Reactivity of Fluorinated Si(100) with F₂

Abstract

The dissociative chemisorption of F 2 on the Si(100)(2 × 1) surface saturated with 1 monolayer (ML) of fluorine is investigated as a function of the incident F 2 translational energy. At energies below 3.8 kcal/mol, no reaction with the Si−Si bonds occurs. Above this threshold, the probability of dissociative chemisorption rises linearly with the normal component of the incident translational energy up to a value of 3.6 × 10 -3 at 13 kcal/mol. The relatively small effect of translational energy implies a late barrier in the potential energy surface for the interaction of F 2 with the Si−Si bonds. These probabilities are measured by exposing the fluorine-saturated surface to supersonic F 2 beams of variable energy, followed by thermal desorption measurements to determine the resulting fluorine coverage. Information regarding the specific Si−Si site (Si−Si dimer or Si−Si lattice bonds) at which the translationally activated reaction occurs is obtained from He diffraction measurements. The intensity of the diffracted beams is monitored after exposing the fluorine-saturated surface to F 2 of variable energy. The intensities remain constant after exposure to low-energy (<3.8 kcal/mol) F 2, whereas they decline monotonically as a function of F 2 normal energy above the 3.8 kcal/mol threshold. Moreover, the similarity of the relative cross sections for diffusive scattering measured after exposure to translationally fast F 2 to those measured after Ar + ion bombardment strongly suggests that the reaction does not occur preferentially at the Si−Si dimer bonds, which are the weakest Si−Si bonds in the system. Reaction at Si−Si lattice bonds also occurs, leading to surface disorder. Additional data show that for submonolayer coverages generated from low energy F 2, no reaction with Si−Si bonds occurs, while exposure to high-energy F 2 leads to reaction with Si−Si bonds.