Effect of Na adsorption on the structural and electronic properties of Si(111)√3 × √3-Au surface.

Adsorption of similar to 0.1 ML of Na onto the Si(111)root 3 x root 3-Au surface held at 300 degrees C has been found to induce pronounced changes in its structural and electronic properties. Domain wall networks, characteristic of the pristine surface, are removed completely, leading to the formation of a highly ordered homogeneous surface. The original atomic arrangement of the Si(111)root 3 x root 3-Au is preserved and Na atoms occupy T-4 adsorption sites at the centers of surface Si trimers. Upon Na adsorption, a pronounced metallic S-1 surface-state band develops. It is characterized by a large spin splitting (momentum splitting at the Fermi level Delta k(parallel to) = 0.027 angstrom(-1) and consequent energy splitting Delta E-F = 110 meV), large electron filling (on the order of 0.5 electrons per root 3 x root 3 unit cell) and small effective electron mass of (0.028 +/- 0.006)m(e). The natural consequence of the latter properties is a high surface conductivity of the Si(111)root 3 x root 3-(Au, Na) surface.