A1(LO)phonon in degenerate InN semiconductor films

We have studied the A$_{1}$(LO) structure of InN thin films from a low (n$_{e}$=6.7x10$^{17}$/cm$^{3}$ ) to a very high (n$_{e}$=9.6x10$^{20}$/cm$^{3 })$ carrier concentration using Raman scattering experiments. Theoretically we investigated this structure using a wavevector dependent dielectric function$\varepsilon (q,\omega )$ which takes into account the coupling of longitudinal-optical (LO) phonon and electrons with non-parabolic energy dispersion. Phonon-plasmon interaction cannot explain the origin of this structure. However, phonon interaction with electron-hole pair excitations forms a well-defined structure in $Im\varepsilon (q,\omega )^{-1}$ which emerges from the electron hole pairs spectrum when higher-energy coupled-mode becomes Landau damped. With increasing values of q, this structure moves towards the experimental value. This peak structure is formed by a weaker (relative to the plasmon) interaction between the LO-phonon and electron hole pair excitations. Experimentally it is observed that the energy of this structure increases with increasing value of electron density.