Toshiyuki Ohdaira

Bio: Toshiyuki Ohdaira is an academic researcher from National Institute of Advanced Industrial Science and Technology. The author has contributed to research in topics: Positron & Vacancy defect. The author has an hindex of 27, co-authored 229 publications receiving 2938 citations. Previous affiliations of Toshiyuki Ohdaira include Lund University & Kyoto University.

Production and recovery of defects in phosphorus-implanted ZnO

Abstract: Phosphorus ions were implanted in ZnO single crystals with energies of 50–380keV having total doses of 4.2×1013–4.2×1015cm−2. Positron annihilation measurements reveal the introduction of vacancy clusters after implantation. These vacancy clusters grow to a larger size after annealing at a temperature of 600°C. Upon further annealing up to a temperature of 1100°C, the vacancy clusters gradually disappear. Raman-scattering measurements reveal the enhancement of the phonon mode at approximately 575cm−1 after P+ implantation, which is induced by the production of oxygen vacancies (VO). These oxygen vacancies are annealed out up to a temperature of 700°C accompanying the agglomeration of vacancy clusters. The light emissions of ZnO are suppressed after implantation. This is due to the competing nonradiative recombination centers introduced by implantation. The recovery of the light emission occurs at temperatures above 600°C. The vacancy-type defects detected by positrons might be part of the nonradiative rec...

Evolution of voids in Al + -implanted ZnO probed by a slow positron beam

Abstract: Undoped ZnO single crystals were implanted with aluminum ions up to a dose of 1015Al+/cm2. Vacancy defects in the implanted layers were detected using positron lifetime and Doppler broadening measurements with slow positron beams. It shows that vacancy clusters, which are close to the size of V8, are generated by implantation. Postimplantation annealing shows that the Doppler broadening S parameter increases in the temperature range from 200°C to 600°C suggesting further agglomeration of vacancy clusters to voids. Detailed analyses of Doppler broadening spectra show formation of positronium after 600°C annealing of the implanted samples with doses higher than 1014Al+/cm2. Positron lifetime measurements further suggest that the void diameter is about 0.8 nm. The voids disappear and the vacancy concentration reaches the detection limit after annealing at 600–900°C. Hall measurement shows that the implanted Al+ ions are fully activated with improved carrier mobility after final annealing. Cathodoluminescence measurements show that the ultraviolet luminescence is much stronger than the unimplanted state. These findings also suggest that the electrical and optical properties of ZnO become much better by Al+ implantation and subsequent annealing.

Microvoid formation in hydrogen-implanted ZnO probed by a slow positron beam

Abstract: ZnO crystals were implanted with 20–80 keV hydrogen ions up to a total dose of 4.4×1015 cm−2. Positron lifetime and Doppler broadening of annihilation radiation measurements show introduction of zinc vacancy-related defects after implantation. These vacancies are found to be filled with hydrogen atoms. After isochronal annealing at 200–500 °C, the vacancies agglomerate into hydrogen bubbles. Further annealing at 600–700 °C causes release of hydrogen out of the bubbles, leaving a large amount of microvoids. These microvoids are annealed out at high temperature of 1000 °C. Raman spectroscopy for the implanted sample shows the enhancement of vibration modes at about 575 cm−1, which indicates introduction of oxygen vacancies. These oxygen vacancies disappear at temperatures of 600–700 °C, which is supposed to contribute to the hydrogen bubble formation. Cathodoluminescence measurements reveal that hydrogen ions also passivate deep level emission centers before their release from the sample, leading to the improvement of the UV emission.

Study of defects in GaN grown by the two-flow metalorganic chemical vapor deposition technique using monoenergetic positron beams

Abstract: Defects in GaN grown using metalorganic chemical vapor deposition were studied through the use of monoenergetic positron beams. For Mg-doped GaN, no large change in the diffusion length of positrons was observed before and after activation of Mg. This was attributed to the scattering of positrons by potentials caused by electric dipoles of Mg–hydrogen pairs. For Si-doped GaN, the line-shape parameter S increased as carrier density increased, suggesting an introduction of Ga vacancy due to the Fermi level effect. Based on these results, we discuss the effects of the growth polar direction of GaN on optical properties in this article. Although the optical properties of a GaN film grown toward the Ga face direction exhibited excitonic features, a film grown toward the N face (−c) direction exhibited broadened photoluminescence and transmittance spectra, and a Stokes shift of about 20 meV was observed. This difference was attributed to extended band-tail states introduced by high concentrations of donors and ...

Luminescence properties of defects in GaN

Abstract: Gallium nitride (GaN) and its allied binaries InN and AIN as well as their ternary compounds have gained an unprecedented attention due to their wide-ranging applications encompassing green, blue, violet, and ultraviolet (UV) emitters and detectors (in photon ranges inaccessible by other semiconductors) and high-power amplifiers. However, even the best of the three binaries, GaN, contains many structural and point defects caused to a large extent by lattice and stacking mismatch with substrates. These defects notably affect the electrical and optical properties of the host material and can seriously degrade the performance and reliability of devices made based on these nitride semiconductors. Even though GaN broke the long-standing paradigm that high density of dislocations precludes acceptable device performance, point defects have taken the center stage as they exacerbate efforts to increase the efficiency of emitters, increase laser operation lifetime, and lead to anomalies in electronic devices. The p...

Dual-comb spectroscopy

Abstract: Dual-comb spectroscopy is an emerging new spectroscopic tool that exploits the frequency resolution, frequency accuracy, broad bandwidth, and brightness of frequency combs for ultrahigh-resolution, high-sensitivity broadband spectroscopy. By using two coherent frequency combs, dual-comb spectroscopy allows a sample’s spectral response to be measured on a comb tooth-by-tooth basis rapidly and without the size constraints or instrument response limitations of conventional spectrometers. This review describes dual-comb spectroscopy and summarizes the current state of the art. As frequency comb technology progresses, dual-comb spectroscopy will continue to mature and could surpass conventional broadband spectroscopy for a wide range of laboratory and field applications.

Defects in ZnO

Abstract: Zinc oxide (ZnO) is a wide band gap semiconductor with potential applications in optoelectronics, transparent electronics, and spintronics. The high efficiency of UV emission in this material could be harnessed in solid-state white lighting devices. The problem of defects, in particular, acceptor dopants, remains a key challenge. In this review, defects in ZnO are discussed, with an emphasis on the physical properties of point defects in bulk crystals. As grown, ZnO is usually n-type, a property that was historically ascribed to native defects. However, experiments and theory have shown that O vacancies are deep donors, while Zn interstitials are too mobile to be stable at room temperature. Group-III (B, Al, Ga, and In) and H impurities account for most of the n-type conductivity in ZnO samples. Interstitial H donors have been observed with IR spectroscopy, while substitutional H donors have been predicted from first-principles calculations but not observed directly. Despite numerous reports, reliable p-t...

Contributions of Phase, Sulfur Vacancies, and Edges to the Hydrogen Evolution Reaction Catalytic Activity of Porous Molybdenum Disulfide Nanosheets

Abstract: Molybdenum disulfide (MoS2) is a promising nonprecious catalyst for the hydrogen evolution reaction (HER) that has been extensively studied due to its excellent performance, but the lack of understanding of the factors that impact its catalytic activity hinders further design and enhancement of MoS2-based electrocatalysts. Here, by using novel porous (holey) metallic 1T phase MoS2 nanosheets synthesized by a liquid-ammonia-assisted lithiation route, we systematically investigated the contributions of crystal structure (phase), edges, and sulfur vacancies (S-vacancies) to the catalytic activity toward HER from five representative MoS2 nanosheet samples, including 2H and 1T phase, porous 2H and 1T phase, and sulfur-compensated porous 2H phase. Superior HER catalytic activity was achieved in the porous 1T phase MoS2 nanosheets that have even more edges and S-vacancies than conventional 1T phase MoS2. A comparative study revealed that the phase serves as the key role in determining the HER performance, as 1T ...