Takefumi Ishikura

Enhancement/depletion MESFETs of diamond and their logic circuits

Abstract Using the p-type surface-conductive layer of diamond film, enhancement mode and depletion mode MESFETs have been fabricated by changing the metals of the gate electrode. The threshold voltages of MESFETs depend on the electronegativity of the metals. A MESFET with a Cr gate was operated in depletion mode and exhibited a peak transconductance of 12.3 mS mm −1 , which is the highest in diamond FETs. This high performance can be obtained by a self-aligned gate fabrication process, which minimizes the spacing between the ohmic contacts and the Schottky contact. By utilizing an E-MESFET for the driver and a D-MESFET for the active load, E/D-type logic circuits such as NOT, NAND and NOR circuits have been fabricated for the first time.

13C NMR study of 13C-enriched single-wall carbon nanotubes synthesized by catalytic decomposition of methane

Abstract 13 C NMR spectra and spin-lattice relaxation times were measured for single-wall carbon nanotubes with 99.9 and 50.0% 13 C enrichments and natural abundance (1.1% 13 C) prepared by catalytic decomposition of CH 4 . The 13 C isotropic shift is about 116 ppm from tetramethylsilane, being estimated from magic-angle-spinning (MAS) spectra. The value does not depend on the degree of the 13 C enrichment. The 13 C MAS NMR spectra show two additional small peaks at 171 and 152 ppm, which are ascribed to carbon species at defects or edges. The line widths of the main isotropic peak in MAS spectra are about 30 ppm, the origin of which is mostly chemical shift dispersion, reflecting a distribution of diameter and helicity. The line width in the 13 C static spectra originates from chemical shift dispersion, chemical shift anisotropy and dipole–dipole interactions between 13 C spins as well as between 13 C and 1 H spins at defects or edges. 1 H NMR spectra confirm the presence of H-containing species. The 13 C spin-lattice relaxation is dominated presumably by interaction with magnetic impurities.

Efficient Free-Exciton Recombination Emission from Diamond Diode at Room Temperature

Free-exciton recombination emission of 235 nm in wavelength is obtained by current injection at room temperature from a diamond-based pn junction diode composed of B-doped crystal grown by high-temperature, high-pressure synthesis and a S-doped homoepitaxial layer grown by the chemical vapor deposition method. The diode shows a clear rectification characteristic and a high external quantum efficiency of excitonic emission, 8×10-5, which indicates that the excitonic emission of diamond is a good candidate for application to semiconductor UV-light-emitting devices. A defect-induced light emission and large leakage current indicate that a higher UV emission efficiency is expected with improvement of the junction quality.

Field emission and structure of aligned carbon nanofibers deposited by ECR-CVD plasma method

Abstract Aligned carbon nanofibers and hollow carbon nanofibers were grown by MW ECR-CVD method using methane and argon mixture gas at a temperature of 550°C. The carbon nanofibers and the hollow carbon nanofibers were deposited perpendicularly on Si substrates and on Si substrates coated with Ni catalyst, respectively. From TEM analysis the diameter and length of the nanofibers are approximately 60 nm and 15 μm, respectively. Raman spectra of these aligned carbon nanofibers showed new bands of 1340 and 1612 cm−1 of the first-order Raman scattering and 2660, 2940 and 3220 cm−1 of the second-order Raman scattering. The second-order Raman scattering bands were assigned to two overtone and one combination bands on the basis of a similar assignment of micro-crystal graphite by Nemanich and Solin. By the measurement of XPS C1s band energies of 284.6 eV for the carbon nanofiber and 284.7 eV for the hollow carbon nanofiber indicate mainly sp2 carbon component in the inclusion of a small amount (

Cathodoluminescence and Hall-effect measurements in sulfur-doped chemical-vapor-deposited diamond

Dominant n-type conductivity in sulfur-doped chemical-vapor-deposited diamond is observed by Hall-effect measurement. The activation energy is estimated at 0.5–0.75 eV above 600 K. Below 600 K, the carrier concentration deviates from the activation energy, and Hall mobility decreases in comparison with that above 600 K. It is considered that hopping conduction takes place. By cathodoluminescence measurement, free-exciton recombination radiation is observed in spite of a very high sulfur doping level of 2.5% during deposition, where boron is not detected by secondary ion mass spectroscopy. Therefore, the n-type conductivity of sulfur-doped diamond is caused by a sulfur-related mechanism.

Diamond Grain Growth on Cu Substrate

Diamond growth on Cu substrates with vapor phase deposition has been studied. Diamond was deposited on Cu substrates using an electron cyclotron resonance chemical vapor deposition apparatus. Its growth process was observed through morphological variation with time in the same field of vision, which showed that diamond grains on Cu substrates grew not only through increase of the size of grains but also through their combination, and that isolated grains at an early stage of growth migrated on the substrate as if they were attracted by each other. This manner of diamond growth is specific to cases where Cu is used as the substrate.

Low-Pressure Diamond Nucleation and Growth on Cu Substrate

We present a new technique for providing nuclei for diamond formation on nondiamond substrates and its application to the growth of diamond on a Cu substrate by chemical vapor deposition (CVD). This is a predeposition process in which the substrate is immersed in a CH4/H2 plasma formed by electron cyclotron resonance at a low pressure (0.1 Torr). The technique provides possibilities of nucleation over an increased area at temperatures lower (about 500°C) than usual, as well as improved process controllability. The grown diamonds on Cu exhibit a morphology significantly different from that of diamonds grown on Si.