Geometry and mode of growth of alkali metal/Si(100)2×1 interfaces

Abstract

The atomic structure of alkali metal (Na,K)/Si(100)2×1 interfaces is investigated by the combination of (i) core-level and valence-band photoemission spectroscopies using synchrotron radiation, (ii) scanning tunneling microscopy, (iii) polarization-dependent photoemission extended x-ray absorption fine-structure (PEXAFS) experiments, and (iv) ab initio all-electron total energy DMol molecular force calculations. The experimental data were taken with extreme care concerning surface preparation and cleanliness. We use the unique ability of PEXAFS to measure the distances between the nearest neighbors of both adsorbate and substrate atoms which allows a double-checking of interatomic distances. We also probe the fine structural changes of the Si(100)2×1 surface and found that the Si-Si dimer is relaxed upon K and Na deposition. We do not find any Na-Na distance consistent with any double layer models. The experimental Na-Si, Si-Na, Si-Si, and Si-K distances are in excellent agreement with the ab initio DMol calculations performed on very large clusters (up to 77 atoms) which also provide the adsorption energy for each site. The results clearly demonstrate that the alkali atoms are adsorbed on a single site, the cave, and form one-dimensional linear chains separated by a distance of 7.68 Å and parallel to the Si dimer rows. The growth and occurrence of a second Na or K layer are clearly related to the presence of very low level of impurities which demonstrates the extreme sensitivity of the atomic structure of these interfaces to the presence of foreign atoms.