Studies of ordering on surfaces and in superlattices by molecular beam epitaxy
Dura, Joseph Andrew
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https://hdl.handle.net/2142/22990
Description
Title
Studies of ordering on surfaces and in superlattices by molecular beam epitaxy
Author(s)
Dura, Joseph Andrew
Issue Date
1991
Doctoral Committee Chair(s)
Flynn, C.P.
Department of Study
Physics, Condensed Matter
Discipline
Physics, Condensed Matter
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Condensed Matter
Language
eng
Abstract
Molecular beam epitaxy, MBE, was used in two different ways to study ordering on atomic scales. In one application the ordering of close packed planes into various stacking sequences was investigated by periodically alternating growth of Ir and Ru to produce single crystal superlattices. In addition, a novel probe of the kinetics by which a surface of Cu$\sb3$Au re-orders was developed. By growing a disordered layer onto a well ordered surface, the kinetics of ordering in layers of almost arbitrarily small thickness can be studied. By eliminating the need to quench the sample this procedure removes the effects of the re-ordering of the bulk and allows for study at temperatures far below the transition temperature.
Single crystal Ir-Ru superlattices have been grown by MBE with their close packed planes perpendicular to the growth direction. The phase diagram which plots structure dependance on the number of Ir monolayers and Ru monolayers in a superlattice has been investigated. The diagram is divided into four regions corresponding to the phases of the superlattices. The types of phases are: bicrystalline, fcc, hcp, and polytype superlattices, determined for each superlattice from qualitative x-ray diffraction features. More detailed computer modeling was also employed. Composition profiles and stacking sequences were determined by means of two distinct x-ray diffraction scans. These simulations make clear that detailed modeling is necessary to fully determine the stacking sequences. The deduction that bicrystal stacking sequences occur has been confirmed.
In a conventional experiment, the order-disorder transition of the (111) surface of Cu$\sb3$Au was studied by means of RHEED intensities. The order parameter was found to go to zero continuously at a temperature equal to that of the discontinuous bulk transition, within experimental uncertainty.
A novel kinetic experiment was performed to examine re-ordering of (111) Cu$\sb3$Au adlayers. It was found that a surface layer re-orders independently on a sublayer frozen by its slower bulk kinetics in its initial disordered state. This re-ordering occurs in two stages. Near the critical temperature it achieves approximately the same degree of order as surfaces under which the subsurface layers have equilibrium order.
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