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Piezoelectric properties of bilayer ferroelectric thin films based on (1−x)[Ba(Zr0.2Ti0.8)O3] –x(Ba0.7Ca0.3TiO3)
Materials Letters 177 (2016) 68–70
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Piezoelectric properties of bilayer ferroelectric thin films based on (1 x)[Ba(Zr0.2Ti0.8)O3] –x(Ba0.7Ca0.3TiO3) Yemei Han n, Zhichao Zhang, Fang Wang, Kailiang Zhang n School of Electronic and Information Engineering, Tianjin Key Laboratory of Film Electronic & Communication Devices, Tianjin University of Technology, Tianjin 300384, China
art ic l e i nf o
a b s t r a c t
Article history: Received 22 February 2016 Received in revised form 17 April 2016 Accepted 20 April 2016 Available online 22 April 2016
We demonstrate that a bilayer heterostructure, comprised of a tetragonal (T) 0.7[Ba(Zr0.2Ti0.8)O3] 0.3 (Ba0.7Ca0.3TiO3) (0.7BZT-0.3BCT) film deposited on a rhombohedral (R) 0.3[Ba(Zr0.2Ti0.8)O3] 0.7 (Ba0.7Ca0.3TiO3) (0.3BZT-0.7BCT)film, on Pt/Ti/SiO2/Si substrates leads to a significant enhancement in piezoelectric properties. The observed enhancement in the piezoelectric properties could be attributed to that ferroelectric domains are tethered only by a soft R under layer, and not by the hard substrate. The piezoelectric constant is approximately 120 pm/V for the bilayer thin films, which is two orders of magnitude larger than what is observed in constrained single-layered 0.7BZT-0.3BCT thin films. Such tailoring of piezoelectric behaviors is very attractive for a variety of electromechanical devices. & 2016 Elsevier B.V. All rights reserved.
Keywords: Piezoelectricity Thin films 0.7[Ba(Zr0.2Ti0.8)O3]-0.3(Ba0.7Ca0.3TiO3) Ferroelectrics 0.3[Ba(Zr0.2Ti0.8)O3]-0.7(Ba0.7Ca0.3TiO3)
1. Introduction Piezoelectric materials have been widely used in energy conversion devices such as sensors, actuators, filters, and ferroelectric systems have been extensively investigated [1–5]. Heterostructured thin film ferroelectrics, multilayers or superlattices have displayed enhanced polarization and piezoelectric properties. These observations have been accounted for on the basis of electric-field-induced coupling, epitaxial strain, and specific polar interactions between the interfacial layers. Such ferroelastic interactions in ferroelectric bilayer lead to a giant piezoelectric response in a tetragonal (T) PbZr0.3Ti0.7O3 film deposited on a rhombohedral (R) PbZr0.7Ti0.3O3 film [2]. Later, Nagarajan et al. use nonlinear thermodynamics to analyze the observed enhancement, and they predict that the analysis can be extended to explain considerable improvements in ferroic properties reported in a number of multilayered ferroelectric systems [3]. In recent years, the environmental and health hazards of lead have been recognized. As a consequence, there is an increasing interest in the development of lead-free piezoelectric materials to develop materials with an equivalent or even higher piezoelectric response than the lead-based materials. Recent reports of a high piezoelectric coefficient d33 of 620 pC/N in the lead-free solid solution n
Corresponding authors. E-mail addresses: [email protected] (Y. Han), [email protected] (K. Zhang). http://dx.doi.org/10.1016/j.matlet.2016.04.154 0167-577X/& 2016 Elsevier B.V. All rights reserved.
0.5[Ba(Zr0.2Ti0.8)O3]-0.5(Ba0.7Ca0.3TiO3) have attracted a significant research interest [4–6], as this value exceeds even the d33 of soft PZT. This result has motivated further investigations of the whole solid solution (1 x)[Ba(Zr0.2Ti0.8)O3]-x(Ba0.7Ca0.3TiO3) thin film samples. Pulse laser deposition, sol–gel method as well as off-axis RF magnetron sputtering [7–11] have been used to synthesize the (1 x)[Ba(Zr0.2Ti0.8)O3]-x(Ba0.7Ca0.3TiO3) films. As is well known, the attempt of the thin film to change strain state has been clamped down by the solid substrate. In this work, bilayer ferroelectric lead-free thin films of (0.3BZT-0.7BCT)/(0.7BZT-0.3BCT) were successfully deposited on Pt/Ti/SiO2/Si substrates by RF magnetron sputtering. Microstructure and piezoelectric effect have been investigated for the novel bilayer thin films.
2. Experimental procedure Bilayer thin-film structures consisting of a 0.7BZT-0.3BCT film ( 200 nm) deposited on top of a 0.3BZT-0.7BCT film ( 200 nm) were prepared in this work. The ferroelectric lead-free 0.3BZT0.7BCT thin films were grown on Pt/Ti/SiO2/Si substrates by RF magnetron sputtering using ceramic targets of 0.3[Ba(Zr0.2Ti0.8) O3] 0.7 Ba0.7Ca0.3TiO3. The films were deposited with RF power of 60 W and the pressure of 2.5 Pa with Ar to O2 ratio of 20:15, the substrates were kept at 500 °C during the sputtering. And then 0.7BZT-0.3BCT films were deposited using ceramic targets of 0.7 [Ba(Zr0.2Ti0.8)O3] 0.3Ba0.7Ca0.3TiO3 with the same sputtering
Y. Han et al. / Materials Letters 177 (2016) 68–70
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conditions. After deposition, thin films were annealed at 800� in atmospheric environment. The phase compositions of the films were characterized by X-ray diffraction (XRD). Surface morphology and cross section of the bilayer thin films were characterized using Field-Emission Scanning Electron Microscopy (FE-SEM). Piezoresponse Force Microscopy (PFM, Agilent 5000) was used to analyze the piezoelectric and the ferroelectric properties.
3. Results and discussions The phase composition of the (0.3BZT-0.7BCT)/(0.7BZT-0.3BCT) bilayer films on the Pt/Ti/SiO2/Si substrates was characterized by XRD. The XRD patterns (shown in Fig. 1) of the layers demonstrate a preferred crystallization in the (111) direction, while minor (100) and (110) crystallization were detected. All the samples show a BaTiO3like perovskite structure, narrow range (37–42) XRD patterns for the films was shown in the inset which indicates that both 0.3BZT0.7BCT(111) and 0.7BZT-0.3BCT (111) were obtained. 0.3BZT-0.7BCT exhibits rhombohedral structure and the 0.7BZT-0.3BCT shows tetragonal structure. The diffraction patterns show no hints of secondary phases. Theoretical calculations for BZT-BCT thin films indicate that the (111) direction is favored for piezoelectric applications [7]. FE-SEM micrograph of cross-section of 0.7BZT-0.3BCT films on top of the 0.3BZT-0.7BCT films was given in Fig. 2. The cross-section SEM micrograph shows bilayer thin films on top of the Pt electrode, the thickness of each layer of films is approximately 200 nm. Crosssectional SEM image confirmed that there was minor intermixing between the functional layers. This is in agreement with the XRD analysis result that both 0.3BZT-0.7BCT(111) and 0.7BZT-0.3BCT (111) were obtained in the prepared bilayer structure. For measurements of converse piezoelectric coefficients d33 in all films, voltage was applied between the conductive tip and bottom electrode in the thickness direction. The piezoelectric constants of the samples were measured with poled ahead of time. In the poling process, the bilayer thin films were applied electric field of 1000 kV/cm for 10 min, the electric field was removed before the PFM measurements were performed. For comparison, we plot the polarization-hysteresis loops and butterfly loops for a standard 0.7BZT-0.3BCT single layer of the same composition as the present top T layer. The ferroelectric hysteresis loops of the bilayer films and the single layer films, shown in Fig. 3, show the typical characteristics of a ferroelectric material. For the bilayer thin films, the remnant polarization PS and coercive field EC obtained from the hysteresis are 16.6 μC/cm2 and 55 kV/cm, respectively. Also, one can
Fig. 1. XRD diffraction curve of (0.3BZT-0.7BCT)/(0.7BZT-0.3BCT) bilayer films on Pt/Ti/SiO2/Si, inlet shows narrow range (37–42) XRD patterns of for the films.
Fig. 2. SEM micrograph of the 200 nm/200 nm (0.3BZT-0.7BCT)/(0.7BZT-0.3BCT) thin films.
Fig. 3. Piezoelectric displacement vs voltage of (0.3BZT-0.7BCT)/(0.7BZT-0.3BCT) thin films.
see that the coercive field from the hysteresis loop for these bilayer thin films is much lower than that of the single layer 0.7BZT-0.3BCT film. It indicates that the presence of the 0.3BZT-0.7BCT layer might release the clamping effect to a great extent. In addition, polarization-hysteresis loop is a square-shaped one compared with that of the single layer 0.7BZT-0.3BCT films, which suggests that the polarization process might be improved with the presence of the 0.7BZT-0.3BCT under layer. However, the coercive field was still higher than the value of 1.68 kV/cm for the bulk BZT-BCT ceramic. The much higher coercive field came from the much smaller grains in comparison with the bulk ceramic and hence much more grain boundaries in thin film samples, and also the clamping effect [8]. The decrease of polarization with increasing field might be attributed to the leakage current through the thin films [12]. The ferroelectric hysteresis loops provide another good evidence for the well-crystallized (0.3BZT-0.7BCT)/(0.7BZT-0.3BCT) thin films, as shown in the XRD results in Fig. 1. The piezoelectric properties of the films are shown in Fig. 4. The PFM amplitudes exhibit a typically well-shaped butterfly loop, the effective piezoelectric constant of the bilayer thin films, obtained from the piezoelectric displacement curve yields an effective piezoelectric constant of nearly 120 pm/V, which is more than two times larger than that of the constrained single-layered thin films. The enhancement in the piezoelectricity can be attributed to
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Y. Han et al. / Materials Letters 177 (2016) 68–70
(T) 0.7BZT-0.3BCT (111) films on top of (R) 0.3BZT-0.7BCT (111) films, which is confirmed by the X-ray analysis and SEM cross sectional micrographs. Compared with the 0.7BZT-0.3BCT single layer films, the ferroelectric and piezoelectric properties have been greatly enhanced, the observed enhancement could be attributed to release of the clamping effect and interaction between the R and T layers. The piezoelectric constant is approximately 120 pm/ V, which is more than two orders of magnitude increase compared to their single-layer constituents.
Acknowledgement
Fig. 4. Ferroelectric hysteresis of (0.3BZT-0.7BCT)/(0.7BZT-0.3BCT) thin films.
the interlayer and interdomain interactions between the T and R layer and the release of the substrate clamping compared to single-layer films [3]. This is similar to the enhancement of piezoelectric properties for PbZrxTi1 xO3 (PZT) bilayer consisting of tetragonal (T) PZT and rhombohedral (R) PZT films. V. Anbusathaiah et al. demonstrated experimentally that ferroelastic interactions in a ferroelectric bilayer lead to a giant piezoelectric response, up to 3 times larger than what is normally observed in constrained single-layered ferroelectric thin films [8]. And they predict that such interactions between the polydomain structures in a ferroic multilayer lead to a significant overall enhancement in the ferroic properties compared to single-layer films. In general, the d33 values differ significantly for bulk materials and for thin films, the later ones being clamped to the substrate. The preceding effective piezoelectric constant of approximately 120 pm/ V is comparable to that of the pure PZT thin films. It has been reported that 0.5BZT-0.5BCT films prepared on Pt substrate by pulsed laser deposition exhibit a piezoelectric constant of about 78 pm/V [7]. 0.5BZT-0.5BCT films prepared by chemical solution deposition show piezoelectric constant of about 50 pm/V [8]. A. Piorra et al. have calculated that a d33 of 190–250 pm/V would be expected for 0.5BZT-0.5BCT films using the piezoelectric and elastic properties of BTO approximately [7]. This means that higher piezoelectric constant would be expected for the BZT-BCT bilayer thin films.
4. Conclusions In summary, we demonstrate enhanced pizeoelectric properties in ferroelectric bilayer thin-films. The bilayer films consist of
This work is supported by the National Natural Science Foundation of China (Grant nos. 51502204, 61274113 and 11204212), and Tianjin Natural Science Foundation (Grant nos. 13JCYBJC15700, 13JCZDJC26100 and 14JCQNJC00800), and Tianjin Science and Technology Developmental Funds of Universities and Colleges (Grant nos. 20100703, 20130701 and 20130702).
References [1] Y. Tian, Y. Gong, D. Meng, et al., Structure and electrical properties of Ir4 þ doped 0.5Ba0.9Ca0.1TiO3 0.5BaTi0.88Zr0.12O3 0.12%La ceramics via a modified Pechini method, Mater. Lett. 153 (2015) 44–46. [2] V. Anbusathaiah, D. Kan, F.C. Kartawidjaja, et al., Labile ferroelastic nanodomains in bilayered ferroelectric thin films, Adv. Mater. 21 (2009) 3497. [3] R. Mahjoub, S.P. Alpay, V. Nagarajan, Theory of giant electromechanical response from ferroelectric bilayers with polydomain structures due to interlayer and interdomain coupling, Phys. Rev. Lett. 105 (2010) 197601. [4] W. Liu, X. Ren, Large piezoelectric effect in Pb-free ceramics, Phys. Rev. Lett. 103 (2009) 257602. [5] M.C. Ehmke, J. Glaum, M. Hoffman, et al., In situ X-ray diffraction of biased ferroelastic switching in tetragonal Lead-free (1-x)Ba(Zr0.2Ti0.8)O3-x (Ba0.7Ca0.3)TiO3 piezoelectrics, J. Am. Ceram. Soc. 96 (9) (2013) 2913–2920. [6] L. Zhang, M. Zhang, L. Wang, et al., Phase transitions and the piezoelectricity around morphotropic phase boundary in Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 lead-free solid solution, Appl. Phys. Lett. 105 (2014) 162908. [7] A. Piorra, A. Petraru, H. Kohlstedt, et al., Piezoelectric properties of 0.5 (Ba0.7Ca0.3TiO3) 0.5[Ba(Zr0.2Ti0.8)O3] ferroelectric lead-free laser deposited thin films, J. Appl. Phys. 109 (2011) 104101. [8] Z. Wang, K. Zhao, X. Guo, et al., Crystallization, phase evolution and ferroelectric properties of sol–gel-synthesized Ba(Ti0.8Zr0.2)O3-x(Ba0.7Ca0.3)TiO3 thin films, J. Mater. Chem. C. 1 (2013) 522. [9] B.C. Luo, D.Y. Wang, M.M. Duan, et al., Orientation-dependent piezoelectric properties in lead-free epitaxial 0.5BaZr0.2Ti0.8O3 0.5Ba0.7Ca0.3TiO3 thin films, Appl. Phys. Lett. 103 (2013) 122903. [10] Z.M. Wang, Z.L. Cai, K. Zhao, et al., In-situ observation of nanomechanical behavior arising from critical-temperature-induced phase transformation in Ba(Zr0.2Ti0.8)O3 0.5(Ba0.7Ca0.3)TiO3 thin film, Appl. Phys. Lett. 103 (2013) 071902. [11] W. Bai, B. Shen, F. Fu, et al., Dielectric, ferroelectric, and piezoelectric properties of textured BZT-BCT lead-free thick film by screen printing, Mater. Lett. 83 (23) (2012) 20–22. [12] Y. Tian, Y. Gong, D. Meng, et al., Low-temperature sintering and electric properties of BCT-BZT and BCZT lead-free ceramics, J. Mater. Sci.: Mater. Electron. 26 (2015) 3750–3756.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Piezoelectric properties of bilayer ferroelectric thin films based on (1 x)[Ba(Zr0.2Ti0.8)O3] –x(Ba0.7Ca0.3TiO3) Yemei Han n, Zhichao Zhang, Fang Wang, Kailiang Zhang n School of Electronic and Information Engineering, Tianjin Key Laboratory of Film Electronic & Communication Devices, Tianjin University of Technology, Tianjin 300384, China
art ic l e i nf o
a b s t r a c t
Article history: Received 22 February 2016 Received in revised form 17 April 2016 Accepted 20 April 2016 Available online 22 April 2016
We demonstrate that a bilayer heterostructure, comprised of a tetragonal (T) 0.7[Ba(Zr0.2Ti0.8)O3] 0.3 (Ba0.7Ca0.3TiO3) (0.7BZT-0.3BCT) film deposited on a rhombohedral (R) 0.3[Ba(Zr0.2Ti0.8)O3] 0.7 (Ba0.7Ca0.3TiO3) (0.3BZT-0.7BCT)film, on Pt/Ti/SiO2/Si substrates leads to a significant enhancement in piezoelectric properties. The observed enhancement in the piezoelectric properties could be attributed to that ferroelectric domains are tethered only by a soft R under layer, and not by the hard substrate. The piezoelectric constant is approximately 120 pm/V for the bilayer thin films, which is two orders of magnitude larger than what is observed in constrained single-layered 0.7BZT-0.3BCT thin films. Such tailoring of piezoelectric behaviors is very attractive for a variety of electromechanical devices. & 2016 Elsevier B.V. All rights reserved.
Keywords: Piezoelectricity Thin films 0.7[Ba(Zr0.2Ti0.8)O3]-0.3(Ba0.7Ca0.3TiO3) Ferroelectrics 0.3[Ba(Zr0.2Ti0.8)O3]-0.7(Ba0.7Ca0.3TiO3)
1. Introduction Piezoelectric materials have been widely used in energy conversion devices such as sensors, actuators, filters, and ferroelectric systems have been extensively investigated [1–5]. Heterostructured thin film ferroelectrics, multilayers or superlattices have displayed enhanced polarization and piezoelectric properties. These observations have been accounted for on the basis of electric-field-induced coupling, epitaxial strain, and specific polar interactions between the interfacial layers. Such ferroelastic interactions in ferroelectric bilayer lead to a giant piezoelectric response in a tetragonal (T) PbZr0.3Ti0.7O3 film deposited on a rhombohedral (R) PbZr0.7Ti0.3O3 film [2]. Later, Nagarajan et al. use nonlinear thermodynamics to analyze the observed enhancement, and they predict that the analysis can be extended to explain considerable improvements in ferroic properties reported in a number of multilayered ferroelectric systems [3]. In recent years, the environmental and health hazards of lead have been recognized. As a consequence, there is an increasing interest in the development of lead-free piezoelectric materials to develop materials with an equivalent or even higher piezoelectric response than the lead-based materials. Recent reports of a high piezoelectric coefficient d33 of 620 pC/N in the lead-free solid solution n
Corresponding authors. E-mail addresses: [email protected] (Y. Han), [email protected] (K. Zhang). http://dx.doi.org/10.1016/j.matlet.2016.04.154 0167-577X/& 2016 Elsevier B.V. All rights reserved.
0.5[Ba(Zr0.2Ti0.8)O3]-0.5(Ba0.7Ca0.3TiO3) have attracted a significant research interest [4–6], as this value exceeds even the d33 of soft PZT. This result has motivated further investigations of the whole solid solution (1 x)[Ba(Zr0.2Ti0.8)O3]-x(Ba0.7Ca0.3TiO3) thin film samples. Pulse laser deposition, sol–gel method as well as off-axis RF magnetron sputtering [7–11] have been used to synthesize the (1 x)[Ba(Zr0.2Ti0.8)O3]-x(Ba0.7Ca0.3TiO3) films. As is well known, the attempt of the thin film to change strain state has been clamped down by the solid substrate. In this work, bilayer ferroelectric lead-free thin films of (0.3BZT-0.7BCT)/(0.7BZT-0.3BCT) were successfully deposited on Pt/Ti/SiO2/Si substrates by RF magnetron sputtering. Microstructure and piezoelectric effect have been investigated for the novel bilayer thin films.
2. Experimental procedure Bilayer thin-film structures consisting of a 0.7BZT-0.3BCT film ( 200 nm) deposited on top of a 0.3BZT-0.7BCT film ( 200 nm) were prepared in this work. The ferroelectric lead-free 0.3BZT0.7BCT thin films were grown on Pt/Ti/SiO2/Si substrates by RF magnetron sputtering using ceramic targets of 0.3[Ba(Zr0.2Ti0.8) O3] 0.7 Ba0.7Ca0.3TiO3. The films were deposited with RF power of 60 W and the pressure of 2.5 Pa with Ar to O2 ratio of 20:15, the substrates were kept at 500 °C during the sputtering. And then 0.7BZT-0.3BCT films were deposited using ceramic targets of 0.7 [Ba(Zr0.2Ti0.8)O3] 0.3Ba0.7Ca0.3TiO3 with the same sputtering
Y. Han et al. / Materials Letters 177 (2016) 68–70
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conditions. After deposition, thin films were annealed at 800� in atmospheric environment. The phase compositions of the films were characterized by X-ray diffraction (XRD). Surface morphology and cross section of the bilayer thin films were characterized using Field-Emission Scanning Electron Microscopy (FE-SEM). Piezoresponse Force Microscopy (PFM, Agilent 5000) was used to analyze the piezoelectric and the ferroelectric properties.
3. Results and discussions The phase composition of the (0.3BZT-0.7BCT)/(0.7BZT-0.3BCT) bilayer films on the Pt/Ti/SiO2/Si substrates was characterized by XRD. The XRD patterns (shown in Fig. 1) of the layers demonstrate a preferred crystallization in the (111) direction, while minor (100) and (110) crystallization were detected. All the samples show a BaTiO3like perovskite structure, narrow range (37–42) XRD patterns for the films was shown in the inset which indicates that both 0.3BZT0.7BCT(111) and 0.7BZT-0.3BCT (111) were obtained. 0.3BZT-0.7BCT exhibits rhombohedral structure and the 0.7BZT-0.3BCT shows tetragonal structure. The diffraction patterns show no hints of secondary phases. Theoretical calculations for BZT-BCT thin films indicate that the (111) direction is favored for piezoelectric applications [7]. FE-SEM micrograph of cross-section of 0.7BZT-0.3BCT films on top of the 0.3BZT-0.7BCT films was given in Fig. 2. The cross-section SEM micrograph shows bilayer thin films on top of the Pt electrode, the thickness of each layer of films is approximately 200 nm. Crosssectional SEM image confirmed that there was minor intermixing between the functional layers. This is in agreement with the XRD analysis result that both 0.3BZT-0.7BCT(111) and 0.7BZT-0.3BCT (111) were obtained in the prepared bilayer structure. For measurements of converse piezoelectric coefficients d33 in all films, voltage was applied between the conductive tip and bottom electrode in the thickness direction. The piezoelectric constants of the samples were measured with poled ahead of time. In the poling process, the bilayer thin films were applied electric field of 1000 kV/cm for 10 min, the electric field was removed before the PFM measurements were performed. For comparison, we plot the polarization-hysteresis loops and butterfly loops for a standard 0.7BZT-0.3BCT single layer of the same composition as the present top T layer. The ferroelectric hysteresis loops of the bilayer films and the single layer films, shown in Fig. 3, show the typical characteristics of a ferroelectric material. For the bilayer thin films, the remnant polarization PS and coercive field EC obtained from the hysteresis are 16.6 μC/cm2 and 55 kV/cm, respectively. Also, one can
Fig. 1. XRD diffraction curve of (0.3BZT-0.7BCT)/(0.7BZT-0.3BCT) bilayer films on Pt/Ti/SiO2/Si, inlet shows narrow range (37–42) XRD patterns of for the films.
Fig. 2. SEM micrograph of the 200 nm/200 nm (0.3BZT-0.7BCT)/(0.7BZT-0.3BCT) thin films.
Fig. 3. Piezoelectric displacement vs voltage of (0.3BZT-0.7BCT)/(0.7BZT-0.3BCT) thin films.
see that the coercive field from the hysteresis loop for these bilayer thin films is much lower than that of the single layer 0.7BZT-0.3BCT film. It indicates that the presence of the 0.3BZT-0.7BCT layer might release the clamping effect to a great extent. In addition, polarization-hysteresis loop is a square-shaped one compared with that of the single layer 0.7BZT-0.3BCT films, which suggests that the polarization process might be improved with the presence of the 0.7BZT-0.3BCT under layer. However, the coercive field was still higher than the value of 1.68 kV/cm for the bulk BZT-BCT ceramic. The much higher coercive field came from the much smaller grains in comparison with the bulk ceramic and hence much more grain boundaries in thin film samples, and also the clamping effect [8]. The decrease of polarization with increasing field might be attributed to the leakage current through the thin films [12]. The ferroelectric hysteresis loops provide another good evidence for the well-crystallized (0.3BZT-0.7BCT)/(0.7BZT-0.3BCT) thin films, as shown in the XRD results in Fig. 1. The piezoelectric properties of the films are shown in Fig. 4. The PFM amplitudes exhibit a typically well-shaped butterfly loop, the effective piezoelectric constant of the bilayer thin films, obtained from the piezoelectric displacement curve yields an effective piezoelectric constant of nearly 120 pm/V, which is more than two times larger than that of the constrained single-layered thin films. The enhancement in the piezoelectricity can be attributed to
70
Y. Han et al. / Materials Letters 177 (2016) 68–70
(T) 0.7BZT-0.3BCT (111) films on top of (R) 0.3BZT-0.7BCT (111) films, which is confirmed by the X-ray analysis and SEM cross sectional micrographs. Compared with the 0.7BZT-0.3BCT single layer films, the ferroelectric and piezoelectric properties have been greatly enhanced, the observed enhancement could be attributed to release of the clamping effect and interaction between the R and T layers. The piezoelectric constant is approximately 120 pm/ V, which is more than two orders of magnitude increase compared to their single-layer constituents.
Acknowledgement
Fig. 4. Ferroelectric hysteresis of (0.3BZT-0.7BCT)/(0.7BZT-0.3BCT) thin films.
the interlayer and interdomain interactions between the T and R layer and the release of the substrate clamping compared to single-layer films [3]. This is similar to the enhancement of piezoelectric properties for PbZrxTi1 xO3 (PZT) bilayer consisting of tetragonal (T) PZT and rhombohedral (R) PZT films. V. Anbusathaiah et al. demonstrated experimentally that ferroelastic interactions in a ferroelectric bilayer lead to a giant piezoelectric response, up to 3 times larger than what is normally observed in constrained single-layered ferroelectric thin films [8]. And they predict that such interactions between the polydomain structures in a ferroic multilayer lead to a significant overall enhancement in the ferroic properties compared to single-layer films. In general, the d33 values differ significantly for bulk materials and for thin films, the later ones being clamped to the substrate. The preceding effective piezoelectric constant of approximately 120 pm/ V is comparable to that of the pure PZT thin films. It has been reported that 0.5BZT-0.5BCT films prepared on Pt substrate by pulsed laser deposition exhibit a piezoelectric constant of about 78 pm/V [7]. 0.5BZT-0.5BCT films prepared by chemical solution deposition show piezoelectric constant of about 50 pm/V [8]. A. Piorra et al. have calculated that a d33 of 190–250 pm/V would be expected for 0.5BZT-0.5BCT films using the piezoelectric and elastic properties of BTO approximately [7]. This means that higher piezoelectric constant would be expected for the BZT-BCT bilayer thin films.
4. Conclusions In summary, we demonstrate enhanced pizeoelectric properties in ferroelectric bilayer thin-films. The bilayer films consist of
This work is supported by the National Natural Science Foundation of China (Grant nos. 51502204, 61274113 and 11204212), and Tianjin Natural Science Foundation (Grant nos. 13JCYBJC15700, 13JCZDJC26100 and 14JCQNJC00800), and Tianjin Science and Technology Developmental Funds of Universities and Colleges (Grant nos. 20100703, 20130701 and 20130702).
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