Toshinobu Chiba

Lifetime Spectra of Positrons in (Ba1-1.5xGd\(_{x}\square_{0.5x}\))TiO3

Positron lifetime spectra were measured at 20°C in Gd-doped BaTiO 3 which contained a large amount of Ba 2+ vacancies. It was found that the intensity of long-lived (τ 2 ) component was proportional to the nominal vacancy concentration and the averaged annihilation rate decreased as the concentration increased. To explain the observed results a model is proposed in which most of Ba 2+ vacancies form pairs with Gd 3+ and these pairs attract, but do not trap positrons, and isolated Ba 2+ vacancies trap positrons giving τ 2 component.

Lifetime Spectra of Positrons in GaAs Deformed by Ga- and As-Bending

Positron lifetime measurements have been performed in plastically bent n -type GaAs single crystals in which either Ga- or As-dislocations were introduced. τ 1 and I 1 were decreased by deformation, while I 2 was increased. Following the trapping model, the trapping rates were deduced experimentally, which in turn gave the cross widths of the Ga- and As-dislocations as 2.2×10 2 A and 2.9×10 2 A, respectively. In terms of Reads model for the electric charge of the dislocation in semiconductors, the fractions of the dangling bonds occupied by electrons for the Ga- and As-dislocation were obtained as 0.65 and 0.75, respectively.

Anisotropy in the angular correlation curves of NiO

The positron angular correlation curves were measured along the [100], [110] and [111] directions of NiO single crystals. A simple calculation based on the localized ion model shows that the measured anisotropy is ascribed to the uncompleted 3d orbitals of Ni with a slight covalent mixing of oxygen 2p orbitals.

Positron lifetime and 2D-ACAR studies of divacancies in Si

We have measured positron lifetime and Two Dimensional Angular Correlation of Annihilation Radiation (2D-ACAR) distributions of Floating-Zone grown (FZ) Si specimens containing divacancies (V2) with the definite charge states, V20, V2−1 or V2−2 from room temperature to about 10 K. These charge states are accomplished by an appropriate combination of dopant species, their concentration and irradiation doses of 15 MeV electrons. with reference to the currently accepted ionization level of divacancies. The positron lifetime of the negatively charged divacancy increases with temperature, while that of the neutral divacancy shows little change with temperature. The positron trapping rate, obtained from lifetime and 2D-ACAR measurements, increases markedly with decreasing temperature. This is found not only for the negative divacancies but also for the neutral divacancy. We need a model which explains this temperature dependence. The 2D-ACAR distribution from positrons trapped at divacancies shows nearly the same distribution for the different charge states, which differs considerably from the case of As vacancies in GaAs studied by Ambigapathy et al. We have observed a small but definite anisotropy in the distribution of trapped positrons in V2− using a specimen containing oriented divacancies.

Positron Annihilation in ZrO2: Angular Correlation

Angular correlation curve of powdered monoclinic ZrO 2 was taken and compared with some calculated curves, It is expected that the wave function of oxygen 2s electron extends more outwards than that used here.

Lifetime Spectra of Positrons in NiS and NiO

Intensities of τ 2 components were small for pyrite (FeS 2 ) and Ni 1- x S. It was tried to explain those by the stoichiometry and the shielding of cation vacancies by s type conduction electrons. Enhancement factors were 1.8 for both NiO and NiS and there was no large enhancement in the conduction band of NiS.

Electronic Structure of Nanosized bcc Cu Precipitates in Fe-Cu Alloys Studied by Positron 2D-ACAR

Based on the finding with the use of the coincidence Doppler measurements that the nanosized Cu precipitates are coherently embedded in the Fe-Cu matrix alloy, taking a bcc structure and acting as efficient positron traps, we measured 2D-ACARs of carefully grown and heat treated single crystals of Fe-Cu. We found that the precipitates have a Fermi surface with 12 necks touching the 110 Bragg planes of the bcc Brillouin zone, which contrasts distinctly with that of the bulk Cu with 8 necks touching the 111 Bragg planes of the fcc Brillouin zone. The 3 dimensional momentum reconstruction of the 2D-ACAR data showed that the Fermi surface cutoff of the Cu precipitates is blurred considerably as compared with that of bulk fcc Cu, suggesting a marked size effect in the momentum distribution.

Polytype-Dependent Vacancy Annealing Studied by Positron Annihilation

In this research we investigated the dependence of annealing beha vior of vacancy defects on SiC polytype using positron annihilation spectroscopy. We found that vacancy defects in 3C SiC induced by electron irradiation and multiple-He implantation a re annealed below 1100C while those in hexagonal SiC above 1200 C. The amount of residual vacancy defects due to post-electron-irradiation annealing at 1000 C increases as hexagonality. From two-dimensional angular correlation of annihilation radiation (2D-ACAR) measurements , it was confirmed that the positron annihilation center in electron-irradiated 3 C SiC should be silicon vacancies with tetrahedral symmetry. Silicon vacancies should be constituents in va ca cy defects responsible for positron trapping in electron-irradiated hexagonal SiC. However, 2D-ACA R experiment shows that the local symmetry is different from that for silicon vacancies in 3 C SiC. Introduction We have so far been studying properties of vacancy defects in SiC induced by bombardment using positron annihilation spectroscopy [1-4]. It was confirm ed that silicon vacancy related defects act as strong positron trapping centers in 3C, 4H and 6H SiC irradiated with electrons. Obtained displacement energy for Si site [4] was in good a reement with radiation damage theories [5]. By irradiation, silicon vacancies are crea ted only in cubic sites in 3 C SiC, while both in cubic and hexagonal sites in hexagonal SiC. In this rese arch, we studied if properties of vacancy defects such as thermal stability and atom ic structure are affected by the existence of inequivalent lattice sites. Experiment Specimens used in this study are chemical-vapor-deposition (CVD) grown n-type 3C, 4H and 6H SiC epilayers doped with nitrogen ([N]=5x10 –1x10 cm). We also used modified-Lely-grown bulk n-type 6H SiC crystals doped with nitrogen ([N]=1x10 17 cm). These specimens were irradiated with 0.5-2 MeV electrons to various doses at room temperature. Multiple He-implantation was also conducted in a similar manner a s ref. [3]. Isochronal annealing was done from 100 to 1700 C for 30 min in vacuum or dry argon ambient. Positron annihilation (the Doppler broadening) measurements were performed using slow positron beams at room temperature. Positron annihilation measurements based on the a gular correlation of annihilation radiation (ACAR) technique were carried out for 3 C and bulk 6H samples using positrons directly emitted from a Na source. Materials Science Forum Online: 2003-09-15 ISSN: 1662-9752, Vols. 433-436, pp 477-480 doi:10.4028/www.scientific.net/MSF.433-436.477

Positron Annihilations in Polycrystalline ReO3 and WO3

The angular correlation curve for ReO 3 , which is a little broader than that for WO 3 , was compared with the results of some model calculations. As the annihilation rates suggest that ReO 3 is ionic as WO 3 is, the wavefunction of 2p electrons is considered to extend more outwards in ReO 3 than in WO 3 .

Lifetime spectra of positrons in ionic compounds

Lifetime spectra of some sulphides, AlN, and SiC consist of two components. Temperature dependences of the spectra of Sb2S3 were investigated from 296–723 K and analyzed with a non-trapping model. It was estimated that cation vacancies due to nonstoichiometry were responsible for the τ2 component.