  Motivated by recent transport experiments and proposed atomic-scale
semiconductor devices, we present measurements that extend the reach of
scanned-probe methods to discern the properties of individual dopants tens of
nanometers below the surface of a silicon sample. Using a capacitance-based
approach, we have both spatially-resolved individual subsurface boron acceptors
and detected spectroscopically single holes entering and leaving these minute
systems of atoms. A resonance identified as the B+ state is shown to shift in
energy from acceptor to acceptor. We examine this behavior with respect to
nearest-neighbor distances. By directly measuring the quantum levels and
testing the effect of dopant-dopant interactions, this method represents a
valuable tool for the development of future atomic-scale semiconductor devices.
