Atomic and electronic structure of the Na/Si(111)-(3×1) surface

We have determined the equilibrium structures and energy bands for the Na/Si(111)-(3×1) surface at two coverages, 1/3 and 2/3 ML, using the pseudopotential density-functional total-energy method. We find that Na atoms induce a large charge and atomic corrugation on the top Si layer at 1/3 ML while the Si substrate at 2/3 ML maintains a near-(1×1) geometry. Based on the calculated electronic structures, we propose the saturation Na coverage is 1/3 ML. The present Na-overlayer model with 1/3 ML can explain the previous experimental results: the semiconducting ground state, double-row scanning-tunneling-microscopy image, chemical passivation for oxidation, and possibly low-energy-electron-diffraction I-V curves indicating a (3×1) substrate reconstruction.