Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session W6: Focus Session: Graphene on SiC: Synthesis and Properties |
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Sponsoring Units: DMP Chair: Randall Feenstra, Carnegie Mellon University Room: 302 |
Thursday, March 21, 2013 2:30PM - 2:42PM |
W6.00001: Spin transport in epitaxial graphene on SiC (0001) Yuchen Du, Adam T. Neal, Mike Capano, Peide Ye Graphene has been identified as a promising material for future spintronics devices due to its low spin orbit coupling and long spin diffusion lengths, even at room temperature [1-2]. However, any device application requires the use of large-area graphene compatible with wafer-scale manufacturing methods, such as graphene grown epitaxially on SiC. We study spin transport in epitaxial graphene grown on SiC (0001) as a step toward future spintronics devices. A non-local spin valve signal of 200m$\Omega $ is observed at 77K, with a signal of 50m$\Omega $ resolved at 145K. Assuming a contact polarization of 10{\%} [1], the measured signal corresponds to a spin diffusion length of 130nm at T$=$77K. Hanle effect spin precession measurements are ongoing. [1] Tombros et al. \textbf{Nature} 448 571 (2007) [2] Maassen et al. \textbf{Nano Lett.} 12, 1498 (2012) [Preview Abstract] |
Thursday, March 21, 2013 2:42PM - 2:54PM |
W6.00002: Scanning tunneling microscopy/spectroscopy study of hydrogen intercalated epitaxial graphene on SiC(0001) S. Rajput, Y. Y. Li, M. Weinert, L. Li In this work, we studied the atomic structures and electronic properties of hydrogen intercalated epitaxial graphene on Si-face SiC(0001) using scanning tunneling microscopy/spectroscopy and density functional theory (DFT) calculations. Hydrogen intercalation was achieved by either annealing graphene/SiC(0001) in hydrogen gas at atmospheric pressure or in hydrogen plasma in ultrahigh vacuum. We found that while the as-grown graphene is n-type, the H-intercalated graphene is p-type, which can be attributed to the saturation of the Si dangling bonds at the interface by hydrogen atoms. These results and the origin of the p-type doping in hydrogen intercalated epitaxial graphene on SiC(0001) will be discussed at the meeting. [Preview Abstract] |
Thursday, March 21, 2013 2:54PM - 3:06PM |
W6.00003: Scanning Tunneling Microscopy and Spectroscopy of Quasi-freestanding Graphene on SiC Tianshuai Guan, Andreas Sandin, J.E. (Jack) Rowe, Daniel Dougherty Epitaxial graphene on SiC(0001) is a promising approach for industrial-scale production of very high quality graphene. Recently, it has been demonstrated by angle-resolved photoelectron spectroscopy (Riedl et al., Phys. Rev. Lett 103, 246804 (2009)) that graphene can be prepared on SiC in almost undoped form by intercalating atomic hydrogen beneath the non-graphitic carbon-rich ``buffer layer.'' We present scanning tunneling microscopy and spectroscopy measurements of quasi-free-standing monolayer graphene on SiC(0001) obtained by atomic hydrogen intercalation. Small hydrogen-intercalated domains formed at the initial stages of quasi-free graphene nucleation exhibit a $\left( {\sqrt {\mbox{3}} \times \sqrt {\mbox{3}} } \right)\mbox{R}30$ corrugation due to the sub-surface hydrogen. Local image potential state spectroscopy on these domains is used to observe changes in local doping due to intercalation. These states show the energetic shift ($\approx $ 0.4 eV) with respect to the usual n-doped single-layer graphene on SiC(0001) that suggests that H-intercalated graphene is almost charge-neutral. [Preview Abstract] |
Thursday, March 21, 2013 3:06PM - 3:42PM |
W6.00004: Spin transport over long distance in epitaxial graphene grown on C-face SiC Invited Speaker: Pierre Seneor Spintronics is a paradigm focusing on spin as the information vector and ranging from quantum information to zero-power non-volatile magnetism. Several spintronics evices (logic gates, spin FET, etc) are based on spin transport in a lateral channel between spin polarized contacts. However while spin is acclaimed for information storage, a paradox is that efficient spin transport as remained elusive. We will present magneto-transport experiments on epitaxial graphene multilayers on SiC showing very large spin signals and spin diffusion length in graphene in the 100$\mu$m range (as high as 285$\mu$m). In the best case, the spin transport efficiency of epitaxial graphene is found to be of 75\% of the ideal channel. Graphene, could turn out as a material of choice for large scale logic circuits and the transport/processing of spin information. Understanding the mechanism of the spin relaxation, improving the spin diffusion length and also testing various concepts of spin gates are the next challenges.\\[4pt] Collaborators: B. Dlubak, M.-B. Martin, A. Anane, C. Deranlot, R. Mattana, H. Jaffr\`es, F. Petroff, A. Fert, B. Servet, S. Xavier, M. Sprinkle, C. Berger, and W. de Heer, Unite Mixte de Physique CNRS/Thales, Palaiseau, France and Universit\'e de Paris-Sud, Orsay, France; Institut N\'eel, Grenoble, France and Georgia Tech, Atlanta, USA.\\[4pt] References:\\[0pt] Dlubak et al. Nature Phys 8 557 (2012)\\[0pt] Seneor et al. MRS Bulletin 37 1245 (2012) [Preview Abstract] |
Thursday, March 21, 2013 3:42PM - 3:54PM |
W6.00005: Imaging stacking faults in epitaxial graphene/buffer layer structures on SiC(0001) Patrick Mende, Guowei He, Randall Feenstra, Michael Widom, Irene Calizo, Guangjun Cheng, Randolph Elmquist, Angela Hight Walker, Mariano Real In characterizing the structure of epitaxial graphene on SiC, the homogeneity of the number of monolayers (MLs) of graphene on the surface is important due to its substantial effect on graphene's electronic properties and, until recently, was not easily controlled. As the processing of samples continues to improve, other structural properties of the films and substrate (e.g., substrate morphology, step density, and grain area) have become important in the pursuit of improved electronic behavior. In this talk, imaging of rotational stacking faults in epitaxial graphene on SiC(0001) using low-energy electron microscopy (LEEM) is described. Using a pattern of fiducial marks on the SiC surface, we have correlated LEEM imaging of these stacking faults with micro-Raman imaging. Additionally, while stacking domains in $\ge $1ML graphene have been studied previously in LEEM [1-2], here we introduce first-principles calculations of low-energy electron reflectivity for various stacking arrangements of 1ML graphene/buffer- layer structures on SiC(0001), and compare these predictions to the reflectivity seen in LEEM.\\[4pt] [1] C. Virojanadara et al., Surface Science 603, L87 (2009).\\[0pt] [2] Hibinio et al., PRB 80, 085406 (2009). [Preview Abstract] |
Thursday, March 21, 2013 3:54PM - 4:06PM |
W6.00006: Control of epitaxial graphene growth by SiC-SiC capping Ismet Kaya, Cem Celebi, Cenk Yanik, Anil Gunay Demirkol The growth of epitaxial graphene on the surfaces of silicon carbide is considered to be one of the most promising techniques for obtaining high quality large scale graphene for electronics applications. Although graphene grown on the C-face has high mobility, its growth under vacuum is too fast, not self limited and produces high concentration of crystalline defects. Therefore a precise control over the Si evaporation rate is required. We demonstrate a new method to reduce the growth rate and yield thin graphene layers with excellent thickness uniformity on the C-face of SiC in ultra high vacuum conditions. The sample is capped by another SiC substrate with a rectangular recess of about one micron depth on its surface which forms a partially open cavity between the surfaces. During the growth by high temperature annealing, silicon atoms sublimated from the capped sample are confined inside the cavity between the two substrates. The confined silicon vapor maintains a high partial pressure at the sample surface which significantly reduces the growth rate of graphene to an easily controllable range. We demonstrate that the growth rate linearly increases with the area of the cavity opening. We investigated the effect of Si confinement on the thickness and morphology of UHV grown epitaxial graphene on C-face SiC by Raman spectroscopy, atomic force microscopy, scanning electron microscopy and low energy electron diffraction. [Preview Abstract] |
Thursday, March 21, 2013 4:06PM - 4:18PM |
W6.00007: Growth and characterization of the graphene and its interface on the SiC (0001) face James Palmer, Ming Ruan, Yike Hu, Zelei Guo, John Hankinson, Rui Dong, Jan Kunc, Claire Berger, Walt de Heer The confinement controlled sublimation method [1] provides a method of producing high quality epitaxial graphene on silicon carbide by controlling the silicon evaporation rate through confinement. Here we present growth studies of the first few graphene layers on the silicon terminated face (SiC (0001)). Surface properties of the grown layers are characterized by Raman spectroscopy, AFM, EFM, ellipsometry, and LEED, along with resistivity measurements of the grown graphene. Together these characterization methods can provide information on the substrate step structure and doping of the first layers of graphene. The growth of the initial buffer layer from SiC, of graphene nanoribbons from the SiC substrate steps (e.g. sidewall growth [2, 3]), and of large-area graphene can be better understood for different growth conditions. Finally, we will present electronic transport data for these well characterized graphene layers. Ultimately, the right growth conditions provide control of the substrate steps and number of graphene layers grown, leading to quality epitaxial graphene devices. [1] PNAS 108, 16900 (2011) [2] Nature Nanotechnology 5, 727 (2010) [3] J. Phys. D: Appl. Phys. 45 154010 (2012) [Preview Abstract] |
Thursday, March 21, 2013 4:18PM - 4:30PM |
W6.00008: Studies of epitaxial graphene growth on vicinal silicon carbide M. Tien Hoang, David B. Torrance, Hsin-Ju Wu, Phillip N. First The growth of epitaxial graphene on SiC has been shown to begin at step edges. Therefore, control of the step-edge density and step bunching on the substrate is important for the production of large-area and high-quality graphene. Additionally, recent experiments [1] have exploited the nucleation of graphene at step edges to produce graphene nanoribbons. Here we study the kinetics of graphene growth as a function of SiC step morphology by using dimple-ground SiC samples. This method of sample preparation allows for the study of a continuous range of miscut angles, prepared under identical growth conditions. Samples are annealed inside a graphite furnace with the flux of silicon controlled via physical confinement and a controlled background pressure of argon or silane. The morphology and graphene coverage of the samples are characterized in situ with LEED and Auger spectroscopy and ex-situ by AFM, SEM, and Raman spectroscopy.\\[4pt] [1] M. Sprinkle, M. Ruan, Y. Hu, J. Hankinson, M. Rubio-Roy, B. Zhang, X. Wu, C. Berger and W. A. de Heer, Nature Nano 5, 727 (2010). [Preview Abstract] |
Thursday, March 21, 2013 4:30PM - 4:42PM |
W6.00009: Correlating low-energy electron microscopy and micro-Raman imaging of epitaxial graphene on SiC Guangjun Cheng, Irene Calizo, Patrick Meade, Guowei He, M.A. Real, R.E. Elmquist, R.M. Feenstra, A.R. Hight Walker Several techniques exist for determining the number of graphene layers grown on SiC such as low-energy electron microscopy (LEEM) and Raman spectroscopy. The method which is arguably the most definitive for SiC-grown graphene isLEEM. Low-energy (0 -- 10 eV) electrons interfere with the graphene layers, yielding minima in the electron reflectivity vs. energy curve that can be used to determine the layer number.1 LEEM also provides the means of collecting selected-area diffraction on ?m-size surface regions (micro-LEED), giving access to further useful structural information. While Raman spectroscopy is also commonly used to determine graphene layer number on SiC substrates; such measurements have no definitive calibration for large-area graphene on SiC, unlike the case of exfoliated graphene on SiO2. In this talk, results of correlated LEEM/micro-Raman imaging of large-area, mono and multilayer graphene samples are presented. These initial findings show that LEEM can show the contrast between terrace regions and step edges at particular areas of monolayer-graphene surfaces. Micro-Raman imaging of these same locations show Raman shifts in the G' (2D) band. The influence of heterogeneities on electrical behavior of graphene will be discussed. Comparative studies of multilayer graphene are in progress, and will also be reported. 1. H. Hibino, et al., Phys. Rev. B 77, 075413 (2008). 2. L. I. Johansson, et al., Phys. Rev. B 84, 125405 (2011). [Preview Abstract] |
Thursday, March 21, 2013 4:42PM - 4:54PM |
W6.00010: Electronic and Magnetic Properties of Epitaxial Graphene Sidewall Nanoribbons John Hankinson, Ming Ruan, James Palmer, Wenlong Yu, Rui Dong, Chao Huan, Zhigang Jiang, Claire Berger, Walt de Heer Confinement controlled sublimation growth of epitaxial graphene on silicon carbide has proven to be a viable method for the production of high quality graphene for use in nanoelectronics. However, patterning of bulk graphene using oxygen plasma leads to rough edges that cause electronic transport in nanostructures to be dominated by edge scattering through localization and quantum dot effects. To overcome this, we have developed a method to create graphene nanostructures directly during growth. For this the SiC substrate is etched to reveal sidewall facets that graphitize more readily than the SiC (0001) face. High temperature growth on such pre-patterned SiC yields graphene nanoribbons only a few tens of nanometers wide with well-controlled edges anchored to the SiC substrate. Here we present these growth techniques as well as experimental evidence showing that the resulting ribbons are metallic with unique spin transport properties. [Preview Abstract] |
Thursday, March 21, 2013 4:54PM - 5:06PM |
W6.00011: Electron flow in polycrystalline graphene on C-face SiC Chockalingam Subbaiah, Abhay Pasupathy, James Hannon, Rudolf Tromp, Frances Ross, Shuaihua Ji Graphene films can be grown both on the Si and C faces of SiC (0001), and the films grown have strikingly different morphologies. Previously, we have used scanning tunneling potentiometry to characterize electron flow in epitaxial graphene grown on the Si face of SiC [1]. Here we will describe recent measurements on nanoscale electronic transport in graphene films grown on the C-face of SiC. In particular, C-face graphene has several topographical features such as pleats, ridges and carbon beads, which determine the quality of the material. We use scanning potentiometry to relate these topographical features to the electron transport in these films at the nanoscale, and discuss the relative impact of different sources of scattering in the epitaxial graphene. \\[4pt] [1] Ji, S.-H. et al. \textit{Nature Mat}. \textbf{2012}, 11, 114 [Preview Abstract] |
Thursday, March 21, 2013 5:06PM - 5:18PM |
W6.00012: Electronic Structure of Self-Organized Graphene Nanostructures on SiC(0001) Yuntao Li, David B. Torrance, James O. Andrews, Phillip N. First Graphene nanostructures directly grown on SiC are appealing for their potential application to nanoscale electronic devices. We use different methods to control the step morphology of the SiC$(0001)$ surface in order to guide the growth of graphene, which initiates at step edges. ``Sidewall'' graphene nanoribbons can be formed on step bunches by limiting the graphene growth. We study such nanostructures via scanning tunneling spectroscopy (STS) in ultra-high vacuum. Significant features are observed in tunneling dI/dV spectra, which we interpret in terms of both strain and quantum confinement. Scanning tunneling microscopy (STM) reveals that the epitaxy between SiC and layer-zero (buffer-layer) graphene on nearby terraces determines the crystalline orientation of the sidewall nanoribbons on step-bunches. We also find a somewhat variable character to the insulating buffer layer, depending on growth conditions and air exposure. [Preview Abstract] |
Thursday, March 21, 2013 5:18PM - 5:30PM |
W6.00013: The metastable chemical gallery of the oxide of epitaxial graphene at room temperature Suenne Kim, Si Zhou, Yike Hu, Claire Berger, Walt de Heer, Angelo Bongiorno, Elisa Riedo Insights in the chemistry of graphene oxide and its response to external stimuli are crucial to control its electronic and optical properties, thus enabling future applications of this material. Here, we present a combined experimental and density functional theory study concerning the compositional and structural properties of the oxide of epitaxial graphene (OeG) as a function of time[1, 2] and temperature. Our result indicates that OeG synthesized by oxidizing epitaxial graphene grown on SiC via the Hummers method is a metastable material whose structure and chemistry evolve with a notable degree at room temperature. XPS studies reveal, metastable OeG reaches a nearly stable reduced O/C ratio of ~0.37 with a featured relaxation time of a month. Initially the most enriched epoxide groups decrease with time while hydroxyl groups increase. In addition to this, further XPS study of OeG as a function of temperature shows heating above 120 C in air can abruptly deteriorate the OeG structure. Our calculations show that the availability of hydrogen atoms could be the key factor in tuning structural and chemical properties at relatively low temperatures. [1] S. Kim et al., Nature Materials 11, 544(2012). [2] Z. Wei et al., Science 328, 1373 (2010). [Preview Abstract] |
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