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Na0.5Bi0.5(Ti0.98Fe0.02)O3 thin films prepared by metal organic decomposition
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CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 859–863 www.elsevier.com/locate/ceramint
Short communication
Structural, ferroelectric, and dielectric properties of bilayered Na0.5Bi0.5(Ti0.98Zr0.02)O3/Na0.5Bi0.5(Ti0.98Fe0.02)O3 thin films prepared by metal organic decomposition C.H. Yanga,b, H.T. Suia, G. Wanga,n, F.J. Genga, C. Fenga a School of Materials Science and Engineering, University of Jinan, Jinan 250022, China Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials, University of Jinan, Jinan 250022, China
b
Received 10 August 2014; accepted 27 August 2014 Available online 3 September 2014
Abstract Lead-free bilayered thin film consisting of Na0.5Bi0.5(Ti0.98Zr0.02)O3 (NBTZr) and Na0.5Bi0.5(Ti0.98Fe0.02)O3 (NBTFe) were deposited on indium tin oxide (ITO)/glass substrate by metal organic decomposition. Na0.5Bi0.5TiO3 and NBTZr thin films were studied for comparison. All the thin films exhibit single polycrystalline perovskite structures. The bilayered NBTZr/NBTFe thin film exhibits a well-defined hysteresis loop with a large remanent polarization (Pr) of 21 μC/cm2 due to the distorted structure caused by Zr4 þ doping, the insulating barrier of NBTFe and the small grain size. The NBTZr/NBTFe film also shows a small dielectric dispersion with an intermediate dielectric constant (εr) of 325 and a minimum dissipation factor (tanδ) of 0.08 at 100 kHz. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Thin films; Microstructure; Ferroelectric; Dielectric
1. Introduction Recently, most studies have been focused on development of the potential replacements for lead zirconate titanate [Pb(Zr, Ti)O3, PZT] [1,2]. Sodium bismuth titanate, Na0.5Bi0.5TiO3 (abbreviated as NBT) is considered among the best lead-free ferroelectric materials. It has a relatively large remnant polarization (Pr) of 38 μC/cm2 and high Curie temperature (Tc) of 320 1C at room temperature for bulk ceramic [3]. However, the intrinsic electrical properties are difficult to be measured, especially for thin film type, mainly due to the stoichiometry-related problem. The high volatility of the A-site elements during thermal treatment can induce the generation of oxygen vacancies, then result in high conductivity in the poling of NBT film [4]. Therefore, the NBT-based film is easy to show breakdown before the electric field reaches the saturated polarization state. n
Corresponding author. Tel.: þ86 531 88374857; fax: þ 86 531 87974453. E-mail address: [email protected] (G. Wang).
http://dx.doi.org/10.1016/j.ceramint.2014.08.115 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
To improve the electrical performance of NBT film, considerable efforts have been made. Based on the previous reports, it can be concluded as the following aspects. (i) NBT-based solid solutions can be formed with other perovskites, such as BiFeO3 [5], BaTiO3 [6], and (Bi0.5K0.5)TiO3–BaTiO3 [7]. In particular, if these binary or ternary systems are near the morphotropic phase boundaries (MPB), the components would be poled easily at lower applied voltage and possess superior electrical response. However, it is not easy to identify the existence of MPB in thin-film form [8,9]. (ii) The crystal quality also affects the electrical properties of ferroelectric film to some extent [10]. For NBT-based films, the researchers are in favor of growing the epitaxial films with reduced charge defects [6,11]. (iii) The heterogeneous multilayer structure can offer opportunity to tailor the electrical properties, such as bilayered or trilayered thin films of (Na0.85K0.15)0.5Bi0.5TiO3/Pb0.8La0.1Ca0.1 Ti0.975O3 or SrTiO3/NBT/SrTiO3. The buffer layer which at least prevents the direct contact of NBT film with the bottom metal electrode can serve as an insulating barrier [4,12]. (iv) Many studies demonstrate that doping technique acts as a
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C.H. Yang et al. / Ceramics International 41 (2015) 859–863
more effective approach, not only for increasing the electric resistance but also for improving the ferroelectric and dielectric properties for NBT film. The leakage current density of NBT film can be dramatically decreased by nearly two orders of magnitude by doping of Fe [13]. The NBT film with a large remanent polarization (Pr) of 29.5 μC/cm2 can be obtained by co-substitution of La and Ce [6]. The NBT with 5 mol% Y substitution shows a maximized relative permittivity of 450 [14]. As for the doping technique, the different ion-doping in NBT film can cause differences in microstructure and defects [6], which in turn induce the variation of electrical results. Therefore, more researches are needed to be implemented to clarify the effects of other ion substitutions. In this work, we report on the influence of Zr doping on the structure, ferroelectric and dielectric properties of NBT film. Higher value of Pr induced by structure distortion can be expected in Na0.5Bi0.5(Ti0.98Zr0.02)O3 (NBTZr) film since the ionic radius of Zr4 þ (r¼ 0.605 Å) is much bigger than that of Ti4 þ (r¼ 0.720 Å). Furthermore, the effect of Na0.5Bi0.5 (Ti0.98Fe0.02)O3 (NBTFe) seed layer on the electrical property of NBTZr is investigated. 2. Experimental procedure The NBTZr film was deposited on substrate by a metal organic decomposition process. CH3COONa and Bi(NO3)3 5H2O were initially dissolved in acetic acid. Zr(NO3)4 5H2O was added into a mixture of acetylacetone and 2-methoxyethanol. Then, both solutions were mixed under constant stirring on a magnetic stirrer. Finally, appropriate amounts of Ti[OCH(CH3)2]4, acetylacetone and ethylene glycol monomer were added to the Na–Bi–Zr complex solution. A transparent and stable precursor solution with the concentration of 0.3 M was obtained. The preparation method of NBTFe film was reported in our previous paper [15]. The precursor solution was deposited onto indium tin oxide (ITO)/glass or NBTFe/ITO/glass substrates by spin coating, and annealed at 500 1C by a rapid thermal annealing (RTA) in a N2 atmosphere. The deposition together with heat-treatment procedure was repeated several times to prepare a certain thickness. The thickness of NBTFe film deposited on ITO/glass was 130 nm measured by a step (step obtained by chemical etching) profilometer made by the Ambios Technology Company of USA. The structures of all the film samples were examined using an X-ray diffractometer (XRD, Bruker D8). The surface morphology and cross-sectional structure of the films were tested by a scanning electron microscope (FeSEM, Hitachi S-4200)). The ferroelectric properties were measured by a standard ferroelectric tester (Radiant Technologies). The dielectric properties were examined by an impedance analyzer (HP4294A). 3. Results and discussion Fig. 1 shows the X-ray diffraction patterns of ITO, NBT, NBTZr and NBTZr/NBTFe films. In order to detect whether there exist impurity peaks in NBT-based films for clarity, the y-axes in the XRD patterns are shown in logarithmic scale but not in absolute scale. The main diffraction peaks of all samples
Fig. 1. XRD patterns of (a) ITO, (b) NBT, (c) NBTZr, and (d) NBTZr/NBTFe thin films.
correspond to the rhombohedral perovskite structure and no preferred orientation is observed, indicating the polycrystalline nature of the prepared films. No obvious secondary phase can be detected in these NBT-based films after a RTA treatment at a low temperature of 500 1C under N2 atmosphere. This may be partly due to the fact that more oxygen vacancies ðV 0 Þ generated in N2 assists in transfer of energy and mass between reactants during the annealing process [16]. Thus the N2 atmosphere does favor to the crystallization of perovskite phase, and is adopted by many researchers when preparing the perovskite thin films [17,18]. The FeSEM micrographs of NBT, NBTZr and NBTZr/ NBTFe are presented in Fig. 2. All the films exhibit relatively dense and smooth surface without evident cracks. It is found that the average grain size for NBT, NBTZr and NBTZr/ NBTFe films decrease gradually. Generally, the grain growth is controlled by the motion of grain boundaries, which depends on the diffusivities of the constituent ions. Thus, compared with that of the undoped NBT film (80 nm), the smaller size for NBTZr film of 70 nm may be due to the lower diffusivity of Zr4 þ compared to that of Ti4 þ . When NBTZr is deposited on NBTFe/ITO, the NBTFe seed layer can offer more nucleation sites and reduce the activation energy for the crystallization of NBTZr film, then result in smaller uniform grain with 50 nm in size. As can be seen from the crosssectional images, the NBT, NBTZr and NBTZr/NBTFe samples are all estimated to be about 450 nm in thickness, which is used for the calculation of the relatively dielectric permittivity. However, some voids or grooves are present in the interfaces between some layers in all the films. These defects may be generated due to the fact that, for the route of the film crystallization with sequential annealing process layer by layer, each deposited layer with different smoothness by firing in RTA furnace would affect the next deposition layer. This phenomenon is popularly present in the sequential-layer annealed films and its elimination needs to be further investigated [4,19]. Fig. 3 shows the typical polarization–electric field (P–E) hysteresis loops of all the film samples. One can find that NBT film shows a poor P–E hysteresis loop due to the large contribution from the leakage current. However, the NBTZr
C.H. Yang et al. / Ceramics International 41 (2015) 859–863
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Fig. 2. Surface morphologies and cross-sectional images of thin films: (a) and (b) NBT, (c) and (d) NBTZr and (e) and (f) NBTZr/NBTFe.
exhibits much improved P–E loop with a slim shaped feature by Zr4 þ doping. In sharp contrast, the bilayered NBTZr/ NBTFe films exhibit a well-defined hysteresis loop with a large remanent polarization (Pr) of 21 μC/cm2 at the applied electric field of 700 kV/cm. In our work, the biggest Pr value of NBTZr/NBTFe film can be ascribed to the following three
aspects. (i) The TiO6 octahedron can be distorted by the introduction of Zr4 þ with larger ionic radius [20]. (ii) The NBTFe seed layer inhibits the charge transport since the Fe substitution can make the NBT film to bear high applied electric field by the formation of defect complexes, as reported in our previous work [13]. (iii) Since the grain boundary acts
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Fig. 3. P–E hysteresis loops of thin films as a function of the electric field at 10 kHz: (a) NBT, (b) NBTZr, and (c) NBTZr/NBTFe.
Fig. 4. Capacitance–voltage curves for thin films as a function of the applied voltage: (a) NBT, (b) NBTZr, and (c) NBTZr/NBTFe.
Fig. 5. Frequency dependence of relative dielectric constant and dissipation factor for NBT, NBTZr and NBTZr/NBTFe thin films.
as the potent leakage current path, the leakage current can be restricted by the small grain size, as displayed in Fig. 2. Fig. 4 illustrates the capacitance–voltage (C–V) characteristics of the films. In the C–V curve, each capacitance value measured at a certain bias voltage point represents the slope of the ferroelectric hysteresis at the corresponding applied voltage. Considering this relationship, the typical butterfly shape
obtained for all films confirms the ferroelectric nature of each film again. For NBTZr/NBTFe, high capacitance variation (i.e., a relatively sharp feature), which is related to the polarization, is observed. This demonstrates that the degree of ferroelectricity of NBTZr/NBTFe is strong [21]. The dielectric constant (εr) and dissipation factor (tanδ) were measured as a function of measuring frequency by applying a small ac voltage of 0.01 V, as shown in Fig. 5. It can be seen that the εr of NBT film is decreased obviously with Zr4 þ substitution, which is consistent with the result of the Zr4 þ modified NBT ceramics [22]. As for NBTZr/NBTFe, it shows an intermediate εr value of 325 and a minimum tanδ of 0.08 at 100 kHz, as compared to NBT and NBTZr films. Also, the NBTZr/NBTFe film shows a small dielectric dispersion, which indicates its relatively low defect concentration.
4. Conclusions In summary, the polycrystalline NBTZr/NBTFe thin film was prepared on ITO/glass by metal organic decomposition. The results show that the NBTFe seed layer plays a significant role on the microstructure and electrical properties of the NBTZr film. The bilayered NBTZr/NBTFe thin film exhibits enhanced
C.H. Yang et al. / Ceramics International 41 (2015) 859–863
ferroelectric (Pr ¼ 21 μC/cm2) and frequency-undispersed dielectric behaviors (εr ¼ 325 and tanδ¼ 0.08), together with small average grain size. Acknowledgments This work was supported by the National Natural Science Foundation of China (no. 51002064) and the Graduate Innovation Foundation (no. YCX13003) of University of Jinan. References [1] J.F. Scott, Leading the way to lead-free, ChemPhysChem 11 (2010) 341–343. [2] S.X. Huo, S.L. Yuan, Z.M. Tian, C.H. Wang, Y. Qiu, Grain size effects on the ferroelectric and piezoelectric properties of Na0.5K0.5NbO3 ceramics prepared by pechini method, J. Am. Ceram. Soc. 95 (2012) 1383–1387. [3] K.A. Razak, C.J. Yip, S. Sreekantan, Synthesis of (Bi0.5Na0.5)TiO3 (BNT) and Pr doped BNT using the soft combustion technique and its properties, J. Alloys Compd. 509 (2011) 2936–2941. [4] T. Šetinc, M. Spreitzer, Š. Kunej, J. Kovač, D. Suvorov, Temperature stable dielectric behavior of sol–gel derived compositionally graded SrTiO3/Na0.5Bi0.5TiO3/SrTiO3 thin films, J. Am. Ceram. Soc. 96 (2013) 3511–3517. [5] C.H. Yang, H.T. Wu, F. Yang, G.D. Hu, Non-lead Ce:Na0.5Bi0.5TiO3– BiFeO3 solid solution thin film with significantly reduced leakage current and large polarization, Ceram. Int. 40 (2014) 4753–4757. [6] D.Y. Wang, N.Y. Chan, S. Li, S.H. Choy, H.Y. Tian, H.L.W. Chan, Enhanced ferroelectric and piezoelectric properties in doped lead-free (Bi0.5Na0.5)0.9Ba0.06TiO3 thin films, Appl. Phys. Lett. 97 (2010) 212901. [7] Y.H. Jeon, E. Patterson, D. Cann, B. Gibbons, Dielectric and ferroelectric properties of (Bi0.5Na0.5)TiO3–(Bi0.5K0.5)TiO3–BaTiO3 thin films deposited via chemical solution deposition, Mater. Lett. 106 (2013) 63–66. [8] H. Zhou, G.H. Wu, N. Qin, D.H. Bao, Improved electrical properties and strong red emission of Pr3 þ -doped xK0.5Bi0.5TiO3–(1 x)Na0.5Bi0.5TiO3 lead-free ferroelectric thin films, J. Am. Ceram. Soc. 95 (2012) 483–486. [9] I. Bretos, D.A. José, R. Jiménez, J. Ricote, M.L. Calzada, Evidence of morphotropiphase boundary displacement in lead-free (Bi0.5Na0.5)1 xBaxTiO3 polycrystalline thin films, Mater. Lett. 65 (2011) 2714–2716.
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[10] D.S.L. Pontes, R.A. Capeli, M.L. Garzim, F.M. Pontes, A.J. Chiquito, E. Longo, Structural, microstructural, optical and electrical properties of (Pb,Ba,Sr)TiO3 films growth on conductive LaNiO3-coated LaAO3(1 0 0) and Pt/Ti/SiO2/Si substrates, Mater. Lett. 121 (2014) 93–96. [11] H. Adachi, Y. Tanaka, T. Harigai, M. Ueda, E. Fujii, Large transverse piezoelectricity in strained (Na,Bi)TiO3–BaTiO3 epitaxial thin films on MgO(1 1 0), Appl. Phys. Express 4 (2011) 051501. [12] Q.G. Chi, F.Y. Yang, C.H. Zhang, C.T. Chen, H.F. Zhu, X. Wang, Q.Q. Lei, Influence of seed layer on crystal orientation and electrical properties of (Na0.85K0.15)0.5Bi0.5TiO3 thin films prepared by a sol–gel process, Ceram. Int. 39 (2013) 9273–9276. [13] C.H. Yang, G.D. Hu, W.B. Wu, H.T. Wu, F. Yang, Z.Y. Lu, L. Wang, Reduced leakage current, enhanced ferroelectric and dielectric properties in (Ce,Fe)-codoped Na0.5Bi0.5TiO3 film, Appl. Phys. Lett. 100 (2012) 022909. [14] D.H. Kang, B.S. Lee, H.K. An, Y.H. Kim, D.S. Paik, H.I. Hwang, N.K. Cho, Characteristics of yttrium substituted sodium bismuth titanate thin films, Mater. Lett. 64 (2010) 2331–2333. [15] H.T. Sui, C.H. Yang, M.S. Zhao, W.Q. Lin, C. Feng, Effects of annealing temperature on the microstructure, electrical properties of Fe-doped Na0.5Bi0.5TiO3 thin films, J. Alloys Compd. 586 (2014) 683–687. [16] C.C. Diao, C.F. Yang, J.J. Lin, Crystallization and electrical characteristics of 0.95(Na0.5Bi0.5)TiO3–0.05BaTiO3 thin films under different annealing temperature and atmosphere, J. Nanosci. Nanotechnol. 11 (2011) 10562–10566. [17] G.Q. Kang, K. Yao, J. Wang, 0.90(Na0.5Bi0.5TiO3)–0.06BaTiO3– 0.04K0.5Na0.5NbO3 ferroelectric thin films derived from chemical solutions, J. Am. Ceram. Soc. 94 (2011) 1331–1335. [18] A. Hieno, W. Sakamoto, M. Moriya, Synthesis of BiFeO3–Bi0.5Na0.5 TiO3 thin films by chemical Solution deposition and their properties, Jpn. J. Appl. Phys. 50 (2011) 09NB04. [19] Y.H. Jeon, E. Patterson, D. Cann, P. Mardilovich, W. Stickel, B. Gibbons, Large piezoresponse and ferroelectric properties of (Bi0.5Na0.5)TiO3–(Bi0.5K0.5) TiO3–Bi(Mg0.5Ti0.5)O3 thin films prepared by chemical solution deposition, J. Am. Ceram. Soc. 96 (2013) 2172–2178. [20] R.E. Cohen, Origin of ferroelectricity in perovskite oxides, Nature 358 (1992) 136–139. [21] N.M. Murari, R. Thomas, R.E. Melgarejo, S.P. Pavunny, R.S. Katiyar, Structural, electrical, and magnetic properties of chemical solution deposited BiFe1 xTixO3 and BiFe0.9Ti0.05Co0.05O3 thin films, J. Appl. Phys. 106 (2009) 014103. [22] B.K. Barick, R.N.P. Choudhary, D.K. Pradhan, Dielectric and impedance spectroscopy of zirconium modified (Na0.5Bi0.5)TiO3 ceramics, Ceram. Int. 39 (2013) 5695–5704.
CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 859–863 www.elsevier.com/locate/ceramint
Short communication
Structural, ferroelectric, and dielectric properties of bilayered Na0.5Bi0.5(Ti0.98Zr0.02)O3/Na0.5Bi0.5(Ti0.98Fe0.02)O3 thin films prepared by metal organic decomposition C.H. Yanga,b, H.T. Suia, G. Wanga,n, F.J. Genga, C. Fenga a School of Materials Science and Engineering, University of Jinan, Jinan 250022, China Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials, University of Jinan, Jinan 250022, China
b
Received 10 August 2014; accepted 27 August 2014 Available online 3 September 2014
Abstract Lead-free bilayered thin film consisting of Na0.5Bi0.5(Ti0.98Zr0.02)O3 (NBTZr) and Na0.5Bi0.5(Ti0.98Fe0.02)O3 (NBTFe) were deposited on indium tin oxide (ITO)/glass substrate by metal organic decomposition. Na0.5Bi0.5TiO3 and NBTZr thin films were studied for comparison. All the thin films exhibit single polycrystalline perovskite structures. The bilayered NBTZr/NBTFe thin film exhibits a well-defined hysteresis loop with a large remanent polarization (Pr) of 21 μC/cm2 due to the distorted structure caused by Zr4 þ doping, the insulating barrier of NBTFe and the small grain size. The NBTZr/NBTFe film also shows a small dielectric dispersion with an intermediate dielectric constant (εr) of 325 and a minimum dissipation factor (tanδ) of 0.08 at 100 kHz. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Thin films; Microstructure; Ferroelectric; Dielectric
1. Introduction Recently, most studies have been focused on development of the potential replacements for lead zirconate titanate [Pb(Zr, Ti)O3, PZT] [1,2]. Sodium bismuth titanate, Na0.5Bi0.5TiO3 (abbreviated as NBT) is considered among the best lead-free ferroelectric materials. It has a relatively large remnant polarization (Pr) of 38 μC/cm2 and high Curie temperature (Tc) of 320 1C at room temperature for bulk ceramic [3]. However, the intrinsic electrical properties are difficult to be measured, especially for thin film type, mainly due to the stoichiometry-related problem. The high volatility of the A-site elements during thermal treatment can induce the generation of oxygen vacancies, then result in high conductivity in the poling of NBT film [4]. Therefore, the NBT-based film is easy to show breakdown before the electric field reaches the saturated polarization state. n
Corresponding author. Tel.: þ86 531 88374857; fax: þ 86 531 87974453. E-mail address: [email protected] (G. Wang).
http://dx.doi.org/10.1016/j.ceramint.2014.08.115 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
To improve the electrical performance of NBT film, considerable efforts have been made. Based on the previous reports, it can be concluded as the following aspects. (i) NBT-based solid solutions can be formed with other perovskites, such as BiFeO3 [5], BaTiO3 [6], and (Bi0.5K0.5)TiO3–BaTiO3 [7]. In particular, if these binary or ternary systems are near the morphotropic phase boundaries (MPB), the components would be poled easily at lower applied voltage and possess superior electrical response. However, it is not easy to identify the existence of MPB in thin-film form [8,9]. (ii) The crystal quality also affects the electrical properties of ferroelectric film to some extent [10]. For NBT-based films, the researchers are in favor of growing the epitaxial films with reduced charge defects [6,11]. (iii) The heterogeneous multilayer structure can offer opportunity to tailor the electrical properties, such as bilayered or trilayered thin films of (Na0.85K0.15)0.5Bi0.5TiO3/Pb0.8La0.1Ca0.1 Ti0.975O3 or SrTiO3/NBT/SrTiO3. The buffer layer which at least prevents the direct contact of NBT film with the bottom metal electrode can serve as an insulating barrier [4,12]. (iv) Many studies demonstrate that doping technique acts as a
860
C.H. Yang et al. / Ceramics International 41 (2015) 859–863
more effective approach, not only for increasing the electric resistance but also for improving the ferroelectric and dielectric properties for NBT film. The leakage current density of NBT film can be dramatically decreased by nearly two orders of magnitude by doping of Fe [13]. The NBT film with a large remanent polarization (Pr) of 29.5 μC/cm2 can be obtained by co-substitution of La and Ce [6]. The NBT with 5 mol% Y substitution shows a maximized relative permittivity of 450 [14]. As for the doping technique, the different ion-doping in NBT film can cause differences in microstructure and defects [6], which in turn induce the variation of electrical results. Therefore, more researches are needed to be implemented to clarify the effects of other ion substitutions. In this work, we report on the influence of Zr doping on the structure, ferroelectric and dielectric properties of NBT film. Higher value of Pr induced by structure distortion can be expected in Na0.5Bi0.5(Ti0.98Zr0.02)O3 (NBTZr) film since the ionic radius of Zr4 þ (r¼ 0.605 Å) is much bigger than that of Ti4 þ (r¼ 0.720 Å). Furthermore, the effect of Na0.5Bi0.5 (Ti0.98Fe0.02)O3 (NBTFe) seed layer on the electrical property of NBTZr is investigated. 2. Experimental procedure The NBTZr film was deposited on substrate by a metal organic decomposition process. CH3COONa and Bi(NO3)3 5H2O were initially dissolved in acetic acid. Zr(NO3)4 5H2O was added into a mixture of acetylacetone and 2-methoxyethanol. Then, both solutions were mixed under constant stirring on a magnetic stirrer. Finally, appropriate amounts of Ti[OCH(CH3)2]4, acetylacetone and ethylene glycol monomer were added to the Na–Bi–Zr complex solution. A transparent and stable precursor solution with the concentration of 0.3 M was obtained. The preparation method of NBTFe film was reported in our previous paper [15]. The precursor solution was deposited onto indium tin oxide (ITO)/glass or NBTFe/ITO/glass substrates by spin coating, and annealed at 500 1C by a rapid thermal annealing (RTA) in a N2 atmosphere. The deposition together with heat-treatment procedure was repeated several times to prepare a certain thickness. The thickness of NBTFe film deposited on ITO/glass was 130 nm measured by a step (step obtained by chemical etching) profilometer made by the Ambios Technology Company of USA. The structures of all the film samples were examined using an X-ray diffractometer (XRD, Bruker D8). The surface morphology and cross-sectional structure of the films were tested by a scanning electron microscope (FeSEM, Hitachi S-4200)). The ferroelectric properties were measured by a standard ferroelectric tester (Radiant Technologies). The dielectric properties were examined by an impedance analyzer (HP4294A). 3. Results and discussion Fig. 1 shows the X-ray diffraction patterns of ITO, NBT, NBTZr and NBTZr/NBTFe films. In order to detect whether there exist impurity peaks in NBT-based films for clarity, the y-axes in the XRD patterns are shown in logarithmic scale but not in absolute scale. The main diffraction peaks of all samples
Fig. 1. XRD patterns of (a) ITO, (b) NBT, (c) NBTZr, and (d) NBTZr/NBTFe thin films.
correspond to the rhombohedral perovskite structure and no preferred orientation is observed, indicating the polycrystalline nature of the prepared films. No obvious secondary phase can be detected in these NBT-based films after a RTA treatment at a low temperature of 500 1C under N2 atmosphere. This may be partly due to the fact that more oxygen vacancies ðV 0 Þ generated in N2 assists in transfer of energy and mass between reactants during the annealing process [16]. Thus the N2 atmosphere does favor to the crystallization of perovskite phase, and is adopted by many researchers when preparing the perovskite thin films [17,18]. The FeSEM micrographs of NBT, NBTZr and NBTZr/ NBTFe are presented in Fig. 2. All the films exhibit relatively dense and smooth surface without evident cracks. It is found that the average grain size for NBT, NBTZr and NBTZr/ NBTFe films decrease gradually. Generally, the grain growth is controlled by the motion of grain boundaries, which depends on the diffusivities of the constituent ions. Thus, compared with that of the undoped NBT film (80 nm), the smaller size for NBTZr film of 70 nm may be due to the lower diffusivity of Zr4 þ compared to that of Ti4 þ . When NBTZr is deposited on NBTFe/ITO, the NBTFe seed layer can offer more nucleation sites and reduce the activation energy for the crystallization of NBTZr film, then result in smaller uniform grain with 50 nm in size. As can be seen from the crosssectional images, the NBT, NBTZr and NBTZr/NBTFe samples are all estimated to be about 450 nm in thickness, which is used for the calculation of the relatively dielectric permittivity. However, some voids or grooves are present in the interfaces between some layers in all the films. These defects may be generated due to the fact that, for the route of the film crystallization with sequential annealing process layer by layer, each deposited layer with different smoothness by firing in RTA furnace would affect the next deposition layer. This phenomenon is popularly present in the sequential-layer annealed films and its elimination needs to be further investigated [4,19]. Fig. 3 shows the typical polarization–electric field (P–E) hysteresis loops of all the film samples. One can find that NBT film shows a poor P–E hysteresis loop due to the large contribution from the leakage current. However, the NBTZr
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Fig. 2. Surface morphologies and cross-sectional images of thin films: (a) and (b) NBT, (c) and (d) NBTZr and (e) and (f) NBTZr/NBTFe.
exhibits much improved P–E loop with a slim shaped feature by Zr4 þ doping. In sharp contrast, the bilayered NBTZr/ NBTFe films exhibit a well-defined hysteresis loop with a large remanent polarization (Pr) of 21 μC/cm2 at the applied electric field of 700 kV/cm. In our work, the biggest Pr value of NBTZr/NBTFe film can be ascribed to the following three
aspects. (i) The TiO6 octahedron can be distorted by the introduction of Zr4 þ with larger ionic radius [20]. (ii) The NBTFe seed layer inhibits the charge transport since the Fe substitution can make the NBT film to bear high applied electric field by the formation of defect complexes, as reported in our previous work [13]. (iii) Since the grain boundary acts
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C.H. Yang et al. / Ceramics International 41 (2015) 859–863
Fig. 3. P–E hysteresis loops of thin films as a function of the electric field at 10 kHz: (a) NBT, (b) NBTZr, and (c) NBTZr/NBTFe.
Fig. 4. Capacitance–voltage curves for thin films as a function of the applied voltage: (a) NBT, (b) NBTZr, and (c) NBTZr/NBTFe.
Fig. 5. Frequency dependence of relative dielectric constant and dissipation factor for NBT, NBTZr and NBTZr/NBTFe thin films.
as the potent leakage current path, the leakage current can be restricted by the small grain size, as displayed in Fig. 2. Fig. 4 illustrates the capacitance–voltage (C–V) characteristics of the films. In the C–V curve, each capacitance value measured at a certain bias voltage point represents the slope of the ferroelectric hysteresis at the corresponding applied voltage. Considering this relationship, the typical butterfly shape
obtained for all films confirms the ferroelectric nature of each film again. For NBTZr/NBTFe, high capacitance variation (i.e., a relatively sharp feature), which is related to the polarization, is observed. This demonstrates that the degree of ferroelectricity of NBTZr/NBTFe is strong [21]. The dielectric constant (εr) and dissipation factor (tanδ) were measured as a function of measuring frequency by applying a small ac voltage of 0.01 V, as shown in Fig. 5. It can be seen that the εr of NBT film is decreased obviously with Zr4 þ substitution, which is consistent with the result of the Zr4 þ modified NBT ceramics [22]. As for NBTZr/NBTFe, it shows an intermediate εr value of 325 and a minimum tanδ of 0.08 at 100 kHz, as compared to NBT and NBTZr films. Also, the NBTZr/NBTFe film shows a small dielectric dispersion, which indicates its relatively low defect concentration.
4. Conclusions In summary, the polycrystalline NBTZr/NBTFe thin film was prepared on ITO/glass by metal organic decomposition. The results show that the NBTFe seed layer plays a significant role on the microstructure and electrical properties of the NBTZr film. The bilayered NBTZr/NBTFe thin film exhibits enhanced
C.H. Yang et al. / Ceramics International 41 (2015) 859–863
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