The etching of (100)-(2 x 1) with Cl and Br was studied by using scanning tunnelling microscopy. Clean surfaces were exposed to molecular halogens at room temperature in order to produce well-defined chemisorption structures with coverages of 0.2 to 1 monolayer. Heating to between 750 and 850K led to etching via thermal desorption, as well as to the diffusion of vacancies and adatoms. Analysis of the halogen concentration, before and after heating, indicated that the desorption rates of SiX2 were greatest for intermediate coverages and that etching was suppressed as saturation was approached. This was explained in terms of an isomerization reaction involving adsorbed species, 2SiX ↔ SiX2 + Si, followed by the transfer of adsorbed Si to a terrace site. The creation of a monomer vacancy, adjacent to the adsorbed SiX2 unit, frustrated the reverse reaction and increased the likelihood that it would desorb. However, vacancy-assisted desorption was impeded when terrace sites were blocked by halogens, and the etch-rate decreased. The decrease was less marked for Br than for Cl because there was a high-concentration phase, Br-Si(100)-(3 x 1), in which Si dimerization could occur after isomerization. The effect was again to increase the lifetime of the adsorbed SiBr2 state. In this case, desorption resulted in lines of single-atom vacancies between dimer rows.

Halogen Etching of Si(100)-2 x 1 - Dependence on Surface Concentration K.Nakayama, C.M.Aldao, J.H.Weaver: Physical Review B, 1999, 59[24], 15893-901