In this work, a thorough study of the phosphorus (P) heavy doping of thin Silicon-On-Insulator (SOI) layers by UV nanosecond Laser Thermal Annealing (LTA) is presented. The melting regimes and the regrowth processes as well as the redistribution and activation of P in the top-Si amorphized layer were studied as a function of the implant dose and laser annealing conditions. The results highlight the crucial role of the thin crystalline silicon layer preserved after amorphization of the top-Si layer, which provides nucleation seeds for the liquid phase recrystallization. The impact of the implant dose on the recrystallization process is investigated in detail, in terms of melt energy thresholds, crystallographic nature of the resolidified layer, defect formation, surface roughness and hillocks formation at the silicon surface. For all the implanted doses, optimized laser annealing conditions were identified, corresponding to the laser energies just preceding the onset of the full melt. Such optimized layers exhibit perfect crystallinity, negligible P out-diffusion, an almost perfectly flat P depth profile located below the segregation-induced surface pile-up peak and dopant active concentrations well above 1×10 21 cm -3 , close to the highest reported values reported for phosphorus in bulk Si substrates.