Advanced Materials Research Online: 2013-04-24 ISSN: 1662-8985, Vol. 684, pp 307-311 doi:10.4028/www.scientific.net/amr.684.307 2013 Trans Tech Publications, Switzerland Characteristics of Lead Free Ferroelectric Thin Films Consisted of (Na 0.5 Bi 0.5 )TiO 3 and Bi 4 Ti 3 O 12 Hee Kwon Ahn 1,a, Young Ho Kim 1,b, Sang Keun Gil 2,c, Dong Soo Paik 3,d and Dong Heon Kang 1,e * 1 Dept. of Electronic Materials Eng.,The University of Suwon, Whaseong, Gyeonggi, 445-743, Korea 2 Dept. of Electronic Eng.,The University of Suwon, Whaseong, Gyeonggi, 445-743, Korea 3 School of Materials Science and Eng., Korea University, Seoul, 136-701, Korea a ahk1222@naver.com, b yhkim@suwon.ac.kr, c skgil@suwon.ac.kr, d dspaik64@hanmail.net, e dhkang@suwon.ac.kr (*corresponding author) Keywords: (Na 0.5 Bi 0.5 )TiO 3, Bi 4 Ti 3 O 12, Na 0.5 Bi 4.5 Ti 4 O 15, Stacking layer thin film, Dielectric property Abstract. Ferroelectric thin films containing the same volume fraction of the perovskite structure (Na 0.5 Bi 0.5 )TiO 3 (NBT) and layered structure Bi 4 Ti 3 O 12 (BIT) were prepared with different stacking sequence. Na 0.5 Bi 4.5 Ti 4 O 15 thin film, solid solution of NBT and BIT, was also synthesized to compare with the alternative layered films. Their crystal structure, microstructure and electrical properties were investigated as a function of number of interface. It was found that the interface area as well as each material property is critical to the electrical properties. The alternative layer thin films between NBT and BIT showed positive effect on the dielectric behavior than the single NBIT compound film. Introduction Pb-based perovskites exhibit the best piezoelectric properties and are widely used in several practical applications. However, the toxity and the high vapor pressure of Pb during the manufacturing processing have led to demand for alternative Pb-free piezoelectric materials. The search for alternative piezoelectric materials is now focused on alkali niobate modified bismuth titanates and their solid solutions [1]. (Na 0.5 Bi)TiO 3 (NBT) has been considered as a good candidate for Pb-free ferroelectric ceramics because of its high dielectric constant, good piezoelectricity and high Curie temperature (near 320 ) [1,2]. However, because of its high coercive field and relatively large conductivity, pure NBT is difficult to be poled and cannot be a good piezoelectric material. These drawbacks limit its application for potential ferroelectric and electromechanical devices. These problems were then improved by forming solid solutions with Bi 0.5 K 0.5 TiO 3, KNbO 3, NaNbO 3, and by doping a small amount of various metal oxides [3-5]. Bismuth titanate (Bi 4 Ti 3 O 12, BIT) belongs to another important class of Bi-based ferroelectric compounds. Its structure, named as Aurivillius compound, is known as a typical layered one consisted of (Bi 2 O 2 ) 2+ fluorite-like layers and perovskite (A m-1 B m O 3m-1 ) 2- slabs where m corresponds to the number of perovskite type units between the bismuthyl layers [6]. Several authors have shown that those processing a higher value of m exhibit stronger piezoelectric responses [7]. The BIT consisted All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (ID: 130.203.136.75, Pennsylvania State University, University Park, USA-06/03/16,14:36:25)
308 Advances in Applied Materials and Electronics Engineering II of three perovskite layers shows definitely high Curie temperature (675 C) and remnant polarization. On the other hand, it has been reported that it exhibits low dielectric constant and the flexibility on the manufacturing process is quite low due to the structural anisotropy. A few studies related to the crystal structures and electrical properties according to the variation of composition have been therefore explored using the bulk type ceramics [8]. Furthermore, complex thin films simultaneously using two materials such as NBT and BIT showing different material properties has been scarcely studied. It is, from this complex system, expected that high composition stability can be maintained on the thin film growing process because NBT and BIT commonly have bismuth as a major constituent. In this study, alternative layers with same volume fraction of NBT and BIT were prepared where the stacking order was varied to change the number of interface. A compound Na 0.5 Bi 4.5 Ti 4 O 15 without interface between NBT and BIT was also prepared to compare the difference of electrical behavior for both the alternative and single layered thin films. The Effect of stacking sequence on the crystal structure, surface morphology, and electrical properties for various typed thin films were investigated. Experimental The precursors for NBT and BIT were prepared by a modified sol-gel processing proposed by X. G. Tang [9]. Bi, Na and Ti precursors, prepared similarly in our previous report [5] were cooled down to R.T and then mixed together followed by reheating and stirring at 70 for 1h. Both stock solutions (0.3 M) for coating were diluted using 2-MOE and then aged for 24h respectively. To prepare alternative layer films, NBT and BIT layers were stacked as follows; 1 layer-nbt/2 layers- BIT/2 layers-nbt/2 layers-bit/1 layer-nbt (NBIT-a, total 4 interface layers), 2 layers-nbt/4 layers-bit/2 layers-nbt (NBIT-b, total 2 interface layers) and 4-layers NBT/4-layers BIT (NBIT-c, total 1 interface layer). Each layer was deposited onto Pt(111)/Ti/SiO 2 /Si(100) wafer by the spin coating and pyrolyzed at 300 for 5 min where the thickness of 1 layer approached to about 370~380 Å. To prepare Na 0.5 Bi 4.5 Ti 4 O 15 (NBIT-N) as a solid solution, containing same amount of NBT and BIT, the above process was used. Finally, the thickness of NBIT-N thin film was adjusted to about 3000 Å as the same with the alternative layer films by coating process. All the films were annealed at 700 for 5 min using an RTA(MILA3000-P-N, Sinku-Riko). The annealed films were subsequently post-annealed at 700 for 10 min to improve the crystallinity. Crystal structure and surface morphology were investigated using an X-ray diffractometer (D/MAX-2500, Rigaku) and a FE-SEM (JSM-6700F, Jeol), respectively. After depositing Pt top electrodes, dielectric properties were measured by employing an impedance analyzer (HP4294A) and a precision pro (Radiant Tech. Inc). Results and discussion Fig. 1 depicts the XRD patterns of NBT-BIT alternative layer (NBIT-a,b,c) and Na 0.5 Bi 4.5 Ti 4 O 15 (NBIT-N) thin films. For the NBT-BIT alternative layer thin films, it was obviously seen that there exists fundamental peaks corresponding to the crystal structures of perovskite NBT and Aurivillius structured BIT despite different stacking sequences and number of interface layer. For the Na 0.5 Bi 4.5 Ti 4 O 15 thin film, the XRD pattern is similar to that of the alternative layer ones
Advanced Materials Research Vol. 684 309 and a peak at 2θ 30 o verifying the existence of Aurivillius compound is observed [7,10]. SEM photographs of NBT-BIT alternative layer (NBIT-a,b,c) and Na 0.5 Bi 4.5 Ti 4 O 15 (NBIT-N) thin films and surface morphology of single NBT and BIT thin films are shown in Fig. 2. The stacking sequences are depicted inside the SEM photographs, indicating the number of interface of 4, 2, 1 and null for the specimens of NBIT-a, NBIT-b, NBIT-c, and NBIT-N, respectively. In comparison with the surface morphologies of pure NBT and BIT films as shown in Fig. 2 (e) and (f), those of the alternative layer films generally depend on what the surface materials are although two different phases coexist as shown in the XRD analysis. However, the morphology of 1-layer NBT based on 2-layer BIT as shown in Fig, 2 (a) was highly affected by that of the BIT bottom layer microstructure due to thinner thickness compared with that in Fig. 2 (b). The surface morphology of Aurivillius compound NBIT-N based on the crystal structure analysis is very similar to that of pure BIT film due to the same crystal structure and the grain sizes of 5~10 and ~100 nm are very closely coexisted. Intensity(arb.unit) (004) (006) (008) (111) (101) (0010) (117) (0012) (0014) (020) (220) (2014) (110) (317) (021) (202) (113) (122) (1311) BNT BIT BIT NBIT-N NBIT-c NBIT-b NBIT-a NBT (220) Fig. 1 XRD patterns of BNT-BIT alternative layer and Na 0.5 Bi 4.5 Ti 4 O 15 thin films as a function of number of interface 10 20 30 40 50 60 70 2theta Fig. 2 SEM photographs of BNT-BIT thin films with different features; (a) NBIT-a, (b) NBIT-b, (c) NBIT-c, (d) NBIT-N, (e) pure BIT, and (f) pure NBT
310 Advances in Applied Materials and Electronics Engineering II Based on the structural investigation, electrical properties were measured and analyzed. Dielectric constant and tan δ at 100 khz for various features of NBIT thin films are shown in Fig. 3. The dielectric constants of pure NBT and BIT thin films were 305 and 154, respectively. 260 Dielectric constant 240 220 200 180 160 140 0.05 0.04 0.03 0.02 tanδ Fig. 3 Dielectric constant and tan δ of NBIT thin films with different stacking sequences 120 NBIT-a NBIT-b NBIT-c NBIT-N Type of thin film structure 0.01 Although all the alternative layer thin films have same volume fraction of NBT and BIT, the dielectric constant increases with the decrease of the number of interface, showing maximum 250 for NBIT-c film. However, the dielectric constant of NBIT-N film is somewhat low since Na 0.5 Bi 4.5 Ti 4 O 15 is a new compound, different with alternative layer films definitely coexisting two different phases. It is therefore clearly seen that the dielectric constant is highly affected by the interface area when two materials with different crystal structures and phases coexist. The tan δ of <0.02 is relatively stable regardless of the film feature. As a result, it was found that the alternative layer process resulted in positive effect on the increase of dielectric constant. Fig. 4 shows P-E hysteresis loops and remnant polarization and coercive field values of NBIT thin films with different stacking sequences. While the coercive field of < 250 kv/cm is relatively constant for the alternative layer films, the remnant polarization gradually increases with the decrease of the number of interface. By contrast, the NBIT-N film exhibits very high remnant polarization of 17.5 µc/cm 2 and coercive field of 328 kv/cm. In comparison with the result of dielectric behavior measured under small electric signal, the difference of variation tendency for the dielectric constant and remnant polarization of the alternative layer and single NBIT films probably resulted from the movement of dipole. Since the dipoles under large electric signal are strongly clamped, the interface between different structures may further affect the electrical properties. Conclusions The structural and electrical properties of (Na 0.5 Bi 0.5 )TiO 3 -Bi 4 Ti 3 O 12 alternative and Na 0.5 Bi 4.5 Ti 4 O 15 single layered thin films were investigated. Ferroelectric thin films containing the same volume fraction of the perovskite structure NBT and layered structure BIT were prepared with
Advanced Materials Research Vol. 684 311 Polarization(µC/cm 2 ) 50 40 30 20 10 0-10 (a) -20 NBIT-a -30 NBIT-b NBIT-c -40 NBIT-N -50-1500 -1000-500 0 500 1000 1500 Electric filed(kv/cm) Pr(µC/cm 2 ) 18 16 14 12 10 8 6 (b) NBIT-a NBIT-b NBIT-c NBIT-N Type of thin film structure 500 450 400 350 300 250 Coercive field(kv/cm) Fig. 4 (a) P-E hysteresis loops and (b) remnant polarization and coercive field of NBIT thin films with different stacking sequences different stacking sequence. NBIT thin film, solid solution of NBT and BIT, was also prepared to compare with the alternative layered films. For the NBT-BIT alternative layer thin films, the crystal structures of perovskite NBT and Aurivillius structured BIT are coexisted despite different stacking sequences and number of interface layer. With decreasing the number of interface, dielectric properties tended to increase and showed 250 (ε r ), 0.014 (tanδ) and 13 µc/cm 2 ) (P r ), 240kV/cm (E c ) for the thin film containing one NBT/BIT interface (NBIT- c). The effect of interface in thin film was more noticeably found for polarization measurement using hysteresis loop, comparing the alternative layered thin film and Na 0.5 Bi 4.5 Ti 4 O 15 single one. References [1] S.J. Zhang, R. Xia and T.R. Shrout: J. Electroceram. Vol.19 (2007), p. 51 [2] H, Nagata and T. Takenaka: Jpn. J. Appl. Phys. Vol.30 (1997), p. 6055 [3] T. Yu, K.W. Kwok and H.L.W. Chan: Mater. Lett. Vol.61 (2007), p. 2117 [4] M.G. Gavshin and V.I. Pastukhov: Ferroelectrics Vol.205 (1998), p. 81 [5] D.H. Kang, B.S. Lee, H.K. Ahn, Y.H. Kim, D.S. Paik, H.I. Hwang and N.K. Cho: Mater. Lett. Vol.64 (2010), p. 2331 [6] A. Sanson and R.W. Whatmore: Jap. J. Appl. Phys. Vol.41 (2002), p. 7127 [7] A. Sanson and R.W. Whatmore: J. Am. Ceram. Soc. Vol.88 (2005), p. 3147 [8] M.L. Zhao, Q.Z. Wu, C.L. Wang, J.L. Zhang, Z.G. Gai and C.M. Wang: J. Appl. Alloy. Comp. Vol.476 (2009), p. 393 [9] X.G. Tang, J. Wang, X.X. Wang and H.L.W. Chan: Chem. Mater. Vol.16 (2004), p. 5293 [10] M.S Tomar, S.L Dussan-Devia and O. P-Perez: NSTI-Nanotech, Vol.3 (2006), p. 210
Advances in Applied Materials and Electronics Engineering II 10.4028/www.scientific.net/AMR.684 Characteristics of Lead Free Ferroelectric Thin Films Consisted of (Na 0.5 Bi 0.5 )TiO 3 and Bi 4 Ti 3 O 12 10.4028/www.scientific.net/AMR.684.307