Raman scattering by wave-vector-dependent coupled plasmon/LO-phonon modes in n-type InN

We study the Raman scattering of n-type InN films grown by molecular beam epitaxy on ZnO substrates. The observed spectral Raman features are found to strongly depend on the carrier concentration as well as the photon energy used for excitation. The corresponding spectral changes are explained by coupled plasmon/LO-phonon excitations which are influenced by the selective resonance enhancement for scattering at large wave vectors as well as wave-vector nonconservation. In particular, a broad Raman band spanning the whole frequency range of optical phonons is demonstrated to originate from plasmon-related excitations, as opposed to the frequently assumed pure phonon scattering and wave-vector nonconservation induced by structural disorder.