Thermal evolution of defects in as-grown and electron-irradiated ZnO studied by positron annihilation

Vacancy-type defects in as-grown ZnO single crystals have been identified using positron annihilation spectroscopy. The grown-in defects are supposed to be zinc vacancy (VZn)-related defects, and can be easily removed by annealing above 600 °C. VZn-related defects are also introduced in ZnO when subjected to 3 MeV electron irradiation with a dose of 5.5×1018 cm−2. Most of these irradiation-induced VZn are annealed at temperatures below 200 °C through recombination with the close interstitials. However, after annealing at around 400 °C, secondary defects are generated. A detailed analysis of the Doppler broadening measurements indicates that the irradiation introduced defects and the annealing induced secondary defects belong to different species. It is also found that positron trapping by these two defects has different temperature dependences. The probable candidates for the secondary defects are tentatively discussed in combination with Raman scattering studies. After annealing at 700 °C, all the vacancy defects are annealed out. Cathodoluminescence measurements show that VZn is not related to the visible emission at 2.3 eV in ZnO, but would rather act as nonradiative recombination centers

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

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

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

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

Multi-layer scintillation detector for the MOON double beta decay experiment: Scintillation photon responses studied by a prototype detector MOON-1

An ensemble of multi-layer scintillators is discussed as an option of the high-sensitivity detector Mo Observatory Of Neutrinos (MOON) for spectroscopic measurements of neutrino-less double beta decays. A prototype detector MOON-1, which consists of 6 layer plastic-scintillator plates, was built to study the sensitivity of the MOON-type detector. The scintillation photon collection and the energy resolution, which are key elements for the high-sensitivity experiments, are found to be 1835+/-30 photo-electrons for 976 keV electrons and sigma = 2.9+/-0.1% (dE/E = 6.8+/-0.3 % in FWHM) at the Qbb ~ 3 MeV region, respectively. The multi-layer plastic-scintillator structure with good energy resolution as well as good background suppression of beta-gamma rays is crucial for the MOON-type detector to achieve the inverted hierarchy neutrino mass sensitivity.Comment: 8 pages, 16 figures, submitted to Nucl.Instrum.Met

Structure determination of the rutile-TiO2(110)-(1 × 2) surface using total-reflection high-energy positron diffraction (TRHEPD)

The exact structure of the rutile-TiO2(110)-(1×2) surface, which had been under debate over the past 30 years, was investigated using the newly developed technique of total-reflection high-energy positron diffraction (TRHEPD), which is a positron counterpart of reflection high-energy electron diffraction (RHEED). The rocking-curves for the 00-spot obtained from the experimental diffraction patterns were compared to the curves for various models calculated with a full-dynamical theory. It was found that the rocking-curves matched those for a surface consisting of a Ti2O3 configuration, originally suggested by Onishi and Iwasawa [H. Onishi and Y. Iwasawa, Surf. Sci., 1994, 313, L783], but with a further modification of atomic positions close to the ones proposed by Wang et al. [Q. Wang, A. R. Oganov, Q. Zhu and X. F. Zhou, Phys. Rev. Lett., 2014, 113, 266101]. This result demonstrates that TRHEPD can distinguish between the existence and absence of the oxygen atoms on the topmost surface, and between the Ti atoms residing in positions at the interstitial-vertical sites and those at interstitialhorizontal sites

Effect of dual implantation with Ag and Ni ions on the optical absorption of silica glass

Effects of dual implantation with Ag and Ni ions on the optical absorption of silica glass were investigated. The surface plasmon absorption spectrum after the dual implantation does not correspond to the sum of the spectrum for only Ag ion implantation and that for only Ni ion implantation. The experimental result suggests that morphology and/or the composition of Ag nanoparticles were changed by the subsequent Ni ion implantation. Furthermore, the result of positron annihilation coincidence Doppler broadening profile of Ag and Ni ion implanted sample also shows that the synthesized Ag nanoparticles is affected by the Ni implantation

N+ ion-implantation-induced defects in ZnO studied with a slow positron beam

Undoped ZnO single crystals were implanted with multiple-energy N+ ions ranging from 50 to 380 keV with doses from 1012 to 1014 cm−2. Positron annihilation measurements show that vacancy defects are introduced in the implanted layers. The concentration of the vacancy defects increases with increasing ion dose. The annealing behaviour of the defects can be divided into four stages, which correspond to the formation and recovery of large vacancy clusters and the formation and disappearance of vacancy–impurity complexes, respectively. All the implantation-induced defects are removed by annealing at 1200 °C. Cathodoluminescence measurements show that the ion-implantation-induced defects act as nonradiative recombination centres to suppress the ultraviolet (UV) emission. After annealing, these defects disappear gradually and the UV emission reappears, which coincides with positron annihilation measurements. Hall measurements reveal that after N+ implantation, the ZnO layer still shows n-type conductivity. © 2004 IOP Publishing Lt