Ion beam crystallography of silicon surfaces III. Si(111)-(7 × 7)

Abstract

We have studied the geometric structure of the clean Si(111)-(7 × 7) surface using Medium Energy Ion Scattering combined with the effects of channeling and blocking. Experimental surface blocking minima have been compared with surface blocking minima calculated with a Monte Carlo simulation program for a number of structural models proposed recently. The trimer model proposed by Himpsel and the stacking fault models proposed by Bennett and by McRae give good agreement with experimental results. Models involving small displacements from the bulklike structure along the surface normal, like Chadi's buckled ring model and the pyramidal cluster model are not compatible with experimental results. We show that the Low Energy Ion Scattering results obtained by Aono et al., which have been interpreted as unique evidence for the pyramidal cluster model, are consistent with various other models as well. We have also studied various versions of the adatom model and find that it does not agree with the experiments, unless the surface contains a stacking fault. We conclude that the triangle-dimer stacking fault is most likely a key ingredient of the structure. We speculate that an additional reconstruction in a hexagonal (√3 × √3) unit cell is needed to fully explain the structure. © 1985.