Raman spectroscopy study of the structure of gallium nitride epitaxial layers of different orientations

Abstract

The composition nonstoichiometry and structural quality of undoped gallium nitride layers grown by the hydride vapor phase epitaxy on sapphire substrates of different orientations have been estimated using Raman spectroscopy. It is found for the first time that the peak position of the phonon mode E₂(high) in the Raman spectra of gallium nitride films at a wave vector of 572 cm^–1 depends on the initial orientation of sapphire substrate and is low-frequency shifted when passing from Ga-polar to partially N-polar orientation. Additional modes are found in the spectra of GaN layers grown on substrates with m and r orientations. It is shown that a decrease in the composition deviation from stoichiometry, caused by reducing the HCl flow through the gallium source during the growth of GaN layers, leads to an increase in the phonon-mode intensity in the Raman spectrum.