Studies of ordering on surfaces and in superlattices by molecular beam epitaxy
Dura, Joseph Andrew
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https://hdl.handle.net/2142/23884
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
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
molecular beam epitaxy (MBE)
ordering of materials
atomic scale
kinetics probe
Language
en
Abstract
Molecular beam epitaxy, MBE, was used in two different ways to study the ordering
of materials on an atomic scale. In one application the ordering of close packed planes into
hcp or fcc structures, or polytype 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 Cu3Au re-orders of 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 a wide range of re-ordering 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 super lattice has been
investigated. The diagram is divided into four regions corresponding to the phases of the
superlattices found in the assigned portions of parameter space. The types of phases are:
bicrystalline, fcc, hcp, and polytype superlattices, as determined for each superlattice from
qualitative features of its x-ray diffraction patterns. More detailed computer modeling was
also employed for some superlattices. 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 by computer
modeling for superlattices in the appropriate region of parameter space.
In a conventional experiment, the order-disorder transition of the (111) surface of
Cu3Au was studied by means of the intensities of RHEED satellite streaks. 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) Cu3Au
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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