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Structural and Analytical Studies of 4H Silicon Carbide MOSFETs with Thermally Grown Oxides

Silicon carbide is the only known wide bandgap semiconductor with native SiO2, which makes SiC suitable for metal-oxide-semiconductor (MOS) high power and high frequency devices [1]. However, the interface state densities for SiO2/SiC remain one to two orders of magnitude higher than those typically found at the SiO2/Si interface. This significantly degrades the channel mobility of conventional SiC–MOSFETs [2]. Recent studies suggest that the likely sources of interface defects related to the reduced channel mobility are (i) excess carbon [3,4] , (ii) interface defects involving threefold coordinated O and C interstitials [5], and (iii) defects (vacancies) that extend some distance into the region below the SiO2/SiC interface [6] . The latter correlates well with results from recent modeling studies for enhanced channel mobility few nm below the SiO2/SiC interface [7]. In the present work, we analyzed the SiO2/SiC interface using high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS). The low magnification TEM images reveal sharp interface, free of voids or inclusions. The HRTEM images (Fig. 1), exhibit a contrast variation in the vicinity of the interface which suggests a possible change in the composition of the near-interface layers. These layers of different contrast are observed on both SiO2 (~ 5 nm) and SiC (~ 3 nm) sides of the interface. EELS spectrum-line measurements across the interface revealed the incorporation of C into the bottom part of the SiO2 layer (Fig. 2). In the SiC, a measurable up-shift (2.2 eV) of the C-K edge is observed between the interior of the substrate and the top surface, suggesting a chemical and/or structural modification of the SiC in the vicinity of the SiC/SiO2 interface. HRTEM images exhibit signs of amorphization near the SiC top surface which would be consistent with the chemical shift of the C-K edge, as identified by EELS; however, more work is needed to verify the origin of this amorphous-like contrast. Overall, our results suggest that thermal oxidation of SiC results in the incorporation of C into SiO2 with formation of a SiOCx interfacial layer. These results are consistent with the recent studies using nuclear resonant reaction which have also suggested a gradual nature of the SiO2/SiC interface due to formation and the consumption of the C clusters and chemical reaction with the SiC substrate [9].