Showing papers by "Hiroshi Yamaguchi published in 2007"

15N−15N J-Coupling Across HgII: Direct Observation of HgII-Mediated T−T Base Pairs in a DNA Duplex

Abstract: N−N J-coupling across a metal center (2JNN) was clearly detected in a biological macromolecule (DNA duplex) for the first time. By using 2JNN, the base pairing mode of mercury-mediated T−T pairs (T−HgII−T) was definitely determined. This pairing mode was found to be a novel metal ion-binding mode for DNA and RNA molecules, in which imino proton−metal exchange processes are included. Accordingly, 2JNN is highly important for the determination of the chemical structures of metal-mediated base pairs.

Conductance modulation by individual acceptors in Si nanoscale field-effect transistors

Abstract: The authors measured low-temperature (6–28K) conductance in nanoscale p-channel field-effect transistors lightly doped with boron. They observed a conductance modulation, which they ascribed to the trapping/detrapping of single holes by/from individual acceptors. The statistics of the appearance of the modulation in a few ten samples indicates that the number of acceptors is small, or even just one, suggesting that what the authors have observed is single-charge-transistor operation by a single-acceptor quantum dot.

Controllable coupling between flux qubit and nanomechanical resonator by magnetic field

Abstract: We propose an active mechanism for coupling the quantized mode of a nanomechanical resonator to the persistent current in the loop of a superconducting Josephson junction (or phase slip) flux qubit. This coupling is independently controlled by an external coupling magnetic field. The whole system forms a novel solid-state cavity quantum electrodynamics (QED) architecture in the strong coupling limit. This architecture can be used to demonstrate quantum optics phenomena and coherently manipulate the qubit for quantum information processing. The coupling mechanism is applicable for more generalized situations where the superconducting Josephson junction system is a multi-level system. We also address the practical issues concerning experimental realization.

Diffusion of transition-metal impurities in silicon

Abstract: Diffusion of 3d transition-metal impurities in silicon has been theoretically studied. The chemical trend in the diffusion barrier over all the elements is well reproduced by ab initio pseudopotential method. However, for heavier elements, the energy barriers are so low that the effect of the kinetic energy of atoms cannot be ignored. Various related energies are investigated by taking Cu as an example. The conditions for the presence of substitutional Cu are given.

Growth of very-high-mobility AlGaSb /InAs high-electron-mobility transistor structure on si substrate for high speed electronic applications

Abstract: The growth of the AlGaSb∕InAs high-electron-mobility transistor (HEMT) epitaxial structure on the Si substrate is investigated. Buffer layers consisted of UHV/chemical vapor deposited grown Ge∕GeSi and molecular beam epitaxy-grown AlGaSb∕AlSb∕GaAs were used to accommodate the strain induced by the large lattice mismatch between the AlGaSb∕InAs HEMT structure and the Si substrate. The crystalline quality of the structure grown was examined by x-ray diffraction, transmission electron microscopy, and atomic force microscopy. Finally, very high room-temperature electron mobility of 27300cm2∕Vs was achieved. It is demonstrated that a very-high-mobility AlGaSb∕InAs HEMT structure on the Si substrate can be achieved with the properly designed buffer layers.

A $\delta$ -Doped InGaP/InGaAs pHEMT With Different Doping Profiles for Device-Linearity Improvement

Abstract: In this paper, delta-doped InGaP/InGaAs pseudomorphic high-electron-mobility transistors (pHEMTs) with doping-profile modifications are investigated in order to improve the device linearity. The proposed modification was based on the third-order intermodulation distortion (IM3) and the third-order intercept point (IP3) analysis using a simple equivalent circuit of the devices. The correlations of the extrinsic transconductance (Gm) with IM3 and IP3 indicate that the flatness of Gm, as a function of gate-bias causes a lower IM3 level. On the other hand, a high Gm with a flatter Gm distribution results in higher IP3 value for the device. Therefore, doping modifications that improve the flatness of the Gm distribution will also improve the device linearity. Doping modifications in the Schottky layer (Schottky layer doped) and in the channel layer (channel doped) of the conventional delta-doped InGaP/InGaAs pHEMT were investigated. It was also found that extra doping, either in the channel region or in the Schottky layer, improved the flatness of the Gm distribution under different gate-bias conditions. This achieved a lower IM3 and a higher IP3 with a small sacrifice in the peak Gm value. The power performances of these devices were tested at different drain biases. Even though it had the lowest electron mobility among the three different types of devices studied, the channel-doped device demonstrated the best overall linearity performance, the highest IP3 value, the lowest IM3 level, and the best adjacent-channel power ratio under code-division multiple-access modulation.

Two-dimensional Arrangement of Vertically Oriented Cylindrical Domains of Diblock Copolymers using Graphoepitaxy with Artificial Guiding Pattern Layout

Abstract: In this paper, we demonstrate that a two-dimensional arrangement of vertical cylindrical domains of asymmetric poly(styrene-b-methyl methacrylate) (PSt-b-PMMA) is achieved by graphoepitaxy using a resist pattern as the guide for alignment. The key to success is the combination of the neutralization of a bottom surface and the introduction of a two-dimensional hydrophilic guiding pattern, which leads to vertical orientation and lateral arrangement of cylindrical domains, respectively.

Infrared detection with silicon nano-field-effect transistors

Abstract: The authors fabricated nanoscale silicon metal-oxide-semiconductor field-effect transistors (MOSFETs) to detect an infrared (IR) signal at room temperature. The IR signal excites conduction-band electrons in an undoped channel of a MOSFET and some of them are injected through an energy barrier into a storage node (SN) electrically formed by the MOSFET. Small signals, originating from electrons, stored in the SN are detected by an electrometer with a single-electron resolution. Additionally, the MOSFET controls the number and energy of electrons injected into the SN. This enables electrical control of the sensitivity and cutoff wavelengths of IR signals, suggesting the possibility of highly functional IR sensors.

Nanogap electrodes on Si cantilever for local conductance measurement

Abstract: We developed a new nanotool of scanning probe microscopy (SPM) for local conductance measurement. Two Pt electrodes with a nanogap fabricated by focused ion beam (FIB) milling are integrated on a Si cantilever with Al electrodes. The minimum gap fabricated on the cantilever is 20 nm. Local conductance measurements of conductive materials, a carbon film and a gold film, were performed using the nanogap probe on a conventional SPM system. Ohmic contacts between the gap electrodes and the conductive samples were established. The resolution of the conductance image is almost the same as the gap distance of the electrodes. A single gold grain was successfully imaged with sub-100-nm resolution.

Field-Effect Transistor with Deposited Graphite Thin Film

Abstract: By using a bottom-gate top-contact field-effect transistor structure, the field effect of graphite-rich carbon nanocrystallite thin films deposited by electron cyclotron resonance sputtering was investigated. An appreciable ambipolar field effect was observed at the film edge where the thickness was vanishing. On–off current ratios of 2 and 7 were attained at 294 and 150 K, respectively.

Modulation of Young's Modulus of Poly(methyl methacrylate) Nanobeam Due to Electron-Beam Exposure

Abstract: We fabricated suspended poly(methyl methacrylate) (PMMA) micro/nanobeams using a novel two-step development process. Young's moduli measured for the fabricated beams were found to decrease with additional electron-beam (EB) exposure. This means that we can locally control Young's modulus in PMMA structure using EB exposure and thus effectively design nanoelectromechanical systems using this organic material. The effect was more significant for smaller beams, suggesting that the modulus of the near-surface region is largely affected.

Photoluminescence Dynamics of GaAs/AlAs Quantum Wells Modulated by Surface Acoustic Waves

Abstract: We have investigated the dynamics of photoluminescence (PL) of GaAs/AlAs quantum wells under strain and piezoelectric modulation introduced by a surface acoustic wave (SAW). Measurements performed using a microscopic excitation spot show that the efficiency of PL quenching due to exciton ionization and the subsequent sweeping of free electrons and holes by piezoelectric potential varies significantly with quantum well thickness. This variation is attributed to the well-thickness dependence of carrier mobility and diffusivity. The relative timing between carrier generation pulse and SAW-induced band structure modulation also changes the delay of PL quenching and PL decay time under the SAW field.

Impact of space-energy correlation on variable range hopping in a transistor.

Abstract: Hopping conduction in transistors, i.e., under a transverse electric field, is addressed using percolation theory with a space-energy correlation in the density of states of the impurity band. The computation of the percolation threshold over an extended range of correlation parameters enables us to derive a formula, which, while giving the classical results in the low field limit, describes the emergence of a specific variable range hopping in the high field case. An application of this formula to experimentally extract the localization radius is also proposed.

Transfer and Detection of Single Electrons Using Metal-Oxide-Semiconductor Field-Effect Transistors

Abstract: A single-electron turnstile and electrometer circuit was fabricated on a silicon-on-insulator substrate. The turnstile, which is operated by opening and closing two metal-oxide-semiconductor field-effect transistors (MOSFETs) alternately, allows current quantization at 20 K due to single-electron transfer. Another MOSFET is placed at the drain side of the turnstile to form an electron storage island. Therefore, one-by-one electron entrance into the storage island from the turnstile can be detected as an abrupt change in the current of the electrometer, which is placed near the storage island and electrically coupled to it. The correspondence between the quantized current and the single-electron counting was confirmed.

Giant Magneto-Piezoresistance and Internal Friction in a Two-Dimensional Electron System

Abstract: We use a micromechanical cantilever with an integrated two-dimensional electron system to show that an extremely small strain of the order of 10-4 induces a localized–delocalized electronic state transition. This strong strain effect improves the piezoresistive gauge factor by more than two orders of magnitude compared to the conventional Si cantilever. Furthermore, we found that the cantilever mechanical motion is affected considerably by friction exerted by the electron systems.

Local Conductance Imaging of Semiconductor Nanowires on an Insulative Substrate Using an Integrated Nanogap Probe

Abstract: We have developed a new scanning probe microscopy (SPM)-based tool that enables local in-plane conductance measurements using a split electrode (nanogap) probe. The nanogap integrated on a silicon cantilever was fabricated by focused ion beam (FIB) milling. The integrated nanotool with 45-nm-gap electrodes was used to perform two-terminal conductance measurements of semiconductor nanowires on an insulator substrate. A local conductance image of InAs nanowires grown on a GaAs substrate was successfully obtained with sub-100-nm resolution.

Control of impurity diffusion in silicon by IR laser excitation

Abstract: Control of the diffusion of a specific impurity species is desirable in Si device processes. IR laser excitation matching the impurity vibration mode is a promising method for this purpose. To illustrate the effectiveness of this method, first-principles molecular dynamics simulation has been applied. Technical issues of the simulation are described in detail. It is seen that resonant effects can be reproduced in adiabatic molecular dynamics simulations by applying an external force on the impurity only. The present study forms the basis for further developments of this approach.

Semiconductor heterostructure studies using emerging technologies

Abstract: Low-temperature scanning tunnelling microscopy (LT-STM), surface acoustic wave (SAW), and mechanical cantilever are used to measure and manipulate quantum characteristics in semiconductor heterostructures. A nanoscale detection of local density of states is demonstrated for electrons confined in semiconductor quantum wells and dots by a combination of InAs-based heterostructures and LT-STM. The SAW is used to realize moving quantum wires and dots in semiconductor heterostructures. A synchronized optical measurement confirms two types of modulation, i.e. bandgap and potential modulations. A combination of mechanical cantilever and quantum heterostructures gives a new mechanical functionality and a novel tool to analyse quantum transport characteristics. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron phase modulation in a high Q nanomechanical resonator

Abstract: The electron transport in a quasi one-dimensional electron system embedded in a high Q nanoelec-tromechanical system is investigated at mechanical resonance whilst being immersed in a uniform magnetic field. The resistance change, as a function of magnetic field (magnetopiezoresistance), in the quasi one-dimensional electron system at resonance showed highly periodic oscillations which arise via strain induced modulation of the electron phase. Our analysis reveal that mechanical activation of the quasi one-dimensional electron system affects only a few electron trajectories and the electron interference in only a single electron loop gives rise to the resistance oscillations in the magnetopiezoresistance.

Control of impurity diffusion by IR excitations

Abstract: IR laser excitation has potential to use for a method of selective diffusion of impurities in the Si device processes. To illustrate the effectiveness of this method, first-principles molecular dynamics simulation has been applied. Effectiveness of IR excitation is for the first time shown for those impurities which have a local mode. The laser power required for this method is estimated.

Low-energy electron emission using a Si/SiO 2 /Si cathode for nano-decomposition

Abstract: An SOS cathode was fabricated using MOSFET fabrication processes. The cathode can operate with low operation voltage in a low vacuum, which promises a simple system as well as molecular-level material engineering in various ambients.

Growth of InAs Channel HEMT Structure on Si substrate and It's Possible Application for Low Power Logic

Abstract: Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan, R.O.C. Phone: 886-3-5131536 E-mail: edc@mail.nctu.edu.tw NTT Basic Research Laboratories, NTT Corporation, 3-1 MorinosatoWakamiya, Atsugi, Kanagawa 243-0198, Japan Department of Physics, Tohoku University NTT Electronics Techno Corporation, SORST-JST, 4-1-8 Honmachi, Kawaguchi, Saitama 331-0012, Japan

Control of Young's Modulus for Carbon Nanopillars Grown by Focused Ion Beam Induced Chemical Vapor Deposition

Abstract: The aim of the paper is to discuss about the carbon nanopillars with different heights fabricated by FIB-CVD, achieved the uniformization of pillar diameter and the homogenization of Young's modulus for growth heights by controlling focus position of FIB.