The effects of carbon codoping on the lattice location of Er atoms in silicon have been investigated using ion beam channeling. A float-zone (FZ) Si (100) wafer was first amorphized to a depth of ∼ 0.3 μm by Si ion implantation at 77 K. The amorphous Si layer was then implanted with carbon ions, and recrystallized via solid phase epitaxial growth. Finally, Er ions were implanted into C-doped and C-free Si samples at 300 °C. Angular scans along three major crystalline directions, that is, 〈100〉, 〈110〉, and 〈111〉, were performed for Er-implanted Si with and without C codoping. In FZ-Si, in which the carbon concentration is very low (<1016 cm−3), a large fraction ( ∼ 50%) of implanted Er atoms are found to occupy the near-tetrahedral (T) interstitial site, while few Er atoms are on the hexagonal (H) interstitial site. It is evident that the incorporation of C into Si tends to decrease the fraction of T-site Er atoms and relocate them to the H site. We have also determined the effective number ( ∼ 1.5) of carbon atoms required for depopulating an Er atom from the T site to H site. Implications on the configuration of Er luminescence centers are discussed. © 2002 American Institute of Physics.