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

Abstract

The potential modulation and interface states of Al 2 O 3 /Al 0.25 Ga 0.75 N/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 Al 2 O 3 /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 Al 2 O 3 /AlGaN interface for the first time. The present ALD-Al 2 O 3 /AlGaN/GaN structure showed relatively high interface state densities with a minimum density of 1×10 12 cm -2 eV -1 or higher.