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Title:Quantum Electronic Effects in Ultrathin Metal Films on Semiconductors;
ultrathin film
Author(s):Czoschke, Peter Jeremy
Director of Research:Chiang, Tai-Chang
Department / Program:Physics
quantum confinement
X-ray Diffraction
theoretical models
Abstract:As the size of a metallic system approaches the atomic scale, deviations from the bulk are expected in a plethora of different physical properties due to quantum size effects. In this work, two of these effects are investigated in detail: the structural distortions that arise due to quantum confinement of a metal's itinerant electrons to an ultrathin film and variations in the surface energy (relative stability) of such films as a function of thickness. These effects are first examined from a theoretical viewpoint, where models based upon a free-electron gas confined to a one-dimensional quantum well are derived to illustrate the basic physical phenomena. These models are engineered such that they are realistic enough to be used in the analysis of empirical data with the adjustment of a small number of phenomenological parameters. These effects are then investigated experimentally using surface x-ray diffraction at a third-generation synchrotron radiation facility (the Advanced Photon Source at Argonne National Laboratory). Extended reflectivity spectra from smooth atomic-scale Pb films prepared on Si(111) substrates at 110 K are obtained for thicknesses of 6-19 atomic layers that exhibit distinctive features indicative of quasibilayer lattice distortions. A detailed analysis shows variations within the layer structure of each film that are correlated with Friedel-like charge density oscillations at the film boundaries. Variations in the lattice distortions are also observed as a function of thickness with a quasibilayer periodicity. This effect is explained in the context of quantum size effects using the theoretical models. A second experiment is also described in which initially smooth Pb films are progressively annealed from 110 K to near room temperature. The film morphology is examined every 5-10 K by scanning the extended x-ray reflectivity, which reveals the initially smooth films breaking up into islands of specific heights. Once the samples reach a state of quasi-equilibrium, the distributions of island heights are measured, which show strong quasibilayer variations in the relative stability of different height islands (film thicknesses). These variations are related to electronic contributions to the surface energy using the free-electron-based theoretical models.
Issue Date:2005-05
Genre:Dissertation / Thesis
Rights Information:© 2005 Peter Jeremy Czoschke
Date Available in IDEALS:2012-07-06
Identifier in Online Catalog:5113305

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