Generalized reconstruction model ofSiC(001)−(n×2)surfaces and Si ad-dimer strings

Abstract

We report ab initio calculations on the atomic and electronic structure of the $7\ifmmode\times\else\texttimes\fi{}2$ and $8\ifmmode\times\else\texttimes\fi{}2$ surfaces of cubic $\mathrm{SiC}(001)$ carried out within local density approximation of the density functional theory. Supercells, nonlocal norm-conserving pseudopotentials and Gaussian orbital basis sets are used to describe the surface systems. Our results for the surface structure are in good accord with a lot of experimental data. These investigations complement our previous ab initio studies on $c(4\ifmmode\times\else\texttimes\fi{}2)$, $3\ifmmode\times\else\texttimes\fi{}2$ and $5\ifmmode\times\else\texttimes\fi{}2$ reconstructions of the $\mathrm{SiC}(001)$ surface. On the basis of all our results, we suggest a general two adlayer asymmetric dimer model (TAADM) for $n\ifmmode\times\else\texttimes\fi{}2$ reconstructions of $\mathrm{SiC}(001)$. The model is based on two structural building blocks. The first contains an asymmetric dimer in one partial adlayer while the second contains an asymmetric dimer configuration of six Si adatoms in two partial adlayers. These are characteristic for the $c(4\ifmmode\times\else\texttimes\fi{}2)$ and $3\ifmmode\times\else\texttimes\fi{}2$ reconstructions, respectively. Employing the model for higher $n$ values allows us to suggest optimal structures for a large host of $n\ifmmode\times\else\texttimes\fi{}2$ reconstructions. Comparing the formation energies of the $c(4\ifmmode\times\else\texttimes\fi{}2)$ and all $n\ifmmode\times\else\texttimes\fi{}2$ structures considered, we find in agreement with experiment that the $3\ifmmode\times\else\texttimes\fi{}2$ and $c(4\ifmmode\times\else\texttimes\fi{}2)$ reconstructions are most favorable for silicon-rich and silicon-poor growth conditions, respectively. The formation energies of all other reconstructions fall in between these two limits. In particular, the general TAADM allows us to rationalize the occurrence of periodic and nonperiodic Si ad-dimer-string arrangements at Si-terminated $3\mathrm{C}\text{\ensuremath{-}}\mathrm{SiC}(001)$ surfaces in very good accord with experimental data.