Medium energy ion scattering has been used to study the kinetics of solid-phase epitaxial regrowth (SPEG) of ultrathin amorphous layers formed by room-temperature implantation of 5 keV energy phosphorus ions into Si (100). The implants create P distributions with peak concentrations up to ∼7×1021 cm-3. SPEG has been driven by rapid thermal annealing, 475 °C≤TA≤600 °C, for times up to 2000 s. At each temperature, the regrowth velocity is enhanced in the early stages due to the presence of phosphorus but then slows sharply to a value more than an order of magnitude below the intrinsic rate. The critical phosphorus concentration at the transition point for TA=475 °C regrowth is ∼6×1020 cm-3 and increases steadily with anneal temperature. Time-of-flight secondary ion mass spectroscopy profiles confirm the onset of phosphorus push out, where the advancing recrystallization front enters the transition region. Supplementary cross-sectional transmission electron microscopy evidence confirms the existence of a local strain field.