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AppliedSurfaceScience255(2008)3466–3469

ContentslistsavailableatScienceDirect

AppliedSurfaceScience

journalhomepage:http://wendang.chazidian.com/locate/apsusc

LettertotheEditor

GrowthandpropertiesofBiFeO3thin?lmsdepositedonLaNiO3-bufferedSrTiO3(001)and(111)substratesbyPLD

J.Zhu*,W.B.Luo,Y.R.Li

StateKeyLaboratoryofElectronicThinFilmsandIntegratedDevices,UniversityofElectronicsScienceandTechnologyofChina,Chengdu610054,PRChina

ARTICLEINFOABSTRACT

Articlehistory:

Received16January2008

Receivedinrevisedform27August2008Accepted1September2008

Availableonline12September2008Keywords:BFOXRDRHEED

Ferroelectricpolarization

BiFeO3(BFO)?lmshavebeenepitaxiallygrownonSrTiO3(001)andSrTiO3(111)substrateswithaLaNiO3conductivelayerasatemplateusingpulsedlaserdeposition.X-raydiffractionu–2uandFscanningrevealedthattheepitaxialrelationshipisBFO[100]//LNO[100]//STO[100];BFO(001)//

¯??==LNO½112¯??==STO½112¯??;BFO(111)//LNO(111)//STO(111)onLNO(001)//STO(001)andBFO½112

SrTiO3(001)andSrTiO3(111)substrates,respectively.Theepitaxialrelationshipwasalsocon?rmedbyexsiture?ectivehigh-energyelectronre?ection(RHEED).FromP–Emeasurements,bothsamplesshowedtypicalferroelectrichysteresisloops,indicatinggoodelectricalproperties.Furthermore,itwasfoundthattheremanentpolarization(2Pr)underthesameappliedelectrical?eldoftheepitaxialBFO(111)thin?lmontheLNO-coated(111)STOsubstrateismuchlargerthanthatoftheBFO(100)?lmontheLNO-coatedSTO(100)substrate.ThelargerpolarizationofBFO(111)?lmsmaybecausedbythattheeasypolarizationaxislayscloseto[111]direction,indicatingtheanisotropicferroelectricpropertyforBFO?lms.Thus,withtheepitaxialLNOelectrodetemplatelayers,theepitaxialBFO?lmsexhibithigh-qualitystructureandgoodelectricalproperties.

ß2008ElsevierB.V.Allrightsreserved.

1.Introduction

Inrecentyears,multiferroicmaterialshaveattractedgreatattentionduetotheirpotentialapplicationsfornewelectronicdevicesaswellasthefascinatingfundamentalphysics[1].BiFeO3(BFO),oneofthewell-knownmultiferroics,exhibitsthecoex-istenceofferroelectricandG-typeantiferromagneticorderaboveroomtemperature.Inbulksinglecrystals,BFOhasaferroelectric

´elCurietemperatureTcof1123K[2],andanantiferromagnetNe

temperatureTNof643K[3].ThelatticestructureofBFOisarhombohedradisorderedpervoskitewiththespacegroupR3c[4].

?andar=0.68.Therhombohedraunitcellparametersarear=3.96A

Inthisdistortedstructure,thehexagonal(001)Hisequivalenttothepseudo-cubic(111)C,alongwhichthereisathreefoldrotation.TheBi3+andFe3+cationsaredisplacedfromtheircentro-symmetricpositionsalongthepseudo-cubic[111]Cdirection.Thisdistortionresultsinthedevelopmentofaspontaneouspolarization(Ps)alonethisdirection.ForaferroelectricmaterialwithsuchhighCurietemperatureandlargedistortion,alargepolarizationwouldbeexpected.However,becauseofseriousleakageproblem,BFOshowsunexpectedlysmallvaluesofthe

*Correspondingauthor.Tel.:+862883202140.E-mailaddress:junzhu@http://wendang.chazidian.com(J.Zhu).

0169-4332/$–seefrontmatterß2008ElsevierB.V.Allrightsreserved.doi:10.1016/j.apsusc.2008.09.008

saturationpolarization(Ps)about3.5mC/cm2along[100]Cand6.1mC/cm2along[111]Ceveninsinglecrystalatliquidnitrogentemperature[5].Itissuggestedthatchemical?uctuations[6,7]andparasiticphases[8]arethemajorcontributorstoleakagecurrentsofBFOmaterials.ThepoorferroelectricpropertyofBFOhaslimitedtheapplicationofthismaterial.ManyeffortshavebeenmadetoimprovetheferroelectricpropertiesofBFOmaterials,includingtheadditionofdonors[9]andformingasolidsolutionofBFOwithotherperovskiteferroelectricmaterials[10].

Itiswellknownthatusingconductiveoxideelectrodes,suchasSrRuO3[11]andLaNiO3(LNO)[12]caninducepreferredorientedgrowthandimproveelectricalpropertiesofBFOthin?lms.LNO,asadesirablecandidateforelectrodematerials,hasattractedmoreandmoreattentioninrecentyears.Thespeci?ccharacteristicsofLNOelectrodearelowpreparationtemperature,goodconductiv-ityandchemicalstability[13,14].Inourrecentwork,epitaxialgrowthofLNOthin?lmswassuccessfullyobtainedonSrTiO3,LaAlO3,MgOsubstrates[15,16].Furthermore,theepitaxialgrowthofBaTiO3thin?lmswasalsocon?rmedbyXRDandexsiture?ectivehigh-energyelectronre?ection(RHEED)[17].Obviously,itisofsigni?cancetoepitaxiallygrowmultiferroicBFO?lmsonLNO-coatedsubstrates.Inthiswork,wereportthemicrostructureandelectricalpropertiesofBFOepitaxial?lmswithLNOasatemplatelayeronSTO(100)andSTO(111)substrates,respectively.Thetwokindsofheterostructuresof

J.Zhuetal./AppliedSurfaceScience255(2008)3466–34693467

BFO[100]//LNO[100]//STO[100]andBFO[111]//LNO[111]//STO[111]havebeenfabricatedandrevealgoodelectricalproperties.Thus,withLNOepitaxial?lms,theepitaxialgrowthofBFO?lmscanbeinduced.Inparticular,BFOthin?lmsonLNO-coatedSTOhavealargevalueofremanentpolarization(Pr)arepromisinginpracticalapplications.2.Experimental

Anon-stoichiometricBi1.1FeO3targetsynthesizedbytherapidliquidphasesinteringtechnique[18]andastoichiometricLNOceramictargetpreparedbythesolid-statereactionmethod[17]wereablatedtodepositBFOandLNO?imsbypulsedlaserdeposition(PLD),usingaKrF(l=248nm)excimerlaser(LAMBDAPHYSIK).Theaverageenergydensitywasadjustedto5JcmÀ2atarepetitionrateof3Hz.Duringablation,thetargetwasrotatedataspeedof15rpmtopreventpossiblestructuraldeteriorationandtobehelpfultogethomogeneous?lms.Thetarget-to-substratedistancewaskeptat4.5cm.Beforedeposition,STOsubstrateswereheatedat8008Cfor10mintogetacarbon-freeandhighcrystallinesurfaceinthechamber.LNO?lmsweredepositedonSTO(100)andSTO(111)substratesundera?owingoxygenpressureof20Paatsubstratetemperaturesof7008C.BFO?lmswerethensubsequentlydepositedontheLNOcoatedSTOsubstrates.Theoxygenpressuresandthesubstratetemperature

Fig.1.X-raydiffractionpatternsoftheBFO(001)?lmsdepositedonLNO-coated(001)substrate:(a)u–2uscansofBFO/LNO/STO(001)heterostructureand(b)F-scansofBFO(202),LNO(202)andSTO(202)planesforBFO/LNO/STO(001)

heterostructure.

forBFOgrowthwerechosentobe1Paand6008C.Afterdeposition,thesampleswereinsituannealedfor30minunder0.5atmoxygenpressureat4508Ctoimprovethequalityof?lmsandtodecreasetheoxygenvacancy.ThethicknessesoftheLNOandBFO?lmsweredeterminedbysteppro?le,respectively.Auwasdepositedasthetopelectrodeswithanareaof3.14Â10À4cm2usingashadowmaskatroomtemperaturebyevaporatingmethod.

Themicrostructureofmultilayer?lmswascharacterizedbyX-raydiffraction(XRD,D1SYSTEM,BEDE)withCuKaradiation

(l=1.5405A

?).Thein-planeepitaxywasinvestigatedbyanXRDFscanandexsituRHEEDwithahigh-sensitivecharge-coupleddevice(CCD)camera-basedvideo-recordingsystem.Theferro-electricpropertieswerecharacterizedbymeasuringthecapacitorstructureofAu/BFO/LNOusingaRadiantPrecisionLCmaterialanalyzer(PrecisionLC2000)atroomtemperature.3.Resultsanddiscussion

Toexaminethestructureof200nmBFOthin?lmsonLNO-coatedSTO(001)substrates,XRDu–2uandF-scanswerecarriedout.TheXRDresultsareshowninFig.1.Inourpreviouswork[16],highly(001)-orientedLNO?lmscouldbeobtainedonaSTO(001)substrateatasubstratetemperaturerangingwidelyfrom5008Cto7008C.WhenBFO?lmsweresubsequentlydepositedonLNO/STO,themultilayerstructurealsorevealedthesamehigh(00

1)-

Fig.2.X-raydiffractionpatternsoftheBFO(111)?lmsfabricatedonLNO-coated(111)substrate:(a)u–2uscansofBFO/LNO/STO(111)heterostructureand(b)F-scansofBFO(202)planeforBFO/LNO/STO(001)heterostructure.

3468J.Zhuetal./AppliedSurfaceScience255(2008)3466–3469

orientation.AsshowninFig.1(a),only(001)-typediffractionpeakswereobserved.Nosecondphaseorotherorientedgrainsweredetected.Thisresultindicateshighlyc-axisorientationofBFOonLNO-coatedSTO(001)substrates.Theout-of-planelattice

?.Thedifferenceoflatticeparameterwascalculatedtobe4.06A

parameterbetweenBFO?lmandBFObulkmaybeattributedtothecompressivestressinducedbytheepitaxialLNOonSTOsubstrate.XRDF-scansofBFO(202),LNO(202)andSTO(202)planesfortheBFO/LNO/STO(001)heterostructureareshownin(b).ThefoursharppeaksoriginatedfromBFOandLNO,indicatingthatboththeBFOandLNO?lmshavegoodin-planeorientation.Thisobserva-tionalsorevealsthatBFO?lmhasafourfoldrotationalsymmetry.Inaddition,thepeaksforBFOandLNO?lmswereshownupatthesamepositionasfourcorrespondingpeaksforSTOsubstrate,respectively,demonstratingacubic-on-cubicgrowthofBFOandLNO?lmswithrespecttoSTOsubstrate.XRDF-scansoftheBFO/STOheterostructurerevealthatBFOthin?lmswereepitaxiallydepositedonLNObuffered(001)STOsubstrateswithacube-on-cubearrangementinthemodeofBFO[100]//LNO[100]//STO[100].HRTEMobservationwasperformedtofurthercon?rmtheepitaxialnatureandinterfacecharactersofBFO?lmsdepositedonLNO-coatedSTO(001)substrate.AcoherentBFO/LNOinterfacewithatomic-levelinterfacialroughnesscanbeseeninthehigh-resolutionimage[19],whichcanbeexpectedbecauseBFO,LNOandSTOhaveasimilarcrystalstructurewiththecloselatticeparameter.

Asacomparison,BFO?lmsweredepositedonLNO-coatedSTO(111)substratesunderthesamefabricationconditionstoinvestigatetheeffectofLNO-bufferedsubstratesontheepitaxialgrowthandtheelectricalpropertiesofBFO?lms.Fig.2istheXRDu–2uandF-scanningresultsofBFO?lmsonLNO-coatedSTO(111)substrates.WhenBFO?lmsweredepositedonLNO-coatedSTO(111)substrates,theywerehighly(lll)-oriented,asshowninFig.2(a).AF-scanwasalsocarriedouttoinvestigatetheepitaxialrelationoftheBFO?lmonLNO-coatedSTO(111)substrates.Threeequallyspacedpeaksseparatedby1208canbeobservedinFig.2(b).Athreefoldsymmetryaxisisclearlyevident,indicatingthattheBFO?lmdepositedonLNO-coatedSTO(111)hasarhombohedralstructure.Thisalsocon?rmsthattheepitaxialstructureofBFO/LNO/STO(111)isinthemodeofBFO[111]//LNO[111]//STO[111].Therefore,itcanbeconcludedthatBFO?lmscanbeepitaxiallygrownonbothSTO(111)andSTO(001)substrateswithLNOconductivelayer.AsfortheBFO?lmdepositedonLNObufferedSTO(111)substrate,thecompressivestresswillbeimposedonthe(111)plane,beingfavoroftheformationofarhombohedralstructure.ConsideringthatbulkBFOhasarhombohedralstructurethatispolarizedalong(111),BFO?lmsdepositedonLNO-coatedSTO(111)shouldhaveasimilarstructuretostrainfree-standingBFObulk.Ontheotherside,BFO?lmsgrownonLNO-coatedSTO(001)shouldbeexpectedtobedistorted,whichisdifferentfromthebulkrhombohedralstructureduetothecompressivestressalong(100)direction.

ExsituRHEEDobservationswereperformedtofurthercon?rmtheepitaxialnatureofBFO?lmsonLNO-coatedSTOsubstrates.Foursampleswerechosen.Theywere50-nmLNO?lmonSTO(001)andSTO(111),50-nmBFO?lmonaLNO-coatedSTO(001)andSTO(111),respectively.Afterdeposition,thesamplesweretransferredtoanothervacuumchamberequippedwiththeRHEEDsystemwithabasepressureof10À5Pa.TheworkingvoltageoftheRHEEDwas?xedat20kV.TheexsituRHEEDpatternsofLNO?lmandBFO?lmareshowninFig.3.ThereareregularlyarrangedspottypatternsinFig.3(a).ThebrightspotsrepresentthediffractionpatternsoftheLNO(001)surfacewiththeincidentbeamalongh100idirection;andeachspotisthediffractionresultofthecorrespondingcrystallineplane.ThespottypatternsuggeststhatthesurfaceoftheLNO/STO(001)thin?lmissmooth,buttherearesomemoundsonthesurface.TheRHEEDpatterninFig.3(a)canappearagainwitha908rotationaroundthesubstratenormal,indicatingafourfoldrotationalsymmetry.Fig.3(b)istheRHEEDpatternfora50-nmBFO?lmonaLNO-coatedSTO(001).Theregularlyarrangedspottypatternsalsoshowafourfoldrotationalsymmetry,whichisthesameasthatofthepre-depositedLNO?lmonSTO(001).Itfurthercon?rmstheepitaxialgrowthofBFO?lmontheLNO-coatedSTO(001)substrate.AsshowninFig.3(c)and(d),theRHEEDpatternscorrespondtoLNO(111)andBFO(111)surfaceswiththeincident

¯idirection.Thesespottypatternsshowathreefoldbeamfromh112

rotationsymmetry,whichisconsistentwiththeresultrevealed

by

Fig.3.ExsituRHEEDpatternsof(a)LNO(001)?lmonSTO(001)substrate;(b)50nmBFO(001)?lmonLNO-coatedSTO(001)substrate,withincidentelectronbeamalong

¯ih100idirection;(c)LNO(111)?lmonSTO(111)substrate;(d)50nmBFO(111)?lmonLNO-coatedSTO(111)substrate,withincidentelectronbeamalongh112

direction.

J.Zhuetal./AppliedSurfaceScience255(2008)3466–34693469

Fig.4.P–Ehysteresisloopsfor300nmBFOthin?lmswithdifferentepitaxialorientationsonLNO-coatedSTOsubstrates:(a)for(001)BFO?lmand(b)for(111)BFO?lm.

XRD.Thus,theepitaxialgrowthrelationshipofBFOthin?lmsonLNO-coatedSTOsubstrateswasfurthercon?rmedbytheexsituRHEEDobservations.

TheferroelectricpropertiesofthetwosamplesofBFO?lmsonLNO-coatedSTO(001)andSTO(111)substrateswerequanti?-cationallyinvestigatedbyP–Emeasurement,respectively.Fig.4showsthepolarizationhysteresiscurvesofBFO(111)andBFO(001)samples.Forbothsamples,typicalferroelectrichysteresisloopswereobserved.2PrvaluefortheBFO(001)sampleis23.6mC/cm2,whichismuchlargerthantheresultreportedbyTeagueetal.[5].Thisdramaticenhancementmayberesultsfromthegoodcrystalstructureandthesharpinterfacebetween?lm/electrodeasrevealedbyXRDandTEM[19].Itwasfoundthat2ProfaBFO(111)?lmonaLNO-coatedSTO(111)substrateare55mC/cm2,whichisobviouslylargerthanthatofaBFO(001)?lmonaLNO-coatedSTO(001)substrate.The2PrdifferencebetweenBFO(001)?lmandBFO(111)?lmcouldbeattributedtothattheeasyaxisofspontaneouspolarizationofBFOmaterialliesclosetonormalto(111)plane,indicatingtheanisotropicferroelectricpropertyforBFO?lms.Asmentionedabove,Bi3+andFe3+cationsaredisplacedalongthenormaltopseudo-cubic(111)planefromtheircentro-symmetricpositions,whichcouldresultinthespontaneouspolarizationorientedalongthenormalto(111)plane.Therefore,itcanbeexpectedthattheferroelectricpolarizationsofBFO(111)?lmswouldbemuchlargerthanthatofBFO(001)?lms.AsshowninFig.4,avisibleenhancementinPsandPrwereobservedinBFO(111)?lms.Thus,theelectricalcharacterizationdemonstratesalargespontaneouspolarizationwhoseeasilypolarizedaxisexistsclosetothenormalto(111)plane.Inaddition,itcanbeobservedinFig.4thattheP–Ehysteresisloopsarenotcompletelysaturated,especiallyforBFO(111)?lm.ItwouldbeattributedtoleakagecurrentswhichmaybecausedbypossiblecoexistenceofFe2+andFe3+inBFO?lms.Asmentionedinourpreviouswork[7],thepresenceofoxygenvacanciesusuallyexistinginoxide?lmscanleadtothe

appearanceofFe2+whichwouldbethemainfactorresultinginleakagecurrent.4.Conclusion

Insummary,(001)and(111)epitaxialBFO?lmsweresuccessfullydepositedonLNO-coated(001)and(111)STOsubstratesbyPLD.XRDscanningandRHEEDpatternsrevealedthattheepitaxialrelationshipforthesesamplesisBFO[100]//LNO[100]//STO[100];BFO(001)//LNO(001)//

STO(001)andBFO½112

¯??==LNO½112¯??==STO½112¯??;BFO(111)//LNO(111)//STO(111),respectively.AlthoughthesmallerlatticeparameterofLNOthantheSTOcouldleadtoalargerlatticemismatchbetweentheBFO?lmandtheLNO-coatedSTOsubstrates,theepitaxialgrowthofBFOcanberealized.RHEEDpatternsindicatethatthein-planelatticeparametersbetweenBFOandLNOarealmostthesame,indicatingacoherencegrowthofBFOonLNO-coatedSTO.Forbothsamples,typicalferroelectrichysteresisloopswithlarge2Prwereobserved.2PrvaluesforBFO(111)andBFO(001)?lmsare55mC/cm2and23.6mC/cm2,respectively.Additionally,2PrandPsofBFO(111)?lmarelargerthanthoseofBFO(001)?lms,whichmaybeattributedtotheeasypolarizationaxisclosetothenormaltoBFO(111)plane.Acknowledgement

ThisworkwassupportedbytheNaturalScienceFoundationofChina(grantno.50772019).References

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