Halogen etching of Si(100)-2x1: Dependence on surface concentration

We have studied the etching of Si(100)-2 x 1 by Cl and Br, using scanning tunneling microscopy to obtain morphological information that can be related to reaction and desorption pathways. Clean surfaces were exposed to molecular halogens at room temperature to produce well-defined chemisorption structures for coverages of 0.2-1.0 ML. Heating to 750-850 K induced etching by thermal desorption, as well as the diffusion of vacancies and adatoms. Analysis of the halogen concentration before and after heating indicated that the rates of desorption for SiX2 were greatest for intermediate coverages and that etching was suppressed as saturation was approached. This unexpected result is discussed in terms of an isomerization reaction involving adsorbed species 2SiX(a)<->SiX2(a) + Si(a), followed by the transfer of Si(a) to a terrace site. The creation of a monomer vacancy adjacent to the SiX2(a) unit frustrated the reverse reaction and increased the likelihood that it would desorb. However, vacancy-assisted desorption is impeded when terrace sites are blocked by halogens, and the etch rate drops. The decrease is less profound for Br than Cl because there is a high-concentration phase, Br-Si(100)-3 x 1, in which Si dimerization can occur after isomerization. The effect is again to increase the lifetime of the SiBr2(a)state. In this case, desorption results in lines of single atom vacancies between dimer rows.