Influence of Oxygen Partial Pressure on Sputtered NiO x Film Characteristics and Deep Interface Trap Density in NiO x /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 NiOx a key technological challenge. Oxygen partial pressure during sputtering strongly influences NiOx thin film quality and ultimately the performance of NiOx/β-Ga₂O₃ heterojunction diodes. This work investigates how varying the oxygen fraction in the sputtering atmosphere affects NiOx film properties and the associated deep trap states at the heterointerface. As the oxygen partial pressure increases (20%-40%), NiOx 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 NiOx deposition and offer valuable insight for the design of high-performance β-Ga₂O₃-based devices.