Microstructure and electrical properties of - Max Planck Institute of ...

JOURNAL OF APPLIED PHYSICS 102, 044111 共2007兲

Microstructure and electrical properties of „120…O-oriented and of „001…O-oriented epitaxial antiferroelectric PbZrO3 thin films on „100… SrTiO3 substrates covered with different oxide bottom electrodes Ksenia Boldyrevaa兲 Max Planck Institute of Microstructure Physics, Weinberg 2, D-06120 Halle (Saale), Germany

Dinghua Bao Max Planck Institute of Microstructure Physics, Weinberg 2, D-06120 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 510275, People’s Republic of China

Gwenael Le Rhun, Lucian Pintilie, Marin Alexe, and Dietrich Hesseb兲 Max Planck Institute of Microstructure Physics, Weinberg 2, D-06120 Halle (Saale), Germany

共Received 12 March 2007; accepted 30 June 2007; published online 23 August 2007兲 Epitaxial antiferroelectric PbZrO3 共PZO兲 thin films of two different crystallographic orientations were grown by pulsed laser deposition on 共100兲-oriented SrTiO3 single crystal substrates. The latter were covered either with SrRuO3 epitaxial bottom electrodes, or with an epitaxial BaZrO3 buffer layer and an epitaxial BaPbO3 bottom electrode, respectively. Their crystal orientation and microstructure were characterized by x-ray diffraction, transmission electron microscopy, and electron diffraction. The orthorhombic 共index O兲 PZO films on SrRuO3 / SrTiO3 were predominantly 共120兲O oriented and consisted of four azimuthal domains forming 90° and 60° boundaries, whereas those grown on BaPbO3 / BaZrO3 / SrTiO3 were 共001兲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 PS and the critical field EC of the 共120兲O-oriented PZO films 共PS = 41 ␮C / cm2; EC = 445 kV/ cm兲 are different from those of the 共001兲O-oriented films 共PS = 24 ␮C / cm2; EC = 500 kV/ cm兲. 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. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2769335兴 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 PbZrO3 共PZO兲 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兲

Electronic mail: [email protected] Electronic mail: [email protected]

b兲

0021-8979/2007/102共4兲/044111/8/$23.00

However, reports on growth and properties of epitaxial, antiferroelectric PbZrO3 films are rather limited so far. Yamakawa et al. studied 共120兲O-oriented 共the index O refers to orthorhombic and the index C to pseudocubic indexing兲 PZO thin films deposited by reactive magnetron cosputtering,12 and Bharadwaja and Krupanidhi deposited columnar 共110兲C-oriented PZO films onto Pt-covered Si substrates by pulsed laser deposition.13 Parui and Krupanidhi investigated the dielectric properties of 共110兲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 LaAlO3 or SrTiO3, 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 共100兲MgO and Pt/共100兲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 SrRuO3-covered

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© 2007 American Institute of Physics

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FIG. 1. 共Color online兲 AFM images of 共a兲 a STO共100兲 substrate with terraces, after etching and annealing, 共b兲 a SRO thin film 共40 nm兲 deposited on a STO共100兲 substrate, and 共c兲 a PZO thin film 共50 nm兲 on STO共100兲 covered with SRO.

共100兲SrTiO3 and on BaPbO3-covered and BaZrO3-buffered 共100兲SrTiO3 single crystal substrates, respectively. PZO has an orthorhombic crystal structure at temperatures below 230 ° C 共in the antiferroelectric phase兲 and a cubic structure above 230 ° C 共in the paraelectric phase兲. On cooling through the phase transition, antiferroelectricity occurs due to an alternate pairwise shift of Pb ions along the ¯¯10兴 directions. The lattice constants of the 关110兴C and 关1 C antiferroelectric, orthorhombic phase at room temperature are a = 5.88 Å, b = 11.787 Å, c = 8.231 Å.18 The antiferroelectric axis is along the 关100兴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 4.137 Å, the idea to consider a perovskite unit with a lattice parameter of aCp = 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 共PLD兲共KrF excimer laser, ␭ = 248 nm兲 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 SrTiO3 共STO兲 substrates 共cubic, lattice parameter a = 3.905 Å兲 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, SrRuO3 共SRO兲共pseudocubic with a = 3.928 Å兲 or BaPbO3 共BPO兲共cubic, a = 4.265 Å兲 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 共of only 0.6%兲. In the case of the BaPbO3 electrodes, BaZrO3 共BZO兲共cubic, a = 4.19 Å兲 was used as a buffer layer. The laser fluence was set at 1.5, 0.9, 1.6, and 1.2 J / cm2 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 共AFM兲 in tapping mode using a Digital Instruments D5000 microscope. Phase contents and crystallographic orientation of the films were characterized by x-ray diffraction 共XRD兲 using a Philips X’Pert MRD four-circle diffractometer with Cu K␣ radiation. Samples for transmission electron microscopy 共TEM兲 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 共HRTEM兲 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 共PFM兲共Ref. 22兲 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. PbZrO3 on „100…SrRuO3 / „100…SrTiO3

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 共120兲O orientation of the PZO film. A corresponding pole figure 共center ␺ = 0; rim ␺ = 90°; ␺ = 90° corresponds to the substrate surface being parallel to the plane defined by the incident and reflected x-ray beams兲, taken at 2␪ = 16.833° 关corresponding to PZO共110兲O兴 is shown in Fig. 3共b兲. Notwithstanding the asymmetric position of the 共120兲O-oriented unit cell, the pole figure shows a fourfold


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FIG. 2. 共Color online兲 X-ray diffraction pattern of a PZO thin film on a STO共100兲 substrate covered with SRO. The peaks of STO共l00兲 are marked with an asterisk. Symbol “O” designates peaks originating from K␤ radiation, and symbol “#” a peak originating from WL radiation due to the contamination of the anode by tungsten from the cathode.

symmetry. This points to a fourfold positioning 共i.e., the presence of four types of azimuthally oriented domains兲 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 共120兲O-oriented domains, as long as the film thickness did not exceed 260 nm. For thicker films, two additional types of 共001兲O-oriented domains grown in the form of pyramids were found.5 Figure 3共a兲 shows the simulation of a pole figure for only one azimuthal 共120兲O domain taken at 2␪ = 16.833. It consists of two peaks at ␺ = 18.42° and ␺ = 71.44°, the ␺ values of which correspond to the angles ¯ 0兲 = 71.447°. It is ⬔共120兲 ; 共110兲 = 18.422° and ⬔共120兲 ; 共11 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 共determined separately兲 into account, the following orientation relation holds for the entire film, where 具100典 ¯ 00兴, 关010兴, and denotes the four different directions 关100兴, 关1 ¯ 关010兴 on the 共001兲STO substrate surface: 共120兲OPZO 储共001兲STO,

¯ 0兴 PZO 储具100典STO. 关21 O

共1兲

The four azimuthally different 共120兲O-oriented domains are obviously formed during the paraelectric-toantiferroelectric phase transition, because the four directions

FIG. 3. 共Color online兲 Pole figures: 共a兲 simulated and 共b兲 measured at fixed 2␪ = 16.833° corresponding to the 共110兲O plane of orthorhombic PZO. Eight peaks are at ␺ = 17.7° and ␺ = 71.7°, respectively.

FIG. 4. 共a兲 Bright field cross-sectional TEM view of a PZO/SRO/STO共100兲 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 共010兲O planes in one of the 共120兲O domains and 共b兲 selected area electron diffraction pattern of PZO/SRO/STO共100兲.

¯ 01兴, 关011兴, and 关01 ¯ 1兴 of the paraelectric cubic phase 关101兴, 关1 are equivalent to each other. Thus each of them may become the antiferroelectric a axis of the 共120兲O-oriented antiferroelectric phase. Concerning the two remaining 具110典 direc¯ 10兴, they are qualitatively different tions, viz., 关110兴 and 关1 from the above four directions, because if one of them becomes the antiferroelectric a axis, this will result in 共001兲O-oriented domains. The domains were also observed by TEM investigations. As an example, Fig. 4共a兲 shows the 共010兲O planes 共d = 11.8 Å兲 within two neighboring 90° domains 共see white lines兲. Selected area electron diffraction 共SAED兲, as shown in Fig. 4共b兲, confirms that PZO is 共120兲O oriented on SRO/ STO共100兲. The c axis of the PZO unit cell lies in plane, i.e., the 共001兲O planes are perpendicular to the substrate. Figure 5 shows a dark-field image taken in the 关120兴O reflection, showing 60°- and 90°-domain boundaries in the PZO film. 90°-domain boundaries were also identified by HRTEM 共inset of Fig. 5兲. 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 ¯ 兴 = 60°. Straight 60° domain boundaries angle ⬔关110兴C; 关101 C running under 45° to the 共120兲O plane were first observed


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FIG. 5. TEM dark-field image of a PZO film obtained by the 共120兲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 crystals19 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共100兲 substrates exhibited small particles 共of approximate size 5 ⫻ 10 nm2兲 situated directly on the PZO/SRO interface. In TEM images, they showed up by Moiré contrast. As systematic experiments 共not shown兲 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 共FFT兲 analyses 共see insets兲 revealed that these particles consist of PZO but have a different orientation from that of the surrounding 共120兲O-oriented film matrix, viz., an 共001兲O orientation 共cf. scheme in the figure兲. It appears that in these particles, the ¯ 10兴 antiferroelectric a axis has formed along the 关110兴 or 关1 axis of the paraelectric phase, i.e., their orientation corresponds to that of the pyramid-shaped 共001兲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共a兲. Three pyramids, two of which are highlighted, are seen in this figure. These pyramids were studied by SAED 共not shown兲 and turned out to, indeed, have a 共001兲O orientation. Correspondingly, on XRD patterns 共00l兲O PZO peaks appear 关Fig. 7共b兲兴. In the figure, only the 共008兲O PZO peak is clearly seen, because the other 共00l兲O PZO peaks overlap with peaks from the 共120兲O-oriented film and the substrate. Evidence of the 共001兲O oriented pyramids was also obtained by XRD ␻ scans 共not shown兲. From the measurements of the macroscopic film properties, Fig. 8共a兲 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共b兲 shows a

FIG. 6. HRTEM image of a particle near the PZO/SRO interface. A comparison of the fast Fourier transform 共FFT兲 of the bare PZO film 共upper left inset兲 with that of a film region containing a particle 共upper right inset兲 revealed the orientation of the PZO particle 共see schemes on the bottom兲. The particle is 共001兲O oriented in a 共120兲O-oriented surrounding. 共The white arrows point to the reflections originating from the particle.兲

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 共120兲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 / cm2. This ferroelectric behavior is even better seen in the switching current curve in Fig. 8共a兲 and in the C-V characteristics 关Fig. 8共b兲兴. 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 / cm2 was observed in PZO ceramics and attributed to a coexistence of ferroelectric and antiferroelectric phonon modes.25


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FIG. 7. 共Color online兲共a兲 TEM cross-sectional view of a 390 nm thick PZO film on SRO/STO共100兲 and 共b兲 XRD pattern of this sample. The STO共l00兲 peaks are marked with an asterisk. The PZO film has a preferred 共120兲O orientation; additional PZO共00l兲 peaks appear. Symbol “O” designates peaks originating from K␤ radiation, and symbol “#” those originating from WL radiation.

Figure 9共a兲 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共100兲 heterostructure of

FIG. 8. 共Color online兲 Macroscopic electric properties of a 共120兲O oriented PZO film. 共a兲 Polarization vs applied voltage 共thick red dots兲 and current vs bias voltage 共thin blue line兲. 共b兲 Capacitance-voltage curve. All measurements were performed at 1 kHz. The saturation polarization PS is 41 ␮C / cm2 and the value of the critical field EC is 445 kV/ cm.

J. Appl. Phys. 102, 044111 共2007兲

FIG. 9. 共Color online兲共a兲 Local hysteresis loop of a PZO thin film on SRO/STO共100兲 obtained by piezoresponse scanning force microscopy. 共b兲 Temperature dependence of the capacitance 共increasing temperature only, 10 kHz兲, revealing a transition temperature of 260 ° C.

390 nm PZO thickness, recorded at 10 kHz during heating in vacuum, is shown in Fig. 9共b兲. 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 BaTiO3 thin films on GdScO3 and DyScO3 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共b兲, the capacitance 共i.e., the dielectric constant兲 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共b兲 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 共10−5 mbar兲 during the slow measurement procedure 共temperature ramping rate of 2 K / min兲, 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.


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FIG. 10. 共Color online兲 XRD pattern of PZO/ BaPbO3 / BaZrO3 / STO. BaPbO3 and BaZrO3 grow 共100兲 oriented. PbZrO3 grows 共001兲O oriented, with c axis out of plane. The STO共l00兲 peaks are marked with an asterisk.

B. PbZrO3 on „100…BaPbO3 / „100…BaZrO3 / „100…SrTiO3

To obtain another crystallographic orientation of the PZO thin films, BaPbO3 with a cubic lattice parameter of 4.265 Å has been chosen as bottom electrode. A detailed discussion of crystallography and interfacial misfit at the 共real and hypothetical兲 PZO共120兲O / SrRuO3共001兲, PZO共001兲O / SrRuO3共001兲, PZO共120兲O / BaPbO3共001兲, and PZO共001兲O / BaPbO3共001兲 interfaces is given in Sec. III C. It will show why the use of BaPbO3 as bottom electrode was promising to achieve an orientation of PZO different from 共120兲O. To exclude a possible reactivity of SrTiO3 with BaPbO3, BaZrO3 was used as a buffer layer in-between. Figure 10 shows a ␪-2␪ scan, indicating a preferred 共001兲O orientation for the PbZrO3 film and 共100兲 orientations for BaZrO3 共BZO兲 and BaPbO3 共BPO兲. The PbZrO3 film has a uniform 共001兲O orientation; a detailed analysis 共not shown兲 revealed two azimuthal domains, in analogy to the four azimuthal domains of the 共120兲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 共determined separately兲 into account, the following orientation relation holds for the entire film, where 具110典 denotes the two different directions ¯ 0兴 on the 共001兲STO substrate surface, 关110兴 and 关11 共001兲OPZO 储共001兲STO,

关100兴OPZO 储具110典STO.

FIG. 11. TEM cross-sectional image of a PZO/ BaPbO3 / BaZrO3 / STO sample.

the transition, the crystal symmetry of PZO transforms from orthorhombic to rhombohedral, involving a ferroelectric axis along the 关111兴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 PS and the critical field EC are different for 共120兲O-oriented PZO films 共PS

共2兲

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 关Fig. 12共a兲兴, and by PFM 关Fig. 12共b兲兴. 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. 共Color online兲共a兲 Macroscopic ferroelectric hysteresis curve of a 共001兲O-oriented PZO film deposited on SrTiO3 with BZO as buffer layer and BPO as bottom electrode. The saturation polarization Ps is 24 ␮C / cm2. The value of the critical field EC is 500 kV/ cm. 共b兲 Local piezoelectric hysteresis loop of a 共001兲O-oriented PZO thin film obtained by PFM.


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= 41 ␮C / cm2; EC = 445 kV/ cm兲 and 共001兲O-oriented PZO films 共PS = 24 ␮C / cm2; EC = 500 kV/ cm兲, 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 共120兲O-oriented on 共001兲SrRuO3 electrodes, but 共001兲O oriented on 共001兲BaPbO3 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 共B兲 PZO共001兲O / 共A兲 PZO共120兲O / SrRuO3共001兲, SrRuO3共001兲, 共C兲 PZO共120兲O / BaPbO3共001兲, and 共D兲 PZO共001兲O / BaPbO3共001兲 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. 共1兲 and 共2兲 we derive the following crystallographic relations regarding crystallographic planes which are perpendicular to the interface plane and to each other, viz, for the cases 共A兲 and 共C兲, ¯ 0兲 PZO 储共100兲E 共12 O

and 共001兲OPZO 储共010兲E,

共3兲

and for the cases 共B兲 and 共D兲, 共100兲OPZO 储共110兲E

¯ 0兲E. and 共010兲OPZO 储共11

共4兲

We consider the following d values of crystallographic planes in Eqs. 共3兲 and 共4兲共i.e., their interplanar spacings or their corresponding fractional values兲, calculating them from standard textbook formulae, •

For PZO, ¯ 0兲 = 4.1625 Å; d共12 O d共002兲O = 4.1155 Å; d共200兲O = 2.94 Å; and d共020兲O = 2.947 Å.

•

For E = SrRuO3, considering a pseudocubic unit cell, d共100兲 = d共010兲 = 3.928 Å; and ¯ 0兲 = 2.78 Å. d共110兲 = d共11

•

For E = BaPbO3, d共100兲 = d共010兲 = 4.265 Å; and

TABLE I. Values of lattice misfits f calculated from Eq. 共5兲 for the four hypothetical interfaces 共A兲–共D兲, taking into account Eqs. 共1兲–共4兲. 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 共in PZO and in the electrode, respectively兲 which are perpendicular to the plane of the interface according to Eqs. 共3兲 and 共4兲.

Interface 共A兲 PZO共120兲O / SrRuO3共001兲共B兲 PZO共001兲O / SrRuO3共001兲共C兲

First perp. planes Second perp. planes f2 f1 ¯ 0兲 P 储共100兲E 共12 O +5.8% 共100兲O P 储共110兲E +5.6%

Gifm

共001兲O P 储共010兲E

55.73

+4.7% ¯ 0兲E 共010兲 P 储共11

65.0

+5.8% 共001兲O P 储共010兲E

18.65 11.79

O

PZO共120兲O / BaPbO3共001兲共D兲

¯ 0兲 P 储共100兲E 共12 O −2.43% 共100兲O P 储共110兲E

−3.57% ¯ 0兲E 共010兲 P 储共11

PZO共001兲O / BaPbO3共001兲

−2.55%

−2.3%

O

¯ 0兲 = 3.016 Å. d共110兲 = d共11 The interfacial lattice misfit f 共in%兲 for each case is calculated from these d values, considering Eqs. 共3兲 and 共4兲, according to the standard textbook formula f = 200共dPZO − dE兲/共dPZO + dE兲,

共5兲

with dPZO for PZO and dE 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 共A兲–共D兲, we define an “interfacial figure of merit” Gifm as follows: Gifm = f 21 + f 22 ,

共6兲

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. 共3兲 and 共4兲, respectively. The smaller value of Gifm 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 共001兲SrRuO3 electrode, cases 共A兲 and 共B兲, the smaller value of Gifm is achieved for the 共120兲O orientation of PZO, viz, case 共A兲. For the 共001兲BaPbO3 electrode, cases 共C兲 and 共D兲, however, the smaller value of Gifm is attained for the 共001兲O orientation of PZO, viz, case 共D兲. This should explain why PZO grows 共120兲O oriented on 共001兲SrRuO3, but 共001兲O oriented on 共001兲BaPbO3, and this was, in fact, our reason to choose BaPbO3 for a second electrode. The relatively small ratio of 65.0/ 55.73= 1.17 of the Gifm values for SrRuO3, compared to the larger ratio of 18.65/ 11.79= 1.6 for BaPbO3 is an indication for the relatively small preference of the 共120兲O PZO orientation compared to the 共001兲O PZO orientation on SrRuO3 electrodes. It can thus explain why 共001兲O-oriented small particles are also present at the 共120兲OPZO/ 共001兲SRO interface.


044111-8

IV. CONCLUSIONS

PbZrO3 epitaxial thin films were grown by pulsed laser deposition. Different orientations were achieved due to epitaxial electrodes with different lattice parameters, viz, SrRuO3 and BaPbO3. The films were investigated by AFM, PFM, 共HR兲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 共120兲O, and 共001兲O for the films deposited on BaPbO3 / BaZrO3 / SrTiO3共100兲. Four variants of domains are present in the 共120兲O-oriented films, and two in the 共001兲O-oriented films. In 共120兲O-oriented films thicker than 260 nm, additional 共001兲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 MartinLuther-Universität Halle-Wittenberg. One of the authors 共D.B.兲 gratefully acknowledges support from the Alexander von Humboldt Foundation, Germany, and also from NSFC 共Nos. U0634006 and 10574164兲 and FANEDD 共No. 200441兲. B. Xu, N. G. Pai, and L. E. Cross, Mater. Lett. 34, 157 共1998兲. X. Li, J. Zhai, and H. Chen, J. Appl. Phys. 97, 024102 共2005兲. 3 K. Yamakawa, K. Wa Gachigi, S. Trolier-McKinstry, and J. P. Dougherty, J. Mater. Sci. 32, 5169 共1997兲. 1 2

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