First-principles-derived dynamics of F₂ reactive scattering on Si(100)-2X1

We have simulated via molecular dynamics the interaction of F₂ with the clean Si(100)-2X1 reconstructed surface. Using a Stillinger-Weber-type many-body potential with the Si-F interactions refit to ab initio data, we find that both vibrational and translational excitation of the incident F₂ can lead to increased reactivity, but they do so in different ways. The dominant reaction channels are (a) F-atom abstraction, leading to the formation of one Si-F bond while the remaining fluorine atom is ejected away from the surface, and (b) dissociative chemisorption, where both fluorine atoms in the incident F₂ molecule form Si-F bonds on the surface. Nonreactive scattering is almost never observed. As a result, enhanced reactivity is mainly characterized by an increase in dissociative chemisorption at the expense of F-atom abstraction and by a corresponding increase in the initial reaction probability S₀. We find S₀ ranges from 0.57±0.04 for the lowest excitation energies to 0.78±0.04 for the largest translational excitation of 20.9 kcal/mol. For cases where F-atom abstraction occurs, the exit velocities of fluorine atoms ejected from the surface are found to be independent of the incident F₂ energy and with kinetic temperatures much higher than the surface temperature, suggesting that the exiting fluorine atom does not equilibrate with the surface, yet loses memory of its initial state. Finally, for dissociative chemisorption trajectories, we find that the adsite location of the two fluorine atoms is strongly dependent on the incident orientation.