Two-Dimensional Carbon Incorporation into Si(001): C Amount and Structure ofSi(001)−c(4×4)

The C amount and the structure of the $\mathrm{S}\mathrm{i}(001)\mathrm{\text{\ensuremath{-}}}c(4\ifmmode\times\else\texttimes\fi{}4)$ surface is studied using scanning tunneling microscopy (STM) and ab initio calculations. The $c(4\ifmmode\times\else\texttimes\fi{}4)$ phase is found to contain $1/8$ monolayer C (1 C atom in each primitive unit cell). From the C amount and the symmetry of high-resolution STM images, it is inferred that the C atoms substitute the fourth-layer site below the dimer row. We construct a structure model relying on ab initio energetics and STM simulations. Each C atom induces an on-site dimer vacancy and two adjacent rotated dimers on the same dimer row. The $c(4\ifmmode\times\else\texttimes\fi{}4)$ phase constitutes the subsurface ${\mathrm{S}\mathrm{i}}_{0.875}{\mathrm{C}}_{0.125}$ $\ensuremath{\delta}$ layer with two-dimensionally ordered C atoms.