SLAB-MINDO CALCULATIONS ON THE SI(111)2X1 SURFACE

In this paper the SLAB-MINDO molecular orbital method is used to investigate the topology of the Si(111)2 × 1 surface. In contrast to the Si(100) surface, and contrary to the results of self-consistent Pseudopotential calculations, buckling of the surface chains is found to lower the energy. The most stable geometry for the Si(111)2 × 1 surface is determined to be that of the π-bonded chain model. Optimization of the total energy with respect to the detailed topology of this model, however, is shown to produce two minima differing in energy by only 0.006 eV surface atom. The less stable of these two configurations is characterised by a positive buckling of the surface chains of 0.15 Å and is very similar to the topology derived from self-consistent pseudopotential calculations. The lowest energy configuration is found to exhibit a negative surface chain buckling of 0.23 Å. Since the energy barrier separating these two configurations is only 0.011 eV surface atom, however, it is reasonable to expect that defects, and thermal and strain effects, would lead to surface chain buckling of both signs being present on the surface. Recently, Haneman has proposed a possible alternative structural model for the Si(111)2 × 1 surface based on the cleavage of three surface bonds per surface atom. Optimization of this structure, however, is found to result in a topology which is slightly less stable than that of the π-bonded chain model. The associated charge transfers also appear to be too large to be compatible with the measured 2p core shifts. © 1990.