Si(100) wafers were implanted with O+at an energy of 180 keV to a dose of 2.3×1018/cm2in the separation by implanted oxygen process. Following implantation, one wafer was annealed at 1275 °C for 2 h. Spectroscopic ellipsometry measurements were then performed on these samples. Effective medium modeling of the measurements was used to nondestructively depth profile the samples. These results show that the superficial Si layer for the unannealed sample includes noncrystalline and crystalline components. In addition, the optical properties of the buried oxide for the unannealed sample were found to be different from those of bulk fused silica or thermal oxides of Si. The superficial Si layer for the annealed sample was crystalline Si, but the buried oxide consisted of a phase‐separated mixture of noncrystalline SiO2and crystalline Si. These results were further substantiated by selective chemical etch‐back studies and additional spectroscopic ellipsometry measurements, and by other techniques, including Raman spectroscopy, infrared transmission measurements, sputter depth profiling using x‐ray photoelectron spectroscopy, and Nomarski microscopy.