Investigation of longitudinal-optical phonon-plasmon coupled modes in highly conducting bulk GaN

Abstract

We report a cross-correlated investigation, performed by means of Raman scattering and infrared spectroscopy, of coupled LO phonon-plasmon modes in bulk GaN. Using different samples with different (high) residual concentrations of free carriers, we find that the high-energy Raman mode follows closely the plasma frequency resolved from the infrared data. On the opposite, the low-frequency modes appears down shifted, with respect to the standard TO phonon frequency, by about 11 cm−1. Both findings agree satisfactorily with predictions of the linear response theory for undamped phonon-plasmon modes and establish Raman scattering as a powerful and nondestructive tool to investigate the residual doping level of GaN up to about 1020 cm−3 .