Spin correlations in semiconductor dangling bonds: Implications for the alkali-metal-covered surfaces.

Abstract

It is well known that an antiferromagnetic (AF) spin arrangement within dimers provides a simple explanation of the gap in the electronic structure of the Si(100)-(2\ifmmode\times\else\texttimes\fi{}1) reconstructed surface. Taking this AF description at its face value, we explore the implications of strong Si on-site repulsions (U) for the alkali-metal-covered surface in the coverage (\ensuremath{\theta}) range up to two potassium atoms per dimer. As coverage (electron doping) increases, the substrate undergoes an insulator-metal transition which, in contrast with the U=0 situation, is gradual and due to weakening and final destruction of the AF ordering and its associated gap. The adsorption bond is strongly ionic in the low-coverage regime, with fractional ionic character (FIC) about 0.9. At \ensuremath{\theta}=0.5 (one-half of the first monolayer), however, a crossover between one-dimensional and two-dimensional behavior is found, with a steep rise of the potassium charge accompanied by a steep drop of the FIC of the bond, both signaling the formation of an occupied potassium band increasingly charged and hybridized with the Si band. This process goes on, but now slowly, along the rest of the coverage range, up to saturation, leading finally to a rather low FIC of the bond (0.28) at \ensuremath{\theta}=2. The above crossover separates the low-coverage regime, insulating and highly ionic, from the high-coverage regime where both the substrate and the adsorbate overlayer are metallic and the bonds predominantly covalent. The present model, therefore, does not sustain either the strong ionic picture along the whole coverage range or the opposite view that the transferred charge on the Si atoms is small even at low coverage.