Silicon-implanted thermal SiO2 layers were studied using depth-resolved positron annihilation spectroscopy (PAS) and transmission electron microscopy (TEM). TEM observations show the presence of silicon nanocrystals (Si-nc) in the region between 200 nm and 300 nm. The defect annealing behavior is studied by means of PAS. For 1 000 degrees C annealed samples at a depth for which Si-nc are observed, a distinctive PAS signal is detected and ascribed to the Si-nc/SiO2 interface.