Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session W40: Focus Session: Morphology and Evolution at Surfaces: Instabilities and Patterned Substrates |
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Sponsoring Units: DMP DCMP Chair: Gary Kellogg, Sandia National Labs Room: LACC 408A |
Thursday, March 24, 2005 2:30PM - 2:42PM |
W40.00001: Buffer Layer Patterning of InAs/GaAs Quantum Dot Superlattices W. Ye, M. Reason, X. Weng, R.S. Goldman The mechanisms of lateral ordering of quantum dots (QDs) are the subject of continued debate. For example, anisotropic ``mounds'' are often observed during GaAs growth, but their formation has not been related to QD alignment. Therefore, we have examined the patterning effects of buffer layers on the growth of InAs/GaAs QD superlattices (SLs). Multi- period InAs/GaAs QD SLs were deposited on GaAs buffer layers grown at 580$^{o}$C and/or 500$^{o}$C, with various annealing steps. Atomic force microscopy reveals that high temperature grown buffers consist of relatively flat surfaces, while low temperature grown buffers contain $[1\bar {1}0]$-elongated ``mound-like'' features. Isotropic distributions of QDs are observed for QD growth on flat buffers. For QD growth on buffers containing mounds, QD alignment along the $[1\bar {1}0]$ direction is observed. This alignment is enhanced as the number of QD SLs increases and is dependent on the density of mounds. We propose a new mechanism for lateral QD alignment, which is based upon patterning by undulated In-enriched GaAs spacer layers following the initials sets of QD SLs. [Preview Abstract] |
Thursday, March 24, 2005 2:42PM - 2:54PM |
W40.00002: Effects of Wetting on Instabilities of Thin Solid Films Margo Levine, Alexander Golovin Effects of wetting interactions on instabilities of thin epitaxial solid films are studied. It is shown that wetting interactions between a film and the substrate suppress the long-wave modes of both Asaro-Tiller-Grinfeld instability and thermodynamic instability caused by anisotorpic surface energy. This yields a short-wave instability spectrum that can lead to the formation of spatially-regular surface structures. Pattern formation in such systems is analyzed by means of weakly nonlinear analysis and numerical simulations of continuum evolution equations describing the film shape. It is shown that pattern formation in such systems is strongly affected by the presence of the Goldstone mode associated with the conservation of mass. [Preview Abstract] |
Thursday, March 24, 2005 2:54PM - 3:30PM |
W40.00003: Self-organizing surface structures as templates for the creation of low-dimensional adsorbate systems Invited Speaker: The energetics of low-coordinated sites at surfaces play a key role in the formation of nanometer-scale structures. Examples to be discussed range from regular step arrays on vicinal silicon surfaces over atomically straight edges of NaCl islands to radiation-induced pits in KBr. All these structures can be imaged with atomic resolution by means of scanning tunneling and scanning force microscopy. These surface structures can be exploited to direct the growth of adsorbates. Evaporating gold on top of vicinal silicon surfaces results in the formation of atomic gold chains, with extra silicon adatoms positioned along the chains. The atomic pits in KBr have been succesfully used to contain organic molecules which show a high mobility on unstructured insulating surfaces. [Preview Abstract] |
Thursday, March 24, 2005 3:30PM - 3:42PM |
W40.00004: Roughening rates of strained–layer instabilities Fumiya Watanabe, David G. Cahill, J.E. Greene We study the evolution of the morphology of Si$_{0.75}$Ge$_{.025}$ strained layers using a wide range of deposition times, 60 $<$ \textit{$\tau $} $<$ 2400 s, at 600 \r{ }C on laser textured substrates with miscuts \textit{$\theta $} $<$ 15 \r{ } off Si(001). Ripple-shaped morphologies form spontaneously on miscuts along the $<$110$>$ directions inside the laser textured dimples. At the shortest deposition times, roughening is suppressed as predicted by a linear stability analysis that uses previously measured values for the mass transport rate on the surface. The origin of the instability is thermodynamic, and not caused by kinetic effects related to step motion, confirmed by the fact that the morphology of the rapidly deposited layers roughens with annealing. The exponential time constant of the roughening measured is $\approx $ 80 s; a factor of 4 larger than that predicted by theory. Linear instability analysis is not only sufficient in describing the morphological changes in the layers qualitatively, but also is adequate in deriving the quantitative rate of the roughening. [Preview Abstract] |
Thursday, March 24, 2005 3:42PM - 3:54PM |
W40.00005: Roughening evolution of the interfaces in two-component surface growth with an admixture of random deposition Alice Kolakowska, Mark Novotny, Poonam Verma We study universal properties of competitive two-component growth models on a one dimensional substrate of $L$ sites. One component is random deposition (RD). The other component generates correlations among lattice sites. We derive the universal scaling function of the interface width for the models where these correlations are either of the Kardar-Parisi-Zhang type [1] or of the Edwards-Wilkinson type, and show that the RD admixture acts as a dilatation mechanism to fundamental time and height scales. The RD blending does not change the universality class of the interface since it does not change the dynamics of mechanisms that are responsible for building correlations. However, such dilatation, when combined with an initial flat- substrate condition, may obscure a clear observation of the universal scaling in experiments. This research has been supported by the NSF grants DMR-0113049 and DMR-0120310, and used resources of the NERSC Center, which is supported by the Office of Science of the US DoE under contract No. DE-AC03-76SF00098.\\[4pt] [1] A. Kolakowska, M. A. Novotny, and P. S. Verma, Phys. Rev. E {\bf 70}, in press. [Preview Abstract] |
Thursday, March 24, 2005 3:54PM - 4:06PM |
W40.00006: Step Evolution Toward Equilibrium: Fokker-Planck Equation and the Wigner Surmise Alberto Pimpinelli, T. L. Einstein, Hailu Gebremariam The generalized Wigner surmise $P_w(s)=as^\varrho\exp(-bs^2)$, drawn from random-matrix theory, has been shown to provide arguably the best (both conceptually and quantitatively) description of the equilibrium terrace-width distribution (TWD) of steps on a vicinal surface,\footnote{Hailu Gebremariam, S.D. Cohen, H.L. Richards, \& T.L. Einstein, Phys. Rev. B {\bf 69}, 125404 (2004) and references therein.} but with limited formal justification for non-special $\varrho$.\footnote{H.L. Richards \& T.L. Einstein, cond-mat/0008089.} Using a mean-field approach to Dyson's Brownian motion model,\footnote{F.J. Dyson, J. Math. Phys. {\bf 3}, 1191 (1962).} we show that $P_w(s)$ can be derived from a Fokker-Planck equation, analogously to the derivation of the Heston model of econophysics.\footnote{A.A. Dragulescu and V.M. Yakovenko, Quantitative Finance {\bf 2}, 443 (2002) [cond-mat/0203046].} From this formulation we can find how the system evolves from some arbitrary initial distribution toward $P_w(s)$. For a simple initial TWD such as uniformly-spaced straight steps, we can find the solution analytically. In parallel we carry out Monte Carlo studies within the terrace-step-kink model$^2$ with such initial distributions and confirm that the variance of the TWD evolves as predicted. [Preview Abstract] |
Thursday, March 24, 2005 4:06PM - 4:18PM |
W40.00007: The Step Bunching Instability during Aqueous Silicon Etching Hailing Bao, Simon Garcia, Melissa Hines STM investigations of vicinal Si(111) surfaces etched in aqueous KOH solutions under controlled flow conditions show that macrostep formation (step bunching) is driven by inhomogeneities that develop in the etchant (not on the surface) as a result of the highly step-site-specific etching reactions. Using a microfabricated test pattern, we confirm that aqueous silicon etching is inherently non-local. Micron-scale inhomogeneities develop in the etchant, and these inhomogeneities lead to a step bunching instability. A kinetic Monte Carlo simulation of this process, which combines an atomistic description of the etching surface with a coarse-grained model of etchant inhomogeneities, is in qualitative agreement with the experimental findings. The position and orientation of the bunches can be controlled using microfabricated etch barriers. This technique allows the creation of large, nearly atomically flat regions even on severely miscut surfaces. [Preview Abstract] |
Thursday, March 24, 2005 4:18PM - 4:30PM |
W40.00008: Theoretical Analysis of Step Bunching Instability in presence of Misfit Strain and Ehrlich-Schwoebel Barrier Lugang Bai, Y.G. Zhao, M. Lagally, J. Tersoff, Feng Liu We develop a generic theoretical model to analyze step-flow of strained films, taking into account effects of both strain-induced step-step interaction and Ehrlich-Schwoebel (ES) step-edge barrier. In general, the strain induces a step bunching instability; a positive ES barrier (a barrier for an adatom to go downward over a step) stabilizes the step flow, while a negative ES barrier (a barrier for an adatom to go upward over a step) destabilizes the step flow. We will discuss the stability of step-flow growth in terms of relative strength of strain and ES barrier effects and compare with computer simulations. [Preview Abstract] |
Thursday, March 24, 2005 4:30PM - 4:42PM |
W40.00009: Morphology evolution in oxygen-induced faceting of Re (12-31) Hao Wang, Wenhua Chen, Ally S.Y. Chan, Theodore E. Madey The adsorption of oxygen on Re ($12\bar {3}1)$ has been studied by low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM). The atomically rough Re ($12\bar {3}1)$ surface remains planar at room temperature after being exposed to oxygen. However, the O/Re ($12\bar {3}1)$ surface can undergo drastic morphological changes to become completely faceted upon annealing at 700K or higher temperatures. With low oxygen coverages ($\sim $0.5ML), the facets form ridge-like structures and grow along the ridge direction [$\bar {2}113$]. The size of the ridges grows with annealing temperatures. The typical dimensions for the ridges are $\sim $8nm wide and $>$50nm long upon annealing at 1000K. The orientations of the two facets of the ridge are identified as ($11\bar {2}1)$ and ($01\bar {1}0)$ by LEED measurements, which are consistent with kinematical simulations of the LEED patterns and confirmed by STM measurements. When the oxygen coverage is about 1ML, the ridge-like structure is found to be truncated by a third set of facets in the annealing temperature range between 900K and 1300K. The faceted O/Re surfaces may not only provide us templates to grow ordered nano-structures but also are possible candidates to study structural sensitivity in catalytic reactions. [Preview Abstract] |
Thursday, March 24, 2005 4:42PM - 4:54PM |
W40.00010: Photoelectron spectromicroscopy study of a misfit layer compound, and its interaction with Cs Hans Starnberg, Matthias Kall\"ane, Sven Stoltz Misfit layer compounds are built of alternating MX and TX$_2$ layers, where M is e.g.~Sn or Pb, T is a transition metal, and X is S or Se. We have studied surfaces of (PbS)$_{1.13}$TaS$_2$ and their interaction with Cs, using the photoelectron microscopy beamline at MAX-lab, Sweden. By imaging the sample using emission from the Pb 5$d$ and Ta 5$d$ core levels, we resolved domains terminated by either PbS or TaS$_2$ layers. Imaging by Cs 4$d$ emission revealed larger sticking of Cs on the PbS terminated domains. Further information was obtained by measuring high-resolution spectra from different domains, before and after Cs deposition. We found that the PbS layers react with the Cs, while the TaS$_2$ layers are more inert. The results also indicated presence of substitutional defects. Cs intercalation of the kind observed in TaS$_2$ polytypes was not observed. [Preview Abstract] |
Thursday, March 24, 2005 4:54PM - 5:06PM |
W40.00011: Relationship between grain boundary structure and faceting: a combined multi-dimensional and coarse grained approach Denis Boyer, David Romeu We jointly apply a higher dimensional crystallography theory and symmetry principles of nonlinear dynamics to study the structure of grain boundaries in solids. These two complementary methods efficiently capture general effects of competition for space in crystals, and can easily deal with long, non-singular boundaries. Faceting can be interpreted by the fact that interfaces tend to be aligned either along the Bollmann O-lattice or a new related lattice. Good agreement is found between our predictions and numbers of microscopy experiments on tilt boundaries, both in metals and ceramics. [Preview Abstract] |
Thursday, March 24, 2005 5:06PM - 5:18PM |
W40.00012: Occupied states of clean Pt(997) and bimetallic Ag/Pt(997) surfaces Serban Smadici, Jerry Dadap, Richard Osgood Jr., Tonica Valla, Peter Johnson Stepped surfaces are ideal material systems that permit the growth of regular nanostructures over large areas. The electronic structure of these systems can be probed with photoemission spectroscopy. At low coverage, silver decorates the steps on Pt(997) surface, forming uniform rows with a coverage-dependent width. We report high-resolution angle-resolved photoemission measurements of occupied states of bare stepped Pt(997) and bimetallic stepped Ag/Pt(997) surfaces. The measurements, at the U13UB beamline at the National Synchrotron Light Source, with a typical energy and angle resolution of $\sim $ 5 meV and 5 mrad, respectively, were made at room and low temperatures. Band-structure measurements were carried out along high-symmetry directions parallel and perpendicular to the step edges. The development of Ag-derived states has been studied as a function of Ag coverage. Comparison of the measurements with results obtained using unoccupied state ultrafast photoemission will be discussed. This work was supported by the DOE under contract numbers DE-FG02-90ER14104 and DE-AC02-98CH10886. [Preview Abstract] |
Thursday, March 24, 2005 5:18PM - 5:30PM |
W40.00013: Kinetic pairing of steps Olivier Pierre-Louis, Jean-Jacques Metois A novel instability of crystal surfaces, the kinetic pairing of identical atomic steps, is investigated both theoretically and experimentally. We show that this instability can appear only when the dynamics are nonlocal. We study in more details the case of Si(111) at high temperature under electromigration. In this case, the non-locality comes from the transparency of the steps. The instability was observed in experiments. Comparison with the theory indicates that the step transparency kinetic coefficient is negative. With the help of a phase field model, we find that this result is the consequence of a faster diffusion of atoms in the step region. [1] O. Pierre-Louis and J.-J. M\'etois, Phys. Rev. Lett. 93, 165901 (2004). [Preview Abstract] |
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