Raman scattering by an inhomogeneous plasma in implanted semiconductors

We compare the LO-phonon–plasmon coupled mode spectra of homogenously doped and Si+-implanted InP samples. Whereas a narrow L− peak similar to that of the uniformly doped sample is observed in the implanted samples, scattering by L+ modes gives rise to a broad weak band. No Raman scattering by coupled modes corresponding to the electron densities that are expected close to the surface is observed, and the L+ band extends up to frequencies well beyond those corresponding to the electron densities present in the Raman scattering probing depth. These features are qualitatively explained by a hydrodynamical model that, taking into account the Gaussian doping profile of the implanted sample, predicts the existence of plasma modes localised near the surface due to the presence of a sizable density gradient in the free electron plasma.