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 geometry. A low-energy diffusion path exists between these sites via a saddle point at the bridge-bonded site, consistent with the proposal of Boland based on scanning tunneling microscopy observations. The bridge-bonded site can also act as the attack site for further chemisorption 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 redimerization yielding a mixture of monochloride and dichloride bonding configurations on the surface. The dichloride unit (SiCl2), in turn, exhibits bond weakening of the Si backbonds and this finding provides strong evidence for the spontaneous etching phenomena observed for halogens on Si(100) by Chander, Li, Rioux, and Weaver.