- Email: [email protected]
Enhancement on effective piezoelectric coefficient of Bi3.25Eu0.75Ti3O12 ferroelectric thin films under moderate annealing temperature
Thin Solid Films 519 (2010) 714–718
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Enhancement on effective piezoelectric coefficient of Bi3.25Eu0.75Ti3O12 ferroelectric thin films under moderate annealing temperature X.J. Zheng a,b,⁎, J.F. Peng a, Y.Q. Chen a, L. He a,b, X. Feng c, D.Z. Zhang a, L.J. Gong a, Q.Y. Wu a a b c
Faculty of Materials, Optoelectronics and Physics, Xiangtan University, Xiangtan, Hunan 411105, People's Republic of China Key Laboratory of Low Dimensional Materials and Application Technology of the Ministry of Education, Xiangtan University, Xiangtan, Hunan 411105, People's Republic of China School of Aerospace, Tsinghua University, Beijing, 100084, People's Republic of China
a r t i c l e
i n f o
Article history: Received 30 October 2009 Received in revised form 20 August 2010 Accepted 25 August 2010 Available online 1 October 2010 Keywords: Bismuth europium titanate Thin films Dielectric properties Piezoelectric properties Annealing Metal organic decomposition X-ray diffraction Scanning electron microscopy
a b s t r a c t Bi3.25Eu0.75Ti3O12 (BET) thin films were deposited on Pt/Ti/SiO2/Si(111) substrates by a metal-organic decomposition method. The effects of annealing temperatures 600–800 °C on microstructure, ferroelectric, dielectric and piezoelectric properties of BET thin films were studied in detail. The spontaneous polarization (87.4 × 10− 6 C/cm2 under 300 kV/cm), remnant polarization (65.7 × 10− 6 C/cm2 under 300 kV/cm), the dielectric constant (992.9 at 100 kHz) and the effective piezoelectric coefficient d33 (67.3 pm/V under 260 kV/cm) of BET thin film annealed at 700 °C are better than those of the others. The mechanisms concerning the dependence of the enhancement d33 are discussed according to the phenomenological equation, and the improved piezoelectric performance could make the BET thin film a promising candidate for piezoelectric thin film devices. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Bismuth (Bi) titanate thin films have been intensively investigated as potential materials for non-volatile random access memories, piezoelectric devices, sensors, and actuators [1]. In the past years, many studies reveal that Bi3+ ions in the Bi4Ti3O12 (BIT) structure can be substituted by trivalent lanthanoid ions, such as Nd3+, La3+, Pr3+, Dy3+ and Sm3+, which improves ferroelectric properties [2–4]. In particular, Eu-substituted BIT lead-free thin films have emerged as ferroelectric materials due to good fatigue endurance and large polarization [5,6], and in our previous work the 2Pr is 66 × 10− 6 C/cm2 for Bi3.25Eu0.75Ti3O12 (BET) lead-free thin film annealed at 700 °C [7]. Recently, more and more efforts have been made to develop nontoxic lead-free piezoelectric materials to apply into piezoelectric microelectromechanical systems (MEMS), and Bi-layered perovskite lead-free ferroelectric thin films are considered to be candidate materials, such as the effective piezoelectric coefficients d33 of 30– 60 pm/V for CaBi4Ti4O15 thin films [8,9], and ~ 38 pm/V for the
⁎ Corresponding author. Faculty of Materials, Optoelectronics and Physics, Xiangtan University, Xiangtan, Hunan 411105, People's Republic of China. Tel.: + 86 731 58293648; fax: + 86 731 58298119. E-mail address: [email protected] (X.J. Zheng). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.08.130
neodymium-doped Bi4Ti3O12 (BNT) thin film [10]. According to the phenomenological equation regarding the intrinsic piezoelectric coefficient, the high spontaneous polarization Ps is favorable for yielding higher piezoelectric displacement [10]. It is well known that the annealing temperature has an important effect on the microstructure, dielectric and ferroelectric properties of Bi-layered perovskite ferroelectric thin films. However, few study has involved the piezoelectric properties of BET thin films annealed at different temperatures, therefore it is imperative for us to improve the piezoelectric properties of the thin films in order to impetus for integrating Bi-layered perovskite ferroelectric thin films into MEMS piezoelectric devices. In this paper, BET lead-free ferroelectric thin films were prepared by metal-organic decomposition (MOD), and the effects of the annealing temperature on microstructure, ferroelectric, dielectric and piezoelectric properties were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), scanning probe microscope (SPM), ferroelectric tester, and impedance analyzer. The ferroelectric, dielectric, and piezoelectric behaviors were established from polarization–electric field (P–E) hysteresis loops, dielectric constant–frequency (εr–f) and dissipation–frequency (tanδ–f) curves, and displacement–electric field (D–E) “butterfly” curves, respectively. We expect that the research can offer useful guidelines to the design of Bi-layered thin film piezoelectric devices.
X.J. Zheng et al. / Thin Solid Films 519 (2010) 714–718
715
2. Experimental details BET thin films were deposited on a Pt(200 nm)/Ti(30 nm)/SiO2/ Si(111) substrate by the MOD method [5,6]. The precursor materials were bismuth (III) acetate {Bi[CH3CO2]3}, europium (III) acetate hydrate {Eu[CH3CO2]3} and tetrabutyl titanium {Ti[OC4H9]4}, and the solvents were acetic acid {CH3CO2H} and acetylacetone {CH3COCH2COCH3}. A 10% excess amount of bismuth acetate was used to compensate Biloss during the annealing process. At first, the solid-state bismuth acetate and europium acetate were dissolved in the acetic acid, and the solution was mixed to obtain the (Bi, Eu) stock solution followed by stirring for 12 h. Secondly, the tetrabutyl titanium was dissolved in the mixture of the stock solution and acetylacetone, and it was magnetically stirred in the air atmosphere for 1 h. Finally, the flaxen, transparent and stable BET precursor solution was prepared. The precursor solutions with a mole ratio (Bi:Eu:Ti) of 3.575:0.75:3 were spun on the substrate at 4000 rpm for 30 s. After the spin-coating procedure, the thin films were kept in a rapid temperature process at 400 °C for 180 s to remove the organic ingredients. The coating/drying circles were repeated 7 times to achieve the desired film thickness, and the pre-baked films were respectively annealed at 600, 650, 700, 750 and 800 °C in an oxygen atmosphere to promote crystallization. Using FE-SEM (1530, LEO, Germany), the surface morphologies of BET thin films were identified with 20 K amplification, and the crosssectional micrograph of the BET thin film annealed at 700 °C was recorded with 20 K amplification. The FE-SEM operating voltage was 20 kV. Phase identification, degree of crystallinity, and crystalline orientation of the thin films were investigated by XRD (D/Max 2500, Rigaku, Japan) using the normal θ–2θ scanning method, and they were scanned at 10°/min with Cu Kα radiation (λ = 1.541 Å, 30 kV and 100 mA). The circular Au top electrodes with a radius of 0.1 mm were deposited on the thin films using a shadow mask by dc magnetron sputtering, and under the electric field 300 kV/cm the P–E hysteresis loops were measured by a ferroelectric tester (Precision Workstation, Radiant Technologies, USA). Frequency dependent dielectric constant (εr) and dissipation factor (tanδ) were measured by the impedance analyzer (HP4194A, Hewlett Packard, America). A commercial available SPM (SPI4000&SPA300HV, Seiko, Japan) was used to characterize the piezoelectric properties of the thin films without the top electrode. The conductive rhodium coated silicon cantilever is of a spring constant of 1.9 N/m, a resonant frequency of 28 kHz, and an integrated tip of about 10 nm in diameter. The stiff cantilever was used to get a large indentation force ensuring that the measurement was in the so-called strong-indentation regime [11]. The piezoelectric measurement was conducted on ten points of scanned scope on the sample surface, because the piezoelectric characterization for SPM is a kind of local method [12]. For each sample, keeping the SPM tip fixed above the interesting point the D–E “butterfly” curve was recorded under a dc voltage applied from −9 to 9 V. With considering the unexpected shift of the intersection from the origin of the D–E curve, the d33-electric field (d33-E) loops can be calculated according to the modified equation of converse piezoelectric effect [12] d33 =
D−DI ; dðE−EI Þ
ð1Þ
where d is the initial film thickness before deformation. On the D–E curve, DI and EI are the piezoelectric displacement and electric field for the intersection, and D and E are the measured values of piezoelectric displacement and electric field for each point. All of the above measurements were carried out at room temperature. 3. Results and discussion Fig. 1(a)–(e) shows the surface SEM images of BET thin films annealed at 600, 650, 700, 750 and 800 °C, and Fig. 1(f) shows the
Fig. 1. The FE-SEM images of surface morphologies for BET thin films annealed at (a) 600 °C, (b) 650 °C, (c) 700 °C, (d) 750 °C, and (e) 800 °C and (f) a FE-SEM image of a cross-sectional micrograph for the thin film annealed at 700 °C.
typical cross-sectional micrograph of the BET thin film annealed at 700 °C. In Fig. 1(a)–(e), the porosity values are respectively 1.82, 0.73 and 0.27, 0.19 and 0.13% for BET thin films annealed at 600, 650, 700, 750 and 800 °C, and they are of many irregularly shaped pores. With the increase of the annealing temperature, the porosity decreases and the grain size enlarges, indicating that the high annealing temperature enhances the crystal growth. In our previous investigation, the ferroelectric, fatigue and retention characteristics of Au/BET/Pt thin film capacitors are increased at a range of 600–700 °C and degraded at a range of 700–800 °C with the annealing temperature [13]. The lower porosity may result in the enhancement of sufficient life time of BET thin films for 600–700 °C, and the insufficient Bi supply degrades the ferroelectric, dielectric and piezoelectric properties for 700–800 °C [13,14]. The crack-free surface seems to be important for ferroelectric thin films because the crack will affect the microstructure, ferroelectric properties and residual stress [15]. The film thickness via the same coating/drying circles is the same as 346 nm for the five kinds of BET thin films, therefore there is only given the cross-sectional micrograph for the thin film annealed at 700 °C, as shown in Fig. 1(f). Indeed, the annealing temperature 700 °C may be a special threshold in the relationship between the properties and annealing temperature [14], and it indicates that the typical behavior for the thin film annealed at 700 °C would be better than those for the thin films annealed at other temperatures. Fig. 2 shows a typical Bi-layered perovskite polycrystalline phase without any pyrochlore phase and other phases related to Eu. The 2θ values of (117), (111) and (024) peaks are respectively 29.81°, 23.69° and 34.84°, and there are slight shifts in the position of the XRD peaks compared to the position of BIT standard XRD peaks [16]. These indicate that the substitution of Eu3+ to Bi3+ occurs, and it is in agreement with the previous results [14]. In fact, the amounts of Eu incorporated in the Bi3.15Eu0.85Ti3O12 thin films were quantified by
716
X.J. Zheng et al. / Thin Solid Films 519 (2010) 714–718
Fig. 2. X-ray diffraction patterns of BET thin films annealed at 600, 650, 700, 750, and 800 °C, respectively.
energy dispersive X-ray spectroscopy, and the effects of the annealing temperature on chemical composition are discussed deeply [14]. At 600 °C, the diffraction peaks are low and broad, indicating poor crystallinity of the sample. With the increase of the annealing temperature, the diffraction peaks become sharp and strong, indicating the possible improved crystallinity. According to I(006)/ [I(006) + I(117)] × 100% (where Ihkl is the relative intensity of the corresponding peak) [17], the degrees of c-axis orientation are 24, 17, 15, 19 and 21% for BET thin films annealed at 600, 650, 700, 750 and 800 °C, respectively. The degree of c-axis orientation of the BET thin film annealed at 700 °C is lower than those of thin films annealed at 600, 650, 750 and 800 °C. Generally the relationship between the preferred orientation and annealing temperature is a monotone function for the bulk crystal. There are two possible reasons for the non-linear behavior of the preferred orientation with the annealing temperature. At first, the crystallization with the preferred orientation is related to the free energy, because the nucleation and growth variations occur in the energy barrier heights [18]. Secondly, the increased annealing temperature results in the defect propagation to affect the preferred orientation during the growth of the thin film [19]. According to the previous investigation on ferroelectric materials with a bismuth-layer structure, the weaker the degree of c-axis orientation, the larger the remanent polarization [20]. From Fig. 2, the weakest degree of c-axis orientation is 15%, thus the BET thin film annealed at 700 °C may have the larger polarization than the others. However, the dependence of polarization on a- and b-axis orientations is different for BIT material family thin films with the different preferred orientation. If BIT material family has its polarization mostly in the (a, b) plane, the c-orientation may be not the largest piezoresponse [21]. In other words, the values of polarization are dependent on not only the present c-orientation, but also the a- and b-axis orientations [22]. The vector of the (major) spontaneous polarization in these layered perovskite materials is along the a-axis, the stronger the degree of a-axis orientation, the larger the remanent polarization [2]. The P–E loops of BET thin films annealed at 600, 650, 700, 750 and 800 °C are shown in Fig. 3(a). The P–E loop of the BET thin film annealed at 600 °C is not saturated well, while BET thin films annealed at temperatures above 650 °C show saturated P–E loops. Under the bipolar driving field of 300 kV/cm, the values of 2Ps and 2Pr are 43.8, 64.4, 87.4, 75.8, and 54.4 × 10− 6 C/cm2 and 38.2, 57.5, 65.7, 61.3, and 43.1 × 10− 6 C/cm2 for BET thin films annealed at 600, 650, 700, 750 and 800 °C, respectively. With the increase of the annealing temperature the values of 2Pr and 2Ps increase at a range of 600– 700 °C and decrease from 700 to 800 °C. Obviously, the maximum of 2Pr (65.7 × 10− 6 C/cm2) for the BET thin film annealed at 700 °C equals to 66 × 10− 6 C/cm2 that was reported in the previous reference
Fig. 3. (a) P–E hysteresis loops and (b) frequency dependent εr and tanδ of BET thin films annealed at 600, 650, 700, 750, and 800 °C.
[7], but is larger than 38 × 10− 6 C/cm2 for the Bi3.4Eu0.6Ti3O12 thin film and 26 × 10− 6 C/cm2 for the Bi1.32Ti2.97V0.03O12 ceramic. [5,20]. From the εr–f and tanδ–f curves shown in Fig. 3(b), under the applied frequency from 1 kHz to 1 MHz the εr values decrease with the increase of frequency, and it is due to the extrinsic resonance behavior resulting from the microstructure deficiency, which may in principle be reduced by optimization of the film growth process [23]. The tanδ increases at higher frequencies, and the increasing tendency of tanδ is also reported for BNT and Bi3.25La0.75Ti3O12 (BLT) ferroelectric thin films [24,25]. There are three possible reasons for the dispersion as follows. One possible cause is the extrinsic resonance behavior which results from the microstructure deficiency [23]. Secondly, the mobile charges contained in the film cannot follow higher frequency electric fields [26], indicating that the higher resistive losses result in the increasing tendency of tanδ. The third one is the hypothesis of the influence of the contact resistance between the probe and the electrode [24]. The typical D–E (black) curves and d33-E (blue) loops of BET thin films annealed at 600, 650, 700, 750 and 800 °C are given in Fig. 4. The d33-E loops are almost symmetric and they are in an agreement with the previous symmetries of BiScO3–PbTiO3 and Zn1 − xVxO ferroelectric thin films [12,27]. On the D–E curves, there are no notable horizontal shifts, and little vertical shifts 0.09–0.28 nm can be observed, indicating no obvious imprint effect. The d33-E loops clearly show that the BET thin film is switchable and the piezoelectric is retained [28]. Under a bipolar driving field of 260 kV/cm, the average
X.J. Zheng et al. / Thin Solid Films 519 (2010) 714–718
717
Fig. 4. The representative D–E curves and d33-E loops of the BET thin film annealed at (a) 600 °C, (b) 650 °C, (c) 700 °C, (d) 750 °C, and (e) 800 °C.
values of d33 are 39.6, 48.4, 67.3, 30.85 and 28.2 pm/V for the BET thin films annealed at 600, 650, 700, 750 and 800 °C, respectively. The d33 67.3 pm/V is larger than 19 pm/V for the BLT thin film and 38 pm/V for the BNT thin film [10], and it is comparative with 40–110 pm/V for PbZrxTi1 − xO3 (PZT) thin films [29,30]. It is safe to say that the d33 value stands in comparison with PZT thin films, therefore BET thin films can be considered as a viable alternative to PZT thin films for selected applications. We summarize the values of Pr, Ps, d33, εr and tanδ as the function of the annealing temperature, and they are listed in Table 1. Fig. 5(a) shows εr and tanδ as function of the annealing temperature at 100 kHz, and the values of εr and tanδ were estimated as 600.7, 823.2, 992.9, 954.9, and 925.2 and 0.02358, 0.02106, 0.02342, 0.03112, and 0.03599 for BET thin films annealed at 600, 650, 700, 750 and 800 °C, respectively. Obviously, the εr increases with the annealing temperature at a range of 600 to 700 °C, and decreases from 700 to 800 °C. The possible reason is the grain size effect on the dielectric properties
of polycrystalline thin films for the former, and the other is the insufficient Bi supply which results in nonstoichiometric structural defects in the thin films for the latter, and it is similar with the previous results [14]. The average values of 2Pr, 2Ps and d33 as a function of the annealing temperature are shown in Fig. 5(b). One can read that with the increase of the annealing temperature all of 2Pr, 2Ps
Table 1 The average values of 2Pr, 2Ps, εr, tanδ and d33 of BET thin films annealed at different annealing temperatures. 600 °C −6
2
650 °C
700 °C
750 °C
800 °C
C/cm ) 43.8 64.4 87.4 75.8 54.4 2Ps (×10 38.2 57.5 65.7 61.3 43.1 2Pr (×10− 6 C/cm2) 600.7 823.2 992.9 954.9 925.2 εr tanδ 0.02358 0.02106 0.02342 0.03112 0.03599 d33 (pm/V) 39.6 48.4 67.3 30.85 28.2
718
X.J. Zheng et al. / Thin Solid Films 519 (2010) 714–718
4. Conclusions In conclusion, the average values of 2Pr, 2Ps, εr and d33 of the BET thin film annealed at 700 °C are respectively 87.4 × 10− 6 C/cm2, 65.7 × 10− 6 C/cm2, 992.9 and 67.3 pm/V, and they are larger than those of the others. With the increase of the annealing temperature, the 2Ps, 2Pr, εr and d33 values obviously increase at a range of 600– 700 °C and decrease from 700 to 800 °C. The mechanism concerning piezoelectric performance is considered that both of εr and Ps induced by the variation of the annealing temperature are contributed to the d33 value of the BET thin film via the phenomenological equation. The improved piezoelectric properties could make Eu-doped BIT a promising candidate for sensors, actuators, and transducers. Acknowledgements This work was supported by the NNSF of China (10825209 and 50872117), the Changjiang Scholar Incentive Program ([2009]17), the Project of Hunan's Prestigious Fu-rong Scholar Award ([2007]362), the Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province, and the Scientific Research Fund of Hunan Provincial Education Department (08C862). References
Fig. 5. (a) At 100 kHz the εr and tanδ as a function of the annealing temperature and (b) 2Ps, 2Pr and d33 of BET films as a function of the annealing temperature.
and d33 increase at a range of 600–700 °C and decrease from 700 to 800 °C. The decreased porosity and the enlarged grain size are shown in Fig. 1(a)–(c), and the enhanced crystallinity degree and the lowered c-axis orientation in Fig. 2, therefore 2Pr and 2Ps increase at a range of 600–700 °C [31]. The average values of 2Pr and 2Ps of BET thin film annealed at 750–800 °C are lower than those annealed at 700 °C. Comparing with Fig. 5(a), the variations of 2Pr and 2Ps with the annealing temperature are similar with that of the relative permittivity εr, and they are also original from the grain size effect and the insufficient Bi supply. The d33 can be expressed by the phenomenological equation for most ferroelectric materials [32] d33 = 2Q eff ε0 εr Ps
ð2Þ
where Q eff is the effective electrostriction coefficient, and ε0 is the permittivity of free space. Because Q eff is less sensitive to film quality than d33 [33], εr and Ps can be regarded as the primary factors to relate with d33. The d33 variation with the annealing temperature should be monotonically corresponding to those of εr and Ps, therefore the dependence of d33 on the annealing temperature should be the same as those of εr and Ps , which are determined by the orientation direction, crystallinity, and chemical composition of the thin films, as shown in Fig. 5. Obviously, the BET thin film annealed at 700 °C is of the highest dielectric constant and effective piezoelectric coefficient, implying that it is suitable for use as ferroelectric capacitors [34].
[1] H. Funakubo, T. Watanabe, T. Kojima, T. Sakai, Y. Noguchi, M. Miyayama, J. Cryst. Growth 248 (2003) 180. [2] H.N. Lee, D. Hesse, N. Zakharov, Science 296 (2002) 2006. [3] U. Chon, G.C. Yi, H.M. Jang, Phys. Rev. Lett. 78 (2001) 658. [4] U. Chon, J.S. Shim, H.M. Jang, J. Appl. Phys. 93 (2003) 4769. [5] W.J. Kim, S.S. Kim, K.J. Jang, J.C. Bae, T.K. Song, Y.I. Lee, J. Cryst. Growth 262 (2004) 327. [6] K.T. Lim, K.T. Kim, D.P. Kim, C.I. Kim, Thin Solid Films 447 (2004) 337. [7] X.J. Zheng, L. He, Y.C. Zhou, M.H. Tang, Appl. Phys. Lett. 89 (2006) 252908. [8] K. Kato, D. Fu, K. Suzuki, K. Tanaka, K. Nishizava, T. Miki, Appl. Phys. Lett. 84 (2004) 3771. [9] A.Z. Simões, M.A. Ramírez, A. Ries, J.A. Varela, E. Longo, R. Ramesh, Appl. Phys. Lett. 88 (2006) 072916. [10] H. Maiwa, N. Iizawa, D. Togawa, T. Hayashi, W. Sakamoto, M. Yamada, S.I. Hirano, Appl. Phys. Lett. 82 (2003) 1760. [11] S.V. Kalinin, D.A. Bonnell, Phys. Rev. B 65 (2002) 125408. [12] Y.C. Yang, C. Song, X.H. Wang, F. Zeng, F. Pan, Appl. Phys. Lett. 92 (2008) 012907. [13] X.J. Zheng, L. He, M.H. Tang, Y. Ma, J.B. Wang, Q.M. Wang, Mater. Lett. 62 (2008) 2876. [14] X.J. Zheng, Q.Y. Wu, J.F. Peng, L. He, X. Feng, Y.Q. Chen, D.Z. Zhang, J. Mater. Sci. 45 (2010) 3001. [15] M. Osada, M. Tada, M. Kakihana, T. Watanabe, Jpn. J. Appl. Phys. 1 40 (2001) 5572. [16] Aurivillius, Ark. Kemi. 1 (1950) 499. [17] C.R. Cho, W.J. Lee, B.G. Yu, B.W. Kim, J. Appl. Phys. 86 (1999) 2700. [18] S.H. Yoon, D.J. Kim, J. Cryst. Growth 303 (2007) 568. [19] X.J. Zheng, W.M. Yi, Y.Q. Chen, Q.Y. Wu, L. He, Scr. Mater. 57 (2007) 675. [20] X.Y. Mao, J.H. He, J. Zhu, X.B. Chen, J. Appl. Phys. 100 (2006) 044104. [21] Y. Adachi, D. Su, P. Muralt, N. Setter, Appl. Phys. Lett. 86 (2005) 172904. [22] C.J. Lu, X.L. Liu, X.Q. Chen, C.J. Nie, G.L. Rhun, S. Senz, D. Hesse, Appl. Phys. Lett. 89 (2006) 062905. [23] Y.P. Guo, D. Akai, K. Sawada, M. Ishida, Solid State Sci. 10 (2008) 928. [24] X.L. Zhong, J.B. Wang, S.X. Yang, Y.C. Zhou, Appl. Surf. Sci. 253 (2006) 417. [25] X.L. Zhong, J.B. Wang, M. Liao, C.B. Tan, H.B. Shu, Y.C. Zhou, Thin Solid Films 516 (2008) 8240. [26] H. Jiang, L.G. Hong, N. Venkatasubramanian, J.T. Grant, K. Eyink, K. Wiacek, S. FriesCarr, J. Enlow, T.J. Bunning, Thin Solid Films 515 (2007) 3513. [27] H. Wen, X.H. Wang, C.F. Zhong, L.K. Shu, L.T. Li, Appl. Phys. Lett. 90 (2007) 202902. [28] Y.C. Yang, C. Song, X.H. Wang, F. Zeng, F. Pan, J. Appl. Phys. 103 (2008) 074107. [29] P. Muralt, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 47 (2000) 903. [30] A.L. Kholkine, C. Wütchrich, D.V. Taylor, N. Setter, Rev. Sci. Instrum. 67 (1996) 1935. [31] U. Chon, H.M. Jang, M.G. Kim, C.H. Chang, Phys. Rev. Lett. 89 (2002) 087601. [32] A.L. Kholkin, E.K. Akdogan, A. Safari, P.F. Chauvy, N. Setter, J. Appl. Phys. 89 (2001) 8066. [33] Y. Hosono, K. Harada, Y. Yamashita, Jpn. J. Appl. Phys. 40 (2001) 5722. [34] B.J. Kuh, W.K. Choo, J. Eur. Ceram. Soc. 21 (2001) 1509.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Enhancement on effective piezoelectric coefficient of Bi3.25Eu0.75Ti3O12 ferroelectric thin films under moderate annealing temperature X.J. Zheng a,b,⁎, J.F. Peng a, Y.Q. Chen a, L. He a,b, X. Feng c, D.Z. Zhang a, L.J. Gong a, Q.Y. Wu a a b c
Faculty of Materials, Optoelectronics and Physics, Xiangtan University, Xiangtan, Hunan 411105, People's Republic of China Key Laboratory of Low Dimensional Materials and Application Technology of the Ministry of Education, Xiangtan University, Xiangtan, Hunan 411105, People's Republic of China School of Aerospace, Tsinghua University, Beijing, 100084, People's Republic of China
a r t i c l e
i n f o
Article history: Received 30 October 2009 Received in revised form 20 August 2010 Accepted 25 August 2010 Available online 1 October 2010 Keywords: Bismuth europium titanate Thin films Dielectric properties Piezoelectric properties Annealing Metal organic decomposition X-ray diffraction Scanning electron microscopy
a b s t r a c t Bi3.25Eu0.75Ti3O12 (BET) thin films were deposited on Pt/Ti/SiO2/Si(111) substrates by a metal-organic decomposition method. The effects of annealing temperatures 600–800 °C on microstructure, ferroelectric, dielectric and piezoelectric properties of BET thin films were studied in detail. The spontaneous polarization (87.4 × 10− 6 C/cm2 under 300 kV/cm), remnant polarization (65.7 × 10− 6 C/cm2 under 300 kV/cm), the dielectric constant (992.9 at 100 kHz) and the effective piezoelectric coefficient d33 (67.3 pm/V under 260 kV/cm) of BET thin film annealed at 700 °C are better than those of the others. The mechanisms concerning the dependence of the enhancement d33 are discussed according to the phenomenological equation, and the improved piezoelectric performance could make the BET thin film a promising candidate for piezoelectric thin film devices. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Bismuth (Bi) titanate thin films have been intensively investigated as potential materials for non-volatile random access memories, piezoelectric devices, sensors, and actuators [1]. In the past years, many studies reveal that Bi3+ ions in the Bi4Ti3O12 (BIT) structure can be substituted by trivalent lanthanoid ions, such as Nd3+, La3+, Pr3+, Dy3+ and Sm3+, which improves ferroelectric properties [2–4]. In particular, Eu-substituted BIT lead-free thin films have emerged as ferroelectric materials due to good fatigue endurance and large polarization [5,6], and in our previous work the 2Pr is 66 × 10− 6 C/cm2 for Bi3.25Eu0.75Ti3O12 (BET) lead-free thin film annealed at 700 °C [7]. Recently, more and more efforts have been made to develop nontoxic lead-free piezoelectric materials to apply into piezoelectric microelectromechanical systems (MEMS), and Bi-layered perovskite lead-free ferroelectric thin films are considered to be candidate materials, such as the effective piezoelectric coefficients d33 of 30– 60 pm/V for CaBi4Ti4O15 thin films [8,9], and ~ 38 pm/V for the
⁎ Corresponding author. Faculty of Materials, Optoelectronics and Physics, Xiangtan University, Xiangtan, Hunan 411105, People's Republic of China. Tel.: + 86 731 58293648; fax: + 86 731 58298119. E-mail address: [email protected] (X.J. Zheng). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.08.130
neodymium-doped Bi4Ti3O12 (BNT) thin film [10]. According to the phenomenological equation regarding the intrinsic piezoelectric coefficient, the high spontaneous polarization Ps is favorable for yielding higher piezoelectric displacement [10]. It is well known that the annealing temperature has an important effect on the microstructure, dielectric and ferroelectric properties of Bi-layered perovskite ferroelectric thin films. However, few study has involved the piezoelectric properties of BET thin films annealed at different temperatures, therefore it is imperative for us to improve the piezoelectric properties of the thin films in order to impetus for integrating Bi-layered perovskite ferroelectric thin films into MEMS piezoelectric devices. In this paper, BET lead-free ferroelectric thin films were prepared by metal-organic decomposition (MOD), and the effects of the annealing temperature on microstructure, ferroelectric, dielectric and piezoelectric properties were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), scanning probe microscope (SPM), ferroelectric tester, and impedance analyzer. The ferroelectric, dielectric, and piezoelectric behaviors were established from polarization–electric field (P–E) hysteresis loops, dielectric constant–frequency (εr–f) and dissipation–frequency (tanδ–f) curves, and displacement–electric field (D–E) “butterfly” curves, respectively. We expect that the research can offer useful guidelines to the design of Bi-layered thin film piezoelectric devices.
X.J. Zheng et al. / Thin Solid Films 519 (2010) 714–718
715
2. Experimental details BET thin films were deposited on a Pt(200 nm)/Ti(30 nm)/SiO2/ Si(111) substrate by the MOD method [5,6]. The precursor materials were bismuth (III) acetate {Bi[CH3CO2]3}, europium (III) acetate hydrate {Eu[CH3CO2]3} and tetrabutyl titanium {Ti[OC4H9]4}, and the solvents were acetic acid {CH3CO2H} and acetylacetone {CH3COCH2COCH3}. A 10% excess amount of bismuth acetate was used to compensate Biloss during the annealing process. At first, the solid-state bismuth acetate and europium acetate were dissolved in the acetic acid, and the solution was mixed to obtain the (Bi, Eu) stock solution followed by stirring for 12 h. Secondly, the tetrabutyl titanium was dissolved in the mixture of the stock solution and acetylacetone, and it was magnetically stirred in the air atmosphere for 1 h. Finally, the flaxen, transparent and stable BET precursor solution was prepared. The precursor solutions with a mole ratio (Bi:Eu:Ti) of 3.575:0.75:3 were spun on the substrate at 4000 rpm for 30 s. After the spin-coating procedure, the thin films were kept in a rapid temperature process at 400 °C for 180 s to remove the organic ingredients. The coating/drying circles were repeated 7 times to achieve the desired film thickness, and the pre-baked films were respectively annealed at 600, 650, 700, 750 and 800 °C in an oxygen atmosphere to promote crystallization. Using FE-SEM (1530, LEO, Germany), the surface morphologies of BET thin films were identified with 20 K amplification, and the crosssectional micrograph of the BET thin film annealed at 700 °C was recorded with 20 K amplification. The FE-SEM operating voltage was 20 kV. Phase identification, degree of crystallinity, and crystalline orientation of the thin films were investigated by XRD (D/Max 2500, Rigaku, Japan) using the normal θ–2θ scanning method, and they were scanned at 10°/min with Cu Kα radiation (λ = 1.541 Å, 30 kV and 100 mA). The circular Au top electrodes with a radius of 0.1 mm were deposited on the thin films using a shadow mask by dc magnetron sputtering, and under the electric field 300 kV/cm the P–E hysteresis loops were measured by a ferroelectric tester (Precision Workstation, Radiant Technologies, USA). Frequency dependent dielectric constant (εr) and dissipation factor (tanδ) were measured by the impedance analyzer (HP4194A, Hewlett Packard, America). A commercial available SPM (SPI4000&SPA300HV, Seiko, Japan) was used to characterize the piezoelectric properties of the thin films without the top electrode. The conductive rhodium coated silicon cantilever is of a spring constant of 1.9 N/m, a resonant frequency of 28 kHz, and an integrated tip of about 10 nm in diameter. The stiff cantilever was used to get a large indentation force ensuring that the measurement was in the so-called strong-indentation regime [11]. The piezoelectric measurement was conducted on ten points of scanned scope on the sample surface, because the piezoelectric characterization for SPM is a kind of local method [12]. For each sample, keeping the SPM tip fixed above the interesting point the D–E “butterfly” curve was recorded under a dc voltage applied from −9 to 9 V. With considering the unexpected shift of the intersection from the origin of the D–E curve, the d33-electric field (d33-E) loops can be calculated according to the modified equation of converse piezoelectric effect [12] d33 =
D−DI ; dðE−EI Þ
ð1Þ
where d is the initial film thickness before deformation. On the D–E curve, DI and EI are the piezoelectric displacement and electric field for the intersection, and D and E are the measured values of piezoelectric displacement and electric field for each point. All of the above measurements were carried out at room temperature. 3. Results and discussion Fig. 1(a)–(e) shows the surface SEM images of BET thin films annealed at 600, 650, 700, 750 and 800 °C, and Fig. 1(f) shows the
Fig. 1. The FE-SEM images of surface morphologies for BET thin films annealed at (a) 600 °C, (b) 650 °C, (c) 700 °C, (d) 750 °C, and (e) 800 °C and (f) a FE-SEM image of a cross-sectional micrograph for the thin film annealed at 700 °C.
typical cross-sectional micrograph of the BET thin film annealed at 700 °C. In Fig. 1(a)–(e), the porosity values are respectively 1.82, 0.73 and 0.27, 0.19 and 0.13% for BET thin films annealed at 600, 650, 700, 750 and 800 °C, and they are of many irregularly shaped pores. With the increase of the annealing temperature, the porosity decreases and the grain size enlarges, indicating that the high annealing temperature enhances the crystal growth. In our previous investigation, the ferroelectric, fatigue and retention characteristics of Au/BET/Pt thin film capacitors are increased at a range of 600–700 °C and degraded at a range of 700–800 °C with the annealing temperature [13]. The lower porosity may result in the enhancement of sufficient life time of BET thin films for 600–700 °C, and the insufficient Bi supply degrades the ferroelectric, dielectric and piezoelectric properties for 700–800 °C [13,14]. The crack-free surface seems to be important for ferroelectric thin films because the crack will affect the microstructure, ferroelectric properties and residual stress [15]. The film thickness via the same coating/drying circles is the same as 346 nm for the five kinds of BET thin films, therefore there is only given the cross-sectional micrograph for the thin film annealed at 700 °C, as shown in Fig. 1(f). Indeed, the annealing temperature 700 °C may be a special threshold in the relationship between the properties and annealing temperature [14], and it indicates that the typical behavior for the thin film annealed at 700 °C would be better than those for the thin films annealed at other temperatures. Fig. 2 shows a typical Bi-layered perovskite polycrystalline phase without any pyrochlore phase and other phases related to Eu. The 2θ values of (117), (111) and (024) peaks are respectively 29.81°, 23.69° and 34.84°, and there are slight shifts in the position of the XRD peaks compared to the position of BIT standard XRD peaks [16]. These indicate that the substitution of Eu3+ to Bi3+ occurs, and it is in agreement with the previous results [14]. In fact, the amounts of Eu incorporated in the Bi3.15Eu0.85Ti3O12 thin films were quantified by
716
X.J. Zheng et al. / Thin Solid Films 519 (2010) 714–718
Fig. 2. X-ray diffraction patterns of BET thin films annealed at 600, 650, 700, 750, and 800 °C, respectively.
energy dispersive X-ray spectroscopy, and the effects of the annealing temperature on chemical composition are discussed deeply [14]. At 600 °C, the diffraction peaks are low and broad, indicating poor crystallinity of the sample. With the increase of the annealing temperature, the diffraction peaks become sharp and strong, indicating the possible improved crystallinity. According to I(006)/ [I(006) + I(117)] × 100% (where Ihkl is the relative intensity of the corresponding peak) [17], the degrees of c-axis orientation are 24, 17, 15, 19 and 21% for BET thin films annealed at 600, 650, 700, 750 and 800 °C, respectively. The degree of c-axis orientation of the BET thin film annealed at 700 °C is lower than those of thin films annealed at 600, 650, 750 and 800 °C. Generally the relationship between the preferred orientation and annealing temperature is a monotone function for the bulk crystal. There are two possible reasons for the non-linear behavior of the preferred orientation with the annealing temperature. At first, the crystallization with the preferred orientation is related to the free energy, because the nucleation and growth variations occur in the energy barrier heights [18]. Secondly, the increased annealing temperature results in the defect propagation to affect the preferred orientation during the growth of the thin film [19]. According to the previous investigation on ferroelectric materials with a bismuth-layer structure, the weaker the degree of c-axis orientation, the larger the remanent polarization [20]. From Fig. 2, the weakest degree of c-axis orientation is 15%, thus the BET thin film annealed at 700 °C may have the larger polarization than the others. However, the dependence of polarization on a- and b-axis orientations is different for BIT material family thin films with the different preferred orientation. If BIT material family has its polarization mostly in the (a, b) plane, the c-orientation may be not the largest piezoresponse [21]. In other words, the values of polarization are dependent on not only the present c-orientation, but also the a- and b-axis orientations [22]. The vector of the (major) spontaneous polarization in these layered perovskite materials is along the a-axis, the stronger the degree of a-axis orientation, the larger the remanent polarization [2]. The P–E loops of BET thin films annealed at 600, 650, 700, 750 and 800 °C are shown in Fig. 3(a). The P–E loop of the BET thin film annealed at 600 °C is not saturated well, while BET thin films annealed at temperatures above 650 °C show saturated P–E loops. Under the bipolar driving field of 300 kV/cm, the values of 2Ps and 2Pr are 43.8, 64.4, 87.4, 75.8, and 54.4 × 10− 6 C/cm2 and 38.2, 57.5, 65.7, 61.3, and 43.1 × 10− 6 C/cm2 for BET thin films annealed at 600, 650, 700, 750 and 800 °C, respectively. With the increase of the annealing temperature the values of 2Pr and 2Ps increase at a range of 600– 700 °C and decrease from 700 to 800 °C. Obviously, the maximum of 2Pr (65.7 × 10− 6 C/cm2) for the BET thin film annealed at 700 °C equals to 66 × 10− 6 C/cm2 that was reported in the previous reference
Fig. 3. (a) P–E hysteresis loops and (b) frequency dependent εr and tanδ of BET thin films annealed at 600, 650, 700, 750, and 800 °C.
[7], but is larger than 38 × 10− 6 C/cm2 for the Bi3.4Eu0.6Ti3O12 thin film and 26 × 10− 6 C/cm2 for the Bi1.32Ti2.97V0.03O12 ceramic. [5,20]. From the εr–f and tanδ–f curves shown in Fig. 3(b), under the applied frequency from 1 kHz to 1 MHz the εr values decrease with the increase of frequency, and it is due to the extrinsic resonance behavior resulting from the microstructure deficiency, which may in principle be reduced by optimization of the film growth process [23]. The tanδ increases at higher frequencies, and the increasing tendency of tanδ is also reported for BNT and Bi3.25La0.75Ti3O12 (BLT) ferroelectric thin films [24,25]. There are three possible reasons for the dispersion as follows. One possible cause is the extrinsic resonance behavior which results from the microstructure deficiency [23]. Secondly, the mobile charges contained in the film cannot follow higher frequency electric fields [26], indicating that the higher resistive losses result in the increasing tendency of tanδ. The third one is the hypothesis of the influence of the contact resistance between the probe and the electrode [24]. The typical D–E (black) curves and d33-E (blue) loops of BET thin films annealed at 600, 650, 700, 750 and 800 °C are given in Fig. 4. The d33-E loops are almost symmetric and they are in an agreement with the previous symmetries of BiScO3–PbTiO3 and Zn1 − xVxO ferroelectric thin films [12,27]. On the D–E curves, there are no notable horizontal shifts, and little vertical shifts 0.09–0.28 nm can be observed, indicating no obvious imprint effect. The d33-E loops clearly show that the BET thin film is switchable and the piezoelectric is retained [28]. Under a bipolar driving field of 260 kV/cm, the average
X.J. Zheng et al. / Thin Solid Films 519 (2010) 714–718
717
Fig. 4. The representative D–E curves and d33-E loops of the BET thin film annealed at (a) 600 °C, (b) 650 °C, (c) 700 °C, (d) 750 °C, and (e) 800 °C.
values of d33 are 39.6, 48.4, 67.3, 30.85 and 28.2 pm/V for the BET thin films annealed at 600, 650, 700, 750 and 800 °C, respectively. The d33 67.3 pm/V is larger than 19 pm/V for the BLT thin film and 38 pm/V for the BNT thin film [10], and it is comparative with 40–110 pm/V for PbZrxTi1 − xO3 (PZT) thin films [29,30]. It is safe to say that the d33 value stands in comparison with PZT thin films, therefore BET thin films can be considered as a viable alternative to PZT thin films for selected applications. We summarize the values of Pr, Ps, d33, εr and tanδ as the function of the annealing temperature, and they are listed in Table 1. Fig. 5(a) shows εr and tanδ as function of the annealing temperature at 100 kHz, and the values of εr and tanδ were estimated as 600.7, 823.2, 992.9, 954.9, and 925.2 and 0.02358, 0.02106, 0.02342, 0.03112, and 0.03599 for BET thin films annealed at 600, 650, 700, 750 and 800 °C, respectively. Obviously, the εr increases with the annealing temperature at a range of 600 to 700 °C, and decreases from 700 to 800 °C. The possible reason is the grain size effect on the dielectric properties
of polycrystalline thin films for the former, and the other is the insufficient Bi supply which results in nonstoichiometric structural defects in the thin films for the latter, and it is similar with the previous results [14]. The average values of 2Pr, 2Ps and d33 as a function of the annealing temperature are shown in Fig. 5(b). One can read that with the increase of the annealing temperature all of 2Pr, 2Ps
Table 1 The average values of 2Pr, 2Ps, εr, tanδ and d33 of BET thin films annealed at different annealing temperatures. 600 °C −6
2
650 °C
700 °C
750 °C
800 °C
C/cm ) 43.8 64.4 87.4 75.8 54.4 2Ps (×10 38.2 57.5 65.7 61.3 43.1 2Pr (×10− 6 C/cm2) 600.7 823.2 992.9 954.9 925.2 εr tanδ 0.02358 0.02106 0.02342 0.03112 0.03599 d33 (pm/V) 39.6 48.4 67.3 30.85 28.2
718
X.J. Zheng et al. / Thin Solid Films 519 (2010) 714–718
4. Conclusions In conclusion, the average values of 2Pr, 2Ps, εr and d33 of the BET thin film annealed at 700 °C are respectively 87.4 × 10− 6 C/cm2, 65.7 × 10− 6 C/cm2, 992.9 and 67.3 pm/V, and they are larger than those of the others. With the increase of the annealing temperature, the 2Ps, 2Pr, εr and d33 values obviously increase at a range of 600– 700 °C and decrease from 700 to 800 °C. The mechanism concerning piezoelectric performance is considered that both of εr and Ps induced by the variation of the annealing temperature are contributed to the d33 value of the BET thin film via the phenomenological equation. The improved piezoelectric properties could make Eu-doped BIT a promising candidate for sensors, actuators, and transducers. Acknowledgements This work was supported by the NNSF of China (10825209 and 50872117), the Changjiang Scholar Incentive Program ([2009]17), the Project of Hunan's Prestigious Fu-rong Scholar Award ([2007]362), the Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province, and the Scientific Research Fund of Hunan Provincial Education Department (08C862). References
Fig. 5. (a) At 100 kHz the εr and tanδ as a function of the annealing temperature and (b) 2Ps, 2Pr and d33 of BET films as a function of the annealing temperature.
and d33 increase at a range of 600–700 °C and decrease from 700 to 800 °C. The decreased porosity and the enlarged grain size are shown in Fig. 1(a)–(c), and the enhanced crystallinity degree and the lowered c-axis orientation in Fig. 2, therefore 2Pr and 2Ps increase at a range of 600–700 °C [31]. The average values of 2Pr and 2Ps of BET thin film annealed at 750–800 °C are lower than those annealed at 700 °C. Comparing with Fig. 5(a), the variations of 2Pr and 2Ps with the annealing temperature are similar with that of the relative permittivity εr, and they are also original from the grain size effect and the insufficient Bi supply. The d33 can be expressed by the phenomenological equation for most ferroelectric materials [32] d33 = 2Q eff ε0 εr Ps
ð2Þ
where Q eff is the effective electrostriction coefficient, and ε0 is the permittivity of free space. Because Q eff is less sensitive to film quality than d33 [33], εr and Ps can be regarded as the primary factors to relate with d33. The d33 variation with the annealing temperature should be monotonically corresponding to those of εr and Ps, therefore the dependence of d33 on the annealing temperature should be the same as those of εr and Ps , which are determined by the orientation direction, crystallinity, and chemical composition of the thin films, as shown in Fig. 5. Obviously, the BET thin film annealed at 700 °C is of the highest dielectric constant and effective piezoelectric coefficient, implying that it is suitable for use as ferroelectric capacitors [34].
[1] H. Funakubo, T. Watanabe, T. Kojima, T. Sakai, Y. Noguchi, M. Miyayama, J. Cryst. Growth 248 (2003) 180. [2] H.N. Lee, D. Hesse, N. Zakharov, Science 296 (2002) 2006. [3] U. Chon, G.C. Yi, H.M. Jang, Phys. Rev. Lett. 78 (2001) 658. [4] U. Chon, J.S. Shim, H.M. Jang, J. Appl. Phys. 93 (2003) 4769. [5] W.J. Kim, S.S. Kim, K.J. Jang, J.C. Bae, T.K. Song, Y.I. Lee, J. Cryst. Growth 262 (2004) 327. [6] K.T. Lim, K.T. Kim, D.P. Kim, C.I. Kim, Thin Solid Films 447 (2004) 337. [7] X.J. Zheng, L. He, Y.C. Zhou, M.H. Tang, Appl. Phys. Lett. 89 (2006) 252908. [8] K. Kato, D. Fu, K. Suzuki, K. Tanaka, K. Nishizava, T. Miki, Appl. Phys. Lett. 84 (2004) 3771. [9] A.Z. Simões, M.A. Ramírez, A. Ries, J.A. Varela, E. Longo, R. Ramesh, Appl. Phys. Lett. 88 (2006) 072916. [10] H. Maiwa, N. Iizawa, D. Togawa, T. Hayashi, W. Sakamoto, M. Yamada, S.I. Hirano, Appl. Phys. Lett. 82 (2003) 1760. [11] S.V. Kalinin, D.A. Bonnell, Phys. Rev. B 65 (2002) 125408. [12] Y.C. Yang, C. Song, X.H. Wang, F. Zeng, F. Pan, Appl. Phys. Lett. 92 (2008) 012907. [13] X.J. Zheng, L. He, M.H. Tang, Y. Ma, J.B. Wang, Q.M. Wang, Mater. Lett. 62 (2008) 2876. [14] X.J. Zheng, Q.Y. Wu, J.F. Peng, L. He, X. Feng, Y.Q. Chen, D.Z. Zhang, J. Mater. Sci. 45 (2010) 3001. [15] M. Osada, M. Tada, M. Kakihana, T. Watanabe, Jpn. J. Appl. Phys. 1 40 (2001) 5572. [16] Aurivillius, Ark. Kemi. 1 (1950) 499. [17] C.R. Cho, W.J. Lee, B.G. Yu, B.W. Kim, J. Appl. Phys. 86 (1999) 2700. [18] S.H. Yoon, D.J. Kim, J. Cryst. Growth 303 (2007) 568. [19] X.J. Zheng, W.M. Yi, Y.Q. Chen, Q.Y. Wu, L. He, Scr. Mater. 57 (2007) 675. [20] X.Y. Mao, J.H. He, J. Zhu, X.B. Chen, J. Appl. Phys. 100 (2006) 044104. [21] Y. Adachi, D. Su, P. Muralt, N. Setter, Appl. Phys. Lett. 86 (2005) 172904. [22] C.J. Lu, X.L. Liu, X.Q. Chen, C.J. Nie, G.L. Rhun, S. Senz, D. Hesse, Appl. Phys. Lett. 89 (2006) 062905. [23] Y.P. Guo, D. Akai, K. Sawada, M. Ishida, Solid State Sci. 10 (2008) 928. [24] X.L. Zhong, J.B. Wang, S.X. Yang, Y.C. Zhou, Appl. Surf. Sci. 253 (2006) 417. [25] X.L. Zhong, J.B. Wang, M. Liao, C.B. Tan, H.B. Shu, Y.C. Zhou, Thin Solid Films 516 (2008) 8240. [26] H. Jiang, L.G. Hong, N. Venkatasubramanian, J.T. Grant, K. Eyink, K. Wiacek, S. FriesCarr, J. Enlow, T.J. Bunning, Thin Solid Films 515 (2007) 3513. [27] H. Wen, X.H. Wang, C.F. Zhong, L.K. Shu, L.T. Li, Appl. Phys. Lett. 90 (2007) 202902. [28] Y.C. Yang, C. Song, X.H. Wang, F. Zeng, F. Pan, J. Appl. Phys. 103 (2008) 074107. [29] P. Muralt, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 47 (2000) 903. [30] A.L. Kholkine, C. Wütchrich, D.V. Taylor, N. Setter, Rev. Sci. Instrum. 67 (1996) 1935. [31] U. Chon, H.M. Jang, M.G. Kim, C.H. Chang, Phys. Rev. Lett. 89 (2002) 087601. [32] A.L. Kholkin, E.K. Akdogan, A. Safari, P.F. Chauvy, N. Setter, J. Appl. Phys. 89 (2001) 8066. [33] Y. Hosono, K. Harada, Y. Yamashita, Jpn. J. Appl. Phys. 40 (2001) 5722. [34] B.J. Kuh, W.K. Choo, J. Eur. Ceram. Soc. 21 (2001) 1509.