  Control of dopants in silicon remains the most important approach to
tailoring the properties of electronic materials for integrated circuits, with
Group V impurities the most important n-type dopants. At the same time, silicon
is finding new applications in coherent quantum devices, thanks to the
magnetically quiet environment it provides for the impurity orbitals. The
ionization energies and the shape of the dopant orbitals depend on the surfaces
and interfaces with which they interact. The location of the dopant and local
environment effects will therefore determine the functionality of both future
quantum information processors and next-generation semiconductor devices. Here
we match observed dopant wavefunctions from low-temperature scanning tunnelling
microscopy (STM) to images simulated from first-principles density functional
theory (DFT) calculations. By this combination of experiment and theory we
precisely determine the substitutional sites of neutral As dopants between 5
and 15A below the Si(001):H surface. In the process we gain a full
understanding of the interaction of the donor-electron state with the surface,
and hence of the transition between the bulk dopant (with its delocalised
hydrogenic orbital) and the previously studied dopants in the surface layer.
