  We report on the structural properties of highly B-doped silicon (> 2 at. %)
realised by nanosecond laser doping. We investigate the crystalline quality,
deformation and B distribution profile of the doped layer by STEM analysis
followed by HAADF contrast studies and GPA, and compare the results to SIMS
analyses and Hall measurements. When increasing the active B concentration
above 4.3 at.%, the fully strained, perfectly crystalline, Si:B layer starts
showing dislocations and stacking faults. These only disappear around 8 at.%
when the Si:B layer is well accommodated to the substrate. When increasing B
incorporation, we increasingly observe small precipitates, filaments with
higher active B concentration and stacking faults. At the highest
concentrations studied, large precipitates form, related to the decrease of
active B concentration. The structural deformation, defect type and
concentration, and active B distribution are connected to the initial increase
and subsequent gradual loss of superconductivity.
