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Materials Chemistry and Physics 124 (2010) 894–899

Contents lists available at ScienceDirect

Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

eview

ffect of oxidizing atmosphere on ferroelectric and piezoelectric response ofaBi2Nb2O9 thin films

.Z. Simõesc,∗, E.C. Aguiara,1, C.S. Riccardia,1, E. Longoa,1,2, J.A. Varelaa,1, B. Mizaikoffb

Chemistry Institute, Department of Chemistry-Physics, UNESP, 14800-900 Araraquara, SP, BrazilSchool of Chemistry and Biochemistry, Georgia Institute of Technology, 901 Atlantic Dr., EST Bldg., Atlanta, GA 30332-0400, USAFederal University of Itajuba- Unifei - Itabira, São Paulo Street, 377, Amazonas, 35900-37 Itabira, MG, Brazil

r t i c l e i n f o

rticle history:eceived 13 November 2009eceived in revised form 16 August 2010

a b s t r a c t

Calcium bismuth niobate (CaBi2Nb2O9, CBNO) thin films were evaluated for use as lead-free piezoelec-tric in micro-electromechanical systems. The films were grown by the polymeric precursor method on

◦

ccepted 19 August 2010

eywords:hin filmsnnealinghemical synthesis

Pt/Ti/SiO2/Si (1 0 0) (Pt) substrates and annealed at 700 C for 2 h in ambient and oxygen atmosphericconditions. The films were characterized by means of XRD, Raman, IR, PFM and electrical measurements.Annealing in ambient condition leads to improved ferroelectric properties, higher fatigue and retentionresistances, and improved piezoelectric response. Furthermore, oxygen atmosphere produces bismuthvacancies (V′′′

Bi) that inhibit the movement of domain walls along the z-axis, and consequently reduces thepiezoelectric coefficient. Independently of the applied electric field, the retained switchable polarization

erroelectricity approached a nearly steady-state value after a retention time of 10 s.© 2010 Elsevier B.V. All rights reserved.

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8942. Experimental procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8953. Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8964. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 898

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 898References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 898

. Introduction

Recently, ferroelectric thin films have been widely investigatedor their use in a variety of devices exploiting their unique piezo-lectric, pyroelectric, polarization switching, and electro-opticroperties. Ferroelectric thin films with large remanent polariza-ion, low coercive field, and low fatigue rate characteristics haveignificant potential for use as memory elements in high-densityT-1C nonvolatile memories [1]. CaBi2Nb2O9 (CBNO) is a mem-

extensively investigated because of its excellent fatigue-resistantproperties and small temperature coefficient of resonance fre-quency (TCF). Huang et al. have investigated the effect of lanthanumdoping on the dielectric properties of SrBi2−xLaxNb2O9 ceram-ics with La content between 0 and 0.35 [4]. The authors haveobserved that the temperature of the maximum dielectric constantdecreases linearly with increasing La content. Moreover, La sub-stitution improves the rectangularity of the hysteresis loop, anddecreases both remanent polarization and coercive field. Compared

er of Bi-based layer-structured perovskite compounds [2] suchs SrBi2Ta2O9 (SBT) and SrBi2TaNb2O9 (SBTN), in which substan-ial research on nonvolatile random access memory applicationsas been performed [3]. Recently, SrBi2Nb2O9 (SBN) has been

∗ Corresponding author. Tel.: +55 31 3834 6472/6136;ax: +55 31 3834 6472/6136.

E-mail address: alezipo@yahoo.com (A.Z. Simões).1 Tel.: +55 16 3301 6643; fax: +55 16 3301 6692.2 Tel.: +55 31 3834 6472/6136; fax: +55 31 3834 6472/6136.

254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2010.08.059

with SBT and SBNT thin films, only few reports discuss the elec-trical properties of CaBi2Nb2O9 (CBNO) thin films [5]. CBNO is alayered perovskite ferroelectric oxide, whose lattice constants are:a = 5.5435 A, b = 5.4658 A, c = 24.9701 A [6].

To obtain a usable ferroelectric memory device exploiting film,

it is required that the materials have excellent ferroelectric prop-erties, low interface state density, high breakdown strength, andlow leakage currents. Usually, metal–ferroelectric-semiconductormaterials are the desired configuration for one-transistor high-density memory arrays applied to nonvolatile memories. In this

A.Z. Simões et al. / Materials Chemistry and Physics 124 (2010) 894–899 895

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ig. 1. Raman spectra for CBNO thin films annealed at 700 ◦C for 2 h at (a) staticir and (b) oxygen atmosphere. The inset of Fig. 1 illustrates the X-ray diffractionattern for CBNO thin films annealed at 700 ◦C for 2 h at (a) static air and (b) oxygentmosphere.

onfiguration, direct deposition of ferroelectric thin films at thei wafer surface is required. However, it is difficult to pre-erve ferroelectricity at Si due to the existence of interfacialraps, and/or interdiffusion of the constituent elements. There-ore, it is interesting to evaluate the ferroelectric properties ofhese films at metal electrodes such as e.g., platinum. In a mem-ry element, a data bit (1 or 0) should be stored, and madevailable for queries at some later time. Retention is the abil-ty of a memory element to maintain a given data state as timelapses [7–11]. Retention loss causes a reduction in the differ-nce between switched (P*) and nonswitched (Pˆ) polarizations�P = (P*) − (Pˆ)), and leads to the inability to distinguish betweenhe two logic states. Therefore, sufficient retention properties areuite important for any ferroelectric material utilized for FRAMpplications. Retention loss in ferroelectric capacitors is gener-lly attributed to the presence of the internal electric field due towo basic effects: incomplete screening of the depolarizing fieldnd formation of a depletion layer at the film/electrode interface8–10,12–17].

In our previous work [18], polar-axis-oriented CBNO thinlms have been synthesized via the chemical solution depositionechnique at Pt-coated silicon substrates revealing preferred ori-ntation of the (2 0 0)/(0 2 0) planes. The polar-axis orientation ofBNO thin films was strongly associated with the preferred ori-ntation of the Pt substrate, and results from sufficient latticeatching between the z-axis of CBNO and the Pt(1 0 0) plane. The

erroelectric and piezoelectric properties of polar-axis-orientedBNO thin films had been improved compared to randomly crys-allized thin films at Si(1 0 0) substrates by the pulsed ablationechnique [19].

Several methods such as chemical vapor deposition, laser abla-ion, sol–gel technique and soft chemical method have been usedo produce thin films of various thicknesses and nanoparticles [20].mong all of these, the soft chemical method presents many advan-

ages, such as the possibility to work in aqueous solutions withhe high stoichiometry control. Moreover, it is a low-temperaturerocess and a cost-effective method (inexpensive precursors and

quipments).

In our previous work we have evaluated the defects createdy ambient oxygen in CBNO thin films by X-ray photoemis-ion spectroscopy [21]. We have observed that some oxygentoms at the perovskite layers are removed, implying that oxy-

Fig. 2. Infrared spectra for CBNO thin films annealed at 700 ◦C for 2 h at (a) static airand (b) oxygen atmosphere.

gen vacancies could be induced in the neighborhood of the Biand Nb ions. The crystallization of the films in the ambientoxygen impacted the structurally perfection in z-axis, and theferroelectric and piezoelectric properties. In this paper, we haveinvestigated the influence of the annealing atmosphere for obtain-ing fatigue and retention free properties of CBNO ferroelectricthin films, thereby confirming the potential as Pb-free ferroelectricmaterials.

2. Experimental procedure

The preparation of the CBNO deposition solution has been described in detailelsewhere [18]. The films were deposited at Pt(1 0 0)/Ti/SiO2 substrates by spin-ning the deposition solution at 5000 rpm for 30 s. After deposition, the films weretreated at 350 ◦C for 2 h to decompose the organic material. The desired thicknesswas obtained by several cycles of deposition and thermal treatment at 350 ◦C for2 h. Finally, the films were crystallized at 700 ◦C for 2 h in air and at oxygen flow(100 mL min−1). After crystallization, the films were characterized by X-ray diffrac-tion (XRD; Rigaku-DMax 2500PC) at 40 kV and 150 mA from 2� (10–50) followingthe phase evolution. The thickness of the annealed films was determined by scan-ning electron microscopy (Topcom SM-300) at the transverse section evaluating theback-scattering electrons. The film annealed at an oxygen atmosphere is 240 nmthick, while the film annealed at a static atmosphere is 280 nm thick. Raman mea-surements were performed using an ISA T 64000 triple monochromator. An opticalmicroscope with 80× objective was used to focus the 514.5-nm radiation from aCoherent Innova 99 Ar+ laser on the sample. The same microscope was used to col-lect the back-scattered radiation. The scattering light dispersed was detected bya charge-coupled device (CCD) detection system = 0.3 mm. The piezoelectric mea-surements were carried out using a setup based on an atomic force microscopein a Multimode Scanning Probe Microscope with Nanoscope IV controller (VeecoFPP-100). The films were first poled at a negative dc bias (−12 V) applied to aconducting probe while scanning over a 2 �m × 2 �m area. Another poling wasperformed, with the probe biased at the opposite voltage (+12 V) during scanningover a 1 �m × 1 �m area inside the previously poled one. Details on the PFM tech-nique can be found in our previous work [22]. Infrared analysis was performed on(Bruker-Equinox 55, Germany) Fourier Transformed Infrared spectrometer (FT-IR),using a 30◦ specular reflectance accessory. FT-IR reflectance spectra of the filmswere recorded at room temperature in the 400–1200 cm−1 range. Top Au electrodes(0.5 mm diameter) were prepared for the electrical measurements by evaporationthrough a shadow mask at room temperature. The electrical properties were mea-sured at an Au/CBNO/Pt(1 0 0)/Ti/SiO2 capacitor structure. The fatigue properties ofthe capacitor were measured on a Radiant Technology RT6000 A tester equippedwith a micrometer probe station in a virtual ground mode. For the fatigue measure-ments, internally or externally generated 8.6 �s wide square pulses were appliedat ±9 V and 500 kHz pulse wave. The retention characteristics of the films weremeasured by independently observing the time-dependent changes of P* (switched

polarization), and Pˆ (nonswitched polarization). For P*, the capacitor was switchedwith a negative write pulse and read by a positive read pulse after retention time t.For P, positive pulses were used for both writing and reading. The pulse width for alltriangular pulses was 1.0 ms. The time delay between the right pulse and the firstread pulse is referred to as retention time.

896 A.Z. Simões et al. / Materials Chemistry and Physics 124 (2010) 894–899

F t diffeo lane (

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ig. 3. Topography and out-of-plane (OP) PFM images of CBNO films deposited aut-of-plane (OP) (static air), (c) topography (oxygen atmosphere), and (d) out-of-p

. Results and discussion

Fig. 1 shows the Raman spectra of CBNO thin films crystallizedt 700 ◦C for 2 h in ambient conditions and at oxygen flow. Ramanpectra in the CBNO thin films show the order–disorder degree ofhe atomic structure at short range. The modes further split intoongitudinal and transverse components due the long electrostaticorces associated with lattice ionicity. Vibrational modes of the filmnnealed in oxygen atmosphere tend to disappear when comparedo the film annealed in static air. This can be related to structuralisordering at short range, as well as a phase transition for an order-

ng crystal structure. The structural changes created by annealing in

xygen ambient create localized levels in the band gap and an inho-ogeneous charge distribution between valence and conduction

ands. In fact, a higher oxygen vacancies concentration is observedn films annealed in oxygen ambient than in films annealed in staticir. This occurs in materials with p-type conductivity increasing Bi

rent bottom electrodes at 700 ◦C for 2 h showing: (a) topography (static air), (b)OP) (oxygen atmosphere).

or Nb vacancies defects. For the films annealed in oxygen ambi-ent the oxygen vacancies are disordered, enabling oxygen ions tomigrate and therefore leading to structural changes, disorder andphase transition. Annealing in ambient conditions causes a shift ofthe active modes to higher frequencies, thereby indicating changesin the crystal structure. The vibrational modes located at 218, 280,295, 456, 595, and 826 cm−1 result from the NbO5 octahedral,while the features located at 68 cm−1 are caused by different posi-tions occupied by bismuth within the perovskite structure. SimilarRaman modes were observed for SrBi2Nb2O9 ceramics obtained atroom temperature [22]. On the other hand, Fuyura and Cuchiarohave observed strong changes in the Curie temperature for thin

films. This is possible because the electrical properties are stronglyinfluenced by factors such as rough film/electrode interfaces andinhomogeneity [23].

The inset of Fig. 1 illustrates the XRD pattern of CBNO thinfilms grown at Pt(1 0 0)/Ti/SiO2 substrates, and crystallized at

istry and Physics 124 (2010) 894–899 897

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A.Z. Simões et al. / Materials Chem

00 ◦C for 2 h in air and at oxygen flow. A high intensity ofhe (2 0 0)/(0 2 0) diffraction line compared to other lines wasbserved, although the other lines could not be distinguished fromach other. The characteristic orientation most probably resultsrom favorable matching of the atomic arrangements in CBNO1 0 0)/(0 1 0), and the underlying Pt planes. A single phase of aayer-structured perovskite with orthorhombic crystallographictructure was observed. The diffraction peaks are matched andndexed based on CBNO structure parameters [24]. The pre-erred orientation is considered to result from sufficient matchingetween the z-axis of the CBNO film, and the (1 0 0) plane of the Ptubstrate. The main peak of the film annealed at ambient oxygens very sharp, therefore suggesting the presence of large crystallinerains in the layer (see Fig. 1a). Ambient oxygen caused a signif-cant reduction in the peak intensities, which indicates reducedrystallinity.

The FT-IR reflection spectra of the CBNO thin films annealed at00 ◦C for 2 h is shown in Fig. 2. The lattice vibration of the filmsas analyzed. A broad reflection peak of the BO6 stretching modeas observed at 600–840 cm−1 suggesting the formation of per-

vskite phase [25,26]. A displacement of reflection peaks at 786 and90 cm−1 is caused by annealing in different atmospheres resulting

n a better crystalline CBNO film. This occurs due the antiresonanceith the longitudinal optic (LO) phonon modes which become

harper and narrower and shift toward higher wavenumbers. Thiss considered to be a network stiffening and a structural rearrange-

ent, which leads to the perovskite phase formation, increasinghe crystallinity of the CBNO thin films. In complete contrast toaman data, annealing in oxygen flow shows slightly higher min-

mum, which can be interpreted by a partial amorphous structuren this film.

Fig. 3a–d illustrates the piezoelectric response of CBNO thinlms crystallized at 700 ◦C for 2 h in air, and at oxygen flow. Here,he negative pole of the 12 V power supply was applied to theubstrate bottom electrode in order to analyze the piezoelectricctivity. Two regions with different contrasts can be distinguished;he light contrast corresponds to the as-grown film, the dark con-rast is a consequence from the application of the dc voltage, whichesults in a polarization vector along the electric field pointing per-endicularly into the film plane. In the out-of-plane image (Fig. 3bnd d) the color contrast indicates domains oriented in directiono the bottom electrode and the AFM tip, respectively. Usually, theark region indicates that the polarization vector is oriented in theip direction. Generally, inside the darker quadratic regions thatxhibit a small piezoelectric response, always unpolarized crys-allites (light regions) are evident. This is a common observationor bismuth-layered ferroelectrics, which exhibit a great polar-zation in a-axis direction, besides a smaller polarization alonghe c-axis [2]. This behavior reflects the different orientation ofhe grains, i.e., the light regions are c-axis oriented crystallites.ince the image does not change even for high voltages, it cane concluded that it is impossible to align their polarization vec-or, so the total polarization in field direction is always moderate.y changing the polarity of the applied voltage it can be easilyemonstrated that the light regions present a polarization vec-or perpendicular to the film surface, whereas in the dark regionshe polarization vector lies in the plane of the film. The piezo-lectric response is reduced by annealing in oxygen atmosphere,s a consequence of strain energy and pinning effects of chargedefects due to the charge transfer to the oxygen. Consequently, weonclude that the oxygen atmosphere is changing the crystal struc-

ure of CBNO, thereby affecting its ferroelectric and piezoelectricesponses.

Among the degradation processes in ferroelectric materials,olarization fatigue has received most attention, especially in thinlms deposited at platinum electrodes. The fatigue endurance of

Fig. 4. Fatigue resistance for CBNO thin films annealed at 700 ◦C for 2 h at (a) staticair and (b) oxygen atmosphere.

CBNO thin films was tested at 1 MHz as a function of switch-ing cycles by applying 8.6 �s wide bipolar pulses with maximumamplitudes of ±9 V (Fig. 4). P* is the switched polarization betweentwo opposite polarity pulses, and Pˆ is the nonswitched polariza-tion between the same two polarity pulses. P* − Pˆ denotes theswitchable polarization, which is an important variable for non-volatile memory applications. Fatigue resistance was observedup to 1 × 108 cycles suggesting that CBNO annealed at ambientatmosphere presents improved polarization ferroelectric fatigueproperty. On the other hand, annealing in oxygen atmospherereduces the fatigue resistance due to the generation of bismuthvacancies (V′′′

Bi) that inhibit the movement of domain walls along thez-axis, and due to inducing oxygen vacancies (V0

••) that inhibit themovement of domain walls for Nb and Ca atoms. As a consequence,the space charge defects can interact with domain boundaries influ-encing the domain switching characteristics. As shown in Fig. 4a,the remanent polarization value of the CBNO thin film annealed inambient atmosphere decreases to 12% of its initial value at 1010

switching cycles, while the Pr value of the CBNO thin annealed inoxygen atmosphere decreases 20% at 1010 switching cycles. Theloss of switching polarization with repeated polarization reversalis due to pinning of the domain wall, which inhibits switching ofthe domain affected by induced internal space charge at domain

boundaries. However, a variety of mechanisms for domain wallpinning have been proposed, including pinning due to electroncharges trapped by oxygen vacancies. However, it is difficult toaccess the relative contributions of the above-mentioned differentmechanisms, and further studies are needed for developing a more

898 A.Z. Simões et al. / Materials Chemistry

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ig. 5. P* and Pˆ of the CBNO thin film for retention times up to 1 × 104 s and anpplied electric field of 150 kV cm−1 as a function of different atmospheres: (a) staticir and (b) oxygen atmosphere.

undamental understanding on the fatigue process in ferroelectricalcium bismuth niobate thin films.

The long-time retention characteristics (P* and Pˆ) of thebtained CBNO films are shown in Fig. 5a and b for films annealedn static air and at oxygen atmosphere, respectively. The overalletention time dependence of the polarization retention for CBNOlms appears within the expected range useful for practical appli-ations. After a retention time of 1 × 104 s, the polarization loss wasnly approximately 7% of the value measured at t = 1.0 s for a voltagef 9 V on films annealed in static air. However, this value increasedo 16% for the film annealed in oxygen atmosphere. Depolariza-ion fields generated by the redistribution of space charge, defects,nd dipole charges could be the mechanisms for the polarizationecay after writing. For the initial period (within 10 s), depolariza-ion fields could be the main contribution to polarization losses. Theong-time retention loss is attributed to effects resulting from theedistribution of defect charges. Considering that CBNO thin filmsre characterized by strong polarization decay in oxygen atmo-phere, it can be assumed that the bismuth and niobium vacanciesan be redistributed and driven by polarization causing domainalls pinning. Also, the redistribution of defect charges driven by

olarization may result in imprint failure and a decrease in polar-

zation by compensating the polarization charges. In this way, theolarization compensation is probably located at the oxygen site,hich would compensate the valence deviation introduced by sub-

tituting Bi3+ by Ca2+. The better retention property of the CBNO

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and Physics 124 (2010) 894–899

thin film annealed at ambient atmosphere is a consequence of lesshybridization with O 2p resulting in a less structural distortion.Hence, fundamental understanding and improvement of the degra-dation behavior of ferroelectric thin films will have an essentialimpact on the future success of these films for device applications.Work is now in progress to understand the effect caused by usingmetal oxide electrodes which provides a source of oxygen ionswhich should result in a reduced built-in field at the interface andenhanced retention.

4. Conclusions

Polycrystalline, homogenous, single phase CBNO thin films wereprepared by the soft chemical method. Raman analyses revealthat the modes for the film annealed in oxygen atmosphere tendto disappear indicating structural disordering as well as a phasetransition. Oxygen atmosphere during crystallization of such filmsaffected the structural perfection along the z-axis, as well as the fer-roelectric and piezoelectric properties resulting in piezoresponseimages that are strongly affected by the annealing atmosphere.Higher fatigue resistance was noted in the CBNO film annealedat ambient atmosphere. On the other hand, annealing in oxygenatmosphere reduces the fatigue resistance due to the generationof bismuth vacancies (V′′′

Bi) that inhibit the movement of domainwalls along the z-axis, and due to inducing oxygen vacancies(V0

••) that inhibit the movement of domain walls for Nb andCa atoms. Retention failure indicate that CBNO films have ratherexcellent long-time retention characteristics retaining 90% of thevalues measured at t = 1 s in films annealed in ambient condi-tions, and 80% in films annealed in oxygen atmosphere. The resultsof these studies are very promising and suggest that CBNO thinfilms annealed in static air are attractive for use as storage ele-ment in nonvolatile ferroelectric random access memories whencompared to other bismuth-layer-structured ferroelectrics (BLSF)compounds.

Acknowledgments

The authors gratefully acknowledge the financial support of theBrazilian financing agencies FAPESP, CNPq and CAPES.

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