Comparison of the interactions of XeF2 and F2 with Si(100)(2 x 1)

The interaction of low-energy XeF2 with Si(100)(2 x 1) has been studied and compared to that of F-2. Helium atom diffraction, beam-surface scattering, and thermal desorption measurements are the major techniques used in this study. It is found that XeF2 dissociatively chemisorbs with high probability solely on the Si dangling bonds up to a coverage of about one monolayer (ML). Molecular fluorine has previously been observed to react similarly, saturating the dangling bonds at 1 ML coverage. The thermal desorption kinetics and products from the fluorinated layer produced by XeF2 exposure are identical to those produced by F-2 exposure. The interactions of XeF2 and F-2 are also strikingly similar with respect to the long-range order of the fluorinated Si up, to about 1. ML coverage. The order is monitored by He diffraction. In both systems, the diffracted He beams exhibit a sharp decrease in intensity because of the disorder produced by the fluorination of random surface-unit cells as the coverage increases from 0 to about 0.3 ML. The intensity then increases until the fluorine overlayer has fully recovered its (2 x 1) periodicity at about 1 ML. This recovery corresponds to the decoration of each Si dangling bond with a fluorine atom. A critical observation of this study is that despite the large exothermicity of the dissociative chemisorption of XeF2 or F-2 the order of the surface is not destroyed in either system. After saturation of the dangling bonds, F-2 ceases to react with the surface whereas XeF2 continues to deposit fluorine by reacting with the Si-Si a dimer bonds and the Si-Si lattice bonds. The order is destroyed as a result of the continued fluorine deposition, and ultimately, etching occurs by the formation of volatile SiF4.