ABSTRACTION OF CHEMISORBED BROMINE FROM THE SI(111) SURFACE BY INCIDENT HYDROGEN-ATOMS

The abstraction of bromine atoms from the Si(lll) surface has been stu died by scanning tunneling microscopy (STM), Vacant dangling bond (DB) sites created by H-atom abstraction of surface Br are identified as a re the subsequent reaction of these DB sites with atomic hydrogen. The kinetics of Br abstraction and the H-atom-DB reaction are shown to be dependent on the local surface structure. Bromine abstraction rates o n the 1x1 regions of the surface are about twice that of the 7x7 regio ns. Surprisingly, we also find that the DB reaction rate is significan tly different on the two regions of the surface: reaction on the 7x7 r egions being about six times more efficient than on the 1x1 regions. I n substantial agreement with earlier studies we find an abstraction-to -DB reaction ratio for H atoms of about 0.2 for the 1x1 regions of the surface. This ratio is much smaller (about 0.01) for the 7x7 regions of the surface, suggesting different types of H-atom-surface interacti ons in these regions. Abstraction of chemisorbed hydrogen is about 50 times more efficient than abstraction of chemisorbed bromine. Although the low reaction barrier (similar to 1 kcal/mol) we find is consisten t with a direct Eley-Rideal mechanism, this mechanism cannot readily a ccount for the structural sensitivity observed. The possibility of a h ot-atom precursor mechanism is discussed; At large exposures definite evidence for an etching reaction is presented, which precludes analysi s of the data over a wide exposure range. (C) 1998 American Institute of Physics.