Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session X30: Nanowires: Electronic Transport, Experimental |
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Sponsoring Units: DCMP Chair: Francois Leonard, Sandia National Laboratories Room: C147/154 |
Thursday, March 24, 2011 2:30PM - 2:42PM |
X30.00001: Resistivity of Endotaxial Silicide Nanowires Measured with UHV-STM Nanoprobe Sam Tobler, Peter Bennett We have measured the resistivity of endotaxial silicide nanowires on silicon using a UHV STM nanoprobe in a 2-point configuration, which allows separation of intrinsic resistivity from contact resistance using a variable probe spacing. A fixed contact is provided by a thin metal film deposited by shadow evaporation with an edge profile 100nm wide and sheet resistance 500 ohms, while the second contact is provided by the STM tip. A controlled approach with 15 Ang displacement beyond the tunneling position allows for reliable and repeatable electrical contact without damage to the tungsten STM tip. Using this method, we have obtained resistivity values of 30 micro-ohm-cm and 120 micro-ohm-cm for CoSi2 nanowires of width 40nm and 20nm, respectively, on Si(110). The increase of resistivity with decreasing width is attributed to boundary scattering along the sidewalls of the nanowires. [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X30.00002: Field Effect Transistor based on Single Crystalline InSb Nanowire Jia Lu, Yennai Wang, Karan Banerjee, Huijun Yao, Thomas Schaepers Semiconductor nanowires have attracted substantial scientific and technological interests due to their unique properties arising from the size confinement effects. Among III-V group, indium antimonide (InSb) has the smallest bandgap energy (170 meV) at room temperature and possess an extremely high bulk electron mobility. It has been widely used in infrared optoelectronics and high-speed devices, and has inspired significant interest for fundamental studies in their nanostructure form. In this work, InSb nanowires with precise stoichiometry and zincblende crystal structure are synthesized via pulsed-laser chemical vapor deposition. Raman spectroscopy shows stoke and anti-stoke peaks of transverse-optical mode with asymmetric broadening. The nanowire demonstrates $n$-type semiconductor behavior. Enhanced surface scattering due to size confinement leads to reduced electron mobility. [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X30.00003: Realizing lateral wrap-gated nanowire FETs: Controlling gate length with chemistry rather than lithography Adam Micolich, Kristian Storm, Gustav Nylund, Lars Samuelson An important consideration in miniaturizing transistors is maximizing the coupling between the gate and the semiconductor channel. A semiconductor nanowire with a coaxial metal gate represents the optimum in gate-channel coupling, but has only been realized for vertically-oriented nanowire transistors. We report a method for producing laterally oriented fully wrap-gated nanowire field-effect transistors that provides exquisite control over the gate length via a single wet etch step, eliminating the need for additional lithography beyond that required to define the source/drain contacts and gate lead. Our design allows the contacts and nanowire segments extending beyond the wrap-gate to be controlled independently by biasing the doped substrate, significantly improving the sub-threshold electrical characteristics. Our devices provide stronger, more symmetric gating of the nanowire, operate at temperatures between 300 to 4 Kelvin, and offer new opportunities in applications ranging from studies of one-dimensional quantum transport through to chemical and biological sensing. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X30.00004: Transport studies of ultrathin YSi$_{2}$ nanowires Saban M. Hus, Hao Hu, Violeta Iancu, Hanno H. Weitering, An-Ping Li Extremely long YSi$_{2}$ nanowires have been fabricated via self-assembly during epitaxial growth of Y on the Si (100) 2x1 surface. The thinnest YSi$_{2}$ nanowires have a cross section of $\sim $0.4 x1.1 nm$^{2}$ and can grow up to 2 $\mu $m long. They are among the closest realizations of a one-dimensional conductor. Their electrical transport properties have been investigated with a variable-temperature four-tip scanning tunneling microscope (STM) using ex-situ fabricated contact pads. These ex-situ investigations indicated that the electrical conductivity of single nanowires is thermally activated, following an inverse Arrhenius law. In-situ contact fabrication has been accomplished via a field-induced atomic emission process from a gold STM tip. Details of the in-situ fabrication method and preliminary transport results will be presented. The research at Oak Ridge National Laboratory's Center for Nanophase Materials Sciences was sponsored by the Division of Scientific User Facilities, US Department of Energy. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X30.00005: Experimental observation of very large magnetoconductance in microbial nanowires Nikhil Malvankar, Madeline Vargas, Derek Lovley, Mark Tuominen Microbial nanowires are 2-5 nm-wide conductive proteinous pili filaments secreted by some bacteria, which can grow tens of micrometers long and may serve as a conduit for long-distance electron transport. Our previous studies demonstrated that pili of Geobacter sulfurreducens exhibit properties akin to disordered metals, and indicated a temperature-driven crossover from the regime of weak localization (WL) to strong localization (SL). Here we report a very large positive magnetoconductance (MC), up to 10,000 \%, at 300K. MC increased exponentially with magnetic field. A crossover from positive MC (WL regime) to negative MC (SL) was observed at $\sim$ 280K when the localization and the phase-breaking lengths are expected to become comparable. We attribute positive MC to destruction of the quantum interference of delocalized electron wavefunctions and negative MC to shrinkage of the localized electron wavefunctions due to applied magnetic field, which is consistent with the temperature dependence of conductivity. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X30.00006: Electron Transport in Gold Nanowires: Stable 1-, 2- and 3-Dimensional Atomic Structures and Non-Integer Conduction States Douglas Smith, Francesca Tavazza, Lyle Levine, Jon Pratt, Anne Chaka We report experimental conductivity measurements made during highly stable tensile deformation of Au nanowires showing a rich variety of behaviors, including non-integer quantum conductance plateaus, transitions and slopes. Using tight binding conductance calculations on simulated nanowires previously deformed using density functional theory calculations, we demonstrate that all of these phenomena can arise from structural transitions between highly stable ordered atomic configurations that self-organize during tensile deformation. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X30.00007: Gate-induced Fermi level tuning and ambipolar conduction control in InP nanowires Kristian Storm, Gustav Nylund, Magnus Borgstr\"om, Jesper Wallentin, Carina Fasth, Claes Thelander, Lars Samuelson Semiconducting nanowires are an interesting platform for studies of fundamental material transport properties in one dimension as well as for building blocks for various types of devices. Most conventional semiconductor devices are based upon doping for its operation, but as device dimensions are decreased, the random position of a few incorporated impurity atoms may come to dominate device characteristics. We present measurements of InP nanowires in which the Fermi level is tuned at efficiency close to the theoretical limit using semi-wrapped gates. Furthermore, we present ambipolar devices in which the Fermi level can be tuned across the entire bandgap of the semiconductor. We believe this will be of considerable importance and serve as a foundation for producing nanowire devices where the device behavior is induced by sequential gates wrapped around the nanowire channel, replacing the need for doping in certain types of devices. This way, the properties can be dynamically tuned using wrapgates, as opposed to statically set using the doping level. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X30.00008: Transport Measurements on Sb doped Silicon Nanowires Prathyusha Nukala, Marzieh Zare, Gopal Sapkota, Pradeep Gali, Usha Philipose Semiconductor nanowires (NWs) present an alternative approach for device scaling. N-type Si NWs are generally grown with silane as source with phosphine and arsenic as dopants, all of which are toxic in nature. We present a safe, cost-effective approach for synthesis of n- doped Si NWs using Sb. Structural and compositional characterization using electron microscopy and X-ray spectroscopy will be presented for crystallographic information on the quality and morphology. Ohmic contacts established to a single and on an array of doped and undoped NWs in an FET type of device configuration will provide information on several parameters such as type of majority carriers, mobility and concentration. We will highlight the promise of Sb doped Si NWs for electronic applications such as nano-scale field effect transistors and sensors. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X30.00009: Role of defect states in charge transport in semiconductor nanowires Dongkyun Ko, Xianwei Zhao, Kongara Reddy, Wolfgang Windl, Nitin Padture, Nandini Trivedi, Fengyuan Yang, Ezekiel Johnston-Halperin Charge transport characteristics are investigated in Se-doped InP nanowires in order to determine the nature of the defect states. I-V curves indicate that transport is limited by trapped space charges rather than by Schottky at high bias. In addition, mobility calculations show that hopping between defect states plays an important role at low bias. A transition between hopping mechanisms as a function of temperature can be determined from the behavior of the temperature-dependent resistance R(T). Nearest neighbor hopping (NNH) is dominant in the high temperature regime ($>$158K) , \textit{R$\sim $exp(T}$_{0}/T)^{1.03}$, and Efros-Shklovskii variable range hopping (ES-VRH) is dominant in the low temperature regime ($<$158K), \textit{R$\sim $exp(T}$_{ES}/T)^{0.49}$. Gate-bias dependence of the transition temperature and hopping parameters are also investigated: these results suggest that applying positive gate-bias changes the strength of electron correlations in these quasi-1D systems. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X30.00010: InAs Nanowire Transistors as Gas Sensor: the Role of Surface States Dong Liang, Juan Du, Hao Tang, Xuan P.A. Gao Utilizing the large surface-to-volume ratio, sensors of quasi one-dimensional semiconductor nanowires based electronic devices have been shown high sensitivity to the adsorption gaseous molecules or the binding of biomolecules in liquid, enabling a label-free sensing modality with high sensitivity and direct electrical readout. We report a study of the response of InAs nanowire field-effect transistor sensor devices to various gases and alcoholic vapors. It is concluded that the change in conductance of the device in response to chemical vapors is a combined result of both the charge transfer and modified electron mobility effects. In particular, we found that surface adsorption of most chemical molecules can reduce electron density in nanowires from $\sim $10$^{4}$ to $\sim $10$^{3}$/$\mu $m and enhance the electron mobility greatly (from tens to a few hundred of cm$^{2}$/(V s)) at the same time. These effects are attributed to the interactions between adsorbed molecules and the electron accumulation layer and rich surface states on the InAs nanowire surface. Journal reference: Nano Letters 9, 4348 (2009). [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X30.00011: Surface passivated pure Indium Oxide nanowires for gas sensors Pradeep Gali, Kiran Shrestha, Fang Lingkuo, Nigel Shepherd, Usha Philipose Indium oxide (In$_{2}$O$_{3})$ nanowires have applications in semiconductor electronics and gas sensing. We report on growth of stoichiometric In$_{2}$O$_{3}$ nanowires with diameter ranging from 40 to 80nm and lengths over 10 $\mu $m. Structural characterization done with SEM, XRD and TEM shows that the nanowires exhibits BCC structure and grow along the (100) direction. Energy Dispersive X-ray spectroscopy shows stoichiometric composition. Transport measurements on a single nanowire shows ohmic behavior and a resistance of about 100 K$\Omega $. Photoluminescence spectrum at room temperature shows strong emission peaks at 370nm and 415nm, corresponding to near band edge and defect related emission respectively. We present a technique of post-growth annealing of these nanowires to eliminate the defect induced emission and enhance band edge emission. Passivating the surface of these nanowires enhances their gas sensing abilities. [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X30.00012: Synthesis and Characterization of InAs / InSb Nanowire Heterojunctions Minkyung Jung, Michael Schroer, Jason Petta InSb is a very promising material for both electronic and optoelectronic devices due to its unique features, including a very small band gap, large bulk mobility, enormous electronic g- factor and strong spin-orbit interaction. In particular, the small effective mass of InSb makes it straightforward to fabricate devices that display effects due to quantum confinement [1,2]. Here InAs/InSb nanowire heterostructures were grown by metal-organic vapor-phase epitaxy on InAs $\left (111\right)$B substrates. We investigated morphology changes of InAs/InSb nanowires with varying growth temperature and V/III ratio. The samples were characterized using scanning electron microscopy and high resolution transmission electron microscopy. In order to study the transport properties of InAs/InSb nanowires, field effect transistors were fabricated on SiO$_2$/Si substrates and characterized at room temperature and 4.2 K. \\[4pt] [1] H. A. Nilsson {\it et al.}, Nano Lett. {\bf 9}, 3151 (2009) \\[0pt] [2] P. Caroff {\it et al.}, Small {\bf4}, 878 (2008) [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X30.00013: Measurement of minority carrier diffusion length in individual silicon nanowires with an in-situ grown p-n junction A.D. Mohite, D.E. Perea, S. Singh, S.A. Dayeh, S.T. Picraux, H. Htoon We report a scanning photocurrent microscopy study across a p-n junction of individual in-situ doped Si nanowires (NWs). The measured photocurrent decreases exponentially as the laser spot is scanned away from the p-n junction in both directions. The photocurrent peak widens with increasing reverse bias, indicating the increase of depletion width. For a 40nm diameter NW, the fit of photocurrent decay to an exponential function gives minority carrier diffusion lengths of $L_{n}$=1.842 $\mu $m and $L_{p}$=1.45 $\mu $m for electrons and holes, respectively. Such relatively long minority carrier diffusion lengths are consistent with the low dopant incorporation we expected for our growth condition. This result further suggests that the diffusion length scales with doping concentration despite the impact of surface states of a 1D system. We will further discuss the dependence of the minority carrier diffusion length on diameter, doping concentrations, and back-gating. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X30.00014: Andreev tunneling enhanced by Coulomb oscillations in superconductor-semiconductor hybrid Ge/Si nanowire devices Xiaojie Hao, Tao Tu, Hai-Ou Li, Cheng Zhou, Gang Cao, Guang-Can Guo, Guo-Ping Guo, Wayne Fung, Zhongqing Ji, Wei Lu We explore the magneto-conductance of Ge/Si core/shell nanowire quantum dot devices contacted by superconducting leads. Significant magneto-conductance peaks around zero field are observed and show a periodic modulation with gate voltage as discrete states of the quantum dot are turned on- and off-resonance with the Fermi energy in the superconducting electrodes. The ability to create and control coherent transport in superconductor-semiconductor hybrid nanostructures allows for new opportunities in the study of various fundamental competing effects such as superconductivity and electron-electron interactions. [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X30.00015: Minority Carrier Lifetimes and Surface Effects in VLS-Grown pn Junction Silicon Nanowires Yeonwoong Jung, Aleksandar Vacic, Daniel Perea, Tom Picraux, Mark Reed We study the minority carrier lifetimes and surface effects of pn junction Si nanowires. Axial pn junction Si nanowires with alternating p-n doped segments are grown based on the Au-catalyzed VLS process by an in-situ exchange of gas-phase dopants. As-grown nanowires display strong current rectification only after surface etching processes. By utilizing the reverse recovery transient of minority carriers, we directly characterize the minority carrier lifetimes and observe the decrease of the lifetimes with a decrease of nanowire diameters. Investigation of the diameter-dependent device ideality factor and current density strongly suggests that the surface recombination with an enhanced surface-to-volume ratio significantly governs the carrier transport. We also characterize the carrier lifetimes of nanowires with and without surface passivation layers, and observe an enhancement of the lifetimes in the surface-passivated ones. These studies elucidate the carrier transport mechanism in VLS pn junction Si nanowires and emphasize the importance of the surface passivation for efficient photovoltaic applications. [Preview Abstract] |
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