Silica glasses (SiO2) were implanted with Zn+ ions of 60 keV to a fluence of 1.0 x 10(17) ions/cm(2), and were annealed in oxygen gas flow at 700 degrees 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 degrees C for 1 h in oxygen gas, ZnO NPs of similar to 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 degrees C does not induce any prominent migration of Zn atoms. Mechanisms of the shallowing and the formation of ZnO NPs are discussed. (c) 2005 Elsevier B.V. All rights reserved.