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Title:Scanning tunneling microscopy and photoemission studies of surface growth and defect formation
Author(s):Hirschorn, Eric Scott
Doctoral Committee Chair(s):Chiang, Tai-Chang
Department / Program:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Physics, Condensed Matter
Abstract:STM, synchrotron core-level photoemission, and RHEED were used to examine the room temperature growth of Au on Ag(110) in two separate studies. In the first study of this system, which used only core-level photoemission and electron diffraction, substitution of Au for Ag in the subsurface layer was found to be an important process for the initial adsorption and the subsequent multilayer growth. This finding will be compared with two recent models: (1) a bilayer growth model based on an ion scattering study, and (2) a model of pure Au growth on a Ag/Au inverted layer based on a previous scanning tunneling microscopy study and a reinterpretation of the ion scattering data.
In the second look at this system, which used only STM, the changes in surface topology during the initial submonolayer growth were examined. Ag atoms forced onto the surface by the adatom-substrate substitution process formed anisotropic 2D islands on large Ag(110) terraces, as well as structures on the steps which had a "serrated edge" appearance. Large multilayered defects or "holes" formed initially near steps, and for higher submonolayer Au coverage spread throughout the surface. These findings are compared with the results of a previous STM study on this system which used a Ag(110) sample of larger miscut.
The room temperature growth of Ag on Ag(110) was examined by STM. It was found that small deviations away from uniform step flow growth observed at submonolayer coverages lead gradually to a highly disordered surface at high coverages. Large and directionally dependent serrations initially appear on step edges. The sharp intersections of the serrations appear to be nucleation sites for point defects which eventually cover the surface and disrupt the step flow.
Unstable monatomic steps of the clean Ag(110) $-$ (1 $\times$ 1) surface were also studied by STM. Consecutive images of the same area show that the steps are in motion relative to fixed positions on the surface. The steps straighten and align along the (1$\overline{1}$0) direction as a result of a "tip-sample" interaction.
STM was employed for an investigation of the adsorption of oxygen of the Ge(111) $-$ c(2 $\times$ 8) surface at room temperature, in which atomic-size oxidized sites were observed to be produced on previously defect-free terraces. Images of the same area of the surface were obtained for various exposures of oxygen. The results show that the oxidation proceeds by nucleation at the oxidized site, followed by expansion of the oxidized area.
Images of annealed Ge(111) show large c(2 $\times$ 8) domains with protrusions that match closely the arrangement of adatom and rest-atom dangling bonds in the simple adatom model. The unit cell, however, appears asymmetric or "buckled"; namely, the two adatoms or the two rest atoms within the unit cell show significant differences in apparent height. Both the adatom and rest-atom dangling bonds can be simultaneously observed, indicating that both are partially filled. These results will be discussed in relation to the proposed charge transfer between the rest atoms and adatoms. (Abstract shortened by UMI.)
Issue Date:1994
Rights Information:Copyright 1994 Hirschorn, Eric Scott
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9512395
OCLC Identifier:(UMI)AAI9512395

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