Journal Article Hidden Defects and Unexpected Properties of Graphene — How Advanced TEM Contributes to Materials Development Get access Benjamin Butz, Benjamin Butz Institute of Micro- and Nanostructure Research & Center for Nanoanalysis and Electron Microscopy (CENEM), Friedrich-Alexander-Universitat Erlangen-Nurnberg Erlangen, GermanyVisiting Scholar, Materials Science & Engineering, Stanford University, Stanford, CA, United States Search for other works by this author on: Oxford Academic Google Scholar Christian Dolle, Christian Dolle Institute of Micro- and Nanostructure Research & Center for Nanoanalysis and Electron Microscopy (CENEM), Friedrich-Alexander-Universitat Erlangen-Nurnberg Erlangen, Germany Search for other works by this author on: Oxford Academic Google Scholar Florian Niekiel, Florian Niekiel Institute of Micro- and Nanostructure Research & Center for Nanoanalysis and Electron Microscopy (CENEM), Friedrich-Alexander-Universitat Erlangen-Nurnberg Erlangen, Germany Search for other works by this author on: Oxford Academic Google Scholar Erdmann Spiecker, Erdmann Spiecker Institute of Micro- and Nanostructure Research & Center for Nanoanalysis and Electron Microscopy (CENEM), Friedrich-Alexander-Universitat Erlangen-Nurnberg Erlangen, Germany Search for other works by this author on: Oxford Academic Google Scholar Konstantin Weber, Konstantin Weber Interdisciplinary Center for Molecular Materials (ICMM) & Computer Chemistry Center (CCC), Friedrich-Alexander-Universitat Erlangen-Nurnberg, Erlangen, Germany Search for other works by this author on: Oxford Academic Google Scholar Bernd Meyer, Bernd Meyer Interdisciplinary Center for Molecular Materials (ICMM) & Computer Chemistry Center (CCC), Friedrich-Alexander-Universitat Erlangen-Nurnberg, Erlangen, Germany Search for other works by this author on: Oxford Academic Google Scholar Daniel Waldmann, Daniel Waldmann Chair for Applied Physics, Friedrich-Alexander-Universitat Erlangen-Nurnberg, Erlangen, Germany Search for other works by this author on: Oxford Academic Google Scholar Ferdinand Kisslinger, Ferdinand Kisslinger Chair for Applied Physics, Friedrich-Alexander-Universitat Erlangen-Nurnberg, Erlangen, Germany Search for other works by this author on: Oxford Academic Google Scholar Heiko B Weber, Heiko B Weber Chair for Applied Physics, Friedrich-Alexander-Universitat Erlangen-Nurnberg, Erlangen, Germany Search for other works by this author on: Oxford Academic Google Scholar Sam Shallcross, Sam Shallcross Lehrstuhl fur Theoretische Festkorperphysik, Friedrich-Alexander-Universitat Erlangen-Nurnberg, Erlangen, Germany Search for other works by this author on: Oxford Academic Google Scholar ... Show more Christian Halbig, Christian Halbig Department of Chemistry and Pharmacy and Central Institute of Materials and Processes (ZMP), Friedrich-Alexander-Universitat Erlangen-Nurnberg, Erlangen, Germany Search for other works by this author on: Oxford Academic Google Scholar Siegfried Eigler, Siegfried Eigler Department of Chemistry and Pharmacy and Central Institute of Materials and Processes (ZMP), Friedrich-Alexander-Universitat Erlangen-Nurnberg, Erlangen, GermanyInstitut fur Chemie und Biochemie, Organische Chemie, Freie Universitat Berlin, Erlangen, Germany Search for other works by this author on: Oxford Academic Google Scholar Colin Ophus Colin Ophus Molecular Foundry/National Center for Electron Microscopy, Lawrence Berkeley National, Laboratory, Berkeley, CA, United States Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 23, Issue S1, 1 July 2017, Pages 1752–1753, https://doi.org/10.1017/S1431927617009424 Published: 04 August 2017

We present a transistor testbed for novel organic and all-carbon electronic materials. Epitaxial graphene on silicon carbide (SiC) is used as source and drain electrodes. The gate is implemented as a bottom gate. Due to Fermi level pinning at the graphene/SiC interface, the gate is effective in the channel area only. The semiconductor above source and drain contacts and the graphene itself are unaffected by the bottom gate. This leads to a clear distinction of electronic properties in the sharply separated channel and contact areas. As an example, we investigate P3HT and fullerene films, representing standard p-type and n-type semiconducting materials. In particular, for P3HT an ohmic contact to graphene was observed, and an accurate and consistent determination of transistor parameters was achieved. The fullerene transistor showed a high on/off ratio of 3 × 103. The testbed offers the opportunity to determine semiconductor parameters of novel materials under very well-defined conditions.

Basal-plane dislocations, identified as fundamental defects in bilayer graphene by transmission electron microscopy and atomistic simulations, reveal striking size effects, most notably a pronounced buckling of the graphene membrane, which drastically alters the strain state and is of key importance for the material’s mechanical and electronic properties.

We fabricate free-standing graphene structures from epitaxial graphene on silicon carbide using a photoelectrochemical (PEC) etching process. A combination of Raman spectroscopy and magnetotransport measurements was used to investigate multiterminal devices in various geometries. From the analysis of Raman data and Shubnikov-de Haas oscillations, we conclude that the buffer layer is converted into a graphene layer and, thus, monolayer graphene on SiC gets converted to a free-standing AB-stacked bilayer. The bilayer exhibits inversion-symmetry breaking because of differential doping between the layers. Additionally, lateral inhomogeneities exist in the form of domains with nonuniform mobility. The same PEC process on a pure buffer layer, however, does not yield monolayer graphene.

We present a fabrication process for freely suspended membranes consisting of bi- and trilayer graphene grown on silicon carbide. The procedure, involving photoelectrochemical etching, enables the simultaneous fabrication of hundreds of arbitrarily shaped membranes with an area up to 500 μm2 and a yield of around 90%. Micro-Raman and atomic force microscopy measurements confirm that the graphene layer withstands the electrochemical etching and show that the membranes are virtually unstrained. The process delivers membranes with a cleanliness suited for high-resolution transmission electron microscopy (HRTEM) at atomic scale. The membrane, and its frame, is very robust with respect to thermal cycling above 1000 °C as well as harsh acidic or alkaline treatment.

Graphene is an outstanding electronic material, predicted to have a role in post-silicon electronics. However, owing to the absence of an electronic bandgap, graphene switching devices with high on/off ratio are still lacking. Here in the search for a comprehensive concept for wafer-scale graphene electronics, we present a monolithic transistor that uses the entire material system epitaxial graphene on silicon carbide (0001). This system consists of the graphene layer with its vanishing energy gap, the underlying semiconductor and their common interface. The graphene/semiconductor interfaces are tailor-made for ohmic as well as for Schottky contacts side-by-side on the same chip. We demonstrate normally on and normally off operation of a single transistor with on/off ratios exceeding 10(4) and no damping at megahertz frequencies. In its simplest realization, the fabrication process requires only one lithography step to build transistors, diodes, resistors and eventually integrated circuits without the need of metallic interconnects.

The project GLOWA-Danube (http://www.glowa-danube.de) aimed at investigating the manifold consequences of Global Change on regional water resources in the Upper Danube Basin. In order to achieve this task, an interdisciplinary, university-based network of experts developed the integrative Decision Support System OpenDanubia (OD). The common base for implementing and coupling the various scientific model components is a generic framework, which provides the coordination of the coupled models that run in parallel exchanging iteratively data via their interfaces. The OD framework takes care of technical aspects, such as ordered data exchange between sub-models, data aggregation, data output, model parallelization and data distribution over the network, which means that model developers do not have to be concerned about complexities evolving from coupling their models. Within this framework the sub-model NaturalEnvironment , representing a land surface model, was developed and implemented. The object-oriented design of this sub-model facilitates a plain, logical representation of the actual physical processes simulated by the sub-model. Physical processes to be modelled are organized in naturally ordered, exchangeable lists that are executed on each spatial computation unit for each modelling time step, depending on their land cover. The type of land cover to be simulated on each freely defined spatial unit is distinguished by one of the three types aquatic , terrestrial and glacier . Additionally, the type terrestrial is influenced by dynamic land use changes which can be triggered e.g. by the socio-economic OD sub-model Farming . This paper presents the basic design of the open source (GPL'ed) OD framework and highlights the implementation of the sub-model NaturalEnvironment within this framework, as well as its interactions with other components included in OD.

Due to the lack of graphene transistors with large on/off ratio, we propose a concept employing both epitaxial graphene and its underlying substrate silicon carbide (SiC) as electronic materials. We demonstrate a simple, robust, and scalable transistor, in which graphene serves as electrodes and SiC as a semiconducting channel. The common interface has to be chosen such that it provides favorable charge injection. The insulator and gate functionality is realized by an ionic liquid gate for convenience but could be taken over by a solid gate stack. On/off ratios exceeding 44000 at room temperature are found.

We present a technique to tune the charge density of epitaxial graphene via an electrostatic gate that is buried in the silicon carbide substrate. The result is a device in which graphene remains accessible for further manipulation or investigation. Via nitrogen or phosphor implantation into a silicon carbide wafer and subsequent graphene growth, devices can routinely be fabricated using standard semiconductor technology. We have optimized samples for room temperature as well as for cryogenic temperature operation. Depending on implantation dose and temperature we operate in two gating regimes. In the first, the gating mechanism is similar to a MOSFET, the second is based on a tuned space charge region of the silicon carbide semiconductor. We present a detailed model that describes the two gating regimes and the transition in between.

A bottom gate scheme is presented to tune the charge density of epitaxial graphene via a gate voltage while leaving the surface open for further manipulation or investigation. Depending on the doping concentration of the buried gate layer, the temperature and illumination, the bottom gate structure can be operated in two regimes with distinct capacitances. A model is proposed, which quantitatively describes the gate operation. The model is verified by a control experiment with an illuminated gate structure using UV light. Using UV illumination the Schottky capacitor (SC) regime, which provides improved gate efficiency, can be used even at low temperatures.

We investigate the magnetotransport in large area graphene Hall bars epitaxially grown on silicon carbide. In the intermediate field regime between weak localization and Landau quantization, the observed temperature-dependent parabolic magnetoresistivity is a manifestation of the electron-electron interaction. We can consistently describe the data with a model for diffusive (magneto)transport that also includes magnetic-field-dependent effects originating from ballistic time scales. We find an excellent agreement between the experimentally observed temperature dependence of magnetoresistivity and the theory of electron-electron interaction in the diffusive regime. We can further assign a temperature-driven crossover to the reduction of the multiplet modes contributing to electron-electron interaction from 7 to 3 due to intervalley scattering. In addition, we find a temperature-independent ballistic contribution to the magnetoresistivity in classically strong magnetic fields.

Graphene has many outstanding electronic properties and a vision of post-silicon electronics based on graphene is discussed. However, due to the absence of an electronic band gap transistors with high on/off ratio are still lacking 1 . Consequently, graphene circuits are currently limited to analogue amplification 2 and further modifications are needed for logic applications. Many efforts have been undertaken to establish a gap in graphene devices, with a band gap induced by bilayer graphene, spatial confinement, localization and chemical modification, resulting in rather small effects remote from technical requirements. For wafer based applications, we consider epitaxial graphene on silicon carbide (0001) as the first choice 3 . We propose a concept that monolithically employs the entire system epitaxial graphene, consisting of graphene, silicon carbide, and their common interface. For a transistor, graphene can be used as source, drain and gate electrode. The wide band gap semiconductor substrate silicon carbide forms the channel. Finally, two differently tailored interfaces introduce transistor functionality. The result is an epitaxial graphene transistor with high on/off ratio exceeding 10 4 (see Fig. 1). Depending on the underlying substrate, both normally-on and normally-off operation is possible. The concept’s particular strength is its capability for integration: within the same processing effort many epitaxial graphene transistors can be combined and complex circuits can be designed. In principle any logic functionality can be implemented.

We report on epitaxial graphene on silicon carbide at high current densities. We observe two distinguished regimes, and a final breakdown. First for low current densities the conductance is enhanced due to desorption of adsorbates. Second with increasing bias the sample locally starts to glow and is strongly heated. The silicon carbide material decomposes, graphitic material is formed and thus additional current paths are created. The graphene layer breaks down, which is, however, not visible in high bias data. The final breakdown is a self-amplifying process resulting in a locally destroyed sample but surprisingly with better conductance than the original sample.

Bottom gates in epitaxial graphene structures can now be fabricated through a technique based on nitrogen implantation. This is an important achievement to increase both the versatility of the material for fundamental studies and the potential for its use in devices.

We have extensively studied the electronic properties of epitaxial graphene grown on the Si face of a 6H silicon carbide substrate by thermal decomposition in an argon atmosphere. Using e-beam lithography, large van der Pauw structures as well as Hall bars were patterned. Their size ranged from millimeters down to submicrometer-sized Hall bars, the latter entirely placed on atomically flat substrate terraces. We found reproducible electronic properties, independent of the sample size and orientation, over a broad temperature range. A comparison of the mobility values indicated no enhanced scattering at the macroscopic step edges of the SiC substrate and due to adsorbed molecules. However, the strong coupling to the substrate results in an elevated charge carrier density n and a reduced mobility mu compared to exfoliated graphene. If n is decreased the mobility rises substantially (up to 29 000 cm(2)/V s at 25 K), and Shubnikov-de Haas oscillations and the graphene-like quantum Hall effect become visible. This leads to the conclusion that the electrons in epitaxial graphene have the same quasi-relativistic properties previously shown in exfoliated graphene and expected from theory. (C) 2011 Elsevier Ltd. All rights reserved.

We report on an investigation of quasi-free-standing graphene on 6H-SiC(0001) which was prepared by intercalation of hydrogen under the buffer layer. Using infrared absorption spectroscopy we prove that the SiC(0001) surface is saturated with hydrogen. Raman spectra demonstrate the conversion of the buffer layer into graphene which exhibits a slight tensile strain and short range defects. The layers are hole doped (p = 5.0-6.5 x 10^12 cm^(-2)) with a carrier mobility of 3,100 cm^2/Vs at room temperature. Compared to graphene on the buffer layer a strongly reduced temperature dependence of the mobility is observed for graphene on H-terminated SiC(0001)which justifies the term "quasi-free-standing".

We investigate the transport properties of high-quality single-layer graphene, epitaxially grown on a 6H-SiC(0001) substrate. We have measured transport properties, in particular charge-carrier density, mobility, conductivity, and magnetoconductance of large samples as well as submicrometer-sized Hall bars which are entirely lying on atomically flat substrate terraces. The results display high mobilities, independent of sample size. The temperature dependence of the conductance indicates a rather strong coupling to the SiC substrate. An analysis of the Shubnikov-de Haas effect yields the Landau-level spectrum of single-layer graphene. When gated close to the Dirac point, the mobility increases substantially and the graphenelike quantum Hall effect occurs.

We report on a comprehensive study of the properties of quasi-freestanding monolayer and bilayer graphene produced by conversion of the (6√3×6√3)R30° reconstruction into graphene via intercalation of hydrogen. The conversion is confirmed by photoelectron spectroscopy and Raman spectroscopy. By using infrared absorption spectroscopy we show that the underlying SiC(0001) surface is terminated by hydrogen in the form of Si-H bonds. Using Hall effect measurements we have determined the carrier concentration and type as well as the mobility which lies well above 1000 cm2/Vs despite a significant amount of short range scatterers detected by Raman spectroscopy.

We report on electrical measurements on epitaxial graphene on 6H-SiC (0001). The graphene layers were fabricated by thermal decomposition in Argon atmosphere. Large van der Pauw structures and Hall bars were patterned by e-beam lithography, the Hall bars ranged from rather large structures down to sub-micrometer sized Hall bars entirely placed on atomically °at substrate terraces. We present Hall measurements in a broad temperature range, Shubnikov de Haas oscillations and quantum Hall steps. The data lead to the conclusion that electrons in epitaxial graphene have the same quasi-relativistic properties previously shown in exfoliated graphene. A remarkable di®erence, however, is the stronger coupling to substrate phonons and the relatively high charging being an intrinsic property of this epitaxial system.