Hidenori Gamo

Liquid-Phase Deposition of Aligned Carbon Nanotubes Using Cobalt Catalyst

We have recently developed a novel catalytic method for synthesizing a wide variety of carbon nanomaterials in the organic liquid. The method enabled us to realize a simple, rapid, and high-purity growth of carbon nanotubes (CNTs) in alcohol liquids. In this study, cobalt (Co) was used as a catalyst metal. In order to control the structure of carbon nanomaterials, we investigated the relationship between the growth conditions and the grown materials. Scanning electron microscopy (SEM) observation revealed that the morphology of the grown carbon nanomaterials strongly depended on the reaction temperature. Under the reaction temperature in the range from 873 to 973 K, fibriform deposits were mainly obtained. Transmission electron microscopy (TEM) revealed that the fibriform deposits were CNTs. The amount of the supported Co catalyst affected the fine tubular structure of the CNTs. We found that the existence of the reaction temperature of 873 K during the reaction time was essential for growing a fibriform structure in this study. The longer duration time for the reaction temperature of 1127 K resulted in a higher crystal quality for CNTs. We also demonstrated that the Co catalyst thermal oxidation at 1173 K resulted in the growth of aligned CNTs with the higher density.

Ultrafine Patterning of Nanocrystalline Diamond Films Grown by Microwave Plasma-Assisted Chemical Vapor Deposition

The growth of nanocrystalline diamond films having an appropriate surface smoothness for nanoscaled ultrafine patterning was investigated. The surface smoothness of polycrystalline diamond films was controlled by introducing nitrogen (N2) gas at concentrations ranging from 0 to 1.0% into the gas mixtures of methane (CH4) and hydrogen (H2) on microwave plasma-assisted chemical vapor deposition (MPCVD). The added 1.0% N2 with 10% CH4 in the gas phase yielded the desired diamond film with the smoothest surface among the conditions. An N-doped nanocrystalline diamond film was fabricated by nanoscaled ultrafine patterning by e-beam lithography followed by reactive ion etching (RIE). As a mask material in RIE, we found that a chemical vapor-deposited amorphous silicon nitride film is appropriate. A mixture consisting of oxygen and a small amount of tetrafluoro carbon was used as the etching gas. We succeeded in achieving a minimum line width of 100 nm in the N-doped nanocrystalline diamond film with our fabrication process.

Surface potential change by oxidation of the chemical vapor deposited diamond (001) surface

We have investigated the surface potential change accompanying the variation in diamond surface chemisorbed structures using scanning Maxwell-stress microscopy. The hydrogen chemisorbed diamond surface was prepared by a hydrogen plasma treatment following the homoepitaxial (001) diamond growth by the microwave plasma-assisted chemical vapor deposition (MPCVD). The surface chemisorbed structure was controlled by increasing the oxidized temperature in the range from 100 °C to 500 °C. The hydrogen chemisorbed diamond surface showed a minimum work function value of 4.8 eV. A thermal oxidation of the hydrogen chemisorbed diamond surface at 500 °C yielded the oxygen chemisorbed diamond surface, which showed a maximum work function value of 5.8 eV. The surface potential value, which corresponds to the surface work function value, was strongly influenced by the species of the chemisorption on the diamond surface.

Fabrication of Field Emitter Arrays with Hydrogenated Amorphous Silicon on Glass

An n-type hydrogenated amorphous silicon ( n+-a-Si:H) field emitter array (FEA) was fabricated on a glass substrate and characterized. The n+-a-Si:H film was deposited by a conventional plasma-enhanced-chemical-vapor-deposition (PECVD) technique using a silane and phosphine mixed gas at a temperature lower than 350° C. The present film showed a low resistivity of 50 Ω cm. The FEA consists of 1-µm-high emitter tips and a gate electrode with 1.8-µm-diameter openings. Using the fabricated FEA (25-tips), an emission current of 11 µ A was obtained at a gate voltage of 100 V.

Composition and Bonding Properties of Carbon Nitride Films Grown by Electrochemical Deposition Using Acrylonitrile Liquid

Carbon nitride films were grown electrochemically through the application of a bias voltage to Si substrates immersed in acrylonitrile liquid. Continuous and uniform films were grown by the application of both negative and positive bias voltages. X-ray photoelectron spectra (XPS) of the grown films were studied to clarify their composition and bonding configurations. The XPS spectra show the presence of C, N, and O atoms as major components in the grown films. The atomic ratios of nitrogen to carbon in the films are estimated as 0.16–0.28, which are comparable to those in amorphous carbon nitride (a-CNx) films grown by conventional vapor deposition techniques. The analysis of C 1s and N 1s spectra reveals that the major bonding states of the grown films are attributed to a mixture of C≡N and hydrogenated C=N bonds.

Surface conductivity change by oxidation of the homoepitaxially grown diamond (100) surface

We have investigated the electrical conductivity change that occurs by oxidation of a homoepitaxially grown diamond (100) surface. For this purpose, atomically flat diamond (100) surfaces homoepitaxially grown by a microwave plasma-assisted chemical vapor deposition (MPCVD) were prepared. It is well-known that a CVD-grown diamond surface is a hydrogen-chemisorbed structure. Surface chemisorbed structures of diamond can be controlled by oxidation in air at temperatures from 25 °C up to 350 °C. The degree of oxygen-chemisorption on the diamond surfaces was analyzed by X-ray photoelectron spectroscopy (XPS). The electrical resistivity (or conductivity) of the diamond surfaces was measured at room temperature under nitrogen flow by using the van der Pauw four-probe system. The surface resistivity of diamond was drastically increased at 300-350 °C, a temperature range corresponding to the beginning point of surface oxidation of a diamond analyzed by the XPS.

Fabrication of a New Field Emitter Array with a Built-in Thin-Film Transistor on Glass

We have designed and fabricated a new amorphous-silicon (a-Si) field emitter array (FEA) monolithically integrated with a thin-film transistor (TFT) on glass. In this FEA, a TFT was made of a-Si film deposited on a glass substrate with a chromium insertion electrode. A-Si emitter tips were formed on the drain of the TFT. The gate was separated into two parts: one was biased to about 140 V and used as an extraction gate for field electron emission, and the other was used as a conventional TFT gate to control the drain current, that is, the emission current. From the experimental results obtained using a 1000-tip FEA, the emission characteristics were found to be well controlled and stabilized by the TFT. The emission current of approximately 0.05 µA and with fluctuations of less than 1% was achieved at the extraction and the TFT gate voltages of 140 V and 30 V, respectively.

Fabrication of Petal-Shaped Vertical Field Emitter Arrays

We have invented a novel field emitter array (FEA) with a petal-shaped design. The emitter has a metal-disk fitted into a silicon (111) pyramidal hole ; viewed from an angle it resembles the petal of a flower. We have elucidated the relationships between the detailed structure and the emission characteristics and have achieved the threshold voltage of 60 V, the emission current of 20 μA at the gate voltage of 150 V and the anode current fraction of 90% in this new structure. We could fabricate the present FEA as easily as the disk-shaped lateral FEA, and it gives a much higher anode current fraction than does the latter. We have also investigated the improvement of the surface condition of the emitter, and obtained the lowest threshold voltage of 17 V with metal-coated FEAs.

Field Emission Characteristics of Well-Aligned Carbon Nanotubes Synthesized in Organic Liquids

We have characterized the field electron emission from well-aligned carbon nanotubes (CNTs) synthesized in methanol using cobalt (Co) as a catalyst on silicon (Si) substrates. In order to prepare CNTs aligned vertically to the substrate, the physicochemical state of the Co catalyst was controlled by annealing in air at a temperature of 900 °C as a thermal preoxidation prior to the liquid-phase growth. For the CNT growth, the Co-loaded Si substrate was heated by applying a direct current through the substrate in methanol. Vertically well-aligned CNTs of different crystallinities were synthesized at temperatures of 600 and 900 °C. Field electron emission characteristics of the synthesized well-aligned CNTs were measured using diode-type electrodes in a high-vacuum chamber. A low-threshold electric field of approximately 0.9 Vµm-1 and a very stable emission were observed in the CNTs with high crystallinity grown at a temperature of 900 °C.

Fabrication and Characterization of Cross-Edge-Structured Vertical Field Emitter Arrays

New field emitter arrays (FEAs), called cross-edge-structured vertical FEAs, were fabricated and characterized. Chromium (Cr) emitters and molybdenum/Cr emitters were tested. The anode-current fraction was 50%, being improved in comparison with that of disk-edge FEAs. Threshold voltage of the anode current was a low voltage of 25 V. From the light emission pattern, divergent angle was estimated to be 9°.