Influence of Oxygen Partial Pressure on Sputtered NiO _<i>x</i> Film Characteristics and Deep Interface Trap Density in NiO _<i>x</i> /β-Ga ₂ O ₃ Heterojunction Diodes

The performance of emerging β-Ga₂O₃ electronic and optoelectronic devices is critically constrained by interface traps and the quality of p-type contact layers, making the controlled deposition of NiO_<i>x</i> a key technological challenge. Oxygen partial pressure during sputtering strongly influences NiO_<i>x</i> thin film quality and ultimately the performance of NiO_<i>x</i>/β-Ga₂O₃ heterojunction diodes. This work investigates how varying the oxygen fraction in the sputtering atmosphere affects NiO_<i>x</i> film properties and the associated deep trap states at the heterointerface. As the oxygen partial pressure increases (20%-40%), NiO_<i>x</i> films exhibit improved structural quality and higher carrier concentration; however, at 50% oxygen, excessive oxidation leads to the formation of additional Ni³⁺ and degraded film characteristics. Devices fabricated under these conditions show a markedly elevated interface-related trap density (∼10¹³ cm^-2/eV), dominated by a deep-level trap at approximately 0.75 eV. Although moderate oxygen enrichment benefits device robustness and breakdown capability, overly oxygen-rich sputtering introduces sputtering-induced surface modification and increases interfacial trap densities. These findings provide critical guidance for optimizing oxygen conditions during NiO_<i>x</i> deposition and offer valuable insight for the design of high-performance β-Ga₂O₃-based devices.