Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session S28: Focus Session: Carbon Nanotubes: Devices |
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Sponsoring Units: DMP Chair: Bhupesh Chandra, Columbia University Room: Colorado Convention Center 302 |
Wednesday, March 7, 2007 2:30PM - 3:06PM |
S28.00001: Electrical Contacts to Nanotubes and Nanowires Invited Speaker: Electrical contacts play a key role in electronics. As new materials such as nanotubes and nanowires are explored for nanoelectronics, the fundamental aspects of electrical contacts need to be re-examined due to the unique properties of these nanostructures. In this talk, recent theoretical and modeling results will be presented on the properties of electrical contacts to carbon nanotubes and nanowires. For these quasi-one dimensional (Q1D) structures, side contact with the metal only leads to weak band realignment, in contrast with bulk metal-semiconductor contacts. Schottky barriers are much reduced compared with the bulk limit, and should facilitate the formation of good contacts. However, the conventional strategy of heavily doping the semiconductor to obtain Ohmic contacts breaks down as the nanowire diameter is reduced. The issue of Fermi level pinning will also be addressed, and I will discuss how the unique density of states of Q1D structures makes them less sensitive to this effect. The results agree with recent experimental work, and should apply to a broad range of Q1D materials. [Preview Abstract] |
Wednesday, March 7, 2007 3:06PM - 3:18PM |
S28.00002: Tunable contact barriers at single wall carbon nanotube/silicon heterojunctions Zhuangchun Wu, Bo Liu, Andrew G. Rinzler Due to their prevalence in modern electronic devices understanding contact barriers at metal-semiconductor junctions remains an important area of research. However, due to the sensitivity of such junctions to surface states modified by parameters like the semiconductor surface preparation, the type of metal used, and its method of deposition, this remains an area rich in complications (viz. Fermi-level pinning). Single wall carbon nanotubes (SWNTs), by virtue of their highly passivated side-walls, provide an opportunity to reduce this complexity by their lack of covalent interaction with the semiconductors on which they can be deposited. The porosity of nanotube film contacts provides further opportunities not available with contiguous metal contacts. We describe experiments in modulating the contact barrier between SWNT/Si heterojunctions using an ionic liquid gate. Modest gate voltages are shown to modify the contact barriers modulating the current across the junction by a factor of 300. [Preview Abstract] |
Wednesday, March 7, 2007 3:18PM - 3:30PM |
S28.00003: Encapsulation of floating carbon nanotubes in SiO$_2$ Leonidas Tsetseris, Sokrates Pantelides In many applications of carbon nanotubes (CNT), it is desirable to have them embedded in a dielectric such as SiO$_2$, without significantly impacting their electronic properties. Here we study the CNT-SiO$_2$\ interface of an embedded CNT using first-principles calculations. Our results suggest that a carbon nanotube can be incorporated inside a SiO$_2$\ matrix that nucleates around it through the formation of Si-O-C bridges. The large distortion associated with the formation of these bridges can be alleviated by hydrogenation of the composite. Introduction of hydrogen in the vicinity of the bridges leads to their elimination, whereby the nanotube loses its anchoring to the matrix and it floats. For CNTs of suitable diameter, the final floating structure has electronic properties very close to the structure in vacuum. Overall, our results provide atomic-scale information that is relevant to a broad range of applications using embedded or adsorbed nanotubes, for example, sensors, electronics, actuators, and CNT coatings. This work was supported in part by DOE Grant DEFG0203ER46096. [Preview Abstract] |
Wednesday, March 7, 2007 3:30PM - 3:42PM |
S28.00004: Carbon Nanotube FET Mixers and High Frequency Applications Zhaohui Zhong, xinjian Zhou, Paul McEuen We have investigated the high frequency electrical properties of single-walled carbon nanotube field effect transistors by operating the devices as microwave mixers. The mixing current amplitude depends linearly on the transconductance and quadratically on the applied AC voltage. On devices with insulating substrates, the response is approximately independent of frequency up to 40 GHz. Two applications of these high frequency-operation carbon nanotube FET mixers will be discussed: the detection of terahertz electrical pulses and nanoscale dielectric spectroscopy of liquids. [Preview Abstract] |
Wednesday, March 7, 2007 3:42PM - 3:54PM |
S28.00005: Capacitance Measurement for FETs of Individual SWNTs with Altra-Thin ALD High-k Dielectric Yuerui Lu, Ryan Tu, Yoshio Nishi, Hongjie Dai Recently, we have been able to approach the ultimate vertical scaling limit of carbon nanotube field effect transistors (FETs) and reliably achieve S $\sim $ 60 mV/decade at room temperature, by non-covalent functionalization of single walled carbon nonotubes (SWNTs) with ploy-T DNA molecules, which can impart functional groups of sufficient density and stability for uniform and conformal ALD of high-$\kappa $ dielectrics (HfO$_{2})$ with thickness down to 2-3 nm on SWNTs. Moreover, very small top gate stack capacitance ($\sim $200aF) of the SWNT FET has been successfully measured directly, using a special technique. The quantum capacitance oscillation with top gate voltage bias is clearly observed, due to the large geometric capacitance comparable to or even larger than the quantum capacitance of the SWNT. Theoretical quantum transport model and calculation fit very well with the experiment data. In addition, the mobility of the SWNT FETs at room temperature is also extracted by the capacitance measured directly. [Preview Abstract] |
Wednesday, March 7, 2007 3:54PM - 4:06PM |
S28.00006: Observing sub-60mV/dec Switching in Carbon Nanotube Tunneling Field Effect Transistor Xinran Wang, Yuerui Lu, Hongjie Dai Single-walled carbon nanotubes (SWNTs) are promising materials for future high performance and nano-electronics due to their high performance of SWNT field effect transistors (FETs) such as Ohmic contact, nearly ballistic transport and ideal switching when integrated with thin high-$\kappa $materials. Yet the ideal structure for SWNT transistors is still unclear due to non-ideal behaviors when transistor size is scaled down. Recently, PIN structure is proposed as an ultimate structure for SWNT-FETs on a single device level. In this work, SWNT-FETs with n- and p- doped source and drain and ultra thin high-$\kappa $gate dielectrics is first experimentally fabricated and characterized. This novel tunneling transistor structure is based on quantum mechanical band-to-band tunneling current between conduction and valence band of semiconducting SWNTs as opposed to the conventional Schottky type devices. Sub-60mV/dec subthreshold slope (SS) is observed reliably in our PIN transistors for $\sim $3 orders of current level on both p- and n- channels in transfer characteristics. The previously reported charge pile-up effect in SWNT-FETs is suppressed in our devices. By comparing the performance of PIN transistors with PIP MOSFETs, we find that PIN could offer lower off-state current and better on/off ratio while maintaining a decent on-state current and thus is suited for low power dissipation applications. [Preview Abstract] |
Wednesday, March 7, 2007 4:06PM - 4:18PM |
S28.00007: Simultaneous electrical transport and STM of carbon nanotubes B.J. LeRoy, I. Heller, C. Dekker, S.G. Lemay We have developed a technique to perform simultaneous electrical transport and scanning tunneling spectroscopy measurements on carbon nanotubes. The combination of these techniques allows the charge states involved in transport through the nanotube to be directly probed. The spectroscopy measurements show peaks due to Coulomb blockade, which split and change energy as a function of the source-drain voltage across the nanotube. These splitting peaks track the Fermi level of the source and drain electrodes. With our combined measurement technique, we are able to show that these peaks in the spectroscopy are correlated with changes in the source-drain current. This demonstrates that the states identified by the spectroscopy measurement are the same delocalized states involved in transport through the nanotube. Unexpectedly, the strength of these spectroscopy peaks depends on position along the nanotube. [Preview Abstract] |
Wednesday, March 7, 2007 4:18PM - 4:30PM |
S28.00008: Electron transport in single-walled carbon nanotube coupled to superconducting leads Yong Zhang, Gang Liu, Jeanie Lau We investigate electron transport through individual single-walled carbon nanotubes(SWCNT). The transport characteristic changes dramatically when the metal electrodes of the devices switch from normal state into superconducting state.~In such a situation, CNT functions as a weak link in between the superconductors(S),constituting a Josephson junction. If the contact is highly transparent, individual SWCNT can carry a supercurrent by means of proximity effect. We performed the transport measurement through S-CNT-S type Josephson junction at 260mK. Only those nanotube devices with room temperature resistance below 15kOhm were examined. The differential conductance of S-CNT-S junction showed periodic oscillations as a function of both drain-source and gate voltage, a signature of Fabry-Perot interference. Moreover, we observed a pronounced zero-bias conductance peak, which is tunable by gate voltage. Such conductance peak is attributed to multiple Andreev reflections at the S-CNT interfaces. Further experimental results in different magnetic fields and temperature will be discussed. [Preview Abstract] |
Wednesday, March 7, 2007 4:30PM - 4:42PM |
S28.00009: Evidence for Gated Spin Transport through Individual Semiconducting Single-Wall Carbon Nanotubes. Tanesh Bansal, Bhaskar Nagabhirava, Aditya Mohite, Prasanth Gopinath, Bruce Alphenaar Using a short-channel fabrication technique we are able to characterize semi-conducting single-wall carbon nanotubes at lower temperature (4.2K) than previously possible. Using this technique we are able to observe evidence for gate controlled spin transport through a semi-conducting SWNT for the first time. At 300K the devices showed typical transistor behavior with a 500{\%} change in conductance between on and off states. As the temperature decreases this behavior persists however, gate dependent conductance fluctuations appear superimposed on the standard semi-conducting gate dependence. SEM images reveal only a single nanotube between the contacts. Magnetic field dependent measurements show that spin mediated magnetoresistance follows the conductance of the device with respect to the gate voltage. Measurements reveal that the spin transport through the nanotube survives up to 30K. [Preview Abstract] |
Wednesday, March 7, 2007 4:42PM - 4:54PM |
S28.00010: Probing the characteristics of carbon nanotube based devices through the Aharonov-Bohm phase Georgy Fedorov, Dmitry Smirnov, Alexander Tselev, David Jim\'enez, Sylvain Latil, Nikolay G. Kalugin, Paola Barbara, Stephan Roche The exceptional low-dimensionality and symmetry of carbon nanotubes (CNT) are at the origin of their spectacular physical properties governed by quantum effects. Ajiki and Ando [1] predicted that an axial magnetic field would tune the bandstructure of a CNT between a metal and a semiconductor, owing to the modulation of the Aharonov-Bohm (AB) phase of the electronic wave function. This remarkable effect of magnetic field leads to a class of new physical phenomena observed in CNT. Here we report on quasimetallic CNT forming a conduction channel of three-terminal devices, which can further operate as CNT field effect transistors, under the modulation of the AB- phase. The off-state conductance varies exponentially with the magnetic flux intensity. We show that the helical symmetries of metallic CNT, as well as the characteristics of Schottky barriers formed at the metal-nanotube contacts, can be obtained by using temperature-dependent magnetoresistance measurements. [1] Ajiki, H. \& Ando, T. J. Phys. Soc. Jpn. 62, 1255-1266 (1993) [Preview Abstract] |
Wednesday, March 7, 2007 4:54PM - 5:06PM |
S28.00011: Density-functional study of Peierls instability in carbon nanotubes using hybrid functionals Guillaume Dumont, Paul Boulanger, Michel Cote, Matthias Ernzerhof We present a first-principles study of Peierls distortions in \emph{trans}-polyacetylene, polyacene and armchair $(n,n)$ carbon nanotubes. All calculations were done within density- functional theory using a gaussian basis set. We show that while density only functionals (LDA, GGA) cannot reproduce the experimentally mesured dimerization in \emph{trans}- polyactetylene, hybrid functionals including Hartree-Fock exchange can give the correct geometry. These findings suggest that armchair $(n,n)$ carbon nanotubes could have a nonsymmetric ground state; in contradiction with what is commonly accepted. Indeed, the B3LYP functional (which includes 20\% of exact exchange) opens a gap of 0.26 eV and 0.12 eV for the $(3,3)$ and $(6,6)$ carbon nanotubes respectively. Accordingly, dimerization amplitudes of 0.005 \AA~and 0.002 \AA~are obtained. It is found that the dimerization and the band gap are proportional to the the amount of exact exchange included in the functional. [Preview Abstract] |
Wednesday, March 7, 2007 5:06PM - 5:18PM |
S28.00012: Asymmetric Noise in Carbon Nanotube Quantum Dots Chris Merchant, Nina Markovic We report on low-temperature noise measurements of carbon nanotube quantum dots. The dots are asymmetric, having one ferromagnetic and one superconducting lead. Our measurements indicate that the noise is also asymmetric with respect to current, apparently depending on the direction of electron tunneling. Additionally, we observe that the gate voltage modulates the noise. We discuss these results relative to the standard model for quantum dots as well as spin blockade. [Preview Abstract] |
Wednesday, March 7, 2007 5:18PM - 5:30PM |
S28.00013: Remote phonon scattering in NT field effect transistors Alexey G. Petrov, Slava V. Rotkin We developed a theory of the remote phonon (RP) scattering for the hot charge carriers in nanotube (NT) field effect devices that use polar dielectric substrates, such as SiO2 or high-kappa materials [JETP Lett 84, 156, 2006]. We calculated the effect of this novel scattering mechanism on the NT conductivity. We stress that in contrast to any other scattering mechanisms studied earlier the RP scattering allows to transfer the excess energy of the hot carriers directly to the substrate (not through the NT lattice). The macroscopic substrate has no limitation of a finite thermal capacity as a single NT has. Therefore, our RP scattering mechanism is advantageous for the high power NT devices, especially when aggressively scaling down the size and scaling up the operational frequency. We obtained a scattering time within a self-consistent quantum mechanical approach for inter- and intra-subband transitions in semiconductor and metallic NTs. The intra-subband transitions with forward scattering are shown to prevail over the inter-subband transitions as well as the backward scattering. We obtained the polaronic effects by solving for the electron energy and life-time self-consistently. We found the upper limit of the spacing between the NT and the polar dielectric for the RP scattering to become ineffective, which is approximately 40 nm for the quartz substrate. [Preview Abstract] |
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