nanoscale switching characteristics of nearly tetragonal...

Nanoscale Switching Characteristics of NearlyTetragonal BiFeO3 Thin FilmsDipanjan Mazumdar,*,† Vilas Shelke,† Milko Iliev,‡ Stephen Jesse,§ Amit Kumar,§Sergei V. Kalinin,§ Arthur P. Baddorf,§ and Arunava Gupta†

†Center for Materials for Information Technology, University of Alabama, Tuscaloosa, Alabama 35487, ‡TexasCenter for Superconductivity, University of Houston, Houston, Texas 77204-5002, and §Center for NanophaseMaterial Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831

ABSTRACT We have investigated the nanoscale switching properties of strain-engineered BiFeO3 thin films deposited on LaAlO3

substrates using a combination of scanning probe techniques. Polarized Raman spectral analysis indicates that the nearly tetragonalfilms have monoclinic (Cc) rather than P4mm tetragonal symmetry. Through local switching-spectroscopy measurements andpiezoresponse force microscopy, we provide clear evidence of ferroelectric switching of the tetragonal phase, but the polarizationdirection, and therefore its switching, deviates strongly from the expected (001) tetragonal axis. We also demonstrate a large andreversible, electrically driven structural phase transition from the tetragonal to the rhombohedral polymorph in this material, whichis promising for a plethora of applications.

KEYWORDS Ferroelectricity, multiferroics, piezoresponse force microscopy, morphotropic phase transition, spintronics,ferroelectric tunnel junction

Since the 2003 report of high polarization in BiFeO3

(BFO) thin films, research activity in multiferroicmaterials have generated interest across many scien-

tific disciplines.1,2 As a single phase material, BFO has showna wealth of properties, which includes simultaneous ferro-electric and magnetic order, and photovoltaic effect.3 Ad-ditional functionalities are expected when used in hetero-structures exploiting the magneto-electric (ME) coupling oras a barrier material in a tunnel junction configuration withor without magnetic electrodes.4-11

From a purely materials point-of-view, BFO possesssignificant challenges. In its bulk form, BFO assumes a low-symmetry rhombohedral (R3c symmetry) structure withpolarization along the (111) direction, implying that the (001)films have both an in-plane and out-of-plane polarizationcomponents and a total of eight possible domains arepossible in the film.12 As a consequence, switching is oftenincomplete, as evidenced through incomplete hysteresisloops. To reduce the number of possible switching variants,one route has been through the use of high-miscut vicinalsubstrates.13-15 This has also been seen to reduce thecoercive field, a critical parameter when it comes to deviceengineering.15

But more promising avenues of BFO domain engineeringhave opened up very recently after a few groups reportedsuccessful fabrication of the tetragonal (or nearly tetragonalto be precise) variant of BFO.16-18 The remarkable structuralcharacteristic of this phase is its extremely large c/a ratio of

around 1.25 achieved through the substrate strain effect (orstrain engineering) on LaAlO3 (LAO) and YAlO3 (YAO)substrates.16,17 The high in-plane compressive strain (BFOhas a lattice mismatch of -4.5 and -6.8% on LAO and YAOsubstrates, respectively) results in a large expansion of theout-of-plane lattice parameter, much higher than expectedbased on normal elastic energy considerations of rhombo-hedral bulk BFO. Therefore, this nearly tetragonal poly-morph has been speculated to be a distinctly different phaseand not a strained variant of the rhombohedral phase.17

Epitaxial stabilization of the tetragonal phase is of significantinterest as it has been theoretically predicted to have a highpolarization value of over 150 µC/cm2, which is about 50%higher than its rhombohedral counterpart.18-20 Additionally,the switching process can, in principle, be simpler and moreefficient as compared to the rhombohedral phase, makingit further desirable.

Films grown on LAO susbtrates are in particular interestingbecause they show a mixed tetragonal (T) and rhombohedral(R) character with the relative fraction dependent on the filmthickness.16 As the strain in the film is gradually released therhombohedral phase content increases, due to its overwhelm-ing lower ground state energy as compared to the tetragonalphase.16,19 Such a mixed phase is well-known in other ferro-electric systems. Common piezoelectric materials, such as PZT(PbZrO3-PbTiO3) and PMN-PT (PbMg2/3Nb1/3O3-PbTiO3), dis-play a structural transition in their phase diagram as a functionof the composition.21-23 Such a boundary, termed as themorphotropic phase boundary (MPB), separates regions oftetragonal and rhombohedral symmetry. Further, thesesystems show enhanced electromechanical properties dueto the existence of such MPBs. However, a single-phase lead-

* To whom correspondence should be addressed.Received for review: 04/5/2010Published on Web: 06/29/2010

pubs.acs.org/NanoLett

© 2010 American Chemical Society 2555 DOI: 10.1021/nl101187a | Nano Lett. 2010, 10, 2555–2561

free material with similar properties is more desirable frompresent day application point of view. Therefore, the pos-sibility of observing such a behavior is an enticing choice inthe mixed tetragonal-rhombohedral BFO grown on LAO.From now on we shall denote the nearly tetragonal phasewith “T” and the rhombohedral phase with “R”.

Epitaxial BFO thin films have been grown in the thicknessrange of 70-200 nm using the pulsed laser deposition (PLD)technique on (001)-oriented LAO substrates with and with-out bottom SrRuO3 (SRO) electrodes of 40 nm thickness.Details of the sample preparation technique have beenreported elsewhere.24 In Figure 1, we show the results of ourX-ray analysis. Detailed θ-2θ X-ray diffraction measurementshave been performed to ascertain the structural phases andout-of -plane lattice constant of chemically single-phase BFO.In Figure 1a, we plot the θ-2θ scans in 35-50 degree rangefor the 70, 100, and 200 nm samples deposited directly onLAO substrate and an 80 nm thick BFO film buffered with40 nm SrRuO3 (SRO). Very strong (00l) peaks (002) peakshown in Figure 1a are observed for films deposited directlyon LAO substrate corresponding to an out-of-plane latticeparameter value of 4.65 ( 0.02 Å. This c-parameter valueagrees very well with the recent reports of Zeches et al. andBèa et al.16,17 Also, the peak intensity is stronger for the 70and 100 nm film compared to 200 nm film, which isconsistent with decreasing T-phase content with thickness.The film with SRO bottom electrode, on the other hand,shows a diffraction peak close to the bulk rhombohedralpeak position of BFO (c ∼3.96 Å). Epitaxy analysis have beencarried on the nearly tetragonal BFO sample by performing

φ-scans on its (101) reflection as shown in Figure 1b. Eachpeak of the φ-scan shows three subfeatures pointing to theexistence of three variants of the tetragonal phase (inset ofFigure 1b). Asymmetric reciprocal-space maps (RSM) of the(103) and (013) tetragonal BFO reflection also show a three-variant structure, implying that the tetragonal structure isdistorted with monoclinic tilts (Figure 1c, RSM of (013)reflection is now shown). Such distorted variants have beenpredicted to be lower in energy compared to the pure,undistorted tetragonal structure.17,25 From the peak positionof the strongest spot, we calculate an in-plane lattice con-stant of 3.73 and 3.75 Å for the 70 and 100 nm films,respectively, quite close to the pseudocubic lattice parameterof the LAO substrate (3.79 Å). Therefore, we find oursamples to be highly strained with a high c/a value of ∼1.25(see Figure 1d), in excellent agreement with existing experi-mental and theoretical values.16-20 However, the presenceof in-plane oriented domains cannot be ruled out as thestrong LAO substrate peak very nearly coincides with thein-plane tetragonal lattice constants. Also, conclusive signa-ture for the rhombohedral structure was not picked up inthese samples from X-ray analysis. But all BFO films grownwith buffered SRO bottom electrodes are almost completelyrelaxed with a c-value of 3.97 ( 0.01 Å, and insensitive tothe film thickness studied here (80-600 nm). The presenceof SRO buffer layer favors faster strain relaxation throughformation of misfit dislocations.26

Even more informative with respect to the type of struc-ture are the polarized Raman spectroscopy data. Figure 2shows the polarized Raman spectra of the films of Figure

FIGURE 1. (a) θ-2θ scans in the 35-50° range for 70, 100, 200 nm BFO films deposited directly on LAO substrates and an 80 nm film on 40nm SRO buffer layer. (b) Complete φ-scan of the (101) reflection of the 70 nm BFO film on LAO substrate. Inset shows the subfeatures of oneof the φ peaks showing a three-variant structure. (c) Reciprocal space map (RSM) for the 70 nm BFO film on LAO substrate near the (103)reflection of the tetragonal phase. (d) Schematic diagram of the tetragonal and pseudocubic rhombohedral unit cell scaled according to thetetragonality c/a ratio.

© 2010 American Chemical Society 2556 DOI: 10.1021/nl101187a | Nano Lett. 2010, 10, 2555-–2561

1a obtained under a microscope (probe spot 1-2 µmdiameter) with xx, xy, x′x′, and x′y′ scattering configurationsusing 488 nm excitation. The first and second letters in thesenotations denote, respectively, the polarization of incidentand scattered radiation along x|[100]c, y|[010]c, x′|[110]c,or y′|[-110]c quasicubic directions of the LAO substrate. TheLAO contribution to the original spectra has been subtractedusing the Spectral Subtract (GRAMS AI) software.

As is clear from Figure 2, the tetragonal-like (70, 100, and200 nm BFO/LAO) and rhombohedral (80 nm BFO/40 nmSRO/LAO) films exhibit significantly different spectra. Thefact that the spectra in the xx-x′x′ and xy-xy pair aredifferent provides clear evidence that all films are epitaxial.On the other hand, the identity of the xx-yy and x′x′-y′y′spectra for the tetragonal phase is consistent with (001)t

tetragonal plane being parallel to the surface. A careful studyof numerous points on the 70-200 nm BFO/LAO films showthat tetragonal-like and rhombohedral domains of microme-ter size coexist on the film surface with the rhombohedralphase gradually becoming the dominant phase with increas-ing thickness. As to the spectra of the rhombohedral phase,they are identical to those previously reported27,28 and theirpolarization properties can be explained accounting for the

coexistence in the scattering volume of twin variants withfour different orientation of the rhombohedral axes. Ramanstudy of the tetragonal-like phase has not been reportedbefore and the observation of more than one mode of B1-type symmetry indicates a more complicated structure. Oncareful analysis based on symmetry and the number ofRaman peaks, we find strong evidence that the nearlytetragonal structure has monoclinic Cc symmetry instead ofsimple P4mm tetragonal symmetry. A detail analysis of theRaman spectra of tetragonal BFO is provided elewhere.29

We will only mention here that for the currently acceptedtetragonal P4mm BFO structure one expects in total eight(3A1 + B1 + 4E) Raman modes. Only A1 and B1 modes canbe observed from the (001) surface. The A1 modes areallowed in all parallel and forbidden in all crossed polariza-tions, and the B1 mode is allowed with parallel polarizationsalong [100]t and [010]t and crossed [110]t[-110]t polariza-tion configuration but forbidden with parallel [110]t [110]t

(or [-110]t[-100]t) and crossed [110]t[-110]t configura-tions. The only configuration where neither A1 nor B1 modesare allowed is [100]t[010]t. Unlike x′y′, no strong spectralstructure are observed in the xy spectra of the tetragonalphase, which strongly suggest that the tetragonal [100]t and[010]t directions are parallel to the quasicubic [100]c and[010]c directions of the LAO substrate.

Ferroelectric (FE) domains in our samples were investi-gated using the piezoresponse force microscopy technique(PFM) performed on a commercial atomic force microscope(Asylum Research, Cypher Model). Nanoscale FE switchingproperties were investigated using the newly developedswitching spectroscopy PFM (SS-PFM) technique30,31 asimplemented in the Cypher AFM. Vertical PFM measure-ments were performed near the contact resonance of thetip-sample configuration in order to obtain high signal-to-noise ratio through resonance-enhancement. In addition,both vertical (out-of-plane) and lateral (in-plane) PFM imageswere performed far from resonance. Pt-coated conductivetips (Olympus AC240TM) were used with the AC-bias driveamplitude maintained typically between 1-2 V. SS-PFMmeasurements were performed in a bandwidth of 50 kHzaround contact resonance.

In Figure 3a, we show the topography and near-reso-nance vertical PFM amplitude and phase data of the 80 nmBFO film on buffered SRO. Mosaic domain patterns (Figure3a(ii),(iii)) are observed with little correlation with topogra-phy and verified by scanning different areas of the sample.Vertical and lateral PFM images taken at frequencies far fromresonance (see Supporting Information Figure S1) furtherconfirm the irregular domain configuration. Interestingly, thelateral PFM data shows a higher contrast than the verticaldata, implying that the domains can have strong in-planeorientation. These observed patterns are reminiscent ofrhombohedral BFO films deposited on SRO-buffered STOsubstrates,32,33 and consistent with our Raman spectra andX-ray diffraction analysis. However, this is unlike the results

FIGURE 2. Polarized Raman spectra of (a) 70 nm BFO/LAO, (b) 100nm BFO/LAO, (c) 200 nm BFO/LAO, and (d) 80 nm BFO/40 nm SRO/LAO thin films. x, y, x′, and y′ are parallel, respectively, to the [100]c,[010]c, [110]c, and [-110]c quasi-cubic directions of the LAO substrate.


obtained with La1-xSrxMnO3 (LSMO) or La1-xSrxCoO3 (LSCO)buffer layer that have been shown to preserve the substratestrain16,17 and tetragonality.

Topography and ferroelectric domains (near-resonancevertical PFM) of the 100 and 200 nm tetragonal-BFO filmsare shown in Figure 3b,c. The surface morphology of BFOdeposited directly on LAO consists of two distinct features(Figure 3b(i),c(i)). The majority of the surface shows ex-tremely flat morphology interjected with one-dimensionalarrays of 250-350 nm long rod-shaped trenches alignedside by side and extending to well over micrometers inlength. These arrays in many instances run approximatelyperpendicular to each other but in general have no fixedorientation with respect to the substrate edges. In addition,uniformly dispersed nanoisland depressions are observed.These trenches have been identified to being consistent withthe rhombohedral phase16 and can be thought of as due totheir low c/a ratio compared to the nearly tetragonal phaseas depicted in the schematic Figure 1d. Interestingly, the flat,nearly tetragonal surface shows a step-and-terrace morphol-ogy in all the films investigated. A straight-line sectionalanalysis gives a step-height value of 4.5 ( 0.8 Å that is veryclose to the lattice constants obtained from X-ray diffractionand RSM analysis (Figure 1).

A weak but distinct PFM contrast is observed in thetetragonal areas with a striped-domain pattern oriented 45°to the substrate [100] and [010] directions, that is, along the[110] and [-110] direction (Figure 3b(ii),(iii), and c(ii),(iii))both in the 100 and 200 nm films. The stripes are ap-

proximately micrometers in length with a period of 70-80nm and form flux-closed states with 90° domain walls asindicated by the black arrows in Figure 3b(iii). The weakvertical contrast suggests that the domains might be pre-dominantly in-plane. For further analysis, we have per-formed detailed vertical and lateral PFM measurements farfrom contact resonance (∼30 kHz), as shown in the Sup-porting Information Figures S2-S3. The vertical responseshows uniform phase while the lateral response has strongphase contrast, strongly suggestive of in-plane orienteddomains in the tetragonal phase. The vertical signal isremarkably enhanced (both for near- and off-resonanceimages) at the interface of the tetragonal and rhombohedralphase. Even though crosstalk interference cannot be ruledout completely given the large changes in topography, theenhancement is consistent with higher electromechanicalresponse at the phase boundary. The inside of the rhombo-hedral trenches have weaker out-of-plane signal and strongerin-plane signal than the interface. Similar features are alsoobserved in the PFM date obtained from the 200 nm thickfilm. The observation of in-plane oriented domains is quitecontrary to the expectation from a nearly tetragonal unit cell.In the work of Bèa et al.,17 they have speculated thepolarization direction to be in the (110) plane. Recenttheoretical work of Hatt et al.25 show that the angle betweenthe polarization vector and the [001] direction to be ex-tremely sensitive to small changes in the in-plane latticeparameter, especially for strains values encountered with

FIGURE 3. (i) Topography, (ii) out-of-plane PFM amplitude, and (iii) PFM phase micrographs of (a) BFO(80 nm)/SRO(40 nm)/LAO; (b) BFO(100nm)/LAO; (c) BFO(200 nm)/LAO. SRO-buffered BFO show mosaic domain feature where as the tetragonal domains show predominantly in-plane, flux-closed stripe domains (see Supporting Information Figures S1-S3). PFM images are taken in the near-resonance mode.


LAO substrate, and that the polarization direction could becloser to (111) direction, similar to the rhombohedral phase.

We find that the nearly tetragonal BFO films show revers-ible, electric-field driven phase transition between the R andthe T phase. Large, hysteretic structural changes are ob-served when alternately a (20 V single voltage pulse of 1 sduration is applied at the topographic center through thescanning probe tip in the sequence shown in Figure 4a-d.R-phase-like trenches are formed on applying a sufficientlystrong negative voltage pulse, which transforms back to theT-phase-like topography upon reversing the polarity (Figure4a-d). On this particular film of 100 nm thickness, switchingis observed only at voltages pulses above 15 V and 250 mspulse width. The R-phase-like trench obtained in Figure 4b,dare measured to be 2.3 nm deep, which is higher than theaverage trench-depth measured in the as-deposited config-uration. Based on this observation, we speculate that theenergy barrier separating the T and R phases is small andaccessible by applying an electric field (Figure 4e right). Thisis unlike the bulk form, where the R-phase is overwhelminglythe favored ground state and can be thought of as having alarge energy barrier with the metastable bulk tetragonalphase, if it can be stabilized (Figure 4e left). Therefore,observing phase transitions between these states is practi-cally impossible. But such is not the case of highly strainedBFO films such as on LAO substrates

To investigate further, we performed additional topo-graphic and PFM investigations to up to (100 V. Theabsence of any bottom electrode allows us to apply largevoltages without creating dielectric breakdown (Figure 5).

Voltage pulses are applied to the topographic center in thefollowing sequence:-25,-50,-75,-100,+25,+50,+75,+100 V and PFM/AFM micrographs are taken after eachpulse. In Figure 5, we show images after the application of-25 (a), -75 (b), +25 (c), +75 V (d). The as-is configurationof this area is the same as shown in Figure 3b. Consistentwith the results of Figure 4, application of negative voltagespulses induces local structural transition in the form oftrenches. The density of such trenches increases steadilyupon increasing the voltage to -100 V (-75 V shown inFigure 5b). The PFM images are qualitatively identical to theas-deposited states, thereby confirming that the trenches areindeed the rhombohedral phase. The tetragonal backgroundretains the striped, flux-closed configuration and to a largeextent insensitive to the applied voltage. Reversing thevoltage polarity to +25 V immediately transforms thetrenches near the center of the image (at the point ofapplication of voltage) back to tetragonal-like morphology(Figure 5c) and this trend continues on increasing the voltageto +100 V. We have to mention that this field-induced phasetransition is also thickness dependent and for the 200 nmfilm the T-R/R-T transitions are observed only at very highvoltages ((50 V or above, data not shown).

We now discuss the local ferroelectric switching proper-ties of the nearly tetragonal domains. As the discussions ofthe preceding paragraphs demonstrate, any voltage depend-ent measurements on nearly tetragonal BFO samples areprone to structural phase transition between the tetragonaland the rhombohedral polymorphs. Therefore precautionsare in order to properly assess the local FE switchingproperties of tetragonal BFO, and correct inferences can onlybe made if the local area being probed remains tetragonalthroughout the voltage sweep. In the switching spectroscopytechnique employed to measure the local ferroelectric loops,a series of dc offset voltages are applied to the tip, which isramped up to a maximum voltage (Vmax). Measurementsperformed at different Vmax are repeated 25 times to improvethe signal-to-noise ratio and also to observe, if any, quasi-static time-dependent behavior. The applied dc voltagesweep is a triangular pattern starting from 0 to +Vmax to-Vmax to 0 with Vmax ) 5, 10, 15, 20, 25 V. Additionally, thetopography is imaged after every hysteresis measurementto monitor any T-R switching. It is important to note thatbefore completion of the amplitude cycle the voltage goesto -Vmax. Therefore, any T-R switching event should becaptured in the postmeasurement topographic image as arhombohedral trench. If not, we can conclude that the loopobtained is a measure of the pure nearly tetragonal phase.Complete switching is observed at (25 V as shown in Figure6a,b where we plot the averaged SS-PFM amplitude andphase loops for the 100 nm tetragonal BFO film. Thesymmetric butterfly loops of the amplitude scans and phasechange from -1.5 radians (-90°) to almost +1.5 radians(+90°) in Figure 6b confirm a near-complete polarizationswitching process in the tetragonal phase BFO. At tip volt-

FIGURE 4. (a) Topographic image of the 100 nm BFO film in the as-deposited condition. (b) Topography after the application of a -20V, 1 s pulse at the image center. (c) Topography after applying +20V in the same area and (d) after -20 V. The reversibility of thetransition is clearly demonstrated. The trenches resemble R-phasetrenches of the as-deposited state. (e) Energy landscape diagramshowing the difference in the energy between BFO-T and BFO-R bulkphases (left) and in the nearly tetragonal thin films (right).


ages below 25 V, the switching process are partial withasymmetric amplitude loops and lower than 180° phasechange. Quite noteworthy is the gradual phase change of theswitching process, again possibly due to the off-normalpolarization direction in the film. We also performed ad-ditional SS-PFM measurements on the 80 nm BFO-R samplewith bottom SRO electrodes, and the results are shown inFigure 6c,d. Extremely sharp polarization switching is ob-served in the phase loop with 180° phase change. Interest-ingly, the butterfly wings of the amplitude have an oppositesense compared to the BFO films deposited directly on LAO.The sharpness of the switching transition in BFO-R films,backed up by the PFM images, clearly indicate that thepolarization switching process is distinctly different between

rhombohedral and the nearly tetragonal BFO films. It alsoneeds to be emphasized that we have probed only the near-surface tetragonal domains. A more complicated switchinginvolving intermediate states is possible if the entire bulk ofthe film is involved, given the distorted nature of tetragonaldomains plus the mixed nature of the films. Phase-fieldsimulations, such as performed on tetragonal BTO,34 andfurther experimental work on this nearly tetragonal phaseare necessary to clarify this issue relating to the polarizationreversal mechanism and its absolute value.

In summary, we have investigated thoroughly and dem-onstrate unambiguously that the nearly tetragonal BiFeO3

films grown on LAO substrates have a number of interestingferroelectric and ferroelastic properties, true to its multifer-roicity. Polarized Raman spectra show that the nearly tet-ragonal films have monoclinic (Cc) symmetry and not purelytetragonal (P4mm), and the rhombohedral phase becomesthe dominant phase with increasing thickness. Through localswitching-spectroscopy PFM measurements, we show com-plete ferroelectric switching in tetragonal BFO, while verticaland lateral PFM measurements show that the ferroelectricdomains of nearly tetragonal BFO have a strong in-planeorientation, forming a striped, flux-closed configuration.Strong ferroelastic properties are demonstrated throughelectric-field driven phase transition between the tetragonaland rhombohedral polymorph.

Acknowledgment. The work at the University of Alabamawas supported by ONR (Grant N00014-09-0119). A portionof this research was conducted at the Center for NanophaseMaterials Sciences, which is sponsored at Oak Ridge NationalLaboratory by the Division of Scientific User Facilities, U.S.Department of Energy. The work of M.I. was supported by

FIGURE 5. Topography and near-resonance out-of-plane PFM amplitude and PFM phase data of a 1 × 1 µm2 area of the 100 nm BFO sampleafter the application of (a) -25, (b) -75, (c) +25, and (d) +75 V at the center. Thick black lines in panel b(ii) are a guide to the striped,flux-closed configuration of the tetragonal BFO film.

FIGURE 6. Local switching-spectroscopy PFM amplitude and phasedata of 100 nm nearly tetragonal BFO film (a,b) and SRO-bufferedrhombohedral BFO films (c,d). Symmetric butterfly loops and phasechange value of 180° imply complete polarization switching in bothfilms.


the State of Texas through the Texas Center for Supercon-ductivity at the University of Houston.

Supporting Information Available. Additional figures.This material is available free of charge via the Internet athttp://pubs.acs.org.

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nanoscale switching characteristics of nearly tetragonal...

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