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The characteristics of sputtered NiO for use in pn heterojunctions with Ga2O3 were investigated as a function of sputtering parameters and postdeposition annealing temperature. The oxygen/ nickel and Ni2O3/NiO ratios, as well as the bandgap and resistivity, increased as a function of O2/Ar gas flow ratio. For example, the bandgap increased from 3.7 to 3.9 eV and the resistivity increased from 0.1 to 2.9 Ω cm for the O2/Ar ratio increasing from 1/30 to 1/3. By sharp contrast, the bandgap and Ni2O3/NiO ratio decreased monotonically with postdeposition annealing temperatures up to 600 °C, but the density of films increased due to a higher fraction of NiO being present. Hydrogen is readily incorporated into NiO during exposure to plasmas, as delineated by secondary ion mass spectrometry measurements on deuterated films. The band alignments of NiO films were type II-staggered gaps with both α- and β-Ga2O3. The breakdown voltage of NiO/β-Ga2O3 heterojunction rectifiers was also a strong function of the O2/Ar flow ratio during deposition, with values of 1350 V for 1/3 and 830 V for 1/30.

There is increasing interest in α-polytype Ga2O3 for power device applications, but there are few published reports on dielectrics for this material. Finding a dielectric with large band offsets for both valence and conduction bands is especially challenging given its large bandgap of 5.1 eV. One option is HfSiO4 deposited by atomic layer deposition (ALD), which provides conformal, low damage deposition and has a bandgap of 7 eV. The valence band offset of the HfSiO4/Ga2O3 heterointerface was measured using x-ray photoelectron spectroscopy. The single-crystal α-Ga2O3 was grown by halide vapor phase epitaxy on sapphire substrates. The valence band offset was 0.82 ± 0.20 eV (staggered gap, type-II alignment) for ALD HfSiO4 on α-Ga0.2O3. The corresponding conduction band offset was −2.72 ± 0.45 eV, providing no barrier to electrons moving into Ga2O3.

The band alignments of two candidate dielectrics for ScAlN, namely, SiO2 and Al2O2, were obtained by x-ray photoelectron spectroscopy. We compared the effect of deposition method on the valence band offsets of both sputtered and atomic layer deposition films of SiO2 and Al2O3 on Sc0.27Al0.73 N (bandgap 5.1 eV) films. The band alignments are type I (straddled gap) for SiO2 and type II (staggered gap) for Al2O3. The deposition methods make a large difference in relative valence band offsets, in the range 0.4–0.5 eV for both SiO2 and Al2O3. The absolute valence band offsets were 2.1 or 2.6 eV for SiO2 and 1.5 or 1.9 eV for Al2O3 on ScAlN. Conduction band offsets derived from these valence band offsets, and the measured bandgaps were then in the range 1.0–1.1 eV for SiO2 and 0.30–0.70 eV for Al2O3. These latter differences can be partially ascribed to changes in bandgap for the case of SiO2 deposited by the two different methods, but not for Al2O3, where the bandgap as independent of deposition method. Since both dielectrics can be selectively removed from ScAlN, they are promising as gate dielectrics for transistor structures.