Interactions of Br with Si(111)-7×7: Chemisorption, step retreat, and terrace etching

The dissociative chemisorption of ${\mathrm{Br}}_2$ on Si(111)-7×7 and the effects of spontaneous etching have been studied with scanning tunneling microscopy as a function of temperature, flux, and fluence. At room temperature, the Si surface retains the 7×7 reconstruction, and bias-dependent imaging reveals Br bonding to adatom and rest-atom dangling-bond sites. For 700≤T≤900 K, terrace etching involves Si removal from adatom sites and conversion to a 1×1 periodicity that is stabilized by Br. In this temperature range, bilayer step flow etching dominates and Si removal is fastest along 〈11¯0〉. Regrowth structures derived from six-membered Si rings terminated by Br appear near the bilayer steps. They are more common near steps that descend along [1¯ 1¯2] than those that descend along [12¯1] or [2¯11], a distribution that reflects differences in the atomic scale bonding at the steps. Step flow continues at 1000 K but terrace pitting is also activated. This produces triangular bilayer pits bounded by 〈11¯0〉 edges. Analysis yields the ratio of the rates of formation of terrace pits and step kink formation, giving a difference in activation energies for these processes of 0.8 eV. Flux-dependent studies at 1000 K show that pit sizes and densities vary dramatically, an effect related to the mean Br content on the terrace. No such dependence was observed at 900 K because pits could not be formed and the terraces were inactive once converted to 1×1. At 1100 K, etching produces disordered vacancy clusters in the adatom layer. The presence of small ordered domains amidst randomly distributed adatoms is attributed to facile local removal. In all cases removal proceeds in a layer-by-layer fashion because of the striking anisotropy in etching energetics.