APPLIED PHYSICS LETTERS 90, 072909 共2007兲
Structure and properties of epitaxial ferroelectric PbZr0.4Ti0.6O3 / PbZr0.6Ti0.4O3 superlattices grown on SrTiO3 „001… by pulsed laser deposition Ionela Vrejoiu,a兲 Yinlian Zhu, Gwenaël Le Rhun, Markus Andreas Schubert, Dietrich Hesse, and Marin Alexe Max Planck Institute of Microstructure Physics, D-06120 Halle, Germany
共Received 11 December 2006; accepted 16 January 2007; published online 16 February 2007兲 Ferroelectric epitaxial superlattices combining PbZr0.4Ti0.6O3 and PbZr0.6Ti0.4O3 ultrathin films were grown by pulsed laser deposition onto vicinal SrTiO3 共001兲 substrates. The superlattices were subjected to structural characterization by means of 共high-resolution兲 transmission electron microscopy and x-ray diffraction, to piezoelectric-force microscopy, and to macroscopic ferroelectric measurements. The thickness of the individual PbZrxTi1−xO3 layers was found to play an important role in the overall structure adopted by the superlattices. Superlattices consisting of 22 bilayers of 5 nm thin PbZr0.4Ti0.6O3 layers and 3 nm thin PbZr0.6Ti0.4O3 layers involve continuous 90° a – c domains, indicating a uniform tetragonal structure. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2643259兴 Heterostructures and superlattices 共SLs兲 involving perovskites are under investigation in pursuing artificial materials with tailored properties.1,2 Ferroelectric perovskites, such as BaTiO3 and PbZrxTi1−xO3 共PZT兲, have been combined with nonferroelectric 关i.e., LaAlO3, SrTiO3, and BaZrO3 共Refs. 3–6兲兴 or with antiferroelectric 共i.e., PbZrO3兲 perovskites.7,8 Many of the reports regarding ferroelectric heterostructures deal, however, with nonepitaxial thin films.8–12 Hence, the measured physical properties are rather different from those of epitaxial SLs, where the strain conditions imposed by the single crystalline substrate13,14 and the growth conditions play an important role. PZT-based heterostructures and SLs received much attention, with the ability that by tuning the Zr/ Ti ratio the PZT crystalline structure changes from tetragonal 共Ti-rich compositions兲 to rhombohedral and orthorhombic 共Zr rich兲, and thus most physical quantities undergo severe changes as well. SLs of 关PbZrxTi1−xO3兴n / 关PbZryTi1−yO3兴n, where x and y are on different sides of the morphotropic phase boundary 共MPB兲 and the average composition 共x + y兲 / 2 lies inside this boundary, have been investigated for the unusual thermodynamic properties and nonergodicity15 and for a very large enhancement of the electromechanical response.16 Their investigation is also interesting with respect to a possible monoclinic phase at the MPB.17 We report on growth and structural characterization of epitaxial SLs of PbZr0.4Ti0.6O3 共PZT 40/ 60兲 and PbZr0.6Ti0.4O3 共PZT 60/ 40兲 fabricated on SrTiO3 共001兲 single crystals by pulsed laser deposition 共PLD兲. At room temperature 共RT兲 bulk PZT 40/ 60 has a tetragonal structure, whereas the structure of bulk PZT 60/ 40 crystals is rhombohedral.18 Vicinal single crystalline SrTiO3 共001兲 共STO兲 substrates with a miscut angle of about 0.1° were used. The SLs were fabricated by PLD, employing a KrF excimer laser closely following the procedure described in Ref. 19. Single crystala兲
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line SrRuO3 共SRO兲 layers, grown in step-flow growth mode, served as bottom electrodes. The PZT 40/ 60 and PZT 60/ 40 layers were grown at a temperature of 575 ° C in 0.3 mbar oxygen, at a laser fluence of 2 J / cm2 and a pulse rate of 5 Hz, alternating the two PZT targets. SRO/Pt top electrodes were deposited through a shadow mask 共diameter of ⬃0.15 mm兲.19 Transmission electron microscopy 共TEM兲 共Philips CM20T, at 200 keV兲 and high-resolution TEM 共HRTEM兲 共JEOL 4010, at 400 keV兲 were performed on 共010兲 STO cross-section samples. X-ray diffraction 共XRD兲 共Philips X’Pert MRD兲 was carried out using Cu K␣ emission. Reciprocal-space mapping 共RSM兲 served to identify the crystallographic phases. Local electromechanical behavior was investigated by a scanning probe microscope 共CP Re-
FIG. 1. TEM cross-section micrographs of a 共a兲 关PZT 40/ 60/ PZT 60/ 40兴10 SL and 共b兲 关PZT 40/ 60/ PZT 60/ 40兴22 SL grown on SrRuO3-coated SrTiO3 共001兲, seen from the 关010兴 STO direction.
0003-6951/2007/90共7兲/072909/3/$23.00 90, 072909-1 © 2007 American Institute of Physics Downloaded 19 Feb 2007 to 85.232.23.177. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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FIG. 2. HRTEM image of the 关PZT 40/ 60/ PZT 60/ 40兴22 taken on an area containing a 90° a domain in between two c domains, the white arrows indicating the direction of the polar c axis. The inset is a FFT of this HRTEM image and the existence of 90° domains is evidenced by the double spots 共marked by white circles兲.
search, Veeco兲 in piezoforce mode 共PFM兲.20 Macroscopic polarization hysteresis curves were acquired using a TF2000 analyzer 共AixaCCT兲. TEM cross-section micrographs of two SLs are shown in Fig. 1. The 关PZT 40/ 60/ PZT 60/ 40兴10 SL, with ten bilayers, with thick individual PZT layers 共8 ± 1 nm兲 and an overall thickness of ⬃170 nm had high structural quality. Nevertheless, misfit dislocations 共MDs兲 at the interface with the SRO bottom electrode and threading dislocations crossing the entire heterostructure are present 关Fig. 1共a兲兴. In this figure the bright layers are PZT 40/ 60 and the dark ones are PZT 60/ 40. At RT cubic STO has a lattice parameter a = 0.3905 nm, whereas bulk tetragonal PZT 40/ 60 has an inplane parameter a = 0.3985 nm, giving rise to a compressive misfit strain of ⬃2%. At RT bulk rhombohedral PZT 60/ 40 has a lattice parameter a = 0.406 nm, resulting in a compressive misfit strain with respect to the STO substrate of ca. 4%. Therefore, the formation of MDs could be expected for rather thick SLs. The cross-section micrograph of the 关PZT 40/ 60/ PZT 60/ 40兴22 SL, with 22 bilayers and thinner individual PZT layers, is displayed in Fig. 1共b兲. This SL has 5 nm thin PZT 40/ 60 and 3 nm thin PZT 60/ 40 layers and an overall thickness of ⬃175 nm. In this case, 90° a – c domains formed and most of them extend over the entire thickness of the heterostructure 关Fig. 1共b兲兴. Such 90° domains were absent in the superlattice with thicker individual PZTs 关Fig. 1共a兲兴, although the overall thickness of the two SLs is about the same. This type of 90° twin domains is typical for the tetragonal phase of epitaxial PZT films.21 Their formation and the fact that they cross the entire thickness of the superstructure indicate the ultrathin PZT 60/ 40 layers of the 关PZT 40/ 60/ PZT 60/ 40兴22 SL to deviate from the rhombohedral structure expected for bulk PZT 60/ 40, adopting a tetragonal structure under the particular growth and strain conditions. Figure 2 is a HRTEM micrograph of the 关PZT 40/ 60/ PZT 60/ 40兴22 SL. It comprises both PZT 40/ 60 and PZT 60/ 40 layers and shows 90° a – c domains. The white arrows indicate the direction of the polar c axis. Fast fourier transform 共FFT兲 of this HRTEM image 共inset of Fig. 2兲 was performed and the marked double spots in the FFT are due to the 90° a – c boundaries. TEM investigations 共not shown兲 performed on the cross section of another
FIG. 3. 共Color online兲 X-ray diffraction reciprocal-space maps around the STO 共004兲 reflection taken on the 共a兲 关PZT 40/ 60/ PZT 60/ 40兴10 SL and 共b兲 关PZT 40/ 60/ PZT 60/ 40兴22 SL.
关PZT 40/ 60/ PZT 60/ 40兴22 superlattice with 3 nm thin PZT 40/ 60 layers and 5 nm thin PZT 60/ 40 layers revealed that 90° a – c domains were present also in this case. XRD-RSM diffraction contour maps of the entire heterostructures 共SLs plus SRO and the substrate, STO兲 were acquired 共Fig. 3兲. Figure 3共a兲 displays the RSM obtained around the STO 共004兲 reflection for the 关PZT 40/ 60/ PZT 60/ 40兴10 SL and 关Fig. 3共b兲兴 that acquired around the same reflection for the 关PZT 40/ 60/ PZT 60/ 40兴22 SL. The striking difference consists in the PZT 共400兲 peaks that appear in the RSM of the latter SL 关see dotted circles in Fig. 3共b兲兴. Their presence along with the stronger PZT 共004兲 peak indicates that this SL is tetragonal, c-axis oriented, and has an important fraction of 90° twin domains.22,23 These diffraction spots are much
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Summarizing, epitaxial strain imposed by single crystal substrates has an important impact on the structure adopted by ferroelectric superlattices grown heteroepitaxially. 关PZT 40/ 60/ PZT 60/ 40兴22 SLs with very thin PZT layers grown on cubic STO 共001兲 have a tetragonal structure and involve 90° domains typical for tetragonal ferroelectrics, thus indicating that the expected rhombohedral structure of PZT 60/ 40 was inhibited by the epitaxial strain conditions. The authors thank S. Schmidt for part of the TEM investigations, S. Senz for discussions on RSM, S. Swatek for TEM sample preparation, and N. Schammelt for PLD system maintenance; this work was partly funded by the Volkswagen Foundation 共“Nanosized ferroelectric hybrids” Project No. I/80897兲. 1
FIG. 4. 共Color online兲 PFM piezoresponse images 共2 ⫻ 2 m2兲 on 共a兲 关PZT 40/ 60/ PZT 60/ 40兴10 SL and 共b兲 关PZT 40/ 60/ PZT 60/ 40兴22 SL. In 共c兲 macroscopic hysteresis loops of the two SLs are shown; circle symbols are for 关PZT 40/ 60/ PZT 60/ 40兴10 SL and triangle symbols for the 关PZT 40/ 60/ PZT 60/ 40兴22 SL; open symbols correspond to polarization hysteresis and solid symbols to switching current curves.
weaker in the RSM of the 关PZT 40/ 60/ PZT 60/ 40兴10 SL, thus confirming the TEM results. PFM investigations were made on the free surface of the as-grown SLs. They confirmed the existence of 90° a – c domain patterns in the 关PZT 40/ 60/ PZT 60/ 40兴22 SL 关see the 90° grid of dark contrast needlelike a domains in Fig. 4共b兲兴 and their absence in the 关PZT 40/ 60/ PZT 60/ 40兴10 SL 关Fig. 4共a兲兴.20,21 In order to increase the contrast of the domain imaging, a 1 m2 area had been previously poled in the center 关both with positive and negative biases in Fig. 4共a兲, only positive in Fig. 4共b兲兴.21 Macroscopic polarization hysteresis and switching current measurements performed at 1 kHz at RT on the two SLs are shown in Fig. 4共c兲. The remnant for the polarization is Pr = 35± 4 C / cm2 关PZT 40/ 60/ PZT 60/ 40兴10 SL and considerably higher, Pr = 50 ± 5 C / cm2, for the 关PZT 40/ 60/ PZT 60/ 40兴22 SL. These Pr values are considerably higher than those reported for PZT-based multilayers;7,9–12,24–27 in addition, the hysteresis loops of our SLs are well saturated, which we consider a consequence of the better microstructure. For the sake of comparison, we have also measured the remnant polarization values for ⬃200 nm thick epitaxial PZT 40/ 60 and PZT 60/ 40 thin films, grown under the same PLD conditions and from the same targets. They were Pr = 70± 7 C / cm2 and Pr = 45± 5 C / cm2, respectively. Pr of our epitaxial PZT 52/ 48 共MPB composition兲 films was Pr = 50± 5 C / cm2. The epitaxial PZT 40/ 60 film was tetragonal and contained 90° a – c domains. The epitaxial PZT 60/ 40 film had a rhombohedral structure and exhibited the typical rhombohedral domain patterns.28,29
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