We have investigated the electrical properties of terbium ions incorporated in crystalline Si. Silicon p(+) -n junctions were realized in n-type epitaxial or Czochralski-grown Si, having an O concentration of similar to 10(15) and 7 x 10(17)/cm(3), respectively. These junctions were implanted with 5 MeV Tb ions to fluences in the range 6 x 10(11)-6 x 10(12)/cm(2). Some epitaxial Si samples were also coimplanted with O in order to provide a concentration of similar to 10(18) O/cm(3) in the region where Tb sits. Annealing at temperatures between 800 and 1000 degrees C and times ranging from 5 s to 30 min was performed. Deep-level transient spectroscopy, current-voltage, capacitance-voltage, and carrier lifetime measurements were used to characterize the levels introduced by Tb ions in the Si band gap. It is found that in a highly pure epitaxial Si, Tb introduces several donor levels at energies comprised between 0.15 and 0.53 eV from the conduction band. Interaction between Tb and O produces severe modifications in the distribution of deep levels related to the rare-earth ions. In particular, in the presence of O the concentration of the deep levels is reduced by more than one order of magnitude and shallower levels, with energies in the range 0.07-0.16 eV from the conduction band, dominate the spectrum. As a result, O co-doping produces an enhancement in the donor activity of Tb, a decrease in the freeze-out temperature of the electrons stored in Tb-related levels, and an increase in the minority-carrier lifetime. We have found that these modifications are produced by the formation of Tb-O complexes. Analysis of the growth and dissolution kinetics of these complexes reveal that they are produced by O diffusion to Tb ions at temperatures around 900 degrees C and annealed by evolution into more complex structures upon increasing thermal budget. (C) 1999 American Institute of Physics. [S0021-8979(99)00304-7].