  We have performed electronic state calculations to clarify the initial stage
of the oxidation of the Si- and C-faces in 4H-SiC based on the
density-functional theory. We investigate how each Si and C atomic site is
oxidized on C- and Si-face, and explore most probable reaction pathways,
corresponding energy barriers, and possible defects generated during the
oxidation. We have found that carbon annihilation process is different between
on Si- and on C-face, and this difference causes different defects in
interface; In C-face case, (1), carbon atoms are dissociated directly from the
substrate as CO molecules. (2), after CO dissociation, 3-fold coordinated
oxygen atoms (called Y-lid) are observed at the interface. (3), high density of
C-dangling bonds can remain at the interface. In Si-face case, (1), C atoms
inevitably form carbon nano clusters (composed of a few atoms) in interface to
reduce the number of dangling bonds there. Moreover, we have found that the
carbon nano clusters are composed of not only single but also double chemical
bonds. (2), We have observed that CO molecules are dissociated from the carbon
nano clusters in MD simulations. Furthermore, we have investigated whether
H$_2$ and NO molecules react with the defects found in this study.
