The performance of a SiC MOSFET has not reached the level predicted based on the physical properties of SiC. One of the reasons is that interface state density (Dit) at SiO2/SiC is more than an order of magnitude higher than that at SiO2/Si.  Therefore, it is a key for improving the performance of SiC MOSFET to clarify the origin of Dit and SiO2/SiC interface structure formed by thermal oxidation, and to control interface structure based on the knowledge. We have reported the results of the changes in chemical bonding state of SiO2/4H-SiC (0001) (C-face) and 4H-SiC (0001(-)) (Si-face) structure with the progress of thermal oxidation using angle-resolved X-ray photoelectron spectroscopy (AR-XPS). [2, 3] In this paper, we report the results the initial stage of thermal oxidation on 4H-SiC (0001) on-axis and 4° off-axis substrates.
2. Experimental Method
4H-SiC (0001) epitaxial films with on-axis and 4° off-axis were used in this study. We used the flattened 4H-SiC(0001) surface with on-axis which is composed of alternating wide and narrow terraces with single-bilayer-height steps.  The samples were prepared as follows. The sample was cleaned in the mixture of H2SO4 and H2O2 (H2SO4:H2O2=4:1) at 80-85 ºC, and the native oxide was removed by dipping in 5% hydrofluoric acid (HF) followed by a rinse in deionized water. The sample was oxidized at 850 °C in dry oxygen with a pressure of 133 Pa. Then, the sample was oxidized in dry oxygen with a pressure of 133 Pa at 900 °C. Subsequently, the sample was oxidized in dry oxygen with a pressure of 133 Pa at 950 °C, 1000 °C and 1050 °C. The Si 2p and C 1s photoelectron spectra, excited by monochromatic AlKα radiation, were measured at a photoelectron take-off angle of 15 and 90 with an energy resolution of 0.37 eV and an acceptance angle of 3.3°, using an ESCA-300 manufactured by Scienta Instruments AB .
3. Result and Discussion
The analyses of Si 2p3/2 photoelectron spectra arising from the sample of on-axis 4H-SiC(0001) (Fig. 1) show that the oxide increases with the increase of oxidation time. The analyses of C 1s photoelectron spectra show that the component with higher binding energy (chemical shift of about 2eV) is reduced by the oxidation. The measured spectra in Si 2p and C 1s regions were also decomposed into peaks convolution of Gaussian and Lorentzian as shown in Fig. 1 (b) and Fig. 2 (b). Fig. 3 shows the oxide thickness dependence of spectral intensity ratio in the case of samples of the on-axis and 4° off-axis 4H-SiC(0001). As seen in Fig. 3, the intensity ratio ISi3+/ISiC in the case of the on axis 4H-SiC(0001) is larger than that in the case of the 4° off-axis 4H-SiC(0001). In the addition, the intensity ratio ISi1+/ISiC and ISi2+/ISiC in the case of the on axis 4H-SiC(0001) is smaller than that in the case of the 4° off-axis 4H-SiC(0001). These imply that there is the defference in SiO2/SiC interfce structure between on-axis 4H-SiC(0001) and 4° off-axis 4H-SiC(0001). Here, ISi1+, ISi2+, ISi3+, and ISiC denote the intensity of component related to Si1+, Si2+, Si3+ and SiC, respectively. Figure 4 shows the oxidation time dependence of oxide thickness in the case of the on-axis and 4° off-axis 4H-SiC(0001) oxidized at the range from 850℃ to 1050℃. As seen in Fig. 4, the oxidation rate decreases at the oxide thickness of 0.6nm in both the on-axis and 4° off-axis 4H-SiC(0001). This implies that the oxidation rate decreases when the 1 ML SiO2was formed by the oxidation. There isn't difference in the oxidation rate between on-axis and 4° off-axis 4H-SiC(0001).
From the changes of the Si 2p3/2 and C 1s photoelectron spectra, a changes in chemical bonding state of SiO2/SiC structure with the progress of thermal oxidation were observed. We found that there is the defference in SiO2/SiC interfce structure between on-axis 4H-SiC(0001) and 4° off-axis 4H-SiC(0001).
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