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On the switching dynamics of epitaxial ferroelectric CeO2–HfO2 thin film capacitors
Cüppers Felix,Hirai Koji,Funakubo Hiroshi 나노기술연구협의회 2022 Nano Convergence Vol.9 No.56
Epitaxial layers of ferroelectric orthorhombic HfO2 are frequently investigated as model systems for industrially more relevant polycrystalline films. The recent success in stabilizing the orthorhombic phase in the solid-solution cerium oxide – hafnium oxide system allows detailed investigations of external influences during fabrication. This report analyzes the ferroelectric properties of two thin film capacitors, which were post-deposition annealed in N2 and O2 atmospheres to achieve the orthorhombic phase after room temperature deposition. The samples, which exhibit very similar constituent phase, appear identical in conventional polarization-field hysteresis measurements. However, a significant switching speed difference is observed in pristine devices. Continued field cycling reduces the difference. Deeper analysis of switching transients based on the Nucleation Limited Switching model suggests that the O2 heat treatment atmosphere results in an altered oxygen vacancy profile, which is reverted during ferroelectric cycling.
양상모,신영재,Yoshitaka Ehara,Hiroshi Funakubo,윤종걸,James F. Scott,노태원 한국물리학회 2019 Current Applied Physics Vol.19 No.4
Recently, in ferroelectric materials, there have been many experimental efforts to find out more intriguing topological objects and their functionalities, such as conduction property. Here we investigated ferroelectric domain structures and related topological defects in the (111)-oriented epitaxial tetragonal PbZr0.35Ti0.65O3 thin film. Systematic piezoresponse force microscopy measurements revealed that the field-induced polarization switching can form thermodynamically stable superdomain structures composed of nano-sized stripe subdomains. Within such superdomain structures, we observed the exotic equilateral triangular in-plane flux-closure domains composed of three stripe domain bundles with 120/120/120 degrees of separation. The conductive- atomic force microscopy measurements under vacuum showed that some vertices have significantly higher conductivity compared to other surrounding regions. This work highlights electric field-driven polarization switching and unique crystallographic symmetry (here, three-fold rotational symmetry) can generate exotic ferroelectric domain structures and functional topological defects, such as conductive vertices.
Takahisa Shiraishi,Hiro Einishi,Shintaro Yasui,Hiroshi Funakubo,Tomohito Hasegawa,Minoru Kurosawa,Mutsuo Ishikawa,Hiroshi Uchida,Yukio Sakashita,김상수,송태권,도달현,김명호 한국물리학회 2013 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.62 No.7
(K0.88Na0.12)NbO3 (KNN) thick films with various thicknesses were prepared on Ni-based metal substrates by hydrothermal method. Although the deposition of KNN films was hardly observed directly on metal substrates until 12 h, the amount of KNN films deposited on both SrRuO3/LaNiO3- and SrRuO3-covered Ni based metal substrates increased to about 5 μm with increasing deposition time above 3 h. X-ray diffraction studies confirmed the formation of {100}c- and randomly-oriented KNN films on SrRuO3/LaNiO3- and SrRuO3-covered metal substrates, respectively. Ferroelecctric property studies revealed an improved remanent polarization and an improved piezoelectric response of {100}c-oriented KNN films over the randomly-oriented KNN films. In addition, the values for {100}c-oriented KNN films were similar to those of {100}c-oriented epitaxial KNN films prepared on (100)cSrRuO3//SrTiO3 substrates.
Seiji Nakashima,Hironori Fujisawa,Masaru Shimizu,Osami Sakata,Tomoaki Yamada,Hiroshi Funakubo,박정민,Takeshi Kanashima,Masanori Okuyama 한국물리학회 2011 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.59 No.31
An electric-field-induced strain at 80 K in a 350-nm-thick BiFeO_3 (BFO) thin film deposited by pulsed laser deposition has been investigated by a time-resolved x-ray diffraction under an electric field using a synchrotron radiation. A 300-nsec-width and 804.09-nsec-period voltage pulses were applied to the BFO thin film during the measurements. The time-resolved-x-ray diffraction profile around BFO (001) diffraction peak indicates that BFO (001) diffraction peak shifts to lower angle side during the voltage application. A strain estimated from the peak shift lineally increases with increasing the amplitude of the voltage pulses. A piezoelectric constant (d_(33)) of (001)-oriented domains in the polycrystalline BFO thin film at 80 K is 20.2 pm/V. The value is smaller than that measured at RT of 27.8 pm/V in our previous report.
Jin-Woong Kim,Hiromi Shima,Ken Nishida,Takashi Yamamoto,Hiroshi Funakubo 한국물리학회 2014 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.65 No.3
Ba(ZrxTi1−x)O3 (BZT) thin films with different Zr contents were deposited on (100)MgO and(100)Pt/(100)MgO substrates by RF-magnetron sputtering using metal targets. The BZT thinfilms epitaxially grew on MgO substrates with only a (001)/(100) orientation and had a singleperovskite phase. In all cases, Ba/Ti ratio was stoichiometric and the BZT films possessed a densemicrostructure. The grain size decreased with increasing Zr content. At room temperature, adielectric constant as a function of the DC bais (tunability) of nearly 30% was achieved at 1 MHz;meanwhile, a relatively low dielectric loss was obtained. Moreover, after a post-annealing process,the tunability was increased significantly. These results indicate that we succeeded in depositinghigh-quality, and potential tunable ferroelectrics.
Composition Control of YSZ Thin Film Prepared by MOCVD
Matsuzaki, Tomokazu,Okuda, Norikazu,Shinozaki, Kazuo,Mizutani, Nobuyasu,Funakubo, Hiroshi The Korean Ceramic Society 2000 The Korean journal of ceramics Vol.6 No.2
Zirconia films stabilized b $Y_2O_3$, YSZ, films were deposition by metal organic chemical vapor deposition (MOCVD) onto various kind of substrates. $Y_2O_3$, $ZrO_2$and the mixtures of these two were deposited and characterized. The deposition rate, the film composition and the structure could be systematically varied through the $Y(C_{11}H_{19}O_2)_3$, Zr(O.t-$C_H_9)_4$source gas ratios and the deposition temperature. The Y/Zr ratio in YSZ film could be adjusted by controlling the ratio of $Y(C_{11}H_{19}O_2)_3$, Zr(O.t-$C_4H_9)_4$partial pressures. This is because the ratios of the deposition rates of Y and Zr atoms in $Y_2O_3$and $ZrO_2$films to those in YSZ films, Ф, are constant irrespective of the input gas concentration. However, the Y/Zr ratio was found to be smaller than that estimated based on the deposition rates of un-mixed $Y_2O_3$and $ZrO_2$films. This is because the Фs of Y and Zr atoms are not equal. The activation energy of $Y_2O_3$component in YSZ films was similar to that of $ZrO_2$component in YSZ films. These YSZ values were more than 4 times larger than those of un-mixed $Y_2O_3$or $ZrO_2$films.