Evolution of Ion Beam Synthesized Au Nanoclusters in SiO2 under Ion Irradiation

Au nanoclusters have been synthesized in a 500 nm thick SiO2 layer on (001)Si by 330 keV Au+ implantation of 2x1016 cm-2 and subsequent annealing at T = 1000 °C for 1h in dry oxygen. XTEM images show arround the projected ion range Rp = 100 nm a layer of crystalline Au clusters. At Rp the Au clusters have a mean size of 5 nm and a broad size distribution resembling approximately the LSW distribution of diffusion controlled Ostwald ripening. The Au nanoclusters were subsequently irradiated by 4.5 MeV Au+ ions with a fluence of (0.5...1.0)x1016 cm-2 at 190...210 °C . The high energy Au+ ions come to rest at a depth of 1 µm, i.e. in the Si substrate far below the Au nanoclusters and even below the SiO2 layer. The high energy Au+ irradiation causes a strong decrease of the mean size and of the width of the size distribution of Au nanoclusters. The observed Au nanocluster evolution under ion irradiation agrees qualitatively with recent theoretical predictions and kinitic lattice Monte-Carlo simulations.