Determination of the Na/Si(100)2×1 surface and interface geometry by polarization-dependent photoemission extended x-ray-absorption fine structure and ab initio total-energy molecular calculations

We investigate the structure of the clean and Na-covered Si(100)2×1 surface by a combination of the polarization-dependent photoemission extended x-ray-absorption fine-structure (PEXAFS) experimental technique and ab initio total-energy molecular method using force calculations (DMOL). We use the unique ability of PEXAFS to measure the distances between the nearest neighbors of both Na adsorbate and Si substrate atoms which allows a double check of interatomic distances. The Na-Si and Si-Si distances are obtained by analyzing the EXAFS signal at the Na 2p and Si 2p core-level lines. The experimental Na-Si and Si-Si bond lengths are found to be in excellent agreement with the distances obtained by ab initio total-energy DMOL calculations performed on very large clusters. The results indicate unambiguously that Na atoms are adsorbed on a single site, the cave. We do not find any Na-Na distance consistent with any of the ''double-layer'' models. Furthermore, the lowest adsorption energy is found for Na on the cave site. We also probe the fine structural changes of the Si(100)2×1 surface upon Na deposition including the Si-Si dimer relaxation. The combination of these experimental investigations and theoretical calculations enables us to get a deeper insight into the understanding of structural properties of alkali-metal/silicon systems and to propose a complete structural model.