The electrical activation of B in Si after ion implantation in the energy range between 5 and 160 keV and rapid thermal annealing processes is investigated. It is found that it critically depends on the purity of the substrate as well as the distance from the surface. In particular, while in very pure epitaxial Si layers (where O and C contents are below∼1×1015/cm3)typically the total B content is electrically active, in Czochralski Si (containing∼1×1018 O/cm3and∼1×1017 C/cm3)the active fraction is very small at doses of1×1012/cm2and increases with increasing dose. For very shallow B implants (∼5 keV), the electrical activation in Czochralski Si further decreases to a few percent of the total amount. These results are interpreted in terms of the formation of boron-impurity complexes deactivating the dopant, the dose effect being a result of trap saturation. Vacancies can eventually dissolve some of the inactive complexes. However, close to the surface an enhanced vacancy annihilation process reduces the dissolution probability producing the observed dramatic effects on the electrically active profiles. Finally, at very low energies (∼5 keV), also in epitaxial Si layers, part of the dopant can be electrically inactive due to B–B interactions, the process being negligible at low doses and increasing with dose. The implications of these results on the formation of ultrashallow junctions are discussed. ©1998 American Institute of Physics.