First-principles study of oxygen adsorption on the nitrogen-passivated4H-SiC (0001) silicon face

Abstract

The effects of oxygen and nitrogen adsorption and interaction on the $4H$-SiC (0001) surface are investigated using the density-functional theory coupled with molecular dynamics. Nitrogen coverages from $\frac{1}{9}$ to $\frac{5}{4}$ monolayer are considered. The adsorption sites for isolated oxygen atoms and molecules are considered as well as the interaction between oxygen and nitrogen on the surface. The primary findings are that adsorbed atomic oxygen and nitrogen repel each other. At sparse coverages ($n+m\ensuremath{\le}\frac{1}{3}$, $n=\text{monolayers}$ of nitrogen and $m=\text{monolayers}$ of oxygen) the oxygen atoms bond to two surface-silicon dangling bonds while the nitrogen atoms consume three. For higher densities of oxygen and nitrogen, pairs of nitrogen atoms may combine to form ${\text{N}}_{2}$ dimers with each pair dangling from the top of a surface-silicon atom, thus increasing the number of silicon-surface bond sites available at the given coverage. However the ${\text{N}}_{2}$ are only weakly bound with respect to gas-phase ${\text{N}}_{2}$ and for higher coverages of oxygen, not bound at all. For equal concentrations of oxygen and nitrogen, this arrangement saturates at $\frac{2}{3}$ monolayer of nitrogen and oxygen above which coverage the ${\text{N}}_{2}$ is displaced by atomic oxygen. A higher-energy surface can be formed from the adsorption of NO molecules. This saturates all of the silicon-surface states at $\frac{1}{3}$ monolayer. Above this coverage a second phase consisting of ${\text{N}}_{2}{\text{O}}_{2}$ saturates all of the surface states at 1 monolayer and could conceivably be formed by adsorbing $\frac{1}{2}$ monolayer of ${\text{N}}_{2}$ onto an oxygen-saturated $4H$-SiC silicon surface. Interestingly our study shows that the 1 monolayer coverage of NO leaves the bulk band gap free of states. This has implications for growth of nitrogen on the silicon surface in the presence of oxygen.