In this study, shallow p+-n junction diodes were formed by implanting BF+2 ions into single-crystal silicon or silicon preamorphized by either Si or Ge implantation. BF+ 2 implantation at energies of 20 or 25 keV and a dose of 1×1015 cm-2 was followed by furnace annealing at 600°C in nitrogen ambient. Most samples received a further nitrogen-ambient anneal at 850°C, with various periods of time. Secondary ion mass spectroscopy was used to measure the B profiles. Cross-sectional transmission electron microscopy was used to study the amorphous layers and the defects remaining after annealing. Electrical characterization of the diodes is described. In preamorphized samples, the residual defect density decreases, and the defect band located at the original amorphous-crystalline interface becomes sharper, as the mass of the amorphizing ion species is increased. Ideal low-leakage shallow junctions can be made following either Si or Ge preamorphization and furnace annealing, without removing all the defects induced by preamorphization. This is achieved by containing the implanted B profile completely within the amorphous layer, and by containing the defect band completely within the final biased junction. However, even without a preamorphization step, ideal low-leakage shallow junctions were obtained after BF+2 implantation and 600°C furnace annealing. This suggests that preamorphization may not ultimately be needed for practical engineering of shallow junctions.