Raman microprobe measurement of under-damped LO-phonon-plasmon coupled mode in n-type GaN

Raman scattering spectra from LO-phonon-plasmon coupled modes in n-type hexagonal GaN epitaxial layers have been measured in detail for different carrier densities in n = 10(17)-10(18) cm(-3). Both the upper and lower branches of the coupled modes were clearly observed, showing striking changes in lineshape and peak frequency with the carrier density. A spectral lineshape analysis shows that n-type GaN belongs to a system of under-damped plasmon satisfying omega(p) tau greater than or similar to 1, where omega(p) and tau are the plasmon frequency aid scattering time, respectively. This is in contrast with other wide band-gap semiconductors such as SiC, which shows clearly the character of overdamped plasmon, omega(p) tau<1. A measurement of spatial distribution of carrier density in epitaxial layers is also presented for an application of the coupled-mode analysis.