Quality of the Oxidation Interface of AlGaN in Enhancement-Mode AlGaN/GaN High-Electron Mobility Transistors

Enhancement-mode AlGaN/GaN high-electron mobility transistors (HEMTs) were realized by applying the O₂ and N₂O plasma oxidation method to the AlGaN Schottky layers. After the postannealing process, the threshold voltage V_th of conventional depletion-mode GaN HEMTs was -3.1 V, and this value was shifted to +0.3 V when either N₂O or O₂ plasma oxidation treatment was used. X-ray photoelectron spectroscopy (XPS) analysis revealed that both treatments formed an Al₂O₃/Ga₂O₃ compound oxide layer. The decomposition of N-O bonds in the N₂O plasma suppressed the formation of nitrogen vacancy traps and ensured low roughness of the AlGaN surface during oxidation. This nitridation process also reduced the 1/f noise.