Naoki Yokoyama

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.

Electrostatically-reversible polarity of dual-gated graphene transistors with He ion irradiated channel: Toward reconfigurable CMOS applications

We found that a transistor with a graphene channel irradiated with He ion beams can have a transport gap of up to 380 meV. We made novel dual-gated transistors using such a channel and obtained an on-off ratio up to 103 at 200 K. This novel device has a channel region between dual gates, and the polarity of the transistor (n- or p-type) can be electrostatically reversed by simply flipping the bias polarity of one of the dual gates.

Sub-10-nm-wide intercalated multi-layer graphene interconnects with low resistivity

We fabricated sub-10-nm-wide intercalated multi-layer graphene (MLG) interconnects and demonstrated a resistivity lower than that of cupper (Cu) interconnects with similar dimensions. The high-quality MLG was synthesized epitaxially by chemical vapor deposition on an epitaxial cobalt film, and intercalated with FeCl3. After narrowing down the width to 8 nm by electron beam lithography, the 8-nm-wide intercalated MLG exhibited a resistivity of 3.2 μΩcm, which is predicted to be lower than that of Cu interconnects with the same dimensions. Our results show that intercalated MLG is really promising for future LSI interconnects.

Monte Carlo simulation of electron transport in a graphene diode with a linear energy band dispersion

Electron transport and energy relaxation in a 100-nm channel n+-n-n+monolayergraphene diode were studied by using semiclassical Monte Carlo particle simulations. A diode with a conventional parabolic band and an identical geometry and scattering process was also analyzed in an attempt to confirm that the characteristictransport properties originated from the linear energy band structure. We took into account two scattering mechanisms: isotropic elasticscattering and inelastic phonon emission. The carrier velocity distributions in the two diodes show remarkable differences reflecting their band dispersions. Electron velocity in the monolayergraphene diode is high in the channel region and remains almost constant until the energy relaxation begins. Inelastic scattering does not reduce electron velocity so severely, whereas elasticscattering significantly decreases it through backscattering of hot electrons with high kinetic energy. Elasticscattering also degrades the ballisticity and the drain current; however, increasing the inelastic scattering offsets these effects. We found that elasticscattering should be suppressed to improve the performance of graphene devices.

Low-temperature catalyst activator: mechanism of dense carbon nanotube forest growth studied using synchrotron radiation.

The mechanism of dense vertically aligned carbon nanotube growth achieved by a recently developed thermal chemical vapor deposition method was studied using synchrotron radiation spectroscopic techniques.

Electrostatically Reversible Polarity of Dual-Gated Graphene Transistors

We developed dual-gated graphene transistors in which the transistor polarity (n-type or p-type) is electrostatically reversible by the gate bias of one of the top gates. In this device, a channel is defined as the region between a pair of top gates, where graphene is irradiated by an accelerated helium ion beam to form a defect-induced transport gap. This device features not only a large current ON-OFF ratio of four orders of magnitude but also unipolarity of transistors, which would otherwise be ambipolar. We also show how these polarity-reversible transistors can be used in logic circuits.

Intercalated multi-layer graphene grown by CVD for LSI interconnects

We have fabricated multi-layer graphene (MLG) wiring and demonstrated a resistivity of the same order as Cu and reliability better than Cu. The MLG was synthesized epitaxially by chemical vapor deposition (CVD) on an epitaxial Co film, resulting in quality and electrical properties as good as those of a graphite crystal. The MLG was further intercalated with FeCl3 to achieve a resistivity as low as 9.1 μΩ cm. Our results show that intercalated MLG is really promising for future LSI interconnects.

Wafer-scale fabrication of transistors using CVD-grown graphene and its application to inverter circuit

Graphene transistors were fabricated by a wafer-scale top-down process using a graphene sheet formed by the chemical vapor deposition (CVD) method. The devices have a dual-gated structure with an ion-irradiated channel, in which transistor polarity can be electrostatically controlled. We demonstrated, at room temperature, an on/off operation of current and electrostatic control of transistor polarity. By combining two dual-gated transistors, a six-terminal device was fabricated with three top gates and two ion-irradiated channels. In this device, we demonstrated an inverter operation.

Intercalated multilayer graphene wires and metal/multilayer graphene hybrid wires obtained by annealing sputtered amorphous carbon

We have fabricated an intercalated multilayer graphene (i-MLG) wire and a cobalt metal/multilayer graphene (Co/MLG) hybrid wire with decreased resistivity relative to a multilayer graphene (MLG) wire by annealing sputtered amorphous carbon. The resistances of the i-MLG and Co/MLG wires decrease by 1 and 2 orders of magnitude relative to that of the MLG wire, respectively. The breakdown tests of the i-MLG and MLG wires indicate that the current tolerance per graphene layer remains almost the same, independent of whether the wires are intercalated. On the other hand, the Co/MLG hybrid wire exhibits not only decreased resistivity but also increased current tolerance. Moreover, with the optimization of the film thickness and annealing temperature, the obtained resistivities of Co/MLG, MLG, and i-MLG are 10, 80, and 6 µΩ cm, respectively.

Evaluation of thermal resistance of carbon nanotube film fabricated using an improved slope control of temperature profile growth

We have successfully improved the weight density of a 40-µm-long carbon nanotube (CNT) to 0.27 g/cm3 by improving the slope control of temperature profile (STEP) growth. It was found that the CNT growth is reaction-limited in the early stage of STEP growth and supply-limited in the late stage. We succeeded in doubling the CNT density over the conventional method by optimizing the raw material supply and temperature slope during the reaction-limited and supply-limited periods. A CNT thermal interface material was fabricated using a CNT film prepared by this method. Thermal resistance was 11% below a conventional indium thermal interface material. Owing to the above reasons, CNTs show promise as heat dissipation materials.