Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session B7: Focus Session: Graphene Devices II |
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Sponsoring Units: DMP Chair: Arthur Hebard, University of Florida Room: 303 |
Monday, March 18, 2013 11:15AM - 11:27AM |
B7.00001: Tunnel magnetoresistance of magnetic junctions based on side-wall epitaxial graphene nanoribbons Chao Huan, John Hankinson, Wenlong Yu, Rui Dong, James Palmer, Owen Vail, Ming Ruan, Claire Berger, Edward Conrad, Walter de Heer, Zhigang Jiang We report on tunnel magnetoresistance (TMR) measurements of magnetic tunnel junctions consisting of cobalt, aluminum oxide barrier, and side-wall epitaxial graphene nanoribbons (GNRs). We find that the measured resistance of tunnel junctions exhibits a spin switch behavior when the magnetic field is applied parallel to the cobalt electrode and sweeping between 1 T and -1 T. This observation indicates that the side-wall GNR is magnetic, with a spin component either parallel or antiparallel with respect to the magnetization direction of cobalt. The largest relative change of TMR observed is about 9{\%} at 6.6 K, corresponding to 14{\%} of spin polarization in GNR. In addition, we find that Rashba effect may play an important role in polarizing the electron spins in GNR; the required electric field could be due to the charge transfer between the carbon atoms on the edge of GNR and the Si atoms of the SiC substrate. [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B7.00002: Superconductor-graphene based quantum entangler, a progress report Ivan Borzenets, Yuya Shimazaki, Juergen Sailer, Russell Deacon, Michihisa Yamamoto, Seigo Tarucha We report on the progress in fabricating a functioning quantum entangler. The device is based on the cooper-pair splitter ``T'' junction with either lead (Pb) or niobium (Nb) acting as the superconductor and graphene acting as the normal metal. Unlike the typically used aluminum (Al), lead and niobium have a superconducting transition at much higher temperatures (meaning a higher superconducting gap $\Delta$), thus increasing the extent of the proximity effect. Proper techniques had to be developed in order to create transparent, superconductivity inducing contacts to graphene; and graphene-based Josephson junctions were fabricated and characterized. Meanwhile, graphene features high mobility, and therefore a high coherence length. We have patterned graphene into constrictions resulting in individually gated quantum dots with consistent characteristics. This is required in order to prevent both electrons from the same cooper pair from traveling into a single normal lead. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B7.00003: C-axis magnetoresistance in epitaxially grown multilayer graphene Srikrishna Bodepudi, Abhay Singh, Sandipan Pramanik Magnetoresistance, the change in electrical resistance of a solid-state system as a function of an external magnetic field, is a key effect in condensed matter physics both for fundamental understanding of charge transport phenomena as well as immense commercial implications. Artificial layered structures, such as metallic or metal-insulator multilayers often exhibit ``giant magnetoresistance'' or ``tunnel magnetoresistance'' effects that are exploited in various state-of-the-art data storage and magnetic field sensing devices.. Graphite is a naturally occurring layered structure in which graphene layers are stacked up on each other. Magnetoresistance in graphitic systems has drawn significant attention in recent years due to the unique crystal structures of these materials, which often lead to novel physics. In this work we consider epitaxial multilayer graphene on nickel and studied c-axis charge transport when the magnetic field is applied normal to the graphene plane. We show that the electrical resistance measured across the graphene stack on nickel can be reduced by two orders of magnitude by applying a relatively small magnetic field of few kilogausses normal to the layer plane. This feature persists even at room temperature and is far stronger than any other magnetoresistance effect reported to date for comparable temperature and field conditions. Existence of such effect makes multilayer graphene an attractive platform for magnetic field sensing, data storage and exploration of fundamental insights into graphene physics. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:27PM |
B7.00004: Graphene barristor for high performance devices Invited Speaker: Jinseong Heo Graphene has unique properties, such as work-function tunability and high intrinsic mobility. Recently, we have introduced a new concept device, a graphene variable-barrier ``barristor'' (GB), based on those properties. In this presentation, I will describe the three-terminal active device, GB, where the key is an atomically sharp interface between graphene and hydrogenated silicon surface. Large modulation on the device current, on/off ratio of 100000, is achieved by adjusting the gate voltage to control the Schottky barrier between graphene and silicon. The barrier height was tuned to 0.2 electron volts by adjusting graphene work function which results in large shifts of diode threshold voltages. For logic application, an inverter and a half-adder were demonstrated using the complementary GB's on 150-mm wafers. In addition, recently developed vertical transistor based on asymmetric junctions will be discussed. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B7.00005: Gate Tunable Graphene-Silicon Ohmic/Schottky Contact Chun Chung Chen, Chia Chi Chang, Zhen Li, Anthony Levi, Steve Cronin We have recently demonstrated gate tunable graphene-silicon Schottky diodes, in which the low bias conductance can be varied by more than three orders of magnitude [1,2]. Here, we deposit graphene on silicon substrates and observe the rectifying $I-V$ characteristics in graphene-silicon junctions, indicating the formation of Schottky junction due to the mismatch of their work functions. By applying a polymer electrolyte gate to the graphene surface, the Fermi energy of the graphene can be shifted $\pm$ 0.85eV from its charge neutrality point ($-$4.6eV) to match the conduction ($-$4.01eV ) or valence band ($-$5.13eV) of silicon to reduce the Schottky barrier and result in Ohmic contacts with both $n$- and $p$-type silicon. The $I-V$ characteristics observed under light illumination also indicate that the short circuit current can be increased or decreased by varying graphene-silicon work function difference, further demonstrating that the graphene-silicon junction and be changed between Schottky and Ohmic contact. \\[4pt] [1] Chen, Aykol, Chang, Levi, and Cronin, ``Graphene-Silicon Schottky Diodes.'' Nano Letters, 11, 1863-1867 (2011).\\[0pt] [2] Chen, Chang, Li, Levi, Cronin, ``Gate Tunable Graphene-Silicon Ohmic/Schottky Contacts.'' Applied Physics Letters, accepted (2012). [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B7.00006: Hole Injection from Silicon to Oxide Using Graphene as Transparent Electrode Rusen Yan, Huili G. Xing, Nhan Van Nguyen We demonstrate a novel application of graphene as a transparent electrode in internal photoemission (IPE) spectroscopy. Owing to its low absorption in the IR/Visible/UV range, graphene enables the direct observation of hole injection, and thus the measurement of both conduction and valence band offsets at the semiconductor-oxide hetero-interface. The photocurrents, consisting of electron or hole transitions between Si substrate and graphene as a function of incident photon energy under various applied gate voltage are measured. The barrier height is further determined from the photoemission quantum yield, which is defined as the ratio of photocurrent and light intensity. As a result, the barrier heights, $\varphi _{e}^{0}$, from the valence band top in Si to the bottom of the conduction band in Al$_{2}$O$_{3}$, and $\varphi _{h}^{0}$, from the bottom of the conduction band in Si to the top of the valence band in Al$_{2}$O$_{3}$ are extracted to be 3.5 eV and 4.1~eV, respectively. Furthermore, the bandgap of Al$_{2}$O$_{3}$ can be simply obtained by$\mathop E\nolimits_{g}^{\mathop {Al}\nolimits_{2} \mathop O\nolimits_{3} } =\mathop \phi\nolimits_{e} +\mathop \phi\nolimits_{h} -\mathop E\nolimits_{g}^{Si} =3.5+4.1-1.1=6.5$eV, close to previously reported values. Similar phenomenon is also observed and confirmed by replacing Al$_{2}$O$_{3}$ with 10 nm SiO$_{2}$. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B7.00007: Thin film barristor: a gate tunable vertical graphene-pentacene-gold device Claudia Ojeda-Aristizabal, Wenzhong Bao, Michael S. Fuhrer Graphene, a one atom thick crystal made of carbon, shows exciting possibilities as a tunable electrode for semiconductors. Graphene's electrochemical potential can be tuned over a span of electron volts, and graphene is expected to have no interface states. Here we explore graphene as a tunable electrode contacting pentacene, a van der Waals molecular semiconductor which should also have no interface states. We fabricate a vertical thin-film barristor device consisting of highly doped silicon (gate), 300 nm SiO$_2$ (gate dielectric), monolayer graphene, pentacene, and gold top electrode. During fabrication an intermediate layer of SiO$_2$ is deposited over the graphene leaving a small hole for the pentacene contact, insuring vertical transport. We show that the current across the device is modulated by the Fermi energy level of graphene, tuned with an external gate voltage. We interpret the device current within thermionic emission theory. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B7.00008: First-Principles Study of Contact Resistance between Graphene and Metal Electrodes Tomoaki Kaneko, Takahisa Ohno Graphene attracts much interest for post-silicon electronics material due to its outstanding electronic transport properties such as considerably high mobility at room temperature. For the application of electronics devices, contacting of metal electrodes is necessary and decreasing of contact resistance between graphene and the metal electrodes is regarded as one of a key issue. In this study, we investigate the contact resistance using DFT+NEGF method. We consider the Ni and Cu electrode within LDA and TM-type norm-conserving pseudo-potential. We employed PHASE code [1] to determine the interface structures. Then, we constructed two terminal device structures in which current flows from metals to graphene. The electron transport properties were calculated using ASCOT code[2]. For Ni electrode, the dependence of the electrode size qualitatively agrees well with that obtained by the experiments. But our results suggest that contact resistance can be reduced considerably. [1] http://www.ciss.iis.u-tokyo.ac.jp/english/project/device/. [2] H. Kondo, J. Nara, H. Kino and T. Ohno, Jpn. J. Appl. Phys. 47, 4792 (2008). [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B7.00009: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 1:27PM - 1:39PM |
B7.00010: A 1D wide band gap graphene metal-semiconductor-metal junction for devices Meredith Nevius, Jeremy Hicks, Antonio Tejeda, Amina Taleb-Ibrahimi, Feng Wang, Edward Conrad Despite many advances in understanding graphene physics, progress towards a working, reproducible graphene-based switch has been nearly stagnant. Mastering obstacles like lithographic limitations, process-induced disorder, scalability, and reproducibility is absolutely crucial. We have successfully grown graphene over patterned steps on silicon carbide and, using angle resolved photoemission spectroscopy, have discovered a one-dimensional metal-semiconducting-metal junction made completely from graphene. The junction is created by inherent graphene-substrate interactions as the graphene grows over the patterned steps. The semiconducting graphene strip is connected on either side by metallic graphene sheets and has a band gap of greater than 0.5 eV.[REF] In addition, experimental results show that the average electronic band structure of thousands of ribbons varies very little even on length scales of tens of microns. We will present results on the growth of these graphene structures along with angle resolved photoemission spectroscopy measurements that reveal the band structure of both the graphene ribbons on the step facets and the 1D semiconducting strip. REF Nature to be published [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B7.00011: Probing Klein tunneling through angle dependence of resistance across graphene p-n junctions Atikur Rahman, Janice Wynn Guikema, Nina Markovic We have studied the angle-dependent resistance characteristics of ``Y''-shaped dual-gated graphene p-n junction devices. Different arms of each device share a common top gate, and the branching of current in the arms at different angles is determined by the transparency of p-n junctions formed under the top gate. For a particular back gate and top gate voltage, we first balanced the voltage drop in the straight and angled arms, and then we studied the variation of the resistance as a function of top gate keeping the back gate voltage fixed. Deviation from the balanced condition with varying top gate voltage measures the transparency of the p-n junctions in the arms. We found that this deviation is large for a p-n*-p or n-p*-n configurations, as compared to p-p*-p or n-n*-n junctions, which provides a direct evidence of the angle-selective transmission of charge carriers in graphene p-n junction. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B7.00012: Chemically functionalized graphene for bipolar electronics Bernard Matis, Jeffrey Baldwin, Brian Houston We discuss the use of chemical functionalization, in particular hydrogenation, to achieve control of the local carrier type and density in graphene, which is a prerequisite for the development of graphene-based bipolar electronics. Transport measurements are used to demonstrate independent carrier types and densities within adjacent semi-metallic graphene and semiconducting hydrogenated graphene regions. Measurements of the Hall coefficient confirm that the graphene and hydrogenated graphene charge carriers change sign about the charge neutrality point, that the graphene carrier density retains its linear dependence on a back gate voltage, and reveal that the hydrogenated graphene carrier density deviates from such a linear relationship. Measurements across the bipolar interface reveal an increasing resistance for higher hydrogen concentrations and a source of constant resistance across a range of back gate voltages for lower hydrogen concentrations. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B7.00013: Resist-free graphene/metal interaction extracted through quantum capacitance measurement R. Ifuku, K. Nagashio, T. Nishimura, A. Toriumi Understanding of the graphene/metal interaction is crucially important from both scientific and practical viewpoints. In the electric device structure, it is reported that graphene under the metal electrodes maintains the linear dispersion regardless of kinds of metals. In case of graphene grown on metals, on the other hand, the modulation of the linear dispersion strongly depends on kinds of metals, e.g. band modulation occurs on Ni and not on Au. The key issue to elucidate this discrepancy can be the resist residual in the device fabrication process. In this study, the resist-free graphene/metal interaction was studied from the density of states (DOS)--energy relation determined by the quantum capacitance measurement of metal/graphene/SiO$_{\mathrm{2}}$/n$^{\mathrm{+}}$-Si stack. Graphene in resist-free contact with Au maintains the linear DOS-energy relation, except near the Dirac point. Graphene contacting Ni shows larger DOS at the Dirac point, resulting in limited gate modulation of E$_{\mathrm{F}}$ in graphene. Resist free process reveals the intrinsic difference in the strength of the graphene/metal interaction ($\pi $-d coupling or van der Waals) for Ni and Au. [Preview Abstract] |
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