Metastable pseudomorphic Ge 0.12 Si 0.88 films of 265 nm thickness were grown on Si(100) substrates by molecular beam epitaxy and then implanted with 100 keV phosphorus ions at room temperature to doses of 5 × 10 13 cm -2 (lightly implanted) and 1.5 × 10 15 cm -2 (heavily implanted), respectively. The damage and strain of the films were measured by MeV 4 He channeling spectrometry, transmission electron microscopy, and double-crystal X-ray diffractometry. The projected range of P ions is about half of the epilayer thickness. In both cases, no noticeable damage at the Si-GeSi interface is observed after the implantation. The top ∼ 190 nm of the heavily implanted samples are amorphized by the implantation, while the lightly implanted samples are not amorphized. Implanted samples, together with non-implanted reference samples, were subsequently annealed in a vacuum furnace for 30 min from 400-800°C. For the lightly implanted samples, most of the implantation-induced damage and strain can be effectively removed by furnace annealing at 400-550°C; the electrical activation of P ions at these low annealing temperatures is poor. Annealing of the lightly implanted samples at higher temperatures improves the dopant activation, but results in strain relaxation of the heterostructure. For the heavily implanted samples annealed at or above 500°C, the top amorphous GeSi regrows by solid-phase epitaxy and the implanted phosphorus ions reach ∼ 100% activation after the completion of solid-phase epitaxial regrowth. The regrown GeSi, however, is fully relaxed with a high density of threading dislocations (∼ 10 10 -10 11 cm -2 ). If a heavily implanted sample is annealed at or above 700°C, misfit dislocations appear at the Si-GeSi interface; at 800°C strain in the entire GeSi becomes fully relaxed. Strain relaxation of the Si/GeSi heterostructure is significantly enhanced by the implantation. This enhancement appears to be a function of the ion dose. We conclude that as a metastable pseudomorphic Ge 0.12 Si 0.88 layer on Si(100) is doped with P implantation, steady-state furnace annealing cannot achieve full dopant activation in the epilayer without the loss of its crystalline perfection and its pseudomorphic strain.