Abstract
The abstraction of bromine atoms from the Si(111) surface has been studied by scanning tunneling microscopy (STM). Vacant dangling bond (DB) sites created by H-atom abstraction of surface Br are identified as are 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 on the 1×1 regions of the surface are about twice that of the 7×7 regions. Surprisingly, we also find that the DB reaction rate is significantly different on the two regions of the surface: reaction on the 7×7 regions being about six times more efficient than on the 1×1 regions. In substantial agreement with earlier studies we find an abstraction-to-DB reaction ratio for H atoms of about 0.2 for the 1×1 regions of the surface. This ratio is much smaller (about 0.01) for the 7×7 regions of the surface, suggesting different types of H-atom–surface interactions in these regions. Abstraction of chemisorbed hydrogen is about 50 times more efficient than abstraction of chemisorbed bromine. Although the low reaction barrier (∼1 kcal/mol) we find is consistent with a direct Eley–Rideal mechanism, this mechanism cannot readily account for the structural sensitivity observed. The possibility of a hot-atom precursor mechanism is discussed. At large exposures definite evidence for an etching reaction is presented, which precludes analysis of the data over a wide exposure range.
Published Version
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