Optimized structures and electronic properties of alkali-metal (Na, K) -adsorbed Si(001) surfaces

The Na and K adsorption on the Si(001)-2×1 surface is studied by first-principles molecular dynamics based on the norm-conserving pseudopotential. The stable adsorption sites are determined for full coverage (Θ=1.0) and half coverage (Θ=0.5). At Θ=0.5, the configuration with Na and K adsorbed along the trough is more stable than predicted by the Levine model. The coverage dependence of the absorption energy suggests that the saturation coverage should be Θ=1.0 for both Na and K and that a qualitative difference between Na and K can be observed in the growth mode of the adsorbed layer. Supplementary calculations for Θ=1/3 and 5/6 are performed in order to obtain an insight into the difference between Na and K in the coverage dependence of the adsorption energy. It is pointed out that the partial core correction for the pseudopotentials of Na and K is very crucial in the structural stability arguments.