Fumiyuki Nihey

Chemical vapor deposition of single-wall carbon nanotubes on iron-film-coated sapphire substrates

We report on chemical vapor deposition (CVD) of single-wall carbon nanotubes (SWNTs) on sapphire substrates coated with thin Fe films. The dependence of the SWNTs growth on orientation of the crystallographic face of the sapphire substrate, i.e., defined as A-, R-, and C-faces, was investigated. It was found when the CVD was performed at 800 °C, the largest quantity of SWNTs was produced on the A-face substrate, followed by the R- and C-face substrates. This result implies that the SWNT growth can be controlled by choosing the orientation of the sapphire substrates.

Electron focusing with multiparallel one‐dimensional channels made by focused ion beam

Electron focusing effect is observed in a two‐dimensional electron gas using samples with simple multiparallel one‐dimensional channels made by a Be focused ion beam (FIB). Subharmonics and harmonics are resolved; their strengths allow a direct determination of the elastic scattering length le =1.8 μm and the specularity coefficient p=0.35 for electron reflection at the boundary defined by the FIB. The temperature dependence of the focusing effect is much weaker than the Shubnikov–de Haas effect.

Aharonov-Bohm effect in antidot structures

Abstract We have observed the Aharonov-Bohm (AB) effect in magnetoresistance of a two-dimensional electron gas with a two-dimensional array of depleted circular regions, called antidots. Fine oscillations with a period of about h/ea2 (a = period of the array) are found in the magnetoresistance peak which originates from the formation of the localized orbits encircling a single antidot. These oscillations are a manifestation of the AB effect accompanied by quantization of the localized orbits.

Purification of single-wall carbon nanotubes by using ultrafine gold particles

Abstract Ultrafine gold particles with a diameter of 20 nm were dispersed in carbonaceous soot containing single-wall carbon nanotubes (SWNTs). These gold particles catalyzed the oxidation of carbonaceous impurities at about 350°C. This selective oxidation enabled us to purify SWNTs.

Diameter‐Dependent Performance of Single‐Walled Carbon Nanotube Thin‐Film Transistors

IO N Single-walled carbon nanotubes (SWCNTs) have attracted much attention as a printable semiconducting or metallic material in various electronic devices. In particular, the network of SWCNTs fabricated by printing techniques has been investigated for the application to SWCNT thin-fi lm transistors (SWCNT-TFTs). [ 1 ] Printed SWCNT-TFTs are promising for fl exible display devices, [ 2 ] chemical sensors, [ 3 ] and so on due to their applicability to large-area devices and facility for preparation on plastic substrates. In SWCNT-TFTs, structural properties of the SWCNTs such as their length and tube diameter can considerably affect the device performance because these should dominate the dispersing characteristic and the way of forming SWCNT networks. However, the relationship between the device performance and structural properties of SWCNTs has not yet been fully explored. Recent research on the separation extracting SWCNTs with the semiconducting-type structure have shown improvement in the device performance of SWCNT-TFTs. [ 4–6 ]

A Top-Gate Carbon-Nanotube Field-Effect Transistor with a Titanium-Dioxide Insulator

We developed a carbon-nanotube field-effect transistor (CNTFET), in which a gate electrode and a gate insulator are located on top of a semiconducting single-wall carbon nanotube channel on a Si/SiO2 substrate. The gate insulator is made of titanium dioxide and is 2–3 nm thick. The transconductance of this device at a drain voltage of 100 mV is 320 nS, which, due to the high dielectric constant and nanoscale thickness of the gate insulator, is higher than that of any back-gate and top-gate CNTFETs reported so far.

Carbon-Nanotube Field-Effect Transistors with Very High Intrinsic Transconductance

We investigated intrinsic transconductance of carbon-nanotube field-effect transistors (CNTFETs) with carbon nanotubes (CNTs) grown by chemical vapor deposition. The measured transconductance at a drain voltage of -1 V was 8.7 µS for a CNT with a diameter of 1.5 nm. Very high intrinsic transconductance of 20 µS was estimated by considering the contribution of parasitic resistance. Apparent and intrinsic transconductance per unit channel width were 5800 µS/µm and 13000 µS/µm, respectively. These are considerably larger than those for the state-of-the-art Si metal oxide semiconductor field effect transistors (MOSFETs). Parasitic resistance is dominated by the resistance of the CNT portions between the gate and contacts. We expect the performance of CNTFETs will advance further by improving CNT quality and by optimizing device structures.

Horizontally directional single-wall carbon nanotubes grown by chemical vapor deposition with a local electric field

We studied the directional distribution of horizontally aligned single-wall carbon nanotubes (SWNTs) grown by thermal chemical vapor deposition (CVD) under an electric field. Electrodes formed on the sample surface were used to locally apply the electric field. We inserted spacer layers between the catalyst for the CVD and the substrate and investigated the spacer thickness dependence of the directional distribution width of the SWNTs. We found that a thicker spacer layer resulted in better alignment of the SWNTs. A sample having a 200-nm-thick spacer layer produced 7° of standard deviation along the electric field. The experimental results imply that suppression of the interaction between the substrate and the SWNT tips is important. We propose a simple model that describes this interaction during the growth. Based on this model, we explain our results and formulate a simple criterion for the experimental parameters to enable perfect controllability of the SWNTs direction.

Highly Uniform Thin-Film Transistors Printed on Flexible Plastic Films with Morphology-Controlled Carbon Nanotube Network Channels

Carbon nanotube (CNT) transistor arrays were fabricated on plastic films by printing. All the device elements were directly patterned by maskless printing without any additional patterning process, and minimum materials were used. During fabrication, the morphology of the CNT random network was controlled by an adsorption mechanism on the surface to be printed, which resulted in excellent and uniform electrical properties. The field-effect mobility was further improved by post-treatment to modify the morphology of the CNT network. These results are promising for realizing printed electronics integrated with CNT transistors.

Estimating the yield and characteristics of random network carbon nanotube transistors

The device characteristics of field-effect transistors featuring a random network of carbon nanotubes (CNTs) as a channel material were investigated by means of a numerical simulation. The simulations of the conductivity of CNTs, contact resistances, and metallic to semiconducting ratio indicated that, even with a random network composed of one-third metallic CNTs, the maximum yield rate can be dramatically increased by shortening the CNTs. The error rate can be reduced to 0.0014% when the length of CNTs is one-tenth of the source-to-drain distance. The conductance of the devices, on/off ratio, and methods to improve these characteristics are also discussed.