The formation and annihilation of defects produced in Si nanocrystals (nc) by ion-beam irradiation were investigated in detail. The luminescence properties of Si nanocrystals embedded in a SiO2 matrix were used as a probe of the damaging effects generated by high-energy ion-beam irradiation. Samples were irradiated with 2MeV He+, Si+, Ge+ and Au+ ions at different doses, in the range between 109/cm2 and 1016/cm2. With increasing the ion dose, the nc-related photoluminescence strongly decreases after a critical dose value, which depends on the ion mass. It was observed that the luminescence drop was accompanied by a concomitant lifetime quenching that marks the rise of new non-radiative phenomena, related to the damage left over by the ion beam. It was shown that

the lifetime quenching alone cannot quantitatively explain the much stronger photoluminescence drop, but the total number of emitting centers has to diminish too. By assuming that a Si nc was damaged when it contains at least one defect inside its volume, a model was developed which related the fraction of quenched nc to the total defect concentration in the film and to the value of the nc volume itself. This model was 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, the recovery of the damaged Si nc was studied by performing both isochronal and isothermal annealing. It was demonstrated that in slightly damaged Si nc a large variety of defects characterized by activation energies between 1 and 3eV existed. On the contrary, the recovery of the photoluminescence properties of completely amorphized Si nc was characterized by a single activation energy, whose value was 3.4eV. Actually, this energy was associated with the transition between the amorphous and the crystalline phases of each Si grain. The recrystallization kinetics of Si nanostructures was demonstrated to be very different from that of a bulk system. These data were presented and explained on the basis of the large surface/volume ratio characterizing low-dimensional Si structures.

Defect Production and Annealing in Ion-Irradiated Si Nanocrystals. D.Pacifici, E.C.Moreira, G.Franzò, V.Martorino, F.Priolo, F.Iacona: Physical Review B, 2002, 65[14], 144109 (13pp)