Low temperature scanning tunneling microscope study of metallic thin films on the semiconductor substrates

The physical properties of materials tend to strongly depend on the material’s dimensions. They usually deviate from their bulk properties when the materials’ sizes become comparable to their Fermi wavelengths. These size-dependent phenomena in the nanometer or sub-nanometer scales are called the quantum size effects (QSE) and have attracted enormous attentions in recent years because it is not only theoretically interesting but also practically important. The metallic thin films grown on semiconductor substrates are especially interesting because metals and semiconductors are quite different from each other in terms of the electrical properties. As mentioned above, when the film’s thickness becomes as small as can be comparable to the Fermi wavelength, the QSE will be apparent in the electrical properties. An ultra high vacuum (UHV) low temperature (LT) scanning tunneling microscope (STM) is ideal for studying this QSE on the electrical properties of metallic thin films because an STM renders us to get the information about both electrical and structural properties simultaneously. In this thesis, I will present the study about the QSE on the electrical properties of metallic thin films using a home-built UHV LT-STM. It includes a study of the superconducting order parameters of epitaxial Pb thin films grown on Si(111)-7x7 substrates. This study shows that the 2-dimensional cooper pair condensates remain extremely robust and quantum oscillations of the order parameters are persistent down to the very thin film (5ML). Another subject regarding Pb films is a novel manipulation scheme of nano-structures using an STM, which result in the mass transfer of a macroscopic number of Pb atoms without changing the local structures. Analysis of the underlying kinetic phenomena like vacancy diffusion and energy barrier will be given. Finally, an STM study of the epitaxial flat Ag films on GaAs(110) surface will be presented. One-dimensional quasi-periodic or non-periodic structural modulations are observed and, therefore, they are providing good sample systems for studying the effect of quasi-periodicity or non-periodicity on the electrical properties. An analysis of structural and electrical properties will be given.