The impact of byproducts formed on a 4H–SiC(0001) surface by substrate oxidation on the interface state density (Dit) of Al2O3/4H–SiC gate stacks was investigated in this study, because some C-related byproducts are predicted to have defect states near the conduction band minimum (EC) of 4H–SiC. At first, by developing an in situ cyclic metal layer oxidation method, we realized the formation of an Al2O3/4H–SiC gate stack without substrate oxidation, which was verified using conventional X-ray photoelectron spectroscopy (XPS). Then, the amount of byproducts on the 4H–SiC surface was controlled by chemical etching of thermally oxidized 4H–SiC. The results showed that the Dit near the EC of 4H–SiC for the Al2O3/4H–SiC gate stack increased with the amount of byproducts until it fully covered the 4H–SiC surface; thereafter, it did not increase. For the sample with byproducts below the detection limit of XPS, a Dit value as low as 5 × 1011 cm−2 eV−1 at around EC − 0.15 eV of 4H–SiC was obtained.

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