Scanning Tunneling Microscopy Study On Ag/Si Low-dimensional Structures

Ag/Si system has attracted the sustained attention due to its importance in basic research and device application. The study on the structures and properties of Ag/Si is conducive to the better understanding the nature of metal/semiconductor systems, such as the interface properties, structures, and formation mechanism, which may further establish a relationship between the atomic structures and electronic properties. By effective controlling of the growth process and epitaxial structures, it is expected to develop new structural systems with superior characteristics, and further develop future application in the micro-and nano-devices. This thesis mainly focuses on the fabrication of low dimensional structures of Ag on the Si(111) surface under ultrahigh vacuum by molecular beam epitaxy. Using scanning tunneling microscopy and other techniques, the structural properties and formation mechanism have been studied at the atomic scale, and the controllable growth of ordered Ag/Si structures has been explored.The initial growth and structure evolution of Ag on Si(111)-7×7surface have been studied firstly. It is found that, at low coverage, Ag atoms adsorb in the7×7HUCs and form nanoclusters. Based on the statistical analysis of the clusters with different sizes, the growth and evolution of the clusters have been followed as the increasing of the coverage. By analyzing the STM images recorded at different bias voltages, it is found that at lower bias voltage, the brightness of clusters is smaller, and some clusters almost disappear, which indicates that the electronic structure of those clusters shows a band gap around Fermi level, exhibiting the semiconductor behavior. While elevating the substrate temperature slightly during the growth process, Ag atoms would cross the potential barrier between7×7HUCs and form nanoclusters with larger sizes. Upon Ag deposition at room temperature followed by a rapid thermal annealing treatment at relatively low temperature around100℃, Ag nanoclusters with identical size and shape form and distribute uniformly on the surface. Further increase the annealing temperature, different reconstructions, such as√3×√3,3×1, would form on the surface. In addition, the nucleation process and epitaxial structures of Ag and Si on the Ag/Si alloyed surfaces have been investigated. It is found that due to the increased mobility, Ag and Si would assemble to form3D islands with large size on the√3×√3reconstruction surface at room temperature. While depositing a small amount of Ag atoms onto the surface that contains a mixture of√3×√3and7×7reconstructions, Ag shows a preference adsorption on the7×7regions and forms nanoclusters and other structures. The surface morphology and distribution of different structures have been modified by controlling of the coverage and annealing conditions. These researches provide scientific data for the fabrication of ordered Ag/Si low dimensional structures.