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Acta Materialia 55 (2007) 4707–4712

Impact of oxygen atmosphere on piezoelectric propertiesof CaBi2Nb2O9 thin films

A.Z. Simoes a, C.S. Riccardi c, L.S. Cavalcante b, E. Longo a,*, J.A. Varela a, B. Mizaikoff c

a Laboratorio Interdisciplinar em Ceramica, Departamento de Fısico-Quımica, Instituto de Quımica, Universidade Estadual Paulista,

R. Francisco Degni, P.O. Box 355, 14801-907, Araraquara, SP, Brazilb Laboratorio Interdisciplinar de Eletroquımica e Ceramica, Departamento de Quımica, Universidade Federal de Sao Carlos,

P.O. Box 676, 13565-905, Sao Carlos, SP, Brazilc School of Chemistry and Biochemistry, Georgia Institute of Technology, 901 Atlantic Drive, Atlanta, GA 30332-0400, USA

Received 7 March 2007; received in revised form 17 April 2007; accepted 19 April 2007Available online 7 June 2007

Abstract

CaBi2Nb2O9 (CBNO) thin films were deposited on platinum-coated silicon substrates by the polymeric precursor method, and wereannealed in air and in an oxygen atmosphere. The structure, surface morphology and electrical properties of CBNO thin films have beeninvestigated. The presence of an oxygen atmosphere during crystallization of the films affected the structure perfection and morphology,as well as ferroelectric and piezoelectric properties. A reduction in Pr and piezoelectric coefficient, an increase of Vc and displacement ofthe Curie point is evident in the films crystallized in an oxygen atmosphere. The impact of exposure to the oxygen atmosphere on thecreation of defects caused by bismuth and oxygen vacancies between layers was also investigated by X-ray photoelectron spectroscopy.� 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Piezoelectricity; Vacancies; Point defects; CaBi2Nb2O9; X-ray photoelectron spectroscopy

1. Introduction

Ferroelectric thin films have been widely investigated fortheir use in a variety of devices that exploit their uniquepiezoelectric, pyroelectric, polarization switching and elec-tro-optical properties [1]. Eletromechanical transducermaterials are required to sense strains, vibrations and noiseunder several thermal conditions. Of the different types ofacoustic and strain sensors, piezoelectric devices arefavored, based on considerations of sensitivity, cost anddesign [2]. When an operating temperature of 400 �C orgreater is required, the choice of materials for high-temper-ature piezoelectric transducers is limited. Modified bismuthtitanate compositions are interesting for sensor applica-tions up to 500 �C [3]. When an operating temperature ofup to 750 �C is required, no suitable commercial ceramic

1359-6454/$30.00 � 2007 Acta Materialia Inc. Published by Elsevier Ltd. All

doi:10.1016/j.actamat.2007.04.030

* Corresponding author. Tel.: +55 16 3361 5215; fax: +55 16 3351 8350.E-mail address: elson@iq.unesp.br (E. Longo).

is currently available. In general, the Aurivillius (perov-skite) phase is represented by (Bi2O2)2+(Am�1BmO3m+1)2�,where A is a divalent ion such as Ca2+ [4], Sr2+ [5] andBa2+ [6], B is a metal of valence 5+, usually, Ta5+ [7] orNb5+ [8], and m is the number of the perovskite unit cellsbetween the Bi2O2 layers. CaBi2Nb2O9 (CBNO) is a mem-ber of the Bi-based layer-structured perovskite compounds[9], such as SrBi2Ta2O9 (SBT) and SrBi2TaxNb2�xO9

(SBTN), which have been the subject of much researchfor applications in nonvolatile random access memory[10]. Compared with SBT and SBNT thin films, only afew reports have discussed the electrical properties ofCBNO thin films [11]. CBNO is a layered perovskite ferro-electric oxide with the following lattice constants:a = 5.5435 A, b = 5.4658 A and c = 24.9701 A [12]. Thespontaneous polarization Ps is along the x-axis direction.Therefore, thin films with x-axis orientation are preferredin a wide variety of devices, such as ferroelectric randomaccess memories, piezoelectric microactuators, and resona-

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Fig. 1. X-ray diffraction of CBNO thin films annealed at 700 �C for 2 h in(a), (d) air and (b), (c) an oxygen atmosphere.The zoom shows the 27.5–32.5 2h range of (200)/(020) peaks with FWHM value.

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tors. In our previous work [13], polar-axis-oriented CBNOthin films have been synthesized via the chemical solutiondeposition technique on Pt-coated silicon substrates show-ing preferred orientation of the (200)/(02 0) planes. Thepolar-axis orientation of CBNO thin films was stronglyassociated with the preferred orientation of the Pt sub-strate, and results from sufficient lattice matching betweenthe z-axis of CBNO and the Pt(100) plane. The ferroelec-tric and piezoelectric properties of polar-axis-orientedCBNO thin films had been improved compared to ran-domly crystallized thin films at Si(1 00) substrates by thepulsed ablation technique [14].

In this paper, the effect of ambient oxygen during heattreatment of polar-axis-oriented CBNO films on the struc-ture quality and its electrical properties are reported. Thecrystallographic properties are thoroughly analyzed, andthe ferroelectric and piezoelectric properties are compared.The potential of CBNO films as Pb-free ferroelectric mate-rials can thus be confirmed.

2. Experimental procedure

The preparation of the CBNO deposition solution hasbeen described in detail elsewhere [13]. The films weredeposited onto Pt(100)/Ti/SiO2 substrates by spinningthe deposition solution at 5000 rpm for 30 s. After deposi-tion, the films were treated at 350 �C for 2 h, for decompo-sition of the organic material. The desired thickness wasobtained by several cycles of deposition and thermal treat-ment at 350 �C for 2 h. Finally, the films were crystallizedat 700 �C for 2 h in air and oxygen flow (100 ml min�1).After crystallization, the films were characterized by X-ray diffraction (XRD; Rigaku-DMax 2500PC) at 40 kVand 150 mA from 2 (10–50�) following the phase evolution.High-resolution photoelectron spectra were collected on aPhysical Electronics PHI 1600/3057 spectrometer equippedwith a monochromated Al–Kr (1486.6 eV) X-ray beamwith detection at the surface normal. The spectrometerwas operated in fixed analyzer energy transmission mode,and the pressure during analysis was maintained at approx-imately 5.0 · 10�10 Torr. Photoelectron spectra of Bi 4f, Ca2p and Nb 3p3/2 core levels were recorded using a com-puter-controlled data collection system. The electron ana-lyzer was set at a pass energy of 10 eV.

The thickness of the annealed films was determined byscanning electron microscopy (Topcom SM-300) at thetransverse section, evaluating the back-scattering electrons.The film annealed under an oxygen atmosphere is 240 nmthick while the film annealed under a static atmosphere is280 nm thick. Atomic force microscopy (AFM; Digital,Nanoscope 3A) was used to analyze the layer roughnessand the average grain size. Top Au electrodes (0.5 mmdiameter) were prepared for the electrical measurementsby evaporation through a shadow mask at room tempera-ture. The electrical properties were measured by an Au/CBNO/Pt(100)/Ti/SiO2 capacitor structure. Ferroelectric-ity was investigated using a Sawyer–Tower circuit attached

to a computer-controlled standardized ferroelectric test sys-tem (Radiant Technology 6000 A). The dielectric character-ization was accomplished with an impedance analyzer(model 4192 HP) during measurements of the capacitanceas function of temperature at a frequency of 100 kHz. TheCurie temperature of the material was obtained from thecapacitance-dependent temperature curve. Out-of-planepiezoelectric response measurements using piezoelectricforce microscopy (PFM) were performed to study the piezo-electric properties of these films. The films were first poledat a negative DC bias (�12 V) applied to a conductingprobe while scanning over a 2 · 2 lm2 area. Another polingwas performed with the probe biased at the opposite voltage(+12 V) during scanning over a 1 · 1 lm2 area inside thepreviously poled one. Details of the PFM technique havebeen reported for calcium bismuth titanate films [15].

3. Results and discussion

Fig. 1 shows the XRD pattern of CBNO thin films,grown on Pt(100)/Ti/SiO2 substrates, crystallized at700 �C for 2 h in air and under an oxygen flow.

A high intensity of the (200)/(02 0) diffraction line com-pared to other lines was observed, although those otherlines could not be distinguished from each other. The char-acteristic orientation is considered to result from favorablematching of the atomic arrangements in CBNO (100)/(01 0) and the underlying Pt planes. A single phase of alayer-structured perovskite with orthorhombic crystallo-

A.Z. Simoes et al. / Acta Materialia 55 (2007) 4707–4712 4709

graphic structure was observed. The diffraction peaks arematched and indexed based on CBNO structure parame-ters [14]. The preferred orientation is considered to resultfrom sufficient matching between the z-axis of the CBNOfilm and the (100) plane of the Pt substrate. The main peakof the film annealed under ambient oxygen is very sharp,suggesting the presence of large crystalline grains in thelayer (see Fig. 1a). Ambient oxygen caused a significantreduction in the peak intensities, indicating reduced crystal-linity. The full width at half-maximum (FWHM) values ofthe (200)/(0 20) diffractions for the CBNO films crystal-lized in oxygen flow and air were 0.266� (see Fig. 1c) and0.169� (see Fig. 1d), respectively. The smaller value indi-cates that the degree of structural perfection of the filmcrystallized in air is much higher. The increase in FWHMvalue for the film treated under ambient oxygen conditionsmay be caused by an increase in stress, rendering the orien-tation of crystallites more difficult.

Fig. 2 shows P–E characteristics of Au/CBNO/Pt capac-itors at an applied voltage of 30 V and a frequency of60 Hz.

The remanent polarization (Pr) and coercive field (Ec) ofthe films crystallized in oxygen flow and air were 1 lC cm�2

and 219.73 kV cm�1, and 4.2 lC cm�2 and 55.94 kV cm�1,respectively. The Pr and Vc values were improved in thefilms annealed under static air compared with the valuesobtained for polycrystalline thin films [15]. The higher Pr

value suggests that the (100) orientation is preferred ratherthan the (010) orientation. The reason for this improve-ment is that the ferroelectric polarization switching occursin the x–y plane in CBNO. In contrast, films with a lower z-axis ratio revealed better-saturated hysteresis loops thanfilms with a higher z-axis ratio, due to the weak ferroelec-tricity of bismuth-layered ferroelectrics along this direc-tion. As is commonly known, annealing in an oxygenatmosphere should decrease the oxygen vacancy concentra-tion and increase electron holes, due to the incorporationof oxygen molecules at positively charged oxygen vacan-cies. However, in materials with p-type conductivity, ther-mal treatment in an oxidizing atmosphere increases defectssuch as bismuth and niobium vacancies, which are located

Fig. 2. P–E hysteresis loops for CBNO thin films annealed at 700 �C for2 h in (a) air and (b) an oxygen atmosphere.

at the grain boundary and at the film–electrode interface,thereby promoting a local stoichiometry deviation thatinfluences the shape of the hysteresis loops. This effect –in addition to the presence of z-axis orientation – leads toa lower remanent polarization and higher coercive field.

Temperature dependence of the dielectric constant ofCBNO thin films at 100 kHz is illustrated in Fig. 3.

The Curie point was 944 �C ± 2 �C for the film annealedin air, and 961 �C ± 2 �C for the film annealed under ambi-ent oxygen. This value of the Curie point is much higherthan the values reported by Ismailzade [16]. The fact thatthe Curie point of our films is significantly higher suggeststhat the results of Ismailzade for Tc may be erroneous, asthey were obtained under the assumption of coincidencewith a phase transformation observed by XRD. Theobserved shift of the Curie temperature, and the reductionof dielectric permittivity in films crystallized under an oxy-gen atmosphere, may be associated with small grain size,structural disorder, microscopic inhomogeneities, and thepresence of a substantial two-dimensional stress or strainin the plane of the film. Thus, the collective effect of the fac-tors mentioned above are likely to play a major role in thephase transition of CBNO thin films.

In spite of their relatively larger Ps, the piezoelectricactivity of bismuth layer-structured ferroelectrics (BLSFs)is limited because of two-dimensional orientation restric-tions of the spontaneous polarization rotation, and theirhigher coercive field Ec. Orientation of films is an effectiveapproach to improve piezoelectric activity, because of theanisotropy of their microstructure and properties. Thed33(V) hysteresis loops are shown in Fig. 4.

The d33 value, �58 pm V�1, obtained for the filmannealed in air is higher than that of single-phase CBNOceramics grown by ordinary firing (OF) and spark plasmasintering (SPS) [17]. The enhancement of polarizationcould be caused by the x/y-axis orientation of the ferroelec-tric films, due to the preferred orientation of the Pt(100)substrates. As is evident, the hysteresis loops show an offsetin vertical direction, which may be caused by clamping

Fig. 3. Temperature dependence of dielectric constant for CBNO thinfilms annealed at 700 �C for 2 h in (a) air and (b) an oxygen atmosphere.

Fig. 4. Piezoresponse loops for CBNO thin films annealed at 700 �C for2 h in (a) air and (b) an oxygen atmosphere.

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effects due to the substrate, and the existence of an ultra-thin air gap between the tip and the sample, which maylower the actual voltage drop in the film [18]. The low d33

value of the film crystallized in an oxygen atmosphere(�19 pm V�1) mainly originated from the degree of struc-tural perfection. Considering that substantial piezoelectric

a

c

Fig. 5. XPS spectra of (a) Bi 4f, (b) Ca 2p and (c) Nb 3d peaks for CaBi2Nb2O

activity results from the contribution of x-axis orientation,the reduced polarizability in the film annealed at ambientoxygen may be caused by the presence of significant peaksoriented along z-axis direction, and broadening of the mainpeak of the CBNO phase.

In order to gain further understanding on the defectscreated by ambient oxygen in CBNO thin films, X-ray pho-toemission analysis was performed. Photoemission of Bi 4f,Ca 2p and Nb 3d core levels were performed, and theresults are shown in Fig. 5.

The Bi 4f peak for the film annealed in air is shiftedtoward the lower-binding-energy side with respect to thatfor the oxidized film. However, the Ca 2p peaks for bothfilms show nearly the same position. These experimentalresults clearly indicate that oxygen vacancies are preferablypresent near the Bi ions at the [Bi2O2]2+ layers. Moreover,Nb 3d peaks for films annealed in air are somewhatbroader than those for oxidized films. Furthermore, thepeaks for states that are less oxidized than Nb5+ are alsolocated in a lower-binding-energy region. The peak broad-ening suggests that some oxygen atoms at the perovskitelayers are removed, implying that oxygen vacancies couldbe induced in the neighborhood of the Bi and Nb ions.Because ferroelectric behavior of Bi-layered perovskite

b

9 thin films annealed at 700 �C for 2 h in air and in an oxygen atmosphere.

Fig. 6. Atomistic model for CaBi2Nb2O9 structure: (a) free of defects and (b) with bismuth and oxygen vacancy type defects.

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materials mainly originates from metal–oxygen octahedra,the reduction of the ferroelectric properties in CBNO filmsis related to oxygen vacancies present at the niobium–oxy-gen octahedra.

Fig. 6 shows the atomistic model for the CaBi2Nb2O9

structure free of defects and with defects for CBNO thinfilms annealed at 700 �C for 2 h in air and in an oxygenatmosphere.

Fig. 6a and b illustrates the crystalline structure modelof CaBi2Nb2O9 structure free of defects and with defects,respectively. Thermal treatment of CBNO thin films in anoxygen atmosphere lead to niobium and calcium clustersassociated with oxygen vacancies. The increase in defectsin the z-axis originates from the incorporation of oxygeninto (Bi2O2)2+ layers promoted by bismuth volatilizationwith formation of (V 000Bi). These defects create oxygen vacan-cies in [NbO6] and [CaO12] clusters, with most of the oxy-gen vacancies being considered as complex vacancies in aorder–disorder structure. The CaBi2Nb2O9 structure forthin films thermal treated in air and in oxygen atmospherecan be represented by the following equations:

½NbO6�x þ ½NbO5 � VxO� ! ½NbO6�0 þ ½NbO5 � V�O� ð1Þ

½NbO6�x þ ½NbO5 � V�O� ! ½NbO6�0 þ ½NbO5 � V��O� ð2Þ½CaO12�x þ ½CaO11 � Vx

O� ! ½CaO12�0 þ ½CaO11 � V�O� ð3Þ½CaO12�x þ ½CaO11 � V�O� ! ½CaO12�0 þ ½CaO11 � V��O� ð4Þ½Bi2O2��� þ ½BiO2 � V000Bi� ! ½Bi2O2�x þ ½BiO2 � V0Bi� ð5Þ½BiO2 � V0Bi� þO2 ! ½BiO2 � Vx

Bi� � O02ðadsÞ ð6Þ

In this structure, the [NbO5 � VxO], [CaO11 � Vx

O] and[BiO2 � V000Bi] clusters are donor candidates and [NbO6]x,[CaO12]x and [Bi2O2]�� are acceptor candidates. Accordingto Damjanovic [19] V000Bi produced by metal defects make

domain wall movement easier, while V��O produced byacceptor doping make difficult. An oxygen vacancy–accep-tor ion dipole may interact with polarization within adomain, making its movement more difficult [20]. How-ever, our proposed CBNO thin film thermally treated inair is free of oxygen vacancies (V��O). The clusters presentedin Eqs. (1), (3) and (5) are predominant for CBNO thinfilms thermally treated in air, and the clusters presentedin Eqs. (2), (4) and (6) for the films thermally treated inoxygen. Neutral (Vx

0) and oxygen vacancies of (V�O) typemake the movement of domain walls easier. A stronganisotropic property and the formation of bismuth (V000Bi)and oxygen vacancies (V��O) in the CBNO-type structurereduces the ferroelectric properties of the thin films ther-mally treated in an oxygen atmosphere. This occurs duethe generation of trapping charges in bismuth and oxygenvacancies at (Bi2O2)2+ layer [21].

4. Conclusions

In summary, polar-axis-oriented CBNO films wereobtained on platinum-coated silicon substrates by softchemical deposition. Thermal treatment of CBNO thinfilms in oxygen atmosphere produce bismuth vacancies(V000Bi) that inhibit the movement of domain walls alongthe z-axis, while oxygen vacancies (V��O) make the move-ment of domain walls difficult for Nb and Ca atoms. Neu-tral vacancies (Vx

O) and e�/V�O for Ca and Nb atoms makethe movement of domain walls easier for CBNO thin filmsannealed in air. The presence of an oxygen atmosphereduring crystallization of films affected the structural perfec-tion along the z-axis, as well as the ferroelectric and piezo-electric properties. The Pr and Vc of the films reduced to1 lC cm�2 and 4.8 V, respectively, at an applied voltage

4712 A.Z. Simoes et al. / Acta Materialia 55 (2007) 4707–4712

of 30 V. Finally, XPS analysis reveals that oxygen vacan-cies could be induced in the neighborhood of the Bi andthe Nb ions, thereby affecting the ferroelectric properties.

Acknowledgements

The authors gratefully acknowledge the financial sup-port of the Brazilian agencies FAPESP, CAPES andCNPq. Furthermore, the authors thank Dr. Dennis Hessat the School of Chemical and Biomolecular Engineering(Georgia Tech) for access to XPS instrumentation.

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