Tomohiko Yamakami

An inorganic anionic polymer filter disc: direct crystallization of a layered silicate nanosheet on a glass fiber filter

An inorganic anionic polymer filter disc was successfully produced from heterogeneous nucleation reactions of layered silicate fine crystals on amorphous silica glass fibers. This hybrid material can be obtained only by immersing the silica fiber filter disc in an aqueous solution containing LiF, MgCl2, and urea at 373 K for 48 h. Silica sol, which was partially dissolved from the silica fibers by hydrolysis of urea, was a source of the layered silicate. Firm immobilization of the layered silicate on the fiber silica was confirmed by immersing in aqueous LiCl solution (12 mM) for three weeks. The layered silicate crystals evenly covered the silica fibers while maintaining the original filter disc shape. Careful design of the layered silicate was performed by changing the molar LiF : MgCl2 : urea : SiO2 ratio. The layered silicate on the fibers became thick on adding increasing amounts of LiF and MgCl2. In addition, by increasing the amount of LiF, the negative charge density of the layered silicate increased, following the trend of caffeine adsorption in water. Furthermore, we found that the amount of urea added to the starting solution was important for preventing a loss in the mechanical strength of the fibers (e.g., fracture due to unnecessary dissolution of the silica fiber) and emerging side-reactions (e.g., polymerization of silica sol to yield spherical silica particles). The resulting layers of silicate on the filter disc acted as adsorption sites for both organic (methylene blue and benzylammonium) and inorganic (sodium, calcium, and europium) cationic species in water, as exemplified by batch and flow tests.

Characterization of Nitride Layer on 6H-SiC Prepared by High-Temperature Nitridation in NH3

The nitride layers were prepared by direct thermal nitridation of 6H-SiC substrates at 1200–1570°C in a NH3 atmosphere. The layer was characterized by using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and Raman scattering spectroscopy. The thickness of the nitride layers prepared at lower than 1400°C was estimated to be less than 10 nm. The higher nitridation temperature resulted in the formation of a thicker surface layer. XPS measurement showed that the surface layer was composed of N, Si, C and O. Peaks corresponding to α-Si3N4 were detected in the Raman spectra and the XRD patterns of the sample prepared at higher than 1500°C, indicating the crystallization of the nitrided layer.

Characterization of Metal–Insulator–Semicomductor Capacitors with Insulating Nitride Films Grown on 4H-SiC

Nitrided layers were grown on a 4H-SiC(0001) by plasma nitridation method using NH3. Nitridation was enhanced with increasing RF power and with decreasing growth pressure. However, the exact capacitance–voltage (C–V) properties of the nitride layer/SiC interface could not be determined because of the leakage current. The SiO2 film was deposited on the nitrided layer by thermal chemical vapor deposition method using tetraethoxysilane (TEOS) omit obtain an insulating film with sufficient thickness and an exact interface property. The interface state density Dit was evaluated from C–V characteristics by the Terman method. It was indicated that Dit near the mid gap of the TEOS oxide/nitride layer structure was higher than those of the TEOS–SiO2 films and thermal oxide film. The Dit of the oxide/nitride layer successfully decreased by post NH3 annealing.

Preparation of Carbon Films by Hot-Filament-Assisted Sputtering for Field Emission Cathode

Carbon thin films on a silicon substrate were prepared by DC magnetron sputtering method with a tungsten hot filament. In order to investigate the effects of the hot filament on film properties, the carbon thin films were characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM) and electron emission measurements. The tungsten atoms were evaporated from the hot filament and incorporated in the carbon film. The field emission measurement showed that the incorporation of tungsten was effective in reducing the turn-on voltage. The value of the turn-on voltage was 3.0 V/µm for the sample prepared with a tungsten filament heated at the temperature of 2000°C. The intentional insertion of a tungsten layer between the carbon film and the substrate was effective in obtaining a carbon film with a low turn-on voltage for the field emission.

Effect of Series Resistance on Field Emission Current

A floating sphere model was used to estimate the effect of series resistance on the electric field distribution at the top of a field emission site. A reduction in field strength was caused by field reduction, not potential drop. Series resistance is considered to be one of the important reasons behind the current saturation in field emission characteristics.

Growth and Characterization of SiC Films by Hot-Wire Chemical Vapor Deposition at Low Substrate Temperature Using SiF4/CH4/H2 Mixture

Microcrystalline SiC films were grown by hot-wire chemical vapor deposition (HW-CVD) at a low substrate temperature using a SiF4/CH4/H2 mixture, and their structural properties were characterized. Low growth pressure resulted in a narrow full width at half maximum (FWHM) of the Si–C peak in Fourier transform infrared absorption spectroscopy (FT-IR) spectra. The deposition rate and the FWHM value increased with increasing filament temperature. This seemed to be caused by change of the concentration ratio of precursors on the growth surface with increasing filament temperature. Also, the film crystallinity depended on the CH4/SiF4 flow rate ratio, and µc-3C-SiC(111) films were successfully obtained at low substrate temperature of 250 °C.

Characterization of Al-Based Insulating Films Fabricated by Physical Vapor Deposition

A novel submount substrate with high thermal conductivity for optoelectronic devices is proposed. The substrate is fabricated by depositing an Al-based insulating film on a copper substrate. AlN films were deposited by RF reactive sputtering using an Al target (5N) in a gas mixture of Ar and N2. Al2O3 films were deposited by oxygen-ion-assisted electron beam (EB) evaporation. Many conductive paths were detected in the AlN films. These defects were introduced in the AlN film during a photolithography process for fabricating electrode patterns because the alkaline developer dissolved the film. An Al–OH peak in Fourier transform infrared spectroscopy (FT-IR) spectra suggested that the Al(OH)3 formation was one of the reasons for the presence of conductive paths in the AlN film. In the case of Al2O3 films, no conducting path was detected in electrical measurements, and no marked change in surface morphology was observed in scanning electron microscopy (SEM) images, after treatment with the alkaline developer. The Al2O3/Cu structure is a candidate for the novel submount substrate with high thermal conductivity.

Deposition of apatite on carbon nanofibers in simulated body fluid

Carbon nanofibers (CNFs) were soaked in 1.5 simulated body fluid (1.5 SBF) in which inorganic ion concentrations are 1.5 times as high as those in the standard SBF. The influence of the CNFs content in 1.5 SBF and pretreatment of the CNFs on the biomimetical deposition of apatite were investigated. The spherical bone-like apatite particles were deposited on the pristine CNFs soaked in 1.5 SBF. Amount of deposited apatite per a unit of CNFs increased with a decrease in the CNFs content in 1.5 SBF, and it decreased markedly when the CNFs were pretreated with concentrated sulfuric acid/nitric acid (3: 1 v/v) mixture for longer periods. Such results suggest that too many nucleation sites of apatite, which were functional groups, such as carboxyl and hydroxyl groups, existed on the CNFs in the 1.5 SBF, and most embryos formed on the sites could not grow to critical nuclei and furthermore did not grow to apatite.

Plasma Nitridation of 4H-SiC by Glow Discharge of N2/H2 Mixed Gases

The mixed gas of nitrogen and hydrogen was used for the plasma nitridation of SiC surface.A small amount of hydrogen was effective to activate the nitridation reaction and suppress the oxidationreaction. The interface properties were improved by using nitride layer as an interfacial bufferlayer of SiC MIS structure.

Preparation and Characterization of Nitridation Layer on 4H SiC (0001) Surface by Direct Plasma Nitridation

A nitride layer was formed on a SiC surface by plasma nitridation using pure nitrogen as the reaction gas at the temperature from 800°C to 1400°C. The surface was characterized by XPS. The XPS measurement showed that an oxinitride layer was formed on the SiC surface by the plasma nitridation. The high process temperature seemed to be effective to activate the niridation reaction. A SiO2 film was deposited on the nitridation layer to form SiO2/nitride/SiC structure. The interface state density of the SiO2/nitride/SiC structure was lower than that of the SiO2/SiC structure. This suggested that the nitridation was effective to improve the interface property.