The ordered SiO2 in the buried oxide (BOX) layer of high-dose, low-dose, and internal-thermal-oxidation (ITOX) separation-by-implanted-oxygen (SIMOX) wafers was investigated by X-ray diffraction. From the results, it was found that the SiO2 molecules in the low-dose and ITOX SIMOX wafers are better ordered than those in the high-dose SIMOX wafer and that the ordered structure of the ITOX layer is different from that of the originally formed BOX layer, suggesting that the ITOX layer has a structure similar to that of the ordered SiO2 in the thermal oxide layer. (c) 2004 Elsevier B.V. All rights reserved.