Two-phonon Raman scattering in InP

We have measured two-phonon Raman spectra of InP for laser energies between 1.58 and 2.71 eV. We describe the spectra in terms of an excitonic model in the low energy range, and within a band picture for higher energies. For the low energies the scattering is dominated by processes involving intermediate resonances with states in the excitonic continuum. Due to the exciton dispersion, resonant enhancement becomes possible for phonons with larger wavevectors as the incident laser energy is increased. This manifests itself as a broadening to the low energy side of the 2LO Raman peak. For excitation energies substantially larger than the E(0) + Delta(0) gap of 1.53 eV, the spectrum broadens considerably to the lower energy side. Furthermore, the high-energy cut-off of the 2LO spectrum occurs at more than twice the measured LO energy at the Gamma point. There are two possible explanations: three-phonon LO(L) + LA(L) + LA(X) scattering, or upward dispersion of the LO mode from the Gamma point by 1.7 +/- 0.2 cm(-1), which is the explanation we believe to be more likely. Copyright (C) 1996 Elsevier Science Ltd