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Reduced leakage current, enhanced ferroelectric and dielectric properties of (La, Fe)-codoped Bi0.5Na0.5TiO3-based thin films
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Reduced leakage current, enhanced ferroelectric and dielectric properties of (La, Fe)-codoped Bi0.5Na0.5TiO3-based thin films Peng Lia, Wei Lia, Shaohui Liua, Yang Zhanga, Jiwei Zhaia,n, Haydn Chenb a
Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education, Functional Materials Research Laboratory, School of Materials Science & Engineering, Tongji University, 4800 Caoan Road, Shanghai 201804, China b Institute of Applied Physics and Materials Engineering, Faculty of Science and Technology, University of Macau, Macau, China Received 26 October 2014; accepted 14 March 2015
Abstract The lead-free Na0.5Bi0.5TiO3–K0.5Bi0.5TiO3–SrTiO3 (BNT–BKT–ST) and (La, Fe)-codoped Na0.5Bi0.5TiO3–K0.5Bi0.5TiO3–SrTiO3 (BNT– BKT–ST–LaFe) thin films were deposited on Pt/Ti/SiO2/Si substrates by the sol–gel method. Both the BNT–BKT–ST and BNT–BKT–ST–LaFe thin films exhibit pseudo-cubic perovskite structure and uniform grain size. The leakage current density of BNT–BKT–ST–LaFe thin film at 400 kV/cm is reduced by approximately two orders of magnitude compared with BNT–BKT–ST thin film. Enhanced ferroelectric property is achieved in BNT–BKT–ST–LaFe film with a large remanent polarization of 13 μC/cm2. At the same time, the BNT–BKT–ST–LaFe film also exhibits a large dielectric constant of 420 and small dielectric loss of 0.055 at 100 kHz. These results indicate that the (La, Fe)-codoped BNT– BKT–ST thin film is a promising candidate in lead-free ferroelectric materials. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: C. Electrical properties; Thin film; Sol–gel method; (La, Fe)-codoped
1. Introduction Sodium bismuth titanate (Na0.5Bi0.5TiO3:BNT) is a lead-free material system that was first reported in the 1960s by Smolenskii et al. [1], and has received increasing attention recently for its potential as lead-free alternatives to lead zirconate titanate-based materials [2–4]. As bismuth sodium titanate Na0.5Bi0.5TiO3 exhibits strong ferroelectricity (Pr=38 μC/cm2) at room temperature and high Curie temperature Tc of 320 1C [5], it has been considered as one of the key lead-free ferroelectric materials. It is difficult to obtain desired ferroelectric and dielectric properties for pure BNT thin film because of its high conductivity and large coercive field (Ec 73 kV/cm) [6,7]. In comparison with pure BNT, BNTbased solid solution modified with Bi0.5K0.5TiO3 or BaTiO3 shows the improved piezoelectric and dielectric properties because of the existent rhombohedral–tetragonal morphotropic n
Corresponding author. E-mail address: [email protected] (J. Zhai).
phase boundary [5,8]. The BNT–BKT system is known to have outstanding piezoelectric property at its morphotropic phase boundary with about 20% BKT [9–11]. At room temperature, SrTiO3 (ST) has a perovskite structure and a cubic symmetry. In previous works, the ST substitution of BNT and KNN-based materials were found to be an effective way to modify the ferroelectric, piezoelectric and dielectric properties [12–14]. So we select BNT–BKT–ST system as the object of study. Although lead-free ferroelectric thin films have attracted a lot of attention during the past few years, however, compared to lead-based counterparts, they suffer from inferior insulating properties. This is one of the main drawbacks facing the development of lead-free thin films for practical application such as capacitors and memories in which lower leakage current, dielectric loss and large remanent polarization are demanded. Similar to the case of PZT-based piezoelectric materials, introducing metal ions additives into BNT-based compositions offer a versatile route to modify electrical properties. Recently, it has been demonstrated that proper
http://dx.doi.org/10.1016/j.ceramint.2015.03.177 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: P. Li, et al., Reduced leakage current, enhanced ferroelectric and dielectric properties of (La, Fe)-codoped Bi0.5Na0.5TiO3-based thin films, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.03.177
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Fig. 1. XRD patterns of the BNT–BKT–ST and BNT–BKT–ST–LaFe thin films.
The crystalline structure of the films was characterized by X-ray diffraction (XRD, D/max 2550 V, Rigaku, Japan) with Cu Kα radiation. The microstructure of the films was observed by the Field Emission Scanning Electron Microscope (FESEM, S-4700, HITACHI, Japan). In order to measure the electrical properties of the films, the gold pads with 0.50 mm in diameter were coated on the thin films surface as the top electrodes by DC sputtering. Dielectric constant and loss at different electric fields and frequencies were measured using the precision LCR meter (E4980A Agilent Inc., USA). Leakage current density–electric field (J–E) curves were obtained using a Keithley 6517A electrometer. The polarization–electric field (P–E) hysteresis loops were obtained using the ferroelectric test system (Radiant Precision Premier II). 3. Results and discussion
amounts of lanthanide oxides can improve the electrical properties of BNT-based solutions, depending on the nature of the lanthanide additives [15]. It is also reported that Fe2 þ , Fe3 þ , Mn2 þ and Mn3 þ doped ABO3 perovskite ferroelectric thin films have shown excellent results in improving the ferroelectric properties, reducing the dielectric loss, enhancing the resistivity of the thin films, and increasing the dielectric tunability [16–20]. However, to our knowledge, little attention has been paid to the impact of A-site and B-site codoping on (ABO3) perovskite structure. Thus, it is important to study the effect of (La, Fe)-codoping on the BNT–BKT–ST thin films.
2. Experimental procedures The lead-free ferroelectric thin films 0.9(0.8Bi0.5Na0.5TiO3– 0.2Bi0.5K0.5TiO3)–0.1SrTiO3 (abbreviated as BNT–BKT–ST) and (La, Fe)-codoped 0.9(Na0.8K0.2La0.005)0.5Bi0.5(Ti0.995Fe0.005) O3 þ x–0.1SrTiO3 thin films (abbreviated as BNT–BKT–ST–LaFe) were prepared by sol–gel method. To synthesize the precursor solution of BNT–BKT–ST and BNT–BKT–ST–LaFe thin films, sodium acetate trihydrate (CH3COONa 3H2O), potassium acetate (CH3COOK), bismuth nitrate pentahydrate [Bi(NO3)3 5H2O], strontium acetate [Sr(CH3COO)2], lanthanum nitrate [La(NO3)3], nine hydrated ferric nitrate [Fe(NO3)3 9H2O] and tetrabutyl titanate [Ti(OC4H9)4] were used as starting materials. Acetic acid (CH3COOH) and 2-methoxyethanol were used as solvent and acetylacetone (CH3COCH2COCH3) was used as polymerizing agent and stabilizer. First of all, the starting materials were dissolved in acetic acid and 2-methoxyethanol. Subsequently, the acetylacetone stable tetrabutyl titanate was added to the solution and the concentration of the final solution was adjusted to 0.3 M by adding acetic acid. Then the precursor solution was stirred at 70 1C for 5 h to make it well blended. After aging the precursor solution for 24 h, it was deposited on the Pt/Ti/SiO2/Si substrates by spin-coating at 3000 rpm for 30 s. After each spin, the gel layer was pyrolyzed at 300 1C for 5 min and prefired at 500 1C for 10 min in order to remove the residual organics. The spin-coating and heat treatment processes were repeated until the thin films reached the thickness of approximately 500 nm. Finally, the samples were annealed at 700 1C for 30 min.
The XRD patterns of the BNT–BKT–ST and BNT–BKT– ST–LaFe thin films are shown in Fig. 1. The diffraction peaks show that all thin films form a single-phase solid solution with a perovskite structure without any pyrochlore phase which will deteriorate the electrical performance of films. The restrain of pyrochlore phase can be attributed to the optimal annealing temperature and decreased nonstoichiometric structural defects [8,21]. Moreover, the XRD patterns of the BNT–BKT–ST and BNT–BKT–ST–LaFe thin films revealed that the (La,Fe)codoping does not change the phase structure. Fig. 2(a)–(d) shows the surface morphologies and crosssectional micrographs of BNT–BKT–ST and BNT–BKT–ST– LaFe thin films, respectively. From Fig. 2(a) and (b), it is observed that the average grain size of BNT–BKT–ST and BNT–BKT–ST–LaFe films is about 100 and 125 nm, respectively, and the surface morphologies of the thin films appear smooth and crack-free. The thickness of the films was approximately 500 nm as observed from the cross-sectional micrographs in Fig. 2(c) and (d). Insulating characteristic of dielectric thin film is determined by the value of leakage current. Fig. 3 shows leakage current density as a function of applied electric field. The leakage current density of all the thin films increased gradually with increasing applied electric field up to 400 kV/cm. In BNTbased thin films, the oxygen vacancies (V O ) should be the origins responsible for the high leakage current [22]. The (La, Fe)-codoped BNT–BKT–ST thin films exhibit enhanced insulating behavior, which can probably be attributed to the following reasons. Firstly, for the case of BNT–BKT–ST– LaFe thin film, due to the higher valence of La3 þ than that of (Na0.5Bi0.5)2 þ and the constrain of electrical charge neutrality, La3 þ ions substitution would suppress the formation of V O to a great extent [20]. Secondly, in the BNT–BKT–ST–LaFe thin film, the Fe3 þ was incorporated at the B-site (titanium sites) 0 0 and formed the defect complexes (FeTi V O FeTi ), which limit the free movement of the oxygen vacancy [17,20]. Fig. 4 shows the typical polarization–electric field (P–E) hysteresis loops of the thin films at the frequency of 10 kHz. From the P–E loops, it can be seen that the Ec þ and Ec- is not entirely symmetrical. This may be mainly due to two factors.
Please cite this article as: P. Li, et al., Reduced leakage current, enhanced ferroelectric and dielectric properties of (La, Fe)-codoped Bi0.5Na0.5TiO3-based thin films, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.03.177
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Fig. 2. The surface morphologies (a), (b) and cross-sectional micrographs (c), (d) of BNT–BKT–ST and BNT–BKT–ST–LaFe thin films.
Fig. 3. Leakage current density as a function of applied electric field for the BNT–BKT–ST and BNT–BKT–ST–LaFe thin films.
Fig. 4. P–E hysteresis loops of BNT–BKT–ST and BNT–BKT–ST–LaFe films as a function of electric field.
Firstly, the stress formed during thermal treatment can lead to preferential orientation of ferroelectric domains. Secondly, the ″0 formation of defect complexes (such as (V O V Bi )) lead to local field in the thin films [22]. The remnant polarization (Pr) and spontaneous polarization (Ps) of BNT–BKT–ST–LaFe is higher than BNT–BKT–ST thin film. In our present work, the improved Pr value of BNT–BKT–ST–LaFe film can be explained through distortion of TiO6 octahedra when Fe replace the B-site cations. Fe substitution may increase the dipole size by lengthening the off-center displacement in the perovskite structure, and hence the polarization is improved. Similar results have been reported for Mn doped BNT-based thin films [16]. The reduced leakage current also played an important role in the enhancement of polarization by increasing the effective applied electric field.
Moreover, a small number of La3 þ cations compensating for the A-site deficiencies due to the volatilization of A-site cations during the annealing process may contribute to the increased Pr. Similar results have been reported for Li þ doped BNT–BT thin films [23]. The measured coercive field (Ec) in these thin films are larger compared to the average bulk values (20 kV/cm) [13]. This could be a consequence of a higher defect concentration, local internal field, fine grain size and film stress [24]. In order to study the effect of (La ,Fe)-codoping on the dielectric performance, the dielectric constant and dielectric loss as a function of DC bias field and frequency are measured, as shown in Fig. 5. The C–V plots appear symmetric with a hysteresis behavior of ferroelectric capacitors. Dielectric constant changes with applied electric field, which indicating that the
Please cite this article as: P. Li, et al., Reduced leakage current, enhanced ferroelectric and dielectric properties of (La, Fe)-codoped Bi0.5Na0.5TiO3-based thin films, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.03.177
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P. Li et al. / Ceramics International ] (]]]]) ]]]–]]]
ferroelectric domain contributes significantly to the polarizability of the thin films [24]. Compared with BNT–BKT–ST, the BNT– BKT–ST–LaFe thin film exhibits enhanced dielectric property with a larger dielectric constant of 425 and lower dielectric loss of 0.055 at zero electric field. The larger dielectric constant for BNT–BKT–ST–LaFe thin film is derived from the distortion of TiO6 octahedral caused by (La, Fe)-codoping, which gives B-site ions greater activity space and in turn creates favorable condition for polarization [16]. In addition, the decreased dielectric loss for BNT–BKT–ST–LaFe thin film can be ascribed to the following reasons. Firstly, it is known that, under an applied electric field, charges tend to accumulate (or even be trapped) at the ferroelectric film and metal electrode interface and/or at the grain boundaries, leading to increase the dielectric loss. However, for BNT–BKT–ST–LaFe thin film, (La, Fe)-codoping decrease the concentration of the carriers by surpressing the oxygen vacancies
Fig. 5. Dielectric constant and dielectric loss as a function of electric field.
0
0
and forming defect complexes (such as FeTi V O FeTi ) [20,25]. Secondly, one of the other origins of dielectric loss is the electron hopping through B-site cations of different valences such as Ti4 þ and Ti3 þ . However, in BNT–BKT–ST–LaFe thin films, Fe doping decreased the dielectric loss by preventing the transition of Ti4 þ –Ti3 þ . Similar results have been reported in Mn-doped BNT-based thin film [16,18]. The dielectric constant and dielectric loss measured at room temperature as a function of frequency from 100 Hz to 1 MHz for BNT–BKT–ST and BNT– BKT–ST–LaFe thin films are shown in Fig. 6. The dielectric constant decreases with the frequency increasing from 100 Hz to 1 MHz due to the decreased contribution of free space charge at elevated frequency [26]. The increase in the dielectric loss at high frequency is related to sample fixturing [27]. The improvement in dielectric properties can be attributed to the effect of La and Fe on the concentration of defects, lattice distortion and formation of defect complexes [16,20]. The temperature and frequency dependence of dielectric constant and dielectric loss for BNT–BKT–ST and BNT– BKT–ST–LaFe thin films are shown in Fig. 7. It can be seen that the Curie temperature (Tc) of BNT–BKT–ST–LaFe thin film is higher than pure BNT–BKT–ST thin film, which make the BNT–BKT–ST–LaFe thin film with high temperature stability and improved dielectric and ferroelectric properties to some extent. This result is consistent with previous result for La2O3 and CeO2 doped BNT-based ceramics [15]. It can be seen from Fig. 7(b) that the dielectric constant and dielectric loss are dependent on the frequency strongly. The first dielectric peak (corresponding to the ferroelctric-relaxation ferroelectric phase transition) shifts to high temperature with the increasing frequency and the Curie temperature Tc (corresponding to the ferroelectric–paraelectric phase transition) shifts to a low temperature with the increasing frequency. The above-mentioned characteristics demonstrate that the BNT–BKT–ST–LaFe thin films are classical relaxor ferroelectrics with the characteristics of diffuse phase transition and frequency dispersion [8,9]. 4. Conclusions
Fig. 6. Dielectric constant and dielectric loss as a function of frequency.
In summary, the BNT–BKT–ST and BNT–BKT–ST–LaFe thin films show the single perovskite structure, smooth surface and uniform grain size. The BNT–BKT–ST–LaFe thin films exhibit improved ferroelectric and dielectric properties with
Fig. 7. Temperature and frequency dependence of the dielectric constant and dielectric loss of BNT–BKT–ST and BNT–BKT–ST–LaFe thin films. Please cite this article as: P. Li, et al., Reduced leakage current, enhanced ferroelectric and dielectric properties of (La, Fe)-codoped Bi0.5Na0.5TiO3-based thin films, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.03.177
P. Li et al. / Ceramics International ] (]]]]) ]]]–]]]
high remanent polarization (Pr 13 μC/cm2), dielectric constant ( 420) and low dielectric loss ( 0.055). The results show that (La, Fe)-codoping is effective to improve the insulating, ferroelectric and dielectric properties by reducing the concentration of oxygen vacancies, forming defect complexes and reduce the pinning effect of ferroelectric domain. So, (La, Fe)-codoped BNT–BKT–ST thin films with enhanced ferroelectric and dielectric properties can be considered as potential candidates for replacement of the lead-based ferroelectric materials. Conflict of interest We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. Acknowledgments This work was supported by the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20120072130001) and the National Natural Science Foundation of China, China under Grant no. 51332003. This work was supported in part by the University of Macau, Macao. References [1] G. Smolenskii, V. Isupov, A. Agranovskaya, N. Krainik, New ferroelectrics of complex composition. 4, Sov. Phys. Solid State 2 (1961) 2651–2654. [2] M. Davies, E. Aksel, J.L. Jones, Enhanced high-temperature piezoelectric coefficients and thermal stability of Fe- and Mn-substituted Na0.5Bi0.5TiO3 ceramics, J. Am. Ceram. Soc. 94 (2011) 1314–1316. [3] C. Dragoi, M. Cernea, L. Trupina, Lead-free ferroelectric BaTiO3 doped(Na0.5Bi0.5)TiO3 thin films processed by pulsed laser deposition technique, Appl. Surf. Sci. 257 (2011) 9600–9605. [4] M. Bousquet, J.R. Duclère, C. Champeaux, A. Boulle, P. Marchet, A. Catherinot, A. Wu, P.M. Vilarinho, S. Députier, M. Guilloux-Viry, A. Crunteanu, B. Gautier, D. Albertini, C. Bachelet, Macroscopic and nanoscale electrical properties of pulsed laser deposited (100) epitaxial lead-free Na0.5Bi0.5TiO3 thin films, J. Appl. Phys. 107 (2010) 034102. [5] S.K. Acharya, T.M. Kim, J.H. Hyung, B.G. Ahn, S.K. Lee, Ferroelectric and piezoelectric properties of lead-free Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3– BaTiO3-thin films near the morphotropic phase boundary, J. Alloy. Compd. 586 (2014) 549–554. [6] X.C. Zheng, G.P. Zheng, Z. Lin, Z.Y. Jiang, Thermal and dynamic mechanical analyses on Bi0.5Na0.5TiO3–BaTiO3 ceramics synthesized with citrate method, Ceram. Int. 39 (2013) 1233–1240. [7] X.C. Zheng, G.P. Zheng, Z. Lin, Z.Y. Jiang, Thermo-electrical energy conversions in Bi0.5Na0.5TiO3–BaTiO3 thin films prepared by sol–gel method, Thin Solid Films 522 (2012) 125–128. [8] G. Yueqiu, D. Hui, Z. Xuejun, P. Jinfeng, L. Xujun, H. Renjie, Large piezoelectric response of Bi0.5(Na(1 x)Kx)0.5TiO3thin films near morphotropic phase boundary identified by multi-peak fitting, J. Phys. D: Appl. Phys. 45 (2012) 305301. [9] W.J. Ji, Y.B. Chen, S.T. Zhang, B. Yang, X.N. Zhao, Q.J. Wang, Microstructure and electric properties of lead-free 0.8Bi1/2Na1/2TiO3– 0.2Bi1/2K1/2TiO3 ceramics, Ceram. Int. 38 (2012) 1683–1686.
5
[10] J.F. Trelcat, C. Courtois, M. Rguiti, A. Leriche, P.H. Duvigneaud, T. Segato, Morphotropic phase boundary in the BNT–BT–BKT system, Ceram. Int. 38 (2012) 2823–2827. [11] A. Hussain, J.U. Rahman, A. Zaman, R.A. Malik, J.S. Kim, T.K. Song, W.J. Kim, M.H. Kim, Field-induced strain and polarization response in lead-free Bi1/2(Na0.80K0.20)1/2TiO3–SrZrO3 ceramics, Mater. Chem. Phys. 143 (2014) 1282–1288. [12] W. Krauss, D. Schütz, F.A. Mautner, A. Feteira, K. Reichmann, Piezoelectric properties and phase transition temperatures of the solid solution of (1 x)(Bi0.5Na0.5)TiO3–xSrTiO3, J. Eur. Ceram. Soc. 30 (2010) 1827–1832. [13] K. Wang, A. Hussain, W. Jo, J. Rödel, D.D. Viehland, Temperaturedependent properties of (Bi1/2Na1/2)TiO3–(Bi1/2K1/2)TiO3–SrTiO3 leadfree piezoceramics, J. Am. Ceram. Soc. 95 (2012) 2241–2247. [14] M.R. Bafandeh, R. Gharahkhani, J.S. Lee, Enhanced electric field induced strain in SrTiO3 modified (K, Na)NbO3-based piezoceramics, J. Alloy. Compd. 602 (2014) 285–289. [15] Q. Xu, M. Chen, W. Chen, H.X. Liu, B.H. Kim, B.K. Ahn, Effect of Ln2O3 (Ln¼La, Pr, Eu, Gd) addition on structure and electrical properties of (Na0.5Bi0.5)0.93Ba0.07TiO3 ceramics, J. Alloy. Compd. 463 (2008) 275–281. [16] M.M. Hejazi, 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. [17] E. Aksel, E. Erdem, P. Jakes, J.L. Jones, R.d.A. Eichel, Defect structure and materials hardening in Fe2O3-doped Bi0.5Na0.5TiO3 ferroelectrics, Appl. Phys. Lett. 97 (2010) 012903. [18] W. Li, H. Zeng, J. Hao, J. 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. [19] Y. Wu, X. Wang, C. Zhong, L. 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. [20] 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. [21] X.J. Zheng, J.Y. Liu, J.F. Peng, X. Liu, Y.Q. Gong, K.S. Zhou, D.H. Huang, Effect of potassium content on electrostrictive properties of Na0.5Bi0.5TiO3-based relaxor ferroelectric thin films with morphotropic phase boundary, Thin Solid Films 548 (2013) 118–124. [22] C.H. Yang, W.B. Wu, F. Yang, H.T. Wu, X.Y. Zhang, Dielectric and ferroelectric properties of A-site non-stoichiometric Na0.5Bi0.5TiO3-based thin films, Mater. Lett. 66 (2012) 86–88. [23] S.K. Acharya, B.G. Ahn, J.H. Hyung, S.K. Lee, Effect of Li doping on ferroelectric and piezoelectric properties of Ba0.5Na0.5TiO3–BaTiO3 (BNT–BT) thin films, J. Korean Phys. Soc. 62 (2013) 794–799. [24] M. Abazari, A. Safari, S.S.N. Bharadwaja, S. Trolier-McKinstry, Dielectric and piezoelectric properties of lead-free (Bi,Na)TiO3-based thin films, Appl. Phys. Lett. 96 (2010) 082903. [25] M. Bousquet, J.R. Duclère, B. Gautier, A. Boulle, A. Wu, S. Députier, D. Fasquelle, F. Rémondière, D. Albertini, C. Champeaux, P. Marchet, M. Guilloux-Viry, P. Vilarinho, Electrical properties of (110) epitaxial lead-free ferroelectric Na0.5Bi0.5TiO3 thin films grown by pulsed laser deposition: macroscopic and nanoscale data, J. Appl. Phys. 111 (2012) 104106. [26] C. Jin, F. Wang, C.M. Leung, Q. Yao, Y. Tang, T. Wang, W. Shi, Enhanced ferroelectric and piezoelectric response in Mn-doped Bi0.5Na0.5TiO3–BaTiO3 lead-free film by pulsed laser deposition, Appl. Surf. Sci. 283 (2013) 348–351. [27] 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.
Please cite this article as: P. Li, et al., Reduced leakage current, enhanced ferroelectric and dielectric properties of (La, Fe)-codoped Bi0.5Na0.5TiO3-based thin films, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.03.177
CERAMICS INTERNATIONAL
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Reduced leakage current, enhanced ferroelectric and dielectric properties of (La, Fe)-codoped Bi0.5Na0.5TiO3-based thin films Peng Lia, Wei Lia, Shaohui Liua, Yang Zhanga, Jiwei Zhaia,n, Haydn Chenb a
Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education, Functional Materials Research Laboratory, School of Materials Science & Engineering, Tongji University, 4800 Caoan Road, Shanghai 201804, China b Institute of Applied Physics and Materials Engineering, Faculty of Science and Technology, University of Macau, Macau, China Received 26 October 2014; accepted 14 March 2015
Abstract The lead-free Na0.5Bi0.5TiO3–K0.5Bi0.5TiO3–SrTiO3 (BNT–BKT–ST) and (La, Fe)-codoped Na0.5Bi0.5TiO3–K0.5Bi0.5TiO3–SrTiO3 (BNT– BKT–ST–LaFe) thin films were deposited on Pt/Ti/SiO2/Si substrates by the sol–gel method. Both the BNT–BKT–ST and BNT–BKT–ST–LaFe thin films exhibit pseudo-cubic perovskite structure and uniform grain size. The leakage current density of BNT–BKT–ST–LaFe thin film at 400 kV/cm is reduced by approximately two orders of magnitude compared with BNT–BKT–ST thin film. Enhanced ferroelectric property is achieved in BNT–BKT–ST–LaFe film with a large remanent polarization of 13 μC/cm2. At the same time, the BNT–BKT–ST–LaFe film also exhibits a large dielectric constant of 420 and small dielectric loss of 0.055 at 100 kHz. These results indicate that the (La, Fe)-codoped BNT– BKT–ST thin film is a promising candidate in lead-free ferroelectric materials. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: C. Electrical properties; Thin film; Sol–gel method; (La, Fe)-codoped
1. Introduction Sodium bismuth titanate (Na0.5Bi0.5TiO3:BNT) is a lead-free material system that was first reported in the 1960s by Smolenskii et al. [1], and has received increasing attention recently for its potential as lead-free alternatives to lead zirconate titanate-based materials [2–4]. As bismuth sodium titanate Na0.5Bi0.5TiO3 exhibits strong ferroelectricity (Pr=38 μC/cm2) at room temperature and high Curie temperature Tc of 320 1C [5], it has been considered as one of the key lead-free ferroelectric materials. It is difficult to obtain desired ferroelectric and dielectric properties for pure BNT thin film because of its high conductivity and large coercive field (Ec 73 kV/cm) [6,7]. In comparison with pure BNT, BNTbased solid solution modified with Bi0.5K0.5TiO3 or BaTiO3 shows the improved piezoelectric and dielectric properties because of the existent rhombohedral–tetragonal morphotropic n
Corresponding author. E-mail address: [email protected] (J. Zhai).
phase boundary [5,8]. The BNT–BKT system is known to have outstanding piezoelectric property at its morphotropic phase boundary with about 20% BKT [9–11]. At room temperature, SrTiO3 (ST) has a perovskite structure and a cubic symmetry. In previous works, the ST substitution of BNT and KNN-based materials were found to be an effective way to modify the ferroelectric, piezoelectric and dielectric properties [12–14]. So we select BNT–BKT–ST system as the object of study. Although lead-free ferroelectric thin films have attracted a lot of attention during the past few years, however, compared to lead-based counterparts, they suffer from inferior insulating properties. This is one of the main drawbacks facing the development of lead-free thin films for practical application such as capacitors and memories in which lower leakage current, dielectric loss and large remanent polarization are demanded. Similar to the case of PZT-based piezoelectric materials, introducing metal ions additives into BNT-based compositions offer a versatile route to modify electrical properties. Recently, it has been demonstrated that proper
http://dx.doi.org/10.1016/j.ceramint.2015.03.177 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: P. Li, et al., Reduced leakage current, enhanced ferroelectric and dielectric properties of (La, Fe)-codoped Bi0.5Na0.5TiO3-based thin films, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.03.177
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Fig. 1. XRD patterns of the BNT–BKT–ST and BNT–BKT–ST–LaFe thin films.
The crystalline structure of the films was characterized by X-ray diffraction (XRD, D/max 2550 V, Rigaku, Japan) with Cu Kα radiation. The microstructure of the films was observed by the Field Emission Scanning Electron Microscope (FESEM, S-4700, HITACHI, Japan). In order to measure the electrical properties of the films, the gold pads with 0.50 mm in diameter were coated on the thin films surface as the top electrodes by DC sputtering. Dielectric constant and loss at different electric fields and frequencies were measured using the precision LCR meter (E4980A Agilent Inc., USA). Leakage current density–electric field (J–E) curves were obtained using a Keithley 6517A electrometer. The polarization–electric field (P–E) hysteresis loops were obtained using the ferroelectric test system (Radiant Precision Premier II). 3. Results and discussion
amounts of lanthanide oxides can improve the electrical properties of BNT-based solutions, depending on the nature of the lanthanide additives [15]. It is also reported that Fe2 þ , Fe3 þ , Mn2 þ and Mn3 þ doped ABO3 perovskite ferroelectric thin films have shown excellent results in improving the ferroelectric properties, reducing the dielectric loss, enhancing the resistivity of the thin films, and increasing the dielectric tunability [16–20]. However, to our knowledge, little attention has been paid to the impact of A-site and B-site codoping on (ABO3) perovskite structure. Thus, it is important to study the effect of (La, Fe)-codoping on the BNT–BKT–ST thin films.
2. Experimental procedures The lead-free ferroelectric thin films 0.9(0.8Bi0.5Na0.5TiO3– 0.2Bi0.5K0.5TiO3)–0.1SrTiO3 (abbreviated as BNT–BKT–ST) and (La, Fe)-codoped 0.9(Na0.8K0.2La0.005)0.5Bi0.5(Ti0.995Fe0.005) O3 þ x–0.1SrTiO3 thin films (abbreviated as BNT–BKT–ST–LaFe) were prepared by sol–gel method. To synthesize the precursor solution of BNT–BKT–ST and BNT–BKT–ST–LaFe thin films, sodium acetate trihydrate (CH3COONa 3H2O), potassium acetate (CH3COOK), bismuth nitrate pentahydrate [Bi(NO3)3 5H2O], strontium acetate [Sr(CH3COO)2], lanthanum nitrate [La(NO3)3], nine hydrated ferric nitrate [Fe(NO3)3 9H2O] and tetrabutyl titanate [Ti(OC4H9)4] were used as starting materials. Acetic acid (CH3COOH) and 2-methoxyethanol were used as solvent and acetylacetone (CH3COCH2COCH3) was used as polymerizing agent and stabilizer. First of all, the starting materials were dissolved in acetic acid and 2-methoxyethanol. Subsequently, the acetylacetone stable tetrabutyl titanate was added to the solution and the concentration of the final solution was adjusted to 0.3 M by adding acetic acid. Then the precursor solution was stirred at 70 1C for 5 h to make it well blended. After aging the precursor solution for 24 h, it was deposited on the Pt/Ti/SiO2/Si substrates by spin-coating at 3000 rpm for 30 s. After each spin, the gel layer was pyrolyzed at 300 1C for 5 min and prefired at 500 1C for 10 min in order to remove the residual organics. The spin-coating and heat treatment processes were repeated until the thin films reached the thickness of approximately 500 nm. Finally, the samples were annealed at 700 1C for 30 min.
The XRD patterns of the BNT–BKT–ST and BNT–BKT– ST–LaFe thin films are shown in Fig. 1. The diffraction peaks show that all thin films form a single-phase solid solution with a perovskite structure without any pyrochlore phase which will deteriorate the electrical performance of films. The restrain of pyrochlore phase can be attributed to the optimal annealing temperature and decreased nonstoichiometric structural defects [8,21]. Moreover, the XRD patterns of the BNT–BKT–ST and BNT–BKT–ST–LaFe thin films revealed that the (La,Fe)codoping does not change the phase structure. Fig. 2(a)–(d) shows the surface morphologies and crosssectional micrographs of BNT–BKT–ST and BNT–BKT–ST– LaFe thin films, respectively. From Fig. 2(a) and (b), it is observed that the average grain size of BNT–BKT–ST and BNT–BKT–ST–LaFe films is about 100 and 125 nm, respectively, and the surface morphologies of the thin films appear smooth and crack-free. The thickness of the films was approximately 500 nm as observed from the cross-sectional micrographs in Fig. 2(c) and (d). Insulating characteristic of dielectric thin film is determined by the value of leakage current. Fig. 3 shows leakage current density as a function of applied electric field. The leakage current density of all the thin films increased gradually with increasing applied electric field up to 400 kV/cm. In BNTbased thin films, the oxygen vacancies (V O ) should be the origins responsible for the high leakage current [22]. The (La, Fe)-codoped BNT–BKT–ST thin films exhibit enhanced insulating behavior, which can probably be attributed to the following reasons. Firstly, for the case of BNT–BKT–ST– LaFe thin film, due to the higher valence of La3 þ than that of (Na0.5Bi0.5)2 þ and the constrain of electrical charge neutrality, La3 þ ions substitution would suppress the formation of V O to a great extent [20]. Secondly, in the BNT–BKT–ST–LaFe thin film, the Fe3 þ was incorporated at the B-site (titanium sites) 0 0 and formed the defect complexes (FeTi V O FeTi ), which limit the free movement of the oxygen vacancy [17,20]. Fig. 4 shows the typical polarization–electric field (P–E) hysteresis loops of the thin films at the frequency of 10 kHz. From the P–E loops, it can be seen that the Ec þ and Ec- is not entirely symmetrical. This may be mainly due to two factors.
Please cite this article as: P. Li, et al., Reduced leakage current, enhanced ferroelectric and dielectric properties of (La, Fe)-codoped Bi0.5Na0.5TiO3-based thin films, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.03.177
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Fig. 2. The surface morphologies (a), (b) and cross-sectional micrographs (c), (d) of BNT–BKT–ST and BNT–BKT–ST–LaFe thin films.
Fig. 3. Leakage current density as a function of applied electric field for the BNT–BKT–ST and BNT–BKT–ST–LaFe thin films.
Fig. 4. P–E hysteresis loops of BNT–BKT–ST and BNT–BKT–ST–LaFe films as a function of electric field.
Firstly, the stress formed during thermal treatment can lead to preferential orientation of ferroelectric domains. Secondly, the ″0 formation of defect complexes (such as (V O V Bi )) lead to local field in the thin films [22]. The remnant polarization (Pr) and spontaneous polarization (Ps) of BNT–BKT–ST–LaFe is higher than BNT–BKT–ST thin film. In our present work, the improved Pr value of BNT–BKT–ST–LaFe film can be explained through distortion of TiO6 octahedra when Fe replace the B-site cations. Fe substitution may increase the dipole size by lengthening the off-center displacement in the perovskite structure, and hence the polarization is improved. Similar results have been reported for Mn doped BNT-based thin films [16]. The reduced leakage current also played an important role in the enhancement of polarization by increasing the effective applied electric field.
Moreover, a small number of La3 þ cations compensating for the A-site deficiencies due to the volatilization of A-site cations during the annealing process may contribute to the increased Pr. Similar results have been reported for Li þ doped BNT–BT thin films [23]. The measured coercive field (Ec) in these thin films are larger compared to the average bulk values (20 kV/cm) [13]. This could be a consequence of a higher defect concentration, local internal field, fine grain size and film stress [24]. In order to study the effect of (La ,Fe)-codoping on the dielectric performance, the dielectric constant and dielectric loss as a function of DC bias field and frequency are measured, as shown in Fig. 5. The C–V plots appear symmetric with a hysteresis behavior of ferroelectric capacitors. Dielectric constant changes with applied electric field, which indicating that the
Please cite this article as: P. Li, et al., Reduced leakage current, enhanced ferroelectric and dielectric properties of (La, Fe)-codoped Bi0.5Na0.5TiO3-based thin films, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.03.177
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ferroelectric domain contributes significantly to the polarizability of the thin films [24]. Compared with BNT–BKT–ST, the BNT– BKT–ST–LaFe thin film exhibits enhanced dielectric property with a larger dielectric constant of 425 and lower dielectric loss of 0.055 at zero electric field. The larger dielectric constant for BNT–BKT–ST–LaFe thin film is derived from the distortion of TiO6 octahedral caused by (La, Fe)-codoping, which gives B-site ions greater activity space and in turn creates favorable condition for polarization [16]. In addition, the decreased dielectric loss for BNT–BKT–ST–LaFe thin film can be ascribed to the following reasons. Firstly, it is known that, under an applied electric field, charges tend to accumulate (or even be trapped) at the ferroelectric film and metal electrode interface and/or at the grain boundaries, leading to increase the dielectric loss. However, for BNT–BKT–ST–LaFe thin film, (La, Fe)-codoping decrease the concentration of the carriers by surpressing the oxygen vacancies
Fig. 5. Dielectric constant and dielectric loss as a function of electric field.
0
0
and forming defect complexes (such as FeTi V O FeTi ) [20,25]. Secondly, one of the other origins of dielectric loss is the electron hopping through B-site cations of different valences such as Ti4 þ and Ti3 þ . However, in BNT–BKT–ST–LaFe thin films, Fe doping decreased the dielectric loss by preventing the transition of Ti4 þ –Ti3 þ . Similar results have been reported in Mn-doped BNT-based thin film [16,18]. The dielectric constant and dielectric loss measured at room temperature as a function of frequency from 100 Hz to 1 MHz for BNT–BKT–ST and BNT– BKT–ST–LaFe thin films are shown in Fig. 6. The dielectric constant decreases with the frequency increasing from 100 Hz to 1 MHz due to the decreased contribution of free space charge at elevated frequency [26]. The increase in the dielectric loss at high frequency is related to sample fixturing [27]. The improvement in dielectric properties can be attributed to the effect of La and Fe on the concentration of defects, lattice distortion and formation of defect complexes [16,20]. The temperature and frequency dependence of dielectric constant and dielectric loss for BNT–BKT–ST and BNT– BKT–ST–LaFe thin films are shown in Fig. 7. It can be seen that the Curie temperature (Tc) of BNT–BKT–ST–LaFe thin film is higher than pure BNT–BKT–ST thin film, which make the BNT–BKT–ST–LaFe thin film with high temperature stability and improved dielectric and ferroelectric properties to some extent. This result is consistent with previous result for La2O3 and CeO2 doped BNT-based ceramics [15]. It can be seen from Fig. 7(b) that the dielectric constant and dielectric loss are dependent on the frequency strongly. The first dielectric peak (corresponding to the ferroelctric-relaxation ferroelectric phase transition) shifts to high temperature with the increasing frequency and the Curie temperature Tc (corresponding to the ferroelectric–paraelectric phase transition) shifts to a low temperature with the increasing frequency. The above-mentioned characteristics demonstrate that the BNT–BKT–ST–LaFe thin films are classical relaxor ferroelectrics with the characteristics of diffuse phase transition and frequency dispersion [8,9]. 4. Conclusions
Fig. 6. Dielectric constant and dielectric loss as a function of frequency.
In summary, the BNT–BKT–ST and BNT–BKT–ST–LaFe thin films show the single perovskite structure, smooth surface and uniform grain size. The BNT–BKT–ST–LaFe thin films exhibit improved ferroelectric and dielectric properties with
Fig. 7. Temperature and frequency dependence of the dielectric constant and dielectric loss of BNT–BKT–ST and BNT–BKT–ST–LaFe thin films. Please cite this article as: P. Li, et al., Reduced leakage current, enhanced ferroelectric and dielectric properties of (La, Fe)-codoped Bi0.5Na0.5TiO3-based thin films, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.03.177
P. Li et al. / Ceramics International ] (]]]]) ]]]–]]]
high remanent polarization (Pr 13 μC/cm2), dielectric constant ( 420) and low dielectric loss ( 0.055). The results show that (La, Fe)-codoping is effective to improve the insulating, ferroelectric and dielectric properties by reducing the concentration of oxygen vacancies, forming defect complexes and reduce the pinning effect of ferroelectric domain. So, (La, Fe)-codoped BNT–BKT–ST thin films with enhanced ferroelectric and dielectric properties can be considered as potential candidates for replacement of the lead-based ferroelectric materials. Conflict of interest We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. Acknowledgments This work was supported by the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20120072130001) and the National Natural Science Foundation of China, China under Grant no. 51332003. This work was supported in part by the University of Macau, Macao. References [1] G. Smolenskii, V. Isupov, A. Agranovskaya, N. Krainik, New ferroelectrics of complex composition. 4, Sov. Phys. Solid State 2 (1961) 2651–2654. [2] M. Davies, E. Aksel, J.L. Jones, Enhanced high-temperature piezoelectric coefficients and thermal stability of Fe- and Mn-substituted Na0.5Bi0.5TiO3 ceramics, J. Am. Ceram. Soc. 94 (2011) 1314–1316. [3] C. Dragoi, M. Cernea, L. Trupina, Lead-free ferroelectric BaTiO3 doped(Na0.5Bi0.5)TiO3 thin films processed by pulsed laser deposition technique, Appl. Surf. Sci. 257 (2011) 9600–9605. [4] M. Bousquet, J.R. Duclère, C. Champeaux, A. Boulle, P. Marchet, A. Catherinot, A. Wu, P.M. Vilarinho, S. Députier, M. Guilloux-Viry, A. Crunteanu, B. Gautier, D. Albertini, C. Bachelet, Macroscopic and nanoscale electrical properties of pulsed laser deposited (100) epitaxial lead-free Na0.5Bi0.5TiO3 thin films, J. Appl. Phys. 107 (2010) 034102. [5] S.K. Acharya, T.M. Kim, J.H. Hyung, B.G. Ahn, S.K. Lee, Ferroelectric and piezoelectric properties of lead-free Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3– BaTiO3-thin films near the morphotropic phase boundary, J. Alloy. Compd. 586 (2014) 549–554. [6] X.C. Zheng, G.P. Zheng, Z. Lin, Z.Y. Jiang, Thermal and dynamic mechanical analyses on Bi0.5Na0.5TiO3–BaTiO3 ceramics synthesized with citrate method, Ceram. Int. 39 (2013) 1233–1240. [7] X.C. Zheng, G.P. Zheng, Z. Lin, Z.Y. Jiang, Thermo-electrical energy conversions in Bi0.5Na0.5TiO3–BaTiO3 thin films prepared by sol–gel method, Thin Solid Films 522 (2012) 125–128. [8] G. Yueqiu, D. Hui, Z. Xuejun, P. Jinfeng, L. Xujun, H. Renjie, Large piezoelectric response of Bi0.5(Na(1 x)Kx)0.5TiO3thin films near morphotropic phase boundary identified by multi-peak fitting, J. Phys. D: Appl. Phys. 45 (2012) 305301. [9] W.J. Ji, Y.B. Chen, S.T. Zhang, B. Yang, X.N. Zhao, Q.J. Wang, Microstructure and electric properties of lead-free 0.8Bi1/2Na1/2TiO3– 0.2Bi1/2K1/2TiO3 ceramics, Ceram. Int. 38 (2012) 1683–1686.
5
[10] J.F. Trelcat, C. Courtois, M. Rguiti, A. Leriche, P.H. Duvigneaud, T. Segato, Morphotropic phase boundary in the BNT–BT–BKT system, Ceram. Int. 38 (2012) 2823–2827. [11] A. Hussain, J.U. Rahman, A. Zaman, R.A. Malik, J.S. Kim, T.K. Song, W.J. Kim, M.H. Kim, Field-induced strain and polarization response in lead-free Bi1/2(Na0.80K0.20)1/2TiO3–SrZrO3 ceramics, Mater. Chem. Phys. 143 (2014) 1282–1288. [12] W. Krauss, D. Schütz, F.A. Mautner, A. Feteira, K. Reichmann, Piezoelectric properties and phase transition temperatures of the solid solution of (1 x)(Bi0.5Na0.5)TiO3–xSrTiO3, J. Eur. Ceram. Soc. 30 (2010) 1827–1832. [13] K. Wang, A. Hussain, W. Jo, J. Rödel, D.D. Viehland, Temperaturedependent properties of (Bi1/2Na1/2)TiO3–(Bi1/2K1/2)TiO3–SrTiO3 leadfree piezoceramics, J. Am. Ceram. Soc. 95 (2012) 2241–2247. [14] M.R. Bafandeh, R. Gharahkhani, J.S. Lee, Enhanced electric field induced strain in SrTiO3 modified (K, Na)NbO3-based piezoceramics, J. Alloy. Compd. 602 (2014) 285–289. [15] Q. Xu, M. Chen, W. Chen, H.X. Liu, B.H. Kim, B.K. Ahn, Effect of Ln2O3 (Ln¼La, Pr, Eu, Gd) addition on structure and electrical properties of (Na0.5Bi0.5)0.93Ba0.07TiO3 ceramics, J. Alloy. Compd. 463 (2008) 275–281. [16] M.M. Hejazi, 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. [17] E. Aksel, E. Erdem, P. Jakes, J.L. Jones, R.d.A. Eichel, Defect structure and materials hardening in Fe2O3-doped Bi0.5Na0.5TiO3 ferroelectrics, Appl. Phys. Lett. 97 (2010) 012903. [18] W. Li, H. Zeng, J. Hao, J. 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. [19] Y. Wu, X. Wang, C. Zhong, L. 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. [20] 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. [21] X.J. Zheng, J.Y. Liu, J.F. Peng, X. Liu, Y.Q. Gong, K.S. Zhou, D.H. Huang, Effect of potassium content on electrostrictive properties of Na0.5Bi0.5TiO3-based relaxor ferroelectric thin films with morphotropic phase boundary, Thin Solid Films 548 (2013) 118–124. [22] C.H. Yang, W.B. Wu, F. Yang, H.T. Wu, X.Y. Zhang, Dielectric and ferroelectric properties of A-site non-stoichiometric Na0.5Bi0.5TiO3-based thin films, Mater. Lett. 66 (2012) 86–88. [23] S.K. Acharya, B.G. Ahn, J.H. Hyung, S.K. Lee, Effect of Li doping on ferroelectric and piezoelectric properties of Ba0.5Na0.5TiO3–BaTiO3 (BNT–BT) thin films, J. Korean Phys. Soc. 62 (2013) 794–799. [24] M. Abazari, A. Safari, S.S.N. Bharadwaja, S. Trolier-McKinstry, Dielectric and piezoelectric properties of lead-free (Bi,Na)TiO3-based thin films, Appl. Phys. Lett. 96 (2010) 082903. [25] M. Bousquet, J.R. Duclère, B. Gautier, A. Boulle, A. Wu, S. Députier, D. Fasquelle, F. Rémondière, D. Albertini, C. Champeaux, P. Marchet, M. Guilloux-Viry, P. Vilarinho, Electrical properties of (110) epitaxial lead-free ferroelectric Na0.5Bi0.5TiO3 thin films grown by pulsed laser deposition: macroscopic and nanoscale data, J. Appl. Phys. 111 (2012) 104106. [26] C. Jin, F. Wang, C.M. Leung, Q. Yao, Y. Tang, T. Wang, W. Shi, Enhanced ferroelectric and piezoelectric response in Mn-doped Bi0.5Na0.5TiO3–BaTiO3 lead-free film by pulsed laser deposition, Appl. Surf. Sci. 283 (2013) 348–351. [27] 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.
Please cite this article as: P. Li, et al., Reduced leakage current, enhanced ferroelectric and dielectric properties of (La, Fe)-codoped Bi0.5Na0.5TiO3-based thin films, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.03.177