Substrate off-angle dependency of Al content in Al x Ga1−x N/GaN high-electron-mobility transistor structures on free-standing GaN substrates

Abstract

Precise control of the Al content x in Al x Ga1−x N/GaN epitaxial structures is a potentially important technical parameter for power/RF devices. We fabricated five 2 inch diameter high-electron-mobility transistor wafers by metal organic vapor phase epitaxy on free-standing (0001) Ga-face GaN as a substrate. The Al content x of the AlGaN layer was ∼0.2, and the substrate off-angle varied in the range 0.2°–1.0°. The Al content was measured as a function of the substrate off-angle using room-temperature optical reflectance. The values of the AlGaN bandgap energy derived from reflectance spectra were calibrated against the Al content determined by X-ray diffraction beforehand. The Al content in the AlGaN layers was found to be unaffected by the substrate off-angle, and a downward bowing was observed in the relation of the bandgap energy and the Al content, with a bowing parameter of 0.57 eV and a stress-corrected bowing parameter of 0.22 eV.