We demonstrate that the depth distribution of defects in MeV implantedn-type andp-type crystalline Si is severely affected by the impurity content of the material. Silicon samples with different concentrations of dopants (P or B) and intrinsic contaminants (i.e., C and O) were implanted with 1 or 2 MeV He ions to fluences in the range 2.5×108–1×1013/cm2. Using deep-level transient spectroscopy and spreading resistance measurements, we have identified the defects and determined their concentration and depth distribution. It is found that less than 4&percent; of the defects generated by the beam escape recombination and are stored in electrically active, room temperature stable defect clusters, such as divacancies and carbon–oxygen pairs. When the concentration of these defects is much smaller than the doping level, their profile mirrors the initial defect distribution, as calculated by transport of ions in matter (TRIM), a Monte Carlo code. In particular, the profile presents a maximum at the same depth predicted byTRIMand a width which is strongly dependent on the impurity content of the substrate. Indeed this width can be as large as 2 &mgr;m when implants are performed on a lightly doped, pure epitaxial substrate and returns to the value predicted byTRIM(∼0.5 &mgr;m) upon increasing the concentration of dopants and intrinsic contaminants which act as traps for the diffusing point defects. The broadening of the concentration profiles is however shown to be unavoidable at high implantation fluences when most of the traps are already full and unable to interrupt the free migration of newly generated defects. Finally, by comparing defect distributions inn-type andp-type samples we have detected the spatial separation between vacancy-type and interstitial-type defects, resulting from the ion momentum transfer. The observed phenomena are explained in terms of a trap limited diffusion of the defects generated by the beam. These effects are only observable for a light ion such as He since direct defect clustering within the diluted collision cascades is expected to be significantly inhibited. ©1997 American Institute of Physics.