The orientation dependence of the backscattered yield of 1.0-MeV helium ions has been used to investigate the lattice characteristics of silicon and germanium implanted at room temperature with 40-keV heavy ions (Ga, As, Sb, In, P). The method gives directly the number of substrate atoms displaced from their lattice sites by more than the Thomas–Fermi screening distance (≈ 0.2 Å) and also the location of the implanted atoms.Each heavy ion is found to displace several thousand lattice atoms in a localized region around its track. At doses where the individual disordered regions do not overlap (~1013 ions/cm2, or less), the silicon lattice reorders at 260 °C and the germanium lattice at 180 °C. At doses greater than ~1014 ions/cm2, a completely amorphous layer is formed in both silicon and germanium. During annealing, the reordering of the lattice is initiated at the interface between the amorphous region and the underlying crystal. The reordering of this amorphous layer occurs at significantly higher temperatures (570 °C in Si and 380 °C in germanium) than those required for the isolated disordered regions. At doses much greater than 1015/cm2, even higher anneal temperatures are required.Following a room-temperature implantation, each implanted atom is embedded in a highly disordered region (at least, for doses > 1014/cm2) and consequently the backscattered spectra from these atoms do not show orientation effects. As the lattice reorders during annealing, the number of implanted atoms lying along the lattice row increases. Following anneal, ~75% of the As atoms and ~90% of the Sb atoms are found on substitutional sites in silicon, while in germanium, ~80% of the In atoms are found on substitutional sites. Annealing to higher temperatures (700–900 °C in the case of silicon) often causes the number of substitutional atoms to decrease again, owing to the effect of internal precipitation or diffusion to the surface.