GEOMETRY AND MODE OF GROWTH OF ALKALI METAL SI(100)2X1 INTERFACES/

The atomic structure of alkali metal (Na,K)/Si (100) 2 X 1 interfaces is investigated by the combination of (i) core-level and valence-band photoemission spectroscopies using synchrotron radiation, (ii) scannin g tunneling microscopy, (iii) polarization-dependent photoemission ext ended x-ray absorption fine-structure (PEXAFS) experiments, and (iv) a b 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 me asure the distances between the nearest neighbors of both adsorbate an d substrate atoms which allows a double-checking of interatomic distan ces. We also probe the fine structural changes of the Si (100) 2 x 1 s urface and found that the Si-Si dimer is relaxed upon K and Na deposit ion. We do not find any Na-Na distance consistent with any double laye r 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 adsorp tion energy for each site. The results clearly demonstrate that the al kali atoms are adsorbed on a single site, the cave, and form one-dimen sional linear chains separated by a distance of 7.68 angstrom and para llel 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 impu rities which demonstrates the extreme sensitivity of the atomic struct ure of these interfaces to the presence of foreign atoms.