  We report scanning tunneling microscopy studies of individual adatoms
deposited on an InSb(110) surface. The adatoms can be reproducibly dropped off
from the STM tip by voltage pulses, and impact tunneling into the surface by up
to ~100x. The spatial extent and magnitude of the tunneling effect are widely
tunable by imaging conditions such as bias voltage, set current and
photoillumination. We attribute the effect to occupation of a (+/0) charge
transition level, and switching of the associated adatom-induced band bending.
The effect in STM topographic images is well reproduced by transport modeling
of filling and emptying rates as a function of the tip position. STM atomic
contrast and tunneling spectra are in good agreement with density functional
theory calculations for In adatoms. The adatom ionization effect can extend to
distances greater than 50 nm away, which we attribute to the low concentration
and low binding energy of the residual donors in the undoped InSb crystal.
These studies demonstrate how individual atoms can be used to sensitively
control current flow in nanoscale devices.
