The effect of post-oxidation annealing in Ar atmosphere (Ar POA) on 4H-SiC-oxide interfaces. has been studied by capacitance to gate-bias voltage (C-V) measurements and photoemission spectroscopy (PES). It was found from the C-V measurements that the shift of the C-V curve disappears when the Ar POA temperature is higher than 600° C. On the other hand, angle-resolved x-ray photoelectron spectroscopy measurements revealed that the thickness of the intermediate layers containing Si1+ oxidation states observed at the interfaces decreases abruptly when the Ar POA temperature exceeds 500° C. In ultraviolet photoelectron spectra, O2p, peaks were changed by Ar POA at temperatures higher than 600° C, which is the temperature where the shift of the C-V curve disappears in C-V measurements. This shows that the change in O2p bonding by Ar POA is the origin of the shift observed in C-V characteristics. A model of structural changes in the interfaces by Ar POA has been proposed based on the results of PES measurements and those of C-V measurements. © 2005 American Vacuum Society.