Overdamped excitations of the free electron gas in GaN layers studied by Raman spectroscopy

Raman spectra of $n$-type GaN on GaAs are compared with line-shape calculations for excitations of the free electron gas. The spectral features in the frequency range of the optical phonons are well explained by plasmon-phonon scattering from an overdamped plasma system taking into account wave-vector nonconservation. The charge-density-fluctuation mechanism is found to be important for off-resonant excitation. For excitation closer to the fundamental band gap of GaN, the impurity-induced Fröhlich mechanism becomes dominant. In the latter case, the observation of a relatively narrow line at the longitudinal-optical phonon frequency is consistent with the presence of a high-density electron gas.