Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session A37: Focus Session: Graphene on SiC: Growth, Structure and Properties |
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Sponsoring Units: DMP Chair: Thushari Jayasekera, Southern Illinois University Carbondale Room: 705/707 |
Monday, March 3, 2014 8:00AM - 8:12AM |
A37.00001: STM Study of Sidewall Graphene Nanoribbons on SiC(0001) Yuntao Li, David B. Torrance, M. Tien Hoang, Meredith S. Nevius, Edward H. Conrad, Phillip N. First Graphene nanoribbons grown on SiC sidewall nanofacets have shown interesting transport and electronic structure. We use scanning tunneling microscopy and spectroscopy (STM/STS) to explore their local atomic and electronic structure. Nanoribbon formation is found to depend critically on nanofacet orientation, nanofacet density and growth conditions. Under some conditions, nanoribbons grow predominantly on the nanofacet, under others, they can be induced to grow only at the edges of nanofacets. Significant electronic density-of-states features, resolved by STS, are determined by the different nanoribbon configurations. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A37.00002: Wide-gap Semiconducting Graphene from Nitrogen-seeded SiC Feng Wang, G. Liu, S. Rothwell, M. Nevius, A. Locatelli, T. Mentes, A. Sala, I. Rodriguez, A. Retana, A. Tejeda, A. Taleb-Ibrahimi, L. Fieldman, P. Cohen, E. Conrad We demonstrate a new approach to produce semiconducting graphene that uses surface nitrogen-seeded SiC substrates to grow graphene. The surface nitrogen atoms pin the graphene to the SiC. The starting material is a sub-monolayer of N produced by NO annealing the SiC surface at 1175C. The oxide is then removed chemically to leave $\sim$0.5ML of N that is stable at the graphene growth temperature. Graphene is grown at 1400C by CCS(confinement controlled sublimation) method. Post growth studies with LEED, ARPES, LEEM, PEEM, micro-ARPES and STM show that this N-graphene is continuous but rippled. No nitrogen defect is included in the graphene film. The most important finding is that both ARPES and PEEM show that the N-graphene has a finite bandgap $\sim$0.5-0.7eV depend on graphene thickness. The origin of the band gap is not yet understood although there are strong experimental reasons to suspect strain gradients play an important role. We will also show that the SiC/nitrogen surface can be pre-patterned to high resolution prior to graphene growth. Post growth, the graphene film becomes a periodic N-graphene/normal-graphene modulated structure. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A37.00003: Unraveling the (3$\times$3)-SiC($\bar{1} \bar{1} \bar{1}$) reconstruction and its role as an interface structure Lydia Nemec, Florian Lazarevic, Patrick Rinke, Volker Blum, Matthias Scheffler To refine the growth quality of epitaxial graphene on the C-side of SiC and improving the resulting electronic character of these films, it is important to understand the atomic and electronic-structure of the interface. A phase mixture of different surface phases is observed just when surface graphitization first sets in. However, the atomic structure of some of the competing surface phases as well as of the SiC-graphene interface is unknown. We performed a density functional theory study on the C-side of the polar SiC($\bar{1} \bar{1} \bar{1}$) surface using the all-electron numeric atom-centered basis function code FHI-aims. The formation energy of different reconstructions and model systems for the interface is presented within the thermodynamically allowed range. The surface energies of the known (2$\times$2) phase is compared with several structural models of the (3$\times$3) phase proposed in the literature. Inorian comparison all the previously suggested (3$\times$3) models are higher in energy than the known (2$\times$2) phase. We present a new model for the (3$\times$3) reconstruction. Its formation energy crosses that of the (2$\times$2) phase just at the carbon rich limit of the chemical potential, which explains the observed phase mixture. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A37.00004: Amorphous carbon for structured step bunching during graphene growth on SiC James Palmer, Jan Kunc, Yike Hu, John Hankinson, Zelei Guo, Claire Berger, Walt de Heer Structured growth of high quality graphene is necessary for technological development of carbon based materials. Specifically, control of the bunching and placement of surface steps under epitaxial graphene on SiC is an important consideration for graphene device production. We demonstrate lithographically patterned evaporated amorphous carbon as a method to pin SiC surface steps. Evaporated amorphous carbon is an ideal step-flow barrier on SiC due to its chemical compatibility with graphene growth and its structural stability at high temperatures, as well as its patternability. The amorphous carbon is deposited in vacuum on SiC prior to graphene growth. In the graphene furnace at temperatures above 1200$^{\circ}$C, mobile SiC steps accumulate at these amorphous carbon barriers, forming an aligned step free region for graphene growth at temperatures above 1330$^{\circ}$C. AFM imaging and Raman spectroscopy support the formation of quality step-free graphene sheets grown on SiC with the step morphology aligned to the carbon grid. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A37.00005: Oxynitride and Silicates at Epitaxial Graphene on SiC (0001) Hansika Sirikumara, Jaime Bohorquez, Thushari Jayasekera Epitaxial graphene, the sp$^{2}$-hybridized network of carbon grown on another material is one way of creating large-scale graphene. Intercalated oxygen at the interface has shown to saturate the Si dangling bonds, and is a promising way of tuning the charge density in epitaxial graphene on SiC [1]. It would be interesting to investigate how oxy-nitrides and silicates at the SiC/graphene interface can change the electronic properties of the graphene layer. Based on the first principles density functional theory calculations, we discuss the electronic and structural properties of epitaxial graphene on SiC with Si$_{2}$O$_{5}$ and SiON layers at the interface. \\[4pt] [1] C. Mathieu, et al, Phys. Rev. B 86, 035435 (2012) [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A37.00006: Effect of Oxygen on Sublimation Growth of Graphene on C-face SiC Zachary Robinson, Glenn Jernigan, Konrad Bussmann, Marc Currie, Rachael Myers-Ward, Virginia Wheeler, Luke Nyakiti, Satoshi Oida, James Hannon, Chip Eddy, D. Kurt Gaskill Graphene grown on Si-face SiC has demonstrated improved thickness uniformity when formed in an argon environment. For C-face growth, expected to yield graphene with superior electronic properties, similar progress has not yet been achieved. A systematic study of C-face SiC surface preparation and graphene growth in an argon environment has been carried out in a high temperature chemical vapor deposition system modified for low pressure sublimation. For all growth conditions investigated, the resulting graphene films were found to have non-uniform thickness. Further, x-ray photoelectron spectroscopy (XPS) measurements reveal significant amounts of oxygen on the surface, which has been suggested to cause the non-uniformity [1]. Thus, a sample was transferred to an ultra-high vacuum (UHV) system equipped with in situ XPS, where a UHV anneal of 1200$^{\circ}$C was shown to be necessary to desorb the oxygen. Post-anneal exposure to atmospheric conditions resulted in the return of only 20\% of the original oxygen concentration, suggesting that a robust oxide may be present during growth. Preliminary low energy electron micscoscopy results confirm that trace amounts of oxygen significantly affects the graphene growth process. \\[4pt] [1] Phys. Rev. B 82, 235406 (2010) [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:48AM |
A37.00007: STM Properties and Manipulation of Epitaxial Graphene Invited Speaker: Paul Thibado Epitaxial graphene grown on SiC has been identified as one of the most likely avenues to graphene-based electronics. Understanding how morphology affects electronic properties is therefore important. In our work, epitaxial graphene was grown on the polar and non-polar a-, m-, and r-crystallographic oriented surfaces of SiC, and was investigated using scanning tunneling microscopy (STM). Bunched nano-ridges ten times smaller than previously recorded were observed throughout the surface. A new STM technique called electrostatic-manipulation scanning tunneling microscopy (EM-STM) was performed to modify the morphology of the nano-ridges. By modeling the electrostatics involved in the EM-STM measurement, we estimate that a force of 5 nN and energy of 10 eV was required to alter the local interfacial bonding. At the atomic scale, STM images of Moire patterns reveal low-angle, twisted bi-layer graphene, grain boundaries, and an apparent lattice constant dilation. We will show that this dilation is due to the STM tip electrostatically dragging the graphene surface. Collaborators: P. Xu, D. Qi, M.L. Ackerman, S.D. Barber, J.K. Schoelz, and J. Thompson, Department of Physics, University of Arkansas, Fayetteville, AR, 72701, USA; V.D. Wheelr, R.L. Myers-Ward, C.R. Eddy, Jr., and D.K. Gaskill, U.S. Naval Research Laboratory, Washington, DC 20375, USA; and L.O. Nyakiti, Texas A\&M University.\\[4pt] Financial Support: P.X. and P.M.T. gratefully acknowledge the financial support of ONR under grant N00014-10-1-0181 and NSF under grant DMR-0855358. Work at the U.S. Naval Research Laboratory is supported by the Office of Naval Research. L.O.N. gratefully acknowledges postdoctoral fellowship support through the ASEE. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A37.00008: Transport in chemically gated graphene p-n junctions Christoph Tegenkamp, Jens Baringhaus, Alexander St\"ohr, Ulrich Starke The chirality of charge carriers in graphene allows them to get through potential barriers without any reflection (known as Klein tunneling). To study this effect the fabrication of well-defined p-n junctions is necessary. We use the intercalation of Ge to convert the buffer layer on the SiC(0001) surface into graphene with local p-type or ntype doping depending on the local Ge coverage. The buffer layer is initially patterned using optical lithography, to fabricate isolated n-p, npn and pnp-structures. The n- and p-type doping (340 meV, -290 meV) is confirmed by STS which also reveals very narrow p-n junctions with a length below 5 nm. The corresponding electric fields are as high as $10^6 V/cm$ and therefore significantly higher than those induced by field effects, providing a perfect environment to study Klein tunneling. Transport experiments are carried out by means of a 4-tip STM system,on n-p-n as well as p-n-p structures. Their resistance was found to be strongly dependent on temperature and the inner barrier length. While short barriers ($<$ 200 nm) appear almost transparent, the resistance increases rapidly for barrier widths exceeding the coherence length ($>$ 600 nm). [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A37.00009: Asymmetric Electron Transport Induced by Friedel Oscillations at Monolayer-Bilayer Heterojunctions of Epitaxial Graphene Kendal Clark, X.-G. Zhang, Gong Gu, Guowei He, Randall Feenstra, An-Ping Li We report asymmetric electron transport upon bias polarity reversal at individual monolayer-bilayer (ML-BL) boundaries in epitaxial graphene on SiC (0001), revealed by multi-probe scanning tunneling potentiometry. A greater voltage drop is observed when the current flows from ML to BL graphene than in the reverse direction, and the difference remains nearly unchanged when bias exceeds a threshold. This is not a typical nonlinear conductance due to electron transmission through an asymmetric potential. Rather, it indicates the opening of an energy gap at the Fermi energy. Our theoretical analysis finds that Friedel charge oscillation opens a gap for electrons with wave vectors perpendicular to the boundary. The Friedel gaps are different on the ML and BL sides, which can shift under bias and lead to asymmetric transport upon reversing the bias polarity. A quantitative agreement is seen between experiment and theory on both the sign and the magnitude of the asymmetry. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A37.00010: Selective Growth of Graphene by Pulsed Laser Annealing Ion Implanted SiC Kara Berke, Xiaotie Wang, Nick Rudawski, Dinesh Venkatachalam, Joel Fridmann, Brent Gila, Fan Ren, Rob Elliman, Arthur Hebard, Bill Appleton We report a method for site-selective graphene growth on SiC for direct nano-scale patterning of graphene. Crystalline SiC was implanted with Si and C ions to amorphize the sample surface, then subjected to pulsed laser annealing (PLA); graphene growth occurred only where ions were implanted. PLA parameters including the fluence, number of pulses, and annealing environment were investigated to optimize the growth process. Our previous work involving Au, Cu, and Ge implants in SiC suggested that both the implanted species and surface amorphization affect graphene growth. In this work, we show that surface amorphization alone, without the presence of foreign ionic species, can be used with PLA to create site-selective graphene growth on SiC. Samples were characterized using Raman spectroscopy and cross-sectional transmission electron microscopy. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A37.00011: Tip-induced Coulomb gap in scanning tunneling microscopy experiment on graphene Yue Zhao, Jungseok Chae, Suyong Jung, Cory Dean, Lei Wang, Yuanda Gao, James Hone, Joao Rodrigues, Shaffique Adam, Takashi Taniguchi, Kenji Watanabe, Kenneth Shepard, Andrea Young, Philip Kim, Nikolai Zhitenev, Joseph Stroscio Graphene is a two-dimensional-electron-gas (2DEG) system exposed at the surface, which allows scanning tunneling microscopy (STM) to investigate the electron-electron interactions associated with the Dirac nature on a local scale, with a variety of tuning knobs, such as carrier density, spatially varying disorder potential, and applied magnetic field. However, the electron-electron interaction in graphene is sensitive to the disorder details. Moreover, a tip induced potential can significantly complicate the interpretation of details in the tunneling spectra. Utilizing high mobility graphene devices with low residual disorder, we can minimize the effect of local potential fluctuation, to better understand the role of tip-induced potentials in the measurement. We report the observation of large energy gaps and modification to Landau level (LLs) spectra, which are due to the spatially inhomogeneous density profile caused by tip gating. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A37.00012: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 10:48AM - 11:00AM |
A37.00013: ABSTRACT WITHDRAWN |
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