Dynamics of Gold Droplet Formation on SiO2/Si(111) Surface

Au droplets are used as a catalyst for the growth of nanowires on Si(111) substrate via the vapor-liquid-solid (VLS) mechanism. The dewetting of a Au thin film is the most common method to obtain these droplets. The control of this step is crucial to adjust the density and the diameter of the nanowires during VLS growth. When the Si(111) substrate is covered with a silicon dioxide layer, the kinetics of Au droplet formation is strongly modified. The dependence of the diameter and spatial distribution of the droplets on the surface have been studied by scanning electron microscopy with respect to the thickness of the silicon dioxide layer, the thickness of the Au film and the temperature of the substrate during deposition and postdeposition annealing. In situ low energy electron microscopy and low energy electron diffraction revealed the dynamics of the Au droplet formation after annealing. The Au droplets are shown to catalyze the decomposition of silicon dioxide at high temperature (>650-700 degrees C) and form a wetting layer of Au-(root 3X root 3)-Si(111). Consequently, the droplets absorb silicon atoms from the substrate, migrate perpendicular to the atomic steps, and grow by the Smoluchowski ripening process.