CHEMISORPTION OF CL ON THE SI(100)-2X1 SURFACE

We have investigated the chemisorption of Cl atoms on the Si(100)-2X1 surface using a semiempirical self-consistent molecular-orbital method . Cl atoms are found to saturate the dangling bonds of the surface Si symmetric dimer atoms in an off-normal direction in the equilibrium ge ometry. A low-energy diffusion path exists between these sites via a s addle point at the bridge-bonded site, consistent with the proposal of Boland based on scanning tunneling microscopy observations. The bridg e-bonded site can also act as the attack site for further chemisorptio n of Cl on the dimer whose dangling bonds are already saturated by Cl atoms. This leads to bond lengthening and weakening of the Si-Si dimer bond and can contribute to subsequent bond breaking and redimerizatio n yielding a mixture of monochloride and dichloride bonding configurat ions on the surface. The dichloride unit (SiCl2), in turn, exhibits bo nd weakening of the Si backbonds and this finding provides strong evid ence for the spontaneous etching phenomena observed for halogens on Si (100) by Chander, Li, Rioux, and Weaver.