The effects of the processing conditions on the formation of buried oxide precipitates in He and O co-implanted Si were investigated by the combination of Fourier transform infrared (FTIR) absorption spectroscopy, depth-resolved positron annihilation Doppler spectroscopy, and transmission electron microscopy (TEM). Silicon wafers were implanted with 50 keV He ions at a fluence of 2 x 10(16) cm(-2) and subsequent 150 key O ions at a fluence of 2 x 10(17) cm(-2). For comparison, reference Si wafers were only implanted with 150 keV O ions. The Si-O-Si stretching frequency increases while the peak width of the Si-O-Si stretching absorption band decreases with an increase in annealing temperature. After the same annealing, the peak width of the Si-O-Si stretching absorption band in the He and O co-implanted sample is significantly larger than that in the reference sample. Two kinds of vacancy-type defects are observed by positrons, i.e., vacancy-type defects and vacancy-oxygen complexes. The characteristic S values of vacancy-type defects and vacancy-type complexes in the He and O co-implanted sample are smaller than those of the reference sample. In addition, the thickness of the buried oxide layer in the He and O co-implanted sample is smaller than that in the reference sample. After annealing at 1473 K, the O content is larger in the He and O co-implanted sample compared to that in the reference sample. (C) 2012 Elsevier Ltd. All rights reserved.