Hikari Tomori

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.

Effect of current annealing on electronic properties of multilayer graphene

While ideal graphene has high mobility due to the relativistic nature of carriers, it is known that the carrier transport in actual graphene samples is dominated by the influence of scattering from charged impurities, which almost conceals the intrinsic splendid properties of this novel material. The common techniques to improve the graphene mobility include the annealing in hydrogen atmosphere and the local annealing by imposing a large biasing current. Although annealing is quite important technique for the experimental study of graphene, detailed evaluation of the annealing effect is lacking at present. In this paper, we study the effect of the current annealing in multilayer graphene devices quantitatively by investigating the change in the mobility and the carrier density at the charge neutrality point. We find that the current annealing sometimes causes degradation of the transport properties.

Introducing uniaxial local strain to graphene encapsulated with hBN

Summary form only given. Strain engineering is a promising method for controlling electron transport in graphene. From our previous experimental result, we found that the observation of gap formation by lattice strain requires large spatial variation of strain and long mean free path of charge carriers. For satisfying above requirements, we developed new method for introducing lattice strain to graphene. In this method, we suspended graphene on hBN films with hole which was formed by reactive ion etching. And then, we pushed suspended part of graphene by hBN pole for introduction of lattice strain to graphene. From micro Raman spectroscopy, we confirmed the spatial variation of lattice strain depending on the geometry of hBN films. And we measured the modulation of transport properties.

Strain-induced semiconducting electron transport in graphene field effect devices

Strain engineering is a promising but unexplored method of inducing transport/band gaps in graphene. So far, a strain-induced band gap has been observed in scanning tunnel spectroscopy studies, while it has not been confirmed in actual field effect devices. This missing gap is presumably due to the relaxation of strain in device fabrication processes. Here, we develop a novel device fabrication method which makes graphene largely strained even in field effect devices. The electron transport in graphene with periodic uniaxial strain exhibits semiconducting behavior with a gap of 2 meV. We expect that optimization of the device structure extends the gap and improves device performance.

Effect of metal contact on electron transport and its removal in graphene field effect devices

Effect of metal contact on the transfer characteristics in graphene field effect devices is studied for several metal species. We find that two Dirac points appear in the transfer characteristics: one corresponding to the graphene portion underneath/near the contact and the other to the graphene portion far from the contact. The difference in the gate voltages of the these Dirac points correlates with the work function of the contact metal, clearly showing that the additional Dirac point originates from the carrier doping from the contact metal. This carrier doping effect is successfully removed by inserting multilayer graphene between metal and graphene, without significantly increasing the contact resistance.

Relationship between Transport Properties and Raman Spectra in Graphene Field Effect Devices

Raman spectroscopy is commonly used to characterize disorder in graphene. Increase of defects leads to raise in the intensity ratio of Raman D to G peaks for low defect densities. Defect also causes the degradation of graphene transport properties. Thus, a certain relationship is expected between the Raman spectra and transport properties in graphene. Here, we investigate Raman spectra and transport properties of graphene as a function of the amount of electron beam irradiation. We show that the carrier mean free path is inversely proportional to square root of the intensity ratio of Raman D to G peaks. Our result may pave the way for evaluating graphene transport properties with Raman spectroscopy.

Reducing Carrier Density Pinning at Graphene/Metal Interfaces Using Multi-layer Graphene

In graphene field effect transistors (FETs), as the channel length becomes shorter, the apparent field effect mobility gets smaller. This is due to the carrier injection from metal electrodes (source and drain) to graphene, which makes the carrier density at the interface insensitive to the gate voltage. This carrier density pinning at the interface is unfavorable for graphene applications to electronics. Here, we show that insertion of multilayer graphene between graphene and metal electrodes improves the apparent mobility of short graphene FETs.

Introducing Nonuniform Strain to Graphene: Toward Strain Engineering

1 Institute of Physics and Tsukuba Research Center for Interdisciplinary Materials Science (TIMS), University of Tsukuba, Tennodai, Tsukuba, Ibaraki 305-8571, Japan Phone and fax : +81-29-853-4345, E-mail: tomori@lt.px.tsukuba.ac.jp 2 CREST-JST, Kawaguchi, Saitama 332-0012, Japan 3 MANA, NIMS, Namiki, Tsukuba 305-0047, Japan 4 Nanotechnology Innovation Center, NIMS, Tsukuba, Ibaraki 305-0047, Japan

Inverse spin valve effect in multilayer graphene device

We report the gate-voltage dependence of the spin transport in multilayer graphene (MLG) studied experimentally by the local measurement. The sample consists of a Ni/MLG/Ni junction, where the thickness of the MLG is 9 nm and the spacing of two Ni electrodes is 300 nm. At zero gate voltage, we observed the normal spin valve effect, in which the resistance for the antiparallel alignment of magnetization in ferromagnetic electrodes is larger than that for the parallel alignment. By applying a large gate voltage, on the other hand, the spin valve effect is reversed: the resistance for the antiparallel alignment becomes smaller than that for the parallel alignment. The result is qualitatively interpreted as a quantum interference effect, indicating that the mean free path and the spin relaxation length of the MLG are longer than the electrode spacing (300 nm).