  The effects of the oxidation atmosphere and crystal faces on the
interface-trap density was examined by using constant-capacitance deep-level
transient spectroscopy to clarify the origin of them. By comparing the DLTS
spectra of the low-mobility interfaces oxidized in a N$_2$O atmosphere with
those of the high-mobility interfaces on C-face oxidized in a wet atmosphere,
it was found that a high density of traps are commonly observed around the
energy of 0.16 eV from the edge of the conduction band ($C1$ traps) in
low-mobility interfaces irrespective of crystal faces. It was also found that
the generation and elimination of traps specific to crystal faces: (1) the $C1$
traps can be eliminated by wet oxidation only on the C-face, and (2) the $O2$
traps (0.37 eV) can be observed in the SiC/SiO$_2$ interface only on the
Si-face. The generation of $O2$ traps on the Si-face and the elimination of
$C1$ traps on the C-face by wet oxidation may be caused by the oxidation
reaction specific to the crystal faces.
