The impact of oxidation agent and post-metallization annealing (PMA) on the quality of oxide-semiconductor interface in AlGaN/GaN metal-oxide semiconductor heterostructure field-effect transistors (MOS-HFETs) with HfO2 grown by atomic layer deposition (ALD) was investigated. About six orders of magnitude lower gate leakage current (similar to 10(-7) mA/mm at -10 V) was observed for MOS-HFETs with HfO2 gate oxide grown by ALD using ozone oxidation agent (O-HfO2), compared with grown using water oxidation agent (T-HfO2'). In addition, the output current frequency dispersion was effectively reduced for O-HfO2 by applying PMA performed at 400 degrees C in N-2 ambient, where the current frequency dispersion measured using 100 ns-long pulses was reduced from 40% for as-deposited gate oxide to only 10% for devices after applied PMA. Frequency dispersion was found to be consistent with density of oxide/semiconductor interface states (D-it) determined near the semiconductor conduction and valence band edge using the capacitance-voltage curves measured at different temperatures (CV-T) and photo-assisted capacitance transient (photo-C-t) technique, respectively. MOS-HFET structures with O-HfO2 showed about one order of magnitude lower D-it near the semiconductor valence band compared with structures with T-HfO 2 (2 x 10(11) compared with 10(12) eV(-1) cm(-2) at E-C-E = 2.5 eV, respectively). Moreover, for devices with O-HfO2, D-it was further reduced in entire energy gap as a result of PMA performed at 400 degrees C. The increased Dit for T-HfO2 oxides was attributed to Hf-Hf bonds at the HfO2/GaN interface, as deduced X-ray photoelectron spectroscopy (XPS) analysis. In contrast, formation of Hf-Hf bonds was negligible in as-deposited O-HfO2 oxide films, while PMA at 400 degrees C led to reduction of the hydroxyl group observed by XPS.