Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session B37: Focus Session: Graphene Growth, Characterization, and Devices: Devices and Contacts |
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Sponsoring Units: DMP Chair: Thomas Seyller, University of Erlanger-Nuernberg Room: C146 |
Monday, March 21, 2011 11:15AM - 11:27AM |
B37.00001: BN / Graphene / BN RF Transistors Han Wang, Thiti Taychatanapat, Allen Hsu, Pablo Jarillo-Herrero, Tomas Palacios In this work we demonstrate the first BN/graphene/BN transistor for high frequency RF applications. This sandwich structure allows a significant improvement in the mobility of graphene, which reaches more than 18,000 cm$^{2}$/Vs at room temperature. Graphene field effect transistors (GFETs) have been fabricated with L$_{DS}$= 800 nm and L$_{G}$=300 nm. The minimum conduction point of these devices is very close to zero, a result of the negligible substrate doping to the graphene. A current density in excess of 1 A/mm and DC transconductance above 200 mS/mm are achieved for both electron and hole conductions. RF characterization is performed for the first time on this device structure and initial results show a current-gain cut-off frequency $f_{T}$=10 GHz. These experimental results have been combined with simulations of the small-signal model to study the scaling potential of these GFETs for high frequency applications. The impact of the access resistances (R$_{s}$, R$_{d})$, the capacitances (C$_{gs}$, C$_{gd}$, C$_{ds})$, and the transconductance (g$_{m}$) on the frequency performance of the GFETs has also been studied. Finally, the fabricated devices have been compared to GFETs fabricated with SiO$_{2}$ substrate and Al$_{2}$O$_{3}$ gate dielectrics. The improved performance obtained by the BN/graphene/BN structure is very promising to enable the next generation of high frequency RF electronics. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B37.00002: Bilayer graphene $p-i-n$ tunnel junction controlled by modulated top gate Hisao Miyazaki, Song-Lin Li, Kazuhito Tsukagoshi, Akinobu Kanda Ambipolar nature of graphene enables us to set charge polarity for electric transport to be $p$-type or $n$-type. We fabricated a bilayer graphene (BLG) with spatially modulated $p$-type and $n$-type regions. The spatial modulation was introduced by a pair of gate electrodes; a uniform back (substrate) gate and a top gate with stepwise geometry. The gate electric field between the top and back gate also induces band gap in the BLG. As a result, an insulating region is inserted between the $p$- and $n$-regions, realizing a $p-i-n$ junction. The current through the junction showed nonlinearity as a function of the source-drain bias. We identified that the origin of nonlinearity is the tunnel current between the $p$- and $n$-regions. The nonlinearity reflects the density of states singularity at the edge of the conduction and the valence band in BLG with the band gap. This observation appends another evidence for electric-field-induced band gap in BLG. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B37.00003: Rectification at the graphene and multi-layer-graphene / semiconductor interface from room temperature up to 900K Sefaattin Tongay, Todd Schumann, Arthur F. Hebard We report on the formation of Schottky diodes on GaN and SiC using a graphite/graphene electrode as a semimetal contact to the semiconductor. The GaN (SiC) /graphene Schottky barriers display rectifying behavior over a wide temperature range with ideality constant close to unity, implying thermionic emission is the dominant transport across the interface. The diodes display larger breakdown voltages (more than 20V) compared to conventional metal junctions (5V). Advantageously, graphite/graphene is stable up to high temperatures and does not diffuse into the semiconductor. We find that these diodes are stable and rectifying up to 900K and are superior to typical metal Schottky diodes reported for the same semiconductors. High temperature measurements are interesting since graphite semimetal contact starts behaving as Boltzmann gas at temperatures well above Fermi energy (T$>>$280K). Our results imply that graphene based junctions fabricated on conventional semiconductors are good candidates for both high and low temperature devices. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B37.00004: Complementary-like semiconducting graphene logic inverters Song-Lin Li, Hisao Miyazaki, Kazuhito Tsukagoshi, Akinobu Kanda The application of graphene as a post-silicon channel material is an interesting but challenging topic due to its metallic nature and low switching ratio. It is expected that the condition would change if a sizeable band gap is introduced. Here we report the electrical characteristics of the first semiconducting graphene-based logic inverters. Free of doping, the \textit{p}- and \textit{n}- branches in the bipolar graphene transistors are delicately used as the complementary components required in logic devices. Within perpendicular electric fields, large transport band gap ($>100$\,meV) and high switching ratio ($\sim200$ at 77\,K) are obtained in bilayer graphene channels. Besides, a simple and high capacitive-efficiency top gate with natural alumina dielectric ($\sim0.9$\,\textrm{$\mu$}F/cm$^2$) is adopted and the operating bias is lowered within 2\,V. For the first time, $>1$ voltage gain are extracted from graphene inverters. Voltage gain up to 8 and 2 are achieved at liquid-nitrogen and room temperatures, respectively. Importantly, a match between input and output voltage levels is realized, indicating the potential for direct cascading between multiple devices for future large-scale integration. [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B37.00005: Ferroelectric gating of CVD graphene devices Guangxin Ni, Yi Zheng, Kui Yao, Barbaros \"Ozyilmaz The recent availability of large area graphene has opened up new possibility in graphene research. We will first discuss experiments, where graphene on the ferroelectric substrate~PZT allows the fabrication graphene field effect transistors (GFETs) and graphene memory within $\pm $1 V operating voltage with maximum doping exceeding 10$^{13}$ cm$^{-2}$. Ferroelectric substrates may also be of importance for large scale applications. Graphene's exceptional optical and mechanical properties make it suitable also for transparent conductors (TCs). While chemical doping has been proven to be an efficient approach to achieving ultra-low sheet resistance, some challenges remain. Here we propose an alternative way to obtain low sheet resistance of graphene using ferroelectric gating. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B37.00006: Electrical noise in graphene FETs Nan Sun, Kristof Tahy, Gerald Arnold, Debdeep Jena, Huili Xing, Steven Ruggiero We report on the low-frequency electrical noise measured in graphene FETs. Samples were created by e-beam lithography using both exfoliated graphene and epitaxial graphene films on SiC. The observed 1/f noise varies as a function of gate bias, where the noise amplitude follows Hooge's empirical relation ($S_V \sim 1/N$), and the noise spectrum deviates from 1/f behavior at low carrier densities. We discuss this behavior in the context of a model including random telegraph noise generated by slow traps. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 1:03PM |
B37.00007: Development of epitaxial graphene based electronics Invited Speaker: Epitaxial graphene (EG) has demonstrated a great potential for novel electronic devices [1]. In micron-sized structures graphene is essentially a gapless semimetal, consequently reasonable on-to-off rations can be achieved, but digital electronics is precluded. There are essentially two methods to introduce a bandgap in graphene. One is to make very small structures [1], and the other is to chemically modify the graphene itself [2]. Electron beam lithography is not commercially viable and the graphene is severely degraded by this method. Graphene's conductivity depends on the doping density. For interconnects, reliable methods need to be developed to highly dope graphene without deteriorating the mobility. Furthermore, metallic interconnects are required for all but the simplest structures and they need to be incorporated without defeating graphene's favorable properties. Finally, in the more distant future, EG device architectures that rely on wave properties of the electrons that go beyond diffusive electronics are envisioned [1]. These will require interconnected, nanoscopic graphene structures. An overview and perspective of these issues will be given. I will present new directions, involving multilayer epitaxial graphene, interconnect schemes, non-conventional patterning methods (templated graphene growth [3] and related methods), as well as methods to chemically modify and dope EG.\\[4pt] [1] Berger et al. ``Ultrathin Epitaxial Graphite: 2D Electron Gas Properties and a Route toward Graphene-based Nanoelectronics'', J. Phys. Chem. B 108, 2004,19912 (2004); W.A.de Heer http://smartech.gatech.edu/handle/1853/31270 \\[0pt] [2] E. Bekyarova, et al, JACS 131, 1336 (2009).\\[0pt] [3] M. Sprinkle, et al., ``Epitaxial graphene: Templated graphene growth'' Nature NanoTechnology 5, 727, (2010)] [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B37.00008: Metal selection criteria for enhancing electrical conductance of metal-graphene junctions Marcelo Kuroda, J. Tersoff, Dennis Newns, Glenn Martyna We study from first principles the electrical conductance of a junction formed by graphitic films in between metal electrodes. We find that for some metals the junction conductance decays exponentially with the number of graphene layers (thickness of the film) while for others it saturates. These different behaviors are attributed to the presence/absence of Fermi-level states in the metal electrode that couple to those of the graphitic thin film. We also find that the bonding between the metal and graphene atoms at the interface has a significant contribution which is dominant for sufficiently thin films. The study may be proven useful for the design and optimization of epitaxially grown electrical contacts. [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B37.00009: Influence of Metal Contact on the Operation and Scalability of Graphene Field-Effect-Transistors Pei Zhao, Qin Zhang, Debdeep Jena, Siyuranga O. Koswatta We explore the effects of metal contacts on the operation and scalability of 2D Graphene Field-Effect-Transistors (GFETs) using detailed numerical device simulations based on the non-equilibrium Green's function formalism at the ballistic limit. Our treatment of metal/graphene (M/G) contacts captures: (1) the doping effect due to the shift of the Fermi level in graphene contacts, (2) the density-of-states (DOS) broadening effect inside the graphene contacts, and (3) the Metal-Induced-States inside the graphene channel. Our results confirm the asymmetric transfer characteristics in GFETs due to the doping effect by metal contacts. Furthermore, the DOS broadening effect will increases the on-current at higher M/G coupling strengths. Finally, with scaling of the channel length, influence on the minimum current in the off-state is also discussed. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B37.00010: Flexible and Transparent Field Emission Devices based on Graphene-Nanowire Hybrid Structures Muhammad Arif, Kwang Heo, Byung Yang Lee, David H. Seo, Sunae Seo, Jikang Jian, Seunghun Hong Recent developments in wafer scale synthesis and transfer of graphene have made it possible to fabricate electrodes for versatile flexible devices. However, a flexible and transparent graphene-based field emission device has not been explored yet. Herein, we report the fabrication of flexible and transparent field emission devices based on graphene-nanowire hybrid structures. In this work, we successfully grew vertically-aligned Au nanowires on graphene surface using an electrochemical method and utilized it as a cathode. We also utilized a graphene electrode for an anode resulting in a transparent and flexible field emission device. Our field emission devices can be bent down to 22 mm radius of curvature without any significant change in its field emission currents. This flexible and transparent field emission device based on graphene-nanowire hybrid structures will utilized for various applications such as field emission displays, x-ray tubes, and pressure sensors. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B37.00011: CVD grown graphene field-effect device arrays with water top gate Bei Wang, B. Koger, J. Zhu We synthesize single-layer graphene sheets by chemical vapor deposition (CVD) on copper foil. Large sheets are transferred to Si/SiO$_{2}$ wafers using poly(methyl methacrylate) (PMMA). Raman spectroscopy of transferred graphene shows the signatures of high-quality graphene with a very small D band. Graphene field-effect device arrays are fabricated using conventional photolithography. A thin SiO$_{2}$ film is deposited on top of the finished devices as the last step. We employ two methods of field effect gating. Gate sweeps of the SiO$_{2}$ back gate show large initial hole doping. When a droplet of water is deposited on the device and used as a top gate, the majority of devices show a Dirac point of $\sim$0.3 V and bipolar behavior. The water top gate injects charges much more efficiently than the 290nm SiO$_{2}$ back gate. The mobility of the devices is estimated to be a few thousand cm$^{2}$/Vs. We discuss transport properties and potential applications of these device arrays. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B37.00012: Graphene p-n Junctions via Molecular Functionalization Ren-Jye Shiue, Hung-Chieh Cheng, Chia-Chang Tsai, Yit-Tsong Chen, Wei-Hua Wang An essential challenge in graphene-based electronics is to engineer the carrier type and density and still preserve the transport properties of graphene. We report an experimental investigation of graphene $p-n$ junctions via molecular functionalization. By developing a generic scheme for the chemical functionalization, we have shown that an effective and uniform chemical doping of graphene can be achieved by non-covalent modification of the molecules. The effectiveness and uniformity of the modification is systematically confirmed by optical microscopy, surface potential measurement, and Raman spectroscopic imaging. Furthermore, the chemical doping by molecules is utilized to fabricate the graphene $p-n$ junctions. The transport characteristics of the graphene $p-n$ junctions are investigated by transport and magnetotransport measurements. The signatures of the graphene $p-n$ junctions are presented with high carrier mobility, energy splitting of Dirac points, and non-conventional quantum Hall effect. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B37.00013: Device fabrication progress on epitaxial graphene on SiC Yike Hu, Zelei Guo, Rui Dong, Claire Berger, Walt deHeer Epitaxial graphene on SiC has been demonstrated to be a viable route toward electronic device fabrication. While a top gate is required to locally change doping density and carrier type, specifically for field effect transistors graphene devices, back gating is relevant to globally change carrier and to address the graphene layer at the SiC-graphene interface. Here we report result on back-gating and top-gating epitaxial graphene grown on SiC by the confinement controlled sublimation method. Post-patterning treatments of graphene devices are also discussed. [Preview Abstract] |
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