  With nanoelectronics reaching the limit of atom-sized devices, it has become
critical to examine how irregularities in the local environment can affect
device functionality. Here, we characterize the influence of charged atomic
species on the electrostatic potential of a semiconductor surface at the
sub-nanometer scale. Using non-contact atomic force microscopy, two-dimensional
maps of the contact potential difference are used to show the spatially varying
electrostatic potential on the (100) surface of hydrogen-terminated
highly-doped silicon. Three types of charged species, one on the surface and
two within the bulk, are examined. An electric field sensitive spectroscopic
signature of a single probe atom reports on nearby charged species. The
identity of one of the near-surface species has been uncertain. That species,
suspected of being boron or perhaps a negatively charged donor species, we
suggest is of a character more consistent with either a negatively charged
interstitial hydrogen or a hydrogen vacancy complex.
