STM- and DFT-study of gadolinium disilicide nanowires on the silicon(001) surface.

摘要

This dissertation provides a description of our studies involving self-assembled GdSi2 nanostructures on Si(001). This study is motivated by interests in utilizing bottom-up fabrication techniques to produce one-dimensional structures compatible with existing silicon technology for the development of nanoscale interconnect systems and novel electronic devices. Although previous investigations of rare earth disilicides on Si(001) systems have provided information on the periodicity of the wetting layer reconstructions and the morphology of nanowires, absent from these reports were detailed information about the structure of the wetting layer, growth mechanism of the overlayer and a quantitative measure of the conductivity of the wires.; Through scanning tunneling microscopy observations, we have identified a variety of reaction products that are produced on the Si(100) surface as a function of the Gd metal coverage and annealing temperature. At very low coverages, the metal produces LLDs in the silicon surface. When additional metal is added a variety of surface features are observed, which mark the beginning of wetting layer formation. Further increase in metal coverage leads to the completion of the wetting layer, which can appear as a 2 x 8 and a 2 x 7 reconstruction. A c(4 x 4) reconstruction has also been identified under sample preparation conditions; however, this reconstruction does not contain Gd metal. With increased coverage and temperature an intermediate growth regime is reached where highly anisotropic islands are formed. These metastable elongated islands, or nanowires, were found to be composed of either hexagonal or orthorhombic silicide. At higher temperatures a second growth regime is reached where only three dimensional islands with an orthorhombic crystal structure are formed.; The features of each regime were investigated to identify their structure and role in the formation of nanowires. Structures were determined from scanning tunneling microscopy images both at room temperature and at wire growth temperatures and through density functional theory calculations. Additionally, the electronic properties of hexagonal silicide nanowires were investigated by scanning tunneling spectroscopy in vacuum and conductive atomic force microscopy under ambient conditions.