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-Ga 2 O 3 electronic and optoelectronic devices is critically constrained by interface traps and the quality of p-type contact layers, making the controlled deposition of NiO x a key technological challenge. Oxygen partial pressure during sputtering strongly influences NiO x thin film quality and ultimately the performance of NiO x /β-Ga 2 O 3 heterojunction diodes. This work investigates how varying the oxygen fraction in the sputtering atmosphere affects NiO x film properties and the associated deep trap states at the heterointerface. As the oxygen partial pressure increases (20%–40%), NiO x films exhibit improved structural quality and higher carrier concentration; however, at 50% oxygen, excessive oxidation leads to the formation of additional Ni 3+ and degraded film characteristics. Devices fabricated under these conditions show a markedly elevated interface-related trap density (∼10 13 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 x deposition and offer valuable insight for the design of high-performance β-Ga 2 O 3 -based devices.