PhysRevB.氧压对BFO影响80.024105

PHYSICALREVIEWB80,024105?2009?

In?uenceofoxygenpressureontheferroelectricpropertiesofepitaxialBiFeO3thin?lms

bypulsedlaserdeposition

SchoolofMaterialsScienceandEngineering,NanyangTechnologicalUniversity,Singapore639798,Singapore

InstituteofMaterialsResearchandEngineering,A*STAR(AgencyforScience,TechnologyandResearch),3ResearchLink,

Singapore117602,Singapore

?Received5April2009;revisedmanuscriptreceived15June2009;published7July2009?

ThegrowthwindowofmultiferroicBiFeO3thin?lmsisverysmall.Bothtemperatureandoxygenpressurewillaffectthe?lmqualityandphasepuritysigni?cantly.WedemonstrateherethatevenwithinthewindowwherephasepureBiFeO3thin?lmscanbeobtained,differentoxygenpartialpressuresstillleadtosubstantialvariationinBi/Feratiointhe?lm,whichcloselylinkwiththecorrespondingferroelectricproperties.Piezo-electricforcemicroscopyalsorevealssigni?cantdifferenceinthedomainstructuresofthese?lms.Adefect-dipolecomplexmodelisproposedtoexplainthedifferenceintheelectricalpropertiesanddomainstructuresfor?lmsgrownunderdifferentoxygenpressures.DOI:10.1103/PhysRevB.80.024105

PACSnumber?s?:77.84.?s,77.80.Dj

1

LuYou,1NgeahThengChua,1KuiYao,2LangChen,1andJunlingWang1,*

2

I.INTRODUCTION

MultiferroicBiFeO3?BFO?hasattractedconsiderableat-tentionsincethedayitsintrinsiclargepolarizationwascon-?rmedinthethin-?lmform.1Althoughthemagneticre-sponseofBFOthin?lmsiscontroversial,2,3todateBFOisstilltheonlyknownsingle-phasematerialwithbothrobustferroelectric?Curietemperature?820°C??Ref.4?andmag-netic?Neéltemperature?370°C??Ref.5?orderingscoex-istingatroomtemperature.Furthermore,thedirectcouplingbetweentheseorderparameters6,7makeselectricalcontrolofmagnetismpossible,whichpavesthewayforbridgingtheelectronicandspintronicindustries.8

BulkBFOhasahighlydistortedperovskitestructurewithrhombohedralsymmetry.9ThelargerelativedisplacementbetweenBiandOisbelievedtooriginatefromthehighstereochemicalactivityofBilonepair,10givingrisetoalargespontaneouspolarization?100?C/cm2alongthepseudocubic?111?pcdirection,11whichmakesitapotentiallead-freesubstitutefortraditionalleadzirconiumtitanate?PZT?.However,thegrowthwindowofphasepureBFOthin?lmisverysmall.Bi2O3andFe2O3arethetwotypicalimpurities.12,13Theformerusuallyresultsinlargeleakagecurrentofthe?lms,whilethelattercausesartifactsinmag-neticmeasurements.Recently,Beaetal.14exploredthein-?uenceofgrowthtemperatureandoxygenpressureonBFOthin?lms,thusmappingoutthepressure-temperaturephasediagramforBFOdeposition.Accordingtotheirresults,low-temperature/high-oxygen-pressureconditionresultsinexcessBiwhiletheoppositeconditionleadstoironoxideforma-tion.Consistentwiththeirresult,wefoundsimilartrendinBi/Feratiowithrespecttothegrowthpressureusingastoi-chiometrictarget.Furthermore,eventhoughsingle-phaseBFO?lmscouldbeobtainedwithincertainoxygenpressurerange,theirpropertiesvarydramaticallyasdepositionpres-surechangeswithintherange.

Inthispaper,theBi/FeratioofBFO?lmsgrownunderdifferentoxygenpressuresandthecorrespondingelectricalpropertieswereinvestigated.Itwasfoundthattheferroelec-tricpropertiesanddomainstructurewerecloselyrelatedto

1098-0121/2009/80?2?/024105?5?

theBi/Feratio.Adefect-dipolecomplexmodelwasproposedtoexplaintheobservedexperimentalresults.

II.EXPERIMENTALPROCEDURE

EpitaxialBFO?lmsweredepositedonSrRuO3coatedSrTiO3?100?csingle-crystalsubstratesbypulsedlaserdepo-sition.AstoichiometrictargetwasablatedbyusingaKrFexcimerlaser??=248nm?withanenergydensityof?1J/cm2andarepetitionrateof10Hz.Thegrowthtem-peraturewas?xedat700°C,whiletheoxygenpartialpres-surewasvariedbetween1and100mTorr.All?lmshadathicknessaround200nmdeterminedbyasteppro?ler.Thestructureandcrystallinequalityofthe?lmswerestudiedusingahigh-resolutionx-raydiffractometer?XRD??Pana-lyticalX-pertPro?.Chemicalanalyseswereperformedbyenergydispersivex-rayspectroscopy?EDS?equippedonaJSM-6360scanningelectronmicroscope.SquarePttopelec-trodesof40?mwerepreparedviaastandardphotolithog-raphyprocessforelectricalmeasurements.FerroelectricanddielectricpropertiesweremeasuredbyPrecisionLCferro-electrictester?RadiantTechnologies?andAgilentE4980APrecisionLCRmeter,respectively.Piezoresponseforcemi-croscopyandlocalpiezoelectrichysteresisloopmeasure-mentswerecarriedoutonanatomicforcemicroscope?Asy-lumResearchMFP-3D?usingPt/Ir-coatedtips.Magneticpropertiesofthethin?lmswerestudiedbyvibratingsamplemagnetometer?LakeShore7400?atroomtemperature.

III.RESULTSANDDISCUSSION

Figure1showstheXRDpatternsof?lmsgrownunderdifferentoxygenpressures.PhasepureBFO?lmscanbeobtainedwithinarelativebroadpressurerangeof1–20mTorrat700°C.Withinthisrange,noFe2O3phaseisob-servedwithinthedetectionlimit.However,onceoxygenpressuregoesabove20mTorr,extrapeaksthatcanbeattrib-utedtoBi2O3appear.Notethatastoichiometrictargetisusedinthisstudy,whichexplainstheslightlydifferentdepo-sitionwindowfromRef.14.Italsodemonstratesthatthe

©2009TheAmericanPhysicalSociety

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YOUetal.PHYSICALREVIEWB80,024105?2009?

FIG.1.?Coloronline?XRDpatternsofBFOepitaxial?lmsgrownunderdifferentoxygenpressures.PeaksofBi2O3secondphaseareindicatedby

arrows.

?lmcompositioncanbecontrolledbyvaryingthedepositionparameterswithoutusingBi-excesstargets.Thecalculatedout-of-planelatticeconstantsof?lmsdepositedat1,5,8,and40mTorrare3.952,3.974,3.972,and3.995Å,respec-tively,increasingwiththeoxygenpressure.TheintensitiesoftheBFOpeaksalsoincrease,indicatingimprovedcrystallin-ity.

Theferroelectrichysteresisloopat1kHzandleakagebehaviorof?lmsgrownunderdifferentoxygenpressuresareshowninFigs.2?a?and2?b?.Asthedepositionpressuredecreases,theremanentpolarizationcontinuestodropandthehysteresisloopbecomesmorecanting.Nosigni?cantchangeinthecoercive?eldisobserved.However,thereisminorimprinteffectforlow-pressuresamples,correspond-ingtoapreferredpolarization“up”state.Accompanyingthechangeinpolarization,the?lmsgrownunderlowoxygenpressureshowsimilarleakagecurrentwhilethosegrownun-der20mTorrorhighershowleakagecurrentdensityabouttwotothreeordersofmagnitudelarger.Thedielectricandpiezoelectriccoef?cientd33hysteresisloopsweremeasuredusing50kHzacoscillationswithamplitudesof100mVand

1V,respectively,superimposedonadcbias.TheresultswereplottedinFigs.2?c?and2?d?,bothshowingsimilardecreasingtrendasthepressuregoesdown.Thedatafor?lmsgrownunderhighoxygenpressurecouldnotbeac-quiredbecausetheirlowresistivitiesledtoelectricalbreak-downduringdcbiassweep.Allthesamplesgrownunderlowoxygenpressureshowaclearloopshiftconsistentwiththepolarizationhysteresisloop.Besides,thedielectricC-Vcurvesshowobviousdouble-loopfeaturewithtwopeaksap-pearingatthecoercive?eldandaroundzerobias.

Inordertounravelthevariationsintheelectricalproper-tiesofBFO?lmsdepositedunderdifferentoxygenpres-sures,compositionalanalyseswerecarriedoutbyEDS.AstandardstoichiometricBFOtargetwasusedforcalibration.Foreachsample,themeasurementwasperformedbymap-pingcertainareaatdifferentlocationsofthe?lm.Theresultsareself-consistentwithineach?lmandareplottedinFig.3.ItisclearthattheBi/Feratiointhe?lmsdecreasesasdepo-sitionpressuredecreases?Fig.3?a??.Surprisingly,theBi/Feratiocangodowntoaslowas?0.6withoutFe2O3forma-tionaccordingtotheXRDpattern,whileasmallBiexcesswillgiverisetoBi2O3phases.ThedecreaseinBi/Feratiowithreducedoxygenpressureisconsistentwiththedecreaseintheout-of-planelatticeconstantandthecrystallinityofthe?lms.Magneticpropertieswerealsomeasuredtocon?rmtheabsenceofmagneticFe2O3phases?notshown?.Allthe?lmsexhibitedweakmagnetizationsbelow0.06?B/Fe,showingnotraceofmagneticFe2O3existence.Thelargebismuthde?cienciesindicatebroadsingle-phase?eldintheBi-poorregion.Thisagreeswiththepreviousreport15andissimilartothePbTiO3system.16

TheremanentpolarizationPrderivedfrompositive-up–negative-downpulsemeasurement,relativepermittivity?r,andresistivityallshowdirectcorrelationwiththeBi/FeratioasdepictedinFig.3?b?.Asweknow,Bidisplacementcon-tributessigni?cantlytothespontaneouspolarizationofBFO.Soitisnotsurprisingthatthepolarizationwill

decrease

FIG.2.?Coloronline??a?Ferroelectricpolar-ization,?b?leakagecurrentdensity,?c?relativepermittivity,and?d?piezoelectriccoef?cientd33asafunctionofelectric?eldforBFO?lmsde-positedunderdifferentoxygenpressures.

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almostconstantasdepositionpressureincreases,con?rmingtheintrinsicpolarizationofBFOtobe?55?C/cm2along?001?pc.

Incontrast,theresistivityshowsreversetrendwithre-specttotheBi/http://wendang.chazidian.comrgeleakagecurrentinBFOthin?lmsiswidelyreported,whichismainlyduetotheoxygenvacanciesorconductiveBi2O3phasesratherthantheintrin-sicpropertyofBFO.21TheBide?ciencyreportedherecanbeconsideredasBivacancydoping,whichwillgreatlycom-pensatetheoxygenvacanciesandobviouslyeliminatethepresenceofBi2O3.Asaresult,theleakagecurrentsinBi-de?cientsamplesaremuchsmallerthanthosestoichiometricorBi-richones?Fig.2?b??.Inthissense,introducingBiva-canciesisequivalenttothewidelyadaptedaltervalent/aliovalentcationdoping,22–25providingawayfortuningelectricalpropertiesofBFOthin?lms.

TherelativepermittivityshowslessdependenceontheBicontentwithvaluesaround60–80.Frequency-dependencedielectricspectroscopy?notshown?indicatesonlysmalldi-electricrelaxationupto2MHz,whichcon?rmstheintrinsiccontribution.Accordingtothesemiempiricalphenomeno-logicalequation,thepiezoelectriccoef?cientinasinglecrys-talcanbeexpressedasafunctionofintrinsicrelativeper-mittivity?int,polarizationP,andelectrostrictivecoef?cientQ,26

d33=2?0?intQeffP,

?1?

FIG.3.?Coloronline??a?PlotofBi/Feratioasafunctionofdepositionoxygenpressure,?b?plotofremanentpolarization,rela-tivepermittivity,andresistivityat100kV/cmasafunctionofBi/Feratio,and?c?plotofremanentd33andelectrostrictivecoef?cientQeffasafunctionofBi/FeratiofordifferentBFOthin-?lmsamples.Theredlinein?a?istoguidetheeyes.

whenmoreBivacancies?VBi??areintroduced.ThegradualcantingoftheferroelectrichysteresisloopwiththedeceaseinBiconcentrationisalsoareminiscenceofferroelectricfatiguewhichisclaimedtoresultfromdomainpinningby

··

?arewidelyreportedpointdefects.17Oxygenvacancies?VO

··

defectsinferroelectricthin?lms.18PositivelychargedVOcancombinewithnegativelychargedVBi?toformdefect-dipolecomplexeswhichareeffectiveinferroelectricdomainpinning.19,20Thiscouldalsobeareasonforthereducedpo-larizationinBi-poorregion.WhenthecompositionentersintotheBi-excessrange,however,thepolarizationremains

where?0isthevacuumpermittivityandQeffiseffectiveelectrostrictivecoef?cientalong?001?direction.Themea-suredremanentd33andcalculatedQeffareplottedagainsttheBi/FeratioshowninFig.3?c?.ThecalculatedQeff?0.07m4/C2formorestoichiometricsamplesisclosetotheQ11ofPZTthin?lms26butdeviatedfrompreviousreport.27

Piezoelectricforcemicroscopywasperformedtostudythedomainstructuresandlocalpiezoelectricresponse.Pt/Ir-coatedhighlydopedSitips?springconstantof?42N/m,resonancefrequencyof?320kHz,andtipradiusof?25nm?wereused.Thescanswereperformedunderam-bientcondition,witha50kHzacvoltageof6Vpeaktopeakappliedtothetip.Yellow?bright?andpurple?dark?contrastsinFig.4representpolarizationcomponentpointing“up”and“down,”respectively.Filmsgrownunderlowoxygenpartialpressure?5mTorr?showtopographicfeaturesconsistentwithislandgrowthmode?Fig.4?a??.AsshowninFig.4?c?,theferroelectricdomainsexhibitafractalcharacteristicwithav-eragedomainsizeof?50nm,similartothosestudiedinthinBFO?lms.28Locald33hysteresisloop?Fig.4?e??re-vealsaslightshiftofthehysteresislooptowardnegativebias,consistentwiththedominatingpreferredpolarizationdirectionintheimage.Asdepositionpressureincreasesto20mTorr,drasticallydifferenttopographicfeaturesareobserved?Fig.4?b??possiblyduetoachangetostep?http://wendang.chazidian.comrgedomainsofmicronsizeareobserved.Grain-boundary-liketrenchesareformedlikelytoreducethemis-matchofelasticdistortionsbetweenindividualdomains.Thedomainboundariescouldbeanothersourceofhighleakagecurrent.29Withineachdomain,preferredpolarizationinupordowndirectionisobserved?Fig.4?d??.Locald33

hyster-

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YOUetal.PHYSICALREVIEWB80,024105?2009?

FIG.4.?Coloronline??a?and?b?3?3?m2topographicimages;?c?and?d?correspondingpi-ezoresponsephaseimagesand?e?and?f?locald33hysteresisloopsofBFO?lmsgrownunder5and20mTorroxygenpressures.Thescalesin?a?and?b?and?c?and?d?are30nmand360°,re-spectively.Yellow?bright?andpurple?dark?loopsin?f?representoriginalupanddowndo-mains,

respectively.

esisloopmeasurementsrevealoppositeimprintbehaviors,consistentwiththepreferredpolarizationdirection.

Towhatextentdoesthegrowthmodeaffecttheferroelec-tricdomainstructureisunknowncurrently.Butwesuggestthatdefectsinthe?lmsalsoplayanimportantrole.For?lmsgrownunderlowoxygenpressure,high-concentrationBiva-··

defect-canciesandoxygenvacanciesmayformVBi?-VO

dipolecomplexes.Suchdefect-dipolecomplexeswillpro-duceaninternal?eld,leadingtoapreferreddirectionoftheferroelectricdomain.Duetotherandomlylocatednatureofthedefect-dipolecomplexes,thepolarizationisalsoran-domlyorientedupanddown.For?lmsgrownunderhighoxygenpressure,largedomainscanformduetothelackofsuchdefect-dipolecomplexes.ThismodelalsoexplainsthedoublepeaksofdielectricresponseasdepictedinFig.5.Underalargedc?eld,allthedomainscanbepoledinonedirection.However,thedefectdipolesareunableorverydif?culttobereorientedbyexternal?eldduetotheirimmo-bilenature.Afterremovaloftheexternal?eldorbyapplyingasmall?eldwithoppositepolarity,partoftheferroelectricdomainswillreversewiththeassistanceoftheinternal?eld,givingrisetoapeakclosetothezerobiasinC-Vcurve.Thedifferenceinthepeakamplitudeisconsistentwiththerela-tivefractionsofupanddowndomainsintheoriginalstate?Fig.4?c??.

IV.CONCLUSIONS

Insummary,wehavestudiedtheeffectofoxygenpres-sureontheelectricalpropertiesofBFOthin?lms.Evenwithinthedepositionwindowwherephasepure?lmscanbeobtained,itisobservedthatBi/Feratiointhe?lms

decreases

FIG.5.?Coloronline?Dielectrichysteresisloopof5mTorrBFOthin?lmwithferroelectricdomaincon?gurationatdifferentstages.Thelightblue?bright?arrowsindicateinternal?eldinducedbydefect-dipolecomplexes.Yellow?bright?andpurple?dark?do-mainsrepresentupanddowndomains,respectively.

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withthedepositionoxygenpressure.Theremanentpolariza-tionofthe?lmdropswiththeBi/Feratio.LeakagecurrentsaregreatlyreducedinBi-de?cient?lms,resultingfromcom-pensationfortheoxygenvacanciesandsuppressionofBi2O3formation.Themorphologyandferroelectricdomainstruc-turesofthese?lmsarealsoverydifferent.TheBi-de?cient?lmstypicallyexhibitsmallsizesofgrainsanddomains,whereasthestoichiometric?lmsconsistoflargemicrometersizedomains.Adefect-dipolecomplexmodelcanbeusedto

explainthedifferentdomainstructuresandthedoublepeaksobservedinC-Vcurves.

ACKNOWLEDGMENTS

TheauthorsacknowledgethesupportfromNanyangTechnologicalUniversityandMinistryofEducationofSin-gaporeunderProjectsNo.AcRFRG30/06andNo.ARC16/08.

*Correspondingauthor.FAX:??65?67909081;jlwang@ntu.edu.sg

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