Takato Watanabe

Optical absorption and Hall effect in (220) and (400) oriented polycrystalline silicon films

Carrier transport properties were investigated for polycrystalline silicon (poly-Si:H:F) films fabricated at 300 °C by 100 MHz plasma enhanced chemical vapor deposition from gaseous mixture of SiF4 and H2. Analysis of free carrier optical absorption (FCA) revealed that 1 μm thick (400) oriented phosphorus-doped poly-Si:H:F films with a carrier concentration of 5×1019 cm−3 had the average electron mobility in crystalline grains at 40 cm2/V s, while the electron mobility of the (220) oriented phosphorus-doped poly-Si:H:F films was only 12 cm2/V s. These results indicated that (400) oriented poly-Si:H:F films had excellent quality crystalline grains. Analyses of the FCA combined with Hall effect current measurements revealed that the electrical conductivity at grain boundaries of top doped films increased as the underlying film thickness increased from 0 to 280 nm for (400) oriented phosphorus-doped/undoped double layered samples, but grain boundaries still acted as large resistive regions limiting the effec...

Interaction between the helicase domain of the tobacco mosaic virus replicase and a tobacco arginine decarboxylase

In a yeast two-hybrid screening test for tobacco proteins that interact with TMV replicase using the helicase (H) domain as bait, a cDNA clone was selected that encodes a polyamine biosynthetic enzyme, arginine decarboxylase (ADC). In yeast cells, the C-terminal internal region of ADC interacted with the H domain. This observation was confirmed in vitro by far-Western blotting. Inhibition of the binding between the H domain and the IRnHEL (I region and N-terminus of helicase domain) region by ADC using a yeast three-hybrid assay suggested possible interference of the heterodimerization of 126 K and 183 K by ADC.

Fabrication of planar-type Ni/vacuum/Ni tunnel junctions based on ferromagnetic nanogaps using field-emission-induced electromigration

Planar-type Ni/vacuum/Ni tunnel junctions based on ferromagnetic nanogaps were fabricated by field-emission-induced electromigration in an approach that we call “activation.” In the activation method, we were simply and easily able to control the tunnel resistance of the nanogaps from 1.6 MΩ to 169 kΩ by adjusting only the magnitude of the field emission current. The resistance of the junction formed by the activation was varied by applying external magnetic fields, and the magnetoresistance (MR) ratio was approximately 12.2% at 16 K with a bias voltage of 0.72 mV. Furthermore, after the bias voltage was increased from 0.72 to 7.3 mV, the MR ratio decreased from 12.2% to 6.2%. When the applied bias voltage was fixed at 1.6 mV, the MR ratio was decreased from 11.6% to 1.2% by increasing the measurement temperature from 16 to 270 K. These results imply that it is possible to easily fabricate planar-type Ni/vacuum/Ni ferromagnetic tunnel junctions via activation.

Magnetoresistance Properties of Planar-Type Tunnel Junctions With Ferromagnetic Nanogap System Fabricated by Electromigration Method

We report electromigration techniques for the fabrication of planar-type tunnel junctions with ferromagnetic nanogap system. In these techniques, by monitoring the current passing through the devices, we are easily able to obtain the planar-type Ni-Vacuum-Ni tunnel junctions. In this paper, magnetoresistance (MR) properties of the planar-type Ni-based tunnel junctions formed by stepwise feedback-controlled electromigration (SFCE) and field-emission-induced electromigration (activation) are studied. We performed the SFCE method for Ni nanoconstrictions connecting asymmetrical butterfly-shape electrodes. Furthermore, the activation technique was applied to Ni nanogaps with separations of 15-45 nm. MR ratio of the devices formed by the SFCE exhibited approximately 4% at 16 K . On the other hand, the devices fabricated by the activation showed MR ratio of above 300% at 16 K. These results suggest that it is possible to fabricate planar-type ferromagnetic tunnel junctions with vacuum barriers by electromigration techniques.

Secondary ion emission from Langmuir-Blodgett (LB) films investigated by time-of-flight secondary ion mass spectrometry

Langmuir-Blodgett (LB) films can be regarded as suitable samples for the basic study of the secondary ion emissions in static secondary ion mass spectrometry (S-SIMS), due to its flexibility for desired sample preparation. In this study, using cadmium arachidate films formed on a silicon substrate, the intensity variations were investigated for various kind of secondary ion species obtained by time-of-flight (TOF) SIMS. The results are discussed with respect to the number of monolayers and the energy loss process of the primary ion beam in the organic films and at the film-substrate interfaces.

Thickness and energy dependence of secondary ion emissions from Langmuir-Blodgett films

Using Cd arachidate films formed on Ag and Si substrates, secondary ion intensities were investigated by TOF-SIMS. The intensities of [M + H] + , [M + Cd] + and Ag cationized ions from samples on the Ag and Si substrates were found to change as a function of LB film thickness, as well as the primary beam energy. The intensities of [M + H] + and [M + Cd] + were seen to be (1) the highest at a one-layer LB film, (2) to decrease as the LB film becomes thicker, (3) change slightly with the primary ion beam energy. On the other hand, the intensities of Ag cationized secondary ions were almost constant with the changes of LB thickness and the energy of the primary ion beam, after normalized by Ag + intensity. The results were discussed in terms of the energy transfer of the primary beam to the LB films, the property of the substrate materials, and the strength of chemical bondings.

Simultaneous tuning of tunnel resistance of integrated nanogaps by field-emission-induced electromigration

We report a simple and easy method for the simultaneous control of electrical properties of multiple Ni nanogaps. This method is based on electromigration induced by a field emission current, which is the so-called “activation.” The tuning of tunnel resistance of nanogaps was simultaneously achieved by passing a Fowler-Nordheim (F-N) field emission current through three initial Ni nanogaps connected in series. The Ni nanogaps having an asymmetrical shape with the separation of 80–110 nm were fabricated by electron-beam (EB) lithography and lift-off process. By performing the activation, current-voltage properties of series-connected nanogaps were simultaneously varied from “insulating” to “metallic” through “tunneling” properties with increasing the preset current of the activation. Furthermore, the tunnel resistance of simultaneously activated nanogaps decreased from the order of 100 TΩ to 100 kΩ with increasing the preset current from 1 nA to 30 µA. Scanning electron microscopy (SEM) images of the nanogaps after performing the activation with the final preset current Is of 30 µA clearly indicated that the separation of the gaps reduces to less than 10 nm after the activation. These results clearly imply that the electrical properties of series-connected nanogaps can be simultaneously tuned by the activation procedure.