Atomic structure of Na-adsorbed Si(100) surfaces

We examine the atomic and electronic structure of Na-adsorbed Si(100)-p(2\ifmmode\times\else\texttimes\fi{}2) surfaces for various Na coverages (FTHETA) through first-principles pseudopotential calculations. At FTHETA=1/4, we find that the 4\ifmmode\times\else\texttimes\fi{}1 structure with linear Na chains adsorbed on the valley bridge sites is energetically most stable, while substrate Si dimers are rearranged by buckling towards the Na chains. At FTHETA=1/2, the adsorption site of Na is the valley bridge site; however, the 2\ifmmode\times\else\texttimes\fi{}2 structure is found to be more stable than the 2\ifmmode\times\else\texttimes\fi{}1 structure. From the calculated formation energies, we suggest that the saturation coverage is one monolayer with the Na atoms occupying the pedestal and valley bridge sites, exhibiting a 2\ifmmode\times\else\texttimes\fi{}1 reconstruction. The coverage dependences of surface geometry and work function are discussed.