SiO2 was grown by dry (O-2) thermal oxidation (at 1175, 1300, or 1400 degrees C) on n-type 4H-SiC substrates. The samples were prepared by subsequently exposing the grown SiO2 film on 4H-SiC to postoxidation annealing (POA) treatment using nitric oxide (NO) gas. The SiC-SiO2 interfaces were characterized by high frequency capacitance-voltage measurements, X-ray photoelectron spectroscopy (XPS), and ellipsometry. The interface trap density of the dry oxide grown at 1300 degrees C was much lower than others. At a higher grown temperature (1400 degrees C), the electrical and physical properties of the oxide were not improved compared to those oxides grown at 1175 degrees C. The XPS measurements provided evidence for the presence of intermediate oxidation states of Si oxycarbide in all samples. The areal densities of the intermediate oxidation states affected the interface trap density. The NO POA treatment significantly improved the interface trap density, the near-interface trap density, and the effective oxide charge density of the oxides grown at 1175 and 1300 degrees C. But, this improvement was not observed for the oxide grown at 1400 degrees C. The electrical properties of the metal-oxide-semiconductor devices fabricated using these oxides have also been discussed in terms of the oxide chemical compositions, which were determined by XPS and an oxide etching test.