High-voltage AlGaN/GaN Schottky barrier diodes (SBDs) were fabricated using on a silicon (111) substrate, and a post-process O-2 treatment was carried out to reduce the leakage current and increase the breakdown voltage. Time-of-flight secondary ion mass spectroscopy revealed that, following the post-process O-2 treatment, oxygen diffused into an AlGaN barrier and AlO was generated. A significant suppression of the leakage current (of approximately 6 orders of magnitude) occurred in the buffer isolation structures following the O-2 treatment. Our method also resulted in suppression of the surface leakage current through the mesa-etched surface. A virgin GaN SBD exhibited a leakage current of 1.76 x 10(-2) A/cm(2), whereas the equivalent O-2-treated device had a leakage current of 1.75 x 10(-4) A/cm(2) (the anode-cathode distance was L-AC = 10 mu m, and the applied bias was -100 V). This reduction in the leakage current was caused by surface passivation at the anode and cathode. The post-process O-2 treatment also increased the breakdown voltage from V-b = 808 V to V-b = 1590 V for a device with L-AC = 10 mu m. GaN SBDs with and without the post-process O-2 treatment exhibited low specific on-resistance of R-on,R-sp = 2.51 m Omega cm(2) and R-on,R-sp = 2.48 m Omega cm(2), respectively, with L-AC = 10 mu m. Devices with the post-process O-2 treatment exhibited a figure of merit of V-b(2)/R-on,R-sp = 1006 MW/cm(2), whereas devices without the O-2 treatment exhibited a figure of merit of V-b(2)/R-on,R-sp = 263 MW/cm(2). These high-voltage GaN SBDs employing the post-process O-2 treatment are suitable for applications including DC-DC converters, inverters, and power factor correction circuits, where high voltage operation is required with low leakage currents. (C) 2014 Elsevier Ltd. All rights reserved.