Impact of oxygen stoichiometry in NiO <i>x</i> / <i>β</i> -Ga2O3 heterojunction diodes

The properties of NiOx/β-Ga2O3 heterojunctions and their impact on the device performance are theoretically investigated for different oxygen compositions x in NiOx. Along with the stoichiometric NiO with a 1:1 ratio in nickel and oxygen, two non-stoichiometric cases (Ni2O3 and NiO2) are examined as well in combination with β-Ga2O3 of two typical crystallographic orientations [i.e., (001) and (010)]. First-principles calculations show good agreement with the available experimental data. The results indicate that the bandgap of NiOx shrinks from ∼3.9 eV (NiO) to ∼1.5 eV (NiO2) as the oxygen content increases. More interestingly, the obtained band offsets reveal that the band alignment makes a transition from type-II (NiO and Ni2O3) to type-I (NiO2) due to the asymmetric nature of the band-edge shifts. These characteristics lead to surprisingly large differences in the p–n junction breakdown voltage with the structures of high oxygen composition exhibiting drastically smaller values. The heterojunctions under consideration are also found to be generally free from the localized interface states even in non-stoichiometric compositions (Ni2O3 and NiO2) with only minor charge polarizations. The results highlight the need for an accurate description of the heterointerfaces, including the crystallographic orientation in the design and optimization of NiOx/β-Ga2O3 based devices for high-performance electronic applications.