Abstract
We have studied plasmon-longitudinal optical (LO) phonon coupled modes by means of Raman scattering in n-type InN layers grown by metalorganic vapor phase epitaxy, for carrier densities in the range of 10(19) cm(-3). A strong mode is observed near the frequency of the A(1)(LO) phonon, despite the high conductivity of the films. It is suggested that the main origin which can be invoked for an effective decoupling of the LO phonon from the plasmon is the participation to the scattering of phonons with wave vectors larger than the Thomas-Fermi wave vector. The line shape of the LO mode is calculated using the Lindhard-Mermin dielectric function taking into account finite wave-vectors, for various light scattering processes. We find that the charge density fluctuations mechanism is involved as the main scattering mechanism in n-type InN, at least for the investigated excitation energies in the 2.54-1.89 eV range. The breakdown of the wave-vector conservation is assigned to electron elastic scattering by impurities.