Influence of STM tip on electronic structure: Si(100) dimers

• Published in: Ultramicroscopy 1992-07, Vol.42 (A), p.97-104
• Main Authors: Huang, Z.-H., Allen, R.E.
• Format: Article
• Language: English
• Subjects: Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; Physics; Solid surfaces and solid-solid interfaces; Surface structure and topography; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties)
• ISSN: 0304-3991; 1879-2723
• DOI: 10.1016/0304-3991(92)90251-E

There are strong reasons for believing that dimers in defect-free regions of the Si(100) surface are asymmetric (tilted), but they appear to be symmetric in STM images. It has been suggested that these symmetric images result from a strong distortion of the dimer's electronic structure (and, as a consequence, its geometry) when the STM voltage is applied. Here we report first-principles calculations for the Si(100) surface in the presence of a voltage applied by a simple model of an STM tip: a uniform sphere of charge, 1.06 Å in diameter and centered 2.5 Å above the surface. For voltages comparable to those used in STM experiments, there is only a very small distortion of the electronic structure, both below and above the Fermi energy E F. Even for large applied voltages, the results indicate that the STM images should be far from symmetric. On the other hand, the time-averaged charge densities for rapidly flipping asymmetric dimers are fully consistent with the STM observations.