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Effect of Fe doping on the crystallization and electrical properties of Na0.5Bi0.5TiO3 thin film
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CERAMICS INTERNATIONAL
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Short communication
Effect of Fe doping on the crystallization and electrical properties of Na0.5Bi0.5TiO3 thin film C. Fenga, C.H. Yanga,b,n, H.T. Suia, F.J. Genga, Y.J. Hana 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 4 September 2014; received in revised form 10 October 2014; accepted 11 October 2014
Abstract Na0.5Bi0.5Ti1 xFexO3 δ (abbreviated as NBTFex, x ¼0.005, 0.01, 0.02 and 0.04) thin films were deposited on indium tin oxide/glass substrates via a chemical solution deposition method. The influence of Fe2 þ doping on the crystallization and electrical properties of Na0.5Bi0.5TiO3 was investigated. All the films crystallize into the single perovskite structure without any secondary phases. The insulating measurement reveals that 2 at% Fe2 þ doping can effectively reduce the leakage current of Na0.5Bi0.5TiO3 thin film by the formation of defect complexes. The enhanced ferroelectricity is obtained in NBTFe0.02 with a large remanent polarization (Pr) of 20 μC/cm2 due to the lowest leakage current and the distortion of TiO6. The normalized capacitance–voltage curves of all films agree with the polarization-electric field hysteresis loops. Also, the relative dielectric constant and dissipation factor of NBTFe0.02 thin film are 450 and 0.094 at 100 kHz, respectively. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Film; C. Dielectric properties; C. Ferroelectric properties; Crystallization
1. Introduction Lead-based compounds, represented by Pb(Zr,Ti)O3, Pb (Mg,Nb)O3, etc. are widely used in electric industry, with applications in ferroelectric memories, sensors, piezoelectric transducers and actuators. However, during the process of production and application, lead volatilization has caused serious environmental problems. Therefore, people have been long involved in searching for lead-free alternative materials. Of all, Bi-based perovskites have good ferroelectric property because Bi3 þ has an electronic configuration similar to Pb2 þ [1]. Sodium bismuth titanate (Na0.5Bi0.5TiO3, NBT) is considered to be a potential candidate 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 for bulk ceramic [2]. But until now, for NBT n Corresponding author at: School of Materials Science and Engineering, University of Jinan, Jinan 250022, China. Tel.: þ 86 531 889736280; fax: þ86 531 87974453. E-mail address: [email protected] (C.H. Yang).
thin film, its intrinsic performance is difficult to be measured due to the high leakage current, which is strongly dependent on oxygen vacancies [ðVO2 Þ ] produced by the volatilization of A-site elements [3,4]. In recent years, people are trying to improve electrical properties for NBT thin film by a variety of methods, such as cation substitution [5], formation of multilayered structure [6], control of orientation [7], and forming solid solutions with other components [8–12]. Among them, site engineering is supposed to be an available way, not only for reducing the leakage current but also for improving the ferroelectric property of NBT film [13]. The leakage current density of NBT film can be dramatically decreased by nearly two orders of magnitudes with Mn doping [14]. The NBT film with a large Pr of 29.5 μC/cm2 can be obtained by co-substitution of La and Ce. Chemical solution deposition (CSD), with the advantage of chemical homogeneity, easy stoichiometry control and integration into devices as well as low cost, has been widely used in the preparing process for NBT-based thin film. In this work, we prepared Na0.5Bi0.5Ti1 xFexO3 δ (abbreviated as NBTFex,
http://dx.doi.org/10.1016/j.ceramint.2014.10.066 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: C. Feng, et al., Effect of Fe doping on the crystallization and electrical properties of Na0.5Bi0.5TiO3 thin film, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.10.066
C. Feng et al. / Ceramics International ] (]]]]) ]]]–]]]
2
x ¼ 0.005, 0.01, 0.02 and 0.04) thin films via CSD. The crystallization, ferroelectric and dielectric properties were investigated. The aim of this work is to find the optimal Fe2 þ doping content for improving the electrical properties of NBT thin film. 2. Experiment NBTFex (x¼ 0.005, 0.01, 0.02 and 0.04) thin films were deposited on indium tin oxide (ITO)/glass substrates via CSD. The precursor solutions of NBTFex were prepared using sodium acetate, bismuth acetate, ferric nitrate, tetrabutyl titanate as starting materials. First, to prevent hydrolysis of Ti, tetrabutyl titanate was stabilized by the addition of acetylacetone. Sodium acetate, bismuth acetate and ferric nitrate were dissolved in the mixture of ethylene glycol and acetic acid. 2% excess bismuth and 5% excess sodium were added to compensate the volatilization of bismuth and sodium. Subsequently, the Na–Bi–Fe complex solution was carefully dropped into the Ti solution. Also, a moderate amount of polyethylene glycol (PEG) was added into the final solution. Then, transparent and stable precursor solutions with the same concentration of 0.3 M were obtained. Each precursor solution was deposited onto ITO/glass by spin coating at 4000 rpm for 30 s. The wet film was pyrolyzed on a hot plate at 250 1C for 5 min and then annealed layer by layer at 500 1C for 10 min in the rapid thermal processor for complete crystallization. Finally, the process was repeated several times to obtain a desired thickness. The thicknesses of four thin films were all estimated to be 300 nm measured by a step (step obtained by chemical etching) profilometer made by the Ambios Technology Company of USA. The crystallization of the films was characterized using an x-ray diffractometer (Bruker D8). The x-ray radiation source of Cu Kα with a wavelength of 1.5406 nm was adopted at 40 kV and 25 mA. For electrical measurements, Au top electrodes were deposited on the films using a sputtering system to form the structure of metalferroelectric-ITO. A standard ferroelectric tester was used to exam the ferroelectricity. The dielectric properties were measured using an impedance analyzer (HP4294A). 3. Results and discussion The x-ray diffraction patterns of ITO and NBTFex (x¼ 0.005, 0.01, 0.02 and 0.04) thin films were scanned at 2.41/min in the range of 2θ ¼ 20–501, as shown in Fig. 1. It can be seen that all detectable diffraction peaks for NBTFex thin films match well with those of the distorted rhombohedral R3c structure. All the films have similar polycrystalline structure without any impurity phases after annealing at the low temperature of 500 1C. This may be attributed to the following factors. First, PEG can promote the formation of perovskite phase, which has been reported in our previous reports [15]. Second, each crystallized layer can serve as seeding layer for the next deposited one when NBTFex thin films are prepared using sequential layer annealing process. Third, the oxide electrode assists in the nucleation and growth of thin films
Fig. 1. XRD patterns of ITO (a) and NBTFex (b) x¼ 0.005, (c) x¼ 0.01, (d) x ¼0.02, (e) x¼ 0.04 thin films.
Fig. 2. Leakage current density as a function of electric field for NBTFex (x¼ 0.005, 0.01, 0.02 and 0.04) thin films.
[16]. Additionally, with the increase of Fe2 þ doping content, the relative intensity of peak decreases and the full width at half-maximum increases gradually, especially for the (1 1 0) orientation. Generally, the grain growth is controlled by the minimization of surface and grain boundary energies. This growth mode induced by the motion of grain boundaries depends on the diffusivities of constituent ions [16]. Therefore, the restriction of grain growth for NBT film by the Fe2 þ substitution may be due to the lower diffusivity of Fe2 þ compared to that of Ti4 þ . Fig. 2 illustrates the insulating characteristic of the NBTFex thin films. It can be seen that the leakage current density decreases as the Fe2 þ doping content increases from 0.5 to 2 at%. As is known, the leakage current for NBT-based thin film is strongly dependent on ðVO2 Þ . When Ti4 þ is occupied by the off-valent acceptor of Fe2 þ , the defect of ðFe2Tiþ4 þ Þ″ can be formed simultaneously. Then, it is not easy for the ðVO2 Þ to be movable due to the formation of defect complexes between ðFe2Tiþ4 þ Þ″ and ðVO2 Þ . The applied field that required for generating the mobile ðVO2 Þ would be increased since more electric energy is required to overcome the electrostatic attraction force of ðFe2Tiþ4 þ Þ″–ðVO2 Þ . Therefore, more ðVO2 Þ would be restricted with the increase of Fe2 þ doping content. However, as Fe2 þ doping content
Please cite this article as: C. Feng, et al., Effect of Fe doping on the crystallization and electrical properties of Na0.5Bi0.5TiO3 thin film, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.10.066
C. Feng et al. / Ceramics International ] (]]]]) ]]]–]]]
reaches 4 at%, the leakage current density increases abruptly. With the Fe2 þ doping content increasing, the grain size decreases gradually, which can be reflected from the XRD patterns. This in turn leads to the formation of more defects at long grain boundaries. Therefore, we can attribute the increasing leakage current density of NBTFe0.04 thin film to the fact that the positive effect of Fe2 þ doping on suppressing ðVO2 Þ is overwhelmed by the restriction effect of Fe2 þ on grain growth. It can be also found that high content of ion doping may result in poor electrical behavior in other perovskite films [17,18]. The polarization–electric filed (P–E) hysteresis loops of all NBTFex thin films were tested at room temperature as shown in Fig. 3. The NBTFe0.005 thin film shows a poor P–E hysteresis loop with round-shaped feature. This is because of the large contribution from the high leakage current resulting from ðVO2 Þ , which lies in the free state due to the lower Fe2 þ doping content. It can be seen that the P–E loops for NBTFex (x¼ 1–2 at%.) thin films become slimmer, in which the electrical leakage current is obviously reduced. Note that the coercive field (Ec) for NBTFe0.02 thin film exhibits a 50% reduction compared with that of NBTFe0.005 thin film. The enhancement of ferroelectricity arises from the decreasing leakage current as the Fe2 þ doping content increases to 2 at%. This is mainly due to the fact that the reorientation of
Fig. 3. Polarization-electric field hysteresis loops of NBTFex thin films: (a) x¼ 0.005, (b) x ¼0.01, (c) x¼ 0.02 and (d) x¼ 0.04.
3
ferroelectric domains may become easier with the increase of effective electric field. Also, the distortion of TiO6 which is brought by Fe2 þ doping partly improves the ferroelectricity of NBT thin film. However, the P–E loop for NBTFe0.04 thin film tends to have a round-shaped feature again, which is consistent with the abruptly increasing leakage current, as shown in Fig. 2. At 500 kV/cm, the Pr of the NBTFe0.02 thin film is 20 μC/cm2, which can be comparable to that of (Na0.5 Bi0.5)1 xBaxTiO3 (Pr ¼ 21.5 μC/cm2), Pr3 þ -doped 0.15K0.5 Bi0.5TiO3–0.85Na0.5Bi0.5TiO3 (Pr ¼ 23 μC/cm2) and Mndoped NBT-based thin films (Pr ¼ 23 μC/cm2) [19,20,3]. Subsequently, we measured the capacitance–voltage (C–V) curves of NBTFex thin films, in which the capacitance value measured at each bias voltage point represents the slop of the ferroelectric hysteresis (polarization–voltage: P–V) at the corresponding applied voltage. As shown in Fig. 4, it can be seen that the normalized C–V curves for all the thin films show typical butterfly pattern. This hysteretic nonlinearity is the characteristic of ferroelectricity. The variation of capacitance increases as the Fe2 þ doping content increases to 2 at%, indicating the enhancement of ferroelectricity. However, the capacitance varies slowly with the further increasing of Fe2 þ doping content to 4 at%, which is consistent with the characteristic of P–E loop for NBTFe0.04 thin film. Note that the normalized C–V for NBTFex thin films show asymmetric in the positive and negative voltage directions. This phenomenon is due to the asymmetric configuration of the electrodes [19,21]. Additionally, the center of the curve of NBTFex thin film is not located at zero voltage, but shift slightly towards the negative side. This can be attributed to the local field (Eloc) led by the defect complexes of ðFe2Tiþ4 þ Þ″–ðVO2 Þ . Fig. 5 shows the relative dielectric constant (εr) and dissipation factor (tanδ) as a function of frequency for NBTFex thin films. With the frequency increasing, the εr for NBTFex thin film first decreases slightly below 100 kHz and then decreases steeply. This phenomenon is mainly due to the fact that the polarization has no sufficient time to occur with the application of the electric field at high frequency [22]. Additionally, the tanδ decreases gradually at the beginning, and then increases rapidly. The tanδ is mainly dominated by the leakage current which decreases with the increase of frequency at lower frequency, and by the dipole inertia above 10 kHz [23]. The value of εr and tanδ of NBTFe0.02 thin film are about 450 and 0.094 at 100 kHz, respectively.
Fig. 4. The normalized capacitance as a function of applied voltage for NBTFex (x¼ 0.005, 0.01, 0.02 and 0.04) thin films. Please cite this article as: C. Feng, et al., Effect of Fe doping on the crystallization and electrical properties of Na0.5Bi0.5TiO3 thin film, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.10.066
C. Feng et al. / Ceramics International ] (]]]]) ]]]–]]]
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Fig. 5. Relative dielectric constant and dissipation factor as functions of frequency for NBTFex (x¼ 0.005, 0.01, 0.02 and 0.04) thin films.
4. Conclusion In summary, the NBTFex (x¼ 0.005, 0.01, 0.02 and 0.04) thin films were successfully fabricated on ITO/glass. The results show that all thin films crystallize into the single perovskite phase at 500 1C. The leakage current obviously decreases as Fe2 þ doping increases from 0.5 to 2 at% due to the formation of defect complexes between ðFe2Tiþ4 þ Þ″ and ðVO2 Þ . The ferroelectric nature for NBTFex thin films can be represented by P–E loop and C–V butterfly curve. The NBTFe0.02 thin film shows a relatively large Pr value (20 μC/cm2) which is mainly attributed to the lower leakage current. The εr and tanδ of NBTFe0.02 thin film are about 450 and 0.094 at 100 kHz, respectively. These findings suggest that the substitution of low-valance ion is of great importance for designing good performance NBT-based thin films. Acknowledgment This work was supported by the National Natural Science Foundation of China (no. 51002064) and the Graduate Innovation Foundation of University of Jinan (no. YCX13003). References [1] P. Baetting, C.F. Schelle, R.L. Sar, U.V. Waghmare, N.A. Spaldin, Theoretical prediction of new high-performance lead-free piezoelectrics, Chem. Mater. 17 (2005) 1376–1380. [2] J. Suchanicz, W.S. Ptak, On the phase transition in Na0.5Bi0.5TiO3, Ferroelectr. Lett. 12 (1990) 71–78. [3] M.M. Heijazi, E. Taghaddos, A. Safari, Reduced leakage current and enhanced ferroelectric properties in Mn-doped Bi0.5Na0.5TiO3-based thin films, J. Mater. Sci. 48 (2013) 3511–3516. [4] X.L. Fang, B. Shen, J.W. Zhai, X. Yao, Preparation and ferroelectric properties of (Na0.5Bi0.5)0.94Ba0.06TiO3 thin films deposited on Pt electrodes using LaNiO3 as buffer layer, Ceram. Int. 38 (2012) s83–s86.
[5] Y.Y. Wu, X.H. Wang, C.F. Zhang, L.T. Li, Effect of anneal conditions on electrical properties of Mn-doped (Na0.85K0.15)0.5Bi0.5TiO3 thin films prepared by sol–gel method, J. Am. Ceram. Soc. 94 (2011) 1843–1849. [6] Y.P. Guo, M. Li, W. Zhao, D. Akai, K. Sawada, M. Ishida, Ferroelectric and pyroelectric properties of (Na0.5Bi0.5)TiO3–BaTiO3 based trilayered thin films, Thin Solid Films 517 (2009) 5172974–5172978. [7] T. Harigai, Y. Tanaka, H. Adachi, E. Fujii, Piezoelectric properties of lead-free (Na0.5Bi0.5)TiO3–BaTiO3 (0 0 1) epitaxial thin films around the morphotropic phase boundary, Appl. Phys. Express 3 (2010) 111501. [8] A. Andrei, N.D. Scarisoreanu, R. Birjega, M. Dinescu, G. Stanciu, 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. [9] H. Adachi, Y. Tanaka, T. Harigai, M. Ueda, E. Fujii, Large transverse piezoelectricity in strained (Na,Bi)TiO3–BaTiO3 epitaxial thin films on MgO, Appl. Phys. Express 4 (2011) 051501. [10] S. Kunejn, A. Veber, D. Suvorov, Electrical characterization of sol–gelderived (1 x)NBT-xNTa (0.05oxo 0.3) thin films, Cream. Int. 39 (2013) 5991–5995. [11] Y.H. Lin, P.S. Cheng, C.C. Wu, T.P. Sun, J.J. Lin, C.F. Yang, Properties of RF magnetron sputtered 0.95(Na0.5Bi0.5)TiO3–0.05BaTiO3 thin films, Cream. Int. 37 (2011) 3765–3769. [12] W. Li, H.R. Zeng, K.Y. Zhao, J.G. Hao, J.W. Zhai, Structural, dielectric and piezoelectric properties of (Bi0.5Na0.5)TiO3–(Bi0.5K0.5)TiO3–Bi (Zn0.5Ti0.5)O3 thin films prepared by sol–gel method, Cream. Int. 40 (2014) 7947–7951. [13] 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)0.94Ba0.06TiO3 thin films, Appl. Phys. Lett. 97 (2010) 212901. [14] 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. [15] C.H. Yang, H.T. Sui, H.L. Yang, X.X. Li, Preparation of perovskite Fedoped Na0.5Bi0.5TiO3 thin film from polyethylene glycol-modified solution precursor on LaNiO3/Si substrate, Mater. Lett. 102-103 (2013) 109–111. [16] 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. [17] H.Y. Zhang, L. Chen, B. Jiang, W. Sun, J.J. Liu, G.D. Hu, Large and uniform piezoresponse of BiFe0.995W0.005O3 thin film annealed at 450 1C, J. Mater. Sci. - Mater. Electron. 23 (2012) 1864–1868. [18] L. Cheng, G.D. Hu, B. Jiang, C.H. Yang, W.B. Wu, S.H. Fan, Piezoelectric properties of lead-free (Na,Bi)TiO3–BaTiO3 (0 0 1) epitaxial thin films around the morphotropic phase boundary, Appl. Phys. Express 3 (2010) 101501. [19] 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 Na0.5Bi0.5TiO3 thin films from metal–organic solution deposition, J. Alloys Compd. 540 (2012) 204–209. [20] H. Zhou, G.H. Wu, N. Qin, D.H. Bao, Improved electric 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. [21] W.F. Liu, S.Y. Wang, C. Wang, Asymmetric electrical properties in Pt/Ba0.5Sr0.5Ti0.99Co0.01O3/Nb-doped SrTiO3 capacitors, Physica B 406 (2011) 3406–3409. [22] H.R. Liu, Z.L. Liu, K.L. Yao, Improved electric properties in BiFeO3 films by the doping of Ti,, J. Sol-Gel Sci. Technol. 41 (2007) 123–128. [23] M. Cheng, G.Q. Tan, X. Xue, A. Xia, H.J. Ren, Propertion of Nb-doped BiFeO3 films and their electrical properties, Physica B 407 (2012) 3360–3363.
Please cite this article as: C. Feng, et al., Effect of Fe doping on the crystallization and electrical properties of Na0.5Bi0.5TiO3 thin film, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.10.066
CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
Short communication
Effect of Fe doping on the crystallization and electrical properties of Na0.5Bi0.5TiO3 thin film C. Fenga, C.H. Yanga,b,n, H.T. Suia, F.J. Genga, Y.J. Hana 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 4 September 2014; received in revised form 10 October 2014; accepted 11 October 2014
Abstract Na0.5Bi0.5Ti1 xFexO3 δ (abbreviated as NBTFex, x ¼0.005, 0.01, 0.02 and 0.04) thin films were deposited on indium tin oxide/glass substrates via a chemical solution deposition method. The influence of Fe2 þ doping on the crystallization and electrical properties of Na0.5Bi0.5TiO3 was investigated. All the films crystallize into the single perovskite structure without any secondary phases. The insulating measurement reveals that 2 at% Fe2 þ doping can effectively reduce the leakage current of Na0.5Bi0.5TiO3 thin film by the formation of defect complexes. The enhanced ferroelectricity is obtained in NBTFe0.02 with a large remanent polarization (Pr) of 20 μC/cm2 due to the lowest leakage current and the distortion of TiO6. The normalized capacitance–voltage curves of all films agree with the polarization-electric field hysteresis loops. Also, the relative dielectric constant and dissipation factor of NBTFe0.02 thin film are 450 and 0.094 at 100 kHz, respectively. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Film; C. Dielectric properties; C. Ferroelectric properties; Crystallization
1. Introduction Lead-based compounds, represented by Pb(Zr,Ti)O3, Pb (Mg,Nb)O3, etc. are widely used in electric industry, with applications in ferroelectric memories, sensors, piezoelectric transducers and actuators. However, during the process of production and application, lead volatilization has caused serious environmental problems. Therefore, people have been long involved in searching for lead-free alternative materials. Of all, Bi-based perovskites have good ferroelectric property because Bi3 þ has an electronic configuration similar to Pb2 þ [1]. Sodium bismuth titanate (Na0.5Bi0.5TiO3, NBT) is considered to be a potential candidate 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 for bulk ceramic [2]. But until now, for NBT n Corresponding author at: School of Materials Science and Engineering, University of Jinan, Jinan 250022, China. Tel.: þ 86 531 889736280; fax: þ86 531 87974453. E-mail address: [email protected] (C.H. Yang).
thin film, its intrinsic performance is difficult to be measured due to the high leakage current, which is strongly dependent on oxygen vacancies [ðVO2 Þ ] produced by the volatilization of A-site elements [3,4]. In recent years, people are trying to improve electrical properties for NBT thin film by a variety of methods, such as cation substitution [5], formation of multilayered structure [6], control of orientation [7], and forming solid solutions with other components [8–12]. Among them, site engineering is supposed to be an available way, not only for reducing the leakage current but also for improving the ferroelectric property of NBT film [13]. The leakage current density of NBT film can be dramatically decreased by nearly two orders of magnitudes with Mn doping [14]. The NBT film with a large Pr of 29.5 μC/cm2 can be obtained by co-substitution of La and Ce. Chemical solution deposition (CSD), with the advantage of chemical homogeneity, easy stoichiometry control and integration into devices as well as low cost, has been widely used in the preparing process for NBT-based thin film. In this work, we prepared Na0.5Bi0.5Ti1 xFexO3 δ (abbreviated as NBTFex,
http://dx.doi.org/10.1016/j.ceramint.2014.10.066 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: C. Feng, et al., Effect of Fe doping on the crystallization and electrical properties of Na0.5Bi0.5TiO3 thin film, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.10.066
C. Feng et al. / Ceramics International ] (]]]]) ]]]–]]]
2
x ¼ 0.005, 0.01, 0.02 and 0.04) thin films via CSD. The crystallization, ferroelectric and dielectric properties were investigated. The aim of this work is to find the optimal Fe2 þ doping content for improving the electrical properties of NBT thin film. 2. Experiment NBTFex (x¼ 0.005, 0.01, 0.02 and 0.04) thin films were deposited on indium tin oxide (ITO)/glass substrates via CSD. The precursor solutions of NBTFex were prepared using sodium acetate, bismuth acetate, ferric nitrate, tetrabutyl titanate as starting materials. First, to prevent hydrolysis of Ti, tetrabutyl titanate was stabilized by the addition of acetylacetone. Sodium acetate, bismuth acetate and ferric nitrate were dissolved in the mixture of ethylene glycol and acetic acid. 2% excess bismuth and 5% excess sodium were added to compensate the volatilization of bismuth and sodium. Subsequently, the Na–Bi–Fe complex solution was carefully dropped into the Ti solution. Also, a moderate amount of polyethylene glycol (PEG) was added into the final solution. Then, transparent and stable precursor solutions with the same concentration of 0.3 M were obtained. Each precursor solution was deposited onto ITO/glass by spin coating at 4000 rpm for 30 s. The wet film was pyrolyzed on a hot plate at 250 1C for 5 min and then annealed layer by layer at 500 1C for 10 min in the rapid thermal processor for complete crystallization. Finally, the process was repeated several times to obtain a desired thickness. The thicknesses of four thin films were all estimated to be 300 nm measured by a step (step obtained by chemical etching) profilometer made by the Ambios Technology Company of USA. The crystallization of the films was characterized using an x-ray diffractometer (Bruker D8). The x-ray radiation source of Cu Kα with a wavelength of 1.5406 nm was adopted at 40 kV and 25 mA. For electrical measurements, Au top electrodes were deposited on the films using a sputtering system to form the structure of metalferroelectric-ITO. A standard ferroelectric tester was used to exam the ferroelectricity. The dielectric properties were measured using an impedance analyzer (HP4294A). 3. Results and discussion The x-ray diffraction patterns of ITO and NBTFex (x¼ 0.005, 0.01, 0.02 and 0.04) thin films were scanned at 2.41/min in the range of 2θ ¼ 20–501, as shown in Fig. 1. It can be seen that all detectable diffraction peaks for NBTFex thin films match well with those of the distorted rhombohedral R3c structure. All the films have similar polycrystalline structure without any impurity phases after annealing at the low temperature of 500 1C. This may be attributed to the following factors. First, PEG can promote the formation of perovskite phase, which has been reported in our previous reports [15]. Second, each crystallized layer can serve as seeding layer for the next deposited one when NBTFex thin films are prepared using sequential layer annealing process. Third, the oxide electrode assists in the nucleation and growth of thin films
Fig. 1. XRD patterns of ITO (a) and NBTFex (b) x¼ 0.005, (c) x¼ 0.01, (d) x ¼0.02, (e) x¼ 0.04 thin films.
Fig. 2. Leakage current density as a function of electric field for NBTFex (x¼ 0.005, 0.01, 0.02 and 0.04) thin films.
[16]. Additionally, with the increase of Fe2 þ doping content, the relative intensity of peak decreases and the full width at half-maximum increases gradually, especially for the (1 1 0) orientation. Generally, the grain growth is controlled by the minimization of surface and grain boundary energies. This growth mode induced by the motion of grain boundaries depends on the diffusivities of constituent ions [16]. Therefore, the restriction of grain growth for NBT film by the Fe2 þ substitution may be due to the lower diffusivity of Fe2 þ compared to that of Ti4 þ . Fig. 2 illustrates the insulating characteristic of the NBTFex thin films. It can be seen that the leakage current density decreases as the Fe2 þ doping content increases from 0.5 to 2 at%. As is known, the leakage current for NBT-based thin film is strongly dependent on ðVO2 Þ . When Ti4 þ is occupied by the off-valent acceptor of Fe2 þ , the defect of ðFe2Tiþ4 þ Þ″ can be formed simultaneously. Then, it is not easy for the ðVO2 Þ to be movable due to the formation of defect complexes between ðFe2Tiþ4 þ Þ″ and ðVO2 Þ . The applied field that required for generating the mobile ðVO2 Þ would be increased since more electric energy is required to overcome the electrostatic attraction force of ðFe2Tiþ4 þ Þ″–ðVO2 Þ . Therefore, more ðVO2 Þ would be restricted with the increase of Fe2 þ doping content. However, as Fe2 þ doping content
Please cite this article as: C. Feng, et al., Effect of Fe doping on the crystallization and electrical properties of Na0.5Bi0.5TiO3 thin film, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.10.066
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reaches 4 at%, the leakage current density increases abruptly. With the Fe2 þ doping content increasing, the grain size decreases gradually, which can be reflected from the XRD patterns. This in turn leads to the formation of more defects at long grain boundaries. Therefore, we can attribute the increasing leakage current density of NBTFe0.04 thin film to the fact that the positive effect of Fe2 þ doping on suppressing ðVO2 Þ is overwhelmed by the restriction effect of Fe2 þ on grain growth. It can be also found that high content of ion doping may result in poor electrical behavior in other perovskite films [17,18]. The polarization–electric filed (P–E) hysteresis loops of all NBTFex thin films were tested at room temperature as shown in Fig. 3. The NBTFe0.005 thin film shows a poor P–E hysteresis loop with round-shaped feature. This is because of the large contribution from the high leakage current resulting from ðVO2 Þ , which lies in the free state due to the lower Fe2 þ doping content. It can be seen that the P–E loops for NBTFex (x¼ 1–2 at%.) thin films become slimmer, in which the electrical leakage current is obviously reduced. Note that the coercive field (Ec) for NBTFe0.02 thin film exhibits a 50% reduction compared with that of NBTFe0.005 thin film. The enhancement of ferroelectricity arises from the decreasing leakage current as the Fe2 þ doping content increases to 2 at%. This is mainly due to the fact that the reorientation of
Fig. 3. Polarization-electric field hysteresis loops of NBTFex thin films: (a) x¼ 0.005, (b) x ¼0.01, (c) x¼ 0.02 and (d) x¼ 0.04.
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ferroelectric domains may become easier with the increase of effective electric field. Also, the distortion of TiO6 which is brought by Fe2 þ doping partly improves the ferroelectricity of NBT thin film. However, the P–E loop for NBTFe0.04 thin film tends to have a round-shaped feature again, which is consistent with the abruptly increasing leakage current, as shown in Fig. 2. At 500 kV/cm, the Pr of the NBTFe0.02 thin film is 20 μC/cm2, which can be comparable to that of (Na0.5 Bi0.5)1 xBaxTiO3 (Pr ¼ 21.5 μC/cm2), Pr3 þ -doped 0.15K0.5 Bi0.5TiO3–0.85Na0.5Bi0.5TiO3 (Pr ¼ 23 μC/cm2) and Mndoped NBT-based thin films (Pr ¼ 23 μC/cm2) [19,20,3]. Subsequently, we measured the capacitance–voltage (C–V) curves of NBTFex thin films, in which the capacitance value measured at each bias voltage point represents the slop of the ferroelectric hysteresis (polarization–voltage: P–V) at the corresponding applied voltage. As shown in Fig. 4, it can be seen that the normalized C–V curves for all the thin films show typical butterfly pattern. This hysteretic nonlinearity is the characteristic of ferroelectricity. The variation of capacitance increases as the Fe2 þ doping content increases to 2 at%, indicating the enhancement of ferroelectricity. However, the capacitance varies slowly with the further increasing of Fe2 þ doping content to 4 at%, which is consistent with the characteristic of P–E loop for NBTFe0.04 thin film. Note that the normalized C–V for NBTFex thin films show asymmetric in the positive and negative voltage directions. This phenomenon is due to the asymmetric configuration of the electrodes [19,21]. Additionally, the center of the curve of NBTFex thin film is not located at zero voltage, but shift slightly towards the negative side. This can be attributed to the local field (Eloc) led by the defect complexes of ðFe2Tiþ4 þ Þ″–ðVO2 Þ . Fig. 5 shows the relative dielectric constant (εr) and dissipation factor (tanδ) as a function of frequency for NBTFex thin films. With the frequency increasing, the εr for NBTFex thin film first decreases slightly below 100 kHz and then decreases steeply. This phenomenon is mainly due to the fact that the polarization has no sufficient time to occur with the application of the electric field at high frequency [22]. Additionally, the tanδ decreases gradually at the beginning, and then increases rapidly. The tanδ is mainly dominated by the leakage current which decreases with the increase of frequency at lower frequency, and by the dipole inertia above 10 kHz [23]. The value of εr and tanδ of NBTFe0.02 thin film are about 450 and 0.094 at 100 kHz, respectively.
Fig. 4. The normalized capacitance as a function of applied voltage for NBTFex (x¼ 0.005, 0.01, 0.02 and 0.04) thin films. Please cite this article as: C. Feng, et al., Effect of Fe doping on the crystallization and electrical properties of Na0.5Bi0.5TiO3 thin film, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.10.066
C. Feng et al. / Ceramics International ] (]]]]) ]]]–]]]
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Fig. 5. Relative dielectric constant and dissipation factor as functions of frequency for NBTFex (x¼ 0.005, 0.01, 0.02 and 0.04) thin films.
4. Conclusion In summary, the NBTFex (x¼ 0.005, 0.01, 0.02 and 0.04) thin films were successfully fabricated on ITO/glass. The results show that all thin films crystallize into the single perovskite phase at 500 1C. The leakage current obviously decreases as Fe2 þ doping increases from 0.5 to 2 at% due to the formation of defect complexes between ðFe2Tiþ4 þ Þ″ and ðVO2 Þ . The ferroelectric nature for NBTFex thin films can be represented by P–E loop and C–V butterfly curve. The NBTFe0.02 thin film shows a relatively large Pr value (20 μC/cm2) which is mainly attributed to the lower leakage current. The εr and tanδ of NBTFe0.02 thin film are about 450 and 0.094 at 100 kHz, respectively. These findings suggest that the substitution of low-valance ion is of great importance for designing good performance NBT-based thin films. Acknowledgment This work was supported by the National Natural Science Foundation of China (no. 51002064) and the Graduate Innovation Foundation of University of Jinan (no. YCX13003). References [1] P. Baetting, C.F. Schelle, R.L. Sar, U.V. Waghmare, N.A. Spaldin, Theoretical prediction of new high-performance lead-free piezoelectrics, Chem. Mater. 17 (2005) 1376–1380. [2] J. Suchanicz, W.S. Ptak, On the phase transition in Na0.5Bi0.5TiO3, Ferroelectr. Lett. 12 (1990) 71–78. [3] M.M. Heijazi, E. Taghaddos, A. Safari, Reduced leakage current and enhanced ferroelectric properties in Mn-doped Bi0.5Na0.5TiO3-based thin films, J. Mater. Sci. 48 (2013) 3511–3516. [4] X.L. Fang, B. Shen, J.W. Zhai, X. Yao, Preparation and ferroelectric properties of (Na0.5Bi0.5)0.94Ba0.06TiO3 thin films deposited on Pt electrodes using LaNiO3 as buffer layer, Ceram. Int. 38 (2012) s83–s86.
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Please cite this article as: C. Feng, et al., Effect of Fe doping on the crystallization and electrical properties of Na0.5Bi0.5TiO3 thin film, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.10.066