Reactivity of fluorinated Si(100) with F-2

The dissociative chemisorption of F-2 On the Si(100)(2 x 1) surface saturat ed with 1 monolayer (ML) of fluorine is investigated as a function of the i ncident Fz translational energy. At energies below 3.8 kcal/mol, no reactio n with the Si-Si bonds occurs. Above this threshold, the probability of dis sociative chemisorption rises linearly with the normal component of the inc ident translational energy up to a value of 3.6 x 10(-3) at 13 kcal/mol. Th e 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 bon ds. These probabilities are measured by exposing the fluorine-saturated sur face to supersonic F-2 beams of variable energy, followed by thermal desorp tion 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 dif fraction measurements. The intensity of the diffracted beams is monitored a fter 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 Fz normal energy ab ove the 3.8 kcal/mol threshold. Moreover, the similarity of the relative cr oss sections for diffusive scattering measured after exposure to translatio nally fast F-2 to those measured after Ar+ ion bombardment strongly suggest s 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 wi th Si-Si bends.