Masataka Inoue

Osaka Institute of Technology, Ōsaka, Ōsaka, Japan

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Publications (99)88.27 Total impact

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    ABSTRACT: Self-switching nanodiodes (SSDs) using zinc oxide (ZnO) were fabricated on glass substrates. The SSDs using ZnO have attracted significant attention as transparent devices because of their low cost, abundance in nature, and so on. Rectification characteristics in the SSDs were resemblance to the characteristics of conventional diodes with use of a doping junction or a barrier structure. The changes in characteristics depending on the shape of SSDs were investigated. Channel widths in the SSD of 230 and 190 nm and turn-on voltages of 5 and 8 V were obtained. On the other hand, it was found that the channel length influences the current strength. Moreover, after coating the devices with HfO2 to enhance the electric field coupling, the rectification behavior was maintained while the device current increased dramatically. The SSDs were fabricated using ZnO on flexible plastic substrates. For channel widths of 250 and 200 nm, turn-on voltages of 4 and 6 V were obtained, respectively. We also obtained clear rectification and observed the dependence of the turn-on voltage on the channel width. (c) 2013 The Japan Society of Applied Physics
    Japanese Journal of Applied Physics 06/2013; 52(6S):06GE09. DOI:10.7567/JJAP.52.06GE09 · 1.06 Impact Factor
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    ABSTRACT: Ultra-fast novel nanodevices, namely the self-switching diode (SSD) were fabricated from the InAs/AlGaSb heterostructures grown by solid-source molecular beam epitaxy on a semi-insulating GaAs (100) substrate. The two lines were etched thorough the 2DEG layer and become insulating. The effective channel width was actually smaller because of the depletion layer at the etched boundaries. Depending on the sign of the applied voltage the effective channel width will increase or reduce, giving rise to the diode-like characteristics. The diode-like characteristics were clearly observed for the InAs SSDs at 300K, and turn on voltage is strongly dependent on the channel width of the SSDs. In the SSD with W = 460 nm, the InAs channel of was fully pinched of under the equilibrium condition, and positive voltage of 1.46 V was needed to drive a current thorough the InAs channel. On the other hand, the positive voltage of 2.32 V was needed to drive a current in the SSD with W = 230 nm. We found to need the higher the turn-on voltage in the SSD of the narrower channel width. Furthermore, multi-channel SSDs were fabricated. I-V characteristics and the AFM images of the SSD array of the symmetric nanowires, which are 1.5 μm long and approximately W = 170 nm. The current densities were clearly increased with increasing the number of the nano-wires. The clear diode-like nonlinear behavior and rectification reflected ballistic nature of electrons in InAs-based SSD were observed.
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    ABSTRACT: We report the fabrication and characterization on high-performance ZnO based TFTs on unheated plastic substrate. ZnO films were grown by pulsed laser deposition (PLD) on polyethylene napthalate (PEN) substrates. Top-gate ZnO-TFTs were fabricated by photolithography and wet chemical etching. The source and drain contacts were formed by lift-off of e-beam deposited Ti(20 nm)/Au(200 nm). An HfO2 with thickness 100 nm was selected as the gate insulator, and top gate electrode Ti(20 nm)/Au(200 nm) was deposited by e-beam evaporation. We prepared a set of the structure with SiO2/TiO2 to investigate the characteristic changes that appear in the film characteristics in response to bending. From the ID-VDS and the transfer characteristics which are affected by bending and return for the ZnO-TFT with SiO2/TiO2 buffers, the TFTs were bent to a curvature radius of 8.5 mm. The transconductance, gm is obtained 1.7 mS/mm on flat, 1.4 mS/mm on bending and 1.3 mS/mm on returning the film, respectively. The ID-VDS characteristics were therefore not changed by bending. All of the devices exhibited a clear pinch-off behavior and a high on/off current ratio of ˜10^6. The threshold voltages, Vth were not changed drastically. Furthermore, TFT structures were changed from a conventional top-gate type to a bottom-gate type. A high transconductance of 95.8 mS/mm was achieved in the bottom-gate type TFT by using Al2O3 oxide buffer.
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    ABSTRACT: In-situ organization of InSb quantum dots on Al0.5Ga0.5Sb is reported. Samples were grown on just (100), 5°off (100) towards [0-1-1],(311)A, and (311)B surfaces of GaAs by molecular beam epitaxy. The growth mechanism and characteristics of quantum dots were analyzed using reflection high-energy electron diffraction, atomic force microscopy, and photoluminescence. Observed photoluminescence peak shift towards lower energy side with InSb thickness was interpreted by the development of quantum dots. Substrate orientation effect was examined and found to be useful to increase the dot density. As a result, as high as ~3xl 09 cm-2 dot density was achieved on the (311 )B substrate whereas the typical value on the (100) was ~2xl 08 cm-2 .
    MRS Online Proceeding Library 01/2012; 448. DOI:10.1557/PROC-448-193
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    ABSTRACT: Electric properties of aqueous grown ZnO nanorods were investigated. Vertical current measurement through the top 20-nm-thick Au electrode/ZnO nanorods/bottom Au film structure showed asymmetric I-V properties and was discussed in terms of different interface properties between ZnO nanorods and Au. Schottky barrier height in top Au electrode and ZnO nanorods was estimated to be 0.22eV based on core level X-ray photoelectron spectroscopy (XPS) spectra. XPS experiment also suggests that aqueous grown ZnO nanorods have Zn-polarity and that might be one of the reasons of lower barrier height. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 02/2011; 8(2):522-524. DOI:10.1002/pssc.201000521
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    ABSTRACT: We report the first microwave performance of single crystalline ZnO/ZnMgO heterostructure field-effect transistors (HFETs). The structure consisted of a 15-nm-thick ZnO channel layer was grown by molecular beam epitaxy (MBE) on an a-sapphire substrate. Two-finger type HFETs with 1 or 2-mu m-long gate were fabricated and measured for microwave performance. The transconductance of the HFETs are 28 and 23 mS/mm for 1 and 2-mu m-gate devices, respectively. The microwave measurement of ZnO-based TFTs revealed that the current gain cutoff frequency f(T) of 1.75 GHz and that of unilateral power gain f(max) of 2.45 GHz for 1-mu m-gate HFET. Electron velocity obtained by two-terminal measurements implies that the structural design is crucial for further improvement of the high-frequency performance of ZnO/ZnMgO HFETs. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    Physica Status Solidi (A) Applications and Materials 02/2011; 208(2):449 - 452. DOI:10.1002/pssa.201000509 · 1.53 Impact Factor
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    ABSTRACT: Terahertz (THz) radiation from InAs thin films grown by molecular-beam epitaxy on closely lattice-matched p-type GaSb (100) substrates and lattice-mismatched semi-insulating GaAs (100) substrates was investigated. The THz radiation intensity was measured from InAs films with thicknesses between 100nm and 1.5μm excited by a femtosecond laser pulse with a wavelength of approximately 780nm. The radiation intensity increased as the InAs film thickness increased and it exceeded that from a bulk n-type InAs substrate with an electron concentration of 2.3 × 1016cm−3 when the InAs film thickness was greater than about 500nm. In addition, the THz intensity from a 1-μm-thick InAs film was greater than that from a bulk p-type InAs substrate. We ascribe this enhanced THz intensity to the wave reflected from the lower interface between the InAs film and the layer grown beneath it. We confirmed this by observing an increased pulse width due to constructive overlap of the reflected wave. The results demonstrate that InAs thin films are promising materials for THz emitting devices. KeywordsTHz radiation–InAs–Thin films–MBE
    Journal of infrared, millimeter and terahertz waves 01/2011; 32(5):646-654. DOI:10.1007/s10762-010-9694-0 · 1.89 Impact Factor
  • Journal of the Society of Materials Science Japan 01/2011; 60(11):976-982. DOI:10.2472/jsms.60.976
  • Journal of the Society of Materials Science Japan 01/2011; 60(5):447-456. DOI:10.2472/jsms.60.447
  • MRS Online Proceeding Library 01/2011; 744. DOI:10.1557/PROC-744-M3.1
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    ABSTRACT: ZnO nanorods were grown on a -, c - and r -plane sapphire substrates using aqueous solution with microwave irradiation heating. ZnO nanorods about 200 nm diameter with c-axis orientation were grown independently on the orientation of the substrates. Photoluminescence (PL) spectrum at 6 K showed a dominant peak at 3.3609 eV (I6) with the width as narrow as 2.2 meV comparable to that of epitaxially grown single-crystalline ZnO films. The PL properties were improved by thermal annealing process without generating structural defects. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 06/2010; 7(6):1592-1594. DOI:10.1002/pssc.200983234
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    ABSTRACT: Aiming at stable biosensing operation, aqueous-grown ZnO nanorods on sapphire substrates were examined their chemical stability against the immersion in HCl solution. Density and diameter of the ZnO nanorods were remained but their height was shortened when immersed with aminosilane modification while bare ZnO nanorods were completely disappeared after 5 min immersion, indicating that the aminosilane modification is useful to improve the chemical stability of ZnO nanolods even though the membrane by the silanezation process is not fully stable to HCl solution. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 06/2010; 7(6):1562 - 1564. DOI:10.1002/pssc.200983233
  • Journal of the Society of Materials Science Japan 01/2010; 59(9):660-665. DOI:10.2472/jsms.59.660
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    ABSTRACT: Characteristics of ion-sensitive operation of electrolyte-solution-gate field-effect transistors (ESG-FETs) are reported with implications for the development of healthcare chips. ZnO-based polycrystalline films grown on glass substrates by sputtering were used for the ESG-FETs with the modification of the gate electrode by amino groups. The equilibrium proton transfer to/from the amino groups was found to change the gate electrode potential at a constant rate of 58 mV/pH, which successfully modulated the current flow in transistors in accordance with a conventional FET theory. A typical pH sensitivity of -0.8 muA/pH with a small time-constant of 5 s was obtained for the ESG-FET operation with 3× 5 mm2 gate area.
    Applied Physics Express 08/2009; 2. DOI:10.1143/APEX.2.087001 · 2.57 Impact Factor
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    ABSTRACT: ZnO nanorod arrays were grown directly on n-Si (111) substrates using an aqueous solution method aiming at integration with Si-based device technology. The growth of the ZnO nanorod arrays was found to be influenced by the thickness of the native oxide layer on Si substrates. A dense array of about 200 nm diameter nanorods was obtained from the thick equimolar aqueous solution containing 100 mM of zinc nitrate hexahydrate and hexamethylenetetramine, while the solution with lower concentrations resulted in scattered corn-shaped ones. Electrochemical potential in electrolyte of the ZnO nanorod arrays on Si (111) substrates was studied and found to have a slope of -50 mV/pH, suggesting a potential application to future sensing nanosystems.
    Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures 05/2009; 27(3):1684-. DOI:10.1116/1.3098503 · 1.36 Impact Factor
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    ABSTRACT: The characteristics of an enzyme-based ZnO/Zn0.7Mg0.3O heterojunction field-effect transistor (HFET) for glucose sensing are reported. The enzyme glucose oxidase (GOD) was immobilized on an amine-modified gate electrode of the ZnO/Zn0.7Mg0.3O HIFET, and glucose sensing was performed by detecting biocatalytically yielded protons using an ion-sensitive function of the gate electrode. The chemical bonding states of the amine-modified and GOD-immobilized surfaces were analyzed by X-ray photoemission spectroscopy, and the enzyme activity of GOD was examined by a colorimetric method. In agreement with the promising results of these experiments, this enzyme-based HFET exhibited stable sensing performance with a linear response in a wide range of glucose concentrations from 0 to 4 mg-cm(-3) and at a short time constant less than 20s. (c) 2009 The Japan Society of Applied Physics
    Japanese Journal of Applied Physics 04/2009; 48. DOI:10.1143/JJAP.48.04C081 · 1.06 Impact Factor
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    ABSTRACT: We report on the fabrication and characterization of top-gate ZnO thin film transistors (TFTs) using glass substrates. High quality ZnO epitaxial films were grown on glass substrates (Corning #1737) by pulsed laser deposition. The thickness of the films was in the range of 50-100 nm. The growth temperature was set to 380^oC. These films were characterized by x-ray diffraction, and Hall effects measurements. Highly c-axis oriented ZnO(0002) reflections corresponding to the wurtzite-phase were observed for all the films, indicating that these films grow epitaxially as a crystalline single phase on a glass substrate. The Hall effects measurements show that we have succeeded in fabricating a ZnO film with an electron mobility of 36 cm^2/Vs on a glass substrate. Top-gate ZnO TFTs were fabricated by photolithography and wet chemical etching. The ohmic contact metal Ti/Au was deposited by electron beam evaporation. The top gate electrodes and the gate insulator SiO2 were finally deposited by electron beam evaporation. A room temperature characteristic of ZnO TFT with 50 μm gate length was an n-channel depletion type with a transconductance of 5.4 mS/mm. The off current was less than 10-9 A and the on/off current ratio was about 10^6 at VDS=5V.
  • Journal of the Society of Materials Science Japan 01/2009; 58(3):243-250. DOI:10.2472/jsms.58.243
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    ABSTRACT: We studied the structures of ZnO/ZnMgO heterostructure field-effect transistors (FETs) to achieve high performance and stability in these devices. Two types of heterostructure were examined. One consisted of a ZnO channel layer and a thin ZnMgO cap layer to form a hetero-metal-insulator-semiconductor (hetero-MIS) structure, and the other had formed a conventional MIS structure. Both Al2O3 and HfO2 were examined as high-k gate dielectrics. The results indicate that high-performance FETs can be obtained using a hetero-MIS structure and that the reduction in access resistance is crucial for further improvements in FET performance. In addition, both an increase in transconductance and a stable FET operation were realized for the hetero-MIS structure by replacing the Al2O3 gate dielectric with a HfO2 gate dielectric. Stable operation was verified from the observation of a markedly reduced hysteresis less than 0.1 V for HfO2, which was lower than that for Al2O3.
    Japanese Journal of Applied Physics 04/2008; 47:2845-2847. DOI:10.1143/JJAP.47.2845 · 1.06 Impact Factor
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    ABSTRACT: We report on the fabrication and characterization of InAs/AlGaSb high electron mobility transistors (HEMTs) with high-k gate insulators (Al2O3 and HfO2). InAs/AlGaSb quantum well structures were grown by molecular beam epitaxy on a semi-insulating GaAs substrate [1]. From Hall measurements at room temperature, the as-grown wafer showed an electron mobility of 20,000-25,000 cm^2/Vs and a sheet carrier density of 1.0-2.0x10^12 cm-2. InAs/AlGaSb HEMTs have demonstrated a maximum extrinsic transconductance of 181mS/mm at room temperature. The gate leakage current has been markedly decreased by using thin high-k gate insulators. A typical gate current density of less than 1 nA/mm at room temperature was achieved by inserting the high-k gate insulator. We also found that the leakage current density was smaller than for other experimental results on InAs HEMTs with a Shottky gate [2]. In addition, we evaluated the electron motility and drift velocity by increasing the electronic field between the sources and the drain. [1] T. Maemoto et al., Journal of Physics: Conference Series 38, 112 (2006). [2] J. Bergman et al., 61th Device Research Conference, June 23-25 (2003).