4°-off 4H-SiC (0001) Si-face wafers covered with 5μm-thick epitaxitial layers were thermally-oxidized at different temperatures in dry-O2 or in wet ambient prepared by flowing O2, N2, and their mixed gas through deionized water kept at different temperatures to control the humidity. After determining oxide thicknesses accurately by x-ray reflectivity or Si2p XPS, infrared absorption spectroscopy with attenuated total reflection mode (ATR-FTIR) of grown oxide film was investigated. The films were etched-back repeatedly in a diluted HF solution to investigate the thickness dependence of the spectrum.
The structural difference of the oxides especially in the region within a few nanometers from the interface (near-interface region) was investigated by ATR-FITR. Since the peak frequency of Si-O-Si asymmetric vibration mode sensitively reflects the strain in SiO2 (structural strains or non-stoichiometry of the film) , the formation of strained structure in near interface region is indicated by a significant shift of the peak frequency when the film thickness was reduced below ~2 nm by chemical etching . For the cases of dry oxidation, we also found that such strain in near-interface region was not affected at all by simply changing the oxidation temperatures . Thus we can conclude that employing H2O as the oxidant is crucial for the relaxation of near-interface SiO2 microscopic structure. It was also found that the peak width was also smaller for H2O-oxidation than O2-oxidation, which indicates a formation of near-interface oxide with more uniformity in microscopic structure. It is a reasonable assumption that the relaxation of locally-strained structures would be beneficial to reduce the density of carrier-trapping sites in oxide. The superior channel performances with wet-oxidation, reported frequently for 4H-SiC (000-1) C-face  will be also attributable to the effects of H2O to modify the strained structure of near-interface SiO2.
Next we fabricated lateral n-MOSFETs on p-type 4H-SiC (0001) with dry oxidation at 1300ºC followed by post-oxidation annealing in H2O/O2 mixed ambient at as low temperature as 800ºC. Even though the low temperature annealing in wet ambient results in additional oxide growth with a small thickness around only 1 nm or less, we successfully observed a significant enhancement of the channel mobility. This result is suggesting the modification of near-interface structure is achievable by an additional oxidation of only a few monolayers of SiC surface, which is expected to reduce the near-interface trap density.
Acknowledgements: This work is partially supported by CSTI, Cross-ministerial Strategic Innovation Promotion Program, “Next-generation power electronics” (funding agency: NEDO), and by JSPS KAKENHI.
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