Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session V10: Focus Session: Growth, Structure, Dynamics, and Function of Nanostructured Surfaces and Interfaces -- Semiconductors |
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Sponsoring Units: DMP Chair: Ray Phaneuf, University of Maryland Room: D221 |
Thursday, March 24, 2011 8:00AM - 8:12AM |
V10.00001: In situ x-ray scattering investigation of Ag/Si(111)7x7 Yiyao Chen, M.W. Gramlich, S.T. Hayden, M.C. Tringidies, P.F. Miceli We have used in situ synchrotron x-ray scattering to investigate the growth of quantum-size-effect (QSE) Ag nanocrystals on Si(111)-7x7. The experiments explore the buried interface and the wetting layer as well as the interlayer spacings and the height distribution of the islands at different coverage and temperatures. The areal density of the wetting layer is found to be 30{\%} of Ag(111) and it is located above the adatom layer at a sharp interface. As the coverage is varied, all Ag layer heights are observed in the height distribution except for 2 atomic layers (measured from the Si surface), which were negligible. The structure of the islands and wetting layer will be discussed in relation to recent work that questions whether Ag/Si(111) is a QSE system. Research funding is supported by NSF DMR-0706278. The experiments were performed at the Advanced Photon Source Sector 6 beam-line at Argonne National Laboratory, which is supported by the US-DOE through Ames Lab under Contract No. W-7405-Eng-82. [Preview Abstract] |
Thursday, March 24, 2011 8:12AM - 8:24AM |
V10.00002: Grazing incidence surface scattering in the Pb/Si(111) system using an Area Detector M.W. Gramlich, S.T. Hayden, Yiyao Chen, P.F. Miceli Geometrical effects are considered when using an area detector for in situ x-ray grazing incidence scattering studies of the Pb/Si(111) system. Rod-like scattering and 3D-crystallite diffraction can both occur during in situ studies and these require different geometrical considerations. The Pb/Si(111) system conveniently exhibits different surface phases that provide useful examples, including randomly oriented 3D-crystallites on the surface that form powder diffraction rings. The shape of a diffraction ring depends on the position of the detector in real space. For rod scattering, the length of the image on the detector depends on resolution as well as domain size. We will discuss methods for obtaining reciprocal space information from area detector images in surface diffraction. Research funding is supported by NSF DMR-0706278. The experiments were performed at the Advanced Photon Source Sector 6 beam-line at Argonne National Laboratory, which is supported by the US-DOE through Ames Lab under Contract No. W-7405-Eng-82. [Preview Abstract] |
Thursday, March 24, 2011 8:24AM - 8:36AM |
V10.00003: Increased structural ordering of the low temperature wetting layer in the Pb/Si(111)-7x7 system P.F. Miceli, M.W. Gramlich, S.T. Hayden, Yiyao Chen, C. Kim, E.H. Conrad, M.C. Tringides The Pb/Si(111)-7x7 system exhibits interesting quantum size effects (QSE) for Pb nano-islands, including anomalously fast island coarsening that is facilitated by the wetting layer between the islands. While it is known that the wetting layer has a disordered 8x8 structure, the exact structure of the layer is still an open question. Our in situ x-ray scattering studies show that the wetting layer structure evolves temporally over a remarkably broad range of temperatures due to \textit{two} physically independent mechanisms. The as grown low temperature wetting layer is found to slowly anneal into a \textit{better-ordered 8x8 structure}, which suggests that it is highly dynamic as it attempts to accommodate the large corrugation of the Si(111)7x7 substrate. This increased order has important implications for the fast atom transport between the QSE-islands. Research funding is supported by NSF DMR-0706278 and the Ministry of Knowledge Economy of Korea 2009-F014-01 (CK). The experiments were performed at the Advanced Photon Source Sector 6 beam-line at Argonne National Laboratory, which is supported by the US-DOE through Ames Lab under Contract No. W-7405-Eng-82. [Preview Abstract] |
Thursday, March 24, 2011 8:36AM - 8:48AM |
V10.00004: Quantum Growth of a Metal/Insulator System Hawoong Hong, Aaron Gray, Ruqing Xu, Longxiang Zhang, Tai-C. Chiang Quantum confinement of electrons in thin metal films can lead to novel effects on the growth, structure, stability, and various other physical and chemical properties, as demonstrated by recent work on metal films grown on semiconductor substrates. We report herein the observation of quantum growth behavior in a metal-on-insulator system; the results show substantial differences. Insulating substrates, with their large band gaps, offer minimal electronic coupling at the interface. This decoupling should maximize quantum confinement effects. Indeed, in a study of Pb film growth and thermal processing on sapphire, we have observed robust preferred island height selection over a wide thickness range -- a hallmark of quantum confinement effects -- for processing temperatures up to 250 degrees C. By contrast, room temperature is the limit for Pb films prepared on the semiconducting substrate Si(111). These results provide the first evidence connecting the quantum growth behavior of overlayers with the substrate band gap. [Preview Abstract] |
Thursday, March 24, 2011 8:48AM - 9:00AM |
V10.00005: Electron coherence in Pb/Ag heterostructures epitaxially grown on Si(111) Jisun Kim, Chendong Zhang, Hongjun Gao, Chih-Kang Shih Along with other metals, Pb and Ag can form globally flat ultra- thin films on the Si(111) surface. Due to electron confinement along the growth direction, such films exhibit distinctive quantum well states (QWS's). Confinement occurs between the vacuum-solid and solid-solid interfaces. It was reported earlier, using angle-resolved photoemission, that quantum confined states existing in Ag thin films can coherently propagate through a Pb overlayer with thickness much thicker than the typical electron mean free path. Here we use scanning tunneling microscopy and spectroscopy to investigate the quantum well states formed in double quantum wells (Pb quantum well and Ag quantum well) formed in Pb/Ag/Si(111) double- heterostructures. Both the growth mechanism and the coherent coupling between the Pb and Ag quantum wells will be reported. [Preview Abstract] |
Thursday, March 24, 2011 9:00AM - 9:12AM |
V10.00006: Surface plasmon excitation in ultrathin Mg films on Si(111) Ao Teng, Geunseop Lee, Saban Hus, Hanno Weitering We investigated the dispersion of the surface plasmon in ultrathin Mg(0001) films, grown on a Si(111)-7$\times $7 surface, as a function of film thickness and parallel momentum ($q_{\vert \vert })$, using angle-resolved high-resolution electron-energy-loss spectroscopy (HREELS). In Mg films thicker than $\sim $ 3 ML, surface plasmon excitations exhibit negative dispersions for small $q_{\vert \vert }$(long wavelength limit). In contrast, the surface plasmons of ultrathin Al(111) films are known to exhibit positive dispersions near $q_{\vert \vert }\sim $ 0. The surface plasmon energies of the Mg films increase as the film thickness decreases. The plasmon line widths reveal similar trends, namely, for a given film thickness the line width decreases initially with increasing $q_{\vert \vert }$ while it increases with film thickness. Possible explanations for the observed thickness dependence of the surface plasmon dispersion and damping will be discussed. [Preview Abstract] |
Thursday, March 24, 2011 9:12AM - 9:24AM |
V10.00007: Plasmon response of a quantum-confined electron gas probed by core-level photoemission Mustafa M. Ozer, Eun Ju Moon, Adolfo G. Eguiluz, Hanno H. Weitering The emergence of the ``bulk'' plasmon in \textit{atomically-smooth} ultrathin Mg(0001) films on Si(111) has been determined using x-ray photoelectron spectroscopy (XPS). Plasmons in this quasi two-dimensional (2D) regime turn out to be excited primarily via the sudden creation of the core hole, as the extrinsic loss channel (which is dominant in bulk XPS spectra) is suppressed by electron confinement. The collective plasmon response of the films is remarkably similar to that of a thin slice of \textit{bulk matter}, subject to quantum-size boundary conditions, in spite of the fact that the one-electron degrees of freedom are quantized. The energy-loss spectra of the thinnest films are characterized by a gradual transfer of spectral-weight from the bulk-like collective modes to the low-energy one-electron excitations, and the plasmon ultimately collapses below six monolayers. Our results represent striking manifestations of the role of electronic confinement on plasmon resonances in precisely-controlled nanostructures. [Preview Abstract] |
Thursday, March 24, 2011 9:24AM - 9:36AM |
V10.00008: Controlled Surface functionalization via self-selective metal adsorption and pattern transformation on vicinal Si(111) surface F.K. Men, A.L. Chin, Feng Liu We demonstrate a self-selective metal adsorption and pattern transformation process on vicinal Si(111) surfaces. When Au atoms are deposited onto the self-organized periodic Si(111) surface patterns, the Au atoms self-select to adsorb predominantly onto one of the two distinct domains, the Si(111) terrace or the step-bunched facet, at different Au coverage. This leads to a systematic transformation of the surface pattern, whose domain population changes while its periodicity remains intact with the increasing Au coverage. A stress-domain model is used to explain the observed phenomenon. Our findings suggest a unique method for controlled functionalization of surfaces at the nanoscale, as illustrated further by domain- selective self-assembly of uniform CoSi$_2$ nanoclusters on the Au-functionalized vicinal Si(111) surface. [Preview Abstract] |
Thursday, March 24, 2011 9:36AM - 9:48AM |
V10.00009: Epitaxial silicene formed on single-crystalline ZrB$_{2}$ thin films: structure and electronic properties Antoine Fleurence, Rainer Friedlein, Ying Wang, Yukiko Yamada-Takamura The experimental realization of extended, two-dimensional sheets of silicene, the silicon counterpart of graphene, has been elusive so far. Here, we demonstrate that such a two-dimensional, epitaxial honeycomb Si layer forms through surface segregation on a metallic zirconium diboride (ZrB$_{2})$ film grown itself epitaxially on Si(111). The honeycomb Si layer uniformly covers the ZrB$_{2}$(0001) surface forming a (2$\times $2) reconstruction. Surface-sensitive core-level photoelectron spectroscopy performed using a photon energy of 130 eV identifies Si atoms in different chemical states that are either in contact with Zr atoms or not, confirming details of the slightly-buckled honeycomb structure obtained through scanning tunneling microscopy. Angle-resolved ultraviolet photoelectron spectra reflect surface electronic states related to the predicted band structure of slightly-buckled, free standing silicene together with those of the uppermost Zr layer. [Preview Abstract] |
Thursday, March 24, 2011 9:48AM - 10:00AM |
V10.00010: Endotaxial Si nanolines in Si(001):H James Owen, Fran\c{c}ois Bianco, Sigrun A. K\"oster, Daniel Mazur, Christoph Renner, David Bowler The study of one dimensional wires is of great interest in the area of low-dimensional physics, and these structures also have potential applications in future nanodevices. A perfectly straight nanoline embedded in a H-terminated silicon surface has been fabricated by a process of hydrogenation of a Bi nanoline surface using an atomic H beam source, and comprises a triangular core of Si embedded in the top five layers of the Si substrate. The defect density of this nanoline is extremely low, and being H- terminated, it is stable in air for limited periods of time. Scanning Tunnelling Microscopy experimental data and Density Functional Theory calculations have been used to determine the atomic structure of this nanoline, so-called the Haiku Stripe, and have revealed that there exists a 1D state localised to the nanoline core, lying just above the conduction band minimum. [Preview Abstract] |
Thursday, March 24, 2011 10:00AM - 10:12AM |
V10.00011: Atomic Layer Epitaxy of Si and Ge on Si(100)-(2x1) Jean-Francois Veyan, Heesung Choi, Joshua Ballard, Stephen McDonnell, Wiley P. Kirk, Robert M. Wallace, John Randall, Kyeongjae Cho, Yves J. Chabal Atomic Layer Epitaxy of Si and Ge on Si(100) surface using disilane (Si$_{2}$H$_{6})$ and digermane (Ge$_{2}$H$_{6})$ as precursors is a critical step for constructing 3-D nano-structures, and is indispensable for Atomically Precise Manufacturing of new devices such as quantum dots. Using IRAS and STM together with DFT calculations, we show that Si$_{2}$H$_{6}$ chemisorbs on clean Si(100)-(2x1) via beta-hydride elimination pathway, involving the intermediate states Si-H and Si-SiH$_{2}$-SiH$_{3}$. Thermal decomposition of the chemisorbed Si$_{2}$H$_{5}$ leads to the formation of Si$_{2}$H$_{2}$ as an added dimer rotated 90 degrees with respect to the initial dimer row. A similar chemisorption pathway is observed for Ge$_{2}$H$_{6}$ on Si(100)x(2x1). The thermal decomposition of Ge$_{2}$H$_{5}$ involves the migration of H from Ge to Si, and Ge ad-dimer formation. Evidence for Ge epitaxial growth on Si(100)x(2x1) using Ge$_{2}$H$_{6}$ will be presented. [Preview Abstract] |
Thursday, March 24, 2011 10:12AM - 10:24AM |
V10.00012: Kinetics-Limited Composition Profile of Semiconductor Alloy Quantum Dots Xiaobin Niu, Gerald Stringfellow, Feng Liu Semiconductor alloy quantum dots (QDs) with controlled composition profile are promising nanoscale building blocks for modern nanophotonic and nanoelectronic devices. The overall composition profile of such low-dimensional nanostructures is usually far from equilibrium, because bulk diffusion is negligible at typical growth conditions. However, local equilibrium may be established in the surface regions via surface diffusion. Consequently, the kinetic growth mode, which dictates the way of surface mass transport and alloy mixing in the growth fronts, becomes a key factor in determining the kinetics-limited composition profile. In this talk, we report our recent discovery of a striking correlation between the composition profiles of the strained semiconductor alloy QDs and their growth modes, based on atomistic-strain-model Monte Carlo simulations of InGaN (GeSi) QDs. The layer-by-layer growth forms core-shell structures with the core-rich unstrained component; while the faceted growth forms the core-rich strained component. Our findings suggest a promising method for the control of composition profile of semiconductor alloy QDs by selecting the growth mode. [Preview Abstract] |
Thursday, March 24, 2011 10:24AM - 10:36AM |
V10.00013: Coarsening and Saturation of Quantum Dot Evolution during Strained Film Heteroepitaxy Champika Gigiriwala Gamage, Zhi-Feng Huang Morphological properties of an epitaxially grown film and the self-organization process of coherent strained islands are analyzed via the development of a continuum elasticity model based on the 2nd order perturbation method. Effects of wetting stress due to film-substrate interactions have been incorporated in the resulting nonlinear dynamic equation governing the film morphological profile. We study the formation and evolution of surface strained islands or quantum dots for different film/substrate misfit strains, via analyzing the time-dependent behavior of the structure factor for surface heights, its various moments, and the surface roughness. Three regimes of island array evolution have been identified, including a film instability regime at early stage, a slow power-law-type coarsening at intermediate time, and the crossover to a saturated state, with detailed behavior dependent on misfit strains but not qualitatively on finite system sizes. The results are compared to previous experimental and theoretical efforts on quantum dots coarsening and saturation. [Preview Abstract] |
Thursday, March 24, 2011 10:36AM - 10:48AM |
V10.00014: Directed Self Assembly and Self-Limiting Growth (SLG) of Mound Formation on Patterned GaAs(001) Surface During MBE Homoepitaxy Chuan-Fu Lin, Hung-Chih Kan, Subramaniam Kanakaraju, Chris Richardson, Ray Phaneuf We present the results of molecular beam epitaxial growth experiments on nanopit-patterned GaAs(001) surfaces at temperatures near 500$^{\circ}$C. We find that in the initial stage of growth, the pattern directs the spontaneous formation of multilayer islands at 2-fold bridge sites between neighboring nanopits along [110], seemingly due to the presence of an Ehrlich-Schwoebel barrier [1]. However, as growth continues, the height of mounds at 2-fold bridge ``self-limits'': the mounds cease to grow. Beyond this point an initially less favored 4-fold bridge site for mounds dominates and a different pattern of self assembled mounds begins. We propose that a minimum, ``critical terrace size'' at the top of each mound is responsible for the observed self-limiting growth. \\[4pt] [1] T. Tadayyon-Eslami, H.C. Kan, L.C. Calhoun {\&} R.J. Phaneuf, \textit{Phys. Rev. Lett. } \textbf{97}, 126101 (2006) [Preview Abstract] |
Thursday, March 24, 2011 10:48AM - 11:00AM |
V10.00015: Spontaneous Microfaceting and Pyramid Formation during Si(100) Etching Melissa Hines, Marc Faggin, Ankush Gupta The spontaneous, etching-induced transformation of an initially flat Si(100) surface to a completely nanofaceted morphology consisting of overlapping pyramidal hillocks has been observed using a combination of morphological and spectroscopic probes and modeled using a kinetic Monte Carlo (KMC) simulation of Si(100) etching. The morphological transformation is driven by highly anisotropic chemical reactions that generate self-propagating pyramidal features with near-perfect microfacets. The atomic-scale mechanism of this etching-induced transformation will be discussed. In contrast to the more commonly studied Si(111) surface, the reactivity of the (100) face is dominated by interadsorbate strain. [Preview Abstract] |
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