Halogen etching on Si(100)-2x1

Abstract

We have studied the etching of Si(100)-2x1 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. Analysis of the halogen concentration before and after heating indicated that the rates of desorption for SiCl2 or SiBr2 were greatest for the intermediate coveragearound 0.8 ML and were suppressed at the higher coverages. Hence, desorption is not simply proportional to the concentration of species that can form adsorbed precursors SiX2(a). We conclude that it is directly coupled to the creation of monomer vacancies adjacent to the SiX2(a) unit because this increases the lifetime of the excited state and increases the likehood of its desorption. Increasing the surface concentration of halogens reduces the rate of vacancy formation. We show that these rates are also affected by a redimerization process in the high temperature Br-stabilized Si(100)-3x1 reconstruction that increases the likehood of SiBr2(a) formation and enhances its desorption.