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Preferred orientation controlling of PZT (52–48) thin films prepared by sol–gel process
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Journal of Crystal Growth 285 (2005) 627–632 www.elsevier.com/locate/jcrysgro
Preferred orientation controlling of PZT (52–48) thin films prepared by sol–gel process Zhu Chen, Chentao Yang, Bo Li, Mingxia Sun, Bangchao Yang College of Microelectronics and Solid State Electronics, University of Electronic Science and Technology of China, Chengdu 610054, People’s Republic of China Received 25 June 2005; received in revised form 12 September 2005; accepted 13 September 2005 Communicated by D.P. Norton
Abstract This paper systematically studied the factors including Pb concentration of precursor, PT seeding layer and TiO2 and ZrO2 seeding layers, which influence greatly the crystal orientation of lead zirconate titanate (PZT, Zr/Ti ¼ 52/48) thin films fabricated by sol–gel process. We find that the PZT films deposited by precursor with 20% mole excess Pb displayed strong (1 1 1) preferred orientation, with 5% mole excess Pb showed a little (1 0 0) orientation and pyrochlore phase. PT seeding layer was found prompting the PZT films phase transformation with (1 1 0) preferred orientation. In addition, the results show that the TiO2 and ZrO2 seeding layers had totally different effects on the preferred orientation of PZT films. The films with TiO2 seeding layer were highly (1 1 1) oriented and exhibited better ferroeletric properties (remnant polarization Pr ¼ 14.2 mC cm2, coercive field Ec ¼ 59.1 kV cm1) than those of the films with ZrO2 seeding layer shown (1 0 0) orientation (Pr ¼ 7.4 mC cm2, Ec ¼ 42.9 kV cm1). r 2005 Elsevier B.V. All rights reserved. Keywords: A1. Preferred orientation; A3. Sol–gel; B1. PZT; B2. Ferroelectric films
1. Introduction PbZr1xTixO3 (PZT) thin films with preferred orientation are currently being considered for applications in electrooptic devices, non-volatile Corresponding author. Tel.: +86 28 83259930/83208048; fax: +86 28 83202139. E-mail addresses: [email protected], [email protected] (Z. Chen).
memory, and dynamic access memory. A wide variety of preparation techniques have been employed to produce PZT thin films such as pulsed laser deposition [1–3], ion plasma sputtering [4], electron beam evaporation [5], metalorganic chemical vapor deposition (MOCVD) [6–8], RF magnetron sputtering [9], molecular beam epitaxy [10,11], sol–gel process [12,13]. Among these methods, the sol–gel process offers numerous advantages, including low processing temperature,
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.09.053
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excellent compositional control, uniform homogeneity, ease of fabrication over large areas, and low cost. Generally, the electrical properties of ferroelectric thin films are strong dependent on the surface morphology and microstructure of the films such as phase structures, orientations of crystalline phase [14–16]. Many factors may affect the orientations in the sol–gel derived PZT thin films, such as substrates, chemistry of the precursor solution, seeding layers, annealing temperature and ambient, heating rate and so on. It is preferable to obtain highly oriented PZT thin films because they are reported to provide higher ferroelectric properties from Yamauchi and Yoshimaru [17]. Therefore, it is necessary for us to systemically investigate the orientations of crystalline phase and develop methods for the control of the preferred orientation of PZT thin films. Especially there have been few reports on the PZT films orientation relationship with Pb concentration and PT, TiO2, ZrO2 seeding layers.
2. Experimental details The raw materials are lead acetate trihydrate [Pb(OCOCH3)2 3H2O, 99.5%], zirconium nitrate [Zr(NO3)4 5H2O, 99.5%] and titanium tetrabutoxide [Ti(OC4H9)4,98%]. Because zirconium alkoxide is relative expensive, it was replaced by inorganic zirconium salt. Precursor-monomers (Pb2+, Zr4+ and Ti4+) and PZT precursor were prepared by the precursor-monomer method of sol–gel. Detailed solution preparation procedures were described elsewhere [18]. The 0.2–0.4 M PZT or PT precursors contained 5–20% mole excess Pb were spin coated onto Si(1 0 0) and Pt(1 1 1)/Ti/SiO2/Si(1 0 0) substrates, respectively, at 4000 rev min1 for 30 s followed by drying at 150 1C for 10 min for solvent volatilization, at 350 1C for 10 min for organic decomposition. The above coating-pyrolysis process cycle was repeated by several times to obtain films with desired thickness. Finally, the films were crystallized through a rapid thermal annealing process at 600–650 1C for 15 min in air to form the perovskite structure PZT thin films.
3. Results and discussion 3.1. Pb concentration It is well known that Pb of the PZT films will volatilize inevitably during the final annealing process. Therefore, when preparing the PZT solution, some amount of excess Pb is adopted intentionally to compensate for the volatilization of Pb. Up to now, there are no reports on analyzing the effect of excess Pb on films orientation by our knowledge. To examine the relationship between the films orientation and the Pb concentration of PZT precursor sols, the PZT precursors contained 5%, 10%, 20% mole excess Pb were deposited onto Si(1 0 0) substrates, respectively, to fabricate the PZT thin films labeled as (a), (b), and (c). And then the films were annealed at 650 1C for 15 min in air to form the perovskite structure. Crystallization structure of the amorphous films into the perovskite phase and preferred orientations were investigated using Xray diffraction (XRD, K-Alpha wavelength 1.54056 A˚). From the X-ray diffraction patterns shown in Fig. 1, all PZT thin films were crystallized into perovskite phase. But there was some difference shown in the preferred orientation and relative intensities of peaks. With the increasing of Pb concentration, the preferred orientation of PZT films changed from (1 0 0), (1 1 0) to (1 1 1). The film (b) with moderate excess-Pb was randomly orientated; the film(c) with relative high concentration of excess-Pb was observed high (1 1 1) orientation. In contrast, film (a) with relative deficient-Pb was observed (1 0 0) orientation with low intensity of all the peaks except (1 0 0) plane, and pyrochlore phase was also detected in film (a). The orientation analysis data (the relative intensity percent) of the examined PZT films are listed in Table 1. For film (c), the relative intensity percent of the strongest peak (1 1 1) plane was 62.6%, whereas those figures for the second and the third strong peaks were about 10%. That also confirmed the film (c) is strong (1 1 1) preferred oriented. During phase transition from amorphous to perovskite phase, intermediate phases played a key
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role as nucleation sites to determine the orientation of PZT thin film [6,7]. When the PZT films were deposited by precursors of high concentration Pb, e.g. 20% mole excess Pb in film(c), Pb–Pt intermetallic phase easily formed and enhanced (1 1 1) oriented films. The film (b) with moderate mole excess Pb that was near to the amount of PbO evaporated under 650 1C annealing treatment inclined to crystallize into (1 1 0) orientation polycrystalline structures as powder and bulk PbZr0.52Ti0.48 materials. When the PZT film (a) with relative deficient-Pb was annealed at 650 1C, PbO started to evaporate, and the composition of PZT films changed quickly with the evaporation of PbO, resulting in formation of pyrochlore phases and (1 0 0) preferred orientation films. For (1 0 0) plane, the grain nucleation energy was substantially low, (1 0 0) phase was energetically favorable by the previous report of Tani et al. [19].
film(a)
Intensity (arb. units)
(110) (111)
(100)
(200)
(211) film(b)
film(c) 20
30
40
50
60
2θ(°)
Fig. 1. The XRD patterns of the PZT films with different mole excess Pb (a) 5%, (b) 10%, and (c) 20%.
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3.2. PT seeding layer PbTiO3 (PT) is ferroelectric material with high coercive field (Ec) and easily transforms into perovskite structure. In contrast, PZT(Zr/ Ti ¼ 0.52/0.48) that possesses high dielectric constanter, low coercive field and high crystallization temperature (700 1C) is difficult to form perovskite structure [20,21]. PT can directly transform into stable perovskite phase from amorphous, whereas zirconium-richer materials, like PZT and PZ, first transform into metastable, nanocrystalline pyrochlore phase, and then transform to perovskite structure at higher temperatures, therefore, PT seeding layers are introduced to our experiment in order to prompt the PZT films transformation. PT and PZT precursor contained 10% mole excess Pb were deposited on Si(1 0 0) substrates. The fabrication process of PZT films was described in experimental details. Fig. 2 compared the XRD patterns of the PZT thin films with and without PT seeding layer. It was shown that film (a) with PT seeding layer was high (1 1 0) preferred orientation, and the relative intensities of (1 0 0), (1 1 0), (1 1 1), (2 0 0) and (2 1 1) peaks in sample (a) were higher than those in sample (b), whereas the intensities of all peaks except (1 0 0) plane in sample (b) were observed rather low, the films (b) showed a little (1 0 0)preferred orientation. The results implied that sample (b) annealed at 600 1C for 15 min in air was not completely transformed into perovskite structure, accordingly, pyrochlore phase was still detected. Therefore, under the relative low annealing temperature condition, e.g. 600 1C, the PZT film (a) deposited on PT seeding layer easily crystallized into perovskite structure with (1 1 0) preferred orientation. Because the (1 0 0) plane is
Table 1 The percentage of relative intensity of PZT films
Film (a) Film (b) Film (c)
Excess (Pb/mol %)
(1 0 0) (%)
(1 1 0) (%)
(1 1 1) (%)
(2 0 0) (%)
(2 1 1) (%)
0 10 20
31.2 13.8 10.3
20.9 42.8 8.1
19.5 19.3 62.6
18.4 12.9 11.6
9.9 11.3 7.5
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layers in film (a) prompted the crystallization of perovskite phase and grains growth.
the lowest surface energy of PZT films, so PZT films without PT seeding layers always transformed into (1 0 0) plane orientation. When adding with PT seeding layers that were easily crystallized into perovskite structure, the PZT films phase transformation was greatly promoted and modified by the PT seeding layers for which offered nucleation sites and reduced active energy. Therefore PZT films with PT seeding showed high (1 1 0)-preferred orientation. Surface morphology and microstructure of the PZT films were investigated by atomic force microscopy (AFM). Fig. 3 shows that film (a) with PZT/PT structure has much bigger grain size than that of film (b). The average grain sizes of film (a) and film (b) are about 84, 49 nm, respectively. It also confirmed that the PT seeding
3.3. TiO2 and ZrO2 seeding layer In order to study the influence of the different metal-oxide seeding layers on the PZT crystallization and preferred orientation, the Ti and Zr precursor-monomers (Zr4+ and Ti4+) were deposited, prior to the PZT precursor on Pt(1 1 1)/Ti/ SiO2/Si (1 0 0) substrate and then annealed at 650 1C for 15 min in O2 flow to form TiO2 and ZrO2 seeding layers, respectively. The fabrication process of PZT films was as previously described. Fig. 4 shows the XRD patterns of PZT thin films with TiO2, ZrO2 seeding layers and without seeding layer respectively labeled (a), (b), and (c). From Fig. 4, we can see that the TiO2 seeding layer and ZrO2 seeding layer have totally different effects on the PZT films crystallization and orientation. The TiO2 seeding layer has clearly a positive impact on the crystallization process. The film (a) with TiO2 seeding layer showed strong (1 1 1) preferred orientation as compared with a little (1 0 0) orientation of film (b) with ZrO2 seeding layer, and random orientation of film (c) without seeding layer. The reasons for the preferential film orientation are not understood completely at present. As for the sol–gel processing, it involves an amorphous layer at the initial stage followed by a crystallization step during the final annealing treatment. The nuclei may form at the interface between the film and the substrate. The physical mechanisms of the thin TiO2 seeding
Film (a)--PZT/PT/Si(100)
(110)
Film (b)--PZT/Si(100) (100) Intensity (arb. units)
(111)
(200)
(210)
(211)
(a)
(b)
20
30
40 2θ(°)
50
60
Fig. 2. The XRD patterns of the PZT films with and without PT seeding layer.
0 0
0 0
500
500
500 (a)
500 [nm]
(b)
[nm]
Fig. 3. AFM images of PZT films with and without PT seeding layer.
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Intensity (arb. units)
(111)
(100)
(110)
Table 2 Rerroelectric properties of PZT films with or without seeding layers
(200)
(210)
(211) (a)
(b)
(c) 20
30
40 2θ(°)
631
50
60
Fig. 4. The XRD patterns of the PZT films with different seeding layers (a) with TiO2, (b) with ZrO2, and (c) without seeding layer.
layer promoting the (1 1 1) perovskite phase crystallization and thin ZrO2 seeding layer promoting the (1 0 0) perovskite phase crystallization seem to be complicated, and are still not fully elucidated. Nonetheless, we speculate that such crystallization is related to the minimization of the surface energies by the TiO2 layer and to lattice matching with Pt (1 1 1) substrate. The thin TiO2 seeding layer increases the number of active sites for PZT nucleation and, hence, allows crystallization of the perovskite phase. Benefited from the nucleation effect of the interface with the (1 1 1) Pt substrate, the PZT films with TiO2 seeding layer easily crystallize into (1 1 1) preferred orientation. This result is consistent with the previous studies from Muralt et al. [22] and Bouregba et al. [23] reporting that the (1 1 1) PZT perovskite phase orientation was obtained with a TiO2 or TiOx buffer layer. On the other hand, the transformation energies of ZrO2 are much higher than that of TiO2. When PZT films were annealed at the same temperature, relative low nucleation energy was obtained for ZrO2 seeding layer, so the effects of ZrO2 layer on promoting PZT films crystallization and transfor-
Pr (mc cm2) Ec (kV cm1) Thickness (nm)
Film (a)
Film (b)
Film (c)
14.2 59.1 300
7.4 42.9 300
9.6 45.7 300
mation was not more obvious than that of TiO2 layer. The (1 0 0) plane has the lowest surface energy by Tani et al. [19] surface energies calculation of (1 0 0), (1 1 0), and (1 1 1) planes for PZT (or PLZT) based on bond strengths and the crystallographic structure. Therefore the PZT film (b) with ZrO2 layer would has a tendency to exhibit (1 0 0) orientation. In order to measure the ferroeletric properties of the PZT films, Au top electrodes were deposited on the substrates by using a shadow mask. Pt was used as the common bottom electrode. The test frequency was 1 KHz at an applied electric voltage of 4 V. The ferroeletric properties of the PZT films were measured by using an RT66A ferroelectric tester from Radiant Technologies. The measured ferroeletric properties of PZT films are listed in Table 2. PZT film (a) with TiO2 seeding layer showed remnant polarization Pr of 14.2 mC cm2 that is near a double amount of that PZT film (b) with ZrO2 seeding layer. The experiment results reveal that the use of an oxidized titanium layer (TiO2) leads to a better growth of (1 1 1) preferred orientation perovskite phase and also has better ferroeletric properties of the obtained PZT films.
4. Conclusion PbZr0.52Ti0.48 thin films were successfully deposited on Si (1 0 0) and Pt (1 1 1)/Ti/SiO2/Si (1 0 0) substrates by the sol–gel process. Crystallization and preferred orientation of PZT films are greatly affected by Pb concentration of PZT precursors and seeding layers. The PZT films with 20% mole excess Pb displayed strong (1 1 1) preferred orien-
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tation, whereas the PZT films with relative low Pb concentration showed a little (1 0 0) orientation and the pyrochlore phase was still observed. It was found that PT seeding layer prompted the PZT films phase transformation with (1 1 0) preferred orientation, accordingly lowered perovskite phase crystallization temperature and decrease Pb loss. The results also indicated that TiO2 and ZrO2 seeding layers had totally different effects on preferred orientation of PZT films. The PZT films with TiO2 seeding layers showed high (1 1 1) orientation and better ferroeletric properties with remnant polarization of 14.2 mC cm2 and coercive field of 59.1 kV cm1.
Acknowledgements This work is supported by Chinese national research grant from 973 projects (no. 5130Z02). References [1] J.S. Horwitz, K.S. Grabowski, D.B. Chrisey, R.E. Leuchtner, Appl. Phys. Lett. 59 (1991) 1565. [2] J. Lee, A. Safari, R.L. Pfeffer, Appl. Phys. Lett. 61 (1992) 1643. [3] O. Auciello, J. Appl. Phys. 73 (1993) 5197. [4] R.N. Castellano, L.G. Feinstein, J. Appl. Phys. 50 (1979) 4406. [5] M. Oikawa, K. Toda, Appl. Phys. Lett. 29 (1976) 491.
[6] (a) C.H. Peng, S.B. Desu, Appl. Phys. Lett. 61 (1) (1992) 16. [7] Y. Sakashita, H. Segawa, J. Appl. Phys. 73 (1993) 7857. [8] W. Brau, B.S. Kwak, A. Erbil, Appl. Phys. Lett. 63 (1993) 467. [9] L. Wang, A. Pignolet, F. Levy, Matt. Res. Bull. 25 (1990) 1495. [10] B.E. Park, I. Sakai, E. Tokumitsu, H. Ishiwara, Appl. Surface Sci. 117/118 (1997) 423. [11] T. Nishinaga, J. Crystal Growth 237–239 (2002) 1410. [12] C. Chen, D.F. Ryder, W.A. Spurgeon, J. Am. Ceram. Soc. 72 (8) (1989) 1495. [13] H.M. Jang, M.K. Cho, J. Am. Ceram. Soc. 79 (6) (1996) 1435. [14] J.F. Scott, C.A. Araujo, B.M. Melnick, L.D. McMillan, R. Zuleeg, J. Appl. Phys. 70 (1) (1991) 382. [15] G.A.C.M. Spierings, J.B.A. Van Zon, M. Klee, P.K. Larsen, Integr. Ferroelectron 3 (3) (1993) 280. [16] E. Defay, M. Leberre, B. Semmache, M. Troccaz, Mater. Sci. Eng. B 55 (1998) 123. [17] S. Yamauchi, M. Yoshimaru, Jpn J. Appl. Phys. 35 (1996) 1553. [18] Z. Chen, B.C. Yang, C.T. Yang, S.R. Zhang, J. Funct. Mater. 35 (4) (2004) 417. [19] T. Tani, Z. Xu, D.A. Payne, in: E.R. Myers, B.A. Tuttle, S.B. Desu, P.K. Larsen (Eds.), Ferroelectric Thin films III, MRS 310, Pittsburgh, PA, 1993, p. 269. [20] M.E. Souni, M. Abed, C.H. Solterbeck, A. Piorra, Mater. Sci. Eng. B 94 (2002) 229. [21] M.E. Souni, A. Piorra, Mater. Res. Bull. 36 (2001) 2563. [22] P. Muralt, T. Maeder, L. Sagalowicz, S. Hiboux, J. Appl. Phys. 83 (1998) 3835. [23] R. Bouregba, G. Poullain, B. Vilquin, H. Murray, Mater. Res. Bull. 35 (2000) 1381.
Journal of Crystal Growth 285 (2005) 627–632 www.elsevier.com/locate/jcrysgro
Preferred orientation controlling of PZT (52–48) thin films prepared by sol–gel process Zhu Chen, Chentao Yang, Bo Li, Mingxia Sun, Bangchao Yang College of Microelectronics and Solid State Electronics, University of Electronic Science and Technology of China, Chengdu 610054, People’s Republic of China Received 25 June 2005; received in revised form 12 September 2005; accepted 13 September 2005 Communicated by D.P. Norton
Abstract This paper systematically studied the factors including Pb concentration of precursor, PT seeding layer and TiO2 and ZrO2 seeding layers, which influence greatly the crystal orientation of lead zirconate titanate (PZT, Zr/Ti ¼ 52/48) thin films fabricated by sol–gel process. We find that the PZT films deposited by precursor with 20% mole excess Pb displayed strong (1 1 1) preferred orientation, with 5% mole excess Pb showed a little (1 0 0) orientation and pyrochlore phase. PT seeding layer was found prompting the PZT films phase transformation with (1 1 0) preferred orientation. In addition, the results show that the TiO2 and ZrO2 seeding layers had totally different effects on the preferred orientation of PZT films. The films with TiO2 seeding layer were highly (1 1 1) oriented and exhibited better ferroeletric properties (remnant polarization Pr ¼ 14.2 mC cm2, coercive field Ec ¼ 59.1 kV cm1) than those of the films with ZrO2 seeding layer shown (1 0 0) orientation (Pr ¼ 7.4 mC cm2, Ec ¼ 42.9 kV cm1). r 2005 Elsevier B.V. All rights reserved. Keywords: A1. Preferred orientation; A3. Sol–gel; B1. PZT; B2. Ferroelectric films
1. Introduction PbZr1xTixO3 (PZT) thin films with preferred orientation are currently being considered for applications in electrooptic devices, non-volatile Corresponding author. Tel.: +86 28 83259930/83208048; fax: +86 28 83202139. E-mail addresses: [email protected], [email protected] (Z. Chen).
memory, and dynamic access memory. A wide variety of preparation techniques have been employed to produce PZT thin films such as pulsed laser deposition [1–3], ion plasma sputtering [4], electron beam evaporation [5], metalorganic chemical vapor deposition (MOCVD) [6–8], RF magnetron sputtering [9], molecular beam epitaxy [10,11], sol–gel process [12,13]. Among these methods, the sol–gel process offers numerous advantages, including low processing temperature,
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.09.053
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excellent compositional control, uniform homogeneity, ease of fabrication over large areas, and low cost. Generally, the electrical properties of ferroelectric thin films are strong dependent on the surface morphology and microstructure of the films such as phase structures, orientations of crystalline phase [14–16]. Many factors may affect the orientations in the sol–gel derived PZT thin films, such as substrates, chemistry of the precursor solution, seeding layers, annealing temperature and ambient, heating rate and so on. It is preferable to obtain highly oriented PZT thin films because they are reported to provide higher ferroelectric properties from Yamauchi and Yoshimaru [17]. Therefore, it is necessary for us to systemically investigate the orientations of crystalline phase and develop methods for the control of the preferred orientation of PZT thin films. Especially there have been few reports on the PZT films orientation relationship with Pb concentration and PT, TiO2, ZrO2 seeding layers.
2. Experimental details The raw materials are lead acetate trihydrate [Pb(OCOCH3)2 3H2O, 99.5%], zirconium nitrate [Zr(NO3)4 5H2O, 99.5%] and titanium tetrabutoxide [Ti(OC4H9)4,98%]. Because zirconium alkoxide is relative expensive, it was replaced by inorganic zirconium salt. Precursor-monomers (Pb2+, Zr4+ and Ti4+) and PZT precursor were prepared by the precursor-monomer method of sol–gel. Detailed solution preparation procedures were described elsewhere [18]. The 0.2–0.4 M PZT or PT precursors contained 5–20% mole excess Pb were spin coated onto Si(1 0 0) and Pt(1 1 1)/Ti/SiO2/Si(1 0 0) substrates, respectively, at 4000 rev min1 for 30 s followed by drying at 150 1C for 10 min for solvent volatilization, at 350 1C for 10 min for organic decomposition. The above coating-pyrolysis process cycle was repeated by several times to obtain films with desired thickness. Finally, the films were crystallized through a rapid thermal annealing process at 600–650 1C for 15 min in air to form the perovskite structure PZT thin films.
3. Results and discussion 3.1. Pb concentration It is well known that Pb of the PZT films will volatilize inevitably during the final annealing process. Therefore, when preparing the PZT solution, some amount of excess Pb is adopted intentionally to compensate for the volatilization of Pb. Up to now, there are no reports on analyzing the effect of excess Pb on films orientation by our knowledge. To examine the relationship between the films orientation and the Pb concentration of PZT precursor sols, the PZT precursors contained 5%, 10%, 20% mole excess Pb were deposited onto Si(1 0 0) substrates, respectively, to fabricate the PZT thin films labeled as (a), (b), and (c). And then the films were annealed at 650 1C for 15 min in air to form the perovskite structure. Crystallization structure of the amorphous films into the perovskite phase and preferred orientations were investigated using Xray diffraction (XRD, K-Alpha wavelength 1.54056 A˚). From the X-ray diffraction patterns shown in Fig. 1, all PZT thin films were crystallized into perovskite phase. But there was some difference shown in the preferred orientation and relative intensities of peaks. With the increasing of Pb concentration, the preferred orientation of PZT films changed from (1 0 0), (1 1 0) to (1 1 1). The film (b) with moderate excess-Pb was randomly orientated; the film(c) with relative high concentration of excess-Pb was observed high (1 1 1) orientation. In contrast, film (a) with relative deficient-Pb was observed (1 0 0) orientation with low intensity of all the peaks except (1 0 0) plane, and pyrochlore phase was also detected in film (a). The orientation analysis data (the relative intensity percent) of the examined PZT films are listed in Table 1. For film (c), the relative intensity percent of the strongest peak (1 1 1) plane was 62.6%, whereas those figures for the second and the third strong peaks were about 10%. That also confirmed the film (c) is strong (1 1 1) preferred oriented. During phase transition from amorphous to perovskite phase, intermediate phases played a key
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role as nucleation sites to determine the orientation of PZT thin film [6,7]. When the PZT films were deposited by precursors of high concentration Pb, e.g. 20% mole excess Pb in film(c), Pb–Pt intermetallic phase easily formed and enhanced (1 1 1) oriented films. The film (b) with moderate mole excess Pb that was near to the amount of PbO evaporated under 650 1C annealing treatment inclined to crystallize into (1 1 0) orientation polycrystalline structures as powder and bulk PbZr0.52Ti0.48 materials. When the PZT film (a) with relative deficient-Pb was annealed at 650 1C, PbO started to evaporate, and the composition of PZT films changed quickly with the evaporation of PbO, resulting in formation of pyrochlore phases and (1 0 0) preferred orientation films. For (1 0 0) plane, the grain nucleation energy was substantially low, (1 0 0) phase was energetically favorable by the previous report of Tani et al. [19].
film(a)
Intensity (arb. units)
(110) (111)
(100)
(200)
(211) film(b)
film(c) 20
30
40
50
60
2θ(°)
Fig. 1. The XRD patterns of the PZT films with different mole excess Pb (a) 5%, (b) 10%, and (c) 20%.
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3.2. PT seeding layer PbTiO3 (PT) is ferroelectric material with high coercive field (Ec) and easily transforms into perovskite structure. In contrast, PZT(Zr/ Ti ¼ 0.52/0.48) that possesses high dielectric constanter, low coercive field and high crystallization temperature (700 1C) is difficult to form perovskite structure [20,21]. PT can directly transform into stable perovskite phase from amorphous, whereas zirconium-richer materials, like PZT and PZ, first transform into metastable, nanocrystalline pyrochlore phase, and then transform to perovskite structure at higher temperatures, therefore, PT seeding layers are introduced to our experiment in order to prompt the PZT films transformation. PT and PZT precursor contained 10% mole excess Pb were deposited on Si(1 0 0) substrates. The fabrication process of PZT films was described in experimental details. Fig. 2 compared the XRD patterns of the PZT thin films with and without PT seeding layer. It was shown that film (a) with PT seeding layer was high (1 1 0) preferred orientation, and the relative intensities of (1 0 0), (1 1 0), (1 1 1), (2 0 0) and (2 1 1) peaks in sample (a) were higher than those in sample (b), whereas the intensities of all peaks except (1 0 0) plane in sample (b) were observed rather low, the films (b) showed a little (1 0 0)preferred orientation. The results implied that sample (b) annealed at 600 1C for 15 min in air was not completely transformed into perovskite structure, accordingly, pyrochlore phase was still detected. Therefore, under the relative low annealing temperature condition, e.g. 600 1C, the PZT film (a) deposited on PT seeding layer easily crystallized into perovskite structure with (1 1 0) preferred orientation. Because the (1 0 0) plane is
Table 1 The percentage of relative intensity of PZT films
Film (a) Film (b) Film (c)
Excess (Pb/mol %)
(1 0 0) (%)
(1 1 0) (%)
(1 1 1) (%)
(2 0 0) (%)
(2 1 1) (%)
0 10 20
31.2 13.8 10.3
20.9 42.8 8.1
19.5 19.3 62.6
18.4 12.9 11.6
9.9 11.3 7.5
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630
layers in film (a) prompted the crystallization of perovskite phase and grains growth.
the lowest surface energy of PZT films, so PZT films without PT seeding layers always transformed into (1 0 0) plane orientation. When adding with PT seeding layers that were easily crystallized into perovskite structure, the PZT films phase transformation was greatly promoted and modified by the PT seeding layers for which offered nucleation sites and reduced active energy. Therefore PZT films with PT seeding showed high (1 1 0)-preferred orientation. Surface morphology and microstructure of the PZT films were investigated by atomic force microscopy (AFM). Fig. 3 shows that film (a) with PZT/PT structure has much bigger grain size than that of film (b). The average grain sizes of film (a) and film (b) are about 84, 49 nm, respectively. It also confirmed that the PT seeding
3.3. TiO2 and ZrO2 seeding layer In order to study the influence of the different metal-oxide seeding layers on the PZT crystallization and preferred orientation, the Ti and Zr precursor-monomers (Zr4+ and Ti4+) were deposited, prior to the PZT precursor on Pt(1 1 1)/Ti/ SiO2/Si (1 0 0) substrate and then annealed at 650 1C for 15 min in O2 flow to form TiO2 and ZrO2 seeding layers, respectively. The fabrication process of PZT films was as previously described. Fig. 4 shows the XRD patterns of PZT thin films with TiO2, ZrO2 seeding layers and without seeding layer respectively labeled (a), (b), and (c). From Fig. 4, we can see that the TiO2 seeding layer and ZrO2 seeding layer have totally different effects on the PZT films crystallization and orientation. The TiO2 seeding layer has clearly a positive impact on the crystallization process. The film (a) with TiO2 seeding layer showed strong (1 1 1) preferred orientation as compared with a little (1 0 0) orientation of film (b) with ZrO2 seeding layer, and random orientation of film (c) without seeding layer. The reasons for the preferential film orientation are not understood completely at present. As for the sol–gel processing, it involves an amorphous layer at the initial stage followed by a crystallization step during the final annealing treatment. The nuclei may form at the interface between the film and the substrate. The physical mechanisms of the thin TiO2 seeding
Film (a)--PZT/PT/Si(100)
(110)
Film (b)--PZT/Si(100) (100) Intensity (arb. units)
(111)
(200)
(210)
(211)
(a)
(b)
20
30
40 2θ(°)
50
60
Fig. 2. The XRD patterns of the PZT films with and without PT seeding layer.
0 0
0 0
500
500
500 (a)
500 [nm]
(b)
[nm]
Fig. 3. AFM images of PZT films with and without PT seeding layer.
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Intensity (arb. units)
(111)
(100)
(110)
Table 2 Rerroelectric properties of PZT films with or without seeding layers
(200)
(210)
(211) (a)
(b)
(c) 20
30
40 2θ(°)
631
50
60
Fig. 4. The XRD patterns of the PZT films with different seeding layers (a) with TiO2, (b) with ZrO2, and (c) without seeding layer.
layer promoting the (1 1 1) perovskite phase crystallization and thin ZrO2 seeding layer promoting the (1 0 0) perovskite phase crystallization seem to be complicated, and are still not fully elucidated. Nonetheless, we speculate that such crystallization is related to the minimization of the surface energies by the TiO2 layer and to lattice matching with Pt (1 1 1) substrate. The thin TiO2 seeding layer increases the number of active sites for PZT nucleation and, hence, allows crystallization of the perovskite phase. Benefited from the nucleation effect of the interface with the (1 1 1) Pt substrate, the PZT films with TiO2 seeding layer easily crystallize into (1 1 1) preferred orientation. This result is consistent with the previous studies from Muralt et al. [22] and Bouregba et al. [23] reporting that the (1 1 1) PZT perovskite phase orientation was obtained with a TiO2 or TiOx buffer layer. On the other hand, the transformation energies of ZrO2 are much higher than that of TiO2. When PZT films were annealed at the same temperature, relative low nucleation energy was obtained for ZrO2 seeding layer, so the effects of ZrO2 layer on promoting PZT films crystallization and transfor-
Pr (mc cm2) Ec (kV cm1) Thickness (nm)
Film (a)
Film (b)
Film (c)
14.2 59.1 300
7.4 42.9 300
9.6 45.7 300
mation was not more obvious than that of TiO2 layer. The (1 0 0) plane has the lowest surface energy by Tani et al. [19] surface energies calculation of (1 0 0), (1 1 0), and (1 1 1) planes for PZT (or PLZT) based on bond strengths and the crystallographic structure. Therefore the PZT film (b) with ZrO2 layer would has a tendency to exhibit (1 0 0) orientation. In order to measure the ferroeletric properties of the PZT films, Au top electrodes were deposited on the substrates by using a shadow mask. Pt was used as the common bottom electrode. The test frequency was 1 KHz at an applied electric voltage of 4 V. The ferroeletric properties of the PZT films were measured by using an RT66A ferroelectric tester from Radiant Technologies. The measured ferroeletric properties of PZT films are listed in Table 2. PZT film (a) with TiO2 seeding layer showed remnant polarization Pr of 14.2 mC cm2 that is near a double amount of that PZT film (b) with ZrO2 seeding layer. The experiment results reveal that the use of an oxidized titanium layer (TiO2) leads to a better growth of (1 1 1) preferred orientation perovskite phase and also has better ferroeletric properties of the obtained PZT films.
4. Conclusion PbZr0.52Ti0.48 thin films were successfully deposited on Si (1 0 0) and Pt (1 1 1)/Ti/SiO2/Si (1 0 0) substrates by the sol–gel process. Crystallization and preferred orientation of PZT films are greatly affected by Pb concentration of PZT precursors and seeding layers. The PZT films with 20% mole excess Pb displayed strong (1 1 1) preferred orien-
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tation, whereas the PZT films with relative low Pb concentration showed a little (1 0 0) orientation and the pyrochlore phase was still observed. It was found that PT seeding layer prompted the PZT films phase transformation with (1 1 0) preferred orientation, accordingly lowered perovskite phase crystallization temperature and decrease Pb loss. The results also indicated that TiO2 and ZrO2 seeding layers had totally different effects on preferred orientation of PZT films. The PZT films with TiO2 seeding layers showed high (1 1 1) orientation and better ferroeletric properties with remnant polarization of 14.2 mC cm2 and coercive field of 59.1 kV cm1.
Acknowledgements This work is supported by Chinese national research grant from 973 projects (no. 5130Z02). References [1] J.S. Horwitz, K.S. Grabowski, D.B. Chrisey, R.E. Leuchtner, Appl. Phys. Lett. 59 (1991) 1565. [2] J. Lee, A. Safari, R.L. Pfeffer, Appl. Phys. Lett. 61 (1992) 1643. [3] O. Auciello, J. Appl. Phys. 73 (1993) 5197. [4] R.N. Castellano, L.G. Feinstein, J. Appl. Phys. 50 (1979) 4406. [5] M. Oikawa, K. Toda, Appl. Phys. Lett. 29 (1976) 491.
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