Depletion layer-induced size effects in ferroelectric thin films: A Ginzburg-Landau model study

A Ginzburg-Landau model is used to demonstrate how depletion layers give rise to thickness-dependent ferroelectric properties in thin films. It is shown that free charge layers at the film-electrode interface can result in an internal electric field in the bulk of the film even when no external voltage is applied. At high values of the donor dopant density and small thicknesses, this internal electric field can be strong enough to lead to the formation of a domain pattern. This causes a drop in the remnant polarization; a direct demonstration of the important role free charge plays in thin ferroelectric films.Comment: 16 pages, 7 figure

Grain-boundary grooving and agglomeration of alloy thin films with a slow-diffusing species

We present a general phase-field model for grain-boundary grooving and agglomeration of polycrystalline alloy thin films. In particular, we study the effects of slow-diffusing species on grooving rate. As the groove grows, the slow species becomes concentrated near the groove tip so that further grooving is limited by the rate at which it diffuses away from the tip. At early times the dominant diffusion path is along the boundary, while at late times it is parallel to the substrate. This change in path strongly affects the time-dependence of grain boundary grooving and increases the time to agglomeration. The present model provides a tool for agglomeration-resistant thin film alloy design. keywords: phase-field, thermal grooving, diffusion, kinetics, metal silicidesComment: 4 pages, 6 figure

Grain-Boundary Kinetics: A Unified Approach

Grain boundaries (GBs) are central defects for describing polycrystalline materials, and playing major role in a wide-range of physical properties of polycrystals. Control over GB kinetics provides effective means to tailor polycrystal properties through material processing. While many approaches describe different GB kinetic phenomena, this review provides a unifying concept for a wide range of GB kinetic behavior. Our approach rests on a disconnection description of GB kinetics. Disconnections are topological line defects constrained to crystalline interfaces with both step and dislocation character. These characteristics can be completely specified by GB bicrystallography and the macroscopic degrees of freedom of GBs. GB thermal fluctuations, GB migration and the ability of GBs to absorb/emit other defects from/into the delimiting grains can be modeled via the nucleation, propagation and reaction of disconnections in the GB. We review the fundamentals of bicrystallography and its relationship to disconnections and ultimately to the kinetic behavior of GBs. We then relate disconnection dynamics and GB kinetics to microstructural evolution. While this review of the GB kinetics literature is not exhaustive, we review much of the foundational literature and draw comparisons from a wide swath of the extant experimental, simulation, and theoretical GB kinetics literature.Comment: 384 pages, 50 figure

Apparent hysteresis in a driven system with self-organized drag

Interaction between extended defects and impurities lies at the heart of many physical phenomena in materials science. Here we revisit the ubiquitous problem of the driven motion of an extended defect in a field of mobile impurities, which self-organize to cause drag on the defect. Under a wide range of external conditions (e.g. drive), the defect undergoes a transition from slow to fast motion. This transition is commonly hysteretic: the defect either moves slow or fast, depending on the initial condition. We explore such hysteresis via a kinetic Monte Carlo spin simulation combined with computational coarse-graining. Obtaining bifurcation diagrams (stable and unstable branches), we map behavior regimes in parameter space. Estimating fast-slow switching times, we determine whether a simulation or experiment will exhibit hysteresis depending on observation conditions. We believe our approach is applicable to quantifying hysteresis in a wide range of physical contexts.Comment: 11 pages (preprint format), 4 color figures in separate file

Asperity contacts at the nanoscale: comparison of Ru and Au

We develop and validate an interatomic potential for ruthenium based on the embedded atom method framework with the Finnis/Sinclair representation. We confirm that the new potential yields a stable hcp lattice with reasonable lattice and elastic constants and surface and stacking fault energies. We employ molecular dynamics simulations to bring two surfaces together; one flat and the other with a single asperity. We compare the process of asperity contact formation and breaking in Au and Ru, two materials currently in use in micro electro mechanical system switches. While Au is very ductile at 150 and 300 K, Ru shows considerably less plasticity at 300 and 600 K (approximately the same homologous temperature). In Au, the asperity necks down to a single atom thick bridge at separation. While similar necking occurs in Ru at 600 K, it is much more limited than in Au. On the other hand, at 300 K, Ru breaks by a much more brittle process of fracture/decohesion with limited plastic deformation.Comment: 10 pages, 13 figure