Elucidating oxygen vacancy passivation mechanism at Al2O3/ β -Ga2O3 interface via supercritical N2O processing

Oxygen vacancies (VO) at the β-Ga2O3 surface are associated with the high interface-trap densities (Dit, 1012–1013 cm−2 eV−1) that limit the threshold-voltage stability in Al2O3/β-Ga2O3 MOS devices. We combine first-principles calculations with Raman spectroscopy, photoluminescence (PL), x-ray photoelectron spectroscopy, and electrical characterization to examine how supercritical-fluid processing modifies these interface states. Perdew–Burke–Ernzerhof calculations with a rigid scissor correction to the experimental 4.8 eV bandgap identify in-gap states associated with the three inequivalent VO sites. Interstitial oxygen removes these states in the calculated model and increases the migration barrier from 0.08 to 1.8 eV. Experimentally, supercritical N2O (SN) treatment narrows the Raman linewidths at 347, 416, and 767 cm−1, suppresses the 2.4 and 2.8 eV defect-related PL bands, and increases the lattice-oxygen fraction from approximately 50% to more than 80%. A near-critical CO2 pre-clean further improves the SN response, consistent with removal of surface adsorbates before oxidation. The resulting NCSN process reduces Dit to 1.5 × 1011 cm−2 eV−1, lowers Nit by a factor of 6.4, and decreases the positive-bias-stress flatband-voltage shift from 0.79 to 0.23 V. Together, the calculations and measurements support oxygen vacancy filling and reduced oxygen migration as the main contributors to interface passivation.