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Applied Surface Science 257 (2011) 9600– 9605

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Applied Surface Science

jou rn al h om epa g e: www.elsev ier .com/ locate /apsusc

Lead-free ferroelectric BaTiO3 doped-(Na0.5Bi0.5)TiO3 thin films processed bypulsed laser deposition technique

Cristina Dragoia,b, Marin Cerneaa,∗, Lucian Trupinaa

a National Institute of Materials Physics, Bucharest-Magurele, 077125, Romaniab University of Bucharest, Faculty of Physics, Bucharest-Magurele, 077125, Romania

a r t i c l e i n f o

Article history:Received 20 May 2011Received in revised form 9 June 2011Accepted 13 June 2011Available online 21 June 2011

Keywords:Lead-free ferroelectricsThin filmsPulsed laser depositionDielectric and piezoelectric properties

a b s t r a c t

The difficulties in synthesizing phase pure BaTiO3 doped-(Na0.5Bi0.5)TiO3 are known. In this work,we reporting the optimized pulsed laser deposition (PLD) conditions for obtaining pure phase0.92(Na0.5Bi0.5)TiO3–0.08BaTiO3, (BNT–BT0.08), thin films. Dielectric, ferroelectric and piezoelectric prop-erties of BNT–BT0.08, thin films deposited by PLD on Pt/TiO2/SiO2/Si substrates are investigated in thispaper. Perovskite structure of BNT–BT0.08 thin films with random orientation of nanocrystallites hasbeen obtained by deposition at 600 ◦C. The relative dielectric constant and loss tangent at 100 kHz, ofBNT–BT0.08 thin film with 530 nm thickness, were 820 and 0.13, respectively. Ferroelectric hysteresismeasurements indicated a remnant polarization value of 22 �C/cm2 and a coercive field of 120 kV/cm.The piezoresponse force microscopy (PFM) data showed that most of the grains seem to be constituted ofsingle ferroelectric domain. The as-deposited BNT–BT0.08 thin film is ferroelectric at the nanoscale leveland piezoelectric.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The Pb(Zr,Ti)O3 (PZT) perovskite is the most used ferroelec-tric material in electronic devices (sensors, actuators, memories,etc.) [1]. Lead is considered as a source of pollution due to itsvolatilization at the conventional processing temperatures of thelead-containing ceramic materials. The processing and proper-ties of ferroelectric lead-free thin films have a high interest forthe present microelectronic industry. Sodium bismuth titanate(Bi0.5Na0.5TiO3) (BNT) and its solid solutions are considered a goodalternative lead-free material to the PZT perovskite ceramics [2].BNT has a perovskite structure and shows strong ferroelectric-ity, a relatively large remnant polarization (Pr = 38 �C/cm2), andhigh Curie temperature (Tc = 320 ◦C) [3]. However, their electrome-chanical properties are much lower than those of PZT ceramics. Ithas been reported that BNT ceramics modified with BaTiO3 (BT)[2] or Bi0.5K0.5TiO3 [4] showed improved dielectric and piezoelec-tric properties. The solid solutions (1 − x)BNT–xBT, (abbreviatedas BNT–BTx) of rhombohedral BNT with tetragonal BaTiO3 (BT)have an area of coexistence of the two phases (rhombohedraland tetragonal), named morphotropic phase boundary (MPB) forx = 0.06–0.10 where bulk ceramics show enhanced dielectric prop-

∗ Corresponding author. Tel.: +40 21 369 01 70x130; fax: +40 21 369 01 77.E-mail addresses: mcernea@infim.ro, marincernea@yahoo.com (M. Cernea).

erties, ferroelectric, piezoelectric, and pyroelectric activities [5–7].There is very scarce literature about BNT–BTx thin films [8–11].These BNT–BTx thin films have been deposited by pulsed laserdeposition onto LaAlO3 and MgO single crystal substrates. Aslightly higher number of publications have reported on pureBNT films prepared by different deposition techniques (RF mag-netron sputtering [12], PLD [13] or chemical solution deposition[14–23]).

The aim of this work was the optimization of the PLDdeposition parameters to prepare BNT–BT0.08 thin films andtheir characterization. We investigated the microstructure andthe phase composition of the crystalline films and, the dielec-tric, ferroelectric and piezoelectric properties of as-preparedBNT–BT0.08 thin films. In this paper, we report also on thepure BNT–BT0.08 thin film domain–structure images obtained byPFM.

2. Experimental procedure

The target of BNT–BT0.08 home-made was elaborated in twosteps process. First, BNT–BT0.08 nanopowder was prepared bysol–gel method [24]. This powder was cold isostatic pressed andsintered at 1150 ◦C, 1 h in air. The BNT–BT0.08 thin films weredeposited on Pt-coated silicon (Pt/Ti/SiO2/Si) substrates by pulsedlaser deposition using a KrF excimer laser (Lambda Physik COMPex

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205) with a wavelength of 248 nm, and a repetition rate of 10 Hz.The laser beam impacts the rotating target with an energy densityof 1 J/cm2. The distance between the target and the substrate wasfixed at 5 cm, while the substrate temperature was maintained at600 ◦C. The BNT–BT0.08 films of thickness ∼600 nm were preparedby a number of 15,000 pulses in an oxygen atmosphere of 0.2 mbar.After deposition, the as-grown thin films were gradually cooleddown to room temperature.

The crystallographic structure of the film was analyzed by X-raydiffraction at grazing incidence at 2◦ using a Bruker D8 Advancediffractometer. The microstructure of the samples was investi-gated using a FEI Quanta Inspect F scanning electron microscope(SEM). For electrical measurements Pt electrodes was depositedon top. Deposition of metallic contacts was made by sputtering,using a metal mask with dimensions of 0.2 mm2. The ferroelec-tric hysteresis (P–E) loop was measured using a TF Analyzer 2000equipped with a FE-Module (aixACCT). Dielectric properties ofPt/BNT–BT0.08/Pt capacitor were measured at 100 kHz frequency,at room temperature, using a HIOKI LCR-meter. The leakage cur-rent through the Pt/BNT–BT0.08/Pt capacitors was measured atroom temperature using a Keithley 6517A programmable elec-trometer. An atomic force microscope (AFM) (MFP 3D SA, AsylumResearch) was employed to obtain high resolution images of thesurface. Olympus AC240-TM cantilevers (l = 240 �m, resonant fre-quency ∼70 kHz, spring constant = 2 N/m, Pt coated) were usedfor simultaneous acquisition of topographic views and domainimagining using a MFP-3D Piezo Force Module. DART (Dual ACResonance Tracking) measurement technique was used for studyof the local electromechanical activity at nanoscale size. Thesmall displacement of the thin film induced by the conversepiezoelectric effect was measured applying a sequence of dcbias up to 13.2 V superimposed on a AC modulation bias viathe PFM tip directly on the film surface without the top elec-trodes.

3. Results and discussion

3.1. Structure

The XRD pattern of BNT–BT0.08 thin film is given in Fig. 1.BNT–BT0.08 thin film is well crystallized and shows a polycrystallinestructure without any preferred orientation. The film exhibits adominant ABO3 perovskite phase (rhombohedral Bi0.5Na0.5TiO3),similar results were reported also by Sanjose et al. [20]. The filmstructure seems to be not textured, the relative intensities of the

Fig. 1. XRD patterns (Cu K� radiation) of BNT–BT0.08 thin film deposited on Pt-coated silicon.

Fig. 2. SEM micrographs of the BNT–BT0.08 thin film: (a) plan-view image and (b)cross-section image.

peaks agree well with those given in ICDD-PDF No 70-9850. TheX-ray penetration depth (90% absorption) in BNT–BT0.08 thin filmfor an incidence angle of 2◦ and 40◦ scattering angle (2�) is around0.7 �m (calculated with the BRUKER–Absorb DX v.1.1.4 software).Thus the beam reaches the substrate but the Pt peaks do not occursbecause of the high degree of texturing of Pt(111). Crystallite sizeand microstrain were derived from the line broadening using theBRUKER–TOPAS software. It was obtained a mean crystallite sizeof 44 nm and a microstrain value of 0.031. In order to correct forthe instrumental line breadth, a heat-treated ceria powder provedto not produce measurable size and strain broadening, has beenused.

3.2. Microstructure

Fig. 2 shows the surface morphology and cross-section micro-graphs of the BNT–BT0.08 thin film.

It can be seen that the film possesses homogeneous microstruc-ture with fine grains of ∼90 nm average size and uniform grainsize distribution (Fig. 2(a)). Cluster aggregation has not beenobserved. The grains are more or less polyhedral shape. The film

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Fig. 3. P–E hysteresis loop of a BNT–BT0.08 thin film under various electrical fields.

deposited at 600 ◦C exhibits a dense structure, with few poresbeing observed at the grain boundaries and junctions. Fig. 2(b)shows a cross-sectional SEM image of a typical columnar thinfilm. The tilted columnar morphology of the columnar thin filmis clearly evident in the image, with the cross-sectional diame-ter of the columns roughly 90 nm. The BNT–BT0.08 film is uniformin thickness (Fig. 2(b)) and its thickness is about 530 nm. Simi-lar value of thickness was measured by ellipsometry (not shownhere).

3.3. Dielectric and piezoelectric characterization

Fig. 3 shows the P–E loops of the BNT–BT0.08 thin film measuredunder different electric fields at a frequency of 10 kHz.

Well-defined P–E hysteresis loops are observed and the satura-tion of the hysteresis loop as a function of voltage is key evidence ofintrinsic ferroelectricity in the BNT–BT0.08 thin film. The observedremnant polarization 2Pr and coercive field Ec, are 22 �C/cm2

and 120 kV/cm, respectively. These values are comparable to thatreported for BNT–BT0.06 thin films deposited by sol–gel [25,26].Fig. 4 demonstrates the small-signal (100 kHz) dielectric constantand loss tan ı dependence on dc bias field in the film. Dielectricconstant exhibits a significant change with applied electric field(Fig. 4(a)). This indicates that ferroelectric domain structure, inaddition to ionic and electronic polarizations, contributes to thepolarizabilty of the thin film. The dielectric constant and loss vs.electric field plots appear symmetric with a hysteresis behaviortypical of ferroelectric capacitors. The maximum dielectric constantoccurs at a field of 20 kV/cm, in both directions. The dielectric con-stant and loss were measured as function of frequency at roomtemperature, as shown in Fig. 4(b). The typical values of dielectricconstant and loss of the BNT–BT0.08 thin film, measured at 100 kHz,are 820 and 0.13, respectively.

Fig. 5 shows the current density (J) as a function of electric fieldfor the BNT–BT0.08 thin film measured at room temperature.

Current density has a value of about 1.5 × 10−9 A/cm2 at lowelectric field but present an exponential increase with electricfield. The value of the current density is 4.7 × 10−5 A/cm2, at anelectric field of 100 kV/cm. This value is comparable to thosereported for BNT thin film deposited by RF-sputtering and bysol–gel (6 × 10−5 A/cm2 [12], respectively, 1.6 × 10−5 A/cm2 [22]at 100 kV/cm). The localized oxygen vacancies trapped at grainsboundaries can pin domains and result in leakage current and polar-ization degradation, common phenomenon for the ferroelectricthin films.

Fig. 4. Electrical field dependence of the dielectric constant and loss, at room tem-perature (a) and frequency dependences of dielectric constant and loss (b).

Vertical piezoresponse force microscopy (VPFM) employs theconverse piezoelectric effect to measure small surface displace-ments resulting from the application of an external AC field atthe contact between a conducting tip and the sample surface. Thetip follows the expansion and contraction of the surface allow-ing the voltage-dependent piezoelectric response to be mappedsimultaneously with topography using a lock-in technique. A morecomprehensive description is provided elsewhere [23]. Fig. 6(a)and (b) are topography and amplitude VPFM images, respec-

Fig. 5. Current density as a function of electric field for the Pt/BNT–BT0.08/Pt thinfilm capacitor at room temperature.

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Fig. 6. Large-scale maps of piezoresponse (a) topography and (b) amplitude without external biasing of BNT–BT0.08 thin film exhibiting spontaneous polarization.

tively. The topographic image of the sample shows a grainedmicrostructure with average roughness Rms = 20.4 nm and, aver-age grain size of approximately 90 nm (Fig. 6(a)). The feature sizesin topography and VPFM images are comparable and, the bound-aries of uniformly polarized regions coincide with topographicfeatures.

Regions of bright contrast can be seen in the amplitudeVPFM image [indicated with a black arrow in Fig. 6(b)], show-ing high piezoelectric response. These regions are outlined bynarrow unpolarized regions which are intergrain boundariesseparating two grains. Since the vertical PFM technique is sen-sitive only to the component of polarization normal to thefilm surface, grains with in-plane polarization (with vanishingout-of-plane polarization) exhibit an intermediate contrast (darkcontrast in the amplitude PFM image). One such region is indi-

cated with a white arrow in Fig. 6(b). It can also be observedthat a large proportion of grains have a multidomain struc-ture.

For switching spectroscopy PFM (SS-PFM) measurement, anadditional function generator applies the switching bias to thescanning tip to obtain piezoelectric switching loops. The switch-ing bias is a series of voltage pulses that are combined withthe small AC piezoresponse bias. The pulse polarizes the localarea when it is on, and then the piezoresponses from the localarea are recorded when the dc pulse is off or on. The resultedpiezoresponse when the dc pulse is off corresponds to the piezo-electricity of the BNT–BT0.08 microstructure at remnant state afterpolarization. The amplitude of the AC bias was 4 V, and themaximum dc pulse for to the switching loop measurement was20 V.

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Fig. 7. Remnant piezoresponse (a) amplitude and (b) phase measured as a functionof applied dc voltage.

Fig. 7(a) and (b) shows typical remnant piezoresponse ampli-tude and phase measurements, respectively, measured as afunction of applied dc voltage.

The polarization switching and hysteresis show clearly that asdeposited the BNT–BT0.08 thin film is ferroelectric at the nanoscalelevel. The average coercive voltage for the well-behaved loopsis approximately 9 V. From this, the determined coercive fieldis ∼150 kV/cm. These results demonstrate the existence of localpiezoelectric and ferroelectric responses, in BNT–BT0.08 thin filmson Pt/TiO2/SiO2/Si substrates synthesized by pulsed laser deposi-tion at 600 ◦C.

4. Conclusions

In summary, this study investigated the dielectric and piezoelec-tric properties of MPB lead free BNT–BT0.08 thin films, grown onPt/Ti/SiO2/Si substrate, in optimized pulsed laser deposition con-ditions. The results show that the BNT–BT0.08 thin film exhibitsa well-defined ferroelectric hysteresis loop at room temperaturewith a remnant polarization 2Pr of 22 �C/cm2 and a coercive fieldEc of 120 kV/cm. The leakage current density in BNT–BT0.08 thinfilm is relatively low. We have demonstrated also the existence oflocal piezoelectric and ferroelectric responses, in BNT–BT0.08 thinfilms on Pt/TiO2/SiO2/Si substrates obtained by pulsed laser depo-sition at 600 ◦C. The results indicate that the BNT–BT0.08 thin film

prepared by PLD is a good candidate for lead-free piezoelectricapplications.

Acknowledgement

This work was supported by Romanian National ProgramPNCDI II, Contract no. 72-153/2008 and Contract No. POS-DRU/6/1.5/S/24.

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