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We observed the peculiar capacitance-voltage (C-V) characteristics with two capacitance steps in the Al2O3/AlGaN/GaN samples prepared by atomic layer deposition where the interface states near the midgap or deeper in energies act as fixed charges. From the voltage shift at the reverse bias in the photo-assisted C-V curve, we estimated the interface state density distribution at the Al2O3 /AlGaN interface for the first time. The effects of the inductively coupled plasma (ICP) etching of AlGaN on the interface properties of the Al2O3/AlGaN/GaN structures were investigated. The transmission electron microscopy and X-ray photoelectron spectroscopy analyses indicated that monolayer-level roughness and disorder of the chemical bonds at the AlGaN surface caused poor C-V characteristics due to high-density interface states at the Al2O3/ICP-etched AGaN interface.