Formation processes of zinc-oxide nanoparticles by ion implantation combined with thermal oxidation

Silica glasses (SiO2) were implanted with Zn+ ions of 60 keV to a fluence of 1.0×1017 ions/cm2, and were annealed in oxygen gas flow at 700 °C for 1 h, to form ZnO nanoparticles (NPs). In as-implanted state, metallic Zn NPs of 10–15 nm in diameter are formed between 10 and 50 nm in depth. NPs are not observed on the surface and down to 10 nm in depth. After the annealing at 700 °C for 1 h in oxygen gas, ZnO NPs of ∼10 nm in diameter are observed around 50 nm in depth. Moreover, droplet-like ZnO NPs larger than 30 nm in diameter grow on the surface of SiO2 substrate. Migration of Zn atoms toward the surface, i.e., shallowing of the depth profile, is observed under the oxygen annealing, while vacuum annealing at the same temperature 700 °C does not induce any prominent migration of Zn atoms. Mechanisms of the shallowing and the formation of ZnO NPs are discussed.