An attempt was made to form a thermally grown SiO2/GaN interface. A Si layer deposited on the c-plane GaN surface was oxidized in an O2 atmosphere to form a SiO2 layer. The formation of SiO2 with a bandgap of 8.6 eV was confirmed by x-ray photoelectron spectroscopy. Metal–oxide–semiconductor diodes were fabricated and tested to characterize the interface by electrical measurements. The capacitance–voltage (C–V) characteristics measured at 1 MHz showed that a longer oxidation time resulted in a steeper slope. However, it was unavoidable that a bump in a C–V curve appeared after a long oxidation time. The electron trap distributions derived from C–V curves exhibited a discrete-level trap at 0.7 eV from the conduction band edge. This discrete-level trap was an acceptor-like trap that can be assigned to a Ga vacancy. An insufficient oxidation led to a high leakage current owing to the asperities of the residual polycrystalline Si layer. Although the leakage current was improved by extending the oxidation time, an excessively long oxidation time resulted in a slight increase in the leakage current. We cannot deny the possibility of the diffusion of Ga atoms into SiO2 during oxidation. Moreover, the cross-sectional transmission electron microscopy and energy-dispersive x-ray spectroscopy of a sample formed with an excessively long oxidation time indicated the formation of a Ga oxide interlayer without a severe disorder. Most possibly, the formation of the Ga oxide interlayer by excess oxidation improved the interface properties.

Control of the plasma-CVD SiO2/InAlN interface by N2O plasma oxidation of the InAlN surface was studied. The interface was characterized by both X-ray photoelectron spectroscopy (XPS) and capacitance-voltage measurement of metal-insulatorsemiconductor (MIS) diodes. An excessively long duration of oxidation led to the deterioration of the stoichiometry of the InAlN surface and plasma oxide, resulting in a high density of interface states in the completed MIS diodes. Meanwhile, the surface-localized oxygen deficiency in the plasma oxide layer was observed by XPS. The intensity ratio of the oxygen-deficient component to the fully oxidized component in the O 1s spectrum decreased with increasing oxidation duration. Consequently, there was an optimum oxidation duration. The interface state density was reduced by almost one order in the case of plasma oxidation for an appropriate duration compared with the case of the direct deposition of SiO2 onto InAlN.