APPLIED PHYSICS LETTERS 86, 082906 共2005兲
Growth, structure, and properties of all-epitaxial ferroelectric „Bi, La…4Ti3O12 / Pb„Zr0.4Ti0.6…O3 / „Bi, La…4Ti3O12 trilayered thin films on SrRuO3-covered SrTiO3„011… substrates Dinghua Bao,a兲 Sung Kyun Lee, Xinhua Zhu, Marin Alexe, and Dietrich Hesse Max Planck Institute of Microstructure Physics, Weinberg 2, D-06120 Halle (Saale), Germany
共Received 19 October 2004; accepted 17 December 2004; published online 18 February 2005兲 All-epitaxial 共Bi, La兲4Ti3O12共BLT兲 / Pb共Zr, Ti兲O3共PZT兲 / 共Bi, La兲4Ti3O12 trilayered ferroelectric thin films were prepared on SrRuO3 共SRO兲-covered SrTiO3共011兲 substrates by pulsed-laser deposition. Epitaxial relationships were identified to be BLT共118兲 储 PZT共011兲 储 SrTiO3共011兲, and ¯¯10兴 储 PZT关100兴 储 SrTiO 关100兴. Atomic force microscopy observation of the surface showed BLT关1 3 that the upper BLT layer is composed of rod-like grains. Cross-sectional transmission electron microscopy investigations revealed ferroelectric 90° domains in the PZT layer, as well as a rather smooth morphology of the BLT/PZT interfaces. Remanent polarization and coercive field of the trilayered films were 28.1 C / cm2 and 33.7 kV/ cm, respectively. The thin films showed a high fatigue resistance at least up to 1010 switching pulse cycles. Obviously, a trilayered structure combines the advantages of PZT and BLT, indicating that the all-epitaxial BLT/PZT/BLT trilayered structure is a promising material combination for ferroelectric memory device applications. © 2005 American Institute of Physics. 关DOI: 10.1063/1.1864248兴 Ferroelectric thin films have attracted much attention for their applications in a variety of devices such as ferroelectric memories, infrared pyroelectric sensors, and microelectromechanical systems.1,2 Pb共Zr, Ti兲O3 共PZT兲 and 共Bi, La兲4Ti3O12 共BLT兲 are two important materials, especially for ferroelectric memory devices. The former exhibits excellent ferroelectric properties, and the latter is essentially fatigue-free.3,4 However, PZT films on Pt electrodes for ferroelectric random access memories have several degradation problems such as severe polarization fatigue after bipolar switching pulses, whereas BLT films show relatively low remanent polarization. In recent years, it was shown that multilayered dielectric/ferroelectric thin films such as BaTiO3 / SrTiO3, PZT/ PbTiO3, and PbTiO3 / PbZrO3 have outstanding electrical properties,5–7 and compositionally graded PZT, 共Pb, Sr兲TiO3, and 共Pb, La兲TiO3 thin films have unconventional ferroelectric properties.8–10 In a previous work, a multilayered structure in which a PZT layer was sandwiched between two BLT layers was proposed by Bao and coworkers, and an attempt to prepare the sandwich structure on Pt electrodes was made using a chemical solution deposition method.11 The authors combined the fatigue-free properties of BLT thin films and the good ferroelectric properties of PZT films within the sandwich structure, i.e., to combine the individual advantages of PZT and BLT thin films. As a result, good fatigue and retention properties of the structure were obtained, but the remanent polarization was not high due to a rather large thickness of the BLT layers. More recently, Yan and co-workers12 prepared 共Ba0.5Sr0.5兲TiO3 / Pb共Zr0.52Ti0.48兲O3 / 共Ba0.5Sr0.5兲TiO3兲 trilayered films on Pt/ Ti/ SiO2 / Si. However, obviously, 共Ba0.5Sr0.5兲TiO3 is not an ideal choice for the trilayer struca兲
Present address: State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics, Science and Engineering, Sun Yat-Sen University, Guangzhou 510275, People’s Republic of China; electronic mail:
[email protected].
ture since it is not in a ferroelectric phase, although it can, to some extent, improve the fatigue properties. The present work is an effort to study all-epitaxial BLT/PZT/BLT thin films. We expected that the epitaxial trilayered structure will exhibit better comprehensive ferroelectric properties compared with single BLT thin films and single PZT thin films. In addition, fundamental research on the combination of these two important ferroelectric materials, PZT and BLT, is still lacking. This letter reports on the preparation of allepitaxial BLT/PZT/BLT trilayered structures on SROcovered 共011兲 STO substrates by pulsed-laser deposition 共PLD兲 and their resultant ferroelectric properties. It was demonstrated that the all-epitaxial trilayered structures showed good ferroelectric properties and high fatigue resistance at least up to 1 ⫻ 1010 switching pulse cycles. The BLT and PZT layers were prepared on SRO-covered STO共011兲 substrates by PLD using a KrF excimer laser 共
FIG. 1. X-ray – 2 diffraction pattern of a BLT/PZT/BLT trilayered thin film with the thickness of BLT layers of 20 nm, and the PZT layer of 520 nm, respectively, on a SRO-covered 共011兲-oriented SrTiO3 substrate. The peaks labeled 共P兲, 共B兲, 共S兲, 共䊐兲, 共兲, and 共兲 are diffraction peaks from PZT Cu K␣1, BLT Cu K␣1, SRO Cu K␣1, STO Cu K␣1, PZT Cu K1, and STO Cu K1, respectively. A small unknown peak is labeled 共ⴱ兲. The inset is the schematic diagram of the trilayered BLT/PZT/BLT thin film capacitor.
0003-6951/2005/86共8兲/082906/3/$22.50 86, 082906-1 © 2005 American Institute of Physics Downloaded 23 Feb 2005 to 195.37.184.165. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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FIG. 2. X-ray pole figures of a BLT/PZT/BLT thin film on 共011兲-oriented SrTiO3 substrate. The fixed 2 angles were 30.13° and 38.4° corresponding to BLT兵117其 and PZT兵111其, respectively. The rim of the pole figures corresponds to = 90°.
= 248 nm兲 and ceramic targets of 共Bi3.25La0.75兲Ti3O12 and Pb1.3共Zr0.4Ti0.6兲O3. The substrates were placed parallel to the targets at a distance of 4.5 cm. The substrate temperature and oxygen pressure for the growth of BLT layers were kept at 750 ° C and 0.4 mbar, respectively, whereas the PZT layer was deposited in 0.2 mbar oxygen pressure at a substrate temperature of 600 ° C. After deposition of all the layers, the samples were cooled down to room temperature at an oxygen pressure of 0.4 mbar to prevent losses of bismuth and lead. The thickness of the PZT layer was between 500 nm and 1 m, and that of each of the BLT layers between 20 and 230 nm, with an emphasis on the lower value of the latter. For the electrical characterization, Pt top electrodes were deposited through a stainless steel shadow mask by rf sputtering. The inset of Fig. 1 shows a schematic diagram of the all-epitaxial BLT/PZT/BLT trilayered thin film capacitor. The crystallographic orientation and epitaxial relations of the trilayered films were characterized by x-ray diffraction 共XRD兲 – 2 scans and pole figure measurements using a Philips X’Pert MRD four-circle diffractometer with Cu K␣ radiation. Figure 1 shows an XRD – 2 scan. Only the 共011兲 and 共022兲 peaks of PZT, SRO, and STO, and the 共2216兲 peak of BLT were observed, indicating that both the PZT layer and the SRO bottom electrode showed a 共011兲 orientation and that the BLT layers were 共118兲 oriented. It is worth mentioning that 共011兲-oriented STO was selected as a substrate in this study, rather than 共001兲 or 共111兲-oriented STO, because BLT on 共001兲 STO grows c-axis oriented with a very low remanent polarization, whereas BLT on 共111兲 STO exhibits a 共104兲 orientation with a rough surface which is not suitable for a trilayered structure.13 As is well-known, epitaxial BLT thin films with 共118兲 orientation have better ferroelectric properties than 共001兲-oriented BLT films.13,14 Pole figure analyses were carried out to determine whether the BLT/PZT/BLT trilayered structure is allepitaxial and to identify the crystallographic orientations. Figure 2 shows two pole figures of a BLT/PZT/BLT thin film on SRO-covered STO共011兲 substrate. The 2 angles for the pole figure measurements were fixed at 共a兲 30.13° and 共b兲 38.4° corresponding to the BLT兵117其 and PZT兵111其 planes, respectively. As shown in Fig. 2共a兲, there are two sets of three peaks at ⬇ 5°, ⬇ 63°, and ⬇ 84°, corresponding to ¯ 17兲 / 共11 ¯ 7兲, and 共1 ¯¯17兲 reflections of BLT, respec共117兲, 共1 tively. The two sets of peaks indicate the presence of double twins as reported previously for 共118兲-oriented BLT films on STO.13 From Fig. 2共b兲, it can be seen that there are two sets of two peaks at ⬇ 35° and ⬇ 45°, corresponding to 共111兲
FIG. 3. 共a兲 AFM image of a BLT/PZT/BLT trilayered film with thicknesses of PZT of 520 nm, and BLT of 20 nm each, on 共011兲 SrRuO3 / SrTiO3 substrate. 共b兲 TEM image of a BLT/PZT/BLT trilayered film with PZT thicknesses of around 1 m, and BLT thickness of around 225 nm each, on 共011兲 ¯ 1 PZT. SrTiO3 substrate. Electron beam direction in 共b兲 is 01
PZT and 共0014兲 BLT reflections, respectively, due to the overlapping of the 2 angles of 共111兲 PZT and 共0014兲 BLT. The two sets of peaks also indicate the epitaxial growth of the PZT layer and the BLT layers with two in-plane orientations with 180° symmetry, the BLT 共118兲 and PZT 共011兲 planes being parallel to the STO 共011兲 substrate surface. Based on the above analyses, the epitaxial relationship was determined as follows: BLT共118兲 储 PZT共011兲 储 SrTiO3共011兲; ¯¯10兴 储 PZT关100兴 储 SrTiO 关100兴. BLT关1 3 An atomic force microscope 共Digital Instruments D5000兲 was used for the characterization of the surface mor-
FIG. 4. P – E hysteresis loop of a trilayered BLT/PZT/BLT film with thicknesses of PZT of 520 nm, and BLT of 20 nm each. Downloaded 23 Feb 2005 to 195.37.184.165. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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FIG. 5. Fatigue curve of a trilayered BLT/PZT/BLT film.
phology, see Fig. 3共a兲. The 共118兲-oriented upper BLT layer shows rod-shaped grains arranged mostly along one direction. The width and length of the rod-shaped grains were about 100 and 380 nm, respectively, resulting in an aspect ratio of about 4. These sizes, and the rod-like shape, have been confirmed by a plan-view transmission electron microscopy 共TEM兲 investigation,15 thus also revealing that the elongated shapes in Fig. 3共a兲 are not due to AFM drift. TEM cross-section analysis confirmed that a thick BLT/PZT/BLT trilayered film had a planar, sharp BLT/STO interface and two fairly smooth BLT/PZT interfaces, Fig. 3共b兲. In the PZT layer, ferroelectric 90° domain patterns are visible in Fig. 3共b兲 as well. A TEM image 共not shown兲 taken along an electron beam direction at 45° to the one used in Fig. 3共b兲 revealed a somewhat higher roughness of the BLT/PZT interfaces, a fact that can be explained by the morphology of the BLT layers, see Ref. 15. The polarization versus electric field 共P – E兲 properties of the trilayer films on 共011兲 SRO/STO substrates were measured by a TF Analyzer 2000 ferroelectric tester 共Aix-ACCT兲 at a frequency of 100 Hz. Figure 4 shows a hysteresis loop of the Pt/BLT/PZT/BLT/SRO capacitor measured at an applied voltage of 6 V. From Fig. 4, the remanent polarization and coercive field were deduced as 28.1 C / cm2 and 33.7 kV/ cm, respectively. The measured remanent polarization value of the trilayered structure is comparable to that of some single epitaxial PZT thin films 共about 30– 40 C / cm2兲16 and much higher than that of single epitaxial 共118兲 BLT thin films 共about 10 C / cm2兲13 and single epitaxial 共116兲 SrBi2Ta2O9 films 共about 5 C / cm2兲 prepared by PLD.17 The coercive field is lower than that of BLT films 共about 80 kV/ cm兲 and is comparable to that of PZT thin films 共about 40 kV/ cm兲 reported previously.13,16 These results show that our trilayered thin films have good ferroelectric properties. Ferroelectric fatigue tests were performed applying pulses of 1 MHz to a Pt/BLT/PZT/BLT/SRO/STO capacitor. Figure 5 shows the normalized polarization as a function of polarization switching cycles. The trilayered structure exhibited good fatigue properties up to 1 ⫻ 1010 switching cycles. Compared with Pt/ PZT/ Pt, which exhibits large polarization degradation after 107 switching cycles, the fatigue properties of the trilayered structure are greatly improved. The origin of the good polarization fatigue characteristics of trilayered
BLT/PZT/BLT structures had been analyzed before in Ref. 11, and can be summarized as follows: 共i兲 oxygen vacancies in the PZT intermediate layer are absorbed by the upper and/or lower BLT layers, since the 共Bi2O2兲2+ layers in the Bi-layered perovskite structure can provide a net “pool” to compensate for the charge accumulation near the PZT/BLT interfaces. 共ii兲 The PZT intermediate layer did not come into contact with the upper Pt electrode; thus, the interfaces between the ferroelectric films and the electrodes are BLT/Pt and BLT/SRO. In turn BLT films on Pt electrode and on oxide electrode are fatigue-free. In conclusion, all-epitaxial trilayered 共Bi, La兲4Ti3O12 / Pb共Zr, Ti兲O3 / 共Bi, La兲4Ti3O12 thin films were grown on SrRuO3-covered 共011兲 SrTiO3 substrates using pulsed-laser deposition. The trilayered structure exhibited well-defined hysteresis loops, with a remanent polarization of 28.1 C cm−2 and a coercive field 共Ec兲 of 33.7 kV cm−1. The films showed fatigue-free characteristics up to 1 ⫻ 1010 switching bipolar pulses. These results revealed that this trilayer structure is a promising material combination for ferroelectric memory applications. The optimization of the processing parameters and the effects of the thickness of the BLT layer on the electrical properties of the trilayered BLT/PZT/BLT thin films are currently under investigation to further improve the ferroelectric properties. This work was financially supported by DFG via the Group of Researches FOR 404 at Martin-Luther-Universität Halle-Wittenberg. D.H.B. and X.H.Z. gratefully acknowledge support from the Alexander von Humboldt Foundation, Germany. J. F. Scott and C. A. Paz de Araujo, Science 246, 1400 共1989兲. B. H. Park, B. S. Kang, S. D. Bu, T. W. Noh, J. Lee, and W. Jo, Nature 共London兲 401, 682 共1999兲. 3 O. Auciello, J. F. Scott, and R. Ramesh, Phys. Today 51, 22 共1998兲. 4 H. N. Lee, D. Hesse, N. Zakharov, and U. Gösele, Science 296, 2006 共2002兲. 5 L. Kim, D. Jung, J. Kim, Y. S. Kim, and J. Lee, Appl. Phys. Lett. 82, 2118 共2003兲. 6 J. C. Jiang, X. Q. Pan, W. Tian, C. D. Theis, and D. G. Schlom, Appl. Phys. Lett. 74, 2851 共1999兲. 7 J. V. Mantese, N. W. Schubring, A. L. Micheli, A. B. Catalan, M. S. Mohammed, R. Naik, and G. W. Auner, Appl. Phys. Lett. 71, 2047 共1997兲. 8 D. H. Bao, N. Wakiya, K. Shinozaki, N. Mizutani, and X. Yao, J. Appl. Phys. 90, 506 共2001兲. 9 J. V. Mantese, N. W. Schubring, A. L. Micheli, M. P. Thompson, R. Naik, G. W. Auner, I. B. Misirlioglu, and S. P. Alpay, Appl. Phys. Lett. 81, 1068 共2002兲. 10 D. H. Bao, N. Mizutani, X. Yao, and L. Y. Zhang, Appl. Phys. Lett. 77, 1041 共2000兲. 11 D. H. Bao, N. Wakiya, K. Shinozaki, and N. Mizutani, J. Phys. D 35, L1 共2002兲. 12 F. Yan, Y. N. Wang, H. L. W. Chan, and C. L. Choy, Appl. Phys. Lett. 82, 4325 共2003兲. 13 H. N. Lee and D. Hesse, Appl. Phys. Lett. 80, 1040 共2002兲. 14 T. Sakai, T. Watanabe, H. Funakubo, K. Aaito, and M. Osada, Jpn. J. Appl. Phys., Part 1 42, 166 共2003兲. 15 X. H. Zhu, D. H. Bao, M. Alexe, and D. Hesse, accepted by Appl. Phys. A: Mater. Sci. Process. Published online, DOI: 10.1007/s00339-0043111-2. 16 W. B. Wu, K. H. Wong, C. L. Choy, and Y. H. Zhang, Appl. Phys. Lett. 77, 3441 共2000兲. 17 H. N. Lee, A. Visinoiu, S. Senz, C. Harnagea, A. Pignolet, D. Hesse, and U. Gösele, J. Appl. Phys. 88, 6658 共2000兲. 1 2
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