Kenji Umezawa

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.

Elastic recoil detection analysis of hydrogen adsorbed on solid surfaces

Abstract An experimental apparatus for quantitative analysis of hydrogen adsorbed on solid surfaces is described. Elastic recoil detection analysis (ERDA) by a 6 MeV F3+ ion beam determines the hydrogen coverage and a LEED-Auger system is used to monitor the order and cleanliness of the specimen surface. The method has been applied to a H/Si(111)-7 × 7 system.

Adsorption of hydrogen on the Pt(111) surface from low-energy recoil scattering

We have shown that low-energy recoil scattering (LERS) can be used to detect hydrogen on a Pt(111) surface at 150 K and that this method provides a direct probe for locating the positions of adsorb ...

Raman spectroscopic study of ion-beam synthesized polycrystalline β-FeSi2 on Si(100)

We examined effects of ion implantation doses, annealing temperature and time on ion-beam synthesis (IBS) of polycrystalline β-FeSi2 using Raman spectroscopy. It was confirmed that at very low Fe concentration doses (up to 1×1016 ions/cm2), fine grains of β-FeSi2 and a small amount of fluorite γ-FeSi2 may precipitate after annealing at 800°C. In the case of high dose (>1×1017 ions/cm2), the clear Raman lines showed that β-FeSi2 grows after annealing at 800°C. However, we observed an outstanding Raman line at 324 cm−1 and broad features at 300–450 cm−1 after annealing at 600 and 700°C. These Raman features can be considered to be due to presence of γ-FeSi2 and lattice imperfections in the samples. Furthermore, we evaluated improvement of crystalline quality with increasing the annealing time and temperature using a clear blue shift of the Raman line and increase of the intensity-ratio of two Raman lines at ∼192 and ∼246 cm−1, θ=(I192/I246).

Low energy ion scattering analysis of the surface compositional change of Au3Cu(001) induced by oxygen chemisorption

The change of the surface composition of Au3Cu(001) induced by oxygen chemisorption was investigated by means of impact collision ion-scattering spectroscopy (ICISS) of time of flight version using 180° scattering of 3 keV Ne+ ions. For the clean surface, the topmost layer was found to be occupied almost completely by Au atoms (∼95%) and the second layer by Cu atoms (∼95%). Oxygen chemisorption was performed by exposing the crystal surface to atomic oxygen produced on hot-W filament. With the oxygen adsorption, Au in the topmost layer was replaced with Cu and Cu in the second layer with Au, respectively. Almost complete replacement of elements in the first layer was found at high coverages. At low coverages, at least two Cu atoms were induced by one oxygen atom. From the analysis of the polar scans of the ICISS signals, the oxygen was suggested to be located deeply in the 4-fold hollow site.

Initial stage of Pb adsorption on Ni(111) studied by LEED and ICISS

Impact-collision ion scattering spectroscopy (ICISS) and low energy electron diffraction (LEED) have been used to determine the surface structure in the initial stage of Pb adsorption on Ni(111). Three kinds of surface structure were observed in the Pb coverage dependence of LEED patterns; 1 × 1(0 < θ < 0.25), 3 × 3(0.25 < θ < 0.45) and (4 × 4)′ (0.54 < θ < 1.0). The coverage of 0.54 ML at the completion of the surface phase change to a (4 × 4)′ structure was in agreement with that observed by RBS measurements and corresponds to a complete one atomic layer thick coverage of Pb atoms. At a coverage of 0.45 < θ < 0.54, the lead-lead distance changed drastically from 3.74 A to 3.39 A. In the (4 × 4)′ structure, the lead-lead distance of 3.39 A is 6% larger than that of 3.2 A for an ideal 4 × 4 structure. Pb islands grow as a single overlayer structure like a one layer thick floating solid on the Ni(111) surface.

Nuclear reaction analysis and elastic recoil detection analysis of the retention of deuterium and hydrogen implanted into Si and GaAs crystals

By means of the 3He nuclear reaction analysis (NRA) and elastic recoil detection analysis (ERDA) techniques, the annealing-temperature dependence of the depth profiles of D and H implanted in Si(100), poly-Si, and GaAs(100) was measured. Implanted D in Si(100) is more tightly trapped than H. The detrapping energy of H-GaAs is larger than that of H-Si.

Real-time observation of Escherichia coli cells under irradiation with a 2-MeV H+ microbeam

A high-energy H+ microbeam generated by tapered glass capillary optics was applied to a single Escherichia coli cell, in order to evaluate the effects of irradiation on the activity of the flagellar motor and cell growth in real time. The flagellar motor of the tethered cells was stopped by irradiation with an average ion fluence of 2.0 × 1012 protons/cm2. When a lower dose was applied to the cells attached to the substrate, an elongated cell, which seemed ready to divide, divided into two daughter cells; however, the daughter cells did not elongate, neither did further cell division occur.

Ion beam synthesis of β-FeSi2 as an IR photosensitive material

The large sized and flat polycrystalline (beta) -FeSi2/n- Si heterojunction can be formed by a triple energy implantation method and the sample annealing at 800 degrees C. The polycrystalline (beta) -FeSi2 gains show good crystalline characteristics, a photoluminescence peak at 0.81 eV at 4.2 K and the optical direct band-gap of 0.84 eV. The (beta) -FeSi2/n-Si heterojunction shows good diode characteristics and high photovoltaic sensitivity for IR light. These results support that the ion beam synthesized (beta) -FeSi2/n-Si heterojunction is a promising IR sensitive materials.

Ion-Beam Synthesized Semiconducting β-FeSi 2 Controlled By Annealing Procedures And Phase-Transitions

The ion beam synthesis (IBS) of β-FeSi 2 was examined by Rutherford backscattering spectroscopy (RBS) and x-ray diffractometry (XRD), and the structural characterization was carried out by Raman spectroscopy and scanning electron microscopy (SEM). We found that the IBS of β- FeSi 2 is controlled by two different processes depending on the annealing temperature (Ta) and Fe surface concentration (Cs); (I) precipitation of β-FeSi 2 on the surface in Cs˜30 at% and Ta⩾700° C and (II) phase transition from γ -FeSi 2 to β-FeSi 2 in Cs 2 . The good crystalline β-FeSi 2 obtained above 800°C showed a clear reflectance maximum at 0.88 eV due to the optical transition at the direct band-gap of 0.84 eV observed in the characteristic plot ((ahv) 2 vs. hv) of the optical absorption.