CHEMISORPTION OF HCL, CL2 AND F2 ON THE SI(100) SURFACE

In an earlier publication we have presented the results of some preliminary calculations on the chemisorption of HF and F2 onto the Si(100) 2 x 1 surface using the SLAB-MINDO molecular orbital method. In this paper we extend this work to the case of HCl and Cl2, and report on some new results for F2. All of the species are assumed to dissociate and adsorb onto the surface silicon atoms, and the optimum configuration appropriate to a uniform monolayer coverage determined by minimising the total energy with respect to variations of the atomic coordinates within the six topmost surface layers. The behaviour of HCl on the Si(100) surface is found to be very similar to that obtained previously for HF. The H and Cl atoms chemisorb onto the dangling bonds of the surface silicon atoms at close to the expected tetrahedral angles whilst retaining a 2 x 1 dimer topology for the silicon substrate as suggested by experiment. The adsorption of Cl2 is also found to yield a 2 x 1 symmetric dimer topology with each Si-Cl bond being tilted away from the surface normal at an angle of 14.8-degrees. The lowest energy configuration for Cl2 on the Si(100) surface, however, is determined to be a 2 x 1 bridge site model in which the two chlorine atoms are sited 0.708 angstrom and 1.132 angstrom above the surface plane. An analogous bridge site model is also found for F2 chemisorption with the fluorine atoms lying 0.390 angstrom and 0.015 angstrom above the silicon surface. While this topology is in complete contrast to the molecular type topology reported earlier for a monolayer coverage of F2 on the Si(100) 2 x 1 surface, both configurations are found to have equivalent energies and might thus be expected to coexist during the early stages of fluorine chemisorption.