Growth and Electric Properties of MPB BiScO3–PbTiO3 Thin Films onLa0.7Sr0.3MnO3-Coated Silicon Substrates

Shuai Zhang,z,y Xianlin Dong,z Ying Chen,z Fei Cao,z Yuanyuan Zhang,z Genshui Wang,w,z

Nossikpendou Sama,z and Denis Remiensz

zGroup of Ferroelectric & Piezoelectric Ceramics and Devices, Shanghai Institute of Ceramics, Chinese Academy of Sciences,1295 Dingxi Road, Shanghai 200050, China

yGraduate University of Chinese Academy of Sciences, Beijing 100049, China

zIEMN-DOAE, CNRS UMR 8520, Cite scientifique, 59655 Villeneuve-d’Ascq, Cedex, France

Morphotropic phase boundary BiScO3–PbTiO3 (BSPT) thinfilms were fabricated using La0.7Sr0.3MnO3 (LSMO) as bottomelectrodes on silicon substrates via a sol–gel method. Pt/BSPT/Pt systems were also prepared as the comparison. Both BSPTand LSMO were well crystallized and pure perovskite phaseswere observed through the X-ray diffraction measurement.Well-saturated polarization hysteresis loops were obtained. Amuch bigger remanent polarization (Pr) of BSPT/LSMO wasacquired than that of BSPT/Pt. The remanent polarization andcoercive field of BSPT/LSMO were 28 lC/cm2

and 200 kV/cm, respectively. The films had a room-temperature dielectricconstant of 720. The room-temperature piezoelectric coefficientd33 of 35 pm/V was observed, which was better than that ofBPST/Pt that had been reported.

I. Introduction

FERROELECTRIC thin films have attracted considerable atten-tion in recent years for their potential applications in

nonvolatile ferroelectric random access memories and micro-electromechanical systems (MEMSs).1 Recently, an attractivepiezoelectric solid solution system of BiScO3–PbTiO3 (BSPT)was reported by Eitel and colleagues,2–4 which exhibited a highTc of 4501C (over 1001C higher than that of PbZr1�xTixO3

(PZT)) and comparable properties with those of PZT at themorphotropic phase boundary (MPB), 0.36BiScO3–0.64PbTiO3. Much work had been carried out to explore theelectric properties of BSPT films on different substrates. TheBSPT films were grown on LaAlO3 single crystal substrates viapulsed laser deposition by Trolier–Mckinstry and colleagues5

and platinized Si wafer and Nb-doped SrTiO3 single crystalsubstrates by Wen et al.6,7 The hydrothermal method and theuse of PbTiO3 seed layer were also introduced to the fabricationof BSPT films.8,9 Few works were focused on the electric prop-erties of BSPT films with different electrodes on silicon sub-strates.

Many conductive oxides, such as La0.7Sr0.3MnO3 (LSMO),SrRuO3, YBa2Cu3O7�x, and LaNiO3, have been used as elec-trodes for PZT thin films due to improved ferroelectric, fatigue,and aging characteristics of these PZT thin films.10–13 LSMO is a

metallic oxide with a lattice constant of about 3.88 nm, close tothat of BSPT (about 3.98 nm), and has attracted great interestrecently due to its colossal magneto-resistance. It can provide animproved interface contact due to the similar perovskite struc-ture and the same oxide nature with BSPT. The order of themagnitude of the room-temperature resistivity of LSMO is 10�3

O � cm,14 comparable to those of the oxide electrodes mentionedpreviously. But the BSPT films with complex oxide electrodes onsilicon substrates have not been fabricated, and their propertiesare unknown until now. In this paper, we prepared BSPT thinfilms with the composition near the MPB on Si substrates usingLSMO as the bottom electrodes by the sol–gel method. Thecomparison system Pt/BSPT/Pt/Si was fabricated through thesame sol–gel processes. And their ferroelectric, dielectric, andpiezoelectric properties were investigated.

II. Experimental Procedure

Manganese acetate tetrahydrate (Mn(OOCCH3)2 � 4H2O), Lan-thanum acetate sesquihydrate (La(OOCCH3)3 � 1.5H2O), andStrontium acetate (Sr(OOCCH3)2) were dissolved into glacialacetic acid and distilled water to derive the LSMO precursor.BSPT precursor solution with a nominal composition of 0.36BiScO3–0.64PbTiO3 with 10 mol% lead excess and 10 mol% Biexcess was synthesized by using Titanium isopropoxide (Ti(OCH(CH3)2)4), lead acetate trihydrate (Pb(OOCCH3)2 � 3H2O),Scandium acetate (Sc(OOCCH3)3), and Bismuth nitrate penta-hydrate (Bi(NO3)3 � 5H2O) as starting materials, with glacialacetic acid and distilled water as solvents. Acetylacetone wereadded to the above two precursors to avoid the appearance ofcracks. A single-layer LSMO film was deposited using spincoating at 3000 rpm for 30 s on a Si (100) substrate, then driedat 1801C, pyrolyzed at 4001C, and finally annealed at 7001C us-ing a rapid thermal processor. Multilayer thin films wereachieved by repeating the spin-coating and heat-treating cycles.The BSPT thin films were deposited on LSMO films through asimilar process but pyrolyzed at 4501C to give a final thicknessof about 180 nm. The BSPT films were also fabricated on plat-inized silicon substrates as the comparison systems. Before theferroelectric and piezoelectric properties measurements, Pt topelectrodes with a diameter of 150 mm were sputtered through aphotolithography process.

X-ray diffraction (XRD) analysis was performed to reveal thestructural properties of the film using a Rigaku D/max 2550 Vinstrument (Rigaku Corporation, Akishima-shi, Tokyo, Japan).The ferroelectric properties of the BSPT films were studiedusing a TF analyzer 2000 (aixACCT Systems GmbH, Aachen,Nordrhein-Westfalen, Germany) ferroelectric test module. TheC–V measurements were performed using HP4192A (Agilent

R. E. Eitel—contributing editor

This work was supported by the National Basic Research Program of China (973 Pro-gramNo. 61363 Z 09.1) and the Hundred Talents Program of Chinese Academy of Sciences.

wAuthor to whom correspondence should be addressed. e-mail: genshuiwang@mail.sic.ac.cn

Manuscript No. 27043. Received November 2, 2009; approved December 21, 2009

Journal

J. Am. Ceram. Soc., 93 [6] 1583–1585 (2010)

DOI: 10.1111/j.1551-2916.2010.03630.x

r 2010 The American Ceramic Society

1583

Technologies, Santa Clara, CA) at room temperature. Thepiezoelectric coefficients of BSPT films were investigated withLaser Doppler vibrometry.15 The experimental arrangementused in the laboratory consists of a modified laser Dopplervibrometer, which is combined with a Nikon (Japan) opticalmicroscope for beam delivery to the sample. A small top elec-trode (150 mm in diameter) was used during displacement mea-surement for a decreasing substrate bending effect.

III. Results and Discussions

Figure 1 shows the XRD patterns of the BSPT/LSMO films,which reveal the well crystallization. Pure perovskite phaseswithout any second phase such as pyrochlore are observed forBSPT films.16 The well crystallization is further confirmed by theSEM image as shown in Fig. 2. It can be seen that the LSMOfilms have ball-like grains and distinct and clean interfaces havebeen obtained.

Figure 3(a) presents the polarization hysteresis loops for Pt/BPST/LSMO measured under different electric fields at a fre-quency of 100 Hz. Well-saturated hysteresis loops were ob-

served. The remanent polarization and coercive field were 28mC/cm2 and 200 kV/cm, respectively. The value of this remanentpolarization is very close to that of the ceramics (about 32 mC/cm2), which have the composition in the MPB region.2 Figure3(b) shows comparison of the polarization hysteresis loops ofPt/BSPT/LSMO and Pt/LSMO/Pt derived under the same mea-surement conditions. The remanent polarization is distinctly im-proved when LSMO, instead of Pt, is used as the bottomelectrodes. The enhancement in Pr may attribute to the LSMObottom electrodes, which could reduce the oxygen vacancies inthe BSPT films. Li and colleagues reported that the remanentpolarization decreases with the increase of oxygen vacancies inPZT thin films.17 Also, the perovskite structure nature of LSMOmay provide a better interface and facilitate the crystallization ofBSPT films, which will contribute to the improvement of the re-manent polarization. But the coercive field is much larger thanthat of the ceramics,3 which may arise from the use of the Pt topelectrodes. It was reported that the top electrodes have a signifi-cant influence on the coercive field of the PZT films. The use of Pttop electrode strongly increases both positive and negative coer-cive fields.18 In addition, the interface capacitors may play a rolein the increasing of the coercive field of the BSPT films. Increasein remanent polarization and coercive field are observed with therise in applied voltage. But the amount of the increase becomessmaller with the augmentation of the applied voltage, which givesthe sign of the saturation of the polarization hysteresis loops.

Typical electric field dependence of dielectric constant char-acteristics for the BSPT thin films are shown in Fig. 4. The di-electric constant of BSPT films is about 720. The magnitude issmaller than that of the MPB BSPT ceramics,3 which may bedue to the existence of interface capacitors between Pt top

Fig. 1. X-ray diffraction patterns of BSPT/LSMO/Si.

Fig. 2. SEM images of BSPT/LSMO/Si.

Fig. 3. (a) Polarization hysteresis loops of Pt/BSPT/LSMO/Si underdifferent electric fields at 100 Hz; (b) comparison of the polarizationhysteresis loops of Pt/BSPT/LSMO/Si and Pt/BSPT/Pt/Si.

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electrodes and the BSPT films. The curve is not offset obviouslyin the horizontal direction; hence, internal bias in this Pt–BSPT–LSMO system may be ignored.19

The piezoelectric coefficient d33 as a function of applied electricfield for Pt/BSPT/LSMO/Si and Pt/BSPT/Pt/Si capacitors areshown in Fig. 5. As shown in the figure, typical well-shaped loopsare observed and the piezoelectric coefficient of BSPT/LSMO isabout 35 pm/V. The d33 obtained here is lower than that observedin the ceramics.2 The BSPT films are clamped by the substratewhen the film thickness is very small, which will decrease the pi-ezoelectric coefficient. It is obvious that the d33 of BSPT/LSMO isbigger than that of BSPT/Pt and the d33 of BSPT/LSMO exhibit ahigher increase rate in the small bias field region. Furthermore, thevalue of the d33 of our BSPT films is larger than that of the Pt/BSPT/Pt capacitor reported (23 pm/v) by Wen et al.6 The reasonsmay be that LSMO can provide a better interface contact andrelease the substrate constraint. Because LSMO electrodes providea better lattice match than the (111)-oriented Pt bottom electrodesLSMO has similar peroskite structure with BSPT. Hence, the Pt/BSPT/LSMO systems may have better potential applications inthe MEMS and other devices operated at high temperature.

IV. Conclusions

MPB 0.36BiScO3–0.64PbTiO3 thin films were fabricated by asol–gel method on LSMO-coated and -platinized silicon wafer.Well-saturated P–E hysteresis loops and typical C–V loop wereobserved, which indicated the good ferroelectric and dielectricproperties. A remanent polarization of 28 mC/cm2 and a coercivefield of 200 KV/cm for Pt/BSPT/LSMO were obtained. The di-electric constant is about 720 at room temperature. The piezo-electric coefficient d33 of 35 pm/v and well-shaped piezoelectrichysteresis loops were observed. Both the ferroelectric and pi-ezoelectric properties of BSPT films were distinctly improvedwhen LSMO instead of Pt was used as the bottom electrodes. Itis suggested that the Pt/BPST/LSMO is a promising candidatefor MEMS application.

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Fig. 4. Electric field dependence of dielectric constant characteristicsfor Pt/BSPT/LSMO/Si.

Fig. 5. Piezoelectric coefficient d33 as a function of electric field for Pt/BSPT/LSMO/Si and Pt/BSPT/Pt/Si.

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