Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session C40: Focus Session: Control of Ultrathin Film Morphology |
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Sponsoring Units: DMP Chair: Shirley Chang, University of California Davis Room: 349 |
Monday, March 18, 2013 2:30PM - 2:42PM |
C40.00001: Quantitative model of heterogeneous nucleation and growth of SiGe quantum dot molecules Hao Hu, Feng Liu We develop a quantitative theoretical model for heterogeneous nucleation and the growth of a quantum dot molecule --- a few islands ``strain bonded'' by a pit in heteroepitaxy of thin films, in contrast to homogeneous nucleation and growth of isolated strain islands on the surface. We use a multiscale approach combining continuum model with first-principles calculation, and show that the critical size and energy barrier for island nucleation next to a pit is substantially reduced with the increasing pit size, but the reduction approaches an upper bound of $\sim$85{\%} and $\sim$72{\%} for the size and barrier, respectively. Our model also predicts a self-limiting effect on island growth, resulting from an intriguing interplay between island-pit attraction and island-island repulsion, that drives the island size to increase linearly with the pit size, which explains a long-standing puzzle of experimental observation. [Preview Abstract] |
Monday, March 18, 2013 2:42PM - 2:54PM |
C40.00002: Influence of Surface Reconstruction on Droplet Epitaxy of InAs/GaAs Quantum Dots for Photovoltaics Simon Huang, Larry Aagesen, Jinyoung Hwang, Alan Teran, Jamie Phillips, Roy Clarke, Katsuyo Thornton, Rachel Goldman Quantum dot (QD) superlattices have been proposed for improving solar cell efficiency by providing intermediate energy bands to allow sub-bandgap photon absorption. Although photocurrent enhancement from QD solar cells has been demonstrated, QD cells exhibit lower open-circuit voltages and efficiencies than the GaAs reference cells, presumably due to the high electron capture rates induced by the elliptically shaped Stranski-Krastanov QDs. To improve the QD aspect ratio, thereby reducing the electron capture rate, we are exploring an alternative QD fabrication approach, droplet epitaxy (DE). To date, we have explored the influence of buffer surface reconstructions on the In exposure dependence of DE QD densities and size distributions. The GaAs (1x1) surfaces lead to higher density of smaller QDs with broad log-normal size distributions, suggesting coalescence dominated QD growth with inhibited In atomic surface diffusion. The c(4x4) surfaces enable the formation of larger QDs with lower density and narrow Gaussian size distributions, suggesting Ostwald ripening dominated growth with enhanced In atomic surface diffusion. Furthermore, we will discuss correlations between the formation, interface structure, and photovoltaic properties of DE QDs. [Preview Abstract] |
Monday, March 18, 2013 2:54PM - 3:06PM |
C40.00003: Formation and coalescence of surface domains introduced by metal deposition on a stepped Si(111) surface F.K. Men, A.L. Chin, C.P. Chang By depositing sub-monolayer Au atoms onto a stepped Si(111)-(7$\times$7) surface at 600$^{\circ}$C, stripes of (5$\times$2) domain form on the upper step edges of most terraces. Upon continued annealing at a higher temperature, most of the terraces transform into either Au-free (7$\times$7) terraces or fully reconstructed (5$\times$2) terraces. After analyzing the distance distribution between neighboring (5$\times$2) terraces we detect the presence of an optimal distance separating (5$\times$2) terraces. This optimal distance, controllable via the Au coverage, can be explained by the minimization of long-range strain relaxation energy of a system consisted of alternating domains. The ability of tuning surface domain structure through metal deposition provides a new way of manipulating surface morphology in the nanometer-scale range. [Preview Abstract] |
Monday, March 18, 2013 3:06PM - 3:42PM |
C40.00004: Probing phase transitions at surfaces with ultrafast electron diffraction Invited Speaker: Michael Horn von Hoegen The multitude of possible processes that can occur at surfaces cover many orders of magnitude in the time domain. While large scale growth and structure formation happens on a timescale of minutes and seconds, diffusion is already much faster, but can still be observed by electron microscopy. Many other processes as chemical reactions, phonon dynamics, or phase transitions, however, take place on the femto- and picosecond timescale and are yet way to fast for imaging techniques. In order to study such ultrafast processes at surfaces we have combined modern surface science techniques with fs laser pulses in a pump probe scheme. We use a RHEED setup with grazing incident electrons of 7 - 30 keV to ensure surface sensitivity. In order to overcome the velocity mismatch between light and electrons a tilted pulse front scheme is used to achieve a time resolution of less than 2 ps. The sample is excited with 800 nm photons with a pulse energy of 0.5 mJ at 5 kHz repetition rate. The huge potential of this technique for the study of transient surface phenomena is demonstrated with the non-equilibrium dynamics of the In induced c(8x2) reconstruction on Si(111). This surface exhibits a Peierls-like phase transition at 100 K from a c(8x2) groundstate, which is accompanied by the formation of a charge density wave (CDW), to (4x1) excited state. Upon excitation by the fs-laser pulse this structural phase transition is driven into the excited (4x1) state at a sample temperature of 20 K. The surface is only excited electronically, the CDW is lifted by photo doping and the surface remains up to 500 ps in a super cooled excited (4x1) state. Relaxation into the c(8x2) groundstate happens delayed through the nucleation of the c(8x2) at defects which triggers a 1-dim. recrystallisation front which propagates with the velocity of sound. Utilizing the Debye Waller effect, the excitation, conversion and relaxation of vibrational excitations in monolayer adsorbate systems like the Pb induced HIC ($\surd $3x$\surd $3) phase on Si(111) was studied. Initially only a high frequency optical mode with an amplitude parallel to the surface is excited. Subsequently, this mode decays into low frequency acoustic modes with an amplitude vertical to the surface which, however, do not couple to the phonons of the Si substrate and survive for many nanoseconds. [Preview Abstract] |
Monday, March 18, 2013 3:42PM - 3:54PM |
C40.00005: Conversion among Co adsorption states on Si(111)-(7$\times $7) by atomic manipulation Qin Liu, Kedong Wang, Guohua Zhong, Fangfei Ming, Xudong Xiao Eight types of adsorption structure of single Co atom have been identified by comparing scanning tunneling microscopy (STM) images of Si(111)-(7$\times $7) surface before and after in situ Co deposition at room temperature. The adsorption of single Co atom causes silicon adatoms appearing dimmer or brighter than their symmetry equivalents. Density functional theory calculations are performed to find the possible adsorption sites of Co atom and the magnetic moments of each type of adsorption structure, showing that the magnetic moments of the eight structures are different. Furthermore, atomic manipulation method has been used to realize conversions among the various Co adsorption structures. We have demonstrated that the single Co atom in eight different structure on Si(111)-(7$\times $7) surface can be converted to each other directly or indirectly. Therefore, it makes this single Co/Si(111)-(7$\times $7) become a promising system for building practical atomic magnetic structures for quantum computing since that each kind of single Co atom structure represents different magnetic states. [Preview Abstract] |
Monday, March 18, 2013 3:54PM - 4:06PM |
C40.00006: Iridium-silicide nanowires on Si(001) surface Dylan Nicholls, Nuri Oncel Iridium (Ir) modified Silicon (Si) (001) surface is studied with low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). The surface exhibits $p(2\times 2)$ domains on LEED intensity images. The STM images show that the basis of the crystal lattice is consists of an Ir atom and a Si dimer and similar to Si(001) dimer rows, they are aligned parallel to the [110] orthogonal directions. [Preview Abstract] |
Monday, March 18, 2013 4:06PM - 4:18PM |
C40.00007: On the Connection between Kinetic Monte Carlo and the Burton-Cabrera-Frank Theory Paul Patrone, Dionisios Margetis, T.L. Einstein In the many years since it was first proposed, the Burton- Cabrera-Frank (BCF) model of step-flow has been experimentally established as one of the cornerstones of surface physics. However, many questions remain regarding the underlying physical processes and theoretical assumptions that give rise to the BCF theory. In this work, we formally derive the BCF theory from an atomistic, kinetic Monte Carlo model of the surface in 1$+$1 dimensions with one step. Our analysis (i) shows how the BCF theory describes a surface with a low density of adsorbed atoms, and (ii) establishes a set of near-equilibrium conditions ensuring that the theory remains valid for all times. [Preview Abstract] |
Monday, March 18, 2013 4:18PM - 4:30PM |
C40.00008: Capture Zone Distributions and Island Morphologies in Organic Epitaxy and Graphene Formation Alberto Pimpinelli, T.L. Einstein Stating that island nucleation is an essential step in the formation of an epitaxial or supported layer may appear trivially obvious. However, less trivial is the observation that the size of the critical nucleus plays a crucial role in that it determines both the island density (and therefore the size of domains) and the evolution of the island morphology. In this talk we will describe recent developments in the analysis of capture zone distributions (CZD) specifically tailored for application to organic materials. We will also describe specific features of organic and graphene island morphologies, and discuss how they are related to the nucleation process and to the size of the critical nucleus. [Preview Abstract] |
Monday, March 18, 2013 4:30PM - 4:42PM |
C40.00009: Electromigration-driven dynamics of single-layer epitaxial islands on substrates Dwaipayan Dasgupta, Georgios I. Sfyris, Dimitrios Maroudas Electromigration-driven dynamics of single-layer epitaxial islands on substrates can lead to surface pattern formation that may have significant impact on nanofabrication. We develop a fully nonlinear model for the driven morphological evolution of single-layer homoepitaxial islands and coherently strained heteroepitaxial islands on crystalline elastic substrates with diffusional mass transport limited to the island periphery. We carry out dynamical simulations of the driven dynamics of such islands and validate the model by comparisons of the simulation results for individual islands with published experimental results. We find that the island migration speed varies linearly with 1/R, where R is the island size, up to a critical size that marks the onset of island morphological transition; further increase in R triggers other morphological or dynamical transitions. We also find an exponential dependence of the island mobility on the misfit strain. We also study the driven dynamics of island pairs with the island sizes and the island center-to-center line misalignment with respect to the electric-field direction being the key parameters. This parametric study identifies several classes of pattern forming dynamical phenomena mediated by island coalescence and break-up. [Preview Abstract] |
Monday, March 18, 2013 4:42PM - 4:54PM |
C40.00010: Electromigration- and thermomigration-driven surface morphological stabilization of coherently strained thin films on elastically deformable substrates Georgios I. Sfyris, Dwaipayan Dasgupta, Dimitrios Maroudas We study the surface morphological stability of a coherently strained thin film grown epitaxially on a substrate and subjected to an external electric field and temperature gradient. Due to its lattice mismatch with the substrate the film may undergo a Stranski-Krastanow (SK) instability, resulting in formation of islands on its surface. We consider various types of substrates placing emphasis on compliant substrates that partly accommodate elastically the lattice-mismatch strain in the epitaxial film. To examine the morphological stability of the film's planar surface state, we conduct a linear stability analysis based on a three-dimensional model of driven film surface morphological evolution. We find that the simultaneous action of properly applied and sufficiently strong external fields is necessary to stabilize the planar film surface morphology; in such cases, surface electromigration and thermomigration can inhibit SK-type instabilities and control the onset of island formation on the film surface. We derive the conditions for synergy and competition of the two external fields for surface stabilization and demonstrate the beneficial effects of the thermal field on reducing the critical electric-field strength required to stabilize the planar film surface morphology. [Preview Abstract] |
Monday, March 18, 2013 4:54PM - 5:06PM |
C40.00011: Strain and Shape-Driven Self-Organization of Atomically Abrupt Junctions on Patterned Ge (001) Surfaces Boris Lukanov, Kevin Garrity, Fred Walker, Sohrab Ismail-Beigi, Eric Altman We employ STM, electron diffraction, and other experimental techniques, complemented by density functional theory, in order to explore the interaction of alkaline-earth metals with the Si and Ge (001) surfaces on the atomic scale. Our results reveal a complex series of phase transitions as the alkaline-earth coverage is varied. Each phase transition is accompanied by significant changes in the surface morphology that can only be explained by mass transfer induced by the formation of alloy surfaces. Through comparison of bias-dependent atomic-resolution STM images with first-principle calculations, we develop atomic structural models of the surface alloy phases. Incorporation of the larger alkaline earth atoms into the Ge surface creates anisotropic strain that is ultimately relieved by the formation of remarkably well-ordered arrays of islands and trenches. With applications in mind, we investigate deposition onto a Ge substrate lithographically patterned with shapes, designed to direct the self-organization of the alkaline-earth induced surface structures. Sr deposition onto a Ge substrate patterned with cross-shaped nano-templates results in phase segregation within the template boundaries and the formation of atomically abrupt junctions between the different surface alloys. [Preview Abstract] |
Monday, March 18, 2013 5:06PM - 5:18PM |
C40.00012: Exploring the Role of Steps: A Collection of Case Studies of Vicinal Metal Surfaces using Density Functional Theory James Westover, Abdelkader Kara We will present results from multiple case studies done using DFT. We have explored the contribution made by step edges when metal surfaces are in contact with organic molecules, specifically, the cases involving pentacene (C$_{22}$H$_{14})$. The cases of vicinal surfaces with terrace geometries of 100 and 111 will be compared and contrasted. The question of terrace width is also addressed by results presented for situations involving diminishing step width. Because of the abundance of experimental data for copper it has been chosen as one metal surface to be considered. Additionally, copper's lattice constant is commensurate with the ring width in the benzene chain that forms pentacene. To contrast copper results for another noble metal, silver, will also be presented. We will present results for both structural and electronic changes in both the substrate and molecule. [Preview Abstract] |
Monday, March 18, 2013 5:18PM - 5:30PM |
C40.00013: Large-scale Molecular Dynamics Simulations of Glancing Angle Deposition Bradley Hubartt, Xuejing Liu, Jacques Amar While a variety of methods have been developed to carry out atomistic simulations of thin-film growth at small deposition angles with respect to the substrate normal, due to the complex morphology as well as the existence of multiple scattering of depositing atoms by the growing thin-film, realistically modeling the deposition process for large deposition angles can be quite challenging. Accordingly, we have developed a computationally efficient method based on the use of a single graphical processing unit (GPU) to carry out molecular dynamics (MD) simulations of the deposition and growth of thin-films via glancing angle deposition. Using this method we have carried out large-scale MD simulations, based on an embedded-atom-method potential, of Cu/Cu(100) growth up to 20 monolayers for deposition angles ranging from 50$^{\circ}$ to 85$^{\circ}$ and for both random and fixed azimuthal angles. Our results for the thin-film porosity, roughness, lateral correlation length, and density vs height will be presented and compared with experiments. Results for the dependence of the microstructure, grain-size distribution, surface texture, and defect concentration on deposition angle will also be presented. [Preview Abstract] |
Monday, March 18, 2013 5:30PM - 5:42PM |
C40.00014: First-principles study of tilted binding and precession motion of diatomic NO adsorbed to Co-porphyrin on Au(111) Yunhee Chang, Howon Kim, Se-Jong Kahng, Yong-Hyun Kim To understand the bright square ring structures observed in scanning tunneling microscopy (STM) experiments of NO adsorption to CoTPP on Au(111), we performed first-principles calculations within the spin-polarized DFT formulation and DFT-D method; which includes the van der Waals interaction between CoTPP and Au(111). With the correction, the calculated electronic structrues of NO adsorbed CoTPP/Au(111) are well consistent with STM and scanning tunneling spectroscopy (STS) results. Upon NO exposure, three-lobed structures of CoTPP were transformed to bright square ring shapes on Au(111). The adsorbed NO molecule is tilted away from the axial direction. Due to the symmetry of the CoTPP, the adsorbed NO molecule have a precession motion with the energy barrier of 33 meV. This energy barrier is small enough to allow a fast precession motion of the NO molecule even in cryogenic temperatures as low as 80 K. We will discuss details about NO adsorption mechanisms and electronic structures. [Preview Abstract] |
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