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2)O3–PbTiO3 thin films
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 6307–6310 www.elsevier.com/locate/ceramint
Short communication
Growth and ferroelectric properties of sol–gel derived Bi(Mg1/2Zr1/2)O3–PbTiO3 thin films Linxing Zhanga, Jun Chena,c,n, Lu Yinb, Hanqing Zhaoa, Longlong Fana, Jiangli Caob, Xianran Xinga,c a Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, China Institute of Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China c State Key Laboratory for Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China
b
Received 30 July 2013; received in revised form 9 September 2013; accepted 27 September 2013 Available online 8 October 2013
Abstract The electrical properties included temperature-dependent polarization of (1 x)Bi(Mg1/2Zr1/2)O3–xPbTiO3 (BMZ–xPT) new ferroelectric films were investigated. The films with 220 nm thickness grown on Pt(111)/Ti/SiO2/Si substrates via sol–gel method were well crystallized with a phase-pure perovskite structure and homogeneous microstructure. Saturated polarization hysteresis loops are observed for all BMZ–xPT compounds, and BMZ–0.85PT films with high (100) orientation show a small leakage and remanent polarization of 36.1 μC cm 2, which is comparable to the (100)-oriented BiScO3–PbTiO3 thin films. The present films have high dielectric constants about 544-833. Furthermore, the polarization with elevated temperature slightly decreases, exhibiting stable ferroelectric properties and potentials for memory applications above room temperature such as non-volatile ferroelectric random access memories. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Films; A. Sol–gel processes; C. Dielectric properties; C. Electrical properties; C. Ferroelectric properties
1. Introduction Recently, great attention has been paid to the ferroelectric thin films with a perovskite structure because of high electrical performances and good temperature stability, which are desired for applications in automotive and aerospace industries such as non-volatile ferroelectric random access memories (FeRAM) and micro-electro-mechanical systems (MEMS) [1,2]. As potential candidates, bismuth-based perovskite ferroelectric materials with high Tc have been extensively studied, including BiMeO3–PbTiO3 (where Me=Sc3 þ , Fe3 þ , etc.) and Bi(Me1,Me2)O3–PbTiO3 (where Me1=Mg2 þ , Zn2 þ ,Ni2 þ , etc., and Me2=Ti4 þ , Zr4 þ , etc.) [3–5]. For instance, BiScO3– PbTiO3 (BS–PT) solid solutions with a morphotropic phase boundary (MPB) composition exhibit a high Tc ( 450 1C) and excellent piezoelectric performance (d33=460 pC N 1) [3]. n Corresponding author at: Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, China. Tel.: þ 86 10 62332525; fax: þ86 63 32525. E-mail address: [email protected] (J. Chen).
BS–PT films were also reported to exhibit good ferroelectric and piezoelectric properties [6]. However, the extensive and potential device applications of BS–PT are severely restricted by the high cost of scandium sources. Furthermore, BiFeO3– PbTiO3 (BF–PT) films exhibit good properties with a large remanent polarization (Pr) of 50 μC cm 2 [4]. But relatively high conductivity of the films makes it difficult to the observation of the ferroelectric polarization–electric field hysteresis loop. (1 x)Bi(Mg1/2Zr1/2)O3–xPbTiO3 (BMZ–xPT) is also another bismuth-based perovskite high Tc ferroelectric materials without expensive elements. It has been reported that BMZ–xPT solutions exhibit the nonmonotonic Tc trends arising from the microscopic coupling between A-site and B-site displacements. And the BMZ–xPT compositions with xZ0.75 have a higher Tc than 450 1C [5]. The BMZ–xPT system, which has the substitution of Mg and Zr ions by replacing Me in BiMeO3–PbTiO3, do not have the ions with variable valency such as Fe3 þ for BF–PT, resulting in small leakage. The piezoelectric ceramics of BMZ–xPT fabricated by the solid state reaction method show a high dielectric constant of 1387 and low coercive field of
0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2013.09.120
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23 kV/cm [7]. Since BMZ–xPT exhibit good performances such as high Tc, piezoelectric properties, low cost and conductivity, it is interesting to investigate the system. Furthermore, the similar system (1 x)Bi(Zn1/2Zr1/2)O3–xPbTiO3 (BZZ–xPT) films had been reported [8]. Although the difference between BZZ–PT and BMZ–PT is only one B-site element (Zn and Mg), acute difference such as solid solubility and polarization were emerged between the two systems [5]. It is noticeable to that the important role of Mg is indispensable in Bi(Me1,Me2)O3–PbTiO3. However, only few literatures reported BMZ–xPT ceramics and studies on thin films are still lacking. In this work, BMZ–xPT films were fabricated on Pt(111)/Ti/SiO2/Si substrates by a sol– gel deposition method. The ferroelectric, leakage current and dielectric properties were investigated at room temperature. For consideration as device applications above room temperature, the temperature dependence of ferroelectric property was also studied.
identical processes and conditions exhibit pure perovskite phase (Fig. 1(a)). Compared with BZZ–xPT, whose solid solubility is only x4 0.3, the x 40.55 of solid solubility for BMZ–xPT exhibit better tunability in composition. The reason comes down to the different role of zinc and magnesium ions. The present films with x ¼ 0.85 and 0.80 have a high (100) orientation, while the others show a random orientation. The BMZ–xPT show different preferential orientation changing from a-axis to random as the BMZ content increases. The reason for the preferential orientation would be the factors that lattice match [9], surface energy of planes [10], dry and pyrolysis temperature and so on. As we know, the c/a of BMZ–xPT solid solutions reduces from 1.065 of PbTiO3 (PT) to 1.02 (MPB of BMZ–xPT in the range of 0.55r x r 0.60)
2. Experimental procedure Lead acetate trihydrate, titanium n-butoxide, bismuth nitrate pentahydrate, magnesium nitrate hexhydrate, and zirconium nitrate pentahydrate were selected as starting reagents for the preparation of precursor solution. And 2-methoxyethanol and glacial acetic acid were used as solvents and stabilizer, respectively. 10 mol% Pb and 5 mol% Bi were added to compensate for the volatilization loss. A similar process of precursor solution was synthesized as reported in previous work [8]. Before spin coating, the precursors were filtrated with a 0.2 μm syringe filter to avoid particulate contamination and then deposited on Pt(111)/Ti/SiO2/Si substrates at a spinning rate of 4000 rpm for 30 s. Subsequently, the gel films were dried and pyrolyzed at 300 1C for 10 min and finally annealed at 700 1C for 1 min by rapid thermal processing (RTP) under oxygen environment. The steps were repeated several times to obtain about 200 nm thick BMZ–xPT thin films. X-ray diffraction (XRD, TTRШ, Rigaku, Japan) was used to characterize the crystalline structure of the films. Surface and cross-section microstructures of samples were examined by a field emission scanning electron microscope (FE-SEM, SUPRA-40, Carl Zeiss, Germany). The ferroelectric and leakage properties were measured by an aixACCT ferroelectric test system (TF-Analyser 1000, aixACCT, Germany) in combination with a temperature control system, which was consisted of a temperature controller, heating plate and insulation cover. The dielectric properties of the films were investigated by using a precision impedance analyser (4294A, Agilent, Palo alto, CA). 3. Results and discussion The as-growth films could present different preferential orientation along with various microstructures. The ability to understand the reasons of crystallographic orientation should be critical to control and create highly oriented films. The XRD of BMZ–xPT (0.55 r x r 0.85) films prepared under
Fig. 1. (a) XRD patterns, (b) scanning electron microscopy surface and (c) cross-sectional image of BMZ–xPT films.
L. Zhang et al. / Ceramics International 40 (2014) 6307–6310
with increasing BMZ content, due to a reducing c axis and increasing a axis [7]. The (100) preferential orientation of BMZ–xPT films (x Z 0.80) could be due to the lower surface energy of the (100) planes. These films (an a axis-oriented film) have a large c/a, and the long (c) axis could easily lie down on the growth face [9]. As the reducing c axis is close to a(b) axis with increasing BMZ content (xr 0.75), the comparative surface energy of the (100) and (001) planes would lead to a random orientation. The level of (100) orientation is about 0.96 for BMZ–85PT, calculated by a semiquantitative method using the Lotgering factor. The high orientation indicates the films have a very good texture. The representative SEM micrograph of the cross-sectional and surface image of BMZ–xPT films show a dense and uniform microstructure (Fig. 1(b)(c)). The clearly visible each layer of films/Pt/SiO2/Si exhibit well growth of the films on the substrate. As opposed to pure PT films, which often fracture through the ferroelectric phase transition during the cooling process, no evident cracks are observed in the BMZ–xPT films. The thickness and grain size of the films are about 220 nm and 50–100 nm, respectively. The ferroelectric hysteresis loops (P–E) of the BMZ–xPT films were measured with the frequency of 100 Hz at room temperature (Fig. 2). The films all exhibit saturated P–E hysteresis loops under electric fields of 12 V. The Pr value is 36.1, 27.8, 10.5 19.9 μC cm 2 as the PT content is 0.85, 0.75, 0.65, 0.55, meanwhile, with a coercive field (Ec) of 113, 106, 80, 125 kV cm 1, respectively. Note that the Pr reduces firstly and then increases with decreasing PT content. In upper PbTiO3 content, high Pr would be due to the unique contribution of strongly tetragonal phase. As reported, the polarization and Tc of the BMZ–xPT reduces as the x decreases from 0.85 to 0.55 by first-principles density functional theory calculations [5]. However, the MPB of BMZ–xPT is in the range of 0.55 r xr 0.60 [7], and the increasing Pr of x ¼ 0.55 could be ascribed to the coexistence contribution of both tetragonal and rhombohedral perovskite phase in the MPB. In comparison, the (100) orientation BS–PT films synthesized on Pt(111)/Ti/ SiO2/Si substrates using a modified sol–gel method showed a Pr of 33 μC cm 2 [8], and the 0.25Bi(Ni1/2Ti1/2)–0.85PbTiO3
Fig. 2. The ferroelectric hysteresis loops of BMZ–xPT thin films with the frequency of 100 Hz at room temperature with the inset showing the curves of J–E characteristics.
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films prepared by pulsed laser deposition method had a Pr between 16.8 and 40.9 μC cm 2 [11]. The BMZ–0.85PT films show a comparable Pr of 36.1 μC cm 2, which would be due to the contribution of only tetragonal phase with large c/a and high Tc. In addition, the high (100) orientation of BMZ– 0.85PT films should be responsible for the good ferroelectric property. Obviously, the property of saturated hysteresis loops reveals a considerable high insulation resistivity that is aligned with the relatively small leakage current of BMZ–0.85PT films (1.5 μA cm 2 under an electric field of 100 kV cm 2) as shown in the inset of Fig. 2. Furthermore, the Pr value is 32.7 μC cm 2 for BMZ–0.8PT with a Ec of 105 kV cm 1 (not shown here). In comparison, the same composition BZZ– 0.8PT shows a larger Pr of 55 μC cm 2 and Ec of 220 kV cm 1 using the same method [8]. Both BMZ–0.8PT and BZZ– 0.8PT have the same elements except zinc and magnesium ions. As reported, zinc ion has larger ferroelectricity activity than magnesium ion by theory calculations [5], which would reflect in Pr of BMZ–0.8PT and BZZ–0.8P. This shows that the above experimental data is consistent with the firstprinciples density functional theory calculations. One must bear in mind that the zinc and magnesium ionic radii are similar. So the diverse ferroelectricity activity is due to the different electronic structures of the two ions. Frequency dependence of the dielectric constant (permittivity) and loss of the BMZ–xPT films were measured at room temperature (Fig. 3). At the frequency of 100 kHz, the films
Fig. 3. The frequency dependence of (a) permittivity and (b) loss for the BMZ–xPT thin films at room temperature.
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4. Conclusions In summary, BMZ–xPT new ferroelectric films were deposited on Pt/Ti/SiO2/Si substrates by sol–gel method. The films with a thickness of 220 nm show a dense and uniform microstructure. With different BMZ content, the films prepared under identical processes and conditions have various orientation and polarization. The ferroelectric and dielectric properties of the present films were investigated, as the Pr value from 10.5 to 36.1 μC cm 2 and permittivity value from 544 to 833. The characteristics of reliable temperature dependent polarization are of much significance for high temperature ferroelectric device applications. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant nos. 21322102, 91022016, 21031005, 21231001), Program for Changjiang Scholars and Innovative Research Team in University (IRT1207), the Foundation for the Author of National Excellent Doctoral Dissertation of PR China (201039), Fok Ying Tung Education Foundation (131047), and Program for New Century Excellent Talents in University (NCET-11–0573). References
Fig. 4. Temperature dependence of Pr (remanent polarization), Ec (coercive field) and Pmax (polarization at maximum field) for BMZ–0.85PT films measured at 1 kHz.
with different BMZ contents have a high permittivity of about 544-833 and a loss tangent of less than 0.1, which could also be improved by Mn-doping [12]. The dielectric constant of the BMZ–xPT films is comparable to the other BiMeO3–PbTiO3 thin films [8,10]. As a device application of ferroelectric films, such as FeRAM, it is involved in the performance stability above room temperature. Comprehensively considering that the BMZ–0.85PT films exhibit better ferroelectric and dielectric properties, the study on temperature dependent ferroelectric properties of the films was performed at the frequency of 1 kHz (Fig. 4). Up to 100 1C, the films totally keep a saturated P–E hysteresis loop under electric fields of 12 V. As a function of temperature, the Pr, Ec and polarization at maximum field (Pmax) just slightly reduce with increasing temperature. The slightly decreasing spontaneous polarization (PS) of the films is relevant to the common temperature– polarization relationship of ferroelectric materials that the cation displacement reduces with increasing temperature. For instance, the PS of PbTiO3 as a function of temperature monotonically decreases up to Tc, which would reduce slowly at low temperature range and have a sharp decline close to Tc [13]. The investigated temperature (100 1C) in present study is adequately lower than its Tc (about 500 1C). Hence, the spontaneous polarization should slightly reduce.
[1] J.F. Scott, C.A.P. Dearaujo, Ferroelectric memories, Science 246 (1989) 1400–1405. [2] S.H. Baek, J. Park, D.M. Kim, V.A. Aksyuk, R.R. Das, S.D. Bu, et al., Giant piezoelectricity on Si for hyperactive MEMS, Science 334 (2011) 958–961. [3] R.E. Eitel, C.A. Randall, T.R. Shrout, S.E. Park, Preparation and Characterization of high temperature perovskite ferroelectrics in the solid-solution (1 x)BiScO3–xPbTiO3, Japanese Journal of Applied Physics 41 (2002) 2099–2104. [4] M.A. Khan, T.P. Comyn, A.J. Bell, Large remanent polarization in ferroelectric BiFeO3–PbTiO3 thin films on Pt/Si substrates, Applied Physics Letters 91 (2007) 032901. [5] I. Grinberg, A.M. Rappe, Nonmonotonic Tc trends in Bi-based ferroelectric perovskite solid solutions, Physical Review Letters 98 (2007) 037603. [6] T. Yoshimura, S. Trolier-McKinstry, Growth and properties of (001) BiScO3– PbTiO3 epitaxial films, Applied Physics Letters 81 (2002) 2065–2066. [7] A.H. Qureshi, G. Shabbir, D.A. Hall, On the synthesis and dielectric studies of (1 x)Bi(Mg1/2Zr1/2)O3–xPbTiO3 piezoelectric ceramic system, Materials Letters 61 (2007) 4482–4484. [8] L.X. Zhang, J. Chen, H.Q. Zhao, L.L. Fan, Y.C. Rong, J.X. Deng, et al., Large remanent polarization and small leakage in sol–gel derived Bi(Zn1/2Zr1/2)O3– PbTiO3 ferroelectric thin films, Dalton Transactions 42 (2013) 585–590. [9] H. Wen, X.H. Wang, X.Y. Deng, L.T. Li, Characterization of (100)oriented BiScO3–PbTiO3 thin films synthesized by a modified sol–gel method, Applied Physics Letters 88 (2006) 222904. [10] P. Verardi, F. Craciun, M. Dinescu, C. Gerardi, Epitaxial piezoelectric PZT thin films obtained by pulsed laser deposition, Thin Solid Films 318 (1998) 265–269. [11] G.H. Wu, H. Zhou, N. Qin, D.H. Bao, Growth and electrical properties of 25%Bi(Ni1/2Ti1/2)O3–75% PbTiO3 thin films on Pt/TiO2/SiO2/Si substrates using pulsed laser deposition method, Journal of the American Ceramic Society 94 (2011) 1675–1678. [12] L.D. Liu, R.Z. Zuo, Q. Sun, Q. Liang, Structure and electrical properties of Mn doped Bi(Mg1/2Ti1/2)O3–PbTiO3 ferroelectric thin films, Applied Surface Science 268 (2013) 327–331. [13] J.P. Remeika, A.M. Glass, The growth and ferroelectric properties of high resistivity single crystals of lead titanate, Materials Research Bulletin 5 (1970) 37–46.
CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 6307–6310 www.elsevier.com/locate/ceramint
Short communication
Growth and ferroelectric properties of sol–gel derived Bi(Mg1/2Zr1/2)O3–PbTiO3 thin films Linxing Zhanga, Jun Chena,c,n, Lu Yinb, Hanqing Zhaoa, Longlong Fana, Jiangli Caob, Xianran Xinga,c a Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, China Institute of Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China c State Key Laboratory for Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China
b
Received 30 July 2013; received in revised form 9 September 2013; accepted 27 September 2013 Available online 8 October 2013
Abstract The electrical properties included temperature-dependent polarization of (1 x)Bi(Mg1/2Zr1/2)O3–xPbTiO3 (BMZ–xPT) new ferroelectric films were investigated. The films with 220 nm thickness grown on Pt(111)/Ti/SiO2/Si substrates via sol–gel method were well crystallized with a phase-pure perovskite structure and homogeneous microstructure. Saturated polarization hysteresis loops are observed for all BMZ–xPT compounds, and BMZ–0.85PT films with high (100) orientation show a small leakage and remanent polarization of 36.1 μC cm 2, which is comparable to the (100)-oriented BiScO3–PbTiO3 thin films. The present films have high dielectric constants about 544-833. Furthermore, the polarization with elevated temperature slightly decreases, exhibiting stable ferroelectric properties and potentials for memory applications above room temperature such as non-volatile ferroelectric random access memories. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Films; A. Sol–gel processes; C. Dielectric properties; C. Electrical properties; C. Ferroelectric properties
1. Introduction Recently, great attention has been paid to the ferroelectric thin films with a perovskite structure because of high electrical performances and good temperature stability, which are desired for applications in automotive and aerospace industries such as non-volatile ferroelectric random access memories (FeRAM) and micro-electro-mechanical systems (MEMS) [1,2]. As potential candidates, bismuth-based perovskite ferroelectric materials with high Tc have been extensively studied, including BiMeO3–PbTiO3 (where Me=Sc3 þ , Fe3 þ , etc.) and Bi(Me1,Me2)O3–PbTiO3 (where Me1=Mg2 þ , Zn2 þ ,Ni2 þ , etc., and Me2=Ti4 þ , Zr4 þ , etc.) [3–5]. For instance, BiScO3– PbTiO3 (BS–PT) solid solutions with a morphotropic phase boundary (MPB) composition exhibit a high Tc ( 450 1C) and excellent piezoelectric performance (d33=460 pC N 1) [3]. n Corresponding author at: Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, China. Tel.: þ 86 10 62332525; fax: þ86 63 32525. E-mail address: [email protected] (J. Chen).
BS–PT films were also reported to exhibit good ferroelectric and piezoelectric properties [6]. However, the extensive and potential device applications of BS–PT are severely restricted by the high cost of scandium sources. Furthermore, BiFeO3– PbTiO3 (BF–PT) films exhibit good properties with a large remanent polarization (Pr) of 50 μC cm 2 [4]. But relatively high conductivity of the films makes it difficult to the observation of the ferroelectric polarization–electric field hysteresis loop. (1 x)Bi(Mg1/2Zr1/2)O3–xPbTiO3 (BMZ–xPT) is also another bismuth-based perovskite high Tc ferroelectric materials without expensive elements. It has been reported that BMZ–xPT solutions exhibit the nonmonotonic Tc trends arising from the microscopic coupling between A-site and B-site displacements. And the BMZ–xPT compositions with xZ0.75 have a higher Tc than 450 1C [5]. The BMZ–xPT system, which has the substitution of Mg and Zr ions by replacing Me in BiMeO3–PbTiO3, do not have the ions with variable valency such as Fe3 þ for BF–PT, resulting in small leakage. The piezoelectric ceramics of BMZ–xPT fabricated by the solid state reaction method show a high dielectric constant of 1387 and low coercive field of
0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2013.09.120
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23 kV/cm [7]. Since BMZ–xPT exhibit good performances such as high Tc, piezoelectric properties, low cost and conductivity, it is interesting to investigate the system. Furthermore, the similar system (1 x)Bi(Zn1/2Zr1/2)O3–xPbTiO3 (BZZ–xPT) films had been reported [8]. Although the difference between BZZ–PT and BMZ–PT is only one B-site element (Zn and Mg), acute difference such as solid solubility and polarization were emerged between the two systems [5]. It is noticeable to that the important role of Mg is indispensable in Bi(Me1,Me2)O3–PbTiO3. However, only few literatures reported BMZ–xPT ceramics and studies on thin films are still lacking. In this work, BMZ–xPT films were fabricated on Pt(111)/Ti/SiO2/Si substrates by a sol– gel deposition method. The ferroelectric, leakage current and dielectric properties were investigated at room temperature. For consideration as device applications above room temperature, the temperature dependence of ferroelectric property was also studied.
identical processes and conditions exhibit pure perovskite phase (Fig. 1(a)). Compared with BZZ–xPT, whose solid solubility is only x4 0.3, the x 40.55 of solid solubility for BMZ–xPT exhibit better tunability in composition. The reason comes down to the different role of zinc and magnesium ions. The present films with x ¼ 0.85 and 0.80 have a high (100) orientation, while the others show a random orientation. The BMZ–xPT show different preferential orientation changing from a-axis to random as the BMZ content increases. The reason for the preferential orientation would be the factors that lattice match [9], surface energy of planes [10], dry and pyrolysis temperature and so on. As we know, the c/a of BMZ–xPT solid solutions reduces from 1.065 of PbTiO3 (PT) to 1.02 (MPB of BMZ–xPT in the range of 0.55r x r 0.60)
2. Experimental procedure Lead acetate trihydrate, titanium n-butoxide, bismuth nitrate pentahydrate, magnesium nitrate hexhydrate, and zirconium nitrate pentahydrate were selected as starting reagents for the preparation of precursor solution. And 2-methoxyethanol and glacial acetic acid were used as solvents and stabilizer, respectively. 10 mol% Pb and 5 mol% Bi were added to compensate for the volatilization loss. A similar process of precursor solution was synthesized as reported in previous work [8]. Before spin coating, the precursors were filtrated with a 0.2 μm syringe filter to avoid particulate contamination and then deposited on Pt(111)/Ti/SiO2/Si substrates at a spinning rate of 4000 rpm for 30 s. Subsequently, the gel films were dried and pyrolyzed at 300 1C for 10 min and finally annealed at 700 1C for 1 min by rapid thermal processing (RTP) under oxygen environment. The steps were repeated several times to obtain about 200 nm thick BMZ–xPT thin films. X-ray diffraction (XRD, TTRШ, Rigaku, Japan) was used to characterize the crystalline structure of the films. Surface and cross-section microstructures of samples were examined by a field emission scanning electron microscope (FE-SEM, SUPRA-40, Carl Zeiss, Germany). The ferroelectric and leakage properties were measured by an aixACCT ferroelectric test system (TF-Analyser 1000, aixACCT, Germany) in combination with a temperature control system, which was consisted of a temperature controller, heating plate and insulation cover. The dielectric properties of the films were investigated by using a precision impedance analyser (4294A, Agilent, Palo alto, CA). 3. Results and discussion The as-growth films could present different preferential orientation along with various microstructures. The ability to understand the reasons of crystallographic orientation should be critical to control and create highly oriented films. The XRD of BMZ–xPT (0.55 r x r 0.85) films prepared under
Fig. 1. (a) XRD patterns, (b) scanning electron microscopy surface and (c) cross-sectional image of BMZ–xPT films.
L. Zhang et al. / Ceramics International 40 (2014) 6307–6310
with increasing BMZ content, due to a reducing c axis and increasing a axis [7]. The (100) preferential orientation of BMZ–xPT films (x Z 0.80) could be due to the lower surface energy of the (100) planes. These films (an a axis-oriented film) have a large c/a, and the long (c) axis could easily lie down on the growth face [9]. As the reducing c axis is close to a(b) axis with increasing BMZ content (xr 0.75), the comparative surface energy of the (100) and (001) planes would lead to a random orientation. The level of (100) orientation is about 0.96 for BMZ–85PT, calculated by a semiquantitative method using the Lotgering factor. The high orientation indicates the films have a very good texture. The representative SEM micrograph of the cross-sectional and surface image of BMZ–xPT films show a dense and uniform microstructure (Fig. 1(b)(c)). The clearly visible each layer of films/Pt/SiO2/Si exhibit well growth of the films on the substrate. As opposed to pure PT films, which often fracture through the ferroelectric phase transition during the cooling process, no evident cracks are observed in the BMZ–xPT films. The thickness and grain size of the films are about 220 nm and 50–100 nm, respectively. The ferroelectric hysteresis loops (P–E) of the BMZ–xPT films were measured with the frequency of 100 Hz at room temperature (Fig. 2). The films all exhibit saturated P–E hysteresis loops under electric fields of 12 V. The Pr value is 36.1, 27.8, 10.5 19.9 μC cm 2 as the PT content is 0.85, 0.75, 0.65, 0.55, meanwhile, with a coercive field (Ec) of 113, 106, 80, 125 kV cm 1, respectively. Note that the Pr reduces firstly and then increases with decreasing PT content. In upper PbTiO3 content, high Pr would be due to the unique contribution of strongly tetragonal phase. As reported, the polarization and Tc of the BMZ–xPT reduces as the x decreases from 0.85 to 0.55 by first-principles density functional theory calculations [5]. However, the MPB of BMZ–xPT is in the range of 0.55 r xr 0.60 [7], and the increasing Pr of x ¼ 0.55 could be ascribed to the coexistence contribution of both tetragonal and rhombohedral perovskite phase in the MPB. In comparison, the (100) orientation BS–PT films synthesized on Pt(111)/Ti/ SiO2/Si substrates using a modified sol–gel method showed a Pr of 33 μC cm 2 [8], and the 0.25Bi(Ni1/2Ti1/2)–0.85PbTiO3
Fig. 2. The ferroelectric hysteresis loops of BMZ–xPT thin films with the frequency of 100 Hz at room temperature with the inset showing the curves of J–E characteristics.
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films prepared by pulsed laser deposition method had a Pr between 16.8 and 40.9 μC cm 2 [11]. The BMZ–0.85PT films show a comparable Pr of 36.1 μC cm 2, which would be due to the contribution of only tetragonal phase with large c/a and high Tc. In addition, the high (100) orientation of BMZ– 0.85PT films should be responsible for the good ferroelectric property. Obviously, the property of saturated hysteresis loops reveals a considerable high insulation resistivity that is aligned with the relatively small leakage current of BMZ–0.85PT films (1.5 μA cm 2 under an electric field of 100 kV cm 2) as shown in the inset of Fig. 2. Furthermore, the Pr value is 32.7 μC cm 2 for BMZ–0.8PT with a Ec of 105 kV cm 1 (not shown here). In comparison, the same composition BZZ– 0.8PT shows a larger Pr of 55 μC cm 2 and Ec of 220 kV cm 1 using the same method [8]. Both BMZ–0.8PT and BZZ– 0.8PT have the same elements except zinc and magnesium ions. As reported, zinc ion has larger ferroelectricity activity than magnesium ion by theory calculations [5], which would reflect in Pr of BMZ–0.8PT and BZZ–0.8P. This shows that the above experimental data is consistent with the firstprinciples density functional theory calculations. One must bear in mind that the zinc and magnesium ionic radii are similar. So the diverse ferroelectricity activity is due to the different electronic structures of the two ions. Frequency dependence of the dielectric constant (permittivity) and loss of the BMZ–xPT films were measured at room temperature (Fig. 3). At the frequency of 100 kHz, the films
Fig. 3. The frequency dependence of (a) permittivity and (b) loss for the BMZ–xPT thin films at room temperature.
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4. Conclusions In summary, BMZ–xPT new ferroelectric films were deposited on Pt/Ti/SiO2/Si substrates by sol–gel method. The films with a thickness of 220 nm show a dense and uniform microstructure. With different BMZ content, the films prepared under identical processes and conditions have various orientation and polarization. The ferroelectric and dielectric properties of the present films were investigated, as the Pr value from 10.5 to 36.1 μC cm 2 and permittivity value from 544 to 833. The characteristics of reliable temperature dependent polarization are of much significance for high temperature ferroelectric device applications. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant nos. 21322102, 91022016, 21031005, 21231001), Program for Changjiang Scholars and Innovative Research Team in University (IRT1207), the Foundation for the Author of National Excellent Doctoral Dissertation of PR China (201039), Fok Ying Tung Education Foundation (131047), and Program for New Century Excellent Talents in University (NCET-11–0573). References
Fig. 4. Temperature dependence of Pr (remanent polarization), Ec (coercive field) and Pmax (polarization at maximum field) for BMZ–0.85PT films measured at 1 kHz.
with different BMZ contents have a high permittivity of about 544-833 and a loss tangent of less than 0.1, which could also be improved by Mn-doping [12]. The dielectric constant of the BMZ–xPT films is comparable to the other BiMeO3–PbTiO3 thin films [8,10]. As a device application of ferroelectric films, such as FeRAM, it is involved in the performance stability above room temperature. Comprehensively considering that the BMZ–0.85PT films exhibit better ferroelectric and dielectric properties, the study on temperature dependent ferroelectric properties of the films was performed at the frequency of 1 kHz (Fig. 4). Up to 100 1C, the films totally keep a saturated P–E hysteresis loop under electric fields of 12 V. As a function of temperature, the Pr, Ec and polarization at maximum field (Pmax) just slightly reduce with increasing temperature. The slightly decreasing spontaneous polarization (PS) of the films is relevant to the common temperature– polarization relationship of ferroelectric materials that the cation displacement reduces with increasing temperature. For instance, the PS of PbTiO3 as a function of temperature monotonically decreases up to Tc, which would reduce slowly at low temperature range and have a sharp decline close to Tc [13]. The investigated temperature (100 1C) in present study is adequately lower than its Tc (about 500 1C). Hence, the spontaneous polarization should slightly reduce.
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