Resonant Raman scattering in GaP: Excitonic and interference effects near the $E_0$ and $E_0$+${\Delta }_0$ gaps

We report on the Raman scattering by LO phonons resonant near the $E_0$ and $E_0$+${\Delta }_0$ gaps of GaP. Dipole-allowed and dipole-forbidden Raman scattering by LO phonons and their interference are observed, together with scattering by two-LO phonons. The dominant features in the resonant profiles arise from the $E_0$ gap. They cannot be interpreted with uncorrelated electron-hole pairs. Martin’s calculations for Coulomb-correlated pairs explain qualitatively the resonance enhancement of one-LO-phonon scattering below the $E_0$ gap, whereas a simple Lorentzian oscillator model accounts for the observed features and the signs in the interference profiles above $E_0$.