Microstructures And Formation Mechanisms Of Au On Si(111)-7×7 Surfaces

As one of the most important semiconductor materials, the surface structures and epitaxial growth of Si have been attracting attention. It is significant to characterize and understand the microstructures of Au/Si interface due to its wide application in the electronic devices and surface catalytic engineering. By combining the ultrahigh vacuum scanning tunneling microscopy, first-principles calculations and kinetic Monte Carlo simulation methods, we have investigated systematically the geometrical properties and formation mechanisms of Au on the Si(111)-7×7 surface.The adsorption behaviors of Au atoms on the Si(111)-7×7 surface have been firstly studied. STM observations show that Au atoms diffusing fast at room temperature are frozen up and appear as single bright spots in the 7×7 half unit cells at low temperature of 78K. Based on the results of first-principles calculations, the energy stabilities and electronic structures of different adsorption configurations have been analyzed and compared. Combined with the theoretical simulated STM images of the adsorbed surface, the high coordination sites near Si adatoms have been determined as the most favorable adsorption places for single Au atoms. Furthermore, the scanning tunneling spectroscopy and local density of states studies indicate that Au atom and its adjacent Si adatom exhibit different contributions to the filled and empty states, which leads to the distinct appearance in the STM images of this adsorbed surface under opposite bias voltages.The structure of Au clusters on 7×7 surfaces have also been studied experimentally and theoretically. Au atoms are found to absorb on six high coordination sites at the center of half unit cells due to the interaction among them, forming a hollow hexagonal planar structure. A model of cluster growth on 7×7 surfaces considering various atomic processes of mobility, diffusion, nucleation and aggregation has been introduced. By simulating the formation and evolution of Au clusters using kinetic Monte Carlo method, we find that surface diffusion is the most important mechanism for the growth of Au clusters, which plays the dominative role in the cluster size and spatial distribution at different coverage and substrate temperature conditions. The simulated results provide a theoretical basis for the controllable growth of clusters.In order to fabricate the ordered crystal lattice structures, the influence of growth parameters on the surface phases of Au/Si(111) has been further investigated. After the treatments at high temperature, the Au and Si atoms diffuse and rearrange on the surface, forming various surface reconstructions. The rapid thermal annealing treatment at relatively low temperature makes the arrangement of Au atoms limited by substrate and the periodicity of 7×7 surfaces reserved. By regulating the coverage and annealing process delicately, we obtain the perfectly ordered Si(111)-7×7-Au reticulated surface.