Atsuo Kawasuso

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...

Production and recovery of defects in phosphorus-implanted ZnO

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...

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.

Intrinsic Defects in Cubic Silicon Carbide

Irradiation of fast particles like 1 MeV electrons and 2 MeV protons was made for single crystalline cubic silicon carbide (3C-SiC) grown epitaxially on Si by chemical vapor deposition in order to introduce point defects in the material. Intrinsic point defects in 3C-SiC have been characterized by electron spin resonance (ESR), positron annihilation spectroscopy (PAS), Hall and photoluminescence (PL) techniques. The structure and annealing behavior of intrinsic defects, e.g. monovacancies at silicon and carbon sublattice sites, are described based on the results obtained by ESR and PAS. The contributions of such point defects to electrical and optical properties of 3C-SiC are discussed using the Hall and PL results, with a brief review of published work.

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...

ANNEALING PROCESSES OF VACANCY-TYPE DEFECTS IN ELECTRON-IRRADIATED AND AS-GROWN 6H-SIC STUDIED BY POSITRON LIFETIME SPECTROSCOPY

Annealing processes of vacancy‐type defects in 3 MeV electron‐irradiated and as‐grown 6H‐SiC have been studied by positron lifetime spectroscopy. Vacancy‐type defects giving rise to a positron lifetime of 183 ps were detected in as‐grown n‐type specimens. They were found to be annealed at around 1400u2009°C and were related to silicon vacancies, possibly complexes of silicon vacancies and nitrogen atoms. Defects related to carbon vacancies, silicon vacancies, and divacancies were found to be created by electron irradiation. The defects related to carbon vacancies and divacancies were found to be annealed up to 500u2009°C. The defects related to silicon vacancies were found to be annealed at around 750 and 1400u2009°C. The former annealing stage was inferred to be due to migration of silicon vacancies to internal sinks or nitrogen atoms to form complexes of silicon vacancies and nitrogen atoms. The latter annealing stage was explained as due to annihilations of the complexes as well as the case of as‐grown specimens.

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.

Effect of hydrogen etching on 6H SiC surface morphology studied by reflection high-energy positron diffraction and atomic force microscopy

Hydrogen-etched 6H SiC (0001) surfaces have been studied by reflection high-energy positron diffraction and atomic force microscopy. It was found that residual damage on the surfaces were effectively removed by the hydrogen etching as compared to the HF etching after the oxidation. The hydrogen-etched surfaces were atomically flat. After the oxidation following the hydrogen etching, the surface roughness was found to increase and an anomalous dip structure appeared in the rocking curve of the reflection high-energy positron diffraction.