THE CHEMISORPTION OF HF AND F2 ON THE SI(100) SURFACE

This paper presents an initial study of the chemisorption of HF and F2 onto the Si(100)2 × 1 surface. In each case, the optimum topology of a uniform monolayer coverage is determined by minimising the total energy with respect to both the atomic coordinates of the adsorbate and the first four layers of the substrate. This is done using the SLAB-MINDO method and employing films consisting of 14 silicon layers and having the periodicity of the 2 × 1 unit cell. The behaviour of HF on the Si(100) surface is found to be analogous to that obtained previously for H2. The isolated H and F atoms attach themselves to the dangling bonds of the silicon dimer atoms. This produces distinct SiH and SiF species and maintains the basic 2 × 1 dimer reconstruction of the Si(100) surface. Very different behaviour is predicted to occur for a uniform monolayer coverage of F2, however. In this case, one of the fluorine atoms prefers to occupy the dimer bridge site and hence leads to a significant weakening of the silicon dimer structure. Moreover, we find that this optimised F2 monolayer topology does not correspond to a complete saturation of the silicon dangling bonds. It thus allows for the further rapid adsorption of fluorine onto the Si(100) surface and the possible formation of the various fluorosilyl species which are experimentally observed. © 1990.