First-principles calculations that clarify energetics and reactions of oxygen adsorption and carbon desorption on 4H-SiC ($11\bar20$) surface

TL;DR

This study uses first-principles calculations to elucidate the atomistic mechanisms and energy barriers involved in oxygen adsorption and carbon desorption during the initial oxidation of the 4H-SiC ($11ar20$) surface.

Contribution

It provides detailed reaction pathways, free-energy barriers, and compares oxidation processes on different SiC surface orientations, revealing the role of surface dangling bonds.

Findings

Oxygen adsorption occurs at surface-bridge sites with a barrier of 0.7 eV.

Carbon desorption involves a free-energy barrier of 2.4-2.6 eV.

Surface dangling bonds influence the catalytic effect on CO desorption.

Abstract

We report static and dynamic first-principles calculations that provide atomistic pictures of the initial stage of the oxidation processes occurring at the ($11\bar20$) surface of 4H-SiC. Our results unveil 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 multi-stability and that the surface-bridge sites are the most stable and crucial sites for subsequent oxidation. We find that an approaching O$_2$ molecule is adsorbed, then dissociated and finally migrates toward these surface-bridge sites with a free-energy barrier of 0.7 eV at the ($11\bar20$) surface. We also find that a CO molecule is desorbed from the metastable oxidized structure upon the overcoming of a free-energy barrier of 2.4$\sim$2.6 eV, thus constituting one of the annihilation process of C during the oxidation. The results of the CO molecule desorption on the ($11\bar20$) surface are compared with the ($000\bar1$) surface. A catalytic effect of dangling bonds at the surface, causing a drastic reduction of the CO desorption energy, is found on the ($000\bar1$) surface and the microscopic picture of the effect is ascribed to an electron transfer from the Si to C dangling bonds. The intrinsic ($11\bar20$) surface does not show this catalytic effect, and this is because the surface consists of an equal amount of Si and C dangling bonds and the electron transfer occurs before the desorption.