Abstract
Polarized Raman spectroscopy has been performed on high-quality bulk gallium nitride substrates with various crystal orientations. The transverse-optic (TO) and longitudinal-optic (LO) phonons shifted to a lower frequency upon changing the observed crystal plane from polar to semi-polar to nonpolar. This result is well explained by the mixing of A1 and E1 optical phonons. In addition, we analyzed in detail the frequency of LO-phonon-plasmon-coupled (LOPC) modes by calculating the peak frequency as a function of carrier concentration and the propagation direction of the LOPC mode. Carrier densities deduced by the analysis showed excellent agreement with the results of Hall measurements. We provide a simple, quick, and nondestructive procedure for identifying nonpolar GaN planes as well as obtaining the carrier concentration.
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