Quantum Electronic Effects in Ultrathin Metal Films on Semiconductors
Czoschke, Peter Jeremy
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https://hdl.handle.net/2142/32126
Description
Title
Quantum Electronic Effects in Ultrathin Metal Films on Semiconductors
ultrathin film
Author(s)
Czoschke, Peter Jeremy
Issue Date
2005-05
Director of Research (if dissertation) or Advisor (if thesis)
Chiang, Tai-Chang
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Semiconductors
quantum confinement
X-ray Diffraction
theoretical models
Language
en
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.
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