INVESTIGATION OF LONGITUDINAL-OPTICAL PHONON-PLASMON COUPLED MODES INHIGHLY CONDUCTING BULK GAN

We report a cross-correlated investigation, performed by means of Rama n scattering and infrared spectroscopy, of coupled LO phonon-plasmon m odes in bulk GaN. Using different samples with different (high) residu al 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, w ith respect to the standard TO phonon frequency, by about 11 cm(-1). B oth findings agree satisfactorily with predictions of the linear respo nse theory for undamped phonon-plasmon modes and establish Raman scatt ering as a powerful and nondestructive tool to investigate the residua l doping level of GaN up to about 10(20) cm(-3). (C) 1995 American Ins titute of Physics.