Silicon wafers have been implanted with C+, N+, and C++N+ ions at low energies to form buried insulating layers. Buried silicon nitride layers with Si or SiC small crystalline clusters were segregated after annealing at high temperature, leaving a high-crystalline-quality overlayer on top. In the samples implanted with C+ alone, after annealing, the silicon overlayer exhibits a lattice contraction of about 0.04%. In the N+-implanted samples this contraction is between 0.08% and 0.13%. The C++N+ implantation leads to relaxation of the lattice after annealing, with no reduction of the lattice constant.