JOURNAL OF APPLIED PHYSICS 102,

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1 JOURNAL OF APPLIED PHYSICS 102, Microstructure and electrical properties of 120 O -oriented and of 001 O -oriented epitaxial antiferroelectric PbZrO 3 thin films on 100 SrTiO 3 substrates covered with different oxide bottom electrodes Ksenia Boldyreva a Max Planck Institute of Microstructure Physics, Weinberg 2, D Halle (Saale), Germany Dinghua Bao Max Planck Institute of Microstructure Physics, Weinberg 2, D Halle (Saale), Germany and State Key Laboratory of Optoelectronic Materials and Technologies, Institute of Optoelectronic and Functional Composite Materials, School of Physics and Engineering, Sun Yat-Sen University, Guangzhou , People s Republic of China Gwenael Le Rhun, Lucian Pintilie, Marin Alexe, and Dietrich Hesse b Max Planck Institute of Microstructure Physics, Weinberg 2, D Halle (Saale), Germany Received 12 March 2007; accepted 30 June 2007; published online 23 August 2007 Epitaxial antiferroelectric PbZrO 3 PZO thin films of two different crystallographic orientations were grown by pulsed laser deposition on 100 -oriented SrTiO 3 single crystal substrates. The latter were covered either with SrRuO 3 epitaxial bottom electrodes, or with an epitaxial BaZrO 3 buffer layer and an epitaxial BaPbO 3 bottom electrode, respectively. Their crystal orientation and microstructure were characterized by x-ray diffraction, transmission electron microscopy, and electron diffraction. The orthorhombic index O PZO films on SrRuO 3 /SrTiO 3 were predominantly 120 O oriented and consisted of four azimuthal domains forming 90 and 60 boundaries, whereas those grown on BaPbO 3 /BaZrO 3 /SrTiO 3 were 001 O oriented. All films showed well-defined double P-E hysteresis loops, four distinct switching peaks in the current-voltage characteristics, and piezoelectric double loops recorded by piezoresponse scanning force microscopy. The values of the saturation polarization P S and the critical field E C of the 120 O -oriented PZO films P S =41 C/cm 2 ; E C =445 kv/cm are different from those of the 001 O -oriented films P S =24 C/cm 2 ; E C =500 kv/cm. A transition temperature to the paraelectric phase of 260 C has been found, which is 30 K higher than the bulk value, probably indicating a stabilization of the antiferroelectric phase by substrate-induced strain American Institute of Physics. DOI: / I. INTRODUCTION Antiferroelectric thin films have attracted increasing attention for their possible applications in microactuators, charge storage devices, and electro-optic devices. 1 5 Lead zirconate PbZrO 3 PZO is an example of a typical antiferroelectric material, which was investigated first by Sawaguchi et al. 6,7 One of its characteristic properties is an antiferroelectric-to-ferroelectric phase transition induced by a sufficiently large applied electric field, and a corresponding double P-E hysteresis loop. This property enables antiferroelectric materials to be used, e.g., in capacitors for highpower energy storage. 1,8 Moreover, a giant electrocaloric effect in PZO doped with 5% of Ti was recently reported. 9 Following Berlincourt, 10 two different types of antiferroelectric hysteresis loops exist, viz., slanted and square loops. For high-energy-storage-capacitor application materials with square hysteresis loops are required. 11 To achieve squareshaped hysteresis loops and to ensure laterally uniform film properties, epitaxial antiferroelectric PZO films should be most advantageous. a Electronic mail: ksenia@mpi-halle.de b Electronic mail: hesse@mpi-halle.mpg.de However, reports on growth and properties of epitaxial, antiferroelectric PbZrO 3 films are rather limited so far. Yamakawa et al. studied 120 O -oriented the index O refers to orthorhombic and the index C to pseudocubic indexing PZO thin films deposited by reactive magnetron cosputtering, 12 and Bharadwaja and Krupanidhi deposited columnar 110 C -oriented PZO films onto Pt-covered Si substrates by pulsed laser deposition. 13 Parui and Krupanidhi investigated the dielectric properties of 110 C -oriented PZO thin films grown by sol-gel deposition. 14 Other groups have reported on the epitaxial growth, microstructure, and morphology of PZO thin films, however, most of these films were directly prepared on single crystal substrates such as LaAlO 3 or SrTiO 3, without bottom electrodes. 5,15,16 Kanno et al. investigated electrical properties such as hysteresis loops and the dielectric constant of PZO thin films prepared by multi-ion beam sputtering on 100 MgO and Pt/ 100 MgO substrates. 17 It appears that measurements of the antiferroelectric properties of epitaxial PZO thin films have rather rarely been reported. 14,17 This paper reports on growth, microstructure, and electrical properties of epitaxial PZO films of two different crystallographic orientations, grown on SrRuO 3 -covered /2007/102 4 /044111/8/$ , American Institute of Physics

2 Boldyreva et al. J. Appl. Phys. 102, FIG. 1. Color online AFM images of a asto 100 substrate with terraces, after etching and annealing, b a SRO thin film 40 nm deposited on a STO 100 substrate, and c a PZO thin film 50 nm on STO 100 covered with SRO. 100 SrTiO 3 and on BaPbO 3 -covered and BaZrO 3 -buffered 100 SrTiO 3 single crystal substrates, respectively. PZO has an orthorhombic crystal structure at temperatures below 230 C in the antiferroelectric phase and a cubic structure above 230 C in the paraelectric phase. On cooling through the phase transition, antiferroelectricity occurs due to an alternate pairwise shift of Pb ions along the 110 C and C directions. The lattice constants of the antiferroelectric, orthorhombic phase at room temperature are a=5.88 Å, b= Å, c=8.231 Å. 18 The antiferroelectric axis is along the 100 O direction. Considering the reduced cubic unit cell of the paraelectric phase above 230 C, which is a cubic perovskite unit with a lattice parameter of Å, the idea to consider a perovskite unit with a lattice parameter of a C p =4.14 Å as an approximation of a primary crystallographic motif also in the orthorhombic, antiferroelectric phase of PZO has proven to be helpful. 19 II. EXPERIMENTAL PROCEDURE The PZO films were prepared by pulsed laser deposition PLD KrF excimer laser, =248 nm at a substrate temperature of 550 C, under an oxygen partial pressure of 0.1 mbar and using a repetition rate of 2 Hz. All SrTiO 3 STO substrates cubic, lattice parameter a=3.905 Å were chemically etched in a buffered HF solution and thermally annealed in air in order to achieve step-terrace structures with only one unit-cell height. 20 To enable electrical measurements, SrRuO 3 SRO pseudocubic with a=3.928 Å or BaPbO 3 BPO cubic, a=4.265 Å thin films were deposited as bottom electrodes before depositing PZO. SRO is a well-established bottom electrode on STO because of its similar structure with STO and low lattice mismatch of only 0.6%. In the case of the BaPbO 3 electrodes, BaZrO 3 BZO cubic, a=4.19 Å was used as a buffer layer. The laser fluence was set at 1.5, 0.9, 1.6, and 1.2 J/cm 2 for depositing PZO, SRO, BZO, and BPO, respectively. To enable a layerby-layer growth mode of SRO on STO, the deposition parameters were chosen according to Hong et al. 21 After deposition of all materials, the samples were cooled down to room temperature in 1 mbar oxygen. Using a stainless steel shadow mask, Pt electrodes were deposited on top of each sample by rf sputtering. The surface morphology of the substrates and the grown films was studied by atomic force microscopy AFM in tapping mode using a Digital Instruments D5000 microscope. Phase contents and crystallographic orientation of the films were characterized by x-ray diffraction XRD using a Philips X Pert MRD four-circle diffractometer with Cu K radiation. Samples for transmission electron microscopy TEM were thinned using mechanical and ion-beam based standard methods. Standard TEM investigation was performed in a Philips CM20T at 200 kev primary energy, and highresolution TEM HRTEM in a JEOL 4010 at 400 kev primary energy of the electrons. Macroscopic ferroelectric properties were determined by an AixAcct TF Analyzer 2000, and local ferroelectric properties by piezoresponse AFM PFM Ref. 22 in a Thermo Microscopes Autoprobe CP Research system modified with respect to the piezoresponse mode. 23 The temperature dependence of the dielectric constant was measured in a vacuum of 10 5 mbar, increasing the temperature at a ramping rate of 2 K/min. III. RESULTS AND DISCUSSIONS A. PbZrO 3 on 100 SrRuO 3 / 100 SrTiO 3 The surface morphology was characterized by atomic force microscopy, as shown in Fig. 1. Both SRO and PZO films have grown in a layer-by-layer mode resulting in stepped terraces. The surfaces are flat, with a rms roughness of 0.38 and 0.47 nm for the SRO and PZO thin film, respectively. From the XRD structure investigations, Fig. 2 shows a -2 scan of a PZO film on SRO/STO, clearly indicating a preferred 120 O orientation of the PZO film. A corresponding pole figure center =0; rim =90 ; =90 corresponds to the substrate surface being parallel to the plane defined by the incident and reflected x-ray beams, taken at 2 = corresponding to PZO 110 O is shown in Fig. 3 b. Notwithstanding the asymmetric position of the 120 O -oriented unit cell, the pole figure shows a fourfold

3 Boldyreva et al. J. Appl. Phys. 102, FIG. 2. Color online X-ray diffraction pattern of a PZO thin film on a STO 100 substrate covered with SRO. The peaks of STO l00 are marked with an asterisk. Symbol O designates peaks originating from K radiation, and symbol # a peak originating from W L radiation due to the contamination of the anode by tungsten from the cathode. symmetry. This points to a fourfold positioning i.e., the presence of four types of azimuthally oriented domains of the PZO film. Moret et al., who studied well-oriented PZO thin films grown by metal organic chemical vapor deposition, found that the films consisted of four different types of 120 O -oriented domains, as long as the film thickness did not exceed 260 nm. For thicker films, two additional types of 001 O -oriented domains grown in the form of pyramids were found. 5 Figure 3 a shows the simulation of a pole figure for only one azimuthal 120 O domain taken at 2 = It consists of two peaks at =18.42 and =71.44, the values of which correspond to the angles 120 ; 110 = and 120 ; 11 0 = It is obvious that the experimental pole figure corresponds to a fourfold positioning of the PZO film, i.e., to four different azimuthal domains, rotated with respect to the simulated azimuthal orientation by angles of 0, 90, 180, and 270, respectively. Taking the azimuthal orientation of the STO substrate determined separately into account, the following orientation relation holds for the entire film, where 100 denotes the four different directions 100, 1 00, 010, and 01 0 on the 001 STO substrate surface: 120 O PZO 001 STO, 21 0 O PZO 100 STO. 1 The four azimuthally different 120 O -oriented domains are obviously formed during the paraelectric-toantiferroelectric phase transition, because the four directions FIG. 3. Color online Pole figures: a simulated and b measured at fixed 2 = corresponding to the 110 O plane of orthorhombic PZO. Eight peaks are at =17.7 and =71.7, respectively. FIG. 4. a Bright field cross-sectional TEM view of a PZO/SRO/STO 100 heterostructure. The interface between the bottom electrode and the PZO film shows strain contrast due to misfit dislocations induced by the lattice misfit. The highlighted crystal planes are 010 O planes in one of the 120 O domains and b selected area electron diffraction pattern of PZO/SRO/STO , 1 01, 011, and 01 1 of the paraelectric cubic phase are equivalent to each other. Thus each of them may become the antiferroelectric a axis of the 120 O -oriented antiferroelectric phase. Concerning the two remaining 110 directions, viz., 110 and 1 10, they are qualitatively different from the above four directions, because if one of them becomes the antiferroelectric a axis, this will result in 001 O -oriented domains. The domains were also observed by TEM investigations. As an example, Fig. 4 a shows the 010 O planes d =11.8 Å within two neighboring 90 domains see white lines. Selected area electron diffraction SAED, as shown in Fig. 4 b, confirms that PZO is 120 O oriented on SRO/ STO 100. The c axis of the PZO unit cell lies in plane, i.e., the 001 O planes are perpendicular to the substrate. Figure 5 shows a dark-field image taken in the 120 O reflection, showing 60 - and 90 -domain boundaries in the PZO film. 90 -domain boundaries were also identified by HRTEM inset of Fig. 5. These 90 domain boundaries separate two such azimuthal domains which are rotated with respect to each other by 180 around the film normal. If two azimuthal domains meet that are rotated with respect to each other by 90 or 270, then a 60 -domain boundary results, cf. the angle 110 C ; 101 C =60. Straight 60 domain boundaries running under 45 to the 120 O plane were first observed

4 Boldyreva et al. J. Appl. Phys. 102, FIG. 5. TEM dark-field image of a PZO film obtained by the 120 O reflection. Stripes under 45 to the film are 60 domains. Vertical stripes are 90 domains. Inset: HRTEM image of a 90 -domain boundary. optically for bulk crystals 19 and then by transmission electron microscopy by Tanaka et al. 24 As shown in Fig. 5, they can also be present in epitaxial PZO thin films. All PZO films deposited on SRO-covered STO 100 substrates exhibited small particles of approximate size 5 10 nm 2 situated directly on the PZO/SRO interface. In TEM images, they showed up by Moiré contrast. As systematic experiments not shown have revealed, their presence does not depend on laser energy or oxygen partial pressure. A HRTEM image of such a particle at the PZO/SRO interface is shown in Fig. 6. Fast Fourier transform FFT analyses see insets revealed that these particles consist of PZO but have a different orientation from that of the surrounding 120 O -oriented film matrix, viz., an 001 O orientation cf. scheme in the figure. It appears that in these particles, the antiferroelectric a axis has formed along the 110 or 1 10 axis of the paraelectric phase, i.e., their orientation corresponds to that of the pyramid-shaped 001 O -oriented domains known for films thicker than 260 nm. All samples described before have thicknesses below this threshold thickness of 260 nm. A TEM cross-sectional image of a sample with a thickness of 390 nm is shown in Fig. 7 a. Three pyramids, two of which are highlighted, are seen in this figure. These pyramids were studied by SAED not shown and turned out to, indeed, have a 001 O orientation. Correspondingly, on XRD patterns 00l O PZO peaks appear Fig. 7 b. In the figure, only the 008 O PZO peak is clearly seen, because the other 00l O PZO peaks overlap with peaks from the 120 O -oriented film and the substrate. Evidence of the 001 O oriented pyramids was also obtained by XRD scans not shown. From the measurements of the macroscopic film properties, Fig. 8 a shows polarization-voltage and switching current curves. The polarization hysteresis consists of a double loop, which is a clear sign of antiferroelectricity. The loop shape is close to a square shape, and the polarization value at zero voltage is close to, although not exactly, zero. The switching current curve has four peaks corresponding to the steep sections of the polarization curve. Figure 8 b shows a FIG. 6. HRTEM image of a particle near the PZO/SRO interface. A comparison of the fast Fourier transform FFT of the bare PZO film upper left inset with that of a film region containing a particle upper right inset revealed the orientation of the PZO particle see schemes on the bottom. The particle is 001 O oriented in a 120 O -oriented surrounding. The white arrows point to the reflections originating from the particle. capacitance-voltage curve which has a modified butterfly shape again a sign of antiferroelectricity. Although the PZO films consist of four domain variants, the latter do not play a role for the macroscopic properties, because all four domain variants have the same perpendicular polarization component. As a consequence, the antiferroelectric axis of each of the four 120 O domains lies under 45 to the film. Considering the nonzero value of the polarization at zero voltage, there is obviously a small ferroelectric subloop present, which shows a remanent polarization of about 1.5 C/cm 2. This ferroelectric behavior is even better seen in the switching current curve in Fig. 8 a and in the C-V characteristics Fig. 8 b. The origin of this unexpected behavior is unclear at the moment. A possible explanation might be a distortion of the antiferroelectric order by defects. This possibility has been explicitly mentioned, e.g., by Tanaka et al. for the case of antiphase boundaries. 24 On the other hand, an intrinsic weak ferroelectricity with a saturation polarization of the order of 0.1 C/cm 2 was observed in PZO ceramics and attributed to a coexistence of ferroelectric and antiferroelectric phonon modes. 25

5 Boldyreva et al. J. Appl. Phys. 102, FIG. 7. Color online a TEM cross-sectional view of a 390 nm thick PZO film on SRO/STO 100 and b XRD pattern of this sample. The STO l00 peaks are marked with an asterisk. The PZO film has a preferred 120 O orientation; additional PZO 00l peaks appear. Symbol O designates peaks originating from K radiation, and symbol # those originating from W L radiation. Figure 9 a shows a piezoelectric hysteresis loop obtained by PFM. It has the shape of a double loop, which confirms the antiferroelectric properties of the PZO films. The temperature dependence of capacitance and dielectric constant of a Pt/PZO/SRO/STO 100 heterostructure of FIG. 8. Color online Macroscopic electric properties of a 120 O oriented PZO film. a Polarization vs applied voltage thick red dots and current vs bias voltage thin blue line. b Capacitance-voltage curve. All measurements were performed at 1 khz. The saturation polarization P S is 41 C/cm 2 and the value of the critical field E C is 445 kv/cm. FIG. 9. Color online a Local hysteresis loop of a PZO thin film on SRO/STO 100 obtained by piezoresponse scanning force microscopy. b Temperature dependence of the capacitance increasing temperature only, 10 khz, revealing a transition temperature of 260 C. 390 nm PZO thickness, recorded at 10 khz during heating in vacuum, is shown in Fig. 9 b. The antiferroelectricparaelectric transition temperature is at 260 C, which is 30 K higher than the normal transition temperature of bulk PZO. 7 This is probably due to some stabilization of the antiferroelectric phase by substrate-induced strain, as known, e.g., for the ferroelectric phase of epitaxial BaTiO 3 thin films on GdScO 3 and DyScO 3 single crystal substrates. 26 An intimate relation between the field-induced antiferroelectric-toferroelectric phase transition and field-induced strain is well known, as recently demonstrated by Kanno et al. 27 Notably in Fig. 9 b, the capacitance i.e., the dielectric constant increases by a factor of about 6 on heating from room temperature to the transition temperature. This is clearly higher than the previously reported increase by a factor of 3 for polycrystalline films, 13 which indicates a high crystalline quality of our films. Above the transition temperature, the curve in Fig. 9 b deviates from a Curie-Weiss behavior, which is in accordance with previous observations. 13 We attribute this to a loss of oxygen due to heating in vacuum 10 5 mbar during the slow measurement procedure temperature ramping rate of 2 K/min, most probably resulting in a high concentration of oxygen vacancies that modifies the electrical properties. Another indication of this fact is the observation that we were not able to record a similar capacitance curve on cooling, because the properties of the films had too much changed during heating in vacuum at temperatures above 260 C. However, after annealing at 350 C in a pure oxygen atmosphere, the initial electrical properties were fully recovered.

6 Boldyreva et al. J. Appl. Phys. 102, FIG. 10. Color online XRD pattern of PZO/BaPbO 3 /BaZrO 3 /STO. BaPbO 3 and BaZrO 3 grow 100 oriented. PbZrO 3 grows 001 O oriented, with c axis out of plane. The STO l00 peaks are marked with an asterisk. B. PbZrO 3 on 100 BaPbO 3 / 100 BaZrO 3 / 100 SrTiO 3 To obtain another crystallographic orientation of the PZO thin films, BaPbO 3 with a cubic lattice parameter of Å has been chosen as bottom electrode. A detailed discussion of crystallography and interfacial misfit at the real and hypothetical PZO 120 O /SrRuO 3 001, PZO 001 O /SrRuO 3 001, PZO 120 O /BaPbO 3 001, and PZO 001 O /BaPbO interfaces is given in Sec. III C. It will show why the use of BaPbO 3 as bottom electrode was promising to achieve an orientation of PZO different from 120 O. To exclude a possible reactivity of SrTiO 3 with BaPbO 3, BaZrO 3 was used as a buffer layer in-between. Figure 10 shows a -2 scan, indicating a preferred 001 O orientation for the PbZrO 3 film and 100 orientations for BaZrO 3 BZO and BaPbO 3 BPO. The PbZrO 3 film has a uniform 001 O orientation; a detailed analysis not shown revealed two azimuthal domains, in analogy to the four azimuthal domains of the 120 O -oriented films. Correspondingly, the c axis of the PZO unit cell lies perpendicular to the film plane and the a and b axes lie in plane. Taking the azimuthal orientation of the STO substrate determined separately into account, the following orientation relation holds for the entire film, where 110 denotes the two different directions 110 and 11 0 on the 001 STO substrate surface, FIG. 11. TEM cross-sectional image of a PZO/BaPbO 3 /BaZrO 3 /STO sample. the transition, the crystal symmetry of PZO transforms from orthorhombic to rhombohedral, involving a ferroelectric axis along the 111 rh -direction in the rhombohedral unit cell. 29 This direction is at an angle of 35 to the film plane. Thus the ferroelectric axis has a component perpendicular to the film plane, which gives rise to the double loop. The values of the saturation polarization P S and the critical field E C are different for 120 O -oriented PZO films P S 001 O PZO 001 STO, 100 O PZO 110 STO. 2 Figure 11 shows a cross-sectional TEM image of such a sample, with thicknesses of the BZO buffer layer of 100 nm, the BPO electrode layer of 175 nm, and the PZO film of 200 nm. The columnar nature of the BZO buffer layer, and the threading dislocations in the epitaxial PZO film are clearly revealed. The dotted contrast of the BPO electrode layer is most probably due to radiation damage during ionbeam thinning and/or in TEM. In these PZO films, the antiferroelectric axis lies parallel to the film surface and has no component perpendicular to the film plane. Nevertheless, surprisingly double hysteresis loops were observed both in macroscopic measurements Fig. 12 a, and by PFM Fig. 12 b. Although the antiferroelectric axis lies in plane, double loops were found, because a structural phase transition takes place under the applied field: 28 If the applied field exceeds the critical value for FIG. 12. Color online a Macroscopic ferroelectric hysteresis curve of a 001 O -oriented PZO film deposited on SrTiO 3 with BZO as buffer layer and BPO as bottom electrode. The saturation polarization P s is 24 C/cm 2. The value of the critical field E C is 500 kv/cm. b Local piezoelectric hysteresis loop of a 001 O -oriented PZO thin film obtained by PFM.

7 Boldyreva et al. J. Appl. Phys. 102, =41 C/cm 2 ; E C =445 kv/cm and 001 O -oriented PZO films P S =24 C/cm 2 ; E C =500 kv/cm, although the fieldinduced antiferroelectric-to-ferroelectric phase transition should result in a rhombohedral phase with similar orientations in these two cases. The reason for the difference is unclear at the moment, and requires further investigation. Defects, or the different strain conditions involved due to the different lattice parameters of the electrode and the different crystallographic orientation of the antiferroelectric phase, may play a role, especially in view of the general role of strain in the antiferroelectric-ferroelectric phase transition. 27 However, a thickness- or interface-related effect may also be present. For specific values of strain-related interfacial lattice misfit, see below. C. Origin of the two different PZO orientations The fact that the PZO films grow predominantly 120 O -oriented on 001 SrRuO 3 electrodes, but 001 O oriented on 001 BaPbO 3 electrodes, can be easily understood from a consideration of the different values of lattice misfit occurring at the different PZO/electrode interfaces. Let us consider four hypothetical PZO/electrode interfaces, viz, the A PZO 120 O /SrRuO 3 001, B PZO 001 O / SrRuO 3 001, C PZO 120 O /BaPbO 3 001, and D PZO 001 O /BaPbO interfaces, and let us calculate the corresponding values of the lattice misfit in two mutually perpendicular azimuthal directions for each of these four interfaces, and finally compare these values. To this end, designating the electrode as E, from Eqs. 1 and 2 we derive the following crystallographic relations regarding crystallographic planes which are perpendicular to the interface plane and to each other, viz, for the cases A and C, 12 0 O PZO 100 E and 001 O PZO 010 E, 3 and for the cases B and D, 100 O PZO 110 E and 010 O PZO 11 0 E. 4 We consider the following d values of crystallographic planes in Eqs. 3 and 4 i.e., their interplanar spacings or their corresponding fractional values, calculating them from standard textbook formulae, For PZO, d 12 0 O = Å; d 002 O = Å; d 200 O = 2.94 Å; and d 020 O = Å. For E=SrRuO 3, considering a pseudocubic unit cell, d 100 = d 010 = Å; and d 110 = d 11 0 = 2.78 Å. For E=BaPbO 3, d 100 = d 010 = Å; and TABLE I. Values of lattice misfits f calculated from Eq. 5 for the four hypothetical interfaces A D, taking into account Eqs P designates PZO and E the electrode, which for each case is specified in the left column. First perp. planes and Second perp. planes are the crystal planes in PZO and in the electrode, respectively which are perpendicular to the plane of the interface according to Eqs. 3 and 4. Interface d 110 = d 11 0 = Å. The interfacial lattice misfit f in% for each case is calculated from these d values, considering Eqs. 3 and 4, according to the standard textbook formula f = 200 d PZO d E / d PZO + d E, with d PZO for PZO and d E for the corresponding electrode, resulting in the misfit values given in Table I. To obtain a figure of merit that allows to compare the overall misfit situation for the four different interfaces A D, we define an interfacial figure of merit G ifm as follows: G ifm = f f 2 2, First perp. planes f 1 Second perp. planes f 2 G ifm A 12 0 O P 100 E 001 O P 010 E PZO 120 O /SrRuO % +4.7% B 100 O P 110 E 010 O P 11 0 E 65.0 PZO 001 O /SrRuO % +5.8% C 12 0 O P 100 E 001 O P 010 E PZO 120 O /BaPbO % 3.57% D 100 O P 110 E 010 O P 11 0 E PZO 001 O /BaPbO % 2.3% where f 1 and f 2 are the misfit values for the same interface but for two different, mutually perpendicular directions, each of which is perpendicular to the crystal planes given in Eqs. 3 and 4, respectively. The smaller value of G ifm should be equivalent to a lower interfacial energy and thus indicate the preferred orientation of the PZO film on the specific electrode. As Table I shows, for the 001 SrRuO 3 electrode, cases A and B, the smaller value of G ifm is achieved for the 120 O orientation of PZO, viz, case A. For the 001 BaPbO 3 electrode, cases C and D, however, the smaller value of G ifm is attained for the 001 O orientation of PZO, viz, case D. This should explain why PZO grows 120 O oriented on 001 SrRuO 3, but 001 O oriented on 001 BaPbO 3, and this was, in fact, our reason to choose BaPbO 3 for a second electrode. The relatively small ratio of 65.0/55.73=1.17 of the G ifm values for SrRuO 3, compared to the larger ratio of 18.65/11.79=1.6 for BaPbO 3 is an indication for the relatively small preference of the 120 O PZO orientation compared to the 001 O PZO orientation on SrRuO 3 electrodes. It can thus explain why 001 O -oriented small particles are also present at the 120 O PZO/ 001 SRO interface. 5 6

8 Boldyreva et al. J. Appl. Phys. 102, IV. CONCLUSIONS PbZrO 3 epitaxial thin films were grown by pulsed laser deposition. Different orientations were achieved due to epitaxial electrodes with different lattice parameters, viz, SrRuO 3 and BaPbO 3. The films were investigated by AFM, PFM, HR TEM, XRD, and macroscopic electrical measurements. They have smooth surfaces with low rms roughness. The preferred orientation of the films deposited on SRO/STO is 120 O, and 001 O for the films deposited on BaPbO 3 /BaZrO 3 /SrTiO Four variants of domains are present in the 120 O -oriented films, and two in the 001 O -oriented films. In 120 O -oriented films thicker than 260 nm, additional 001 O -oriented parts in the form of pyramids were observed. Macroscopic electrical measurements and PFM investigations demonstrate the antiferroelectric behavior of the investigated PZO films of both orientations. An antiferroelectric-to-paraelectric transition temperature of 260 C has been found, which is 30 K higher than the bulk value, probably indicating a stabilization of the antiferroelectric phase by substrate-induced strain. ACKNOWLEDGMENTS The authors are thankful to Dr. S. Schmidt for help with TEM work, and to Ms. S. Swatek and Ms. S. Hopfe for TEM sample preparation. This work is financially supported by DFG via the Group of Researchers FOR 404 at Martin- Luther-Universität Halle-Wittenberg. One of the authors D.B. gratefully acknowledges support from the Alexander von Humboldt Foundation, Germany, and also from NSFC Nos. U and and FANEDD No B. Xu, N. G. Pai, and L. E. Cross, Mater. Lett. 34, X. Li, J. Zhai, and H. Chen, J. Appl. Phys. 97, K. Yamakawa, K. Wa Gachigi, S. Trolier-McKinstry, and J. P. Dougherty, J. Mater. Sci. 32, B. Xu, Y. Ye, and L. E. Cross, J. Appl. Phys. 87, M. P. Moret, J. J. Schermer, F. D. Tichelaar, E. Aret, and P. R. Hageman, J. Appl. Phys. 92, E. Sawaguchi, G. Shirane, and Y. Takagi, J. Phys. Soc. Jpn. 6, G. Shirane, E. Sawaguchi, and Y. Takagi, Phys. Rev. 84, K. Singh, Ferroelectrics 94, A. S. Mischenko, Q. Zhang, J. F. Scott, R. W. Whatmore, and N. D. Mathur, Science 311, D. Berlincourt, IEEE Trans. Sonics Ultrason. 13, B. Jaffe, Proc. IRE 49, K. Yamakawa, S. Trolier-McKinstry, and J. P. Dougherty, Appl. Phys. Lett. 67, S. S. N. Bharadwaja and S. B. Krupanidhi, J. Appl. Phys. 86, J. Parui and S. B. Krupanidhi, J. Appl. Phys. 100, G. R. Bai, H. L. M. Chang, D. J. Lam, and Y. Gao, Appl. Phys. Lett. 62, C. J. Lu, H. M. Shen, and Y. N. Wang, J. Cryst. Growth 191, I. Kanno, S. Hayashi, M. Kitagawa, R. Takayama, and T. Hirao, Appl. Phys. Lett. 66, D. L. Corker, A. M. Glazer, J. Dec, K. Roleder, and R. W. Whatmore, Acta Crystallogr., Sect. B: Struct. Sci. 53, F. Jona, G. Shirane, and R. Pepinsky, Phys. Rev. 97, G. Koster, B. L. Kropman, G. J. H. M. Rijnders, D. H. A. Blank, and H. Rogalla, Appl. Phys. Lett. 73, W. Hong, H. N. Lee, M. Yoon, H. M. Christen, D. H. Lowndess, Z. Suo, and Z. Zhang, Phys. Rev. Lett. 95, Nanoscale Characterization of Ferroelectric Materials Scanning Probe Microscopy Approach, edited by M. Alexe and A. Gruverman Springer, Berlin C. Harnagea, A. Pignolet, M. Alexe, D. Hesse, and U. Gösele, Appl. Phys. A: Mater. Sci. Process. 70, M. Tanaka, R. Saito, and K. Tsuzuki, Jpn. J. Appl. Phys., Part 1 21, X. H. Dai, J.-F. Li, and D. Viehland, Phys. Rev. B 51, K. J. Choi, M. Biegalski, Y. L. Li, A. Sharan, J. Schubert, R. Uecker, P. Reiche, Y. B. Chen, X. Q. Pan, V. Gopalan, L.-Q. Chen, D. G. Schlom, and C. B. Eom, Science 306, I. Kanno, T. Inoue, T. Suzuki, H. Kotera, and K. Wasa, Jpn. J. Appl. Phys. 45, G. A. Smolenskii, V. A. Bokov, V. A. Isupov, N. N. Krainik, R. E. Pasynkov, and A. I. Sokolov, Ferroelectrics and Related Materials Gordon and Breach, New York, 1984, Vol O. E. Fesenko, R. V. Kolesova, and Yu. G. Sindeyev, Ferroelectrics 20,