Crystallization and surface morphology of Au/SiO2 thin films following furnace and flame annealing

A crystallization and surface evolution study of Au thin film on SiO₂ substrates following annealing at different temperatures above the eutectic point of the Au/Si system are reported. Samples were prepared by conventional evaporation of gold in a high vacuum (10⁻⁷ mbar) environment on substrates at room temperature. Thermal treatments were performed by both furnace and flame annealing techniques. Au thin films can be crystallized on SiO₂ substrates by both furnace and flame annealing. Annealing arranges the Au crystallites in the (1 1 1) plane direction and changes the morphology of the surface. Both, slow and rapid annealing result in a good background in the XRD spectra and hence clean and complete crystallization which depends more on the temperature than on the time of annealing. The epitaxial temperature for the Au/SiO₂ system decreases in the range of 350-400 °C. Furnace and flame annealing also form crystallized gold islands over the Au/SiO₂ surface. Relaxation at high temperatures of the strained Au layer, obtained after deposition, should be responsible for the initial stages of clusters formation. Gold nucleation sites may be formed at disordered points on the surface and they become islands when the temperature and time of annealing are increased. The growth rate of crystallites is highest around 360 °C. Above this temperature, the layer melts and gold diffuses from the substrate to the nucleation sites to increase the distance between islands and modify their shapes. Well above the eutectic temperature, the relaxed islands have hexagonally shaped borders. The mean crystallite diameters grow up to a maximum mean size of around 90 nm. The free activation energy for grain boundary migration above 360 °C is 0.2 eV. Therefore the type of the silicon substrate changes the mechanism of diffusion and growth of crystallites during annealing of the Au/Si system. Epitaxial Au(1 1 1) layers without formation of islands can be prepared by furnace annealing in the range of 300-310 °C and by flame annealing of a few seconds and up to 0.5 min.