First-Principles Calculations That Clarify Energetics and Reactions of Oxygen Adsorption and Carbon Desorption on 4H-SiC (11(2)over-bar0) Surface

We report static and dynamic first-principles calculations that provide atomistic pictures of the initial stage of the oxidation processes occurring at the (11 (2) over bar0) surface of 4H-SiC. Our results reveal reaction pathways and their associated free-energy barriers for the adsorption of oxygen and the desorption of carbon atoms. We find that oxygen adsorption shows structural multistability and that the surface-bridge sites are the most stable and crucial sites for subsequent oxidation. We find that an approaching 02 molecule is adsorbed, then is dissociated, and finally migrates toward these surface bridge sites with a free-energy barrier of 0.7 eV at the (11 (2) over bar0) surface. We also find that a CO molecule is desorbed from the metastable oxidized structure upon overcoming a free-energy barrier of 2.4-2.6 eV, thus constituting one of the annihilation processes of C during the oxidation. The results of the CO molecule desorption on the (11 (2) over bar0) surface are compared with that of the (000 (1) over bar) surface. A catalytic effect of dangling bonds at the surface, causing a drastic reduction of the CO desorption energy, is found on the (000 (1) over bar) surface, and the microscopic picture of the effect is ascribed to an electron transfer from the Si to C dangling bonds. The intrinsic (11 (2) over bar0) surface does not show this catalytic effect; this is because the surface consists of an equal amount of Si and C dangling bonds and the electron transfer occurs before the desorption.