LEED analysis and energy minimization calculations for Si(111) (7×7) surface structures

Abstract

The low energy electron diffraction patterns from a variety of surface models of the (7×7) structure of Si(111) surfaces have been computed. Kinematic (single scattering) theory was used, together with a 10 phase‐shift atomic scattering factor, Debye–Waller factor, and depth correction. Comparison with limited experimental data currently available was made by averaging data over the voltage range 95–140 V, a procedure which is justified by the similarity of kinematic and dynamical analysis in the case of Si. In addition quantitative spot ratio parameters were defined as an aid in evaluating comparison with experiment. The structures tested included the Lander vacancy, Harrison adatom, and Levine–McFarlane–Mark distortion models, together with combinations of these. In addition, developments of these models were tried, as well as a Peierls instability structure and three new models. Several of the models were part specified, and then allowed to relax to a minimum surface energy configuration. This involved atom shifts down to four layers with a maximum shift in general of about 0.5 Å. The results showed that vacancies or adatoms are not required to match LEED data but a surface distortion is necessary. The computed averaged LEED screens outstrip current experimental data but at this stage the best fit appears to be achieved with a hexagon‐based distortion model. This has a physical basis in chemical bond rehybridization considerations and is shown to be compatible with a variety of other surface data.