### Session V16: Nanowires: Electrical and Thermal Properties

 Thursday, March 18, 2010 8:00AM - 8:12AM V16.00001: Nonlinear electron transport in quantum wires Danhong Huang , Godfrey Gumbs When impurity and phonon scattering coexist, the Boltzmann equation has been solved exactly for nonlinear electron transport in a quantum wire. The scattering effects on mobilities of electrons as functions of temperature and dc field were demonstrated. For the non-differential mobility of electrons, it is switched from a linearly increasing function of temperature to a parabolic-like temperature dependence as the quantum wire is changed from an impurity-dominated system to a phonon-dominated one. A maximum has also been obtained in the dc-field dependence of the differential mobility of electrons. The low-field mobility is dominated by the impurity scattering, whereas the high-field mobility is limited by the phonon scattering. As a quantum wire is dominated by elastic scattering, the peak of the momentum-space distribution function becomes sharpened and both tails of the equilibrium electron distribution centered at the Fermi edges are raised by the dc field after a redistribution of the electrons is fulfilled in a symmetric way. If a quantum wire is dominated by inelastic scattering, on the other hand, the peak of the momentum-space distribution function is unchanged while both shoulders centered at the Fermi edges shift leftward correspondingly with increasing dc field through an asymmetric redistribution of the electrons. Thursday, March 18, 2010 8:12AM - 8:24AM V16.00002: Interaction Effects in the Conductance of Quantum Wires: Crossover from weakly-disordered Fermi liquid to Luttinger liquid Matthew Bell , Andrei Sergeev , Jonathan Bird , Vladimir Mitin , Aleksandr Verevkin We investigate the electrical conductance of long, high-mobility quantum wires and observe a crossover from the weakly-disordered multi-channel Fermi liquid to a Luttinger liquid when the width of the channel is adjusted such that the number of electron subbands $n$ in the quantum wire is less than 3. The quantum wires with adjustable widths were formed from an AlGaAs/GaAs heterostructure using the split-gate technique. In the range of channel widths where $3 \leq n \leq8$, a logarithmic temperature dependence of the conductance is observed for 1 $< T <$ 10 K. This dependence is adequately explained by effects of electron-electron interaction in weakly-disordered quasi-one dimensional (with respect to the interaction) Fermi liquid. In more narrow wires the logarithmic dependence changes to a power-law variation. These observations are shown to be in good agreement with recent theoretical works that attribute the logarithmic term to the interaction effects in a weakly disordered quasi-one-dimensional conductor and associate this interaction correction with the beginnings of a crossover to the multi-channel Luttinger liquid. Thursday, March 18, 2010 8:24AM - 8:36AM V16.00003: Correlating the nanostructure and electronic properties of InAs nanowires M. D. Schroer , J. R. Petta III-V nanowires have many attractive electrical properties, but only recently has control over the planar defect density been achieved. We correlate the nanowire defect structure with electronic properties by growing InAs nanowires with a growth temperature tunable defect density in a home-built MOVPE reactor$^1$. Multiple field effect transistors (FETs) were fabricated along the length of these nanowires to allow the measurement of field-effect mobility as a function of defect density$^2$. Nominally defect-free segments have a 4.2 K mobility up to $\sim$4$\times$ higher than defect-rich segments on the same nanowire and approach $\mu$=16,000-18,000 cm$^2$/V s, comparable to values reported for InAs/InP core shell FETs$^3$. At low temperatures, the presence of these defects leads to the accidental formation of quantum dots along the length of the wire. These results suggest that control of the rotational twinning defect density in InAs nanowires will be of crucial importance in order to allow the fabrication of locally gated nanowire quantum dots.\\ \noindent References:\\ \noindent $^1$ M. D. Schroer, S. Y. Xu, A. Bergman and J. R. Petta, arXiv:0911.0845v1\\ \noindent $^2$ M. D. Schroer and J. R. Petta, (in preparation)\\ \noindent $^3$ X. Jiang, Q. Xiong, S. Nam, F. Qian, Y. Li and C. M. Lieber, \textit{Nano Lett.} \textbf{7}, 3214 (2007) Thursday, March 18, 2010 8:36AM - 8:48AM V16.00004: Coulomb drag in quantum circuits Alex Levchenko , Alex Kamenev We study drag effect in a system of two electrically isolated quantum point contacts (QPC), coupled by Coulomb interactions. Drag current exhibits maxima as a function of QPC gate voltages when the latter are tuned to the transitions between quantized conductance plateaus. In the linear regime this behavior is due to enhanced electron-hole asymmetry near an opening of a new conductance channel. In the nonlinear regime the drag current is proportional to the shot noise of the driving circuit, suggesting that the Coulomb drag experiments may be a convenient way to measure the quantum shot noise. Remarkably, the transition to the nonlinear regime may occur at driving voltages substantially smaller than the temperature. Thursday, March 18, 2010 8:48AM - 9:00AM V16.00005: Anisotropic Suppression of One Dimensional Weak Localizatio In a Single InAs Nanowire Dong Liang , Mohammed R. Sakr , Juan Du , Xuan P.A. Gao The magneto-conductance of a InAs nanowire with 20nm diameter is investigated with respect to temperature (2-40K), and magnetic field at an arbitrary angle. The nanowire exhibits a positive magneto-conductance whose magnitude is the largest (smallest) when the field is perpendicular (parallel) to the wire axis. Magneto-conductances in the perpendicular and parallel field are consistent with the anisotropic suppression of one-dimensional (1D) weak localization, with the same electron phase coherence length $L_{\phi}$. By fitting the magneto-conductance data to the theory, we extract $L_{\phi}$, which has an approximate $T^{-1/3}$ temperature dependence, indicating electron-electron scattering as the dephasing mechanism. Moreover, the measured anisotropic magneto-conductance at an arbitrary angle between the magnetic field and nanowire is well described by the modified theory using the fitting parameters obtained from the transverse and longitudinal magneto-conductance analysis. Our results show that the low temperature magneto-conductance in a InAs nanowire stems from the suppression of 1D weak localization effect. Thursday, March 18, 2010 9:00AM - 9:12AM V16.00006: Weak Localization and Electron-Electron Interactions in Indium-Doped ZnO Nanowires Richard S. Thompson , Dongdong Li , Gerd Bergmann , Christopher M. Witte , Jia G. Lu Single crystal ZnO nanowires doped with indium are synthesized via the laser-assisted chemical vapor deposition method. The conductivity of the nanowires is measured at low temperatures in magnetic fields with directions both perpendicular and parallel to the wire axes. A quantitative fit of our data is obtained, consistent with the theory of a quasi-one-dimensional metallic system with quantum corrections due to weak localization and electron-electron interactions. The anisotropy of the magneto-conductivity agrees with theory. The two quantum corrections are of approximately equal magnitude with respective temperature dependences of $T^{-1/3}$ and $T^{-1/2}$. The alternative model of quasi-two-dimensional surface conductivity is excluded by the absence of oscillations in the magneto-conductivity in parallel magnetic fields. Thursday, March 18, 2010 9:12AM - 9:24AM V16.00007: Metal-insulator transition in individual nanowires of doped-V2O5 Tailung Wu , C.J. Patridge , S. Banerjee , G. Sambandamurthy Recent studies on doped vanadium oxide bronzes ($M_{x}V_{2}O_{5}$) have shown remarkable transport and magnetic properties including charge and spin ordering, paramagnetism, gapless states and superconductivity. Here we present the results of transport measurements in single nanowires of $M_{x}V_{2}O_{5}$ ($M=K$, $Cu$, $Na$ or $W$). Individual nanowire devices are prepared by standard lithographic techniques and resistance ($R$) is measured as a function of temperature ($T$). $R$ vs. $T$ behavior of a typical nanowire shows multiple jumps in $R$ and hysteresis between 300 and 400 K, suggesting that metal-insulator transitions in individual nanowires may occur in several steps, changing parts of the wire from one phase to another as $T$ is varied. Metal-insulator transition in isolated nanowires is studied when the dopant composition, diameter or length is varied. Thursday, March 18, 2010 9:24AM - 9:36AM V16.00008: Magnetic and Electrical Properties of GaN and GaMnN Nanowires Dillon McDowell , Olena Bolila , Mohamed AbdElmoula , Latika Menon GaN and GaMnN nanowires grown by means of chemical vapor deposition method on a catalyst-deposited substrate are typically known to exhibit a vertical growth pattern and crystallize in the hexagonal wurtzite structure. By controlling the catalyst dimensions, novel growth patterns, such as epitaxial growth can be obtained. Transport measurements indicate good current-carrying capacity for epitaxial GaN nanowires with potential applications in advanced nanoelectronic devices. Magnetic properties of GaMnN nanowires synthesized by means of chemical vapor deposition on Au catalysts will also be discussed. Such nanowires will have potential applications in spintronic devices. Thursday, March 18, 2010 9:36AM - 9:48AM V16.00009: Tuning the thermal conductivity of InAs nanowires with carrier density Mandar Deshmukh , Sajal Dhara , Hari S. Solanki , Arvind P. Ravikumar , Vibhor Singh , Arnab Bhattacharya We measure thermal conductivity of four probe suspended InAs nanowires, using 3-omega technique. The InAs wires are ~3 micron long and ~60 nm in diameter. Measurements are done in the temperature range from 10K to 180K. Our VLS grown n-type InAs nanowires have a measurable density of twin defects, which could give rise to the measured low thermal conductivity. Our four probe devices are in field effect transistor geometry and the carrier density in the nanowire can be tuned by the gate electrode. We observe two regimes -- one above 60K, where the thermal conductivity increases with decreasing carrier density and in the second regime, below 60K, the thermal conductivity of nanowire increases with increasing carrier density. We discuss the role of electrons and phonons to understand our results. Thursday, March 18, 2010 9:48AM - 10:00AM V16.00010: Electron-phonon relaxation in weakly disordered AuPd wires due to inelastic scattering from defects Juhn-Jong Lin , Yuan-Liang Zhong , Chii-Dong Chen , Andrei Sergeev To identify and investigate mechanisms of the electron-phonon relaxation in weakly disordered metallic conductors, we study the relaxation in a series of suspended and supported 15-nm thick AuPd wires. In a wide temperature range, from 8 K up to above 20 K, the measured relaxation rate reveals quadratic temperature dependence. Our observations are shown to be in agreement with the theory, which predicts that inelastic electron scattering from vibrating impurities and defects strongly dominates over ordinary electron-phonon interaction even in weakly disordered metallic conductors. Due to inelastic electron-boundary scattering this mechanism plays a leading role in the electron relaxation in nanosctructures with metallic components. Thursday, March 18, 2010 10:00AM - 10:12AM V16.00011: Charge transport in single crystalline indium nitride Nanowires Jia G. Lu , Sheng Chu , Paichun Chang , Dongdong Li , Igor Beloborodov Single crystalline InN nanowires have been synthesized via vapor-liquid-solid growth mechanism in a chemical vapor deposition system. It exhibits wurtzite hexagonal crystal structure with lattice constant a = 3.5{\AA} and c = 5.7{\AA}. Energy dispersive spectroscopy measurement indicates indium and nitrogen concentrations with stoichiometry of 1:1. The as-synthesized nanowires (average diameter of 100nm) are then configured into field effect transistor devices and measured in a variable temperature cryostat. From the electrical transport data, the nanowire channel shows typical n-type semiconductor behavior. As temperature increases from 4.2 K to room temperature, a metal-semiconductor transition is observed at a temperature around 80 K. It is found that in the semiconductor regime, the resistivity follows a power law dependence due to electron hopping mechanism. In addition, this is confirmed by the negative magneto-resistances observed for magnetic fields applied with directions both perpendicular and parallel to the nanowire axes. Thursday, March 18, 2010 10:12AM - 10:24AM V16.00012: Diameter Dependence of the Transport Properties of Antimony Telluride Nanowires Yuri Zuev , Jin Sook Lee , Hongkun Park , Philip Kim We report measurements of electronic, thermoelectric, and galvanometric properties of individual semimetallic single crystal antimony telluride (Sb2Te3) nanowires. Microfabricated heater and thermometer electrodes were used to probe the transport properties of the nanowires with diameters in the range of 22 - 95nm and temperatures in the range of 2 - 300K. Temperature dependent resistivity varies depending on nanowire diameter. Thermoelectric power (TEP) measurements indicate hole dominant diffusive thermoelectric generation, with an enhancement of the TEP for smaller diameter wires. The large surface-to-volume ratio of Sb2Te3 nanowires makes them an excellent platform to explore novel phenomena in this predicted topological insulator. We investigate mesoscopic magnetoresistance effects in magnetic fields both parallel and perpendicular to the nanowire axis. Thursday, March 18, 2010 10:24AM - 10:36AM V16.00013: Temperature- and time-dependence of hot-electron injection and accumulation in vertical Si nanowires studied with nm-resolution ballistic electron emission microscopy J.P. Pelz , W. Cai , Y. Che , E.R. Hemesath , L.J. Lauhon Semiconducting nanowires (NWs) are of great interest for new devices, but the influence of quantum and geometry-related size effects on NW carrier injection and transport must be better understood. We report ballistic electron emission microscopy (BEEM) measurements of hot-electron injection into individual end-on'' metal Schottky contacts to vertical Si-NWs at 80-300K. We observe increasing \textit{suppression} of BEEM current with increasing hot-electron flux compared to a regular Au/Si junction, which we propose to be due to a \textit{steady-state (SS) charge accumulation} in the NW that increases with the amplitude of injected current. The suppression varies greatly for different NWs (suggesting an extrinsic defect-related mechanism) and increases strongly at lower temperature, likely due to increased SS charge accumulation. Dynamic charge trapping or de-trapping behavior (with time scale in the range of 50 to 200 ms) is observed when the tunnel current is abruptly changed, supporting that the suppression is due to (temperature-dependent) SS charge accumulation. Electrostatic simulations of carrier trapping at the Si/SiO$_{2}$ interface at NW walls [1] are consistent with the observed suppression. Work supported by NSF Grant No. DMR-0805237. \\[4pt] [1] Y. Cui \textit{et al}., Nano Lett. \textbf{3}, 149 (2003). Thursday, March 18, 2010 10:36AM - 10:48AM V16.00014: Imaging Minority Carrier Diffusion in Nanowires using Near Field Scanning Optical Microscopy L.G. Baird , R.A. Cole , N.M. Haegel A novel system integrating a near-field scanning optical microscope with a scanning electron microscope is used to image minority carrier diffusion in GaN nanowires. Luminescence associated with carrier recombination is collected with high spatial resolution to image the recombination of carriers generated by use of an electron beam as an independent point source. Light is collected in the near field from a scanning fiber using tuning fork feedback in an open architecture combined AFM/NSOM. The instrument allows for independent scanning of the NSOM probe. With a single image, it is possible to obtain a direct measure of minority carrier diffusion length from the recombination profile. Measurements are presented for n-type GaN-AlGaN core-shell nanowires, grown via Ni-catalyzed MOCVD, as well as unintentionally doped and Mg-doped GaN nanowires. For the core/shell wires, we measure a hole diffusion length of $\sim$ 1.2 $\mu$m. This work is supported by NSF Grant DMR 0804527. Thursday, March 18, 2010 10:48AM - 11:00AM V16.00015: Direct measurements of lateral variations of Schottky barrier height across end-on'' metal contacts to Si nanowires by ballistic electron emission microscopy W. Cai , Y. Che , J.P. Pelz , E.R. Hemesath , L.J. Lauhon Semiconducting nanowires (NWs) are of great interest for future electronic devices, but much needs to be understood about how carrier injection and transport at the contacts are affected by small-size effects [1] and local defects. Here we report measurements of \textit{lateral variations} of the local barrier height across individual end-on'' Au Schottky contacts to vertical $\sim$100 nm diameter Si NWs using ballistic electron emission microscopy (BEEM). Vertical Si NWs were grown on Si (111) substrates, embedded in spin-on-glass, and planarized with a chemical mechanical polish. A brief BHF etch and thin Au film deposition were then used to make end-on NW contacts. BEEM measurements show that the local Schottky barrier height (SBH) at the edge of two separate NWs is $\sim$23$\pm$3 meV lower than at the NWs center, most likely due to a stronger local electric field at the contact edge. Electrostatic simulations of a variety of possible mechanisms indicate that positive charge ($\sim$3$\times$10$^{11} \quad e$/cm$^{2})$ near the NW surface (due to near-surface defects or donors) together with geometry-enhanced electric fields effect at the NW edge could produce the stronger local electric field and resulting lower SBH at the contact edge. Work supported by NSF Grant No. DMR-0805237. [1] F. Leonard\textit{ et al}., Phys. Rev. Lett. \textbf{84}, 4693 (2000).