Journal of Alloys and Compounds Preferential grain growth and ...

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Apr 7, 2009 - Preferential grain growth and improved fatigue endurance in Sr substituted ... PZT film showed high degree of (0 0 1) type preferential grain.
Journal of Alloys and Compounds 482 (2009) 253–255

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Preferential grain growth and improved fatigue endurance in Sr substituted PZT thin films on Pt(1 1 1)/TiOx /SiO2 /Si substrates N.K. Karan, R. Thomas ∗ , S.P. Pavunny, J.J. Saavedra-Arias, N.M. Murari, R.S. Katiyar ∗ Department of Physics and Institute for Functional Nanomaterials, University of Puerto Rico, P.O. Box 23343, San Juan, PR 00931, USA

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Article history: Received 24 December 2008 Received in revised form 27 March 2009 Accepted 27 March 2009 Available online 7 April 2009 Keywords: Ferroelectrics Thin films Perovskites Electrical properties Sol–gel preparation

a b s t r a c t Pb(Zr0.5 Ti0.5 )O3 and (Pb0.9 Sr0.1 )(Zr0.5 Ti0.5 )O3 thin films were grown on Pt/TiOx /SiO2 /Si substrates by chemical solution deposition. 10% Sr substituted PZT film showed high degree of (0 0 1) type preferential grain growth. Surface morphology revealed a clear correlation between preferred grain orientation and grain size. Room temperature dielectric constant was 1200 and 700 for the PZT and PSZT films, respectively. Dielectric loss reduced with Sr substitution. PZT film showed severe fatigue, and hence the polarization reduced to 20% of the initial value (24 ␮C/cm2 ) after 108 cycles where as PSZT showed less fatigue, 75% of the initial polarization (12 ␮C/cm2 ) was retained after 108 switching cycles. © 2009 Published by Elsevier B.V.

Integration of ferroelectric films into Ferroelectric Random Access Memory (FeRAM) non-volatile memories has attracted intense research for more than a decade [1]. In the perovskite oxide family lead zirconium titanate (PZT) is a polymorphic ferroelectric material and is the most studied material for the FeRAM application [2,3]. Ferroelectric PZT exists in two forms at ambient temperatures: a tetragonal phase, in which the polar vector is aligned parallel to the 0 0 1 direction (or c-axis) and a rhombohedral phase in which the polar axis is aligned along the 1 1 1 direction. Much of the ferroelectric PZT thin film research is devoted to composition near the morphotropic phase boundary (MPB), Zr/Ti = 53/47. It is well known that electrical (dielectric constant), ferroelectric (remnant polarization and coercive field) and piezoelectric (piezoelectric coupling factor) properties of PZT located at MPB show superior values [4]. In tetragonal PZT [e.g. Pb(Zr0.5 ,Ti0.5 )O3 ] anisotropy exists in a number of electric properties, including dielectric constant, remanent polarization, and pyroelectric coefficient. Hence, several applications benefit by manipulation of these properties. For example, the 1 1 1 orientation provides the largest polarization needed for non-volatile memory, while the 1 0 0 orientation doubles the piezoelectric coefficient (d31 ) of the film. PZT films with Pt bottom electrode exhibit ferroelectric fatigue after 106 cycles. For most applications 106 cycles, that is equal to 17 min at 1 kHz, is not sufficient for the realization of reliable integrated

∗ Corresponding authors. Tel.: +1 787 751 4210; fax: +1 787 764 2571. E-mail addresses: [email protected] (R. Thomas), [email protected] (R.S. Katiyar). 0925-8388/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.jallcom.2009.03.170

memory devices. Fatigue can be minimized either by using conductive oxide electrodes (e.g., RuO2 ) that prevent space charge formation at the interfaces, or by addition of donor dopants (e.g., Nb, La in Pb(Zrx Ti1−x )O3 ) that reduce the oxygen vacancy concentration and allow increasing number of cycles to amazing 1012 [5,6]. Strong cation–oxygen bond can be another way to reduce the oxygen vacancies and in this respect high stability associated with Sr–O bonding may be interesting to reduce oxygen vacancies [7]. In this work, we present Sr substitution in PZT and its influence on the microstructure and electrical properties.(Pb0.9 Sr0.1 )(Zr0.5 ,Ti0.5 )O3 (PSZT) ferroelectric thin films were successfully prepared by the chemical solution deposition (CSD). Lead (II) acetate trihydrate (99% Alfa Aesar), strontium acetate (Alfa Aesar), zirconium (IV) n-propoxide (Alfa Aesar) and titanium (IV) iso-propoxide (97% Alfa Aesar) were used as precursors. 2-Methoxyethnol (99.8%, Alfa Aesar) and acetic acid (99.99% Alfa Aesar) were used as solvents and acetyl acetone was used as chelating agent. Based on the above precursors, a variety of routes have been developed for the preparation of Pb-based thin films by CSD process [8–10]. Separate solutions of lead acetate and strontium acetate were prepared by dissolving stoichiometric amount of lead acetate trihydrate and strontium acetate in a mixture of 2-methoxy ethanol and acetic acid at ∼100 ◦ C. Zr-propoxide (0.50 mol) and Ti-isopropoxide (0.50 mol) were then added to this solution and refluxed at 100 ◦ C for 2 h. The clear PSZT sol thus obtained aged for 24 h before coating. The concentration of the sol was about 0.3 M. The solution was filtered by 0.2 ␮m filters and spin cast onto Pt/TiO2 /SiO2 /Si at 2500 rpm for 30 s. Resultant films were first heat treated at 150 ◦ C for 2 min and then at 500 ◦ C for 5 min for organic removal. These steps were repeated

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Fig. 3. Variation of dielectric constant and loss tangent with measuring temperature at 100 kHz of CSD prepared Pb(Zr0.5 Ti0.5 )O3 and (Pb0.9 Sr0.1 )(Zr0.5 Ti0.5 )O3 thin films on Pt/TiOx /SiO2 /Si substrate. Fig. 1. X-ray diffraction patterns of CSD prepared (a) Pb(Zr0.5 Ti0.5 )O3 and (Pb0.9 Sr0.1 )(Zr0.5 Ti0.5 )O3 thin films on Pt/TiOx /SiO2 /Si substrate annealed at 600 ◦ C, showing the preferred orientation with Sr substitution at the A-site.

for obtaining thicker films and finally the films were annealed at various temperatures for crystallization. The structural properties of the films were examined by Bragg–Brentano X-ray diffraction [XRD, Siemens D5000] using Cu K␣1 radiation. The surface morphology was investigated by atomic force microscopy [AFM, Digital instruments]. MIM capacitors were processed by sputter depositing circular Pt top electrodes ( = 250 ␮m) using shadow mask. Temperature dependent dielectric and ferroelectric properties were measured with HP4294A Precision Impedance Analyzer and RT500 V Radiant technologies systems, respectively. Further details can be seen elsewhere [11,12]. Films without Sr substitution had randomly oriented polycrystalline structure as can be seen from Fig. 1. PZT film annealed at 600 ◦ C thus shows an XRD pattern similar to its ceramic bulk counterpart, of which (1 1 0) is the strongest diffraction peak (ref. JCPDS #33-0784). It is evident from the figure that films crystallized in perovskite structure with no distortion of the unit cell corresponding to the tetragonal phase. Similar observation has been reported on PZT film near the MPB [13]. In the case of thin films, broadening of the diffraction peaks, mainly due to the small grain size leading to overlapping of the (0 0 l) (h 0 0) peaks and hence it is difficult to index the individual peaks [14]. Therefore the XRD patterns have been indexed as cubic. Interestingly, the XRD pattern of the 10% Sr substituted PZT film shows high degree of (0 0 1) type preferential growth, without any other reflection except a very weak degree of (1 1 0) growth. As these films were deposited and annealed under the same condition, the observed difference in the microstructure is due to the substitution of Sr at the A-site. This observation was

in contrast to the recent report on (Pb1−x Srx )(Zr0.52 Ti0.48 )O3 films prepared by sol–gel process on the same type of substrate, where PZT films preferred the (1 0 0) orientation and this preference disappeared with Sr substitution [7]. Atomic force microscopy was used to study the morphology of the films and Fig. 2 shows the AFM plane view of the un-doped and 10% substituted PZT films annealed at 600 ◦ C. In the case of polycrystalline and randomly orientated PZT, grains were small, rounded and uniform with an average size around 20 nm. The root mean square roughness of the 300 nm thick film was around 4 nm, higher than the surface roughness (1–1.5 nm) of the underlying Pt surface. In contrast, surface morphology of 10% Sr substituted PZT, which showed preferred orientation along (0 0 1) direction, was very different compared to pure PZT. Grains were bigger and flat with average size more than 300 nm, with surface roughness (3 nm) lower than the PZT films. Interestingly, at some of the grain boundary of these flat bigger grains, accumulation of smaller grain was visible with average size comparable with the randomly oriented pure PZT films. In PZT thin films, second phase is often encountered and has been assigned to the so-called ‘oxygen deficient pyrochlore’ phase of Pb2 (Zr,Ti)2 O7−x and ‘lead deficient pyrochlore’ type phase of Pb(Zr,Ti)3 O7 [15]. Generally finer grains of pyrochlore phase with comparatively bigger perovskite phase co-exist in such films. Compositional analysis (EDX) of these two region gave same (Pb + Sr)/(Zr + Ti) ratio and hence additional phase due to pyrochlore phase can be ruled out. XRD patterns showed a small contribution of (1 1 0) reflection and hence these small grains can be of (1 1 0) orientation and the flat bigger grains can be of (0 0 1) orientation. These structural and morphological studies clearly suggest that grain growth has profound influence on the orientation of the grains in PZT films. A relative increase in perpendicular/or in-plane nucle-

Fig. 2. AFM surface morphology (x–y = 1 ␮m × 1 ␮m; z = 100 nm) of CSD prepared (a) Pb(Zr0.5 Ti0.5 )O3 and (b) (Pb0.9 Sr0.1 )(Zr0.5 Ti0.5 )O3 thin films on Pt/TiOx /SiO2 /Si substrate annealed at 600 ◦ C. Thickness of the films was 300 nm.

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Fig. 4. (a) Polarization–electric field (P–E) hysteresis and (b) % polarization with increasing switching cycles at room temperature for the CSD prepared Pb(Zr0.5 Ti0.5 )O3 and (Pb0.9 Sr0.1 )(Zr0.5 Ti0.5 )O3 films (t ∼ 300 nm) on Pt/TiOx /SiO2 /Si.

ation sites over random nucleation sites due to A-site Sr substitution contributes to the improvement in c-axis/a-axis orientation for the PZT films. The electrical and ferroelectric properties of randomly oriented PZT and 1 0 0/0 0 1 oriented PSZT films were measured and compared. Fig. 3 shows the variation of dielectric constant (εr ) and loss tangent (tan ı) of PZT and PSZT films as a function of temperature. No anomaly corresponding to the phase transition temperature was observed within the studied temperature range (100–600 K), suggesting Tc above 600 K in both cases. εr reduced as the temperature reduced and in the case of tan ı it reduced first and then increased with the lowering of temperatures. The room temperature εr of pure PZT was around 1200 with 2.4% loss factor at a frequency of 100 kHz, where as 10 mol% Sr substituted films have the εr of 700 and loss factor of 2.1% at the same frequency. The ferroelectric properties of the films were studied by plotting P–E hysteresis loop and are shown in Fig. 4a. The average remnant polarization (Pr) and the average coercive field (Ec) were 24 ␮C/cm2 and 50 kV/cm, respectively, for the PZT films. However, upon Sr substitution remnant polarization reduced and coercive field increased, the corresponding values were 12 ␮C/cm2 and 66 kV/cm, respectively, for the PSZT films. The rectangular ratio (remnant polarization/maximum polarization) of hysteresis loops is 48% and 33% for randomly oriented PZT and (0 0 1) oriented PSZT films, respectively. The more rectangular hysteresis loop means the less amount of 90◦ domain wall and in-plane direction of polarization [16]. Hence, less squareness of the PSZT films may be due to some in-plane orientation of the grains. Fatigue is one of the most important factors in determining the reliability of ferroelectric devices. Fatigue tests were carried out on the randomly oriented PZT and (0 0 1) oriented PSZT films by applying 1 ␮s wide pulses with a frequency of 100 Hz and the result is shown in Fig. 4b. For both films, the cycling voltage was 200 kV/cm that was sufficiently high for the complete switching of polarization state (see Fig. 4a). PZT without Sr substitution exhibits a significant polarization degradation after 104 switching cycles and polarization reduced to 20% of the initial value after 108 cycles, which is similar to the fatigue behavior typically seen in Pb(Zr,Ti)O3 [17]. Although various mechanisms for the polarization fatigue have been proposed, such as 90◦ domains, microcracking, formation of interfacial layers, vacancy migration, interface nucleation inhibition mechanisms induced by charge injection, etc. but a complete understanding is still lacking [18]. In contrast, 10% Sr substituted PZT films showed lower fatigue, 75% polarization was retained after 108 cycles, significantly higher than the polarization (20%) of pure PZT after the same number cycles. Hence the Sr substitution of Pb improves the chemical stability of perovskite PZT structure and

which in turn reduces the oxygen vacancies. This enhanced oxygen stability, and lower concentration and mobility of oxygen vacancy may be the reason for improved fatigue endurance in the PSZT films [7]. To summarize, ferroelectric PSZT thin films (Sr = 0% and 10%) were grown on Pt/TiOx /SiO2 /Si substrates by chemical solution deposition method. A relative increase in the perpendicular/inplane nucleation sites over random nucleation sites due to A-site Sr substitution contributes to the improvement in the 1 0 0/0 0 1 orientation for PZT films. PZT film showed severe fatigue and the polarization reduced to 20% of the initial value after 108 cycles where as PSZT showed less fatigue, 75% of the polarization was retained after 108 switching cycles. Hence, Sr-substitution significantly improved the fatigue endurance. Acknowledgements The financial support under the Grant DOD (W911NF-06-10030) is acknowledged. One of us (N. K. Karan) is grateful to the NSF for the graduate fellowship. References [1] J.F. Scott, C.A. Araujo, Science 246 (1989) 1400. [2] J.F. Scott, L.D. McMillan, C.A. Araujo, Ferroelectrics 116 (1991) 147. [3] R. Ramesh, Thin Film Ferroelectric Materials and Devices, Kluwer Academic Press, Boston, 1997. [4] B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics, Academic Press, New York, 1971. [5] S.B. Desu, Phys. Status Solidi (a) 151 (2006) 467. [6] J.F. Scott, M. Dawber, Appl. Phys. Lett. 76 (2000) 3801. [7] Y. Wang, Q.Y. Shao, J.-M. Liu, Appl. Phys. Lett. 88 (2006) 122902. [8] R. Thomas, S. Mochizuki, T. Mihara, T. Ishida, Mater. Res. Soc. Symp. Proc. 718 (2002), D10.5.1. [9] D. Zhu, Q. Li, T. Lai, D. Mo, Y. Xu, J.D. Mackenzie, Thin Solid Films 313–314 (1998) 210. [10] N. Tohge, S. Takahashi, T. Minami, J. Am. Ceram. Soc. 74 (1991) 67. [11] N.K. Karan, J.J. Saavedra-Arias, M. Perez, R. Thomas, R.S. Katiyar, Appl. Phys. Lett. 92 (2008) 012903. [12] N.M. Murari, A. Kumar, R. Thomas, R.S. Katiyar, Appl. Phys. Lett. 92 (2008) 132904. [13] G.A.C.M. Spierings, G.J.M. Dormans, W.G.J. Moors, M.J.E. Ulenaers, P.K. Larsen, J. Appl. Phys. 78 (1995) 1926. [14] M. Klee, R. Eusemann, R. Waser, W. Brand, H. VanHal, J. Appl. Phys. 72 (1992) 1566. [15] R. Thomas, S. Mochizuki, T. Mihara, T. Ishida, Thin Solid Films 413 (2002) 65. [16] D.J. Wouters, G. Willems, E.G. Lee, H.E. Maes, Integr. Ferroelectr. 15 (1997) 79. [17] H.N. Alshareef, A.I. Kingon, X. Chen, K.R. Bellur, O. Auciello, J. Mater. Res. 9 (1994) 2968. [18] F. Yang, M.H. Tang, Y.C. Zhou, F. Liu, Y. Ma, X.J. Zheng, J.X. Tang, H.Y. Xu, W.F. Zhao, Z.H. Sun, J. He, Appl. Phys. Lett. 92 (2008) 022908.