Comparison of the Interactions of XeF₂ and F₂ with Si(100)(2 × 1)

The interaction of low-energy XeF 2 with Si(100)(2 × 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 XeF 2 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 XeF 2 exposure are identical to those produced by F 2 exposure. The interactions of XeF 2 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 × 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 XeF 2 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 XeF 2 continues to deposit fluorine by reacting with the Si−Si σ 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 SiF 4 .