S. Odaka

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.

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.

Anisotropic transport in graphene on SiC substrate with periodic nanofacets

Anisotropic transport in graphene field-effect transistors fabricated on a vicinal SiC substrate with a self-organized periodic nanofacet structure is investigated. Graphene thermally grown on a vicinal substrate contains two following regions: atomically flat terraces and nanofacets (atomically stepped slopes). The graphene film at a nanofacet is continuously connected between two neighboring terrace films. Anisotropic transport properties are clearly observed, indicating a difference in the graphene properties of the two regions. The observed anisotropic properties are discussed in terms of the effects of nanofacet structures on conductivity and electron mobility.

Role of atomic terraces and steps in the electron transport properties of epitaxial graphene grown on SiC

Thermal decomposition of vicinal SiC substrates with self-organized periodic nanofacets is a promising method to produce large graphene sheets toward the commercial exploitation of graphenes superior electronic properties. The epitaxial graphene films grown on vicinal SiC comprise two distinct regions of terrace and step; and typically exhibit anisotropic electron transport behavior, although limited areas in the graphene film showed ballistic transport. To evaluate the role of terraces and steps in electron transport properties, we compared graphene samples with terrace and step regions grown on 4H-SiC(0001). Arrays of field effect transistors were fabricated on comparable graphene samples with their channels parallel or perpendicular to the nanofacets to identify the source of measured reduced mobility. Minimum conductivity and electron mobility increased with the larger proportional terrace region area; therefore, the terrace region has superior transport properties to step regions. The measured elect...

Coulomb Blockade Oscillations in Narrow Corrugated Graphite Ribbons

We report Coulomb blockade oscillations in an atomically thin graphite ribbon fabricated by the micromechanical cleavage technique. Aperiodic current oscillations as a function of the gate voltage indicate the formation of multiple Coulomb islands inside the thin graphite ribbon. We conclude that the Coulomb islands originate from puddles of electrons and holes caused by the inhomogeneous interface between the ribbon and the substrate.

Thickness-dependent Resistance Change of Dual-gated Thin Graphite Films

We present fundamental researches on thin graphite film, with the goal of realizing future nanometer scale electronic applications. Because thin graphite films are by nature nanometer scale materials with remarkable electrical conductions, they are expected to be an important element in nano-carbon electronics. For a control of the conduction of the thin graphite channel, gating effect must be fully clarified. Our starting materials are thin layers (thickness 1-10 nm) of graphite films pealed off from bulk graphite on SiO2/doped-Si substrate. The thin film is connected to two or multiple metallic electrodes. In general, conduction of the graphite can be changed in gate voltage applied to the doped-Si substrate. In this configuration, the gate electric field can be applied from the substrate side (back-gate configuration). Observed resistance in the gate-voltage change shows ambipolar behavior based on clear carrier polarity change. We attached a front gate, which was directly formed on the surface of the graphite film. We deposit an Al electrode on the graphite film (Fig. 1). The graphite channel and the Al electrode are naturally insulated by exposed in air. Then the Al electrode can be used as a front gate. The front gate also changes the conduction of the thin graphite film. A scan of the top gate voltage (Vtg) generates a resistance peak in the ambiploar response. In relatively thicker film (~7 nm), the back gate voltage (Vbg) shifts the ambipolar peak depending only slightly (Fig. 2). On the other hand, the shift is clear in thinner film (~1 nm) (Fig. 2). The thickness-dependent peak shift is clarified in terms of the inter-layer screening length to the electric field in the dual-gated graphite film. We assume that the gate-induced carriers decay exponentially from both surfaces, and that the conductivity in each layer increases proportionally to the induced carrier density. The ambipolar peak corresponds to the situation that the carriers induced by the back gate is maximally ejected by the top gate. Then the condition for the ambipolar resistance peak in Vtg scan is obtained as a function of Vbg, , and the graphite film thickness d. Then, we estimated a screening length of 1.2 nm [1]. In films thicker than 2 nm, ambipolar resistance peak decreases at large Vbg. This is because the carriers induced by the back gate cannot be ejected completely Fig.1 Optical microscope image of a thin graphite film with source-drain electrodes and a Al top gate on SiO2/Si substrate. Extended Abstracts of the 2008 International Conference on Solid State Devices and Materials, Tsukuba, 2008,

Observation of gate-controlled superconducting proximity effect in microfabricated thin graphite films

We investigated the influence of the oxygen plasma etching on the electron transport in thin graphite films. The semimetallic temperature dependence of zero-bias resistance was observed for samples microfabricated with both Al mask and resist mask, but the possible damage by e-beam irradiation was observed in films with Al mask. In thin graphite films microfabricated by O2 plasma with resist mask, the proximity-induced superconductivity was observed and the critical supercurrent and temperature strongly depend on the gate voltage.

Coulomb Blockade Oscillations in Patterned Ultrathin Graphite Films

Nanoscale channel formation and characterization in ultrathin graphite film are performed. Two narrow channels with a submicron square connected in series are patterned by electron-beam lithography followed by oxygen plasma etching. The patterned film showed Coulomb blockade oscillations in the source–drain current upon a gate voltage change at 4.2 K. The Coulomb blockade effect occurred due to the electric potential modulation in the narrow graphite channels.