Enhancing polarization by electrode-controlled strain relaxation in PbTiO3heterostructuresJ. Peräntie, M. S. Stratulat, J. Hannu, H. Jantunen, and M. Tyunina Citation: APL Mater. 4, 016104 (2016); doi: 10.1063/1.4939790 View online: http://dx.doi.org/10.1063/1.4939790 View Table of Contents: http://scitation.aip.org/content/aip/journal/aplmater/4/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Challenges in the stoichiometric growth of polycrystalline and epitaxial PbZr0.52Ti0.48O3/La0.7Sr0.3MnO3multiferroic heterostructures using pulsed laser deposition J. Appl. Phys. 112, 064101 (2012); 10.1063/1.4751027 Octahedral tilt transitions in relaxed epitaxial Pb(Zr1− x Ti x )O3 films J. Appl. Phys. 109, 094104 (2011); 10.1063/1.3580328 Investigation of biferroic properties in La 0.6 Sr 0.4 MnO 3 / 0.7 Pb ( Mg 1 / 3 Nb 2 / 3 ) O 3 – 0.3 PbTiO 3epitaxial bilayered heterostructures J. Appl. Phys. 106, 054103 (2009); 10.1063/1.3211315 Spectroscopic ellipsometry study of epitaxially grown Pb(Mg 1/3 Nb 2/3 ) O 3 – PbTiO 3 / MgO/TiN/Siheterostructures Appl. Phys. Lett. 83, 1599 (2003); 10.1063/1.1603339 Phase development and electrical property analysis of pulsed laser deposited Pb(Mg 1/3 Nb 2/3 )O 3 –PbTiO3 (70/30) epitaxial thin films J. Appl. Phys. 84, 5147 (1998); 10.1063/1.368809

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APL MATERIALS 4, 016104 (2016)

Enhancing polarization by electrode-controlled strainrelaxation in PbTiO3 heterostructures

J. Peräntie,a M. S. Stratulat, J. Hannu, H. Jantunen, and M. Tyuninab

Microelectronics and Materials Physics Laboratories, University of Oulu,P.O. Box 4500, FI-90014 Oulu, Finland

(Received 21 August 2015; accepted 30 December 2015; published online 12 January 2016)

A large remanent polarization close to theoretical value 80 µC/cm2 of bulk PbTiO3is achieved in epitaxial heterostructures of (120–600)-nm-thick PbTiO3 films grownby pulsed laser deposition on (001) SrTiO3 substrate using a 100-nm-thick SrRuO3bottom electrode layer. The heterostructures employing a 50-nm-thick electrodeexhibit a significantly smaller polarization of ≤60 µC/cm2. A detailed x-ray diffrac-tion analysis of the crystal structure allows for relating this large polarization toelectrode-controlled relaxation of epitaxial strain in PbTiO3. Based on the observedresults, we anticipate that the electrode-promoted strain relaxation can be used toenhance polarization in other epitaxial ferroelectric films. C 2016 Author(s). Allarticle content, except where otherwise noted, is licensed under a Creative CommonsAttribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4939790]

Advanced ferroelectric (FE) miniature devices (e.g., piezomotors, pyroelectric detectors, andFE memories) employ FE films between top and bottom electrodes.1,2 The optimal performance ofthe device often requires FE films with a large and/or stable spontaneous polarization in the direc-tion normal to the substrate surface (out-of-plane direction). In particular, the out-of-plane polar-ization in single-crystal-type epitaxial perovskite oxide FE films is larger than in polycrystallinefilms and it may be further enhanced by biaxial in-plane compressive strain due to film-substratemismatch of lattice parameters and thermal expansion coefficients.3,4 For example, high polarizationvalues of around 105 µC/cm2 and 130 µC/cm2 have been demonstrated in strained (001)-orientedepitaxial films of PbZr0.2Ti0.8O3 and tetragonal-like BiFeO3, respectively.5,6

Although significant polarization enhancements can be achieved in epitaxial coherently strainedFE films, an inevitable strain relaxation is encountered above the misfit-dependent critical thickness.This relaxation may proceed through the formation of defects, domains, and surface roughening7,8

and it restricts the usable thickness range of coherently strained films. Moreover, films with thicknessbelow the critical one often possess undesirable scaling effects, such as large leakage currents,strong depolarization fields, and degraded response functions.9–11 Importantly, the strain relaxationin tetragonal FEs may give rise to a formation of a-domains with in-plane polarization that may bedifficult to switch. Although a controlled nano-sized polydomain configurations and related domainwall properties are particularly interesting in future nanoelectronics,12,13 such features are generallyundesirable in applications requiring large out-of-plane polarization.

In general, polarization appears to be less sensitive to the lattice strain in FEs with intrinsicallyhigh polarization (e.g., PbTiO3) compared to other FEs.4,14 A large out-of-plane polarization couldbe principally obtained in PbTiO3 (PTO) and PbZrxTi1−xO3 films if a-domain formation is sup-pressed therein. The formation, characteristic dimensions, and configuration of a/c-domain patternsare controlled by FE film thickness, stress state, and material-specific correlation strength.13,15 Inparticular, a-domain formation in PTO on SrTiO3 (STO) can be suppressed by growing a suffi-ciently thin PTO film below or close to the Curie temperature Tc of the film.16 However, a wider use

aElectronic mail: jani@ee.oulu.fibElectronic mail: marinat@ee.oulu.fi

2166-532X/2016/4(1)/016104/6 4, 016104-1 ©Author(s) 2016

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016104-2 Peräntie et al. APL Mater. 4, 016104 (2016)

of this approach is restricted by small thickness of PTO and low temperatures may be insufficientfor other growth techniques.

In this letter, we show that a large out-of-plane polarization (close to the bulk crystal value)can be achieved in epitaxial PTO films with thickness up to 600 nm grown on (001) STO sub-strates at high deposition temperature of Td = 700 ◦C. PTO films with thicknesses of 120–600 nmwere deposited by pulsed laser ablation using (50–100)-nm-thick SrRuO3 (SRO) bottom electrodes.Parallel plate Pt/PTO/SRO capacitors were formed by shadow-mask deposition of 0.2 mm2 Pt topelectrodes. It is demonstrated that increasing thickness of the bottom SRO layer can promote strainrelaxation and consequent polarization enhancement in PTO.

The SRO-STO lattice mismatch is small with an in-plane strain of s = (aSTO/aSRO − 1)× 100% ≈ −0.6% at Td = 700 ◦C, which allows for a coherent cube-on-cube-type growth of SROon STO.17 A theoretical in-plane strain in PTO on STO is compressive with s = (aSTO/aPTO − 1)× 100% ≈ −1.4% at Td = 700 ◦C and its magnitude decreases on cooling. Thus, it is possible toobtain a room-temperature tetragonal c-PTO film if the misfit strain is not relaxed (i.e., if the filmthickness is small enough).16 When assuming a thicker film with fully relaxed misfit strain at thegrowth temperature, the mismatch between thermal expansion coefficients of PTO and STO willlead to a tensile in-plane strain in PTO above Tc and a change of strain from s ≈ 0.14% to 0.4%at Tc. This tensile strain in PTO increases on cooling below Tc and can reach s ≈ 1.5% at roomtemperature. The resulting relaxation of the misfit and thermal strains, and the magnitude of residualstrain can be influenced by the thickness of PTO as well as by the presence and thickness of SRO,and therefore this relaxation on cooling is theoretically difficult to predict.

The room-temperature crystal structure and strain in the grown Pt/PTO/SRO heterostructureswere studied by x-ray diffraction (XRD) using Cu Kα radiation with D8 Discover diffractometer(Bruker Corporation). Figure 1(a) shows Θ-2Θ XRD pattern for a 120-nm-thick PTO films on a50-nm-thick SRO bottom electrode (PTO120/SRO50 for brevity). Qualitatively similar Θ-2Θ pat-terns were observed for all studied heterostructures. The main diffractions are assigned to parallel(00l)-planes of perovskite PTO, SRO, and STO. A weak diffraction around 2Θ = 40◦ is due tothe used Pt electrodes. In addition, all PTO films showed a nanocolumnar microstructure with anaverage column width of around 50–80 nm (inset of Figure 1(a)).

The average out-of-plane lattice parameters c of the PTO films were determined from the (004)diffractions using a pseudo-Voigt single-peak fitting. The (004) STO substrate diffraction (aSTO

= 3.905 Å)was used as a reference. The extracted parameters are c = 4.097 − 4.129 Å (Table I).Compared to bulk PTO (c = 4.156 Å),18 the c-PTO films experience an out-of-plane compres-sion of approximately sc = −(0.6% − 1.4%) in agreement with the above-discussed in-plane latticestrain. This observation suggests an efficient relaxation of misfit strain at Td. Nevertheless, PTOfilms grow epitaxially in a cube-on-cube-type manner as evidenced by fourfold symmetry of (101)

FIG. 1. (a) Θ−2Θ x-ray diffraction pattern and (b) φ-scans on STO (101) and PTO (101) crystal planes of Pt/PTO/SROheterostructures on STO substrates for PTO120/SRO50 structure. Inset of (a): surface morphology of 120-nm-thick PTOfilm.

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016104-3 Peräntie et al. APL Mater. 4, 016104 (2016)

TABLE I. Structural and ferroelectric characteristics of Pt/PTO/SRO heterostructures.

Structure PTO120/SRO50 PTO200/SRO50 PTO120/SRO100 PTO600/SRO100

PTO thickness, nm 120 200 120 600SRO thickness, nm 50 50 100 100Average c, Å 4.097 4.129 4.123 4.119Vc, % 82 91 100 852Pqs, µC/cm2 120 114 160 1502Pd, µC/cm2 (1 kHz) 120 110 155 1082Ecqs, kV/cm 240 200 300 1302Ecd, kV/cm (1 kHz) 500 315 500 230J, µA/cm2 (0.5 V) 13 120 50 0.5c1//c2, Å 4.122//4.097 4.129//4.106 4.124//4.120 4.119

diffractions in φ-scans (Figure 1(b)). The epitaxial relationship of c-PTO[100](001)||STO[100](001) is confirmed.

Reciprocal space mapping (RSM) was performed for (103) and (002) diffractions in order todetect possible presence of a-domains and to study epitaxy in more detail.19 The RSM results wereplotted by using reciprocal lattice units (r.l.u.) of STO substrate (r.l.u. = 2π/aSTO ≈ 1.609 Å−1).Typical space maps in proximity of (103) STO lattice point are shown in Figures 2(b) and 2(d).Presence of a-domains is detected in the PTO films except in PTO120/SRO100. The volume frac-tion Vc of c-domains was estimated from the ω-rocking curves along the PTO (200) and (002)diffraction peaks (see typical curves in Figures 2(a) and 2(c)) by taking into account the fourfoldsymmetry of PTO (200) domains.20 A large c-domain fraction Vc = 80%−100% (Table I) evidencesthat the PTO films are predominantly c-oriented. The formation of a/c-polydomain state in thetetragonal FE phase is accompanied with tilting of domains away from 90◦. In PTO, this domaintilting can be significant due to large tetragonality (∼1.06 in a relaxed bulk PTO).21,22 Here, tilted

FIG. 2. (a) and (c) ω-rocking curves along the PTO diffractions and (b) and (d) space maps around the (103) reciprocallattice point for (a) and (b) PTO120/SRO50 and (c) and (d) PTO120/SRO100 structures. Black and red symbols in (a) and (c)correspond to rocking curves at 2Θ of PTO (200) and (002) diffractions, respectively.

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016104-4 Peräntie et al. APL Mater. 4, 016104 (2016)

FIG. 3. Quasi-static polarization Pqs as a function of electric field in Pt/PTO/SRO heterostructures with (a) 100 nm and(b) 50 nm thick SRO bottom electrode. Different PTO thicknesses shown in the plots.

PTO a-domains (tilting angle of 2.15◦–2.4◦) with the out-of-plane lattice constant close to that ofSTO are found (Figures 2(a) and 2(b)). Overall, the XRD analysis shows that the grown PTO filmsare pure perovskite, highly c-oriented, cube-on-cube-type epitaxial, with a residual in-plane tensilestrain.

The leakage currents together with quasi-static and dynamic polarization loops were measuredin the PTO capacitors using TF 2000E Analyzer (aixACCT Systems GmbH) and Precision LCIIFerroelectric Tester (Radiant Technologies, Inc.).19 The leakage current density at 0.5 V (in prox-imity of typical Schottky barrier) was (0.5–120) µA/cm2, which is excellent for ferroelectric thin-film capacitors.23 All loops are characterized by double polarization [2P = |P(Ec

+)| + |P(Ec−)|] and

coercive field [2Ec = |Ec+| + |Ec

−|] (Table I). The quasi-static polarization (Figure 3) is the largestand close to the theoretical bulk value24,25 in the c-domain PTO120/SRO100 structure. The polar-ization decreases slightly with increasing PTO thickness to 600 nm, but it drops significantly withsmaller thickness of SRO. Compared to the quasi-static hysteresis, the dynamic loops are influencedby switching kinetics. Importantly, a very large dynamic polarization (2Pd = 160 µC/cm2 at 10 Hz)is achieved in the PTO600/SRO100 structure.19 It is noticed that the large out-of-plane polariza-tion is not directly related to the large c-domain volume fraction Vc. For instance, the quasi-staticpolarization is 2Pqs = 150 µC/cm2 for Vc = 83% (PTO600/SRO100) and 2Pqs = 114 µC/cm2 forVc = 91% (PTO200/SRO50). Also a direct correlation between the polarization and the latticeparameter c is absent. We stress that polarization is larger in heterostructures using 100-nm-thick bottom SRO electrode. A typical thickness of commonly employed SRO electrodes is small∼4 − 40 nm in contrast to thicker SRO layers of this study. In the following, we show that thick SROlayers can enable efficient strain relaxation in PTO, resulting in polarization enhancement.

A detailed inspection of perovskite (004) PTO and SRO diffractions is shown in Figure 4.The SRO layers on STO are cube-on-cube-type epitaxial with excellent crystal quality indicated byLaue satellites. The extracted SRO out-of-plane lattice parameters of 3.958 Å (50 nm) and 3.956 Å(100 nm) are in good agreement with those previously reported for coherent SRO films on STO.17

The (004) PTO diffractions are broad and asymmetric in the films on the 50-nm-thick SROlayer (Figures 4(b) and 4(c)). The obtained good fits by two pseudo-Voigt peaks indicate a coexis-tence of two c-domain groups in these films: first one with average out-of-plane lattice parameter ofc1 ≈ 4.12 − 4.13 Å and another with lattice parameter of c2 ≈ 4.10 Å. The small lattice parameterc2 ≈ 4.10 Å suggests that the c2-domains may be caused by almost complete relaxation of misfitstrain at growth temperature and a consequent buildup of tensile strain on cooling. The PTO filmson the 100-nm-thick SRO layer contain the c1-domains mainly (Figures 4(e) and 4(f)). Becauseof many factors influencing formation of a/c domain patterns,13 a direct correlation between theformation of c2-and a-domains is absent. Our observations imply that the c1-domains are respon-sible for larger polarization, whilst the presence of c2-domains leads to polarization decrease inthe PTO films. Although the mechanism of the decrease is unclear, one may speculate that thec2-domains are non-switchable26 or difficult to switch.

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016104-5 Peräntie et al. APL Mater. 4, 016104 (2016)

FIG. 4. Θ−2Θ x-ray diffraction patterns around (a) and (d) SRO (004) and ((b), (c), (e), and (f)) PTO (004) diffractions for(a) and (b) PTO120/SRO50, (c) PTO200/SRO50, (d) and (e) PTO120/SRO100, and (f) PTO600/SRO100 structures. Solidlines show the used pseudo-Voigt fits.

The formation of undesirable tensile-strained c2-domains is suppressed for thicker SRO layers.The smaller tension in PTO on thicker SRO can be qualitatively understood from mechanical equi-librium, which requires equal traction on both sides at the SRO-PTO interface.27,28 This elasticcoupling distributes the total stress of the stack between SRO and PTO according to their respectedthicknesses. For a fixed PTO thickness and by assuming the misfit strain relaxation being indepen-dent of SRO during the deposition of PTO, the increasing thickness of SRO may lead to a smallertensile strain in PTO.19 Experimental indications of such elastic coupling have been found in otherheterostructures.29 Although further investigations are required to clarify the corresponding mecha-nisms, our experimental results strongly suggest that the electrode-controlled strain relaxation canbe employed for the FE polarization enhancement.

In summary, large polarization close to theoretical bulk value is achieved in epitaxial Pt/PTO/SRO heterostructures with (120–600)-nm-thick PTO films grown on (001) STO substrateusing a 100-nm-thick SRO bottom electrode layer. The observed large polarization in PTO filmsis attributed to relaxation of epitaxial strain promoted by relatively thick bottom SRO layer. Ourresults indicate that electrode-controlled strain relaxation could be used to enhance polarization inepitaxial ferroelectric films.

This work was supported by the Finnish Funding Agency for Innovation (GrantNo. 400.31.2013).

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