Yoshiharu Onuma

Carbon Fibers Obtained by Thermal Decomposition of Vaporized Hydrocarbon

Carbon fibers are successfully grown on ceramic and/or graphite substrates through thermal decomposition of vaporized hydrocarbons at temperatures from 1100°C to 1300°C. The diameter ranges from 3 to 100 µm and the length 10 to 25 cm. Appropriate conditions for the growth of such long fibers are described. Some branched and crossed structures were observed in the fibers. Morphological and structural studies are made by means of optical microscope, scanning electron microscope, and X-ray diffraction. It leads to a conclusion that these carbon fibers are composed of cylindrical layers and scrolls of thin aromatic films aligned parallel to the fiber axis. There are two kinds of fibers with smooth and rough surfaces as illustrated in the accompanying photographs. The electrical resistivity, the tensile strength and the Youngs modulus are found to be 1–2×10-3 ton-cm2, 4–34 ton/cm2 and 2–4×103 ton/cm2 respectively.

Piezoresistive Properties of Polycrystalline Silicon Thin Film

Intending to use semiconductor films for piezoresistive elements, polycrystalline silicon films are prepared by electron bombardment method on insulating substrates and electrical properties of the films are investigated. Gage factor of the film is in the vicinity of 25, temperature coefficient of resistance and gage factor is about 1.8×10-4/°C, 1.1×10-3, respectively, for the resistivity of an order of 10-3 Ω cm. Much smaller coefficients are obtained by heat treament.

Preparation of polycrystalline SiC films for sensors used at high temperature

Abstract Polyerystalline silicon carbide (SiC) films have been prepared by plasma-assisted chemical vapour deposition. Pressure sensors are fabricated using the polycrystalline SiC films as piezoresistive sensing elements to show their applicability for a pressure sensor. The elements are formed by plasma etching. The sensitivity is 2.2 mV/ (kg/cm 2 ), and the non-linearity (deviation from linear relation) is less than 1% of full scale.

Preparation and Piezoresistive Properties of Polycrystalline SnO2 Films

Tin oxide thin films were deposited by dc magnetron sputtering in a gas mixture of Ar and O2 using a target containing antimony. The films were characterized using X-ray diffraction. The films showed preferred orientation in a or plane. The properties of films depended on the substrate temperature and the gas flow ratio of Ar/O2. The piezoresistive properties of these films have been measured using a conventional cantilever method. The gauge factor was measured to be around negative 5–20 at room temperature, which is comparable to the gauge factor of polycrystalline silicon films.

Preparation of carbon nanofibers by hot filament-assisted sputtering

Abstract Crystalline carbon thin films containing carbon nanofibers were obtained by using a hot filament-assisted sputtering system. Pure argon gas was employed as the sputtering gas. The crystal structure of the carbon thin films was investigated by XRD, XPS, TEM and TED. XPS profiles show the compositions of the films, which consisted of over 90% carbon atoms. From the results of observations using TEM and TED, it was revealed that the crystalline carbon films consisted of nanocrystal grains and nanofibers. To determine the valence state (sp 3 , sp 2 and sp hybridization) of the carbon atoms in the films, laser raman spectra were examined and it was found that the films showed three Raman bands at 1581, 1368 and 1979 cm −1 . The first two peaks belong to polycrystalline graphite and the last peak corresponds to carbyne, one of the allotropes of carbon.

Preparation and properties of boron thin films

Abstract Boron thin films were deposited by pyrolysis of decaborane. The substrate was heated up to 1200 °C by infrared irradiation from a halogen lamp. The films were characterized by electron beam diffraction, XPS and electrical measurements. Spots and rings were observed in the electron beam diffraction patterns, which indicates that the films consist of α-rhombohedral boron. The activation energy of the conductivity was 0.1–0.3 eV from R.T. to 500 °C and 1.0–1.4 eV above 500 °C. This demonstrates that infrared radiation is an effective method in obtaining high quality boron films via the pyrolysis of decaborane.

Y-Ba-Cu-O Thin Films Formed on Alumina Ceramic Substrates Coated with Yttria Stabilized Zirconia Layer

Y–Ba–Cu–O thin films have been deposited by DC magnetron sputtering on alumina ceramic substrate coated with yttria stabilized zirconia layer (YSZ). The YSZ layer acted as the buffer layer between the alumina ceramic substrate and the Y–Ba–Cu–O thin film. The temperatures of superconducting onset and zero resistance were 95 K and 47 K, respectively.

Contact Resistance of SnO2 Films Determined by the Transmission Line Model Method

The specific contact resistance of SnO2 thin films was estimated by the transmission line model (TLM) method. The values were on the order of 10-6–10-4 Ωm2. The sample was annealed in N2 atmosphere at 200–650° C. The TLM measurement before and after annealing indicated that annealing was not effective in reducing the contact resistance. The high-temperature annealing resulted in the increase in both the specific contact resistance and the sheet resistance of the SnO2 thin film.

Thermoelectric power of polycrystalline Si films prepared by microwave plasma chemical vapour deposition

The thermoelectric properties of silicon (Si) thin films prepared by microwave plasma chemical vapour deposition were studied. For preparation, monosilane (SiH4) was used as the source gas, and phosphine (PH 3 ) and diborane (B 2 H 6 ) were used as the doping gases. X-ray diffraction shows that Si films were polycrystalline and that the average grain size of these films was approximately 100 nm. Measurement of the thermoelectromotive force between Si films and the counter electrodes revealed that the thermoelectric power depends on the flow rate of the doping gas, and that the Si films prepared at the B 2 H 6 /SiH 4 flow rate of 0.05% showed a maximum thermoelectric power of approximately 0.9 mV K -1 .

Some Investigations on Vacuum-Evaporated Al-CdSe Thin Film Diodes

The Schottky barrier formed by deposition of CdSe on Al has been examined by temperature-dependence measurements of the current-voltage (I-V) characteristics and capacitance-voltage (C-V) characteristics. The failure to determine the built–in potential Vd from the simple Schottky approximation can be remedied by assuming two regions of donor concentration in the space-charged region. The value of Vd is 0.2 V. The current transport mechanism is due to the tunneling current arising from field emission.