Absence of boron aggregates in superconducting silicon confirmed by atom probe tomography

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.