Ryo Kuroda

Nanometer scale conductance change in a Langmuir‐Blodgett film with the atomic force microscope

A nanometer scale metal/Langmuir‐Blodgett (LB) film/metal structure is realized with an atomic force microscope combined with scanning tunneling microscope (AFM/STM). Even in this nanometer scale configuration, increase in conductance can be induced at any point in the LB film by application of a voltage pulse. The AFM/STM observation shows little surface modification has occurred by the voltage application, which shows that the conductance of the LB film changes without pit formation in the LB film or metal cluster deposition from the tip of the probe.

In-line phase-contrast imaging of a biological specimen using a compact laser-Compton scattering-based x-ray source

Laser-Compton scattering (LCS) x-ray sources have recently attracted much attention for their potential use at local medical facilities because they can produce ultrashort pulsed, high-brilliance, and quasimonochromatic hard x rays with a small source size. The feasibility of in-line phase-contrast imaging for a “thick” biological specimens of rat lumbar vertebrae using the developed compact LCS-X in AIST was investigated for the promotion of clinical imaging. In the higher-quality images, anatomical details of the spinous processes of the vertebrae are more clearly observable than with conventional absorption radiography. The results demonstrate that phase-contrast radiography can be performed using LCS-X.

Reduction of iron diffusion in silicon during the epitaxial growth of β-FeSi2 films by use of thin template buffer layers

We fabricated continuous highly (110)/(101)-oriented β-FeSi2 films on Si (111) substrates by the facing-target sputtering method. An epitaxial thin β-FeSi2 template buffer layer preformed on the silicon substrate was found to be essential in the epitaxial growth of thick β-FeSi2 films. It was proved that the template reduced the iron diffusion into the silicon substrate during thick β-FeSi2 film fabrication. Even though the annealing was performed at high temperature (880u200a°C) for a long duration (10 h), iron diffusion was effectively hindered by the template. By introducing this template buffer layer, an abrupt interface without appreciable defects between the β-FeSi2 film and the silicon substrate formed. The mechanism for the reduction of iron diffusion by the template buffer layer is discussed.

Switching Characteristics of Composite Magnetic Thin Films

The effects of the strong exchange coupling between magnetic spins within a composite film, composed of multiple layers of ferromagnetics, are represented by biaxial and triaxial anisotropy terms to be added to the ordinary effective magnetic energy. The influences of these multiaxial terms on the form of the critical switching curves are studied theoretically and experimentally. An experimental result is given showing a considerable improvement of the pulse coincidence switching characteristic of an ordinary uniaxially anisotropic permalloy film by adding a very thin Ni-Co alloy layer thereupon.

Radially polarized terahertz waves from a photoconductive antenna with microstructures

The generation of radially polarized terahertz (THz) waves from a photoconductive antenna (PCA) with radial microstructures was investigated. Microstructured electrodes with 10u2009μm lines and spaces were fabricated on a semi-insulating InP wafer using photolithography. The PCA with three layers was driven by an amplified femtosecond laser and DC power supply. Radially polarized THz waves from the PCA were measured by a Michelson interferometer with a 4.2u2009K silicon bolometer. The polarization components of THz waves were analyzed by a wire grid polarizer.

Influence of Si∕Fe ratio in multilayer structures on crystalline growth of β-FeSi2 thin film on Si substrate

The deposited Si∕Fe ratio has been found to have a significant influence on crystalline growth of β-FeSi2 film on Si substrate. Stoichiometry is always satisfied by interdiffusion of Si, under both Si-lean and Si-rich conditions. However, Si diffusion from the substrate into the deposited layer, which compensates for deficient Si, induces an undulated interface, as well as Fe and Si vacancies. On the other hand, excess Si is driven to the β-FeSi2∕Si interface, which results in a lamellar structure with a large number of small grains. Fe and Si vacancies are significantly reduced by excess Si. These results suggest that precise control of the Si∕Fe ratio is essential for good crystallinity and fewer vacancies.

Boron doping for p-type β-FeSi2 films by sputtering method

High quality epitaxial β-FeSi2 thin films prepared by alternate Fe/Si multilayers stacking were doped for p-type by co-sputtering of silicon and boron, in which elemental boron chips were placed on silicon target. The starting β-FeSi2 films before doping were n-type with residual electron concentration of about 2 ×1017 cm-3 and mobility of about 200 cm2/Vs. After doping with boron, β-FeSi2 films showed the same epitaxial crystallinity with continuous structure as that of non-doped one. Doping level of p-type β-FeSi2 films with net hole concentration from 3 ×1017 to 1 ×1019 cm-3 and mobility from 100 to 20 cm2/Vs were successfully achieved. Desired net hole concentration was obtained by varying the area ratio of boron chips on silicon target.

Localized Surface Plasmon Resonant Metal Nanostructures as Refractive Index Sensors

The sensitivities of localized surface plasmon resonant sensors can be improved by changing the shape, size, and arrangement of nanosized metal dots on a substrate. In this study, we fabricated nanosized gold structure arrays on a silica substrate by electron beam lithography combined with a liftoff process, and studied their performance as refractive index sensors. We found that the sensitivity of single gold square dot arrays increases with the pitch while the absorbance decreases. On the other hand, both the sensitivity and absorbance of twin gold square dot arrays increase as the gap distance decreases for a particular incident polarization. We discuss the sensitivity of these structures for practical sensor applications.

Arsenic doping of n-type β-FeSi2 films by sputtering method

High quality epitaxial n-type β-FeSi2 thin films prepared by alternate Fe/Si multilayer deposition were doped with arsenic as impurity by sputtering method. Doping sources were heavily arsenic-doped silicon chips placed on the surface of silicon target. The starting β-FeSi2 films before doping were typically n-type with a residual electron concentration of about 1.0 ×1017 cm-3 and a mobility of about 260 cm2/Vs. When arsenic concentration changed from 7.0 ×1017 to 1.9 ×1018 cm-3, net electron concentration increased from about 2.0 ×1017 to 4.0 ×1017 cm-3, and electron mobility decreased from 250 to 160 cm2/Vs. Secondary ion mass spectroscopy (SIMS) measurements showed a homogeneous arsenic distribution in β-FeSi2 films and a small diffused region (about 50 nm) at the interface between β-FeSi2 and a Si substrate. Arsenic-doped β-FeSi2 films exhibited epitaxial growth in the (110)/(101) orientation and a continuous structure without cracks. However, other crystalline orientations together with pinholes appeared when the arsenic doping concentration increased.

Search for Quarks in Sea Water by Ion-Exchange Chromatography

Sea water was pre-enriched in quark concentration by ion-exchange chromatography under the assumption that quarks behave like fractionally charged ions in aqueous solution. A phenomenological arguments in support of this assumption is disclosed. Millikans experiment was performed on about 700 salt grains obtained from the pre-enriched sea water. None of them beared fractional charges, which sets an upper limit: less than one quark per 4.5×10 -3 gram or per 3.0×10 21 nucleons in sea water.