Tuning of electrical properties in correlated transition metal oxides for electronics

손준우 한국공업화학회 2016 한국공업화학회 연구논문 초록집 Vol.2016 No.0

Correlated transition metal oxides have attracted considerable attention due to a remarkable array of functionalities that stem from the strong correlations between the localized transition metal valence d electrons. One of the representative functionalities in transition metal oxides includes abrupt phase transition under external stimuli, e.g. metal-insulator transition, in correlated electron systems. The uniques property can be applicable to the electronic devices for steep electronic switch, such as the logic and memory devices with high on/off ratio, the selector devices for 3D cross point memory. In this presentation, I will represent our recent result to realize unconventional electronic devices, such as the steep selector devices and synaptic analog devices for neuromorphic application, using correlated oxides, such as NbO2, NdNiO3 etc.

Oxygen stoichiometry controlled sharp insulator-metal transition in highly oriented VO₂/TiO₂ thin films

Im, Ji-Seok,Anoop, Gopinathan,Sohn, Min Kyun,Kang, Dae Joon,Jeong, Sang Yun,Lee, Sanghan,Jo, Ji Young ELSEVIER 2018 Current Applied Physics Vol.18 No.6

<P><B>Abstract</B></P> <P>The insulator-metal transition (IMT) in vanadium dioxide (VO<SUB>2</SUB>) which occurs above room temperature (67 °C) is highly sensitive to atomic defects caused by oxygen stoichiometry. The strained growth and the degree of oxygen deficiency in VO<SUB>2</SUB> epitaxial films result in lowering of transition temperature below room temperature as well as the broadening of transition parameters such as transition width and hysteresis width, which limit its application potential. Here we demonstrate the growth of highly oriented strain-relaxed VO<SUB>2</SUB> thin films on (001)-oriented TiO<SUB>2</SUB> substrates at various oxygen partial pressures, exhibiting the narrow transition and hysteresis width. The cross-sectional transmission electron microscopy and x-ray diffraction analyses of the films reveal the highly oriented growth of insulating monoclinic VO<SUB>2</SUB>. The IMT parameters associated with temperature-dependent phase transition vary with the oxygen partial pressure used during the deposition. The presence of multiple and mixed valence states of vanadium in the films was confirmed by Raman and XPS analyses. We have achieved a narrow transition width (2.3 °C) and hysteresis width (1.2 °C) through controlling the oxygen stoichiometry during the growth of VO<SUB>2</SUB>/TiO<SUB>2</SUB> films.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Highly oriented epitaxial VO<SUB>2</SUB> films were grown on (001)-oriented TiO<SUB>2</SUB> substrate. </LI> <LI> Insulator to metal transition (IMT) parameters of the VO<SUB>2</SUB> film varies with the oxygen stoichiometry. </LI> <LI> IMT in VO<SUB>2</SUB> varied depending on the degree of crystallinity and microstructure of the films. </LI> <LI> The VO<SUB>2</SUB> film grown at high deposition oxygen pressure showed a sharp transition with a narrow hysteresis width. </LI> </UL> </P>

Role of surface oxidation for thickness-driven insulator-to-metal transition in epitaxial MoO₂ films

Ahn, Eunyoung,Min, Taewon,Lee, Jaekwang,Lee, Inwon,Kim, Younghak,Jeen, Hyoungjeen Elsevier 2018 APPLIED SURFACE SCIENCE - Vol.459 No.-

<P><B>Abstract</B></P> <P>Interfaces in transition metal oxides play critical roles for tuning physical properties. In thin film form, multiple interfaces can be created in between a film and a substrate, in between a film and air, and within a thin film. The role of each interface has been rarely studied. In this research, we used MoO<SUB>2</SUB> as a model system to study the role of the oxidized layer at film-air interface in thickness-driven metal-insulator transition. The oxidized layer at the surface is likely to be the main cause in positive temperature coefficient of resistivity in MoO<SUB>2</SUB> thin films thinner than about 20 nm. To find the origin of this insulating behavior in electronic transport measurements, we used x-ray diffraction, density functional theory and various spectroscopic methods. We observed the formation of oxidized MoO<SUB>2+</SUB> <I> <SUB>x</SUB> </I> at the film-air interface and its thickness explain the peculiar insulating behavior in the thinner films and even nanoparticles from the literature.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Size dependent metal-to-insulator transition in MoO<SUB>2</SUB> was unveiled by systematic study with various spectroscopic methods. </LI> <LI> Electronic transport measurements showed similar metal-to-insulator transition by film thickness. </LI> <LI> Surface oxidation in epitaxial thin films would be the main driver to trigger this transition. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Metal-to-Insulator Transitions in La_2/3Sr_1/3MnO₃/LaMnO₃ (LSMO/LMO) Superlattices

Ryu, Sang-Woo,Jang, Hyun-M. The Korean Ceramic Society 2006 한국세라믹학회지 Vol.43 No.11

A series of manganite-based superlattices composed of half-metallic $La_{2/3}Sr_{1/3}MnO_3/LaMnO_3$ and insulating LaMnO$_3$ stacking layers were fabricated by employing pulsed laser deposition method. The dc resistivity increased drastically by simply reducing the stacking periodicity. The resistivity enhancement was accompanied by a gradual decrease in the temperature (T$_c$) of the Metal-to-Insulator Transition (MIT). This observation was interpreted as a small decrease in the effective metallic fraction near the percolation threshold. For the stacking periodicity less than a certain critical value, there appeared another transition to an insulating state at temperatures far below T$_c$. This low-temperature transition seems to be closely related to the AF-type (C-type) orbital ordering in newly formed insulating domains.

금속-절연체 전이 임계온도센서를 이용한 보상식 화재 감지기 개발

정순규,김현탁 한국화재소방학회 2014 한국화재소방학회논문지 Vol.28 No.1

A Compensation-type fire detector (CFD) is operated with two functions of a differential-temperature detector and as afixed-temperature detector. The differential-temperature detector observes a rate of temperature increase, and the fixedtemperaturedetector measures a given fixed temperature. The differential-temperature detector does not observe the outbreakof fire in slowly increasing temperature conditions, whereas the fixed-temperature detector is not able to observe theoutbreak of fire in conditions under predetermined temperature level. We developed a CFD to compensate for weaknessesof both detectors. To compensate for the disadvantages, a sensor of the sensor metal-insulator-transition criticaltemperaturesensor was used. Temperature coefficient of resistance is the sensitivity for sensor. At 55 oC, temperaturecoefficient of resistance of metal-insulator-transition critical-temperature sensor was 14.15%. Temperature coefficient ofresistance of thermistor was about 0.5%. This CFD was operated as two ways that fixed-temperature detector and differential-temperature detector in one sensor. 보상식 화재 감지기는 일정한 온도 상승률 이상으로 빠르게 상승하는 경우에 작동하는 차동식 화재 감지기의 기능과정온점에 이르렀을 때 작동하는 정온식 화재 감지기의 기능을 모두 가진 감지기다. 이 화재 감지기는 정온점 이하에서발생하는 화재를 감지하지 못하는 정온식 화재 감지기의 단점과 불꽃없이 천천히 타는 현상과 같이 천천히 열이 증가하는 화재를 감지하지 못하는 차동식 화재 감지기의 단점을 보완하기 위해 개발된다. 이것을 위해 우리는 이 감지기의 센서로 금속-절연체 전이 임계온도센서를 이용했는데 이 센서의 감도를 결정하는 저항온도계수가 55 oC일 때 14.15%로써,서미스터(약 0.5%)보다 큰 값이다. 이 센서는 하나의 센서가 정온점 이하에서는 차동 기능, 정온점 이상에서는 정온 기능을 모두 가진다.

이산화바나듐의 금속-절연체 전이와 구조 상전이

한상욱 ( Sang-wook Han ) 전북대학교 과학교육연구소 2020 과학과 과학교육 논문지 Vol.45 No.2

VO₂ is a typical metal-to-insulator transition (MIT) material. VO₂ shows insulating and metallic (or semiconducting) properties below and above approximately 68℃, respectively. The direct bandgap of insulating VO₂ is approximately 0.65 eV, the resistance ratio between insulator and metal is approximately 10⁴, and the MIT transition abruptly occurs by a little change of temperature. The MIT of VO₂ is accompanied by a first-order structural phase transition (SPT). We examined the relationship between the MIT and the SPT of VO₂ using EXAFS (extended x-ray absorption fine structure) at V K edge and resistance measurements. The measurements revealed that the MIT does not simultaneously occur with the SPT at the same temperature, however, the MIT is related to the SPT. Furthermore, the measurements confirmed that a straight alignment of V atoms in VO₂ is a prerequisite condition of the metallic phase of VO₂.

Non-stoichiometry-induced metal-to-insulator transition in nickelate thin films grown by pulsed laser deposition

이종민,최경순,이태권,정일석,김상모,송재선,박정웅,이주형,정종훈,이주한,김태헌,이상한 한국물리학회 2018 Current Applied Physics Vol.18 No.12

While controlling the cation contents in perovskite rare-earth nickelate thin films, a metal-to-insulator phase transition is reported. Systematic control of cation stoichiometry has been achieved by manipulating the irradiation of excimer laser in pulsed laser deposition. Two rare-earth nickelate bilayer thin-film heterostructures with the controlled cation stoichiometry (i.e. stoichiometric and Ni-excessive) have been fabricated. It is found that the Ni-excessive nickelate film is structurally less dense than the stoichiometric film, albeit both of them are epitaxial and coherent with respect to the underlying substrate. More interestingly, as a temperature decreases, a metal-to-insulator transition is only observed in the Ni-excessive nickelate films, which can be associated with the enhanced disproportionation of the Ni charge valence. Based on our theoretical results, possible origins (e.g. anti-site defects) of the low-temperature insulating state are discussed with the need of future work for deeper understanding. Our work can be utilized to realize unusual physical phenomena (e.g. metal-to-insulator phase transitions) in complex oxide films by manipulating the chemical stoichiometry in pulsed laser deposition.

BLDC 모터-배터리 시스템의 MIT(Metal-Insulator-Transition) 과온도 보회회로 및 전동휠체어 적용에 관한 연구

배진용(Jin-Yong Bae),장광호(Kwhng-Ho Jang),배동권(Dong-Gwon Bae) 한국자동차공학회 2019 한국자동차공학회 학술대회 및 전시회 Vol.2019 No.11

This paper discusses the application of OTP(Over-temperature Protection) circuit and EW(Electric Wheelchair) in BLDC motor-battery system using MIT(Metal-Insulator-Transition) device. Recently, a battery system has a problem in that a fire occurs due to inadequate battery design, over charge, over voltage, cell balancing failure, and inadequate BMS(Battery Management System) circuit. Therefore, in this study, the protection circuit that protects the entire system before the battery fire and explosion temperature is studied for proper protection of the battery system. In particular, the metal-insulator instantaneous transition device proposes an over temperature protection circuit that stops the BLDC motor and battery charging when an over temperature occurs to the battery based on a new material called VO₂. The validity of the proposed over temperature protection circuit is applied to 500[W] class BLDC motor and 1500[Wh] classbattery system, and its applicability is discussed.

Oxygen stoichiometry controlled sharp insulator-metal transition in highly oriented VO2/TiO2 thin films

임지석,아눕,손민균,강대준,정상윤,이상한,조지영 한국물리학회 2018 Current Applied Physics Vol.18 No.6

The insulator-metal transition (IMT) in vanadium dioxide (VO2) which occurs above room temperature (67 °C) is highly sensitive to atomic defects caused by oxygen stoichiometry. The strained growth and the degree of oxygen deficiency in VO2 epitaxial films result in lowering of transition temperature below room temperature as well as the broadening of transition parameters such as transition width and hysteresis width, which limit its application potential. Here we demonstrate the growth of highly oriented strain-relaxed VO2 thin films on (001)-oriented TiO2 substrates at various oxygen partial pressures, exhibiting the narrow transition and hysteresis width. The cross-sectional transmission electron microscopy and x-ray diffraction analyses of the films reveal the highly oriented growth of insulating monoclinic VO2. The IMT parameters associated with temperature-dependent phase transition vary with the oxygen partial pressure used during the deposition. The presence of multiple and mixed valence states of vanadium in the films was confirmed by Raman and XPS analyses. We have achieved a narrow transition width (2.3 °C) and hysteresis width (1.2 °C) through controlling the oxygen stoichiometry during the growth of VO2/TiO2 films.

Pressure-induced insulator-metal transition in Ca2Ru0.92Fe0.08O4 investigated by infrared microspectroscopy

S.H. Park,M.S. Kim,G. Cao,K.I. Kim,B.N. Chae,이종석 한국물리학회 2018 Current Applied Physics Vol.18 No.1

We investigated pressure-induced insulator-metal transition in Ca2Ru0.92Fe0.08O4 by using infrared microspectroscopy. As the pressure is increased up to 1.7 GPa, we observed a large increment of the reflectivity in the entire mid-infrared range. Accompanied by such a clear signature of the insulatormetal transition, we found an evidence of the structural transition from the frequency shift of the Ru- O stretch phonon mode which is attributed to the shortening of the in-plane Ru-O bond length. When we compared these pressure-dependent changes with the corresponding temperature-dependent results, we found that the pressure-induced metallic state has a higher reflectivity as well as the higher phonon frequency. Indeed, it turns out that the pressure-induced metallic state of Ca2Ru0.92Fe0.08O4 looks very similar with the metallic state of Sr-substituted Ca2RuO4 not only in the reflectivity level but also in the phonon frequency. This suggests that the electronic properties are closely related to the structural degree of freedom, and the pressure can be a useful parameter to induce the transitions from the Mottinsulator to the metal and further to the superconductor as observed for Sr2RuO4.