Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session D12: Morphological Evolution and Instabilities |
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Sponsoring Units: DCMP DMP Chair: Ted Einstein, University of Maryland Room: Baltimore Convention Center 304 |
Monday, March 13, 2006 2:30PM - 2:42PM |
D12.00001: Langevin Equation for the Morphological Evolution of Strained Epitaxial Films Dimitri Vvedensky, Christoph Haselwandter A stochastic partial differential equation for the morphological evolution of strained epitaxial films is derived from an atomistic master equation. The transition rules in this master equation are based on previous kinetic Monte Carlo (KMC) simulations of a model that incorporates the effects of strain through local environment-dependent energy barriers to adatom detachment from step edges. The morphological consequences of these rules are seen in the transition from layer-by-layer growth to the appearance of three-dimensional islands with increasing strain. The regularization of the exact Langevin description of these rules yields a continuum equation whose lowest-order terms provide a coarse-grained theory of this model. The coefficients in this equation are expressed in terms of the parameters of the original lattice model, so a direct comparison between the morphologies produced by KMC simulations and this Langevin equation are meaningful. Comparisons with previous approaches are made to provide an atomistic interpretation of a similar equation derived by Golovin {\it et al.} based on classical elasticity. [Preview Abstract] |
Monday, March 13, 2006 2:42PM - 2:54PM |
D12.00002: Crystal Surfaces out of Equilibrium: Instabilities, Multiscaling and Non-locality Alberto Pimpinelli, Florin Nita We have recently proposed a novel scenario for the step meandering instability on vicinal surfaces, in which the instability is ascribed to unhindered diffusion of atoms along the step edges.\footnote{F. Nita and A. Pimpinelli, Phys. Rev. Lett. 95, 106104 (2005)} Then, in a rather counterintuitive way, step meandering appears due to the very mechanism - step edge diffusion - which may be expected to oppose it. We discuss the detailed comparison of the scaling properties of unstable surfaces driven by different atomistic mechanisms. We have discovered an unsuspected richness of behaviors: anomalous scaling, multiaffinity, $\ldots$ We discuss a number of continuum equations that share several features with the simulations. Implications for experiments are also discussed. [Preview Abstract] |
Monday, March 13, 2006 2:54PM - 3:06PM |
D12.00003: Criticality of morphological instability of a strained film growing on a patterned substrate Hangyao Wang, Feng Liu We show that the morphological instability of a strained film on a patterned substrate is fundamentally different from that on a flat substrate. It exhibits a film thickness ($t$) dependent critical wavelength, which takes a simple form as $\lambda_{c}=\lambda_{0}/2+\pi t$ for a very thin film, where $\lambda_{0}$ is the critical wavelength on a flat substrate. It also defines three distinct regimes of growth stability depending on the wavelength of substrate undulation ($\lambda_{s}$): for $\lambda_{s}\leq\lambda_{0}/2$, growth is stable; for $\lambda_{s}\geq\lambda_{0}$, growth is unstable; in between, growth is unstable below a critical film thickness $t_{c}$, and stable above it. [Preview Abstract] |
Monday, March 13, 2006 3:06PM - 3:18PM |
D12.00004: First-principles Investigation of the Stability of Surface Gold Oxides on Au(111) Hongqing Shi, Catherine Stampfl In contrast to the long held view that gold is catalytically inert and as such uninteresting, it is now well known that Au is significantly more active than Pt in the catalytic oxidation of CO under basic environments. Au can also promote many other reactions in the form of nanoparticles on metal oxide and activated carbon supports [1].$^{ }$This has simulated huge efforts in an attempt to understand the mechanisms responsible for the high activity, including investigations into the nature of oxygen on gold surfaces [2]. In the present work we have investigated the relative stability of oxygen adsorbed on and under the Au(111) surface, as well as thin surface oxides. We identify structures in which the binding of atomic oxygen is stronger than that at under-coordinated surface Au atoms (e.g. at steps). To determine the stability of the structures for different pressure and temperature conditions, we use the approach of \textit{ab initio} thermodynamics [3], which indicates that these structures should be stable under certain catalytic conditions. \newline [1] Haruta, Catal. J. New. Mater. Electro. Sys. \textbf{7}, 163 (2004). \newline [2] R. Meyer,\textit{ et al}., Gold Bull. \textbf{37}, 72 (2004), and references therein. \newline [3] K. Reuter, C. Stampfl and M. Scheffler, in Handbook of Materials Modeling, Volume 1, Fundamental Models and Methods, Sidney Yip (Ed) 2005, 149-194; K. Reuter and M. Scheffler, Phys. Rev. B \textbf{65}, 035406 (2002). [Preview Abstract] |
Monday, March 13, 2006 3:18PM - 3:30PM |
D12.00005: Modeling Nanoscale Dynamics for Film Growth Alexandra ten Bosch Small scale particle motion controls the onset of a phase transition. A general method is developed which links atomic and mesoscopic dynamics in a nanoscale description reminiscent of the classical theory of fluid flow. Derived from a Fokker Planck equation for the non-equilibrium particle distribution, the dynamic equation includes inertia terms essential for high frequency fluctuations. Film nucleation and growth are modeled by the spatially inhomogeneous evolution of the instantaneous density profile which measures the average number of particles at a given time and position. The method is used to show how an alteration in the equilibrium distribution of particles at the boundary between parent and product phases induces transient film growth and/or damped vibrations at the surface. To illustrate, condensation of a simple fluid on a surface is considered. [Preview Abstract] |
Monday, March 13, 2006 3:30PM - 3:42PM |
D12.00006: Interface Roughening Dynamics of Spreading Droplets Haim Taitelbaum, Avraham Be'er, Inbal Hecht, Aviad Frydman, Yossi Lereah We review our recent experimental data of interface roughening dynamics of spreading mercury droplets on thin films (silver or gold), obtained using optical microscopy and other techniques (AFM, SEM). We discuss the various results obtained for the roughness and growth exponents associated with the interface dynamics, and their universality classes. We analyze the temporal width fluctuations, obtained for single interfaces, and show that these fluctuations contain information on the lateral correlation length of these interfaces. We show how to extract this length from experimental data, and demonstrate the validity of this method in a wide range of growing interfaces (droplet spreading experiments as well as water imbibition on paper). References: 1. A. Be'er, Y. Lereah and H. Taitelbaum, Physica A, 285, 156 (2000). 2. A. Be'er, Y. Lereah, I. Hecht and H. Taitelbaum, Physica A, 302, 297 (2001). 3. A. Be'er, Y. Lereah, A. Frydman and H. Taitelbaum, Physica A, 314, 325 (2002). 4. A. Be'er and Y. Lereah, J. of Microscopy, 208, 148 (2002). 5. I. Hecht and H. Taitelbaum, Phys. Rev. E, 70, 046307 (2004). 6. A. Be'er, I. Hecht and H. Taitelbaum, Phys. Rev. E, 72, 031606 (2005). 7. I. Hecht, A. Be'er and H. Taitelbaum, Fluctuation and Noise Letters, 5, L319 (2005). [Preview Abstract] |
Monday, March 13, 2006 3:42PM - 3:54PM |
D12.00007: Evolution of the morphology and composition of metal germanosilicide thin films Mathieu Bouville, Dongzhi Chi, David J. Srolovitz Metal germanosilicide thin films, formed from the reaction of metals on silicon-germanium substrates, are attractive for use in advanced metal-oxide-semiconductor field-effect transistors (MOSFETs). However, metal germanosilicide films are less stable than silicides films at elevated temperatures, easily agglomerating into isolated islands following severe grain boundary grooving. The resultant germanosilicide islands are noticeably different from both silicides and germanides. Agglomerated germanosilicide films consist of small, regular islands with faceted interfaces, whereas agglomerated silicides form irregular islands and uniformly curved island/substrate interfaces. Experimental observations show that the germanium composition is inhomogeneous both in the film and in the substrate. We use phase-field simulations to study the interplay between morphology, composition inhomogeneities, and strain during grain boundary grooving and agglomeration of polycrystalline metal germanosilicide films on silicon-germanium alloy substrates. By simulating the evolution of germanosilicide films on compressive and relaxed substrates, we demonstrate the important role played by misfit stress on agglomeration morphologies. [Preview Abstract] |
Monday, March 13, 2006 3:54PM - 4:06PM |
D12.00008: Effects of shadowing in oblique incidence epitaxial growth J.G. Amar, Y. Shim, V. Borovikov The results of kinetic Monte Carlo simulations of a simplified model of oblique incidence epitaxial growth on a metal fcc(100) surface are presented. Results for the effects of shadowing on the mound morphology, surface roughness, and nanoscale pattern formation as a function of the angle of incidence $\theta$ relative to the surface normal as well as the azimuthal angle $\phi$ relative to the high-symmetry direction are presented. In good agreement with experiment, we find that shadowing has little effect for deposition angles up to approximately $55^o$. However, for higher angles there is a significant effect. In particular, the formation of ripples perpendicular to the incident beam is found at $\theta \simeq 78^o$ while at even higher angles the formation of ripples parallel to the incident beam is observed. The coarsening behavior of the resulting ripple structures is also studied. Results are also presented for azimuthal angles away from the high-symmetry directions. [Preview Abstract] |
Monday, March 13, 2006 4:06PM - 4:18PM |
D12.00009: Multiscale simulations of oblique-incidence Cu/Cu(100) epitaxial growth V. Borovikov, Y. Shim, J.G. Amar We present an improved method for multiscale simulations of homoepitaxial growth of metal thin films in which the effects of oblique incidence on the deposition process may be accurately taken into account. Our method combines a kinetic Monte Carlo (KMC) simulation for the thermal surface diffusion with a small scale molecular-dynamics simulation of every deposition event, in order to account correctly for both shadowing as well as the detailed atom-surface interaction of depositing atoms. To speed-up the simulations, that are very time consuming computationally, we have utilized a parallel algorithm that involves simultaneous depositions in different areas of the surface along with parallel kinetic Monte Carlo. Simulation results for the growth of Cu/Cu(100) at 160 and 200 K are presented and compared with both ordinary KMC simulations and experiments. Results will be presented for the case of both normal incidence as well as oblique angle incidence. [Preview Abstract] |
Monday, March 13, 2006 4:18PM - 4:30PM |
D12.00010: Evolution of Roughness and Wavelength Selection during Fluorocarbon Plasma Etching of Nanoporous Silica Taesoon Kwon, Hung-Chih Kan, Xuefeng Hua, Gottlieb Oehrlein, Ray Phaneuf Nanoporous silica is a technologically appealing candidate as a low-k dielectric material for high speed nano device applications. A crucial issue during plasma pattern transferring processes is the stability of the plasma/NPS interface. Induced surface/interface roughness may ultimately limit the minimum feature sizes attainable in devices using this material. Our results show a monotonic increase with porosity in the roughening rate, and pronounced disagreement with the predictions of simple models based upon self-affine behavior. In addition we find direct evidence for spontaneous pattern formation during etching. [Preview Abstract] |
Monday, March 13, 2006 4:30PM - 4:42PM |
D12.00011: Wavelength Tunability of Ion-bombardment Induced Surface Ripples on Sapphire Hua Zhou, Lan Zhou, Yi-Ping Wang, Randall L. Headrick, Ahmet S. Ozcan, Yi-Yi Wang, Gozde Ozaydin, Karl F. Ludwig Jr., David P. Siddons Energetic particle bombardment on surfaces is known to produce well ordered 2-D (ripples or wires) and 1-D (dots) structures at submicron/nanoscale by a self-organization process. Recently, significant experimental and theoretical effort has been expended to develop methods to produce self-organized nanostructures on diverse substrates from semiconductors to metals. These studies have shown potential in tailoring surface morphology in order to exploit novel physical properties, and contributed much to reveal the mechanisms of the instability-driven self-organization process. In this work, a study of ripple formation on sapphire surfaces by ion sputtering is presented. Surface characterization by in-situ synchrotron grazing incidence small angle x-ray scattering (GISAXS) and ex-situ atomic force microscopy (AFM) for the wavelength, shape and amplitude of sapphire ripples is performed. The wavelength can be varied over two orders of magnitude by changing the ion incidence angle. The linear Bradley-Harper (B-H) theory with ion induced viscous flow (IVF) relaxation fits the general trends of the data. However, anomalous smoothing not predicted by current models is observed near normal incidence. [Preview Abstract] |
Monday, March 13, 2006 4:42PM - 4:54PM |
D12.00012: Atomistic calculations of mass redistribution and surface morphology evolution due to medium energy ion bombardment Harley Johnson, Nagarajan Kalyanasundaram, Benjamin Davidovitch, Michael Brenner, Mike Aziz, Jonathan Freund Nanoscale mass redistribution mechanisms and dynamics near a surface due to ion bombardment are studied using molecular dynamics (MD). In addition to sputter erosion, as described by the well-known Bradley-Harper theory, ion assisted surface mass redistribution is identified as an important contributor to surface morphology changes. MD simulations yield a response function based on the change in surface height at any point on the surface due to impact at an arbitrary point on the surface. We derive a linear continuum equation for surface morphology evolution using the new response function and relate the results to pattern formation. We conclude that consideration of both sputtering and mass redistribution could explain the experimentally observed limits on the slopes of ripples formed in this process. [Preview Abstract] |
Monday, March 13, 2006 4:54PM - 5:06PM |
D12.00013: Measurement and modeling of temperature-dependent step bunching on Si(111) Brian Gibbons, Jonathan Pelz Direct Current (DC) induced step bunching on Si(111) is a long-standing puzzle, with the required DC direction for bunching (relative to the ``step-down'' vicinal surface direction) reversing multiple times with increasing temperature. It was recently proposed [1,2] that this could be explained if step attachment is \textit{faster (slower)} than terrace diffusion in Temperature Regime II (Regimes I and III). We have numerically simulated a similar model and directly compared with measurements of how the step bunching depends on the initial terrace width $l_{0}$ in all three regimes [3]. Using realistic parameter values for terrace diffusion and step attachment, this model can account for the bunching behavior in all three temperature regimes, provided there indeed exist modest (0.2 -- 0.4 eV) temperature-dependent variations in the relative activation barriers for attachment and diffusion, and/or modest changes in the respective activation attempt rates. Work supported by NSF. [1] N. Suga \textit{et al.}, \textit{Jpn. J. Appl. Phys.} \textbf{39}, 4412 (2000). [2] T. Zhao \textit{et al.}, \textit{Phy. Rev. B} \textbf{71}, 155326 (2005). [3] B.J. Gibbons \textit{et al.}, submitted to Surf. Sci; Surf. Sci. Lett. \textbf{575}, L51-56 (2005). [Preview Abstract] |
Monday, March 13, 2006 5:06PM - 5:18PM |
D12.00014: Oxygen-induced faceting of NiAl(111) E. Loginova, W. Chen, N. M. Jisrawi, F. Cosandey, T. E. Madey Our research is focused on the adsorption of oxygen and oxygen-induced faceting of NiAl(111), as studied by means of LEED, AES, SEM, AFM and high-resolution soft XPS (HRSXPS, using synchrotron radiation at NSLS). The atomically rough NiAl(111) surface remains planar at room temperature when exposed to oxygen. However, the surface changes its morphology and becomes faceted upon annealing at 1100K and higher; nucleation and growth of facets are studied. The adsorption and reaction of oxygen are characterized by HRSXPS measurements of Al 2p and Ni 3p core levels for the faceted and planar surfaces. The data indicate that a well-ordered thin aluminum oxide film can be formed on the faceted NiAl surface. After extensive annealing in oxygen, symmetric oxide features of micrometers in length are observed on the surface. We investigate the stoichiometry of these features by X-Ray mapping and propose a model for their formation. Our work is motivated by the possibility that alumina thin-film-covered NiAl facets might be used as templates for growth of metallic nanostructures with controlled size and spacing. The Al$_{2}$O$_{3}$/NiAl(111) system is a good model for studying catalytic reactions over Al$_{2}$O$_{3}$-supported metal catalysts. [Preview Abstract] |
Monday, March 13, 2006 5:18PM - 5:30PM |
D12.00015: Faceting of Re ($11\bar {2}1)$ induced by oxygen Hao Wang, Wenhua Chen, Theodore E. Madey The oxygen-induced nanoscale faceting of Re ($11\bar {2}1)$ has been studied by low energy electron diffraction (LEED) and Auger electron spectroscopy (AES); the results are compared with recent STM and LEED studies of O-induced faceting of Re($12\bar {3}1)$. The evolution of surface morphology depends on oxygen exposures and deposition temperatures. Re($11\bar {2}1)$ remains planar after oxygen deposition at 300K. Annealing O-covered Re($11\bar {2}1)$ between 800K-1200K leads to the formation of ($01\bar {1}0)$ and ($10\bar {1}0)$ facets that coexist with the ($11\bar {2}1)$ surface. Under oxidation conditions, i.e. dosing a large amount of oxygen at high temperatures (900-1000K), the ($11\bar {2}1)$ surface is completely covered by 4-sided nanoscale pyramidal structures whose facets are identified as ($01\bar {1}0)$, ($10\bar {1}0)$, ($01\bar {1}1)$ and ($10\bar {1}1)$. The fact that the ($11\bar {2}1)$ surface becomes completely faceted only after oxidation is consistent with our previous data for O-induced faceting of Re($12\bar {3}1)$, where one facet has the ($11\bar {2}1)$ orientation and is unstable against oxidation. The faceted O/Re surfaces may be potential templates to grow nano-structures with narrow size distribution, and may also be substrates to study structural sensitivity in catalytic reactions. [Preview Abstract] |
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