Silicon oxynitride (Si x O y N z ) buried insulating layers were synthesized by SIMNOX (separation by implanted nitrogen-oxygen) process by 14 N + and 16 O + ion implantation to high fluence levels 1 • 10 17 , 2.5 • 10 17 and 5 • 10 17 ions cm À2 sequentially in the ratio 1:1 at 150 keV into p-type (1 0 0) silicon wafers. The identification of structures and defects in the ion beam synthesized buried layers were carried out by FTIR, XRD and ESR measurements before and after RTA treatments at different temperatures in nitrogen ambient. The FTIR spectra show single broad absorption band in the wavenumber range 1250-600 cm À1 confirming the formation of silicon oxynitride. The integrated absorption band intensity is found to increase with increasing ion fluence and on annealing indicating gradual chemical transformation of the ion implanted layer into silicon oxynitride. The XRD data of the implanted samples show the formation of Si 2 N 2 O (O) phase of silicon oxynitride. On annealing the samples, SiO 2 (H)/Si 3 N 4 (H) phases are also formed in addition to Si 2 N 2 O (O) phase. The concentration of the formed phases is found to increase with increase in the ion fluence as well as the annealing temperature. The ESR studies both at room temperature and at low temperatures reveal the presence of a defect center associated with silicon dangling bonds. The increase in ion fluence gives rise to small variations in g-values and increase in the spin density. The spin density decreases in general with increasing the annealing temperature.