Sub-bandgap-light-assisted capacitance–voltage study of gap states near valence band maximums at AlSiO/GaN interfaces with and without an AlN interfacial layer

Gap states at AlSiO/GaN interfaces with and without an AlN interfacial layer (IL) have been investigated by the sub-bandgap-light-assisted capacitance–voltage (C–V) method. The thickness of the crystalline AlN IL was 0.8 nm, which is below the critical thickness. The metal–oxide–semiconductor structures formed on n-type GaN were used for the investigation. In sub-bandgap-light-assisted C–V measurements, the bias voltage was swept from the negative bias end to the positive bias end after the monochromatic sub-bandgap light was irradiated at the negative bias end and turned off. The AlN IL showed a reduced interface state density (Dit) especially around the midgap, which is clarified by sub-bandgap-light-assisted C–V measurements with the irradiation time fixed. On the basis of the irradiation time dependence of sub-bandgap-light-assisted C–V characteristics, we investigated the behavior of near-interface traps (NITs). The flatband shift (ΔVFB) of C–V characteristics in the backward direction was dependent on irradiation time. In the sample without the AlN IL, the irradiation time dependence of ΔVFB indicated the Fowler–Nordheim tunneling injection of holes into NITs at the 2.2 and 2.6 eV photon energies of the irradiated monochromatic light. Upon the insertion of the AlN IL, however, the irradiation time dependence of ΔVFB was negligible for the 2.2 eV light irradiation and completely different from the Fowler–Nordheim tunneling injection for the 2.6 eV light irradiation, indicating a possibility that a portion of photoinduced holes accumulated at the AlSiO/AlN interface.