Confocal Raman depth-scanning spectroscopic study of phonon−plasmon modes in GaN epilayers

Coupled longitudinal-optical (LO)-phonon-plasmon excitations were studied using confocal micro-Raman spectroscopy. The high-quality Si-doped GaN epilayers were grown in a Gunn diode structure on (0001) sapphire substrates by plasma assisted molecular beam epitaxy. Depth-profiled Raman spectra exhibit a spatial variation of both low, ω-, and high, ω+, frequency coupled phonon−plasmon modes (CPPMs) in the n-GaN layers. To describe the features of the CPPMs in the Raman spectra a self-consistent model that includes both the electro-optic and deformation-potential as well as charge-density fluctuation mechanisms as important processes for light scattering in n-GaN has been proposed. An agreement between the theoretical and experimental line shapes of the Raman spectra is obtained. From the best line-shape fitting of the CPPMs the depth profiles of the plasmon and phonon damping, plasmon frequency, free carrier concentrations, and electron mobility as well as the contributions of the electron−phonon interactio...