Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session G7: Focus Session: Graphene Devices V |
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Sponsoring Units: DMP Chair: Masa Ishigami, University of Central Florida Room: 303 |
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G7.00001: Coarse-grained quantum transport simulation for analyzing leakage-mobility antagonism in GNRFET Masakatsu Ito, Shintaro Sato, Naoki Yokoyama, Christian Joachim Since it became clear that graphene transistors based on the classical MOSFET principle suffer from serious performance problems, researchers have explored new graphene device design using quantum transport simulations. A first-principle quantum transport simulation, however, still takes unaffordable computational cost to deal with a realistic size of graphene transistor ($> 10^4$ atoms). This motivated us to import ESQC (elastic scattering quantum chemistry) technique from the research field of molecular electronics and to develop its coarse-grained version. To eliminate the atomic scale details, we reformulated ESQC technique using the continuum limit description of graphene charge carriers, which is given by the massless Dirac equation. Since the potential function in this Dirac equation is electrostatic potential distribution, it can be obtained from Poisson equation with the boundary conditions of gate voltages in a self-consistent manner. We are now applying this coarse-grained quantum transport simulation to GNRFETs (graphene nanoribbon field effect transistors) for resolving the mobility-leakage antagonism, where opening a bandgap in a graphene channel improves its switching ability but at the same time deteriorates the electron channel mobility. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G7.00002: Density of States and Its Local Fluctuations Determined by Capacitance of Strongly Disordered Graphene Xiaolong Chen, Wei Li, Lin Wang, Yuheng He, Zefei Wu, Yuan Cai, Mingwei Zhang, Yang Wang, Yu Han, Rolf W. Lortz, Zhao-Qing Zhang, Ping Sheng, Ning Wang We demonstrate that local fluctuations of the density of states (DOS) in strongly disordered graphene play an important role in determining the quantum capacitance of the top-gate device geometry. Depending on the strength of the disorder induced by metal-cluster decoration, the measured quantum capacitance of disordered graphene could dramatically decrease in comparison with pristine graphene (previous work on transport of metal-cluster decoration has been published on Phys. Rev. B 84, 045431, 2011). A quantitative model for correlating fluctuations of local density of states with the disorder strength and quantum capacitance is presented and discussed. The DOS of disordered graphene obeys a non-universal power law. By measuring the quantum capacitance of disordered graphene, we simultaneously determined both the DOS and its local fluctuations, which is in agreement with the lognormal distributions reported previously for localized samples. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G7.00003: Understanding Quantum Transport and the Kondo Effect in 2D Carbon Systems Ross McIntosh, Dmitry Churochkin, Somnath Bhattacharyya The rich physics surrounding correlations between conduction electrons and local spins in quantum dot systems is of significant interest towards the development of spintronic quantum information devices. In this study we establish the Kondo effect in reduced graphene oxide (RGO) films through a metal-insulator transition in resistance versus temperature interpreted within the Fermi liquid description of the Kondo effect and negative magnetoresistance which scales with a Kondo characteristic temperature. With a microstructure consisting of intact graphene nano-islands embedded within residual functionalized regions where local magnetic moments may form, RGO is effectively a disordered quantum dot system. This work is augmented with a theoretical study of transport through nano-scale multiple quantum dot devices. Solving within a Keldysh formalism we scrutinize quasi-bound state formation in a range of geometrical quantum dot configurations in order to interpret coherent quantum interference effects. We demonstrate negative differential conductance and control over device parameters such as the characteristic time. This tandem approach illustrates the promise of innovative low dimensional carbon spintronic devices. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G7.00004: An Essential Mechanism of Heat Dissipation in Nanocarbon Electronics Slava V. Rotkin, Alexey G. Petrov Nanocarbon materials were proposed and have been already used for fabricating electronic devices. These nanocarbon devices are not unlike other semiconductor devices and are subject to Joule losses. However the physics of heat dissipation in those materials is unlike the classical thermal physics. This talk focuses on near-field radiative heat dissipation mechanism in nanocarbon materials, the associated component of the heat transport across the interface with the substrate and remote Joule losses. Analytical theory was derived which allows one to trace the origin of anomalously large strength of the effect and predict dependence on the materials parameters. It was predicted to be very substantial and for certain surface morphology dominate over other mechanisms, as it was recently shown experimentally. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G7.00005: Determination of dominant scatterer in Graphene on SiO$_2$ using atomic hydrogen Jyoti Katoch, Duy Le, Talat Rahman, Masa Ishigami We have measured the impact of low energy atomic hydrogen (\textless\ 250meV) on the transport property of graphene sheets as a function of hydrogen coverage and initial, pre-hydrogenation field-effect mobility. In order to understand the correlation between the field effect mobility and the apparent affinity of atomic hydrogen to graphene, we have performed a detailed temperature programmed desorption study on hydrogen-dosed graphene sheets. Atomic hydrogen is found to be desorbing with three different desorption energies. The physisorbed atomic hydrogen on graphene with desorption energy of 60 $\pm$ 30meV (consistent with our theoretical calculations), is found to be correlated to the native scatterers in graphene. The associated charge transfer expected for such small desorption energy indicates that atomic-scale defect sites are not responsible for determining the mobility of graphene on SiO$_{2}$ and that charged impurities, presumably in substrates, define the transport property of graphene on SiO$_{2}$. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G7.00006: The zero-voltage conductance of nano-graphenes: Simple rules and quantitative estimates Matthias Ernzerhof, Yongxi Zhou, Didier Mayou Zero-dimensional graphenes, also called nano-graphenes ($NGs$), consist of fragments of graphene with a finite number of hexagons. $NGs $ can be viewed as a subset of the polycyclic aromatic hydrocarbons (PAHs) that continue to attract chemists attention. We develop a simple theory for the ballistic electron transport through $NGs$ which combines elements of the electronic structure theory of graphene, intuitive methods for the calculation of the molecular conductance, and chemical concepts such as Kekul\'{e} structures. This theory enables one to analyze the relation between the structure of $NGs$ and their conductance. General formulas and rules for the zero-voltage conductance as a function of the contact positions are derived. These formulas and rules require at most simple paper and pencil calculations in applications to systems containing several tens of carbon atoms. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 1:03PM |
G7.00007: Graphene devices and its performance limitations and opportunities Invited Speaker: Tony Low Being a two-dimensional membrane, its mechanical properties such as morphology, strains, phonons can significantly modifies the electronic properties of graphene. In fact, the coupling between the mechanical and electronic properties of graphene plays a key limiting role in the performance of electronics and optoelectronics devices. Modeling studies of several in-house experiments will be discussed to exemplify this point. Next, I will discuss how the modification of electronic properties of graphene via mechanical strains might lead to interesting electronics and electromechanical devices. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G7.00008: Epitaxial Graphene on SiC for Ultra-high Frequency Transistors Zelei Guo, Rui Dong, Partha Sarathi Chakraborty, Nelson Lourenco, James Palmer, Yike Hu, Ming Ruan, John Hankinson, Jan Kunc, John Cressler, Claire Berger, Walt deHeer Electronic devices and systems operating at ultra-high frequencies have recently generated significant interest. Graphene is considered a promising candidate material for high-frequency electronics, due to its intrinsic low dimensionality, high carrier mobility and large carrier velocity. Field effect transistors made of exfoliated graphene flakes as the channel material have shown cut-off frequency (f$_{\mathrm{T}})$ above 400 GHz. However, the maximum oscillation frequency (f$_{\mathrm{max}})$ of graphene transistors, which sets the practical limit on useful circuit operation, to date have not exceeded 45 GHz. We report here record intrinsic f$_{\mathrm{max}}$ of 70 GHz, with f$_{\mathrm{T}}$ exceeding 100 GHz, for transistors based on epitaxial graphene on SiC. In addition to setting a new performance record for graphene technology, these epitaxial graphene transistors were fabricated using well-developed, robust, top-down processes compatible with a mass-production-compatible platform. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G7.00009: Effect of back-gate bias on Graphene RF device performance Wenjuan Zhu, Damon Farmer, Yanqing Wu, Bruce Ek, Keith Jenkins, Phaedon Avouris Graphene is very promising for RF devices due to its high carrier mobility. High cut-off frequency graphene RF devices using CVD grown graphene and epitaxially grown graphene have been reported. Here we report the effect of the back-gate bias on the FET cut-off frequency and current saturation. We found that there are two peak cut-off frequencies corresponding to electron peak trans-conductance and hole peak trans-conductance maxima respectively, as we sweep the top-gate bias. The electron peak cut-off frequency can be significantly increased by applying a positive back-gate bias. The higher the voltage, the larger the maximum cut-off frequency. This can be explained by the additional electron doping introduced by the back-gate bias in the under-lap region, which forms an n-n$+$-n configuration. Similarly, the hole peak cut-off frequency can be significantly enhanced by applying negative back-gate bias to form the p-p$+$-p configuration. The shorter the channel, the more pronounced this effect. We also found that the current saturation is also improved by introducing the same type of carrier as the channel in the under-lap region. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G7.00010: Can we reduce the OFF currents of graphene without hurting their ON currents? Frank Tseng, Gianluca Fiori, Avik Ghosh The current-voltage characteristics of graphene can be understood from Landauer-Keldysh theory. The low bias conductivity is dominated by tunneling through closely spaced modes. The mid-voltage regime is dominated by Coulomb scattering from charge puddles and remote optical phonons that compete with each other towards quasi-saturation. Finally, the high bias physics is given by band-to-band (Zener) tunneling. The overall I-V, consistent with experiments is limited by the lack of a band-gap that compromises the overall gain and inverter characteristics, also seen experimentally. Conversely, structural band-gaps increase the effective mass of the electrons as well as their phase space for scattering, reducing their overall mobility. We show that a way around this dilemma is to engineer sequences of gates that stagger the Dirac point regions in the separately gated graphene segments (equivalently, bandgapped regions for nanoribbons and nanotubes) so as to effectively increase the transmission gap and suppressing subthreshold conduction by two orders of magnitude and extending current saturation without overall ON-current. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G7.00011: Electrical Transport Properties of Graphene Oxide Transistor Using Step-by-Step Reduction Seung Jae Baek, Min Park, Byung Hoon Kim, Yongseok Jun, Yung Woo Park Step by step reduced graphene oxide(GO) thin film transistors were electrically characterized as a temperature and gate voltage. The GO transistors were prepared by thermally reduced using step by step method in same samples. The reduction temperature were subtracted from the inflection point of thermogravimetric analysis(TGA) plot and their points are 88, 158, 185, 215, 250, 300 degree Celsius. All GO condition at various reduction temperatures were defined using Raman spectroscopy and atomic force microscopy(AFM). Temperature dependence electrical measurements were carried out using two terminal technique and various temperatures up to unmeasurable condition. Our charge transport behaviors well fitted to 2 dimensional variable-range hopping(2D VRH) mechanism and fluctuation induced tunneling(FIT) model. Also the conductivity level of each step was increased more than 10$^{4\, }$times. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G7.00012: Mechanism of the doping dependence of 2D Raman band: Dirac-cone migration Ken-ichi Sasaki, Yasuhiro Tokura, Satoru Suzuki, Tetsuomi Sogawa The Raman G and 2D bands are informative characterization tools. The G band can be used to determine whether or not the position of the Fermi energy $\mu$ is close to the Dirac point, since the width broadens when $\mu\simeq 0$, which is known as the Kohn anomaly effect. By contrast, the width of the 2D band sharpens when $\mu\simeq 0$ [1]. We have explored the origin of the difference between the $\mu$ dependencies of the G and 2D bands, first intuitively by employing a concept of a shifted Dirac cone, and then more rigorously in terms of self-energy taking electron-phonon coupling into account[2]. By considering a direct transition in shifted Dirac cones, we clarified that the spectral features of a phonon show varieties of behavior that depend strongly on the value of the phonon momentum $q$. In particular, the resonance decay of a phonon satisfying $v_{\rm F} q > \omega$ ($\omega$ is phonon's frequency) into an electron-hole pair is suppressed when $2\mu < \hbar v_{\rm F}q-\hbar\omega$. The idea of shifted Dirac cone can be applied to a general phonon with a nonzero $q$, including the defect induced D and D$'$ bands, which are of prime importance in recent studies on graphene edges.\\[4pt] [1] Das et al., Nature Nano. (2008).\\[0pt] [2] Sasaki et al., arXiv:1204.4543, PRB (RC) in press. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G7.00013: Effect of gate-induced doping on the Raman spectra of disordered graphene Isaac Childres, Luis A. Jauregui, Yong P. Chen We report a Raman spectroscopy study of graphene field-effect transistors (GFET) after exposure to electron-beam irradiation, used to introduce a controlled amount of defects in graphene. Raman spectra are taken over a range of temperatures (4-300 K), back gate voltages and electron-beam exposures. We observe that the intensity ratio between Raman ``D'' and ``G'' peaks,$ I_{D}/I_{G}$, commonly used to determine the amount of disorder in graphene, not only changes with the irradiation dosage, but also with gate-induced doping. At low temperature (4 K), we observe a peak in the plot of $I_{D}/I_{G}$ versus back gate voltage at the Dirac point of the GFET. As the temperature increases, the back gate voltage at which this peak occurs decreases relative to the Dirac point. Our findings may be valuable for understanding the Raman spectra and electron-phonon physics in doped and disordered graphene. [Preview Abstract] |
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