Photoluminescence (PL), electron spin resonance (ESR), and high resolution transmission electron microscopy (HRTEM) were used to investigate the luminescence mechanism in Si nanocrystals. Si ions were implanted inSiO2films at 190 keV to a dose of3×1017/cm2.An intense photoluminescence (PL) band at 755 nm (1.65 eV) was observed when the implanted films were annealed above 800 °C in air or in nitrogen. HRTEM images showed Si nanocrystals of sizes between 1 and 6 nm from these annealed samples. ESR indicated Si dangling bonds. Upon annealing at 900 °C in air a few times, the particle sizes were reduced to less than 2 nm due to oxidation. The red PL band is attributed to emission from Si nanocrystals. ©1997 American Institute of Physics.