||Masafumi Inaba, Takumi Ochiai, Kazuyoshi Ohara, Ryogo Kato, Tasuku Maki, Toshiyuki Ohashi, Hiroshi Kawarada, Correlation between the Carbon Nanotube Growth Rate and Byproducts in Antenna-Type Remote Plasma Chemical Vapor Deposition Observed by Vacuum Ultraviolet Absorption Spectroscopy, Small, 10.1002/smll.201901504, 15, 48, 2019.11, For sp2 or sp3 carbon material growth, it is important to investigate the precursors or intermediates just before growth. In this study, the density of ethylene (C2H4) outside the plasma discharge space and just before reaching the carbon nanotube (CNT) growth region is investigated by vacuum ultraviolet absorption spectroscopy for plasma discharge in an antenna-type remote plasma chemical vapor deposition with a CH4/H2 system, with which the growth of very long (≈0.5 cm) CNT forests is achieved. Single-wall CNT forests have the potential for application as electrodes in battery cells, vertical wiring for high current applications, and thermal interface materials. It is observed that the plasma discharge decomposes the CH4 source gas and forms C2Hx species, which reversibly reform to C2H4 in the plasma-off state. In addition, the density of the formed C2H4 has a strong correlation to the CNT growth rate. Therefore, the C2H4 density is a good indicator of the density of C2Hx species for CNT growth in the CH4/H2 plasma system..
||Masafumi Inaba, Hiroshi Kawarada, Yutaka Ohno, Electrical property measurement of two-dimensional hole-gas layer on hydrogen-terminated diamond surface in vacuum-gap-gate structure, Applied Physics Letters, 10.1063/1.5099395, 114, 25, 2019.06, Hydrogen-terminated diamond metal-insulator-semiconductor field-effect transistors are candidates for power devices that require a high breakdown field and stable, high-frequency operation. A two-dimensional hole-gas layer can form on H-terminated diamond surfaces. To understand the electrical properties of bare H-terminated diamond surfaces, we investigate the surface impurities on a H-terminated diamond surface in a vacuum-gap gate structure, which uses a H-terminated diamond channel and a vacuum gap as gate dielectrics. To obtain a bare surface without surface adsorbate, the device is annealed in a vacuum. The transconductance is increased by removing adsorbates. The mobility and interface-state density at the H-terminated diamond surface with no adsorbates are 25 cm^2 V^-1 s^-1 and 1 × 10^12 cm^-2 eV^-1, respectively..
||Nobutaka Oi, Masafumi Inaba, Satoshi Okubo, Ikuto Tsuyuzaki, Taisuke Kageura, Shinobu Onoda, Atsushi Hiraiwa, Hiroshi Kawarada, Vertical-type two-dimensional hole gas diamond metal oxide semiconductor field-effect transistors, Scientific reports, 10.1038/s41598-018-28837-5, 8, 1, 2018.12, Power semiconductor devices require low on-resistivity and high breakdown voltages simultaneously. Vertical-type metal-oxide-semiconductor field-effect transistors (MOSFETs) meet these requirements, but have been incompleteness in diamond. Here we show vertical-type p-channel diamond MOSFETs with trench structures and drain current densities equivalent to those of n-channel wide bandgap devices for complementary inverters. We use two-dimensional hole gases induced by atomic layer deposited Al2O3 for the channel and drift layers, irrespective of their crystal orientations. The source and gate are on the planar surface, the drift layer is mainly on the sidewall and the drain is the p+ substrate. The maximum drain current density exceeds 200 mA mm^-1 at a 12 μm source-drain distance. On/off ratios of over eight orders of magnitude are demonstrated and the drain current reaches the lower measurement limit in the off-state at room temperature using a nitrogen-doped n-type blocking layer formed using ion implantation and epitaxial growth..
||Masafumi Inaba, Kazuyoshi Ohara, Megumi Shibuya, Takumi Ochiai, Daisuke Yokoyama, Wataru Norimatsu, Michiko Kusunoki, Hiroshi Kawarada, Electrical contact properties between carbon nanotube ends and a conductive atomic force microscope tip, Journal of Applied Physics, 10.1063/1.5027849, 123, 24, 2018.06, Understanding the electrical contact properties of carbon nanotube (CNT) ends is important to use the high conductance of CNTs in the CNT on-axis direction in applications such as through-silicon via structures. In this study, we experimentally evaluated the contact resistivity between single-/multi-walled CNT ends and a metal nanoprobe using conductive atomic force microscopy (C-AFM). To validate the measured end contact resistivity, we compared our experimentally determined value with that obtained from numerical calculations and reported values for side contact resistivity. The contact resistivity normalized by the length of the CNT ends was 0.6-2.4 × 10^6 Ω nm for single-walled CNTs. This range is 1-2 orders of magnitude higher than that determined theoretically. The contact resistivity of a single-walled CNT end with metal normalized by the contact area was 2-3 orders of magnitude lower than that reported for the resistivity of a CNT sidewall/metal contact. For multi-walled CNTs, the measured contact resistivity was one order of magnitude higher than that of a CNT forest grown by remote plasma-enhanced chemical vapor deposition, whereas the contact resistivity of a top metal electrode was similar to that obtained for a single-walled CNT forest..
||Masafumi Inaba, Tsubasa Muta, Mikinori Kobayashi, Toshiki Saito, Masanobu Shibata, Daisuke Matsumura, Takuya Kudo, Atsushi Hiraiwa, Hiroshi Kawarada, Hydrogen-terminated diamond vertical-type metal oxide semiconductor field-effect transistors with a trench gate, Applied Physics Letters, 10.1063/1.4958889, 109, 3, 2016.07, The hydrogen-terminated diamond surface (C-H diamond) has a two-dimensional hole gas (2DHG) layer independent of the crystal orientation. A 2DHG layer is ubiquitously formed on the C-H diamond surface covered by atomic-layer-deposited-Al2O3. Using Al2O3 as a gate oxide, C-H diamond metal oxide semiconductor field-effect transistors (MOSFETs) operate in a trench gate structure where the diamond side-wall acts as a channel. MOSFETs with a side-wall channel exhibit equivalent performance to the lateral C-H diamond MOSFET without a side-wall channel. Here, a vertical-type MOSFET with a drain on the bottom is demonstrated in diamond with channel current modulation by the gate and pinch off..
||Masafumi Inaba, Chih Yu Lee, Kazuma Suzuki, Megumi Shibuya, Miho Myodo, Yu Hirano, Wataru Norimatsu, Michiko Kusunoki, Hiroshi Kawarada, Contact Conductivity of Uncapped Carbon Nanotubes Formed by Silicon Carbide Decomposition, Journal of Physical Chemistry C, 10.1021/acs.jpcc.5b11815, 120, 11, 6232-6238, 2016.03, Understanding of the contact conductivity of carbon nanotubes (CNTs) will contribute to the further application of CNTs for electronic devices, such as thin film transistors whose channel or electrode is made of dispersed CNTs. In this study, we estimated the contact conductivity of a CNT/CNT interface from the in-plane conductivity of an uncapped CNT forest on SiC. Investigation of the electrical properties of dense CNT forests is also important to enable their electrical application. The in-plane conductivity of a dense CNT forest on silicon carbide normalized by its thickness was measured to be 50 S/cm, which is two to three orders of magnitude lower than the conductivity of a CNT yarn. It was also found that both the CNT cap region and the CNT bulk region exhibit in-plane conductivity. The contact conductivity of CNTs was estimated from the in-plane conductivity in the bulk region. Dense and uncapped CNT forest can be approximated by a conductive mesh, in which each conductive branch corresponds to the CNT/CNT contact conductance. The evaluated contact conductivity was in good agreement with that calculated from the tunneling effect..
||Masafumi Inaba, Kazuma Suzuki, Megumi Shibuya, Chih Yu Lee, Yoshiho Masuda, Naoya Tomatsu, Wataru Norimatsu, Atsushi Hiraiwa, Michiko Kusunoki, Hiroshi Kawarada, Very low Schottky barrier height at carbon nanotube and silicon carbide interface, Applied Physics Letters, 10.1063/1.4916248, 106, 12, 2015.03, Electrical contacts to silicon carbide with low contact resistivity and high current durability are crucial for future SiC power devices, especially miniaturized vertical-type devices. A carbon nanotube (CNT) forest formed by silicon carbide (SiC) decomposition is a densely packed forest, and is ideal for use as a heat-dissipative ohmic contact in SiC power transistors. The contact resistivity and Schottky barrier height in a Ti/CNT/SiC system with various SiC dopant concentrations were evaluated in this study. Contact resistivity was evaluated in relation to contact area. The Schottky barrier height was calculated from the contact resistivity. As a result, the Ti/CNT/SiC contact resistivity at a dopant concentration of 3 × 10^18 cm-3 was estimated to be ∼1.3 × 10^-4 Ω cm2 and the Schottky barrier height of the CNT/SiC contact was in the range of 0.40-0.45 eV. The resistivity is relatively low for SiC contacts, showing that CNTs have the potential to be a good ohmic contact material for SiC power electronic devices..