Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session S41: Steps, Facets, and Evolution of Surface Structures |
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Sponsoring Units: DCMP Chair: Alberto Pimpinelli, Universite Blaise Pascal Room: Colorado Convention Center 504 |
Wednesday, March 7, 2007 2:30PM - 2:42PM |
S41.00001: Temperature Dependence of the Step Line Tension and Island Decay on the Si(111) (1x1) Surface M.S. Altman, K.L. Man, A.B. Pang, T. Stasevich, F. Szalma, T.L. Einstein Atomic steps are common defects at surfaces that can play an important role in many phenomena. Advances in the fabrication of nanostructures at surfaces depend largely upon the degree to which one can understand and control factors that affect step morphology. The step line tension is a crucial element in the Gibbs-Thomson relation, which describes the dependence of the chemical potential of an surface step upon its radius of curvature. This dependence can have a notable influence on step morphology. A proper description step morphological phenomena therefore requires accurate knowledge of line tension, including its temperature dependence. The step line tension on the Si(111) (1x1) surface was determined from a capillary wave analysis of two-dimensional island edge fluctuations that were observed with low energy electron microscopy. The line tension decreases by nearly 20{\%} between 1145 K and 1233 K. The role of desorption in island decay varies from negligible to dominant in the temperature range, 1145 -- 1380K, that island decay was measured. A general model of island decay is presented that takes account of desorption. Evaluation of the island decay time with this model referenced to the temperature-dependent line tension accurately determines activation energies that are relevant to island decay and sublimation. [Preview Abstract] |
Wednesday, March 7, 2007 2:42PM - 2:54PM |
S41.00002: Phase Field Model for Step Dynamics Including Elastic Interactions Between Steps and the Ehrlich-Schwoebel Barrier Dong-Hee Yeon, Katsuyo Thornton Understanding the evolution of steps on a vicinal surface is crucial in many important problems involving surfaces. Elastic interactions between steps and the preferential incorporation of adatoms into the upper step due to an asymmetric energy barrier, so-called the Ehrlich-Schwoebel(ES) barrier, greatly influence the step dynamics, often generating morphological instabilities of steps. For example, in the step bunching instabilities, the elastic interactions invoke the progressive coalescence of steps, while the ES barrier has a stabilizing effect. We will present a phase-field model for step dynamics including effects of elastic interactions and the ES barrier, and its application to investigate the effects of these factors on step dynamics. The results of the linear stability analysis will also be presented and are compared with those obtained by the phase-field model. In our simulation, it is shown that the flux is an important factor limiting the growth of step bunches through the debunching process. We will also present the analyses of step meandering instabilities resulting from the interplay among the elastic interaction, the ES barrier, and the step line energy. [Preview Abstract] |
Wednesday, March 7, 2007 2:54PM - 3:06PM |
S41.00003: Distinctive Fluctuations of Facet Edges M. Degawa, T. J. Stasevich, W. G. Cullen, Alberto Pimpinelli, T. L. Einstein, E. D. Williams Spurred by theoretical predictions of distinctive static scaling of the step bounding a facet,\footnote{P.L.~Ferrari et al., Phys.~Rev.~E {\bf 69} (2004) 035102(R) } we extend the results to dynamic scaling, also rederiving the static results heuristically\footnote{A.~Pimpinelli et al., Surf.~Sci.~Lett.~{\bf 598} (2005) L355 } and we measure this behavior using STM line scans.\footnote{M. Degawa et al., Phys.~Rev.~Lett.~{\bf 97}, 080601 (2006)} The correlation functions go as $t^{0.15 \pm 0.03}$ decidedly different from the $t^{0.26 \pm 0.02}$ behavior for fluctuations of isolated steps. From the exponents, we categorize the universality, confirming the prediction that the non-linear term of the KPZ equation, long known to play a central role in non-equilibrium phenomena, can also arise from the curvature or potential-asymmetry contribution to the step free energy. We study a simple model with Monte Carlo simulations to illustrate the novel scaling of fluctuations in an asymmetric potential. [Preview Abstract] |
Wednesday, March 7, 2007 3:06PM - 3:18PM |
S41.00004: Evolution of patterned step structure on vicinal Si(111) surface during high temperature annealing Hung-Chih Kan, Taesoon Kwon, Raymond Phaneuf We present the results of numerical simulations of the evolution of patterned step structures on vicinal Si(111) surfaces during high temperature annealing, which presumably drives the surface far away from equilibrium. We use a mesoscopic model [1] to describe the motion of individual steps under the effects of sublimation, step stiffness (line tension), and step-step interaction. The qualitative consistency between our simulation and experiment [2] suggest that thermodynamic driving force, such as the step-stiffness and step-step interaction dominate the evolution of the step structure during high temperature annealing. [1] J. D. Weeks, D.-J. Lui, and H.-C. Jeong, in Dynamics of Crystal Surfaces and Interfaces, edited by P. M. Duxbury and T.J. Pence (Plenum Press, New York and London 1997), pp. 199-216 [2] T. Kwon, H-C. Kan, R. J. Phaneuf, Appl. Phys. Lett. \textbf{88}, 071914 (2006) . [Preview Abstract] |
Wednesday, March 7, 2007 3:18PM - 3:30PM |
S41.00005: Step Density Minimum on Vicinal Surfaces and Surface Cusps in Epitaxial Regrowth on Patterned Substrates Tian Li, A. Ballestad, T. Tiedje In kinetic Monte Carlo (KMC) simulations of a solid-on-solid model of epitaxial growth, we measure the step density as a function of surface slope and find a special slope where the step density has a minimum. This slope occurs where the surface step density changes from two dimensional (islands) to one dimensional (staircase). The minimum in the step density results from the fact that one dimensional steps are more efficient at capturing adatoms than two dimensional steps, so that a small vicinal angle tends to suppress nucleation of islands. The slope for minimum step density is most sharply defined at low growth rates and high temperatures where the step edges tend to be smooth. The special slope goes away at low temperatures where the steps have a convoluted fractal shape. The minimum in the step density generates characteristic features in KMC simulations for stable (negative Ehrlich Schwoebel barriers) growth on patterned substrates, namely cusps on the top and shoulders of ridges. These features are also found in a continuum growth model which includes the step density minimum. Simulated surface shapes are in good agreement with experimental data for MBE growth of GaAs on patterned substrates, and with data in the literature. [Preview Abstract] |
Wednesday, March 7, 2007 3:30PM - 3:42PM |
S41.00006: The reconstructions of Si(15 3 23) Teresa Davies, Cristian Ciobanu Among the few remaining puzzles in the physics of heteroepitaxial quantum dots, there remains the atomic structure of the facets that bound the dome-shape islands. For a large range of germanium concentrations in the deposited Si- Ge alloy, the (15 3 23) orientation appears to be ubiquitous as the highest-index facet present on the dots at the latest stages of their coherent growth. We present here two approaches to the determination of the reconstructions of Si(15 3 23), one based on a systematic elimination of bonding possibilities and the other being a structural search performed via a genetic algorithm. Both approaches lead to classes of reconstruction models that have much lower surface energies than the only proposal currently available in the literature. Furthermore, the models allow for the presence of low-energy edges between the (15 3 23)-orientations and other facets present on the quantum dots. [Preview Abstract] |
Wednesday, March 7, 2007 3:42PM - 3:54PM |
S41.00007: Faceting of Re ($11\bar {2}1)$ induced by ammonia Hao Wang, Wenhua Chen, Theodore E. Madey, Timo Jacob The ammonia-induced nanoscale faceting of Re ($11\bar {2}1)$ has been studied by LEED and STM; the results are compared with recent studies of O-induced faceting of Re($11\bar {2}1)$. After exposure to ammonia at 700K, the Re($11\bar {2}1)$ surface only shows a (1$\times $2) reconstruction and remains planar (ammonia dissociates on Re, and only N remains on the surface at T$>$700K). By exposure to ammonia at 900K, Re($11\bar {2}1)$ becomes completely faceted, forming 2-sided ridge-like structures with ($13\bar {4}2)$ and ($31\bar {4}2)$ facets. However, this morphology is different from that in O-induced faceting of Re($11\bar {2}1)$. The two ridge sides, ($13\bar {4}2)$ and ($31\bar {4}2)$, are vicinal surfaces of closed-packed ($01\bar {1}0)$ and ($10\bar {1}0)$ respectively; these latter two surfaces appear as facets in O-induced faceting of Re($11\bar {2}1)$. DFT calculations are implemented to understand the origin of the different morphologies. Our work demonstrates that it is possible to tailor the surface morphology by choosing appropriate adsorbate and annealing conditions, which in turn provides model systems to study structural sensitivity in catalytic reactions as well as potential templates to grow nanostructures. [Preview Abstract] |
Wednesday, March 7, 2007 3:54PM - 4:06PM |
S41.00008: LEEM study of nucleation, growth, and decay of Ag nanowires on Cu(110) Indrajith Senevirathne, Ezra Bussmann, Gary Kellogg, Richard Kurtz, Phillip Sprunger Low energy electron microscopy (LEEM) has been used to study the nucleation, growth, and ripening of Ag nanowires on Cu(110). Previous STM and LEED studies of Ag on the Cu(110) surface have shown that for a Ag coverage of below 0.3 ML, Ag forms a surface alloy, followed by the formation of a 2D Ag(111) flat superstructure through dealloying at one monolayer. For Ag coverages above 1.3 ML , nanowires of Ag(110), with widths/heights of 12 nm / 2nm, grow along the [1\underline {1}0] crystallographic direction with highly anisotropic aspect ratios. LEEM reveals that Ag initially alloys at Cu/Ag step edges producing a distortion of the steps. Upon deposition above 1 ML, nucleation of Ag nanowires was observed across terraces, however nucleation still occurred at defect and step edges. LEEM showed that the nanowires grow to micron lengths and have highly anisotropic aspect ratios. Annealing above 573K resulted in rapid Oswald ripening of nanowires to Ag clusters of several micron dimensions. Quantitative details of the growth and decay mechanisms will be discussed. Sandia Corporation is a Lockheed Martin Company, for the US DOE's NNSA under Contract DE-AC04-94AL85000. Work was supported CINT-U2006A123 and NSF-DMR-0504654. [Preview Abstract] |
Wednesday, March 7, 2007 4:06PM - 4:18PM |
S41.00009: Studies of Cu adatom island ripening on Cu(100) by LEEM Ezra Bussmann, Gary L. Kellogg Simple metal surfaces are model systems for characterizing kinetic processes governing the growth and stability of nanoscale structures. It is generally presumed that diffusive transport of adatoms across terraces determines the rate of these processes. However, STM studies in the temperature range T$\sim $330-420 K reveal that transport between step edges on the Cu(100) surface is limited by detachment barriers at the step edges, rather than by the adatom diffusion barrier.$^{1}$ This is because on the Cu(100) surface, mass transport is mediated primarily by vacancies, instead of adatoms. We have used low energy electron microscopy (LEEM) movies to characterize coarsening of Cu islands on the Cu(100) surface in the range T$\sim $460-560 K. By measuring the temperature dependence of the island decay rate we find an activation barrier of 0.9$\pm $0.1 eV. This value is comparable to the 0.80$\pm $0.03 eV barrier found in STM studies.$^{1 }$However, we are not able to conclude that transport is entirely detachment limited at these elevated temperatures. This work serves as background to establish whether or not Pd alloying in the Cu(100) surface will slow Cu surface transport. $^{2}$ 1. C. Kl\"{u}nker, \textit{et al.}, PRB \textbf{58}, R7556 (1998). 2. M. L. Grant, \textit{et al.}, PRL \textbf{86}, 4588 (2001). Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. DOE NNSA, Contract No. DE-AC04-94AL85000. [Preview Abstract] |
Wednesday, March 7, 2007 4:18PM - 4:30PM |
S41.00010: Growth of Au on Si(111) surface A.-L. Chin, F.-K. Men We have studied the growth of sub-monolayer Au atoms on Si(111)-(7$\times $7) surface using scanning tunneling microscopy. By heating the Si substrate after room-temperature Au deposition, we have observed the formation of two types of the (5$\times $2) structure on a (7$\times $7) terrace: one is the (5$\times $2) depressions with an apparent height slightly lower than that of the terrace, the other is the protruding (5$\times $2) islands on the terrace. Comparing total areas occupied by the two types of the (5$\times $2) structure we have obtained the number of Si atoms in the reconstructed layers of a (5$\times $2) unit cell. The surface steps act as good sinks for Au adatoms coming either from the up or down terraces. Widths of denuded zones have been investigated by analyzing the spatial distribution of the (5$\times $2) structure. We will discuss relevant surface diffusion parameters related to the growth of Au. [Preview Abstract] |
Wednesday, March 7, 2007 4:30PM - 4:42PM |
S41.00011: Nucleation, Structure, Morphology of Co on Ag(110): temperature dependent subsurface migration. Phillip Sprunger, Indrajith Senevirathne, Orhan Kizilkaya, Richard Kurtz STM, LEED and Auger spectroscopy have been used to study the nucleation, structure, and morphology of Co on Ag(110) as a function of both coverage and temperature. STM has shown that films grown at coverages of $\theta $ $<$ 1ML and low substrate temperatures ($\sim $150K) give rise to segregation of Co to the subsurface layer. Furthermore it is possible to observe the nucleation of displaced Ag into small monatomic height islands on top of the substrate. For higher coverages of Co, $\theta \quad >$ 1ML a 3-D cluster growth mode takes place giving islands. These islands have a typical $\sim $20 {\AA} diameter and the height of $\sim $3 {\AA}. However, upon annealing the surface to $\sim $673K, profound changes are observed on the morphology of the surface. STM shows that there is a drastic reduction of the density and height of Co islands on the surface, presumably due to subsurface Co cluster growth. Further verification of this was obtained by sequential LEED and Auger spectra obtained while progressive annealing. It was observed the onset of this process occurs at $\sim $673K and flattens out at $\sim $873K. This work is funded by NSF NSF-DMR-0504654 [Preview Abstract] |
Wednesday, March 7, 2007 4:42PM - 4:54PM |
S41.00012: Morphology Evolution of Cobalt Thin Films on Al$_2$O$_3$ (110) Above the Roughening Transition: Formation of Gigantic Multilayered Islands Jorge Espinosa, Leonardo Golubovic, David Lederman Co films, 4.0 nm thick, were grown on Al$_2$O$_3$ (110) at 315~$^\circ$C via molecular beam epitaxy. Their surfaces were imaged via atomic force microscopy while annealing at $T\geq 535$~$^\circ$C for several hours. The films exhibited a striking formation of multilayered islands that reach heights more than ten times larger than the initial film thickness. At the early stages of the annealing process ($t< 2 $~hr) the islands' height $h$ grows exponentially with time $t$, which is consistent with height instabilities of the film surface. For $t> 2$~hr, $h$ continues increasing at a slower rate with a power law $h \sim t^\gamma$ with $\gamma = 0.20-0.25$, whereas, interestingly, the base areas of the multilayer islands do not appreciably change with time. This behavior is independent of $T$ in the 535~$^o$C to 590~$^o$C range. These phenomena are discussed within an interface dynamics model incorporating both surface diffusion relaxation and de-wetting forces. The model is used to elucidate the physical origin of the observed island height growth in terms of strong up-hill surface currents caused by long range Casimir-like forces acting across the film. [Preview Abstract] |
Wednesday, March 7, 2007 4:54PM - 5:06PM |
S41.00013: First-principles studies of the $\surd $ 7 $\times \quad \surd $ 7 R19.1$^{\circ}$ structure of sulfur on the Pd(111) surface Dominic Alfonso Density functional theory is used to investigate the ($\surd $ 7 $\times $ $\surd $ 7) R19.1$^{\circ}$ structure of sulfur on the Pd(111) surface. Applying the concepts of first-principles atomistic thermodynamics, we analyze the stability of various ($\surd $ 7 $\times \quad \surd $ 7) models in equilibrium with arbitrary H$_{2}$ and H$_{2}$S environment. Among the different models that were considered, the densely packed mixed sulfur-metal overlayer structure proposed by Berndt \textit{et al}. [Surf. Sci. \textbf{393}, L119 (1997)] was found to be the most energetically favorable. This model consists of arrangement of Pd triangles and pentagons on top of Pd(111) with the sulfur atoms at 3/7 monolayer coverage. The dominant mechanism for sulfur interaction with Pd in the overlayer region is the rehybridization of sulfur 3 p and metal 4 d bands. Simulated scanning tunneling microscopy image for this structure shows some similarity with that obtained from experiment. Our study confirms that surface sulfide with no structural resemblance to its bulk counterparts can form on Pd(111). [Preview Abstract] |
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