Kenya Ohashi

Photovoltaic properties of ion-beam synthesized β-FeSi2/n-Si heterojunctions

We present the first evident photovoltaic responses from ion-beam synthesized (IBS) polycrystalline p-type β-FeSi2/n-Si(100) heterojunctions. The triple ion implantation and subsequent annealing at 800°C provided polycrystalline continuous layers ∼60-nm thick with large crystalline grains of ∼10 μm. The high temperature and long annealing time were very effective in amplifying the photovoltaic responses from the heterojunctions. We achieved a maximum open-circuit voltage of 0.34 V by 5 mW/cm2 of white light illumination. Furthermore, we confirmed that the annealing procedure at 500°C induced the precipitation of the psuedomorphic metallic γ phase, which is detrimental to both rectification and the photovoltaic voltage at the p–n heterojunction.

Formation of iron film by ion beam deposition

Abstract A low-energy, mass-separated ion beam deposition (IBD) system was developed to investigate fundamental ion-solid interactions which play major roles in thermal non-equilibrium ion-assisted film growth. Using the system, high purity and highly corrosion resistant iron films were formed on silicon substrates at room temperature with ion energies of 50 and 100 eV. The reason why the IBD Fe films had such a high corrosion resistance compared with other high purity Fe metals and stainless steel is discussed from the viewpoint of ion-solid interactions. There is a purification effect due to mass separation and also enhanced surface atom migration due to low energy ion bombardment which seem to be responsible for the formation of a stable surface iron oxide layer, which contributes to the high corrosion resistance of the IBD Fe films.

Superior Corrosion Resistance of Ion Beam Deposited Iron Film

Superior corrosion resistance was obtained for 56Fe+-IBD (ion beam deposited) films on Si and Ge substrates. The iron films deposited at an ion energy of 50 eV showed the highest corrosion resistance in 0.001 M NaCl solution. The estimated current density at its corrosion potential was four orders of magnitude smaller, 10-6 A/m2, than that of 2N-purity iron bulk materials. Ion bombardment induced formation of finely-grained polycrystalline structure, and coupled with its purifying action, these were considered to account for the superior corrosion resistance.

Infrared-Photovoltaic Responses of Ion-Beam Synthesized β-FeSi 2 / n -Si Heterojunctions

Photoresponses of photovoltaic cells using ion-beam synthesized (IBS) polycrystalline p + -β-FeSi 2 /n-Si heterojunctions were examined in an infrared (IR) wavelength region. At room temperature, an evident photoresponse due to an internal photoemission from trap levels in β-FeSi 2 with the threshold energy Φ=0.62 eV was observed at 0.6-0.87 eV. The pronounced increase of a photoresponse corresponding mostly to an interband transition in β-FeSi 2 was observed at 0.87-1.1 eV. The maximum dominated by a surface recombination process appeared around ∼1.2 eV. The surface recombination rate of ∼10 4 cm/s was estimated. The quantum efficiency was ∼60 % in the 0.8-1.0 µm wavelength region and ∼14 % around the band-gap of βFeSi 2 .

Improved ion beam deposition system with RF sputter-type ion source

Abstract A newly developed RF sputter-type ion source has been introduced into our ion beam deposition (IBD) system for high purity Fe film formation. Vacuum environment during deposition has been much improved compared with our previous IBD system, in which a conventional CCl 4 method was employed to produce Fe + ions in a microwave ion source. With no use of CCl 4 gas, an inert argon background has been achieved. The ion source is composed of an RF (13.56 MHz) Cu coil and an Fe sputter target located inside. An Ar discharge was generated at an RF power of 100–300 W and a negative DC bias voltage of 500–1000 V was applied to the Fe target. The tip of the target was heated almost up to the melting temperature and simultaneous sputtering and evaporation took place. Major ion species extracted were Ar + and Fe + ions. With an increase in the DC bias voltage up to 1 kV, a relative ratio of Fe + to Ar + ion intensities became one order larger. This efficient Fe + ion production is thought to be due to a Penning ionization effect of metastable Ar ∗ atoms. It is expected to form extremely high purity Fe films, in contrast to our former Fe films which contained C and Cl as impurities.

Suppression of Radiation Buildup on Stainless Steel in a Boiling Water Reactor

Abstract Deposition of radioactive corrosion products, such as Co-60, causes radiation field buildup in boiling water reactors (BWR) plants. In this study, deposition kinetics of Co-60 and elemental cobalt ions on stainless steels (SSs) and the effects of pre-oxidation on the suppression were evaluated in actual reactor water and laboratory autoclaves. Cobalt ions are incorporated as divalent cobalt oxides such as cobalt ferrite at the oxide/metal interface through corrosion. Therefore, a protective oxide film, which is preformed under suitable circumstances such as high temperature water with a certain amount of dissolved oxygen, can suppress the deposition. The oxide film containing aluminum has greater protectiveness.

Molecular dynamics simulations of low energy atomic collisions between an atom and a substrate: Effect of incident angle and energy

Abstract Atomic collisions during the ion beam deposition of Al thin films are simulated by a molecular dynamics method, assuming that the incident atoms are neutralized. The kinetic energies of the incident atoms range from 10 to 50 eV and the angles of incident to Al substrate are controlled between π 6 and π 3 . The simulation results show reflection or rapid trapping of the incident atoms as well as a surface migration for a longer distance. As a response to the collision, atoms in the substrate show highly directional or much more diffused propagation of thermal vibration, depending on the incident angle or timing of the collision.

Developments of nanoimprint technologies and applications

The nanoimprint technology is attractive for the fabrication of nano-scale structures in view of cost and mass production. There are several points for the industrial applications such as pattern formation area, resolution, residual layer thickness, precise control of pattern transfer, lifetime of mold, alignment and so on. A thermal nanoimprint system, which can imprint fine dots on a 300 mm diameter wafer in a single step, is developed. The narrow pith patterns for future storage and IT devices are formed on a polymer layer. A high-aspect nanoprint (Hi-NP) technology forms polymer nanopillars with high aspect ratio. The nanopillars are applied to a bio-chip for the fluorescence immunoassay. The chip is effective in the enhancement of the fluorescence intensities, since the nanopillars enlarge the surface area.

Effect of ambient vacuum pressure on the corrosion properties of pure iron films formed by the ion beam deposition method

Abstract High purity iron films deposited on Si substrates by the low-energy, mass-separated ion beam deposition (IBD) method showed the highest corrosion resistance against NaCl solution than any other iron materials. The corrosion properties of the films depend on the ambient vacuum conditions during film formation. High pressure surroundings seem to bring gaseous compounds into the films, which make the surfaces unstable owing to a high density of defects in the films. Surface observations by transmission electron microscopy indicate the presence of a stable iron oxide layer on the IBD iron film which had formed under suitable vacuum conditions. The presence of fewer defects in IBD iron film seems to be a prerequisite of this protective oxide layer.

Electronic structure analysis of oxygen chemisorption on a Fe surface

Abstract The initial stage of oxidation, oxygen chemisorption, on the Fe surface has been investigated by the tight binding extended Huckel method, employing the Fe(110) and (211) surfaces that are often observed in thin film growth of Fe. Chemical bonding between oxygen and a Fe atom has been studied on the basis of the model that an oxygen molecule approaches on the hollow site of the Fe surface. Results are summarized as follows; (1) Oxygen is combined with only the surface layer when approaching on the Fe(110) surface; (2) Not only the surface but also the second layer can form bonding with oxygen on the Fe(211) surface. These results can lead to the fact that the Fe(110) surface is less oxidized than the Fe(211) surface.