INTERACTIONS OF BR WITH SI(111)-7X7 - CHEMISORPTION, STEP RETREAT, AND TERRACE ETCHING

The dissociative chemisorption of Br-2 on Si(111)-7 X 7 and the effect s of spontaneous etching have been studied with scanning tunneling mic roscopy as a function of temperature, flux, and fluence. At room tempe rature, the Si surface retains the 7 X 7 reconstruction, and bias-depe ndent imaging reveals Br bonding to adatom and rest-atom dangling-bond sites. For 700 less than or equal to T less than or equal to 900 K, t errace etching involves Si removal from adatom sites and conversion to a 1 X 1 periodicity that is stabilized by Br. In this temperature ran ge, bilayer step flow etching dominates and Si removal is fastest alon g [1 (1) over bar 0]. Regrowth structures derived from six-membered Si rings terminated by Br appear near the bilayer steps. They are more c ommon near steps that descend along [<(11)over bar>2] than those that descend along [1 (2) over bar 1] or [(2) over bar 11], a distribution that reflects differences in the atomic scale bonding at the steps. St ep flow continues at 1000 K but terrace pitting is also activated. Thi s produces triangular bilayer pits bounded by [1 (1) over bar 0] edges . Analysis yields the ratio of the rates of formation of terrace pits and step kink formation, giving a difference in activation energies fo r these processes of 0.8 eV. Flux-dependent studies at 1000 K show tha t 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 onc e converted to 1 X 1. At 1100 K, etching produces disordered vacancy c lusters in the adatom layer. The presence of small ordered domains ami dst 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.