The effects of implant dose and postimplant annealing treatment on the microstructure of nitrogen‐implanted silicon‐on‐insulator were studied by cross‐sectional transmission electron microscopy techniques. In the lower dose case (0.75×1018/cm2) an amorphous silicon layer forms after implantation. Annealing at 1200 °C or higher results in a buried polycrystalline α‐Si3N4 layer containing many randomly oriented silicon particles. Higher dose implantation results in an amorphous silicon‐nitride layer. A porous layer also forms in the middle of the amorphous layer if the implant dose is 1.2×1018/cm2 or higher. The crystallization of the amorphous layer in the higher dose cases is shown to happen in two steps. In the first step nucleation and growth of α‐Si3N4 grains occur in the amorphous nitride region to form a spherulitic polycrystalline structure. The second step is the cellular growth of the spherulitic nitride grains into the crystalline silicon regions. Silicon particles are trapped at the cell walls as the cellular reaction advances. These particles are conglomerated and sphereodized but retain the same orientation as the substrate silicon at higher temperatures. The quality of the top silicon film is excellent after annealing at 1200 °C or higher, irrespective of the implant dose.