Adsorption and coadsorption of hydrogen and fluorine on the Si(100)-(2×1) surface

We have performed density functional calculations using cluster models containing two dimers along a dimer row of the Si(100)-(2×1) surface. Calculations were performed for half-monolayer, one-monolayer and two-monolayer coverages of hydrogen, fluorine and coadsorbed hydrogen and fluorine, the latter at a 1:1 stoichiometry. In all three cases, we found a rather strong attractive pairing-interaction between adsorbed atoms on the same dimer. We also found a weak attractive clustering-interaction between pairs of dimers with two adsorbed atoms each. For paired-fluorine dimers, this attractive clustering interaction is approximately 0.1 eV. In the case of coadsorption, we find that mixed dimers are energetically favored. Thus, our calculations suggest that, in the case of coadsorption, the hydrogen and fluorine atoms do not segregate.