Papers by Author: Katsunori Danno

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Authors: Yoshihiro Sugawara, Y. Yao, Yukari Ishikawa, Katsunori Danno, Hiroshi Suzuki, Takeshi Bessho, Yoichiro Kawai, Yuichi Ikuhara
Abstract: We developed the transmission electron microscopy (TEM) sample preparation technique for the low dislocation density of 4H-SiC by combining the KOH+Na2O2 (KN) etching and the focused ion beam (FIB) microsampling technique. The dislocation under sea-shell pit was then characterized by large-angle convergent-beam electron diffraction (LACBED). It is demonstrated that this method is powerful for evaluating Burgers vectors of dislocations. Burgers vector of the measured basal plane dislocation (BPD) is determined to be b=1/3[-12-10]. Two-beam bright-field (TBBF) imaging identified the rotating direction of the threading screw dislocation (TSD) is counter-clockwise.
Authors: Yukari Ishikawa, Yoshihiro Sugawara, Hiroaki Saitoh, Katsunori Danno, Yoichiro Kawai, Noriyoshi Shibata, Tsukasa Hirayama, Yuichi Ikuhara
Abstract: The structures of defects that form different types of etch pits on highly N-doped 4H-SiC substrates, that were produced by a sublimation method, after molten KOH etching were characterized. It was found that most of the dislocations in the epitaxial layer originated from defects at the surface of substrate whose etch pit structures were clearly different from the conventional structures. The etch pits were classified into drop, oval, round and caterpillar pits. The drop and oval pits were concluded to be formed by the deformation of conventional etch pits. Round pits were concluded to originate from half loop dislocations and were transformed to complex dislocations by epitaxial growth. Analysis by transmission electron microscopy measurement indicates that slipped edge dislocations (or screw dislocations) on the basal plane form caterpillar pits.
Authors: Yoshihiro Sugawara, Yong Zhao Yao, Yukari Ishikawa, Katsunori Danno, Hiroshi Suzuki, Takeshi Bessho, Satoshi Yamaguchi, Koichi Nishikawa, Yuichi Ikuhara
Abstract: A threading dislocation (TD) in 4H-SiC, which was currently interpreted as a perfect threading edge dislocation (TED) by synchrotron monochromatic-beam X-ray topography (SMBXT) and molten KOH etching with Na2O2 additive, was performed comparative characterization using weak-beam dark-field (WBDF) and large-angle convergent-beam electron diffraction (LACBED) methods. The TD was suggested to be dissociated into a dislocation pair which can be observed in the WBDF image of g=-12-10. The TD, which was identified as b//[-12-10] by SMBXT observation, was unambiguously determined as b=1/3[-12-10] by LACBED analysis. In the case of perfect TED, it was found that the direction of Burgers vector derived from SMBXT observation corresponds to LACBED analysis.
Authors: Tsunenobu Kimoto, Katsunori Danno, Keiko Fujihira, Hiromu Shiomi, Hiroyuki Matsunami
Authors: Katsunori Danno, Tsunenobu Kimoto
Abstract: Deep levels in as-grown p-type 4H-SiC epilayers have been investigated by DLTS. Three deep hole traps (HK2, HK3 and HK4) can be detected by DLTS in the temperature range from 350K to 700K. They are energetically located at 0.84 eV (HK2), 1.27 eV (HK3) and 1.44 eV (HK4) above the valence band edge. The activation energy of the traps does not show any meaningful change regardless of applied electric field, indicating that the charge state of the deep hole traps may be neutral after hole emission (donor-like). By the low-energy electron irradiation, the HK3 and HK4 concentrations are significantly increased, suggesting that the origins of the HK3 and HK4 may be related to carbon displacement. Study on the thermal stability of these hole traps has revealed that the trap concentrations of HK3 and HK4 are reduced to below the detection limit (1-2 × 1011 cm-3) by annealing at 1350°C. The HK2 is thermally more stable than HK3 and HK4, and becomes lower than the detection limit by annealing at 1550°C.
Authors: Katsunori Danno, Tsunenobu Kimoto
Abstract: The authors have investigated deep levels in electron-irradiated n- and p-type 4H-SiC epilayers by deep level transient spectroscopy (DLTS). By low-energy electron irradiation at 116 keV, the Z1/2 and EH6/7 concentrations are increased in n-type samples, and the concentrations are almost unchanged after annealing at 950°C for 30 min. In p-type samples, the unknown centers, namely EP1 and EP2, are introduced by irradiation. By annealing at 950°C, the unknown centers are annealed out. The HK4 center (EV + 1.44 eV) is increased by the electron irradiation and subsequent annealing at 950°C. The dependence of increase in the trap concentrations by irradiation (NT) on the electron fluence reveals that NT for the Z1/2 and EH6/7 centers is in proportional to the 0.7 power of electron fluence, while the slope of the plot is 0.5 for the HK4 center. The Z1/2 and EH6/7 centers show similar annealing stage and are thermally stable up to 1500-1600°C, while the HK4 center is annealed out at about 1350°C. The Z1/2 and EH6/7 centers may be derived from a same origin (single carbon vacancy: VC) but different charge state. The HK4 center may be a complex including VC.
Authors: Yong Zhao Yao, Yoshihiro Sugawara, Yukari Ishikawa, Katsunori Danno, Hiroshi Suzuki, Takeshi Bessho, Yoichiro Kawai, Noriyoshi Shibata
Abstract: Electron beam induced current (EBIC) and etch pit method have been used to study the dissociation behavior of basal plane dislocations (BPDs) in 4H-SiC under electron beam irradiation. When 20 kV scanning electron beam was applied for 1 h, it has been found that BPDs whose dislocation lines were along [11-20] off-cut direction dissociated into partial dislocations (PDs) forming a stacking fault (SF) between them; while no dissociation was found for BPDs extending along other directions. These results are discussed in terms of different formation energy of SFs expanding from a pure screw type and a mixed type BPD. In addition, the angle between dislocation line of a BPD and the [11-20] off-cut direction might also play a role in determining the minimum energy for SF formation.
Authors: Katsunori Danno, H. Saitoh, Akinori Seki, T. Shirai, Hiroshi Suzuki, T. Bessho, Yoichiro Kawai, Tsunenobu Kimoto
Abstract: Diffusion of transition metals in 4H-SiC has been investigated by secondary ion mass spectroscopy using epilayers and substrates implanted with titanium (Ti), chromium (Cr), iron (Fe), or nickel (Ni). In the epilayers, Cr, Fe, and Ni atoms have diffused by argon (Ar) annealing at 1780°C for 30 min. In n+ substrates, the diffusivity of the metals is smaller than that in the epilayers, and only Ni has diffused by the annealing. By the Ar or helium implantation following the implantation of transition metals, diffusion of transition metals can be successfully suppressed.
Authors: Yong Zhao Yao, Yoshihiro Sugawara, Yukari Ishikawa, Hiroaki Saitoh, Katsunori Danno, Hiroshi Suzuki, Yoichiro Kawai, Noriyoshi Shibata
Abstract: Dislocations in highly doped n-type 4H-SiC (n+-SiC, n>1019 cm-3) substrate have been studied by means of electron beam induced current (EBIC). Ni/n-SiC/n+-SiC/Al structure was fabricated in order to simultaneously observe the dislocations in n-SiC epilayer and n+-SiC substrate. We have found that dark dots in the EBIC image correspond to threading screw dislocations (TSDs) and threading edge dislocations (TEDs) with the former being relatively darker. Short dark lines along off-cut are attributed to basal plane dislocations (BPDs) in the epilayer; and the randomly oriented long dark lines are caused by the BPDs in the substrate. The classification of the dislocations by EBIC has been examined by wet etching in KOH+Na2O2.
Authors: Yong Zhao Yao, Yukari Ishikawa, Yoshihiro Sugawara, Koji Sato, Katsunori Danno, Hiroshi Suzuki, Hidemitsu Sakamoto, Takeshi Bessho, Satoshi Yamaguchi, Koichi Nishikawa
Abstract: In this paper, we report a newly developed dislocation-revealing etch pit method for 4H-SiC single crystal, which can distinguish edge (TED, Burgers vector b=a), elementary screw (TSD, b=1c) and mixed (TMD, b=c+a) threading dislocations. In this method, vaporized NaOH gas was used to etch the Si-face of a SiC wafer at substrate temperature around 950 °C. By a side-by-side comparison between the optical images of the etch pits and the X-ray topographic (XRT) images, it has been found that threading dislocations (TDs) in SiC could be revealed as hexagonal etch pits with distinct geometrical features (shape, size and facet orientation) depending on their Burgers vectors. Based on these results, we consider this etch pit method as an easily-operated and inexpensive technique to categorize TDs, and it may help to promote our understanding on the different roles that these types of TDs have played in the performance degradation of SiC power devices.
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