Masahito Niibe

Identification of B-K near edge x-ray absorption fine structure peaks of boron nitride thin films prepared by sputtering deposition

Four π∗ resonance peaks were observed in the B-K near edge x-ray absorption fine structure spectra of boron nitride thin films prepared by magnetron sputtering. In the past, these peaks have been explained as the K-absorption of boron atoms, which are present in environment containing nitrogen vacancies, the number of which is 1–3 corresponding to the three peaks at higher photon energy. However, the authors found that there was a strong correlation between the intensities of these three peaks and that of O-K absorption after wide range scanning and simultaneous measurement of nitrogen and oxygen K-absorptions of the BN films. Therefore, the authors conclude that these three peaks at the higher energy side correspond to boron atoms bound to one-to-three oxygen atoms instead of three nitrogen atoms surrounding the boron atom in the h-BN structure. The result of the first-principles calculation with a simple cluster model supported the validity of this explanation.

Ar+-irradiation-induced damage in hydride vapor-phase epitaxy GaN films

The authors have investigated the electrical characteristics of hydride vapor-phase epitaxy GaN films exposed to Ar+ irradiation, employing Schottky barrier diodes. The Ar+ irradiation tends to largely increase the effective carrier concentration in the near surface region of GaN up to ∼25 nm, due to the generation of donor-type N vacancy defects, compared to the original value before the irradiation. More interestingly, acceptor-type deep-level defects are found to be formed at ∼2.1, ∼2.9, and ∼3.2 eV below the conduction band in the subsequently deeper region, in which Ga vacancies introduced by the Ar+ irradiation are considered to be in-diffused and immediately combined with hydrogen. These N vacancies and hydrogenated Ga vacancies formed are dominantly responsible for changing the depth profiles of the effective carrier concentration via the carrier generation, the carrier trapping, and/or carrier compensation.

Optical and electrical investigation of Ar+-irradiated GaN

The generation behavior of deep-level defects in Ar+-irradiated GaN has been investigated by photoluminescence, capacitance–voltage, and photocapacitance techniques. Ar+ irradiation induces a significant increase in near-band edge emission peak intensity due to donor-bound excitons, in agreement with the largely increased effective carrier concentration in a shallow-lying region near the surface. More interestingly, with increasing irradiation time, the carrier concentration tends to decrease in a deep-lying region, compared with that in the inner bulk region, which is considered to stem from carrier trapping and/or carrier compensation via the diffusion and formation of acceptor-type deep-level defects, such as Ga vacancies and residual C.

Low energy soft x-ray emission spectrometer at BL-09A in NewSUBARU

A compact soft X-ray emission spectrometer for the energy region of 50-600 eV has been designed and constructed for the long undulator beamline BL-09A in the NewSUBARU synchrotron radiation (SR) facility. The optical design of the spectrometer is based on a grazing incidence flat-field spectrometer using a valid line-spacing grating. The average groove density of the grating is 2000 L/mm, and the angle of incidence to the grating is 86.5 deg. The distances from the slit to the grating and from the grating to the CCD are 355 mm and 650 mm, respectively. The energy resolution, E/ΔE, was estimated to be greater than 1000 in the energy range of 50-600 eV. Spectra of K-emission X-rays of several light elements, such as B, C, N, and O, from various samples were successfully obtained.

Development of the surface-sensitive soft x-ray absorption fine structure measurement technique for the bulk insulator

We have succeeded in measuring X-ray absorption fine structure (TEY-XAFS) spectra of insulating plate samples by total electron yield. The biggest problem is how to suppress the charge-up. We have attempted to deposit a gold stripe electrode on the surface and obtained a TEY-XAFS spectrum. This indicates that the metal stripe electrode is very useful in the TEY-XAFS measurement of the insulating plate samples. In the detailed analysis, we have found that the effective area for suppressing charge-up was approximately 120 μm from the edge of the electrode.

Fabrication and characterization of fine-grained 316L steel with 2.0 mass% TiC

ABSTRACT Development of fine grain 316L with small amount of TiC for high radiation-tolerant performance was tried considering the fabrication process of thermo-mechanical treatments. The materials obtained are UFG316L+2.0 mass% TiC with the grain size of 90–270 nm, depending on the final annealing temperature from 700 ℃ to 900 ℃. The materials were examined by transmission electron microscopy observation, X-ray absorption near-edge structure spectroscopy, high voltage electron microscope electron irradiation and stress corrosion cracking by crevice bent beam method in high-temperature water with 8 ppm dissolved oxygen. The test results showed that the material is generally of excellent quality. Especially void swelling induced by the electron irradiation at 400 ℃ is less than 1/10 compared to the commercial SUS316L.

Recovery of x-ray absorption spectral profile in etched TiO2 thin films

Near edge x-ray absorption fine structure (NEXAFS) spectra of plasma-etched TiO2 thin films were observed using the total fluorescence yield method involving visible emission. The disrupted spectrum recovered its as-grown (nonetched) profile, upon soft x-ray (SX) irradiation. This recovery was investigated by ultraviolet (UV) irradiation, spatial distribution measurements, exposing recovered samples to air, and NEXAFS measurements of ultrafine TiO2 particles. The spectral profile recovered upon UV irradiation, and at sample positions outside of the SX irradiation site. The recovered spectral profiles were disrupted again, upon exposure to air. Nonetched ultrafine TiO2 particles also exhibited a disrupted spectral profile, which was recovered upon SX irradiation. The spectral recovery is explained by a model involving electrons trapped in oxygen vacancies generated by etching.