  Superconducting boron-doped silicon films prepared by gas immersion laser
doping (GILD) technique are analyzed by atom probe tomography. The resulting
three-dimensional chemical composition reveals that boron atoms are
incorporated into crystalline silicon in the atomic percent concentration
range, well above their solubility limit, without creating clusters or
precipitates at the atomic scale. The boron spatial distribution is found to be
compatible with local density of states measurements performed by scanning
tunneling spectroscopy. These results, combined with the observations of very
low impurity level and of a sharp two-dimensional interface between doped and
undoped regions show, that the Si:B material obtained by GILD is a well-defined
random substitutional alloy endowed with promising superconducting properties.
