Symposium Organizers
Margit Zacharias Max-Planck-Institute of Microstructure Physics
Walter Riess IBM Research GmbH
Peidong Yang University of California-Berkeley
Younan Xia University of Washington
P1: Methods for Templating and Nanostructuring
Session Chairs
Tuesday PM, April 18, 2006
Room 2024 (Moscone West)
11:30 AM - **P1.1
Template Approaches to Nanowire Growth.
Ulrich Goesele 1 , Danilo Zschech 1 , Martin Steinhart 1 , Woo Lee 1 , Kornelius Nielsch 1 , Silke Christiansen 1 , Peter Werner 1
1 Experimental Dept. II, Max Planck Institute of Microstructure Physics, Halle Germany
Show AbstractFor many technical applications it is desirable to grow nanowires ofdefined diameter and length at predefined positions. The talk willdescribe a number of approaches using templates either as mask fordefining the location of catalytic particles or as growth reactorwithin which the nanowires are grown. Special emphasis will be put ontemplates based on ordered pore arrays of alumina ordiblock-copolymers as well as those based on dislocation arraysfabricated by twist wafer bonding. The state of the art of theseapproaches will be discussed and potential future approaches will beoutlined.
12:00 PM - P1.2
Large Area Si Nanowire Arrays Fabricated Using Nano-Imprint Lithography.
Luke Hunter 1 , A. Talin 1 , Bhavin Rokad 2 , Francois Leonard 1 , Blake Simmons 1 , Paul Dentinger 1
1 Nanoscale Science & Technology, Sandia National Labs, Livermore, California, United States, 2 , Cornell University, Ithica, New York, United States
Show Abstract12:15 PM - P1.3
Defined Preparation and Optical as well as Structural Characterization of ZnO Nanorods.
Thomas Buesgen 1 , Michael Hilgendorff 1 , Witold Kandulski 1 , Peter Karageorgiev 1 , Michael Giersig 1
1 nanoparticle technology, caesar research center, Bonn Germany
Show AbstractWe present structural and optical properties of ZnO nanorods grown by chemical vapor deposition (CVD) and wet-chemical syntheses. The CVD-growth is catalyzed by gold islands, pre-patterned on sapphire substrates by use of the nanosphere lithography (NSL) technique, resulting in laterally ordered, upright ZnO nanowires of diameters less than 100 nm and a length of up to several micrometers. By modifying the NSL mask using annealing or chemical treatment, the holes between adjacent nanospheres can be reduced, which results in smaller, well-separated catalytic islands on the substrate. Tilting and turning the sample holder during catalyst evaporation produces many different structures.Rods grown by the wet chemical approach from zinc organics or zinc salts as precursors are much thinner with diameters less than 10 nm and aspect ratios up to 10. We show, that the use of long-chain amines induces the one-dimensional growth. Currently, we are carrying out doping experiments on both CVD and wet-chemical synthetic routes, to influence the conductivity, magnetism or luminescence of ZnO. We are going to present first results on these doped ZnO nanorods, which probably have applications as light emitting devices, sensors, bio-labels or piezoelectric devices, etc. in the near future.All produced rods are characterized structurally by electron microscopy (SEM, TEM, HRTEM) and optically by absorbance and photo-luminescence spectroscopy. Furthermore, we show results obtained by scanning near-field optical microscopy of the ZnO nanowires.
12:30 PM - P1.4
Synthesis and Characterization of Silicon Nanorod Arrays for Solar Cell Applications
Brendan Kayes 1 , Joshua Spurgeon 1 , Nathan Lewis 1 , Harry Atwater 1
1 , California Institute of Technology, Pasadena, California, United States
Show AbstractAs a photovoltaic material, nanorods have the potential to enhance carrier collection and hence increase efficiency, in the case that the minority carrier diffusion length in the material is much less than the material’s “optical thickness”, i.e., thickness of material required to absorb 90% of the energy from solar photons above the band gap of the material. This can be achieved by creating an array of nanorods aligned normal to a substrate, either embedded inside another material, or with each nanorod having a radial pn junction (see Kayes, Lewis, and Atwater, in Jour. Appl. Phys. 97 (11): Art. No. 114302 JUN 1 2005). Silicon nanorods for photovoltaic applications have been grown by chemical vapor deposition (silane diluted to 5% concentration in argon), using either gold or indium as a catalyst for the vapor liquid solid (VLS) process. Rod morphology improves with increasing substrate temperature and decreasing silane partial pressure in the range T=300-600 C and for silane partial pressure = 50-1000 mTorr. Best results were achieved with substrate temperatures of 600 C and silane partial pressure of 50 mTorr. Flow rate was varied between 40 and 200 sccm. Growth was achieved on both silicon and germanium substrates. In most cases, catalyst particles were formed by partial de-wetting of vapor deposited films of the catalytic material from the substrate to form droplets with diameters of tens to hundreds of nanometers. Periodic arrays of catalyst particles with controlled size and spacing were achieved by both the use of porous alumina membranes and by e-beam lithography. Using these techniques, silicon nanorods were grown with diameters of 100nm to microns and lengths of microns to tens of microns. Using a gold catalyst with or without templating, and using an indium catalyst with a template, the goal of fabricating dense arrays of silicon nanorods aligned normal to the substrate was achieved.Nanorod morphology was investigated primarily by scanning electron microscope (SEM). It was found that indium acts as a VLS catalyst and can seed growth of nanorods that are straight for up to tens of microns. However, without templating, the growth of nanorods did not occur across the entire substrate, apparently due to both the high mobility of indium on the substrate surface and also the high indium vapor pressure at the deposition temperature. These problems were alleviated by the templating methods described above. Gold does not suffer from either of these qualities, making it apparently a more suitable catalyst. However, it appears that the incorporation of gold into the nanorod as it grows quenches luminescence (private communication with Prof. Mark Brongersma of Stanford University). Gold is well-known to form a deep-level impurity in silicon. We will present photoluminescence (PL) intensity measurements illustrating the effect that changing catalyst has on the optical properties of the nanorods.
P2: Nanowire Growth
Session Chairs
Tuesday PM, April 18, 2006
Room 2024 (Moscone West)
2:30 PM - **P2.1
Silicon Nanowire Growth Kinetics from In situ Transmission Electron Microscopy.
Frances Ross 1 , Suneel Kodambaka 1 , James Hannon 1 , Ruud Tromp 1 , Mark Reuter 1 , Jerry Tersoff 1
1 TJ Watson Research Center, IBM Research Division, Yorktown Heights, New York, United States
Show AbstractNanowires grown by the vapour-liquid-solid process have exciting possible uses as elements of vertical transistors, sensors or microelectromechanical devices. Most applications of nanowires require the diameter to be uniform, the surface structure to be well defined, and the growth to initiate at specific positions on a substrate. A detailed understanding of the effects of different growth conditions on the growth kinetics and surface structure would be helpful in achieving the necessary control. We have therefore measured in real time the growth kinetics of individual Si and Ge nanowires formed using Au as the catalyst, observing nucleation and measuring growth direction, growth rate and surface structure as a function of temperature, gas ambient and wire diameter. This data was obtained by recording wire growth at video rate in an ultra high vacuum electron microscope which has deposition capabilities enabling wires to be grown in situ. We show that the kinetics of Si wire growth display interesting features which are not included in the basic vapour-liquid-solid growth mechanism. In particular, surface diffusion of the Au catalyst leads to droplet coarsening, changing the diameter of wires during growth, while surface oxidation slows this diffusion process and can also change the wire growth direction. We will compare these results for Si with kinetic and structural data obtained during Ge wire growth, and will also discuss in situ observations of the growth of heterostructures. Finally we will consider the implications of such real-time results on forming wirelike structures of controlled orientation and uniformity in Si and in other systems.
3:00 PM - P2.2
A New Understanding of Metal-assisted Growth of 1D Nanowire.
Zhenyu Ryu 1 , Judith Yang 1 , Kyeongjae Cho 2
1 Department of Materials Science and Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania, United States, 2 Department of Mechanical Engineering, Stanford University, Stanford, California, United States
Show AbstractOne-dimensional (1D) nanostructures have attracted a considerable attention, due to their potential application as building blocks for electronic and photonic nanodevices. A remarkably elegant method of producing nanowires is through a metallic nanoparticle which catalyzes a solid nanowire from a vapor phase precursor molecules. However, detailed mechanistic understanding of the catalytic growth process is still lacking. Such understanding is needed for further control and development of nanowires. The interface between metal nanoparticle and nanowire is the growth front of the nanowire, and its detailed structure plays a crucial role in the nanowire formation. A detailed experimental analysis for metal nanoparticle-nanowire interface is being carried out by ex-situ and in-situ electron microscopy, including Z-contrast imaging, energy dispersive X-ray emission (EDX) and electron energy-loss spectroscopy (EELS) techniques, electron diffraction and high-resolution electron microscopy (HREM). The experimental data will be systematically compared with a multi-length scale modeling of nanowire growth from metal nanoparticles. The observed results showed that active sites or catalyst facets on the metal-nanoparticles are essential to nucleate the nanowires. Single active site would lead to one nanowire growth and two active sites would lead to formation of twin nanowires or biaxial nanowires, even branched heterostructured nanowires. These understandings on reaction mechanisms may be helpful for the controlling synthesis of 1D nanowire structures and further application development.
3:15 PM - P2.3
What Makes the Generation of Silicon Nanowires by Molecular Beam Epitaxy so Special
Peter Werner 1 , Nikolai Zakharov 1 , Gerhard Gerth 1 , Luise Schubert 1 , Ulrich Goesele 1
1 , MPI of Microstructure Physics, Halle (Saale) Germany
Show AbstractSilicon nanowires (Si NW) can be successfully grown by applying the vapor-liquid-solid process (VLS). Small metal particles, e.g. of gold, deposited on a substrate are used as a seed for the subsequent NW growth. In the case of the mainly used chemical vapor deposition technique (CVD) a Si containing gas/precursor is cracked at the Au droplets and single Si atoms are subsequently solved in the liquid metal. Due to a supersaturation within this droplet, Si precipitates predominantly at the liquid-solid interface – a nanowire growth. At the substrate surface no Si deposition is observed. In the case of the low-pressure CVD technique, the surface is even covered by carbon or oxide layers, which favors the growth only at the metal tip of the NW. A partly completely different situation occurs, if NW are grown by molecular beam epitaxy (MBE) via the VLS mechanism. We will describe the differences between CVD and MBE generated NW. This concerns the role of the metal seed, the morphology of the NW and the aspect ratio of length and width. Especially surface diffusion including the metal used as well as Si strongly influences the growth process. Surface contamination (oxygen, carbon) have a significant influence on the MBE growth and on a further technological application.
3:30 PM - P2.4
Silicon Nanowire Epitaxial Growth Dependence on Substrate Orientation.
Pavan Aella 1 3 , W. Petuskey 1 3 , S. Picraux 2 3 4
1 Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona, United States, 3 Science and Engineering Materials Graduate program, Arizona State University, Tempe, Arizona, United States, 2 Department of Chemical and Materials Engineering, Arizona State University, Tempe, Arizona, United States, 4 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractSilicon nanowires were grown on different substrates to study the substrate influence on the growth characteristics of the wires. In contrast to recent reports we show a strong dependence of the nanowire growth rate and orientation on the substrate. Initially, catalytic, 1nm thick gold films were thermally evaporated onto hydrogen terminated Si (100), Si (111), and Si (110) substrates, as well as on 500 nm thick SiO2 films, heated to 250oC in a UHV deposition system. Silicon nanowires were grown in a low pressure chemical vapor deposition system by the catalytic vapor-liquid-solid (VLS) technique using 5% SiH4 diluted in H2 at temperatures ranging from 450 to 600oC. All substrates were placed side by side during VLS growth to allow a direct comparison of the nanowire morphology under identical growth conditions. Field emission SEM images show that the nanowires grow predominantly in the <110> and <111> directions. Clear differences in nanowire nucleation density are observed as a function of both substrate type and growth conditions with the nanowire density being much lower on the SiO2 substrate. The length of the nanowires is dependent on the substrate orientation and the growth temperature, with nanowires growing increasingly longer for epitaxial seeding from (100) vs (110) vs (111) substrates, and at higher temperatures. We also show the anomalous high seeding of very small diameter nanowires (≤20nm) on (110) oriented substrate, suggesting the presence of an easier nucleation configuration in this case.
3:45 PM - P2.5
Growth and Passivation of Vertically Aligned Germanium Nanowires for Three Dimensional Nanoelectronics
Hemant Adhikari 1 , Philippe Rouffignac 3 , Kevin Kim 3 , Roy Gordon 3 , Christopher Chidsey 2 , Paul McIntyre 1
1 Materials Science and Engineering, Stanford University, Stanford , California, United States, 3 Chemistry and Chemical Biology, Harvard University, Cambridge, California, United States, 2 Chemistry, Stanford University, Stanford, California, United States
Show AbstractIn the emerging technology of 3-dimensional (3-D) nanoelectronics, vertically aligned nanowires have been proposed to provide a solution to attain ultra high density nanoscale device arrays. Germanium nanowire (GeNW) transistors are very promising components for active device layers above a single crystal silicon substrate because of: (a) the relatively low growth temperature of these nanowires, which is compatible with sub-400°C temperatures expected to be required for 3-dimensional integrated circuits and (b) the high intrinsic hole and electron mobilities of Ge compared to Si. In this paper, we present results of growth of vertically aligned single-crystal germanium nanowires at temperatures of 350°C or less by metal nanoparticle-catalyzed chemical vapor deposition. Single crystal Ge (111), Ge (110), Ge (001) and an epitaxially-grown Ge film on a Si (001) wafer were used to explore the epitaxial relation between the nanowires and the substrate. We have observed homoepitaxial growth of GeNWs along <111> and <110> growth orientations on these substrates. Because wires grown at higher temperatures (350°C) are significantly tapered, a two-temperature growth procedure was devised to obtain epitaxial Ge nanowires of constant diameter. A short high-temperature step to promote nucleation is followed by a longer lower temperature nanowire growth step. Our results indicate that single crystal growth of GeNWs can occur at temperatures substantially less than those required for efficient nucleation of epitaxial nanowires. The defect free nature of the germanium nanowires and their homoepitaxial relationship with the substrate has been studied by high resolution transmission electron microscope imaging and diffraction. Detailed investigation of the surface chemistry of as-grown and air-exposed GeNWs and exploration of various chemical passivation pathways is valuable for understanding and controlling the behavior of devices made from these GeNWs. With the photoemission studies of the surface composition of GeNWs, using a low energy synchrotron source, we find that the wires are initially free of oxide, oxidize relatively slowly in air and can be cleaned of oxide by various aqueous treatments. This is, to our knowledge, the first report of as-grown GeNWs that are free of surface oxide. The control of surface composition demonstrated in this paper forms a sound basis for the deposition of high-quality gate-dielectric layers on the nanowires. We have deposited conformal layers of high dielectric constant hafnium nitride and hafnium oxide layers on the free standing germanium nanowires using atomic layer deposition. In an effort to embed and isolate the nanowire device layers from other layers in a 3 D circuit, the space between the nanowires was filled in with silica by a novel atomic layer deposition technique. The tips of these nanowires, when exposed by chemical mechanical polishing, can serve as seeds for epitaxial growth of another Ge device layer.
P3: Si Nanowires - Doping and Devices
Session Chairs
Tuesday PM, April 18, 2006
Room 2024 (Moscone West)
4:30 PM - P3.1
Resistivity Measurements of Intentionally Doped Silicon Nanowire Arrays.
Sarah Dilts 1 , Alexana Cranmer 1 , Suzanne Mohney 1 , Joan Redwing 1
1 Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania, United States
Show AbstractResistivity measurements of individual nanowires can be difficult, requiring the use of advanced assembly and lithography techniques. Additionally, contact resistance can dominate nanowire resistance measurements carried out using a simple two-point geometry. In this study, resistivity measurements were performed on high-density vertical arrays of intentionally doped silicon nanowires (SiNWs) synthesized by vapor-liquid-solid growth in nanoporous alumina membranes. Alumina membranes provided a support structure for the aligned growth of the nanowires and also assisted in the formation of electrical contacts via the top and bottom membrane surfaces. The alumina membranes used as templates in this study had a nominal thickness of 60 µm and 200 nm diameter pores. The membrane structures were prepared by initially sputtering a thin layer of silver on the back-side of the membrane followed by the sequential electrodeposition of 5 µm of silver, 30-50 µm of cobalt and 0.25 µm of gold within the pores. Vapor-liquid-solid growth of SiNWs within the pores was then carried out at 500 °C and 13 Torr using SiH4 as the silicon source and trimethylboron (TMB), and phosphine (PH3) for p-type and n-type doping, respectively. The dopant/SiH4 ratio was varied from 2E-2 to 2E-4 for p-type and from 2E-3 to 2E-5 for n-type doping. Circular top-side electrical contacts to the SiNW arrays were formed by e-beam evaporation of Al for p-type and Ti/Pt/Au for n-type SiNWs. For measurements of nanowire resistivity and contact resistance, a series of samples were prepared in which the length of the SiNWs was varied from 5 to 25 µm using a constant dopant/SiH4 inlet gas phase ratio during growth. Plots of total resistance versus SiNW length were used to extract the resistance of the SiNW arrays and average contact resistance. Measurements of nominally undoped nanowire arrays yielded an average resistivity of 2.84 +/- 0.30 Ω-cm. This result was similar to the resistivity of 2.1 Ω-cm determined for individual undoped SiNWs grown out the top of the membrane and assembled into a gated 4-point electrical testbed. The gated measurements reveal that an unintentional p-type impurity is present in SiNWs grown in nanoporous alumina membranes. The addition of dopant precursors during growth resulted in a decrease in the average nanowire resistivity to 0.22 +/- 0.07 Ω-cm for highly doped (TMB/SiH4= 2E-2) p-type wires and 0.034 +/- 0.002 Ω-cm for highly doped (PH3/SiH4= 2E-3) n-type wires.
4:45 PM - P3.2
Post Growth Doping of CVD Grown Silicon Nanowires with Boron
Sarang Ingole 1 , Teresa Clement 1 , Jacob Thorp 1 , Pavan Aella 2 , S Picraux 3 1
1 Department of Chemical and Materials Engineering, Arizona State University, Tempe, Arizona, United States, 2 Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona, United States, 3 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractBeing able to obtain the desired electrical conductivity in silicon nanowires (SiNWs) via controlled doping is critical to the development of SiNW-based devices. From studies reported so far, the introduction of dopant gases during growth has been the preferred approach for electrically doping SiNWs. However in current silicon device technology the most commonly used doping methods involve diffusion and ion implantation. It is thus of technical and scientific interest to explore post growth techniques for doping SiNWs. In the present work we report a newly developed diffusion-based approach for electrical doping of SiNWs with boron. In this approach Spin-On-Dopant spun on an inert substrate serves as a boron source. The boron source and nanowire sample are kept in proximity to each other (~300 µm distance) during the predisposition stage. In a separate stage rapid thermal annealing is performed for drive-in diffusion. By using a separate source wafer for the predeposition stage we are able to better control the extent of boron oxide deposition and thus limit both chemical formation of difficult to remove surface layers as well as sacrificial oxidation of the SiNWs. SIMS analysis on clusters of nanowires doped in this fashion allows qualitative detection of the presence of boron. For studying electrical transport characteristics, the SiNWs are aligned using electric field between previously deposited Cr/Au contacts. These contacts are deposited on an oxidized Si substrate (oxide thickness 500 nm) with 2 µm separation. We also use contacts with 300 nm separation defined by Focused Ion Beam machining for short channel SiNW and four probe measurements. Linear I-V characteristics are observed after low temperature sintering of the contacts. To further confirm doping characteristics, gate voltage and temperature dependent I-V measurements are conducted. From the present I-V characteristics doping concentrations thus obtained are estimated to be in the 1017 to 1019/cm3 range. Work to improve the metal–SiNW contact formation and better control over the number of nanowires between electrodes is underway. We conclude that the present doping approach of Spin-On-Dopant in combination with proximity deposition is promising for post-growth diffusion doping of SiNWs.
5:00 PM - P3.3
Thermally-Oxidized Silicon Nanowires: Structural and Electrical Properties.
Yanfeng Wang 1 , Bangzhi Liu 2 , Tsung-Ta Ho 1 , Sarah Dilts 2 , Suzanne Mohney 2 , Joan Redwing 2 1 , Theresa Mayer 1
1 Department of Electrical Engineering, The Pennsylvania State University, University Park, Pennsylvania, United States, 2 Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania, United States
Show Abstract There has been considerable interest in bottom-up integration of semiconductor nanowires for their applications in future logic, memory, and sensor circuits. The ability to grow p- and n-type silicon nanowires (SiNWs) of varying carrier density has been demonstrated. However, the subthreshold properties of SiNW field effect transistors (FETs) often exhibit severe hysteresis partly due to their large unpassivated surface area. In this presentation, we will discuss the structural properties of thermally-oxidized p- and n- SiNWs and the electrical results of top-gated FETs fabricated using them, which show improved operational stability as compared to unpassivated SiNW FETs. The SiNWs used in these studies were synthesized by vapor-liquid-solid (VLS) growth from Au catalyst particles using 10% SiH4 in H2 as the silicon gas source, trimethylboron (TMB) as the p-type dopant, and phosphine (PH3) as the n-type dopant. The ratio of TMB or PH3 to SiH4 was varied from 0 to 10-2 to modulate the hole or electron carrier concentration in the SiNWs. Dry oxidation of the as-grown SiNWs was carried out at temperatures between 700 and 1000oC after removing the Au catalyst particles by wet etching and cleaning the NW surface to remove residual metal contaminants. The oxide thickness and interfacial roughness were investigated using transmission electron microscopy. These measurements show that the oxidation rate of the SiNWs is considerably faster than that observed on planar silicon substrates, and that the interface between the SiNW and the thermal oxide is very smooth. As-grown and thermally-oxidized SiNWs were integrated into top- and back-gated test structures. The breakdown field strength of a 10-nm thick layer of SiO2 was measured by probing between the top gate and the source of the FET, and was found to be approximately 5×106 V/cm, which is comparable to that observed for planar silicon samples. Significantly less hysteresis in the subthreshold characteristics was observed for different sweep rates and directions in top-gated, thermally-oxidized SiNW FETs as compared to back-gated, as-grown SiNW FETs. This improvement could be due in part to the improved passivation provided by the SiO2 shell as well as the smaller exposed surface area of the top-gated FETs. Top-gated p- and n- SiNW FETs have small subthreshold slope of ~ 0.25 V/dec and high ON/OFF ratios of ~ 105. These results demonstrate that SiO2 grown by thermal oxidation of the as-grown SiNWs can be used as a gate dielectric that improves the operational stability of SiNW FETs.
5:15 PM - P3.4
Vertical Silicon Nanowire Field Effect Transistors.
Joshua Goldberger 1 , Allon Hochbaum 1 , Rong Fan 1 , Peidong Yang 1
1 Chemistry, UC Berkeley, Berkeley, California, United States
Show AbstractSilicon nanowires have received considerable attention as transistor components because they represent a facile route towards sub-100 nm single-crystalline Si features with minimal surface roughness. Typically, silicon nanowire transistors have a horizontal planar layout with either a top or back gate geometry. However, the difficulty in reliably assembling ultra-high density planar nanowire circuits, combined with the performance limitations of the horizontal device geometry may ultimately hinder nanowire-based electronics from realizing their full potential. Pushing the transistor geometry into the third dimension would result in ultra-high transistor densities without the need for multi-step post-growth nanowire alignment processes. In addition, a vertical nanowire geometry promises enhanced transistor performance due to the enhanced gate control efficiency in its surround-gate design. Herein we demonstrate the integration of vertically grown Si nanowire arrays into vertical field effect transistors with a surround-gate architecture. These first-generation vertically-integrated nanowire field-effect-transistors (VINFETs) exhibit electronic properties that are comparable to traditional metal-oxide silicon field effect transistors, suggesting that further optimization of this device structure may make them competitive with advanced solid state electronic devices, e.g. double-gate Fin field-effect transistors (FINFET), for future nanoelectronic devices. This presentation will focus on the fabrication and properties of our VINFET devices.
5:30 PM - P3.5
A Novel Cross-bar Structure Toward Ultrahigh-density Nanowire Devices.
Dunwei Wang 1 , Bonnie Sheriff 1 , James Heath 1
1 , Caltech, Pasadena, California, United States
Show AbstractNanowire or nanotube electronic devices, such as field effect transistors, memory and logic, units have been built by various groups on a wide range of materials with different structures. The research focus, however, have been mostly devoted to single unit functionality demonstration. For large scale integration toward comprehensive functionalities, routing of individual units requires conventional photo or e-beam lithography techniques. In this context, scaling of nanostructure devices still remains challenging due to the lack of advantageous integration schemes despite the small dimensions offered by nanostructures themselves. Here we present a novel cross-bar structure for ultrahigh-density devices. This structure is realized on highly order Si nanowires ~10nm in diameter and a spacing pitch less than 20nm, fabricated through superlattice nanowire pattern transfer (SNAP). Selectively doped p- and n-type Si nanowires building blocks as parallel arrays are fabricated on SiO2 substrates. Ohmic contacts and gate electrodes are formed perpendicular to the arrays, serving as power supply, input and output. Nanowire selection for different functionalities is realized by altering gate dielectric materials, either high-k or low-k to select or deselect certain nanowires. Complementary logic devices are realized using this scheme and it is also demonstrated that cross-bar structure is a generic approach toward ultrahigh-density nanowire devices.
5:45 PM - P3.6
Fabrication and Post-growth Doping of Silicon Nanowire for Novel Nano-electronic Devices.
Kumhyo Byon 1 , D. Tham 1 , A. Johnson 2 , J. Fischer 1
1 Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 2 Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Show AbstractOne-dimensional silicon nanowires (SiNWs) are attractive materials for future nanoelectronic applications. Reliable control of the carrier type and concentration is crucial for the application of these nanowires to working devices and integrated systems. In this work, field effect transistors (FETs) were fabricated using both as-grown p-SiNWs and post-growth n-doped SiNWs. Single crystalline silicon nanowires (SiNWs) sheathed with oxide were synthesized by thermal evaporation without the use of catalyst. FET devices from p-type source materials behave as p-channel devices with channel mobilities 1 - 10 cm2 V -1 s -1. Using bismuth vapor, the as-grown SiNWs were doped into n-type materials with various doping concentrations depending on the oxide sheath thickness. The majority carriers in SiNWs can therefore be controlled by choosing proper vapor phase species for dopant. We anticipate that the presented fabrication and doping technique can be used to make more sophisticated devices such as diodes or bipolar transistors by selective patterning and doping of the SiNWs in the future.
Symposium Organizers
Margit Zacharias Max-Planck-Institute of Microstructure Physics
Walter Riess IBM Research GmbH
Peidong Yang University of California-Berkeley
Younan Xia University of Washington
P4: Si Nanowires - Structure
Session Chairs
Wednesday AM, April 19, 2006
Room 2024 (Moscone West)
10:00 AM - P4.1
Surface Control of Si Nanowire Electronic Structure.
Paul Leu 1 , Kyeongjae Cho 1
1 , Stanford University, Stanford, California, United States
Show AbstractSemiconductor nanowires (NWs) are promising nanomaterials for diverse nanodevice applications. NW electronic and photonic devices have been experimentally demonstrated, and NW photonic devices can be used to develop tunable sub-wavelength photonic devices. NW band gaps show strong dependence on their diameters due to quantum confinement effects, and it is possible to tune the band gap by controlling NW diameter. The silicon nanowire (SiNW) is a well known example of NWs, and SiNW diameters can be controlled by controlling the size of the Au nanoparticle catalysts used during chemical vapor deposition (CVD) growth of SiNWs. However, the effects of chemical modification of SiNW surface is not well investigated even though silicon surface modifications are frequently used in microelectronic device processes such as HF treatment of Si surface to remove silica and terminate the surface with Si-H bonds. In this study, we have performed detailed ab initio simulations using density functional theory (DFT) method to investigate the effects of oxide, hydrogen, and halogen surface species on the SiNW electronic structure [1]. The result shows that the surface chemical bonding change has stronger effects on the NW electronic structure (comparable to those of quantum confinement effects or even larger for some cases). A detailed analysis of the electronic structure and energetics is performed for different sized nanowires, and the results predict that the NW electronic structure can be controlled by controlling the chemical modification of NW surface. This prediction opens up new possibilities for controlling nanowire band gap and optical properties by through surface chemistry. Furthermore, this result indicates a possibility of developing SiNW chemical sensor based optical detection (rather than more commonly known electronic detection) by monitoring the optical response of NWs under the presence of chemical species [2, 3]. [1] S. Peng and K. Cho, “Chemical Control of Nanotube Electronics,” Nanotechnology 11, 57 (2000).[2] S. Peng and K.Cho, “Ab Initio Study of Doped Carbon Nanotube Sensors,” Nano Lett. 3(4), 513-517 (2003).[3] S. Peng, K. Cho, P. Qi, and H. Dai, “Ab initio Study of CNT NO2 gas sensor,” Chem. Phys. Lett. V.387 p.271-276 (2004).
10:15 AM - P4.2
Raman Spectroscopy of Silicon Nanowires.
Andrea Ferrari 1 , Stefano Piscanec 1 , Mirco Cantoro 1 , Stephan Hofmann 1 , Alan Colli 1 , John Robertson 1
1 Engineering, University of Cambridge, Cambridge United Kingdom
Show AbstractWe measure the Raman spectra of silicon nanowires [1] produced by plasma enhanced chemical vapor deposition and oxide assisted growth [2,3]. In contrast to what often assumed, we show that local heating plays a major role in Raman spectroscopy of Si nanostructures. The Raman spectra strongly change as a function of laser power, since the low thermal conductivity of the nanowires results in intense heating under the laser beam [1]. This contrasts with bulk Si, where no heating is observed for similar laser power. The local temperature on SiNWs can reach several hundreds K for an excitation power of ~2 mW [1,4], which is a typically power level in micro-Raman measurements in other materials, such as carbon nanotubes. Only by using extremely low laser power and low ambient temperatures we can eliminate the thermal effects and study the effects of phonon confinement on the Raman spectra [1]. Unlike dots, wires are not confined along the axis, whilst nano-ribbons are confined only in one dimension. Thus, the trends in the peak position and width differ and allow us to identify these nanostructures. The diameter derived by phonon confinement is in good agreement with TEM measurements. For high power measurements the peak positions are significantly lower than what predicted by the confinement theory. To fully account for the measured spectra it is necessary to include an-harmonic phonon effects [5] and consider the in-homogeneous nature of the heating under the Raman microscope [1]. The Raman induced heating effects are not peculiar to Si nanostructures, but are expected to be a general finding in semiconductor nanowires and nanocrstals.1.S. Piscanec et al. Phys. Rev. B 68, 241312(R) (2003)2.S. Hofmann et al. J. Appl. Phys. 94, 6005 (2003)3.R. Q. Zhang et al., Adv. Mater. 15, 635 (2003).4.R. Gupta et al., Nano Lett. 3, 627 (2003).5.M. Balkanski et al., Phys. Rev. B 28, 1928 (1983).
10:30 AM - P4.3
Strain in Semiconductor Nanowire Heterostructures
S. Picraux 1 3 , A Batwal 3 , P Peralta 3 , J Taraci 3 , M Hytch 2 , T Clement 3 , D Smith 3 , M McCartney 3 , Jeff Drucker 3
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 3 , Arizona State University, Tempe, Arizona, United States, 2 , Centre National de Recherche Scientifique, Vitry-sur-Seine France
Show Abstract10:45 AM - P4.4
Simulation of Semiconducting Nanowires with Core-shell Structures.
Rana Biswas 1 , Bicai Pan 2
1 Dept. of Physics & ECpE, MRC, Ames Lab, Iowa State University, Ames, Iowa, United States, 2 Physics, Univ of Science and Technology of China, Hefei China
Show AbstractP5: Transport and Devices
Session Chairs
Wednesday PM, April 19, 2006
Room 2024 (Moscone West)
11:30 AM - **P5.1
Quantum Coherent Transport in Semiconductor Nanowires.
Jorden van Dam 2 , Yong-Joo Doh 2 , Aarnoud Roest 3 , Erik Bakkers 3 , Leo Kouwenhoven 2 , Silvano De Franceschi 1 2
2 Kavli Institute of Nanoscience , Delft University of Technology, Delft Netherlands, 3 , Philips Research Laboratories, Eindhoven Netherlands, 1 , CNR TASC-INFM , Trieste Italy
Show Abstract12:00 PM - P5.2
Electrical Characteristics of Epitaxially Integrated InP Nano-bridges Between Silicon Electrodes.
M. Saif Islam 1 , Ibrahim Kimukin 1 , T. I. Kamins 2 , Sung Soo Yi 3 , G. Girolami 3 , Jun Amano 3
1 Department of Electrical and Computer Engineering, University of California Davis, Davis, California, United States, 2 Quantum Science Research, Hewlett-Packard Laboratories, Palo Alto, California, United States, 3 Molecular Technology Laboratory, Agilent Technologies, Palo Alto, California, United States
Show AbstractHeteroepitaxial growth of III-V compound semiconductors on Si can open exciting opportunities for Si optoelectronics. Almost 8% lattice and large thermal expansion mismatch along with differences in crystal structures, have hindered progress in epitaxial integration of III-V materials in the form of thin films on Si. However, metal catalyzed III-V nanowires were found to epitaxially grow on Si due to the small cross-sections of nanowires that accommodates large lattice mismatch and help relieve strain. This work presents a bridging technique that connects metal-catalyzed InP nanowires between pre-fabricated Si electrodes. Two opposing vertical and electrically isolated Si surfaces are fabricated using coarse optical lithography, along with wet and dry etching. Lateral InP nanowires are then grown from one vertical surface by metal-catalyst-assisted chemical vapor deposition (CVD) and connected to the other vertical surface during growth, forming mechanically robust 'nanobridges'. The InP nano-bridges make mechanically strong and robust connection at both vertical Si surfaces and resist considerable force. By forming the structure on a silicon-on-insulator substrate, electrical isolation is achieved. High-resolution TEM measurements show that InP nanowires are single-crystalline and of high quality. Electrical measurements indicate a potential barrier between the Si electrode and the p-doped InP nanowires. The Si-InP interface characteristics are studied to evaluate the barrier height. Photo-excited carriers were observed in the nanowires under optical illuminations. The growth and bridging of these InP nanowires can be integrated with existing silicon processes and can offer a new degree of freedom in the design of heterojunction nanodevices combined with Si technology.
12:15 PM - P5.3
Vertical Silicon Nanowire Surround-Gate Field-Effect Transistor Realized.
Heike Riel 1 , Volker Schmidt 2 , Siegfried Karg 1 , Stephan Senz 2 , Heinz Schmidt 1 , Ute Drechsler 1 , Oliver Hayden 1 , Walter Riess 1 , Ulrich Goesele 2
1 Science & Technology, IBM Research GmbH, Rueschlikon Switzerland, 2 , Max Planck Institute of Microstructure Physics, Halle Germany
Show AbstractSemiconducting nanowires have recently attracted considerable attention as the ongoing miniaturization in microelectronics demands new, innovative fabrication and device concepts. Owing to their potential compatibility with existing CMOS technology, in particular, epitaxially grown silicon (Si) nanowires are considered to be one of the most promising candidates for future logic and memory elements.In this paper a generic process flow for fabricating vertical surround-gate field-effect transistors (VS-FET) from epitaxially grown Si nanowires is described and device characteristics are presented. The catalyst for the nanowire growth was patterned by electron beam lithography resulting in well defined arrays of vertical Si nanowires grown on Si (111) substrates.We demonstrate the fabrication processes using n-type silicon nanowires grown on a p-type substrate in ultra-high vacuum using gold as catalyst and silane as precursor gas. The VS-FET fabrication consists of various deposition and etching steps, and has the advantage that no chemical mechanical polishing is required. Moreover, the process can be used to fabricate individual as well as arrays of nanowire VS-FETs. Electrical characterization was carried out on vertical ungated n-doped and p-doped two-terminal devices and gated nanowire FETs. Single nanowires as well as parallel contacted arrays of about 10^4 nanowires were tested. The measured transistor output and transfer characteristics indicate the behavior of an inversion mode driven FET similar to a conventional p-channel MOSFET. Temperature-dependent measurements are also reported.
12:30 PM - P5.4
Low-Temperature Solid-Phase Epitaxy of Defect-Free Aluminum p+-doped Silicon for Nanoscale Device Applications.
Yann Civale 1 , Lis Nanver 1 , Peter Hadley 2 , Egbert Goudena 1 , Henk van Zeijl 1 , Hugo Schellevis 1
1 Laboratory of ECTM - DIMES, Delft University of Technology, Delft Netherlands, 2 Kavli Institute of Nanoscience, Delft University of Technology, Delft Netherlands
Show AbstractThe growth of semiconducting nanowires sometimes takes place below the eutectic temperature of the catalyst particle [1]. In these cases, a solid-phase epitaxy (SPE) mechanism has been suggested to describe the growth of the nanowires [2]. In SPE, atoms of the semiconductor diffuse through a solid transport metal and attach themselves to crystal at the metal/semiconductor interface. Here we describe the growth of nanoscale silicon islands by SPE using aluminum as the transport metal. Contact windows down to 100 nm in size are opened through a thermally oxidized mono-crystalline <100> Si substrate. A thin layer of aluminum and amorphous silicon is then sputtered in the same vacuum system. This stack is annealed in nitrogen at temperatures between 350°C and 500°C, far below the 577°C eutectic point of the Al/Si alloy. Silicon crystals grow preferably on the exposed silicon rather than on the surrounding oxide. In the initial stages of growth, crystals grow along the edges of the contact windows presumably driven by stress until a single crystal fills the entire contact window. The height of SPE-Si was determined by the initial Al thickness. This material, that is p+-doped due to aluminum, has been used to fabricate ultra-abrupt ultra-shallow p+n diodes, contacts, and p+np bipolar transistors. Despite the low-processing temperature, near-ideal device characteristics were reproducibly obtained, indicating an exceptionally defect-free epitaxial process. The quality of the SPE-Si was extensively investigated by fabricating electrical devices. The contact resistance to both p- and p+-bulk silicon regions was found to be low-ohmic and the total contact resistivity was measured to be at most 10-7 Ω.cm2. On n-Si, nearly-ideal p+n diodes are formed, with an ideality factor of 1.02. A SPE-Si region was used as emitter in p+np bipolar transistors. An analysis of the transistor characteristics indicates near-ideal forward base and collector currents and an Al-doping around 1.2 × 1018 cm-3.In view of the processing temperatures, the quality of the SPE p+-Si presented here is remarkable. The electrical characterization shows clearly that very controllable growth conditions have been found whereby the contact window is entirely filled with an exceptionally low-defect density at the interface with bulk-Si. Ultra-abrupt ultra-shallow junctions with a 25 nm deep p-doped region were obtained. All in all, these properties make this a very versatile fully CMOS compatible module for the integration of nanoscale silicon devices.[1] T. I. Kamins, R. Stanley Williams, D. P. Basile, T. Hesjedal and J. S. Harris, J. Appl. Phys., Vol. 89, pp. 1008-1016, 2001.[2] A. I. Persson, M. W. Larsson, S. Stenström, B. J. Ohlsson, L. Samuelson, and L. R. Wallenberg, Nature Materials, Vol. 3, pp. 677-681, 2004.
12:45 PM - P5.5
1D Hole Gas in Ge/Si Nanowire Heterostructures and Demonstration of High Performance Field Effect Transistors.
Jie Xiang 1 , Wei Lu 1 , Yongjie Hu 1 , Yue Wu 1 , Hao Yan 1 , Charles Lieber 1 2
1 Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, United States, 2 Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States
Show AbstractNanowires and nanotubes are promising building blocks for nanoelectronics because their unique 1D electronic structure could yield improved performance compared to conventional planar devices. For example, long carrier mean-free-paths and ballistic transport have been demonstrated in carbon nanotubes. Yet, the fundamental 1D quantum confinement effect on transport in semiconducting nanowires has not been observed before primarily due to poor contacts and dopant scattering. Here we employed the idea of band structure engineering to create a 1D hole gas in undoped epitaxial Ge/Si core/shell nanowire heterostructures. Similar to 2D electron and hole gases in planar semiconductor heterostructures, the valence band offset between Si shell and Ge core confines holes in the Ge channel, forming a quantum well structure. We show that transparent contacts to the hole gas can be made in a reproducible fashion and demonstrate ballistic transport through discrete 1D subbands for the first time in free-standing semiconductor nanowires. To show the potential of these clean 1D quantum well nanostructures for device applications, we further fabricated single Ge/Si nanowire field-effect transistors (FETs) using high-k dielectrics with a metal top gate geometry. The clean hole-gas system and enhanced gate coupling from the high-k dielectric afford very high device performance. Significantly, these results are the best achieved in nanowire FETs, and moreover, the raw performance data is several times better than state-of-the-art planar Si MOSFETs. In addition, studies investigating the scaling of device speed as a function of channel length and the effect of novel gate structures to control ambipolar behavior will also be discussed. Our Ge/Si core/shell nanowire heterostructures exhibit great potential as a new platform for the fundamental study of 1D transport as well as new building blocks for nanoscale electronics.
P6: Nanowire Heterostructures
Session Chairs
Wednesday PM, April 19, 2006
Room 2024 (Moscone West)
2:30 PM - **P6.1
Nanowires as Building Blocks in Quantum-based Devices.
Lars Samuelson 1
1 Solid State Physics / the Nanometer Structure Consortium, Lund University, Lund Sweden
Show AbstractIn this talk I will discuss growth of semiconductor nanowires based on epitaxial nucleation of nanowires from lithographically defined nanoparticles, allowing arrays of position- and dimension-controlled nanowires to be formed. Of special value for studies of physics of quantum dot systems and for applications in electronics and photonics is the opportunity to form designed and abrupt heterostructures within nanowires. I will give examples of our studies of electrical and optical properties of nanowire structures as well as devices implemented in this technology, for instance for single-electronics, resonant tunneling and for wrap-gate FET applications. I will conclude with examples of new opportunities offered by nanowires, e.g. in neuroscience and for nanoelectromechanical applications.
3:00 PM - P6.2
Three-dimensional Nanoscale Composition Mapping of Semiconductor Nanowires.
Daniel Perea 1 , Lincoln Lauhon 1 , Jonathan Allen 1 , Steven May 1 , Bruce Wessels 1 2 , David Seidman 1
1 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States, 2 Electrical Engineering and Computer Science, Northwestern University, Evanston, Illinois, United States
Show Abstract3:15 PM - P6.3
Nanowire Radial Heterostructures as High Electron Mobility Transistors.
Yat Li 1 , Jie Xiang 1 , Fang Qian 1 , Silvija Gradecak 1 , Yue Wu 1 , Hao Yan 1 , Charles Lieber 1 2
1 Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, United States, 2 Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States
Show AbstractSemiconductor nanowires are attractive building blocks for nanoscale electronic devices. The ability to assemble, improve and control the properties of these building blocks is critical in moving toward nanoscale integrated electronic circuits. In this regard, we report a general strategy to synthesize GaN/AlN/AlGaN nanowire heterostructures and their implementation as nanoscale high electron mobility transistors (HEMTs). GaN/AlN/AlGaN nanowire heterostructures were synthesized with atomic-level control using metal-organic chemical vapor deposition. Electron microscopy studies revealed that the nanowire heterostructure are dislocation-free single crystals, with radial modulation of composition and thickness well-controlled during synthesis. Electrical transport measurements on undoped GaN/AlN/AlGaN nanowire heterostructures demonstrate the existence of quantum confined electron gas, and moreover, yield high electron mobilities of 3100 cm2/V-s at room temperature and 21,000 cm2/V-s at 5 K. These new nanowire heterostructures have been used to assemble field-effect transistors with high-k dielectrics that exhibit outstanding device performance. GaN/AlN/AlGaN nanowire HEMTs offer great promise as a new building block for nanoscale integrated electronic circuits, high-sensitivity detectors and complementary macroelectronics on unconventional substrates.
3:30 PM - P6.4
Growth and Characterization of InP/InAs/InP Core-multishell Heterostructure Nanowires by Selective-area Metalorganic Vapor Phase Epitaxy.
Junichi Motohisa 1 2 , Premila Mohan 1 , Katsuhiro Tomioka 1 2 , Takashi Fukui 1 2
1 Research Center for Integrated Quantum Electronics, Hokkaido University, Sapporo Japan, 2 Graduate School of Information Science and Technology, Hokkaido University, Sapporo Japan
Show Abstract3:45 PM - P6.5
MOCVD Synthesis and Characterization of Aligned III-Nitride Nanowire and Heterostructure Nanowire Arrays.
George Wang 1 , J. Randall Creighton 1 , A. Alec Talin 1 , Paula Provencio 1 , Don Werder 2
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractNanowires based on the direct bandgap semiconductor Group III nitride (AlGaInN) materials system are attractive due to their potential in novel optoelectronic applications, including LEDs, lasers, high power transistors, and sensors. We have employed a metal-organic chemical vapor deposition (MOCVD) process to synthesize highly aligned arrays of single-crystalline GaN nanowires in a standard cold-wall rotating disk reactor on 2-inch diameter sapphire wafer substrates without the use of a template. SEM and TEM analysis indicate that the nanowires share a common growth direction and have aligned facets. Interestingly, the majority of the nanowires do not have a catalyst droplet at the tip, suggesting the growth differs from the standard vapor-liquid-solid process. Building on this technique, we have also been able to synthesize radial heterostructure nanowire arrays consisting of a GaN cores and various III-nitride shell materials, including AlN, InN, and AlGaN, and InGaN. In this presentation, several challenges and issues regarding control of the nanowire and heterostructure growth process will be discussed. We have found that the growth conditions, particularly temperature, have a strong effect on the structural, optoelectronic, and electrical properties of the nanowires. Additionally, the choice of substrate and the catalyst preparation play critical roles in the density, uniformity, and alignment of the nanowire arrays. Preliminary data on the use of lithographically patterned templates to control the growth of GaN nanowires will also be presented. The growth processes and reactor environment employed in this study are typical of those used to synthesize device-quality III-nitride films and should be scalable to larger commercial reactors and substrates.
P7: Sensors and Devices
Session Chairs
Wednesday PM, April 19, 2006
Room 2024 (Moscone West)
4:30 PM - P7.1
General and Powerful Platform for Large-scale, Label-free, Parallel Electrical Detection of Biomolecules by Ultrasensitive Nanowire Transistor Arrays.
Gengfeng Zheng 1 , Fernando Patolsky 1 , Charles Lieber 1 2
1 Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, United States, 2 Division of Engineering and Applied Science, Harvard University, Cambridge, Massachusetts, United States
Show AbstractNanoscale materials offer unique and powerful opportunities for detection of biological and chemical species central to many areas of healthcare and the life science, ranging from disease diagnosis to the discovery and screening of new drug molecules. Here we demonstrate a general and powerful platform using nanowire transistor arrays for large-scale, label-free, real-time, parallel electrical detection of a variety of biomolecules ranging from proteins, nucleic acids to viruses. Composed of hundreds of individually electrically addressable nanowire devices with highly sensitive and reproducible performances, these nanowire arrays can be controllably modified by solution arrays of antibodies or cell-surface receptors with precise device registration, and show discrete conductance changes characteristic of highly selective binding and unbinding of multiple target biomolecules, thus providing a high-throughput, real-time parallel detection and rapid screening of libraries of biomolecules. Studies show that proteins, nucleic acids and viruses can be simultaneously detected at femtomolar concentrations with high selectivity even in undiluted serum samples, and that simultaneous incorporation of control nanowires in a single array enables discrimination against false positive/negative signals. Moreover, both electrokinetic effects and noise analysis to demonstrate the diversified applications of our nanowire sensor system such as extracting single molecule binding kinetics. The integrated nanowire sensor array platform opens up substantial opportunities for diagnosis and treatment of complex diseases such as cancer, detection of biological threats, and fundamental proteomic and biophysical studies.
4:45 PM - P7.2
Synthesis and Applications of Single-crystalline Indium Oxide Nanowires.
Daihua Zhang 1 , Bo Lei 1 , Koungmin Ryu 1 , Fumiaki Ishikawa 1 , Chongwu Zhou 1
1 Electrical Engineering, University of Souther California, Los Angeles, California, United States
Show AbstractSingle-crystalline indium oxide nanowires were synthesized using a laser ablation method and characterized using various techniques. Precise control over the nanowire diameter down to 7 nm was achieved by using monodispersed gold clusters as catalytic nanoparticles. In addition, field effect transistors with on/off ratios up to 104 were fabricated based on individual nanowires. Detailed electronic measurements revealed that the In2O3 nanowires are n-type semiconductors with a typical electron mobility of ~ 100 cm2/Vs. Furthermore, we studied the chemical sensing properties of our In2O3 nanowire transistors at room temperature. Upon exposure to a small amount of NO2, the nanowire transistors showed a decrease in conductance of up to six orders of magnitude, in addition to substantial shifts in the threshold gate voltage. Our devices exhibit significantly improved chemical sensing performance compared to existing solid-state sensors in many aspects, such as the sensitivity, the selectivity, the response time and the lowest detectable concentrations. We have also demonstrated the use of UV light as a “gas cleanser” for In2O3 nanowire chemical sensors, leading to a recovery time as short as 80 seconds.
5:00 PM - P7.3
Three-Armed Cadmium Sulfide Nanowires
Oliver Hayden 1 3 , David C. Bell 2 , Andrew B. Greytak 1
1 Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, United States, 3 , IBM Research Laboratory, Rueschlikon Switzerland, 2 Center for Imaging and Mesoscale Structures, Department of Physics, Harvard University, Cambridge, Massachusetts, United States
Show AbstractHere, we present the synthesis of single crystalline three-armed cadmium sulphide (CdS) nanowires using pulsed laser ablation. The growth process and composition of these symmetrically-shaped CdS wires were studied by (S)TEM, EDS and SEM from the initial stages (nanocrystal) until the fully micrometer-long branched nanostructure. In contrast to tetrapod-shaped nanocrystals which are reported to have a zinc blende nuclei and wurzite arms, the “Mercedes”-star like CdS structure that we have synthesized is a single crystal zinc blende.The single crystalline tripods are efficient waveguides. Emission can be observed from the central part as well as from the end of each individual arm. Additional optical pumping experiments were performed to study this unusual waveguiding in detail. Nanoscale LEDs were fabricated with single tripods showing electroluminescence from multiple arms. [1] L. Manna, E. C. Scher, A. P. Alivisatos, J. Am. Chem. Soc. 122, 12700, (2000)
5:15 PM - P7.4
Three dimensional nanowire networks and complex nanostructures of In2O3.
D. Alina Magdas 1 2 , Ana Cremades 1 , Javier Piqueras 1
1 Fisica de Materiales, Universidad Complutense de Madrid, Facultad de Ciencias Fisicas, Madrid Spain, 2 Faculty of Physics, Babes-Bolyai University, 400084 Cluj-Napoca Romania
Show Abstract5:30 PM - P7.5
Single Crystal InSb Nanowires: Synthesis, Characterization, Properties and Applications.
Qi Ye 1 2 , Toshishige Yamada 2 , Hongbing Liu 1 , Natalio Mingo 2 , Raymond Scheffler 1 2 , Ryan Leverenz 1 2 , Hidenori Yamada 1 2
1 , Eloret Corporation, Sunnyvale, California, United States, 2 Center for Nanotechnology, NASA Ames Research Center, Mountain View, California, United States
Show Abstract5:45 PM - P7.6
Recombination Dynamics in Metal-oxide Nanobelts: Study and Application in All Optical Gas Sensor Devices.
Alfredo Bismuto 1 , Stefano Lettieri 1 , Pasqualino Maddalena 1 , Camilla Baratto 2 , Elisabetta Comini 2 , Guido Faglia 2 , Giorgio Sberveglieri 2
1 physics, CNR-INFM, Naples Italy, 2 , Sensor Lab- CNR-INFM, Brescia Italy
Show AbstractP8: Poster Session: Semiconductor Nanowires - Fabrication, Properties and Devices I
Session Chairs
Walter Riess
Younan Xia
Peidong Yang
Margit Zacharias
Thursday AM, April 20, 2006
Salons 8-15 (Marriott)
9:00 PM - P8.1
Electrochemical Fabrication of Heterojunction Nanowires
Cengiz Ozkan 1 , Xu Wang 2 , Ibrahim Khan 1 , Xiaoye Jing 3
1 Mechanical Engineering, University of California at Riverside, Riverside, California, United States, 2 Chemical and Environmental Engineering, University of California, Riverside, California, United States, 3 Electrical Engineering, University of California, Riverside, California, United States
Show AbstractSynthesis of nanowires or 1-D nanostructures and fabrication of nanowire based devices has been gaining more interest within the last decade. In a bottom-up approach, it is necessary to have nanoscale building blocks with precisely controlled dimensions and tunable chemical composition, structure and morphology. In this presentation, we will describe the fabrication of composite Au-CdTe-Au, Au-ZnO-Au, Au-InSb-Au and Au-BiSb-Au nanowires for optoelectronic, nanoelectronic and thermoelectric applications. Stand alone and monolithically integrated porous alumina membranes with pore sizes ranging from 20nm to 100nm have been used to fabricate the composite nanowires. The structure and composition of the nanowires are characterized by transmission electron microscopy, energy-dispersive spectroscopy and X-ray diffraction. In addition, the I-V characteristics of the composite nanowires have been determined with a four-point-probe technique.
9:00 PM - P8.10
Tungsten Oxide Nanowires and Their Gas-Sensing Properties.
Alexander Gurlo 1 , Nicolae Barsan 1 , Udo Weimar 1 , Julien Polleux 2 , Markus Niederberger 2
1 Institute of Physical Chemistry, University of Tuebingen, Tuebingen Germany, 2 , Max-Planck-Institute of Colloids and Interfaces, Potsdam Germany
Show Abstract9:00 PM - P8.12
Growth Control and Characterization of Indium Nitride One-dimensional Nanostructures and Its Core-shell Heterostructure
Wei-Jung Lai 1 , Chia-Te Chien 4 , Jhih-Siang Lin 4 , Li-Chyong Chen 2 , Kuei-Hsein Chen 3 , Chun-Wei Chen 1 , Zhe-Chuan Feng 4
1 Department of Materials Science and Engineering, National Taiwan University, Taipei Taiwan, 4 Graduate Institute of Electro-Optical Engineering & Department of Electrical Engineering , National Taiwan University, Taipei Taiwan, 2 Center for Condensed Matter Sciences, National Taiwan University, Taipei Taiwan, 3 Institute of Atomic and Molecular Sciences, Academia Silica, Taipei Taiwan
Show AbstractWe have successfully grown various indium nitride (InN) one-dimensional nanostructures, including nanorods, nanotips and InN@GaN core-shell heterostructure by metalorganic chemical vapor deposition (MOCVD). The InN nanorods and nanotips grown on GaN(0001) substrate reveal vertical alignment and have InN(0001)/GaN(0001) epitaxial relationship. High-resolution transmission electron micrographs (HRTEM) and selected-area electron diffraction (SAED) patterns indicate the single-crystalline quality and long axis along InN[0001] direction for the InN 1D materials. Through the two-step growth method, epitaxial GaN has been clad on InN nanorod, making the formation of coaxial InN@GaN core-shell heterostructure. The optical and electrical characterizations of the pure InN and InN@GaN 1D structure has been carried out. Temperature-dependent photoluminescence (PL) measurement has been utilized to characterize the infrared (IR) photoemission and the quantum efficiency of the 1D materials. InN@GaN core-shell exhibits significant blue-shift as compared to bare InN nanoros, suggesting the effective quantum confinement in this 1D heterostructure. The potential application as a field emitter for the vertically aligned InN nanotips has been also studied.
9:00 PM - P8.13
Resonant Tunneling in GaAs/AlAs Nanocolumns Improved by Quantum Collimation.
Jakob Wensorra 1 , Klaus Indlekofer 1 , Mihail Lepsa 1 , Arno Foerster 2 , Hans Lueth 1
1 Instiute of Thin Film and Interfaces , Research Centre Juelich, Juelich, NRW, Germany, 2 , Aachen University of Applied Sciences, Juelich, NRW, Germany
Show Abstract9:00 PM - P8.14
Er-doped GaN nanowires
Joan Carvajal 1 , Magdalena Aguilo 2 , Francesc Diaz 2 , J. Carlos Rojo 1
1 Department of Materials Science & Engineering, State University of New York, Stony Brook, New York, United States, 2 Fisica i Cristal.lografia de Materials (FiCMA), Universitat Rovira i Virgili, Tarragona Spain
Show AbstractThe recent demonstration of strong visible (green) and infrared (1.54 µm) electroluminescence at room temperature on Er-doped gallium nitride (GaN) thin layers and powders brings significant interest to this material for possible applications in photonics, including solid state laser, telecommunications, optical storage devices, and full color displays. GaN is a very important wide bandgap semiconductor that finds application in the fabrication of UV and blue emitters, detectors, high-speed field-effect transistors (FETs), and high-power/high-temperature devices.During the last decade much effort has been directed toward the synthesis of GaN nanowires, with means of controlling their diameters, and to probe their functionality in different devices such a light emitting diodes (LEDs), diode lasers, and FETs. Incorporating Er in GaN nanowires could enable the development of new functional devices through hybrid integration with a variety of other passive and active optical materials.Despite the considerable progress reported on the synthesis of Er-doped GaN, its application is currently hindered by the poor quantum yield observed at both the wavelengths characteristic of the bound-exciton radiation, 358 nm, and the wavelength in the IR region, 1.54 µm. The identification of absorption bands near the bandgap of the semiconductor might, however, make possible enhancing the quantum yields of the emissions corresponding to Er3+ or other rare-earth ions by carefully designing the host environment. This engineered environment can be achieved by tailoring the bandgap and lattice parameter of the host nitride material via the selection of the appropriate solid solution of GaN, InN, and AlN. Here we present the first successful synthesis of Er doped GaN nanowires on silicon (100) substrates by the catalyst-assisted chemical vapor deposition (CVD) technique. Confocal microscopy characterization has revealed emission from synthesized Er-doped GaN nanowires in the 490-540 nm and 665 nm-IR channels after excitation at 488 and 543 nm, respectively. In addition, several absorption bands of Er3+ in these nanowires, including the hypersensitive bands have been identified. This results open up new avenues for designing novel opto-electronic devices nased on rare-earth doped III-nitride low-dimensional structures.
9:00 PM - P8.15
GaN-based Nanodot Arrays Synthesized by Nano-selective Area Growth.
Keyan Zang 1 2 , Ya Dong Wang 1 , Soo Jin Chua 1 2
1 , Singapore-MIT Alliance, Singapore Singapore, 2 , Institute of Materials Research and Engineering, Singapore Singapore
Show Abstract9:00 PM - P8.16
Controlled Growth of Phase-separated GaN/AlGaN core/sheath Nanowires.
Jung-Chul Lee 1 , Myeong-Ki Lee 1 , Yun-Mo Sung 1
1 Materials Sci. & Eng., Korea University, Seoul Korea (the Republic of)
Show AbstractSemiconductors nanowire (NW) structures have attracted considerable attention due to their novel electronic and optical properties, originated from high crystallinity, low defect concentration, quantum size effects, etc. GaN NW’s have been spot lighted due to their superior optical properties such as blue or UV light emission. However, only a few number of results were reported on the synthesis and characterization of AlxGa1-xN alloy nanowires in which the energy band gap can be tuned and thus the light emission characteristics can be engineered. Moreover, only one group has reported the phase separation phenomenon in the AlxGa1-xN alloy nanowires so far. In this study, AlGaN NW’s were grown by the vapor-liquid-solid (VLS) mechanism using chemical vapor deposition (CVD) transporting trimethlygallium (TMG) and/or aluminum chloride (AlCl3) onto the nickel-coated sapphire substrate under a flow of ammonia (NH3). The formation of in situ GaN/AlxGa1-xN core/sheath NW’s via phase separation was identified using high-resolution transmission electron microscopy (HRTEM) and energy dispersive x-ray spectroscopy (EDS) analyses. The morphological features of one-dimensional GaN/AlxGa1-xN NW’s were identified by scanning electron microscopy (SEM). X-ray diffraction (XRD) was performed to analyze crystallinity of GaN/ AlxGa1-xN NW’s and approximate alloy composition of AlxGa1-xN sheath. The growth mechanism of GaN/AlxGa1-xN NW’s was proposed as the gradual phase separations from homogenious AlxGa1-xN alloy NW’s during processing at ~1000 oC. The effects of compositional and size (diameter) variation on the phase separation behavior of GaN/AlxGa1-xN NW’s were carefully examined and discussed in detail. Also, the roles of substrates and heat treatment condition including temperature and time period were investigated for the phase separation in GaN/AlxGa1-xN NW’s. Owing to the quantum well structure created by the band gap difference between the two components, we could enhance quantum efficiency as well as control the photoluminescence (PL) characteristics of GaN/AlxGa1-xN nanowires, thus making them an ideal candidate structure for blue or UV light-emitting diodes.
9:00 PM - P8.18
Selective Area Growth of Gallium Oxide Nanowires on Gold Patterned Gallium Arsenide Using Implantation-assisted Technique.
Kwong-Chun Lo 2 , Ho-Pui Ho 2 , Chi-man Wu 1 , K.Y. Fu 1 , Paul Chu 1
2 Electronic Engineering, Chinese University of Hong Kong, Hong Kong Hong Kong, 1 Physics and Materials Science, City University of Hong Kong, Hong Kong Hong Kong
Show AbstractIn this paper, the selective area synthesis of gallium oxide (Ga2O3) nanowires on gold patterned gallium arsenide (GaAs) substrate by ion implantation will be reported. The GaAs substrate was first treated with implantation of acetylene ions, nitrogen ions or nitrogen and oxygen co-implantation, followed by coating the surface with a gold film of 40nm in thickness. After rapid thermal annealing at 950oC for 30 seconds, Ga2O3 nanowires were found on the gold patterned surface. The Ga2O3 nanowires with diameters of 50-500nm were examined and characterized by Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, photoluminescence spectroscopy and cathodoluminescence spectroscopy. Nanoparticles of gold were found at the tip of the nanowires, suggesting that a vapor-liquid-solid mechanism was involved. However, the fact that other implanted species such as argon and oxygen would not lead to the formation of Ga2O3 nanowires also suggests that the nitrogen and carbon play an important role as a catalyst. The implanted nitrogen forms intermediate gallium nitride nano-grains, which may in term lead to the formation of nano-grains of Ga2O3 upon reacting with ambient oxygen. These Ga2O3 nano-grains then act as a growth template for nanowires. Carbon has previously been reported to be a reduction agent for the formation of group III sub-oxides. Such sub-oxides provide the vapor source for the growth of nano-materials through further oxidation. In addition, when we annealed the implanted samples in an ambient with oxygen concentration higher than 1%, a thick Ga2O3 film formed and nanoribbons was inhibited.
9:00 PM - P8.19
Spectroscopy of Single InGaAs/GaAs Nanowires Grown by Selectrive-Area Metalorganic Vapor Phase Epitaxy
Junichi Motohisa 1 2 , Shinjiro Hara 2 1 , Takashi Fukui 1 2
1 Research Center for Integrated Quantum Electronics, Hokkaido University, Sapporo Japan, 2 Graduate School of Information Science and Technology, Hokkaido University, Sapporo Japan
Show AbstractSemiconductor nanowires have been intensively investigated because of their enormous potential for new functional electronic and photonic device applications in next generation. We here report on the growth and spectroscopy of high-quality InGaAs embedded in a single GaAs nanowire to study their electronic states and to demonstrate a possibility for efficient light sources. GaAs/InGaAs/GaAs double-heterostructured nanowires were grown by selective-area metaloganic vapor phase epitaxy on GaAs (111)B substrates partially masked with SiO2. After successive growth of GaAs, InGaAs, and GaAs at appropriate growth conditions, free-standing hexagonal nanowires were formed selectively in the circular opening area of the mask. One of the advantages of our approach is that it is free from catalyst to realize high-quality nanowires. Furthermore, we can grow nanowires in predetermined positions of the substrates by the design of the mask pattern. To facilitate the spectroscopy for a single nanowire, we defined holes of the mask in 3 μm pitch arranged in a triangular lattice. Diameter of the nanowires investigated here was in the range from 150 to 200 nm, which was mainly determined by the size of the mask opening. Though the alloy content of In in InGaAs was 2.4 % for reference (001) substrates, actual amount of In incorporated into nanowires seemed to be more than 10 %. In addition, we estimated the thickness of InGaAs layer to be about 10 nm, which is twice of that on a reference substrate. These differences between nanowires and the reference originate from the diffusion kinetics of source and constituent materials in selective epitaxy, resulting in the difference of the growth rate between them. Spectroscopy of individual nanowires was performed at 4K in a micro-photoluminescence setup with an excitation and correction spot of emission in a diameter of approximately 2 μm. HeNe laser was used for excitation source. Typical spectrum from the sample consisted of a single emission of InGaAs together with emssion peaks of GaAs band-to-band and impurity-related transitions. The position as well as the width of the InGaAs emission differs from nanowires to nanowires, presumably due to alloy fluctuations inside and in between the nanowires. At the lowest excitation we could perform (~1.4 W/cm2), integrated intensity of the InGaAs photoluminescence was 20% of those of GaAs-related peaks. Thus, considering the size of the nanowires and excitation spot, the emission is reasonably bright. As the excitation power was increased, InGaAs emission in some nanowires showed considerable blue shift and exhibited an additional peak which was attributable to the excited states. This is explained by the band filling, and indicates efficient carrier injection into InGaAs from GaAs barriers and good optical quality of InGaAs embedded in GaAs naowoires.
9:00 PM - P8.20
Interplay Between Strain and Effective Electron Mass on the Absorption Strength of Dilute Nitride Semiconductor Quantum Wires.
Andrea Feltrin 1 , Andenet Alemu 1 , Alex Freundlich 1
1 Physics, University of Houston, Houston, Texas, United States
Show AbstractIn the last decade semiconductor materials with diluted nitrogen content have attracted growing attention because of their unusual electronic properties. The incorporation of small amounts of nitrogen in III-V semiconductor produces a large reduction of the optical band gap with a huge bowing parameter. A further effect is the modification of the electron dispersion in the reciprocal space and the increase of the conduction band electron effective mass. Masses as high as 0.4 free electron mass have been measured in quantum confined structures and calculations seem to support these first experimental results.These properties give to diluted nitrogen materials, as GaNAs or GaInNAs (GINA), promising potentials for applications in the optical domain. Recently, it has been suggested that nano-structures made out of these materials might have a beneficial application in nano-engineered solar cells. Especially dilute nitride quantum wire based solar cells, apart from extending the absorption spectrum towards the infrared region, could potentially improve the charge extraction in comparison to solar cells containing quantum wells, leading to increased efficiencies. In this perspective it is important to assess possible advantages of diluted nitrogen materials in comparison to their non nitrogenated counterparts. In this work we carry out a systematic comparison between the quantum wire absorption spectra of dilute nitride materials with varying content of nitrogen and indium. The calculations are carried out in the dipole and effective mass approximation, using parabolic dispersions. We take into account both, heavy and light hole bands. We compute the single particle allowed transitions and take into account consistently the Coulomb interaction between electrons and holes for each of these transitions. The inclusion of excitonic features in our model leads to the well-known appearance of an excitonic resonance below the single particle transition energy and to the disappearance of the divergence of the absorption at single particle transition energy.We show that compounds with varying fractions of indium and nitrogen, but similar band gaps have different absorption patterns. This behavior is related to the interplay between different effects as strain, which mainly affects the band offsets, and the increased electron mass in dilute nitride III-V semiconductors. We also study how the influence of these parameters changes by varying the optical band gap. For larger band gaps quaternary compounds containing both In and N show the strongest absorption, whereas by progressively decreasing the optical band gap, the strongest absorption shift to compounds containing mainly N. These results suggest that depending on the desired band gap, a proper design of the material composition is necessary in order to obtain optimum device performances.
9:00 PM - P8.21
Ultrahigh-Density Silicon Nanowire Logic Circuits
Bonnie Sheriff 1 , Dunwei Wang 1 , James Heath 1
1 Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California, United States
Show AbstractSemiconducting nanowires (NWs) have great potential for creating novel nanoelectronic and sensing devices. Achieving aligned and regular NW arrays have remained challenging using conventional NW growth mechanisms. However, uniform arrays of semiconducting and metal NWs at 35 nm pitch have been fabricated using the superlattice nanowire pattern transfer (SNAP) process, which is a top-down, template-based fabrication approach. Doped silicon NWs made from the SNAP process have already been used to demonstrate field-effect transistors (FETs), a binary tree demultiplexer and crossbar molecular memory structures. This technology also enables the high-density placement of several FETs with routing using a novel architecture. Here we report the development of crossbar logic circuits that combine complementing p- and n-type Si NW FETs to demonstrate fundamental logic functions. The fabrication and characterization of these circuits will be presented.
9:00 PM - P8.22
Multiple Routes for Si Nanowires Synthesis.
Alan Colli 1 , Stephan Hofmann 1 , Peyman Servati 1 , Youngqing Fu 1 , Paul Beecher 1 , Andrea Fasoli 1 , M. Rafiq 2 , Zahid Durrani 1 , Caterina Ducati 3 , John Robertson 1 , William Milne 1 , Andrea Ferrari 1
1 Engineering, Cambridge University , Cambridge United Kingdom, 2 Cavendish Laboratory, Cambridge University, Cambridge United Kingdom, 3 Department of Materials Science and Metallurgy, Cambridge University, Cambridge United Kingdom
Show AbstractSilicon nanowires (SiNWs) are particularly attractive due to the central role of the silicon semiconductor industry. Here we present and analyse complementary approaches to SiNWs production by Chemical Vapour Deposition (CVD), plasma-enhanced CVD (PECVD), Au-catalysed thermal evaporation and oxide-assisted vapour transport. Thermal and plasma-enhanced CVD using SiH4 as precursor gas and Au as catalyst are effective ways to produce surface-bound, thin (~10nm), highly crystalline SiNWs at low temperatures (300-400°C) [1-3]. PECVD also allows the growth of Si nano-cones, by using low-power (~10W) plasma and low SiH4 partial pressure. The cone tips are very sharp and their radius of curvature is determined by the initial Au nanoparticle size. Si nanocrystals ~8 nm in diameter can also be prepared by plasma decomposition of SiH4, with no catalyst. Vapour transport growth in a furnace allows high yield SiNWs production. This is achieved by evaporating Si powder at high temperature (1200-1400°C) in a horizontal tube furnace. The Si vapour then condenses at ~800°C on substrates coated with Au. In this case, the SiNW morphology is found to be substrate-dependent. Very long (~10-20 µm), thin (~10 nm), bent nanowires nucleate when Au is deposited on SiO2. Shorter, straight wires, ~30-40 nm in diameter, are obtained if the catalyst is deposited on metal surfaces. Finally, nano-beads composed of ~10nm Si nano-crystals separated by SiO2 barriers are obtained by metal-free oxide-assisted growth.Electron transport in these nanowires and nanocrystals is investigated using lithographically defined contacts. In 8nm Si nanocrystals we find that transport is dominated by single-electron charging at low temperatures, and by a space charge limited current with an exponential distribution of trapping states above 200 K [4].1. J. Westwater et al., J. Vac. Sci. Technol. B 15, 554(1997).2. S. Hofmann et al., J. Appl. Phys. 94, 6005 (2003).3. Y. Wu et al., Nano Lett. 4, 433 (2004).4. M. A. Rafiq et al., Appl. Phys. Lett. 87, 182101 (2005).
9:00 PM - P8.23
Electronic Interface Between Nanowires and Neurons.
Brian Timko 1 , Fernando Patolsky 1 , Guihua Yu 1 , Andrew Greytak 1 , Charles Lieber 1 2
1 Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, United States, 2 Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States
Show AbstractThe interface between nanoscale semiconductors and biological systems represents a powerful means for molecular-scale communication between these two distinct yet complementary components of information processing systems. In this work, we report the assembly and electrical properties of nanowire-based device arrays integrated with mammalian neurons. Discrete hybrid structures enable neuronal recording and stimulation at the axon, dendrite, or soma with high sensitivity and spatial resolution. Aligned arrays of these electronic nanostructures are used to measure the speed and shape evolution of action potentials as well as to interact with a single cell as multiple inputs and outputs. Additionally, we have demonstrated the assembly of hybrid n- and p-type structures enabling the generation of bipolar signals that could form the basis of logic gates and other integrated neuron-based computing structures. The flexible assembly of arrays of these structures creating tens of inputs or outputs to a single cell could prove useful for fundamental neurophysiological studies, real-time cellular interaction with chemical species, and the creation of hybrid cell / semiconductor computational networks.
9:00 PM - P8.24
Surface Depletion Thickness and Charge Density of CVD Grown p-doped Silicon Nanowires.
Ibrahim Kimukin 1 , M. Saif Islam 1 , S. Sharma 2 , T. I. Kamins 2 , R. Stanley Williams 2
1 Department of Electrical and Computer Engineering, University of California Davis, Davis, California, United States, 2 Quantum Science Research, Hewlett-Packard Laboratories, Palo Alto, California, United States
Show AbstractAn accurate evaluation of depletion profile in a nanowire is crucial for designing future nanoscale devices synthesized using bottom-up techniques. The nanowires were grown between pre-fabricated highly conductive Si electrodes on silicon-on-insulator wafer. We developed a very slow wet chemical etchant for gradually reducing the diameters of metal catalyzed boron-doped silicon nanowires with varying diameter and lengths. Nitric acid based etchant was used for isotropic atch. Particular care has been taken to perform the experiment in room temperature to prevent dopant segregation which is common in high temperature processes. By ensuring identical surface conditions subsequent to diameter reduction, the resistance of the nanowires was measured and, as anticipated, was found to increase with decreasing diameters. As the diameters were shrunk using wet-chemical etching, nanowires exhibited a non-linear intensification in the magnitude of the resistance when diameter was reduced to ~50nm. This is an indication of near-complete depletion in the nanowires caused by nanowire surface charges. The dopant concentration of the nanowires was calculated to be 2x10^18 1/cm^3 and corresponding surface charge density was found to be 2.5x10^12 1/cm^2
9:00 PM - P8.25
Mechanical Properties of Si Nanowires and Nanowire Arrays Fabricated by the VLS Method with Control of Size, Density and Location.
Alvaro San Paulo 1 , Rongrui He 3 , Noel Arellano 1 , Carlo Carraro 2 , Roger Howe 4 1 , Roya Maboudian 2 , Peidong Yang 3 , Jeffrey Bokor 1
1 Electrical Engineering and Computer Sciences, UC Berkeley, Berkeley, California, United States, 3 Chemistry, UC Berkeley, Berkeley, California, United States, 2 Chemical Engineering, UC Berkeley, Berkeley, California, United States, 4 Electrical Engineering, Stanford University, Stanford, California, United States
Show Abstract9:00 PM - P8.26
Synthesis of Silica Nanospring using Gold-catalyzed Chemical Vapor Deposition
Lidong Wang 1 , David Major 1 , Pradeep Paga 1 , David McIlroy 1
1 Deaprtment of Physics, University of Idaho, Moscow, Idaho, United States
Show AbstractNanospring is a member of one dimensional nanomaterials. With their unique geometric and mechanic properties, the potential applications of silica nanosprings are in nanomechanical system areas, given that we can produce high quality nanosprings with high yield (ratio between straight wires and helical springs). In this study, we have developed a new technology based on chemical vapor deposition to synthesis silica nanosprings with very high yield (80 - 90%). The synthesis of nanosring is fulfilled by vapor-liquid-solid growth mechanism, where sputtered thin Au film served as catalyst, tri-methyl silane as precursor, and oxygen as oxidization agent. We found that growth temperature can be as low as 300oC to obtain nanosprings with similar size and yield as those grown under 1000oC. The thickness of Au catalyst has trivial effects on the properties of silica nanosprings. TEM examination reveals that one nanospring is composed of several small nanowires which are tangled together. Further study of these phenomena is required to understand the mechanism of this revolution. Some carbon bonded to silicon was found in the nanospring using XPS. We present a likely reaction mechanism to explain the synthesis process for nanosprings in this study. The carbon was believed to be the product from the decomposition of precursor ligand.
9:00 PM - P8.27
Strained Silicon Nanowire Field Effect Transistors by Bottom-up Device Integration.
Rongrui He 1 , Alvaro San Paulo 2 , Di Gao 3 , Carlo Carraro 3 , Roya Maboudian 3 , Jeffrey Bokor 2 , Peidong Yang 1
1 Chemistry, Univ. of California, Berkeley, Berkeley, California, United States, 2 Electrical Engineering and Computer Sciences, Univ. of California, Berkeley, Berkeley, California, United States, 3 Chemical Engineering, Univ. of California, Berkeley, Berkeley, California, United States
Show AbstractDevice fabrication and integration remains a challenge for bottom-up grown semiconductor nanowires. In this study, we realized the device fabrication in the Vapor-Liquid-Solid growth of Si nanowires by bridge-in-trench structures. Based on these unique structures, piezoresistance effect of Si nanowires was studied and applied to build strained nanowire field effect transistors.The device integration is based on the epitaxial growth of Si nanowires. Si nanowires grow preferentially along <111> directions. If the vertical {111} planes contained in a (110) Si wafer are exposed by vertical etching, Si nanowires can be grown laterally, bridging the two face-to-face {111} surfaces. Particularly, heavily doped (110) silicon-on-insulator (SOI) wafers were used as substrates to realize electrical insulation and interfacing. Structural characterizations and Mechanical tests indicated that the as grown nanowires were self-weld into the {111} side walls and form ideal double clamped beams, i.e., mechanical and electrical connection was automatically formed for device operation during the growth. To achieve various applications, diameters and densities of nanowires were controlled by using Au colloidal as catalyst, and conductivities were controlled by Boron doping for p-type nanowires. These suspended nanowire structures with mechanically rigid clamps and integrated electrical interface provide us great opportunities for studies and applications. As an example, we studied the piezoresistance effect of Si nanowires. The piezoresistance effect of Si had been commercialized to make mechanical sensors, and currently it is widely investigated to increase the mobility of field effect transistors. The piezoresistance effect of Si nanowires was first qualitatively confirmed by deflection experiments in an atomic force microscope. The four point bending scheme was then used to further quantify the piezoresistance coefficients. It was found that the piezoresistance coefficients of nanowires were generally much higher than that of bulk Si and they increased as the diameters decreased, which implies that the strain engineering would be more important for nanoscale devices. Finally, field effect transistors were made by polyelectrolyte gating. The measurements on transconductance showed the change in conductance under strains is indeed due to the change in mobility. The mobility enhancement factor can be as large as 2 under 0.05% compression for p-type devices.
9:00 PM - P8.28
Growth Phenomena of Si and Si/Ge Nanowires on Si (111) by Molecular Beam Epitaxy
Nikolai Zakharov 1 , Leonid Sokolov 1 2 , Tamas Marek 1 , Ulrich Goesele 1 , Peter Werner 1
1 , MPI of Microstructure Physics, Halle (Saale) Germany, 2 , Institute of Semiconductor Physics, Novosibirsk Russian Federation
Show AbstractThe growth mechanism of Si and SiGe nanowires (NW) by molecular beam epitaxy (MBE) initiated via gold droplets was investigated. The behavior essentially differs from the classical vapor-liquid-solid-mechanism observed for chemical vapor deposition (CVD). In the MBE case the growth of NW occurs due to consumption of the ad-atoms from the surrounding substrate, while in the CVD mechanism Si ad-atoms are generated at the gold droplet surface due to the cracking of the precursor gas. The supersaturation for NW growth by MBE is determined by relaxation of elastic energy generated in Si substrate due to Au intrusions. An adding of Ge decreases the growth rate of Si NW or can even results in their dissolution. This effect is interpreted in term of elastic energy accumulation due to differences in atomic radii of Si and Ge.
9:00 PM - P8.3
Solution Based Straight and Branched Semiconductor Nanowires.
Masaru Kuno 1
1 Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana, United States
Show Abstract9:00 PM - P8.30
Integrated Silicon Nanowire Logic and Memory Arrays for Nanocomputing.
Guihua Yu 1 , Yajie Dong 1 , Yue Wu 1 , Hao Yan 1 , Wei Lu 1 , Charles M. Lieber 1 2
1 Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, United States, 2 Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States
Show AbstractThe development of organized and interconnected arrays of nanoscale materials, such as semiconductor nanowires, are central to efforts directed towards the realization of assembled nanocomputing systems. Here we report progress on the assembly of logic and memory arrays from crossed silicon and metal nanowires. High density silicon nanowire arrays with the nanowire pitch less than 100 nm were assembled by Langmuir-Blodgett technique and dense metal nanowires were subsequently defined by high resolution electron beam lithography. We show that the crossed silicon/metal nanowire junctions can serve as both field effect transistors and diode switches. In addition, we have used patterned atomic layer deposition of distinct dielectric constant layers to both improve the nanowire field-effect transistor performance and produce inversion stage in the logic arrays. The crossed silicon/metal nanowire arrays have been used to construct dense nonvolatile random access memory arrays, as well as AND and NAND logic structures. Progress towards increasingly complex,integrated array structures for nanocomputing will be presented.
9:00 PM - P8.31
Nanoparticle-Coated Silicon Nanowires
Jami Hafiz 1 , Rajesh Mukherjee 1 , Xiaoliang Wang 1 , Joachim Heberlein 1 , Peter McMurry 1 , Steven Girshick 1
1 Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractHybrid nanostructures consisting of nanotubes or nanowires that are decorated or completely coated with nanoparticles are of interest for a variety of potential applications, including catalysis, sensors, electronic devices, magnetics, optics and others. Here we report the synthesis of Si nanowires that are densely coated with Si nanoparticles. During growth the nanowires are exposed to bombardment by nanoparticles from the gas phase, and we believe that this is the source, or at least the primary source, of the nanoparticle coating. While Si nanowires coated with Si nanoparticles may not in itself be a fruitful system for applications, to our knowledge coating of nanowires with nanoparticles that bombard the nanowires from the gas phase has not previously been reported, and this approach may be promising for other material combinations.These structures were deposited using a process known as hypersonic plasma particle deposition (HPPD). In HPPD nanoparticles are synthesized by gas-phase nucleation in the rapid nozzle expansion of a thermal plasma with injected chemical reactants. The particles are accelerated in the hypersonic expansion, and impact the substrate to be coated at velocities up to about 2 km/s, creating a dense nanoparticle film.Synthesis of the nanoparticle-coated nanowires involved a two-step process. In the first step, a Ti-Si nanoparticle film was deposited. In the second step the Ti-source was switched off, and a dense network of nanoparticle-coated nanowires grew under the simultaneous action of Si vapor deposition and bombardment by Si nanoparticles. The Mo substrate temperature during both steps equalled 1230 K. Total process time, including both steps, equalled five minutes, and resulted in formation of a dense network of randomly oriented nanowires covering approximately 1.5 cm2 of substrate area.The nanoparticle film deposited in the first step was characterized by scanning electron microscopy (SEM) and by X-ray diffraction (XRD). The nanowires deposited in the second step were examined by SEM, by transmission electron microscopy, and by energy dispersive spectrometry. The size distribution of the impacting nanoparticles was determined by replacing the film substrate with a sampling probe interfaced to a scanning mobility particle sizer.The nanoparticle film initially deposited consisted of particles with diameters in the range 30 to 40 nm. XRD examination of this film showed crystalline phases of TiSi2 and Si, with a smaller amount of Ti5Si3. The nanowires were found to have diameters ranging from 100 to 800 nm, to consist of single-crystal Si, and to be completely covered with nanoparticles whose diameters lie in the range 5 to 25 nm. Each nanowire has a faceted crystalline TiSi2 catalyst particle, believed to have been solid during nanowire growth, at its tip.
9:00 PM - P8.32
Growth Dynamics of Silicon Nanowires as a Function of the Metal Catalyst.
Anna Fontcuberta i Morral 1 2 , Billel Kalache 2 3 , Pere Roca i Cabarrocas 2 , Didier Pribat 2 , Costel Cojocaru 2 , Serge Palacin 3 , Muriel Firon 3 , Jordi Arbiol 4 , Joan Ramon Morante 4
1 Walter Schottky Institut, Technical University of Munich, Garching Germany, 2 Laboratoire de Physique des Interfaces et Couches Minces, Ecole Polytechnique, Palaiseau France, 3 DRT/LITEN/DSEN/GENEC/L2C, CEA, Gif sur Yvette France, 4 Enginyeria i Materials Electronics, Departament d'Electronica, University of Barcelona, Barcelona Spain
Show AbstractHere we present a study on the effect of the metal catalyst on the growth dynamics of silicon nanowires. Silicon nanowires with sizes ranging from 10 to 100 nm were synthesized by Thermal Chemical Vapor Deposition. As metal catalysts, gold and copper were simultaneously used in separate samples. The morphology evolution at various stages of growth was determined by Scanning Electron Microscopy. Growth dynamics of the wires and structural properties of the nanowires were compared as a function of the metal catalyst. In general, there is an optimum temperature at which the wires grow straight with very little kinks and with very high crystalline quality. At lower and higher temperatures the wires present more kinks and ever a worm-like structure. This optimal range of temperature is between 500 and 650 °C for gold, while for Cu the optimal range is smaller and lies between 600 and 650oC. The initial phase of the growth is studied with particular interest. Specifically, we measure the time for the nanowires to start growing as a function of temperature. This time is the sum of the time necessary for silane molecules to decompose at the surface of the metal catalyst plus the time necessary for the resulting silicon atoms to diffuse inside the metal nanoparticle, saturate it and precipitate in the form of nanowire. Under our operating conditions, we measure incubation times between 10s and 200s at temperatures ranging from 700oC to 400oC respectively, or activation energy of 0.5eV. A comprehensive model of the incubation and nucleation process is presented.
9:00 PM - P8.33
Growth and Study of Silicon Nanowires to Obtain Devices Based on Nanobridges.
Pascal Gentile 1 , Pierre Ferret 2 , Thiery Baron 3 , Florian Dallhuin 3 , Mike Gordon 3
1 DRFMC/SPMM/SiNaPS, CEA, Grenbole France, 2 DRT/LETI, CEA , Grenoble France, 3 LTM, CNRS, Grenoble France
Show Abstract9:00 PM - P8.34
Growth of Si/Ge Strained Heterostructures using In Situ Optical Reflectometry.
Teresa Clement 1 , Sarang Ingole 1 , Jeff Drucker 2 , S.T. Picraux 1 3
1 Department of Chemical & Materials Engineering, Arizona State University, Tempe, Arizona, United States, 2 Department of Physics & Astronomy, Arizona State University, Tempe, Arizona, United States, 3 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractWe exploit in situ optical reflectometry to study the growth of both axial and core-shell Si/Ge nanowire heterostructures. These vapor-liquid-solid growth studies are carried out in a cold-walled UHV CVD system (base pressures 3x10-10 Torr) and combine in situ gold seed evaporation on atomically clean Si (111) surfaces, in situ growth from Si and Ge gaseous precursors (1-5 mTorr), and real-time monitoring of nanowire growth via optical reflectometry. Additional control over heterostructure growth is provided by using (di)silane or (di)germane precursors. For a given growth temperature or pressure, the more reactive digermane or disilane favor lateral growth compared to the less reactive silane or germane. As a consequence, axial heterowires form more easily by catalyzing silane or germane decomposition at the Au eutectic seed. Core/shell structures most easily grow by stabilizing lateral growth using disilane or digermane. Glancing angle optical reflectometry from a 635nm laser monitors the nanowire growth process by recording the change in reflected intensity as it optically scatters from the bulk silicon surface and the growing nanowire layer. The observed optical interference, plotted in real-time as a function of the nanowire length, shows an inherent nucleation time followed by a well defined growth rate, depending on precursor pressure and substrate temperature. Utilizing the optical reflectometry, we create nanowires of desired lengths, with control over the length of the heterojunctions. This method allows us to understand the nanowire growth kinetics for nucleation time and growth rate for the various gasses and effectively control the heterostructure growth process. In addition to field emission SEM and RBS characterization, we use our recently demonstrated strain mapping technique based on geometric phase analysis of HRTEM lattice images to reveal the large strains present at the heteroepitaxial interfaces of the nanowires. The combined information adds insight into the mechanisms of heterostructure nanowire growth and enables more systematic design, control and reproducibility of Si-Ge heterostructured nanowires.
9:00 PM - P8.35
The Growth of Self-organized GaAs Nanostructures by Microscale Droplet Epitaxy in Solid-source MBE.
kazuhiro matsuda 1 , Hideyuki Imanaka 1 , Naokatsu Sano 1 , Tadaaki Kaneko 1
1 physics, Kwansei Gakuin University ARCS, sanda Japan
Show Abstract9:00 PM - P8.4
One-Step Preparation of Core-Shell CdS-CdO Nanorod Arrays from a Single-Source Precursor.
Shih-Yuan Lu 1 , Yi-Feng Lin 1 , Yung-Jung Hsu 1 , Wei-Shan Chiang 1
1 Department of Chemical Engineering, National Tsing-Hua University, Hsin-Chu Taiwan
Show AbstractDensely packed and well aligned coaxial (core-shell) CdS-CdO nanorod arrays were prepared with a one-step, non-catalytic, template-free metallorganic chemical vapor deposition (MOCVD) process, using a single-source molecular precursor, Cd(S2COCH2CH3)2. The coaxial CdS-CdO nanorod array was formed at a collection temperature of 180 oC. The coaxial nanostructure was characterized and confirmed with high-resolution TEM, showing CdS core of 20-30 nm in diameter sheathed with CdO particle film of 5 nm thick. The CdS core grew as single crystal of hexagonal structure in the [0002] direction. In photoluminescence study, near-band edge emissions at around 524 and 551 nm attributable to CdS and CdO, respectively were observed. The effective surface passivation of CdS core by CdO shell led to significant enhancement in quantum yield of photoluminescence.
9:00 PM - P8.6
Application of Nanosphere Lithography for the Growth of Nanowires.
Hartmut Leipner 1 , Bodo Fuhrmann 1 , Hans-Rainer Hoeche 1
1 Center of Materials Science, Martin Luther University, Halle Germany
Show AbstractWhile length and aspect ratio of semiconductor nanowires depend mainly on the growth technique used, size and location of the nanowires are strongly related to the deposition of metal particles used as mediators of nanowire growth.The control of the position and the tuning of the dimensions is of big topical interest. The obvious way to do this is the preparation of ordered metal particle arrays by a suitable lithographic method. Optical lithography on the one hand is not applicable because the desired dimensions lie in the nanometer range. Electron beam lithography on the other hand would be suitable regarding the resolution, but is cost intensive and hardly available.We have used in our work nanosphere lithography to fabricate ordered arrays of metal nanoparticles. In nanosphere lithography, hexagonal closed packed monolayers or bilayers of monodisperse particles are used as a shadow mask for the subsequent processing of the substrate, which is in our case the metal deposition by thermal evaporation. In a subsequent thermal treatment step, the triangular metal islands are transfered into semispheres. Size and distance of the metal particles depend mainly on the size of the particles used for the mask. We have used suspensions of monodisperse polystyrene particles with diameters in the range 287 nm to 1.32 µm. The spectrum of applicable substrates could be extended to hydrophobic substrates like GaAs and GaN by applying a mask transfer technique. In this technique, the mask of polystyrene spheres is deposited on a hydrophilic substrate, then stabilized by deposition of a metal layer and finally transfered onto the desired substrate. The advantage of this technique is that the holes between the spheres become smaller by increasing the thickness of the stabilization layer. This opens the possibility to control the size of the metal particles independently from their distance, which is mainly determined by the diameter of the polystyrene particles. Several examples of the production of arrays of nanowires of ZnO, GaAs, and Si will be shown.
9:00 PM - P8.7
Low-temperature Templateless Synthesis and Thermoelectric Properties of Single-crystal Bismuth Telluride Nanorods.
Arup Purkayastha 1 , Makala Raghuveer 2 , Fabio Lupo 2 , Seong-yul Kim 2 , Theodorian Borca-Tasciuc 2 , Ganapathiraman Ramanath 2
1 Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Max Planck Institute for Metals Research, Max Planck Institute for Metals Research, Stuttgart Germany
Show AbstractNanostructuring bismuth telluride is attractive for significantly enhancing the thermoelectric figure of merit by size-scaling-induced thermal conductivity decrease and quantum confinement effects. With this motivation, recent works have demonstrated nanorod array growth of this material by physical templating , , which is easily adaptable to forming electrical contacts. The nanorods, however, are polycrystalline and exhibit low carrier mobility. Template removal could also be a potential issue that may introduce contamination and limit process scaling. Here, we report low-temperature (100 °C) templateless synthesis of single- crystals bismuth telluride nanorods capped with selected molecular moieties that facilitate assembly on a variety of chemically functionalized surfaces and matrices. We obtain nanorods by hydrazine reduction of aqueous bismuth chloride and orthotelluric acid in the presence of a difunctional molecular surfactant, namely, thioglycolic acid or L-cysteine. The diameter and length of the nanorods were tunable in the 27-130 nm, and 210-810 nm, ranges respectively, by choice of surfactant and adjusting growth time. High resolution transmission electron microscopy and electron diffraction show that each nanorod is a trigonal single-crystal. X-ray photoelectron spectroscopy and X-ray diffraction revealed that sulfur was incorporated in the trigonal lattice as well as the nanorod surfaces. Room temperature measurements of thin films of dispersed nanorods on prefabricated microelectrode structures reveal ohmic behavior and high Seebeck coefficient of ~-100 µV/K. The negative sign indicates n-type behavior, most likely arising due to sulfur doping. Based upon our results, we present a phenomenological model of the templateless growth mechanism and compositional evolution, and the consequent effects on thermoelectric properties. References: Chen, G.; Dresselhaus, M. S.; Dresselhauss, G.; Fleurial, J. P.; and Caillat, T.; International Materials Reviews, (2003), 48, 45. Prieto, A. L.; Sander, M. S.; Gonzalez, M. M.; Gronsky, R.; Sands, T.; Stacy, A. M. J. Am. Chem. Soc 2001, 123, 7161 Sander, M. S.; Prieto, A. L.; Gronsky, R.; Sands, T.; Stacy, A. M. Adv.Mater. 2002, 14, 665
9:00 PM - P8.8
Template Assisted Electrochemical Synthesis of Copper Sulphide Nanowires: Potential Candidate for Solar Cell Applications.
Krishna Singh 1 , Mihrimah Ozkan 2
1 Chemical Engineering, UC Riverside, Riverside, California, United States, 2 Electrical Engineering, UC Riverside, Riverside, California, United States
Show Abstract9:00 PM - P8.9
A Novel Fabrication Method of Cu Coaxial Nanocable With Al2O3 Insulation by Using Atomic Layer Deposition and Electrodeposition.
Moon Chul Kang 1 , Myung Mo Sung 1
1 Chemistry, Kookmin University, Seoul Korea (the Republic of)
Show Abstract
Symposium Organizers
Margit Zacharias Max-Planck-Institute of Microstructure Physics
Walter Riess IBM Research GmbH
Peidong Yang University of California-Berkeley
Younan Xia University of Washington
P9: Nanowires Based on Nitrides
Session Chairs
Thursday AM, April 20, 2006
Room 2024 (Moscone West)
10:00 AM - P9.2
Core/multishell Nanowire Heterostructures As High-efficiency, Multicolor Light-emitting Diodes.
Fang Qian 1 , Silvija Gradecak 1 , Yat Li 1 , Charles Lieber 1 2
1 Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts, United States, 2 Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States
Show AbstractSemiconductor nanowires represent attracting building blocks for the assembly of functional photonic and optoelectronic devices. To this end we report the growth and characterization of III-nitride based core/multishell (CMS) nanowire radial heterostructures, and their implementation as efficient and synthetically tunable multicolor nanophotonic sources. CMS nanowires were prepared by metal-organic chemical vapor deposition with an n-GaN core and InxGa1-xN/GaN/p-AlGaN/p-GaN shells, where variation of indium mole fraction is used to tune emission wavelength. Electroluminescence measurements demonstrate that in forward bias the CMS nanowires function as high brightness light-emitting diodes, with tunable emission from 365 to 600 nm and high quantum efficiencies. The ability to synthesize rationally CMS nanowire heterostructures as electrically-driven, efficient and color-tunable light sources should open up significant potential for integrated nanoscale photonic systems, including multicolor lasers.
10:15 AM - P9.3
Self-Assembled MBE-grown Nitride Nanowires.
Ralph Meijers 1 , Thomas Richter 1 , Ratan Debnath 1 , Toma Stoica 1 , Raffaella Calarco 1 , Michel Marso 1 , Hans Lueth 1
1 Institute of Thin Films and Interfaces (ISG1), Research Centre Juelich, Juelich, North Rhine-Westphalia, Germany
Show AbstractAmong different types of nanostructures, semiconductor nanowires and nanotubes are extremely interesting as building blocks for nanoelectronics, due to their suitability for fabricating both nanoscale devices and interconnects. Although there have been a lot of investigations on these semiconductor nanowires, fundamental physical properties are still unclear. The large surface to volume ratio can produce interesting physical effects compared to the bulk material [1]. Also the growth mechanism and especially the nucleation of the wires, which is very important for producing ordered arrays of nanowires, remains an issue of debate.The self-assembled growth of GaN, InN as well as InxGa1-xN nanowires on Si(111) substrates by molecular beam epitaxy (MBE) was investigated by means of several characterization methods [2-5]. Scanning electron microscopy (SEM) images showed the influence of growth parameters on column shape and density, whereas optical methods (photo- (PL) and cathodoluminescence (CL)) provided the information about the quality of the grown wires. It was even possible to get spatially-resolved information by combining SEM and CL. By introducing doping materials (Si and Mg) in the nanowires column morphology can be changed considerably, depending on the concentration of the dopants. To gather further information about intrinsic physical properties of the nanowires, current-voltage measurements were performed on a pre-patterned host substrate and then individual contacts were defined by e-beam lithography. The results showed a pronounced influence of the column’s surface on the electrical conductivity inside the wire, which can be related to Fermi-level pinning.[1] R. Calarco, M. Marso, T. Richter, A. I. Aykanat, R. Meijers, A. v.d. Hart, T. Stoica, and H. Lüth, Size-dependent Photoconductivity in MBE-Grown GaN-Nanowires, Nano Letters 2005, Vol. 5, No. 5, p.981-984[2] Calarco, R.; Marso, M.; Meijers, R.; Richter, T.; Aykanat, A.I.; Stoica T.; Lüth, H. Proceedings of the ASDAM ’04 Conference, Smolenice, Slovakia, 2004, 9.[3] Meijers, R.; Richter T.; Calarco, R.; Stoica T.; Bochem H.-P.; Marso M.; Lüth, H., GaN-nanowhiskers: MBE-growth conditions and optical properties, submitted to Journal of Crystal Growth[4] Stoica T.; Meijers, R.; Calarco, R.; Richter T.; Lüth, H., Growth properties and IR photoluminescence of MBE-grown InN nanowires, to be published[5] Stoica T.; Meijers, R.; Calarco, R.; Richter T.; Lüth, H., Photoluminescence and intrinsic properties of MBE-grown InN nanowires, to be published
10:30 AM - P9.4
Ohmic Contacts to GaN Nanowires: Focused Ion Beam Deposited Pt
Chang-Yong Nam 1 , Douglas Tham 1 , John Fischer 1
1 Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Show AbstractGaN is an important class of wide and direct bandgap semiconductor for short wavelength and high power applications. Recently, one-dimensional GaN nanowires have drawn extensive research as potential building blocks for nano-scale optoelectronic devices such as light emitting diode and laser. Meanwhile, complex electrical contact fabrication process involving atomic force microscopy and electron beam lithography often causes difficulties in nanowire device fabrication. We previously demonstrated the potentials of direct Pt deposition by focused ion beam (FIB) for making contacts on n-GaN nanowires. In addition to the convenience and versatility of the process, the deposited Pt, which is generally Schottky barrier metal for n-GaN, exhibits surprisingly low resistance ohmic contact. Here, we investigate the electrical and structural nature of these contacts by current-bias-temperature (I-V-T) measurement and cross-sectional transmission electron microscopy. The I-V characteristics evolve from linear to rectifying as the diameter increases, and both exhibit strongly non-metallic T dependence. The small-diameter (66 nm) T-dependent resistance is explained by two-dimensional variable range hopping with a small characteristic energy, ensuring low resistance at 300 K. For the large diameter (184 nm), back-to-back Schottky barriers explain the nonlinear I-V at all T, and the doping concentration is estimated from the bias-dependent barrier height. Both behaviors can be understood by accounting for the role of FIB-induced disorder in GaN underneath the contact, as confirmed by cross-sectional transmission electron microscopy.
10:45 AM - P9.5
Self-organized (In,Ga)N Nanorod Heterostructure Arrays Fabricated Without Catalysts or Nanolithography.
Parijat Deb 1 , Mark Oliver 1 , Eric Stach 1 , Timothy Sands 1 2
1 School of Materials Engineering, Purdue University, West Lafayette, Indiana, United States, 2 School of Electrical and computer engineering, Purdue University, West Lafayette, Indiana, United States
Show AbstractThe lateral relaxation of lattice misfit strain in heterostructures grown on nanoscale substrates substantially increases the range of lattice misfit and overlayer thickness that can be accommodated without the introduction of extended defects. In the case of (In,Ga)N, nanoheteroepitaxy offers the possibility of increasing the maximum InN mole fraction or the InN quantum dot size that can be accommodated in quantum confined structures, thereby promising a broader range of emission wavelengths from GaN-based LEDs. To take advantage of this aspect of nanoscale strain engineering for white light emitters or tunable monolithic sources, it will be necessary to develop processes for fabricating arrays of (In,Ga)N nanorod heterostructures with spatially varying diameters in specific patterns. In this presentation, we describe a process for attaining control of (In,Ga)N nanorod dimensions without nanolithography or foreign catalysts. The process yields monocrystalline, vertically aligned and faceted (In,Ga)N nanorods with diameters ranging from 50 nm to 100 nm distributed in the form of microscale subarrays, each exhibiting uniform feature sizes. The process begins with electron-beam evaporation of a 60 nm SiOx film followed by a 1 micrometer Al film onto a (0001) GaN film. The Al film is subjected to a two-step anodization process resulting in a porous anodic alumina (PAA) film approximately 250 nm in thickness. Pore widening in phosphoric acid is then used to increase the PAA pore diameter. The PAA pore pattern is then transferred into the SiOx film using reactive ion etching. Finally, the alumina template is selectively etched away. The remaining porous silica template defines the positions and diameters of the GaN nanorods or (In,Ga)N nanorod heterostructures grown selectively within the pores by organometallic vapor phase epitaxy. A low V/III ratio along with hydrogen as the carrier gas results in <0001> oriented nanorods with prismatic {1bar100} facets, terminated by a cap defined by pyramidal {1bar101 } facets. Diameter control of the nanorods is achieved by varying the anodization potential and pore widening time in selected areas of the wafer through photolithography, resulting in controlled variation of nanorod diameter across the wafer. The small radius of curvature at the nanorod apex facilitates the relaxation of misfit strain during subsequent growth of InN or (In,Ga)N, yielding self-organized quantum dots along the nanorod axis. These axial quantum dots act as radiative recombination centers. The relationships between the nanorod structure, as assessed by transmission electron microscopy, and the luminescent properties of these nanoscale heterostructures will be discussed. This work was supported in part by the National Science Foundation (ECS-0424161).
P10: ZnO and Related Materials
Session Chairs
Thursday PM, April 20, 2006
Room 2024 (Moscone West)
11:30 AM - **P10.1
Semiconducting and Piezoelectric Nanostructures - From Scientific Discovery to Road Maps for Nanomanufacturing.
Zhong Wang 1
1 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractResearch in synthesizing semiconducting nanostructures is a forefront area in nanotechnology due to their applications in nanoelectronics, photonics, data storage, and sensing. To meet the needs of large scale, controlled and designed synthesis of nanostructures, it is desperate to systematically find experimental conditions under which the desired nanostructures are synthesized reproducibly, at large quantity and with controlled morphology. It is necessary to systematically investigate the underlying mechanisms that determine the morphology and dimensionality of 1D nanostructures. This is an important step towards nanomanufacturing for the applications in future industry. In this talk, I will start with the discovery of nanosprings [1], nanorings [2], nanobows [3] and nanohelices [4] of ZnO. Then, the key parameters for improving the yield of nanosprings will be demonstrated [5, 6]. Finally, the “phase diagram” for high-yield growth of aligned ZnO nanowires [7] on nitrides surfaces, and the controlled synthesis of nanosaws, nanobelts and nanowires of CdSe [8] will be described for illustrating the road map for nanomanufacturing [9, 10].[1] X.Y. Kong and Z.L. Wang, Nano Letters, 2 (2003) 1625 + cover.[2] X.Y. Kong, Y. Ding, R.S. Yang, Z.L. Wang, Science, 303 (2004) 1348.[3] W.L. Hughes and Z.L. Wang, J. Am. Chem. Soc., 126 (2004) 670[4] P.X. Gao, Y. Ding, W.J. Mai, W.L. Hughes, C.S. Lao and Z.L. Wang, Science, 309 (2005) 170[5] P.X. Gao and Z.L. Wang, Small, 1 (2005) 945-949.[6] W.L. Hughes and Z.L. Wang, Appl. Phys. Letts., 86 (2005) 043106 + cover.[7] J.H. Song, X.D. Wang, E. Riedo and Z. L. Wang, J. Phys. Chem. B, 109 (2005) 9869.[8] C. Ma and Z.L. Wang, Advanced Materials, 17 (2005) 1.[9] Thanks the support from NSF, DARPA, NASA and Airforce.[10] for details see: http://www.nanoscience.gatech.edu/zlwang/
12:00 PM - P10.2
Growth of ZnO Nanotubes without catalysts and templates
Samuel Mensah 1 , Vijaya Kayastha 1 , Yoke Khin Yap 1
1 , Michigan Technological University, Houghton, Michigan, United States
Show AbstractNanotubular structures of oxides materials have recently gained attention for their hydrophilic properties. These oxide nanotubes are attracting for biological applications including nanofluidic devices for single DNA molecule sensing, rapid diseases diagnosis, and DNA sequencing. Here, we show that nanotubular cavities of ZnO can be directly grown on substrates without the use of catalysts and templates.Various types of growth techniques have been used for the growth of ZnO nanostructures. Thermal chemical vapor deposition (CVD) is a relatively simpler approach for the growth of ZnO nanowires, nanobelts, nanocombs, and nanorods. In the thermal-CVD process, ZnO and graphite powders are often used as the raw materials for generating desied growth species for the formation of ZnO nanostructures according to vapor-liquid-solid (VLS) and vapor-solid (VS) processes. By this technique, we found that vertical ZnO nanotubes can be grown on Si and oxidized substrates without the use of catalysts and templates. These experiments were conducted in a double-tube configuration. The substrates were inserted into a small quartz tube (2cm in diameter and 60cm long, closed at one end) with a mixture of ZnO and graphite powders in a ratio of 2:1, respectively. This tube is then inserted into the cylindrical quartz tube chamber of the thermal CVD furnace such that the mixed powder is at the center of the horizontal furnace. In this arrangement, the mixed powder will be combusted at 1100 °C and the substrates are at a temperature of ~650 °C. Oxygen gas was introduced into the quartz tube at a flow rate of 40 sccm. Tubular ZnO nanostructures are formed on the substrates when appropriate cooling rate was then applied.Characterization of these ZnO nanotubes was conducted by X-ray powder diffraction, high-resolution transmission electron microscopy (HRTEM), Field-emission scanning electron microscopy (FESEM), Raman spectroscopy, and photoluminescence (PL). Results shown that these ZnO nanotubes were single crystals of pure hexagonal wurtzite structure. Our results show that rapid cooling rate and deficiency of oxygen contributed to the formation of tubular ZnO nanostructures. Details of our growth model (VS mechanism) and possible reasons for the growth of ZnO nanotubes will be discussed at the meeting. Y.K.Y acknowledges supports from the Michigan Tech Research Excellence Fund, Army Research Office (W911NF-04-1-0029), CNMS at ORNL, and NSF CAREER Award (0447555).
12:15 PM - P10.3
Metal-Oxide-Metal (MOM) Heterojunction Nanowires for Chemical-Sensor Devices.
Jason Tresback 1 , Alexander Vasiliev 1 , Jing-Jong Shyue 1 , Nitin Padture 1
1 Materials Science and Engineering, The Ohio State University, Columbus, Ohio, United States
Show AbstractThere is growing interest in the field of nanoelectronic devices, where nanoscale building blocks, such as nanowires (metals, semiconductors, oxides), are fabricated in isolation and assembled into nanocircuits. In the case of functional oxides, currently all-oxide nanowires are assembled across metal contact-pad electrodes to create devices, where the active region of the oxide is determined by the distance spanning the electrodes. In this context, metal-oxide-metal (MOM) heterojunction nanowires, where two metal nanowires (50 to 100 nm diameter; Au or Pt) are separated by a nanoscale segment (50-100 nm length) of a functional oxide, offer several advantages over all-oxide nanowires. We have introduced a generic, template-based method for the synthesis of MOM nanowires in systems Au-oxide-Au (where oxide=SnO2, ZnO, NiO, CdO, FeO, or CuO). Here the metal interconnects are integrated within the building block making them more suitable for large-scale assembly, as well as providing Schottky junctions and catalysis sites at the nanoscale. Furthermore, our synthesis method provides great control over the geometry and composition of the MOM nanowires. Also, this synthesis method allows us to introduce aliovalent dopants into the oxide segment, which can be used to tailor the functional properties of the oxide. By virtue of the nanoscale nature of the MOM nanowires, the chemical-sensor devices made from MOM nanowires are expected to be highly sensitive and selective, and they can be operated at low voltages and temperatures. Results from the synthesis, characterization, device fabrication, and sensor-properties measurements of various MOM nanowires will be presented, together with a discussion of relevant mechanisms.
12:30 PM - P10.4
Electrical Characterization of Tin-oxide Nanowires Contacted with a Focused-ion-beam.
Francisco Hernandez-Ramirez 1 , Jordi Rodriguez 1 , Olga Casals 1 , Anna Vila 1 , Albert Romano-Rodriguez 1 , Juan Ramon Morante 1 , Sven Barth 2 , Sanjay Mathur 2 , Tae-Youl Choi 3
1 Department of Electronics, University of Barcelona, Barcelona Spain, 2 Leibniz Institute of New Materials, University of Saarbruecken, Saarbruecken Germany, 3 Institute of Energy Technology, Eidgenoessiche Technische Hochshule Zuerich, Zurich Switzerland
Show Abstract12:45 PM - P10.5
Shape-Selective Synthesis of II-VI Semiconductor Nanowires.
Andrea Fasoli 1 , Alan Colli 1 , Stephan Hofmann 1 , Caterina Ducati 2 , John Robertson 1 , Andrea Ferrari 1
1 Engineering, Cambridge University, Cambridge United Kingdom, 2 Materials Science and Metallurgy, Cambridge University, Cambridge United Kingdom
Show AbstractPolar II-VI semiconductors can nucleate in complex shapes ranging from nanowires to nanoribbons, nanosaws and multipods [1-2]. The full potential of these nanostructures can be realized if they are directly grown into devices. There is thus the need for general approaches to achieve shape-selectivity combined with position-selectivity. Shape-selectivity is the deterministic control of the nano-structure morphology. Position-selectivity is the deterministic growth into specific locations. Here we demonstrate the deterministic and fully reproducible shape- and position-selective growth of several morphologies of CdSe and ZnTe nanocrystals by a steady-state vapour transport process [3]. The key step in order to achieve reproducible shape selectivity for a given set of deposition parameters is to exclude any effects of the temperature ramping. We show how to implement a simple precursor-flow shutter by changing the total pressure in the furnace reactor. This can be easily done for any vapour transport growth process in a quartz-tube and requires no mechanical parts. This step is essential in order to ensure that the selected growth parameters are fully representative of the observed nanostructure morphology [3], but is very often neglected in literature [4,5]. Once thermal gradients are eliminated, we show that the transition from one nanocrystal shape to another is controlled just by the interplay of precursor impinging on the substrate and sample surface kinetics. These two general processes are ruled by the powder and substrate temperatures, respectively, and are the same controlling the growth kinetics in semiconductor epitaxy [6]. Once the desired nanoshape is selected, position selectivity is simply achieved by pre-patterning a metal catalyst by photo- or e-beam- lithography.1. D. J. Milliron et al., Nature 430, 190 (2004).2. Z. L. Wang, Materials Today 7, 26 (2004).3. A. Colli et al., Nanotechnology (submitted).4. C. Ma et al., J. Am. Chem. Soc. 126, 708 (2004).5. Z. R. Dai et al., Adv. Funct. Mat. 13, 9 (2003).6. M. A. Herman and H. Sitter, Molecular beam epitaxy, 1989 Springer, Berlin.
P11: Colloidal and Solution Growth
Session Chairs
Thursday PM, April 20, 2006
Room 2024 (Moscone West)
2:30 PM - **P11.1
Colloidal Synthesis of Inorganic Nanocrystals with Complex Shapes, Connectivities, and Topologies.
A. Paul Alivisatos 1
1 Department of Chemistry, University of California, Berkeley, Berkeley, California, United States
Show AbstractThis talk will review the selective adhesion approach for controlling the shapes of nanocrystals grown in colloidal solution. The production of rod, disk, and branched nanocrystals will be described. In addition, very recent results on the synthesis of nanocrystals with entirely new connectivity will be presented.
3:00 PM - P11.2
Zero-Phonon Line and Phonon Sidebands in CdSe Nanorods.
Sasha Tavenner Kruger 1 , Young-Shin Park 1 , Mark Lonergan 2 , Hailin Wang 1
1 Physics, University of Oregon, Eugene, Oregon, United States, 2 Chemistry, University of Oregon, Eugene, Oregon, United States
Show Abstract3:15 PM - P11.3
Fluorescence Intermittency in CdSe Quantum Wires
John Glennon 1 2 , Rui Tang 1 2 , Richard Loomis 1 2 , William Buhro 1 2
1 Department of Chemistry, Washington University, St. Louis, Missouri, United States, 2 Center for Materials Innovation, Washington University, St. Louis, Missouri, United States
Show AbstractQuantum-dot fluorescence intermittency or "blinking," in which individual dots cycle between on (bright) and off (dark) states over time scales ranging from milliseconds to minutes, is of great fundamental and practical interest. The mechanism(s) of blinking and methods for controlling blinking in 3D-confined quantum dots are under active investigation. One may wonder if 2D-confined quantum wires also exhibit fluorescence intermittency, and if so how the extended dimension in wires will influence blinking behavior. We are studying blinking in CdSe quantum wires by single-wire imaging and spectroscopy. We have found that most wires exhibit a "twinkling" phenomenon, in which the fluorescence intensity in small, localized wire domains fluctuates independently of similar fluctuations in nearby domains. However, a persistent fraction of the wires exhibits synchronous blinking over large wire segments (lengths > 500 nm), or even over whole wires (lengths > 2 μm). Significantly, the blinking dynamics follow inverse-power-law statistics comparable to previous findings for quantum dots, suggesting mechanistic similarities between dot and wire blinking. As blinking in dots is believed to result from single-charge fluctuations, a remarkable and surprising implication is that, in ideal cases, single-charge changes may switch whole, multi-micrometer-length wires between on and off states. These findings and their possible mechanistic implications will be discussed.
3:30 PM - P11.4
Low-temperature solution growth of ZnO nanowires ripened from ZnO nanodots.
Frank Jones 1 , A. Talin 1 , Nelson Bell 2 , Timothy Boyle 2
1 , Sandia National Laboratories, Livermore, California, United States, 2 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractDue to unique electrical, optical, and physical characteristics, zinc oxide nanostructures have been the focus for numerous applications, including chemical sensing, UV emission, and physical resonators. However, the most common routes for creating zinc oxide nano- wires or rods involve high temperature vapor processes, limiting the available substrates and material combinations, or require the use of a catalyst that may remain in the final product and interfere with device operation. In contrast to these methods, we report a catalyst-free, low temperature solution-grown method for creating high aspect ratio ZnO nanowires, and characterize these wires electrically and optically. In this growth method, ZnO nanodots are fully formed in solution using a solution of zinc acetate dihydrate and tetramethylammonium hydroxide at room temperature. The ZnO nanodots were ripened into nanowires by heating in a pressure vessel at 150°C, then collected and dispersed into ethanol for testing. By allowing the ZnO nanodots to form before ripening, in contrast to heating the precursor solution directly, we see a more uniform set of ZnO nanowire with a narrower range of size distribution. The nanowires used in electrical measurement had an average diameter of 70 ± 10 nm and an aspect ratio of 16:1. In testing individual wires across identical addressable Au/Ni electrodes, the electrical conductances were often non-linear and non-uniform. Photoluminescent spectra of a dispersed film of the ZnO nanowires on silicon showed a strong exciton peak at 375 nm (3.31 eV) and a weaker, broad peak at 507 nm (2.45 eV) that is associated with oxygen vacancies in the structure. Having previous success improving device performance with carbon nanotubes and GaN devices by vacuum annealing, we annealed the ZnO nanowire structures up to 500°C in vacuum and found near uniform electrical performance of the structures, with >100 pA at 3V. As an exhibit for device application, these nanowires were found to be photosensitive to UV light; exhibiting greater than two orders of magnitude increase in current when exposed to 325 nm He-Cd laser light. Finally, the photoluminescent peaks in the spectra diminished in an annealed sample, and nearly extinguished for the exciton peak. This last result is intriguing, in that it is a direct contrast to previous reports where ZnO nanowires were annealed in vacuum and showed strong increases in both peaks.Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
3:45 PM - P11.5
Piezoelectric and Electrical Properties of Solution Grown Semicondutor ZnO Nanorods.
David Scrymgeour 1 , Thomas Sounart 1 , Julia Hsu 1
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractSemiconducting zinc oxide is a key nanomaterial because it can be grown as nanostructures via solution and vapor phase methods and it possesses a wide range of important and useful physical properties. These include a wide band gap (~3.44 eV) as well as interesting optoelectronic, pyroelectric, and piezoelectric properties. Because of these properties, zinc oxide nanostructures are of great interest for the next generation applications in UV lasers, solar energy conversion, gas and biological sensors, and actuator materials.Piezoelectric zinc oxide nanocrystals are grown by solution techniques on highly textured Ag (111) films in patterned arrays. These ZnO nanocrystals form hexagonal crystal rods with diameters of 100-600 nm and heights of 400-1200 nm with their <0001> polar axis perpendicular to the substrate. However, the specific growth direction from the substrate, either along [0001] or [0001], and hence the crystal polarity, is unclear. It is known that impurity incorporation, optical and electrical properties, and reactivity to chemicals depend strongly on the crystal polarity. Thus, it is important to determine the polarity of the ZnO nanorods. Since inversion symmetry is broken along the polar directions, the sign of the piezoelectric coefficient (d33) is opposite for [0001] direction and for [0001] direction. Using piezoelectric force microscopy (PFM), the polarity of these ZnO nanorods has been determined from the phase response of the piezoelectric effect. Measurements were performed on single nanorods, and results were compared to single crystal ZnO with known orientation. These results establish that the nanorods are [0001] oriented, which contrasts with preliminary morphological examinations of ZnO nanocrystals. Additionally, the magnitude of the piezoelectric response from individual nanorods was determined. It was found that the amplitudes varied from rod to rod and are not strongly correlated to nanorod height, radius, or embedding depth, which indicate another factor is responsible for the variation. These nanorod values are compared to those obtained on n-type single crystal ZnO of known conductive properties. The piezoelectric response values measured on ZnO nanorods and n-type single crystal are smaller than literature values for Li compensated ZnO. To investigate the relationship between conductive and piezoelectric properties, results from parallel measurements on the conductive properties of these individual ZnO nanorods taken using conductive atomic force microscopy will be presented and discussed. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
P12: Nanowire Growth and Characterization
Session Chairs
Thursday PM, April 20, 2006
Room 2024 (Moscone West)
4:30 PM - P12.1
Growth and Characterization of GaN/AlN Multilayer Nanocolumns Grown by RF Plasma Assisted Molecular Beam Epitaxy.
Akihiko Kikuchi 1 2 , Takuya Nakazato 1 , Hiroto Sekiguchi 1 , Katsumi Kishino 1 2
1 Electrical & Electronics Engineering, Sophia University, Tokyo Japan, 2 CREST, Japan Science and Technology Agency, Kawaguchi, Saitama Japan
Show AbstractGaN based nanocolumns are self-organized columnar nanocrystals with average diameter of from tens to hundreds nanometers. Each nanocolumn individually grows in a high density of ~10E10cm-2 with their c-axis normal to the substrate surface. We reported the first growth of GaN nanocolumns in 1996-1997[1,2]. The GaN nanocolumns had superior optical property that is a strong photoluminescence (PL) and a low threshold stimulated emission due to their dislocation free nature and low surface non-radiative recombination rate[3]. We also demonstrated InGaN/GaN nanocolumn LEDs emitting in whole visible color region[4].In this study, GaN/AlN multilayer nanocolumns were successfully grown by an rf-plasma assisted molecular beam epitaxy (RF-MBE) on (0001)Al2O3 and (111)Si substrates. The thickness of GaN and AlN layers were varied in a large scale of 0.9-71nm and 1.7-50nm, respectively.For GaN/AlN short period multilayer nanocolumns (superlattice nanocolumns), strong room temperature (RT) PL emission was observed at 358nm for GaN(1.1nm)/AlN(3.4nm) sample and 420nm for (3.7nm)/(2.8nm) samples. The peak intensity was several hundred times stronger than that of RF-MBE grown GaN/AlN superlattice film with similar layer structure. The integrated PL intensity at RT was as high as 46% of that of 15K. And the RT time resolved PL measurement shown long recombination life time of 6ns. These results indicated that non-radiative recombination was very low for the GaN/AlN superlattice nanocolumns. From a cross-sectional high resolution transmission electron microscope observation, it was confirmed that the GaN/AlN superlattice nanocolumns are almost free from threading dislocations and the interface of GaN/AlN was atomically smooth.For long period GaN/AlN multilayer nanocolumns, we designed 15 pairs of quarter wavelength layer stacks (distributed Bragg reflector (DBR) nanocolumns). The thickness of GaN and AlN layers of the DBR for green light were designed to be 59nm and 67nm, respectively. From the SEM observation, growth of well aligned GaN/AlN multilayer stack was confirmed keeping nanocolumn structure. The reflection peak was observed in the blue (472nm) to green (537nm) range by slight change of the layer thickness.These results revealed that GaN/AlN multilayer nanocolumns are relatively high optical and structural quality in spite of large lattice mismatching. It was confirmed that the nanocolumns are attractive nano-materials to use for various functional device applications, especially for large lattice mismatching hetero-system which is difficult to realize a high quality heterostructure films.References: [1] M. Yoshizawa et al., 23nd Int. Symp. Compound Semiconductors, St. Petersburg, Russia, 02.1.03 (1996). [2] M. Yoshizawa et al., Jpn. J. Appl. Phys. 36 (1997) L459. [3] A. Kikuchi et al. phys. stat. sol. (b) 241 (2004) 2754. [4] A. Kikuchi et al., Jpn. J. Appl. Phys., 43 (2004) L1524.
4:45 PM - P12.2
Effect of Growth Temperature on the Electrical Characteristics of GaN Nanowires.
E. Lai 1 , A. A. Talin 1 , G. Wang 2 , J.R. Creighton 2 , F.E. Jones 1 , R.J. Anderson 1 , E. Marquis 1
1 , Sandia National Labs, Livermore, California, United States, 2 , Sandia National Labs, Albuquerque, New Mexico, United States
Show AbstractDespite impressive progress in the synthesis of semiconductor nanowires, the systematic electrical characterization of these 1-D nanostructures remains a formidable challenge. Thus, the majority of data on nanowire transport published to date is based on one or a few individual devices fabricated using slow and painstaking e-beam or FIB methods. Alternatively, electrical characteristics are measured simultaneously on large aggregates of nanowires, which typically vary in dimension and composition. Therefore, there is a need for reliable data which correlates individual nanowire electrical and optical characteristics with microstructure and growth conditions. In this paper, we describe current-voltage characteristics of GaN and GaN/InN nanowires grown by metal-catalyzed MOCVD. We use conventional optical lithography to define arrays of individually addressable parallel electrodes with 1, 2, and 4 um CDs. This approach results in several hundred nanowires on a 4" wafer. In addition, we measure PL spectra from collections of nanowires and correlate PL images from individual nanowires with conductivity. We find that growth temperature, rather than substrate choice, has an overwhelming effect on both conductivity and PL intensity. GaN nanowires grown at 900οC have an average resistance of 10^4Ω, and a clear band edge emission PL peak in addition to yellow luminescence. However, GaN nanowires grown at 800oC but under otherwise identical conditions, exhibited resistance ranging from 10^8 to >10^13 and no band edge PL.
5:00 PM - P12.3
Form-birefringence of III-V Semiconductor Nanowires.
Otto Muskens 1 , Maarten van Weert 1 , Magnus Borgstrom 2 , Erik Bakkers 2 , Jaime Gomez Rivas 1
1 , FOM intitute AMOLF, Eindhoven Netherlands, 2 , Philips Research Laboratories, Eindhoven Netherlands
Show AbstractThe quasi-one dimensional geometry of semiconductor nanowires leads to a large optical anisotropy. Composite materials consisting of closely stacked, oriented semiconductor nanowires therefore will exhibit large form birefringence for optical wavevectors perpendicular to the wire axis. This birefringence depends on the diameter, volume fraction, and refractive index contrast of the nanowires in the effective medium. In this contribution we will present results on the form birefringence of epitaxially grown nanowire layers made of GaP. Highly oriented nanowire ensembles were grown in the vapour-liquid-solid (VLS) growth mode using metal-organic vapour phase epitaxy (MOVPE). A GaP wafer with a <111> crystal orientation was used as substrate for the epitaxial growth. A thin gold film deposited onto the wafer acts as a catalyst for nanowire growth. The thickness of the film was varied between 0.3 and 5 nm, resulting in different wire diameters in the range 15 to 55 nm and high nanowire volume fractions, as determined from high resolution scanning electron microscopy (SEM) images. Optical measurements were performed on dense ensembles of nanowires in order to characterize the coherent and diffuse transmission, as well as the specular and diffuse reflection of linearly polarized light from a He:Ne laser (λ = 632 nm), which was incident at different angles with respect to the wires axis. Changes in the transmission and reflection characteristics are observed that can be attributed to form birefringence of the highly anisotropic nanowires.
5:15 PM - P12.4
Controlled Growth of Highly Uniform, Individually Addressable InAs Nanowires by Selective-Area Metalorganic Vapor Phase Epitaxy.
Katsuhiro Tomioka 1 2 , Premila Mohan 1 , Shinjirou Hara 1 2 , Junichi Motohisa 1 2 , Takashi Fukui 1 2
1 , Graduate School of Information Science and Technology, Hokkaido University, Sapporo Japan, 2 , Research Center for Integrated Quantum Electronics(RCIQE),Hokkaido University, Sapporo Japan
Show Abstract5:30 PM - P12.5
Atomic Scale Structure and Growth of Nanowires and Nanowire Heterostructures Studied by STM.
Anders Mikkelsen 1 , Niklas Skoeld 2 , Lassana Ouatara 1 , Jessica Eriksson 1 , Emelie Hilner 1 , Werner Seifert 2 , Lars Samuelson 2 , Edvin Lundgren 1
1 Synchrotron Radiation Research, Lund University, Lund Sweden, 2 Solid State Physics, Lund Technical University, Lund Sweden
Show AbstractFree-standing semiconductor nanowires have attracted much attention in recent years because of their potential as tools and devices in physics, chemistry and biology. These nanowires offer a unique possibility to create confined structures with a high degree of flexibility - mixing quantum dots, core-shell structures of a wide range of materials. However a high degree of control of the nanowire growth and its effect on the substrate is needed in order to tailor the nanowires structures for specific purposes. The nanowire growth proces is also highly interesting in itself, as it differs significantly from the more well understood Stranski-Krastanov and Volmer-Weber growth modes.In the present study we have investigated nanowire heterostructures on the atomic scale using Scanning Tunnelling Microscopy (STM). We show high resolution atomically resolved STM images of GaAs nanowires with an AlGaAs shell and an AlGaAs segment. From such images we derive information about the interdiffusion of the Al due to the presence of the Au seed particle, as well as on differences in growth speed between wire segment, substrate and wire shell. In addition we have studied the influence of the Au seed particles on the GaAs(111) surface, a materials system which is often used for nanowire growth. The annealing of Au films and Au nanoparticles gives rise to significant changes in surface morphology as an Au wetting layer is formed.
P13: Poster Session: Semiconductor Nanowires - Fabrication, Properties and Devices II
Session Chairs
Walter Riess
Younan Xia
Peidong Yang
Margit Zacharias
Friday AM, April 21, 2006
Salons 8-15 (Marriott)
9:00 PM - P13.1
Tailor-made Catalysts for the Growth of SiC Whiskers.
Ilya Mazov 1 , Vladimir Kuznetsov 1 , Ann Usoltseva 1
1 , Boreskov Institute of Catalysis, Novosibirsk Russian Federation
Show Abstract9:00 PM - P13.10
Sterically Varied Germanium alkoxides/silanols/thiols for Solution Synthesis of Nanocrystals and Nanowires.
Louis Tribby 1 , Henry Gerung 1 , Timothy Boyle 2 , Sang Han 1
1 Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, New Mexico, United States, 2 Advanced Material Laboratory, Sandia National Laboratories, Albuquerque, New Mexico, United States
Show Abstract9:00 PM - P13.11
Phase-diagram-guided Fabrication and Characterization of Sn/Ge/ZnS axial and Ge/ZnS side-to-side Nanowire Heterostructures.
Huang Qing 1 , bando yoshio 1 , Golberg Dmitri 1
1 , national institute for materials science, tsukuba, Ibaraki Japan
Show AbstractSn/Ge/ZnS axial and Ge/ZnS side-to-side nanowire heterostructures have been successfully fabricated via one-step chemical vapor deposition method using different catalyst. The growth mechanism of heterostructures is preliminarily explained in term of thermodynamical properties and knowledge of phase diagram. This synthesis stratagem open a new road to fabricate multiphase nanowire heterostructures. Abrupt composition change at the junctions and epitaxy growth characterazation were revealed via high-resolution transmission electron microscopy observation. Interestingly, wurtzite ZnS is always sandwiched between Ge and sphalerite ZnS in the axial heterostructures. tetragonal Ge, predicted as semiconductor with directive band gap of 1.47 eV, is also been found stable in the heterostructures. epitaxy growth mechanism is well illustrated via high resolution transmission microscopy (HRTEM) images, and simulated by crystallographic structures. finally, the optical properties of these nanowire heterostructures are investigated and explained by energy-band model.
9:00 PM - P13.12
A Novel Fabrication Technique for developing Metal Nanodroplet Arrays with Uniform Size, Shape and Periodicity for Synthesis of Metal-catalyzed Semiconductor Nanowire Arrays
Christopher Edgar 1 , Chad Johns 1 , M. Saif Islam 1
1 Electrical & Computer Engineering, University of California, Davis, Davis, California, United States
Show AbstractWe report the successful demonstration of a novel technique for the synthesis of two dimensional arrays of micro and nanoscale metal droplets with uniform period, size and shape. A linear array of metal wires was fabricated onto a (100)-Si surface using conventional optical lithography and a lift-off process subsequent to an e-beam evaporation of 10-20nm of Au. The sample was then annealed at 800C in a furnace for 2 hours and the Au wires were found to disintegrate into arrays of identical Au droplets. Variations in the temperature, thickness and width of the Au wires was found to influence the breakdown process of the wires and work is in progress for correlating the physical growth parameters with the formation of a uniformly spaced linear array of Au droplets. The origin of the disintegration of these Au wires into metal droplets can be attributed to Rayleigh instabilities induced in the Au wires at elevated temperatures. By using angular deposition techniques, metal wires with nanoscale widths are generated to produce linear arrays of nanoscale metal droplets. This novel technique of forming regular arrays of metal nanoscale droplets can offer exciting opportunities in plasmonics, bio-chemical sensing and ultra-high density information storage. The uniformly spaced metal nanodroplets can also be used to develop metal-catalyzed nanowire arrays that have uniform diameter and spacing thus offering new and exciting opportunities for positioning nanowires “in place” for future nano-electronics, photonics and energy conversion.
9:00 PM - P13.13
Focused Ion Beam-based Synthesis of Semiconductor Nanowires and Functional Structures – An Atomistic Computer Simulation Study.
Lars Roentzsch 1 , Karl-Heinz Heinig 1
1 FWIT, Research Center Rossendorf, Dresden Germany
Show Abstract9:00 PM - P13.14
Effect of Surface Roughness and Phonon Scattering on Electron Transport in Silicon Nanowires.
Alexei Svizhenko 1 , Paul Leu 1 , Kyeongjae Cho 1
1 Mechanical Engineering, Stanford University, Stanford, California, United States
Show Abstract9:00 PM - P13.15
Atomistic Design of Multicomponent Nanowires
Traian Dumitrica 1 , Ming Hua 2 , Boris Yakobson 2
1 Mechanical Engineering, Univ of Minnesota, Minneapolis, Minnesota, United States, 2 Mechanical Engineering and Materials Science, Rice University, Houston, Texas, United States
Show AbstractAlong with carbon nanotubes, semiconductor-based nanowire (seamless solid rods with nanometer size widths and micrometer lengths) represent the most critical components of molecular electronics assemblies. While carbon forms narrow nanotubes (with diameters as small as 3 Angstroms), another critical element, silicon, can form nanowires with diameters one order of magnitude larger. A new class of epitaxial growth experiments [1] produced subnanometer-height nanowires of silicon combined with metal. Besides introducing novel optical, electric, or magnetic properties, we argue, based on density functional theory calculations, that metal (M) addition has a major role in the nanowire stability. We discuss new metal-encapsulated silicon nanotube (M@SiNT, where M = Be, Ca, Sc, Ti, Zr, Cr, Fe, Ni) structures [2], which show remarkable structural connection with the produced metal-silicide wires. Theoretical identification with the limit of small-diameter, rigid one-dimensional atomic structures that can be experimentally grown, the shown M@SiNTs structural versatility, computed electronic properties, and possibility of obtaining comprehensive thermodynamic maps for MxSi1-x nanowires from ab initio calculations, hold promise for connecting the theory with experiments. 1. G.M. Ribeiro, A.M. Bratkovski, T.I. Kamins, D.A.A. Ohlberg, and R.S. Williams, Science 279, 353 (1998).2. T. Dumitrica, M. Hua, and B. I. Yakobson, Physical Review B70, 01448 (2004).
9:00 PM - P13.16
Nano-structured Materials Fabricated by Lateral Phase Separation.
Kazuhiko Fukutani 1 , Youhei Ishida 1 , Koichi Tanji 1 , Tohru Den 1
1 Leading-Edge Technology Development Headquarters, Canon Inc., Tokyo Japan
Show AbstractIn recent years, one-dimensional structures such as nanowires and nanotubes have received considerable interest for their potential applications in many fields. In particular, nanowire arrays are considered to be promising materials for high-density magnetic recording media, sensors and thermoelectric devices. Among various fabrication techniques for one-dimensional materials, direct fabrication methods using lateral phase separation (spinodal decomposition) during film growth are elegant and simple, because conventional technologies used to obtain nanostructures, such as electron beam lithography and x-ray lithography, are expensive and not always suitable for commercial applications. Various one-dimensional materials embedded in a matrix, such as Fe-LaSrFeO4 films1, BaTiO3-CoFe2O4 films2, Al-Si films3 have been developed by using the lateral phase separation methods.The phase separated Al-Si films composed of Al nano-cylinders (nanowires) embedded in an amorphous Si matrix are interesting materials because the fabrication method, sputtering deposition at a low substrate temperature, is quite simple and allows us to use various substrates.3 In addition, the materials are very useful to fabricate ultrahigh density nanowires by template-assisted growth.3 However, similar nanostructures haven’t been reported by using the simple fabrication technique although the growth mechanism has been reported.4Phase separated Al-Ge films fabricated by the sputtering deposition have been investigated as a possible binary material that can give us nanowires embedded in a matrix. In case of well-controlled deposition conditions, phase separation of the Al-Ge system during the film deposition creates the Al nanowires, perpendicular to the substrate and parallel to each other, in the Ge matrix. In this presentation, the differences between phase separated Al-Si films and Al-Ge films are discussed. Finally, a method that is able to fabricate different types of nano-structured materils such as nano-porous films and various nanowire arrays is also discussed.1L. Mohaddes-Ardacili, H. Zheng, S. B. Ogale, B. Hannoyer, W. Tian, J. Wang, S. E. Lofland, S. R. Shinde, T. Zhao, Y. Jia, L. Salamanca-Riba, D. G. Schlom, M, Wuttig, R. Ramesh, Nature Materials, 3, 533 (2004)2H. Zheng, J. Wang, S. E. Lofland, Z. Ma, L. Mohadded-Ardabili, T. Zhao, L. Salamanca-Riba, S. R. Shinde, S. B. Ogale, F. Bai, D. Viehland, Y. Jia, D. G. Schlom, M. Wuttig, A. Roytburd, R. Ramesh, Science 303, 661 (2004).3K. Fukutani, K. Tanji, T. Motoi, T. Den, Adv. Mater. 16, 1456 (2004).4K. Fukutani, K. Tanji, T. Saito, T. Den, J. Appl. Phys. 98, 033507 (2005)
9:00 PM - P13.17
Formation of MoO3 Nano Rods by a Simple Atmospheric Pressure Microplasma Technique.
Arumugam Chandra Bose 1 , Yoshiki Shimizu 1 , Takeshi Sasaki 1 , Naoto Koshizaki 1
1 Nanoarchitectonics Research Center, National Institute Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan
Show Abstract9:00 PM - P13.18
Localized States in Boron Nanobelts Probed by Photocurrent Measurements.
Kazuhiro Kirihara 1 , Kenji Kawaguchi 1 , Yoshiki Shimizu 1 , Takeshi Sasaki 1 , Naoto Koshizaki 1 , Kohei Soga 2 , Kaoru Kimura 3
1 Nanoarchitectonics Research Center, National Institute of Advanced Industrial Science and Technology, Tsukuba Japan, 2 Department of Materials Science and Technology, Tokyo University of Science, Noda Japan, 3 Department of Advanced Materials Science, University of Tokyo, Kashiwa Japan
Show Abstract9:00 PM - P13.2
Electrical and Optical Properties of SiC Nanowires Grown by Various Methods.
Jae-Soo Kim 1 , Dae-Ho Rho 1 , Jae-Hoon Lee 2 , Jae-Woong Yang 3 , Na-Ri Kim 1 , Myoung-Won Ahn 3
1 Metal Processing Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 2 Advanced Materials Development Center, Korea Institute of Industrial Technology, Inchon Korea (the Republic of), 3 Devision of Advance Materlas, Daejin University, Pochon Korea (the Republic of)
Show Abstract9:00 PM - P13.20
Kinetics and Thermodynamics in the Synthesis of Semiconducting Nanowires By the Chemical Vapor Routes
Jae-Hwan Park 1 , In-Sung Hwang 1 2 , Jong-Heun Lee 2 , Jae-Gwan Park 1
1 Materials Division, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 2 Department of Materials Science and Engineering, Korea University, Seoul Korea (the Republic of)
Show AbstractAlthough there have been a number of reports on the synthesis of semiconducting nanowires, the growth mechanisms and related thermodynamics are still quite unclear. In this presentation, the nanowire growth mechanisms will be reviewed and thermodynamics and kinetics in the various routes of ZnO nanowire synthesis will be discussed. Especially, in a carbothermal reduction process, it will be proved theoretically and experimentally that a critical limit of oxygen with addition to a conventional Ar gas is necessary to get ZnO nanostructures. More than 1ppm of oxygen was required to fabricate ZnO nanowires. This oxygen-assisted carbothermal reduction process enables an evolution of not only ZnO nanowires but also interesting nanorods and nanosheets in series with controlling oxygen partial pressure in the range of 10-6 to 10-2. [Refs.]1. J. H. Park et al, J. Cryst. Growth, 280, p161-167 (2005).2. Y. J. Choi et al, J. Mater. Res., 20, p959~964 (2005)
9:00 PM - P13.21
Fabrication And I-V Characterization of ZnO Nanorod Based Metal-Insulator-Semiconductor Junction.
Quang Le Hong 1
1 SMA, National University of SIngapore, singapore, singapore, Singapore
Show AbstractWe report on the characteristics of a ZnO based metal insulator semiconductor (MIS) diode comprised of a heterostructure of n-ZnO nanorods/n-GaN. The MIS structure consisted of unintentional - doped n type ZnO nanorods grown on n-GaN sample using hydrothermal synthesis at low temperature (1000C). The ZnO nanorod layer was vertically grown from the GaN sample, having the diameter 100nm and length 2µm. Then, an insulator layer for electrical isolation was deposited on the top of ZnO nanorod layer by using spin coating method. A metal layer (gold) was finally deposited on the top. The I-V dependences show a rectifying diode like behavior with a leakage current of 2.10-5 A and a threshold voltage of about 3V. Depend on the thickness of the insulator, the I-V dependences of the n-ZnO/n-GaN heterostructure was varied from rectifying behavior to Ohmic and nearly linear.
9:00 PM - P13.22
Hierarchical 3D ZnO Nanowire Networks.
Deli Wang 1 , Bin Xiang 1 , Shadi Dayeh 1 , David Aplin 1
1 ECE, Unversity of California - San Diego, La Jolla, California, United States
Show AbstractSemiconductor nanowires has been broadly researched as building blocks for assembly of a wide range of nanodevices for electronics and photonics. The hierarchical arrangement of nanoscale components is essential to successful manufacture of nanoelectronic/photonic devices and systems. Herein we report the growth of well aligned 3D ZnO nanowire network on a-sapphire substrates using hydrothermal CVD. First, we (a) demonstrate very large area uniform vertical epitaxial ZnO nanowire growth on sapphire and the nanowire are grown on ZnO islands. (b) By changing the growth substrate geometry in furnace, we achieved reproducible growth of hierarchically aligned 3D ZnO nanowire networks in large area. The X-ray diffraction showed two crystallographic orientations of <0001> and <10-11>. SEM and TEM studies on individual nanowires indicated single crystal structures with uniform diameters. Cross sectional SEM and TEM studies illustrate the island growth mechanism. Potential application on 3D electronic circuits and 3D photonic crystal will be also addressed.
9:00 PM - P13.23
Nanowire Growth: A Transmission Electron Microscopy Study
Julia Deneen 1 , Joysurya Basu 1 , Divakar Ramachandran 1 , C. Barry Carter 1
1 Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractThe growth mechanisms and properties of whiskers of various sizes have been studied for decades, and in recent years there has been a growing interest in nanowires, nanowhiskers and nanobelts. One-dimensional structures of semiconductors and functional oxides have become of particular interest due to their unique properties which make them candidates for various optoelectronic devices, and the ease with which they can be produced using vapor-liquid-solid growth techniques. The most common method of characterizing these structures is through post-growth studies, in which particles are synthesized and subsequently characterized in the transmission electron microscope (TEM). This method has shed some light on the growth mechanism of nanostructures, but is somewhat speculative since critical aspects of the growth process could easily be overlooked. To more fully characterize the nanowire growth mechanism it is desirable to view a particular particle during growth. In this study a technique of relocating an individual nanowire at various stages during the growth process is investigated. A TEM sample, capable of withstanding multiple processing stages, has been designed such that nanowire growth is perpendicular to the electron beam and the interface between the nanowire and the substrate is electron transparent. After each subsequent processing step the same nanowire is relocated in the TEM for further characterization. The use of in-situ heating techniques is also explored.
9:00 PM - P13.24
Growth of Pure ZnO Nanowires Without the Mixture of Nanobelts and Nanocombs.
Samuel Mensah 1 , Vijaya Kayastha 1 , Yoke Khin Yap 1
1 , Michigan Technological University, Houghton, Michigan, United States
Show AbstractZnO nanowires with small diameters (< 50nm) are desired for nanoelectronic devices including field effect transistors and chemical / biological sensors. These nanowires can offer very high surface area to volume ratios, which is especially important for these devices. Thermal CVD has proven to be an efficient technique for the growth of various ZnO nanostructures. In most cases, a mixture of ZnO and graphite powders were used as the raw materials and result in the growth of nanobelts, nanocombs, and nanorods when Au catalysts are used. Here, we show that similar technique can be used for the growth ZnO nanowires without the mix of nanobelts and nanocombs. Our experiments were conducted in a double-tube configuration. The oxidized Si substrates were inserted into a small quartz tube (2cm diameter and 60cm long, closed at one end) together with a mixture of ZnO and graphite powders in a ratio of 2:1, respectively. This tube is then inserted into the cylindrical quartz tube chamber of the thermal CVD furnace such that the mixed powder is at the center of the horizontal furnace. In this arrangement, the mixed powder will be combusted at 1100 °C and the substrates are at a temperatures between 500 to 850 °C, depends on their relative positions from the ZnO/graphite powders. Oxygen gas was introduced into the quartz tube at a flow rate of 10 to 120 sccm.We found that nanobelts and nanocombs are grown when oxidized Si substrates coated with Au thin films were use. Nanobelts are often grown when these substrates are placed at the temperature zone between 750-850 °C. Nanocombs are detected by placing the substrates at 700-550 °C. It was noticed that small diameter nanowires were always formed with the mixtures of nanocombs and nanobelts.However, pure ZnO nanowires can be grown on catalyst-free substrates. This is obtained at the temperature zone of 500-600 °C, a region beyond those for growing nanobelts and nanorods. The crucial step is the need of an Au-coated substrate adjacent to the plain substrates. We have found that when there is no Au-coated substrate present the wires that were formed had a rod-like base. Whereas, long nanowires with uniformed diameter as small as 15-20 nm were formed in the cases with an Au-coated substrate present. Field-emission scanning electron microscopy (FESEM) analysis shows that the gold vapors from the adjacent substrates help in the formation of nucleation sites on the catalyst free substrates to initiate nanowire growth. Characterization of these ZnO nanostructure was conducted by X-ray powder diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, and photoluminescence (PL). Details of these results will be discussed in the meeting. Y.K.Y acknowledges supports from the Michigan Tech Research Excellence Fund, Army Research Office (W911NF-04-1-0029), CNMS at ORNL, and NSF CAREER Award (0447555).
9:00 PM - P13.25
Photoconductivity of ZnSe One Dimension Nanostructures
Wallace Choy 1 , Y.P. Leung 1
1 Department of Electrical & Electronic Engineering, the University of Hong Kong, Hong Kong Hong Kong
Show AbstractZinc-based II-VI compound semiconductors have already been utilized in a wide range of applications and zinc selenide (ZnSe) is an attractive candidate for IR optics, optoelectronic and microelectronic devices. Recently, various types of one dimension (1-D) ZnSe nanostructures have been fabricated such as nanowires, nanorods, nanobelts and nanorings. The growing mechanisms and optical properties of the 1-D ZnSe nanostuctures have been discussed. However, there is no clear investigation on the photoconductivity of ZnSe 1-D nanostructures. In this report, we will investigate the photoconductivity of single ZnSe nanowire.The nanowires were grown through tube furnace using ~1g ZnSe powder (99.99% Sigma Aldrich) in an alumina boat placed at the center of the furnace. The temperature was increased to 1100 oC and maintained for 1 hour. The carrier gas was 95% Ar mixed with 5% H2 and the pressure was kept at 100 torr. The substrate is Au (~5nm) coated Si. From the results of EDS, SAED, SEM and TEM, we find that the nanostructures are cubic ZnSe nanowires. Electrodes were made to individual nanowires and the photoconductivity properties were measured using He-Cd laser. The results show that the photocurrent linearly increases with the light power density and the photocurrent increases by 20 times at the power density of 2.8W/cm2 as compared to the current measured at dark. The time response of the photoconductivity will also be discussed.
9:00 PM - P13.26
Fabrication and Photoluminescence Properties of Heteroepitaxial ZnO/Mg0.2Zn0.8O Coaxial Nanorod Heterostructures.
Jinkyoung Yoo 1 , Won Il Park 1 , Dong-Wook Kim 1 , Gyu-Chul Yi 1 , Miyoung Kim 2
1 Materials Sci. & Eng., POSTECH, Pohang Korea (the Republic of), 2 Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of)
Show AbstractOne-dimensional nanorod heterostructures which show composition modulation along the various directions have attracted as versatile building blocks for electronic and photonic devices. For instance, a nanorod heterostructure consisted of a core layer and a shell layer having a wider band gap and a lower refractive index than a core can confine both carriers and emitted photons in the core nanorod. This confinement of carriers and photons can enhance the quantum efficiency of photonic devices. Among several techniques for fabrication of coaxial nanowire/nanorod heterostructures, a catalyst-free metal-organic vapor phase epitaxy has some merits: precise thickness and composition control, and absence of spontaneous phase separation. Here we report on synthesis and optical properties of ZnO/Mg0.2Zn0.8O coaxial nanorod heterostructures. Synthesized ZnO/Mg0.2Zn0.8O coaxial nanorod heterostructures were characterized by field-emission transmission electron microscopy (FE-TEM), synchrotron x-ray diffraction (SR-XRD), and photoluminescence (PL) spectroscopy. From FE-TEM analyses, we confirmed that our ZnO/Mg0.2Zn0.8O coaxial nanorod heterostructures have a clean and abrupt interface between ZnO and Mg0.2Zn0.8O. SR-XRD θ-2θ scan exhibited the peak shift originated from difference between ZnO and Mg0.2Zn0.8O lattice constants. Room temperature PL spectra were obtained to characterize optical properties of ZnO/Mg0.2Zn0.8O coaxial nanorod heterostructures. The dominant PL peak intensity of ZnO/Mg0.2Zn0.8O coaxial nanorod heterostructures is much higher than that of bare ZnO nanorods while the PL spectra shape and the peak positions were not affected by Mg0.2Zn0.8O coating layer. The enhanced PL properties can be explained by carrier confinements and suppression of luminescence quenching. Furthermore, ZnO/Mg0.2Zn0.8O/ZnO/Mg0.2Zn0.8O multishell nanorod heterostructures were fabricated on c-sapphire and Si (100) substrates, and their optical properties will also be discussed.
9:00 PM - P13.27
Synthesis of ZnO Nanodisks by Solution-based Method and their Optical Properties.
Yong-Jin Kim 1 , Gyu-Chul Yi 1 , Chinkyo Kim 2
1 Environmental Science and Engineering, POSTECH, Pohang, Gyeongbuk, Korea (the Republic of), 2 Physics, Kyunghee University, Seoul Korea (the Republic of)
Show Abstract Due to direct and wide bandgap characteristics with a large binding exciton energy, ZnO has potential applications to short wavelength optoelectronic devices. In addition to single crystalline ZnO thin films, ZnO nanowires and nanorods with perfect crystallinity were recently fabricated and newly developed ZnO-based nanostructures demonstrated the possibility for nanoscale optoelectronic devices. Reducing the size of photonic devices, however, results in decrease of luminescence intensity in such a way that careful design of intensity enhancement mechanism should be introduced. For this reason, whispering gallery mode has attracted much attention because it is a very efficient mechanism of luminescence enhancement even in small scale resonators. In contrast with one-dimensional nanostructures such as nanorods, hexagonal nanodisk resonators employing whispering gallery modes have smaller effective volume of gain medium, so that it is easier to fabricate compact photonic nano devices. In addition, nanodisks are more connected to the surface of substrate and mechanical stability of nanodisks for post growth process is superior to that of nanorods. Top-down process such as lithographic etching has a few generic disadvantages over bottom-up approach in fabricating nano-resonators; (1) unavoidable damage to films associated with lithographic etching process, (2) an unwanted strain effect due to the limited choice of substrates to accommodate lattice misfit. In contrast, a bottom-up approach typically does not depend on the choice of substrates, which is a major advantage in integration with current silicon technology. Here we report the first observation of whispering-gallery-mode-like enhanced emission from ZnO nanodisks grown on Si substrates.
9:00 PM - P13.28
Spatially Resolved Cathodoluminescence Excitation Spectroscopy of Individual ZnO Nanowires
Sven Ruhle 1 , Lambert van Vugt 1 , Jaime Gomez Rivas 2 , Kobus Kuipers 2 , Albert Polman 2 , Daniel Vanmaekelbergh 1
1 Condensed Matter & Interfaces, University Utrecht, Utrecht Netherlands, 2 Center of Nanophotonics, FOM Institute AMOLF, Amsterdam Netherlands
Show AbstractCathodoluminescence imaging spectroscopy is a high-resolution tool to investigate the light emission of nanostructures, excited with an electron beam. Due to the strong spatial confinement of the electron beam, nanostructures can be excited with nanometer resolution; the emitted light is collected over a macroscopic area by a parabolic mirror. Here, we present new data on the cathodoluminescence characterization of ZnO nanowires. The wires are grown on a sapphire substrate by carbothermal reduction of ZnO powder using a gold assisted vapour-liquid-solid mechanism. The typical diameter and length of the hexagonal-shaped wires is 50-400 nm and 1-10 micron, respectively. The cathodoluminescence spectrum of the ZnO nanowires shows a strong exciton peak in the UV and a broad luminescence peak in the green region, the latter attributed to defect luminescence. Here, we present spatially-resolved cathodoluminescence spectra measured on single ZnO nanowires. We show that the cathodoluminescence intensity and spectrum depend strongly on the wire diameter. Furthermore, periodic oscillations of the luminescence intensity are observed along the length of the wires, which is an indication for standing wave formation inside a single wire. We discuss our results in terms of confined photonic and polaritonic modes that result from the strong light-matter interaction in the nanowires. The data are important for application of single nanowires as building blocks in nanophotonic integrated circuits.
9:00 PM - P13.29
MOCVD Growth of ZnO Nanostructures and Thin Films on Silicon Dioxide Surface.
Hui Wang 1 , Kwong-chun Lo 1 , Ho-pui Ho 1
1 Electronic Engineering, The Chinese University of Hong Kong, Hong Kong China
Show AbstractThe MOCVD growth of ZnO nanostructure and thin films has been investigated using diethylzinc (DEZn) and N2O on SiO2/Si substrate. The structural and morphological properties of ZnO nanostructures were found to be strongly dependent on growth conditions, including growth temperature, flow ratio of VI/II, and growth rate. Low VI/II ratio and high growth rate favour the growth of ZnO nanostructures (nanowires, nanobelts), while high VI/II ratio and low growth rate favour the growth of ZnO thin films. As the adhesion of ZnO on SiO2/Si substrate is low at high temperature, a low temperature seed-layer grown at 350 deg C was found to be effective to enhance the initial nucleation process and to achieve high quality ZnO layers on it at high temperatures (450-600 deg C). The ZnO epilayers have been characterized by scanning electron microscopy, X-ray diffraction and photoluminescence.
9:00 PM - P13.3
Energy Decomposition Analysis of Metal Silicide Nanowires.
Nevill Gonzalez Szwacki 1 , Boris Yakobson 1
1 Dept. of ME&MS, and Dept. of Chemistry, Rice University, Houston, Texas, United States
Show AbstractAlthough yttrium-silicide nanowires are not yet produced experimentally they could be of great technological interest since the bulk YSi2 has a very small lattice mismatch with Si(111) surface and exhibits very low Schottky barrier on n-type Si (0.3-0.4 eV) [1]. First principle calculations using density functional theory and the gradient-corrected LSDA approximation have been performed to examine the structural, electronic, and elastic properties of Y and Ni encapsulated silicon clusters and (2,2) nanotubes. Ultrasoft pseudopotentials and plane wave basis set has been used and complete ionic optimization has been performed. The wires have AlB2-type structure similar to that of bulk YSi2. Both finite M3Si28, M5Si44 (M = Y or Ni), and infinite MSi8 structures are found to be stable. The results will be presented on cohesive energies E, HOMO-LUMO gaps, or band structure, and the Young modulus. We also examined the stability of Y@Ge clusters and wires. The stability of larger in diameter nanowires built from MSi8 tube-components has further been examined. The total energy is decomposed into the bulk, surface, and edge contributions [2] and a simple equation proposed for the cohesive energy E(n, m) of arbitrary wire as a function of its cross-section dimension n and m. The bulk, surface, and edge energies found are common for all studied nanowires. Some limitations of the equation are discussed based on obtained theoretically and also reported experimentally changes in lattice parameters with the width of the slubs-wires. [1] L. Magaud et al., Phys. Rev. B 55, 13479 (1997). [2] Y. Zhao and B.I. Yakobson, Phys. Rev. Lett. 91, 035501 (2003).
9:00 PM - P13.30
Structural and Optical Properties of ZnO Nanowires Grown by Atmospheric Pressure Chemical Vapor Deposition Using ZnCl2-H2O and Zn-H2O as Source Materials.
Tomoaki Terasako 1 , Kosuke Umeki 1 , Yuuki Miyazaki 1 , Youhei Yagishita 1 , Masakazu Yagi 2 , Sho Shirakata 1
1 Electrical and Electronic Engineering, Faculty of Engineering, Ehime University, Matsuyama Japan, 2 , Takuma National College of Technology, Takuma Japan
Show Abstract A variety of zinc oxide (ZnO) nanostructures, such as nanowires (NWs), nanorods (NRs), and nanobelts (NBs), are synthesized using various growth techniques with and without a catalyst because of their importance in basic scientific research and technological applications. A mixture of ZnO and graphite powders has been widely used for obtaining ZnO NWs with a high crystalline quality. However, stoichiometory control, the first prerequisite for impurity doping, seems to be difficult for ZnO NWs grown by using ZnO powder. Therefore, in this study, we attempted to clarify the possibility of ZnO NW growth on the catalyst-coated substrate by atmospheric pressure chemical vapor deposition (AP-CVD) using ZnCl2-H2O and Zn-H2O as source materials. ZnO NWs were grown on the Ni-coated SiO2/Si(100) substrate by the AP-CVD method. The Ni coating was done by dropping an ethanol solution of Ni(NO3)6H2O on the substrate heated at 80°C in the atmosphere. The horizontal furnace of our CVD system has two temperature zones. One is for heating the substrate (700-750°C) and the other for vaporizing ZnCl2 powder (475°C)or Zn powder (700°C). The oxygen source H2O is vaporized at 54°C and transported into a quartz tube reactor by N2 carrier gas. Typical flow rates of the N2 carrier gas for the Zn sources and H2O were 40-240 and 40-320 sccm, respectively. Growth time was two hours. The NWs were characterized by x-ray (XRD) diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoacoustic spectroscopy (PAS) and photoluminescence (PL). In the ZnCl2-H2O system, SEM and TEM observations revealed that the ZnO NWs grown using ZnCl2 and H2O sources under the different feeding ratios exhibited different morphologies and growth directions, reflecting the difference in growth mechanism, i.e., vapor-liquid-solid (VLS) growth and vapor-solid (VS) growth. PL spectra of the ZnO NWs exhibited a dominant near-band-edge emission, indicating that their high crystalline quality. In the Zn-H2O system, it was found that the use of Zn powder is effective in achieving the reproducibility of NW growth. This is because HCl gas is not generated during the growth process. The NWs with diameters ranging from 80 to 800 nm were obtained. The wide diameter distribution for the NWs is probably due to coalescence among adjacent Ni particles. Growth experiments using Ni particles supported on zeolites were also attempted for suppressing the coalescence. The resultant NWs have smaller diameters ranging from 35 to 95 nm. This fact suggests that the coalescence among adjacent Ni particles is essentially suppressed using the zeolite support. Further investigations are required for the control of such morphology parameters as diameter, length, and position.
9:00 PM - P13.31
Piezoelectricity in Single Li Doped ZnO Nanowire.
Scott Mao 1 , Minhua Zhao 1 , C.B. Jiang 2 , S.X. Li 2
1 Mechanical Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania, United States, 2 IMR, Shenyang Interfacial Material Center, Shenyang China
Show Abstract9:00 PM - P13.32
Photoluminescence Blueshift of ZnO Nanowires Grown by Physical Vapor Deposition on GaN/Sapphire (0001).
Fu-Chun Tsao 1 , B. J Pong 1 , C. H Kuo 2 , C. J Pan 3 , G. C. Chi 1
1 Department of Physics, National Central University, Jhongli, Taoyuan, Taiwan, 2 Institude of Optical Science, National Central University, Jhongli, Taoyuan, Taiwan, 3 Optical Science Center, National Central University, Jhongli, Taoyuan, Taiwan
Show Abstract9:00 PM - P13.33
Nitride and Oxide Based Nanowires Grown by Plasma-Assisted Molecular Beam Epitaxy.
Chito Kendrick 1 3 , Damain Carter 2 3 , Paul Miller 2 3 , Roger Reeves 2 3 , Steven Durbin 1 3
1 Department of Electrical and Computer Engineering, University of Canterbury, Christchurch New Zealand, 3 , Institute for Advanced Materials and Nanotechnology, Christchurch New Zealand, 2 Department of Physics and Astronomy, University of Canterbury, Christchurch New Zealand
Show AbstractThe growth of nanostructured material continues to attract attention for a number of applications, including highly sensitive gas sensors (due to the increased surface area), and photonic crystals (which require arrays of nanostructures). Even though nanostructures can be formed through self-assembly, they typically do not possess the high crystal quality of those grown using vapour liquid solid (VLS) techniques; also, with VLS the feature size and placement can be easily controlled. To achieve VLS growth, however, several parameters have to be considered specific to the material of interest.ZnO has experienced a resurgence in interest mainly due to its free exciton energy of 58 meV. Still, perhaps more is the broad range of nanostructures that can be fabricated from ZnO, including wires, rods, and more complex sharp structures. Generally such structures are grown either in a furnace or using an MOVPE-type process; in this study an MDP21 oxygen plasma source in conjunction with molecular beam epitaxy (MBE) was employed, with the growth unaided by ozone. A gold coated substrate with a thickness gradient from 2 nm to 30 nm was used to achieve VLS growth. Growth on the 2 nm thick region resulted in the formation of ribbon-like features on the order of 100 nm in length. With increased gold thickness the growth turns towards a mixture of short nanorods and ribbons, and at the maximum gold thickness investigated, only 31 nm diameter rods are visible. Photoluminescence in each region shows features at ~3.2 eV and 3.35 eV, with the peak at higher energy becoming for significant with transformation of the morphology from ribbons to rods.Another material of interest is InN, under renewed scrutiny since 2002 when it was proposed the band gap was lower than the well established value of 1.9 eV. As a binary endpoint of the commercially interesting alloy InGaN, it remains poorly understood, although it has potential for applications requiring bandgap tunability over a wide energy range. Nanoscale structures can, in theory, lead to enhanced optical emission and sensitivity, although again initial reports of such structures have been of ammonia-based, not plasma assisted, approaches. Using a plasma-assisted MBE technique, low temperature (450oC) growth of InN on an untreated/nonpatterned (111) germanium wafer has been shown to lead to the growth of sharply facetted noncontiguous columns measuring 10-20 nm in diameter. At higher temperatures (~500oC) a single layer, albeit very rough, is formed; if a ~10 nm thick indium metal layer is first deposited, site selected growth of nanowires is observed. Similar site-selected growth is seen in the case of annealed gold-coated silicon substrates. In all cases, PL in the vicinity of 0.7 eV is observed from the nanostructures, and no signal is detected from the substrate.
9:00 PM - P13.34
Well-aligned ZnO Nanorods with Variable Diameters on Fused Silica Substrate.
Song Yang 1 , Hsu-Cheng Hsu 1 , Wei-Ren Liu 1 , Hsin-Min Cheng 1 , Wen-Feng Hsieh 1
1 Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu Taiwan
Show AbstractWe report the study of fabricating the well-aligned ZnO nanorods with variable diameters by the simple physical vapor deposition method with NiO particles deposited on the fused silica substrate. The results of HRTEM and XRD indicate that the ZnO nanords have single crystal quality and preferentially align in the c-axis direction. The SEM images show a ZnO buffer-layer between the flat top-facet nanorods and substrate. The formation and influence of this buffer layer was discussed in this report. By comparing with the sample without NiO particles, the NiO particles precoated on the substrate are presumed a nucleation center during the Zn vapor deposition process that improves the quality of the buffer-layer for growing nanorods. The diameter control of the well-aligned ZnO nanorods on the fused silica substrate is available by varying the growth pressure. The PL spectra of the ZnO nanorods at room temperature exhibit strong exciton emission of 3.25eV. The sample made in higher growth pressure shows the exciton emission peak red shifting toward 3.22eV which is attributed to the influence of the electron-acceptor-level transition.
9:00 PM - P13.36
Seeded Growth of Semiconductor Nanowires from Colloidal Nanocrystals.
Alan Colli 1 , Andrea Fasoli 1 , Stefan Kudera 2 , Liberato Manna 2 , Caterina Ducati 3 , John Robertson 1 , Andrea Ferrari 1
1 Engineering, Cambridge University, Cambridge United Kingdom, 2 , National Nanotechnology Laboratory (NNL), Lecce Italy, 3 Materials Science and Metallurgy, Cambridge University, Cambridge United Kingdom
Show AbstractSolution-synthesis of colloidal nanocrystals is a widely used technique to achieve quantum dots, wires, tetrapods and multi-branched heterostructures [1-3]. It also allows the growth of Au nanoparticles at the ends of semiconductor nanorods [4,5]. An advantage of this approach, compared to high-vacuum deposition, is that it does not require an expensive experimental set-up. Vapor-transport is another widely available technique, which, in principle, combines the advantages of both wet-chemistry and molecular beam synthesis. Surface bound vapour-phase growth of semiconductor nanowires is commonly achieved by pre-patterning metal nanoparticles on the substrate to define the nucleation points and promote anisotropic growth [6]. Here we demonstrate hetero-epitaxial growth of microns-long Si nanowires on CdSe nanocrystals by using Au particles grown at the ends of CdSe nanorod or multipods as catalysts for Si nanowires nucleation. Si nanowires stemming from CdSe nanocrystals can be used to probe the electrical properties of such molecular-scale structures, which are challenging to contact directly by lithography techniques. For devices applications, however, the metal catalyst may be a source of contamination. We then show that metal-free, seeded epitaxial growth of nanowires is possible by using CdSe colloidal nanocrystals as seeds. These allow position-selective vapor-transport growth of CdSe nanowires up to several microns in length. By tuning the deposition conditions, branched nanostructures can also be grown directly on the substrate. No growth is detected on the regions where no seed is present.1. W. U. Huynh et al., Science 295, 2425 (2002).2. L. Manna et al., Nat. Mat. 2, 382 (2003).3. D. J. Milliron et al., Nature 430, 190 (2004).4. T. Mokari et al., Science 304, 1787 (2004).5. S. Kudera et al., Nano Lett. 5,445 (2005).6. R. S. Wagner and W. C. Ellis, Appl. Phys. Lett. 4, 89 (1964).
9:00 PM - P13.4
Growth GeO2 Nanowires by Thermal Annealing in Sub-atmospheres.
Chun-I Wu 1 , Timothy Hogan 1
1 Department of Electrical and Computer Engineering , Michigan State University, East Lansing, Michigan, United States
Show AbstractGermanium dioxide nanowires have gained considerable interest lately [1,2], in part this is due to the bandgap of 2.44 eV, and high index of refraction, n = 1.63 [3]. In this paper we report a simple fabrication technique for making large quantities of GeO2 wires with diameters ranging from 30nm to >500nm. The wires tend to exhibit square cross-sectional structure, and show strong preferential growth at gold catalyst locations. These nanowires were grown on silicon substrates at locations where ~10nm of Au was deposited. Growth and diameter of the wires are strongly dependent on the background gas (Ar2/H2 (95/5), or room air) and the length of time exposed to air at the growth temperature. Pre-patterned substrates were also used to demonstrate the growth of nanowire bridges across 60µm wide trenches. We will present the growth conditions, where in all cases there is a flow Ar2/H2 but with varying amounts of exposure time to air in the range of 0 to 15 minutes, which results in varying diameter nanowires starting from no growth to >500nm diameter nanowires. 1. P. Hidalgo, B. Mendez and J. Piqueras, “GeO2 nanowires and nanoneedles grown by thermal deposition without a catalyst,” Nanotechnology, 16, pp. 2521–2524, 2005.2. Y. H. Tang, Y. F. Zhang, N. Wang, I. Bello, C. S. Lee, and S. T. Lee, “Germanium dioxide whiskers synthesized by laser ablation,” Applied Physics Letters, 74 (25), pp. 3824-3826, 1999.3. Zhong-Yi Yin and B. K. Garside, “Low-loss GeO2 optical waveguide fabrication using low deposition rate rf sputtering,” Applied Optics, 21 (23), pp. 4324-4328, 1982.
9:00 PM - P13.5
Ge and Core-Shell Ge/Si-C-N Nanowires: Mechanistic Investigations on Auto-Catalytic Growth
Sanjay Mathur 1 , Hao Shen 1 , Thomas Ruegamer 1 , Vladimir Sivakov 1 , Sven Barth 1
1 , Leibniz Institute of New Materials, Saarbruecken Germany
Show AbstractSemiconductor materials (Ge, Si) in confined 1D (nano)geometries display interesting electronic and optical properties due to the partial quantization of electronic states, which has led to the proposed use of Ge nanowires as building elements in future nanoscale electronic devices, in particular due to the high carrier mobilities. However, implementation of bottom-up concepts for the fabrication of functional devices demands synthetic methods offering precise control over the size and composition of nanowires. In addition, means for the chemical passivation of nanowires surface are currently required to provide them the necessary insulation.We have performed catalyst-free and high yield synthesis of single crystalline Ge and Ge/Si-C-N core-shell nanowires by employing thermally labile molecular precursors. The chemical vapor deposition (CVD) of precursor [Ge(C5H5)2] with weak Ge-C interactions produced Ge nanowires at extremely low temperatures (~ 275 °C). Whereas the decomposition of [Ge{N(SiMe3)2}2] resulted in core-shell morphology in which single crystalline Ge core is wrapped with a Si-C-N amorphous overlayer. The nanowires could be grown on both single and polycrystalline substrates without performing any surface treatment or application of catalysts. In contrary to the popular VLS technique, nanowires in our case grow through an auto-catalytic mechanism, which is initiated by a defect-assisted diffusion of precursor species and tendency for a 3D growth. The growth model was supported by following experiments: (i) preferable growth on artificial defects created by nano-indenting a sapphire substrate and (ii) cross-sectional HR-TEM of a single nanowire to reveal the nucleation stages of nanowires on the substrate. The microstructure, morphology and chemical composition of pure Ge and Ge/Si-C-N core-shell nanostructures were characterized by XRD, SEM, TEM and XPS analyses. The EDX line scan on Ge/Si-C-N system, revealed a Ge-enriched crystalline core whereas Si was locally distributed in the shell. A comparison of Si 2p binding energy proved that Si exists in the Si-C-N composition and not in the elemental form. In comparison to bulk Ge (~ 300 cm-1), the micro-Raman spectra of both systems revealed a low field shift (< 300 cm-1) which excluded the possible formation of GeSi alloy in the later case.
9:00 PM - P13.6
Erbium-Doped Silicon-Germanium Nanowires: Structure and Photophysics
Jeffery Coffer 1 , Ji Wu 1 , Leandro Tessler 2 , Danilo Mustafa 2
1 Chemistry, Texas Christian University, Fort Worth, Texas, United States, 2 Institute for Applied Physics, UNICAMP, Campinas Brazil
Show AbstractCrystalline alloys and layered structures comprised of silicon (Si) and germanium (Ge) possess a number of fundamentally-useful properties that translate into significant advantages in state-of-the-art device platforms. Fabricating these structures into a nanowire geometry adds the perspective of dimensional confinement to their investigation; furthermore the incorporation of erbium ions (Er3+) into these semiconductors is a promising method to prepare electronic devices with efficient optical functions since the 4I13/2→ 4I15/2luminescence transition at 1.54 μm from Er3+lies at a transmission maximum for silica-based waveguides. Depending on composition, bonding, and local structure, the presence of Er3+ into these nanowires can enhance the photophysical utility of the SiGe. This presentation outlines a strategy designed to fabricate a family of SiGe nanowire platforms that incorporate the rare earth ion erbium (III) into a given structure. By probing the effect of average erbium location in the structure along with its first coordination sphere, we propose to elucidate and tune the optical behavior of these Group IV nanostructures grown via a bottom-up synthetic process. The fundamental issue of size dependent charge carrier confinement along the width of the wire is an intriguing tunable parameter to be explored here as well. A number of experimental tools will be described which determine the structure and physico-chemical characteristics of the above. These include scanning electron microscopy (SEM), conventional TEM imaging, high resolution transmission electron microscopy (HREM), energy dispersive x-ray analysis (including mapping), extended x-ray absorption fine structure measurements (EXAFS), vibrational spectroscopies (micro- Raman), as well as photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies.
9:00 PM - P13.8
Characterization of Strain and Epitaxy of Ge Nanowires.
Irene Goldthorpe 1 , Paul McIntyre 1
1 Department of Materials Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractGe nanowires (NWs) have been less extensively studied than Si NWs, however, they offer advantages such as a lower synthesis temperature and higher intrinsic carrier mobilities. In this work, epitaxial Ge NWs have been grown on single crystal Si substrates of different orientations in a cold-wall CVD reactor. To obtain an array of wires with similar diameters, monodisperse Au nanoclusters were used as the catalysts. We will describe how pre-growth sample preparations dictate the density of wires and the quality of the epitaxial relationship between the wires and the substrate. By using a two-step temperature profile, untapered cylindrical wires were synthesized. We will also report on and compare different strategies for heteroepitaxially depositing crystalline Si shells around the Ge NWs in order to produce structures which may confine carriers in the NW interior, thus reducing the influence of surface defects in carrier scattering.The crystallographic relation between the Si substrate and heteroepitaxial Ge nanowires was studied by x-ray diffraction, including detailed symmetric and asymmetric diffraction scans, and x-ray pole figures. The strongest peaks in all Ge pole figures matched the pole pattern of the substrate, indicating that the majority of wires were heteroepitaxial. However, evidence of twinning defects was also found, most noticeably in samples grown on Si(111) substrates where twinning occurs at the Ge/Si interface at the bases of both vertically oriented wires and wires oriented along inclined <111> directions. Since strain in NWs may affect a number of electronic properties, x-ray diffraction was used to characterize the strain state of Ge NWs. Even though the wires are believed to have a thin native oxide layer, no axial component of strain in the Ge NWs was measured. A grazing incidence synchrotron x-ray scattering (GIXS) technique was used to quantify the radial strain of the wires. A compressive radial strain of -0.25% was measured near the tips of the Ge NWs, nearest to where the gold catalyst nanoparticles are attached after completion of Ge NW growth. As the effective x-ray penetration depth into the NW array was increased, the radial plane spacing approached that of bulk Ge.
9:00 PM - P13.9
Shape-controlled Growth of Ge Nanostructures.
Chang-Beom Jin 1 , Jee-Eun Yang 1 , Moon-Ho Jo 1
1 Materials Science and Engineering, POSTECH, Pohang Korea (the Republic of)
Show AbstractWe explored shape-controlled growth of single-crystalline Ge nanostructures by Au catalyst-assisted chemical vapor syntheses using GeH4 as a precursor. We first synthesized single-crystalline Ge nanowires, whose growth can be well explained by vapor-liquid-solid mechanism. Then, by carefully tuning growth temperature and GeH4 partial pressure near the Au-Ge eutectic temperature, we further controlled geometry of Ge nanowires, which continuously evolves from nanowires to nanocone shapes. We discuss a phenomenological model of the growth of Ge nanostructures in contrast with that of Si nanowires around the role of thermal decomposition of GeH4 and SiH4 precursors at their eutectic temperatures with Au catalysts. The controllable variation of Ge nanostructures in our study provides unique opportunities to explore material properties that are shape-dependent at the nanometer scale, and we report optical and electrical properties observed based on individual Ge nanostructure.
Symposium Organizers
Margit Zacharias Max-Planck-Institute of Microstructure Physics
Walter Riess IBM Research GmbH
Peidong Yang University of California-Berkeley
Younan Xia University of Washington
Friday AM, April 21, 2006
Room 2024 (Moscone West)
9:30 AM - **P14.1
Photoluminescent Characteristics of ZnO/ZnMgO Nanorod Quantum Structures.
Gyu-Chul Yi 1
1 Materials Science and Eng., POSTECH, Pohang Korea (the Republic of)
Show AbstractOne-dimensional semiconductor nanorods are potentially ideal functional components for nanometer-scale electronics and optoelectronics, due to both well-controlled composition modulation and their high aspect ratio offering easy fabrication of nanodevices. In particular, heteroepitaxial nanorod heterostructures with well-defined crystalline interfaces are essentially useful for the fabrication of devices on a single wire or a rod, which in principle permit extremely small size and ultrahigh density. Furthermore, embedding quantum structures in a single nanorod would enable novel physical properties including quantum confinement to be exploited, leading to fabrications of sophisticated quantum nanodevices. Here, I present on synthesis and photoluminescent characteristics of ZnO/Zn0.8Mg0.2O coaxial nanorod heterostructures as well as ZnO/Zn0.8Mg0.2O quantum well structures with modulated composition along their axial direction. The ZnO/ZnMgO nanorod quantum well structures with composition modulation along the axial direction exhibited a blue-shift in the bandedge photoluminescence (PL) peak, depending on the well layer thickness. The PL blue-shift is explained in terms of a quantum confinement effect in ultrathin ZnO well layers between ZnMgO barrier layers. Meanwhile, quantum confinement along radial direction was also observed for both ultrafine ZnO nanorods with diameters less than 10 nm and coaxial nanorod quantum structures with the ultrafine ZnO core nanorod. Furthermore, we fabricated coaxial nanorod quantum structures with composition modulation along their radial direction. We also measured time-resolved PL and near-field scanning optical microscopy offering near-field PL images and spatially resolved PL spectra of ZnO nanorod quantum structures, and will discuss near-field PL characteristics of the isolated nanorod quantum structures.
10:00 AM - P14.2
Standing Polariton Waves in ZnO Nanowires
Sven Ruhle 1 , Lambert van Vugt 1 , Prasanth Ravindran 2 , Kobus Kuipers 3 , Hans Gerritsen 2 , Daniel Vanmaekelbergh 1
1 Condensed Matter & Interfaces, University Utrecht, Utrecht Netherlands, 2 Molecular Biophysics, University Utrecht, Utrecht Netherlands, 3 Center for Nanophotonics, FOM-Institute AMOLF, Amsterdam Netherlands
Show AbstractZnO nanowires have attracted much interest recently due to their compelling optical properties. Such wires can be grown in high crystal quality with diameters ranging from ~50 to 500 nm, with a length of several hundred nanometers up to tens of micrometers, where the length axis of the nanowires corresponds to the c-axis of the ZnO wurtzite structure. Waveguiding has been observed in ZnO nanowires,[1] which makes them a hot candidate for photonic applications, e.g. in photonic circuits, even though the nature of waveguiding in nanowires is poorly understood. ZnO has furthermore been proposed to be the most suitable material for the technological realization of a semiconductor laser,[2] based on Bose-Einstein condensation of polaritons.[3] However a comprehensive understanding of exciton-polaritons in ZnO nanowires is still missing.We have studied cavity effects in ZnO nanowires by two different types of spatially resolved excitation spectroscopy, monitoring the spectrally resolved emission of the entire wire. Spatial resolution of about 300nm is achieved by scanning over individual ZnO nanowires with two-photon or electron beam excitation (cathode luminescence). We observe strong spatial oscillations in the total wire luminescence, which depend on the polarization and intensity of the excitation beam. In the vicinity of the exciton resonances the results indicate the existence of standing exciton-polariton modes, confined in the ZnO nanowire, which is forming a cavity for polaritons. Our results provide evidence for a group of longitudinal and a group of transverse polariton modes, separated by a gap of more than 100 meV. Further research is needed to confirm if this forms the first example of a giant vacuum Rabi-splitting. References:[1]J. C. Johnson, H. Yan, P. Yang, and R. J. Saykally, J. Phys. Chem. B 107, 8816 (2003).[2]M. H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, and P. Yang, Science 292, 1897 (2001).[3]M. Zamfirescu, A. Kavokin, B. Gil, G. Malpuech, and M. Kaliteevski, Phys. Rev. B 65, 161205 (2002).
10:15 AM - P14.3
Fracture Strength of ZnO Nanowires in Tensile and Bending Experiments.
Samuel Hoffmann 1 , Johann Michler 1 , Christophe Ballif 2 , Margit Zacharias 3 , Hong Jin Fan 3
1 , EMPA Materials Science and Technology, Thun Switzerland, 2 Institute of Microtechnology, University of Neuchatel, Neuchatel Switzerland, 3 , Max Planck Institute of Microstructure Physics, Halle Germany
Show Abstract10:30 AM - P14.4
Selective Lateral Growth and Electrical Properties of ZnO Nanowires Between Two Isolated Electrodes.
JongSoo Lee 1 , Sangtae Kim 2 , M. Saif Islam 3
1 Chemical engineering and materials science, UCDAVIS, DAVIS, California, United States, 2 Chemical engineering and materials science, UCDAVIS, DAVIS, California, United States, 3 Electrical and Computer Engineering, UCDAVIS, DAVIS, California, United States
Show Abstract10:45 AM - P14.5
High-Performance ZnO Nanowire Field-Effect Transistors with Self-Assembled Organic Gate Nanodielectrics
Sanghyun Ju 1 , Kangho Lee 1 , David Janes 1 , Myung-Han Yoon 2 , Antonio Facchetti 2 , Tobin Marks 2 , Jianye Li 3 , R. P. H. Chang 3
1 Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana, United States, 2 Department of Chemistry and the Materials Research Center, Northwestern University, Evanston, Illinois, United States, 3 Department of Materials Science and Engineering, INAC and NASA funded Center, Northwestern University, Evanston, Illinois, United States
Show Abstract Development of nanowire field effect transistors (NW-FETs) enabled by appropriate dielectrics would be of great interest for flexible electronic and display applications. One promising candidate is ZnO nanowire field-effect transistors (ZnO NW-FETs) because ZnO is a transparent material with a wide bandgap (3.37 eV), and nanowires are known to have inherent flexibility. The significant challenges in realizing such devices include control of the interface between the nanowire and the gate insulator and of the contacts to the devices and optimization of the post-treatment processes. In this study, high performance single ZnO NW-FETs using a self-assembled superlattice (SAS) as the gate insulator have been fabricated and characterized in terms of conventional device performance metrics. The ~15 nm SAS film consists of interlinked layer-by-layer self-assembled organic monolayers and exhibits excellent insulating properties with a large specific capacitance, ~180 nF/cm2, and a low leakage current density, ~1×10-8 A/cm2. The SAS-based ZnO NW-FETs showed drain current saturation at Vds = 0.5 V, a threshold voltage of -0.4 V, a channel mobility of ~ 200 cm2/V-sec, an on-off current ratio of ~104, and a subthreshold slope of 400 mV/dec. The operating voltages and mobility are significantly higher than comparable devices using thicker SiO2 gate insulators (40 nm). The nanowires used in this study are either non-intentionally doped, which are nominally n-type, or Mg doped, which should yield p-type wires. To further optimize the device performance and understand the interface properties between ZnO nanowire and metal contacts, ZnO NW-FETs were fabricated with various source/drain metal contacts having different work functions. The devices were finally exposed to ozone, which has previously been reported to increase the density of donor oxygen vacancies. In each case, the modified device performance metrics can be explained consistently according to an electrostatic model. Ozone-treatment of SAS-based ZnO NW-FETs with aluminum contacts enhances device performance displaying a saturated on-current of ~2 μA at Vds = 1 V, a threshold voltage shift to 0.2 V, a steeper subthreshold slope of ~150 mV/dec, and a much higher on-off current ratio of ~107.
P15: ZnO - Doping and Special Methods
Session Chairs
Friday PM, April 21, 2006
Room 2024 (Moscone West)
11:30 AM - P15.1
Transition Metal-Doped Zinc Oxide Nanowires.
Benjamin Yuhas 1 , David Zitoun 1 , Peter Pauzauskie 1 , Rongrui He 1 , Peidong Yang 1 2
1 Chemistry, University of California, Berkeley, Berkeley, California, United States, 2 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractThe ability to reliably and controllably dope semiconductor nanostructures is vital to the future integration of nanomaterials in device applications and engineering. In addition, it is also important that the dopant be well characterized and its local environment and interaction with the host semiconductor be well understood.This is quite apparent in the field of dilute magnetic semiconducting (DMS) materials, which are II-VI or III-V semiconductors that have been doped with transition metals. These materials are the subject of considerable research interest owing to their unique magnetic properties, and they are thought to have great potential use for integration into spintronic devices.One such material is transition-metal doped zinc oxide (Zn1-xMxO). Although this system has been theoretically predicted to have room-temperature ferromagnetism, experimental reports show a large variance in observed magnetic behavior, ranging from ferromagnetic ordering above room temperature to no magnetic ordering at all. In addition, the vast majority of experimental studies done on transition-metal doped ZnO are performed on thin films grown at high temperatures. There are very few reports of one-dimensional ZnO nanostructures doped with transition metals.We have recently developed a novel, solution-based method for the synthesis of Zn1-xMxO nanowires. We are able to dope a wide variety of transition metals into the ZnO lattice (e.g., Co, Mn, Fe, Cu), with the majority of our studies to date being carried out on the Co-doped system (Zn1-xCoxO). We have performed extensive characterization of the nanowires in an effort to precisely determine the local environment of the dopant ions and relate our characterizations to observed magnetic behavior.Drawing from a variety of techniques (electron microscopy, XRD, EELS, EPR, etc), we observe that the doping process in our reaction is very uniform during nanowire synthesis, and that the Co2+ dopants substitute directly for Zn2+. We see no evidence of any secondary phase formation, nor of a concentration gradient of the dopants in the axial or longitudinal direction. The magnetic behavior of our Co-doped nanowires, however, is striking in that it does not resemble a simple paramagnetic or ferromagnetic mechanism. This stands in stark contrast to our characterization efforts that describe our nanowires as pure, single-crystalline materials. Further characterization efforts are underway in an effort to explain more extensively the origins of the observed magnetic behavior, as well as to compare with different transition metal dopants.
11:45 AM - P15.2
Fabrication Of Room Temperature Diluted Magnetic Semiconductor Zno Nanowire Arrays.
Weilie Zhou 1 , Jingjing Liu 1 , Jiajun Chen 1 , Minhui Yu 1
1 AMRI/Chemistry, University of New Orleans, New Orleans, Louisiana, United States
Show AbstractRoom temperature diluted magnetic semiconductor (DMS) ZnO nanowire arrays, doped with Mn and Co, have been successfully fabricated through chemical vapor deposition and pulsed laser deposition, respectively. The nanostructures were characterized using X-ray diffractometer (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The nanowire arrays are all c-axis growth with stacking faults near surface area. The doping elements were detected homogeneously distributed along the DMS nanowires by electron energy loss spectroscopy (EELS). Superconducting quantum interference device (SQUID) magnetometer was employed to measure ferromagnetism of the DMS nanowire arrays. Room temperature ferromagnetism was observed in both doped nanowires. The DMS nanowire arrays with room temperature ferromagnetic ordering have strong applications in spintronic nanodevices.
12:00 PM - P15.3
Optical Properties of Implanted Single ZnO Nanowires.
Carsten Ronning 1 , Sven Mueller 1 , Daniel Stichtenoth 1 , Lars Wischmeier 2 , Chegnui Bekeny 2 , Tobias Voss 2
1 II. Institute of Physics, University of Goettingen, Goettingen Germany, 2 Institute for Solid State Physics, University of Bremen, Bremen Germany
Show Abstract12:15 PM - P15.4
Periodic, ZnO Single Crystal, Hexagonal Posts Grown in Water at 90°C
Frederick Lange 1 , Jin Hyeok Kim 2 , David Andeen 1 , Jacob Richardson 1
1 Materials, UCSB, Santa Barbara, California, United States, 2 Materials Science and Engineering, Mat Sci & Eng, Kwangju Korea (the Republic of)
Show AbstractPrevious work showed that lateral epitaxial growth (LEO) of ZnO could be achieved in aqueous Zn-nitrate solutions at 90°C by masking the substrate with strips of photoresist and controlling the growth rate of the ZnO in the <0001> direction using specifically adsorbing ions. Here it is shown that periodic ZnO posts can be grown using a photoresist mask containing periodic holes. The size of the holes and the control of the in-plane and out-of-plane growth rates controls the diameter (submicron to micron) and length (> 10 microns) of the posts. Without the addition of the citrate ions, the posts are needle shaped. Subsequent growth cycles with added citrate ions converts the needles to flat-ended, hexagonal posts. The role of the specifically adsorbing ions will also be presented.
12:30 PM - P15.5
Advanced Chemical Bath Deposition and Application of ZnO Nanostructures on Various Substrate Materials.
Marc Kreye 1 , Bianca Postels 1 , Khaled Aljasem 1 , Hergo Wehmann 1 , Erwin Peiner 1 , Andreas Waag 1
1 Institute of Semiconductor Technology, Technical University of Braunschweig, Braunschweig Germany
Show Abstract12:45 PM - P15.6
Large-Scale “Surface-Programmed Assembly” of Zinc Oxide Nanowires for Device Applications
Kwang Heo 1 , Sung Myung 1 , Narae Cho 1 , Jinkyoung Yoo 3 , Gyu-Chul Yi 3 , Seunghun Hong 1 2
1 Interdisciplinary Program in Nano-Science and Technology, Seoul National University, Seoul Korea (the Republic of), 3 Materials Science and Engineering, Pohang University of Science and Technology, Pohang Korea (the Republic of), 2 Physics, Seoul National University, Seoul Korea (the Republic of)
Show Abstract Zinc oxide nanowires have been attracting an attention due to its possible future device applications such as sensors, electronic circuits etc. However, a major bottleneck holding back their practical applications can be a lack of mass production method of such devices. Since ZnO nanowires are first synthesized in a solution or powder form, one has to assemble individual nanowires onto the substrate to build devices, which can be an extremely time-consuming task. Herein, we present a massive assembly process of ZnO nanowire-based device structures, where molecular patterns on the substrate direct the assembly of ZnO nanowires. Significantly, unlike previous directed assembly methods, ZnO nanowires in our process ‘self-align’ along molecular patterns without relying on any external forces such as liquid flow, electric or magnetic fields, etc. Using this method, we demonstrated the wafer-scale assembly of ZnO nanowire-based device structures on Au and SiO2 substrates, which allows us to envision a large scale integrated devices based on ZnO nanowires. The self-alignment mechanism of ZnO nanowires on molecular patterns also will be discussed.