Akinobu Kanda

Carbon nanotube devices for nanoelectronics

We present a review of recent fundamental research on carbon nanotubes, with the goal of realizing future nanometer-scale electronic applications. Since nanotubes are by nature nanometer-scale devices with remarkable electrical properties, they are expected to be an important element in nanometer-scale electronics. As preliminary steps towards realizing nanotube-electronics, we have investigated the formation mechanism of metal contacts to carbon nanotubes, nanotube device fabrication possibilities, and nanotube spin electronics.

Experimental Evidence for Giant Vortex States in a Mesoscopic Superconducting Disk

The response of a mesoscopic superconducting disk to perpendicular magnetic fields is studied by using the multiple-small-tunnel-junction method, in which transport properties of several small tunnel junctions attached to the disk are measured simultaneously. This allows us to make the first experimental distinction between the giant vortex states and multivortex states. Moreover, we experimentally find a magnetic-field induced rearrangement and combination of vortices. The experimental results are well reproduced in numerical results based on the nonlinear Ginzburg-Landau theory.

Low Operating Bias and Matched Input−Output Characteristics in Graphene Logic Inverters

We developed a simple and novel method to fabricate complementary-like logic inverters based on ambipolar graphene field-effect transistors (FETs). We found that the top gate stacks (with both the metal and oxide layers) can be simply prepared with only one-step deposition process and show high capacitive efficiency. By employing such a top gate as the operating terminal, the operating bias can be lowered within 2 V. In addition, the complementary p- and n-type FET pairs can be also simply fulfilled through potential superposition effect from the drain bias. The inverters can be operated, with up to 4-7 voltage gains, in both the first and third quadrants due to the ambipolarity of graphene FETs. For the first time, a match between the input and output voltages is achieved in graphene logic devices, indicating the potential in direct cascading of multiple devices for future nanoelectronic applications.

Influence of Disorder on Conductance in Bilayer Graphene under Perpendicular Electric Field

Electron transport in bilayer graphene placed under a perpendicular electric field is revealed experimentally. Steep increase of the resistance is observed under high electric field; however, the resistance does not diverge even at low temperatures. The observed temperature dependence of the conductance consists of two contributions: the thermally activated (TA) conduction and the variable range hopping (VRH) conduction. We find that for the measured electric field range (0-1.3 V/nm) the mobility gap extracted from the TA behavior agrees well with the theoretical prediction for the band gap opening in bilayer graphene, although the VRH conduction deteriorates the insulating state more seriously in bilayer graphene with smaller mobility. These results show that the improvement of the mobility is crucial for the successful operation of the bilayer graphene field effect transistor.

Inter-Layer Screening Length to Electric Field in Thin Graphite Film

Electric conduction in thin graphite film was tuned by two gate electrodes to clarify how the gate electric field induces electric carriers in thin graphite. The graphite was sandwiched between two gate electrodes arranged in a top and bottom gate configuration. A scan of the top gate voltage generates a resistance peak in ambiploar response. The ambipolar peak is shifted by the bottom gate voltage, where the shift rate depends on the graphite thickness. The thickness-dependent peak shift was clarified in terms of the inter-layer screening length to the electric field in the double-gated graphite film. The screening length of 1.2 nm was experimentally obtained.

Introducing Nonuniform Strain to Graphene Using Dielectric Nanopillars

A method for inducing nonuniform strain in graphene films is developed. Pillars made of a dielectric material (electron beam resist) are placed between graphene and the substrate, and graphene sections between pillars are attached to the substrate. The strength and spatial pattern of the strain can be controlled by the size and separation of the pillars. Application of strain is confirmed by Raman spectroscopy as well as from scanning electron microscopy (SEM) images. From SEM images, the maximum stretch of the graphene film reaches about 20%. This technique can be applied to the formation of band gaps in graphene.

Gate control of spin transport in multilayer graphene

We experimentally studied the gate voltage dependence of spin transport in multilayer graphene (MLG) using the nonlocal spin detection technique. We found that the spin signal is a monotonically decreasing linear function of the resistance of MLG, which is characteristic of the intermediate interfacial transparency between the MLG and the ferromagnetic electrodes (Co). The linear relation indicates a large spin relaxation length significantly exceeding 8μm. This shows the superiority of MLG for the utilization of the graphite-based spintronic devices.

Electron transport in metal/multiwall carbon nanotube/metal structures (metal=Ti or Pt/Au)

We report observation of the Coulomb blockade effect in a metal-on-tube metal/multiwall carbon nanotube (MWNT)/metal structure (metal=Ti or Pt/Au). We find that the tunnel barrier is located in the interface between the MWNT and the metal electrode, and that the tunnel resistance depends crucially on the electrode material. A possible origin of the high contact barrier is discussed.

Complementary-Like Graphene Logic Gates Controlled by Electrostatic Doping

Realization of logic circuits from graphene is very attractive for high-speed nanoelectronics. However, the intrinsic ambipolar nature hinders the formation of graphene logic devices with the conventional complementary architecture. Using electrostatic doping modulation, we show here a facile method to control the charge neutrality points and form a complementary-like structure, in which the ambipolar conduction is used as a benefit rather than a drawback to construct logic devices. A band gap is also introduced in the channels to improve the switching ratio of the graphene transistors. For the first time, complementary-like NOR and NAND logic gates were demonstrated. This method provides a possible route for logic circuits from ambipolar graphene and, in principle, can be also extended to other ambipolar semiconductors, such as organic compounds and carbon nanotube thin films.

Local current injection into mesoscopic superconductors for the manipulation of quantum states.

We perform strategic current injection in a small mesoscopic superconductor and control the (non)equilibrium quantum states in an applied homogeneous magnetic field. In doing so, we realize a current-driven splitting of multiquanta vortices, current-induced transitions between states with different angular momenta, and current-controlled switching between otherwise degenerate quantum states. These fundamental phenomena form the basis for the electronic and logic applications discussed, and are confirmed in both theoretical simulations and multiple-small-tunnel-junction transport measurements.