Wave-Vector-Dependent Raman Scattering from Coupled Plasmon-Longitudinal Optical Phonon Modes and Fano Resonance in n-type Scandium Nitride

Raman scattering from coupled plasmon-longitudinal optical (LO) phonon modes in polar semiconductors is an effective tool to determine electronic properties, such as carrier concentration, mobility, carrier freeze-out, relaxation times, etc., as well as to understand different types of electron (hole)-phonon interactions. The physics of such coupling mechanism traditionally utilizes the Drude dielectric permittivity that predicts an increase in coupled plasmon-LO phonon mode (LPP+) frequencies with an increase in carrier concentrations. Herein, it is demonstrated that for n-type epitaxial scandium nitride (ScN) thin films, the frequencies of the coupled plasmon-LO phonon Raman modes exhibit red-shift with increasing carrier concentrations, which is contrary to the predictions from the Drude theory. Utilizing the generalized Lindhard dielectric function that considers both the frequency and wave-vector components of free-electron plasma, it is demonstrated that such a decrease in the frequencies of the coupled plasmon-LO phonon mode in Raman spectra is related to the nonconserved wave vectors due to inelastic scattering from magnesium (Mg) impurities. Modeling of the experimental Raman line shape, intensity, and frequencies illustrates that the wave-vector dependence of the coupled modes decreases with increasing electron concentrations. An asymmetric broadening of LO Raman modes is observed in films having large electron concentrations (>10(20) cm(-3)) that are explained by Fano resonance.