Production and Characterization of Novel Nanostructure Materials

The research presented here was conducted in the Department of Chemistry at the University of Pittsburgh, in the Surface Science Center, under the supervision of Prof. John T. Yates, Jr. The work described can be divided into two main parts. As the size of materials decreases, they experience confinement. The electronic properties of materials are strongly correlated to the confinement experienced by the material or device being studied. New tools and experimental techniques need to be developed in order to probe the properties of such objects at sub-micrometer scales in a non-interfering way. The development of such an instrument and its usefulness in measuring electronic properties of confined objects is the topic of the first part of this dissertation, covering sections 1, 5 and 6. The Nanoworkbench is a unique multiple-probe instrument operating under ultrahigh vacuum conditions. It combines a molecular-beam epitaxy chamber for nucleation of quantum dots or the growth of thin films, a preparation and analysis chamber equipped with standard surface science tools such as Auger electron spectroscopy, X-ray photoelectron spectroscopy and mass spectrometry. The core of the system is the multiple-probe STM chamber combined with a scanning electron microscope, allowing precise controlled motion of the probes and the realization of sub-micrometer scale four-point probe measurements. This instrument was fully conceived, designed and assembled in the Department of Chemistry, at the University of Pittsburgh. The application of the Nanoworkbench to sub-micrometer four-point probe conductivity measurements and the formation of a roaming field-effect transistor on a silicon-on-insulator surface is demonstrated in section 5. Additionally the Nanoworkbench was used to electrically sense the normal motion of the interface between a metal (Al(111)) and its oxide coating, and a semiconductor (Si(100)) during its oxidation, finding that Al(111) oxidizes in conformance with the Mott-Cabrera model. Here Al3+ ions move away from the Al/Al2O3 interface to the outer Al2O3 surface when oxide film growth occurs. In contrast, oxygen penetration into covalent Si(100) occurs during oxide film formation. The second part of this dissertation - sections 7 to 10 - deals with the formation of dense arrays of sub-10nm Ge islands on Si(100). Upon deposition of Ge on Si(100), Ge islands are randomly nucleated on the surface by self-assembly after the formation of a wetting layer, whose thickness depends on the experimental conditions. These islands have rather large diameters and a broad size distribution and are therefore not suitable for application in quantum computing architecture for instance. The object of this study is the formation of arrays of Ge islands with control over both the island diameter (10nm, ± 2nm) and the islands spacing (as small as 35nm, ± 1nm). We will show that the pre-adsorption of small islands of carbon on Si(100) leads to the formation of Ge islands that are smaller than Ge islands nucleated on a clean Si(100) substrate. We can use this carbon effect" by creating a carbon template on the Si(100) surface by electron-beam-induced deposition. These carbon deposits transform into SiC after annealing under UHV conditions. Upon subsequent Ge deposition, ultra-small Ge islands will nucleate by directed self-assembly on the surface in perfect registry with the carbon template.Conceptual ideas will be provided as a background for the research described in sections 5 to 10. These conceptual ideas include a description of the four-point probe measurement technique, the framework of the Mott-Cabrera theory for oxide film growth on metal surfaces, a description of heteroepitaxy, and finally the description of the formation of Ge islands in Si/Ge and Si/C/Ge system along with the attempts at ordering these islands