Papers by Author: Toshiyuki Isshiki

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Authors: Takahiro Sato, Yoshihiro Ohtsu, Yoshihisa Orai, Toshiyuki Isshiki, Munetoshi Fukui
Abstract: A peculiar surface defect on a silicon carbide (SiC) epitaxial wafer, found to be associated a basal plane dislocation (BPD), was studied using a low energy scanning electron microscope (LESEM), and a novel method we are calling multi directional scanning transmission electron microscopy (MD-STEM). We have confirmed that an etch pit with double cores neighboring a peculiar surface defect is derived from the extended BPD. The BPD consisted of two partial dislocations with a stacking fault width of about 100 nm. Observation of only one viewing direction in a previous study missed the extended dislocation but through the use of the MD-STEM method in the current study, the dislocation has been confirmed to be extended into a stacking fault.
Authors: Toshiyuki Isshiki, Hiroshi Saijo, Shigehiro Nishino, M. Shiojiri
Authors: Nobuyuki Tatemizo, Saki Imada, Yoshio Miura, Koji Nishio, Toshiyuki Isshiki
Abstract: Band structure calculations for radiofrequency-sputtered AlN-films doped with various 3d-transition-metals (TM: V, Cr, and Mn) were conducted to investigate the origin of the characteristic optical absorption structures. Experimentally evaluated crystal structures and lattice constants of the synthesized films were adopted for supercells. The model calculations showed that additional energy bands mainly consisting of 3d e and t states of TMs are formed in the band gap of AlN (6.2 eV), and that their potentials depend on the TM species. It was also shown that the Fermi levels of Cr- and Mn-doped AlN lie within the spin-up t band, while the Fermi level of V-doped AlN lies between the spin-up e and t bands. These findings imply that the materials have TM species-dependent, multiple absorption paths with lower energy than the band gap energy of AlN, resulting in optical absorption in the near-ultraviolet, visible, and infrared regions.
Authors: Takahiro Sato, Yoshihisa Orai, Toshiyuki Isshiki, Munetoshi Fukui, Kuniyasu Nakamura
Abstract: Cross section and plan view dislocation analysis at the conversion point of a basal plane dislocation (BPD) into a threading edge dislocation (TED) in a silicon carbide epitaxial wafer was developed using a newly modified multi directional scanning transmission electron microscopy (STEM) technique. Cross section STEM observation in the [-1100] direction, found a conversion point located 5.5 μm from the surface, where two dislocation lines in the basal plane convert into one dislocation line nearly along the hexagonal c axis was observed. Using plan view STEM observation along the [000-1] direction, it is confirmed that the dislocation lines are two partial dislocations of a BPD and one TED by g·b invisibility analysis. This new technique is a powerful tool to evaluate the fundamental dislocation characteristics of power electronics devices.
Authors: Takahiro Sato, Hiroaki Matsumoto, Seiichi Suzuki, Toshiyuki Isshiki, Nakamura Kuniyasu
Abstract: The dislocation analysis of latent scratch induced chemical mechanical polishing process on 4H-silicon carbide (SiC) using the multi directional scanning transmission electron microscopy (STEM) method and elastic stain measurement were performed. A scanning electron microscope image shows that a latent scratch extended toward the [30] direction and the width is about 50 nm. Cross sectional STEM images shows that the depth of latent scratch due to distortion is about 20 nm. From the result of plan view STEM observation along [000] direction, it was observed that a latent scratch had two defect lines toward the [30] direction, which were a loop type on upper side and a linear type on the lower side. The Burgers vector of each defect have a component in basal plane. Elastic strain mapping was performed using transmission electron microscope equipped with a procession diffraction system. Inside the latent scratch indicates stain-free field, however around latent scratch indicates compressive strain field. About 1.5 % compressive strain field x, y direction and shear strain along latent scratch exists on typical area. As a results of STEM and elastic strain analysis, the atomic arrangement in basal plane seems to be related with the compressive strain around latent scratch.
Authors: Takahiro Sato, Yuya Suzuki, Hiroyuki Ito, Toshiyuki Isshiki, Munetoshi Fukui
Abstract: The conventional KOH etching method at elevated temperatures is an easy way to study SiC dislocations, but presents problems due to an increased etch rate. Here, we examine the application of low temperature KOH treatment for the analysis of dislocation cores and etch pits in SiC. A low energy scanning electron microscope (SEM) is effective to classify dislocation kinds. The scanning transmission electron microscope (STEM) observation of thick samples prepared by the in situ micro-sampling technique enables evaluation of detailed dislocation properties.
Authors: Atsushi Nakahira, Y. Fujita, T. Kubo, S. Eguchi, S. Nishida, Toshiyuki Isshiki
Abstract: By the usage of the hydrolysis of Ti alkoxide, various types of TiO2 were obtained through the addition of catalyst (HCl, NH4OH, and CH3COONH4) and some additives into Ti alkoxide solution at room temperature. In special, the effect of two additives, diethylene glycol (DEG) and hexamethylphospheric triamide (HMPA), on hydrolysis behaviors and microstructure with various catalyst for hydrolysis of Ti-tetraisopropoxide was in detail investigated. In case of TiO2 powder with HMPA as an additive, the crystallinity of TiO2 except with NH4OH as a catalyst decreased in XRD patterns, compared to those of products with no additive and with DEG as an additive. The addition of HMPA prohibited the growth of TiO2 and led to fine TiO2 with the average particle size of approximately 10nm.
Authors: Shinkichi Hamada, Hisashi Yoshioka, Hiroshi Kawami, Nobuhiko Nakamura, Yoshitaka Setoguchi, Toru Matsunami, Kimito Nishikawa, Toshiyuki Isshiki
Abstract: We have been trying to improve a quality of crystal, using the metastable solvent epitaxy (MSE) method, one of the solution methods. In MSE, a Frank-type fault is formed by conversion of a threading screw dislocation (TSD) in the substrate. To study the status of the growth, we performed plane-viewed TEM observation. Analysis of Burgers vectors in the TEM image showed Frank PDs (Partial Dislocations) which do not include a components and Frank PDs which include a components. The total Burgers vectors of Frank-type fault including a components are represented as b=a/3+c, which indicates some TSDs in the substrate also include a components.
Authors: Toshiyuki Isshiki, Koji Nishio, Yoshihisa Abe, Jun Komiyama, Shunichi Suzuki, Hideo Nakanishi
Abstract: Epitaxial growth of AlN was carried out by MOVPE method on SiC/Si buffered substrates prepared by using various Si surfaces of (110), (211) and (001). Cross-sectional HRTEM analyses of the interfaces between SiC buffer layer and AlN epitaxial layer disclosed characteristic nanostructures related growth mechanism on the each substrate. In the case of Si(110) and Si(211) substrate, hexagonal AlN grew directly on SiC(111) plane with AlN(0001) plane parallel to it. In contrast, growth on Si(001) substrate gave complicate structure at AlN/SiC interface. Hexagonal AlN didn’t grow directly but cubic AlN appeared with a pyramidal shape on SiC(001). When the cubic AlN grew 10nm in height, structure of growing AlN crystal changed to hexagonal type on the pyramidal {111} planes of cubic AlN.
Authors: Toshiyuki Isshiki, Mitsutaka Nakamura, Taro Nishiguchi, Koji Nishio, Satoru Ohshima, Shigehiro Nishino
Abstract: Interfaces between a Si(110) substrate and 3C-SiC crystals grown hetero-epitaxially by CVD were investigated by cross-sectional transmission electron microscopy. Gas flow condition during the carbonization process affects the roughness of the substrate surface and there is an optimum condition to preserve the flat surface. High quality 3C-SiC crystals grew only on the flat substrate, with crystallographic relationship of Si[1-10]//SiC[1-10] and Si[001]//SiC[1-1 - 2], because the well-lattice-match relationship was limited in a two-dimensional region at the SiC(111)/Si(110) interface. Using the optimum condition, some kinds of roughness at an atomic scale remained on the surface of the substrate. Nanoscopic observation of the crystals grown on an off-axis substrate revealed the influence of the roughness on the epitaxial growth and the defects generation at the interface.
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