Abstract
The spontaneous formation of epitaxial three-dimensional transition-metal-silicide nanostructures is an active subject in experimental and theoretical research in recent years. The small size and the crystalline perfection of these nanostructures imply the possibility of using them as self-assembled metallic quantum dots or quantum wires. Well-defined shapes and uniform sizes are the most critical issues concerning both optical and electronics applications of nanostructures. To control the morphology, we must truly understand the growth mechanisms of these nanostructures. Most of the growth mechanisms used to explain the growth process of the transition-metal-silicide nanostructures are quoted from the growth mechanisms of other epitaxial systems, however, there is no confirmation that these theories are suitable for the epitaxial silicide nanostructures. To decide the growth mechanism of the epitaxial NiSi2 nanostructures, the structures, the energy, and the shape distribution of the epitaxial NiSi2 nanostructures are investigated exhaustively in this thesis. Several kinds of epitaxial NiSi2 nanostructures were found in this experiment, and the epitaxial relationship with the Si substrates were analyzed for each structure. The structures of the NiSi2 nanostructures were found affected by the deposition rate; by controlling the deposition rate, the rectangular hut structures could exist alone. This method is helpful in controlling the structure of the epitaxial NiSi2 nanostructures. Finite-element method was used to simulate the strain energy of the epitaxial NiSi2 nanostructures. After combining with the surface energy, the stable shape of the NiSi2 hut structures was determined. Because the shape distribution of the NiSi2 clusters is not consistent with the stable shape of the NiSi2 nanostructures calculated here, we infer that the elongation process of the NiSi2 clusters should be mainly governed by the growth kinetics. A growth process based on the kinetic model was used to explain the existence of the elongated NiSi2 clusters and the shape distribution of the clusters observed in this experiment. Finite-element method was also used to simulate the strain energy of three-dimensional epitaxial structures which grow under different conditions. The evolution of strain energy with the aspect ratio (length/width) of the epitaxial structures was found to be affected by the height, the elastic constant of the substrates and the epilayers, and the symmetry of the lattice mismatch of these structures. Asymmetric structures will have smaller strain energy if the height of the epitaxial structures is fixed or growing very slowly, or the ratio of the elastic constant of the epilayer to that of the substrate is smaller, or the lattice mismatch between the substrates and the epilayers is asymmetric on the epitaxial surface. For different growth and surface energy conditions, the shape-transition process between symmetric and asymmetric structures is discussed.