Experimental evidence for orientation property of Pb

APPLIED PHYSICS LETTERS 96, 102905 共2010兲

Experimental evidence for orientation property of Pb„Zr0.35Ti0.65…O3 by manipulating polar axis angle using CaF2 substrate Satoru Utsugi,1 Takashi Fujisawa,1 Yoshitaka Ehara,1 Tomoaki Yamada,1 Masaaki Matsushima,1 Hitoshi Morioka,2 and Hiroshi Funakubo1,a兲 1

Department of Innovative and Engineered Material, Tokyo Institute of Technology, Yokohama 226-8503, Japan 2 Application Laboratory, Bruker AXS, 3-9-A Moriya-cho, Kanagawa-ku, Yokohama 221-0022, Japan

共Received 8 January 2010; accepted 16 February 2010; published online 9 March 2010兲 Perfectly oriented 共001兲, 共101兲, and 共111兲 Pb共Zr0.35Ti0.65兲O3 共PZT兲 films were grown on identical 共111兲CaF2 substrates by metal-organic chemical vapor deposition. These films exclude domains parallel to the surface; therefore, all domains are actively switchable under the electric field between top and bottom electrodes. Saturation polarization values, Psat共001兲, Psat共101兲, and Psat共111兲, for these PZT films were 75, 50, and 43 ␮C / cm2, respectively. This orientation dependency was in a good agreement with the theoretical relationship for a tetragonal PZT single crystal, where Psat共001兲 = Psat共101兲 / 冑2 = Psat共111兲 / 冑3. © 2010 American Institute of Physics. 关doi:10.1063/1.3357421兴 Ferroelectric films have received much attention for a wide variety of applications, including nonvolatile ferroelectric random access memories,1 sensors, and actuators in microelectrical mechanical systems.2 Pb共Zr, Ti兲O3 has been actively researched because of its superior properties, such as large spontaneous polarization and piezoelectricity.3 A large orientation dependency is expected in film form since ferroelectricity and piezoelectricity emerge only in a particular orientation in case of the single crystal.4,5 However, because of the difficulty for making a Pb共Zr, Ti兲O3 single crystal, there are almost no experimental reports on their orientation dependence. Epitaxial film growth has been attempted to investigate this subject,6 but the coexistence with domains oriented parallel to the surface that are not switchable under the electric field except for the switch to horizontal along substrate 关for example, existence of both 共101兲 and 共110兲 orientations in the film兴 hidden the direct observation of the orientation dependency, especially for the films above 100 nm in thickness. It is thought that these domains appear via the relaxation of internal strain at the phase transition temperature during cooling after the deposition, so that the control of the strain applied to the film is key issues for growing films with single orientation. Fujisawa et al.8 reported perfectly 共001兲-oriented Pb共Zr0.35Ti0.65兲O3 共PZT兲 films grown on CaF2 with larger thermal expansion coefficient 共18.9⫻ 10−6 K−1兲 than that of PZT 关5.4⫻ 10−6 K−1 irrespective of the Zr/ 共Zr + Ti兲 ratio7兴 by metal-organic chemical vapor deposition 共MOCVD兲. Here, we report perfectly 共001兲, 共101兲, and 共111兲 tetragonal Pb共Zr, Ti兲O3 films, grown on the same 共111兲CaF2 substrate, without domains oriented parallel to the surface. These films showed a clear orientation dependency of the saturation polarization 共Psat兲. This dependency was similar to the theoretical value given by the vector model for the single crystal. Epitaxial 共001兲, 共101兲, and 共111兲 single-oriented PZT films were grown at 600 ° C on 共100兲cSrRuO3 / / 共111兲Pt/ / 共111兲CaF2, 共110兲cSrRuO3 / / 共111兲CaF2, and a兲

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共111兲cSrRuO3 / / 共111兲Pt/ / 共111兲CaF2 substrates, respectively, by pulsed-MOCVD. Film thicknesses of SrRuO3 and 共111兲Pt layers were 50 and 15 nm, respectively. CaF2共111兲-cut single crystals with 5 ⫻ 5 mm2 were used as a common substrate because their thermal expansion coefficient is large enough to induce large compressive thermal strain during cooling after the deposition. rf magnetron sputtering was used to grow the SrRuO3 and Pt layers. For comparison, 共100兲/共001兲, 共101兲/共110兲, and 共111兲-oriented PZT films were also grown on 共100兲cSrRuO3 / / 共100兲SrTiO3, 共110兲cSrRuO3 / / 共110兲SrTiO3, and 共111兲cSrRuO3 / / 共111兲SrTiO3 substrates, respectively. Details of the PZT film growth on SrTiO3 substrates have already been reported elsewhere.9 The crystal structure and the orientation of the films were investigated by high-resolution x-ray diffraction 共XRD兲 using a four-axis diffractometer 共PANalytical X’Pert MRD兲. XRD reciprocal space mapping 共XRD-RSM兲 technique was used to estimate the volume fraction of 共hkl兲 orientation, V共hkl兲. In the present paper, the volume fraction of 共001兲, 共101兲, and 共111兲 orientations in the mixed orientations of 共001兲/共100兲, 共101兲/共110兲, and 共111兲 / 共11− 1兲, 关V1, V2, and V3兴, were defined by V1 = V共001兲 / 关V共100兲 + V共001兲兴, V2 = V共101兲 / 关V共101兲 + V共110兲兴, and V3 = V共111兲 / 关V共111兲 + V共11 − 1兲兴, respectively. Note that, however, because 共111兲 and 共11–1兲 could be considered to be the same in tetragonal phase 关same thing can be mentioned about 共111兲/共1–11兲兴, 兵111其 PZT has only one orientation perpendicular to the surface normal regardless of the strain in the film.10 That means that V3 always equals to 100%. The ferroelectricity was measured using a ferroelectric tester 共Toyo Corp. FCE-1兲, after preparing ⬃100 ␮m diameter Pt top electrodes by electronbeam evaporation. Figures 1共a兲–1共c兲 show ␪-2␪ scans of the films grown on 共111兲 CaF2 substrates covered with 共001兲c, 共110兲c, and 共111兲c-oriented SrRuO3. There were no secondary phases on these XRD charts, and only 00l, 101, and 111 diffraction peaks were detected from the PZT layers. XRD pole figure measurements confirmed the in-plane alignment and epitaxial growth of these films. Figures 1共d兲–1共f兲 show the XRD-

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FIG. 1. 共Color online兲 XRD ␪-2␪ results 关共a兲, 共b兲, and 共c兲兴 and XRD reciprocal space mapping near PZT 共d兲 002, 共e兲 202, and 共f兲 222 for PZT films grown on 共111兲CaF2 substrates covered with 共001兲c 关共a兲 and 共d兲兴, 共101兲c 关共b兲 and 共e兲兴, and 共111兲c 关共c兲 and 共f兲兴-oriented SrRuO3 layers.

RSM results near the PZT 002, 202, and 222, respectively. Only a PZT 002, 202, and 222 peaks were observed 共no detections of 200 and 220 peaks兲, suggesting perfect 共001兲, 共101兲, and 共111兲 orientations, i.e., V1 = V2 = V3 = 100%. These results are different from the films grown on SrTiO3 substrates covered with 共100兲, 共110兲, and 共111兲 SrRuO3 grown in the same experimental batch, giving 共100兲 and 共001兲, 共101兲 and 共110兲, and 共111兲 orientations, respectively. V1, V2, and V3 values on CaF2 and SrTiO3 substrates are summarized in Table I. Larger V1, V2, and V3 values on CaF2 than on SrTiO3 can be explained by the larger thermal compressed strain in the film on CaF2 at the phase transition temperature estimated to be about 430 ° C from the Zr/ 共Zr+ Ti兲 ratio11 because of the larger thermal expansion coefficient of CaF2 than that of SrTiO3 共11.2⫻ 10−6 K−1兲. Under high compression, orientations with smaller in-plane unit cell area are preferable: 共001兲 is better than 共100兲, and 共101兲 is better than 共110兲. It must be emphasized that this single-axis orientations are achieved via the same SrRuO3-covered 共111兲 CaF2 substrates by changing the buffer layers. Moreover, we expect these oriented films to show significant orientation dependency in their ferroelectricity, similar to single crystals. Figures 2共a兲–2共f兲 show the room temperature polarization-electric field 共P-E兲 hysteresis loops measured at 1 kHz for the ⬃200 nm thick films listed in Table I. Wellsaturated hysteresis loops originated from ferroelectricity were observed, regardless of substrate or orientation. The saturation polarization, 共Psat兲 values for 共001兲, 共101兲, and 共111兲-oriented PZT films grown on 共111兲CaF2, Psat共001兲, Psat共101兲, and Psat共111兲 were 75, 50, and 43 ␮C / cm2, respectively. Here, Psat was estimated from the cross point of the extrapolated line of the polarization from the slope near maximum applied electric field. However, Psat values were

FIG. 2. 共Color online兲 Room temperature P-E hysteresis loops of PZT films measured at 1 kHz. 共a兲共001兲, 共b兲共101兲, and 共c兲共111兲 PZT films grown on 共111兲CaF2 substrates. 共d兲共100兲/共001兲, 共e兲共101兲/共110兲, and 共f兲共111兲-oriented PZT films grown on SrTiO3 substrates.

50, 34, and 42 ␮C / cm2, respectively for 共100兲-/共001兲-, 共101兲-/共110兲-, 共111兲-oriented PZT films grown on SrTiO3 substrates. The Psat values for the films on 共111兲SrRuO3 were almost the same for both substrates, but the values for the films on 共100兲 and 共110兲-oriented SrRuO3 were larger for CaF2 substrates compared to SrTiO3 ones. This variation of Psat as a function of substrate was caused by the existence of the 共100兲 and 共110兲 orientations in the films on SrTiO3, i.e., the existence of 90° domains.12 Figures 3共a兲 and 3共b兲 show the tilt angle dependency of the saturation polarization on CaF2 and SrTiO3 substrates, respectively. This tilt angle of the 共100兲c plane of the pseudocubic SrRuO3 layers from the in-plane direction is defined to be ␪. Thus 共001兲c orientation corresponds to ␪ = 0°, 共110兲c to ␪ = 45°, and 共111兲c to ␪ = 55°. From simple vector rotation, Psat is expected to have a cosine form for 共001兲, 共101兲, and 共111兲 single-oriented PZT films grown on the 共100兲, 共110兲, and 共111兲 SrRuO3 layers 关continuous lines in Figs. 3共a兲 and 3共b兲兴. As shown in Fig. 3共a兲, Psat values were in good agreement with the expected cosine curve for the CaF2 substrate case; Psat共001兲 = Psat共101兲 / 冑2 = Psat共111兲 / 冑3. The estimated spontaneous polarization values, 共Ps兲, correspond to the Psat for the polar axis, Psat共001兲, Ps = Psat / V1, or Psat / V2 or Psat / V3, are shown as open square and dashed line in Fig. 3共a兲, taking into consideration the tilt angle of the polar axis. The estimated Ps values were

TABLE I. Volume fraction of PZT films grown with various orientations and kinds of substrates.

Substrate CaF2 SrTiO3

V1 = V共001兲 / 兵V共100兲 + V共001兲其共%兲

V2 = V共101兲 / 兵V共101兲 + V共110兲其共%兲

V3 = V共111兲 / 兵V共111兲 + V共11− 1兲其共%兲

100 74

100 80

100 100


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square and dashed line in Fig. 3兲. It must be emphasized that PZT films on CaF2 substrates show single-crystal-like orientation dependency, and their Ps can be easily estimated from the measured Psat values. The present success of the various epitaxial PZT films opens the way for fundamental understanding of the orientation dependency of PZT single-crystal. In summary, 共001兲, 共101兲, and 共111兲 single-oriented, epitaxial PZT films were grown on 共111兲CaF2 substrates by pulsed MOCVD. Relatively large Psat values were observed for 共001兲 and 共101兲 PZT films grown on CaF2 compared to those on SrTiO3 substrates, due to the absence of domains oriented parallel to substrate in the former case. Furthermore, the orientation dependency of Psat values was on the theoretically predicted cosine curve, although Psat of conventional PZT films grown on SrTiO3 substrates shows only a weak dependence. The present results directly show that epitaxial PZT films using CaF2 substrate to manipulating polar axis orientation showed single-crystal-like behavior of Psat with orientation. O. Auciello, J. F. Scott, and R. Ramesh, Phys. Today 51, 22 共1998兲. P. Muralt, J. Micromech. Microeng. 10, 136 共2000兲. 3 I. Vrejoiu, G. L. Rhun, L. Pintilie, D. Hesse, M. Alexe, and U. Gosele, Adv. Mater. 18, 1657 共2006兲. 4 X. Du, J. Zheng, U. Belegundu, and K. Uchino, Appl. Phys. Lett. 72, 2421 共1998兲. 5 D. V. Taylor and D. Damjanovic, Appl. Phys. Lett. 76, 1615 共2000兲. 6 S. Yokoyama, Y. Honda, H. Morioka, S. Okamoto, H. Funakubo, T. Iijima, H. Matsuda, K. Saito, T. Yamamoto, H. Okino, O. Sakata, and S. Kimura, J. Appl. Phys. 98, 094106 共2005兲. 7 K. S. Lee, Y. M. Kang, and S. Baik, J. Mater. Res. 14, 132 共1999兲. 8 T. Fujisawa, H. Nakaki, R. Ikariyama, H. Morioka, T. Yamada, K. Saito, and H. Funakubo, Appl. Phys. Express 1, 085001 共2008兲. 9 K. Nagashima, M. Aratani, and H. Funakubo, Jpn. J. Appl. Phys., Part 2 39, L996 共2000兲. 10 S. Utsugi, T. Fujisawa, Y. Ehara, T. Yamada, S. Yasui, H. Morioka, T. Iijima, and H. Funakubo 共to be published兲. 11 D. Berlincourt, IEEE Trans. Sonics Ultrason. SU-13, 116 共1966兲. 12 C. S. Ganpule, V. Nagarajan, B. K. Hill, A. L. Roytburd, E. D. Williams, R. Ramesh, S. P. Alpay, A. Roelofs, R. Waser, and L. M. Eng, J. Appl. Phys. 91, 1477 共2002兲. 13 M. J. Haun, Z. Q. Zhuang, E. Furman, S. J. Jang, and L. E. Cross, J. Am. Ceram. Soc. 72, 1140 共1989兲. 14 K. Saito, T. Kurosawa, T. Akai, T. Oikawa, and H. Funakubo, J. Appl. Phys. 93, 545 共2003兲. 1

FIG. 3. 共Color online兲共a兲 Tilt angle dependency of PZT films grown on 共111兲CaF2 substrates. The tilt angle of the 共100兲c plane of the pseudocubic SrRuO3 layers from the in-plane direction was defined as ␪. Hence the 共001兲c orientation corresponds to ␪ = 0°, 共110兲c to ␪ = 45°, and 共111兲c to ␪ = 55°. 共Closed circles兲: measured Psat values. 共Open squares兲: Estimated Ps values, calculated from the polarization vector and the 共b兲 tilt angle dependency in PZT films grown on SrTiO3 substrates. 共Closed circles兲: Measured Psat values. 共Open squares兲: Ps values estimated from the polarization vector and the volume fractions, V1, V2, and V3 in Table I. Continuous lines in 共a兲 and 共b兲 show the data for polar-axis oriented films 共Ref. 7兲.

⬃75 ␮m / cm2, regardless of orientation, and were similar to the value predicted for the single crystal by Haun et al.13 Nevertheless, as shown in Fig. 3共b兲, Psat did not show an obvious trend for the SrTiO3 substrate. The reason for this is the existence of 90° domains along the normal direction: namely, 共100兲 and 共110兲 orientations coexist with respective 共001兲 and 共101兲 orientations for the films on 共100兲c and 共110兲c SrRuO3-covered substrates.14 The estimated value of Ps, taking into account the volume fraction of V1 and V2, as shown in Table I, is almost identical to that of CaF2 共open

2
