Session Q22: Graphene Junctions

Au/Graphene/Nb Tunnel Structures

We report on work with Au/Graphene/Nb FET-type structures. Samples are created by e-beam lithography on electronic-grade oxidized Si substrates, using commercially prepared graphene flakes and epitaxial films on SiC. Raman scattering is used to verify the single-layer nature of samples under study. We discuss noise studies in these systems and the status of experiments designed to observe predicted oscillations in the tunnel conductance of samples, associated with Klein tunneling in the graphene films and Andreev reflections at graphene/lead interfaces.   

Direct measurements of the current-phase relation in graphene Josephson junctions

The current-phase relation (CPR) of a Josephson junction can provide key information about the microscopic processes and symmetries that influence the~supercurrent. However, CPR has not been previously measured in junctions with graphene barriers, which is a system that exhibits unusual electronic properties and symmetries. In this talk we present CPR results on Josephson junctions containing single-layer graphene as a weak link. The measurements are based on a phase-sensitive SQUID technique in which we determine the~supercurrent~amplitude and phase as a function of both temperature and electrostatic doping (gate voltage). We discuss evidence for a non-sinusoidal shape of the CPR, which is in agreement with some theoretical predictions.   

Effects of metallic contacts on electron transport through graphene

Despite their undoubted importance in eventual graphene electronics, theoretical studies of the specific features of electron transport through graphene between metal contacts are in their first stages. In order to bridge this gap we perform a first-principles based, non-equilibrium Green's functions study of the conductance through graphene junctions suspended between noncovalent aluminum contacts as a function of the distance L between metal leads and the width W (up to 100 nm) of the junction. Electron-hole asymmetry is obtained as a consequence of doping at the leads. Furthermore, the doping in graphene originated by charge transfer from metals at the leads results in two conductance minima at the energies of the crossing of the linear bands in suspended and clamped graphene, for sufficiently large L. We present a tight-binding model that accounts for the first-principles results and can be employed for larger lengths and widths of the junctions up to experimental accessible values and for arbitrary noncovalent-bonding metal leads.   

Spectroscopy measurements of superconductor-graphene-normal metal junctions

We discuss results on single layer graphene junctions made by a graphene sheet in contact with a superconducting tunnel probe and a normal ohmic contact. The superconducting gap of the tunnel probes is well formed at 250 mK. However, we observe an additional conductance gap at smaller biases (a ``subgap'' feature), which is symmetric in bias voltage and oscillates as function of the applied backgate voltage. Even though no supercurrent flow is observed, due to the large spacing of the leads, the observed effects may be due to Andreev bound states. We discuss the possible role of Andreev bound states in determining the inner subgap features of the conductance, and potential spectroscopic signatures arising from the ballistic transport of normal electrons in graphene that travel between the superconducting and the normal contacts.   

Tunneling measurements of clean metal/graphene junctions

The tunneling density of states of graphene offers valuable information to understand its transport properties. Hall bars fabricated from graphene can only probe the states at the fermi level, while STM studies are subject to tip effects, and have greater limits on temperature and magnetic field than a similar experiment that can be done with a device. Here, we present magnetotransport measurements on clean, micron scale junctions between metal and graphene at cryogenic temperatures. In our fabrication process, the tunneling lead is made to contact an exfoliated graphene flake without the use of lithography and accompanying polymer residues. These experiments compliment similar efforts with STM local probes, and comparisons will be made with these systems.   

Quantum Hall effect in graphene: the role of the contacts and the device geometry

We investigate the motion of electronic wave-packets in the mean-field potential of a graphene Hall bar. The presence of Ohmic contacts enforces metallic boundary conditions along the source and drain contacts and affects the global form of the Hall potential in small devices, offering a possible explanation for the difficulty to detect the QHE in graphene in a four-terminal setup. Our injection-model of the QHE takes into account the existence of hot-spots in the device, which break any translational invariance. References: http://www.quantumdynamics.de/publications.html   

Conductance Fluctuations~in the Quantum Hall Regime by Graphene\textit{ pnp} Junctions

We investigated quantum hall conductance in high quality graphene \textit{pnp} junctions with suspended top gates. In high magnetic fields, in addition to well-developed integer conductance plateaus for the first 5~Landau levels, we also observe prominent conductance fluctuations on transitions between the plateaus as the top gate voltage is varied.~ Latest experimental progress regarding the interpretation of these conductance fluctuations will be discussed.   

Graphene/Carbon Nanotube Cross-Junction Devices

We have built crossed carbon nanotube/graphene junctions from CVD graphene and aligned arrays of carbon nanotubes. Large-area single-layer graphene was grown on a copper film and transferred to silicon oxide, then lithographically patterned and electrically contacted. Highly aligned arrays of single-walled carbon nanotubes were CVD-grown on quartz and transferred to complete the devices. We probed these new geometries using electrical measurements, studied their optoelectronic response with scanning photocurrent microscopy, and explored the temperature and gate dependence of the junctions. We found that graphene acts as a very good electrode for carbon nanotubes, pointing to the possibility of creating fully-integrated, transparent, flexible transistors purely from carbon nanomaterials.   

Manipulation of Electron Beam Propagation by Hetero-Dimensional Graphene Junctions

Manipulation of electron beam propagation in low-dimensional nanostructures is the fundamental process required for quantum information processing and quantum computation in the future. Recently, graphene has been proposed as a candidate system for splitting and focusing electron beam by applying an external field or potential. Here, we demonstrate theoretically a new mechanism for the energy-selective manipulation of electron beam by nanostructured hetero-dimensional graphene junctions (HDGJs) without external field. Electron beam splitting, collimation, and beam-guide can all be realized by designing HDGJs of different dimensionality, size and orientation. Importantly, these different functions can be combined together by pre-designed patterning of multiple HDGJ units in one graphene sheet, making it feasible for large-scale integration of graphene-based quantum devices. Furthermore, we show an effective method for mapping the electron beam propagation in graphene by scanning probe microscopy (SPM), as being done in two-dimensional electron gas (2DEG), which will be very useful for fundamental study of electron transport and quantum phenomena in graphene.   

Investigating Classical and Quantum Behavior of Graphene-based Josephson Junctions

We present work investigating the classical and quantum nature on the switching from the superconducting to the normal state in graphene Josephson junction devices. These devices consist of two parallel superconducting leads deposited onto single- and few-layer graphene flakes. In current-biased graphene Josephson junctions, we predict a potential similar to the tilted washboard potential in conventional Josephson junctions, which is a function of the gauge-invariant phase difference. A switching event can be thought of as the escape of a fictitious ``phase particle'' out of a local minimum. It can escape due to classical resonant activation over the potential barrier or by quantum tunneling through the potential barrier. The switching properties of these devices depend on many factors such as thermal and current noise. We explore these factors and consider the implications of ballistic versus diffusive charge carriers. We also present our ongoing experimental progress studying these devices.   

Direct observation of a reconfigurable graphene p-n junction through surface potential imaging

Graphene p-n junctions have tremendous potential for future carbon-based electronic devices. Various theoretical models predict that graphene p-n junctions can be configured to guide electrons analogous to an optical wave guide or focus electrons as from a negative index lens, depending on abruptness of the electrostatic doping profile across the p-n junction. In this work, mechanically exfoliated graphene has been deposited on buried split-gate test structures to form pristine graphene p-n junctions not exposed to photoresist or lithographic patterning. An electrostatically formed p-n junction was created on this structure through the application of a voltage differential between the buried `split' gates. Scanning Kelvin Probe Microscopy was used to directly and simultaneously image p-type, n-type and intrinsic regions on the monolayer graphene deposited across the p-n gate structure with nanometer spatial resolution and potential resolution in mV range. The electrostatic doping in graphene is seen to change according to gate polarity when varied from positive to negative values. This observation is reproducible for multiple samples studied. Graphene differential surface potential is measured as a function of split-gate electrode voltage and displays the expected square root behavior. Measurements of the junction profile are also presented and discussed.   

Formation of graphene p-n superlattices on Pb quantum wedged islands

Based on first-principles calculations within density functional theory, we propose a novel scheme to create graphene p-n superlattices on Pb wedged islands with quantum stability. Pb(111) islands grown on vicinal Si(111) extend over several Si steps to form a wedged structure with atomically flat tops. The monolayer thickness variation caused by the underlying substrate steps is a sizeable fraction of the total thickness of the wedged islands and gives rise to a bi-layer oscillation in the Pb(111) work function due to quantum size effect. When a graphene sheet is placed on the surface of such a Pb wedged island, the work-function variations caused by the steps result in an oscillatory shift in the Fermi energy relative to the Dirac point of the graphene. By applying an appropriate external electrical field in the direction perpendicular to the substrate, the Fermi energy of a graphene sheet can be globally tuned to form a well-defined p-n superlattice with potentially intriguing applications in nanoelectronics.   

Temperature dependence of proximity-induced supercurrent in single and multi-layer graphene

Graphene is an attracting material for the superconducting proximity effect. In single layer graphene (SLG), the peculiar band structure leads to the relativistic Josephson effect, while in multilayer graphene (MLG), the layered structure with large modulation of carrier density from negative to positive values provides a novel situation of conventional proximity effect. Here we present experimental study on superconducting proximity effect in SLG and MLG. For SLG with junction length of 220 nm, we observed gate-voltage dependent critical supercurrent $I_c$, and its temperature dependences for all gate voltages were well explained by a conventional theory for short and dirty junctions (KO1 theory). On the other hand, in MLG junctions, $I_c(T) \propto \exp (-(T/T_0)^2)$, where $T_0$ is a sample- and gate- dependent constant. This behavior can be explained by a successive transition model, in which a graphene layer with larger carrier density has a higher temperature for the onset of supercurrent.   

Tuning the Schottky barrier across graphite/semiconductor junctions by bromine intercalation

We report \textit{in situ} tuning of the Schottky barrier height (SBH) formed at graphite/semiconductor (semiconductor = $n$-Si, 4H-SiC) interfaces by exposing completed junctions to Br vapor. The Br, which acts like an acceptor, intercalates into the graphite and hole dopes the graphene planes. The studied Schottky diodes display lower forward/reverse current density and higher depletion capacitance after the Br intercalation. Capacitance-voltage measurements confirm 0.3~-~0.4 eV increases in the SBH, consistent with Br-induced changes in the graphite work function deduced from the Mott-Schottky relations and measured by X-ray photoemission spectroscopy. The lowering of the graphite Fermi energy, or equivalently the raising of the graphite work function, is attributed to the increase in the density of mobile hole carriers resulting from electron transfer from the carbon planes to the Br intercalates. These results have implications for HEMTs, MESFET devices, sensing, and high power applications as well as graphene electronics, since the outermost layer of graphite in contact with the semiconductor is a single sheet of carbon atoms.