Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session D40: Focus Session: Morphology and Evolution at Surfaces: Epitaxy |
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Sponsoring Units: DMP DCMP Chair: Jim Hannon, IBM Research Room: LACC 408A |
Monday, March 21, 2005 2:30PM - 3:06PM |
D40.00001: Effects of Steering and Shadowing in Epitaxial Growth Invited Speaker: While shadowing has been known to play a role in some thin-film deposition processes, until recently it has been assumed that in epitaxial growth the effects of steering and shadowing are negligible. Here we present analytical and molecular dynamics results describing the effects of steering due to the short-range and long-range van der Waals (vdW) attraction in metal (100), (111) and (110) epitaxial growth. Our results lead to a general picture of the process of deposition near step-edges$^{1,2}$ which is quite different from the standard downward funneling picture. In particular, we find that short-range attraction plays an important role not only {\it before} but also {\it after} collision with the step. As a result, it can significantly enhance the uphill current, selected mound angle, and surface roughness in epitaxial growth. In the case of deposition on metal (111) and (110) surfaces we also find a significant asymmetry between the interaction at A and B steps which may be explained by differences in the step geometry. General expressions for the surface current and selected mound angle valid for arbitrary crystal geometry are also presented. We have also calculated the vdW constant describing the long-range interaction between a Cu atom and a Cu(100) surface.$^3$ Our result is large enough to explain recent observations$^4$ of a significant increase in mound angle in Cu/Cu(100) growth for large angles of incidence ($\theta > 50^{\circ}$) and also indicates that for smaller angles of incidence the dominant effects are due to the short-range rather than to the long-range interaction. Finally, we discuss the effects of shadowing in oblique incidence epitaxial growth and its implications for the control of nanoscale patterning.\\ 1. J. Yu and J.G. Amar, Phys. Rev. Lett. {\bf 89}, 286103 (2002).\\ 2. J. Yu and J.G. Amar, Phys. Rev. B {\bf 69}, 045426 (2004).\\ 3. J.G. Amar, Phys. Rev. B {\bf 67}, 165425 (2003).\\ 4. S. van Dijken et al, Phys. Rev. B {\bf 61}, 14047 (2000). [Preview Abstract] |
Monday, March 21, 2005 3:06PM - 3:18PM |
D40.00002: Growth instability in Cu multilayer films due to fast edge/corner diffusion Y. Shim, Jacques G. Amar The instability observed in Cu/Cu (100) epitaxial growth [Ernst et al, Phys. Rev. Lett. 72, 112 (1994)] is studied by using a recently proposed parallel algorithm for kinetic Monte Carlo (KMC) simulations with realistic parameters obtained from effective medium theory. The parallel algorithm allows us to simulate longer time scales which are not easily accessible by a serial Monte Carlo simulation. Unlike the proposed mechanism for the change in the value of the growth exponent from 0.26 to 0.56 with increasing temperature from 160 K to 200 K, simulation results show that an enhancement of edge and corner diffusion with temperature is responsible for the change during the low temperature growth. A similar parallel simulation study using parameters obtained from embedded atom methods is also presented. [Preview Abstract] |
Monday, March 21, 2005 3:18PM - 3:30PM |
D40.00003: Short range steering effect in off-normal incidence epitaxial growth on dense steps on Cu(001): a molecular dynamics study Frits Rabbering, Herbert Wormeester, Bene Poelsema, Oleg Trushin, Abdelkader Kara, Talat S. Rahman Analysis of high-resolution LEED data of 40 ML Cu on Cu(001) at various polar deposition angles led to the conclusion that attractive forces between the surface and the incoming particles have a dramatic steering effect on the trajectory of the particle such that incoming particles are attracted towards protruding structures leading to increased roughness and changes in morphology for glancing incidence. In connection with these results, we have carried out molecular dynamics simulations to examine in particular the influence of two important parameters: the polar angle and the velocity of the incident particles. Using a uniform grid of incident particles, we find a surplus of particles at the top of the step which increases dramatically as the polar angle is increased from normal to glancing incidence. We also find a novel shadowing effect which is expected to impact growth patterns. Details, including the influence of the incident energy will be presented in the context of the on going experiments. [Preview Abstract] |
Monday, March 21, 2005 3:30PM - 3:42PM |
D40.00004: Step Permeability on the Pt(111) Surface Michael Altman, Michael Man Surface morphology will be affected, or even dictated, by kinetic limitations that may be present during growth. Asymmetric step attachment is recognized to be an important and possibly common cause of morphological growth instabilities. However, the impact of this kinetic limitation on growth morphology may be hindered by other factors such as the rate limiting step and step permeability. This strongly motivates experimental measurements of these quantities in real systems. Using low energy electron microscopy, we have measured step flow velocities in growth on the Pt(111) surface. The dependence of step velocity upon adjacent terrace width clearly shows evidence of asymmetric step attachment and step permeability. Step velocity is modeled by solving the diffusion equation simultaneously on several adjacent terraces subject to boundary conditions at intervening steps that include asymmetric step attachment and step permeability. This analysis allows a quantitative evaluation of step permeability and the kinetic length, which characterizes the rate limiting step continuously between diffusion and attachment-detachment limited regimes. This work provides information that is greatly needed to set physical bounds on the parameters that are used in theoretical treatments of growth. The observation that steps are permeable even on a simple metal surface should also stimulate more experimental measurements and theoretical treatments of this effect. [Preview Abstract] |
Monday, March 21, 2005 3:42PM - 3:54PM |
D40.00005: Spatially-resolved Thin-film Alloy Compositions J.B. Hannon, J. Sun, K. Pohl, G.L. Kellogg Controlling the composition of thin-film alloys is critical in a wide range of technologies. However, measuring alloy compositions at surfaces is difficult. Quantitative information on surface alloy compositions can be obtained from analysis of low energy electron diffraction (LEED) I-V curves, but, in nearly all LEED studies it is implicitly assumed that the structure and composition are spatially uniform. In this talk we describe low-energy electron microscopy (LEEM) experiments showing that surface steps can make alloy compositions inherently inhomogeneous. Specifically, we have investigated the formation of the well-known PdCu surface alloy phase on Cu(001). We show that surface steps introduce strong variation the electron reflectivity (i.e. in the LEED-IV curves) due to inhomogeneous Pd alloying. By analyzing the reflectivity using multiple-scattering LEED calculations we have determined the random surface alloy composition by using the average t-matrix approximation, with a spatial resolution of about 10 nm. We find that regions of the surface overgrown via step flow have excess Pd in the third layer, while regions far from steps have virtually no Pd in the third layer. We conclude that step flow buries Pd, which then becomes immobile. Surprisingly, post-growth annealing above 300 C is required to form a laterally homogeneous alloy film. [Preview Abstract] |
Monday, March 21, 2005 3:54PM - 4:06PM |
D40.00006: Quantum Beating Patterns Observed in the Energetics of Pb Film Nanostructures Peter Czoschke, Hawoong Hong, Leonardo Basile, Tai-Chang Chiang We have studied the nanoscale structural evolution of Pb films grown at 110 K on a Si(111) substrate as they are annealed to increasingly higher temperatures. Surface x-ray diffraction from a synchrotron source is used to observe the morphology evolve from an initial smooth film through various metastable states before reaching a state of local equilibrium, at which point the coverage of different height Pb structures is analyzed and related to the thickness-dependent surface energy. Beating patterns are observed in the resulting energy landscape consistent with the Friedel oscillation wavelength of the electronic contribution. The form of the surface energy is explained by a simple model calculation based on the confinement of free electrons to a quantum well. [Preview Abstract] |
Monday, March 21, 2005 4:06PM - 4:18PM |
D40.00007: The Pb/Si(111) phase diagram and the thermal stability of a ``Devil's Staircase'' S. Stepanovskyy, M. Hupalo, M. Yakes, V. Yeh, M. Tringides The T-$\theta $ phase diagram for Pb/Si(111) was determined in the range 6/5$<\theta <$4/3ML from complementary STM and SPA-LEED experiments. Within this coverage a ``Devil's Staircase'' (DS) was found with numerous phases formed as a result of the long range elastic Pb interactions. The presence of the DS phases can account for previous contradictory observations in the literature about the Pb/Si(111) phases. In addition we study the thermal stability of the different linear DS phases and determine that the transition temperature is a monotonic function of the phase periodicity. Because of additional complexity in the experimental system (i.e. 2-dimensionality and 3-fold symmetry) the DS phases transform at higher temperature into commensurate phases of 3-fold symmetry ``HIC''. Different types of ``HIC'' phases have been discovered differing in the size of their supercell within $\sim $0.04ML coverage variation. The presence of such large number of different ordered phases within a very small range of control parameters demonstrate the unusual degree of self organization in Pb/Si(111). [Preview Abstract] |
Monday, March 21, 2005 4:18PM - 4:30PM |
D40.00008: Initial stages of Mn deposition on Si(001) studied by STM Michael R Krause, Andrew Stollenwerk, Chaffra Awo-Affouda, Vincent P LaBella Semiconductor devices which utilize the spin of the electron (spintronic devices) have the potential to achieve higher speeds with lower power consumption than conventional devices. The integration of ferromagnetism into these device structures is needed to couple to electron spin. Diluted magnetic semiconductors (DMS) have been demonstrated as a successful method for integrating ferromagnetism through doping of a semiconductor crystal such as Si with an additional transition metal impurity such as Mn. Therefore the study of Mn deposition onto Si(001) in the submonolayer regime can give detailed insight into the bonding and energetics of Mn with Si. We present results of both clean Si(001) surfaces and submonolayer coverages of Mn on Si(001). The STM images have been taken at room temperature as well as 77 K. [Preview Abstract] |
Monday, March 21, 2005 4:30PM - 4:42PM |
D40.00009: Structure and thermal stability of thin Fe films grown on Al(001) surfaces with Ti as an interface stabilizer R.J. Smith, C.V. Ramana, G. Bozzolo, B.-S. Choi Achieving thermal and chemical stability of thin metal film device structures at elevated operating temperatures is very important, and becomes more difficult as the film thickness is reduced to the nanometer regime. We investigated the structure and thermal stability of Fe films grown on Al(001) surfaces with thin Ti interlayers at the interface. Using Rutherford backscattering and channeling (RBS/c), we identify the bcc structure of the Fe(001) film on the fcc Al(100) substrate. RBS/c and low-energy ion scattering (LEIS) were used to evaluate the thermal stability of the interface. BFS model calculations were used to understand the stability of the Ti interlayer, and its evolution at increased temperatures. The epitaxial Fe structure is observed to be stable up to $\sim $400 $^{\circ}$C, although Al atoms, apparently captured in the Fe film during growth, begin to diffuse to the Fe surface at $\sim $200 $^{\circ}$C. [Preview Abstract] |
Monday, March 21, 2005 4:42PM - 4:54PM |
D40.00010: Surface Freezing in Binary Liquid Gold-Silicon Alloy Oleg Shpyrko, Alexei Grigoriev, Reinhard Streitel, Peter Pershan, Ben Ocko, Moshe Deutsch We present experimental x-ray studies of formation of a 2D solid monolayer at the liquid-vapor interface of AuSi eutectic alloy above bulk melting point $T_{melt}=363^{\circ}$C. Additionally, at temperature $T=T_{melt}+12^{\circ}C$ the 2D surface-frozen layer undergoes a solid-solid surface phase transition. Surface-induced atomic layering structure normal to the surface was found to be significantly enhanced for low-temperature 2D phase (layering length $\approx$ 3~nm), while reverting to classical layering length of $\approx$ 1~nm above surface transition temperature. The Gold-Silicon eutectic is the first miscible binary metallic system for which such surface freezing behavior has been observed. [Preview Abstract] |
Monday, March 21, 2005 4:54PM - 5:06PM |
D40.00011: The growth of Xe on the 10-fold Al-Ni-Co Quasicrystal Surface Renee Diehl, Nicola Ferralis, Raluca Trasca, Milton Cole, Wahyu Setyawan, Stefano Curtarolo The growth of Xe films on tenfold decagonal Al-Ni-Co was studied using low-energy electron diffraction and He-atom scattering. Adsorption isobars indicate layer-by-layer growth of Xe in the temperature range of 60K to 120K. The first layer apparently possesses the symmetry of the substrate, but at the onset of the second layer, the film reorders into a 6-fold structure. This 6-fold structure has domains that are aligned along the 10-fold directions of the quasicrystal, leading to a diffraction pattern having 30-spot rings. The domain size of the Xe is at least 160 angstroms. The bilayer is consistent with two layers of bulk fcc(111) Xe. Further adsorption produces further ordered growth of the film. A Xe-substrate potential computed using Lennard-Jones potentials was used as the basis for Monte Carlo simulations that agree substantially with these measurements. Such studies provide powerful insight into growth processes on quasicrystal surfaces. [Preview Abstract] |
Monday, March 21, 2005 5:06PM - 5:18PM |
D40.00012: 5 to 6-fold symmetry transition and T-dependent stacking of Xe adsorbed on a quasicrystal surface Wahyu Setyawan, Stefano Curtarolo, Renee Diehl, Nicola Ferralis, R. A. Trasca, Milton Cole The ordering of Xe films on an Al$_{73}$Ni$_{10}$Co$_{17}$ quasicrystals surface is studied using Grand Canonical Monte Carlo simulations with Lennard-Jones interatomic potentials and an empirical adsorption potential. Isotherms and layer density profiles show interesting phenomena. The study confirms the experimental layer-by-layer growth in the explored temperature (T) range (70-280K). By increasing the pressure at low temperatures (70-110K) the first layer, which forms following the 5-fold ordering of the substrate, evolves continuously assuming a 6-fold hexagonal symmetry before the condensation of the second layer. Thereafter, further layers have 6-fold symmetry. The 6-fold axes are aligned with the 5-fold axes of the quasicrystal, in agreement with experiment. However, at low T (70K) the layer stacking is ABC, consistent with fcc(111) experimental observations, while at higher T (160K) the stacking is AB, indicating a possible fcc to hcp transition of the adsorbate. Research supported in part by NSF. [Preview Abstract] |
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D40.00013: ``Epi-nucleation'' on Reconstructed Surface and a Model Study of Si(001) Homoepitaxy Raj Ganesh Pala, Feng Liu We introduce the concept of ``epi-nucleation'' to distinguish nucleation and growth on a reconstructed surface from that on a non-reconstructed surface. The `critical nucleus' defined in the classical nucleation theory refers to a nucleus stable against dissociation, which is also implicitly assumed to have an epi-relation to the substrate. However, such assumption may fail on a reconstructed surface because although the ``classical'' nucleus is stable against dissociation, it may not yet establish the epi-relation with the substrate as the substrate surface underneath the nucleus remains reconstructed. Therefore, on a reconstructed surface, in addition to the stable ``classical'' nucleus, there must exist another ``epi-nucleus'' that is not only stable against dissociation but also has a epi-relation with the substrate (breaking the underlying substrate surface reconstruction). We illustrate the general concept of ``epi-nucleus'' using first-principles calculations of homoepitaxial nucleation on a reconstructed Si(001) surface and we find that here the epi-nucleus consists of six adatoms, in contrast to the classical nucleus of two adatoms. [Preview Abstract] |
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