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) [1], 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 [2]. 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 [2]. 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 [3] 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 1300^ºC followed by post-oxidation annealing in H₂O/O₂ mixed ambient at as low temperature as 800^ºC. 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.

References:

[1] H. Hirai and K. Kita, Appl. Phys. Lett. 103, 132106 (2013).

[2] H. Hirai and K. Kita, Appl. Phys. Lett. 110, 152104 (2017).

[3] M. Okamoto et al., Appl. Phys. Exp. 5, 041302 (2012).