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Enhanced ferroelectric and dielectric properties of Nb5+-doped Na0.5Bi0.5TiO3 thin film deposited under nitrogen annealing atmosphere
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CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 10272–10275 www.elsevier.com/locate/ceramint
Short communication
Enhanced ferroelectric and dielectric properties of Nb5 þ -doped Na0.5Bi0.5TiO3 thin film deposited under nitrogen annealing atmosphere C.H. Yanga,b,n, H.T. Suia, H.T. Wua, C. Fenga, F.J. Genga, Y.Y. Yaoa 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 21 January 2015; received in revised form 5 March 2015; accepted 5 March 2015 Available online 11 March 2015
Abstract Na0.5Bi0.5(Ti0.98Nb0.02)O3 þ δ (NBTNb) thin films are fabricated under N2 and O2, respectively. For comparison, the Na0.5Bi0.5TiO3 (NBT) thin film under O2 is also prepared. The co-effects of Nb5 þ -doping and annealing atmosphere on microstructure, ferroelectric and dielectric properties of NBT thin film are investigated. The XRD results show that all the thin films have crystallized into phase-pure polycrystalline perovskite structure. Under O2 annealing ambience, compared with NBT thin film, enhanced ferroelectricity is achieved for NBTNb, which can be ascribed to the reduced leakage current and TiO6 octahedral distortion. Furthermore, the NBTNb annealed under N2 exhibits a well-defined polarization– electric field hysteresis loop with a larger remanent polarization (Pr) of 10.7 μC/cm2, which can be due to the well-crystallized structure with smooth surface and high densification. Also, the dielectric constant (εr) and dissipation factor (tanδ) versus frequency show smaller dispersion with a εr of 402 and tanδ of 0.065 at 100 kHz. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Films; C. Ferroelectric properties; D. Perovskite
1. Introduction Lead-free sodium bismuth titanate [Na0.5Bi0.5TiO3, abbreviated as NBT] is considered principally important in theoretical and experimental research since it possesses strong ferroelectricity with a remanent polarization (Pr) of 38 μC/cm2 and Curie temperature (Tc) of 320 1C [1]. Especially, NBT thin film shows its promising potential for the practical applications in electronic industry. However, it is difficult for NBT thin film to be polarized, which arises from the large coercive field and high conductivity [2]. This can be mainly attributed to the nonstoichiometry-related problem, which is originated from the formation of oxygen vacancies [ðVO2 Þ ] brought by the volatilization of Na þ or Bi3 þ during an annealing process [3].
n Corresponding author at: School of Materials Science and Engineering, University of Jinan, Jinan 250022, China. Tel.: þ86 531 88374857; fax: þ86 531 87974453. E-mail address: [email protected] (C.H. Yang).
http://dx.doi.org/10.1016/j.ceramint.2015.03.022 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Based on this fact, many studies have concentrated on the modifications of NBT with acceptor or donor dopants. Generally, by doping low valence ions, leakage current can be reduced within a certain degree due to the formation of defect complex. It is reported that, by doping Mn2 þ , 3 þ with 6 mol% concentration into (Na0.85K0.15)0.5Bi0.5TiO3 thin film, Pr value can be increased to 19.2 μC/cm2 [4]. Our previous work demonstrates that with Fe3 þ doping, the leakage current density is reduced by about two orders of magnitudes due to the formation of defect complexes between ðVO2 Þ and ðFe3Tiþ4 þ Þ0 [5]. Whereas, in fact, doping low valence ions could result in some problems, for example, aging effect. This may cause asymmetric coercivity and double hysteresis loop [6]. Also, as the electric field applied to the thin film increasing, the electrostatic attraction force between defect complexes may be overcome, which in turn lead to the gradual release of ðVO2 Þ . On the other hand, doping isovalent ions may not generate any additional charge carriers and the performance can be tailored by the introduction of lattice distortion. In our recent study, enhanced ferroelectricity is achieved in Zr4 þ -doped NBT thin film with a Pr value of 19.2 μC/cm2 [7]. However, doping isovalent ions cannot
C.H. Yang et al. / Ceramics International 41 (2015) 10272–10275
do any help to the decrease of leakage current. For high valence doping, it possesses more positive charge carriers than Ti4 þ , which will cause the formation of defects with extra positive charge carriers, for example, ðNb5Tiþ4 þ Þ , then resulting in the elimination of ðVO2 Þ due to the charge-neutrality principal. For other thin films, such as BiFeO3, high valence substitution of W6 þ with 1 at% concentration can effectively suppress the formation of ðVO2 Þ and improve the piezoelectric property with the piezoelectric coefficient of 132 pm/V [8]. Nevertheless, to our knowledge, NBT with high valence ions has rarely been reported. Therefore, the Nb5 þ -doped NBT thin film attracts our attention. For ferroelectric thin film prepared by chemical solution decomposition (CSD), the parameters of preparing process, especially the annealing atmosphere may have a direct effect on the ðVO2 Þ content [9]. Therefore, in this work, we choose Na0.5Bi0.5(Ti0.98Nb0.02)O3 þ δ (NBTNb) thin films deposited on indium tin oxide (ITO)/glass annealed under N2 and O2 for investigated and NBT thin film annealed under O2 is studied for comparison. The co-effects of Nb5 þ -doping and annealing atmosphere on microstructure, ferroelectric and dielectric properties for NBT thin film are discussed. 2. Experimental procedure NBT and NBTNb thin films were fabricated on ITO/glass via CSD. The precursor solutions were prepared by dissolving sodium acetate (CH3COONa), bismuth nitrate [Bi(NO3)3 5H2O], titanium isopropoxide {Ti[OCH(CH3)2]4} and niobium ethoxide [Nb (OC2H5)5] according to the stoichiometric ratio into the mixture of ethylene glycol and acetic acid. Acetylacetone was added to the mixture to make the precursor solutions stable. Subsequently, 20% by weight of ethylene glycol monomer with molecular weighs of 600 was added on the basis of the final product of metal oxide. The precursor solutions for NBT and NBTNb were both deposited on ITO/glass by spin coating and annealed at 500 1C under selected atmospheres (NBT was annealed under O2, NBTNb were annealed under O2 and N2, respectively) by a rapid thermal annealing process until a certain thickness for each film was obtained. The structure of the films was examined by an X-ray diffractometer (Bruker D8). The surface morphology and crosssectional structure were detected by a field-emission scanning electron microscope (FeSEM, Hitachi S-4200). The ferroelectric property was measured by a standard ferroelectric tester (Radiant Technologies). The dielectric property was texted by an impedance analyzer (HP4294A). 3. Results and discussions Fig. 1 shows XRD patterns of NBT and NBTNb thin films on ITO/glass substrates annealed under modified atmospheres. All the films are crystallized into phase-pure perovskite structure, which can be attributed to the high uniformity of precursor solution, as well as the favorable effects of nucleation and growth from the oxide electrode [10,11]. Regardless of the annealing atmosphere, the films have no preferential orientation, which indicates the polycrystalline nature. As calculated by the Lotgering factor, the degrees for (110) and (100) orientations of the NBTNb film under
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Fig. 1. XRD patterns of ITO, as well as NBT and NBTNb thin films annealed under modified atmospheres. The inserts show two magnified patterns in the vicinity of 2θ¼ 321 and 471.
N2 (O2) are 0.423 (0.495) and 0.354 (0.270), respectively. Although the NBTNb thin films show predominant (110)-orientation despite the annealing atmosphere, a slightly competitive growth mode between (110) and (100) orientations has been reflected. This can be ascribed to two possible factors. (i) The minimization of the surface energy for the crystalline orientation is dependent on the annealing ambience [12]. (ii) The nucleation and growth rates along the surface normal may be diverse, resulting in the changeable predominant orientation [13]. Two subtle XRD patterns are shown in the inserts of Fig. 1. With Nb5 þ -doping, the (l10) and (100) peaks for NBTNb thin film shift to lower diffraction angle due to the expansion of the unit-cell as Ti4 þ is substituted by bigger Nb5 þ in the host lattice (r4Tiþ ¼ 0.605 Å, r5Nbþ ¼ 0.64 Å). The FeSEM micrographs of NBT and NBTNb thin films annealed under modified ambience are displayed in Fig. 2(a)–(f). All the films exhibit surfaces without any cracks and consist of evenly distributed grain size, except for a few voids scattered in the films due to the decomposition of the residual organic component during annealing [14]. One can note that, compared with the NBT thin film, grain sizes for both the NBTNb films tend to decrease obviously. The NBTNb thin film annealed under N2 possesses densified structure and exaggerated grain growth with an average grain size of 70 nm, while that of the NBTNb under O2 is approximately 50 nm. Usually, the grain size is highly dependent on the rates for the nucleation and grain growth. Compared with the NBTNb thin film annealed under O2, the bigger grain size for the one annealed under N2 is originated from the higher ðVO2 Þ content, which favors the transfer of mass and energy between reactants during annealing [15]. As can be seen from the crosssectional images, all the thin films are estimated to be 500 nm in thickness. Especially, the NBTNb thin film annealed under N2 exhibits uniform grain size and less content of voids, compared with the other two films annealed under O2. Fig. 3 shows the typical polarization–electric field (P–E) hysteresis loops for NBT and NBTNb thin films annealed under modified atmospheres. The applied electric field is the maximum that the film can withstand. One can find that the NBT film exhibits a poor P–E loop reflected by the round shape and with
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C.H. Yang et al. / Ceramics International 41 (2015) 10272–10275
Fig. 2. Surface morphologies and cross-sectional images of thin films: (a) and (d) NBT under O2, (b) and (e) NBTNb under O2, (c) and (f) NBTNb under N2.
Fig. 3. P–E hysteresis loops of NBT and NBTNb thin films annealed under modified atmospheres as a function of the electric field at 10 kHz: (a) and (b) O2, (c) N2. The inserts are the applied voltage dependence of capacitance.
Nb5 þ -doping, the ferroelectricity is obviously improved. Meanwhile, at 700 kV/cm, the remanent polarization (Pr) for the NBTNb annealed under N2 is 10.7 μC/cm2, which is larger than that of the film under O2 (7.3 μC/cm2). As is reported, the Pr value of ferroelectric thin film is dependent on several factors, such as the matching degree of the substrate and thin film [15], orientation [16], crystalline quality [17] and so on. For the NBTNb film annealed under N2, the enhanced ferroelectricity can be attributed to the following factors. (i) Nb5 þ -doping could lead to less content of ðVO2 Þ , resulting in the decrease of leakage current. (ii) The introduction of Nb5 þ may bring in distortion of TiO6 octahedron. (iii) The densified structure without obvious porosity makes the reorientation of ferroelectric domains easier under applied electric field [14]. The inserts of Fig. 3 illustrate the capacitance-voltage (C–V) curves for all the film samples. All the films exhibit butterfly-shaped variation of capacitance with the applied voltage, providing the evidence of ferroelectricity of the NBT and NBTNb
thin films. Usually, a higher capacitance variation is directly related to the polarization [18]. And the value of ΔC/C0 (where ΔC is the change in capacitance relative to the capacitance at zero voltage C0), which is defined as dielectric tunability, for the NBTNb film under N2 is calculated to be 40.36%, while those of the NBT and NBTNb in O2 are 31.58% and 36.10%, respectively. This is consistent with the results as demonstrated in Fig. 3(a)–(c). The dielectric constant (εr) and dissipation loss (tanδ) of NBT and NBTNb thin films annealed under modified atmospheres as a function of operating frequency, measured at room temperature, are shown in Fig. 4. The εr for all the film samples tend to decline in the whole frequency range, while the tanδ values decrease firstly before 50 kHz and then ascend. Obviously, the NBTNb thin film annealed under N2 shows a small dispersion, compared with those of the NBT and NBTNb films under O2, indicating the existence of relatively lower defect concentration [19]. Also, at 100 kHz, the NBTNb thin film annealed under N2 possesses the
C.H. Yang et al. / Ceramics International 41 (2015) 10272–10275
Fig. 4. Frequency dependences of dielectric constant and dissipation factor for NBT and NBTNb thin films annealed under modified atmospheres.
biggest εr of 402 and the smallest tanδ of 0.065. As known that, the microstructure affects the performance significantly [20]. The biggest εr for NBTNb thin film under N2 can be ascribed to the largest grain sizes and few voids as can be seen from Fig. 2 [21]. 4. Conclusions In summary, we have prepared polycrystalline NBTNb thin films under N2 and O2, respectively, and NBT thin film under O2 on ITO/glass. The NBTNb thin film annealed under N2 exhibits better ferroelectricity with a Pr of 10.7 μC/cm2 due to the reduced leakage current, high densification and TiO6 octahedral distortion. Also, stable dielectric properties with smaller dispersion tendency is obtained (at 100 kHz, εr ¼ 402, tanδ¼ 0.065, dielectric tunability of 40.36%). 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] A. Andrei, N.D. Scarisoreanu, R. Birjega, M. Dinescu, G. Stanciu, F. Craciun, C. Galassi, Pulsed laser deposition of lead-free (Na0.5Bi0.5)1 xBaxTiO3 ferroelectric thin films with enhanced dielectric properties, Appl. Surf. Sci. 278 (2013) 162–165. [2] S.K. Acharya, S.-K. Lee, J.-H. Hyung, Y.-H. Yang, B.-H. Kim, B.-G. Ahn, Ferroelectric and piezoelectric properties of lead-free BaTiO3 doped Bi0.5Na0.5TiO3 thin films from metal–organic solution deposition, J. Alloy. Compd. 540 (2012) 204–209. [3] 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.
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[4] Y.Y. Wu, X.H. Wang, C.F. Zhong, L.T. Li, Effect of Mn doping on microstructure and electrical properties of the (Na0.85K0.15)0.5Bi0.5TiO3 thin films prepared by sol–gel method, J. Am. Ceram. Soc. 94 (2011) 3877–3882. [5] C.H. Yang, G.D. Hu, W.B. Wu, H.T. Wu, F. Yang, Reduced leakage current, enhanced ferroelectric and dielectric properties in (Ce, Fe)codoped Na0.5Bi0.5TiO3 film, Appl. Phys. Lett. 100 (2012) 022909. [6] X. Wang, G.D. Hu, L. Cheng, C.H. Yang, W.B. Wu, Comparative study on aging effect in BiFeO3 thin films substituted at A- and B-sites, Appl. Phys. Lett. 99 (2011) 262901. [7] H.T. Sui, C.H. Yang, F.J. Geng, C. Feng, Effects of Zr doping content on microstructure, ferroelectric and dielectric properties of Na0.5Bi0.5TiO3 thin films, Mater. Lett. 139 (2015) 284–287. [8] L. Cheng, G.D. Hu, B. Jiang, C.H. Yang, W.B. Wu, S.H. Fan, Enhanced piezoelectric properties of epitaxial W-doped BiFeO3 thin films, Appl. Phys. Express 3 (2010) 101501. [9] 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. [10] F. Remondiere, B. Malič, M. Kosec, J.-P. Mercurio, Study of the crystallization pathway of Na0.5Bi0.5TiO3 thin films obtained by chemical solution deposition, J. Sol-Gel. Sci. Technol. 46 (2008) 117–125. [11] J. Yan, G.D. Hu, X.M. Chen, W.B. Wu, C.H. Yang, Ferroelectric properties, morphologies, and leakage currents of Bi0.97La0.03FeO3 thin films deposited on indium tin oxide/glass substrates, J. Appl. Phys. 104 (2008) 076103. [12] J. Yan, G.D. Hu, Z.M. Liu, S.H. Fan, Y. Zhou, C.H. Yang, Enhanced ferroelectric properties of predominantly (100)-oriented CaBi4Ti4O15 thin films on Pt/Ti/SiO2/Si substrates, J. Appl. Phys. 103 (2008) 056109. [13] X.M. Chen, G.D. Hu, J. Yan, X. Wang, C.H. Yang, W.B. Wu, Enhanced multiferroic properties of (110)-oriented BiFeO3 film deposited on Bi3.5Nd0.5Ti3O12-buffered indium tin oxide/Si substrate, J. Phys. D: Appl. Phys. 41 (2008) 225402. [14] S.H. Yu, K. Yao, S. Shannigrahi, F.T.E. Hock, Effects of poly(ethylene glycol) additive molecular weight on the microstructure and properties of sol–gel-derived lead zirconate titanate thin films, J. Mater. Res. 18 (2003) 737–741. [15] H.Y. Liu, Y.P. Pu, X. Shi, Q.B. Yuan, Dielectric and ferroelectric properties of BiFeO3 ceramics sintered in different atmospheres, Ceram. Int. 39 (2013) S217–S220. [16] X.J. Zeng, S.H. Dai, X. Feng, T. Zhang, D.Z. Zhang, Y.Q. Gong, Y.Q. Chen, L. He, Structural and electrical properties of (Na0.85K0.15)0.5Bi0.5TiO3 thin films deposited on LaNiO3 and Pt bottom electrodes, Appl. Surf. Sci. 256 (2010) 3316–3320. [17] 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. [18] J.G. Wu, D.Q. Xiao, Y.Y. Wang, J.G. Zhu, J.L. Zhu, R.S. Xie, High tunability of highly (100)-oriented lead zirconate titanium thin films, J. Am. Ceram. Soc. 91 (2008) 3786–3788. [19] Y.J. Ren, X.H. Zhu, C.Y. Zhang, J.L. Zhu, J.G. Zhu, D.Q. Xiao, High stable dielectric permittivity and low dielectric loss in sol–gel derived BiFeO3 thin films, Ceram. Int. 40 (2014) 2489–2493. [20] Y. Ye, T.L. Guo, Dielectric properties of Fe-doped Ba0.65Sr0.35TiO3 thin films fabricated by the sol–gel method, Ceram. Int. 35 (2009) 2761–2765. [21] W. Li, H.R. Zeng, J.G. Hao, J.W. Zhai, Enhanced dielectric and piezoelectric properties of Mn doped (Bi0.5Na0.5)TiO3–(Bi0.5K0.5)TiO3– SrTiO3 thin films, J. Alloy. Compd. 580 (2013) 157–161.
CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 10272–10275 www.elsevier.com/locate/ceramint
Short communication
Enhanced ferroelectric and dielectric properties of Nb5 þ -doped Na0.5Bi0.5TiO3 thin film deposited under nitrogen annealing atmosphere C.H. Yanga,b,n, H.T. Suia, H.T. Wua, C. Fenga, F.J. Genga, Y.Y. Yaoa 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 21 January 2015; received in revised form 5 March 2015; accepted 5 March 2015 Available online 11 March 2015
Abstract Na0.5Bi0.5(Ti0.98Nb0.02)O3 þ δ (NBTNb) thin films are fabricated under N2 and O2, respectively. For comparison, the Na0.5Bi0.5TiO3 (NBT) thin film under O2 is also prepared. The co-effects of Nb5 þ -doping and annealing atmosphere on microstructure, ferroelectric and dielectric properties of NBT thin film are investigated. The XRD results show that all the thin films have crystallized into phase-pure polycrystalline perovskite structure. Under O2 annealing ambience, compared with NBT thin film, enhanced ferroelectricity is achieved for NBTNb, which can be ascribed to the reduced leakage current and TiO6 octahedral distortion. Furthermore, the NBTNb annealed under N2 exhibits a well-defined polarization– electric field hysteresis loop with a larger remanent polarization (Pr) of 10.7 μC/cm2, which can be due to the well-crystallized structure with smooth surface and high densification. Also, the dielectric constant (εr) and dissipation factor (tanδ) versus frequency show smaller dispersion with a εr of 402 and tanδ of 0.065 at 100 kHz. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Films; C. Ferroelectric properties; D. Perovskite
1. Introduction Lead-free sodium bismuth titanate [Na0.5Bi0.5TiO3, abbreviated as NBT] is considered principally important in theoretical and experimental research since it possesses strong ferroelectricity with a remanent polarization (Pr) of 38 μC/cm2 and Curie temperature (Tc) of 320 1C [1]. Especially, NBT thin film shows its promising potential for the practical applications in electronic industry. However, it is difficult for NBT thin film to be polarized, which arises from the large coercive field and high conductivity [2]. This can be mainly attributed to the nonstoichiometry-related problem, which is originated from the formation of oxygen vacancies [ðVO2 Þ ] brought by the volatilization of Na þ or Bi3 þ during an annealing process [3].
n Corresponding author at: School of Materials Science and Engineering, University of Jinan, Jinan 250022, China. Tel.: þ86 531 88374857; fax: þ86 531 87974453. E-mail address: [email protected] (C.H. Yang).
http://dx.doi.org/10.1016/j.ceramint.2015.03.022 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Based on this fact, many studies have concentrated on the modifications of NBT with acceptor or donor dopants. Generally, by doping low valence ions, leakage current can be reduced within a certain degree due to the formation of defect complex. It is reported that, by doping Mn2 þ , 3 þ with 6 mol% concentration into (Na0.85K0.15)0.5Bi0.5TiO3 thin film, Pr value can be increased to 19.2 μC/cm2 [4]. Our previous work demonstrates that with Fe3 þ doping, the leakage current density is reduced by about two orders of magnitudes due to the formation of defect complexes between ðVO2 Þ and ðFe3Tiþ4 þ Þ0 [5]. Whereas, in fact, doping low valence ions could result in some problems, for example, aging effect. This may cause asymmetric coercivity and double hysteresis loop [6]. Also, as the electric field applied to the thin film increasing, the electrostatic attraction force between defect complexes may be overcome, which in turn lead to the gradual release of ðVO2 Þ . On the other hand, doping isovalent ions may not generate any additional charge carriers and the performance can be tailored by the introduction of lattice distortion. In our recent study, enhanced ferroelectricity is achieved in Zr4 þ -doped NBT thin film with a Pr value of 19.2 μC/cm2 [7]. However, doping isovalent ions cannot
C.H. Yang et al. / Ceramics International 41 (2015) 10272–10275
do any help to the decrease of leakage current. For high valence doping, it possesses more positive charge carriers than Ti4 þ , which will cause the formation of defects with extra positive charge carriers, for example, ðNb5Tiþ4 þ Þ , then resulting in the elimination of ðVO2 Þ due to the charge-neutrality principal. For other thin films, such as BiFeO3, high valence substitution of W6 þ with 1 at% concentration can effectively suppress the formation of ðVO2 Þ and improve the piezoelectric property with the piezoelectric coefficient of 132 pm/V [8]. Nevertheless, to our knowledge, NBT with high valence ions has rarely been reported. Therefore, the Nb5 þ -doped NBT thin film attracts our attention. For ferroelectric thin film prepared by chemical solution decomposition (CSD), the parameters of preparing process, especially the annealing atmosphere may have a direct effect on the ðVO2 Þ content [9]. Therefore, in this work, we choose Na0.5Bi0.5(Ti0.98Nb0.02)O3 þ δ (NBTNb) thin films deposited on indium tin oxide (ITO)/glass annealed under N2 and O2 for investigated and NBT thin film annealed under O2 is studied for comparison. The co-effects of Nb5 þ -doping and annealing atmosphere on microstructure, ferroelectric and dielectric properties for NBT thin film are discussed. 2. Experimental procedure NBT and NBTNb thin films were fabricated on ITO/glass via CSD. The precursor solutions were prepared by dissolving sodium acetate (CH3COONa), bismuth nitrate [Bi(NO3)3 5H2O], titanium isopropoxide {Ti[OCH(CH3)2]4} and niobium ethoxide [Nb (OC2H5)5] according to the stoichiometric ratio into the mixture of ethylene glycol and acetic acid. Acetylacetone was added to the mixture to make the precursor solutions stable. Subsequently, 20% by weight of ethylene glycol monomer with molecular weighs of 600 was added on the basis of the final product of metal oxide. The precursor solutions for NBT and NBTNb were both deposited on ITO/glass by spin coating and annealed at 500 1C under selected atmospheres (NBT was annealed under O2, NBTNb were annealed under O2 and N2, respectively) by a rapid thermal annealing process until a certain thickness for each film was obtained. The structure of the films was examined by an X-ray diffractometer (Bruker D8). The surface morphology and crosssectional structure were detected by a field-emission scanning electron microscope (FeSEM, Hitachi S-4200). The ferroelectric property was measured by a standard ferroelectric tester (Radiant Technologies). The dielectric property was texted by an impedance analyzer (HP4294A). 3. Results and discussions Fig. 1 shows XRD patterns of NBT and NBTNb thin films on ITO/glass substrates annealed under modified atmospheres. All the films are crystallized into phase-pure perovskite structure, which can be attributed to the high uniformity of precursor solution, as well as the favorable effects of nucleation and growth from the oxide electrode [10,11]. Regardless of the annealing atmosphere, the films have no preferential orientation, which indicates the polycrystalline nature. As calculated by the Lotgering factor, the degrees for (110) and (100) orientations of the NBTNb film under
10273
Fig. 1. XRD patterns of ITO, as well as NBT and NBTNb thin films annealed under modified atmospheres. The inserts show two magnified patterns in the vicinity of 2θ¼ 321 and 471.
N2 (O2) are 0.423 (0.495) and 0.354 (0.270), respectively. Although the NBTNb thin films show predominant (110)-orientation despite the annealing atmosphere, a slightly competitive growth mode between (110) and (100) orientations has been reflected. This can be ascribed to two possible factors. (i) The minimization of the surface energy for the crystalline orientation is dependent on the annealing ambience [12]. (ii) The nucleation and growth rates along the surface normal may be diverse, resulting in the changeable predominant orientation [13]. Two subtle XRD patterns are shown in the inserts of Fig. 1. With Nb5 þ -doping, the (l10) and (100) peaks for NBTNb thin film shift to lower diffraction angle due to the expansion of the unit-cell as Ti4 þ is substituted by bigger Nb5 þ in the host lattice (r4Tiþ ¼ 0.605 Å, r5Nbþ ¼ 0.64 Å). The FeSEM micrographs of NBT and NBTNb thin films annealed under modified ambience are displayed in Fig. 2(a)–(f). All the films exhibit surfaces without any cracks and consist of evenly distributed grain size, except for a few voids scattered in the films due to the decomposition of the residual organic component during annealing [14]. One can note that, compared with the NBT thin film, grain sizes for both the NBTNb films tend to decrease obviously. The NBTNb thin film annealed under N2 possesses densified structure and exaggerated grain growth with an average grain size of 70 nm, while that of the NBTNb under O2 is approximately 50 nm. Usually, the grain size is highly dependent on the rates for the nucleation and grain growth. Compared with the NBTNb thin film annealed under O2, the bigger grain size for the one annealed under N2 is originated from the higher ðVO2 Þ content, which favors the transfer of mass and energy between reactants during annealing [15]. As can be seen from the crosssectional images, all the thin films are estimated to be 500 nm in thickness. Especially, the NBTNb thin film annealed under N2 exhibits uniform grain size and less content of voids, compared with the other two films annealed under O2. Fig. 3 shows the typical polarization–electric field (P–E) hysteresis loops for NBT and NBTNb thin films annealed under modified atmospheres. The applied electric field is the maximum that the film can withstand. One can find that the NBT film exhibits a poor P–E loop reflected by the round shape and with
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C.H. Yang et al. / Ceramics International 41 (2015) 10272–10275
Fig. 2. Surface morphologies and cross-sectional images of thin films: (a) and (d) NBT under O2, (b) and (e) NBTNb under O2, (c) and (f) NBTNb under N2.
Fig. 3. P–E hysteresis loops of NBT and NBTNb thin films annealed under modified atmospheres as a function of the electric field at 10 kHz: (a) and (b) O2, (c) N2. The inserts are the applied voltage dependence of capacitance.
Nb5 þ -doping, the ferroelectricity is obviously improved. Meanwhile, at 700 kV/cm, the remanent polarization (Pr) for the NBTNb annealed under N2 is 10.7 μC/cm2, which is larger than that of the film under O2 (7.3 μC/cm2). As is reported, the Pr value of ferroelectric thin film is dependent on several factors, such as the matching degree of the substrate and thin film [15], orientation [16], crystalline quality [17] and so on. For the NBTNb film annealed under N2, the enhanced ferroelectricity can be attributed to the following factors. (i) Nb5 þ -doping could lead to less content of ðVO2 Þ , resulting in the decrease of leakage current. (ii) The introduction of Nb5 þ may bring in distortion of TiO6 octahedron. (iii) The densified structure without obvious porosity makes the reorientation of ferroelectric domains easier under applied electric field [14]. The inserts of Fig. 3 illustrate the capacitance-voltage (C–V) curves for all the film samples. All the films exhibit butterfly-shaped variation of capacitance with the applied voltage, providing the evidence of ferroelectricity of the NBT and NBTNb
thin films. Usually, a higher capacitance variation is directly related to the polarization [18]. And the value of ΔC/C0 (where ΔC is the change in capacitance relative to the capacitance at zero voltage C0), which is defined as dielectric tunability, for the NBTNb film under N2 is calculated to be 40.36%, while those of the NBT and NBTNb in O2 are 31.58% and 36.10%, respectively. This is consistent with the results as demonstrated in Fig. 3(a)–(c). The dielectric constant (εr) and dissipation loss (tanδ) of NBT and NBTNb thin films annealed under modified atmospheres as a function of operating frequency, measured at room temperature, are shown in Fig. 4. The εr for all the film samples tend to decline in the whole frequency range, while the tanδ values decrease firstly before 50 kHz and then ascend. Obviously, the NBTNb thin film annealed under N2 shows a small dispersion, compared with those of the NBT and NBTNb films under O2, indicating the existence of relatively lower defect concentration [19]. Also, at 100 kHz, the NBTNb thin film annealed under N2 possesses the
C.H. Yang et al. / Ceramics International 41 (2015) 10272–10275
Fig. 4. Frequency dependences of dielectric constant and dissipation factor for NBT and NBTNb thin films annealed under modified atmospheres.
biggest εr of 402 and the smallest tanδ of 0.065. As known that, the microstructure affects the performance significantly [20]. The biggest εr for NBTNb thin film under N2 can be ascribed to the largest grain sizes and few voids as can be seen from Fig. 2 [21]. 4. Conclusions In summary, we have prepared polycrystalline NBTNb thin films under N2 and O2, respectively, and NBT thin film under O2 on ITO/glass. The NBTNb thin film annealed under N2 exhibits better ferroelectricity with a Pr of 10.7 μC/cm2 due to the reduced leakage current, high densification and TiO6 octahedral distortion. Also, stable dielectric properties with smaller dispersion tendency is obtained (at 100 kHz, εr ¼ 402, tanδ¼ 0.065, dielectric tunability of 40.36%). 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] A. Andrei, N.D. Scarisoreanu, R. Birjega, M. Dinescu, G. Stanciu, F. Craciun, C. Galassi, Pulsed laser deposition of lead-free (Na0.5Bi0.5)1 xBaxTiO3 ferroelectric thin films with enhanced dielectric properties, Appl. Surf. Sci. 278 (2013) 162–165. [2] S.K. Acharya, S.-K. Lee, J.-H. Hyung, Y.-H. Yang, B.-H. Kim, B.-G. Ahn, Ferroelectric and piezoelectric properties of lead-free BaTiO3 doped Bi0.5Na0.5TiO3 thin films from metal–organic solution deposition, J. Alloy. Compd. 540 (2012) 204–209. [3] 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.
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