研究者詳細 - 田中　貴久

論文 - 田中　貴久

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Detection of PPB-Level H<inf>2</inf>S Concentrations in Exhaled Breath Using Au Nanosheet Sensors with Small Variability, High Selectivity, and Long-Term Stability

Kato T., Tanaka T., Uchida K.

ACS Sensors （ACS Sensors） 9 （ 2 ） 708 - 716 2024年02月

The continuous monitoring of hydrogen sulfide (H2S) in exhaled breath enables the detection of health issues such as halitosis and gastrointestinal problems. However, H2S sensors with high selectivity and parts per billion-level detection capability, which are essential for breath analysis, and facile fabrication processes for their integration with other devices are lacking. In this study, we demonstrated Au nanosheet H2S sensors with high selectivity, ppb-level detection capability, and high uniformity by optimizing their fabrication processes: (1) insertion of titanium nitride (TiN) as an adhesion layer to prevent Au agglomeration on the oxide substrate and (2) N2 annealing to improve nanosheet crystallinity. The fabricated Au nanosheets successfully detected H2S at concentrations as low as 5.6 ppb, and the estimated limit of detection was 0.5 ppb, which is superior to that of the human nose (8-13 ppb). In addition, the sensors detected H2S in the exhaled breath of simulated patients at concentrations as low as 175 ppb while showing high selectivity against interfering molecules, such as H2, alcohols, and humidity. Since Au nanosheets with uniform sensor characteristics enable easy device integration, the proposed sensor will be useful for facile health checkups based on breath analysis upon its integration into mobile devices.

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Understanding of a Pt thin-film H<inf>2</inf> sensor under working conditions using AP-XPS and XAFS

Toyoshima R., Tanaka T., Kato T., Abe H., Uchida K., Kondoh H.

Chemistry Letters （Chemistry Letters） 53 （ 2 ） 2024年02月

ISSN 03667022

The operating principle of a Pt thin-film H2 gas sensor was investigated using a combination of surface sensitive ambient-pressure X-ray photoelectron spectroscopy and bulk sensitive X-ray absorption fine structure techniques, which provided chemical and structure information under working conditions, coupled with electric resistivity measurements. It is shown that the sensor response was in a linear relation with both coverages of H and O atoms on the Pt surface. Moreover, the bulk structure of Pt remains unchanged under H2 exposure. These observations support that the resistivity change is associated with electron scattering in the near-surface region.

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A 65nm Cryogenic CMOS Design and Performance at 4.2K for Quantum State Controller Application

Tada M., Okamoto K., Tanaka T., Miyamura M., Ishikuro H., Uchida K., Sakamoto T.

IEEE Journal of the Electron Devices Society （IEEE Journal of the Electron Devices Society） 12 28 - 33 2024年

A performance evaluation of cryogenic CMOS circuit at liquid-helium temperature (4.2K) is conducted using a standard 65nm bulk CMOS for quantum state controller (QSC) applications. The ON-current (Ion) of the core n/pMOSFET are increased by 25% and 9% with excellent gate modulation (Ion/Ioff=~109). The cryogenic characteristics of copper interconnects in the back end of the line (BEOL), including line and via resistances, capacitances, and Joule-heating effect (JHE) are accurately assessed. The interconnect and via resistances decrease with temperature due to a reduction of electron-phonon scattering, resulting in resistances that are 75% and 20% lower at 4.2K compared to those at room temperature(RT). No significant change in inter-line capacitance and no severe JHE are observed in the Cu BEOL at 4.2K. The developed cell libraries for Simulation Program with Integrated Circuit Emphasis (SPICE) model and the technology file, which includes RC interconnect parameters, enable precise design of CMOS circuits at 4.2K. This results in a demonstrated +18.3% increase in speed or -16% reduction in power consumption for ring-oscillator (ROSC) at 4.2K, aligning well with the simulation results obtained from the developed model.

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Hydrogen gas and the gut microbiota are potential biomarkers for the development of experimental colitis in mice

Yuta Fujiki, Takahisa Tanaka, Kyosuke Yakabe, Natsumi Seki, Masahiro Akiyama, Ken Uchida, Yun-Gi Kim

Gut Microbiome （Cambridge University Press (CUP)） 1 - 24 2023年11月

筆頭著者, 査読有り

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Alkyl-Chain-Length and Temperature Dependencies of Sensor Responses to Aliphatic Alcohols in Nanostructured Pt-Based Sensors

Hamanaka Y., Tanaka T., Uchida K.

ACS Applied Nano Materials （ACS Applied Nano Materials） 6 （ 18 ） 16433 - 16441 2023年09月

査読有り, ISSN 2574-0970

Long-alkyl-chain alcohols are promising biofuel alternatives to fossil fuels; however, their detection has been a challenging task. In this study, the responses of nanoscale Pt-based sensors to aliphatic alcohols were examined, focusing on alkyl-chain-length and temperature dependencies. Atomistic simulations with a universal neural network potential were used to analyze the reaction and activation energies. Two types of Pt-based sensors were fabricated: a Pt nanosheet resistor sensor (PtNS) and a Pt nanoparticle-decorated graphene field-effect transistor sensor (PtNP-GFET). PtNP-GFET exhibited considerably higher sensor responses to 1-heptanol than to ethanol at 225 °C; moreover, PtNS showed the opposite alkyl-chain-length dependence compared to the response of PtNP-GFET at the same temperature. At a lower temperature of 150 °C, the sensor response toward ethanol and 1-heptanol in the case of PtNP-GFET was nearly identical, indicating that the response was primarily influenced by the concentration of hydroxyl groups. Atomistic simulation analysis revealed that the dehydrogenation reaction of the hydrocarbons near the hydroxyl groups was thermodynamically more favorable on the Pt147 nanocluster than on the Pt(111) surface. The dehydrogenation from the alkyl chain by PtNP was confirmed in alkanes; this dehydrogenation caused a higher sensor response of PtNP-GFET to 1-heptanol than to ethanol at 225 °C. Overall, employing nanomaterials, namely nanostructured Pt catalysts and a graphene-FET transducer, and using multiple sensing temperatures are key factors for realizing long-alkyl-chain alcohol recognitions. A low temperature of 150 °C is suitable for hydroxyl group sensing, while a high temperature of 225 °C is preferable for hydrocarbon group sensing.

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Efficient electrocatalytic H<inf>2</inf>O<inf>2</inf> evolution utilizing electron-conducting molecular wires spatially separated by rotaxane encapsulation

Chou S.Y., Masai H., Otani M., Miyagishi H.V., Sakamoto G., Yamada Y., Kinoshita Y., Tamiaki H., Katase T., Ohta H., Kondo T., Nakada A., Abe R., Tanaka T., Uchida K., Terao J.

Applied Catalysis B: Environmental （Applied Catalysis B: Environmental） 327 122373 - 122373 2023年06月

査読有り, ISSN 09263373

Immobilization of a catalytically active metal complex onto an electrode surface in highly dispersed form is crucial to assure high catalytic activity observed in solution. In this report, mononuclear CoII chlorin complexes were successfully immobilized on a conductive metal-oxide electrode by using electron conducting molecular wires comprising phenylene–ethynylene-based π-conjugation covered by a linked permethylated α-cyclodextrin to enhance electrocatalysis for selective two electron reduction of molecular oxygen to hydrogen peroxide. First, the rotaxane-encapsulation effect of the molecular wire on electron transfer was examined by the comparison of electrochemical behaviors of ferrocene (Fc) molecules immobilized on an ITO substrate using the molecular wires with or without rotaxane encapsulation. Then, electrodes were modified with metalloporphyrinoids, i.e., RhIII(OEP)Cl (OEP = 2,3,7,8,12,13,17,18-octaethylporphyrin) and CoII chlorin through coordination with the molecular wires with pyridine moieties at the end. The electrocatalytic O2 reduction was performed with CoII chlorin immobilized on an electron-conductive metal-oxide substrate by molecular wires. The turnover frequency for H2O2 production using the CoII chlorin coordinated by molecular wires with rotaxane encapsulation was 331 ± 75, which is significantly higher than that of 82 ± 8 obtained for CoII chlorin immobilized by that without rotaxane encapsulation. These results clearly indicate that the molecular wires with rotaxane encapsulation are beneficial for CoII chlorin complexes to exhibit high electrocatalytic activity and selectivity to H2O2.

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Study on Sub-100-NM-Scale Measurement of Temperature Distribution in Joule-Heated Au Nanosheet Gas Sensors Using Self-Assembled Monolayers as Temperature Probes

Kato T., Tanaka T., Miyagishi H., Terao J., Uchida K.

7th IEEE Electron Devices Technology and Manufacturing Conference: Strengthen the Global Semiconductor Research Collaboration After the Covid-19 Pandemic, EDTM 2023 （7th IEEE Electron Devices Technology and Manufacturing Conference: Strengthen the Global Semiconductor Research Collaboration After the Covid-19 Pandemic, EDTM 2023） 2023年

We present a new method for temperature distribution measurement with sub-100-nm spatial resolution using molecular modification on device surfaces. In this method, a Au nanosheet modified with self-assembled monolayers (SAM) is locally heated by Joule heating. The SAM molecules are desorbed due to the local heating, and the temperature is estimated from the molecular distribution evaluated by Kelvin probe force microscopy (KFM). The validity of the temperature estimation is confirmed by finite-element-analysis (FEA) results. This method is useful for visualizing thermal distribution in nanoscale devices such as metal nanosheet gas sensors.

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Low-resistance (NH<inf>4</inf>)xWO<inf>3</inf>Nanowire Sensors for Acetone Recognition Operating at Low Voltage with Low Power Consumption

Narita Y., Tanaka T., Uchida K.

A small-size and low-power consumption acetone sensor for the possible integration with LSIs was realized by a low-resistance (NH4) x WO3 nanowire. By utilizing hydrothermal synthesis, a large amount of NH4+ was inserted in the hexagonal tunnel of h-WO3 nanowires. The insertion and the activation of NH4+ were confirmed by FT-IR and XPS. The low nanowire resistance of 1.9kΩ made it possible to realize self-Joule heating effective to activate catalytic reaction with low applied voltage of 0.60 V and low power consumption of 0.11 mW. With the self-Joule heating, selective acetone sensing against ethanol was successfully demonstrated.

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Origin of the High Selectivity of the Pt-Rh Thin-Film H<inf>2</inf>Gas Sensor Studied by Operando Ambient-Pressure X-ray Photoelectron Spectroscopy at Working Conditions

Toyoshima R., Tanaka T., Kato T., Uchida K., Kondoh H.

Journal of Physical Chemistry Letters （Journal of Physical Chemistry Letters） 13 （ 36 ） 8546 - 8552 2022年09月

査読有り, ISSN 1948-7185

The Pt-Rh thin-film sensors exhibit excellent sensitivity and selectivity for H2 gas detection. Here, we studied the mechanism of highly selective detection of H2 by the Pt-Rh thin-film sensors with ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) measurements at working conditions, which were paralleled with electric resistivity measurements. The elemental composition and chemical state of surface Pt and Rh drastically change depending on the background gas environments, which directly link to the sensor response. It is revealed that surface segregated Pt atoms accelerate dissociative adsorption of H2, resulting in a reduction of the sensor surface and then a decrease of electric resistivity of the film, whereas a thin oxidized Rh layer blocks dissociation of the other reducing agent, that is, NH3. This is supported from the adsorption energetics obtained by the density functional theory (DFT) calculations.

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Experimental study on shallow and deep dopant properties at the interface of PtO <inf>x</inf> /ZnO Schottky diodes

Matsumura M., Tanaka T., Uchida K.

Japanese Journal of Applied Physics （Japanese Journal of Applied Physics） 61 （ SD ） 2022年06月

査読有り, ISSN 00214922

The donor properties at the interface of the PtO x /ZnO Schottky diodes on hydrothermally grown ZnO were investigated. From the capacitance-voltage (C-V) characteristics of the diode and Hall effect measurements of the bulk substrate, we revealed that the carrier concentration at the ZnO Schottky diodes interface is much higher than that in the bulk. By utilizing the C-V characteristics and the deep level transient spectroscopy, we demonstrated that the interfaces have two kinds of ionized donors; the shallow and deep donors, whose activation energies were 4 meV and 0.33 eV, respectively. Both of the two types of donors were ionized near the interface, whereas only the low-concentration shallow donors were ionized in the bulk. The shallow and deep donors were assigned to hydrogen interstadials and E3 defects, respectively. Since both the donor types are related to hydrogens, annealing in the oxygen atmosphere was effective to greatly suppress the donor concentration at the diode interface.

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Simultaneous Detection of Mixed-Gas Components by Ionic-Gel Sensors with Multiple Electrodes

Tanaka T., Hamanaka Y., Kato T., Uchida K.

ACS Sensors （ACS Sensors） 7 （ 3 ） 716 - 721 2022年03月

査読有り, ISSN 2379-3694

The sensing of gas components in a mixed gas is required for breath-based health monitoring and diagnosis. In this work, we report the simultaneous detection of mixed-gas components using a sensor consisting of [EMIM][BF4]-based ionic gel with four electrodes made of Au, Pt, Rh, and Cr. The voltage between any given pair of electrodes depends on the gas molecules absorbed in the ionic gel and the elements the electrodes are made of. When the voltage signals between all pairs of electrodes were used, H2, NH3, and C2H5OH concentrations were simultaneously estimated by a neural-network-based inference. From molecular dynamics simulations, the origin of the voltage signal was attributed to the catalytically generated adsorbates on the electrodes.

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Cryogenic CMOS Performance Analysis Including BEOL Characteristics at 4K for Quantum Controller Application

Okamoto K., Tanaka T., Miyamura M., Ishikuro H., Uchida K., Sakamoto T., Tada M.

2022 IEEE International Interconnect Technology Conference, IITC 2022 （2022 IEEE International Interconnect Technology Conference, IITC 2022） 139 - 141 2022年

Cryo-CMOS circuit performance at 4K including both BEOL and FEOL characteristics has been investigated in a 65nm bulk CMOS for the first time. ON-current (Ion) of n/pMOSFET are improved +25%/+9% with excellent gate modulation (Ion/Ioff=∼109). Cu line/via resistances decrease with temperature due to reduction of phonon scattering, and -75%/-20% lower resistances are obtained at 4K. It is revealed that there is no inter-line capacitance change and no severe Joule heating effect (JHE) of Cu BEOL at 4K. The newly developed 4K-SPICE model including BEOL characteristics enables accurate CMOS circuit design at 4K, giving 5 ∼ 40% faster operation of RC line with clear dependence on driver-size and interconnect-load.

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Cryogenic operation of NanoBridge at 4 K for controlling qubit

Okamoto K., Tanaka T., Miyamura M., Ishikuro H., Uchida K., Sakamoto T., Tada M.

Japanese Journal of Applied Physics （Japanese Journal of Applied Physics） 61 （ SC ） SC1049 - SC1049 2022年

査読有り, ISSN 00214922

A nonvolatile resistive switching of NanoBridgeTM (NB) at 4 K has been demonstrated for realizing the Quantum-classical interface (QCI), in which the challenging of reset operation at cryogenic temperature is successfully achieved. The set voltage of the NB is increased with decreasing temperature, saturated around 150 K and to be 2.55 V at 4 K. The on-state resistances tuned at 1 k-5 kω show small temperature dependence down to 4 K due to high residual resistivity. The increased reset current of the NB at 4 K is compensated by the process optimization with thermal engineering and the increased I dsat of the select transistor at 4 K, resulting in stable switching. The low-power QCI featuring NBs is a strong candidate for controlling a large number of qubits at cryogenic temperature.

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Enhanced Drain Current in Transient Mode due to Long Ionization Time of Shallow Impurities at 4 K in 65-nm bulk Cryo CMOS Transistors

Miyao T., Tanaka T., Imanishi I., Ichikawa M., Nakagawa S., Ishikuro H., Sakamoto T., Tada M., Uchida K.

Device Research Conference - Conference Digest, DRC （Device Research Conference - Conference Digest, DRC） 2022-June 1 - 2 2022年

ISSN 15483770

Despite the importance of cryo CMOS technologies in quantum computing systems, the transient behaviors of cryo MOS transistors have been less studied. In this work, in advanced CMOS transistors we observed sub-us transient drain current (IdTrans) that was much greater than the static drain current (IdStatic) at 4 K (Fig. 6); the transient-to-static ratio r=IdTrans/IdStatic reached as large as 2.7 (Fig. 9), whereas r stays at one in the same device at 20 K. The observed transient characteristics are not due to the self-heating effects, but due to the long emission time of holes from acceptors at 4 K. After applying biases, more electrons flow into the channel than those in static conditions to mitigate the frozen acceptors. IdTrans. goes down to IdStatic because of gradual ionization of acceptors. We consider that the observed transient behavior needs to be considered in cryo MOSFET model to accurately design cryo LSI circuits.

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In Situ Monitoring Technique of Self-Heating in Bulk MOSFETs at Cryogenic Temperatures using Subthreshold Current

Ichikawa M., Tanaka T., Uchida K., Miyao T., Tada M., Ishikuro H.

2022 IEEE Latin America Electron Devices Conference, LAEDC 2022 （2022 IEEE Latin America Electron Devices Conference, LAEDC 2022） 2022年

This work proposes an evaluation technique of self-heating (SH) in a 65-nm bulk CMOS transistor at cryogenic temperature (4.2K). For the in-situ monitoring of self-heating in cryo-CMOS circuits, transient response of the subthreshold drain current is used, which makes it possible to estimate the temperature at on-state of MOSFETs. The relation between the temperature increase by self-heating and power consumption of MOSFET has been experimentally obtained, which will help a design of high performance cryo-CMOS circuits for quantum computer application.

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Atomistic Simulation Study of Impacts of Surface Carrier Scatterings on Carrier Transport in Pt Nanosheets

Tanaka T., Kato T., Yajima T., Uchida K.

IEEE Electron Device Letters （IEEE Electron Device Letters） 42 （ 7 ） 1057 - 1060 2021年07月

査読有り, ISSN 07413106

The understanding of carrier transport in metal nanostructures is indispensable for the development of nanoelectronics. In particular, Pt nanostructures have been intensively studied to realize gas sensors based on adsorbate-induced surface electron scattering. Conventionally, electron scattering at the surface of metal nanostructures has been phenomenologically described by a single specularity parameter. In this work, surface electron scattering was quantitatively studied through molecular dynamics simulations, followed by density functional nonequilibrium Green's function calculations. Although the extracted specularity parameters qualitatively agreed with empirically treated diffusive scattering at the O-covered Pt surface and specular scattering at the H-covered Pt surface, our atomistic calculation revealed an increase in resistivity owing to H adsorption on thin Pt(111) nanosheets.

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Temperature dependence of resistivity increases induced by thiols adsorption in gold nanosheets

Kato T., Tanaka T., Yajima T., Uchida K.

Japanese Journal of Applied Physics （Japanese Journal of Applied Physics） 60 （ SB ） SBBH13 - SBBH13 2021年05月

査読有り, ISSN 00214922

By utilizing stable thiol self-assembled monolayers (SAMs) on gold (Au) nanosheets, we investigated the temperature dependence of the amount of resistivity increases Drad by SAM modification. Drad increased with increasing temperature from 5 to 375 K in all the measured nanosheets for various thicknesses (9-20 nm). We consider from the DFT calculation that the temperature dependence originates from unoccupied states of sulfur atoms above the Fermi level of Au. The available unoccupied states increase at higher temperatures, which enhances scattering rates and thus increases the resistivity. Furthermore, we calculated the temperature dependence of Drad using the Bloch-Grüneisen theory and Fuchs-Sondheimer (FS) model. Based on the results, we conclude that for ultra-thin metal films, the temperature-dependent specularity parameter PT which represents the fraction of electrons scattered specularly should be introduced into the conventional FS model to predict Drad over a wide temperature range.

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Regulating phase transformation kinetics via redox reaction in ferroelectric Ge-doped HfO2

T. Yajima, T. Nishimura, S. Migita, T. Tanaka, K. Uchida, A. Toriumi

Applied Physics Letters （AIP Publishing） 117 （ 18 ） 182902 - 182902 2020年11月

査読有り, ISSN 0003-6951

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In situ AP-XPS analysis of a Pt thin-film sensor for highly sensitive H2 detection

Ryo Toyoshima, Takahisa Tanaka, Taro Kato, Ken Uchida, Hiroshi Kondoh

Chemical Communications （Royal Society of Chemistry (RSC)） 56 （ 70 ） 10147 - 10150 2020年07月

査読有り, ISSN 1359-7345

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Impact of defects in self-assembled monolayer on humidity sensing by molecular functionalized transistors

T. Tanaka, T. Yajima, K. Uchida

Jpn. J. Appl. Phys. 2020年03月

筆頭著者, 責任著者, 査読有り

Modulation of VO2 Metal–Insulator Transition by Ferroelectric HfO2 Gate Insulator

T. Yajima, T. Nishimura, T. Tanaka, K. Uchida, A. Toriumi

Advanced Electronic Materials （Wiley） 6 （ 5 ） 1901356 - 1901356 2020年03月

査読有り, ISSN 2199-160X

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Co-porphyrin functionalized CVD graphene ammonia sensor with high selectivityto disturbing gases: hydrogen and humidity

K. Sawada, T. Tanaka, T. Yokoyama, R. Yamachi, Y. Chiba, H. Masai, J. Terao, K. Uchida

Jpn. J. Appl. Phys （IOP Publishing） 59 （ SG ） SGGG09 - SGGG09 2020年02月

査読有り, ISSN 0021-4922

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Low-Power and ppm-Level Multimolecule Detection by Integration of Self-Heated Metal Nanosheet Sensors

T. Tanaka, K. Tabuchi, K. Tatehora, Y. Shiiki, S. Nakagawa, T. Takahashi, R. Shimizu, H. Ishikuro, T. Kuroda, T. Yanagida, K. Uchida

IEEE Trans. Electron Devices （IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC） 66 （ 12 ） 5393 - 5398 2019年10月

筆頭著者, 査読有り, ISSN 0018-9383

H-2 and NH3 detection with low power consumption was demonstrated by integrated chemiresistive Pt and PtRh nanosheet sensors on glass substrates. The self-heating effects realized low power and local heating of metal nanosheet sensors, enabling the integration of sensors with different operating temperatures. Based on different resistance changes in Pt and PtRh nanosheets toward H-2 and NH3, the concentration of each gas was detected from a gas mixture by consuming around 1-mW power. For decreasing the power consumption and further integration of sensors, sensor scaling and pulsed operations were numerically and experimentally studied. In addition to good connectivity of metal nanosheet sensors to large-scale integration (LSI) circuits, improvements of the power consumption by sensor scaling were proven. The pulsed operations required for integrated sensor arrays maintained a sensor response, or a resistance change, of approximately 60, even when the power consumption was reduced by 20.

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Nanoscale, low-energy molecular sensors for health care and environmental monitoring

K. Uchida, T. Tanaka

AAPPS Bulletin 29 16 - 20 2019年06月

Enhancement of sensor response in graphene gas sensors by gate-induced field

T. Tanaka, T. Yokoyama, K. Uchida

IEEE Electron Device Lett. 39 1924 - 1927 2018年10月

筆頭著者, 査読有り

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Pd-functionalized, suspended graphene nanosheet for fast, low-energy multimolecular sensors

T. Yokoyama, T. Tanaka, Y. Shimokawa, R. Yamachi, Y. Saito, K. Uchida

ACS Appl. Nano Mater. 1 3886 - 3894 2018年07月

査読有り

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Numerical analysis of band tails in nanowires and their effects on the performance of tunneling field-effect transistors

Takahisa Tanaka, Ken Uchida

Japanese Journal of Applied Physics （Japan Society of Applied Physics） 57 （ 6 ） 06HC04 2018年06月

筆頭著者, 査読有り, ISSN 1347-4065

Band tails in heavily doped semiconductors are one of the important parameters that determine transfer characteristics of tunneling field-effect transistors. In this study, doping concentration and doing profile dependences of band tails in heavily doped Si nanowires were analyzed by a nonequilibrium Green function method. From the calculated band tails, transfer characteristics of nanowire tunnel field-effect transistors were numerically analyzed by Wentzel-Kramer-Brillouin approximation with exponential barriers. The calculated transfer characteristics demonstrate that the band tails induced by dopants degrade the subthreshold slopes of Si nanowires from 5 to 56mV/dec in the worst case. On the other hand, surface doping leads to a high drain current while maintaining a small subthreshold slope.

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Nanoscale Pt thin film sensor for accurate detection of ppm level hydrogen in air at high humidity

Takahisa Tanaka, Shinsuke Hoshino, Tsunaki Takahashi, Ken Uchida

Sensors and Actuators, B: Chemical （Elsevier B.V.） 258 913 - 919 2018年04月

筆頭著者, 査読有り, ISSN 0925-4005

Hydrogen is an important biomarker for the human digestive system. However, accurate detection of ppm-level hydrogen in breath is difficult due to the competing detection of high concentration water. We fabricated Pt thin films that respond to hydrogen in air at concentrations as low as 500 ppb. In both dry and humid air, these films have almost identical response to hydrogen, i.e., their resistance decreases linearly with increasing hydrogen concentration regardless of relative humidity. Even at high relative humidity, these Pt thin films can detect ppm-level hydrogen. Furthermore, it was strongly suggested that these films can be applied to low-level hydrogen in the air expired by a healthy human. Based on the chemical kinetics, namely the adsorption and desorption of hydrogen and oxygen, the sensor response is quantitatively described by relating the hydrogen surface coverage to the magnitude of electron scattering at the Pt surface. The proposed model successfully reproduces the effects of hydrogen concentration and time on the sensor response, particularly at hydrogen concentrations below 20 ppm. Based on this model, these Pt thin film sensors have the potential to detect 1 ppm hydrogen in expired air within 30 s.

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Experimental Study on Deformation Potential (D-ac) in MOSFETs: Demonstration of Increased D-ac at MOS Interfaces and Its Impact on Electron Mobility

Teruyuki Ohashi, Takahisa Tanaka, Tsunaki Takahashi, Shunri Oda, Ken Uchida

IEEE JOURNAL OF THE ELECTRON DEVICES SOCIETY （IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC） 4 （ 5 ） 278 - 285 2016年09月

査読有り, ISSN 2168-6734

Deformation potential (D-ac), which is one of the most important parameters determining the rate of electron-acoustic phonon scattering, in Si around MOS interfaces is thoroughly studied with regard to the dependences on surface carrier densities, back-gate biases, and device structures. It is demonstrated that D-ac increases sharply at the MOS interface. To investigate the impact of the increased D-ac on mu(e), thick body-channel SOI MOSFETs, where drain current flows in the entire SOI layers, was fabricated. The carrier transport experiments reveal that mu(e) of greater than 1100 cm(2) V-1 s(-1) is obtained in body-channel SOI MOSFETs with the SOI thickness of greater than 70 nm. By taking into account the D-ac profile around the MOS interface, experimental mu(e) of SOI MOSFETs is numerically reproduced over a wide range of SOI thicknesses. mu(e) of the body-channel SOI MOSFETs is also well reproduced using the same D-ac profile. Thus, it is concluded that D-ac increases sharply at the Si/SiO2 interface. The accurate modeling of the increased D-ac around the Si/SiO2 interface is indispensable for designing high-performance and/or low-power 3-D MOSFETs including FinFETs, extremely thin SOI MOSFETs, and nanowire MOSFETs, because these types of MOSFETs have greater interface-to-volume ratios.

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Deionization of Dopants in Silicon Nanofilms Even with Donor Concentration of Greater than 10(19) cm(-3)

Takahisa Tanaka, Yuya Kurosawa, Naotoshi Kadotani, Tsunaki Takahashi, Shunri Oda, Ken Uchida

NANO LETTERS （AMER CHEMICAL SOC） 16 （ 2 ） 1143 - 1149 2016年02月

筆頭著者, 査読有り, ISSN 1530-6984

Understanding the dopant properties in heavily doped nanoscale semiconductors is essential to design nanoscale devices. We report the deionization or finite ionization energy of dopants in silicon (Si) nanofilms with dopant concentration (N-D) of greater than 10(19) cm(-3), which is in contrast to the zero ionization energy (E-D) in bulk Si at the same N-D. From the comparison of experimentally observed and theoretically calculated E-D, we attribute the deionization to the suppression of metal-insulator transition in highly doped nanoscale semiconductors in addition to the quantum confinement and the dielectric mismatch, which greatly increase E-D in low-doped nanoscale semiconductors. Thus, for nanoscale transistors, N-D should be higher than that estimated from bulk Si dopant properties in order to reduce their resistivity by the metal insulator transition.

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Impact of hydrogen on carrier mobility and concentration in graphene decorated with Pd nanoparticle

Akihito Goto, Go Takeuchi, Ryosuke Yamachi, Takahisa Tanaka, Tsunaki Takahashi, Ken Uchida

ECS Transactions （Electrochemical Society Inc.） 72 （ 14 ） 7 - 12 2016年

査読有り, ISSN 1938-6737

As a candidate of highly-sensitive hydrogen sensors, Palladium (Pd)-decorated graphene sensors have been investigated. Thanks to high reactivity of Pd nanoparticles to hydrogen and high surface-tovolume ratio of two-dimensionally layered graphene, graphene decorated with Pd nanoparticles can be used as a highly sensitive hydrogen sensor. Although the conductivity of Pd-graphene sensor is known to be greatly changed in hydrogen atmosphere, the mechanisms of conductivity change have not been fully understood. In this work, both carrier concentration and mobility in hydrogen are investigated using Hall effect measurement. It is shown that relative change of carrier concentration is greater when carrier concentration is lower. On the other hand, mobility is not systematically changed as a function of carrier concentration. This mobility behavior is attributed to the competition between greater effective mass and shorter screening length at higher carrier concentration. Based on these observations, we develop strategy for achieving high sensitivity in Pd graphene hydrogen sensor.

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Upper limit of two-dimensional hole gas mobility in strained Ge/SiGe heterostructures

T. Tanaka, Y. Hoshi, K. Sawano, N. Usami, Y. Shiraki, K. M. Itoh

APPLIED PHYSICS LETTERS （AMER INST PHYSICS） 100 （ 22 ） 222102 2012年05月

筆頭著者, 査読有り, ISSN 0003-6951

High two-dimensional hole gas (2DHG) mobility (mu(2DHG) > 10000 cm(2)/Vs at T < 100 K) strained Ge/Si1-xGex structures with x = 0.5 and 0.65 were fabricated, and temperature dependence of their 2DHG mobility was obtained experimentally by the mobility spectrum analysis of the conductivity under magnetic fields. The theoretically calculated 2DHG mobility was compared to experimental data to determine the effective deformation potentials for scattering by acoustic and optical phonons. Using empirically confirmed parameters, the upper theoretical limit of room temperature 2DHG mobility mu(2DHG) in strained Ge as a function of strain was calculated. The possibility to achieve mu(2DHG) > 5000 cm(2)/Vs at room temperature is presented. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4723690]

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Experimental and theoretical analysis of the temperature dependence of the two-dimensional electron mobility in a strained Si quantum well

Takahisa Tanaka, Go Tsuchiya, Yusuke Hoshi, Kentarou Sawano, Yasuhiro Shiraki, Kohei M. Itoh

JOURNAL OF APPLIED PHYSICS （AMER INST PHYSICS） 111 （ 7 ） 073715 2012年04月

筆頭著者, 査読有り, ISSN 0021-8979

The temperature dependence of the mobility of the two-dimensional electron gas (2DEG) in a silicon quantum well strained by Si0.7Ge0.3 relaxed buffer layer is determined precisely by a mobility spectrum analysis. The 2DEG mobility is 2780 cm(2)/V s at room temperature and, upon cooling, increases continuously to reach mu(2DEG) = 7.4 x 10(4) cm(2)/V s at 7K. A back gate installed on the sample changes the 2DEG concentration n successfully to establish mu(2DEG) alpha n(1.4) at the constant temperature T = 10K, implying that the scattering at such low temperature is limited solely by the remote ionized impurity scattering. Based on this finding, theoretical analysis of the temperature dependence of mu(2DEG) is performed based on the relaxation time approximation using 2DEG wavefunctions and subband structures determined self-consistently and including three major scatterings; by intravalley acoustic phonons, intervalley g-processes of longitudinal optical (LO) phonons, and remote ionized impurities. The calculation included only three fitting parameters, the shear deformation potential (Xi(u) = 9.5 eV), LO phonon deformation potential for g-process scattering (D-0 = 9.0 x 10(8) eV/cm), and sheet density of remote ionized impurities that have been determined by quantitative comparison with our experimental results. The temperature dependence of mu(2DEG) calculated theoretically show excellent agreement with experimentally determined mu(2DEG). (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3702464]

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