The potential modulation and interface states of Al2O3/Al0.25Ga0.75N/GaN structures prepared by atomic layer deposition (ALD) were characterized by capacitance–voltage (C–V) measurements. We observed the peculiar C–V characteristics with two capacitance steps in forward and reverse bias regions, corresponding to the electron accumulation or depletion behavior at the Al2O3/AlGaN and AlGaN/GaN interfaces. From the experimental and calculated C–V characteristics, it was found that the charging and discharging of interface states near the AlGaN conduction-band edge mainly caused the stretch-out and hysteresis of the C–V curve at the forward bias. On the other hand, it is likely that the interface states near the midgap or deeper in energies act as fixed charges. From the bias-dependent hysteresis voltage in the forward bias region and the photo-induced voltage shift at the reverse bias, we estimated the interface state density distribution at the Al2O3/AlGaN interface for the first time. The present ALD-Al2O3/AlGaN/GaN structure showed relatively high interface state densities with a minimum density of 1×1012 cm-2 eV-1 or higher.

The impact of states at the insulator/AlGaN interface on the capacitance-voltage ͑C-V͒ characteristics of a metal/insulator/AlGaN/GaN heterostructure ͑MISH͒ capacitor was examined using a numerical solver of a Poisson equation and taking into account the electron emission rate from the interface states. A parallel shift of the theoretical C-V curves, instead of the typical change in their slope, was found for a MISH device with a 25-nm-thick AlGaN layer when the SiN x / AlGaN interface state density D it ͑E͒ was increased. We attribute this behavior to the position of the Fermi level at the SiN x / AlGaN interface below the AlGaN valence band maximum when the gate bias is near the threshold voltage and to the insensitivity of the deep interface traps to the gate voltage due to a low emission rate. A typical stretch out of the theoretical C-V curve was obtained only for a MISH structure with a very thin AlGaN layer at 300°C. We analyzed the experimental C-V characteristics from a SiN x / Al 2 O 3 / AlGaN/ GaN structure measured at room temperature and 300°C, and extracted a part of D it ͑E͒. The relatively low D it ͑ϳ10 11 eV −1 cm −2 ͒ in the upper bandgap indicates that the SiN x / Al 2 O 3 bilayer is applicable as a gate insulator and as an AlGaN surface passivant in high-temperature, high-power AlGaN/GaN-based devices.