  The ability to control single dopants in solid-state devices has opened the
way towards reliable quantum computation schemes. In this perspective it is
essential to understand the impact of interfaces and electric fields, inherent
to address coherent electronic manipulation, on the dopants atomic scale
properties. This requires both fine energetic and spatial resolution of the
energy spectrum and wave-function, respectively. Here we present an experiment
fulfilling both conditions: we perform transport on single donors in silicon
close to a vacuum interface using a scanning tunneling microscope (STM) in the
single electron tunneling regime. The spatial degrees of freedom of the STM tip
provide a versatility allowing a unique understanding of electrostatics. We
obtain the absolute energy scale from the thermal broadening of the resonant
peaks, allowing to deduce the charging energies of the donors. Finally we use a
rate equations model to derive the current in presence of an excited state,
highlighting the benefits of the highly tunable vacuum tunnel rates which
should be exploited in further experiments. This work provides a general
framework to investigate dopant-based systems at the atomic scale.
