Mechanism for SiCl₂ Formation and Desorption and the Growth of Pits in the Etching of Si(100) with Chlorine

A mechanism for SiCl₂ formation and desorption in the etching of Si(100 - (2 × 1) at low chlorine coverages is analyzed using first-principles calculations. We find that the two monochlorinated Si atoms of a surface dimer can rearrange into a metastable SiCl₂(a) adsorbed species plus a Cl-free Si atom. Desorption of SiCl₂ occurs via a two-step mechanism, in which the adsorbed species is preliminarily stabilized by the diffusion away of the free Si atom. The energy barrier to form SiCl₂(a) is lower on a dimer next to a dimer vacancy than in an undamaged region of the surface, consistent with recent STM observation of preferential linear growth of etch pits along dimer rows.