Defect production and annealing in ion-irradiated Si nanocrystals

In this paper the formation and annihilation of defects produced in Si nanocrystals (nc) by ion-beam irradiation are investigated in detail. The luminescence properties of Si nanocrystals embedded in a ${\mathrm{SiO}}_2$ matrix were used as a probe of the damaging effects generated by high-energy ion-beam irradiation. Samples have been irradiated with 2 MeV ${\mathrm{He}}^{+},$ ${\mathrm{Si}}^{+},$ ${\mathrm{Ge}}^{+},$ and ${\mathrm{Au}}^{+}$ ions at different doses, in the range between $1\times {10}^9/{\mathrm{cm}}^2$ and $1\times {10}^{16}/{\mathrm{cm}}^2.$ With increasing the ion dose, the nc-related photoluminescence (PL) strongly decreases after a critical dose value, which depends on the ion mass. We have observed that the luminescence drop is accompanied by a concomitant lifetime quenching that marks the rise of new nonradiative phenomena, related to the damage left over by the ion beam. It is shown that the lifetime quenching alone cannot quantitatively explain the much stronger PL drop, but the total number of emitting centers has to diminish too. By assuming that a Si nc is damaged when it contains at least one defect inside its volume, we developed a model that relates the fraction of quenched nc to the total defect concentration in the film and to the value of the nc volume itself. This model is shown to be in good agreement with the experimental value of the quenched fraction of Si nc extracted from the luminescence and lifetime measurements. Moreover, we studied the recovery of the damaged Si nc by performing both isochronal and isothermal annealings. It is demonstrated that in slightly damaged Si nc a large variety of defects characterized by activation energies between 1 and 3 eV exists. On the contrary, the recovery of the PL properties of completely amorphized Si nc is characterized by a single activation energy, whose value is 3.4 eV. Actually, this energy is associated with the transition between the amorphous and the crystalline phases of each Si grain. The recrystallization kinetics of Si nanostructures is demonstrated to be very different from that of a bulk system. These data are presented and explained on the basis of the large surface/volume ratio characterizing low-dimensional Si structures.