Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session A32: Focus Session: Directed Self Assembly of Dots, Islands and Wires on Templates |
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Sponsoring Units: DMP Chair: Ray Phaneuf, University of Maryland Room: E142 |
Monday, March 15, 2010 8:00AM - 8:36AM |
A32.00001: SiGe self-assembled quantum dots and quantum dot molecules patterned using a focused ion beam Invited Speaker: Many proposed nanoelectronic device architectures rely on the ability to place quantum dots at specific locations. However, the ability to arrange quantum dots with specific compositions and in any arbitrary pattern or layout is extremely challenging at these dimensions. We have explored using the combination of a gallium focused ion beam (FIB) and limited growth kinetics as a method of laterally controlling the nucleation positions of quantum dot arrangements on silicon substrates. The FIB can be used to directly modify surface topography at specific sites on the surface of a substrate in order to create a topographical template. Shallow features on the order of only a few nanometers in depth, will remain even after ex-situ chemical cleaning and deposition of a thin Si buffer layer. This template can be used to influence nucleation of islands during subsequent SiGe epitaxial growth on the template. Using kinetically limited growth conditions that reduce surface diffusion, island formation can be suppressed, that would otherwise occur at random locations on the surface in order to relax the strain. A shallow faceted pyramidal pit with four edges will instead form first at each previously modified FIB site, followed by nucleation of islands only at the energetically favorable pit edge sites. The size of the islands and the pit, can be tuned by varying the strain in the film. If the size of the islands can be reduced sufficiently to allow for quantum confinement effects, the resulting structure can be considered a ``quantum dot molecule". This ``molecule" consists of four closely spaced quantum dots arranged precisely around a central pit that are remarkably uniform in size. The characteristics of these self-assembled arrangements of quantum dots are of particular interest for potential applications where electron tunneling between dots would be required. [Preview Abstract] |
Monday, March 15, 2010 8:36AM - 8:48AM |
A32.00002: Heterogeneous Nucleation of Quantum Dot Molecules in Heteroepitaxy Hao Hu, Hongjun Gao, Feng Liu We develop a theoretical model to elucidate the heterogeneous nucleation of quantum dot molecules (QDMs) in heteroepitaxial growth of strained thin films. We show that critical size and energy barrier for nucleation of a QDM (two islands ``strain bonded'' by a pit in between) are reduced relative to those for isolated strained islands on flat surface. This is caused by an attractive strain-induced interaction between the islands and the pit. Consequently, islands prefer to heterogeneously nucleate next to a pit forming QDMs. For a fixed pit size, island-island interaction is shown to increase with increasing island size, giving rise to a self-limiting effect to prevent the island from further lateral growth (perpendicular to the pit edge), so that the islands elongate along the pit edge to form a ``mature'' structure of QDM. Our theory explains the most salient features of experimental results of QDMs. [Preview Abstract] |
Monday, March 15, 2010 8:48AM - 9:00AM |
A32.00003: Directing Self Assembly of Nanostructures Kinetically: Patterning and the Ehrlich Schwoebel Barrier Chuan-Fu Lin, Ajmi BH Hammouda, Hung-Chih Kan, Ray Phaneuf We present the results of Kinetic Monte Carlo simulation, which show that an extra diffusion barrier (Ehrlich Schwoebel Barrier) to an atom crossing a step can lead to self assembly of a variety of ordered arrangements of nanometer-sized ``mounds'' during epitaxial growth on a patterned substrate. Interestingly, in different temperature windows of epitaxial growth, the pattern-mound interaction acts as an important factor to assemble specific periodic nanostructures, with mound unit cells of 1/3 x 1/3 , 1/$\surd $2 x 1/$\surd $2 and 1x1, with respect to the pattern period, as the temperature is increased. Our previous work on epitaxial growth of GaAs on patterned GaAs(001) substrates also showed evidence that an Ehrlich-Schwoebel barrier might play a role in a transient growth instability we observe[1-3]. We anticipate that this phenomenon could find application in the fast, controlled assemblies of nanostructures. 1. T. Tadayyon-Eslami et al., \textit{PRL} \textbf{97}, 126101 (2006) 2. H.-C. Kan et al., \textit{PRB} \textbf{73}, 195410 (2006) 3. H.-C. Kan et al., \textit{PRL} \textbf{92}, 146101 (2004) [Preview Abstract] |
Monday, March 15, 2010 9:00AM - 9:12AM |
A32.00004: ABSTRACT WITHDRAWN |
Monday, March 15, 2010 9:12AM - 9:48AM |
A32.00005: Epitaxial growth and design of semiconductor nanowires Invited Speaker: Semiconductor nanowires represent an important technology for future applications in electronics and optoelectronics. In order to realize this potential, precise control of their synthesis must be achieved in terms of morphology, size, position, chemical composition and crystal structure. In this presentation, recent progress on controlled fabrication of III-V semiconductor nanowire structures will be discussed. The most common and versatile technique using for nanowire fabrication today uses metal alloy particles as templates for one-dimensional crystal growth. These particles may be fabricated in vapor or colloidal suspension or by agglomeration of thin films. Position controlled arrays may be achieved using lithographically-patterned thin metal films as growth seeds. The advantages and limitations of these techniques will be compared. As well, the effect of alloy particle metal choice on nanowire morphology will be discussed. Crystal structure control is also essential to applications of semiconductor nanowires. This is particularly important for III-V materials, which often exhibit an intermixing of cubic and hexagonal structures. The ability to selectively tune the structure, and to combine these structures in a precise manner, will be discussed in terms of accessible growth parameters. Finally, more complex structures can be fabricated once controlled fabrication of simple nanowires is achieved. Different semiconductor materials can be combined either axially or radially to produce heterostructures. Lattice matching is of lesser importance in nanowire systems due to lateral strain relaxation. However, growth from seed particles introduces new challenges, which will be discussed here. More complex structures may also be fabricated by incorporating sequential generations of nanowires into complex networks. [Preview Abstract] |
Monday, March 15, 2010 9:48AM - 10:00AM |
A32.00006: Atomistic mechanisms and diameter selection during nanorod growth Da-Jun Shu, Xiang Xiong, Zhao-Wu Wang, Zhenyu Zhang, Mu Wang, Nai-Ben Ming We report that two growth modes of nanorods can be realized, depending on a characteristic radius which increases with the ratio of the interlayer hopping rate limited by the two-dimensional Ehrlich-Schwoebel barrier (ESB) to the deposition rate of the growth units. When the radius of the initial island is larger than this characteristic radius, the growth morphology evolves from a taper-like structure to a nanorod with radius equal to the characteristic radius after some transient layers. Otherwise the nanorod morphology can be maintained during the growth, with stable radius being limited by both the radius of the initial island and the three-dimensional ESB. The theoretical predictions are in good agreement with experimental observations of ZnO growth. This work was supported in part by MOST of China (2004CB619005 and 2006CB921804), NSF of China (10625417 and 10874068) and Jiangsu Province (BK2008012). Z. Zhang acknowledges partial support by USDOE (grant No. DE-FG02-05ER46209, the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences), and USNSF grant No. DMR-0906025. [Preview Abstract] |
Monday, March 15, 2010 10:00AM - 10:12AM |
A32.00007: Spontaneous Formation of Nanopillar Arrays in Ultrathin Viscous Films: Critical Role of Thermocapillary Stresses Sandra Troian, Mathias Dietzel Nanoscale structures manifest exceedingly large surface to volume ratios and are therefore highly susceptible to control by surface stresses. Actuation techniques which can exploit this feature provide a key strategy for construction and self-organization of large area arrays. During the past decade, several groups have reported that molten polymer nanofilms subject to an ultra-large transverse thermal gradient undergo spontaneous formation of nanopillar arrays. The prevailing explanation is that coherent interfacial reflection of acoustic phonons causes periodic modulation of the radiation pressure leading to instability and pillar growth. We demonstrate instead that thermocapillary forces play a crucial if not dominant role in the formation process due to the strong modulation of surface tension with temperature. Any nanoscale viscous film is prone to such formations, not just polymeric films. Analysis of the governing interface equation reveals the mechanism controlling the growth, spacing and symmetry of these self-assembling arrays. We discuss how these findings are being used in our laboratory to construct nanoscale components for optical and photonic applications. [Preview Abstract] |
Monday, March 15, 2010 10:12AM - 10:24AM |
A32.00008: Tensile-strained self-assembled III-V nanostructures Paul Simmonds, Minjoo Larry Lee We present, for the first time, growth of self-assembled nanostructures via a novel combination of tensile-strained GaP on (110) GaAs. Several material systems exhibit self-assembling behavior driven by strain relief. To date, much of the work on quantum dots formed by such processes has focused on situations combining (001) surfaces with compressive strain; notably Stranski-Krastanov growth-mode InAs/GaAs and Ge/Si. Attempts to grow similar dots on other low-index planes often results in heavily dislocated 2D films. We demonstrate that 3D GaP nanostructures form spontaneously on (110) GaAs, even at submonolayer thickness, implying a Volmer-Weber growth-mode. These features exhibit high shape and size uniformity, with smaller dots showing no evidence of dislocations in cross-sectional TEM. Tuning of MBE parameters enables control of dot densities from 10$^{6}$ - 10$^{8}$ cm$^{-2}$. Since formation occurs solely at terrace edges, further engineering of dot coverage may be possible using off-cut (110) substrates to vary step-edge density. It is anticipated that this work will form the first step towards a more general description of self-assembled nanostructure growth under tensile strain. [Preview Abstract] |
Monday, March 15, 2010 10:24AM - 10:36AM |
A32.00009: Horizontally-aligned carbon nanotube films with nanotube diameter controlled by growth temperature Ali Almaqwashi, Josh Kevek, Ethan Minot Chemical vapor deposition (CVD) growth of carbon nanotubes (CNTs) on quartz substrates yields dense, horizontally aligned CNTs which are ideally suited for manufacturing electronic devices. We have examined the role of growth temperature in this CVD process using 0.15 nm thick iron catalyst on ST-cut quartz. Diameter and wall number of aligned CNTs were characterized by AFM imaging using a range of tapping forces [1]. We find that as growth temperature is reduced from 900 C to 800 C the average CNT diameter (measured at low tapping force) drops from 2.6 nm to 1.1 nm and the fraction of single walled CNTs increases from 45{\%} to 80{\%}. We observed no significant variation of CNT density as growth temperature was varied from 800 C to 900 C. [1] DeBorde, Leyden, Joiner {\&} Minot, Nano Letters 8, 3568 (2008) [Preview Abstract] |
Monday, March 15, 2010 10:36AM - 10:48AM |
A32.00010: Nano-ripple Formation on TiO2(110) by Grazing Incidence Low Energy Ion Beams Tim Luttrell, Wei-Kun Li, Xue-Qing Gong, Matthias Batzill Structure formation due to surface sputtering by grazing incident ion beams is investigated by scanning tunneling microscopy. Higher local sputter yield at monoatomic step edges compared to atomically flat terraces causes preferential erosion of step edges. This results in the destruction of step edges normal to the ion beam and consequently a preferential alignment of step edges along the azimuthal direction of the ion beam. We show that this kinetic method allows the preparation of high-energy step edges that are thermodynamically unstable. In addition for long irradiation times the surface self organizes in a nano-ripple structure with ripple-alignment in the ion beam direction. Studying the surface evolution as a function of ion fluence enables us to extract information of local sputter yields at step edges and terraces. This information will form the basis for the analysis of the mechanisms that give rise to the surface structuring by grazing, low energy ion beams. [Preview Abstract] |
Monday, March 15, 2010 10:48AM - 11:00AM |
A32.00011: Spinodal nanotechnology as a new class of bottom-up one and applications Hiroshi Katayama-Yoshida, Tetsuya Fukushima, Hidetoshi Kizaki, Masamune Oshitani, Kazunori Sato We discuss the nano-materials design of spinodal nano-decomposition as a new class of bottom-up nanotechnology by combining ab initio calculations and kinetic Monte Carlo simulations. We include all the complexity in the fabrication process of spinodal nano-decomposition (Konbu- and Dairiseki-phase) into advanced materials design with inhomogeneous materials. We compare the theoretical predictions with available experiments, such as (i)semiconductor nano-spintronics in dilute magnetic semiconductors, (ii)colossal thermoelectric-power responses of spincaloritronics, (iii)self-repaired nano-catalysis in La(Fe,Pd)O3, (iv)high-efficiency solar-cells, (v)high-efficiency light-emitting diode and Lasers. (1) K. Sato, et al., Reviews of Modern Physics, in printing (2009). (2) H. Katayama-Yoshida, et al.,Handbook of Spintronic Semiconductors, (Pan Stanford Pub.), p.1-79, (2009). (3) H. Katayama-Yoshida, et al.,Semiconductors and Semimetals, 82,433 (2008). (4) H. Katayama-Yoshida, et al.,Jpn. J. Appl. Phys. 46, L777 (2007). (5) H. Kizaki, et al.,Applied Physics Express 1, 104001, (2008). [Preview Abstract] |
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