Quantum behavior of graphene transistors near the scaling limit

Graphene-Based Analog of Single-Slit Electron Diffraction

This work reports the experimental demonstration of single-slit diffraction exhibited by electrons propagating in encapsulated graphene with an effective de Broglie wavelength corresponding to their attributes as massless Dirac fermions. Nanometer-scale device designs were implemented to fabricate a single-slit followed by five detector paths. Predictive calculations were also utilized to readily understand the observations reported. These calculations required the modeling of wave propagation in ideal case scenarios of the reported device designs to more accurately describe the observed single-slit phenomenon. This experiment was performed at room temperature and 190 K, where data from the latter highlighted the exaggerated asymmetry between electrons and holes, recently ascribed to slightly different Fermi velocities near the K point. This observation and device concept may be used for building diffraction switches with versatile applicability.

A ballistic graphene superconducting microwave circuit

Josephson junctions (JJ) are a fundamental component of microwave quantum circuits, such as tunable cavities, qubits, and parametric amplifiers. Recently developed encapsulated graphene JJs, with supercurrents extending over micron distance scales, have exciting potential applications as a new building block for quantum circuits. Despite this, the microwave performance of this technology has not been explored. Here, we demonstrate a microwave circuit based on a ballistic graphene JJ embedded in a superconducting cavity. We directly observe a gate-tunable Josephson inductance through the resonance frequency of the device and, using a detailed RF model, we extract this inductance quantitatively. We also observe the microwave losses of the device, and translate this into sub-gap resistances of the junction at μeV energy scales, not accessible in DC measurements. The microwave performance we observe here suggests that graphene Josephson junctions are a feasible platform for implementing coherent quantum circuits.

Observation of Electron Coherence and Fabry-Perot Standing Waves at a Graphene Edge

Electron surface states in solids are typically confined to the outermost atomic layers and, due to surface disorder, have negligible impact on electronic transport. Here, we demonstrate a very different behavior for surface states in graphene. We probe the wavelike character of these states by Fabry-Perot (FP) interferometry and find that, in contrast to theoretical predictions, these states can propagate ballistically over micron-scale distances. This is achieved by embedding a graphene resonator formed by gate-defined p-n junctions within a graphene superconductor-normal-superconductor structure. By combining superconducting Aharanov-Bohm interferometry with Fourier methods, we visualize spatially resolved current flow and image FP resonances due to p-n-p cavity modes. The coherence of the standing-wave edge states is revealed by observing a new family of FP resonances, which coexist with the bulk resonances. The edge resonances have periodicity distinct from that of the bulk states manifest in a repeated spatial redistribution of current on and off the FP resonances. This behavior is accompanied by a modulation of the multiple Andreev reflection amplitude on-and-off resonance, indicating that electrons propagate ballistically in a fully coherent fashion. These results, which were not anticipated by theory, provide a practical route to developing electron analog of optical FP resonators at the graphene edge.

Contact gating at GHz frequency in graphene

The paradigm of graphene transistors is based on the gate modulation of the channel carrier density by means of a local channel gate. This standard architecture is subject to the scaling limit of the channel length and further restrictions due to access and contact resistances impeding the device performance. We propose a novel design, overcoming these issues by implementing additional local gates underneath the contact region which allow a full control of the Klein barrier taking place at the contact edge. In particular, our work demonstrates the GHz operation of transistors driven by independent contact gates. We benchmark the standard channel and novel contact gating and report for the later dynamical transconductance levels at the state of the art. Our finding may find applications in electronics and optoelectronics whenever there is need to control independently the Fermi level and the electrostatic potential of electronic sources or to get rid of cumbersome local channel gates.

Nonlinear Ballistic Transport in an Atomically Thin Material

Ultrashort devices that incorporate atomically thin components have the potential to be the smallest electronics. Such extremely scaled atomically thin devices are expected to show ballistic nonlinear behavior that could make them tremendously useful for ultrafast applications. While nonlinear diffusive electron transport has been widely reported, clear evidence for intrinsic nonlinear ballistic transport in the growing array of atomically thin conductors has so far been elusive. Here we report nonlinear electron transport of an ultrashort single-layer graphene channel that shows quantitative agreement with intrinsic ballistic transport. This behavior is shown to be distinctly different than that observed in similarly prepared ultrashort devices consisting, instead, of bilayer graphene channels. These results suggest that the addition of only one extra layer of an atomically thin material can make a significant impact on the nonlinear ballistic behavior of ultrashort devices, which is possibly due to the very different chiral tunneling of their charge carriers. The fact that we observe the nonlinear ballistic response at room temperature, with zero applied magnetic field, in non-ultrahigh vacuum conditions and directly on a readily accessible oxide substrate makes the nanogap technology we utilize of great potential for achieving extremely scaled high-speed atomically thin devices.

Conductance enlargement in picoscale electroburnt graphene nanojunctions

Provided the electrical properties of electroburnt graphene junctions can be understood and controlled, they have the potential to underpin the development of a wide range of future sub-10-nm electrical devices. We examine both theoretically and experimentally the electrical conductance of electroburnt graphene junctions at the last stages of nanogap formation. We account for the appearance of a counterintuitive increase in electrical conductance just before the gap forms. This is a manifestation of room-temperature quantum interference and arises from a combination of the semimetallic band structure of graphene and a cross-over from electrodes with multiple-path connectivity to single-path connectivity just before breaking. Therefore, our results suggest that conductance enlargement before junction rupture is a signal of the formation of electroburnt junctions, with a picoscale current path formed from a single sp(2) bond.

Tailoring 10 nm scale suspended graphene junctions and quantum dots

The possibility to make 10 nm scale, and low-disorder, suspended graphene devices would open up many possibilities to study and make use of strongly coupled quantum electronics, quantum mechanics, and optics. We present a versatile method, based on the electromigration of gold-on-graphene bow-tie bridges, to fabricate low-disorder suspended graphene junctions and quantum dots with lengths ranging from 6 nm up to 55 nm. We control the length of the junctions, and shape of their gold contacts by adjusting the power at which the electromigration process is allowed to avalanche. Using carefully engineered gold contacts and a nonuniform downward electrostatic force, we can controllably tear the width of suspended graphene channels from over 100 nm down to 27 nm. We demonstrate that this lateral confinement creates high-quality suspended quantum dots. This fabrication method could be extended to other two-dimensional materials.

Contact properties to CVD-graphene on GaAs substrates for optoelectronic applications

The optimization of contacts between graphene and metals is important for many optoelectronic applications. In this work, we evaluate the contact resistance and sheet resistance of monolayer and few-layered graphene with different metallizations using the transfer length method (TLM). Graphene was obtained by the chemical vapor deposition technique (CVD-graphene) and transferred onto GaAs and Si/SiO₂ substrates. To account for the quality of large-area contacts used in a number of practical applications, a millimeter-wide TLM pattern was used for transport measurements. Different metals--namely, Ag, Pt, Cr, Au, Ni, and Ti--have been tested. The minimal contact resistance Rc obtained in this work is 11.3 kΩ μm for monolayer CVD-graphene, and 6.3 kΩ μm for a few-layered graphene. Annealing allows us to decrease the contact resistance Rc and achieve 1.7 kΩm μm for few-layered graphene on GaAs substrate with Au contacts. The minimal sheet resistance Rsh of few-layered graphene transferred to GaAs and Si/SiO₂ substrates are 0.28 kΩ/□ and 0.27 kΩ/□. The Rsh value of monolayer graphene on the GaAs substrate is 8 times higher (2.3 kΩ/□), but it reduces for the monolayer graphene on Si/SiO₂ (1.4 kΩ/□). For distances between the contacts below 5 μm, a considerable reduction in the resistance per unit length was observed, which is explained by the changes in doping level caused by graphene suspension at small distances between contact pads.

Ballistic collective group delay and its Goos-Hänchen component in graphene

We theoretically construct an experimental observable for the ballistic collective group delay (CGD) of all the particles on the Fermi surface in graphene. First, we reveal that lateral Goos-Hänchen (GH) shifts along barrier interfaces contribute an inherent component in the individual group delay (IGD). Then, by linking the complete IGD to spin precession through a dwell time, we suggest that the CGD and its GH component can be electrostatically measured by the conductance difference in a spin precession experiment under weak magnetic fields. Such an approach is feasible for almost any Fermi energy. We also indicate that it is a generally nonzero self-interference delay that relates the IGD to the dwell time in graphene.

Sub-10 nm gate length graphene transistors: operating at terahertz frequencies with current saturation

Radio-frequency application of graphene transistors is attracting much recent attention due to the high carrier mobility of graphene. The measured intrinsic cut-off frequency (f_T) of graphene transistor generally increases with the reduced gate length (L_gate) till L_gate = 40 nm, and the maximum measured f_T has reached 300 GHz. Using ab initio quantum transport simulation, we reveal for the first time that f_T of a graphene transistor still increases with the reduced L_gate when L_gate scales down to a few nm and reaches astonishing a few tens of THz. We observe a clear drain current saturation when a band gap is opened in graphene, with the maximum intrinsic voltage gain increased by a factor of 20. Our simulation strongly suggests it is possible to design a graphene transistor with an extraordinary high f_T and drain current saturation by continuously shortening L_gate and opening a band gap.

Chemically modulated graphene diodes

We report the manufacture of novel graphene diode sensors (GDS), which are composed of monolayer graphene on silicon substrates, allowing exposure to liquids and gases. Parameter changes in the diode can be correlated with charge transfer from various adsorbates. The GDS allows for investigation and tuning of extrinsic doping of graphene with great reliability. The demonstrated recovery and long-term stability qualifies the GDS as a new platform for gas, environmental, and biocompatible sensors.

A review of carbon nanotube- and graphene-based flexible thin-film transistors

Carbon nanotubes (CNTs) and graphene have attracted great attention for numerous applications for future flexible electronics, owing to their supreme properties including exceptionally high electronic conductivity and mechanical strength. Here, the progress of CNT- and graphene-based flexible thin-film transistors from material preparation, device fabrication techniques to transistor performance control is reviewed. State-of-the-art fabrication techniques of thin-film transistors are divided into three categories: solid-phase, liquid-phase, and gas-phase techniques, and possible scale-up approaches to achieve realistic production of flexible nanocarbon-based transistors are discussed. In particular, the recent progress in flexible all-carbon nanomaterial transistor research is highlighted, and this all-carbon strategy opens up a perspective to realize extremely flexible, stretchable, and transparent electronics with a relatively low-cost and fast fabrication technique, compared to traditional rigid silicon, metal and metal oxide electronics.

Large-scale mesoscopic transport in nanostructured graphene

Through exponential sample-size scaling of conductance, we demonstrate strong electron localization in three sets of nanostructured antidot graphene samples with localization lengths of 1.1, 2, and 3.4 μm. The large-scale mesoscopic transport is manifest as a parallel conduction channel to 2D variable range hopping, with a Coulomb quasigap around the Fermi level. The opening of the correlation quasigap, observable below 25 K through the temperature dependence of conductance, makes possible the exponential suppression of inelastic electron-electron scatterings and thereby leads to an observed dephasing length of 10 μm.

How close can one approach the Dirac point in graphene experimentally?

The above question is frequently asked by theorists who are interested in graphene as a model system, especially in context of relativistic quantum physics. We offer an experimental answer by describing electron transport in suspended devices with carrier mobilities of several 10(6) cm(2) V(-1) s(-1) and with the onset of Landau quantization occurring in fields below 5 mT. The observed charge inhomogeneity is as low as ≈10(8) cm(-2), allowing a neutral state with a few charge carriers per entire micrometer-scale device. Above liquid helium temperatures, the electronic properties of such devices are intrinsic, being governed by thermal excitations only. This yields that the Dirac point can be approached within 1 meV, a limit currently set by the remaining charge inhomogeneity. No sign of an insulating state is observed down to 1 K, which establishes the upper limit on a possible bandgap.