Publications

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The quantification of the production of primary defects via displacement cascades is an important ingredient in the prediction of the influence of radiation on the performance of electronic components in radiation environments. Molecular dynamics simulations of displacement cascades are performed for GaAs The interatomic interactions are described using a recently proposed Bond Order Potential, and a simple model of electronic stopping is incorporated. The production of point defects is quantified as a function of recoil energy and recoil species. Correlations in the point defects are examined. There are a large number of anti-site defects nearest-neighbor pairs as well as di-vacancies and larger order vacancy clusters. Radiation damage and ion implantation in materials have been studied via molecular dynamics for many years. A significant challenge in these simulations is the detailed identification and quantification of the primary defect production. For the present case of a compound semiconductor, GaAs, there are a larger number of possible point defects compared to elemental materials; two types of vacancies, two types of interstitials and antisite defects. This is further complicated by the fact that, in addition to the formation of point defects, amorphous zones may also be created. The goal of the current work is to quantify the production of primary defects in GaAs due to radiation exposures. This information will be used as part of an effort to predict the influence of radiation environments on the performance of electronic components and circuits. The data provide the initial state for continuum-level analysis of the temporal evolution of defect populations. For this initial state, it is important to know both the number of the various point defects that may be produced as well as the initial spatial correlations between the primary defects. The molecular dynamics simulations employ a recently developed Bond Order Potential (BOP) for GaAs. The analysis of the resulting atomic configurations follows a generalization of methods presented previously for elemental Si. The number of point defects of various types, exclusive of the amorphous zones, is predicted as a function of recoil energy. It is also shown that certain primary point defects are initially formed in binary or larger clusters.

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We have performed molecular dynamics simulations of cascade damage in the gadolinium pyrochlore Gd{sub 2}Zr{sub 2}O{sub 7}, comparing results obtained from traditional methodologies that ignore the effect of electron-ion interactions with a 'two-temperature model' in which the electronic subsystem is modeled using a diffusion equation to determine the electronic temperature. We find that the electron-ion interaction friction coefficient {gamma}{sub p} is a significant parameter in determining the behavior of the system following the formation of the primary knock-on atom (here, a U{sup 3+} ion). The mean final U{sup 3+} displacement and the number of defect atoms formed is shown to decrease uniformly with increasing {gamma}{sub p}; however, other properties, such as the final equilibrium temperature and the oxygen-oxygen radial distribution function show a more complicated dependence on {gamma}{sub p}.

Interfaces are a critical determinant of the full range of materials properties, especially at the nanoscale. Computational and experimental methods developed a comprehensive understanding of nanograin evolution based on a fundamental understanding of internal interfaces in nanocrystalline nickel. It has recently been shown that nanocrystals with a bi-modal grain-size distribution possess a unique combination of high-strength, ductility and wear-resistance. We performed a combined experimental and theoretical investigation of the structure and motion of internal interfaces in nanograined metal and the resulting grain evolution. The properties of grain boundaries are computed for an unprecedented range of boundaries. The presence of roughening transitions in grain boundaries is explored and related to dramatic changes in boundary mobility. Experimental observations show that abnormal grain growth in nanograined materials is unlike conventional scale material in both the level of defects and the formation of unfavored phases. Molecular dynamics simulations address the origins of some of these phenomena.

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Like other interfaces, equilibrium grain boundaries are smooth at low temperature and rough at high temperature; however, little attention has been paid to roughening except for faceting boundaries. Using molecular dynamics simulations of face-centered cubic Ni, we studied two closely related grain boundaries with different boundary planes. In spite of their similarity, their boundary roughening temperatures differ by several hundred degrees, and boundary mobility is much larger above the roughening temperature. This has important implications for microstructural development during metallurgical processes.

To effectively integrate nanotechnology into functional devices, fundamental aspects of material behavior at the nanometer scale must be understood. Stresses generated during thin film growth strongly influence component lifetime and performance; stress has also been proposed as a mechanism for stabilizing supported nanoscale structures. Yet the intrinsic connections between the evolving morphology of supported nanostructures and stress generation are still a matter of debate. This report presents results from a combined experiment and modeling approach to study stress evolution during thin film growth. Fully atomistic simulations are presented predicting stress generation mechanisms and magnitudes during all growth stages, from island nucleation to coalescence and film thickening. Simulations are validated by electrodeposition growth experiments, which establish the dependence of microstructure and growth stresses on process conditions and deposition geometry. Sandia is one of the few facilities with the resources to combine experiments and modeling/theory in this close a fashion. Experiments predicted an ongoing coalescence process that generates signficant tensile stress. Data from deposition experiments also supports the existence of a kinetically limited compressive stress generation mechanism. Atomistic simulations explored island coalescence and deposition onto surfaces intersected by grain boundary structures to permit investigation of stress evolution during later growth stages, e.g. continual island coalescence and adatom incorporation into grain boundaries. The predictive capabilities of simulation permit direct determination of fundamental processes active in stress generation at the nanometer scale while connecting those processes, via new theory, to continuum models for much larger island and film structures. Our combined experiment and simulation results reveal the necessary materials science to tailor stress, and therefore performance, in nanostructures and, eventually, integrated nanocomponents.

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Numerous molecular dynamics simulation studies of radiation cascades in Si have elucidated many of the general features of the initial defect production. However, the resulting defect structures have been analyzed with techniques that are not sensitive to changes in the local bonding topology. Here the results of analyzing the ring content in Si cascades, in addition to more traditional defect characterization such as Wigner-Seitz cell analysis, will be presented for recoil energies ranging from 25 eV up to 25 keV. The ring content of local amorphous regions in the cascades will be compared to the ring content in simulations of bulk amorphous Si. The number of atoms in the amorphous regions and the number of point defects as a function of recoil energy are determined. © 2006 Elsevier B.V. All rights reserved.

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Grain boundary stiffness and mobility determine the kinetics of curvature driven grain growth. Here the stiffness and mobility are determined using a computational approach based on the analysis of fluctuations in the grain boundary position during molecular dynamics simulations. This work represents the first determination of grain boundary stiffness. The results indicate that the boundary stiffness for a given boundary plane has a strong dependence on the direction of the boundary distortion. The mobility deduced is in accord with previous computer simulation studies.

The ability to integrate metal and semiconductor micro-systems to perform highly complex functions, such as RF-MEMS, will depend on developing freestanding metal structures that offer improved conductivity, reflectivity, and mechanical properties. Three issues have prevented the proliferation of these systems: (1) warpage of active components due to through-thickness stress gradients, (2) limited component lifetimes due to fatigue, and (3) low yield strength. To address these issues, we focus on developing and implementing techniques to enable the direct study of the stress and microstructural evolution during electrodeposition and mechanical loading. The study of stress during electrodeposition of metal thin films is being accomplished by integrating a multi-beam optical stress sensor into an electrodeposition chamber. By coupling the in-situ stress information with ex-situ microstructural analysis, a scientific understanding of the sources of stress during electrodeposition will be obtained. These results are providing a foundation upon which to develop a stress-gradient-free thin film directly applicable to the production of freestanding metal structures. The issues of fatigue and yield strength are being addressed by developing novel surface micromachined tensile and bend testers, by interferometry, and by TEM analysis. The MEMS tensile tester has a ''Bosch'' etched hole to allow for direct viewing of the microstructure in a TEM before, during, and after loading. This approach allows for the quantitative measurements of stress-strain relations while imaging dislocation motion, and determination of fracture nucleation in samples with well-known fatigue/strain histories. This technique facilitates the determination of the limits for classical deformation mechanisms and helps to formulate a new understanding of the mechanical response as the grain sizes are refined to a nanometer scale. Together, these studies will result in a science-based infrastructure to enhance the production of integrated metal--semiconductor systems and will directly impact RF MEMS and LIGA technologies at Sandia.

An experimental technique was developed to perform isentropic compression of heated liquid tin samples at the Z Accelerator, and multiple such experiments were performed to investigate solidification under rapid compression. Preliminary analyses, using two different methods, of data from experiments with high uncertainty in sample thickness suggest that solidification can begin to occur during isentropic compression on time scales of less than 100 ns. Repeatability of this result has not been confirmed due to technical issues on the subsequent experiments performed. First-principles molecular-dynamics calculations based on density-functional theory showed good agreement with experimentally-determined structure factors for liquid tin, and were used to investigate the equation of state and develop a novel interatomic pseudo-potential for liquid tin and its high-pressure solid phase. Empirical-potential molecular-dynamics calculations, using the new potential, gave results for the solid-liquid interface velocity, which was found to vary linearly with difference in free energy between the solid and liquid phases, as well as the liquidus, the maximum over-pressurization, and the solid-liquid interfacial energy. These data will prove useful in future modeling of solidification kinetics for liquid tin.