Epitaxial Nb (011) surfaces as a template for Cu3Au
Appleton, Randal Scott
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https://hdl.handle.net/2142/46442
Description
Title
Epitaxial Nb (011) surfaces as a template for Cu3Au
Author(s)
Appleton, Randal Scott
Issue Date
2001
Department of Study
Physics
Discipline
Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Nb films, low energy electron microscopy, growth of Cu3Au
Language
en
Abstract
The symmetry breaking between ABC and ACB stacking twins in the growth of
Cu3Au (111) thin films on Nb (011) is investigated. Nb films, similar to the buffer
layers upon which Cu3Au is grown, are studied with low energy electron microscopy
for the first time. Microscopy of the Nb surface reveals the organization of an oxygeninduced Nb surface which reconstructs into a stripe phase at and just below T0 =
1230 ± 30 °C. Above this temperature the reconstruction lifts. At temperatures more
than 100 K below T 0 the surface is dominated by a single reconstruction variant.
This symmetry breaking is attributed to shear from anisotropic thermal contraction
of the Al20 3 substrate. The response of stripes to in-plane shear is also observed
within strain fields caused by dislocations in the Nb film. These behaviors lead to
a model of stripe behavior based on a competition between surface and bulk elastic
energy. Additionally, LEEM studies show steps on the Nb surface which coalesce into
bee {110} nanofacets. The nanofacets intersect the (011) plane at 90° and 60°
angles and completely accommodate sample miscut at low temperature. At high
temperature the steps are of single height and occur in all orientations. A phase
diagram is proposed for the surface facets as a function of temperature and miscut
azimuth. Cu3Au films are grown under a number of conditions, which isolates step nudeation
as the key to the stacking bias. The stacking ratio has a sinusoidal dependance
on miscut azimuth. The ratio depends on miscut magnitude first linearly but with a
rapid increase for miscuts near 1 °. This behavior fits well with a model of selective
step nucleation for adatoms within diffusion distance of surface steps. Variations in average adatom chemistry and size have little effect, leaving the best explanation for
stacking selection as an adatom affinity for step nucleation due to increased coordination
number and reduced energy at step sites.
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