APPLIED PHYSICS LETTERS 86, 142903 共2005兲
Reducing azimuthal domains in epitaxial ferroelectric lanthanum-substituted bismuth titanate films using miscut yttria-stabilized zirconia substrates Sung Kyun Lee,a兲 Dietrich Hesse, and Ulrich Gösele Max Planck Institute of Microstructure Physics, Weinberg 2, D-06120 Halle/Saale, Germany
Ho Nyung Lee Condensed Matter Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
共Received 14 December 2004; accepted 17 February 2005; published online 31 March 2005兲 We report the effect of a definite miscut of yttria-stabilized zirconia 共YSZ兲 共100兲 single-crystal substrates onto the number of azimuthal domain variants within epitaxial La-substituted Bi4Ti3O12 共BLT兲 ferroelectric thin films as well as within SrRuO3 electrode layers, both grown on these substrates. YSZ substrates with a miscut angle of 5° were studied, with two different directions of the miscut, viz., YSZ关001兴 and YSZ关011兴. A reduction of the number of azimuthal domain variants by 50% was attained on substrates with a 关011兴-directed miscut. Due most probably to the reduced number of azimuthal domain boundaries, larger remanent polarization values were attained in BLT films when grown on miscut substrates. © 2005 American Institute of Physics. 关DOI: 10.1063/1.1897044兴 Bismuth-layered perovskite thin films are important candidates for a variety of applications as, e.g., nonvolatile random access memories.1,2 Among them, non-c-axis oriented La-substituted ferroelectric Bi4Ti3O12 films are most promising for high-density memories, due to their good fatigue resistance, high and uniform remanent polarization, and rather low process temperatures.3–5 For growing non-c-axis-oriented epitaxial thin films of ferroelectric bismuth-layered perovskites such as SrBi2Ta2O9 and La-substituted Bi4Ti3O12 on silicon substrates, simple perovskite-type SrRuO3 films have been used as a conducting epitaxial template.4,6 However, when 共110兲-oriented SrRuO3 films are grown on 共100兲-oriented yttria-stabilized zirconia 共YSZ兲-buffered Si共100兲 substrates, there are four equivalent azimuthal domain variants formed in the SrRuO3 films due to a diagonal-type rectangle-on-cube epitaxy.6,7 Therefore, subsequently deposited bismuth-layered perovskite films on the SrRuO3 electrodes also grow with the equivalent number of azimuthal domain variants, resulting in 20°-, 70°-, and 90°-azimuthal domain boundaries.4,5 The electrical properties of the films may be influenced by the domain boundaries, which contain defects and are not fully crystallized.5 In order to enhance the growth of single-domain, perovskite-type thin films including, e.g., SrRuO3,8,9 various miscut substrates such as SrTiO3,8–12 LaAlO3,13 MgO,14,15 and YSZ16 single crystals have been used. In this work, we demonstrate the effective reduction of the number of azimuthal domain variants in Bi3.25La0.75Ti3O12 共BLT兲 and SrRuO3 films using miscut YSZ共100兲 substrates. We investigate two different miscut directions, viz., 关001兴 and 关011兴. In order to elucidate the influence of the miscut, both commercially exactly cut 共i.e., less than 0.5° miscut兲 and 5° miscut 共100兲-oriented YSZ single crystals were used. Before deposition, the YSZ substrates were thermally treated at 1200 ° C for 10 min in air. The deposition of the SrRuO3 electrode a兲
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layers and the BLT thin films was carried out in situ by pulsed-laser deposition 共PLD兲. More details on the film growth by PLD can be found elsewhere.4–6 The crystallographic structure of the epitaxial films was analyzed by x-ray
FIG. 1. XRD scans and pole figures of SrRuO3 thin films grown on exactly cut 关共a兲 and 共b兲兴, 关001兴 miscut 关共c兲 and 共d兲兴, and 关011兴 miscut 关共e兲 and 共f兲兴 YSZ共100兲 substrates using the SrRuO3 200 reflection 共2 = 46.18° 兲. The arrows in 共d兲 and 共f兲 indicate that the SrRuO3共110兲 lattice plane is parallel to the YSZ共100兲 lattice plane, which is tilted with respect to the top and bottom surfaces of the substrate.
0003-6951/2005/86共14兲/142903/3/$22.50 86, 142903-1 © 2005 American Institute of Physics Downloaded 31 Mar 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. 3. Plan-view TEM image of a BLT film grown on 共110兲 SrRuO3 on a 关011兴 miscut YSZ共100兲 substrate. Two predominant grain orientations separated by ⬃70° are seen. The inset is an electron diffraction pattern consisting of two rows of BLT 共00l兲 reflections.
FIG. 2. 共a兲 Schematic drawing of the rectangle-on-cube epitaxy of SrRuO3共110兲 on YSZ共100兲 共see Refs. 4–6兲. 关共b兲 to 共e兲兴 XRD scans of BLT films grown on SrRuO3 layers on 关001兴 miscut 关共b兲 and 共c兲兴 and 关011兴 miscut 关共d兲 and 共e兲兴 YSZ共100兲 substrates using the BLT 111 reflection 共2 = 23.31° 兲. The scans are recorded at = 41° – 51° 关共b兲 and 共d兲兴 and = 31° – 41° 关共c兲 and 共e兲兴, respectively, in correspondence to the 共100兲- and 共118兲-oriented parts of the BLT films.
diffraction 共XRD兲 and transmission electron microscopy 共TEM兲. The ferroelectric properties were evaluated using a ferroelectric tester 共TF Analyzer 2000, AixACCT兲. Sputtered Pt with a diameter of 100 m was used as the top electrode for electrical characterizations. Figure 1 shows XRD scans and pole figures of SrRuO3 films deposited on 共i兲 exactly cut 关Figs. 1共a兲 and 1共b兲兴, 共ii兲 关001兴 miscut 关Figs. 1共c兲 and 1共d兲兴, and 共iii兲 关011兴 miscut 关Figs. 1共e兲 and 1共f兲兴 YSZ共100兲 substrates. These figures were recorded using the SrRuO3 200 reflection 共2 = 46.18° 兲. On the exactly cut substrates, eight diffraction peaks arranged with a fourfold symmetry are seen. These peaks are grouped into four sets of two reflection peaks each, with a angle difference ⌬ of ⬃20°. This arrangement of the peaks originates from the diagonal-type rectangle-oncube epitaxy relation of SrRuO3共110兲 on YSZ共100兲4–6 as ¯ 11兴 direction schematically shown in Fig. 2共a兲. The SrRuO3关1 ¯¯1兴, 关011 ¯ 兴, may be parallel to any of the four YSZ关011兴, 关01 ¯ and 关011兴 directions due to misfit minimization, resulting in four types of azimuthal SrRuO3 domains.17 Pairwise comparing peaks with the angle difference of ⬃20° at = 45° 关Figs. 1共a兲 and 1共b兲兴, we observe that the intensities of them are almost the same. This means that the four azimuthal SrRuO3 domain variants on exactly cut YSZ共100兲 substrates are of approximately equal population. On the other hand, SrRuO3 films grown on miscut 共100兲 YSZ substrates as shown in Figs. 1共c兲 and 1共e兲 revealed an asymmetric intensity between two neighboring peaks, indicating that the fraction of two azimuthal domain variants 共out of four兲 is reduced. Finally, SrRuO3 films grown on YSZ with the 关011兴 miscut direction showed an almost entire elimination of two 共out of four兲 azimuthal domain variants. The
¯ 11兴 direction of the remaining dominant domain SrRuO3 关1 ¯¯1兴 direction, which variants is parallel to the YSZ关011兴 or 关01 is identical with the substrate miscut direction. The YSZ关011兴 miscut direction thus effectively reduces the number of the azimuthal domain variants in SrRuO3 thin films by 50%. Another interesting feature is observed in the pole figures. As shown in Figs. 1共d兲 and 1共f兲, a tilt of the crystallographic orientation of the films was observed. The shift of all reflection peaks in the SrRuO3 films on the miscut YSZ substrates towards a certain direction means that the SrRuO3 films were grown with the SrRuO3共110兲 plane parallel to the YSZ共100兲 lattice plane, which is tilted with respect to the top and bottom surfaces of the substrate. In distinction, the peaks from SrRuO3 films grown on exact 共100兲 YSZ substrates are centrosymmetrically positioned at = 45° 兵i.e., SrRuO3共110兲 储 YSZ共100兲; SrRuO3关110兴 储 YSZ关100兴其. Due to this fact, the XRD scans 共for miscut substrates兲 mentioned earlier had to be performed through finding the maxima of all peaks after several optimization scans in and . Similar phenomenon of crystallographic tilting has also been theoretically studied in heteroepitaxy of semiconductors.18,19 In order to investigate whether the miscut substrate also affects the domain variants of subsequently grown films, ferroelectric BLT films were deposited on SrRuO3共110兲 electrodes on miscut YSZ共100兲 single-crystal substrates. We used growth conditions that result in the growth of a mix of 共100兲 and 共118兲 orientations.4,5 Figures 2共b兲 and 2共c兲 show XRD scans of a BLT film on 关001兴 miscut YSZ substrates, whereas Figs. 2共d兲 and 2共e兲 exhibit those of a BLT film on 关011兴 miscut YSZ. These scans were recorded using the BLT 111 reflection in the angle range between ⬃41° and ⬃51° 关Figs. 2共b兲 and 2共d兲兴 for the 共100兲-oriented part of the film and between ⬃31° and ⬃41° 关Figs. 2共c兲 and 2共e兲兴 for the 共118兲-oriented part, respectively. 关Note the angles ⬔共100兲 : 共111兲 = 45.6° and ⬔共118兲 : 共111兲 = 36.4° for a BLT film on an exactly cut YSZ substrate兴. For the reason mentioned before, angle deviations of the BLT 111 reflection are also attributed to the miscut analogous to the situation in the SrRuO3 films. Four sets of reflection peaks in all scans were observed. For the 共100兲-oriented part of the BLT film 关Figs. 2共b兲 and 2共d兲兴, each set of peaks shown in Fig. 2共b兲 is composed of two subsets 共overlapping into three peaks兲, with
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FIG. 4. Remanent polarization as a function of the applied electric field of Pt/ BLT/ SrRuO3 capacitors using three different YSZ共100兲 single-crystal substrates. The inset represents corresponding polarization-electric field hysteresis loops.
an offset between the two subsets of ⌬ ⬃ 20°, whereas in Fig. 2共d兲 each set consists of only one subset. The analoguous situation occurs in the 共118兲-oriented part of the BLT film 关Figs. 2共c兲 and 2共e兲兴. Overall, Fig. 2 indicates that the effect of miscut substrate on the reduction of the number of azimuthal domain variants has taken place not only in the SrRuO3 electrodes 关Figs. 1共c兲–1共f兲兴, but also in the subsequently grown BLT films. TEM investigations clearly confirmed the reduction of the number of the azimuthal domain variants in ferroelectric BLT thin films. Figure 3 shows a plan-view TEM image of a BLT film on a 关011兴 miscut YSZ共100兲 substrate covered with an SrRuO3共110兲 electrode layer. Most of the BLT grains have two specific azimuthal orientations, separated by ⬃70°. On account of these two predominant azimuthal orientations, the electron diffraction pattern shown in the inset of Fig. 3 consists of only two rows of BLT 共00l兲 reflections, in distinction to that of a BLT film grown on SrRuO3共110兲 共having four azimuthal domains兲 showing four rows of BLT 共00l兲 reflections 共not shown兲. The darkest grains in Fig. 3 are a-axis-oriented grains, which at higher magnification 共not shown兲 reveal the BLT共002兲 lattice fringes with a spacing of half of the c lattice parameter. From the XRD and TEM results it can be concluded that the respective reduced azimuthal domain characteristics of the SrRuO3 films were inherited by the BLT films for both cases of 关001兴 miscut and 关011兴 miscut YSZ共100兲 substrates. It should be noted that the BLT films on three different substrates were deposited under identical growth conditions exhibiting almost the same 共100兲:共118兲 volume ratio of 20%:80% 共with less than 2% volume tolerance兲. For example, the volume ratios of the BLT films on 关001兴 and 关011兴 miscut YSZ共100兲 substrates were calculated from scans of the BLT 111 reflection 关Figs. 2共b兲–2共e兲兴. The inset of Fig. 4 shows ferroelectric hysteresis loops recorded from BLT films grown on SrRuO3 electrode layers on exactly cut, 关001兴 miscut, and 关011兴 miscut YSZ共100兲 substrates. The corresponding measured remanent polarization 共Pr兲 of the BLT films are
9.5, 10.3, and 12.5 C / cm2, respectively, for a maximum applied electric field of 433.3 kV/ cm. Figure 4 summarizes the recorded remanent polarization of the BLT films grown on the three different YSZ substrates as a function of the applied electric field. Larger values of the remanent polarization were recorded in BLT films on 关011兴 miscut YSZ substrates. This is most probably due to the reduction of the number of the azimuthal domains, which results in a lower areal density of azimuthal domain boundaries, thus reducing domain pinning or clamping. In summary, a reduction of the number of azimuthal domain variants in SrRuO3共110兲 electrode layers as well as in 共100兲 / 共118兲-oriented BLT films has been achieved by using miscut YSZ共100兲 substrates. As a consequence, improved ferroelectric properties were achieved for BLT films with less azimuthal domain variants in the film. This research was supported by Deutsche Forschungsgemeinschaft 共DFG兲 via the Group of Researchers FOR 404 at Martin Luther University Halle-Wittenberg. One of the authors 共H.N.L.兲 was supported by the U.S. Department of Energy under contract with the Oak Ridge National Laboratory, managed by UT-Battelle, LLC, as part of a BES NSET initiative on Nanoscale Cooperative Phenomena and of the LDRD program. 1
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