Effect of ion-irradiation and annealing on the luminescence of Si nanocrystals in SiO₂

SiO₂ layers containing Si nanocrystals were irradiated with either 400 keV or 3 MeV Si ions to determine the effect of nuclear and electronic energy loss processes on defect production and luminescence. Irradiation reduced the nanocrystal-related luminescence at 806 nm and produced a well-known defect emission at 640 nm. Irradiation had a similar dose dependence for both 400 keV and 3 MeV ions. despite significant differences in the magnitude and nature of their energy loss. This was reconciled by assuming that the defect production rate from electronic energy loss processes was approximately 10% of that for nuclear processes. The nanocrystal emission was particularly sensitive to irradiation, being quenched to 4% of its initial value following irradiation to 5×10¹² Si cm^-2 and saturating for fluences ≥5×10¹³ Si cm^-2 (0.18 dpa). This is discussed in terms of a previously proposed model in which point defects produced by irradiation accumulate at the nanocrystal surface leading to amorphization at low displacement rates (0.1-0.2 dpa). In this model, quenching of the nanocrystal emission and its sensitivity to dose are assumed to result from the preferential accumulation of point-defect at the nanocrystal-SiO₂ interface, an effect which is predicated on the assumption that such defect act as non-radiative recombination centres. The existence of such defects is shown to be supported by the annealing behaviour of the nanocrystal and defect emissions.