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Functional Inorganic Materials and Devices
Flexible, fatigue-free and large-scale Bi3.25La0.75Ti3O12 ferroelectric memories Liushuai Su, Xubing Lu, Lang Chen, Yaojin Wang, Guoliang Yuan, and Jun-Ming Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04781 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018
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ACS Applied Materials & Interfaces
Flexible, Fatigue-free and Large-scale Bi3.25La0.75Ti3O12 Ferroelectric Memories
Liushuai Su1, Xubing Lu2, Lang Chen3, Yaojin Wang1, Guoliang Yuan1,* and J. –M. Liu2,4,* 1
School of Materials Science and Engineering, Nanjing University of Science and Technology,
Nanjing 210094, China 2
Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, and Key
Laboratory of Quantum Engineering and Quantum Materials, South China Normal University, Guangzhou 510006, China 3
Department of Physics, Southern University of Science and Technology, Shenzhen 518055, China
4
National Laboratory of Solid State Microstructures and Innovative Center for Advanced
Microstructures, Nanjing University, Nanjing 210093, China
ABSTRACT: The flexible, fatigue-free, large-scale, and nonvolatile memory is an emerging technological goal in a variety of fields, including electronic skins, wearable devices, and other flexible electronics. The perovskite oxide films deposited on rigid substrates (e.g. Si and SrTiO3) at 500~700 oC and >1.0 Pa oxygen ambience have been widely used in electronic industries. However, their applications in flexible electronics become challenging if not impossible. Here, the Bi3.25La0.75Ti3O12 ferroelectric films with SrRuO3 or Pt electrodes were prepared on the two-dimensional mica substrates and then the flexible Pt/SrRuO3/Bi3.25La0.75Ti3O12/Pt memories have been achieved through reducing the mica to ~10 µm thickness. These memories show the saturated polarization of Ps ~ 20 µC/cm², and either the 0.1 mm thickness, the as-prepared FeRAMs are usually non-flexible as organic memories do. Flexible,9-23 fatigue-free, and large-scale13-14,20 memories have attracted considerable attention because of the development of electronic skins, wearable devices, and other flexible electronics. Highly favored memories should be thin, weight-light, printable, foldable, and stretchable to satisfy the requirements of miniature, flexibility, and super-integration with flexible electronics. Organic ferroelectrics12-13,15-17 or MoS2-based17 memories are flexible enough, however so far reported numbers of ferroelectric write/erase cycles are ~103 and the writing speed is much slower than those of BLT- and PZT-based memories.1,5-8,23 Although many oxide films are thin enough to be flexible, they are clamped by the rigid substrates, making a sufficient bending impossible. In addition, most high-quality perovskite oxide films can’t be obtained unless the deposition is done in a high substrate temperature (Ts) in an ambient of sufficient oxygen pressure (Po) (e.g. Ts > 600 oC and Po > 1.0 Pa).24-26 There are serious issues on the thermal stability of substrate against the ruthless circumstance for depositing the ferroelectric films and bottom electrodes etc. However, almost all flexible organic substrates, e.g. poly(ethylene terephthalate) (PET), polyethylene naphthalate (PEN), polyimide (PI), poly(methylmethacrylate) (PMMA), will decompose at such high temperature. Thus, it is still a big challenge to design and prepare the flexible oxide ferroelectric memories. Recently, some researchers peeled off oxide films (e.g. PZT and BaTiO3) from rigid substrates (e.g. Si and sapphire) by etching an intermediate layer and transferred these films to an organic ACS Paragon Plus Environment
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ACS Applied Materials & Interfaces
flexible substrate, enabling the flexible oxide ferroelectric memories and piezoelectric nano-generators/sensors.5,21-24 These memories or sensors can be bent forward-and-backward between the flat shape and curved shape with a bending radius R as small as ~ 5 mm. However, the traditional methods of peeling rigid substrate such as etching and delamination under laser illumination are too complex and expensive to produce large-size, reliable, and cost-effective FeRAMs. In order to settle down this problem, large-scale oxide ferroelectric memories directly deposited on inorganic flexible substrates in a simple and reliable way would be highly favored. Here, the large-scale Pt/SrRuO3(SRO)/BLT/Pt films were prepared on a thick mica substrate, and then the flexible mica/Pt/SRO/BLT/Pt nonvolatile ferroelectric memories are achieved through reducing the mica thickness to ~10 µm by mechanical force. These memories can not only undergo 109 write/erase cycles without any fatigue but also endure the bending with R = 1.4 mm for 10,000 cycles without any performance deterioration in 20-200 oC or under light illumination.
2. EXPERIMENTAL SECTION The 50-µm-thick mica substrates were separated from the (001) single-crystal fluorophlogopite mica (AlF2O10Si33Mg, Changchun Taiyuan Co., China) by mechanical exfoliation,13,27 and the ~200 nm Pt as the bottom electrode was grown at 700 oC and 10-4 Pa oxygen pressure by pulsed laser deposition (PLD) with a KrF excimer laser (248 nm wavelength). The ~20-nm-thick SRO film as a buffered layer and the 100-300-nm-thick BLT ferroelectric film were grown on mica/Pt at 700 oC and 13 Pa oxygen pressure with 110 mJ per laser pulse, and then the films were cooled to 1015 Ω⋅m), and high chemical and thermal stability (e..g. ACS Paragon Plus Environment
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ACS Applied Materials & Interfaces
the melting temperature of ~1375 oC, and the coefficient of linear expansion of ~1.1×10-4 oC-1), however its Young’s modulus is ~57 GPa and its Mohs hardness is 3-3.4 and a ≥0.1 mm thick mica cannot be safely bent to mm-scale radius (Table S1 in Supporting Information). Here the flexible mica crystal is separated from the mica plate by mechanical exfoliation along the (001) plane (Figure 1a and Figure S1 in Supporting Information).13,27 The thin mica crystal still keeps an ultra-flat surface with surface roughness less than 1.0 nm (Figure S1), which is an important factor as an ideal substrate. When an h1-thickness Pt/SRO/BLT/Pt multilayer grown on the h2-thickness mica substrate is bent to R radius, the maximum bending strain (δmax) satisfies the formula δmax ~ (h1+h2)/R. Since h1 109 cycles),1,2,5 high-temperature resistance (20-200 oC) and anti-radiation capability,29,30 compared with those memories based on PVDF11, PVDF:P3HT,12 P3HT:Au15, and MoS217-18 films in which information are commonly written/erased for only ≤104 cycles. Besides, many oxide thin films are separated from hard substrates and then are transferred to organic substrates to prepare flexible electronics nowadays. Although excellent properties can be achieved in a small region, it is still a big challenge to prepare flexible, large-scale and reliable
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ACS Applied Materials & Interfaces
electronics with this method. In many cases, the break of a local circuit will result in the complete failure of an expensive electronic device; however a film transferred from hard substrates to organic substrates cannot achieve 100% reliability on a large scale. Here the large-scale Pt/SRO/BLT/Pt memories are directly prepared on a cheap mica substrate in a simple way, thus they are reliable and cost-effective which are dominant factors in the memory market.
4. CONCLUSIONS The Pt/SRO/BLT/Pt memories are prepared on the two-dimensional mica crystal and then its flexibility is acquired through reducing mica to a ~10 µm thickness. The flexible memories show a Ps of ~20 µC/cm2 and a high dielectric tunability of ~86%. Most importantly, they do not show obvious deterioration even after they were written/erased for 109 cycles or were bent to 1.4 mm radius for 10,000 times. In these bending processes, the corresponding bending strain of
