The energy spectrum of interface state density, D_it(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 Al_xGa_1–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) D_it(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, D_it(E) for SiO₂/Al_xGa_1–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 D_it(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 (Q_F) which is not related to D_it(E) and which almost compensates the negative polarization charge ($Q_{\mathit{p}\mathit{o}\mathit{l}}^{-}$ ).

The main goal of the carried out tests was to analyze the influence of the surface modification of a substrate by depositing composite ZnO layers by the Atomic Layer Deposition (ALD) method. The samples were subjected to preliminary surface modification consisting of being sandblasted and electropolished. A ZnO layer was applied to the prepared substrates by the ALD method. As a precursor of ZnO, diethylzinc (DEZ) was used, which reacted with water, enabling the deposition of the thin films. The chamber temperature was as follows: T = 100–300 °C. The number of cycles was 500 and 1500. As part of the assessment of the physicochemical properties of the resulting surface layers, the tests of chemical composition of the layer, pitting corrosion, impedance corrosion, adhesion to the metal substrate, morphology surface, and wettability were carried out. On the basis of the obtained research, it was found that a composite ZnO layer deposited onto a substrate previously subjected to the electrochemical polishing process has more favorable physicochemical properties. Moreover, an influence of temperature and the number of cycles of the deposition process on the obtained properties was observed, where the ZnO layer was characterized by more favorable properties at a temperature of 200–300 °C at 1500 cycles of the deposition process.