Surface and interface electronic properties of AlGaN(0001) epitaxial layers

AlGaN layers with Al content varying over the whole range of compositions were grown by molecular beam epitaxy (MBE) on n-6H-SiC substrates. The band gap energy is obtained from the vanishing of Fabry–Perot oscillations in a fit to optical reflection spectra near the band gap absorption edge. The surface potential was determined by in-situ X-ray photoemission spectroscopy (XPS) and is found to increase as a function of the Al content from (0.5±0.1) eV to (1.3±0.1) eV, from GaN to AlN. A Si3N4 thin passivation layer was formed in-situ onto a 2DEG AlGaN/GaN structure. The mechanism underlying the passivation of high electron mobility transistor (HEMT) structures is suggested to be based on the formation of interface states, which keep the Fermi level fixed at a position close to that of the free AlGaN surface.