Flexible Memristors Based on Single-Crystalline Ferroelectric Tunnel

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Flexible Memristors Based on Single-Crystalline Ferroelectric Tunnel Junctions Zheng-Dong Luo,* Jonathan J. P. Peters, Ana M. Sanchez, and Marin Alexe* Department of Physics, University of Warwick, CV4 7AL, Coventry, United Kingdom

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S Supporting Information *

ABSTRACT: Ferroelectric tunnel junction (FTJ) based memristors exhibiting continuous electric field controllable resistance states have been considered promising candidates for future high-density memories and advanced neuromorphic computational architectures. However, the use of rigid single crystal substrate and high temperature growth of the epitaxial FTJ thin films constitutes the main obstacles to using this kind of heterostructure in flexible computing devices. Here, we report the integration of centimeterscale single crystalline FTJs on flexible plastic substrates, by wateretching based epitaxial oxide membrane lift-off and the following transfer. The resulting highly flexible FTJ membranes retain the single-crystalline structure along with stable and switchable ferroelectric polarization as the grown-on single crystal substrate state. We show that the obtained flexible memristors, i.e., FTJs on plastic substrates, present high speed and low voltage mediated memristive behaviors with resistance changes over 500% and are stable against shape change. This work is an essential step toward the realization of epitaxial ultrathin ferroelectric oxide film-based electronics on large-area, flexible, and affordable substrates. KEYWORDS: ferroelectrics, functional oxides, epitaxial oxide film transfer, flexible nonvolatile memory, ferroelectric tunnel junction

1. INTRODUCTION Flexible electronics are on the verge of an innovative breakthrough toward smart, multifaceted, interacting systems comprising sensing, communication, information storage, and processing, all in one unit, which require advanced functional components.1−6 A key building block of such multifunctional devices is the computing cell for which a lightweight, high performance, and even wearable memory element is required. Despite significant efforts to fabricate memory devices on flexible substrates, there are continuous requests for new devices and architectures with superior properties in terms of the key memory performance parameters such as low-power consumption, high data density, nonvolatility, fast operating speed, etc. Memristors or memristive devices exhibiting continuous electric field controllable resistance states have long been considered as promising candidates, which could fulfill the aforementioned requirements.7−9 Among the various types of the memristors, the ferroelectric tunnel junction (FTJ) memristor consisting of two metal electrodes separated by an ultrathin ferroelectric barrier layer10−16 exhibits polarization dependent resistance states and can present a large number of robust intermediate resistance values corresponding to the electric field mediated ferroelectric domain configuration.8,17−19 The multiple resistance switching as a function of the applied voltage in an FTJ can be well described by the electrically modulated ferroelectric domain reconfiguration.8,18 The capability of scaling down to less than 5 nm in thickness © 2019 American Chemical Society

and achieving the ultrafast, low energy, nonvolatile electrical modulation along with the predictive, well-established understanding of operating mechanism thus make ferroelectric tunnel memristors highly promising for the implementation of future high density memory applications and the advanced neuromorphic computational architectures.8,18 Therefore, the successful fabrication of ferroelectric tunnel memristors on soft substrates could present an essential leap toward the aforementioned high performance flexible memories. Despite the recent breakthrough by growing ferroelectric layers directly on soft mica substrates,20 a feasible way to achieve the mechanical flexibility of the FTJ devices is to integrate such heterostructures on mainstream flexible polymer substrates. Unfortunately, the synthesis of ferroelectric tunnel memristors using conventional oxides materials like Pb(Zr,Ti)O3 (PZT) and BaTiO3 (BTO) requires the high-temperature epitaxial growth on rigid single-crystal substrates in order to ensure that nonvolatile and switchable polarization could persist in the ultrathin ferroelectric layer. Although there are reports demonstrating the successful fabrication of FTJ devices using nonepitaxial thin films, the growth temperature is higher than the decomposition temperature of most of the polymer flexible substrates.21−23 The strict Received: March 16, 2019 Accepted: June 10, 2019 Published: June 10, 2019 23313

DOI: 10.1021/acsami.9b04738 ACS Appl. Mater. Interfaces 2019, 11, 23313−23319

Research Article

ACS Applied Materials & Interfaces

Figure 1. Fabrication process of the flexible ferroelectric tunnel memristors. (a) Schematic drawing of the process of BTO/LSMO thin film lifted off by water etching the SAO sacrifice layer and transfer onto metal-coated PET flexible substrate. (b) Optical images of the BTO/LSMO flexible memristor devices.

2. RESULTS AND DISCUSSION 2.1. Device Fabrication Process. Figure 1a schematically illustrates the key steps involved in the fabrication of the BTO/ LSMO/SAO ferroelectric tunnel memristors on a flexible polyethylene terephthalate (PET) substrate. The presented epitaxial lift-off process is based on the possibility to epitaxially grow the water-soluble sacrificial oxide buffer layer, i.e., the SAO which is compatible with epitaxial functional oxide overlayer growth.27 We started by growing high quality epitaxial BTO (3.6 nm)/LSMO (40 nm)/SAO (30 nm) heterostructures on (001)-oriented SrTiO3 single crystal substrates using pulsed laser deposition (PLD). The sample was then immersed in deionized (DI) water in order to etch the SAO buffer layer and release the BTO/LSMO overlayer; see also the Supporting Information (SI) Figure S1. After water etching the SAO sacrifice layer away, the top BTO/ LSMO single-crystalline membrane was lifted off from the growth substrate and subsequently transferred onto a metalcoated PET substrate using the spin-coated poly(methyl methacrylate) (PMMA) layer as the transfer stamp. After baking treatment of the transferred sample, the PMMA layer was then washed away and the BTO/LSMO membrane was left on the PET substrate. As can be seen from the optical images, centimeter-scale BTO/LSMO film was completely transferred onto the PET foil from the STO substrate. Finally, top electrodes were patterned through standard photolithography and e-beam metal evaporation to form the ferroelectric tunnel junctions (SI Figure S2). The resulting memristor devices are highly flexible and bendable as can be seen from the optical image in Figure 1b. Additionally, we have also transferred BTO/LSMO membranes onto Si substrates using the same method in order to carry out the transmission electron microscopy (TEM) measurements. 2.2. Material Structure Properties. To check the single crystallinity of the transferred BTO/LSMO membranes, we conducted HR-XRD measurements. It is worth noting, in order to obtain significant XRD signal, that we used BTO thin film in thickness of 6 nm for measurements; however, the BTO

growth conditions thus strongly hamper the development of ferroelectric tunnel memristors toward flexible electronics applications. Very recently, a strategy involving release of single-crystalline thin films from the growth substrates by chemical etching of the sacrificial buffer layer has been shown capable of obtaining membranes thin enough to be flexible while retaining the original structural quality after lift-off, which significantly stimulates the development toward epitaxial oxide film based flexible electronics.24−30 Using such a method, epitaxial transfer of relatively thick epitaxial PZT (60 nm) and BiFeO3 (400 nm) functional ferroelectric oxide films onto polymer or Si substrates have been achieved.25,31 However, there has been no reports regarding the transfer of ultrathin ferroelectric thin films for devices. Moreover, whether the spontaneous ferroelectric polarization can be preserved in the freestanding ultrathin ferroelectric membranes remains an open question. In this work, we report the integration of single-crystalline ferroelectric tunnel memristor heterostructures on plastic substrates for ultrathin ferroelectrics based flexible memory applications. The ferroelectric tunnel memristors consisting of single-crystalline BTO/La0.7Sr0.3MnO3 (LSMO) heterostructures can be lifted off from the water-soluble Sr3Al2O6 (SAO) buffered SrTiO3 (STO) substrate, and subsequently attached to arbitrary host substrates. We address that BTO singlecrystalline membranes of only 3.6 nm in thickness can still preserve the switchable ferroelectric polarization after the liftoff and exhibit over 500% tunneling electroresistance (TER) at room temperature. Our results therefore suggest an easy way to integrate high quality epitaxial ferroelectric tunnel memristors onto a wide range of substrates as the hardware basis for future high-density memory systems and human-centric intelligent computing platforms with mechanical flexibility. Moreover, given that ultrathin ferroelectric films can still retain robust ferroelectricity even in the freestanding state, our results could boost ultrathin functional ferroelectric based new applications which can be compatible with various surfaces rather than constrained on the rigid substrates.10,32 23314

DOI: 10.1021/acsami.9b04738 ACS Appl. Mater. Interfaces 2019, 11, 23313−23319

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Figure 2. Structural properties of the transferred BTO/LSMO films. (a) 2θ−ω scans of the BTO/LSMO after transfer. The right panel shows the magnified (002) peak of the BTO/LSMO. (b) ϕ-Scan around (103) diffraction peak of LSMO of the transferred BTO/LSMO heterostructure. (c) ADF-STEM image of the transferred BTO/LSMO stack on SiO2/Si substrate and the interface region with atomic resolution (the right panel). (d) Distribution of BTO tetragonality as a function of distance from the LSMO/BTO interface (left in the plot) to the BTO surface (right) before and after transfer process. The dashed line indicates the tetragonality value of bulk BTO.

crystallographic direction using annular dark field (ADF). The transferred BTO/LSMO heterostructure on SiO2/Si substrate can be clearly identified as shown in Figure 2c, the coherent structure indicates that the transferred BTO/LSMO membrane is free from any intermixing or defect formation during the entire fabrication process. The higher magnification ADFSTEM image on the right panel of Figure 2c further demonstrates the near-atomically sharp interface in the transferred BTO/LSMO heterostructures. In ultrathin ferroelectric films, the misfit strain imposed by the single crystal substrates is believed to be a key ingredient in stabilizing the ferroelectricity.11 Whether the strain state or stable ferroelectricity can be maintained in freestanding ultrathin ferroelectric single crystalline membranes remains an open question and is crucial for the implementation of ultrathin ferroelectrics based flexible electronic devices. To check the strain state, we then examined the lattice constants c and a of the transferred epitaxial BTO/LSMO heterostructure from the ADF-STEM image, which are well matched with the XRD results. The calculated c/a ratios of the as-grown and transferred BTO layer are shown in Figure 2d. We found that the tetragonality of the transferred BTO layer is slightly lower than its as-grown state but still remains a high value of ∼1.05 on average compared to that of the bulk BTO (c/a = 1.01). In comparison, the thicker

diffraction signal is still too weak to be recorded in the (103) peak reciprocal space mapping (SI Figure S3). As shown in Figure 2a, the 2θ−ω XRD scan clearly indicates the single phase and epitaxial (00l) orientation of the BTO/LSMO heterostructure after transfer. The epitaxial quality of the transferred membranes was examined by ϕ-scan around the (103) diffraction of the LSMO. As shown in Figure 2b, the sharp peaks with fourfold symmetry in the ϕ-scan pattern suggest that the transferred membrane remains an intact singlecrystalline structure after the transfer process. The right panel in Figure 2a shows the magnified (002) Bragg peaks of the transferred BTO/LSMO membrane. Surprisingly, the BTO layer is still compressively strained with the prolonged c lattice constants of 4.14 Å compared to its bulk value of 4.04 Å. In addition, the LSMO layer preserves its original crystal structures with only a slight change of lattice parameters after lifting off (SI Figure S3a). Overall, these results unambiguously demonstrate that the single-crystalline structure of the BTO/LSMO membranes is well preserved after the epitaxial lift-off process. To further check the strain state and atomic-scale structural coherency of transferred BTO/LSMO heterostructures, we performed cross-sectional aberration-corrected scanning transmission electron microscopy (STEM) imaging along [001] 23315

DOI: 10.1021/acsami.9b04738 ACS Appl. Mater. Interfaces 2019, 11, 23313−23319

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Figure 3. Ferroelectricity characterizations of the BTO/LSMO films. (a) Topography of BTO/LSMO on metal-coated PET. (b) PFM image of the same area with electrically written ferroelectric domains; the writing voltages are 4 V and −4 V, respectively. Scale bars for (a) and (b): 1 μm. (c) Hysteresis dependence of the PFM phase and amplitude. (d) IV curves of the FTJ at different polarization states. The inset sketches the barrier for the metal/BTO/LSMO tunnel junction at different states.

further confirms the role of the preservation of high tetragonality for retaining the ferroelectricity in freestanding ultrathin BTO membranes with a buffer layer (LSMO in this case). 10 μm × 10 μm Pt top electrodes were deposited on the BTO/LSMO membranes (SI Figure S2), and the resistive switching of the resulting Pt/BTO/LSMO flexible FTJs was then investigated by measuring the device current−voltage (IV) characteristics upon BTO polarization switching. Figure 3d shows the IV curves of a fabricated Pt/BTO/LSMO junction after applying 100 μs long voltage pulses with amplitudes of +3 V and −4 V, respectively. These results indicate an OFF/ON ratio of 5 or 500% change of TER in our flexible FTJ. In addition, the IV curve of a Pt/LSMO/Pt junction shows no TER effect (see SI Figure S6), which confirms the role of the ferroelectric BTO layer in the observed resistive switching effect. The IV curves of the FTJ can be well fitted using the Brinkman model for tunnel transport across trapezoidal potential barriers in the Wentzel−Kramers− Brillouin (WKB) approximation;12 see SI Note 5 for fitting details. A sketch of the barrier shapes using the obtained fitting results at different BTO ferroelectric polarization states is shown in the inset of Figure 3d. Overall, the results shown here clearly indicate the ferroelectric character retained in the transferred epitaxial BTO layer and the ferroelectric polarization dependent tunneling transport nature of the flexible FTJ devices. In the following, we present the device properties of the prototype flexible ferroelectric tunnel memristors. Figure 4a shows the plot of the device resistance as a function of the voltage pulse amplitude while fixing the pulse duration at 10 μs. A clear hysteretic resistance cycle between high and low resistance states was observed, which demonstrates the junction resistance as a history of the applied voltages pulses, i.e., memristive behaviors.33 Note that the resistance of the flexible memristors in the following experiments was recorded at 0.2 V. The observed multilevel resistance states of the

BTO (50 nm) layer with the LSMO buffer layer after transfer undergoes a rapid strain relaxing and is of bulk-like lattice parameters (SI Figure S3). After the epitaxial lift-off process, although the freestanding membranes tend to be relieved from the strain state imposed by the growth substrate,31 if the film is attached to a buffer layer, the strain relaxation normally diminishes by decreasing the film thickness, being coherent to the buffer layer underneath.11,32 This is confirmed by the relationship between the lattice parameters and the thickness of the transferred BTO layer (see the SI Figure S3f). Therefore, we conclude that the high tetragonality of the transferred BTO membrane is related to its ultrathin nature which could maintain the strain imposed by the LSMO buffer layer. We note that this high tetragonality in the transferred BTO layer (∼1.05) is even comparable to that of ultrathin BTO films grown directly on single crystal substrate; for example, c/a = 1.051 for BTO grown on LSMO buffered NdGaO3 substrate.11 2.3. Functional Properties of the Flexible Ferroelectric Tunnel Memristor. After the confirmation of the epitaxial structure of the transferred single-crystalline oxide membranes, we now investigate the most important functional properties, i.e., ferroelectricity of the BTO/LSMO membranes by piezoresponse force microscopy (PFM). As can be seen from Figure 3b and c, the electrically writing of oppositely polarized ferroelectric domains and the clear hysteretic behavior of out-of-plane piezoresponse signal in phase and amplitude acquired as a function of the d.c. voltage unambiguously illustrate the ferroelectric nature for the BTO layer integrated in the flexible junction devices. As indicated by the amplitude loop, the local coercive voltages of the BTO layer are +2 V and −3 V (see also SI Figure S5). The written antiparallel polarized ferroelectric domain patterns shown by the PFM phase image remains stable after several hours (data not shown). In contrast, a bare 6-nm-thick BTO membrane transferred on Pt/PET substrate shows no ferroelectric behaviors as investigated by PFM (data not shown) which 23316

DOI: 10.1021/acsami.9b04738 ACS Appl. Mater. Interfaces 2019, 11, 23313−23319

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fixed duration of 500 ns and the ReSET voltage of 3 V, 100 μs to tune the junction resistance. The final resistance states of the flexible memristor are dependent on the number of the SET voltage pulses applied which correspond to the cumulative effect.36 For practical applications of the flexible memories, the device reliability is crucial. We thus carried out bending and retention tests to investigate the mechanical flexing performance and time stability of the PET substrates hosted BTO/ LSMO ferroelectric tunnel memristors, respectively. The bending tests with 1.5 mm curvature of these flexible devices were carried out up to 100 cycles, which shows no sizable degradation of the functional properties, see Figure 4d. Furthermore, the devices present stable resistance state within several hours at respective OFF, intermediate, and ON states as shown in Figure 4e, which indicate excellent retention characteristics as nonvolatile memories. Overall, the presented results indicate that the BTO/LSMO ferroelectric tunnel memristor transferred on PET flexible substrate is of excellent memristive properties as the grown-on single crystal substrate memristor does and added up with the mechanical flexibility. Moreover, these demonstrated characteristics, especially the cumulative effect, indicate that the presented flexible memristors can be used as artificial synapses in which the synaptic transmission can be modified through spike-timingdependent plasticity, thus are appealing for future flexible and portable neural computing applications.18,37,38

Figure 4. Memristive properties of the flexible BTO/LSMO ferroelectric tunnel memristors. (a) Hysteretic resistance as a function of operating voltage pulses reading at 200 mV. (b) Evolution of the memristor resistance states as a function of pulse duration and amplitude. (c) Response of the device resistance state to multiple 0.5μs-long SET voltage pulses followed by the 3 V ReSET pulses. (d) Bending test up to 100 cycles of the device at high and low resistance states, respectively. The inset shows the bending test configuration. (e) Retention time of the device at three different resistance states.

3. CONCLUSIONS In summary, we have fabricated high quality flexible memristors based on the single-crystalline BTO/LSMO ferroelectric tunnel junction heterostructures. By etching the water-soluble and highly strained SAO sacrifice layer, ultrathin epitaxial BTO layer along with the LSMO buffer can be lifted off and transferred onto flexible plastic substrates while retaining the robust ferroelectricity. The ferroelectric polarization dependent resistive switching in plastic substrate hosted BTO/LSMO FTJ memristors demonstrates predictive functionalities required for implementation of the flexible and portable high-density memories and neuromorphic architectures. Moreover, our work presents a practical and generic approach to integrate systems with ultrathin ferroelectric oxide film on arbitrary surfaces, which suggests exciting opportunities for future large area, affordable, and flexible substrate hosted ferroelectric thin film devices with multifunctional characteristics arising from the compatibility of such materials with magneto-electric coupling,39 optical control,40 strain modulation,41 and so on.

flexible FTJ can be attributed to the ferroelectric domain reconfiguration in the BTO ferroelectric barrier during applying the voltage pulses, i.e., the expansion of these existing downward- or upward-polarized ferroelectric domains as well as the nucleation of new domains.8,18,34 Starting from the uniform upward ferroelectric polarization state in the BTO layer (after applying −4 V voltage pulse), we gradually increased the ratio of downward/upward polarization domains from 0 to 100% (uniform downward polarization, ON state) by applying a set of positive voltage pulses. The reversed process, i.e., from OFF to ON resistance state, was also carried out by applying negative voltage pulses. The deterministic electric control of the multiple resistance states in one single junction indicates that our flexible FTJs are capable of multilevel data storage and can be exploited for flexible high density memories.35 Next, we show that the resistance of the flexible memristors can be tuned by the voltage pulse number, amplitude, and duration. A detailed modulation of the junction resistance by voltage pulse duration and amplitude is depicted in Figure 4b. The device was switched from the high resistance state toward low resistance state by applying voltage pulses of different amplitude and duration. Clearly, a wide range of intermediate resistance states in our flexible memristor can be obtained through control of the shape of the applied voltage pulses. Figure 4c shows the junction resistance evolution from low resistance state to high resistance state upon applying consecutive pulses varying amplitude. To obtain the results in Figure 4c, we used a series of negative SET voltage pulses with

4. EXPERIMENTAL SECTION 4.1. Sample Fabrication. The BTO/LSMO/SAO heterostructures were grown on STO (001) substrates by PLD using a 248 nm wavelength KrF excimer laser. The 30-nm-thick SAO layers were first grown at 670 °C using 1.5 J cm−2 laser fluence at 5 Hz repetition rate in 1 × 10−3 mbar O2 atmosphere. For the bottom electrode and supporting layer of the freestanding membrane, LSMO of 40 nm in thickness was subsequently grown at 600 °C using 0.8 J cm−2 laser fluence at 2 Hz repetition rate in 0.1 mbar O2 atmosphere. BTO ferroelectric layers were finally deposited at 600 °C using 0.5 J cm−2, 2 Hz laser fluence with O2 atmosphere at 0.1 mbar. The as-grown samples were spin-coated with PMMA (EM resist Ltd.) and baked at 130 °C for 2 min. The PMMA coated samples were then immersed into DI water for 24 h to ensure that the SAO layer was fully etched. Next, the PMMA/BTO/LSMO membranes were cleaned by DI water 23317

DOI: 10.1021/acsami.9b04738 ACS Appl. Mater. Interfaces 2019, 11, 23313−23319

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(2) Koo, J. H.; Kim, D. C.; Shim, H. J.; Kim, T.-H.; Kim, D.-H. Flexible and Stretchable Smart Display: Materials, Fabrication, Device Design, and System Integration. Adv. Funct. Mater. 2018, 28, 1801834. (3) Lee, H. E.; Park, J. H.; Kim, T. J.; Im, D.; Shin, J. H.; Kim, D. H.; Mohammad, B.; Kang, I.-S.; Lee, K. J. Novel Electronics for Flexible and Neuromorphic Computing. Adv. Funct. Mater. 2018, 28, 1801690. (4) Melzer, M.; Monch, J. I.; Makarov, D.; Zabila, Y.; Canon Bermudez, G. S.; Karnaushenko, D.; Baunack, S.; Bahr, F.; Yan, C.; Kaltenbrunner, M.; Schmidt, O. G. Wearable Magnetic Field Sensors for Flexible Electronics. Adv. Mater. 2015, 27, 1274−1280. (5) Chu, Y.-H. Van der Waals Oxide Heteroepitaxy. npj Quantum Mater. 2017, 2, 67. (6) Rogers, J. A.; Lagally, M. G.; Nuzzo, R. G. Synthesis, Assembly and Applications of Semiconductor Nanomembranes. Nature 2011, 477, 45−53. (7) Yang, J. J.; Strukov, D. B.; Stewart, D. R. Memristive Devices for Computing. Nat. Nanotechnol. 2013, 8, 13−24. (8) Chanthbouala, A.; Garcia, V.; Cherifi, R. O.; Bouzehouane, K.; Fusil, S.; Moya, X.; Xavier, S.; Yamada, H.; Deranlot, C.; Mathur, N. D.; Bibes, M.; Barthelemy, A.; Grollier, J. A Ferroelectric Memristor. Nat. Mater. 2012, 11, 860−864. (9) Borghetti, J.; Snider, G. S.; Kuekes, P. J.; Yang, J. J.; Stewart, D. R.; Williams, R. S. ’Memristive’ Switches Enable ’Stateful’ Logic Operations via Material Implication. Nature 2010, 464, 873−876. (10) Garcia, V.; Bibes, M. Ferroelectric Tunnel Junctions for Information Storage and Processing. Nat. Commun. 2014, 5, 4289. (11) Garcia, V.; Fusil, S.; Bouzehouane, K.; Enouz-Vedrenne, S.; Mathur, N. D.; Barthelemy, A.; Bibes, M. Giant Tunnel Electroresistance for Non-Destructive Readout of Ferroelectric States. Nature 2009, 460, 81−84. (12) Gruverman, A.; Wu, D.; Lu, H.; Wang, Y.; Jang, H.; Folkman, C.; Zhuravlev, M. Y.; Felker, D.; Rzchowski, M.; Eom, C.-B. Tunneling Electroresistance Effect in Ferroelectric Tunnel Junctions at the Nanoscale. Nano Lett. 2009, 9, 3539−3543. (13) Pantel, D.; Goetze, S.; Hesse, D.; Alexe, M. Room-temperature Ferroelectric Resistive Switching in Ultrathin Pb (Zr0. 2Ti0. 8) O3 Films. ACS Nano 2011, 5, 6032−6038. (14) Tsymbal, E. Y.; Kohlstedt, H. Applied physics. Tunneling Across a Ferroelectric. Science 2006, 313, 181−183. (15) Yin, Y. W.; Burton, J. D.; Kim, Y. M.; Borisevich, A. Y.; Pennycook, S. J.; Yang, S. M.; Noh, T. W.; Gruverman, A.; Li, X. G.; Tsymbal, E. Y.; Li, Q. Enhanced Tunnelling Electroresistance Effect due to a Ferroelectrically Induced Phase Transition at a Magnetic Complex Oxide Interface. Nat. Mater. 2013, 12, 397−402. (16) Zhuravlev, M. Y.; Sabirianov, R. F.; Jaswal, S. S.; Tsymbal, E. Y. Giant Electroresistance in Ferroelectric Tunnel Junctions. Phys. Rev. Lett. 2005, 94, 246802. (17) Boybat, I.; Le Gallo, M.; Nandakumar, S. R.; Moraitis, T.; Parnell, T.; Tuma, T.; Rajendran, B.; Leblebici, Y.; Sebastian, A.; Eleftheriou, E. Neuromorphic Computing with Multi-Memristive Synapses. Nat. Commun. 2018, 9, 2514. (18) Boyn, S.; Grollier, J.; Lecerf, G.; Xu, B.; Locatelli, N.; Fusil, S.; Girod, S.; Carrétéro, C.; Garcia, K.; Xavier, S. Learning Through Ferroelectric Domain Dynamics in Solid-State Synapses. Nat. Commun. 2017, 8, 14736. (19) Chanthbouala, A.; Crassous, A.; Garcia, V.; Bouzehouane, K.; Fusil, S.; Moya, X.; Allibe, J.; Dlubak, B.; Grollier, J.; Xavier, S.; Deranlot, C.; Moshar, A.; Proksch, R.; Mathur, N. D.; Bibes, M.; Barthelemy, A. Solid-State Memories Based on Ferroelectric Tunnel Junctions. Nat. Nanotechnol. 2012, 7, 101−104. (20) Hou, P.; Yang, K.; Ni, K.; Wang, J.; Zhong, X.; Liao, M.; Zheng, S. An Ultrathin Flexible Electronic Device Based on the Tunneling Effect: A Flexible Ferroelectric Tunnel Junction. J. Mater. Chem. C 2018, 6, 5193−5198. (21) Hou, P.; Wang, J.; Zhong, X. Investigation of Multilevel Data Storage in Silicon-based Polycrystalline Ferroelectric Tunnel Junction. Sci. Rep. 2017, 7, 4525.

and transferred onto the desired substrates, e.g., Si and metal-coated PET. The PET substrates were coated with Al2O3 (10 nm) and the following Pt (30 nm) or Ni (30 nm) by e-beam evaporation. After baking the samples at 60 °C for 2 h, the PMMA was washed by acetone. The top electrodes of 100 nm Au capped Pt (30 nm) were fabricated on the BTO/LSMO/metal-coated PET through standard photolithography, e-beam evaporation, and lift-off. 4.2. Thin Film Characterization and Electrical Measurements. 10 μm × 10 μm Au/Pt/BTO/LSMO tunnel junction devices were electrically characterized by measuring IV characteristics using Keithley 2635 electrometer. To switch the polarization orientation of the BTO barrier, voltage pulses with various amplitude and duration were applied by using a Tektronix AFG 3102 function generator. To ensure the genuine contact between the probe and top electrode, the capacitance of the device was checked by using a Keisight E4980 LCR meter prior to any electric measurement. The resistance states of the devices were read by using a small dc voltage of 0.2 V. The PFM characterizations were performed using Dual AC Resonance Tracking piezoresponse force microscopy (MFP-3D, Asylum Research). Conductive NSC14/Pt cantilevers were used. PFM hysteresis loops were recorded at off-field model. The structural information on the films was collected by high resolution X-ray diffraction (HR-XRD), using a Panalytical X’pert Pro diffractometer. 4.3. Scanning Transmission Electron Microscopy. TEM specimens were prepared using an FEI Scios focused ion beam (FIB) with standard lift-out procedures. STEM images were acquired using a double CEOS corrected (to third order), Schottky emission JEOL ARM-200F microscope operating at 200 kV. To reduce scan distortion and sample drift effects, each image is the sum of 20 short exposure images. Lattice parameters were measured from the difference between atom peak positions found by fitting 2D Gaussians. Values for each unit cell were then averaged along the in-plane direction with errors given as the standard error of the means.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b04738. More detailed set of data about transfer process, structural information and electrical properties for the flexible ferroelectric tunnel junction memristors (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zheng-Dong Luo: 0000-0003-3725-4912 Ana M. Sanchez: 0000-0002-8230-6059 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Z.D.L. acknowledges the Chancellor’s international scholarship of University of Warwick. We are grateful to Dr. M. Crouch and C. Maltby for technical support of device fabrication with photolithography. M.A. acknowledges the Wolfson Research Merit and Theo Murphy Blue-sky Awards of Royal Society. The work was partly supported by the EPSRC (UK) through grants no. EP/M022706/1, EP/P031544/1, and EP/P025803/ 1.

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REFERENCES

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DOI: 10.1021/acsami.9b04738 ACS Appl. Mater. Interfaces 2019, 11, 23313−23319

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ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.9b04738 ACS Appl. Mater. Interfaces 2019, 11, 23313−23319