  During fabrication and scanning-tunneling-microscope (STM) imaging of
dangling bonds (DBs) on a hydrogenated silicon surface, we consistently
observed halo-like features around isolated DBs for specific imaging
conditions. These surround individual or small groups of DBs, have abnormally
sharp edges, and cannot be explained by conventional STM theory. Here we
investigate the nature of these features by a comprehensive 3-dimensional model
of elastic and inelastic charge transfer in the vicinity of a DB. Our essential
finding is that non-equilibrium current through the localized electronic state
of a DB determines the charging state of the DB. This localized charge distorts
the electronic bands of the silicon sample, which in turn affects the STM
current in that vicinity causing the halo effect. The influence of various
imaging conditions and characteristics of the sample on STM images of DBs is
also investigated.
