We demonstrate that He can be a powerful tool to control B diffusion both in crystalline (c-Si) and preamorphized Si (PA-Si). By means of positron annihilation spectroscopy (PAS), we showed in He-implanted c-Si the formation after annealing of large open-volume defects at the implant projected range R-p of He (voids) and of smaller vacancy-type defects toward the surface (nanovoids). In particular, these nanovoids locally suppress the amount of self-interstitials (Is) generated by B implantation, as verified by PAS, eventually reducing B diffusion and leading to a boxlike shape of the B-implanted profile. On the other hand, for B implantation in PA-Si, the authors demonstrated that if He-induced voids are formed between the end-of-range (EOR) defects and the surface, they act as a diffusion barrier for Is coming from the EOR defects. Indeed, this barrier strongly reduces diffusion of B placed in proximity of the surface. (C) 2008 American Vacuum Society.