Capacitance–Voltage Characteristics of Al₂O₃/AlGaN/GaN Structures and State Density Distribution at Al₂O₃/AlGaN Interface

The potential modulation and interface states of Al₂O₃/Al_0.25Ga_0.75N/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.