Silicon surfaces : STM, theory and experiment
The fundamental atomic and electronic behaviour of clean silicon surfaces has been studied within a simple tight-binding picture of bonding in solids. Of the various contributions to the surface binding energy, the lowering in the promotion energy (i.e. rehybridization) which accompanies localized Jahn-Teller distortions has been identified as a major electronic driving force underlying the stability of silicon surfaces. The structure of Si(113) has been experimentally determined by the technique of scanning tunnelling microscopy (STM). Despite its high index, the Si(113) surface is found to be highly stable. STM images of both empty and filled states provide strong evidence for a particular structural model with a 3x2 unit cell. The STM results are explained in terms of a general rehybridization principle, suggested by the earlier theoretical study, which accounts for the low surface energy as well as the observed spatial distribution of empty and filled states. In addition, the STM images reveal a high density of domain boundaries which introduce energy states that pin the Fermi level and explain earlier reports of a 3x1 reconstruction for this surface. Voltage-dependent STM image simulations for the Si(113)3x2 surface have been carried out using a simple tight-binding description of surface electronic structure. Quantitative agreement with experiment is obtained confirming the qualitative rehybridization arguments used previously. The local barrier for tunnelling electrons is shown to have an important effect on the interpretation of STM images. The high stability of clean Si(l 13) is shown by STM to be disrupted by adsorption of submonolayer amounts of atomic hydrogen which saturates dangling bonds. Mass transport of silicon occurs and structural models are proposed for the resultant mixed 2x2 and 2x3 surface.