Adsorption geometry and saturation coverage of Na on the Si(100)-(2x1) surface: First-principles calculations

We have performed first-principles total-energy calculations for Na on the Si(100)-(2x1) surface at different coverages. At half-monolayer Na coverage [one Na atom per Si(100)-(2x1) unit cell], the most favorable adsorption site of Na is found to be the valley bridge site. At one-monolayer Na coverage [two Na atoms per Si(100)-(2x1) unit cell], the Na atoms occupy both the valley bridge sites and the pedestal sites. The one-monolayer-coverage models have substantially lower surface energies than the half-monolayer-coverage models, indicating that the saturation coverage should be one monolayer. We also found that the surface is metallic at half-monolayer Na coverage and semiconducting at one-monolayer coverage