Kyushu University Academic Staff Educational and Research Activities Database
List of Papers
Inaba Masafumi Last modified date:2019.09.13

Assistant Professor / Applied Energy Engineering / Department of Electrical Engineering / Faculty of Information Science and Electrical Engineering


Papers
1. Wenxi Fei, Masafumi Inaba, Haruka Hoshino, Ikuto Tsuyusaki, Sora Kawai, Masayuki Iwataki, Hiroshi Kawarada, Point-Arc Remote Plasma Chemical Vapor Deposition for High-Quality Single-Crystal Diamond Selective Growth, Physica Status Solidi (A) Applications and Materials Science, 10.1002/pssa.201900227, 2019.06, Selective growth techniques are believed to be the most effective approaches in the fabrication of diamond devices. Herein, the contamination from antenna and mask damage that causes poor performance in electronic applications is overcome with a high-quality diamond film on Ib (100) substrate, obtained by utilizing point-arc remote microwave plasma chemical vapor deposition (MPCVD). Scanning electron microscope (SEM) images and energy dispersive X-ray (EDX) mapping suggest that the diamond nucleation and selective growth occurred only in unmasked regions. Crystalline quality is evaluated by Raman spectra. Mo concentration from the antenna is detected with secondary ion mass spectroscopy (SIMS) at only a background level in the selectively grown diamond, indicating that the films are free of contaminant from the antenna, at three orders of magnitude lower than the impurity concentration in films selectively grown by typical hot filament CVD. Moreover, the moderate growth rate (approximately 50 nm h−1) enables high reproducibility, which is of great importance for the precise control thickness of the selective growth layer, and thus the proposed method offers significant potential for the architecture modification of diamond devices..
2. 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..
3. Nobutaka Oi, Takuya Kudo, Masafumi Inaba, Satoshi Okubo, Shinobu Onoda, Atsushi Hiraiwa, Hiroshi Kawarada, Normally-OFF Two-Dimensional Hole Gas Diamond MOSFETs Through Nitrogen-Ion Implantation, IEEE Electron Device Letters, 10.1109/LED.2019.2912211, 40, 6, 933-936, 2019.06, Diamond is a promising material for power applications owing to its excellent physical properties. Two-dimensional hole gas (2DHG) diamond metal-oxide-semiconductor field-effect transistors (MOSFETs) with hydrogen-terminated (C-H) channel have high current densities and high breakdown fields but often show normally- ON operation. From the viewpoint of safety, normally- OFF operation is required for power applications. In this letter, we used ion implantation to form a shallow and thin nitrogen-doped layer below the C-H channel region, which realized normally- OFF operation. Nitrogen-ion implanted length is fixed at 5 or 10 \mu \text{m}. Nitrogen is a deep donor (1.7 eV) and the nitrogen-doped layer prevents hole accumulation near the surface. The threshold voltage was as high as -2.5 V and no obvious dependence on the threshold voltage of nitrogen-ion implanted length is observed. The breakdown field was 2.7 MV/cm at room temperature. Of 64 devices with a common gate length, 75% showed normally- OFF operation. We confirmed the threshold voltage shift by a thin and shallow nitrogen-doped layer formed by ion implantation..
4. Shozo Kono, Taisuke Kageura, Yuya Hayashi, Sung Gi Ri, Tokuyuki Teraji, Daisuke Takeuchi, Masahiko Ogura, Hideyuki Kodama, Atsuhito Sawabe, Masafumi Inaba, Atsushi Hiraiwa, Hiroshi Kawarada, Carbon 1s X-ray photoelectron spectra of realistic samples of hydrogen-terminated and oxygen-terminated CVD diamond (111) and (001), Diamond and Related Materials, 10.1016/j.diamond.2019.01.017, 93, 105-130, 2019.03, Key factors in C 1s photoelectron spectroscopy for realistic samples of single crystal diamonds are remarked. Basic equations for angle-dependent photoelectron spectroscopy applied to single crystal diamond samples are described in Appendix A. Carbon 1s photoelectron spectroscopic works so far reported for hydrogen-terminated and oxygen-terminated diamond (001) and (111) samples were reviewed placing special attention on surface C 1s components with reference to the key factors. The results showed diversity in C 1s photoelectron spectra so far reported. We had three specific subjects of the study in C 1s XPS; the first is that we have reconfirmed the phenomenon that surface conductive layers resumed when smooth non-doped CVD C(111)-O samples were annealed in vacuum [Diam.Rela.Mate.18(2009)206]. A single C 1s XPS surface component was found for a smooth C(111)-O sample before the vacuum-anneal, which was attributed to surface carbon atoms in C–O–H bonding. The second subject is that dependence of C 1s XPS spectra on surface sensitivity has been measured for all the samples with different surface roughness of C(001)-O, C(111)-O, C(001)-H, and C(111)-H. The results were converted to the energy difference between the Fermi-level (Ef) and valence band maximum (Ev) on the probing depth from the surface. All the samples showed downward bending of Ev toward the surface. For the C(001)-H samples, this was a reconfirmation of previous work [Surf.Sci.604(2010)1148]. For the C(001)-H and C(111)-H samples, various degrees of downward band bending toward surface were observed and analyzed with two-dimensional band simulation. It was concluded that another source of holes such as shallow acceptors is present in a deeper region of the surface in addition to holes very close to surface caused by the charge-transfer-doping. The third subject is that C 1s XPS spectra for superconducting C(111)-O samples showed a lattice distortion of ~9 monolayers near the surface..
5. Keita Nishimura, Takuya Ushiyama, Nguyen Xuan Viet, Masafumi Inaba, Shigeru Kishimoto, Yutaka Ohno, Enhancement of the electron transfer rate in carbon nanotube flexible electrochemical sensors by surface functionalization, Electrochimica Acta, 10.1016/j.electacta.2018.10.147, 295, 157-163, 2019.02, Thin films of single-walled carbon nanotubes (SWNTs) possess high potential for use in flexible electrochemical sensors in wearable medical devices. However, the electrochemical activity of such materials is not yet fully understood. We studied the enhancement in electrochemical activity of SWNT-based flexible electrochemical sensors. To exclude the effect of surface contaminations from the fabrication processes on the electrochemical activity, we introduced the dry process for high-quality and clean SWNT film formation and the clean device fabrication process with a protective layer. So-fabricated SWNT electrodes exhibited a low electron transfer rate. Electrochemical functionalization with an H2SO4 solution successfully enhanced the electrochemical activity of the SWNT electrode for the inner sphere probes such as [Fe(CN)6]4−/3−. This method is quite gentle and controllable, but also effective at increasing the electron transfer rate without either degrading the potential window. We found out that there was a correlation between the electron transfer rate and the amount of defects evaluated from Raman scattering spectroscopy. XPS analysis showed that the functionalization process introduced C–O and C[dbnd]O species, suggesting that these species constituted active sites for inner sphere probes. The electrochemical functionalization was also effective for enhancing the limit of detection in dopamine detection with the flexible SWNT electrode. The limit of detection was ∼100 nM for functionalized electrode whereas it was ∼1 μM for as-fabricated electrodes..
6. 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..
7. Ryosuke Fukuda, Priyadharshini Balasubramanian, Itaru Higashimata, Godai Koike, Takuma Okada, Risa Kagami, Tokuyuki Teraji, Shinobu Onoda, Moriyoshi Haruyama, Keisuke Yamada, Masafumi Inaba, Hayate Yamano, Felix M. Stürner, Simon Schmitt, Liam P. McGuinness, Fedor Jelezko, Takeshi Ohshima, Takahiro Shinada, Hiroshi Kawarada, Wataru Kada, Osamu Hanaizumi, Takashi Tanii, Junichi Isoya, Lithographically engineered shallow nitrogen-vacancy centers in diamond for external nuclear spin sensing, New Journal of Physics, 10.1088/1367-2630/aad997, 20, 8, 2018.08, The simultaneous control of the number and position of negatively charged nitrogen-vacancy (NV) centers in diamond was achieved. While single near-surface NV centers are known to exhibit outstanding capabilities in external spin sensing, trade-off relationships among the accuracy of the number and position, and the coherence of NV centers have made the use of such engineered NV centers difficult. Namely, low-energy nitrogen implantation with lithographic techniques enables the nanoscale position control but results in degradation of the creation yield and the coherence property. In this paper, we show that low-energy nitrogen ion implantation to a 12C (99.95%)-enriched homoepitaxial diamond layer using nanomask is applicable to create shallow NV centers with a sufficiently long coherence time for external spin sensing, at a high creation yield. Furthermore, the NV centers were arranged in a regular array so that 40% lattice sites contain single NV centers. The XY8-k measurements using the individual NV centers reveal that the created NV centers have depths from 2 to 12 nm, which is comparable to the stopping range of nitrogen ions implanted at 2.5 keV. We show that the position-controlled NV centers are capable of external spin sensing with a ultra-high spatial resolution..
8. Shaili Falina, Sora Kawai, Nobutaka Oi, Hayate Yamano, Taisuke Kageura, Evi Suaebah, Masafumi Inaba, Yukihiro Shintani, Mohd Syamsul, Hiroshi Kawarada, Role of carboxyl and amine termination on a boron-doped diamond solution gate field effect transistor (SGFET) for PH sensing, Sensors (Switzerland), 10.3390/s18072178, 18, 7, 2018.07, In this paper, we report on the effect of carboxyl-and amine terminations on a boron-doped diamond surface (BDD) in relation to pH sensitivity. Carboxyl termination was achieved by anodization oxidation in Carmody buffer solution (pH 7). The carboxyl-terminated diamond surface was exposed to nitrogen radicals to generate an amine-terminated surface. The pH sensitivity of the carboxyl-and amine-terminated surfaces was measured from pH 2 to pH 12. The pH sensitivities of the carboxyl-terminated surface at low and high pH are 45 and 3 mV/pH, respectively. The pH sensitivity after amine termination is significantly higher—the pH sensitivities at low and high pH are 65 and 24 mV/pH, respectively. We find that the negatively-charged surface properties of the carboxyl-terminated surface due to ionization of –COOH causes very low pH detection in the high pH region (pH 7–12). In the case of the amine-terminated surface, the surface properties are interchangeable in both acidic and basic solutions; therefore, we observed pH detection at both low and high pH regions. The results presented here may provide molecular-level understanding of surface properties with charged ions in pH solutions. The understanding of these surface terminations on BDD substrate may be useful to design diamond-based biosensors..
9. 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..
10. S. Kono, T. Sasaki, Masafumi Inaba, A. Hiraiwa, H. Kawarada, Sheet resistance underneath the Au ohmic-electrode on hydrogen-terminated surface-conductive diamond (001), Diamond and Related Materials, 10.1016/j.diamond.2017.09.020, 80, 93-98, 2017.11, The sheet resistance (Rsk) underneath ohmic Au-electrodes on hydrogen-terminated surface-conductive diamond (001) surfaces was examined by a special current-voltage measurement. It has been found that Rsk is about ~ 200 times larger than the sheet resistance, Rsh, in the open areas without Au-electrodes. The specific contact resistance, RC, of ohmic Au-electrodes on H-terminated surface-conductive diamonds as determined by linear transmission line models must be corrected accordingly. As the results of this, RC is suggested to be of the order of 1 × 10− 3 – 1 × 10− 4 Ωcm2..
11. Masafumi Inaba, Akinori Seki, Kazuaki Sato, Tomoyoshi Kushida, Taisuke Kageura, Hayate Yamano, Atsushi Hiraiwa, Hiroshi Kawarada, Vertical edge graphite layer on recovered diamond (001) after high-dose ion implantation and high-temperature annealing, Physica Status Solidi (B) Basic Research, 10.1002/pssb.201700040, 254, 9, 2017.09, A vertical edge graphite layer (VEG) fabricated on a diamond (001) substrate and the recovery of the crystallinity of the diamond substrate following high-dose ion implantation and high-temperature annealing (HTA) was investigated. The Al ions were implanted into the diamond (001) surface at 773 K (500 °C), followed by HTA at 1973 K (1700 °C). The graphite edges were vertically oriented, but each domain was randomly rotated in the in-plane direction, which was confirmed via multiple cross-sectional transmission electron microscopy images obtained from different directions rotated 2, 5, 10, and 15° around the [001] axis. The Raman and photoluminescence exhibited no significant peaks. The initial sp2 structure state of the VEG was nucleated in an early stage of the HTA and the surface diamond was subsequently reconstructed, which was confirmed using stopping-and-range-of-ions-in-matter calculations and Rutherford backscattering/channeling (RBS-C) measurements. The RBS-C spectra indicate that the crystal is maintained after hot implantation and is recovered by HTA. This VEG structure may be useful for ohmic contact with diamond electrical devices..
12. Takuro Naramura, Masafumi Inaba, Sho Mizuno, Keisuke Igarashi, Eriko Kida, Shaili Falina Mohd Sukri, Yukihiro Shintani, Hiroshi Kawarada, Threshold voltage control of electrolyte solution gate field-effect transistor by electrochemical oxidation, Applied Physics Letters, 10.1063/1.4991364, 111, 1, 2017.07, Diamond electrolyte solution-gate-field effect transistors (SGFETs) are suitable for applications as chemical ion sensors because of their wide potential window and good physical and chemical stabilities. In this study, we fabricated an anodically oxidized diamond SGFET from a full hydrogen-terminated diamond SGFET and demonstrated control of the device threshold voltage by irreversible anodic oxidation. The applied anodic bias voltage (VAO) was varied gradually from low to high (1.1-1.7 V). As the anodic oxidation proceeded, the threshold voltage shifted to more negative values with no degradation of hole mobility. Thus, anodic oxidation is a useful method for controlling the threshold voltage of diamond SGFETs..
13. Taisuke Kageura, Kanami Kato, Hayate Yamano, Evi Suaebah, Miki Kajiya, Sora Kawai, Masafumi Inaba, Takashi Tanii, Moriyoshi Haruyama, Keisuke Yamada, Shinobu Onoda, Wataru Kada, Osamu Hanaizumi, Tokuyuki Teraji, Junichi Isoya, Shozo Kono, Hiroshi Kawarada, Effect of a radical exposure nitridation surface on the charge stability of shallow nitrogen-vacancy centers in diamond, Applied Physics Express, 10.7567/APEX.10.055503, 10, 5, 2017.05, A nitridation process of a diamond surface with nitrogen radical exposure far from the radio-frequency plasma for the stabilization of a negatively charged nitrogen-vacancy (NV%) centers near the surface is presented. At a nitrogen coverage of as high as 0.9 monolayers, high average Rabi contrasts of 0.40 + 0.06 and 0.46 + 0.03 have been obtained for single NV% centers formed by shallow nitrogen implantation with acceleration voltages of 1 and 2 keV, respectively. This indicates that nitrogen termination by a radical exposure process produces an electric charge state suitable for single NV% centers near the surface compared with the states obtained for alternatively terminated surfaces..
14. Hayate Yamano, Sora Kawai, Kanami Kato, Taisuke Kageura, Masafumi Inaba, Takuma Okada, Itaru Higashimata, Moriyoshi Haruyama, Takashi Tanii, Keisuke Yamada, Shinobu Onoda, Wataru Kada, Osamu Hanaizumi, Tokuyuki Teraji, Junichi Isoya, Hiroshi Kawarada, Charge state stabilization of shallow nitrogen vacancy centers in diamond by oxygen surface modification, Japanese Journal of Applied Physics, 10.7567/JJAP.56.04CK08, 56, 4, 2017.04, We investigated the charge state stability and coherence properties of near-surface single nitrogen vacancy (NV) centers in 12C-enriched diamond for potential use in nanoscale magnetic field sensing applications. The stability of charge states in negatively charged NV centers (NV-) was evaluated using one of the pulsed optically detected magnetic resonance measurements, Rabi oscillation measurements. During the accumulation of Rabi oscillations, an unstable shallow NV- was converted to a neutral state. As a result, the contrast of Rabi oscillations degraded, depending on charge state stability. We stabilized the NV- state of very shallow NV centers (∼2.6 ± 1.1nm from the surface) created by 1.2 keV nitrogen ion implantation by diamond surface modification, UV/ozone exposure, and oxygen annealing. This improvement indicates that we can suppress the upward surface band bending and surface potential fluctuations through Fermi level pinning originating from oxygen-terminated diamond surfaces..
15. Yuya Kitabayashi, Takuya Kudo, Hidetoshi Tsuboi, Tetsuya Yamada, Dechen Xu, Masanobu Shibata, Daisuke Matsumura, Yuya Hayashi, Mohd Syamsul, Masafumi Inaba, Atsushi Hiraiwa, Hiroshi Kawarada, Normally-Off C-H Diamond MOSFETs with Partial C-O Channel Achieving 2-kV Breakdown Voltage, IEEE Electron Device Letters, 10.1109/LED.2017.2661340, 38, 3, 363-366, 2017.03, Diamond has unique physical properties, which show great promise for applications in the next generation power devices. Hydrogen-terminated (C-H) diamond metal-oxide-semiconductor field-effect transistors (MOSFETs) often have normally-on operation in devices, because the C-H channel features a p-type inversion layer; however, normally-off devices are preferable in power MOSFETs from the viewpoint of fail safety. We fabricated hydrogen-terminated (C-H) diamond MOSFETs using a partially oxidized (partial C-O) channel. The fabricated MOSFETs showed a high breakdown voltage of over 2 kV at room temperature and normally-off characteristics with a gate threshold voltage Vth of -2.5 - 4 V..
16. Hiroshi Kawarada, Tetsuya Yamada, Dechen Xu, Hidetoshi Tsuboi, Yuya Kitabayashi, Daisuke Matsumura, Masanobu Shibata, Takuya Kudo, Masafumi Inaba, Atsushi Hiraiwa, Durability-enhanced two-dimensional hole gas of C-H diamond surface for complementary power inverter applications, Scientific reports, 10.1038/srep42368, 7, 2017.02, Complementary power field effect transistors (FETs) based on wide bandgap materials not only provide high-voltage switching capability with the reduction of on-resistance and switching losses, but also enable a smart inverter system by the dramatic simplification of external circuits. However, p-channel power FETs with equivalent performance to those of n-channel FETs are not obtained in any wide bandgap material other than diamond. Here we show that a breakdown voltage of more than 1600 V has been obtained in a diamond metal-oxide-semiconductor (MOS) FET with a p-channel based on a two-dimensional hole gas (2DHG). Atomic layer deposited (ALD) Al 2 O 3 induces the 2DHG ubiquitously on a hydrogen-terminated (C-H) diamond surface and also acts as both gate insulator and passivation layer. The high voltage performance is equivalent to that of state-of-the-art SiC planar n-channel FETs and AlGaN/GaN FETs. The drain current density in the on-state is also comparable to that of these two FETs with similar device size and V B..
17. Yukihiro Shintani, Shoji Ibori, Keisuke Igarashi, Takuro Naramura, Masafumi Inaba, Hiroshi Kawarada, Polycrystalline boron-doped diamond with an oxygen-terminated surface channel as an electrolyte-solution-gate field-effect transistor for pH sensing, Electrochimica Acta, 10.1016/j.electacta.2016.06.104, 212, 10-15, 2016.09, A polycrystalline boron-doped diamond (BDD) electrolyte solution-gate field effect transistor (SGFET) for use as a pH sensor was developed. The polycrystalline diamond films with a boron-doped layer possessed semiconducting properties that were comparable to hydrogen-terminated non-doped diamond. The hydrogen-terminated BDD surface was successfully transferred to a partially oxygen-terminated surface by ozone exposure, and its SGFET current–voltage (I–V) characteristics were evaluated with bias voltages within the potential window of diamond. The drain-source current(Ids)–drain-source voltage(Vds) characteristics showed pinch-off and saturation. In addition, they stably operated in electrolyte solutions with pH values from 2 to 12. The transfer characteristics exhibited a pH sensitivity of approximately 30 mV/pH, which is comparable with the pH sensitivity of the conventional oxygen-terminated non-doped SGFET and the single-crystal BDD SGFET investigated in our previous work. Furthermore, the BDD SGFET exhibited improved long-term stability, and the coefficient of variation (CV) of Ids for 10 months was up to 10%..
18. H. Kawarada, T. Yamada, D. Xu, Y. Kitabayashi, M. Shibata, D. Matsumura, M. Kobayashi, T. Saito, T. Kudo, Masafumi Inaba, A. Hiraiwa, Diamond MOSFETs using 2D hole gas with 1700V breakdown voltage, 28th International Symposium on Power Semiconductor Devices and ICs, ISPSD 2016 Proceedings of the 2016 28th International Symposium on Power Semiconductor Devices and ICs, ISPSD 2016, 10.1109/ISPSD.2016.7520883, 483-486, 2016.07, More than 1600V breakdown voltages have been obtained in hydrogen terminated (C-H) diamond planar p-channel MOSFETs with gate-drain distance of 16-22 μm. The drain current density exceeds 100mA/mm in the FETs. The blocking voltage and drain current characteristics are comparable to those of n-channel AlGaN/GaN FETs and planar SiC MOSFETs in a similar device size. Atomic layer deposited Al2O3 works as gate insulator and passivation layer. It also induces the 2 dimensional hole gas ubiquitously on C-H diamond surface not only in planar, but in a trench gate structure. The first diamond vertical MOSFET has also operated using the trench structure..
19. 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..
20. 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..
21. Masafumi Inaba, Kazuma Suzuki, Yu Hirano, Wataru Norimatsu, Michiko Kusunoki, Hiroshi Kawarada, Ohmic contact for silicon carbide by carbon nanotubes, 16th International Conference on Silicon Carbide and Related Materials, ICSCRM 2015 Silicon Carbide and Related Materials 2015, 10.4028/www.scientific.net/MSF.858.561, 561-564, 2016.01, The electrical contact properties of silicon carbide (SiC) and carbon nanotubes (CNTs) were measured by conductive atomic force microscopy (C-AFM). A CNT forest was synthesized by SiC surface decomposition. Trenches, which electrically separate the conduction area, were fabricated using a focused ion beam (FIB) without a cover layer, and the resistance of each island was measured by C-AFM. From the dependence of the resistance on the CNT forest island size, the contact resistance between the CNTs and the SiC substrate was measured. By varying the dopant density in the SiC substrate, the Schottky barrier height was evaluated to be ~0.5 eV. This is slightly higher than a previously reported result obtained from a similar setup with a metal covering the CNT forest. We assumed that the damaged region existed in the islands, which is due to the trench formation by the FIB. The commensurate barrier height was obtained with the length of the damaged region assumed to be ~3 μm. Here, we could estimate the resistivity of a CNT/SiC interface without a cover layer. This indicates that a CNT forest on SiC is useful as a brief contact electrode..
22. Miho Myodo, Masafumi Inaba, Kazuyoshi Ohara, Ryogo Kato, Mikinori Kobayashi, Yu Hirano, Kazuma Suzuki, Hiroshi Kawarada, Large-current-controllable carbon nanotube field-effect transistor in electrolyte solution, Applied Physics Letters, 10.1063/1.4921454, 106, 21, 2015.05, Large-current-controllable carbon nanotube field-effect transistors (CNT-FETs) were fabricated with mm-long CNT sheets. The sheets, synthesized by remote-plasma-enhanced CVD, contained both single- and double-walled CNTs. Titanium was deposited on the sheet as source and drain electrodes, and an electrolyte solution was used as a gate electrode (solution gate) to apply a gate voltage to the CNTs through electric double layers formed around the CNTs. The drain current came to be well modulated as electrolyte solution penetrated into the sheets, and one of the solution gate CNT-FETs was able to control a large current of over 2.5 A. In addition, we determined the transconductance parameter per tube and compared it with values for other CNT-FETs. The potential of CNT sheets for applications requiring the control of large current is exhibited in this study..
23. 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..