The energy spectrum of interface state density, Dit(E), was determined at oxide/III-N heterojunction interfaces in the entire band gap, using two complementary photo-electric methods: (i) photo-assisted capacitance-voltage technique for the states distributed near the midgap and the conduction band (CB) and (ii) light intensity dependent photo-capacitance method for the states close to the valence band (VB). In addition, the Auger electron spectroscopy profiling was applied for the characterization of chemical composition of the interface region with the emphasis on carbon impurities, which can be responsible for the interface state creation. The studies were performed for the AlGaN/GaN metal-insulator-semiconductor heterostructures (MISH) with Al2O3 and SiO2 dielectric films and AlxGa1–x layers with x varying from 0.15 to 0.4 as well as for an Al2O3/InAlN/GaN MISH structure. For all structures, it was found that: (i) Dit(E) is an U-shaped continuum increasing from the midgap towards the CB and VB edges and (ii) interface states near the VB exhibit donor-like character. Furthermore, Dit(E) for SiO2/AlxGa1–x/GaN structures increased with rising x. It was also revealed that carbon impurities are not present in the oxide/III-N interface region, which indicates that probably the interface states are not related to carbon, as previously reported. Finally, it was proven that the obtained Dit(E) spectrum can be well fitted using a formula predicted by the disorder induced gap state model. This is an indication that the interface states at oxide/III-N interfaces can originate from the structural disorder of the interfacial region. Furthermore, at the oxide/barrier interface we revealed the presence of the positive fixed charge (QF) which is not related to Dit(E) and which almost compensates the negative polarization charge (Qpol−).

We developed a method for determining of the deep donor-like interface state density distribution D it ðEÞ at the insulator/wide bandgap semiconductor interface in metal/insulator/semiconductor structures from the measurements of photocapacitance vs. ultraviolet light intensity C L ðUÞ. From the comparison of theoretical and experimental C L ðUÞ curves we obtained the continuous donor D it ðEÞ in the energy range between 0.15 eV and 1 eV from the valence band top for a metal/ Al 2 O 3 =n-GaN device. In addition, the acceptor-like interface state D it ðEÞ in the upper part of the bandgap was determined from the capacitance-voltage method. V