Flexible, Temperature-Resistant, and Fatigue-Free Ferroelectric

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Functional Inorganic Materials and Devices

Flexible, Temperature-Resistant, and Fatigue-Free Ferroelectric Memory Based on Bi(Fe0.93Mn0.05Ti0.02)O3 Thin Film Changhong Yang, Yajie Han, Jin Qian, Panpan Lv, Xiujuan Lin, Shifeng Huang, and Zhenxiang Cheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01464 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 19, 2019

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ACS Applied Materials & Interfaces

Flexible, Temperature-Resistant, and Fatigue-Free Ferroelectric Memory Based on Bi(Fe0.93Mn0.05Ti0.02)O3 Thin Film Changhong Yang,† Yajie Han,† Jin Qian,† Panpan Lv,‡ Xiujuan Lin,‡ Shifeng Huang,,‡ and Zhenxiang Cheng,§ † School

of Materials Science and Engineering, University of Jinan, Jinan 250022, China

‡ Shandong

Provincial Key Laboratory of Preparation and Measurement of Building Materials,

University of Jinan, Jinan 250022, China § Institute

for Superconducting and Electronic Materials, Australian Institute for Innovative

Materials, University of Wollongong, Innovation Campus, North Wollongong, NSW 2500, Australia KEYWORDS: flexible, ferroelectric memory, BiFeO3 film, temperature-stable, fatigue-free

ABSTRACT: A recent hot-spot topic for flexible and wearable devices involves highperformance nonvolatile ferroelectric memories operating under compressive or tensile mechanical deformations. This work presents the direct fabrication of a flexible (Mn,Ti)codoped multiferroic BiFeO3 film capacitor with Pt bottom and Au top electrodes on mica

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substrate. The fabricated polycrystalline Bi(Fe0.93Mn0.05Ti0.02)O3 film on mica exhibits superior ferroelectric switching behavior with robust saturated polarization (Ps ~ 93 μC/cm2) and remanent polarization (Pr ~ 66 μC/cm2), and excellent frequency stability (1 - 50 kHz) and temperature resistance (25 - 200 C) as well as reliable long-lifetime operation. More saliently, it can be safely bent to a small radius of curvature, as low as 2 mm, or go through repeated compressive/tensile mechanical flexing for 103 bending times at 4 mm radius without any obvious deterioration in polarization, retention time at 105 s, or fatigue resistance after 109 switching cycles. These findings demonstrate a novel route to the design of flexible BiFeO3based ferroelectric memories for information storage and data processing, with promising applications in next-generation smart electronics.

1. INTRODUCTION Long viewed as an important topic in functional materials, the ferroelectrics relying on robust electrical polarization, and excellent pyroelectric and piezoelectric properties are being utilized in electronic components, such as ferroelectric random access memories (FeRAM), pyroelectric detectors, piezoelectric sensors and transducers, etc.1-5 The nanoscale ferroelectric elements integrated onto Si chips have greatly promoted the development of the semiconductor industry when ferroelectrics are deposited in thin film embodiments. At present, as the modern era of “Internet of Things” calls for flexible, light-weight and portable smart electronics, there is a growing demand for the design and development of ferroelectric memories with outstanding bendability and stable polarization to achieve fully flexible and compact integrated circuits for information storage and data processing in the areas of flexible displays, wearable devices and intelligent sensors.6-8


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Over the past several decades, the ferroelectric polymers, which are typically based on poly(vinylidene fluoride) (PVDF) and its copolymer with trifluoroethylene [P(VDF-TrFE)], have received much attention in memory devices. Such as for P(VDF-TrEE) with space-chargemediated ferroelectric switching, it possesses a remanent polarization (Pr) of ~ 8.2 μC/cm2, data retention of 104 s, and write/erase of 50 cycles.9 The organic ferroelectrics are favorable in manufacturing flexible electronics, as they are lightweight, mechanically flexible, and easy to process. As compared with the organic ferroelectric films, inorganic ferroelectrics show more superior ferroelectric properties, especially larger remanent polarization. Furthermore, improved fatigue endurance and thermal stability have been achieved in traditional perovskite oxide ferroelectrics. For example, large remanent polarization of 55-80 μC/cm2 and 107 switching cycles were presented in BiFeO3-based thin films.10,11 Meanwhile, a good thermal stability up to 140 °C and excellent fatigue endurance of 4×108 cycles were reported in Bi3.25La0.75Ti3O12 film.12 However, it is difficult for most inorganic films to be bendable because of the bottleneck of the large clamping effect from rigid substrates. The currently commercialized technique of “grow-transfer” can be employed to render the thin-film devices flexible, but this method requires the tedious multi-step process of first growing ferroelectric thin films on rigid substrates and then transferring the free-standing films onto plastic substrates.13,14 In addition, it is quite difficult to achieve a simple one-step fabrication of high-performance oxide films on flexible organic substrates because these substrates with relatively low temperature endurance (~ 200 C) are unable to survive the high-temperature growth of perovskite ferroelectric films (500 - 700 C). Hence, it will be highly favorable to realize cost-effective and reliable fabrication of highperformance inorganic ferroelectric films on temperature-stable flexible substrates.


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Motivated by the aforementioned challenge, the concept of MICAtronics has been developed, opening a new pathway for creating novel flexible device applications.15 As a newlydeveloped inorganic flexible substrate, layer-structured mica offers salient features such as high transparency, mechanical flexibility, high operating temperature (700 C), and easy peeling to yield an atomically smooth surface for film growth.15,16 With the assistance of mica, the research into flexible memory elements has begun to bloom. Jiang et al. reported that single-crystalline PbZr0.2Ti0.8O3 film on flexible mica substrate exhibited remarkable charge retention capability after 105 s, even when bent 103 times at a radius (r) of 5 mm.17 Gao et al. found that PbZr0.52Ti0.48O3 capacitor fabricated on 10 μm thick mica possessed stable capacities for writing, reading, and keeping of polarization information in spite of a repeated bending process down to a radius of 2.2 mm for 104 times.18 Yang et al. demonstrated that the resistive-switching properties of a BaTi0.95Co0.05O3 memory based on mica showed no obvious change at a small bending radius of 1.4 mm.19 Seen in this light, these ferroelectric films on flexible mica are endowed with high crystalline quality and excellent electrical performances under mechanical flexing, giving them the potential for applications in next-generation flexible electronic devices. As the only known room-temperature multiferroic material, perovskite BiFeO3 (BFO) exhibits both an ultrahigh Curie temperature (TC ~ 830 C) and a robust theoretical remanent polarization (Pr ~ 100 μC/cm2), so that it is considered to be a promising lead-free material for information storage and data processing.10,20,21 Considering that the pure BFO film usually suffers from a severe leakage problem originated from oxygen vacancies ( VO ) by the volatilization of Bi and valence fluctuation of Fe, which increases the difficulty in various applications of the material. In this regard, Kawae et al. demonstrated that (Mn,Ti) codoping is conductive to improving the ferroelectric property (Pr ~ 75 μC/cm2) and obtaining the well


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saturated hysteresis curves in BFO film.22 In the present state-of-the-art flexible memory devices, however, including the Pb(Zr,Ti)O3 (PZT) and BaTiO3 systems mentioned above, BFO-based ferroelectric functional devices have not been reported as yet. In this case, the combination of flexible technology and extraordinary electrical performance in BFO-based thin film is essential for the design of novel bendable integrated chips in ferroelectric memory applications that could meet the demand for flexibility in smart electronics. Herein,

we

present

a

feasible

approach

to

the

fabrication

of

a

flexible

Bi(Fe0.93Mn0.05Ti0.02)O3 (BFOMnTi) ferroelectric element supported by a mica substrate. The film exhibits large saturated polarization, with Ps ~ 93 μC/cm2, and remanent polarization, with Pr ~ 66 μC/cm2. The electrical polarization is stable in the frequency range of 1 - 50 kHz and at working temperatures varying between 25 - 200 C. Furthermore, the polarization characteristic, retention capability and fatigue endurance of the flexible BFOMnTi display no obvious deterioration even down to a small bending radius of 2 mm or repeated mechanical flexing for 103 cycles at 4 mm radius. 2. EXPERIMENTAL SECTION The mica substrate with a clean and smooth surface, with a root mean square roughness (Rrms) of 0.104 nm, was obtained from fluorophlogopite mica [KMg3(AlSi3O10)F2] (Changchun Taiyuan Fluorophlogopite Co., Ltd. Changchun, China) via mechanical exfoliation using tweezers (Figure S1, Supporting Information). The Pt bottom electrode was deposited on the fresh cleaved mica by sputtering with a Pt target and then annealed at 600 C for 10 min in a rapid thermal processor. The deposition procedure was performed in Ar atmosphere at 0.05 mbar and 30 mA.


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The BFOMnTi thin film was deposited by a chemical solution deposition technique combined with annealing layer by layer. Firstly, bismuth nitrate pentahydrate (Bi(NO3)3·5H2O, Sigma-Aldrich, 99.99%), iron (III) nitrate nonahydrate (Fe(NO3)3·9H2O, Sigma-Aldrich, 99.95%) and manganese (II) acetate tetrahydrate (CH3COO)2Mn·4H2O, Sigma-Aldrich, 99.99%) were dissolved in ethylene glycol (HOCH2CH2OH, Sigma-Aldrich, 99%) and acetic acid (CH3COOH, Sigma-Aldrich, 99.7%). Here, 5 mol% excess bismuth was added to compensate for volatilization in the high-temperature treatment. Secondly, titanate isopropoxide (C12H28O4Ti, Aladdin, 99.9%) was added into the solution drop by drop, followed by an appropriate amount of acetylacetone (C5H8O2, Aladdin, 99.6%) added as stabilizing agent. The concentration of the final BFOMnTi precursor was 0.3 M. Thereafter, the precursor was deposited on mica substrate with Pt electrode by spin coating at 3000 rpm for 30 s, followed by a pyrolysis process at 400 °C for 10 min on a hot plate to decompose the organics, and then the film was annealed at 500 °C for 5 min with a heating rate of 40 °C per second in N2 atmosphere in a rapid thermal processor. Then the sample was cooled naturally to room temperature. According to the FTIR spectra (Figure S2, Supporting Information), our heat treatment process can not only effectively avoid the incorporation of contaminants that come from the precursors such as nitrogen and carbon but also ensure sufficient crystallization of the film. On repeating the procedure of spin coating and annealing, the ~ 300 nm-thick BFOMnTi film was obtained. Finally, Au top electrodes, with ~ 200 m in diameter, were sputtered on the film surface through a custom-designed shadow mask for electrical measurements. Several pieces of the mica layer were peeled off until the whole thickness was down to ~ 10 m to make the Pt/BFOMnTi/Au capacitor fully flexible. The cross-sectional structure and surface morphology of BFOMnTi film were observed by field-emission scanning electron microscope (FESEM, Hitachi S-4200) and atomic force


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microscope (AFM, Bruker dimension icon), respectively. The crystal structure of BFOMnTi was analyzed through X-ray diffraction (XRD, Bruker D8). The functional groups and chemical bonds for BFOMnTi samples were examined using the Fourier transform infrared spectrometer (FTIR, Nicolet 380). An ultraviolet-visible spectrometer (Shimadzu UV-2550) was utilized to investigate the transmittance of the sample. The piezoelectric phase and amplitude were studied using piezoresponse force microscope (PFM) measurements conducted on the AFM system in piezoelectric mode. Ferroelectric, insulating, and positive-up and negative-down (PUND) measurements of the film were conducted with a standard ferroelectric tester (Precision Pro. Radiant Technologies). The temperature dependent ferroelectricity was measured supported by a temperature-controlled probe station (Linkam-HFS600E-PB2). 3. RESULTS AND DISCUSSION To obtain freshly cleaved mica with a clean and smooth surface, a piece of transparent mica can be peeled off via mechanical exfoliation using tweezers, as shown in Figure 1a, (also Figure S1, Supporting Information), which is due to the weak inter-layer interactions in the stacked-layer structure of mica.15 After thinning the two-dimensional mica down to tens of micrometers, the substrate is capable of being flexible, with no cracks or fractures observed in bent state, as displayed in Figure 1b. After depositing a Pt metal bottom electrode via sputtering, the mica/Pt still remains excellent flexibility (Figure 1c). Subsequently, the BFOMnTi film is fabricated on the mica substrate with Pt electrode by spin coating, and the attractive flexible mica/Pt/BFOMnTi ferroelectric element can be obtained (Figure 1d). For understanding the sample’s crystal structure, Figure 1e presents the XRD pattern of BFOMnTi film on mica. As comparison, the diffraction pattern of mica crystal, and the corresponding PDF cards of mica (No. 16-0344, monoclinic phase, a =5.307 Å, b = 9.192 Å, c = 10.142 Å), Pt (No. 04-0802, cubic


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phase, a = b = c = 3.9231 Å), and BFO (No. 74-2016, rhombohedral phase, a = b = c = 3.962 Å) are also included to make distinctions between the diffraction peaks. Obviously, except for the peaks of Pt electrode and mica substrate, the results show the pure polycrystalline perovskite structure of BFOMnTi film. As shown in Figure S2, the (110) diffraction peak in the BFOMnTi film shifts to lower diffraction angle, indicating that Mn and Ti codoping modifies the lattice parameter of the film. The surface morphology of the BFOMnTi film was probed by an AFM in an area of 500 × 500 nm2 as shown in Figure 1f. An average surface roughness (Ra) of 1.74 nm and a root mean square roughness (Rrms) of 2.15 nm can be derived. Figure 1g presents the crosssectional FESEM image of Pt/BFOMnTi/Au capacitor. The microstructure of the film is relatively compact and homogeneous without obvious pinholes. There are limited cation diffusions at the film/electrodes interfaces, which can be further identified through the crosssectional structure accompanied by a typical element mapping using energy-dispersive spectrometer (EDS) (Figure S3, Supporting Information). The thicknesses of the Pt layer, Au electrode, and the BFOMnTi film are approximately 90 nm, 120 nm and 300 nm, respectively. Understanding the changes in the capacitor performance under curved circumstances is crucial for designing flexible smart electronics. To validate the feasibility of BFOMnTi thin film in flexible ferro- and piezoelectric devices, measurements of BFOMnTi film in the flat state are first performed, as shown in Figure 2a. Then, the mica/Pt/BFOMnTi was attached to a bending mould 4 mm in radius with either a concave or a convex surface to generate compressive or tensile strain in the film, as depicted in Figure 2b and c, and then the microstructure and microscale ferro- and piezoelectric properties in the same regions were detected. As the tf-thickness Pt/BFOMnTi layer on the ts-thickness mica substrate was mounted on the mould with radius r, the maximum bending strain  of the Pt/BFOMnTi on mica substrate in the bending direction


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can be approximately calculated through  = (tf+ts)/2r.23 Since the capacitor thickness (~ 510 nm) is far less than that of the mica substrate (~ 10 μm), the formula can be simplified to  ≈ ts/2r. By calculation, the induced bending strain is only about 0.5 %, even when the sample is bent to r = 1 mm. Therefore, the mica substrate is thinned down to about 10 μm in our experiment for safely keeping the BFOMnTi thin film in the bent condition. Figure 2d-f show the surface morphologies of the same region of 5×5 m2 of the BFOMnTi film in its flat, compressive and tensile bending states (r = 4 mm), respectively. On comparison, the morphology of the thin film remains unchanged within experimental error margin regardless of the tiny bending strains in the film. To characterize the microscopic ferroelectric and piezoelectric behaviors of BFOMnTi film on mica substrate, PFM measurements are ideal for switching the local polarization, which is conducted in an AFM system in piezoelectric mode. Here, a box-in-box polarization configuration in flat BFOMnTi film was written by a PFM tip poling a 33 m2 region with a dc bias of +10 V and subsequently scanning a 11 m2 area in the central area with an opposite bias of -10 V. Figure 2g presents an out-of-plane PFM phase image of BFOMnTi in the flat state, in which obvious dark and bright contrasts corresponding to downward and upward polarizations can be observed, demonstrating excellent ferroelectric switching behavior on the micro-scale.17 Moreover, the contrast in the as-grown background region outside the 33 m2 square is similar to that of an area polarized with negative bias, demonstrating that the as-prepared BFOMnTi thin film has an upward self-polarization. Such a preferred polarization state is mainly induced by the built-in electric field at the film/electrode interface.24 Interestingly, whether the BFOMnTi film on mica is under compressive or tensile condition, the previous region with its box-in-box polarization configuration can be found, and the upward self-polarization can be well maintained, as shown by the phase images in Figure 2h


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and i. This suggests that moderate mechanical bending would not damage the ferroelectric polarization and piezoelectric phases. Figure 2j shows a representative local PFM phase hysteresis loop, measured in the as-grown region, and amplitude as functions the tip bias in flat mica/Pt/BFOMnTi. The square loop with a 180 change in PFM phase and the typical butterfly piezoelectric amplitude indicate its ferroelectric switching property. A large converse piezoelectric constant d33 of ~ 387 pm/V for flat BFOMnTi film on a 10 m-thick mica substrate can be obtained from the piezoelectric amplitude, which is much higher than its rigid counterpart prepared on Si substrate in this experiment (d33 ~ 134 pm/V) (Figure S4, Supporting Information) and a reported BFO film on SrTiO3 single crystal (d33 ~ 70 pm/V).10 Such a phenomenon is a result of the strong mechanical coupling between the BFOMnTi film and the flexible mica substrate due to the considerably reduced substrate clamping effect. Similar results were also observed in flexible mica/SrRuO3/PZT and mica/LaNiO3/PZT heterostructures.18,25 Although the sample has been subjected to compressive and tensile bending down to a 4 mm radius, the local piezoelectric phase hysteresis loops and amplitude in the as-grown regions, as shown in Figure 2k and l, only present minor changes compared with those in the unbent state. The similar d33 values are ~ 403 pm/V and ~ 395 pm/V under compressive and tensile bending strains, respectively. The results of superior mechanical flexing tolerance in ferroelectric switching and piezoelectric response suggest promising applications of BFO-based films in flexible ferroelectric memories and piezoelectric sensors. The detailed ferroelectric properties of the unbent BFOMnTi film fabricated on mica substrate were investigated and are shown in Figure 3. As shown in Figure 3a, the BFOMnTi film exhibits relatively saturated polarization versus electric field (P-E) hysteresis loops with


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applied electric field up to 2000 kV/cm at 10 kHz (also in Figure S5, Supporting Information). It is worth noting that the saturated polarization (Ps) and remanent polarization (Pr) are about 93 μC/cm2 and 66 μC/cm2, respectively, which are comparable to those in BFO-based films on rigid substrates,10 and even not inferior to flexible PZT film capacitors.18,25 The P-E hysteresis loops exhibit obvious horizontal positive shift with asymmetrical coercive field (Ec) values. For P-E loop with maximum electric field of 2000 kV/cm, the positive coercive field (Ec+) and the negative one (Ec-) are ~ 844 kV/cm and ~ 276 kV/cm, respectively. The horizontal positive shift of P-E indicates a preferred upward polarization state, which is consistent with the PFM results. Such asymmetry characteristic can be ascribed to the internal electric field at film/electrode interface, which is determined by the different work functions of top (Au) and bottom (Pt) electrodes, and the movable ions or space charges accumulation at the interface.24,26,27 Figure 3b shows the P-E loops measured at frequencies ranging from 1 to 50 kHz. It can be observed that increasing the measurement frequency results in a slight decrease in the polarization values (Figure S6, Supporting Information). Such weak dependence on frequency indicates only a small contribution of the leakage current, nonlinear dielectric effects, and fast switching of domains. The P-E loops of BFOMnTi thin film measured under 1250 kV/cm at 1 kHz and at temperatures ranging from 25 to 200 °C. The temperature heating rate was 30 °C/min from one measuring temperature point to the next. It should be pointed that the deterioration of the ferroelectric property occurs to a less marked degree, taking the form of fluctuations of the Pr value in the range of 51 to 43 μC/cm2, with temperature varying from 25 to 200 °C (Figure S7, Supporting Information). The thermal stability can be partly attributed to a small increase of the leakage current density over a broad temperature range from 25 to 200 °C (Figure S8, Supporting Information). Based on the analysis above, it is entirely possible to achieve excellent


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ferroelectric performance for BFO-based ferroelectric film grown on flexible mica substrate, paving the way for fixed frequency operation and high-temperature electronic applications. To rationalize the effects of mechanical flexing on the electrical performance of BFOMnTi thin film, measurements were conducted at different compressive and tensile strain states. As shown in Figure 4a, Pt/BFOMnTi/Au ferroelectric capacitors were prepared for electrical measurements, during which one probe was in contact with the Au top electrode, while the other probe was in contact with the Pt bottom electrode. The bent state of Pt/BFOMnTi/Au capacitors on mica was maintained by fixing the capacitors on a series of specific moulds with different bending radii. Correspondingly, the different concave and convex bending provided by the moulds induced varying compressive and tensile strains in the film. Figure 4b shows the P-E hysteresis loops of the BFOMnTi film measured while flat (r) and at a series of compressive/tensile bending radii at 10 kHz and room temperature. Figure 4c provides the variations in Ps, Pr, and the coercive field Ec as functions of the bending radius. Compared with the saturated polarization (Ps ∼ 93 μC/cm2), remanent polarization (Pr ~ 66 μC/cm2), and coercive fields (Ec+ ~ 844 kV/cm, Ec- ~ 276 kV/cm) of the flat BFOMnTi film, negligible changes can be found for the values of these three parameters when measured at various bent states, revealing the stable ferroelectric property of BFOMnTi film on mica, irrespective of the physical bending state (Figure S9, Supporting Information). Generally, the flexoelectric effect is defined as the coupling of the polarization and the strain gradient, which can be inevitably induced by the inhomogeneous strain in a film due to substrate bending. For one consideration, the large Ec of the BFOMnTi film plays a vital role in stabilizing the domains against being switched by the flexoelectric field, contributing greatly towards the resistance against mechanical bending. For another, the switching behavior of the polarization in response


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to the flexoelectric effect can be usually detected in ultrathin films, such as 4.8 nm-thick BaTiO3 and 50 nm-thick BFO thin films.28,29 Across a film thickness of hundreds of nanometers, the strain at the film/substrate interface may be relaxed,30 and the BFOMnTi film with thickness of ~ 300 nm in our experiment may fall into this category. Additionally, the leakage current density versus electric field (J-E) plots of BFOMnTi sample under flat and compressively/tensilely flexed situations were shown in Figure 4d. The J of BFOMnTi is relatively low, which is several orders of magnitude lower than that in pure BFO film.31,32 As is well known to all, the pure BFO films suffer from a severe leakage problem. The high concentration of oxygen vacancies induced is generally regarded as the main contributor to the large leakage in BFO-based materials.33,34 In the BFOMnTi film with Bi excess, the generation of oxygen can be inhibited and thus to decrease the leakage current, according to the following defect equations:

Mn 2  VFe  MnFe

(1)

1  2 MnFe  VO  O2  2 MnFe  OO 2

(2)

 Ti 4  VFe  TiFe

(3)

Also, note that the substrate bending has no obvious negligible effects on the insulating property. Almost identical J-E curves with ~ 410-4 A/cm2 at 400 kV/cm are observed in different states, reflecting the slight impact of bending strain on film leakage. The pulsed switchable polarization P, [P = P*(switched polarization) –P^(nonswitched polarization)] is measured using a PUND method, which is an important parameter to evaluate the reliability of ferroelectric memories over their long-lifetime operation. Figure 4e displays the


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normalized P as a function of retention time for the BFOMnTi sample in the original unbent state and in the bent state in a series of compressive/tensile bending radii. It is worth noting that the flat BFOMnTi film exhibits excellent charge-retaining capability at room temperature with a loss of P of ~ 8.6 % for retention time up to 105 s. Subsequently, the film was subjected to bent processing at different radii of curvature of 12 mm to 2 mm, with the P value recorded in the corresponding bending state. With the plots of P versus retention time almost overlapping with each other, no evident decay in the polarization retention can be found, indicating a stable charge-retaining capability against mechanical flexing. Figure 4f presents the normalized P as a function of switching cycles for flat and bent BFOMnTi film. It can be seen that the BFOMnTi thin film in the flat state exhibits almost no loss of P after 109 switching cycles, and no obvious decay can be found in the P-E loop after 109 polarization switching (Figure S10, Supporting Information). The results suggest that the flexible BFOMnTi film is fatigue-free during the polarization switching process. Also, such good fatigue resistance can be obtained even when a higher electric field of 1.5Ec is applied (Figure S10, Supporting Information). The fatigue-free behavior of the BFOMnTi film capacitor remains nearly the same when it is bent from a radius of curvature of 12 mm down to a small radius of 2 mm. The insulating properties also show no evident deteriorations even after 109 fatigue cycles at 1.2Ec and 1.5Ec (Figure S11, Supporting Information). Such excellent polarization retention and strong fatigue resistance of the BFOMnTi film capacitor in Figure 4e and f prove its reliable performance under a small radius of curvature, satisfying the demand for flexible ferroelectric memories that are capable of long-term operation. Overall, it is clear from Figure 4 that the BFOMnTi thin film grown on mica substrate possesses superior stable electrical performances against mechanical bending, which provides more


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freedom to design highly suitable BFO-based ferroelectric elements for flexible memory device applications. For further scrutinizing the practicability of flexible BFOMnTi thin film subjected to repeated bending processes, electrical measurements were performed on a film that was flattened again after mechanical flexing for 1 and 103 bending cycles at r = 4 mm, as shown in Figure 5, where the electrical properties in the original unbent state are provided as a comparison. The P-E hysteresis loops before and after compressive/tensile bending for 1 and 103 times are presented in Figure 5a, from which the similar Ps ∼ 93 μC/cm2 and Pr ~ 66 μC/cm2 can be noticed, implying stable ferroelectricity. With little changes being found in the J-E plots in Figure 5b, the conclusion can be drawn that the insulating behavior in the flexible BFOMnTi sample is nearly immune to compressive or tensile strain as demonstrated by the repeated bending process at 4 mm radius. Figure 5c shows the normalized P versus retention time for BFOMnTi film before and after mechanical bending. On the whole, the sample can remain its excellent polarization retention capability, even though the P value of the film flattened after 103 cycles of mechanical bending is reduced by ~ 19 % of its initial value at 105 s for the polarizations at negative E. In addition, Figure 5d shows the normalized P versus switching cycle for the BFOMnTi sample while flat and after undergoing bent processing at r = 4 mm. It is evident that the fatigue behavior is not dependent on the compressive or tensile treatment, with 1 as well as 103 bending cycles (Figure S12, Supporting Information). In any case, the flexible BFOMnTi thin film on mica substrate exhibits excellent mechanical stability, indicating its wide prospects in bendable ferroelectric memory applications under some special curved circumstances. To show the progress to date in flexible ferroelectric memories, comparisons of some basic parameters are made between this work and several reported ones,13,17, 35-37 as shown in Table 1.


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It should be noted that the flexible BFO-based memory element is first achieved with the aid of mica substrate, and the following salient merits make it promising in flexible memory device applications. (i) The flexible PZT achieved by the “grow-transfer” technique suffers from expensive and tedious multi-step process,13 while the BFOMnTi ferroelectric memory with excellent flexibility in this work is obtained through a cost-effective one-step method. (ii) Compared with the organic ferroelectrics,35,36 or PZT based on flexible plastic substrate,13 the BFOMnTi film on mica shows superior thermal stability, working normally at 200 C, together with better fatigue resistance of 109 switching cycles and mechanical stability up to 103 bendings. (iii) In comparison with the currently available inorganic ferroelectric memories based on mica, i.e., PZT and Bi3.25La0.75Ti3O12,17,18,37 the flexible BFOMnTi exhibits robust ferroelectric polarization with Ps ~ 93 μC/cm2 and Pr ~ 66 μC/cm2. 4. CONCLUSIONS In summary, a high-performance flexible polycrystalline BFOMnTi film capacitor was successfully fabricated on mica substrate through a simple and cost-effective chemical solution deposition approach. The BFOMnTi film shows large saturated polarization of Ps ~ 93 μC/cm2 and remanent polarization of Pr ~ 66 μC/cm2. The film retains excellent ferroelectric properties at variable frequencies from 1 to 50 kHz and elevated temperature up to 200 C. Most importantly, no evident decay in the electrical polarization, charge-retaining ability at 105 s, and fatigue-free behavior at 109 switching cycles can be found even when the film is subjected to compressive/tensile bending to a small radius of 2 mm or after repeated bending for 103 times at 4 mm radius. The work reported here supports the feasibility of manufacturing flexible nonvolatile memory devices by direct growth of BFO-based film on mica substrate, and it is


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anticipated to advance the state-of-the-art with respect to flexibility and portability for smart wearable devices. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:10.1021/acsami.XXXXXXX. Flexibility, AFM surface morphology and transmittance of layer-structured mica substrate, FTIR spectra, XRD, EDS element mapping, piezoelectric amplitudes, J-E plots, P-E loops, and switching current versus electric field curves under different conditions of BFOMnTi film. (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] ORCID Shifeng Huang: 0000-0001-8693-3570 Zhenxiang Cheng: 0000-0003-4847-2907 Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (Grants Nos. 51632003, U1806221, and 51702120), Shandong provincial key research and development plan (Grant No. 2016JMRH0103), and Shandong Provincial Natural Science Foundation of China (ZR2017LEM008). Z. X. Cheng thanks the Australian Research Council for support (DP190100150). REFERENCES (1) Scott, J. F. Applications of Modern Ferroelectrics. Science 2007, 315, 954-959. (2) Owczarek, M.; Hujsak, K. A.; Ferris, D. P.; Prokofjevs, A.; Majerz, I.; Szklarz, P.; Zhang, H. C.; Sarjeant, A. A.; Stern, C. L.; Jakubas, R.; Hong, S.; Dravid, V. P.; Stoddart, J. F. Flexible Ferroelectric Organic Crystals. Nat. Commun. 2016, 7, 13108. (3) Martin, L. W.; Rappe, A. M. Thin-Film Ferroelectric Materials and Their Applications. Nat. Rev. Mater. 2016, 2, 16087. (4) Han, S.-T.; Zhou, Y.; Roy, V. A. L. Towards the Development of Flexible Non-Volatile Memories. Adv. Mater. 2013, 25, 5425-5449. (5) Baek, S. H.; Park, J.; Kim, D. M.; Aksyuk, V. A.; Das, R. R.; Bu, S. D.; Felker, D. A.; Lettieri, J.; Vaithyanathan, V.; Bharadwaja, S. S. N.; Bassiri-Gharb, N.; Chen, Y. B.; Sun, H. P.; Folkman, C. M.; Jang, H. W.; Kreft, D. J.; Streiffer, S. K.; Ramesh, R.; Pan, X. Q.; Trolier-


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Page 19 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


McKinstry, S. ; Schlom, D. G.; Rzchowski, M. S.; Blick, R. H.; Eom, C. B. Giant Piezoelectricity on Si for Hyperactive MEMS. Science 2011, 334, 958-961. (6) Gupta, S.; Navaraj, W. T.; Lorenzelli, L.; Dahiya, R. Ultra-Thin Chips for HighPerformance Flexible Electronics. npj Flex. Electron. 2018, 2, 8. (7) Ding, R.; Liu, H.; Zhang, X. L.; Xiao, J. X.; Kishor, R.; Sun, H. X.; Zhu, B.; Chen, G.; Gao, F.; Feng, X. H.; Chen, J. S.; Chen, X. D.; Sun, X. W.; Zheng, Y. J. Flexible Piezoelectric Nanocomposite Generators Based on Formamidinium Lead Halide Perovskite Nanoparticles. Adv. Funct. Mater. 2016, 26, 7708-7716. (8) Gao, W. X.; Chang, L.; Ma, H.; You, L.; Yin, J.; Liu, J. M.; Liu, Z. G.; Wang, J. L.; Yuan, G. L. Flexible Organic Ferroelectric Films with a Large Piezoelectric Response, NPG Asia Mater. 2015, 7, e189. (9) Hu, W. J.; Wang, Z. H.; Du, Y. M.; Zhang, X.-X.; Wu, T. Space-Charge-Mediated Anomalous Ferroelectric Switching in P(VDF-TrEE) Polymer Films, ACS Appl. Mater. Interfaces 2014, 6, 19057-19063. (10) Wang, J.; Neaton, J. B.; Zheng, H.; Nagarajan, V.; Ogale, S. B.; Liu, B.; Viehland, D.; Vaithyanathan, V.; Schlom, D. G.; Waghmare, U. V.; Spaldin, N. A.; Rabe, K. M.; Wuttig, M.; Ramesh, R. Epitaxial BiFeO3 Multiferroic Thin Film Heterostructures. Science 2003, 299, 17191722. (11) Cheng, Z. X.; Wang, X. L.; Dou, S. X.; Kimura, H.; Ozawa, K. Improved Ferroelectric Properties in Multiferroic BiFeO3 Thin Films through La and Nb Codoping. Phys. Rev. B 2008, 77, 092101.


19


Page 20 of 30

(12) Yang, B. B.; Guo, M. Y.; Song, D. P.; Tang, X. W.; Wei, R. H.; Hu, L.; Yang, J.; Song, W. H.; Dai, J. M.; Lou, X. J.; Zhu, X. B.; Sun, Y. P. Bi3.25La0.75Ti3O12 Thin Film Capacitors for Energy Storage Applications. Appl. Phys. Lett. 2017, 111, 183903. (13) Bakaul, S. R.; Serrao, C. R.; Lee, O.; Lu, Z. Y.; Yadav, A.; Carraro, C.; Maboudian, R.; Ramesh, R.; Salahuddin, S. High Speed Epitaxial Perovskite Memory on Flexible Substrates. Adv. Mater. 2017, 29, 1605699. (14) Park, K.-I.; Son, J. H.; Hwang, G.-T.; Jeong, C. K.; Ryu, J. ; Koo, M.; Choi, I.; Lee, S. H.; Byun, M.; Wang, Z. L.; Lee, K. J. Highly-Efficient, Flexible Piezoelectric PZT Thin Film Nanogenerator on Plastic Substrates. Adv. Mater. 2014, 26, 2514-2520. (15) Bitla, Y.; Chu, Y.-H. MICAtronics: A New Platform for Flexible X-tronics. FlatChem. 2017, 3, 26-42. (16) Yang, C. H.; Lv, P. P.; Qian, J; Han, Y. J.; Ouyang, J.; Lin, X. J.; Huang, S. F.; Cheng, Z. X. Fatigue-Free and Bending-Endurable Flexible Mn-doped Na0.5Bi0.5TiO3-BaTiO3-BiFeO3 Film Capacitor with An Ultrahigh Energy Storage Performance, Adv. Energy Mater. 2019, DOI: 10.1002/aenm.201803949. (17) Jiang J.; Bitla, Y.; Huang, C.-W.; Do, T. H.; Liu, H.-J.; Hsieh, Y.-H.; Ma, C.-H.; Jang, C.-Y;. Lai, Y.-H.; Chiu, P.-W.; Wu, W.-W.; Chen, Y.-C.; Zhou, Y.-C.; Chu, Y.-H. Flexible Ferroelectric Element Based on Van der Waals Heteroepitaxy. Sci. Adv. 2017, 3, e1700121. (18) Gao, W. X.; You, L.; Wang, Y. J.; Yuan, G. L.; Chu, Y.-H.; Liu, Z. G.; Liu, J.-M. Flexible PbZr0.52Ti0.48O3 Capacitors with Giant Piezoelectric Response and Dielectric Tunability. Adv. Electron. Mater. 2017, 3, 1600542.


20

Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(19) Yang, Y. X.; Yuan, G. L.; Yan, Z. B.; Wang, Y. J.; Lu, X. B.; Liu, J.-M. Flexible, Semitransparent, and Inorganic Resistive Memory Based on BaTi0.95Co0.05O3 Film. Adv. Mater. 2017, 29, 1700425. (20) Catalan, G.; Scott, J. F. Physics and Applications of Bismuth Ferrite. Adv. Mater. 2009, 21, 2463-2485. (21) Zeches, R. J.; Rossell, M. D.; Zhang, J. X.; Hatt, A. J.; He, Q.; Yang, C.-H.; Kumar, A.; Wang, C. H.; Melville, A.; Adamo, C.; Sheng, G.; Chu, Y.-H.; Ihlefeld, J. F.; Erni, R.; Ederer, C.; Gopalan, V.; Chen, L. Q.; Schlom, D. G.; Spaldin, N. A.; Martin, L. W.; Ramesh, R. A Strain-Driven Morphotropic Phase Boundary in BiFeO3. Science 2009, 326, 977-980. (22) Kawae, T.; Terauchi, Y.; Tsuda, H.; Kumeda, M.; Morimoto, A. Improved Leakage and Ferroelectric Properties of Mn and Ti Codoped BiFeO3 Thin Films. Appl. Phys. Lett. 2009, 94, 112904. (23) Zhang, Y.; Shen, L.; Liu, M.; Li, X.; Lu, X. L.; Lu, L.; Ma, C. R.; You, C. Y.; Chen, A. P.; Huang, C. W.; Chen, L.; Alexe, M.; Jia, C.-L. Flexible Quasi-Two-Dimensional CoFe2O4 Epitaxial Thin Films for Continuous Strain Tuning of Magnetic Properties. ACS Nano 2017, 11, 8002-8009. (24) Miao, P. X.; Zhao, Y. G.; Luo, N. N.; Zhao, D. Y.; Chen, A. T.; Sun, Z.; Guo, M. Q.; Zhu, M. H.; Zhang, H. Y.; Li, Q. Ferroelectricity and Self-Polarization in Ultrathin Relaxor Ferroelectric Films. Sci. Rep.-UK. 2016, 6, 19965. (25) Wang, D.; Yuan, G. L.; Hao, G. Q.; Wang, Y. J. All-Inorganic Flexible Piezoelectric Energy Harvester Enabled by Two-Dimensional Mica. Nano Energy 2018, 43, 351-358.


21


Page 22 of 30

(26) Simões, A. Z.; Ramírez, M. A.; Riccardi, C. S.; Gonzalez, A. H. M.; Longo, E.; Varela, J. A. Synthesis and Electrical Characterization of CaBi2Nb2O9 Thin Films Deposited on Pt/Ti/SiO2/Si Substrates by Polymeric Precursor Method. Mater. Chem. Phys. 2006, 98, 203-206. (27) Lin, Q. R.; Ding, R.; Li, Q.; Tay, Y. Y.; Wang, D. Y.; Liu, Y.; Huang, Y. Z.; Li, S. Large Piezoelectricity and Ferroelectricity in Mn-doped (Bi0.5Na0.5)TiO3-BaTiO3 Thin Film Prepared by Pulsed Laser Deposition. J. Am. Ceram. Soc. 2016, 99, 2347-2353. (28) Lu, H.; Bark, C.-W.; Esque de los Ojos, D.; Alcala, J.; Eom, C. B.; Catalan, G.; Gruverman, A. Mechanical Writing of Ferroelectric Polarization. Science 2012, 336, 59-61. (29) Jeon, B. C.; Lee, D.; Lee, M. H.; Yang, S. M.; Chae, S. C.; Song, T. K.; Bu, S. D.; Chung, J.-S.; Yoon, J.-G.; Noh, T. W. Flexoelectric Effect in the Reversal of Self-Polarization and Associated Changes in the Electronic Functional Properties of BiFeO3 Thin Films. Adv. Mater. 2013, 25, 5643-5649. (30) Wang, Z. H.; Zhang, X. X.; Wang, X. B.; Yue, W. S.; Li, J. Q.; Miao, J. M.; Zhu, W. G. Giant Flexoelectric Polarization in a Micromachined Ferroelectric Diaphragm. Adv. Funct. Mater. 2013, 23, 124-132. (31) Cheng, L.; Hu, G. D.; Jiang, B.; Yang, C. H.; Wu, W. B.; Fan, S. H. Enhanced Piezoelectric Properties of Epitaxial W-doped BiFeO3 Thin Films. Appl. Phys. Express 2010, 3, 101501. (32) Zhong, Z. Y.; Ishiwara, H. Variation of Leakage Current Mechanisms by Ion Substitution in BiFeO3 Thin Films. Appl. Phys. Lett. 2009, 95, 112902. (33) Hu, G. D.; Fan, S. H.; Yang, C. H.; Wu, W. B. Low Leakage Current and Enhanced Ferroelectric Properties of Ti and Zn Codoped BiFeO3 Thin Film. Appl. Phys. Lett. 2008, 92, 192905.


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(34) Guo, Y. Q.; Xiao, P.; Wen R.; Wan, Y.; Zheng, Q. J.; Shi, D. L.; Lam, K. H.; Liu, M. L.; Lin, D. M. Critical Roles of Mn-Ions in Enhancing the Insulation, Piezoelectricity and Multiferroicity of BiFeO3-Based Lead-Free High Temperature Ceramics. J. Mater. Chem. C 2015, 3, 5811-5824. (35) Singh, D.; Deepak; Garg, A. An Efficient Route to Fabricate Fatigue-Free P(VDF-TrFE) Capacitors with Enhanced Piezoelectric and Ferroelectric Properties and Excellent Thermal Stability for Sensing and Memory Applications. Phys. Chem. Chem. Phys. 2017, 19, 7743-7750. (36) Kim, K. L.; Lee, W.; Hwang, S. K.; Joo, S. H.; Cho, S. M.; Song, G.; Cho, S. H.; Jeong, B.; Hwang, I.; Ahn, J.-H.; Yu, Y.-J.; Shin, T. J.; Kwak, S. K.; Kang, S. J.; Park, C. Epitaxial Growth of Thin Ferroelectric Polymer Films on Graphene Layer for Fully Transparent and Flexible Nonvolatile Memory. Nano Lett. 2016, 16, 334-340. (37) Su, L. S.; Lu, X. B.; Chen, L.; Wang, Y. J.; Yuan, G. L.; Liu, J.-M. Flexible, FatigueFree, and Large-Scale Bi3.25La0.75Ti3O12 Ferroelectric Memories. ACS Appl. Mater. & Interfaces 2018, 10, 21428-21433.


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Figure 1. (a) Mechanical exfoliation process to obtain mica crystal with a clean surface. Photographs of flexible (b) mica substrate, (c) mica/Pt and (d) mica/Pt/BFOMnTi. (e) XRD patterns of mica substrate and mica/Pt/BFOMnTi, where the data base of PDF cards of mica crystal, Pt, and BFO are included to verify the diffraction peaks. (f) AFM surface morphology and (g) cross-sectional FESEM image of Pt/BFOMnTi/Au capacitor.


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Figure 2. Schematic diagrams of BFOMnTi film in (a) flat, (b) compressive, and (c) tensile bending states. Surface morphologies (d, e, f), out-of-plane PFM phase images (g, h, i), and local piezoelectric phase hysteresis loops and amplitudes (j, k, l) for the flexible BFOMnTi film on mica under (d, g, j) flat, (e, h, k) compressively bent, and (f, i, l) tensilely bent conditions with a 4 mm radius, which are measured from the as-grown regions.


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Figure 3. (a) Room temperature P-E hysteresis loops of flat BFOMnTi film measured at 10 kHz. (b) Frequency dependence of P-E loops at room temperature. (c) P-E loops at various temperatures.


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Figure 4. (a) Schematic illustration of the procedure for testing the electrical properties of BFOMnTi film in different bent states. (b) P-E loops of BFOMnTi thin film measured under flat conditions and with various compressive and tensile bending radii at 10 kHz. (c) Variations of Ps, Pr, and Ec as functions of the bending radius. (d) Insulating, (e) switchable polarization retention, and (f) fatigue characteristics for BFOMnTi sample under flat and bent conditions for a series of compressive/tensile bending radii.


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Figure 5. (a) P-E loops, (b) J-E plots, (c) switchable polarization retention, and (d) fatigue characteristics for BFOMnTi film in the unbent and flattened state after compressive/tensile bending for 1 and 103 times at a radius of 4 mm.


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Table 1. Comparision of some recently reported flexible ferroelectric memories. Ferroelectric material

BFOMnTi

PbZr0.2Ti0.8O3*

PbZr0.52Ti0.48O3

Bi3.25La0.75Ti3O12

PZT#

P(VDFTrFE)

P(VDFTrFE)

Mica

Mica

Mica

Mica

PETa)

PETa)

PMMAb)

Method

Spin coating

Sputtering

Sputtering

Sputtering

Sputtering

Spin coating

Spin coating

Transfer

No

No

No

No

Yes

No

No

Ps (μC/cm2)

93

75

~60

~ 20

89

~ 10

~8

Pr (μC/cm2)

66

60

38

~ 10

75

8.9

~7

Ec (kV/cm)

Ec+ ~ 844

100

~50

80

-

520

~ 744

＞ 200

＞ 175

＞ 170

Pr ~ 30 % loss at 200

-

110

-

Retention (s)

105

105

-

-

105

-

104

Fatigue (cycles)

109

~ 33 % loss at 1010

~ 50 % loss at 1010

109

~ 50 % loss at 1010

108

125

2

2.5

1.4

1.4

10

-

4

103 at 4 mm

103 at 5 mm

104 at 2.2 mm

104

102

-

103 at 5 mm

This work

(17)

(18)

(37)

(13)

(35)

(36)

Flexible substrate

Ec- ~ 276 Working (C)

temperature

Minimum radius (mm) Bending cycles Reference

*Single-crystalline; #Highly

oriented; a)Polyethylene terephthalate; b)poly(methyl methacrylate).


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TABLE OF CONTENTS GRAPHIC


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Flexible, Temperature-Resistant, and Fatigue-Free Ferroelectric

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