Imprint Control of Nonvolatile Shape Memory with Asymmetric

Ferroelectric BiFeO3 nanodots formed in non-crystallized BiFeO3 thin-films via a local heating process using a heated atomic force microscope tip. Hyu...
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Imprint Control of Nonvolatile Shape Memory with Asymmetric Ferroelectric Multilayers Woo-Hee Kim,† Jong Yeog Son,*,‡ Young-Han Shin,*,§ and Hyun Myung Jang∥ †

Department of Chemical Engineering, Stanford University, 381 North-South Mall, Stanford, California 94305, United States Department of Applied Physics, Kyung Hee University, 1732 Deogyeong-daero, Yongin 446-701, Republic of Korea § Department of Physics and EHSRC, University of Ulsan, Ulsan 680-749, Republic of Korea ∥ Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), 77 Cheongam-Ro, Nam-Gu, Pohang 790-784, Republic of Korea ‡

S Supporting Information *

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artificial control of the imprint effect in ferroelectric hysteresis. More specifically, we show that the shift in ferroelectric hysteresis could be effectively modulated by adjusting the thickness of each ferroelectric layer or the thickness ratio in the ferroelectric multilayered schemes. On the basis of a theoretical firstprinciples calculation, we estimated characteristics of the depolarization field arising from an internal electric field in a Pt/PbTiO3 (PTO) BaTiO3 (BTO)/Pt multilayer structure. To experimentally achieve adjustable piezoelectric shape memories, we fabricated well-defined and epitaxially multilayered PTO/ BTO capacitors with the different thickness ratios, and we observed the asymmetric ferroelectric and piezoelectric hysteresis behaviors with different ratios of PTO and BTO layers. We start by schematically describing our desired shape memory scheme. To begin with, ferroelectric multilayered capacitors consisting of PTO and BTO are selected because both the ferroelectric materials possess the simple crystal structure of ABO3-type perovskite made up of the same B site atom of Ti and two different A site atoms of Pb and Ba with the same atomic valences, as shown in Figure 1a. In the multilayered ferroelectric structures, we speculate that internal electric fields originating from their asymmetric structures also exist. As Figure 1b shows, the induced internal electrical fields, however, can decrease with an increase in the ratio of PTO to BTO. Accordingly, the imprinted ferroelectric hysteresis loops switch back and forth from zero electric field to the magnitudes of the internal electric fields in accordance with the ratio of multilayers described in Figure 1c. The imprinted ferroelectric hysteresis loops then respond to the external strains of the ferroelectric multilayers. Figure 1d shows the strain curves of the imprinted ferroelectric multilayers as a function of the external electric fields, where the nonvolatile shape memory window emerges based on a shift in the strain curve. This result suggests that the control and optimization of the internal electric fields, which are associated with the thickness ratio of the multilayer ferroelectrics, is essential to obtain the maximized nonvolatile shape memory window. Besides that, it is worth noting that the PTO and BTO multilayer structures currently under investigation can be replaced by another pair of ferroelectric materials such as

erroelectric materials exhibit multifunctional properties such as piezoelectricity, nonvolatile charge, and electrooptic functions.1−5 Accordingly, ferroelectric materials have been an interesting subject of intensive scientific investigation, especially for ferroelectric nonvolatile random access memories (FeRAM) and actuators.5−10 Basically, ferroelectric memories utilize the remanent polarization originating from the employed ferroelectric materials themselves, enabling suitable operation by charge detection.11,12 Once electric polarization is changed by an external electric field, piezoelectric strain is generated under the biased states.13,14 Even though the applied electric field is switched off, two different polarized states still remain to be observed and thereby can be utilized for nonvolatile digital information. In contrast, piezoelectric strains cannot possess these nonvolatile characteristics with the two observable different states because the generated piezoelectric strains are returned to the same positions without an applied electric field.5 In other words, a symmetric piezoelectric butterfly curve is expected with regard to the piezoelectric strain axis, indicating that no shape memory window is available. Meanwhile, an imprint, which is a defect (e.g., fatigue) in a ferroelectric material, is known to generate a shift in the ferroelectric hysteresis loop, leading to a failure of stored ferroelectric information.15 Therefore, the symmetric piezoelectric butterfly curve can be shifted to be asymmetric in piezoelectric thin films by virtue of an electrical imprint field.16−18 It is interesting to note that the shifted hysteresis loop with an asymmetric piezoelectric strain provides a potential operating source for nonvolatile piezoelectric shape memory.5 The origin of this imprint effect in ferroelectric materials is the internal electric field induced by electrode configurations, trapped charge, and external processes such as optical and thermal processes.18−22 In terms of realizing the use of piezoelectric strains in ferroelectric materials, Morita et al. demonstrated that control of the imprint electric field, which is induced by treatment with a high electrical field of 3.5 kV/mm at 150 °C, could be used to produce viable piezoelectric shape memory devices.5 Motivated by these intriguing possibilities in innovative memory applications, we believe that the artificial manipulation of imprint effects is essential to achieve highly efficient nonvolatile shape memories.23 Herein, we propose that the ferroelectric multilayer structure composed of two different ferroelectric thin films enables © XXXX American Chemical Society

Received: August 12, 2014 Revised: November 19, 2014

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Figure 1. Schematic illustrations of ferroelectric multilayer capacitors, internal electric fields, imprinted ferroelectric hysteresis, and shape memory window. (a) Multilayered capacitors composed of PTO and BTO ferroelectrics with ratios of 1:1 and m:1, respectively. (b) Internal electric field plots versus ratio of PTO and BTO ferroelectric layers (1:1, 2:1, and 3:1). (c) Polarization curves versus electric field for the PTO and BTO ferroelectric layers with ratios of 1:1, 2:1 and 3:1, where imprinted ferroelectric hysteresis loops are generated by internal electric fields. (d) Strain curves versus electric field for the PTO and BTO ferroelectric layers with ratios of 1:1, 2:1, and 3:1. Once the external electric fields are switched off, a nonvolatile piezoelectric shape memory window arises from the shifts in strain curves.

Figure 2. Macroscopic-averaged electrostatic potentials (black curves) and average depolarization fields (blue arrows) obtained from firstprinciples calculations for (a) positively polarized PTO, (b) positively polarized BTO, (c) positively polarized BTO/PTO, and (d) negatively polarized BTO/PTO capacitors with Pt electrodes. Depolarization fields of multilayer TiO2-(BaO-TiO2)5-(PbO-TiO2)5 are not symmetric with respect to the polarization direction, which induces a negative internal field to shift its hysteresis loop in the positive field direction.

BiFeO3 and BiAlO3 for the purpose of shape memory applications.24,25 Next, we carried out first-principles calculations to better understand the feasibility of the proposed imprint characteristics at the atomic level in these multilayer structures of Pt/PTO/ BTO/Pt. Specifically, the estimations were performed by a localdensity approximation with a plane wave basis set and projector augmented wave method using the Vienna Ab-initio Simulation Package.26−28 The projector augmented wave (PAW) method was utilized with a cutoff energy of 500 eV.29 The exchange correlation functional was given by a generalized gradient approximation (GGA) of the Perdew−Burke−Ernzerhof version revised for solids (PBEsol).30 The Brillouin zone was divided by a 8 × 8 × 1 Monkhorst−Pack mesh.31 A vacuum region of about 15 Å was placed with the periodic boundary condition along z direction. First, we confirmed the macroscopic-averaged electrostatic potentials of two symmetric Pt5/TiO2-(PbO-TiO2)5/Pt5 and Pt5/TiO2-(BaO-TiO2)5/Pt5 capacitors as a reference in Figure 2a.32,33 For both the PTO system and the BTO system, 5 unit cells were utilized along the normal direction, and the top and bottom were terminated with 2 unit cells of Pt. The in-plane lattice parameters were set to 3.92 Å in a tetragonal structure, and the atoms were fully relaxed until the force on each atom was less than 0.001 eV/Å. Additionally, the ratio of the asymmetric ferroelectric capacitor was set to 1:1 with Pt5/TiO2-(BaOTiO2)5-(PbO-TiO2)5/Pt5 in Figure 2b. For the PTO−BTO mixed system, each 5 unit cells of BTO and PTO were employed

in conjunction with the same number of Pt layers. Its in-plane lattice parameter was also set to 3.92 Å. Here, the depolarization fields were calculated from the average slope of the macroscopicaveraged electrostatic potential. In the case of symmetric capacitors, the depolarization fields of PTO and BTO are −5.0 and −4.7 MV/cm, respectively, and their depolarization fields should be symmetric with respect to the polarization direction (Figure 2a,b). Considering the symmetry of depolarization fields in the single layer capacitors, the imprint effect does not seem to occur therein. On the contrary, depolarization fields in the asymmetric two-layer Pt5/TiO2-(BaO-TiO2)5-(PbO-TiO2)5/Pt5 capacitor are found to be asymmetric with respect to its polarization direction and thereby induce an internal electric field resulting in a shift in the hysteresis loop (Figure 2c,d). We fabricated ferroelectric multilayer capacitors similar to the ferroelectric multilayer capacitors employed for the firstprinciples calculations. These capacitors consisted of ferroelectric PTO and BTO layers with top and bottom SrRuO3 (SRO) electrodes on SrTiO3 (STO) substrates. Although Pt electrodes were utilized as a simple electrode in first-principles density-functional calculations, Pt cannot provide epitaxial ferroelectric thin films. In addition, the SRO electrode has B

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been known to exhibit a high endurance against polarization fatigue, allowing for the improved electrode−ferroelectric interfaces in comparison with the elemental metal electrodes such as Pt.34,35 For these reasons, we adopted epitaxial SRO electrodes grown by pulsed laser deposition (PLD) on the STO substrate, which is the base material for the epitaxial growth of upper PTO and BTO thin films. The same SRO is chosen as the top electrode material as well; the other purpose of this adoption is to remove the imprint effect that can originate from different top and bottom electrode configurations.36,37 First, the detailed structural properties were characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM) analyses for multilayer structures of PTO/BTO/SRO/STO. Figure 3a,b shows the theta−two theta (θ−2θ) XRD patterns for the multilayered structure of PTO (50 nm)/BTO (50 nm)/

Figure 4. Experimental characterization of epitaxial multilayer SRO/ PTO/BTO/SRO capacitors with thickness ratios of 5:1, 9:1, and 13:1, respectively. (a) Ferroelectric hysteresis loops of the three PTO/BTO capacitors at a measurement frequency of 10 kHz. (b) Internal electric fields of the three PTO/BTO capacitors as a function of thickness ratio. (c) Piezoelectric butterfly curves of the three PTO/BTO capacitors as a function of electric field.

Figure 3. Structural properties of the PTO/BTO/SRO/STO multilayer by (a−c) XRD and (d) TEM analyses. (a, b) out of plane θ−2θ patterns and (c) in-plane Φ-scan patterns of the PTO (50 nm)/BTO (50 nm)/ SRO (50 nm)/STO structure. Herein, the spectra of Φ-scans were obtained by keeping the Bragg angle at (102), (101), (111), and (110) for PTO, BTO, SRO, and STO, respectively. (d) Cross-sectional TEM image of the PTO (65 nm)/BTO (5 nm)/SRO (50 nm)/STO structure.

polarization versus electric field (P−E) curves of the three SRO/ PTO/BTO/SRO capacitors at a measurement frequency of 10 kHz. All the P−E hysteresis loops of the films exhibit a welldefined saturation with remnant polarizations (2Pr) over 33 μC/ cm2 and asymmetry in the direction of the positive electric fields due to the imprint effect, as expected from the theoretical calculations. These shifts in hysteresis loops indicate the existence of internal electric fields in the PTO/BTO multilayer structures. Estimations of the internal electric fields in the multilayered structures are shown in Figure 4b. Here, we can see a linear decrease in the internal fields with respect to the increasing thickness ratio. The highest value of the internal electric field was observed for the 5:1 sample. To further confirm the feasibility of these devices for nonvolatile shape memory applications, we investigated applied electric field dependence of the shape memory displacement based on piezoelectric force microscopy (PFM) measurements. Here, the shape memory displacement is defined as the difference between the displacements for the two stable states at the point of zero voltage. Figure 4c shows the asymmetric

SRO (50 nm)/STO. These data indicate that the PTO/BTO/ SRO films are highly (001)-oriented without any secondary phases along the (001) orientation of the STO substrate. The inplane heteroepitaxial relationship between the STO substrate and the multilayer PTO/BTO/SRO was further investigated via a phi (Φ)-scan shown in Figure 3c. From the relative positions of all in-plane peaks in the Φ-scan, it is clear that the three consecutive thin films of PTO, BTO, and SRO were grown epitaxially on the STO substrate, and the unit cells of PTO, BTO, and SRO were placed in parallel with the unit cell of the STO substrate along the in-plane direction. Figure 3d shows the crosssectional TEM image of the multilayer structure of PTO (65 nm)/BTO (5 nm)/SRO (50 nm)/STO, also indicating the epitaxial growth relationship constrained on the STO substrate. We then prepared epitaxially multilayered SRO/PTO/BTO/ SRO capacitors with three different thickness ratios of PTO to BTO (5:1, 9:1, and 13:1). Figure 4a shows the ferroelectric C

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(5) Morita, T.; Kadota, Y.; Hosaka, H. Appl. Phys. Lett. 2007, 90, 082909. (6) Scott, J. F. Science 2007, 315, 954−959. (7) Son, J. Y.; Lee, G.; Jo, M.-H.; Kim, H.; Jang, H. M.; Shin, Y.-H. J. Am. Chem. Soc. 2009, 131, 8386−8387. (8) Son, J. Y.; Shin, Y.-H.; Ryu, S.; Kim, H.; Jang, H. M. J. Am. Chem. Soc. 2009, 131, 14676−14678. (9) Son, J. Y.; Ryu, S.; Park, Y.-C.; Lim, Y.-T.; Shin, Y.-S.; Shin, Y.-H.; Jang, H. M. ACS Nano 2010, 4, 7315−7320. (10) Son, J. Y.; Lee, J.-H.; Song, S.; Shin, Y.-H.; Jang, H. M. ACS Nano 2013, 7, 5522−5529. (11) Cohen, R. E. Nature 1992, 358, 136−138. (12) Catalan, G.; Seidel, J.; Ramesh, R.; Scott, J. F. Rev. Mod. Phys. 2012, 84, 119−156. (13) Saito, Y.; Takao, H.; Tani, T.; Nonoyama, T.; Takatori, K.; Homma, T.; Nagaya, T.; Nakamura, M. Nature 2004, 432, 84−87. (14) Lee, J.-W.; Park, C.-S.; Jo, J.-H.; Kim, H.-E. Appl. Phys. Lett. 2007, 91, 072903. (15) Warren, W. L.; Dimos, D.; Pike, G. E.; Tuttle, B. A.; Raymond, M. V.; Ramesh, R.; Evans, J. T., Jr. Appl. Phys. Lett. 1995, 67, 866−868. (16) Kim, S.-H.; Lee, D.-S.; Hwang, C. S.; Kim, D.-J.; Kingon, A. I. Appl. Phys. Lett. 2000, 77, 3036−3038. (17) Schorn, P.; Ellerkmann, U.; Bolten, D.; Boettger, U.; Waser, R. Integr. Ferroelectr. 2003, 53, 361−369. (18) Morita, T.; Cho, Y. Appl. Phys. Lett. 2006, 88, 112908. (19) Lee, E. G.; Wouters, D. J.; Willems, G.; Maes, H. E. Appl. Phys. Lett. 1996, 69, 1223−1225. (20) Kim, S.-H.; Lee, D.-S.; Hwang, C. S.; Kim, D.-J.; Kingon, A. I. Appl. Phys. Lett. 2000, 77, 3036−3038. (21) Ye, Z.; Tang, M. H.; Zhou, Y. C.; Zheng, X. J.; Cheng, C. P.; Hu, Z. S.; Hu, H. P. Appl. Phys. Lett. 2007, 90, 042902. (22) Mokry, P.; Tagantsev, A. K.; Fousek, J. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 094110. (23) Kadota, Y.; Morita, T. Smart Mater. Struct. 2012, 21, 045002. (24) 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. Science 2003, 299, 1719−1722. (25) Zylberberg, J.; A. Belik, A.; Takayama-Muromachi, E.; Ye, Z.-G. Chem. Mater. 2007, 19, 6385−6390. (26) Kresse, G.; Hafner, J. Phys. Rev. B 1993, 48, 13115. (27) Kresse, G.; Furthmüller, J. Phys. Rev. B 1996, 54, 11169. (28) Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59, 1758. (29) Blöchl, P. E. Phys. Rev. B 1994, 50, 17953. (30) Perdew, J. P.; Ruzsinszky, A.; Csonka, G. I.; Vydrov, O. A.; Scuseria, G. E.; Constantin, L. A.; Zhou, X.; Burke, K. Phys. Rev. Lett. 2008, 100, 136406. (31) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188−5192. (32) Balderschi, A.; Baroni, S.; Resta, R. Phys. Rev. Lett. 1988, 61, 734. (33) Sai, N.; Kolpak, A. M.; Rappe, A. M. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 72, 020101. (34) Eom, C. B.; Van Dover, R. B.; Phillips, J. M.; Werder, D. J.; Marshall, J. H.; Chen, C. H.; Cava, R. J.; Fleming, R. M.; Fork, D. K. Appl. Phys. Lett. 1993, 63, 2570−2572. (35) Do, D.-H.; Evans, P. G.; Isaacs, E. D.; Kim, D. M.; Eom, C. B.; Dufresne, E. M. Nat. Mater. 2004, 3, 365−369. (36) Friessnegg, T.; Aggarwal, S.; Ramesh, R.; Nielsen, B.; Poindexter, E. H.; Keeble, D. J. Appl. Phys. Lett. 2000, 77, 127−129. (37) Wenbin, W.; Wang, Y.; Pang, G. K. H.; Wong, K. H.; Choy, C. L. Appl. Phys. Lett. 2004, 85, 1583−1585. (38) Shan-Tao, Z.; Alain Brice, K.; Emil, A.; Helmut, E.; Jurgen, R. Appl. Phys. Lett. 2007, 91, 112906. (39) Zhou, Z. H.; Gao, X. S.; John, W.; Fujihara, K.; Ramakrishna, S.; Nagarajan, V. Appl. Phys. Lett. 2007, 90, 052902.

butterfly curves, and notable shifts in the piezoelectric strains are clearly observed therein, which reflect the influence of internal electric fields in the ferroelectric multilayered structures. These ferroelectric multilayer structures possess two different strain states depending on the polarity of the applied electric fields prior to removal of the voltage. It is interesting to note that the windows in nonvolatile shape memories indicated at zero voltage were found to be about 1.7%, 0.6%, and 0.2% for thickness ratios of 5:1, 9:1, and 13:1, respectively. These strain values are significantly higher than previous publications on piezoelectric strains.38,39 Therefore, it is worth noting that the controllable strain gaps formed by the effective modulation of thickness ratios in PTO/BTO multilayer structures make these structures suitable for practical applications of nonvolatile shape memory. In summary, we proposed controllable piezoelectric shape memories composed of PTO/BTO multilayered structures, enabling artificial modulation for imprint characteristics induced by internal electric fields. On the basis of our first-principles calculations, we confirmed that the imprint characteristics could be triggered by the internal electric fields originating from asymmetric depolarization fields in the Pt/PTO/BTO/Pt multilayer structures. Then, PTO/BTO/SRO multilayers were prepared on a base STO substrate by PLD, and structural XRD and TEM analyses indicated the epitaxial growth relationship of the multilayered films on the (001) STO. To experimentally clarify the imprint effects, we fabricated SRO/PTO/BTO/SRO capacitors with three different thickness ratios of PTO to BTO (5:1, 9:1, and 13:1), and thereby shape memory windows with controllable strain gaps were demonstrated by the effective modulation of the thickness ratio in PTO/BTO multilayer structures. This new insight potentially provides opportunities for implementation of emerging piezoelectric shape memories.

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ASSOCIATED CONTENT

S Supporting Information *

(1) Experimental methods, including the preparation of epitaxial multilayer thin films and capacitors, characterization, and firstprinciples calculations. (2) Surface morphology of the epitaxially multilayered thin films composed of PTO(65 nm)/BTO(5 nm)/SRO(50 nm) on a STO substrate. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J. Y. Son). *E-mail: [email protected] (Y.-H. Shin). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (Nos. 2009-0093818, 2012-014007, and 2012R1A2A2A01046451) and by a grant from the Kyung Hee University in 2013 (KHU20130691).

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REFERENCES

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dx.doi.org/10.1021/cm5029782 | Chem. Mater. XXXX, XXX, XXX−XXX