Nature of bonding of alkali metals to Si(111)

The nature of the chemisorption bond between alkali metals, Li to Cs, on the Si(l 11) surface has been studied by means of the ab initio Hartree-Fock cluster-model approach. A comparative and systematic study has been carried out for a variety of cluster models simulating the high-symmetry sites of this sur­face. In all cases we found the bond highly ionic with a small participation of covalent effects to the in­teraction energy, which ranges from ,:::,,20% for chemisorbed Li to less than 10% for Rb and Cs above the different active sites. This result is consistent with several analyses of the interaction focused on the interaction energy, the final Hartree-Fock wave function, the analysis of the dipole moment, and of its variation with the adsorbate-surface distance. We show that the dipole moment for chemisorbed alkali metals is smaller than the one expected from an ionic bond beca use of the substrate polarization. Conse­quently we argue that changes in the measured work function are not adequate to extract information about the ionicity of a given interaction. This is in agreement with previous works considering a metal substrate. Here we show that the same mechanism holds for semiconductor surfaces as well