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Structural, Dielectric andFerroelectric Properties of Ti-Modified0.72BiFeO3–0.28PbTiO3 MultiferroicThin Films Prepared by Pulsed LaserDepositionLing Chen a b , Wei Ren a , Weimin Zhu b , Zuo-Guang Ye a b , PengShi a , Xiaofeng Chen a , Xiaoqing Wu a & Xi Yao aa Electronic Materials Research Laboratory, Key Laboratory of theMinistry of Education , Xi'an Jiaotong University , Xi'an, 710049,Chinab Department of Chemistry and 4D LABS , Simon Fraser University ,Burnaby, British Columbia, V5A 1S6, CanadaPublished online: 20 Jan 2011.

To cite this article: Ling Chen , Wei Ren , Weimin Zhu , Zuo-Guang Ye , Peng Shi , Xiaofeng Chen ,Xiaoqing Wu & Xi Yao (2010) Structural, Dielectric and Ferroelectric Properties of Ti-Modified0.72BiFeO3–0.28PbTiO3 Multiferroic Thin Films Prepared by Pulsed Laser Deposition, Ferroelectrics,410:1, 42-49, DOI: 10.1080/00150193.2010.492058

To link to this article: http://dx.doi.org/10.1080/00150193.2010.492058

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Ferroelectrics, 410:42–49, 2011Copyright © Taylor & Francis Group, LLCISSN: 0015-0193 print / 1563-5112 onlineDOI: 10.1080/00150193.2010.492058

Structural, Dielectric and Ferroelectric Propertiesof Ti-Modified 0.72BiFeO3–0.28PbTiO3 Multiferroic

Thin Films Prepared by Pulsed Laser Deposition

LING CHEN,1,2 WEI REN,1,∗ WEIMIN ZHU,2

ZUO-GUANG YE,1,2,∗ PENG SHI,1 XIAOFENG CHEN,1

XIAOQING WU,1 AND XI YAO1

1Electronic Materials Research Laboratory, Key Laboratory of the Ministryof Education, Xi’an Jiaotong University, Xi’an 710049, China2Department of Chemistry and 4D LABS, Simon Fraser University, Burnaby,British Columbia V5A 1S6, Canada

Ti-modified 0.72BiFeO3–0.28PbTiO3 thin films were prepared on Pt/Ti/SiO2/Si sub-strates by pulsed laser deposition under different oxygen pressures from 2 Pa to 15 Pa.The microstructures and electrical properties were investigated. Tetragonal (001) prefer-ential orientation was observed in all films under different oxygen atmosphere pressures.The electrical properties of the thin films have been investigated for various depositionconditions. The dielectric constant and double remanent polarization were found to be550 and 80 µC/cm2, respectively, for the thin film deposited at an oxygen atmosphereof 10 Pa and a substrate temperature of 680◦C.

Keywords Ti modified 0.72BiFeO3–0.28PbTiO3 thin films; Pulsed laser deposition;Dielectric and ferroelectric properties.

1. Introduction

As a promising candidate for room temperature multiferroic materials, BiFeO3 and its solidsolution have become the focus of attention recently. BiFeO3, with a high Curie temper-ature of TC = 850◦C and a Neel temperature of TN = 310–370◦C, exhibits simultaneousferroelectric and magnetic orderings below TN [1, 2]. However, because BiFeO3 is anincongruent-melting compound and an impurity phase of Bi2Fe4O9 appears more favor-ably, synthesizing pure and dense BiFeO3 ceramics is a difficult task. In order to improvethe multiferroic performance of BiFeO3, several kinds of chemical modifications have beeninvestigated, such as the doping of Mn4+, Co5+ or La3+ on the A or B site of the perovskitestructure in BiFeO3 [3–6]. On the other hand, other materials with a perovskite structurewere introduced to form solid solutions with BiFeO3 [7–9]. Among them, PbTiO3 appearsto be one of the most promising end-materials because the introduction of PbTiO3 not onlystabilizes the perovskite phase but also forms a morphotropic phase boundary (MPB) withBiFeO3 due to the difference in crystal symmetry between PbTiO3 and BiFeO3. This solid

Received August 23, 2009.∗Corresponding authors. E-mail: wren@mail.xjtu.edu.cn; zye@sfu.edu

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solution was proven to present interesting results [10, 11]. An MPB was indeed observedin (1–x)BiFeO3–xPbTiO3 at the composition of x≈0.3, and an unusually large tetragonal-ity (c/a) was found in the tetragonal phase region of the perovskite system [12]. Recentwork by Zhu et al. on the single crystal of the (1–x)BiFeO3–xPbTiO3 solid solution with acomposition in the MPB region revealed the coexistence of multiple phases, which affectthe magnetic properties [13]. BiFeO3 and the (1–x)BiFeO3–xPbTiO3 solid solution appearto be promising for practical multiferroic applications. Thin films of BiFeO3 and B-sitesubstituted systems were prepared by various methods [14–17]. However, there have beenmuch fewer reports on the thin films of the (1–x)BiFeO3–xPbTiO3 system, in particular,the Ti-modified thin films [18, 19]. The aliovalent substitution of Ti4+ for Fe3+ was foundto be an effective way to reduce the electric conductivity and thereby improve the dielectricand ferroelectric properties of the BiFeO3-PbTiO3 solid solution [21].

In this paper, we report the preparation and characterization of 0.72Bi(Fe0.98Ti0.02)O3–0.28PbTiO3 (BFPT1+0.02) thin films deposited on Pt/Ti/SiO2/Si substrates under differ-ent oxygen atmosphere pressure (PO2), using pulse laser deposition (PLD) method.

2. Experiments

A stoichiometric 0.72Bi(Fe0.98Ti0.02)O3–0.28PbTiO3 (BFPT1+0.02) target synthesized bya conventional solid solution method was ablated to deposit BFPT1+0.02 thin films onPt/Ti/SiO2/Si substrates by PLD. The substrate temperature was set at 680◦C and the oxygenatmosphere pressures were chosen in the range of 2–15 Pa. The typical deposition rate was4 nm/min. The thicknesses of the BFPT1+0.02 films varied from 380 to 470 nm according tothe different oxygen pressures. After deposition, the samples were post-annealed at 700◦Cfor 30 min in a rapid thermal annealing (RTA) furnace.

The crystal structure and surface morphologies of the BFPT1+0.02 films were examinedby X-ray diffraction (XRD) using Cu Kα radiation (Philips 1700) and atomic force mi-croscopy (AFM: Nanoscope, Digital Instrument). X-ray photoelectron spectroscopy (XPS)was performed to analyze the Fe element using an XPS instrument (Axis Ultra DLD, KratosAnalytial, U.K.) and the peaks of spectra were fitted using the software of XPS PeakFit 4.Gold upper electrodes were deposited on the thin films. Frequency dependence of dielectricproperties were measured by an HP4294 LCR meter. Polarization-electric field hysteresisloops (P-E loops) were displayed using a TF Analyzer2000 standardized ferroelectric testsystem (Germany, AixACCT Systems).

3. Results and Discussion

Figure 1 presents the XRD patterns on a log scale for the BFPT1+0.02 thin films preparedunder different oxygen pressures (from 2 to 15 Pa) at the fixed substrate temperature of680◦C. All the films mostly exhibit a tetragonal perovskite structure with a preferential(001) orientation. Several weak peaks near T(001) and T(011) are also observed for theBFPT1+0.02 films deposited under 10 Pa and 15 Pa, which might belong to the rhombohedralphases since the composition of BFPT1+0.02 falls into the MPB region of the BiFeO3-PbTiO3

solid solution where the tetragonal and rhombohedral phases coexist [11, 13]. The absenceof any other peaks, which are observed in the target, reveals that the T(001) orientation isthe optimized orientation, which will contributes to the enhancement of the FE and FMproperties. The degree of preferential (001) orientation can be calculated according to theformula α001 = I(001)/{I(001)+I(011)+I(111)} by using the integrated intensity (I) of the

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Figure 1. XRD patterns of the BFPT1+0.02 thin films deposited under different oxygen pressures.The “#” denotations indicate the peak position of the rhombohedral phase.

corresponding diffraction peaks. All the films show the values of α001 higher than 75%,confirming the highly preferential (100) orientation.

The AFM images of the film morphology are shown in Fig. 2. It can be seen thatthe grain sizes of the films increase from 150 nm to 500 nm with the increasing oxygenpressure. For the films prepared by PLD, in general, the oxygen pressure is one of thekey factors affecting the crystal structure and properties of the films. The BFPT1+0.02 filmsdeposited under an oxygen pressure of 10 Pa appear to be uniform and dense.

The binding energies of Fe 2p electrons measured by XPS are plotted and fitted to foursplitting peaks for all the films, as shown in Fig. 3(a)–(c). The reported binding energies are709 eV for Fe2+ 2P3/2 and 711 eV for Fe3+ 2P3/2 [20]. In this study, for the films depositedunder the oxygen pressures of 6 Pa, 10 Pa, 15 Pa, the valence states of Fe3+ and Fe2+ areobserved at 710.8 eV and 709.6 eV, respectively, from Fe 2P3/2 peaks. Accordingly, theother peaks are assigned to Fe3+ 2P1/2 at 724.6 eV and Fe2+ 2P1/2 at 723.1 eV. Because ofthe comparable thermodynamic stabilities of Fe3+ and Fe2+, Fe2+ ions always exist duringthe synthetic process of BiFeO3 and its solid solutions, resulting in oxygen vacancies for thecharge balance [21]. Our XPS results confirm that for the films deposited at 6 Pa, 10 Pa and15 Pa, both charge states of Fe3+ and Fe2+ are indeed simultaneously present (the peaksare not obvious from the spectrum of the film deposited at 2 Pa, which are not shown here).However, from the fitting curves, the area of the Fe3+ peaks is greater than that of the Fe2+

peaks for all the films. As the area ratio of the fitting peaks represents the content ratioof the different valence states of Fe ions, a Fe3+/Fe2+ area ratio > 1.0 for all the filmsindicates that the Fe3+ state is the predominant valence state. Further calculation shows thatthe ratio of the Fe3+ to Fe2+ ions is the largest for the film deposited under 10 Pa amongall of the films, which suggests that the oxygen vacancies are minimized when the film isdeposited under that oxygen pressure. Notablely, there is a peak between the Fe 2P3/2 andFe 2P1/2 peaks, located approximately at 8 eV higher than the main Fe 2P3/2 peak, whichis associated with the Fe 2P3/2 state as a satellite peak [22]. In addition, there appears to beanother peak at 730 eV, which may be a satellite peak for Fe 2P1/2.

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Figure 2. AFM images of the BFPT1+0.02 thin films deposited under (a) 2 Pa, (b) 6 Pa, (c) 10 Pa, and(d) 15 Pa, respectively. (See Color Plate IV)

The frequency dependences of the dielectric constant (ε′) and dielectric dissipationfactor (tanδ) of the BFPT1+0.02 films under different oxygen pressures are shown in Fig. 4.The dielectric constants of the films deposited under 10 Pa and 15 Pa are larger than thoseof the films deposited under 2 Pa and 6 Pa. The values of the dielectric constant and losstangent (tanδ) measured at 1 kHz are 550 and 5%, respectively, for the films depositedunder 10 Pa. Thereinto, the small loss value might be attributed to a low concentration ofthe oxygen vacancies, as revealed by the XPS results. These results also indicate that thedielectric properties of the BFPT1+0.02 films can be enhanced by optimizing the oxygenpressure during the deposition.

Figure 5 shows the ferroelectric hysteresis (P-E) loops measured at room temperatureand 1 kHz for the BFPT1+0.02 films. The measurements were carried out at differentvoltages for the BFPT1+0.02 films deposited at different oxygen pressures. The P-E loopsof the BFPT1+0.02 films deposited under 6 Pa and 15 Pa are very flat, indicating that nearlyno polariztion reverse occurred when a voltage up to ±15 V is applied on the films. TheP-E loop of the films deposited under 2 Pa is asymmetry, which suggests possible domain

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Figure 3. XPS spectra of the binding energy and the relative fitting curves of Fe 2p for the filmsdeposited under different oxygen pressures. (See Color Plate V)

pinning due to the presence of oxygen vacancies in BFPT1+0.02 films [23]. Moreover, theelliptical-like loop indicates that the films have serious electrical leakage which also iscaused by the oxygen vacancies [25]. The remnant polarization of the BFPT1+0.02 filmsunder 10 Pa is the largest in all the films. The remnant polarization and the coercive electricfield are 2Pr = 80 µC/cm2 and 120 kV/cm, respectively. Although the Pr value is not as largeas that reported for the films grown on the Pt/Si substrates by PLD (2Pr up to 100 µC/cm2)[18] and the 0.7BiFeO3–0.3PbTiO3 films prepared by chemical solution deposition with2Pr = 100 µC/cm2 at −190◦C [24], it is much higher than the bulk material (a Pr of17 µC/cm2 is reported for the 0.67BiFeO3–0.33PbTiO3 ceramic doped with 2% Ti on the

Figure 4. Frequency dependences of the dielectric constant (ε′) and dissipation factor (tanδ) of theBFPT1+0.02 thin films deposited under different oxygen pressures.

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Figure 5. P-E hysteresis loops at room temperature and 1 kHz for the BFPT1+0.02 films depositedunder different oxygen pressures.

Fe site) [21]. Thus, the high polarization must arise from the larger lattice distortion inducedin the films compared to that in the bulk material and some sol-gel derived films [19]. Inaddition, the preferential (001) orientation with tetragonal structure and the greater ratio ofthe Fe3+ and Fe2+ ions, as detected by XRD and XPS, respectively, should contribute tothe considerable remanent polarization in the film deposited under 10 Pa oxygen pressure.

4. Conclusions

The BFPT1+0.02 thin films prepared by PLD under different oxygen pressures exhibit apreferential (001) tetragonal structure. The coexistence of Fe2+ and Fe3+ ions implies theformation of oxygen vacancies in the films. The dielectric and ferroelectric propertiesof the BFPT1+0.02 thin films have been significantly improved by optimizing the oxygenpressure, and thereby minimizing the concentration of the oxygen vacancies. Althoughthe ferroelectric hysteresis (P-E) loops are not completely saturated, a large remanent

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polarization at room temperature has been observed in the BFPT1+0.02 film deposited underan oxygen pressure of 10 Pa, with 2Pr = 80 µC/cm2 and Ec = 120 kV/cm.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grants Nos.50728201 and U0634006) and the Shaanxi Province International Collaboration Program(Grant No. 2009KW-12), the Natural Science and Engineering Research Council of Canada(NSERC), and the U.S. Office of Naval Research.

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