests itself through phonon mode modifications. Several new modes are detected and their frequency and intensity behaviors are properly analyzed. In addition to the folded longitudinal-acoustic mode on Raman spectra, we observed the folded transverse-acoustic mode. Both folded acoustic modes display narrow lines (∼10 ${\mathrm{cm}}^{\mathrm{-}1}$) superposed to the disorder-induced acoustic bands DALA and DATA of the alloy. In the ‘‘optical mode’’ frequency range one clearly observes a doubling of the longitudinal GaP-type and InP-type modes, never reported before to our knowledge. The magnitude of the splitting between the two components of the LO doublet is measured precisely in the case of GaP-type modes and is an increasing function of the atomic ordering degree. So it is with intensities of the TO modes which become Raman active due to the trigonal symmetry. Finally the ‘‘valley depth’’ which had been empirically assigned as a significant parameter of the atomic ordering degree, is now precisely interpreted in the light of the whole Raman study. © 1996 The American Physical Society.