Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session A12: Directed Organization of Nanostructured Films |
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Sponsoring Units: DMP DCMP Chair: Ray Phaneuf, University of Maryland Room: 308 |
Monday, March 16, 2009 8:00AM - 8:36AM |
A12.00001: Controlled Synthesis of Functional Nanostructures Invited Speaker: |
Monday, March 16, 2009 8:36AM - 8:48AM |
A12.00002: Kinetic Monte Carlo Simulations of Nanostructure Evolution During Unstable Growth on Patterned GaAs(001) Chuan-Fu Lin, Krista Cosert, Ajmi Hammouda, Hung-Chih Kan, Ray Phaneuf We present results of kinetic Monte Carlo simulations, which include a diffusion barrier, lateral atom interaction energy, and Ehrlich-Schwoebel barrier to investigate unstable growth for comparison with our observations on patterned GaAs(001) surfaces at typical growth conditions [1-3]. Our results show a profound change in the mode by which an initial lithographic pattern evolves during growth, with growth mounds dominating at low temperatures and island nucleation and growth at higher temperatures. We describe the use of height-height correlation maps as a tool to facilitate the statistical characterization of the evolution of periodic patterns during growth, and correlate peaks in the maps with the change in growth mode with temperature. \\[0pt] [1] T. Tadayyon-Eslami et al., \textit{PRL} \textbf{97}, 126101 (2006) \\[0pt] [2] H.-C. Kan et al., \textit{PRB} \textbf{73}, 195410 (2006) \\[0pt] [3] H.-C. Kan et al., \textit{PRL} \textbf{92}, 146101 (2004) [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A12.00003: Nanopatterning as a Probe of Unstable Growth on GaAs(001) Krista Cosert, Chuan-Fu Lin, Ajmi Hammouda, Hung-Chih Kan, Kanakaraju Subrumaniam, Chris Richardson, Ray Phaneuf We report on observations of unstable growth on nanopatterned GaAs(001) surfaces. For growth at 500$^{o}$C, 1 ML/sec and an As$_{2}$/Ga beam equivalent pressure ratio of 10:1, we find that grooves oriented at right angles to [110] produce a build up of ridges of GaAs at the upper edges, while for grooves oriented at right angles to [1\underline {1}0] no ridges form; instead cusps evolve at the bottoms of such grooves [1]. The cusp-forming grooves show a pronounced initial amplification of depth during growth which changes with length/width ratio, and become more narrow. The ridge-forming grooves instead broaden during growth. We compare these experimental observations with kinetic Monte Carlo simulations in which a small anisotropic Ehrlich-Schwoebel barrier is included. [1] T. Tadayyon-Eslami, H.-C. Kan, L. C. Calhoun and R. J. Phaneuf, \textit{Phys. Rev. Lett. }\textbf{97}, 126101 (2006) [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A12.00004: Optimization of air-assisted CVD growth of vertically-aligned ZnO nanowires, guided by structural analysis using X-ray scattering Jong G. Ok, A. John Hart ZnO nanowires (ZNWs) are of significant interest for applications ranging from optical sensors to vibrational energy harvesters, due to properties including UV photoluminescence and piezoelectricity. We have studied low-pressure growth of ZNWs using a vapor transport method in air flowing within a tube furnace, giving vertically-aligned ZNW arrays on sapphire substrates seeded by Au catalysts. The growth rate and the average length of ZNWs depend on the flow rate of air and the total growth time, while multiple parameters such as catalyst thickness, pressure, and temperature also interdependently affect the ZNW characteristics. Grazing incidence small-angle X-ray scattering (GI-SAXS) measurements enable non-destructive quantification of ZNW diameter and alignment. By fitting GI-SAXS images using analytical models of the array as a population of solid cylinders having a Gaussian diameter distribution, we establish precise relationships between the structural characteristics and the growth conditions; for example, we determine rates of radial growth and size distribution broadening in comparison to axial growth. Control of the temperature gradient within the furnace also enables growth of well-aligned arrays at substrate temperatures as low as 600 $^{\circ}$C. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A12.00005: Influence of impurities on phase transition in quasi-one-dimensional nanowires on Si surface Geunseop Lee, Woosang Lee, Hyungjoon Shim, Sang-Yong Yu, Ja-Yong Koo We investigated using low-energy electron diffraction the influence of impurity doping on the structural phase transition in an array of quasi-one dimensional In nanowires on Si(111). A clean Si(111)4$\times$1-In surface, in its pristine form, undergoes a structural phase transition into a 8$\times$2 phase below 120 K. Introducing various impurities (hydrogen, oxygen, and alkali metals) on the surface at room temperature was found to affect the 4$\times$1-to-8$\times$2 structural phase transition by changing the transition temperature (T$_c$). Adsorption of the two types of the gases affected the transition in opposite ways: hydrogen adsorption lowered the T$_c$, whereas oxygen adsorption raised the T$_c$. Dosing of different alkali metals (Na, K, and Li) all decreased the T$_c$. Usually, impurities are expected to suppress the phase transition into the symmetry-broken phase (the low-temperature phase) by acting as random fluctuations in structure. In this sense, the increase in T$_c$ by the oxygen adsorption is an exceptional case enhancing the phase transition. Possible mechanisms leading to different influences of the various impurities on the structural phase transition of this In/Si(111) will be discussed. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A12.00006: Numerical simulations of VLS heteroepitaxial nanowire growth Vivek Shenoy, Klaus Schwarz, Jerry Tersoff Nanowires are particularly attractive for designing heterostructures, as effective radial strain relaxation allows heterostructures with a wider range of material combinations. The electrical, optical, and thermal properties of the nanowire are highly dependent on the accurate control of the locations and thicknesses of such heterostructures. However, in the case of nanowire growth from a metal seed particle, the composition of the seed particle will vary for growth of different materials due to alloying, which may cause problems in controlling interface abruptness. Also, recent experiments have shown that in many cases, growth instabilities do not allow for the formation of nanowires with desired morphology and material combinations. We have developed a continuum model for the growth of heteroepitaxial nanowires, and we use it to study the factors that control interface abruptness and instabilities during growth. Our model includes the following features that are critical for capturing the composition profiles in nanowires: 1) the differences of the attachment rates of the alloy components at the catalyst-wire interface, 2) the possibility of a miscibility gap in the alloy phases of the catalyst and the nanowire, 3) composition dependence of the surface energies of the nanowire and nanowire-catalyst interface and 4) anisotropy in surface energies leading to faceted morphologies. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A12.00007: Three Dimensional, Single-crystal, Oxide NANOFENCES for Epitaxial Growth of Electronic, Magnetic or Electromagnetic Nanoscale-Devices Amit Goyal, Sung-Hun Wee, Karren More, Eliot Specht A unique, three-dimensional (3D), single-crystal, MgO, NANOFENCE comprised of single crystal MgO nanowire units was synthesized via epitaxial growth on (100) SrTiO3 substrates. Individual single crystal MgO nanowire units comprising the nanofence were observed to have high aspect ratios with small diameters of 10-20 nm and long lengths from 100 nm up 1 $\mu $m. X-ray diffraction shows that the 3D MgO nanofence has an epitaxial relation with (100) SrTiO3 substrates with only a single {\{}100{\}}$<$100$>$ orientation and with full-width-half-maximum values of (200) $\omega $-scan and (110) $\phi $-scan with 4.5o and 5.5o, respectively. Such nanofences offer a single crystal, 3D nanotemplate for epitaxial growth of wide-ranging, 3D, electronic, magnetic and electromagnetic nanodevices. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:24AM |
A12.00008: Hierarchical Assembly of Epitaxial Quantum Dot Nanostructures on Templated Substrates Invited Speaker: Using the focused ion beam (FIB), we have modified the local topography and chemistry of Si(100) surfaces, and demonstrated control of the geometry, size, location and proximity of epitaxial Ge(Si) quantum dot (QD) nanostructures which are nucleated on these templated surfaces. We show how QDs can be located with a precision $\sim $ 10 nm using local Ga$^{+}$ FIB doses $\sim $ 10$^{14}$ cm$^{-2}$, and how QD size and morphology can be modified by local surface chemistry. We further describe how growth kinetics can control formation of more complex nanostructures with internal length scales bridging the $\sim $ 10 nm dimensions necessary for application to potential nanoelectronic device architectures and dimensions that are accessible through external lithography. In particular, we describe the self assembly of ``quantum dot molecule'' (QDM) Ge$_{x}$Si$_{1-x}$ nanostructures where a four-fold QD structures form around shallow strain relieving pits. Positional control of these QDMs using external lithographic templating allows formation of hierarchically assembled systems with length scales ranging from $\sim $ 10 nm in QD size and proximity, through the $\sim $ 100 nm dimensions of the QDM, to the micro/macro-scopic dimensions accessible with external lithography. We also describe methods for electronic and magnetic functionalization of these nanostructures by separation of ion species from alloy liquid metal sources in a mass selecting FIB column. This allows generation of ion beams comprising electronically non-invasive species for nanoscale surface templating (e.g. Si, Ge), electronic doping (e.g. As, B), or spin doping (e.g. Mn). Application of such structures to potential novel nanoelectronic device structures will be discussed. This work is done in collaboration with J. Floro, J. Graham, M. Gherasimova, J. Thorp (UVa), F. Ross (IBM), A. Portavoce (CNRS), M. Kammler (U. Duisburg) and J. Gray (U. Pittsburgh). [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A12.00009: Fabrication Methods for Positioning of Quantum Dots Rebecca Kramer, Rupert Oulton, Volker Sorger, Nitipat Pholchai, Xiang Zhang Quantum dot positioning is highly useful in terms of integrating nanoemitters into nanostructures, such as nanocavities and quantum dot waveguides. Demonstration of control over the positioning of quantum dots has proven difficult, and consequently construction of single-photon emitting systems has been hindered. We report the ability to reliably position nanoscale functional objects, specifically quantum dots, within a well-defined location. Programmed assembly of DNA linked quantum dots on both gold and silver substrates is obtained by Electron Beam Lithography patterning and a series of surface chemical functionalizations. A single quantum dot was successfully positioned within 100 nm of the desired location in 36 percent of the experiments. Furthermore, the method was completely reproducible within 500 nm accuracy. This method has the potential to functionalize quantum dots in even smaller pattern sizes. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A12.00010: Stress-driven self-assembly of Si-based nanomembranes for on-chip applications Francesca Cavallo, Rudeesan Songmuang, Yongfeng Mei, Armando Rastelli, Oliver Schmidt A new field of Si technology based on transferable and engineered nanomembranes has developed with the realization of the fact that properties of bulk Si are preserved in nm-thin layers released from the substrate surface. We demonstrate the ability to pattern Si-based films with nano- scale features, and fold them into a predetermined 3D configuration by finely tuning the strain distribution in the membranes by well-established deposition processes, i.e., MBE, PVD, and thermal oxidation. Our major contributions are the fabrication of integrated microtube resistors based on Si:B/SiGe:B tubes; the use of the Ge condensation technique to tailor the strain distribution in SiGe films on insulator; the manufacturing of fully scalable and CMOS compatible all- semiconductor and hybrid tubes; the fabrication of linear and circular networks formed by interconnected wrinkled structures; the experimental demonstration of light emission from Ge and Si nanoparticles integrated in a tube wall; the observation and investigation of the waveguiding effect along the axis of SiOx/Si tubes. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A12.00011: Isotropic and anisotropic strain-induced self-assembled oxide nanostructures Marta Gibert, Patricia Abellan, Alessandro Benedetti, Felip Sandiumenge, Teresa Puig, Xavier Obradors The apparition of new functionalities based on size- and shape-dependent properties requires strategies for the formation of well-defined structures at nanometric scale. We present a bottom-up low-cost chemically-derived methodology based on the control of strain and surface energies anisotropies in CeO2/LAO system to tune the lateral aspect ratio, orientation and kinetics of interfacial oxide nanostructures. Self-organized uniform square-based nanopyramids form under isotropic strain [1]. In contrast, highly elongated nanostructures (long/short axis $\sim $20) grow induced by biaxial anisotropic strain and anisotropic surface energies. Island's distinct crystallographic orientation is the clue of their differentiated shape, and also influences their distinct evolution. The kinetically-limited coarsening of isotropic nanodots contrasts with the ultrafast kinetics of anisotropic islands. Experimental analyses are based on AFM, TEM, XRD and RHEED, and simulations based on a thermodynamic model enables us to confirm the equilibrium shape of each sort of island's shape in relation to its misfit strain and surface characteristics. [1] Gibert, M. et al., \textit{Adv.Materials} \textbf{19} (22), 3937 (2007). [Preview Abstract] |
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