Enhanced breakdown voltage and thermal stability in β-Ga2O3 heterojunction power diodes through Al-NiO alloy engineering

β -Ga 2 O 3 is a promising ultra-wide-bandgap semiconductor for next-generation power electronics; however, device performance remains limited by interface-related defects, leakage currents, and thermal reliability challenges in heterojunction architectures. Here, we demonstrate that Al-mediated alloy engineering of NiO provides an effective route for tailoring the interfacial properties of Al-NiO/ β -Ga 2 O 3 vertical heterojunction diodes. Quantitative XPS analysis revealed controlled Al incorporation from 0 to 11.86 at%, resulting in lattice contraction, Al-O bond formation, and significant modification of the Ni 3+ /Ni 2+ ratio. Among the investigated samples, the heterojunction containing 2.84 at% Al exhibited the most favorable combination of electrical and interfacial properties, including the highest hole concentration (1.3 × 10 18 cm -3 ), the lowest resistivity (17.8 Ω·cm), and the lowest interface-trap density. Consequently, the optimized device achieved a reverse leakage current as low as 4.59 × 10 − 8 A/cm 2 at -500 V and maintained excellent thermal stability up to 473 K. Most notably, the breakdown voltage increased by approximately 217%, from 1.2 kV for the undoped heterojunction to 3.8 kV for the optimized device. TCAD simulations reveal that this enhancement originates from reduced defect-assisted conduction and a more uniform electric-field distribution, lowering the peak electric field from 6.6 to 3.3 MV/cm. These findings establish moderate Al incorporation as an effective strategy for simultaneously controlling interfacial chemistry, band alignment, leakage-current suppression, thermal stability, and breakdown performance in β -Ga 2 O 3 power devices.