Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session C47: Surface, Growth, and Morphology |
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Sponsoring Units: DCMP Room: 312 |
Monday, March 14, 2016 2:30PM - 2:42PM |
C47.00001: Observation of heterodyne and homodyne mixing in X-ray photon correlation spectroscopy during thin film deposition Randall Headrick, Jeffrey Ulbrandt, Meliha Rainville, Christa Wagenbach, Suresh Narayanan, Alec Sandy, Hua zhou, Karl Ludwig The properties of artificially grown thin films are often strongly affected by the dynamic relationship between surface growth processes and subsurface structure. Coherent mixing of X-ray signals promises to provide an approach to better understand such processes. Continuously variable mixing of surface and bulk scattering signals during real-time studies of sputter deposition of a-Si and a-WiSi$_2$ films has been observed by controlling the X-ray penetration and escape depths in coherent grazing incidence small angle X-ray scattering (Co-GISAXS). Under conditions where the X-ray signal comes from both the growth surface and the thin film bulk, oscillations in temporal correlations arise from coherent interference between scattering from stationary bulk features and from the advancing surface. The absence of oscillations at larger in-plane wavevector transfer is interpreted as evidence that elongated bulk features propagate upward at the same velocity as the surface. Additionally, a highly surface sensitive mode is demonstrated that can access the surface dynamics independently of the subsurface structure. [Preview Abstract] |
Monday, March 14, 2016 2:42PM - 2:54PM |
C47.00002: Unusual ``Explosive'' Nucleation and Superdiffusion in Pb/Si(111)-7x7 M. T. Hershberger, M. Hupalo, P. A. Thiel, H. Hattab, M. Horn von Hoegen, M. C. Tringides The study of the recently found ``explosive'' nucleation on Pb/Si(111) was further investigated to understand the origin of the sharp transition in coverage and the presence of superdiffusive motion. After small stepwise depositions of \textasciitilde 0.03 ML spatial correlations in the growth direction of neighboring islands are observed. The island growth rates are much higher than what is expected in classical nucleation. Islands collect material many times larger than the amount deposited in the surrounding Voronoi areas contrary to classical expectations. Their centers of mass shift by large amounts, \textasciitilde 10nm, again confirming the directionality in their growth and that material must be arriving over mesoscale distances. The island size distributions do not agree with the expected sharply peaked classical distributions since only fully completed islands are observed. Further depositions show additional nucleation of smaller islands and with higher densities thus conforming that when the critical coverage is reached locally the island nucleation is still active. Comparisons will be made between the diffusion length deduced in these experiments and the one extracted from earlier LEEM experiments monitoring the refilling of an initial vacant area. [Preview Abstract] |
Monday, March 14, 2016 2:54PM - 3:06PM |
C47.00003: Submonolayer island growth with anomalous diffusion Jacques Amar, Mikhael Semaan Island nucleation and growth play an important role in the early stages of thin-film growth. Of particular interest is the exponent $\chi$ which describes the dependence of the peak island density on deposition flux, and which also depends sensitively on the critical island-size $i$. While the dependence of $\chi$ on $i$ is known for normal diffusion, the case of anomalous monomer diffusion is also of interest, since this appears to play a role in recent experiments. Here we derive general expressions for $\chi$ which are valid for arbitrary substrate dimension, island fractal dimension, critical island size, and monomer diffusion exponent $\mu$. Excellent agreement is obtained between our predictions and kinetic Monte Carlo simulations carried out for the case of irreversible growth ($i=1$), and monomer superdiffusion with $1<\mu\le 2$, although unusually large crossover effects are also observed. These results also confirm and generalize a previous prediction for the case of ballistic diffusion ($\mu=2$). We also consider the case of monomer subdiffusion corresponding to $0\le\mu < 1$. Good agreement with our predictions for $\chi(\mu)$ is also found in this case, although the general scaling behavior is more complex due to the presence of large fluctuations. [Preview Abstract] |
Monday, March 14, 2016 3:06PM - 3:18PM |
C47.00004: Interaction of Dysprosium with the Basal Plane of Graphite. Patricia Thiel, Emma Kwolek, Huaping Lei, Yinghui Zhou, Ann Lii-Rosales, Mark Wallingford, Cai-Zhuang Wang, Michael Tringides We have studied adsorption, nucleation, growth, intercalation, and carburization of dysprosium (Dy) on graphite surfaces, and the way that these phenomena are influenced by surface defects. The experiments consist of scanning tunneling microscopy in ultrahigh vacuum. The condensation coefficient of Dy is strongly enhanced by surface defects. In the absence of defects, at room temperature, homogeneous nucleation can occur on terraces, consistent with DFT calculations of energetics. At room temperature, island shapes consist of a flat three-atom base, decorated by single-atom upper layers. Layer populations in such islands are analyzed in the context of a tailored analytic model, which provides information about interlayer mobility. At elevated temperature, Dy can intercalate with the graphite substrate and can form carbide. [Preview Abstract] |
Monday, March 14, 2016 3:18PM - 3:30PM |
C47.00005: Thermal evolution of Fe on $Ge(111)-c(2\times8)$ surface and the effect of $(\sqrt{3}\times\sqrt{3})$ Ag-Ge buffer layer Tsu-Yi Fu, Hung-Chang Hsu, Ming-Kuan Jhou, Jia-Yuan Wu Using scanning tunneling microscopy, two systems of Fe deposition on a clean $Ge(111)-c(2\times8)$ surface and surfaces with a $(\sqrt{3}\times\sqrt{3})$ $R30^{o}$ Ag-Ge buffer layer were compared. Complex surface alloy structures were easily formed on Fe/Ge systems through annealing at 300-650 K. On clean Ge (111) surfaces, similar surface morphology evolution was observed when two different amounts of Fe were deposited. To reduce the complexity, $(\sqrt{3}\times\sqrt{3})$ $R30^{o}$ Ag-Ge interface were used as buffer layers. The growth morphologies differed in the presence and absence of the buffer layers. After heat treatment to 570 K, $(2\times2)$ reconstruction platform islands were formed in the Fe/Ge system, which transformed to three-dimensional (3D) islands at 640 K. With Ag buffer layer, only nanoparticle growth occurred and 3D islands were formed early at 570 K. Generally, $\sqrt{19}$ ring clusters increased to break the order $c(2\times8)$ reconstruction by increasing the temperature and disappeared at 640 K in Fe-Ge system, but only $\sqrt{7}$ ring clusters appeared at 390 K with $(\sqrt{3}\times\sqrt{3})$ $R30^{o}$ Ag-Ge buffer layer. [Preview Abstract] |
Monday, March 14, 2016 3:30PM - 3:42PM |
C47.00006: Free energy of steps using atomistic simulations Rodrigo Freitas, Timofey Frolov, Mark Asta The properties of solid-liquid interfaces are known to play critical roles in solidification processes. Particularly special importance is given to thermodynamic quantities that describe the equilibrium state of these surfaces. For example, on the solid-liquid-vapor heteroepitaxial growth of semiconductor nanowires the crystal nucleation process on the faceted solid-liquid interface is influenced by the solid-liquid and vapor-solid interfacial free energies, and also by the free energies of associated steps at these faceted interfaces. Crystal-growth theories and mesoscale simulation methods depend on quantitative information about these properties, which are often poorly characterized from experimental measurements. In this work we propose an extension of the capillary fluctuation method for calculation of the free energy of steps on faceted crystal surfaces. From equilibrium atomistic simulations of steps on (111) surfaces of Copper we computed accurately the step free energy for different step orientations. We show that the step free energy remains finite at all temperature up to the melting point and that the results obtained agree with the more well established method of thermodynamic integration if finite size effects are taken into account. [Preview Abstract] |
Monday, March 14, 2016 3:42PM - 3:54PM |
C47.00007: Low temperature Silicon epitaxy on bare Si (100) and H terminated Si (100) surfaces Xiao Deng, Pradeep Namboodiri, Kai Li, Xiqiao Wang, Tongbao Li, Richard Silver Silicon on Silicon growth morphology is studied using an ultrahigh vacuum scanning tunneling microscopy (UHV-STM) and transmission electron microscopy (TEM). Sub-monolayer to 18 nm of silicon was evaporated using an all-silicon sublimation source (SUSI) onto a UHV prepared Si (100) sample at 250\textdegree C. The results are compared with the growth characteristics on hydrogen passivated surfaces (H: Si) under identical experimental conditions. STM images indicate that growth morphology of both Si on Si and Si on H: Si is of epitaxial nature at temperatures as low as 250\textdegree C. For Si on bare Si growth at 250\textdegree C, there exists a stable thickness regime where Si epitaxial growth front keeps the same morphology. Although the mobility of silicon is modestly affected on the H: Si surface because of the H atoms during the initial sub-monolayer regime, the growth proceeds epitaxially with the 3D island growth mode and noticeable surface roughening. [Preview Abstract] |
Monday, March 14, 2016 3:54PM - 4:06PM |
C47.00008: A diffuse interface model of grain boundary faceting Fadi Abdeljawad, Douglas Medlin, Jonathan Zimmerman, Khalid Hattar, Stephen Foiles Incorporating anisotropy into thermodynamic treatments of interfaces dates back to over a century ago. For a given orientation of two abutting grains in a pure metal, depressions in the grain boundary (GB) energy may exist as a function of GB inclination, defined by the plane normal. Therefore, an initially flat GB may facet resulting in a hill-and-valley structure. Herein, we present a diffuse interface model of GB faceting that is capable of capturing anisotropic GB energies and mobilities, and accounting for the excess energy due to facet junctions and their non-local interactions. The hallmark of our approach is the ability to independently examine the role of each of the interface properties on the faceting behavior. As a demonstration, we consider the $\Sigma 5 \ \langle 001 \rangle$ tilt GB in iron, where faceting along the $\{ 310 \}$ and $\{ 210 \} $ planes was experimentally observed. Linear stability analysis and numerical examples highlight the role of junction energy and associated non-local interactions on the resulting facet length scales. On the whole, our modeling approach provides a general framework to examine the spatio-temporal evolution of highly anisotropic GBs in polycrystalline metals. [Preview Abstract] |
Monday, March 14, 2016 4:06PM - 4:18PM |
C47.00009: External-Field-Driven Nanopatterning on Crystalline Substrate Surfaces Ashish Kumar, Dwaipayan Dasgupta, Dimitrios Maroudas Current-driven dynamics of single-layer epitaxial islands on fcc crystalline substrates can lead to surface pattern formation with significant implications for nanofabrication. We have developed and validated a fully nonlinear model of driven island evolution on $\{110\}$, $\{100\}$ and $\{111\}$ substrate surfaces due to diffusional mass transport along the island edge and accounting for edge diffusional anisotropy. We find that the migration speed of a morphologically stable island is inversely proportional to the island size, $R$, up to a critical size that marks the onset of island morphological transition; further increase in $R$ triggers edge fingering and/or necking or dynamical transitions. We report formation of complex nanopatterns emerging from individual larger-than-critical islands with two different types of initial configuration: a slender, high-aspect-ratio island shape and an equilibrium, rounded morphology. We have developed a linear stability theory that explains the observed morphological instabilities. We characterize the nanopatterns formed and study the dependence of the nanopattern features on the duration of application of the electric field and the misorientation angle between a fast edge diffusion direction and the electric field direction. [Preview Abstract] |
Monday, March 14, 2016 4:18PM - 4:30PM |
C47.00010: Progress in Application of Generalized Wigner Distribution to Growth and Other Problems T.L. Einstein, Josue Morales-Cifuentes, Alberto Pimpinelli, Diego Luis Gonz\’alez We recap the use of the (single-parameter) Generalized Wigner Distribution (GWD) to analyze capture-zone distributions associated with submonolayer epitaxial growth \footnote{TLE, AP, DLG, J. Cryst. Growth 401, 67 (2014); TLE, JRM-C, AP, DLG, J. Phys. Conf. Ser. 640, 012024 (2015).}. We discuss recent applications to physical systems, as well as key simulations. We pay particular attention to how this method compares with other methods to assess the critical nucleus size characterizing growth. The following talk discusses a particular case when special insight is needed to reconcile the various methods. We discuss improvements that can be achieved by going to a 2-parameter fragmentation approach. At a much larger scale we have applied this approach to various distributions in socio-political phenomena (areas of secondary administrative units [e.g., counties] and distributions of subway stations). [Preview Abstract] |
Monday, March 14, 2016 4:30PM - 4:42PM |
C47.00011: Role of Transient Mobility on Submonolayer Island Growth: Extensions and Testing Josue Morales Cifuentes, Theodore Einstein, Alberto Pimpinelli In studies of epitaxial growth a major goal is assessing the smallest stable cluster ($i+1$ monomers, with $i$ the critical nucleus size), by analyzing the capture zone distribution (CZD) or the scaling of incident flux $F$ to the density of stable islands $N$ ($N \propto F^\alpha$, with $\alpha$ the growth exponent). As noted in the previous talk, the GWD has well described the data in several experiments, including submonolayer para-hexaphenyl (6P) on amorphous mica (i $\approx$ 3). Different scaling ($F^\alpha$) for 6P at (small) large $F$ is attributed to (DLA) ALA dynamics, i.e. $i = (5) 7 \pm 2$.\footnote{L. Tumbek & A. Winkler, Surf. Sci. 606, L55 (2012)} Our recent theoretical work considered monomers propagating ballistically before thermalizing or attaching to islands, leading to scaling, non-monotonic crossover, and activation energies that account for the data and reconciling the values of $i$ \footnote{J. R. Morales-Cifuentes, T. L. Einstein, and A. Pimpinelli. Phys. Rev. Lett. 113, 246101 (2014)}. We present applications to other experimental systems: 6P on SiO$_2$ and pentacene (5A) on amorphous mica. We describe useful simplifying approximations, and preliminary kinetic Monte Carlo simulations including transient effects on growth. [Preview Abstract] |
Monday, March 14, 2016 4:42PM - 4:54PM |
C47.00012: Unravelling the ``Silicene'' Growth Mechanism Based on a Seeding Layer Approach Wei Jiang, Miao Zhou, Feng Liu Unlike \textit{sp}2 graphene, silicon atoms prefer to form \textit{sp}3 hybridized state that gives silicene a buckled geometry. To study how to grow flat silicene, we have investigated the structure and stability of multi-layer ``silicene'' using \textit{ab initio }methods by introducing a ``seeding layer'' of silicene on which additional ``silicene'' layers are grown. The buckling height and the isotropic strain of the seeding layer is shown to play a key in affecting the structure, in particular the flatness of the growing layers. A phase diagram in the parameter space of buckling height and in-plane strain of the seeding layer is constructed to guide the growth of additional ``silicene'' layer. Furthermore, in contrast to monolayer silicene growth on Ag substrate which exhibits various patterns, only the $\surd $3-$\surd $3 pattern is found stable using large supercell calculations. Our calculations suggest that thermodynamically no silicene structures can survive beyond three layers. These results will shed useful lights on experimental growth of flat and low-buckled silicene and help explain existing experimental results. [Preview Abstract] |
Monday, March 14, 2016 4:54PM - 5:06PM |
C47.00013: Current-Driven Nanowire Formation on Crystalline Conducting Substrate Surfaces Dwaipayan Dasgupta, Ashish Kumar, Dimitrios Maroudas Using a simulation study, we demonstrate a new, driven-assembly-based approach to single-layer nanowire formation on fcc crystalline substrate surfaces. In this approach, we manipulate individual epitaxial islands using an external electric field to drive the formation of single nanowires or arrays of them. We have developed and validated a fully nonlinear model of current-driven island evolution mediated by diffusional mass transport along the island edge and accounting for edge diffusional anisotropy and island coalescence and breakup. Using a linear stability theory, we analyze the morphological stability of islands with equilibrium shapes and predict the occurrence of morphological instability for islands larger than a critical size under the action of an electric field along the slowest edge diffusion direction on $\{110\}$, $\{100\}$, and $\{111\}$ substrate surfaces. Consistent with the theoretical prediction, dynamical simulations show that large-size islands undergo a fingering instability which, following finger growth and, depending on the substrate orientation, necking instability, leads to formation of single or multiple nanowires. We find that the nanowires have constant widths, on the order of tens of nanometers, and explain analytically the nanowire dimensions. [Preview Abstract] |
Monday, March 14, 2016 5:06PM - 5:18PM |
C47.00014: Nonlinear Analysis of the Surface Morphological Stability of Stressed Crystalline Materials and Coherently Strained Epitaxial Thin Films Lin Du, Dwaipayan Dasgupta, Dimitrios Maroudas The competition between surface energy and elastic strain energy in surfaces of stressed solids may cause the Asaro-Tiller/Grinfeld (ATG) instability leading to surface cracking, which can be predicted by linear stability theory (LST). Self-consistent dynamical simulations based on a fully nonlinear surface evolution model reveal that, in addition to the ATG instability, long-wavelength perturbations from the planar surface morphology can also trigger a tip-splitting instability, causing the formation of a pattern of secondary ripples that cannot be predicted by LST. We have developed a weakly nonlinear stability theory that can explain the occurrence of such nonlinear rippling instabilities and predict the critical wavelength for secondary ripple formation as well as the number of secondary ripples that form on the surface as a function of perturbation wavelength. We also have applied the weakly nonlinear theory to study the surface morphological stability of a coherently strained epitaxial thin film on a crystalline elastic substrate. We find that, in addition to the Stranski-Krastanow instability, secondary rippling instabilities may also occur on the film surface, leading to formation of smaller-sized quantum dots (QDs) through tip splitting of larger QDs. [Preview Abstract] |
Monday, March 14, 2016 5:18PM - 5:30PM |
C47.00015: Triple line kinetics in solid-state dewetting Ashwani Tripathi, Olivier Pierre-Louis Solid-state dewetting has been studied in a large number of experimental systems, such as SOI (Si on amorphous Si$O_2$) \footnote{E Bussmann, F Cheynis, F Leroy, P Müller and O. Pierre-Louis, New J. Phys. 13 043017 (2011).}, or metal films on various substrates \footnote{C.V. Thompson, Annu. Rev. Mater. Res. {\bf 42}, 399 (2012).}. Several theoretical approaches have been proposed in the past 10 years to understand this phenomena, ranging from Kinetic Monte Carlo to phase field, and continuum Mullins-like models. We present a continuum model \footnote{ A. Tripathi, O. Pierre-Louis, preprint.} which addresses the question of the limit of validity of the usual approximation of a constant contact angle at the triple line between the solid, the film, and the vacuum (or vapor). Our results suggest that the Young relation is subject to systematic deviations, which could be measured in experiments. In addition, the so-called mass shedding effect, which leads to the breakup of the film at a finite distance from the triple-line, can be accelerated by orders of magnitude due to wetting effects. [Preview Abstract] |
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