Eiki Watanabe

Visible photoluminescence from Si clusters in γ-irradiated amorphous SiO2

Visible photoluminescence (PL) bands around 2 eV were studied in 60Co γ‐irradiated (dose<1 MGy) oxygen‐deficient‐type amorphous SiO2 (a‐SiO2) excited by 2–4 eV photons. In addition to the well‐known 1.9 eV PL band due to nonbridging oxygen hole centers, another PL band was observed at 2.2 eV when excited by 3.8 eV photons. The intensity of the 2.2 eV band increases with decreasing oxygen partial pressure during the sample preparation. Electron‐spin‐resonance measurements show that the intensity of the 2.2 eV band is correlated with the concentration of the Eδ′ center, a paramagnetic state of a cluster of silicons. After much higher γ irradiation with a dose up to 10 MGy, a new PL band was induced at 1.75 eV under excitation by 2.5 eV photons, as well as the 1.9 and 2.2 eV PL bands. By comparing its spectral shape and excitation energy with known PL band in Si‐implanted a‐SiO2, it is suggested that the 1.75 eV band is associated with Si nanocrystals formed from Si clusters in a‐SiO2 by the high‐dose γ irra...

Photoluminescence study of defects in ion‐implanted thermal SiO2 films

Photoluminescence (PL) study was performed on B or P ion‐implanted thermal SiO2 films. Two PL bands at 4.3 and 2.6 eV were observed. For the 4.3 eV bands, two PL excitation (PLE) bands were observed at 5.0 and 7.4 eV. Based on the close similarities of the PL and PLE bands to those observed in oxygen‐deficient‐type bulk silica, the 4.3 and 2.6 eV PL bands are ascribed to the oxygen‐deficient‐type defects induced by ion implantation. While the 4.3 eV PL band in the bulk SiO2 decays exponentially, the decay of the corresponding PL band in the implanted thermal SiO2 films follows a power low or stretched exponential, suggesting the distribution of PL lifetimes. This suggests that the oxygen‐deficient‐type defects induced by the ion implantation in thermal SiO2 films are perturbed by the structural distribution of the surrounding Si‐O‐Si network, including the concentration of PL quenching centers.

Photoluminescence of oxygen-deficient-type defects in a-SiO2

Abstract Oxygen-deficient-type defects in a-SiO 2 were studied by means of photoluminescence (PL) measurements. Various properties of the 4.4-eV PL such as the decay lifetime and the temperature dependence in oxygen-deficient-type a-SiO 2 can be explained in terms of an energy diagram involving two configurations of the oxygen-deficient-type defect. The 4.4-eV PL observed from the ion-implanted thermal oxides and the oxides prepared by the plasma-enhanced CVD method, has a stretched-exponential decay, suggesting a large structural distribution in the local network structures. A PL band at ∼ 1.8 eV associated with highly oxygen-deficit states is also observed in oxygen-deficient-type a-SiO 2 after high-dose γ-irradiation (dose: 10 MGy).

Ultrasonic visualization method of electrical trees formed in organic insulating materials

The only practical method to visualize electrical trees formed in organic insulating materials has been to section a specimen with a microtome or a diamond cutter, especially in translucent or opaque materials. In our experiment, a 25 MHz ultrasonic transducer was used to scan a specimen within which an electrical tree had been formed. Ultrasonic echo signals reflected from the treeing region were detected by the same transducer and fed to a electronic measuring system. By determining the horizontal coordinates at which each echo signal from the tree was detected, two-dimensional ultrasonic images were obtained for various types of electrical trees in polyethylene. Furthermore, three-dimensional ultrasonic images of bush-type trees were obtained by means of a new detection or measurement method developed by the authors. These images agree well with those obtained microscopically by sectioning the specimen with a microtome.

Changes in the optical properties of Ge-doped silica glass during exposure to a KrF excimer laser

Abstract Refractive-index changes ( Δn ) in Ge-doped silica glass exposed to a KrF excimer laser were investigated by measurements of the diffraction efficiency from a grating formed using a phase mask. The Δn of 3.2 × 10 −3 was achieved for a hydrogen-treated (150 atm, 2 weeks, room temperature) sample after a KrF laser irradiation (25 Hz, 0.5 J/cm 2 /pulse, 27 kJ/cm 2 ), while the Δn obtained for a non-treated sample is less than 10 −4 . The depth profile of the Δn for the hydrogen-treated sample is limited by the distribution in the hydrogen concentration inside the sample, while it is determined by the attenuation of the ultraviolet light for the non-treated sample.

The effect of filler particle size on ultrasonic image of electrical breakdown region in filler-added insulating materials

The ultrasonic visualization method proposed provides an effective way to detect or visualize the electrical breakdown region in specimens of organic insulating materials even when filled with powder materials. However, there exist a few problems on ultrasonic visualization for such kinds of resins, mainly due to their peculiar acoustic characteristics, i.e. specific acoustic impedance, heavy signal decay within the material and scattering noises from filler particles. Above all, scattering noises prevent the formation of clear ultrasonic images of the electrical breakdown region. In this paper we report the dependence of filler particle size on the magnitudes of scattering noises reflected from filler particles, and recommend the use of filler materials with smaller particle size for convenient future insulation diagnosis.

Kinetics of enhanced photogeneration of E′ centers in oxygen-deficient silica

Abstract The kinetics of the generation of E′ centers induced by a 6.4 eV excimer laser were investigated. Enhanced defect generation was observed in OH-containing oxygen-deficient silicas. The increased E′ centers were found to be correlated with an absorption band at 5.7 eV. The intensity of the 5.7 eV band α, as a function of the laser fluence, F, follows a simple formula of α(F) = αs[1 − exp(− DF)], where the values of αs and the decay coefficient, D, depend on the concentrations of SiH and SiOH bonds.

High Signal-to-Noise Ratio Ultrasonic Point Detection Method using a Fused Quartz Rod as a Pulse Compression Filter and a Sensor

In this paper we propose a method to detect ultrasound with high signal-to-noise ratio (S/N) at a point on the surface of a material. To obtain high S/N, we applied an frequency-modulation (FM) pulse compression method as used in radar. We also used a fused quartz rod for point detection, at the same time using it for the pulse compression filter. The concept is demonstrated with an image reconstructed using a computer tomography algorithm.

Characterization of ion-implanted silica glass by vacuum ultraviolet absorption spectroscopy

AbstractWeinvestigatethemechanismsofdefectformationandopticalabsorptioninducedbyionimplantation,forfab-rication of optical devices by radiation effects. High-purity silica implanted by H þ or He 2þ was characterized usingvacuum-ultraviolet spectroscopy and electron-spin-resonance measurement. Defect formation is suppressed by OHgroups,possiblybythereleaseofatomichydrogen.TheE 0 centerandnonbridgingoxygenholecenterwerecreatedthroughpairgenerationfromthenormalSiAOASibond.TheperoxyradicalwasgeneratedthroughthereactionoftheE 0 center with interstitial oxygen, which is a Frenkel-defect pair with an oxygen vacancy. By the Kramers–Kroniganalysis on the MeV-ion implantation-induced defects, a refractive index increase of the order of 10 4 was esti-mated. 2002ElsevierScienceB.V.Allrightsreserved. Keywords:Silicaglass;Ionimplantation;Vacuumultraviolet;Electronspinresonance 1. IntroductionFabrication of optical devices such as opticalfibergratingshasbeenreportedbyourgroupus-ing mega-electron-volt-order ion implantation onsilica-coreopticalfibers[1].Inordertoextendthistechniqueforthefabricationofsilica-basedplanaroptical devices, it is important to understand themechanismsbywhichchangesinopticalpropertiessuch as refractive index are induced by the im-plantedions.Attentionisfocusedontheelectronicstopping energy loss, since it accounts for morethan 99% of the total loss and most defects areinducedthroughthisprocessinthecaseofMeV-orderionimplantation.2. Experimental proceduresSamplesaretwotypesofhigh-puritysilicaglas-ses containing different OH contents, EDC (OHcontent <1 ppm) and ES (OH content¼ 1200ppm).Theyareplateswithanareaof100mm

Ultrasonic image formation of electrical breakdown region in epoxy resin blended with silica powder as filler material

The only practical way to visualize the electrical breakdown region in insulating materials has long been to section a specimen with a cutting machine like microtome, since they are usually opaque, or at best, translucent. We already demonstrated that electrical trees in simple materials, e.g., polyethylene, could be inspected by an ultrasonic visualization method. As the next step of our study, we began to try to obtain ultrasonic images of the electrical breakdown region that was formed in compound material blended with filler powder. However, it was difficult by far, as was expected, to make the ultrasonic image because the reflection signal from the target (electrical breakdown region) was screened by heavy scattering noises dispersed by filler particles. So we prepared the specimens in which silica powder with different particle sizes and contents were blended. From the experimental results, it became clear that when the mean diameter of filler particle was smaller than the value determined by the theory of Rayleigh scattering, the influence of scattering was reduced abruptly and clear ultrasonic images could be obtained.