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Bendable Bi(Fe0.95Mn0.05)O3 ferroelectric film directly on aluminum substrate
Journal Pre-proof Bendable Bi(Fe0.95Mn0.05)O3 ferroelectric film directly on aluminum substrate Jin Qian, Yingzi Wang, Ranran Liu, Xiangfu Xie, Xu Yan, Jinfeng Leng, Changhong Yang PII:
S0925-8388(20)30744-1
DOI:
https://doi.org/10.1016/j.jallcom.2020.154381
Reference:
JALCOM 154381
To appear in:
Journal of Alloys and Compounds
Received Date: 25 December 2019 Revised Date:
11 February 2020
Accepted Date: 14 February 2020
Please cite this article as: J. Qian, Y. Wang, R. Liu, X. Xie, X. Yan, J. Leng, C. Yang, Bendable Bi(Fe0.95Mn0.05)O3 ferroelectric film directly on aluminum substrate, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154381. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Credit author statement Jin Qian: Investigation, Writing - Original Draft. Yingzi Wang: Resources. Ranran Liu: Investigation. Xiangfu Xie: Investigation. Xu Yan: Investigation. Jinfeng Leng: Resources. Changhong Yang: Conceptualization, Writing - Original Draft, Writing - Review & Editing. All authors have given approval to the final version of the manuscript.
Bendable Bi(Fe0.95Mn0.05)O3 ferroelectric film directly on aluminum substrate Jin Qiana, b, Yingzi Wangb, Ranran Liub, Xiangfu Xieb, Xu Yanb, Jinfeng Lengb, Changhong Yanga, * a
Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials,
University of Jinan, Jinan 250022, China b
School of Materials Science and Engineering, University of Jinan, Jinan 250022, China
Abstract With the rapid development of intelligent electronic system, portable, wearable, and smart electronics with excellent electrical performance and mechanical deformation are needed. This work presents a simple one-step way to construct a bendable Bi(Fe0.95Mn0.05)O3 (BFOMn) film capacitor by the assistant of aluminum metal substrate, which can be used as the substrate and bottom electrode simultaneously. The BFOMn film shows a polycrystalline structure and dense surface morphology. The BFOMn ferroelectric element shows a large remanent polarization (Pr~68 µC/cm2) and saturated polarization (Ps~80 µC/cm2) due to the low leakage current (~2×10−5 A/cm2 at 150 kV/cm). A high dielectric constant of 196 and a small loss tangent of 0.05 are obtained at a frequency of 100 kHz. The film also displays outstanding thermal stability over a wide temperature range from 25 to 150 °C. More significantly, even the capacitor is bent to a minor radius of 4 mm, there is no obvious deterioration in polarization, and the film possesses excellent fatigue resistance (109 cycles). These findings are respected to promote the advancement of bendable BiFeO3-based ferroelectric memories for information storage and data processing, which will exhibit an anticipating future in next-generation smart flexible electronics.
Keywords: bendable; ferroelectric; BiFeO3 film; temperature endurance * Corresponding author. E-mail addresses: [email protected] (Changhong Yang)
1
1. Introduction As hot research topic in functional materials, ferroelectrics are widely used in electron devices, such as ferroelectric random access memories, pyroelectric detectors, and piezoelectric sensors due to their large electrical polarization, outstanding thermoelectric and piezoelectric properties [1-5]. For decades, it has dramatically accelerated the semiconductor industry for the rapid advancement in silicon-based microelectronics. Meanwhile, nanoscale ferroelectric element has sparked a technological revolution and vastly changed the production and life of humans. Now, in the present era of “Internet-of-Things”, as the novel material towards being flexible, lightweight, and portable, flexible electronics with superior bendability and stable polarization, which can realize compact integrated circuits for information storage and data processing, arouse the research interest around the world [6-8]. Moreover, the advancement space of flexible electronics is broader than that of the traditional Si-based in the area of implantable biosensors, roll-up displays, and smart mobile devices [9]. It is foreseeable that the ferroelectric memories which combine superior ferroelectric property with good bending endurance will promote the development of next-generation intelligent electronics. Attributed to the advantages of low cost, easy combination, and good mechanical flexibility, organic ferroelectrics have been widely studied. The typical representative is poly(vinylidene fluoride) (PVDF) and its copolymer with trifluoroethylene [P(VDF-TrFE)] [10,11]. However, the inferior thermal stability and fatigue life of ferroelectric polymer hinder its future applications in devices. Compared with the organic ferroelectric films, the inorganic ferroelectrics show larger remanent polarization, superior thermal stability, and fatigue endurance, such as the BiFeO3 (BFO)-based thin film and Bi3.25La0.75Ti3O12 film [12-14]. A fly in the ointment is that the perovskite film usually grows on rigid substrates. To realize the flexibility of the ferroelectric element, the method of “grow-transfer” was adopted. Though the method of transferring the film from a rigid substrate to a flexible plastic one is capable of solving the flexible problem, it experiences the extravagant and subtle setups as well as tedious multi-step processes [15,16]. Meanwhile, the method has a low output and poor mechanical stability. Considering the inferior temperature endurance of plastic substrate (~200 °C), it is impossible to grow film on the flexible plastic substrate directly due to the fact that the plastics can't bear the high crystallization temperature of perovskite oxide film (~600 °C). There is still a challenge on how to obtain a high-performance bendable inorganic ferroelectric film in an economical-efficient and simple one-step way. Encouraged by the challenge as mentioned above, some reports are focused on the flexible functional element with mica substrate [17-19]. 2
Nevertheless, the ferroelectric memory element on inorganic aluminum foil substrate has not been reported, and it enjoys unique advantages. The aluminum substrate, with a melting point of 660 °C, takes on the advantages such as recycling, favorable mechanical bendability, and high-temperature endurance, can fit the requirement for the growth of oxide film. Besides, excellent electrical conductivity makes it possible to be used as both substrate and bottom electrode [20], saving the electrode preparation procedure. At the same time, using the aluminum as substrate can greatly reduce the cost since it is an abundant resource. BFO, a room-temperature multiferroic material, shows a super-high Curie temperature (Tc~830 °C) and large remanent polarization (Pr~100 µC/cm2). It is considered to be a potential ferroelectric to be used in the information storage and data processing [12,21,22]. For pure BFO thin film, its deficiency is the leakage problem. As described by the following equations, the volatilization of Bi and valence fluctuation of Fe can produce oxygen vacancies, causing a serious leakage problem, which can deteriorate its ferroelectric performances: 2 2
+
→2
′
+
+3
→2
′′′
+3
•• ••
+
+
(1) (2)
It has been proved that Mn-doping is beneficial to improve the ferroelectric properties by reducing the leakage current for BFO film [23]. Accordingly, the Mn-doped BFO ferroelectric film can be fabricated on aluminum substrate to further enhance the ferroelectric performance, which will greatly promote the development of intelligent and flexible ferroelectric components. In this study, we present a practical method of preparing bendable Bi(Fe0.95Mn0.05)O3 (BFOMn) ferroelectric element with the support of aluminum substrate. The film shows a large saturated polarization (Ps) of 80 µC/cm2 and a Pr of 68 µC/cm2 as well as good fatigue endurance (109 cycles). Moreover, its ferroelectric properties display no visible deterioration after 109 cycles fatigue when the film is bent to a small bending radius of 4 mm. 2. Material and methods The BFOMn film was deposited on aluminum substrate by spin coating method followed by a layer-by-layer annealing process. Bismuth nitrate pentahydrate [Bi(NO3)3·5H2O], iron(III) nitrate nonahydrate [Fe(NO3)3·9H2O], and manganese (II) acetate tetrahydrate [(CH3COO)2Mn·4H2O] were selected as the raw materials, and then they are dissolved in the solvents of ethylene glycol (HOCH2CH2OH) and acetic acid (CH3COOH). To compensate the loss of bismuth at high temperature, 5 mol% excess bismuth was added. The final concentration of the precursor solution was 0.3 3
M. The precursor solution was spin-coated onto the clean aluminum substrate at 3000 rpm for 30 s firstly. A step of pyrolysis at 250 °C for 3 min was followed to volatilize organics. Then the sample was put into a rapid thermal annealing processor and annealed at 480 °C for 5 min. After that, the sample was cooled down to room temperature rapidly. By repeating the process of spin-coating and annealing, the BFOMn film with desirable thickness on the aluminum substrate was obtained. Finally, Au top electrodes were sputtered on the film surface. The crystallinity of BFOMn film was analyzed by X-ray diffraction (XRD, Bruker D8). The microscopic surface image of the film was characterized by field emission scanning electron microscopy (FESEM, Hitachi S-4200). The ferroelectric properties were measured using a ferroelectric tester (Precision Pro. Radiant Technologies). The thickness of BFOMn film was estimated to be 600 nm using a step profiler made by the Ambios Technology Company in USA. The insulating property was detected through a semiconductor characterization system (Keithley 4200). An impedance analyzer (HP4294A) was used to verify the dielectric properties. 3. Result and discussion Figure 1 Figure 1a shows the schematic diagrams of the preparation of bendable Au/BFOMn/Al ferroelectric memory element. In brief, the precursor solution was spin-coated onto the clean aluminum substrate. After the annealing step, sputtering the Au on the sample as top electrode to obtain the Au/BFOMn/Al structure. Figure 1b shows a clean aluminum substrate with an excellent mechanical bending property. The prepared bendable Au/BFOMn/Al ferroelectric element is shown in Figure 1c. It is noted that no crevice or breakage was seen in the bending state, suggesting that the fabricated ferroelectric memory with aluminum substrate is feasible for mechanical deformations. To comprehend the crystal structure of the sample, the crystal structure of BFOMn film was characterized by XRD pattern. The prepared BFOMn film exhibits a polycrystalline structure with a random orientation. Moreover, except for the diffraction peaks of the aluminum substrate, all the diffraction peaks match well with the rhombohedral distorted perovskite structure [24]. As shown in the FESEM surface morphology image (Figure 1e), the film possesses a relatively dense structure without visible local microcracks, suggesting the good crystallinity of BFOMn film grown on aluminum. 4
Figure 2 The ferroelectric property of BFOMn film in the original unbent state was investigated. And the polarization versus electric field (P-E) loops are shown in Figure 2a, which were measured from low E to 1600 kV/cm. It can be seen that the Pr and Ps are about 68 and 80 µC/cm2 respectively, and they are comparable to those BFO-based films grown on rigid counterparts, such as Sm-doped BFO on Nb-doped SrTiO3 [25], Bi0.85Yb0.15FeO3 on Pt/Ta/glass [26], and BFO on indium tin oxide/Si [27]. These parameters even no less than that of flexible PZT film grown on mica (Ps~78 µC/cm2, Pr~30 µC/cm2) [28]. The room-temperature P-E and corresponding switching current-electric field (I-E) loops for the flat BFOMn film at 10 kHz are shown in Figure 2b. There exist two sharp current peaks that can be attributed to the fast domain switching in the ferroelectric film [29]. In the I-E curve, the peaks of switching current corresponding electric field is the coercive field (EC), which is consistent with the EC obtained in the P-E loop. Moreover, the leakage current density versus electric field (J-E) curve of BFOMn film is presented in Figure 2c. It is visible that the leakage curves are slightly asymmetric between the positive and negative applied electric field. This may be because of the different interface state between the top Au/BFOMn and the bottom BFOMn/Al. The leakage current density of BFOMn (2×10−5 A/cm2 at 150 kV/cm) is one order of magnitude lower than other pure BFO film and is similar to other Mn-doped BFO film under the same electric field [30,31]. As is well-known, pure BFO film has serious leakage behavior due to the high concentration of oxygen vacancies. [32,33]. In this work, Mn substitution decreases the leakage current density of the BFOMn film by reducing the oxygen vacancies, as described by the following chemical equation: 1 ' (3) 2 Mn Fe + VO•• + O2 → 2 MnFe + OO 2 Figure 2d presents the frequency dependence of dielectric constant (εr) and loss
tangent (tanδ) for BFOMn film. With the frequency increasing from 1 kHz to 1 MHz, the εr shows a slight decrease (~196 at 100 kHz). Compared with Bi(1+x)Fe0.95Mn0.05O3 (x=-0.05-0.1) [34], the values of tanδ are relatively small (~0.05 at 100 kHz), which is of importance to address the problems of self-heating and thermal run away for dielectric capacitors [35]. Figure 3 Thermal stability is an important factor to determine the applicability of the materials in extreme environments. Considering that the ferroelectric memory works at various temperatures, the P-E loops over a wide temperature range from 25 to 5
150 °C are measured at a fixed electric field of 1200 kV/cm and 10 kHz. As shown in Figure 3a-f, with the temperature increasing from room temperature to 150 °C, the shape of the P-E loop has no appreciable changes. Figure 3g provides the temperature evolution of Ps and Pr. The values of Ps fluctuate between 35.8 and 38.3 µC/cm2, and Pr varies between 26.4 and 31.3 µC/cm2. By calculation, the changes of Ps and Pr are within 7% and 15%, respectively, suggesting good thermal stability and promising the application in wide range temperature. The thermal stability of the Au/BFOMn/Al structure is almost equaled to lead-based PZT flexible ferroelectric element [36]. Figure 4 A series of electrical measurements under different bending states are carried out to detect the bendability of the ferroelectric memory. The capacitor is rolled on the molds with various radii to produce different bending degrees, as shown in Figure 4a. Figure 4b shows that P-E loops of the BFOMn film measured at 1600 kV/cm and 10 kHz with different compressive/tensile bending radii (from 12 to 2 mm). It is evident that all loops almost overlap with each other under a set of bending conditions. Figure 4c shows the changes of Ps, Pr, and EC with bending radius. It is found that the values of the three parameters, namely Pr~68 µC/cm2, Ps~80 µC/cm2, and coercive field (EC+~1187 kV/cm, EC-~1036 kV/cm), are almost consistent with those in the unbent state, indicating that mechanical bending has little effect on the electrical polarization. The J-E plots of BFOMn sample under various bending radii are shown in Figure 4d. At 400 kV/cm, almost the same J can be observed under different bending states, which indicates that the influence of physical bending on the leakage behavior can be ignored. Figure 5 Fatigue stability is one of the crucial properties of the ferroelectric film. Figure 5a-c illustrate the P-E hysteresis loops before and after 109 fatigue cycles for the samples under flat and bent conditions with a small radius of 4 mm. The values of Pr, Ps, and EC show slight changes with small variation range (less than 0.4%) after bending deformations. Figure 5d shows the normalized ∆P versus switching cycles for the BFOMn sample under unbent and compress/tensile strains at a radius of 4 mm. The polarization also maintains well even after 109 switching cycles, which indicates the BFOMn film is fatigue endurable and is immune to the bending process. These superior performances are resulted from the stable insulating characteristic under different bending states. In brief, the BFOMn film grown on aluminum substrate possesses outstanding fatigue endurance and mechanical stability and it will have broad future in ferroelectric memory applications for some special curved circumstances. 6
4. Conclusion In summary, the flexible polycrystalline BFOMn film capacitor was deposited on the aluminum substrate successfully through a chemical solution deposition approach. The fabricated film exhibits robust Ps~80 µC/cm2 and Pr~68 µC/cm2 and excellent thermal stability. Furthermore, it can be safely bent to a series of radii from 12 to 2 mm without any obvious deterioration in the values of Ps, Pr, and leakage current density. Meanwhile, strong ferroelectric fatigue resistance up to 109 cycles is realized in the film even it under compressive/tensile bending state at r=4 mm. These results demonstrate that it is feasible to grow bendable BFOMn film directly on aluminum substrate. This work will significantly promote the state of the art of portability for smart electric devices and reduce their costs. Conflict of interest All authors declare that there is no conflict of interests regarding the publication of this paper. Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 51972144, 51871111, 51803109), the Key Research and Development Program of
Shandong
Province
Undergraduate
Training
(2019GGX102015), Program
for
and
the
Innovation
Shandong and
Provincial
Entrepreneurship
(S201910427014).
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Figure captions Figure 1. (a) Schematic diagrams of preparation and characterization of the bendable Au/BFOMn/Al ferroelectric memory element. (b) Photograph of the bendable aluminum substrate. (c) The prepared BFOMn film on aluminum substrate. (d) XRD pattern of BFOMn film on Al substract. (e) SEM image of the surface morphology of BFOMn film. Figure 2. (a) Room-temperature P-E hysteresis loops of flat Au/BFOMn/Al capacitor measured at 10 kHz. (b) P-E loop and corresponding switching current curve under 1600 kV/cm at 25°C and 10 kHz. (c) Insulation property of Au/BFOMn/Al capacitor under flat condition. (d) The frequency dependence of dielectric content and loss tangent of Au/BFOMn/Al capacitor. Figure 3. (a-f) P-E loops measured at 10 kHz at temperatures from 25 to 150 °C of the BFOMn film capacitor. (g) Saturation and remanent polarizations as functions of temperatures. Figure 4. (a) Schematic diagrams of electrical measurements for the BFOMn film capacitor in compressive and tensile states. (b) P-E loops of the Au/BFOMn/Al capacitor under various bending radii. (c) Ps, Pr and EC changes as functions of bending radii. (d) J-E plots of the bendable Au/BFOMn/Al capacitor under different bending conditions. Figure 5. The P-E loop evolution process of Au/BFOMn/Al structure before and after 109 fatigue cycles under (a) flat state; (b) compressive bending state at r=4mm; (c) tensile bending states at r=4 mm. (d) Fatigue characteristics for BFOMn sample under flat and bent conditions at a radius of 4 mm.
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Highlights ► The bendable Bi(Fe0.95Mn0.05)O3 film exhibits superior ferroelectric property with Ps~80 µC/cm2 and Pr ~68 µC/cm2. ►In a wide temperature range from 25 to 150 °C, the film shows excellent thermal stability. ► The ferroelectric performance maintains well under various bending radii of 12-2mm. ►Outstanding stability of the film against cycle fatigue (109) is also realized in the film.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0925-8388(20)30744-1
DOI:
https://doi.org/10.1016/j.jallcom.2020.154381
Reference:
JALCOM 154381
To appear in:
Journal of Alloys and Compounds
Received Date: 25 December 2019 Revised Date:
11 February 2020
Accepted Date: 14 February 2020
Please cite this article as: J. Qian, Y. Wang, R. Liu, X. Xie, X. Yan, J. Leng, C. Yang, Bendable Bi(Fe0.95Mn0.05)O3 ferroelectric film directly on aluminum substrate, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154381. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Credit author statement Jin Qian: Investigation, Writing - Original Draft. Yingzi Wang: Resources. Ranran Liu: Investigation. Xiangfu Xie: Investigation. Xu Yan: Investigation. Jinfeng Leng: Resources. Changhong Yang: Conceptualization, Writing - Original Draft, Writing - Review & Editing. All authors have given approval to the final version of the manuscript.
Bendable Bi(Fe0.95Mn0.05)O3 ferroelectric film directly on aluminum substrate Jin Qiana, b, Yingzi Wangb, Ranran Liub, Xiangfu Xieb, Xu Yanb, Jinfeng Lengb, Changhong Yanga, * a
Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials,
University of Jinan, Jinan 250022, China b
School of Materials Science and Engineering, University of Jinan, Jinan 250022, China
Abstract With the rapid development of intelligent electronic system, portable, wearable, and smart electronics with excellent electrical performance and mechanical deformation are needed. This work presents a simple one-step way to construct a bendable Bi(Fe0.95Mn0.05)O3 (BFOMn) film capacitor by the assistant of aluminum metal substrate, which can be used as the substrate and bottom electrode simultaneously. The BFOMn film shows a polycrystalline structure and dense surface morphology. The BFOMn ferroelectric element shows a large remanent polarization (Pr~68 µC/cm2) and saturated polarization (Ps~80 µC/cm2) due to the low leakage current (~2×10−5 A/cm2 at 150 kV/cm). A high dielectric constant of 196 and a small loss tangent of 0.05 are obtained at a frequency of 100 kHz. The film also displays outstanding thermal stability over a wide temperature range from 25 to 150 °C. More significantly, even the capacitor is bent to a minor radius of 4 mm, there is no obvious deterioration in polarization, and the film possesses excellent fatigue resistance (109 cycles). These findings are respected to promote the advancement of bendable BiFeO3-based ferroelectric memories for information storage and data processing, which will exhibit an anticipating future in next-generation smart flexible electronics.
Keywords: bendable; ferroelectric; BiFeO3 film; temperature endurance * Corresponding author. E-mail addresses: [email protected] (Changhong Yang)
1
1. Introduction As hot research topic in functional materials, ferroelectrics are widely used in electron devices, such as ferroelectric random access memories, pyroelectric detectors, and piezoelectric sensors due to their large electrical polarization, outstanding thermoelectric and piezoelectric properties [1-5]. For decades, it has dramatically accelerated the semiconductor industry for the rapid advancement in silicon-based microelectronics. Meanwhile, nanoscale ferroelectric element has sparked a technological revolution and vastly changed the production and life of humans. Now, in the present era of “Internet-of-Things”, as the novel material towards being flexible, lightweight, and portable, flexible electronics with superior bendability and stable polarization, which can realize compact integrated circuits for information storage and data processing, arouse the research interest around the world [6-8]. Moreover, the advancement space of flexible electronics is broader than that of the traditional Si-based in the area of implantable biosensors, roll-up displays, and smart mobile devices [9]. It is foreseeable that the ferroelectric memories which combine superior ferroelectric property with good bending endurance will promote the development of next-generation intelligent electronics. Attributed to the advantages of low cost, easy combination, and good mechanical flexibility, organic ferroelectrics have been widely studied. The typical representative is poly(vinylidene fluoride) (PVDF) and its copolymer with trifluoroethylene [P(VDF-TrFE)] [10,11]. However, the inferior thermal stability and fatigue life of ferroelectric polymer hinder its future applications in devices. Compared with the organic ferroelectric films, the inorganic ferroelectrics show larger remanent polarization, superior thermal stability, and fatigue endurance, such as the BiFeO3 (BFO)-based thin film and Bi3.25La0.75Ti3O12 film [12-14]. A fly in the ointment is that the perovskite film usually grows on rigid substrates. To realize the flexibility of the ferroelectric element, the method of “grow-transfer” was adopted. Though the method of transferring the film from a rigid substrate to a flexible plastic one is capable of solving the flexible problem, it experiences the extravagant and subtle setups as well as tedious multi-step processes [15,16]. Meanwhile, the method has a low output and poor mechanical stability. Considering the inferior temperature endurance of plastic substrate (~200 °C), it is impossible to grow film on the flexible plastic substrate directly due to the fact that the plastics can't bear the high crystallization temperature of perovskite oxide film (~600 °C). There is still a challenge on how to obtain a high-performance bendable inorganic ferroelectric film in an economical-efficient and simple one-step way. Encouraged by the challenge as mentioned above, some reports are focused on the flexible functional element with mica substrate [17-19]. 2
Nevertheless, the ferroelectric memory element on inorganic aluminum foil substrate has not been reported, and it enjoys unique advantages. The aluminum substrate, with a melting point of 660 °C, takes on the advantages such as recycling, favorable mechanical bendability, and high-temperature endurance, can fit the requirement for the growth of oxide film. Besides, excellent electrical conductivity makes it possible to be used as both substrate and bottom electrode [20], saving the electrode preparation procedure. At the same time, using the aluminum as substrate can greatly reduce the cost since it is an abundant resource. BFO, a room-temperature multiferroic material, shows a super-high Curie temperature (Tc~830 °C) and large remanent polarization (Pr~100 µC/cm2). It is considered to be a potential ferroelectric to be used in the information storage and data processing [12,21,22]. For pure BFO thin film, its deficiency is the leakage problem. As described by the following equations, the volatilization of Bi and valence fluctuation of Fe can produce oxygen vacancies, causing a serious leakage problem, which can deteriorate its ferroelectric performances: 2 2
+
→2
′
+
+3
→2
′′′
+3
•• ••
+
+
(1) (2)
It has been proved that Mn-doping is beneficial to improve the ferroelectric properties by reducing the leakage current for BFO film [23]. Accordingly, the Mn-doped BFO ferroelectric film can be fabricated on aluminum substrate to further enhance the ferroelectric performance, which will greatly promote the development of intelligent and flexible ferroelectric components. In this study, we present a practical method of preparing bendable Bi(Fe0.95Mn0.05)O3 (BFOMn) ferroelectric element with the support of aluminum substrate. The film shows a large saturated polarization (Ps) of 80 µC/cm2 and a Pr of 68 µC/cm2 as well as good fatigue endurance (109 cycles). Moreover, its ferroelectric properties display no visible deterioration after 109 cycles fatigue when the film is bent to a small bending radius of 4 mm. 2. Material and methods The BFOMn film was deposited on aluminum substrate by spin coating method followed by a layer-by-layer annealing process. Bismuth nitrate pentahydrate [Bi(NO3)3·5H2O], iron(III) nitrate nonahydrate [Fe(NO3)3·9H2O], and manganese (II) acetate tetrahydrate [(CH3COO)2Mn·4H2O] were selected as the raw materials, and then they are dissolved in the solvents of ethylene glycol (HOCH2CH2OH) and acetic acid (CH3COOH). To compensate the loss of bismuth at high temperature, 5 mol% excess bismuth was added. The final concentration of the precursor solution was 0.3 3
M. The precursor solution was spin-coated onto the clean aluminum substrate at 3000 rpm for 30 s firstly. A step of pyrolysis at 250 °C for 3 min was followed to volatilize organics. Then the sample was put into a rapid thermal annealing processor and annealed at 480 °C for 5 min. After that, the sample was cooled down to room temperature rapidly. By repeating the process of spin-coating and annealing, the BFOMn film with desirable thickness on the aluminum substrate was obtained. Finally, Au top electrodes were sputtered on the film surface. The crystallinity of BFOMn film was analyzed by X-ray diffraction (XRD, Bruker D8). The microscopic surface image of the film was characterized by field emission scanning electron microscopy (FESEM, Hitachi S-4200). The ferroelectric properties were measured using a ferroelectric tester (Precision Pro. Radiant Technologies). The thickness of BFOMn film was estimated to be 600 nm using a step profiler made by the Ambios Technology Company in USA. The insulating property was detected through a semiconductor characterization system (Keithley 4200). An impedance analyzer (HP4294A) was used to verify the dielectric properties. 3. Result and discussion Figure 1 Figure 1a shows the schematic diagrams of the preparation of bendable Au/BFOMn/Al ferroelectric memory element. In brief, the precursor solution was spin-coated onto the clean aluminum substrate. After the annealing step, sputtering the Au on the sample as top electrode to obtain the Au/BFOMn/Al structure. Figure 1b shows a clean aluminum substrate with an excellent mechanical bending property. The prepared bendable Au/BFOMn/Al ferroelectric element is shown in Figure 1c. It is noted that no crevice or breakage was seen in the bending state, suggesting that the fabricated ferroelectric memory with aluminum substrate is feasible for mechanical deformations. To comprehend the crystal structure of the sample, the crystal structure of BFOMn film was characterized by XRD pattern. The prepared BFOMn film exhibits a polycrystalline structure with a random orientation. Moreover, except for the diffraction peaks of the aluminum substrate, all the diffraction peaks match well with the rhombohedral distorted perovskite structure [24]. As shown in the FESEM surface morphology image (Figure 1e), the film possesses a relatively dense structure without visible local microcracks, suggesting the good crystallinity of BFOMn film grown on aluminum. 4
Figure 2 The ferroelectric property of BFOMn film in the original unbent state was investigated. And the polarization versus electric field (P-E) loops are shown in Figure 2a, which were measured from low E to 1600 kV/cm. It can be seen that the Pr and Ps are about 68 and 80 µC/cm2 respectively, and they are comparable to those BFO-based films grown on rigid counterparts, such as Sm-doped BFO on Nb-doped SrTiO3 [25], Bi0.85Yb0.15FeO3 on Pt/Ta/glass [26], and BFO on indium tin oxide/Si [27]. These parameters even no less than that of flexible PZT film grown on mica (Ps~78 µC/cm2, Pr~30 µC/cm2) [28]. The room-temperature P-E and corresponding switching current-electric field (I-E) loops for the flat BFOMn film at 10 kHz are shown in Figure 2b. There exist two sharp current peaks that can be attributed to the fast domain switching in the ferroelectric film [29]. In the I-E curve, the peaks of switching current corresponding electric field is the coercive field (EC), which is consistent with the EC obtained in the P-E loop. Moreover, the leakage current density versus electric field (J-E) curve of BFOMn film is presented in Figure 2c. It is visible that the leakage curves are slightly asymmetric between the positive and negative applied electric field. This may be because of the different interface state between the top Au/BFOMn and the bottom BFOMn/Al. The leakage current density of BFOMn (2×10−5 A/cm2 at 150 kV/cm) is one order of magnitude lower than other pure BFO film and is similar to other Mn-doped BFO film under the same electric field [30,31]. As is well-known, pure BFO film has serious leakage behavior due to the high concentration of oxygen vacancies. [32,33]. In this work, Mn substitution decreases the leakage current density of the BFOMn film by reducing the oxygen vacancies, as described by the following chemical equation: 1 ' (3) 2 Mn Fe + VO•• + O2 → 2 MnFe + OO 2 Figure 2d presents the frequency dependence of dielectric constant (εr) and loss
tangent (tanδ) for BFOMn film. With the frequency increasing from 1 kHz to 1 MHz, the εr shows a slight decrease (~196 at 100 kHz). Compared with Bi(1+x)Fe0.95Mn0.05O3 (x=-0.05-0.1) [34], the values of tanδ are relatively small (~0.05 at 100 kHz), which is of importance to address the problems of self-heating and thermal run away for dielectric capacitors [35]. Figure 3 Thermal stability is an important factor to determine the applicability of the materials in extreme environments. Considering that the ferroelectric memory works at various temperatures, the P-E loops over a wide temperature range from 25 to 5
150 °C are measured at a fixed electric field of 1200 kV/cm and 10 kHz. As shown in Figure 3a-f, with the temperature increasing from room temperature to 150 °C, the shape of the P-E loop has no appreciable changes. Figure 3g provides the temperature evolution of Ps and Pr. The values of Ps fluctuate between 35.8 and 38.3 µC/cm2, and Pr varies between 26.4 and 31.3 µC/cm2. By calculation, the changes of Ps and Pr are within 7% and 15%, respectively, suggesting good thermal stability and promising the application in wide range temperature. The thermal stability of the Au/BFOMn/Al structure is almost equaled to lead-based PZT flexible ferroelectric element [36]. Figure 4 A series of electrical measurements under different bending states are carried out to detect the bendability of the ferroelectric memory. The capacitor is rolled on the molds with various radii to produce different bending degrees, as shown in Figure 4a. Figure 4b shows that P-E loops of the BFOMn film measured at 1600 kV/cm and 10 kHz with different compressive/tensile bending radii (from 12 to 2 mm). It is evident that all loops almost overlap with each other under a set of bending conditions. Figure 4c shows the changes of Ps, Pr, and EC with bending radius. It is found that the values of the three parameters, namely Pr~68 µC/cm2, Ps~80 µC/cm2, and coercive field (EC+~1187 kV/cm, EC-~1036 kV/cm), are almost consistent with those in the unbent state, indicating that mechanical bending has little effect on the electrical polarization. The J-E plots of BFOMn sample under various bending radii are shown in Figure 4d. At 400 kV/cm, almost the same J can be observed under different bending states, which indicates that the influence of physical bending on the leakage behavior can be ignored. Figure 5 Fatigue stability is one of the crucial properties of the ferroelectric film. Figure 5a-c illustrate the P-E hysteresis loops before and after 109 fatigue cycles for the samples under flat and bent conditions with a small radius of 4 mm. The values of Pr, Ps, and EC show slight changes with small variation range (less than 0.4%) after bending deformations. Figure 5d shows the normalized ∆P versus switching cycles for the BFOMn sample under unbent and compress/tensile strains at a radius of 4 mm. The polarization also maintains well even after 109 switching cycles, which indicates the BFOMn film is fatigue endurable and is immune to the bending process. These superior performances are resulted from the stable insulating characteristic under different bending states. In brief, the BFOMn film grown on aluminum substrate possesses outstanding fatigue endurance and mechanical stability and it will have broad future in ferroelectric memory applications for some special curved circumstances. 6
4. Conclusion In summary, the flexible polycrystalline BFOMn film capacitor was deposited on the aluminum substrate successfully through a chemical solution deposition approach. The fabricated film exhibits robust Ps~80 µC/cm2 and Pr~68 µC/cm2 and excellent thermal stability. Furthermore, it can be safely bent to a series of radii from 12 to 2 mm without any obvious deterioration in the values of Ps, Pr, and leakage current density. Meanwhile, strong ferroelectric fatigue resistance up to 109 cycles is realized in the film even it under compressive/tensile bending state at r=4 mm. These results demonstrate that it is feasible to grow bendable BFOMn film directly on aluminum substrate. This work will significantly promote the state of the art of portability for smart electric devices and reduce their costs. Conflict of interest All authors declare that there is no conflict of interests regarding the publication of this paper. Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 51972144, 51871111, 51803109), the Key Research and Development Program of
Shandong
Province
Undergraduate
Training
(2019GGX102015), Program
for
and
the
Innovation
Shandong and
Provincial
Entrepreneurship
(S201910427014).
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Figure captions Figure 1. (a) Schematic diagrams of preparation and characterization of the bendable Au/BFOMn/Al ferroelectric memory element. (b) Photograph of the bendable aluminum substrate. (c) The prepared BFOMn film on aluminum substrate. (d) XRD pattern of BFOMn film on Al substract. (e) SEM image of the surface morphology of BFOMn film. Figure 2. (a) Room-temperature P-E hysteresis loops of flat Au/BFOMn/Al capacitor measured at 10 kHz. (b) P-E loop and corresponding switching current curve under 1600 kV/cm at 25°C and 10 kHz. (c) Insulation property of Au/BFOMn/Al capacitor under flat condition. (d) The frequency dependence of dielectric content and loss tangent of Au/BFOMn/Al capacitor. Figure 3. (a-f) P-E loops measured at 10 kHz at temperatures from 25 to 150 °C of the BFOMn film capacitor. (g) Saturation and remanent polarizations as functions of temperatures. Figure 4. (a) Schematic diagrams of electrical measurements for the BFOMn film capacitor in compressive and tensile states. (b) P-E loops of the Au/BFOMn/Al capacitor under various bending radii. (c) Ps, Pr and EC changes as functions of bending radii. (d) J-E plots of the bendable Au/BFOMn/Al capacitor under different bending conditions. Figure 5. The P-E loop evolution process of Au/BFOMn/Al structure before and after 109 fatigue cycles under (a) flat state; (b) compressive bending state at r=4mm; (c) tensile bending states at r=4 mm. (d) Fatigue characteristics for BFOMn sample under flat and bent conditions at a radius of 4 mm.
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Highlights ► The bendable Bi(Fe0.95Mn0.05)O3 film exhibits superior ferroelectric property with Ps~80 µC/cm2 and Pr ~68 µC/cm2. ►In a wide temperature range from 25 to 150 °C, the film shows excellent thermal stability. ► The ferroelectric performance maintains well under various bending radii of 12-2mm. ►Outstanding stability of the film against cycle fatigue (109) is also realized in the film.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: