Theoretical study on diffusion mechanism of fluorine atom adsorbed on Si(111) reconstructed surface

We study diffusion mechanisms of fluorine atom adsorbed on Si(111) reconstructed surface by means of the first-principles density functional calculation. Recently, it was revealed experimentally that the fluorine diffusion is assisted by extra silicon atoms diffusing freely on the surface, whereas the detailed mechanism of the Si assist has not been understood. We first investigate F atom adsorption structures with and without a Si atom and find that extra Si atoms do not affect them. Next, we investigate the diffusion mechanism, considering four models in which extra silicon atoms act a different role, and find that SiF-complex diffusion model with activation energy of 1.34 eV is appropriate to explain the experiment. In this model, the Si-F bond is kept during diffusion, while in the others, it repeats breaking and re-bonding. This diffusion mechanism is also understood as the Si diffusion with carrying an F atom. We analyze why this diffusion mechanism is preferable to the others and find that two important features play roles. One is the strong Si-F bond that favors to keep the complex form, and the other is the existence of an extra silicon atom, which takes over the adatom position to passivate dangling bonds left behind after the SiF-complex removal.