Junji Iihara

Local structure analyses around Er 3+ in Er-doped fiber with Al co-doping

The local structure around Er(3+) in Er-doped optical fiber (EDFs) was explored by X-ray absorption fine structure (XAFS) measurements. Using several new approaches such as a novel sample preparation, we successfully measured the XAFS. The intensities near the 8.36 keV peaks were observed for the first time using X-ray Absorption Near Edge Structure (XANES) analysis. The intensities increased in the order of Er, Er(2)O(3), and EDF samples, indicating that Er(3+) in the EDFs existed as an oxide state. Extend X-ray Absorption Fine Structure (EXAFS) analysis also showed that the oxygen coordination number of Er(3+) increased as the Al concentration increased and that the Er-O distances of EDF with Al co-doping is longer than that of EDF without Al co-doping.

A high temperature reduction cleaning (HTRC) process: a novel method for conductivity recovery of yttrium-doped barium zirconate electrolytes

Proton conducting Y-doped BaZrO3 (BZY) and nickel oxide (NiO) are currently the most promising electrolyte and anode catalyst for protonic ceramic fuel cells, respectively. However, during the co-sintering process to fabricate the fuel cells, Ni cations diffuse from the anode into the lattice of the BZY electrolyte, resulting in significant degradation of the electrolyte conductivity and fuel cell performance. With the aim to solve such a problem, in this work, we report a novel method, named as high temperature reduction cleaning (HTRC) process, which is composed of several sequential heat-treatments in controlled atmospheres. The most interesting point is that after heat-treating the NiO-contaminated BZY at 1400 °C in a Ti-deoxidized Ar atmosphere for 100 h, Ni cations were observed to be expulsed from the BZY lattice and segregated at the grain boundary as Ni metal particles. And the conductivity of the BZY electrolyte was recovered. However, delamination along the grain boundary of the BZY electrolyte was introduced when the segregated Ni metal particles were oxidized to NiO particles in an oxygen atmosphere. And a series of sequential heat-treatments were designed to solve such a problem.

Element-Selective Observation of Electronic Structure Transition between Semiconducting and Metallic States in Boron-Doped Diamond Using Soft X-ray Emission and Absorption Spectroscopy

The electronic structure transition between the semiconducting and metallic states in boron (B)-doped diamonds was element-selectively observed by soft X-ray emission and absorption spectroscopy using synchrotron radiation. For lightly B-doped diamonds, the B 2 p-density of states (DOS) in the valence band was enhanced with a steep-edge feature near the Fermi level, and localized acceptor levels, which are characteristic of semiconductors, were clearly observed both in B 2 p- and C 2 p-DOS in the conduction bands. For heavily B-doped diamonds, the localized acceptor levels developed into extended energy levels and the new energy levels generated formed an extended conduction band structure that overlapped with the valence band. Thus, the metallic energy band structure is actually formed by heavy boron doping. These valence and conduction band structures observed by soft X-ray emission and absorption spectroscopy accounted for the electrical properties of B-doped diamonds.

Bismuth-doped silicate glass fiber for ultra-broadband amplification media

Bismuth (Bi) -doped silicate glass for future photonics applications was proposed by Y. Fujimoto et al. in 2001 [1]. And its broadband fluorescence characteristic at 1.3um band was demonstrated for the first time. From this report, several studies such as clarifying the fluorescence mechanism, surveying the optical properties of Bi-doped glass with various host glass materials and so on have been done. However, there was no report on fabricating Bi-doped silicate glass fiber (BiDF) for the demonstration of optical amplification and laser oscillation. In 2005, Haruna et al. succeeded in fabricating the BiDF by Modified Chemical Vapor Deposition (MCVD) method for the first time, and evaluated its absorption and fluorescent characteristics [2]. In this report, the absorption bands around 0.5 um, 0.7 um, 0.8 um and 1.0um were shown, and the very wide fluorescence band with a FWHM of 192 nm centered at 1.06um was also indicated. The background loss of the fiber was as low as <0.05 dB/m at 1.55 um because of the MCVD method that is well-established method for the conventional optical fibers. Furthermore, the 1.3 um band amplification using the BiDF prepared by the similar MCVD method was reported by V. V. Dvoyrin et al. in 2005 [3], and the laser oscillation by this fiber was also demonstrated in 2006 [4]. In this paper, a current research progress on the Bi-doped glasses and fibers is reviewed. By looking at the optical properties such as fluorescence characteristics, future possible applications are explained and its fluorescence mechanism is also discussed.

Structural analysis of sulfuric acid solutions containing Ti and Mn using x-ray diffraction, x-ray absorption fine structure, and molecular dynamics simulation

Structural analyses have been performed for sulfuric acid (H2SO4) aqueous solutions containing Ti and/or Mn. These solutions are used as the positive electrolyte in the Ti-Mn redox flow (RF) battery, in which it had been found that adding Ti4+ in the positive electrolyte is very effective to reduce the MnO2 precipitation at a high state of charge. X-ray diffraction and x-ray absorption fine structure (XAFS) measurements were employed in order to obtain total correlation functions, T(r)s, and coordination numbers around Mn and Ti in the solutions, respectively. The T(r)s showed some peculiar peaks that were assigned to correspond to S-O, O-O, Mn-O, and Ti-O pairs in the solutions. The XAFS analysis demonstrated that both Mn and Ti have 6-coordinating oxygen atoms in the solutions. The classical molecular dynamics simulation was also carried out to obtain structural models of the solutions. By tuning the Born-Mayer type potential parameters, the T(r)s calculated from the models showed good agreement with the experimental ones. Regarding the coordination number, the 6-coordinated Mn-O was reproduced successfully, while we need further investigation to find parameters that can reproduce the 6-coordinated Ti-O in the solutions. The simulation results also indicated the existence of Ti-SO4 bonds, which should promote the H+ dissociation from HSO4- and increase the H+ concentration in the solutions. This may be effective to suppress the MnO2 precipitation at a high state of charge in the RF battery.

Time-Resolved Micro-Beam X-Ray Absorption Fine Structure (XAFS) Measurement to Investigate the Cause of a Current Collapse of GaN-HEMTs

The purpose of our work was to carry out a direct measurement of a GaNs surface condition under a voltage stress, and to identify a cause of a current collapse. Using pulsed S-parameters measurements immediately after the voltage stress was applied for the GaN HEMT, equivalent circuit parameters were extracted, and we estimated the virtual gate length which induced the current collapse. And we carried out time resolved micro-beam X-ray Absorption Fine Structure (XAFS) measurement after the voltage stress was applied. As a result, by the applied voltage stress, an X-ray Absorption Near Edge Structure (XANES) spectrum of gallium, Ga, was found to be changed. This means the surface condition of GaN was changed in the virtual gate region by the voltage stress.

Development of the surface-sensitive soft x-ray absorption fine structure measurement technique for the bulk insulator

We have succeeded in measuring X-ray absorption fine structure (TEY-XAFS) spectra of insulating plate samples by total electron yield. The biggest problem is how to suppress the charge-up. We have attempted to deposit a gold stripe electrode on the surface and obtained a TEY-XAFS spectrum. This indicates that the metal stripe electrode is very useful in the TEY-XAFS measurement of the insulating plate samples. In the detailed analysis, we have found that the effective area for suppressing charge-up was approximately 120 μm from the edge of the electrode.

Analyses of Chemical States at SiN x /GaN Interface by HAXPES

We have analyzed the chemical state at the SiNx/GaN interface, which affects collapse phenomenon in GaN-HEMT. In the specimen fabrication, we employed O2and N2-plasma for GaN surface cleaning prior to the SiNx deposition, since the O2-plasma had been confirmed to reduce the collapse compared to the N2-plasma. HAXPES analyses demonstrated that the O2-plasma grew more metallic Ga at the interface than N2-plasma. The metallic Ga is thought to be a good leakage path, through which the trapped electrons can escape from the interface to the electrodes.

Novel Technique for Improving the Signal-to-Background Ratio of X-ray Absorption Near-Edge Structure Spectrum in Fluorescence Mode and Its Application to the Chemical State Analysis of Magnesium Doped in GaN

A novel measurement technique for an X-ray absorption near-edge structure (XANES) for magnesium (Mg) doped in gallium nitride (GaN) has been developed. XANES spectra from Mg at very low concentrations of 1 ×1018/cm3 doped in GaN have successfully been obtained by optimizing the region of interest (ROI) and by using highly brilliant synchrotron radiation X-rays of SPring-8. The ROI is the limited energy region from an X-ray fluorescence spectrum to elicit signals of particular atoms. Using this new technique, we have investigated the effect of the annealing process for Mg-doped GaN on the XANES spectra. It has been found that the XANES spectra of Mg significantly changed as the annealing temperature increased. This indicates that the local structure around Mg atoms in GaN was modified by the annealing process.

Structure analyses for Er‐doped SiO2 fibers (EDFs) by X‐ray scattering, XAFS, and MD simulation.

The structures of Er‐doped SiO2 fibers (EDFs) were analyzed by X‐ray scattering, XAFS, and molecular dynamics (MD) simulations. The experiments and simulations have demonstrated that the Al‐codoping expands Er‐O distance and increases Er‐O coordination number. These effects should cause a variation in the electric field around Er3+ in the SiO2.