  We report on tuning the carrier capture events at a single dangling bond (DB)
midgap state by varying the substrate temperature, doping type, and doping
concentration. All-electronic time-resolved scanning tunneling microscopy
(TR-STM) is employed to directly measure the carrier capture rates on the
nanosecond time scale. A characteristic negative differential resistance (NDR)
feature is evident in the scanning tunneling microscopy (STM) and scanning
tunneling spectroscopy (STS) measurements of DBs on both n and p-type doped
samples. It is found that a common model accounts for both observations.
Atom-specific Kelvin probe force microscopy (KPFM) measurements confirm the
energetic position of the DB's charge transition levels, corroborating STS
studies. It is shown that under different tip-induced fields the DB can be
supplied from two distinct reservoirs: the bulk conduction band and/or the
valence band. We measure the filling and emptying rates of the DBs in the
energy regime where electrons are supplied by the bulk valence band. By adding
point charges in the vicinity of a DB, Coulombic interactions are shown to
shift observed STS and NDR features.
