Masaki Maekawa

Postgrowth annealing of defects in ZnO studied by positron annihilation, x-ray diffraction, Rutherford backscattering, cathodoluminescence, and Hall measurements

Defects in hydrothermal grown ZnO single crystals are studied as a function of annealing temperature using positron annihilation, x-ray diffraction, Rutherford backscattering, Hall, and cathodoluminescence measurements. Positron lifetime measurements reveal the existence of Zn vacancy related defects in the as-grown state. The positron lifetime decreases upon annealing above 600u200a°C, which implies the disappearance of Zn vacancy related defects, and then remains constant up to 900u200a°C. The Rutherford backscattering and x-ray rocking curve measurements show the improvement of crystal quality due to annealing above 600u200a°C. Although the crystal quality monitored by x-ray diffraction measurements is further improved after annealing at above 1000u200a°C, the positron lifetime starts to increase. This is due to either the formation of Zn vacancy related defects, or the change of the Zn vacancy charge state occupancy as a result of the Fermi level movement. The electron concentration increases continuously with increa...

Fluorine-doping in titanium dioxide by ion implantation technique

Abstract We implanted 200 keV F+ in single crystalline titanium dioxide (TiO2) rutile at a nominal fluence of 1xa0×xa01016 to 1xa0×xa01017 ionsxa0cm−2 and then thermally annealed the implanted sample in air. The radiation damage and its recovery process during the annealing were analyzed by Rutherford backscattering spectrometry in channeling geometry and variable-energy positron annihilation spectroscopy. The lattice disorder was completely recovered at 1200 °C by the migration of point defects to the surface. According to secondary ion mass spectrometry analysis, the F depth profile was shifted to a shallower region along with the damage recovery and this resulted in the formation of an F-doped layer where the impurity concentration steadily increased toward the surface. The F doping proved to provide a modification to the conduction-band edge of TiO2, as assessed by theoretical band calculations.

Annealing process of ion-implantation-induced defects in ZnO: Chemical effect of the ion species

ZnO single crystals implanted with O+ and B+ ions were studied by positron annihilation and Raman scattering measurements. Positron annihilation results show that vacancy clusters are generated by implantation. For the B+-implanted sample, the vacancy clusters have a sufficient increase in size and evolve into microvoids after annealing up to 500°C. These microvoids need a high temperature of 900–1000°C to be annealed out. However, for the O+-implanted sample, the size of the vacancy clusters shows only a slight increase during annealing process, and they are removed at much lower temperature of 700–800°C. The different annealing process is supposed to be due to the chemical effect of boron impurities. Raman measurements reveal the production of oxygen vacancies by implantation. In the B+-implanted sample they have high thermal stability up to 700°C, while in the O+-implanted sample they are annealed out early at 400°C. It is thus suggested that the boron impurities might form complexes with oxygen inters...

Interaction of nitrogen with vacancy defects in N+-implanted ZnO studied using a slow positron beam

ZnO crystals were implanted with N+, O+, and Al+, and co-implanted with O+∕N+ and Al+∕N+ ions. Positron annihilation measurements indicate introduction of vacancy clusters upon implantation. In the N+-implanted and Al+∕N+ co-implanted samples, these vacancy clusters are only partially annealed at 800°C, as compared with their entire recovery in the O+- and Al+-implanted samples at 800–900°C, suggesting a strong interaction between nitrogen and vacancy clusters. However, in the O+∕N+ co-implanted sample, most vacancy clusters disappear at 800°C. Probably oxygen scavenges nitrogen to enhance the annealing of the vacancy clusters. Upon further annealing at 1000–1100°C, nitrogen also forms stable complexes with thermally generated vacancies. These nitrogen-related vacancy complexes need high-temperature annealing at 1200–1250°C to be fully removed.

A coherent positron beam for reflection high-energy positron diffraction

A 10u2002keV positron beam has been developed using coaxially symmetric electromagnetic lenses for reflection high-energy positron diffraction (RHEPD) experiments. The beam brightness is ∼107u2002e+/s/cm2/rad2/V which is comparable to that obtained using brightness enhancement technique. The beam parallel and normal coherence lengths are over 100 and 40u2002A, respectively, which are long enough to observe diffraction patterns associated with large surface super-structures. RHEPD patterns from a Si(111)-(7×7) reconstructed surface have been successfully observed with a much better quality than previously reported.

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.

Structure of SiO 2 ∕ 4 H − SiC interface probed by positron annihilation spectroscopy

The structure of the

SiO2/SiC interface proved by positron annihilation

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Application of Positron Microprobe for Nuclear Materials

interface produced by dry oxidation has been studied using positron annihilation spectroscopy using energy-variable slow positron beams. Based on the Doppler broadening shape and wing parameter (

Ion-Implantation Induced Defects in ZnO Studied by a Slow Positron Beam

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