Interaction of F and Cl with silicon surfaces

We have performed local-density-approximation calculations on clusters modeling the Si(111) surface with adsorbed and subsurface F and Cl atoms. These calculations explicitly show that the anomalously low effective binding energy U₀ of SiF and SiCl radicals to the Si substrate in plasma etching conditions is entirely due to the presence of subsurface halogen atoms. The different behavior of subsurface Cl and F is discussed; in the latter case we also discuss the dependency of the SiF U₀ on the flux of incoming F atoms.