Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session L8: Modeling and Measurement of Surface Morphology |
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Sponsoring Units: DCMP Chair: Mina Yoon, Oak Ridge National Laboratory Room: 006C |
Wednesday, March 4, 2015 8:00AM - 8:12AM |
L8.00001: A Computational Investigation of Random Angle Grain Boundaries for CdTe Solar Cells Christopher Buurma, Maria Chan, Robert Klie, Sivalingam Sivananthan Grain boundaries (GB) in poly-CdTe solar cells play an important role in species diffusion, segregation, defect formation, and carrier recombination. Many studies on GBs in CdTe focus on either entire grain-boundary networks found in complete poly-CdTe devices, those exhibiting high symmetry such as the coincident site lattice (CSL) or symmetric tilt or twist, or on very small scale Scanning-Tunneling Electron Microscopse (STEM) viewable interfaces and dislocations. The topic of this talk is a comprehensive survey of the grain boundary parameter space regardless of the degree of symmetry found and whether the STEM channeling condition is satisfied. Our survey encompasses both near-CSL or vicinal grain boundaries decorated with nearby dislocations, as well as mixed tilt and twist interfaces with all possible symmetrically inequivalent grain boundary planes. Atomistic calculations using a Stillinger-Weber potential will be presented on a large representative sample of random-angle GBs. Trends in interfacial energies and atomistic structures as a function of tilt/twist/displacement parameters will be investigated. First principles density functional theory (DFT) calculations will be performed on a subset of these GBs to reveal their electronic structures and their implications towards PV performance. DoE Sunshot program contract DOE DEEE005956. Use of the Center for Nanoscale Materials was supported by the USDoE, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Wednesday, March 4, 2015 8:12AM - 8:24AM |
L8.00002: The surface morphology of CuFeS$_2$: A hybrid-exchange density functional study Vincent Huair-Yu Chen, Ruth Martinez Casado, Giuseppe Mallia, Nicholas Harrison The surface morphology of CuFeS$_2$ has been determined using hybrid-exchange density functional theory calculations. The (110) surface is identified to be the most stable non-polar surface with a surface energy of 0.58 Jm$^{-1}$. The polar (112)/($\overline{112}$) surface pair is shown to be remarkably stable and thermodynamically preferred to the (110) surface if certain defects are included. The stability of the polar (112)/($\overline{112}$) surface pair is attributed to a combination of geometric and electronic mechanisms localised to the (112) surface which combine to neutralise the electrostatic dipole perpendicular to the surface. The former entails a partial reversal of the surface atomic layer sequence while the latter involves dispersion of charge from the subsurface anions to neighbouring cations. Because of its stability, (112) and ($\overline{112}$) facets always form a significant portion of the Wulff construction regardless of the growth conditions. [Preview Abstract] |
Wednesday, March 4, 2015 8:24AM - 8:36AM |
L8.00003: Characterizing Submonolayer Growth of 6P on Mica: Capture Zone Distributions vs. Growth Exponents and the Role of Hot Precursors T.L. Einstein, Josue Morales-Cifuentes, Alberto Pimpinelli Analyzing capture-zone distributions (CZD) using the generalized Wigner distribution (GWD) has proved a powerful way to access the critical nucleus size $i$.\footnote{TLE, AP, Diego Luis Gonz\'alez, J. Cryst. Growth 401, 67 (2014)} Of the several systems to which the GWD has been applied, we consider 6P on mica, for which Winkler's group found $i \approx 3$. Subsequently they measured the growth exponent $\alpha$ (island density $\propto F^\alpha$, for flux $F$) of this system and found good scaling but different values at small and large $F$, which they attributed to DLA and ALA dynamics, but with larger values of $i$ than found from the CZD analysis. We investigate this result in some detail. The third talk of this group describes a new universal relation between $\alpha$ and the characteristic exponent $\beta$ of the GWD. The second talk reports the results of a proposed model that takes long-known transient ballistic adsorption into account, for the first time in a quantitative way.\footnote{JRM-C, TLE, \& AP, PRL, in press} We find several intermediate scaling regimes, with distinctive values of $\alpha$ and an effective activation energy. One of these, rather than ALA, gives the best fit of the experimental data and a value of $i$ consistent with the CZD analysis. [Preview Abstract] |
Wednesday, March 4, 2015 8:36AM - 8:48AM |
L8.00004: How Hot Precursor Modify Island Nucleation: A Rate-Equation Model Josue Morales-Cifuentes, T.L. Einstein, Alberto Pimpinelli We describe the analysis, based on rate equations, of the hot precursor model mentioned in the previous talk.\footnote{J.R. Morales-Cifuentes, T.L. Einstein, \& A. Pimpinelli, PRL, in press} Two key parameters are the competing times of ballistic monomers decaying into thermalized monomers vs.\ being captured by an island, which naturally define a ``thermalization" scale for the system. We interpret the energies and dimmensionless parameters used in the model, and provide both an implicit analytic solution and a convenient asymptotic approximation. Further analysis reveals novel scaling regimes and nonmonotonic crossovers between them. To test our model, we applied it to experiments on parahexaphenyl (6P) on sputtered mica.\footnote{L. Tumbek \& A. Winkler, Surf. Sci. 606, L55 (2012)} With the resulting parameters, the curves derived from our analytic treatment account very well for the data at the 4 different temperatures. The fit shows that the high-flux regime corresponds not to ALA (attachment-limited aggregation) or HMA (hot monomer aggregation) but rather to an intermediate scaling regime related to DLA (diffusion-limited aggregation). We hope this work stimulates further experimental investigations. [Preview Abstract] |
Wednesday, March 4, 2015 8:48AM - 9:00AM |
L8.00005: Scaling and Exponent Equalities in Island Nucleation: Novel Results and Application to Organic Films Alberto Pimpinelli, Levent Tumbek, and Adolf Winkler Alberto Pimpinelli, Levent Tumbek, Adolf Winkler As discussed in the first talk, the scaling of the island density with the flux $F$ and/or the capture zone distribution (CZD) can be used to determine the size of the critical nucleus $i$, but so far an analytic function for CZD exists only for diffusion-limited aggregation (DLA). For CZD the scaling function is $P_\beta(s) = a_{\beta} s^{\beta}\exp(-b_{\beta} s^2)$, with $\beta = i+2$. We have extended the analytic description of the CZD in terms of $P_\beta$ also to attachment-limited aggregation (ALA); in this case we obtain $\beta = (i+3)/2$. Furthermore, we could demonstrate that the general relationship $\alpha \beta = i$ holds, independent of the aggregation mechanism.\footnote{A. Pimpinelli, L. Tumbek, \& A. Winkler, J. Phys. Chem. Lett. 5, 995 (2014)} This important exponent equality should help to better characterize nucleation and growth of thin films. [Preview Abstract] |
Wednesday, March 4, 2015 9:00AM - 9:12AM |
L8.00006: A spatio-temporal model of wrinkling in photopolymerised networks Matthew Hennessy, Alessandra Vitale, Paul Stavrinou, Omar Matar, Joao Cabral Photopolymerisation is a common solidification process whereby crosslinked polymer networks are created by illuminating a monomer-rich bath with collimated light. In addition, photopolymerisation is extensively employed industrially and shows exceptional promise for advanced three-dimensional patterning of functional surfaces. Under conditions of strong optical attenuation and limited mass and thermal diffusion, polymerisation occurs in a localised region which propagates from the illuminated surface into the bulk as a travelling wave with a planar wavefront. Under specific conditions that we set out to map, this planar wavefront may become unstable and the surface of the resulting gel can acquire a wrinkled morphology. We believe this instability is mechanical in nature and arises from compressive stresses that are generated during frontal photopolymerization. In this talk, we will present a novel mathematical model that captures both the photopolymerisation with wrinkling processes. We show that by coupling photopolymerisation with wrinkling in a controlled manner, a number of interesting and industrially relevant patterns can be achieved. [Preview Abstract] |
Wednesday, March 4, 2015 9:12AM - 9:24AM |
L8.00007: Ginzburg-Landau theory of the bcc-liquid interface kinetic coefficient Kuo-An Wu, Jeffrey Hoyt, Alain Karma We extend the Ginzburg-Landau (GL) theory of atomically rough bcc-liquid interfaces outside of equilibrium to derive an analytical expression for the kinetic coefficient $\mu(\hat n)$. The kinetic coefficient is expressed as a spatial integral along the normal direction of a sum of gradient square terms corresponding to different nonlinear density wave profiles. Anisotropy arises naturally from the dependence of those profiles on the angles between the principal reciprocal lattice vectors $\vec K_i$ and $\hat n$. Values of the kinetic coefficient for the $(100)$, $(110)$ and $(111)$ interfaces are compared quantitatively to the prediction of linear Mikheev-Chernov (MC) theory and previous MD simulation studies of crystallization kinetics for a classical model of Fe. The GL theory predicts a similar expression for $\mu$ as the MC theory but yields a better agreement with MD simulations for both its magnitude and anisotropy due to a fully nonlinear description of density wave profiles across the solid-liquid interface. In particular, the overall magnitude of $\mu$ predicted by GL theory is an order of magnitude larger than predicted by the MC theory. GL theory is also used to derive an inverse relation between $\mu$ and the solid-liquid interfacial free-energy. [Preview Abstract] |
Wednesday, March 4, 2015 9:24AM - 9:36AM |
L8.00008: Island Dynamics Model for Mound Formation: Effect of a Step-Edge Barrier Christian Ratsch, Dionisios Margetis, Frederic Gibou We have developed an island dynamics model for epitaxial growth with the level-set technique, where islands are treated as continuous in the x-y-plane, while individual atomic layers are resolved in the z-direction. Adatoms are treated as a mean field quantity by solving a diffusion equation. We will discuss an analytic derivation for the proper expression for the equilibrium adatom density at the step edge in the presence of a step-edge barrier. The effect of an additional step-edge barrier is incorporated via a mixed Robin-type boundary condition for the diffusion equation. We will present a numerical scheme to solve such a boundary condition on a fixed grid with moving boundaries. We will show how the inclusion of the step-edge barrier leads to the formation of mounds that become progressively steeper as the step-edge barrier increases. Finally, we will discuss how we can include the effect of downward funneling in our model, and how it leads to the stabilization of the slope of the mounds. [Preview Abstract] |
Wednesday, March 4, 2015 9:36AM - 9:48AM |
L8.00009: In-situ probing and modeling atomic layer deposition processes on Ge surface Yuanxia Zheng, Sungwook Hong, Bruce Rayner, Suman Datta, Adri Van Duin, Roman Engel-Herbert Germanium (Ge) is a promising CMOS compatible channel material with a low effective-mass of holes. One of the major challenges in developing Ge-FETs is integrating a high-quality gate-stack on Ge. A direct high-k dielectric deposition like HfO$_{2}$ on Ge has resulted in poor electrical characteristics of the semiconductor-dielectric interface.$^{1}$ GeO$_{2}$/Ge interface has been found low in interface-trap density, but its quality rapidly degraded when scaling down to ultrathin GeO$_{2}$.$^{2}$ Takagi \textit{et al }showed that such interface quality can be preserved using an ultrathin Al$_{2}$O$_{3}$ layer on GeO$_{2}$/Ge, but the detailed mechanism has not been addressed and remained elusive.$^{3}$ In this work, we studied this problem by combining (a) \textit{in-situ} spectroscopic ellipsometry for real-time monitoring of atomic-layer-deposition (ALD) processes on Ge, (b) \textit{ex-situ} X-ray photoelectron spectroscopy (XPS) to probe the interface chemistry, and (c) reactive force field (ReaxFF) simulations to directly model the growth kinetics and interface formation. A strong surface-chemistry dependence (hydrogenated Ge vs oxidized Ge) has been found in the Al$_{2}$O$_{3}$-ALD nucleation (Trimethylaluminum$+$H$_{2}$O), which is well reproduced by ReaxFF simulation. Furthermore, both experiments and simulations revealed that the Al$_{2}$O$_{3}$ capping on GeO$_{2}$/Ge interface prevents oxygen diffusion into Ge, and therefore stabilizes the interface. [1] Appl. Phys. Lett. 87, 032107 (2005). [2] J. Appl. Phys. 106, 104117 (2009). [3] 2012 Symp. VLSI Technol. 2011--2012 (2012). [Preview Abstract] |
Wednesday, March 4, 2015 9:48AM - 10:00AM |
L8.00010: Structural evolution of Ag nanoparticles during electron driven synthesis of Ag filaments on Ag$_{2}$WO$_{4}$: \textit{In situ} observation and theoretical supporting evidence Edison Z. da Silva, Wyllamanney da Silva Pereira, Juan Andr\'es, Lourdes Gracia, Miguel San-Miguel, Elson Longo, Valeria M. Longo $\alpha -$Ag$_{2}$WO$_{4}$ crystals irradiated by an electron beam from an electron microscope under high \textit{vacuum}, nucleate metallic Ag, and form Ag metallic nanowires on the $\alpha $ crystals surface. In order to understand this interesting and complex behavior of the formation and growth of Ag nanowires on $\alpha $-Ag$_{2}$WO$_{4}$ we investigated by detailed in situ transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM) studies, density functional theory calculations and \textit{ab initio} molecular dynamics (MD) simulations. First principle calculations point out that Ag-3 and Ag-4 atoms, located on the (100) surface, are the most energetically favorable to undergo the diffusion process to form metallic Ag. \textit{Ab initio} MD simulations and nudged elastic band (NEB) method were used to investigate the minimum energy pathways for diffusion of Ag atoms to outward sites on the (100) surface. The results point out that the injection of electrons decreases the activation barrier for this diffusion step and this unusual behavior results from the presence of a lower energy barrier process. [Preview Abstract] |
Wednesday, March 4, 2015 10:00AM - 10:12AM |
L8.00011: Kinetic Transition of Crystal Morphology from Nanoparticles to Dendrites during Electron Beam Induced Deposition of Gold Jeung Hun Park, Nicholas Schneider, Haim Bau, Suneel Kodambaka, Frances Ross We studied the kinetic transition from compact nanoparticle to dendritic morphology during electron beam-induced Au deposition using \textit{in situ} liquid cell-based transmission electron microcopy. Radiolysis of water by electrons generates radicals and molecular species. Hydrated electrons and hydrogen and hydroxide radicals can act as reducing agents and initiate the reduction of the water-soluble precursor, HAuCl$_{4}$, resulting in the precipitation of Au as nanostructures. We tracked nucleation, growth, and morphological transition of Au from movies recorded \textit{in situ}, as a function of irradiated dose and liquid thickness. We identified several distinct regimes that depend on the irradiation time: (1) nucleation; (2) linear volumetric growth; (3) formation of dendritic structures; (4) coalescence and dissolution. A diffusion and reaction model for the radiolytic species and metal ions in the confined geometry of the irradiated volume is used to understand the nucleation sites and morphological transitions. We finally describe how nanoparticles can be made to grow in a stepwise manner by switching the supply of Au ions on and off electrochemically, and discuss possibilities for creating more complex nanostructures. [Preview Abstract] |
Wednesday, March 4, 2015 10:12AM - 10:24AM |
L8.00012: Possible detection of surface melting on solid hydrogen by TOF-SIMS Taku Suzuki The molecular hydrogens form the simplest of all molecular solids. Thus, the irradiation effect of ion beams on quench-condensed hydrogen film is conceptually the simplest of all molecular condensed systems and therefore is an ideal benchmark system for testing theories. However, the number of studies concerning ion beam irradiation effect on solid hydrogen is quite limited. In the present study, we have investigated secondary ion emission from quench-condensed hydrogen films under ion beam irradiation. To prepare the quench-condensed hydrogen films, we developed ultra-high vacuum (UHV) -- compatible cryostat, which enables sample cooling to 4 K. The UHV chamber was equipped with an ion gun with an electrostatic deflector for chopping and an electrostatic energy analyzer. The continuous 2 keV He$+$ ion beam was utilized for ion scattering spectroscopy (ISS), while the ion beam was chopped for time-of-flight secondary ion mass spectroscopy (TOF-SIMS). The sample was prepared by the exposure of a polycrystalline We found the enhancement of the H$+$ ion emission with decreasing the sample temperature. The enhancement of the secondary H$+$ ion emission is most likely related with the solid-liquid phase transition. [Preview Abstract] |
Wednesday, March 4, 2015 10:24AM - 10:36AM |
L8.00013: Surface nanopatterning using electric-field-driven assembly of single-layer epitaxial islands Ashish Kumar, Dwaipayan Dasgupta, Dimitrios Maroudas We report a systematic simulation study of an approach to surface nanopatterning based on electric-field-driven assembly of single-layer epitaxial islands on face-centered cubic crystalline substrates. We have developed and validated a fully nonlinear driven island evolution model with diffusional mass transport limited to the island edges and accounting for edge diffusional anisotropy and island coalescence and break-up. For islands on $\left\langle {110} \right\rangle $-, $\left\langle {100} \right\rangle $-, and $\left\langle {111} \right\rangle $-oriented substrate surfaces, we report formation of complex nanopatterns starting from two different types of initial configurations: a single island with larger-than-critical size and an assembly of relatively small islands, which undergo a sequence of coalescence and break-up events. For both initial configurations, we study the dependence of the nanopattern features on the duration of application of the electric field, the strength of edge diffusional anisotropy, and the misorientation angle between a fast edge diffusion direction and the applied electric field direction. For assemblies of islands, we also study the resulting nanopattern dependence on the intrinsic geometrical parameters of the assembly. We report entire classes of complex patterns formed as the above parameters are varied. [Preview Abstract] |
Wednesday, March 4, 2015 10:36AM - 10:48AM |
L8.00014: Nonlinear Analysis of Secondary Ripple Formation on Surfaces of Stressed Crystalline Solids Lin Du, Dwaipayan Dasgupta, Dimitrios Maroudas The competition between surface energy and elastic strain energy in surfaces of stressed crystalline solids is responsible for Asaro-Tiller/Grinfeld (surface cracking) morphological instabilities. Using linear stability theory (LST), we have predicted that properly directed and sufficiently strong electric fields and thermal gradients can inhibit such instabilities; we validate the LST predictions based on self-consistent dynamical simulations according to a fully nonlinear surface evolution model. The simulations also reveal that long-wavelength perturbations from the planar surface morphology can trigger a tip-splitting instability, which causes formation of a pattern of secondary ripples that is beyond the scope of LST. Based on weakly nonlinear analysis, we have developed a theory that can explain the occurrence of such rippling instabilities and predict the number of ripples that form on the surface as a function of perturbation wavelength. The theory predicts the critical wavelength for the onset of secondary ripple formation and the external field strength requirement for planar surface stabilization. The conclusions of the theory are validated by comparisons with the results of the self-consistent numerical simulations. [Preview Abstract] |
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