Hisayoshi Itoh

Band gap narrowing of titanium dioxide by sulfur doping

Titanium dioxide (TiO2) doped with sulfur (S) was synthesized by oxidation annealing of titanium disulfide (TiS2). According to the x-ray diffraction patterns, TiS2 turned into anatase TiO2 when annealed at 600 °C. The residual S atoms occupied O-atom sites in TiO2 to form Ti–S bonds. The S doping caused the absorption edge of TiO2 to be shifted into the lower-energy region. Based on the theoretical analyses using ab initio band calculations, mixing of the S 3p states with the valence band was found to contribute to the band gap narrowing.

Analysis of electronic structures of 3d transition metal-doped TiO2 based on band calculations

Abstract The electronic structures of titanium dioxide (TiO 2 ) doped with 3d transition metals (V, Cr, Mn, Fe, Co and Ni) have been analyzed by ab initio band calculations based on the density functional theory with the full-potential linearized-augmented-plane-wave method. When TiO 2 is doped with V, Cr, Mn, Fe, or Co, an electron occupied level occurs and the electrons are localized around each dopant. As the atomic number of the dopant increases the localized level shifts to lower energy. The energy of the localized level due to Co is sufficiently low to lie at the top of the valence band while the other metals produce midgap states. In contrast, the electrons from the Ni dopant are somewhat delocalized, thus significantly contributing to the formation of the valence band with the O p and Ti 3d electrons. Based on a comparison with the absorption and photoconductivity data previously reported, we show that the t 2g state of the dopant plays a significant role in the photoresponse of TiO 2 under visible light irradiation.

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.

Electron spin resonance in electron-irradiated 3C-SiC

Electron‐irradiation‐induced defects in epitaxially grown 3C‐SiC crystals have been studied by electron‐spin‐resonance (ESR) measurements. The results indicate the presence of an isotropic ESR center that consists of five lines equally spaced at about 1.5 G and has a g value of 2.0029±0.0001. Isochronal and isothermal annealing of electron‐irradiated 3C‐SiC showed that this center was annealed at three stages (150, 350, and 750 °C) and that the 750 °C stage exhibited first‐order reaction with an activation energy of 2.2±0.3 eV.

Development of a new data collection system and chamber for microbeam and laser investigations of single event phenomena

Abstract A new target chamber and control system for temperature-based transient-IBIC and transient-LBIC measurements using the same experimental chamber is outlined. The system has been designed for both ultra-fast and relatively slow transient measurements as a function of temperature from 77 K to 450 K. The control system, implemented in the Labview environment, allows single ion scanning and transient acquisition on a set of oscilloscopes, for an array of temperatures and bias. The modularity of the system allows its use for a broad range of experiments from single event upset transient current measurements to scanning ion deep level transient spectroscopy charge transient measurements. In this paper, we describe the overall system and illustrate its potential by way of example.

Radiation induced defects in CVD-grown 3C-SiC

Radiation-induced defects in 3C-SiC epitaxially grown by a chemical vapor deposition method were studied with the electron spin resonance (ESR) technique. A 15-line ESR spectrum was observed in 2-MeV proton and 1-MeV electron irradiated 3C-SiC when the magnetic field was applied parallel to the axis. This spectrum, T1, which has an isotropic g-value of 2.0029+or-0.0001, was interpreted by simultaneous hyperfine interactions of a paramagnetic electron with the surrounding /sup 13/C at four carbon sites and /sup 29/Si at 12 silicon sites. The T1 spectrum appeared to arise from a point defect at a silicon site. The observed hyperfine interactions with neighboring /sup 13/C and /sup 29/Si nuclei are discussed in terms of a simple molecular-orbital treatment of the defect. >

High-radiation-resistant InGaP, InGaAsP, and InGaAs solar cells for multijuction solar cells

The radiation response of 3 MeV proton-irradiated InGaP, InGaAsP and InGaAs solar cells was measured and analyzed in comparison with those of InP and GaAs. The degradation of the minority-carrier diffusion length was estimated from the spectral response data. The damage coefficient KL for the 3 MeV proton-irradiated InGaP, InGaAsP and InGaAs was also determined. The radiation resistance increases with an increase in the fraction of In–P bonds in InGaP, InGaAsP and InGaAs. Differences in the radiation resistance of InGaP, InGaAs and InGaAs materials are discussed. Minority-carrier injection under forward bias is found to cause partial recovery of the degradation on irradiated InGaP and InGaAsP cells.

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 1400 °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 500 °C. The defects related to silicon vacancies were found to be annealed at around 750 and 1400 °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.

Suppressed diffusion of implanted boron in 4H–SiC

Transient-enhanced diffusion of boron (B) during anneals at 1700 °C is experimentally observed in B-implanted 4H–SiC samples. This enhanced diffusion can strongly be suppressed by coimplantation of carbon or by a preanneal at 900 °C. It is proposed that B in 4H–SiC diffuses via the kick-out mechanism with the assistance of silicon interstitials in analogy to the B diffusion in Si. From the Fickian diffusion tail into the undamaged bulk, the preexponential factor D0 and the activation energy EA of the B diffusion coefficient D(B,T) are determined.

Temperature dependence of heavy ion-induced current transients in Si epilayer devices

We report on the temperature dependence of the heavy-ion transient-ion beam induced current response of Si epilayer devices from 80 to 300 K. The measurements were performed on a heavy-ion microbeam in conjunction with the new transient-ion beam induced current system developed at the Japan Atomic Energy Research Institute. Furthermore, we perform a detailed comparison with technology computer-aided design (TCAD) simulations and discuss the results in terms of TCAD modeling, experimental procedure, and the implications for temperature-related single-event upset modeling.