Transparent Antiradiative Ferroelectric Heterostructure Based onFlexible Oxide HeteroepitaxyChun-Hao Ma,†,§ Jie Jiang,⊥ Pao-Wen Shao,§ Qiang-Xiang Peng,⊥ Chun-Wei Huang,§,# Ping-Chun Wu,§

Jyun-Ting Lee,‡ Yu-Hong Lai,§ Din-Ping Tsai,∇ Jyh-Ming Wu,‡ Shen-Chuan Lo,# Wen-Wei Wu,§

Yi-Chun Zhou,⊥ Po-Wen Chiu,*,†,○ and Ying-Hao Chu*,§,∥,#,¶

†Department of Electrical Engineering and ‡Department of Materials Science and Engineering, National Tsing Hua University,Hsinchu 30013, Taiwan§Department of Materials Science and Engineering and ∥Center for Emergent Functional Matter Science, National Chiao TungUniversity, Hsinchu 30010, Taiwan⊥Key Laboratory of Low Dimensional Materials and Application Technology of Ministry of Education, Xiangtan University, Hunan411105, China#Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 31040, Taiwan∇Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan○Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan¶Institute of Physics, Academia Sinica, Taipei 11529, Taiwan

*S Supporting Information

ABSTRACT: In the era of Internet of Things, the demand for flexibleand transparent electronic devices has shifted to the forefront of materialsscience research. However, the radiation damage to key performance oftransparent devices under radiative environment remains as a criticalissue. Here, we present a promising technology for nonvolatiletransparent electronic devices based on flexible oxide heteroepitaxy. Adirect fabrication of epitaxial lead lanthanum zirconate titanate ontransparent flexible mica substrate with indium tin oxide electrodes ispresented. The transparent flexible ferroelectric heterostructures not onlyretain their superior performance, thermal stability, reliability, andmechanical durability, but also exhibit remarkably robust propertiesagainst to a strong radiation exposure. Our study demonstrates anextraordinary concept to realize transparent flexible nonvolatile electronicdevices for the design and development of next-generation smart deviceswith potential application in electronics, automotive, aerospace, and nuclear systems.

KEYWORDS: ferroelectric, transparent, flexible, antiradiative, PLZT, mica, van der Waals epitaxy

■ INTRODUCTION

With increasing demand on Internet of Things, wearable andflexible electronics becomes an important research direction tointegrate more functionalities. Along this research direction,transparent components serve as a necessary ingredient in lotsof devices, such as panels, glasses, windshield, windows, etc.Thus, transparent flexible electronics have attracted muchinterest as novel technical solution for next-generationconsumer electronics toward a convenient and smart livingfuture.1−4 To realize these transparent flexible electronicsystems, it is necessary to compose the system with ubiquitouscomponents, such as image displays,5 logics,6 informationstorages,7 and sensors.8 Moreover, most transparent compo-nents are required to use under radiation exposure, theradiation loss of key performance becomes a critical feature.9,10

Among the components of transparent electronics, an

important integral part of the electronic circuits is an elementof nonvolatile memory, which can be used to store and retrieveinformation as required. Several types of nonvolatile memoriesthat feature transparency in visible light have been reported,such as a memory with In−Ga−Zn−O semiconductor charge-storage layer,11,12 the polymer ferroelectric memory based onpoly(vinylidene fluoride trifluoroethylene),13,14 graphene-based resistive switching memory, etc.15,16 They all showsuperior advantages with tremendous potentials in applica-tions. However, none of them showed promising featureagainst radiation yet. Oxide materials are ubiquitous in modernscience due to their fascinating physical properties and

Received: June 20, 2018Accepted: August 17, 2018Published: August 17, 2018

Research Article

www.acsami.orgCite This: ACS Appl. Mater. Interfaces 2018, 10, 30574−30580

© 2018 American Chemical Society 30574 DOI: 10.1021/acsami.8b10272ACS Appl. Mater. Interfaces 2018, 10, 30574−30580

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promising applications in next-generation technologies. Forinstance, ferroelectric oxides have gained great interest due totheir large polarization, fast switching time, low coercive field,and a high piezoelectric coefficient.17,18 More importantly, theyalso show superior radiation tolerance.19 Thus, it is promisingto realize high-speed, high storage density, low-powerconsumption, and low-cost nonvolatile memory based onferroelectric oxides. However, no transparent flexible ferro-electric memory based on complex oxides has been reported.Therefore, to build up a transparent flexible oxide-basedferroelectric memory is a big challenge and an important stepin transparent flexible applications.To construct a ferroelectric memory featuring optical

transparency and mechanical flexibility, three key ingredientshave to be integrated: (1) transparent ferroelectric layer. In thesearch of transparent ferroelectrics, perovskite lead lanthanumzirconate titanate (Pb1−xLax(ZryTi1−y)O3; PLZT) is animportant one showing high transmittance, relatively lowcoercive field, and high remnant polarization.20−22 (2)Transparent conducting oxides.23 Among them, indium tinoxide (ITO) is the most common one due to its highconductivity and good optical transmittance. It can also serveas a potential solution to solve the issue of ferroelectric fatiguein the heterostructure since oxide electrodes are favorable.24,25

However, typically the process for ferroelectric oxides requiresa thermal treatment at high oxygen pressure to reduce leakagecurrent, whereas transparent conducting oxides need arelatively reducing atmosphere to produce oxygen vacanciesas the source for the generation of charge carriers. Thus, theoptimization of growth process to satisfy the needs of bothmaterials is an essential step. (3) Transparent and flexiblesubstrates. Recently, due to its atomically smooth surface, highthermal stability, high transparency, and mechanical flexibility,muscovite mica has been reported to be a proper substrate foroxide heteroepitaxy.26,27 Various flexible heterostructures

composed oxide heteroepitaxy were delivered, providing anew platform for flexible electronics.28−30 In this study, webuild up a transparent ferroelectric system composed of ITO/PLZT/ITO/muscovite heteroepitaxy to combine these keyparts for transparent flexible memory (Figure 1a). In ourinvestigation, the ITO/PLZT/ITO/mica heteroepitaxy notonly retains the superior optical and ferroelectric propertiesbut also exhibits good mechanical flexibility, durability, andthermal stability. As shown in the inset of Figure 1b, thephotograph of ITO/PLZT/ITO/mica clearly highlights highoptical transparency of the heterostructure with mechanicalflexibility. The optical transmittance spectrum of theheterostructure in the ultraviolet−visible−infrared regime isshown in Figure 1b. An average visible transmittance of 80% ofthe heterostructure can be observed. Our results represent animportant milestone in the advancement of transparent flexibleferroelectric applications via oxide heteroepitaxy.

■ RESULTS AND DISCUSSION

In the heterostructure, the local ferroelectricity was probed bypiezoresponse force microscopy (PFM) based on the patternswritten on the PLZT layer with an application of electric fieldvia a conducting tip. A tip bias of +6 V was applied to pole theregion of 3 μm × 3 μm, followed by another poling with a tipbias of −8 V in the central area of 1 μm × 1 μm. The surfacetopography and the corresponding out-of-plane polarizationsignal are shown in Figure 1c,d, respectively. In Figure 1d, aPFM phase image shows the regions with clear bright and darkcontrast corresponding to downward and upward polarizations,respectively. After the poling, the areas of 3 μm × 3 μm and 1μm × 1 μm show uniform bright contrast and dark contrast,respectively. This result indicates that the polarization of thePLZT layer is switchable, delivering the local and uniformferroelectric feature of the heterostructure. Figure S1 showsrepresentative local hysteresis loops extracted from the

Figure 1. (a) Schematic representation of PLZT/ITO/mica heterostructure. (b) Optical spectrum of ITO/PLZT/ITO/mica. The (c) surfacetopography and the (d) out-of-plane phase image. (e) P−E hysteresis loops at various temperatures. (f) Remnant, saturation polarizations, andcoercive field as functions of temperature.

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amplitude and phase signals of PFM. The square loop showingan 180° change in phase signal and a clear butterfly loop inamplitude signal confirms the good ferroelectric switchingnature of the PLZT/ITO/mica system. The advances inemerging transparent flexible electronic devices motivateresearchers to explore additional requirements in harshenvironments. Well-saturated and symmetric polarization-electric field (P−E) hysteresis loops of the heterostructuremeasured at 1 kHz and the temperatures ranging from 25 to200 °C on a virgin device are shown in Figure 1e. The typicalhysteresis loop with a change in polarization shows effectiveswitching of dipoles. The ferroelectric capacitor exhibits asaturated polarization (Psat) of ∼84 μC/cm2, a remnantpolarization (Pr) of ∼41 μC/cm2, and a coercive field (Ec) of∼290 kV/cm at 25 °C, and the temperature evolution is shownin Figure 1f. The superior ferroelectric properties of theheteroepitaxy against temperature can be attributed to highquality and thermal stability of the oxide heteroepitaxy,delivering a potential solution for high-temperature transparentflexible applications.The heteroepitaxy and phase identification of the structure

were characterized by X-ray diffraction (XRD). In theheterostructure, a very thin ZnO layer (∼10 nm) was inserted.Without this ZnO layer, it is very difficult to obtain epitaxialITO bottom layer since the variation of ITO surface energiesalong different orientations is not very large. However, due tothe lowest surface energy of ZnO(00l) and the same symmetrywith muscovite, high-quality epitaxial ZnO thin film can beobtained and served as the seeding layer for the sequentialgrowth of high-quality heteroepitaxy. Then, an epitaxial ITOlayer as the transparent bottom electrode was inserted forelectrical characterizations. After the ITO layer, a PLZT layerwas grown on top of the heterostructure. The correspondinggrowth conditions for each layer can be found in Method

session. Figure 2a shows a typical out-of-plane L-scan of theheterostructure. The observations of only PLZT(ll0), ITO(lll),and ZnO(00l) diffraction peaks with muscovite (00l) suggestthe epitaxial nature of the heterostructure without othersecondary phases. Furthermore, the Φ-scans of PLZT andITO, ZnO, and muscovite reflections were used to analyze thein-plane structural relationships as shown in Figure 2b. Theobservation of one primary and two secondary muscovitepeaks at 120° intervals indicates different stacking sequencesbetween sheet units along c axis, whereas ZnO{102},ITO{004}, and PLZT{200} exhibit multiple peaks thatindicates the growth of epitaxial ZnO, ITO, and PLZT filmswith a multidomain feature on mica substrate. On the basis ofthe XRD results, the epitaxial relationship can be determinedas (110)PLZT//(111)ITO//(001)ZnO//(001)mica and(001)PLZT//(11̅0)ITO//(010)ZnO//(010)mica for the hetero-structure. The multidomain feature and epitaxial relationshipof PLZT on ITO are schematically shown in Figure 2c. Therocking curves measured to obtain the critical informationabout the crystallinity resulted in the full width at halfmaximum of ∼0.71, ∼1.28, and ∼1.77° for ZnO(002),ITO(222), and PLZT(110) peaks, respectively, as shown inFigure 2d. To characterize the detailed microstructure on thePLZT/ITO/mica heterostructure and further confirm theheteroepitaxy, the interfaces of the heterostructures wereexamined by transmission electron microscopy (TEM). Figure2e shows high-resolution cross-sectional TEM images takenalong the zone axis of [010] mica, revealing PLZT/slater-typeorbital (STO)/ITO and ITO/ZnO/mica interfaces. The STObuffer layer (∼10 nm) was chosen to realize high-qualityepitaxial PLZT on ITO due to the excellent structuralcompatibility with both PLZT and ITO layers. In addition, itcan serve as a transition layer for oxygen stoichiometry sinceoxygen vacancy is not favorable in the PLZT layer but it is

Figure 2. (a) Typical 2θ−θ scan of the heterostructure. (b) Φ-scans at PLZT{200} and {002}, ITO{004}, ZnO{102}, and muscovite {203}diffraction peaks. (c) Schematic describing the relationship between the multidomain structures of PLZT grown on ITO layer. (d) Rocking curvesof PLZT(110), ITO(222), and ZnO(002). (e) Cross-sectional TEM image of PLZT/ITO/mica interface along with the fast Fourier transformpatterns of PLZT, STO, ITO, and mica.

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required in the ITO layer. Without this layer, only polycrystal-line PLZT could be obtained, and all of the heterostructuressuffered from a severe leakage problem. The corresponding fast

Fourier transform patterns of selected areas of PLZT, STO,ITO, and mica are shown in the right panels. The reciprocallattices are also clearly indexed, and the consistency of epitaxial

Figure 3. (a) P−E hysteresis loops under various tensile and compressive bending radii. (b) Pr, Psat, and Ec variation as a function of bending radius.(c) P−E hysteresis loops under tensile and compressive bending of 5 mm before and after 1000 bending cycles. (d) Pr, Psat, and Ec variation beforeand after 1000 bending cycles. (e) Retention and (f) fatigue for the ITO/PLZT/ITO capacitors in unbent and compressively and tensilely bentconditions.

Figure 4. (a) P−E hysteresis loops under various bending conditions after the heterostructure absorbed radiation at the dose level of 5 Mrad. (b)Pr, Psat, and Ec variations as a function of bending radius after the samples exposed to radiation. (c) Retention and (d) fatigue for theheterostructure, which were exposed in unbent and compressively and tensilely bent conditions.

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relationships with the XRD results is further confirmed. Theinterfaces without observable interdiffusion of species acrossthe interfaces indicate high quality of the heterostructure.According to the XRD and TEM results, a good oxideheteroepitaxy system based on PLZT/ITO/muscovite isdelivered, which is a key to attain excellent thermal andmechanical stabilities.In the push to practical applications, a series of tests on

cyclability of the heteroepitaxy was carried out. The macro-scopic ferroelectric performance of the heterostructure againstmechanical flexing was evaluated under both tensile andcompressive bending modes on the capacitor structure withthe top electrode size of 200 μm in diameter. Figure 3a showsthe P−E hysteresis loops of the ITO/PLZT/ITO capacitorsunder various compressive and tensile bending radii (r),whereas Figure 3b shows the change in Psat, Pr, and Ec values asa function of bending radius. For the bending radius down to 5mm, an observation of constant Psat, Pr, and Ec values withinexperimental errors suggests that the ITO/PLZT/ITOcapacitor shows stable electrical properties even undermechanical constraints. Moreover, the P−E hysteresis loopsunder tensile and compressive bending radius of 5 mm beforeand after 1000 bending cycles and the changes in Psat, Pr, andEc values obtained from the heterostructure exhibit nonoticeable change (Figure 3c,d). As shown in Figure S2, novisible cracks or exfoliation show up after 1000 repeatedbending cycles. It is easy to understand that due to therelatively weak interaction between film and substrate and thethickness ratio of film to substrate is completely small, theactual strain applied to the capacitor is too small to influenceits performances. Furthermore, for these ferroelectric capaci-tors to be used in nonvolatile memory applications, ferro-electric reliability issues, such as imprint, retention, and fatigue,have to be addressed for their long lifetime operation. TheITO/PLZT/ITO/mica heterostructure exhibits excellentpolarization retention (Figure 3e) after 105 s without andunder the bending situations. The polarization fatigue, which isthe reduction in the amount of remnant polarization withrepeated switching cycles, of a ITO/PLZT/ITO/micacapacitor is displayed in Figure 3f. The fatigue behavior isstable in the bent states and remains identical before and after1000 compressive bending cycles. Almost no polarizationfatigue is detected after 1010 switching cycles. It is clear fromthe aforementioned results that the transparent flexible ITO/PLZT/ITO/mica system exhibits stable and superior perform-ance against mechanical bending highly desirable for trans-parent flexible ferroelectric applications. More importantly, theoptical transmittance remains >80% after the mechanicalbending tests. Such a feature is attributed to the superior oxideheteroepitaxy built on muscovite substrates. In addition, thepiezoelectric response of the heterostructure was tested underbending condition (see the schematic diagram in Figure S3aleft side). As shown in Figure S3a,b, the corresponding open-circuit voltage and short-circuit current generated by bendingthe sample are 0.3 V and 4 nA, respectively, implying itspotential application for transparent piezoelectric devices, self-powered sensors,31 and nanogenerators.32

A key advantage of ferroelectric memory is the fearlessnessof radiation. However, for the applications in aerospace andnuclear, transparent components actually expose to radiation.Thus, it is important to study the ferroelectric performance ofthe heterostructure after an exposure to strong radiation.Figure 4a shows the P−E hysteresis loops of the ITO/PLZT/

ITO/mica system after given a dose of 5 Mrad. Theferroelectric performance of the heterostructure was alsostudied under both compressive and tensile bending modes,and Figure 4b shows the changes in Psat, Pr, and Ec values as afunction of bending radius. The ferroelectric properties retainconstant regardless of the mechanical constraints, and theperformance also remains the same before and after exposed tothe radiation. These results indicate that the ITO/PLZT/ITO/mica system is robust against radiation. Figure 4c displaysstable polarization retention of the ITO/PLZT/ITO capacitorsafter absorbed 5 Mrad of dose level. Furthermore, Figure 4dshows the evolution of the normalized polarization as afunction of the number of switching cycles. The fatiguebehavior is both stable in the unbent and bent states, and evenmaintains the same after 1000 compressive bending cycles.The heterostructure, which absorbed the radiation, shows nopolarization fatigue up to 1010 switching cycles. Therefore, it isevident that the transparent flexible ferroelectric PLZT/micasystem exhibits stable performance and can be applied underradiative environment, which can play an important role innext-generation aerospace and nuclear applications.

■ CONCLUSIONS

In conclusion, high-quality transparent flexible ferroelectricelement based on the ITO/PLZT/ITO/mica heteroepitaxyhas been successfully demonstrated. Furthermore, thiscapacitor retains its superior electrical performance againsttemperature variation. Meanwhile, the heterostructure isrobust against mechanical and radiation constraints. Thisstudy paves the way for an exciting new avenue to next-generation transparent flexible smart electronics.

■ METHOD

The epitaxial ITO/PLZT/ITO/mica heterostructure wasfabricated via the pulsed laser deposition using commercialITO (90% In2O3 and 10% SnO2) and PLZT(Pb0.9La0.1(Zr0.7Ti0.3)O3) targets. A freshly cleaved muscovitemica (1 cm × 1 cm) without any surface treatment orprecleaning process was adopted in this study. A KrF excimerlaser (λ = 248 nm, Coherent) was operated at 10 Hz laserrepetition and laser fluence of 3 J/cm2. The depositionchamber was initially evacuated to a base pressure of 10−6

Torr. The deposition process of 100 nm PLZT was carried outat a substrate temperature of 550 °C in 100 mTorr oxygenpressure. Both the top (diameter of 200 μm) and bottom ITOlayers (50 nm) were deposited at the substrate temperature of400 °C and in the oxygen pressure of 5 × 10−4 Torr.Moreover, prior to PLZT and bottom ITO deposition, STO(∼10 nm) and ZnO (∼10 nm) were deposited as buffer layers,respectively. The STO was grew at a substrate temperature of550 °C in 100 mTorr oxygen pressure, meanwhile the ZnOwas deposited at 400 °C with oxygen pressure of 5 × 10−4

Torr.The 2θ−θ scan along normal direction and Φ scans were

performed at the BL17A at the National SynchrotronRadiation Research Center in Hsinchu to obtain the structuralinformation. The film−substrate interface microstructure wasstudied by cross-sectional TEM. Cross-sectional TEM speci-mens were prepared by the focused ion beam technique. Theoptical spectra were collected in the transmission mode using aPerkinElmer Lambda-900 spectrometer.

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The surface morphology and PFM study were investigatedvia Asylum Research MPF-3D. The phase−voltage hysteresisloops were measured with a bias window up to 10 V. A home-made bending stage was used to perform the evolution offerroelectricity while the sample was under bending.The ferroelectric properties were characterized by a

ferroelectric test system (Radiant Technologies Precisionsworkstations RT66A). The P−E hysteresis loops weremeasured by two tips at a frequency of 1 kHz, and differentradii were checked by using the bending stage. A heating stagewas used to change the measurement temperature to estimatethe P−E variation along with the temperature ranging from 25to 200 °C.Predesigned Teflon molds of fixed bending radii were used

to induce the reported compressive and tensile bending strains.For the bending-cycle measurement, a computer-aided home-built bending setup combined with optical microscope wasused. The ITO/PLZT/ITO/mica was bent from one end,whereas the other end was fixed, and the vertical distance wasmeasured using the microscope, as shown in Figure S4.Considering to the radiation environment in outer space, a

series of tests on the ITO/PLZT/ITO/mica heterostructurewas carried out after the absorbed radiation. The radiation wasgiven by a 60Co γ-ray, which is at a dose level of 5 Mrad with adose rate of 50 rad/s.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.8b10272.

Local PFM study; cross-section SEM images; energygeneration measurement; and information of home-made bending stage (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: pwchiu@ee.nthu.edu.tw (P.-W.C.).*E-mail: yhc@nctu.edu.tw (Y.-H.C.).ORCIDDin-Ping Tsai: 0000-0002-0883-9906Jyh-Ming Wu: 0000-0001-9244-6621Wen-Wei Wu: 0000-0002-8388-8417Po-Wen Chiu: 0000-0003-4909-0310Ying-Hao Chu: 0000-0002-3435-9084NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work is supported by the Ministry of Science andTechnology, Taiwan (Grant nos. MOST 106-2119-M-009-011-MY3, 106-2628-E-009-001-MY2, and 106-2923-M-009-003-MY2) and The SPROUT project of Ministry ofEducation, Taiwan.

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ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.8b10272ACS Appl. Mater. Interfaces 2018, 10, 30574−30580

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