Optoelectronic properties and application of <i>p</i>-type ultrawide bandgap Zn0.7Ni0.3O1+δ thin films in p–n heterojunction diodes

p-type ultrawide bandgap oxide semiconductors play a crucial role in developing optoelectronic and electronic devices. Our previous studies have identified rock salt-structured Zn1−xNixO (∼0.27 ≤ x ≤ 1) alloys as promising wide bandgap oxides for achieving p-type doping. This is attributed to their high valence band maximum position, which favors the formation of native acceptors, such as nickel vacancies (VNi). However, the application of p-type O-rich Zn1−xNixO1+δ alloys in bipolar devices remains unexplored. In this study, we synthesized rock salt-structured p-type Zn0.7Ni0.3O1+δ thin films with a bandgap ∼4.4 eV using room-temperature magnetron sputtering in varying oxygen flow ratios (0%–30%). The structural and optoelectronic properties of films were characterized by x-ray diffraction, spectroscopic ellipsometry, and variable-temperature Hall-effect measurements. We observed a significant increase in subgap absorption with higher oxygen flow ratios. Subsequently, p-Zn0.7Ni0.3O1+δ/n-ZnO heterojunction diodes were fabricated on ITO glass. These p–n diodes exhibited high rectification ratio up to ∼3.1 × 104 and an ideality factor of ∼3.1. The band diagram of the p–n heterojunction was simulated using SCAPS-1D. These findings underscore the potential of p-type ultrawide bandgap Zn0.7Ni0.3O1+δ semiconductors in bipolar device applications, demonstrating their promising performance for future optoelectronic and electronic technologies.