Papers by Author: Takeshi Ohshima

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Authors: Gerhard Pensl, Frank Schmid, Sergey A. Reshanov, Heiko B. Weber, M. Bockstedte, Alexander Mattausch, Oleg Pankratov, Takeshi Ohshima, Hisayoshi Itoh
Abstract: Nitrogen (N) donors in SiC are partially deactivated either by Si+-/N+-co-implantation or by irradiation with electrons of 200 keV energy and subsequent annealing at temperatures above 1450°C; simultaneously the compensation is decreased. The free electron concentration and the formation of energetically deep defects in the processed samples are determined by Hall effect and deep level transient spectroscopy. A detailed theoretical treatment based on the density functional theory is conducted; it takes into account the kinetic mechanisms for the formation of N interstitial clusters and (N-vacancy)-complexes. This analysis clearly indicates that the (NC)4-VSi complex, which is thermally stable up to high temperatures and which has no level in the band gap of 4HSiC, is responsible for the N donor deactivation.
Authors: Viktor Ivády, Joel Davidsson, Nguyen Tien Son, Takeshi Ohshima, Igor A. Abrikosov, Ádám Gali
Abstract: Point defects in wide band gap semiconductors have recently shown outstanding potential for implementing room temperature quantum bits and single photon emitters. These atomic scale tools can be used in various quantum information processing, sensing, and imaging applications. Silicon vacancy related photoluminescence centers in 4H, 6H, and 15R-SiC are among the most studied quantum bits that possess a particular spin-3/2 ground and excited state. The microscopic structures of these defects have been recently identified as isolated negatively charged silicon vacancy defects at the symmetrically non-equivalent silicon sites in SiC. Relying on this identification, here we carry out high precision ab initio simulations on negatively charged silicon vacancies in 4H and 6H-SiC and calculate the most important magneto-optical data, such as the zero-phonon photoluminescence energies, the zero-field-splitting, and the hyperfine tensors for the nearest and farther nuclear spins.
Authors: Li Cai, Toshio Hirao, Hiroaki Yano, Zong Fan Duan, Hideharu Takayanagi, Hideharu Ueki, Takeshi Ohshima, Yasushiro Nishioka
Abstract: Electrical characterization of 60Co γ-ray radiation effects on pentacene-based organic thin-film-transistors having two kinds of gate insulator have been carried out. For transistors with SiO2 gate insulator, the threshold voltage shifts are consistent with positive charge trapping in the oxide and a “rebound” effect is observed. This “rebound” effect is attributed to the negatively charged interface traps generated during irradiation. For polyimide gate insulator, the threshold voltage continually decreases with an increasing total-dose. At total-dose of 1200 Gy (Si), for the SiO2 gate insulator, the field-effect mobility decreased by almost 80%, and for polyimide gate insulator, it decreased by 40%.
Authors: Jun Kajihara, Shinichiro Kuroki, Seiji Ishikawa, Tomonori Maeda, Hiroshi Sezaki, Takahiro Makino, Takeshi Ohshima, Mikael Östling, Carl Mikael Zetterling
Abstract: 4H-SiC pMOSFETs with Al-doped S/D and NbNi silicide ohmic contacts were demonstrated and were characterized at up to a temperature of 200 °C. For the pMOSFETs, silicides on p-type 4H-SiC with Nb/Ni, NbNi alloy, Ni and Nb/Ti were investigated, and the Nb/Ni silicide with the contact resistance of 5.04×10-3 Ωcm2 were applied for the pMOSFETs.
Authors: Shinichiro Kuroki, Tatsuya Kurose, Hirofumi Nagatsuma, Seiji Ishikawa, Tomonori Maeda, Hiroshi Sezaki, Takamaro Kikkawa, Takahiro Makino, Takeshi Ohshima, Mikael Östling, Carl Mikael Zetterling
Abstract: For logic gate with higher voltage swing, 4H-SiC pseudo-CMOS logic inverter with four nMOS was suggested and demonstrated, and a high voltage swing of 4.4 V was achieved at VDD=5 V. Simple nMOS inverters were also investigated. Both of pseudo-CMOS and nMOS inverters were operated at a high temperature of 200°C. For future SiC large integrated circuits, junction leakage current between n+ regions were also investigated with the comb-shaped test elements.
Authors: Takeshi Ohshima, Akira Uedono, Osamu Eryu, Kin Kiong Lee, Koji Abe, Hisayoshi Itoh, Kenshiro Nakashima
Authors: Walter M. Klahold, Robert P. Devaty, Wolfgang J. Choyke, Koutarou Kawahara, Tsunenobu Kimoto, Takeshi Ohshima
Abstract: Ultra-pure n-type (8×1013 cm-3), 99 μm thick epitaxial films of 4H SiC were electron irradiated at 170 keV with a fluence of 5×1016 cm-2 or at 1 MeV with a fluence of 1×1015 cm-2 in various geometries. Low temperature photoluminescence (LTPL) spectra and microwave photoconductance (μPCD) lifetime measurements were obtained on all samples prior to annealing and after annealing in Argon in free standing mode or on a POCO carbon platform, every 50°C from 1100°C to 1500°C. No improvement in carrier lifetime was obtained. Spurious lines attributable to the use of a Genesis CX 3550Å laser are also reported.
Authors: Takuma Matsuda, Takashi Yokoseki, Satoshi Mitomo, Koichi Murata, Takahiro Makino, Hiroshi Abe, Akinori Takeyama, Shinobu Onoda, Yuki Tanaka, Mikio Kandori, Toru Yoshie, Yasuto Hijikata, Takeshi Ohshima
Abstract: Radiation response of 4H-SiC vertical power Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs) was investigated at 150°C up to 10.4 MGy. Until irradiation at 1.2 MGy, the drain current – gate voltage curves of the SiC MOSFETs shifted to the negative voltage side, and the leakage of drain current at gate voltages below threshold voltage increased with increasing absorbed dose. However, no significant change in the electrical characteristics of SiC MOSFETs was observed at doses above 1.2 MGy. For blocking characteristics, there were no degradations of the SiC MOSFETs irradiated at 150°C even after irradiated at 10.4 MGy.
Authors: Hiroaki Yano, Li Cai, Toshio Hirao, Zong Fan Duan, Yutaro Takayanagi, Hirokuni Ohuchi, Hideharu Ueki, Takeshi Ohshima, Yasushiro Nishioka
Abstract: P-channel pentacene field effect transistorswith a Si/SiO2/pentacene/Au structure were fabricated, and were gamma-ray irradiated with a Co60 source. The changes of the drain current ID vs. source/drain voltage VSD (ID - VSD) characteristics were measured after every 200 Gy in silicon (GySi) irradiations up to the total dose of 1200 GySi. The drain current ID continuously decreased to less than 10 % of that before irradiation after 1200 GySi irradiation. The threshold voltage Vth continuously decreased up to 800 GySi, started to saturate above 800 GySi, and recovered above 1000 GySi. The mobility m continued to decrease up to 1200 GySi. Those behaviors were explained by accumulation of positive trapped charge within the gate insulator SiO2 near the interface, continuous increase of interface traps near the interface between the SiO2 and pentacene, and build up of electrons in the channel regions. These behaviors were discussed in comparisons with previously reported results on ultraviolet (UV) light irradiation experiments on similarly structured pentacene-based transistors.
Authors: Shinichiro Kuroki, Hirofumi Nagatsuma, Milantha de Silva, Seiji Ishikawa, Tomonori Maeda, Hiroshi Sezaki, Takamaro Kikkawa, Takahiro Makino, Takeshi Ohshima, Mikael Östling, Carl Mikael Zetterling
Abstract: Characteristics of 4H-SiC nMOSFETs with arsenic-doped S/D and NbNi silicide contacts in harsh environments of high-temperature up to 450°C, and high gamma-ray radiation up to over 100 Mrad, were investigated. At high temperature, field effect mobility increased as proportional to T3/2, and threshold voltage was shifted with temperature coefficients of -4.3 mV/K and -2.6 mV/K for oxide thicknesses of 10 nm and 20 nm, respectively. After Co60 gamma-ray exposure of 113 Mrad, the field effect mobility was varied within 8% for oxide thickness of 10 nm, however for 20 nm oxide thickness, this variation was 26%. The threshold voltage shifts were within 6%.
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