Received 1 June 2006; accepted 9 August 2006 Available online 11 September 2006

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1 Materials Research Bulletin 42 (2007) Ferroelectric and piezoelectric properties of bismuth titanate thin films grown on different bottom electrodes by soft chemical solution and microwave annealing A.Z. Simões a, *, M.P. Cruz b,c,d, A. Ries a, E. Longo a, J.A. Varela a, R. Ramesh c,d a Chemistry Institute, Universidade Estadual Paulista (UNESP), Rua Prof. Francisco Degni s/n, Araraquara, SP, Brazil b Centro de Ciencias de la Materia Condensada (CCMC), UNAM, Km 107, Carretera Tijuana-Ensenada, Ensenada, BC, C.P , Mexico c Department of Materials Science and Engineering, University of California, Berkeley, CA 94720, United States d Department of Physics, University of California, Berkeley, CA 94720, United States Received 1 June 2006; accepted 9 August 2006 Available online 11 September 2006 Abstract Bismuth titanate (Bi 4 Ti 3 O 12, BIT) films were evaluated for use as lead-free piezoelectric thin films in micro-electromechanical systems. The films were grown by the polymeric precursor method on LaNiO 3 /SiO 2 /Si (1 0 0) (LNO), RuO 2 /SiO 2 /Si (1 0 0) (RuO 2 ) and Pt/Ti/SiO 2 /Si (1 0 0) (Pt) bottom electrodes in a microwave furnace at 700 8C for 10 min. The domain structure was investigated by piezoresponse force microscopy (PFM). Although the converse piezoelectric coefficient, d 33, regardless of bottom electrode is around (40 pm/v), those over RuO 2 and LNO exhibit better ferroelectric properties, higher remanent polarization (15 and 10 mc/ cm 2 ), lower drive voltages (2.6 and 1.3 V) and are fatigue-free. The experimental results demonstrated that the combination of the polymeric precursor method assisted with a microwave furnace is a promising technique to obtain films with good qualities for applications in ferroelectric and piezoelectric devices. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Thin films; B. Chemical synthesis; D. Ferroelectricity 1. Introduction The superior ferroelectric properties of Bi 4 Ti 3 O 12 (BIT) thin films among bismuth-layer-structured ferroelectric compounds have attracted intense interest, making these films some of the most competitive candidates for piezoelectric devices against lead containing materials [1]. Bismuth titanate is composed of a triple perovskite unit sandwiched between (Bi 2 O 2 ) 2+ layers. The pseudo-orthorhombic BIT unit possesses the lattice parameters of a = , b = , and c = nm, and exhibits the spontaneous polarizations P s = 50 and 4 mc/cm 2 along a- and c-axes, respectively [2]. Therefore, films having a larger fraction of a-axis-oriented grains should exhibit better ferro- and piezoelectric properties. However, BIT films prefer to grow with c-axis perpendicular to the film surface when common Pt (1 1 1) electrodes are used. A suitable electrode with better lattice matching with the long c-axis is required in order to fabricate a- or a/b-axis orientation [3,4]. Because of their complex chemical composition and * Corresponding author. Tel.: ; fax: address: alezipo@yahoo.com (A.Z. Simões) /$ see front matter # 2006 Elsevier Ltd. All rights reserved. doi: /j.materresbull

2 976 A.Z. Simões et al. / Materials Research Bulletin 42 (2007) crystallographical structure, single crystals of these materials are difficult to be grown. Thus, most of the current studies are focused on the applications of thin films [5]. Ferroelectric thin films are constrained by substrates and therefore their properties can be affected by many factors, such as orientation, properties of the substrate (lattice parameters and thermal expansion coefficient), and film thickness. For some applications, as for example in ferroelectric memories, large remanent polarization and good fatigue-free characteristics are required [6]. Unfortunately, films of BIT grown over common platinum coated silicon substrates do not satisfy these requirements. This might be due to an interfacial reaction between platinum and bismuth which can lead to undesired electrical properties [7]. Therefore, the substitution of metallic electrodes based on noble metals, like platinum, with conductive oxides is an alternative to reach better electrical properties caused by the high oxygen affinity of these electrodes [8]. For this purpose, it is essential to understand the ferroelectric properties of the BIT films deposited on electrodes based on metallic oxides such as LNO and RuO 2. Some attempts have been made to enhance the crystallization ability of ferroelectric thin films and metallic oxide electrodes [3,9]. For obtaining good crystallized films by chemical solution deposition, heat treatments at high temperatures for a long time, 2 h, are normally necessary. These long heat treatments can cause several damages to the stack, leading to interdiffusion between the film and the substrate, and sometimes loss of stoichiometry (due to the loss of volatile cation). So, it is important to decrease the temperature and time of thermal treatment. Recently, the use of a domestic microwave furnace has been developed as a way to process materials and has opened an opportunity to enhance crystallization with a lower annealing processing time. This leads to a decrease the interfacial reactions between ferroelectric thin films and electrodes and also improves the control over the crystallographic orientation of the thin films [10]. Among various methods such as metal-organic chemical vapour deposition, pulsed laser deposition and sol gel, the polymeric precursor method has its advantages over the other production techniques include its low cost, good compositional homogeneity, relatively low processing temperatures and the ability to coat large substrate areas [11,12]. In this work, we present our findings on the preparation of piezoelectric BIT films on LaNiO 3, RuO 2 and Pt/Ti/SiO 2 / Si substrates by the polymeric precursor method combined with the domestic microwave oven with the advantage of reducing the time of thermal treatment. 2. Experimental The bottom electrodes thin films (LNO and RuO 2 ) were spin-coated on SiO 2 /Si (1 0 0) substrates by a commercial spinner at 5000 revolutions/min for 30 s (spin coater KW-4B, Chemat Technology). Bottom electrodes of commercial platinum coated silicon substrates were also used. Each deposited layer was pre-fired at 400 8C for 2 h in a conventional oven. After the pre-firing, each layer was crystallized in a microwave oven at 700 8C for 10 min using a SiC susceptor, which absorbs the microwave energy and rapidly transfers the heat to the film. No post-annealing treatment was performed after crystallization. Using the same procedure, BIT thin films were deposited by spinning the precursor solution on the desired substrates. Through this process, we obtained thicknesses of about 150 nm for the bottom electrodes and about 300 nm for BIT, by repeating the spin-coating and heating treatment cycles. The microwave oven used here was a simple domestic model similar to that described in literature [10]. Phase analysis was performed at room temperature by X-ray diffraction (XRD) in Bragg Brentano geometry (Rigaku 2000) at Cu Ka radiation. Furthermore, topography and thickness were examined using atomic force microscopy (AFM) (Digital Instruments, Nanoscope IIIa) and scanning electron microscopy (Topcom SM-300), respectively. The top Pt electrodes were prepared by photolithography with mm 2 dot area. The ferroelectric properties of the capacitors were measured by a Radiant Technology Tester RT6000 A in a virtual ground mode. The piezoelectric measurements were done using a setup based on an atomic force microscope [13] in a Multimode Scanning Probe Microscope with Nanoscope IV controller. 3. Results and discussion Fig. 1 shows the XRD results for BIT films deposited on different bottom electrodes. All peaks were assigned to a BIT-type structure. No reflections were detected that would be indicative of second phases. Film orientations depend in general on surface and interface energies. The (1 1 7) and (2 0 0) orientations for the films deposited on LNO and RuO 2 bottom electrodes are expected to be due to good lattice matching. In contrast, the predominant c-oriented

3 A.Z. Simões et al. / Materials Research Bulletin 42 (2007) Fig. 1. X-ray diffraction patterns of Bi 4 Ti 3 O 12 films deposited at 700 8C for 10 min on: (a) LaNiO 3, (b) RuO 2 and (c) Pt. crystallites in the film deposited on Pt are thought to be driven by their low surface energies [14]. In the case of the (2 0 0) oriented film, the lattice mismatch between bottom electrode and the peculiar long c-axis of BIT-type structure is proposed to have its origin in the different O O bond lengths along the c-axis and other (h kl) directions [15]. From the application point of view, random-oriented BIT films are more favourable than (0 0 l) oriented films. Therefore, this suggest that the films deposited on oxides electrodes have a suitable orientation to reach excellent ferroelectric and piezoelectric properties. Fig. 2 shows polarization hysteresis loops of the BIT films deposited on different bottom electrodes. Remanent polarizations (P r ) of 10 and 15 mc/cm 2 with drive voltages of 2.3 and 1.6 V were observed in the films deposited on LNO and RuO 2 bottom electrodes, respectively. For the films deposited on Pt substrates the remanent polarization is reduced to 8.5 mc/cm 2 due the stronger contribution of the crystallites grown in the c-axis direction. These results clearly demonstrate that controlling the orientation of grains in a film is a key point to improve the ferroelectric properties. Another factor can be the inhibition of domain wall movement caused by a high concentration of dipole complexes at the film substrate interface. This can originate from the thermal shock caused by the rapid heating of the SiC susceptor on platinum which favours the accumulation of the static charges at the interface film substrate. It is supposed that the real temperature in the susceptor may be some degrees higher, which, allied to the effect of the microwave energy may cause degradation of the film electrode interface and hence a loss of the ferroelectric properties [16]. This effect is more evident in the films deposited on platinum since it does not act as sink for oxygen vacancies compared to the oxides which accommodate relatively large concentration of oxygen defects [16]. For this film the drive voltage is around 4 Vand is considered large for use as ferroelectric memories. From hystereses curves, it is clear that the remanent polarization of (1 1 7)- and (2 0 0)-oriented film is larger than that of (0 0 l)- and (1 1 7)- oriented films. Since LNO and RuO 2 electrodes give better ferroelectric properties, it is believed that if oxygen vacancies accumulate near the film electrode interface, the conductive oxide can consume the vacancies by changing its nonstoichiometry. Therefore, the accumulation of oxygen vacancies near the interface is reduced. This implies less Fig. 2. P V hystereses loops of Bi 4 Ti 3 O 12 films deposited on LaNiO 3, RuO 2 and Pt bottom electrodes at 700 8C for 10 min.

4 978 A.Z. Simões et al. / Materials Research Bulletin 42 (2007) Fig. 3. (P * P^) as a function of polarization cycles for Bi 4 Ti 3 O 12 films deposited on different bottom electrodes at 700 8C for 10 min: (*) LaNiO 3,(&) RuO 2 and (~) Pt. charge trapping and domain wall pinning in the interface region. Therefore, the effect of the LNO and RuO 2 electrodes may be attributed to their function as an oxygen vacancy sink [17,18]. As a consequence of oxygen vacancies accumulated at the film electrode interface, a shift of the hysteresis loop along the electric field axis towards the positive side was observed and may lead to a failure of the capacitor. These charges may originate during the heat treatment process due to the decomposition of the polymeric precursor [12]. According to the electrostatic model proposed by Robels et al. [19], this horizontal shift of the curve represents the internal bias, which is closely connected to the electrode/film interface. These findings suggest that the slightly higher voltage shift observed in the films deposited on Pt substrates resulted from the nature of the bottom electrode, since the only difference in the film preparation is the type of substrate used. In our case it is observed that microwave crystallization with platinum as bottom electrode induces a permanent voltage shift in the hysteresis loops of the BIT films due to the generation of trapped charges at the defect sites near the electrode film interface. These results indicate the unsuitability of the films for use as memories owing to the significant difference between +V c and V c, respectively. Fig. 3 presents the fatigue endurance of the BIT thin films as a function of switching cycles. P * is the switched polarization between two opposite polarity pulses and P^ is the nonswitched polarization between the same two polarity pulses. The P * P^ or P * ( P^) denote the switchable polarization, which is an important variable for nonvolatile memory application. Fatigue resistance was observed up to cycles with oxide electrodes indicating that the conductive oxide can consume the oxygen vacancies accumulated near the film electrode interface. On the other hand, for the films deposited on platinum coated silicon substrates no fatigue resistance was observed indicating that the sparks formed by the high electric field in the microwave environment damaged the film electrode interface and therefore affect the switching characteristics of BIT films. Fig. 4 shows the out-of-plane (OP) and in-plane (IP) piezoresponse images of the as-grown films after applying a bias of 12 V, on an area of 2 mm 2 mm, and then an opposite bias of +12 V in the central 1 mm 1 mmarea.to obtain the domain images of the BIT films, a high voltage that exceeds the coercive field was applied during scanning. The contrast in these images is associated with the direction of the polarization [13]. The white regions in the out-of-plane PFM images correspond to domains with the polarization vector oriented toward the bottom electrode hereafter referred to as down polarization (Fig. 4a, d and g) while the dark regions correspond to domains oriented upward referred to as up polarization. Grains which exhibit no contrast change is associated with zero outof-plane polarization. Although the films have a certain degree of preferred orientation as indicated by the XRD spectra in Fig. 1, they still preserve their polycrystalline nature. This can be concluded from the piezo-contrast given by the as-grown state. After a negative bias was applied, grains labeled A did not change their contrast. However in most cases, such as in grains labeled B and C (Fig. 4a), a polarization pointing predominantly out-of-plane was observed.asimilarsituationwasobservedwhenapositive bias was applied to the film. We noticed that some of the grains exhibit a white contrast associated to a component of the polarization pointing toward the bottom electrode. On the other hand, in the in-plane PFM images (Fig. 4b, e and h) the contrast changes were associated with changes of the in-plane polarization components. In this case, the white contrast indicates polarization, e.g. in the positive direction of the y-axis while dark contrast are given by in-plane polarization components pointing to the negative part of the y-axis. The d 33 (V) hysteresis loops are shown in Fig. 4c, f and i. The maximum d 33 value, 40 pm/v, is

5 A.Z. Simões et al. / Materials Research Bulletin 42 (2007) Fig. 4. Out-of-plane (OP) and in-plane (IP) PFM images, as well as piezoresponse loops of Bi 4 Ti 3 O 12 films deposited on different bottom electrodes at 700 8C for 10 min: (a) LaNiO 3 (OP), (b) LaNiO 3 (IP) and (c) LaNiO 3 hysteresis loop; (d) RuO 2 (OP), (e) RuO 2 (IP) and (f) RuO 2 hysteresis loop; (g) Pt (OP), (h) Pt (IP) and (i) Pt hysteresis loop.

6 980 A.Z. Simões et al. / Materials Research Bulletin 42 (2007) Fig. 4. (Continued). similar for all samples and approaches the reported value for a BIT single crystal [19]. The enhancement of polarization could be caused by the microwave annealing whose role is to avoid interfacial reactions and increase the films crystallization.as can be seen, the hysteresis loop for the film deposited on the Pt substrate (Fig.4i) shows an offset in the vertical direction which can be probably causedbyclampingeffectduetothesubstrateandthe generation of trapped charges at the defect sites near the electrode film interface caused by strong interaction between microwave energy with platinum electrode [20]. Although the PZT films still have higher d 33 values, rangingfrom40to110pm/v[21], the presented values reported for our BITfilmssuggestthatthismaterialcanbe considered as a viable alternative for lead-free piezo-ferroelectric devices. Also, the microwave annealing for the processing of materials provides the advantages of low investment, rapid and uniform heating, low sintering temperatures and times and improved product quality. In comparison with other lead-free ferroelectrics, 40 pm/v is much higher than the d 33 value of SrBi 2 Ta 2 O 9 films (17 pm/v) and close to the reported value of Nd-doped Bi 4 Ti 3 O 12 (38 pm/v) [22]. 4. Conclusions In conclusion, BIT thin films were successfully crystallized using a low power microwave oven, with no postannealing treatment. The electrical measurements indicate that the use of platinum substrate as bottom electrode is inappropriate to obtain good BIT films. This effect was not observed for films deposited on oxide electrodes. Regularly shaped hysteresis is observed for the films deposited on the LaNiO 3 and RuO 2 electrodes. Furthermore, high fatigue resistance was observed for films deposited on LaNiO 3 and RuO 2 electrodes which shows that our films are promising

7 candidates for nonvolatile random access memories. Our results indicate that the BIT films exhibit a good piezoelectric response of (40 pm/v). BIT films crystallized in the microwave oven using conductor oxides as bottom electrodes present good ferroelectric properties and piezoelectric coefficients and can be used for ferroelectric random access memories and piezoelectric devices. Acknowledgments The authors gratefully acknowledge the financial support of the Brazilian agencies FAPESP, CNPq, CAPES. References A.Z. Simões et al. / Materials Research Bulletin 42 (2007) [1] M. Chu, M. Ganne, M.T. Caldes, E. Gautier, L. Brohan, Phys. Rev. B 68 (2003) [2] S.E. Cummins, L.E. Cross, J. Appl. Phys. 39 (1968) [3] H.N. Lee, D. Hesse, N. Zahharov, U. Gosele, Science 296 (2003) [4] H. Matsuda, S. Ito, T. Iijima, Appl. Phys. Lett. 83 (2003) [5] J.K. Lee, C.H. Kim, H.S. Suh, K.S. Hong, Appl. Phys. Lett. 80 (2002) [6] K. Kim, C. Kim, Surf. Coat. Technol. 177 (2004) [7] D.H. Bao, X. Yao, N. Wakiya, K. Shinozaki, N. Mizutani, J. Phys. D 36 (2003) [8] R. Ramesh, H. Gilchrist, T. Sands, V.G. Keramidas, R. Haakenaasen, D.K. Fork, Appl. Phys. Lett. 63 (1993) [9] R. Ramesh, D.G. Schlom, Science 296 (2002) [10] N.S.L.S. Vasconcelos, J.S. Vasconcelos, V. Bouquet, M. Guilloux-Viry, M.I. Bernardi, J.A. Varela, Thin Solid Films 436 (2003) [11] A.Z. Simões, B.D. Stojanovic, A. Tangastev, N. Setter, J.A. Varela, Integrated Ferroelectrics 43 (2002) [12] A.Z. Simões, A. Ries, F.M. Filho, J.A. Varela, E. Longo, Appl. Phys. Lett. 85 (2004) [13] A. Gruverman, O. Auciello, H. Tokumoto, Annu. Rev. Mater. Sci. 28 (1998) [14] Y.M. Sun, Y.C. Chen, J.Y. Gan, J.C. Hwang, Jpn. J. Appl. Phys. (Part 2) 41 (2002) L892 L894. [15] W.J. Takei, N.P. Formigoni, M.H. Francombe, Appl. Phys. Lett. 15 (1969) [16] S.M. Zanetti, J.S. Vasconcelos, N.S.L.S. Vasconcelos, E.R. Leite, E. Longo, J.A. Varela, Thin Solid Films 466 (2004) [17] H.N. Al-Shareef, D. Dimos, W.L. Warren, B.A. Tuttle, J.A. Voigt, R.D. Nasby, J. Appl. Phys. 79 (1996) [18] F.M. Pontes, E.R. Leite, E. Longo, J.A. Varela, J.A. Eiras, Appl. Phys. Lett. 76 (2000) [19] U. Robels, J.H. Calderwood, G. Arlt, J. Appl. Phys. 77 (1995) [20] A.L. Kholkin, K.G. Brooks, N. Setter, Appl. Phys. Lett. 71 (1997) [21] P. Muralt, IEEE Trans. Ultrason. Ferroelectr. Freq. Control. 47 (2000) [22] L. Kholkine, C. Wuetchrich, D.V. Taylor, N. Setter, Rev. Sci. Instrum. 67 (1996)