Insight into the initial oxidation of 4H-SiC from first-principles thermodynamics

The initial oxidation of 4H-SiC is investigated under realistic temperature and pressure conditions in order to better understand the mechanism of defect creation at the SiO sub(2)/SiC interface. The oxidation reaction commences on a clean 4H-SiC (0001) surface with chemisorption of the on-surface oxygen atoms at high-symmetry coordinated sites. Our calculations show that oxygen adsorption on the 4H-SiC (0001) surface occurs preferentially at the bridge site when the coverage is low of 1/9 monolayer (ML). As oxygen coverage increases to 3/4 ML, the first oxygen atom is incorporated into the subsurface region that actuates the formation of a surface oxide on the SiC surface. Above 1 ML, oxidation mechanisms in the subsurface layer are explored by taking into account oxygen incorporation and substitutions. To determine the stability of these O/SiC structures for real oxidation conditions, a surface phase diagram of 4H-SiC initial oxidation is constructed over a coverage range of 1/9-2 ML. With the increase of oxygen chemical potential, the 1/9-, 1/4-, 3/2-, and 2-ML configurations become thermodynamically favorable. Moreover, a C sub(2) dimer defect is observed in the 2-ML configuration. This can be extended to the SiO sub(2)/SiC interface as a thin SiO sub(2) layer and may provide a possible mechanism for the creation of a C-cluster defect in the interface.