We have used conversion electron emission channeling to investigate the lattice sites of following implantation of the radioactive isotope into CZ Si and FZ Si at varying doses (6 $\times 10^{12} – 5 \times 10^{13}$ cm$^{−2}$). In all cases isothermal annealing at 900°C caused Er to leave its preferred near-tetrahedral sites in favour of random lattice sites, but this process occurred by orders of magnitude faster in CZ Si. Furthermore, in CZ Si the incorporation of Er on random lattice sites was fastest in samples implanted with low doses of Tm+Er. We compare our experimental results to a simple numerical model which accounts for the diffusion of Er and O and the formation of Er$_{n}$O$_{m}$ complexes. On the basis of this model, our experimental data indicate that only a few (probably between 1 and 2) O atoms are required in order to remove an Er atom from its tetrahedral site.