Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session V2: Kavli Foundation Symposium on Nanowire Quantum Devices |
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Sponsoring Units: DCMP Chair: Lars Samuelson, Lund University Room: Colorado Convention Center Four Seasons 4 |
Thursday, March 8, 2007 11:15AM - 11:51AM |
V2.00001: Engineering the electronic properties of nanowires for device applications Invited Speaker: Semiconductor nanowires have recently been recognized as a possible add-on technology to silicon CMOS. Successful integration of nanowires may push the miniaturization of components further and could also bring improved, and completely new, device functions to a chip. In particular, nanowires composed of III-V materials are of interest for applications as they benefit from a small and/or direct bandgap. We will present results from electrical measurements on InAs/InP nanowires grown by chemical beam epitaxy. Changes in the precursors fed to the growth chamber can be made to control the electronic properties of the grown material. In this way it is possible to create atomically sharp heterostructure interfaces, as well as to change the carrier concentration along the wire. The latter can be achieved by controlling the carbon incorporation from the In precursor. It will be shown that heterostructure nanowires can be used in memory cells, and also as single-electron transistors for electrostatic read-out of such cells. Finally, we will discuss the design and application of InAs nanowire-based field-effect transistors, where issues related to lateral and vertical processing of nanowires will be addressed. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:27PM |
V2.00002: Kinetic measurements during the vapor-liquid-solid growth of Si and Ge nanowires Invited Speaker: Growth of nanowires using vapour-liquid-solid (VLS) process has been successfully demonstrated over the past 40 years, but the exact mechanisms are not well understood. In this talk, we will present \textit{in situ} transmission electron microscopy studies of Si and Ge nanowire growth kinetics as a means to develop a fundamental understanding of the mechanisms governing their shape and structure. From the images of the wires, collected at video rates as a function of growth pressure, temperature, and gas environment, we identify several novel aspects of wire growth: Ostwald ripening of catalyst droplets on top of the wires, effect of oxygen on Si wire morphology, and VLS growth of Ge wires at temperatures below the bulk Au-Ge alloy eutectic temperature. We will consider the generality and applicability of these results for the growth of nanowires of other materials. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 1:03PM |
V2.00003: Spin properties of strongly interacting quantum wires Invited Speaker: A number of recent experiments on quantum wires report deviations from perfect conductance quantization at low densities. These conductance anomalies manifest themselves as quasi-plateaus in the conductance as a function of gate voltage at about 0.5 to 0.7 of the conductance quantum $G_0=2e^2/h$, depending on the device. Most commonly the experimental findings are attributed to non-trivial spin properties of quantum wires. In particular, spontaneous spin polarization of the ground state has been proposed as a possible origin of the conductance anomalies. The issue has generated a lot of interest in the community as this interpretation is in apparent contradiction with the Lieb-Mattis theorem, which forbids spontaneous spin polarization in one dimension. However, the spin properties may change dramatically when the system becomes quasi-one-dimensional. We show [1] that sufficiently strong interactions between electrons induce deviations from the strictly one-dimensional geometry and indeed give rise to a ferromagnetic ground state in a certain range of electron densities. A novel phase with more complicated spin interactions generated by four-particle ring exchanges is identified at higher densities. \smallskip [1] A.D. Klironomos, J.S. Meyer, and K.A. Matveev, Europhys. Lett. {\bf 74}, 679 (2006). [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:39PM |
V2.00004: Mesoscopic Josephson Junctions Employing Ge/Si Core/Shell Nanowires Invited Speaker: Semiconductor nanowires are finding increased importance in nanoelectronics due to their controlled growth and reduced dimensions. Band structure engineering of heterostructure nanowires is proving to be instrumental in creating low-dimensional carrier gases with enhanced mobility, low scattering and reproducible contacts. We present low temperature transport measurements of one-dimensional hole gases formed in (undoped) germanium/silicon (Ge/Si) core/shell heterostructure nanowires. The Ge core diameter of the nanowires is 15 nm with a 2 nm Si shell. The length of the nanowire between the contacts is typically 150 nm. Transparent contacts to the nanowires allow observation of transport through one-dimensional subbands arising from radial confinement. When connected to superconducting aluminum leads, a dissipationless supercurrent flows through the semiconductor nanowire due to proximity-induced superconductivity. By using a Au top gate, which modulates the carrier density of the nanowire and the number of one-dimensional subbands populated, the critical current of these mesoscopic Josephson junctions can be tuned from zero to greater than 100 nA. Resonant multiple Andreev reflections in the superconductor-nanowire-superconductor system is also observed. Finally, we investigate the interplay between one-dimensional quantum confinement and superconductivity. [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 2:15PM |
V2.00005: Imaging Few-Electron Quantum Dots in InAs/InP Nanowires Invited Speaker: Heterostructure semiconducting nanowires provide an excellent system to make high quality, ultra-small quantum dots for future applications in nano-electronics, spintronics, and quantum information processing. We use a liquid helium cooled scanning probe microscope (SPM) as a movable gate to image electrical conduction through an InAs quantum dot grown inside an InAs/InP heterostructure nanowire. Electrical transport measurements in the few-electron Coulomb-blockade regime exhibit the shell structure of quantum dot states down to the last electron. SPM images are formed by recording nanowire conductance as the charged SPM tip is scanned above the nanowire. The images display rings of peaked conductance centered on the quantum dot; the rings correspond to Coulomb-blockade oscillations of the quantum dot. In this way the tip locates the quantum dot and can be used as a movable gate to change the induced charge on a single dot in a spatially dependent way, down to zero electrons. We have also imaged homogeneous InAs nanowires. At 4K, the wires exhibit Coulomb blockade oscillations in conductance versus backgate voltage that are indicative of multiple quantum dots in series. The images reveal the location of the quantum dots along the wire and the tip voltage can tune their charge state. [Preview Abstract] |
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