Ferroelectric properties of highly a-oriented polycrystalline Bi2WO6 thin films grown on glass substrates Yoonho Ahn and Jong Yeog Son Citation: AIP Advances 6, 105008 (2016); doi: 10.1063/1.4965925 View online: http://dx.doi.org/10.1063/1.4965925 View Table of Contents: http://aip.scitation.org/toc/adv/6/10 Published by the American Institute of Physics AIP ADVANCES 6, 105008 (2016) Ferroelectric properties of highly a-oriented polycrystalline Bi2 WO6 thin films grown on glass substrates Yoonho Ahna and Jong Yeog Sona Department of Applied Physics and Institute of Natural Sciences, Kyung Hee University, Yongin 446-701, South Korea (Received 17 August 2016; accepted October 2016; published online 17 October 2016) Polycrystalline Bi2 WO6 (BWO) thin films were deposited on Pt/Ta/glass substrates by pulsed laser deposition (PLD) In this study, we comparatively investigate the influence of oxygen partial pressure on structural and ferroelectric properties of the BWO films In comparison with the BWO films deposited at oxygen partial pressure of 100 and 300 mTorr, the BWO film deposited at 300 mTorr exhibits a highly a-oriented crystalline structure The highly a-oriented polycrystalline BWO thin film shows good ferroelectric properties with a remnant polarization of about 21.5 µC/cm2 The piezoresponse force microscope study reveals that the highly aoriented BWO thin film possesses larger ferroelectric domain patterns due to smaller domain wall energy © 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4965925] I INTRODUCTION Ferroelectric thin films have been widely investigated for various applications including nonvolatile random access memories, power generators, solar cells, actuators, sensors, etc,.1–7 Representative ferroelectric thin films are lead titanate zirconate (Pb(Ti,Zr)O3 , PZT) thin films; these films possess the best physical properties in terms of their piezoelectric coefficients, pyroelectric coefficients, and ferroelectric polarizations.1–7 However, PZT thin films have serious problems related to ferroelectric fatigue and the toxicity of lead.8–10 The ferroelectric fatigue phenomenon can be overcome by using optimum electrodes such as IrO2 and SrRuO3 11,12 To avoid the toxicity of lead in PZT thin films, many researchers have developed lead-free ferroelectric thin films such as BiFeO3 and SrBi2 Ta2 O9 13,14 In particular, layered perovskite thin films, such as SrBi2 Ta2 O9 and Bi2 WO6 (BWO), are well–known, fatigue-free ferroelectric thin films (although they have lower ferroelectric polarizations than PZT thin films).15,16 Since layered perovskite thin films have an inversion symmetry along the c-axis, the highly a-oriented layered perovskite thin films can have ferroelectric polarizations that are significantly higher than those of the c-oriented layered perovskite thin films.15 In this work, we fabricated highly a-oriented BWO thin films on Pt/Ta/glass substrates by pulsed laser deposition (PLD) by controlling the oxygen partial pressure We confirmed that the highly a-oriented polycrystalline BWO thin films had enhanced ferroelectric properties with a remnant polarization of about 21.5 µC/cm2 We further analyzed the ferroelectric domain structures of the BWO thin films in terms of the ferroelectric domain size and ferroelectric domain wall energy II EXPERIMENTAL PROCEDURE BWO thin films were deposited on Pt/Ta/fused silica glass substrates via PLD For the PLD ablation, Bi2 WO6 bulk targets were prepared by a conventional solid-state reaction method aAuthors to whom correspondence should be addressed Electronic addresses: yahn@khu.ac.kr and jyson@khu.ac.kr 2158-3226/2016/6(10)/105008/5 6, 105008-1 © Author(s) 2016 105008-2 Y Ahn and J Y Son AIP Advances 6, 105008 (2016) High purity powders of the chemicals, which were commercially-available Bi2 O3 (99.99% pure) and W2 O3 (99.99%) (MTI Co., USA), were mixed in molar ratios The inch pellet of the Bi2 WO6 mixtures were well ground, palletized, and annealed in an air environment at 450 ◦ C for 10 h, at 750 ◦ C for 12 h, and finally at 950 ◦ C for h with intermediate grinding The PLD laser of a KrF excimer laser had a wavelength of 248 nm and an energy density of 0.5 J/cm2 The distance between the target and substrate was maintained to be ∼4 cm Once the base pressure reached at 5×10-7 Torr and the substrate temperature was set to 900 ◦ C with an oxygen partial pressure of 100 mTorr After the deposition, all of the thin films were subjected to slow cooling down to room temperature at a rate of 100 ◦ C/h in oxygen ambient at 300 Torr The structural analysis of the thin films was carried out by X-ray diffraction (XRD) experiments The surface morphology, root-mean-square (RMS) roughness, and ferroelectric domain structure of the thin films were observed by AFM and PFM measurements To fabricate circular-shaped top electrodes with a radius of 100 µm, 100 nm-thick Pt was deposited on the BWO thin film by RF magnetron sputtering through a dot-patterned shadow mask Subsequently, all of the samples were annealed at 400 ◦ C for prior to obtaining the ferroelectric hysteresis loops, which were measured using an RT66A (Radiant Technologies, Inc.) test system III RESULTS AND DISCUSSION Figure (a) shows a schematic drawing of a 100-nm-thick BWO thin film grown on a Pt/Ta/glass substrate The Ta buffer layer was used to improve the crystallinity of the Pt bottom electrode for the deposition of the BWO thin film We first checked the crystal structures of the BWO thin films on the Pt/Ta/glass substrates Figure (b) shows the θ-2θ XRD pattern of the BWO/Pt/Ta/glass substrate Two BWO thin films, deposited at 100 and 300 mTorr, have (002), (113), (004), (0010), and (226) XRD peaks, indicating the polycrystalline structures of the two films The BWO2 thin film (300mTorr) has (113) and (226) peaks that are much higher than the BWO1 thin film (100mTorr) Alternatively, the BWO1 film has higher (00l) peaks than the BWO2 film Thus, it is inferred that the BWO2 film has a highly a-oriented crystal structure In addition, the BWO2 film has smaller full width at half maximums for the (113) and (226) peaks than the BWO1 film, indicating that the BWO2 film has larger grains than the BWO1 film The crystalline structures of the BWO thin films were influenced by adjusting the oxygen partial pressure Generally, layered perovskite oxides thin films such as SrBi2 Ta2 O9 and YBi2 Cu3 O7 thin films have highly c-oriented crystal structures under the deposition conditions of a high substrate temperature and a high deposition rate.14,17,18 On the other hand, the low substrate temperature and the low deposition rate made it possible to deposit the a-axis oriented layered perovskite oxides thin films A high oxygen partial pressure gives a low deposition rate because the number of collisions between adatoms and oxygen atoms increases with increasing an oxygen partial pressure.19 Under a low deposition rate, adatoms have time enough to achieve optimum crystal positions corresponding to FIG (a) Schematic drawing of a BWO thin film grown on a Pt/Ta/glass substrate (b) The X-ray diffraction pattern of the BWO thin films grown on the Pt/Ta/glass substrates 105008-3 Y Ahn and J Y Son AIP Advances 6, 105008 (2016) the crystal structures of substrates.19 The BWO2 film had the deposition rate of 0.32 nm/pulse slightly larger than that (0.4 nm/pulse) of the BWO2 Thus, it is inferred that the highly a-oriented crystal structure of the BWO2 originates from the high oxygen partial pressure giving the low deposition rate Figure show the surface morphology and ferroelectric domain structure of the polycrystalline BWO thin films Figures 2(a) and (b) show AFM images of the BWO1 and BWO2 films, respectively For AFM measurement, we used a silicon nitride (Si3 N4 ) cantilever (f ∼ 67 kHz, k ∼ 0.32 N/m) under non-contact operating mode There are longish grains on both the BWO1 and BWO2 films, which are affected by the crystallinity of the layered perovskite materials As expected from the XRD results, the BWO2 film had larger grains than the BWO1 film In addition, it is inferred that the large grains of the BWO2 film result from the low deposition rate The BWO1 and BWO2 thin films had root-mean-square (RSM) roughnesses of 6.5 and 7.5 nm, respectively The ferroelectric domain structures were also acquired by PFM measurement, which was supported by an Rh-coated tip (f ∼ 25 kHz, k ∼ N/m) at applied 10 kHz AC voltage The BWO1 and BWO2 thin films had mosaic ferroelectric domain structures with domain sizes of about 80 and 100 nm, respectively (Figures 2(c) and (d)) To investigate the ferroelectric properties, we observed the hysteresis loops of the polycrystalline BWO thin films at a measurement frequency of 1kHz, as shown in Figure (a) To measure the hysteresis, we prepared a Pt/BWO/Pt capacitor Both the BWO1 and BWO2 films exhibited good ferroelectric properties with high remnant polarizations of 13.8 and 21.5 µC/cm2 , respectively Since the single crystal BWO had the polar axis perpendicular to the c-axis, it inferred the large remnant polarization of the BWO1 film is due to the highly a-oriented crystal structure of the BWO1 film.15,16 Figure (b) shows the piezoelectric d 33 hysteresis loops of the BWO1 and BWO2 films The BWO2 film has a larger remnant piezoelectric d 33 value The highly a-oriented BWO thin film (BWO2) had better ferroelectric properties than the BWO1 film Figure (c) shows the switching FIG (a), (b) AFM images of the BWO1 and BWO2 films, respectively There are longish grains on the surface of the BWO thin films (c), (d) PFM images of the BWO1 and BWO2 films, respectively 105008-4 Y Ahn and J Y Son AIP Advances 6, 105008 (2016) FIG (a) Ferroelectric hysteresis loops of the BWO1 and BWO2 films (b) Piezoelectric d 33 hysteresis loops of the BWO1 and BWO2 films (c) Switching current curves as a function of time under switching bias of 5V current curves of the BWO1 and BWO2 thin films as a function of time under switching bias of 5V The BWO2 film had the switching time of about 113 ns faster than that (127 ns) of the BWO1 film To analyze the difference in the switching time, we evaluated ferroelectric domain wall energies of the two BWO thin films by Landau, Lifshitz, and Kittel’s (LLK) scaling law In the LLK law, the scaling exponent γ is proportional to the domain wall energy.20,21 The relationship between the domain width and the thickness is well defined by the LLK law, which states that the domain width (w) is proportional to dγ , where d is the film thickness and γ is the scaling exponent with a value close to 1/2.20,21 We obtained the domain widths of the polycrystalline BWO thin films as a function of the thickness (Figure 4) The fitted scaling exponent of the BWO2 film is about 0.52, which is smaller than that of the BWO1 film (0.56), indicating that the BWO2 film has a smaller domain wall energy FIG Domain sizes of the BWO thin films as a function of the thickness The scaling exponent (γ) of the BWO2 thin films (0.52) is smaller than that of the BWO1 thin films (0.56) 105008-5 Y Ahn and J Y Son AIP Advances 6, 105008 (2016) than the BWO1 thin film Thus, due to the fact that the domain wall energy of the BWO2 film is smaller than that of the BWO1 film, the BWO2 film has a larger ferroelectric domain pattern Since BWO films have a polar axis perpendicular to the c-axis, the highly a-oriented BWO2 film has a domain wall energy that is lower than the BWO1 film.15,16 It is suggested that the BWO2 film has the fast polarization switching characteristics resulting from the low domain wall energy of the BWO thin films IV CONCLUSIONS We fabricated BWO thin films on Pt/Ta/glass substrates by PLD The BWO thin film deposited at 300 mTorr had a highly a-oriented crystal structure with longish grains that were influenced by the crystallinity of the layered perovskite materials The highly a-oriented BWO thin film exhibited good ferroelectric properties with a high remnant polarization of 21.5 µC/cm2 The Landau, Lifshitz, and Kittel’s scaling law revealed that the highly a-oriented BWO thin film had a scaling exponent of about 0.52, which is smaller than the polycrystalline BWO thin film, indicating that highly a-oriented BWO thin film has smaller domain wall energy than the polycrystalline BWO2 thin film ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (No 2015R1A2A2A05027951) M Dawber, K M Rabe, and J F Scott, Rev Mod Phys 77, 1083 (2005) V Nagarajan, A Roytburd, A Stanishevsky, S Prasertchoung, T Zhao, L Chen, J Melngailis, O Auciello, and R Ramesh, Nat Mater 2, 43 (2003) G Schlom, L Q Chen, C B Eom, K M Rabe, S K Streiffer, and J M Triscone, Annu Rev Mater Res 37, 589 (2007) N Setter, D 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thin film grown on a Pt/Ta /glass substrate (b) The X-ray diffraction pattern of the BWO thin films grown on the Pt/Ta /glass substrates. .. perovskite thin films have an inversion symmetry along the c-axis, the highly a- oriented layered perovskite thin films can have ferroelectric polarizations that are significantly higher than those of