Optimized photoluminescence of Si nanocrystals produced by ion implantation

We have produced Si nanocrystals by implanting Si ions at 90 keV into 430 nm thick SiO2 films, to fluences from 6 x 10(16) to 1.4 x 10(17) ions/cm(2) (or 9 - 20 at. % excess Si). High temperature anneals at 1100 C and 1200 C in an N-2 ambient followed, to coalesce the excess silicon into nanocrystalline precipitates. Samples were further annealed for 1 hr at 450 degrees C in a 5% H-2 95% N-2 ambient to passivate dangling bonds and reduce non-radiative electron-hole recombinations. The films were then characterized by Photoluminescence (PL), and the PL intensity is maximized at 17 at. % excess silicon. This suggests that quantum confinement and/or the total number of nanocrystals is optimized at a Si excess of 17 at. % for high temperature annealing conditions. Annealing at 1100 C results in a greater number of smaller nanocrystals than annealing at 1200 degrees C, and consequently a greater PL intensity. The peak wavelength of light emission reaches equilibrium after a much shorter annealing time than does the PL intensity, suggesting that the nanocrystals attain their final size before the film reaches equilibrium. Positron Annihilation Spectroscopy (PAS) was used to investigate defects in the film. The majority of defects introduced by ion implantation annealed away very quickly, in correlation with the rapid equilibration of the peak wavelength of PL emission.