The structure of (111) oriented, unheated oxygen-implanted silicon (dose 7. 3 multiplied by 10**1**6cm** minus **2) has been studied by transmission electron microscopy (TEM). The as-implanted material exhibited four structurally different layers: defect-free monocrystalline silicon, amorphous silicon, monocrystalline silicon with a high defect density, and the perfect crystalline substrate. After heat-pulse annealing for 20s at 800 degree C, 900 degree C, or 1000 degree C, the amorphous layer recrystallized resulting in polycrystalline silicon, rich in oxygen. The uniform insulator buried layer was not formed under these specific implantation and annealing conditions.