Coherent gold nanoislands were prepared directly on (100)-oriented Si substrates by a physical methodology, consisting of the thermal evaporation of a very thin Au film (t<2 nm) and its successive annealing in the temperature range 350°C<T<814°C. We found that at annealing temperature of 814 °C and in the presence of residual oxygen during the annealing process, epitaxial monocrystalline gold nanoislands embedded in the Si lattice are formed. The crystallographic orientation and epitaxial relationship between the Au nanoislands and the Si lattice are well defined. In contrast, at lower annealing temperatures, namely at 350 °C and 626 °C, the nanoislands are randomly oriented without epitaxial relationships. The morphology, orientation, and crystalline structure of Au nanoislands were investigated by scanning and high-resolution transmission electron microscopy and grazing-incidence x-ray diffraction. A model of the epitaxial Au nanoisland formation on (100)Si is presented in which the Si atom out-diffusion and the formation of a liquid Au-Si droplet during the annealing process (increasing temperature) and the Si redeposition and oxidation (i.e., SiOx complex formation and removing of the excess Si in the gold islands) during the cooling process (decreasing temperature) play a fundamental role.
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