Hisao Miyazaki

Origin of the relatively low transport mobility of graphene grown through chemical vapor deposition

The reasons for the relatively low transport mobility of graphene grown through chemical vapor deposition (CVD-G), which include point defect, surface contamination, and line defect, were analyzed in the current study. A series of control experiments demonstrated that the determinant factor for the low transport mobility of CVD-G did not arise from point defects or surface contaminations, but stemmed from line defects induced by grain boundaries. Electron microscopies characterized the presence of grain boundaries and indicated the polycrystalline nature of the CVD-G. Field-effect transistors based on CVD-G without the grain boundary obtained a transport mobility comparative to that of Kish graphene, which directly indicated the detrimental effect of grain boundaries. The effect of grain boundary on transport mobility was qualitatively explained using a potential barrier model. Furthermore, the conduction mechanism of CVD-G was also investigated using the temperature dependence measurements. This study can help understand the intrinsic transport features of CVD-G.

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.

Determination of the Number of Graphene Layers: Discrete Distribution of the Secondary Electron Intensity Stemming from Individual Graphene Layers

Using a scanning electron microscope, we observed a reproducible, discrete distribution of secondary electron intensity stemming from an atomically thick graphene film on a thick insulating substrate. We found a distinct linear relationship between the relative secondary electron intensity from graphene and the number of layers, provided that a low primary electron acceleration voltage was used. Based on these observations, we propose a practical method to determine the number of graphene layers in a sample. This method is superior to the conventional optical method in terms of its capability to characterize graphene samples with sub-micrometer squares in area on various insulating substrates.

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.

Volatile/Nonvolatile Dual-Functional Atom Transistor

We demonstrate a conceptually new atom transistor operation by electric-field control of the nanoionic state. The new atom transistor possesses novel characteristics, such as dual functionality of selective volatile and nonvolatile operations, very small power consumption (pW), and a high ON/OFF ratio [106 (volatile operation) to 108 (nonvolatile operation)], in addition to complementary metal oxide semiconductor (CMOS) process compatibility enabling the development of future computing systems that fully utilize highly-integrated CMOS technology. Cyclic endurance of 104 times switching was achieved with the prototype.

Influence of electrode size on resistance switching effect in nanogap junctions

The size dependence of the resistance switching effect in nanogap junctions was investigated to determine the nature of the local structural changes responsible for the effect. The maximum current, during resistance switching, decreased with the total emission area across the nanogap to an average of 146u2002μA at a linewidth of 45 nm. This implies that the resistance switching effect stems from changes in the gap width at multiple local sites on the metal surface.

Gate-Controlled P–I–N Junction Switching Device with Graphene Nanoribbon

A graphene P–I–N junction switching device with a nanoribbon is proposed, which was aimed at finding an optimized operation scheme for graphene transistors. The device has two bulk graphene regions where the carrier type is electrostatically controlled by a top gate, and these two regions are separated by a nanoribbon that works as an insulator, resulting in a junction configuration of (P or N)–I–(P or N). It is demonstrated that the drain current modulation strongly depends on the junction configuration, while the nanoribbon is not directly top-gated, and that the device with a P–I–N or N–I–P junction can exhibit better switching properties.

Unipolar transport in bilayer graphene controlled by multiple p-n interfaces

Unipolar transport is demonstrated in a bilayer graphene with a series of p-n junctions and is controlled by electrostatic biasing by a comb-shaped top gate. The OFF state is induced by multiple barriers in the p-n junctions, where the band gap is generated by applying a perpendicular electric field to the bilayer graphene, and the ON state is induced by the p-p or n-n configurations of the junctions. As the number of the junction increases, current suppression in the OFF state is pronounced. The multiple p-n junctions also realize the saturation of the drain current under relatively high source-drain voltages.

Toward sub-20 nm hybrid nanofabrication by combining the molecular ruler method and electron beam lithography

It is of great interest and importance to develop new nanofabrication processes to fabricate sub-20 nm structures with sub-2 nm resolution for next-generation nanoelectronic devices. A combination of electron beam lithography (EBL) and a molecular ruler is one of the promising methods to make these fine structures. Here we successfully develop a hybrid method to fabricate sub-20 nm nanogap devices at the desired positions with a complex structure by developing a post-EBL process, which enabled us to avoid damaging the molecular ruler with the high-energy electron beam, and to fully utilize the EBL resolution. It was found that slight etching of the Ti adhesion layer of the parent metal (Pt) by ACT935J solution assisted the removal of molecular rulers, resulting in improved enhancement in the product yield (over 70%) of nanogap devices.