Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Z48: Graphene Transport: Role of Defects and Interfaces |
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Sponsoring Units: DCMP Chair: Masahiro Ishigami, University of Central Florida Room: Mile High Ballroom 1A-1B |
Friday, March 7, 2014 11:15AM - 11:27AM |
Z48.00001: Negative correlation between charge carrier density and mobility fluctuations in graphene Jie Pan, Jianming Lu, Ping Sheng By carrying out simultaneous longitudinal and Hall measurements in graphene, we find that the 1/f noise for the charge carrier density is negatively correlated to that of mobility, with a governing behavior that differs significantly from the relation between their mean values. The correlation in the noise data can be quantitatively explained by a single parameter theory whose underlying physics is the trapping and de-trapping of the fluctuating charge carriers by the oppositely charged Coulomb scattering centers. This can alter the effective density of long-range scattering centers in a transient manner, with the consequent fluctuating effect on the mobility. The longitudinal noise turns out to be dominated by the remaining component of the mobility fluctuations, and display no correlation to the Hall noise. Due to the negative correlation between charge carrier density and mobility fluctuations, the normalized PSD is smaller than that of the Hall noise. [Preview Abstract] |
Friday, March 7, 2014 11:27AM - 11:39AM |
Z48.00002: Current densities due to electron-hole puddles in graphene flakes at the charge neutrality point Leandro Lima, Caio Lewenkopf Graphene flakes show a typical conductivity minimum of about $e^2/h$, almost independent of sample mobility, at the charge neutrality point. This is at odds with the notion that as the mobility increases, and graphene becomes more ballistic, its density of states (DOS) and conductivity at the charge neutrality point should vanish. The observed conductivity minimum is often attributed to the presence of electron-hole charge puddles, that give rise to an effective local-dependent chemical potential. In this way, the local chemical potential fluctuates creating p and n-doped regions and the electronic transport is facilitated by Klein tunneling through the p and n-doped domains. Although very attractive, there is little quantitative support for this this picture. We revisit this problem and analyze the transport properties using a self-consistent recursive Green's functions technique with spin resolution that includes the electronic interaction modeled by a mean field Hubbard term. We calculate electronic current densities between neighboring carbon sites near the p-n interface and relate the electronic propagation to the puddles charge, size and shapes. [Preview Abstract] |
Friday, March 7, 2014 11:39AM - 11:51AM |
Z48.00003: Ballistic interference in ultraclean suspended monolayer graphene Christian Schonenberger, Peter Rickhaus, Romain Maurand, Peter Makk, Samuel Hess, Endre Tovari, Markus Weiss, Ming-Hao Liu, Klaus Richter We have developed a versatile technology that allows to suspend graphene and complement it with arbitrary bottom and top-gate structures. Using current annealing we demonstrate exceptional high mobililties in monolayer graphene approaching $100$ m$^2$/Vs. These suspended devices are ballistic over micrometer length scales and display intriguing interference patterns in the electrical con-ductance when different gate potentials are applied. Specifically we will discuss different types of Fabry-Perot resonances that appear in different gate voltage regimes of ballistic pn devices [1]. We will go beyond our recent publication [1] and also show electric transport measurements in magnetic field, where intriguing features appear in the intermediate field range in between the low-field Klein-tunneling regime and the quantum Hall regime. We observe a large number of non-dispersing states which might be due to so-called snake states confined to the pn interface. We will also discuss first results on electron guiding in ultraclean monolayer graphene.\\[4pt] [1] P. Rickhaus et al., Nature Communications 4, 2342 (2013) [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z48.00004: Multiple Scattering of Dirac Fermions in Two Dimensions Mahmoud M. Asmar, Sergio E. Ulloa The low energy dispersion of electrons in graphene-as well as surface states of three dimensional topological insulators- are characterized by a linear dispersion, leading to interesting dynamical properties. The presence of potential scattering centers, such as impurities in real samples or artificially created gated regions, also reflect the ``massless'' nature of electrons in these materials. The study of Dirac fermion scattering from single potential obstacles is made possible through partial wave methods. In the case of closely-spaced potential obstacles (high defect concentration), one should consider multiple scattering effects. Using separation of variables, Graf's addition rules, and far field matching, one can generalize the partial wave method to the case of many scatterers, and obtain physical observables for such problem. We present our study of the scattering problem of Dirac fermions from multiple potential obstacles, with focus on the two-center problem. We discuss the dependence of the differential cross section on the separation, and different potential shifts caused by these obstacles, and compare these results with the differential cross section for a single scattering center. We also study the minimal conditions that allow the observation of Klein tunneling. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z48.00005: Temperature-dependent Transport Properties of Graphene Bochen Zhong, Amol Singh, Ahsan Uddin, Goutam Koley, Richard Webb Temperature-dependent transport properties of graphene synthesized by chemical vapor deposition (CVD) on a Cu thin sheet have been investigated. Raman spectra of our samples show good quality of the CVD graphene. We have measured the temperature dependence of conductivity, charge-carrier density and Hall mobility of graphene by patterning them into micrometer-sized Hall bars. Quantum Hall effect has been observed when the temperature is about 60 Kelvin, which is the evidence for single-layer graphene. Furthermore, the results of temperature dependence of Hall mobility indicate that impurity and defect scattering is the primary scattering mechanism at low temperature, while substrate surface polar phonon scattering is dominant at high temperature. [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z48.00006: The Low Frequency Noise Spectrum in Gated Epitaxial Graphene Field Effect Transistors D. Kurt Gaskill, H.K. Chan, V.D. Wheeler, V.K. Nagareddy, L.O. Nyakiti, A. Nath, R.L. Myers-Ward, Z.R. Robinson, N.Y. Garces, M.V. Rao, J.P. Goss, N.G. Wright, C.R. Eddy, Jr., A.B. Horsfall The low frequency noise (LFN) spectrum characteristics in ungated and gated \textit{ca.} 1 ML graphene field effect transistor structures are presented. Synthesis was via the Ar ambient method in a commercial reactor on semi-insulating on-axis 6H(0001)SiC. Samples were processed using photolithography before dielectric deposition; Ti/Au stack was used for ohmic and gate contacts. High-$\kappa $ dielectric deposition used F-functionalization followed by atomic layer deposition of 15 nm Al$_{2}$O$_{3}$ or HfO$_{2}$. The LFN data was averaged over 5 different samples on the same substrate for each oxide case. The LFN spectrum, proportional to 1/f, was similar in magnitude for both bare and dielectric covered graphene, implying the F-functionalization process does not appreciably add noise generation-recombination centers. Both gate oxides showed noise hysteresis ($\sim$ 15{\%}) although it was more pronounced for the HfO$_{2}$ devices. The LFN increased with increasing carrier concentration but decreased with increasing mobility implying that the empirical Hooge model cannot explain the origin of the noise and points to carrier scattering mechanisms as the noise source. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z48.00007: Monitoring the electrical property of graphene transistor by the oxygen vacancy generation of top oxide layer Tae Kwang Kim, Hye Won Du, So Myeong Shin, Jong-Hyuk Yoon, Eun-Kyu Lee, Seungmin Cho, Sunae Seo Fermi level tuning in graphene is crucial for the applications such as conducting electrode or semiconducting electronic device. It is generally achieved by both non-covalent and covalent molecular doping. Former is related with weak Vander Waals interaction which keeps electronic band structure of graphene intact. However, the molecular doping is sensitive to the air exposure so that the degradation of electrical property induce reliability issue without passivation. Here, we suggest self-passivated and well-controlled graphene doping by changing the resistivity of interfacial oxide. The oxygen in transition metal oxide is released at the high temperature under vacuum due to the concentration gradient at the interface leaving electrons which is probable to be conduction electrons by thermal activation. This indicates oxygen vacancy generates impurity level close to conduction band minimum and forms insulating oxide into N-type semiconductor. We monitored the annealing time dependent electron doping concentration of graphene under vacuum. Along with the change of charge neutrality point, to understand the conduction mechanism of graphene at room temperature with Fermi level increase, we investigated the mobility variation of electron and hole carrier versus doping concentration. [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z48.00008: The exploration of bandgap opening in graphene oxides by electrical measurements Wen-Bin Jian, Sheng-Tsung Wang, Yen-Fu Lin, Pei-Ching Yeh, Baruch Rosenstein, A. Torgeman, Lain-Jong Li, Xufeng Zhou, Zhaoping Liu Chemical or electrochemical exfoliation with post reduction is a vital, mass-production method to make few-layer graphene or reduced graphene oxides (rGOs). It was argued that several structures, including C-O-C and C-OH groups, are formed at graphene surface. The rGO flakes are reduced by hydrazine or thermal annealing whereas a small ratio of residue graphene oxides remains on surface. Unlike the metallic graphene, graphene oxides and rGOs show a bandgap with low conductivity. Several atomic models of rGOs were proposed but lacking experimental corroborations. We prepared rGOs with different ratios of remaining oxides. Two ohmic contacts and one tunneling junction were fabricated on each flake. The oxygen coverage, decided by the resistivity of rGO flakes, is in the range from 8 to 23\%. Electron transport was studied from measurements of $R$-$T$ data and fitted to two-dimensional Mott's variable range hopping. The tunneling junction and differential conductance measurements disclosed the density of states (DOS) in rGO flakes. Both transport and DOS measurements indicate an electronic phase transition at an oxygen coverage of ~15\%. The DOS variation and bandgap opening are reproduced from theoretical calculation. [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z48.00009: Kinetic and chemical stability of graphene oxide layers Si Zhou, Angelo Bongiorno Chemical functionalization of graphene holds great promise to open new applications of graphene in technology. Here we combine density functional theory (DFT) and Monte Carlo calculations to study both the stability and structure of graphene layers functionalized with epoxide and hydroxyl species. Our calculations show that sparse functionalizations of graphene are unstable in air at room temperature. However, oxygen groups diffuse and are prone to form dense agglomerates. To investigate these phenomena, we use DFT calculations to first map the interaction of functionalities on graphene, and then to device a simple energy scheme to both compute the Gibbs free energy of formation of arbitrary functionalizations of graphene and predict the structure resulting from diffusion and agglomeration processes. We find that the stability of graphene oxide increases for increasing both the O:C ratio and ageing time. The structure of the aged layers consists of a non-homogeneous phase of highly oxidized regions surrounded by areas of pristine graphene. Within the oxidized domains, formation of energetically stable motifs reduces the likelihood of occurrence of decomposition reactions, thereby enhancing the kinetic stability of the oxidized layer. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z48.00010: Vertical Graphene-base transistor on GaN substrate Ahmad Zubair, Omair Saadat, Yi Song, Jing Kong, Mildred Dresselhaus, Tomas Palacios The high carrier mobility, saturation velocity and thermal conductivity make graphene an attractive candidate for RF electronics. In addition to conventional lateral transistors, several alternative vertical device structures like hot electron transistors have been demonstrated to be promising for RF applications. The unique combination of sub-nanometer thickness and high conductivity makes graphene an excellent base material for hot electron transistors by lowering the base transit time in these vertical devices. The demonstrated graphene-base hot electron transistor performance is limited by low current density and low common-base current gain. In this work, we fabricated a graphene-base transistor on GaN/AlGaN heterostructure. We studied the tunneling from GaN/AlGaN heterojunction to graphene and compared with other demonstrated vertical graphene-base devices. We also investigated the effect of AlGaN thickness and different filtering barriers on both room temperature and low temperature transport characteristics of the fabricated devices. With careful design and optimization of the structure, graphene-base transistors on GaN substrate can be a potential candidate for future graphene RF electronics. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z48.00011: Graphene junction field-effect transistor Tzu-Min Ou, Tomoko Borsa, Bart Van Zeghbroeck We have demonstrated for the first time a novel graphene transistor gated by a graphene/semiconductor junction rather than an insulating gate. The transistor operates much like a semiconductor junction Field Effect Transistor (jFET) where the depletion layer charge in the semiconductor modulates the mobile charge in the channel. The channel in our case is the graphene rather than another semiconductor layer. An increased reverse bias of the graphene/n-silicon junction increases the positive charge in the depletion region and thereby reduces the total charge in the graphene. We fabricated individual graphene/silicon junctions as well as graphene jFETs (GjFETs) on n-type (4.5x10$^{\mathrm{15}}$ cm$^{\mathrm{-3}})$ silicon with Cr/Au electrodes and 3$\mu $m gate length. As a control device, we also fabricated back-gated graphene MOSFETs using a 90nm SiO$_{\mathrm{2}}$ on a p-type silicon substrate (10$^{\mathrm{19}}$ cm$^{\mathrm{-3}})$. The graphene was grown by APCVD on copper foil and transferred with PMMA onto the silicon substrate. The GjFET exhibited an on-off ratio of 3.75, an intrinsic graphene doping of 1.75x10$^{\mathrm{12}}$ cm$^{\mathrm{-2}}$, compared to 1.17x10$^{\mathrm{13}}$ cm$^{\mathrm{-2}}$ in the MOSFET, and reached the Dirac point at 13.5V. Characteristics of the junctions and transistors were measured as a function of temperature and in response to light. Experimental data and a comparison with simulations will be presented. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z48.00012: Electrical transport in graphene-carbon nanotube hybrid junctions Jhao-Wun Huang, Cheng Pan, Hang Zhang, Fenglin Wang, Son Tran, Lei Jing, Marc Bockrath, Jeanie Lau We performed transport experiments in 1D-2D hybrid systems consisting of graphene and single-walled carbon nanotube junctions. We fabricated suspended graphene-carbon nanotube junctions by transferring monolayer graphene sheets onto single-walled carbon nanotubes and etching the SiO2/Si substrates in hydrofluoric acid. We measured the transport properties as a function of magnetic field and gate voltage and electric field. Coulomb blockade feature at 260mK was observed. More of our latest data will be presented with theoretical models. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z48.00013: Fabrication of Ultra-low-contact-resistance Graphene Devices Wei Sun Leong, John T.L. Thong Reactive graphene surface is important to facilitate carriers transport from graphene to other contacting material or vice versa. In this work, we present a technique that is both simple and complementary metal-oxide-semiconductor (CMOS) compatible to generate a significant amount of nano-sized pores in the graphene surface. Edge termination of the created pores in graphene was verified to be of pure zigzag configuration. A number of graphene field-effect transistors were fabricated such that the graphene channel remained intact and the graphene under metal contacts were made porous. These graphene devices exhibit very low contact resistance which is about 60{\%} better than that required for the silicon-based technology at 22 nm node with much higher mobility. In addition, approaches to control the size of pores created in graphene down to sub-nanometer regime and their density will be discussed. In short, the findings suggest that the creation of well-defined nanopores in graphene could be a promising method to enhance interaction of graphene with the contacting metal and thus optimizing the performance of graphene devices. [Preview Abstract] |
Friday, March 7, 2014 1:51PM - 2:03PM |
Z48.00014: Reducing Carrier Density Pinning at Graphene/Metal Interfaces Using Interfacial Multilayer Graphene Akinobu Kanda, Kenta Katakura, Yu Ito, Shintaro Nihei, Rineka Hiraide, Hirokazu Tanaka, Youiti Ootuka, Hikari Tomori In graphene field effect transistors, as the channel length becomes shorter, the apparent field effect mobility gets smaller. One of the origins of the mobility reduction is carrier injection from metal electrodes (source and drain) to graphene mainly due to the difference in the work function, which makes the carrier density at the interface insensitive to the gate voltage. This carrier density pinning at the interface is unfavorable not only for graphene applications to field effect devices but for the observation of some kinds of Dirac Fermionic behaviors of electrons in graphene, such as specular Andreev reflection at graphene/superconductor interfaces. One of the possible solution for lifting the carrier density pinning is to insert an interfacial layer between the graphene film and electrode metals. Here, we report our attempt to form multilayer graphene at the interface and the observed modulation of the transport property by the interfacial layer. [Preview Abstract] |
Friday, March 7, 2014 2:03PM - 2:15PM |
Z48.00015: ABSTRACT WITHDRAWN |
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