article

JournaloftheKoreanPhysicalSociety,Vol.57,No.6,December2010,pp.1875?1878

E?ectsofBiContentontheFerroelectricPropertiesofBiFeO3ThinFilms

DalhyunDo,JinWonKim,SangSuKim?andWon-JeongKim

DepartmentofPhysics,ChangwonNationalUniversity,Changwon641-773

MyangHwanLeeandTaeKwonSong

SchoolofNanoandAdvancedMaterialEngineering,ChangwonNationalUniversity,Changwon641-773(Received12January2010,in?nalform25April2010)

TwopolycrystallineBiFeO3thin?lmsweredepositedonPt(111)/Ti/SiO2/Si(100)substratesbyusingpulsedlaserdepositionfromnominalcompositionBi1.0FeO3andBi1.1FeO3targets.X-raydi?ractionresultscon?rmedthatdi?erentimpurityphaseswereformedintheBi1.0FeO3andtheBi1.1FeO3thin?lms.ThepreferredorientationoftheBi1.0FeO3thin?lmsisthe<110>directionwhilethatoftheBi1.1FeO3thin?lmsisthe<100>direction.Thevaluesoftheremnantpolarization(2Pr)oftheBi1.0FeO3andtheBi1.1FeO3?lmcapacitorswere49µC/cm2atanappliedelectric?eldof500kV/cmand40µC/cm2at467kV/cm.Asimilarleakagecurrentpropertywasobservedinboth?lms.

PACSnumbers:77.80.-e,77.80.Fm

Keywords:MultiferroicBiFeO3thin?lm,Ferroelectrichysteresisloop,PulsedlaserdepositionDOI:10.3938/jkps.57.1875

I.INTRODUCTION

Bismuthferrite(BiFeO3(BFO))isoneofthemultifer-roicmaterials,inwhichelectric,magnetic,andstructuralorderparametersarecoupledtoeachother,resultinginsimultaneousferroicpropertiessuchasferroelectric,ferromagnetic,andferroelasticproperties[1,2].Unlikemostothermultiferroics,BFOexhibitsmultiferroicprop-ertiesatroomtemperatureduetoferroelectricitywithaCurietemperatureof820?CandantiferromagnetismwithaN´eeltemperatureof380?C,whichgivesgreatpotentialforindustrialapplications[3].Inaddition,a?rst-principlesstudypredictedalargeferroelectricpo-larization,90?100µC/cm2,alongthe[111]directionofthepseudo-cubicperovskiteunitcell[4].BFOthin?lmsgrownonSrTiO3(STO)singlecrystalsexhibitedthevalueofpolarizationpredictedbyRef.4.Therem-nantpolarizationPrwas100µC/cm2for(111)BFOthin?lmsdepositedonSTObyusingpulsedlaserdeposition(PLD)[5,6].AsimilarresultwasobtainedinBFOthin?lmsgrownonSTObysputtering[7].

DespitethefactthatalargeremnantpolarizationwasobservedinepitaxialBFOthin?lmsdepositedonSTOsubstrates,experimentalresultsoftenshowedasmallremnantpolarizationinbulkBFOandinBFOthin?lmsgrownonaPt-coatedSiwafers.BFOsinglecrystalsex-hibitedaspontaneouspolarizationof3.5µC/cm2along

?E-mail:

the<100>direction,correspondingto6.1µC/cm2alongthe<111>direction[8].Highly(100)-orientedBFOthin?lmsdepositedonPt/Ti/SiO2/Sisubstratesbyrfmag-netronsputteringshowedaremnantpolarizationof5.2µC/cm2atanelectric?eldof360kV/cm[9].ThemostconsiderableproblemisthelowelectricalresistivityasaresultofdefectsandnonstoichiometryduetothehighvolatilityofBi.ImprovedferroelectricpropertieswereobtainedbysubstitutingasmallnumberofdonorsinBFO,whichhelpedtoincreasetheresistivitybyreduc-ingtheconcentrationofoxygenvacancies[10].There-fore,controloftheBicontentisoneoftheimportantissuestomakephase-pureBiFeO3.Inthisstudy,theef-fectsoftheinitialBicontentonthecrystalstructureandtheferroelectricpropertiesofBFOthin?lmsdepositedbyusingPLDwereinvestigated.

II.EXPERIMENTALDETAILS

BFOthin?lmsweredepositedonPt(111)/Ti/SiO2/Si(100)substratesbyusingapulsedlaserdepositionmethod.TwoPLDtargetscontainingdi?erentBicon-tentswerepreparedbyusingatypicalsolid-stateprocesswithrawpowdersofBi2O3(Aldrich,99%)andFe2O3(Aldrich,99%).ThenominalcompositionsofthePLDtargetswereBi1.0FeO3andBi1.1FeO3,respectively.AKrFexcimerlaserwithawavelengthof248nmwasusedtoablatethetargetatarepetitionrateof5Hz.The

sskim@changwon.ac.kr

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Fig.1.XRDpatternsoftheBi1.0FeO3andtheBi1.1FeO3thin?lms.Symbolsindicatethesubstrate()andtheim-purityphasessuchasFe-richphases(??)and?

Bi-richphases(??).

laser’senergydensitywasabout1.1J/cm2perpulse.DuringthedepositionoftheBFOthin?lms,thesub-stratetemperaturewaskeptat550?Cunderanoxy-genpartialpressureof30mTorr.Afterdeposition,the?lmswerecooledtoroomtemperatureunderanoxygenpartialpressureof760Torr.Pttopelectrodeswerede-positedbyionsputteringthroughashadowmask,200µmindiameter.

ThecrystalstructureandthemicrostructureoftheBFOthin?lmswerecon?rmedbyusingX-raydi?rac-tion(XRD,Rigaku,MiniFlexII)andscanningelectronmicroscopy(SEM,Tescan,MIRAIILMH).Ferroelec-trichysteresisloopsandleakagecurrentweremeasuredbyusingaferroelectrictester(RadiantTech.Inc.,Pre-cisionLC)andanelectrometer(Keithley,6517A).

III.RESULTSANDDISCUSSION

Figure1showstheXRDpatternsoftheBi1.0FeO3andtheBi1.1FeO3thin?lms.Forthemeasurements,X-rayswithCuKαradiationfromadi?ractometerop-eratedat30kVand15mAwereused.AscanbeseeninFig.1,inboththin?lms,polycrystallineBFOisamajorphase,alongwithotherimpurityphases.InBi-basedoxideferroelectricmaterials,thevolatilityofBiresultsinnon-stoichiometricstructuraldefects.Thus,theimpurityphasesmarkedby“??”mayberelatedtoFe-richphasessuchasFe2O3and/orBi2Fe4O9[11].Ontheotherhand,theimpurityphasesmarkedby“??”,appearinginBi1.1FeO3?lms,mayberelatedtoBi-richphasessuchasBi2O3[11].

Thevolumefractionsof(100),(110),and(111)-orientedgrains,whicharethemajorre?ectionpeaksinbothBFOthin?lms,werecalculatedfromV/I(hkl)=I(hkl)total,whereI(hkl)isthepeakintensityofthe(hkl

)

Fig.2.SEMimagesofthesurfacemorphologiesof(a)theBi1.0FeO3and(b)theBi1.1FeO3thin?lms.

re?ectionandItotalisthesumofallIisdirectly(hklrelated)used[12].Thecrystallographicorientationtothepolarizationvectorinmostferroelectricmaterialsandprovidesusefulinformationtoestimatethevalueofpo-larization.Thevolumefractionsof(100)-,(110)-,and(111)-orientedgrainsintheBi1.0FeO3thin?lmswerelessthan1%,77%,and22%,respectively,andthoseintheBi1.1FeO3?lmswere73%,13%,and14%,respectively.TheresultindicatesthatthepreferredorientationoftheBi1.0FeO3?lmsisthe<110>directionwhilethatoftheBi1.1FeO3?lmsisthe<100>direction.

Figures2(a)and2(b)showSEMimagesofthesurfacemorphologiesoftheBi1.0FeO3andtheBi1.1FeO3thin?lms.Theimagesshowedthatgrainsof100?150nminaveragesizearehomogeneouslydistributedacrossthesurfacesofboththin?lms.Thevaluesofthe?lmthick-nessmeasuredfromthecross-sectionalSEMimageswere400nmand300nmfortheBi1.0FeO3andtheBi1.1FeO3thin?lms,respectively.

Ferroelectrichysteresisloopsweremeasuredbyapply-ingafrequencyof1.25kHzwithtriangularvoltagepulse.TheloopsoftheBi1.0FeO3andtheBi1.1FeO3?lmca-pacitorsweretakenat20V(500kV/cm)and14V(467kV/cm),respectively.Boththin?lmsshowedasymmet-

E?ectsofBiContentontheFerroelectricPropertiesofBiFeO3ThinFilms–DalhyunDoetal.

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Fig.3.FerroelectrichysteresisloopsoftheBi1.0FeO3andtheBi1.1FeO3thin?lms.richysteresisloops,indicatingthateasypolarizationdo-mainswitchingoccurredwhenapositiveelectric?eldwasappliedwhilepolarizationswitchingwasdi?cultun-deranegativeelectric?eld.Thise?ectmaybeexplainedbystressinducedbythe?lm/electrodeinterfaceduetoclampingorlatticemismatchand/ordomainpinningin-ducedbyoxygenvacancies[13].OurBFOthin?lmsweredepositedonepitaxialPtbottomelectrodes,whichmighthavegivenaclampinge?ectonthe?lmswhiletheinterfacebetweenthePttopelectrodesandthe?lmlayerwasfreefromaclampinge?ect.Also,thevolatilityofBiionsresultsintheformationofoxygenvacancies.Theremnantpolarization(2Pr)andthecoerciveelec-tric?eld(2Ec)wereestimatedfromthehysteresisloops.Thevaluesof2Prand2EcoftheBi1.0FeO3andtheBi1.1FeO3?lmcapacitorswere49µC/cm2and526kV/cmatanappliedelectric?eldof500kV/cm,respec-tively,and40µC/cm2and582kV/cmatanappliedelectric?eldof467kV/cm,respectively.Thedevicesbrokedownforappliedelectric?eldsabove500kV/cmduetothepossiblyhighleakagecurrent.Thus,itwasimpossibletoobtainthevaluesof2Prand2Ecfromthesaturationhysteresisloop.

Theremnantpolarizationestimatedfromtheloopsissomewhatanaveragevaluefromseveralcrystallographicorientations.ThismakesdirectcomparisonofremnantpolarizationswithP√each√otherdi?cult.Itwasassumedthat√

r(111)=(3/2)Pr(110)=(3)Pr(100)inBFObecausethespontaneouspolarizationvectorliesalongthe<111>directionsintherhombohedralBFOphase[5].FromthevolumefractionofgrainswitheachorientationcalculatedfromtheXRDpatternsandthe2Probtainedfromthehysteresisloops,thevaluesof2Pr(111)oftheBi1.0FeO3andtheBi1.1FeO3thin?lmscouldbecloseto57µC/cm2and60µC/cm2,respec-tively.Thissimplyimpliesthatboththin?lmshavesimilarferroelectricpropertiesintermsofpolarization,regardlessoftheformationofdi?erentimpurityphases,asshowninFig.1.However,identifyingthe

impurity

Fig.4.Leakagecurrentdensityasafunctionofanappliedelectric?eldfortheBi1.0FeO3andtheBi1.1FeO3thin?lms.

phasesisnecessary.

Theleakagecurrentwasmeasuredatroomtempera-tureunderadcappliedelectric?eld.Figure4showstheleakagecurrentdensityasafunctionoftheappliedelec-tric?eldforBi1.0FeO3andBi1.1FeO3?lms.Overall,theBi1.0FeO3?lmcapacitorshowedaslightlylowerleakagecurrentdensitythantheBi1.1FeO3?lmcapacitor.How-ever,theleakagecurrentdensityistoohightoachievetheremnantpolarizationpredictedbythe?rst-principlesstudyandobservedinepitaxialBFOthin?lmsgrownonSTOsubstrates.

IV.CONCLUSION

TheBFOthin?lmswerepreparedfromBFOtargetswithnominalcompositionsofBi1.0FeO3andBi1.1FeO3byusingPLD.DuetothevolatilityofBiions,impurityphaseswereformedinadditiontomajorBiFeO3phase.TheimpurityphasesoftheBi1.0FeO3thin?lmmayberelatedtoFe-richphaseswhilethoseoftheBi1.1FeO3thin?lmmayberelatedtoBi-richphases.BothBFOthin?lmsshowedalmostsimilarferroelectricandleakagecurrentproperties,regardlessoftheformationofthedif-ferentimpurityphases.Ontheotherhand,comparedtotheepitaxialBFOthin?lmsgrownonSTOsubstrates,arelativelysmallremnantpolarizationwasobserved.Theoriginmayberelatedtothehighleakagecurrent.

ACKNOWLEDGMENTS

ThisworkwassupportedbythePriorityResearchCentersProgramthroughtheNationalResearchFoun-dationofKorea(NRF)fundedbytheMinistryofEdu-cation,ScienceandTechnology(2009-0094064).

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