Epitaxial Nb (011) surfaces as a template for Cu3Au

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.U of I Onlythesi