Effect of strontium addition on ferroelectric phase transition of PZT thin films prepared by chemical route

Transcription

1 Appl Phys A (2009) 95: DOI /s Effect of strontium addition on ferroelectric phase transition of PZT thin films prepared by chemical route F.M. Pontes M.S. Galhiane L.S. Santos A. Gavino L.A. Petit D.S.L. Pontes E. Longo A.J. Chiquito C.E.F. Costa Received: 5 November 2008 / Accepted: 16 November 2008 / Published online: 5 December 2008 Springer-Verlag 2008 Abstract The dependence of ferroelectric phase transition temperature as a function of strontium substitution in lead titanate zirconate thin films (referred here as PSZT) on platinum-coated silicon substrates was investigated. The dielectric study reveals that the material undergoes a diffuse type ferroelectric phase transition that depends on the substitution of Sr for Pb in PZT. At 100 khz, the phase transition temperatures were 633, 613 and 516 K for PSZT10, PSZT20 and PSZT30 thin films, respectively. On the other hand, the results showed that the dependence of the dielectric constant upon the frequency does not reveal any relaxor behavior. The diffusivity increases with increasing Sr contents in the studied composition range. The experimental data obtained from measurements of the dielectric constant as a function of temperature and frequencies showed a classical behavior of ferroelectric phase transition in PSZT thin films, rather than a relaxor ferroelectric phase transition. The F.M. Pontes ( ) M.S. Galhiane L.S. Santos A. Gavino L.A. Petit LAMAC-Department of Chemistry, Universidade Estadual Paulista (UNESP), P.O. Box 473, Bauru, São Paulo, Brazil fenelon@fc.unesp.br Fax: D.S.L. Pontes E. Longo Chemistry Institute, Universidade Estadual Paulista (UNESP), P.O. Box 355, Araraquara, São Paulo, Brazil A.J. Chiquito Department of Physics, UFSCar, Via Washington Luiz, km 235, CEP São Carlos, SP, Brazil C.E.F. Costa LOC-Institute of Natural Science, Universidade Federal do Pará (UFPA), Av. Augusto Corrêa, 01, Belém, Pará, Brazil transition temperature decreases with increasing Sr contents due to the decrease in grain size, lattice decrease and local structural disorder. PACS Bh f Dj d 1 Introduction Lead titanate zirconate [Pb(Zr x Ti 1 x )O 3 ], also known as PZT, has received special attention because of its piezoelectric and ferroelectric properties [1 3]. It is well known that homovalent and heterovalent substitutions of lead, zirconium or titanium ions give rise to various behaviors, including relaxor properties, which may appear for some composition temperature ranges [4, 5]. One of the characteristics of the relaxor ferroelectrics is the strong influence of the frequency f of the electric field on the ε T curves, i.e., when f increases, ε decreases and the ε maximum temperature increases. The PZT system has a wide range of ferroelectric phase transition temperature and different ferroelectric structures, depending on the Zr:Ti composition ratio [6]. PZT is a ferroelectric compound with one of the highest phase transition temperatures. The phase diagram clearly shows that Curie temperatures of a PZT system increase as the Ti content increases [6]. However, it is required for memories, piezoelectric and electro-optical devices to have a wide range of the operating temperature within which the dielectric, electro-optical and ferroelectric properties remain constant. Some studies have investigated the substitution of the cations Pb 2+,Zr 4+ or Ti 4+. This substitution can induce lattice defects in order to maintain the charge neutrality [7]. The substitution of the sites is facilitated when the cations have the same charge and size. In addition,

2 694 F.M. Pontes et al. lanthanum, calcium, strontium, and barium can be employed in PZT in order to lower the Curie temperature for several applications and maintain a fairly small temperature dependence of the dielectric constant. Recently, we reported that PbTiO 3 and SrTiO 3 can form continuous solid solution system, and the Curie temperature of Pb 1 x Sr x TiO 3 thin films was found to decrease with increasing strontium concentration [8, 9]. Wang et al. [10] reported a study on Pb 0.91 La 0.09 Zr 0.65 Ti 0.35 O 3 (PLZT) thin films prepared by sol gel method. The dielectric permittivity versus temperature data at 1 khz showed a broad peak at around 378 K associated with the ferroelectric paraelectric phase transition temperature. Nb doped PZT thin films processed via chemical solution deposition on platinum-coated silicon substrate show diffuse peaks corresponding to paraelectric to ferroelectric transition [11]. The structural, microstructural and ferroelectric details of lead zirconate titanate (Pb(Zr x Ti 1 x )O 3 ) thin films, especially close to the morphotropic phase boundary (MPB, corresponding to x = 0.52), have been keenly studied during the last few years. From these studies, the ferroelectric properties are maximized because of the co-existence of rhombohedral and tetragonal phases [12 15]. The PbZr 1 x Ti x O 3 near the MPB has a high ferroelectric to paraelectric phase transition temperature around 658 K. Recently, appropriate control of the Sr 2+ in lead zirconate titanate allowed the PZT material to be used for a wide range of device applications [16 19]. Ansari et al. have reported humidity sensors based on the strontium-doped PZT, referred as PSZT [20]. Shao et al. reported that Pb 1 x Sr x Zr 0.52 Ti 0.48 O 3 (PSZT) thin films prepared by sol gel exhibit a paraelectric nature in room temperature region with increasing strontium concentration [21]. Dalakoti et al. have used Zn, Y and Sr dopants on PZT thin films with composition near the morphotropic phase boundary, and they were able to demonstrate an increase of piezoelectric properties [22]. This work is focused on investigation of the temperature dependence of the dielectric permittivity and ferroelectric properties of Pb 1 x Sr x Zr 0.30 Ti 0.70 O 3 thin films where x = 10, 20 and 30 mol% prepared by a soft chemical method on Pt-coated silicon substrate. 2 Experimental procedure Pb 1 x Sr x Zr 0.30 Ti 0.70 O 3 thin films studied in this work were prepared using a polymeric precursor method (PPM) with x = 10, 20 and 30 mol% (referred here as PSZT10, PSZT20 and PSZT30). Details of the preparation method can be found in the literature [23]. The PSZT thin films were deposited on commercial Pt/Ti/SiO 2 /Si substrates by a commercial spinner operating at 7400 rev/min for 30 s (spin Coater kw-4 B Chemat Technology Northridge, CA), via a syringe filter to avoid particulate contamination. After deposition, each layer was dried in ambient air at 150 o C on a hot plate for 10 minutes, to remove residual solvents. The heat treatment was carried out in two stages: initial heating at 400 o C for 4 h at a heating rate of 5 o C/min in an air atmosphere to pyrolize the organic material, and finally by heating at 700 o C for 2 h for crystallization. The film thickness was controlled by adjusting the number of coatings, and each layer crystallized before the next layer was coated. These operations were repeated until the desired thickness was obtained. The film thickness was measured by a thin film cross-section analysis made by SEM (Zeiss, DSM94OA). The thin films were structurally characterized by X-ray diffraction (XRD) (Cu K α radiation) in the mode of θ 2θ scan, recorded on a Rigaku diffractometer. In all samples the perovskite structure was confirmed by X-ray diffraction (not show here). The samples deposited on Pt-coated silicon substrates by PPM are polycrystalline as reported elsewhere [23]. In order to measure electrical properties, Au dot electrodes of mm area were deposited through a shadow mask by thermal evaporation process on the surfaces of the films as top electrodes. The dielectric constant of the thin films was measured using a Keithley 3330 (LCR) meter, over a wide range of temperature, K, and in the frequency range of 1, 10 and 100 khz. The capacitance voltage (C V) curves were measured using an HP4194A impedance/gain phase analyzer at the temperature range of K. 3 Results and discussions The dielectric constant was measured as a function of temperature for various compositions of the PSZT thin films, at 100 khz (see Fig. 1). As the Sr +2 content increases from 10% to 30% mol, the transition temperature shifts from 633 to 516 K. In our case, all the phase transitions were noticed to be of diffuse type. Furthermore, it was observed that the Curie temperature varies linearly with composition, as showed in Fig. 2. This means that the solubility limit was reached in the whole interval of composition studied for the PSZT thin films prepared by soft chemical method. It is known that below phase transition temperature the PZT (PbZr 0.30 Ti 0.70 O 3 ) structure is tetragonal and all the PbO 12,TiO 6 and ZrO 6 polyhedra become increasingly distorted as temperature decreases. In addition, the dipolar moments are due to the cations Ti 4+ and Zr 4+ in the B sites (BO 6 ) that are all ferroelectrically active, so these cations are off-centered in the octahedral site and give rise to a polarization at room temperature. When Sr 2+ is substituted in dodecahedral coordination sites (PbO 12 to SrO 12 ),

3 Effect of strontium addition on ferroelectric phase transition of PZT thin films prepared by chemical route 695 Fig. 1 The variation of dielectric constant with temperature of Pb 1 x Sr x Zr 0.30 Ti 0.70 O 3 thin films at 100 khz Fig. 2 The dependence of the Curie temperature of Pb 1 x Sr x Zr 0.30 Ti 0.70 O 3 thin films as a function of the concentration, measured at 100 khz the AO 12 units are more regular, and the PSZT structure is slowly transformed from tetragonal to cubic. From previous structural analyses [23], an increase of Sr contents means the shrinkage of oxygen octahedrals, which depresses the ionic displacement responsible to induce a dipolar moment whose behavior leads to a decrease in transition temperature of the PSZT thin films. Shao et al. [21] studied the influence of strontium content on phase transition temperature of Pb 1 x Sr x Zr 0.52 Ti 0.48 O 3 with x = thin films where the T c of the thin films was observed to linearly decrease with increasing the content of Sr. A similar result was observed in Pb 1 x Ba x Zr Ti O 3 ceramic compositions for x = 0 to 40 mol%. Recent work by Shao et al. has revealed that PSTZ60 (Pb 0.40 Sr 0.60 Zr 0.52 Ti 0.48 O 3 ) thin films prepared on Nb SrTiO 3 substrates show higher phase transition temperature ( 256 K) compared to thin films on Ptcoated silicon substrates ( 204 K) [16]. The authors associated the T c shift to the different stress levels between PSZT60 thin films on Nb SrTiO 3 and on Pt-coated silicon substrates, arising from lattice and thermal expansion coefficient mismatch between the interface PSZT60 thin films and substrate. In summary, the Table 1 shows the results of the phase transition temperature compared with the literature. It is interesting, as it is always for a complex system, to find out whether there is a frequency dispersion of the dielectric constant. For our thin films, the measurements of the dielectric constant at different frequencies for PSZT thin films are shown in Fig. 3. A small shift was observed in the maxima of dielectric constant curves at different frequencies, indicating a classical rather than relaxor behavior in this system. In perovskite-type ferroelectrics thin films, an enhanced diffuse behavior is expected when the amount of different substitute cations on the AO 12 site is increased. Irrespective of the Sr +2 concentrations, a broad maximum of dielectric constant is observed. Qualitatively, the strongly broadened dielectric constant peak indicates that the phase transition is of a diffuse type in the vicinity of the ferroelectric to paraelectric phase transition. The temperature variation of the dielectric constant at a fixed frequency is known to follow a well-known equation which has been proposed to describe and quantify the diffuseness parameters [26]: 1 ε = 1 [1 + (T T c) γ ε max 2δ 2 ], where γ is an empirical exponent which is equals to 1 for ideal Curie Weiss law behavior and has values between 1 and 2 for diffuse phase transitions. In order to determine the diffuseness parameter δ, the exponent γ was obtained from the slope of ln[1/ε 1/ε max ] versus ln[t T c ] at 100 khz for different compositions. The results are shown in Fig. 4. Aplotofγ and δ as a function of composition for the PSZT thin films is shown in Fig. 5. The values of γ at 100 khz are found to be 1.52, 1.79 and 1.86 for PSZT10, PSZT20 and PSZT30 thin films, respectively, indicating that the transitions are of a diffuse nature. Pokharel and Pandey reported values of γ in the range of for the diffuse behaviorofthe(pb 1 x Ba x )ZrO 3 system [24]. In addition, Ganesh and Goo reported values of γ in the range of for the diffuse behavior of the paraelectric ferroelectric transition region for the (Pb 1 x Ca x )TiO 3 system [25]. Recently, Halder et al. [26] have performed electrical characterization studies of films of BaTiO 3 substituted with Zr. It is noticed that for a higher amount of Zr substitution the γ has the value 2. The strong diffuseness of the thin films can be attributed to the microscopic inhomogeneity and/or smaller grain size

4 696 F.M. Pontes et al. Table 1 Phase transition temperature of PSZT thin films obtained herein and according to the literature Samples Frequency Phase transition temperature Process Substrate PNZT(10/65/35) [32] 10 khz 623 K Mixed oxide PNZT(30/65/35) [32] 10 khz 543 K Mixed oxide PBZT(10/53.2/46.8) [13] 1 khz 588 K Mixed oxide PBZT(20/53.2/46.8) [13] 1 khz 548 K Mixed oxide PBZT(30/53.2/46.8) [13] 1 khz 498 K Mixed oxide PSZT(10/80/20) [33] 1 khz 464 K Mixed oxide PLZT(9/65/35) [10] 100 khz 653 K Sol gel MgO PSZT(60/52/48) [16] 1 MHz 256 K Sol gel Nb SrTiO 3 PSZT(60/52/48) [16] 1 MHz 204 K Sol gel Pt/Ti/SiO 2 /Si PSZT(20/52/48) [17] 100 khz 455 K Sol gel Pt/Ti/SiO 2 /Si PSZT(20/52/48) [17] 100 khz 464 K Sol gel Pt/Ti/SiO 2 /Si PSZT(30/52/48) [21] 100 khz 390 K Sol gel Pt/Ti/SiO 2 /Si PSZT(80/52/48) [21] 100 khz 118 K Sol gel Pt/Ti/SiO 2 /Si PSZT(10/30/70) * 100 khz 633 K Chemical solution Pt/Ti/SiO 2 /Si PSZT(20/30/70) * 100 khz 613 K Chemical solution Pt/Ti/SiO 2 /Si PSZT(30/30/70) * 100 khz 516 K Chemical solution Pt/Ti/SiO 2 /Si * This study; Pb 1 x Nb x Zr 0.65 Ti 0.35 O 3 (PNZT); Pb 1 x Ba x Zr Ti O 3 (PBZT); Pb 1 x La x Zr 0.65 Ti 0.35 O 3 (PLZT); Pb 1 x Sr x Zr 1 y Ti y O 3 (PSZT) that may have stress-induced the region near the film electrode and/or grain boundary interfaces. In addition, the large broadness observed in thin films can also occur due to structural disordering in the arrangement of cations in one or more crystallographic sites of the perovskite structure. Following Shao et al. [16, 21], the structural disorder will cause a nanoscale composition heterogeneity in thin films and will therefore induce a broadening of the phase transition temperature. The diffuse character of the transition increases as the grain size decreases from 170 in PSZT0 to 100 nm in PSZT30 thin films. Therefore, we believe that the presence of a submicron size grain in PSZT thin film causes intergranular stress, which leads to a broadened maximum in the temperature dependence of the dielectric constant. This behavior was observed mainly in the PSZT30 thin films with a grain size of approximately 100 nm. According to Park et al. [27], the particle size influences the ferroelectric transition of the BaTiO 3.ThevalueofT c decreases gradually, but the transition becomes diffuse as the particle size decreases from 81 to 40 nm. Similar diffuse transitions were observed for Ca-doped barium titanate thin films reported by Victor et al. [28]. These results were interpreted due to the larger compositional and structural disorder. Jimenez and Jimenez [29] recently predicted a diffusive nature for the ferroelectric phase transition of x = 0.45 for PCT ceramic using dielectric data. The dielectric result for x = 0.45 was γ = In addition, Zhai et al. [30] reported a diffusive phase transition accompanied by relaxor behavior for Ba(Zr x Ti 1 x )O 3 thin films grown by a sol gel process. To analyze the broad phase transition around 516, 613 and 633 K as explained above, we have plotted the ε V curves at different temperatures. Figures 6, 7, and 8 show the dielectric constant voltage (ε V ) curves of PSZT thin films deposited on Pt/Ti/SiO 2 /Si substrate. The ε V curves were obtained at different temperatures at 100 khz, with oscillation amplitude of 50 mv. The capacitance or dielectric constant of the films showed a strong dependence on the applied voltage. The ε V curves showed two broad peaks at 298 K, and were symmetric about the zero-bias axis. All the thin films displayed a hysteresis behavior at room temperature. The gap between the ε V curves of the positive and negative bias was reduced; the curves showed absence of splitting (butterfly-type) at decreasing/increasing voltages for both polarities at about 623, 573 and 523 K for PSZT10, PSZT20 and PSZT30 thin films, respectively. This fact indicates that the temperatures range of 623 K to 523 K corresponds to the diffuse type ferroelectric paraelectric phase transition temperatures. Venkateswarlu et al. [31] observed similar behavior for lead lanthanum titanate thin films prepared by the laser ablation technique. 4 Conclusions The dielectric study reveals that the PSZT thin films grown by a soft chemical process undergo a diffuse type ferroelectric phase transition. The broadness in dielectric constant

5 Effect of strontium addition on ferroelectric phase transition of PZT thin films prepared by chemical route 697 Fig. 4 Aplotofln(1/ε 1/ε max ) as a function of ln(t T c )for Pb 1 x Sr x Zr 0.30 Ti 0.70 O 3 thin films with different compositions Fig. 5 The variation of γ and δ as a function of composition for the Pb 1 x Sr x Zr 0.30 Ti 0.70 O 3 thin films Fig. 3 The temperature dependence of the dielectric constant for the PSZT thin films as a function of the frequency Fig. 6 Dielectric constant voltage characteristic curves of the PSZT10 thin films at different temperatures at 100 khz

6 698 F.M. Pontes et al. Acknowledgements The authors gratefully acknowledge the financial support of the Brazilian financing agencies FAPESP/CEPID. FAPESP process No. 06/ and No. 08/ References Fig. 7 Dielectric constant voltage characteristic curves of the PSZT20 thin films at different temperatures at 100 khz Fig. 8 Dielectric constant voltage characteristic curves of the PSZT30 thin films at different temperatures at 100 khz versus temperature curves occurs due a variety of possible factors, including the cationic disorder induced by A site substitution of the perovskite structure, grain size and compositional fluctuation. This type of behavior confirms the diffuse type phase transitions in the here studied thin films in the Curie temperature range. It is due to the coexistence of paraelectric (non polar) and ferroelectric (polar) phases in this temperature range. On the other hand, the dielectric constant values as a function of temperature and frequency revealed that the ferroelectric phase transition is not relaxortype, taking into account that the transition temperature was independent on the frequency (classical ferroelectric). Also, the diffusivity was observed to increase with increasing Sr 2+ contents in the studied composition range. 1. S. Muensit, P. Sukwitsu, P. Khaenamkeaw, S.B. Lang, Appl. Phys. A 92, 659 (2008) 2. T. Wei, Y. Wang, C. Zhu, X.W. Dong, Y.D. Xia, J.S. Zhu, J.M. Liu, Appl. Phys. A 90, 185 (2008) 3. U. Ellerkmann, T. Schneller, C. Nauenheim, U. Böttger, R. Waser, Thin Solid Films 516, 4713 (2008) 4. S.B. Majumder, B. Roy, R.S. Katiyar, S.B. Krupanidhi, J. Appl. Phys. 90, 2975 (2001) 5. P.Goel,K.L.Yadav,J.Mat.Sci.42, 3928 (2007) 6. R. Eitel, C.A. Randall, Phys. Rev. B 75, (2007) 7. W.S. Kim, S.M. Ha, H.H. Park, C.E. Kim, Thin Solid Films 355, 531 (1999) 8. F.M. Pontes, S.H. Leal, E.R. Leite, E. Longo, P.S. Pizani, A.J. Chiquito, J.A. Varela, J. Appl. Phys. 96, 1192 (2004) 9. S.H. Leal, F.M. Pontes, E.R. Leite, E. Longo, P.S. Pizani, A.J. Chiquito, M.A.C. Machado, J.A. Varela, Math. Chem. Phys. 87, 353 (2004) 10. D.Y. Wang, Y.L. Cheng, J. Wang, X.Y. Zhou, H.L.W. Chan, C.L. Choy, Appl. Phys. A 81, 1607 (2005) 11. M. Es-Souni, A. Piorra, C.H. Solterbeck, S. Iakovlev, M. Abed, J. Electroceram. 9, 127 (2002) 12. P. Gerber, U. Bottger, R. Waser, J. Appl. Phys. 100, (2006) 13. K. Ramam, M. Lopez, J. Phys. D Appl. Phys. 39, 4466 (2006) 14. C. Yang, J. Liu, S. Zhang, Z. Chen, Math. Sci. Eng. B 99, 356 (2003) 15. J. Ouyang, J. Slusker, I. Levin, D.M. Kim, C.B. Eom, R. Ramesh, A.L. Roytburd, Adv. Funct. Mater. 17, 2094 (2007) 16. Q.Y. Shao, A.D. Li, Y. Dong, F. Fang, J.Q. Jiang, Z.G. Liu, J. Phys. D Appl. Phys. 40, 3793 (2007) 17. Y. Wang, Q.Y. Shao, J.-M. Liu, App. Phys. Lett. 88, (2006) 18. P.W. Leech, A.S. Holland, S. Sriram, M. Bhaskaran, App. Phys. A 91, 679 (2008) 19. Z.A. Ansari, T.G. Ko, J.-H. Oh, Surf. Coat. Technol. 179, 182 (2004) 20. S. Sriram, M. Bhaskaran, A.S. Holland, Semicond. Sci. Technol. 21, 1236 (2006) 21. Q.-Y. Shao, A.-D. Li, Y.-D. Xia, D. Wu, Z.-G. Liu, N.-B. Ming, J. Appl. Phys. 100, (2006) 22. A. Dalakoti, A. Bandyopadhyay, S. Bose, J. Am. Ceram. Soc. 89, 1140 (2006) 23. C.E.F. Costa, F.M. Pontes, A.G. Souza, E.R. Leite, P.S. Pizani, E. Longo, Appl. Phys. A. 79, 593 (2005) 24. B.P. Pokharel, D. Pandey, J. Appl. Phys. 88, 5364 (2000) 25. R. Ganesh, E. Goo, J. Am. Ceram. Soc. 80, 653 (1997) 26. S. Halder, T. Schneller, U. Bottger, R. Waser, Appl. Phys. A 81, 25 (2005) 27. Y. Park, W.J. Lee, H.G. Kim, J. Phys. Condens. Matter 9, 9445 (1997) 28. P. Victor, R. Ranjith, S.B. Krupanidhi, J. Appl. Phys. 94, 7702 (2003) 29. B. Jimenez, R. Jimenez, Phys. Rev. B 66, (2002) 30. J.W. Zhai, D. Hu, X. Yao, Z.K. Xu, H. Chen, J. Eur. Ceram. Soc. 26, 1917 (2006) 31. P. Venkateswarlu, S.S.N. Bharadwaja, S.B. Krupanidhi, Thin Solid Films 389, 84 (2001) 32. M.A. Mohiddon, A. Kumar, K.L. Yadav, Physica B 395, 1 (2007) 33. Y. Yu, R.N. Singh, J. Appl. Phys. 88, 7249 (2000)