Fluorine atom abstraction by Si(100). I. Experimental

Abstract

In the interaction of low energy F 2 with Si͑100͒ at 250 K, a dissociative chemisorption mechanism called atom abstraction is identified in which only one of the F atoms is adsorbed while the other F atom is scattered into the gas phase. The dynamics of atom abstraction are characterized via time-of-flight measurements of the scattered F atoms. The F atoms are translationally hyperthermal but only carry a small fraction ͑ϳ3%͒ of the tremendous exothermicity of the reaction. The angular distribution of F atoms is unusually broad for the product of an exothermic reaction. These results suggest an ''attractive'' interaction potential between F 2 and the Si dangling bond with a transition state that is not constrained geometrically. These results are in disagreement with the results of theoretical investigations implying that the available potential energy surfaces are inadequate to describe the dynamics of this gas-surface interaction. In addition to single atom abstraction, two atom adsorption, a mechanism analogous to classic dissociative chemisorption in which both F atoms are adsorbed onto the surface, is also observed. The absolute probability of the three scattering channels ͑single atom abstraction, two atom adsorption, and unreactive scattering͒ for an incident F 2 are determined as a function of F 2 exposure. The fluorine coverage is determined by integrating the reaction probabilities over F 2 exposure, and the reaction probabilities are recast as a function of fluorine coverage. Two atom adsorption is the dominant channel ͓ P 2 ϭ0.83 Ϯ0.03(95%, Nϭ9)͔ in the limit of zero coverage and decays monotonically to zero. Single atom abstraction is the minor channel (P 1 ϭ0.13Ϯ0.03) at low coverage but increases to a maximum (P 1 ϭ0.35Ϯ0.08) at about 0.5 monolayer ͑ML͒ coverage before decaying to zero. The reaction ceases at 0.94Ϯ0.11(95%, Nϭ9) ML. Thermal desorption and helium diffraction confirm that the dangling bonds are the abstraction and adsorption sites. No Si lattice bonds are broken, in contrast to speculation by other investigators that the reaction exothermicity causes lattice disorder.

Determination of dissociative fragment-adsorbate interaction energy during chemisorption of the diatomic molecule HCl on Si(100)

“…[15][16][17] For example, the reaction probability of single F abstraction of F 2 on Si͑100͒ increases from ϳ0.1 at zero F coverage ͑ F ͒ to a peak value of ϳ0.3 at F Х 0.5 ML and then falls to 0 at saturation coverage. Before Su-A or Su-B bonds are formed, in either of the two chemisorption mechanisms, fragment A or B can readily interact not only with the substrate but also with other fragments or adsorbates, which are previously adsorbed fragments.…”

Dissociation of a Product of a Surface Reaction in the Gas Phase:XeF2Reaction with Si

“…Molecular F 2 has been recently shown to dissociate on Si1002 1 by an atom abstraction mechanism [1][2][3]. This mechanism is distinct from that of classic dissociative chemisorption because a bond of an incident molecule is cleaved upon formation of only a single bond to the surface.…”

“…The velocities of the incident and scattered beams are determined by cross correlation time-of-flight (TOF) methods. The beam is incident at 20 from the normal on a p-type Si(100) crystal that is held at 150 K. The Si surface purity, structure, and fluorine coverage are determined as previously described [2,8,9].…”

“…However, TOF spectroscopy unambiguously distinguishes between XeF arising from XeF radicals and XeF arising from the cracking of XeF 2 on the basis of the different velocities with which a XeF radical and a XeF 2 molecule scatter from the surface [2,10]. In addition, because the fragmentation ratio, XeF =XeF 2 , is known from analysis of the incident beam, the contribution of XeF 2 to the m=e 148 TOF spectrum can be subtracted. The net XeF spectra measured at scattering angles of 75 and 60 and for a coverage range of 0 -0.22 monolayer (ML) F atom are shown in Fig.…”

“…Error bars represent the propagated statistical uncertainty. The solid line is a fit to a Maxwell-Boltzmann function[2] from which average energies shown ( 2) are obtained.…”