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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Enhancement of Ferroelectric Properties of Superlattice-Based Epitaxial BiFeO Thin Films via Substitutional Doping Effect 3

Jaesun Song, Kyoung Soon Choi, Sejun Yoon, Woonbae Sohn, Seung Pyo Hong, Tae Hyung Lee, Hyunji An, Sam Yeon Cho, So-Young Kim, Do Hyun Kim, Taemin Ludvic Kim, Sang Yun Jeong, Chung Wung Bark, Byoung Hun Lee, Sang Don Bu, Ho Won Jang, Cheolho Jeon, and Sanghan Lee J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00156 • Publication Date (Web): 19 Apr 2019 Downloaded from http://pubs.acs.org on April 19, 2019

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The Journal of Physical Chemistry

Enhancement of Ferroelectric Properties of Superlattice-Based Epitaxial BiFeO3 Thin Films via Substitutional Doping Effect Jaesun Song,†,# Kyoung Soon Choi,§,# Sejun Yoon,‡ Woonbae Sohn, ∥ Seung Pyo Hong, ∥ Tae Hyung Lee,∥ Hyunji An,‡ Sam Yeon Cho,⊥ So-Young Kim,‡ Do Hyun Kim,※ Taemin Ludvic Kim,† Sang Yun Jeong,‡ Chung Wung Bark,※ Byoung Hun Lee,‡ Sang Don Bu,⊥ Ho Won Jang,∥ Cheolho Jeon,§ and Sanghan Lee*,‡ †R&D

Division, SK hynix Inc., Icheon 17336, Republic of Korea.

‡School

of Materials Science and Engineering, Gwangju Institute of Science and Technology,

Gwangju 61005, Republic of Korea. §The

Advanced Nano Surface Research Group, Korea Basic Science Institute, Daejeon 34133,

Republic of Korea. ∥

Department of Materials Science and Engineering, Research Institute of Advanced

Materials, Seoul National University, Seoul 08826, Republic of Korea. ⊥Department

of Physics, Chonbuk National University, Jeonju 54896, Republic of Korea.

※Department

of Electrical Engineering, Gachon University, Seongnam 13120, Republic of

Korea.

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

Although there is considerable interest in BiFeO3 owing to its versatile physical properties, which make it suitable for a wide range of applications, its high leakage current is a significant limitation. Among the various methods for reducing the leakage current, substitution with transition-metal or rare-earth elements is widely recognized as the most effective approach. Herein, to enable in-depth studies of the physical properties of BiFeO3, high-quality epitaxial BiFeO3 thin films with a low leakage current must be formed. However, owing to the difficulty of controlling the element doping when pulsed laser deposition is used for epitaxial thin-film growth, studies on substitutional doping based on epitaxial BiFeO3 thin films have not been systematically studied. In this regard, we establish an innovative approach for overcoming the high leakage current of BiFeO3 by fabricating artificially engineered superlattice-based epitaxial BiFeO3 thin films, in which there is a significant reduction of the leakage current. The control of the element doping in epitaxial BiFeO3 thin films is easily regulated precisely at the atomic-scale level. The results of this study strongly suggest that superlattice-based epitaxial BiFeO3 thin films can be a cornerstone for exploring the reliable fundamental physical properties of substitutional doping in epitaxial BiFeO3 thin films.

INTRODUCTION Multiferroics are interesting materials with coupled primary ferroic properties such as ferromagnetism, ferroelectricity, and ferroelasticity. The coupling between ferroic-order parameters has attracted considerable attention owing to the potential of the novel physical ACS Paragon Plus Environment

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phenomena enabling application of a single stimulus, including electric and magnetic fields or strain, for controlling another ferroic-order parameter.1-4 Among the multiferroic materials, bismuth ferrite (BiFeO3) has been extensively studied as the most prototypical single-phase multiferroic because it exhibits ferroelectricity and antiferromagnetism simultaneously at room temperature.5 In regard to the ferroelectricity of BiFeO3, there is considerable advantage in having a large spontaneous polarization (~100 μC cm-2) along the diagonal direction of the distorted rhombohedral perovskite structure.6 However, BiFeO3 has weak magnetization owing to the weak spin canting induced by the tilting of oxygen oxtahedrons.7 In addition to these multiferroic properties, recently, BiFeO3 has been found to offer various research opportunities and potential in optical devices, such as solar water splitting8,9 and photovoltaics10, because of its relatively narrow optical bandgap in the range of 2.2–2.7 eV11,12 and large ferroelectric spontaneous polarization. Although BiFeO3 has versatile physical properties, its high leakage current limits its technological applications, such as in spintronics, actuators, ferroelectric memories, and optical devices, owing to the existence of various oxidation states of the transition-metal Fe cations and O vacancies.6,13-15 Hence, numerous attempts have been made to reduce the leakage current of BiFeO3, and substitution with transition-metal16,17 or rare-earth elements1820

has been widely recognized as an effective method from the perspective of defect

chemistry. Specifically, the crystalline phase of BiFeO3 has a distorted rhombohedral ABO3 perovskite structure. The ferroelectricity of BiFeO3 originates from the non-centrosymmetric lattice structure induced by the a consequence of the localized lone electron pair (6s2) on Asite atoms, whereas its magnetization arises from the partially filled 3d-orbitals of the Fe cations at the B-sites of the perovskite crystalline structure.21 Thus, in this distorted rhombohedral ABO3 perovskite structure, the A-site substitution is strongly related to the ferroelectricity, whereas the B-site substitution affects the magnetic properties of BiFeO3. Consequently, studies on various elemental substitutions in BiFeO3 have been conducted for each property, and most studies on substitutional doping have been based only on polycrystalline BiFeO3 thin films prepared using sol–gel synthesis, because of the ease of ACS Paragon Plus Environment


control of the elemental doping content.16-20,22-25 However, the ferroelectric properties of these polycrystalline BiFeO3 thin films have been characterized by slanted polarization– electric (P-E) hysteresis loops, exhibiting the non-uniformity of the ferroelectric domains, instead of the typical square-like P-E loops. Therefore, to ensure reliability, research on the ferroelectric and magnetic properties of BiFeO3 thin films should be performed by growing epitaxial BiFeO3 thin films. Such thin films have different fundamental physical properties depending on the lattice parameters of the substrate, crystallographic orientations, and miscut angles.26-29 Nevertheless, investigations on substitutional doping based on epitaxial BiFeO3 thin films have not been systematically studied, owing to the challenge of controlling the elemental doping content in the pulsed laser deposition (PLD) employed for their growth. Thus, a deep understanding of the substitutional doping of epitaxial BiFeO3 thin films is required. In this regard, we establish an innovative strategy to overcome the high leakage current of BiFeO3 by fabricating artificially engineered superlattice-based epitaxial BiFeO3 thin films (BiFeO3 SL) via PLD, as shown in Figure 1. Ce-doped LaMnO3 (LCMO) is chosen as the insertion layer in the BiFeO3 SL thin films because of the following two advantages. First, there is excellent lattice matching between rhombohedral BiFeO3 (a = 3.963 Å and α = 89.40°)30,31 and orthorhombic LaMnO3 (pseudocubic a0 = 3.95 Å)32 thin films, which is expected to be sustained in high-quality epitaxial BiFeO3 thin films. Second, within the LCMO material, various dopants, such as La, Mn, and Ce, are included to reduce the leakage current of BiFeO3. Especially, Ce is widely considered as a highly promising A-site dopant in BiFeO3 thin films owing to the similar ionic radii of Bi3+ (1.17 Å) and Ce3+ (1.14 Å). Moreover, the effects of Ce substitution on the BiFeO3 thin films could significantly reduce their leakage current and improve ferroelectric properties by preventing the volatility of the Bi cations and suppressing the reduction of Fe3+ to Fe2+.14,33 Because of these two advantages of LCMO as the insertion layer, our epitaxial BiFeO3 SL thin films enable reliable research on substitutional doping based on epitaxial BiFeO3 thin films. These films exhibit a significantly reduced leakage current and enhanced ferroelectric properties of BiFeO3. ACS Paragon Plus Environment

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Additionally, the Ce-doping content in the epitaxial BiFeO3 thin films is easily controlled precisely at the atomic-scale level via the control of the ratio and period between the BiFeO3 and LCMO layers. According to the results, we expect our epitaxial BiFeO3 SL thin films to be a cornerstone for in-depth investigations of the fundamental physical properties of substitutional doped epitaxial BiFeO3 thin films with variations in the biaxial strain, crystallographic orientation, and doping profile.

METHODS Preparation of Epitaxial BiFeO3 and BiFeO3 SL Thin Films. Epitaxial BiFeO3 SL thin films were fabricated by alternately depositing BiFeO3 and LCMO layers via the PLD method using a KrF excimer laser (Coherent COMPexPro 205F). Epitaxial BiFeO3 SL thin films were grown with SrRuO3 thin films as the conducting buffer layer on an SrTiO3 (001) substrate at a substrate temperature of 680 °C and an O pressure of 200 mTorr. BiFeO3 and LCMO targets were used for the growth of the epitaxial BiFeO3 SL thin films. First, a BiFeO3 ceramic target was synthesized via a solid-state reaction using Bi2O3 (99.9%, Kojundo Chemical Lab.) and α-Fe2O3 (99.9%, Kojundo Chemical Lab.) powders. The mixed powders were calcined at 700 °C for 2 h and sintered at 820 °C for 2 h. Subsequently, the LCMO ceramic targets were synthesized via a solid-state reaction using La2O3 (99.9%, Kojundo Chemical Lab.), CeO2 (99.9%, Kojundo Chemical Lab.), and MnO2 (99.9%, Kojundo Chemical Lab.) powders. The mixed powders were calcined at 1,050 °C for 15 h and sintered at 1,350 °C for 15 h. Characterizations. The crystalline structural properties were determined via X-ray diffraction (XRD, PANalytical X’Pert Pro diffractometer) with Cu Kα radiation (λ = 1.5418 Å) and high-resolution transmission electron microscopy (HRTEM, JEOL, JEM-2100F). Reciprocal space mapping (RSM) was performed using the 3A beamline at the Pohang Light Source with a six-circle Paul Scherrer Institute diffractometer. The chemical properties were measured via X-ray photoelectron spectroscopy (XPS, KRATOS AXIS Ultra DLD model). ACS Paragon Plus Environment


The ferroelectric properties, including the P-E hysteresis loops and fatigue behaviors, were examined using a ferroelectric analyzer (Radiant Precision LC). The frequency dependence of the dielectric constant was estimated using an impedance analyzer (Agilent 4294A), and capacitance–voltage measurements were performed using an LCR meter (Agilent, 4284A)

RESULTS AND DISCUSSION Epitaxial BiFeO3 SL thin films were fabricated by alternately depositing BiFeO3 and LCMO layers using PLD. Herein, the epitaxial BiFeO3 SL thin films are denoted as p (m, n), where m and n are the thicknesses (nm) of the BiFeO3 and LCMO layers, respectively, and p represents the number of periods in the epitaxial BiFeO3 SL thin films. First, to investigate the crystalline structural properties of the epitaxial BiFeO3 and BiFeO3 SL thin films, including the epitaxial and crystallographic orientations, and the effect of the introduction of LCMO as the insertion layer on the crystalline structure, XRD and HRTEM were performed. Figure 2a shows the XRD θ–2θ scans for the epitaxial BiFeO3 and BiFeO3 SL - 30 (12.8, 0.2) thin films on a SrRuO3-buffered SrTiO3 (001) substrate. Both the XRD patterns exhibit only the diffraction peaks of rhombohedral BiFeO3 (00l), without any other noticeable second phases or orientations. This indicates that both the thin films were grown epitaxially along the c-axis orientation, regardless of the presence or absence of the insertion layer. Additionally, both the BiFeO3 thin films have similar crystalline quality and out-of-plane lattice parameters (Figure S1). This suggests that the insertion layer had little effect on the crystallinity and nano-strain of the BiFeO3 layers in the epitaxial BiFeO3 SL thin films. The additional inplane epitaxial and macroscopic domain structure of the epitaxial BiFeO3 SL thin film was examined via high-resolution XRD RSM, as shown in Figure 2b. In general, it is well-known that the domain structure of BiFeO3 is directly related to the leakage current and conductivity characteristics of its thin films. BiFeO3 thin films have also been reported to exhibit more conducting properties in the domain walls than in the BiFeO3 domains.34,35 For this reason, studies have been performed to simultaneously improve the ferroelectric properties and ACS Paragon Plus Environment

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leakage current of BiFeO3 thin films by controlling their domain structure.36 However, in the present study, the domain engineering of BiFeO3 thin films may be a disturbing factor for verifying the difference in the leakage current of the BiFeO3 thin films based on the precise doping of transition metals. In this regard, Figure 2b presents the RSM patterns for the epitaxial BiFeO3 SL thin film around the (103) diffraction peak of the SrTiO3 (001) substrate, and the (103) diffraction pattern of the epitaxial BiFeO3 SL thin film exhibits a wider pattern. This pattern exhibits four structural domains denoted as r1, r2, r3, and r4, as previously reported.31,36 In conclusion, despite the introduction of LCMO as the insertion layer, the domain structure of the epitaxial BiFeO3 SL thin film was maintained in the form of the four structural domains of the previously known c-axis-oriented epitaxial BiFeO3 thin films. This suggests that the effect of the domain structure of the epitaxial BiFeO3 thin films with and without the insertion layers can be disregarded as the cause of the difference in the leakage current between the two thin films. Subsequently, HRTEM was performed to directly identify the presence of LCMO as the insertion layer in the epitaxial BiFeO3 SL thin film, as shown in Figures 2c and d. First, it can be discerned that the LCMO layers were periodically grown between the epitaxial BiFeO3 layers in a parallel direction on the substrate. Second, the epitaxial BiFeO3 layers were consistently grown with well-aligned lattice fringes, despite the introduction of LCMO as the insertion layer. Thus, the overall crystalline structural analysis proves that the crystalline structural properties of the epitaxial BiFeO3 thin films, including the epitaxial, crystallographic orientations, crystallinity, and macroscopic domain structure, were similar regardless of the presence of the LCMO layers. This can be attributed to the excellent lattice matching between the rhombohedral BiFeO3 and orthorhombic LaMnO3 thin films, which allowed the sustenance of high-crystalline epitaxial BiFeO3 layers despite the introduction of LCMO as the insertion layer. To confirm the chemical valence states of the epitaxial BiFeO3 layers under the effect of LCMO as the insertion layer, the chemical properties of the epitaxial BiFeO3 and BiFeO3 SL thin films were estimated using angle-resolved XPS. Generally, the depth resolution of conventional XPS analysis is approximately 10 nm, even though it is dependent on the ACS Paragon Plus Environment


kinetic energy of the electrons and the specimen to be analyzed. However, the depth resolution of the XPS analysis can be adjusted via simple sample tilting, as shown in Figures 3a and b. For example, the depth resolution of the XPS analysis at an X-ray incident angle of 60°, which was defined as the angle between the surface normal of the sample and the XPS detector, was approximately 3–4 nm. This is more sensitive to the surface analysis of the sample depending on the X-ray incident angle. Consequently, angle-resolved XPS analysis can validate the uniformity of the chemical valence states of the epitaxial BiFeO3 thin films in the perpendicular direction. The wide-scan XPS spectra of the epitaxial BiFeO3 and BiFeO3 SL thin films at different X-ray incident angles revealed the equal presence of all the elements without any other peaks (Figure S2). Figures 3c–f show the high-resolution XPS spectra of the Fe 2p core level of the epitaxial BiFeO3 and BiFeO3 SL thin films at different X-ray incident angles, which obviously reveal two symmetric peaks corresponding to Fe 2p1/2 and Fe 2p3/2 for both of the thin films. In the additional fitted curves for the Fe 2p3/2 peaks, the ratio of the Fe2+ ions in the FeO phase (709.13 eV) to the Fe3+ ions in the Fe2O3 phase (710.27 eV) exhibits a similar tendency for both films and depends on the X-ray incident angle. The reduction of Fe3+ to Fe2+ was relatively suppressed in the epitaxial BiFeO3 SL thin films compared with the epitaxial BiFeO3 thin films. This discrepancy is ascribed to the Ce in the insertion layers. The stable chemical valence states of Ce ions are commonly +3 and +4 (Figure 3g),37 and the replacement of Ce4+ ions with A-site dopants in the BiFeO3 layers results in a reduction of the concentration of O vacancies, which suppresses the reduction of Fe3+ to Fe2+.14 In this regard, the similarity of the ratio of Fe2+ to Fe3+ between the two thin films, depending on the X-ray incident angle, implies that the O vacancies were uniformly distributed in both films. Moreover, in the case of the epitaxial BiFeO3 SL thin film, it can be inferred that the Ce ions were evenly substituted in the epitaxial BiFeO3 layers. The overall volume fractions for the charge valence states of Fe ions in both films are plotted in Figure 3h. According to the foregoing crystalline structural and chemical analysis of the epitaxial BiFeO3 SL thin films, the LCMO layer as the insertion layer did not affect the crystalline ACS Paragon Plus Environment

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structural properties of the epitaxial BiFeO3 thin films, including the crystallinity and the ferroelectric domain structure. Moreover, the precise doping of Ce via the insertion layer reduced the defects, such as the O vacancies, in the epitaxial BiFeO3 thin films. On the basis of these results, the ferroelectric P-E hysteresis loops were measured to investigate the effect of the Ce substitution of the insertion layers on the ferroelectric properties of the epitaxial BiFeO3 SL thin films, as shown in Figure 4. First, the frequency-dependent P-E hysteresis loops of the epitaxial BiFeO3 and BiFeO3 SL thin films were characterized to verify the effect of the Ce substitution on the ferroelectric domain motion (Figures 4a and b). Physically, ferroelectric domain switching is caused by the movement of the domain walls, and as the frequency increases, the stable ferroelectric domain switching becomes more challenging owing to the limitations of the speed of the domain walls.38 Hence, in general, a decay of the ferroelectric polarization occurs as the frequency increases. However, epitaxial BiFeO3 SL thin films exhibit reliable square-like P-E hysteresis loops with the increase of the frequency owing to the effect of Ce substitution. The remnant polarization (Pr) value of the thin films was measured to be 85 μC cm-2, indicating complete ferroelectric domain switching behavior compared with that reported for BiFeO3 thin films.39-42 This relatively high Pr value of the epitaxial BiFeO3 SL thin films is attributed to the reduction of the leakage current due to the Ce substitution. In contrast to the epitaxial BiFeO3 SL thin films, the epitaxial BiFeO3 thin films exhibited open P-E hysteresis loops, particularly at a low frequency, and significantly low Pr values of 67 μC cm-2. The open shape indicates a high leakage current in the epitaxial BiFeO3 thin films.43,44 Leakage-current measurements confirmed that the leakage current density of the epitaxial BiFeO3 SL thin films was two orders of magnitude lower than that of the epitaxial BiFeO3 thin films at 100 kV cm-1 (Figure 4c). Additionally, the ferroelectric P-E hysteresis loops of the epitaxial BiFeO3 SL thin films with various thicknesses and Ce ratios of the insertion layer were evaluated to optimize the leakage current of the epitaxial BiFeO3 thin films, as shown in Figures 4d and e. Among the various thicknesses of the insertion layer, the epitaxial BiFeO3 SL - 30 (12.8, 0.2) thin film exhibited the best ferroelectricity, and thick insertion layers are considered as fatal defects in the thin films. The epitaxial BiFeO3 ACS Paragon Plus Environment


SL thin films with 10% and 15% Ce-doped LaMnO3 layers as the insertion layer were found to have similar excellent ferroelectricity. However, when the LaMnO3 layer was used as the insertion layer in the epitaxial BiFeO3 SL thin film, the ferroelectricity was poor (Figure 4f). This suggests that Ce cations among various transition metals in the LCMO layers performed the principal function of reducing the leakage current of the epitaxial BiFeO3 thin films, which is consistent with the aforementioned XPS results. To evaluate the impact of the reduction of the leakage current due to the Ce substitution in the insertion layers on the fatigue behavior of the epitaxial BiFeO3 SL thin films, fatigue measurements of the epitaxial BiFeO3 and BiFeO3 SL thin films were performed by applying rectangular pulses with a repetition rate of 200 Hz and an amplitude of 20 V. Figures 5a and b show the P-E hysteresis loops before and after the fatigue cycles for the epitaxial BiFeO3 and BiFeO3 SL thin films, respectively. The two films exhibit significantly different fatigue behavior. In the case of the epitaxial BiFeO3 SL thin films, the reliable square-like P-E hysteresis loops were similar before and after 105 fatigue cycles. However, the epitaxial BiFeO3 thin films exhibited open P-E hysteresis loops both before and after 105 fatigue cycles. These loops had severely unclosed shapes after the fatigue cycles, suggesting that the epitaxial BiFeO3 thin films had a relatively high leakage current, which increased after the fatigue cycles. Additionally, the trend of the measured switched polarization (Psw) values of the epitaxial BiFeO3 and BiFeO3 SL thin films with increasing switching cycles agreed with the above P-E hysteresis loops (Figure 5c). With increasing switching cycles, the Psw values of the epitaxial BiFeO3 SL thin films remained steady, whereas those of the epitaxial BiFeO3 thin films tended to deteriorate continuously to approximately 80% of the initial value after 105 fatigue cycles. Additionally, the trend of the measured switched polarization values of the epitaxial BiFeO3 SL thin films remained steady after 105 cycles (Figure S3). It is concluded that the epitaxial BiFeO3 SL thin films achieved a significant reduction of the leakage current owing to the Ce substitution introduced by the insertion layers, which are consistent with previously reported results.45,46


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Finally, Figure 6a shows the frequency dependence of the dielectric constant for the epitaxial BiFeO3 and BiFeO3 SL thin films measured in the frequency range of 10 kHz–1 MHz. Here, for both films, the dielectric constant decreases smoothly with the increase of the frequency. This phenomenon can be attributed to the space-charge relaxation. Generally, electric dipoles respond to the frequency of the applied electric field; if the frequency is shorter than the relaxation time, the electric dipoles are more likely to not follow the oscillation rate. Thus, for both films, the dielectric constant tended to decrease as the frequency increased.47 Similar to the trend of these graphs, the epitaxial BiFeO3 SL thin film had a larger dielectric constant (approximately 120) than the epitaxial BiFeO3 thin film over the entire frequency range. This indicates that the epitaxial BiFeO3 SL thin film had an enhanced dielectric response because of the reduction of the concentration of the O vacancies due to the Ce substitution.14 Figures 6b and c present the curves of the dielectric constant versus the electric field for the epitaxial BiFeO3 and BiFeO3 SL thin films, which were obtained via electric-field sweeping from a positive bias to a negative bias and vice versa. The measured butterfly shape for both films confirms their typical ferroelectric behavior. Similar to the trend in Figure 6a, the dielectric constant of the epitaxial BiFeO3 SL thin film was estimated to be larger than that of the epitaxial BiFeO3 thin film during the electric-field sweeping.

CONCLUSIONS We fabricated artificially engineered epitaxial BiFeO3 SL thin films by alternately depositing BiFeO3 and LCMO layers using PLD and found that the leakage current of the epitaxial BiFeO3 SL thin films was significantly reduced compared with that of the epitaxial BiFeO3 thin films. The origin of the reduction of the leakage current in the epitaxial BiFeO3 SL thin films was systematically investigated. First, the excellent lattice matching between the BiFeO3 and LaMnO3 thin films allowed the high-quality epitaxial BiFeO3 thin films to be sustained. Second, the Ce substitution in the epitaxial BiFeO3 thin films made it possible to significantly reduce the leakage current and improve the ferroelectricity of the films by ACS Paragon Plus Environment


reducing the formation of volatile Bi cations and O vacancies. These two advantages facilitated a reliable study of substitutional doping using epitaxial BiFeO3 thin films. Furthermore, the Ce doping content in the epitaxial BiFeO3 thin films was easily controlled precisely at the atomic-scale level. Our results indicate that the epitaxial BiFeO3 SL thin films could be a cornerstone for exploring the reliable fundamental physical properties of substitutional doping.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI:

Detailed information about XRD reflection rocking curves, XPS spectra, and fatigue behavior.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions #These

authors contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS J.S and K.S.C. contributed equally to this work. This research was supported by Basic Science Research Program (2016R1D1A1B03931748) and Creative Materials Discovery ACS Paragon Plus Environment

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Program (2017M3D1A1040828) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education and the Ministry of Science and ICT, and by GRI (GIST Research Institute) project through a grant provided by GIST.

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FIGURE CAPTIONS Figure 1. Schematic of the epitaxial BiFeO3 SL thin film on the SrTiO3 (001) substrate fabricated by alternately depositing BiFeO3 and LCMO layers. Figure 2. (a) Out-of-plane θ–2θ XRD patterns for the epitaxial BiFeO3 and BiFeO3 SL thin films. (b) RSM patterns for the epitaxial BiFeO3 SL thin film around the (103) diffraction ACS Paragon Plus Environment


peak of SrTiO3 (001). (c) HRTEM measurements for the epitaxial BiFeO3 SL thin film. (d) Magnified image of the area near the surface of the epitaxial BiFeO3 SL thin film. Figure 3. Schematic of the angle-resolved XPS technique at X-ray incident angles of (a) 0° and (b) 60°. The high-resolution XPS Fe 2p core-level spectra for the epitaxial BiFeO3 thin films at X-ray incident angles of (c) 0° and (d) 60°. The high-resolution XPS Fe 2p core-level spectra for the epitaxial BiFeO3 SL thin films at X-ray incident angles of (e) 0° and (f) 60°. (g) High-resolution XPS Ce 3d core-level spectra for the epitaxial BiFeO3 SL thin films. (h) Overall volume fractions for the charge valence states of the Fe ions in the epitaxial BiFeO3 and BiFeO3 SL thin films. Figure 4. Frequency-dependent P-E hysteresis loop measurements of the epitaxial (a) BiFeO3 and (b) BiFeO3 SL thin films. (c) Current density versus electric field curves for the epitaxial BiFeO3 and BiFeO3 SL thin films. The P-E hysteresis loop measurements of the epitaxial BiFeO3 SL thin films for (d) various thicknesses and (e) Ce ratios of the insertion layer. (f) PE hysteresis loop measurement of the epitaxial BiFeO3 SL thin film with the LaMnO3 layer introduced as the insertion layer. Figure 5. P-E hysteresis loop measurements taken before and after the fatigue cycles (~105) for the epitaxial (a) BiFeO3 and (b) BiFeO3 SL thin films. (c) Fatigue behavior of the epitaxial BiFeO3 and BiFeO3 SL thin films. Figure 6. (a) Frequency dependence of the dielectric constant for the epitaxial BiFeO3 and BiFeO3 SL thin films, measured in the frequency range of 10 kHz–1 MHz. Dielectric constant versus electric field curves for the epitaxial (b) BiFeO3 and (c) BiFeO3 SL thin films.


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Figure 1.



Figure 2.


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Figure 3.



Figure 4.


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Figure 5.



Figure 6.


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