电调控La0.5Sr0.5CoO3CeY2Fe5O12氧化物异质结的反射性能.pdf 8页

'中国科技论文在线http://www.paper.edu.cnElectricalControlofReflectancein#La0.5Sr0.5CoO3/CeY2Fe5O12OxideHeterostructure1,2,31,2,3**5ZHUYinlong,BILei(1.NationalEngineeringResearchCenterofElectromagneticRadiationControlMaterials,Chengdu610054;2.StateKeyLaboratoryofElectronicThinFilmsandIntegratedDevices,Chengdu610054;3.SchoolofMicroelectronicsandSolid-StateElectronics,UniversityofElectronicScienceand10TechnologyofChina,Chengdu610054)Abstract:Electricallycontrollingmaterials’propertiescontributestoextendingfunctionsandapplicationsofelectronicdevicesduetointroducinganewdegreeoffreedomthatcanbemanipulatedbyanelectricfield.Here,theelectricalproperties,magneto-opticaleffect,andelectricfieldcontrolofthereflectanceintheLa0.5Sr0.5CoO3/CeY2Fe5O12oxideheterostructurehavebeenexperimentally15investigatedanddemonstrated.Byapplyingavoltageof±5Vacrosstheinterfaceatroomtemperature,thereflectanceoftheheterostructurecanbereversiblymodulatedby0.12%.Simultaneously,theelectricalresistanceandcapacitanceofthestructurewerereversiblymodified.OxygenionmigrationneartheLa0.5Sr0.5CoO3/CeY2Fe5O12interfaceundertheappliedelectricfieldisconsideredtobethemechanismofelectricfieldcontrolledreflectanceinthisstructure.Thisworkbasedonelectrochemical20gatingatoxygenlevelprovidesanewmechanismtoelectricallyreconfiguretheopticalpropertiesofmagneto-opticaldevicesforintegratedphotonicdeviceapplications.Keywords:electricfieldcontrolledreflectance;electrochemicalgating;yttriumirongarnet;integratedphotonicdevice250IntroductionElectricalcontrolofmaterials’propertieshasattractedintenseresearchinterestrecently.Thecapabilityofcontrollingmaterials’propertiesenablestodesignandrealizeavarietyofcompactandreconfigurabledevices.Comparedtopassivedevices,functionsandapplicationsoftunabledevicesareexpandedduetothefactthatanewdegreeoffreedomisintroducedtorealize30multi-functionsinonedeviceabletoreplacemultiplesinglefunctionalones.Therefore,areasofchipsandvolumesofsystemscanberemarkablysaved,whichmakesminiaturizationand[1,2]integrationofelectronicdevicesmorepromising.Electrochemicalgatingisanemergentandnovelmethodtomodifythepropertiesof+2++2-materials.Theprincipleofelectrochemicalgatingisdrivingtheion(e.g.,Ag,Cu,Li,O)35migrationtoachievetheredistributionofionsinaspecificmaterialorexchangeofionsbetween[3,4]differentmaterials.Controllingmaterials’propertiesusingthismechanismhasbeen[5-7]demonstratedinCoFe2-xO4,NiFe2O4,andIn2O3/YSZsystems,amongwhichoxygenionsplayanimportantrole.Infunctionaloxides,oxygenionsinfluenceordeterminetheirphysicalor[6,8,9]chemicalpropertiessuchasresistivity,energystoragecharacteristics,magnetism.In40particular,Oxygenconcentrationsignificantlyinfluencesthevalencestateanddorbitaloccupancyofthetransitionmetalionsinmagneticoxides(e.g.,CoFe2O4,Y3Fe5O12).Therefore,iftheoxygenion’sconcentrationordistributioncanbecontrolledbyavoltageorotherexternalstimuli,theFoundations:NationalNaturalScienceFoundationofChina(61475031,51302027,51522204);theFundamentalResearchFundsfortheCentralUniversities(ZYGX2013J028,ZYGX2014Z001);theScienceFoundationforYouthsofSichuanProvince(2015JQO014);theMinistryOfEducationProgramofIntroducingTalentsofDiscipline(111project,B13042);theNationalResearchFoundationfortheDoctoralProgramofHigherEducationofChina(20130185120009)Briefauthorintroduction:ZHUYinlong(1992-),Male,Master,Mainresearch:ElectricfieldControlofmaterials"propertiesCorrespondanceauthor:BILei（1982-),Male,Professor,Mainresearch:Magneticoxidethinfilmsandintegratedmagneto-opticaldevices.E-mail:bilei@uestc.edu.cn-1- 中国科技论文在线http://www.paper.edu.cnmaterials’propertiescloselyrelatedtooxygenionswillbetuned.Therearetwomechanismsforcontrollingthepropertiesoffunctionaloxidesbasedon45electrochemicalgating:(i)utilizingintrinsicpropertiesofmaterials:someoxidesaregoodionconductorsinwhichanelectricfieldcanaltertheion’sdistribution.It’sreportedthatthesizeofmagneticdomainwithdifferentmagnetizationcanbemodulatedupto70%byanelectricfieldinCoFe2-xO4.First-principlecalculationindicatesthatnanoscalemagnetizationreversaliscausedby2+migrationofCobetweenFevacanciesandassistedbyoxygenvacancies.Consequently,the50unidirectionalmagneticanisotropyalong<110>directionscanbeinducedtoachievethemagnetizationreversal[ref.6];(ii)designingtheheterostructurebasedonoxideswithoxygeniontransparencyattheinterface:Anelectricfield4V/cmwasusedtomaketheconductanceofIn2O3/YSZheterostructureenhancedbyafactorof190comparedtoitspristinevalue,whichismeasuredat370°C,withthesampleequilibratedatpO2=150Torr.IonicallyconductingYSZ55substratecanaccepttheoxygenionsfromO2atmosphereatnegativeelectrode.DuetotheinequalityofresistanceinPtelectrode/YSZinterfaceandPtelectrode/In2O3interface,thevoltageprofilesinIn2O3andYSZaredifferentsothataverticalelectricfieldisformedinIn2O3/YSZ2-interfaceorientingfromYSZtoIn2O3.OcanmigratefromIn2O3toYSZandsubsequently,2-chargeneutralityofYSZismaintainedbytransformingOtoO2atPtpositiveelectrode.60Therefore,theelectricalconductionofIn2O3canbeenhancedbyoxygenvacancydoping[ref.7].Thefirstmechanismislimitedtosomefunctionaloxideswithalowmigrationenergybarrierofions.It’shardtobeappliedtoalargeamountofoxideswithlowionconductivities.Thesecondmechanismisamoregeneralmethodtocontroltheinterfacialcharacteristicseveninaheterostructurewhereapoorionconductorisincluded.65Inthispaper,westudiedtheelectricfieldeffectonanoveloxideheterostructureofLa0.5Sr0.5CoO3/CeY2Fe5O12thinfilms.La0.5Sr0.5CoO3(LSCO)isaconductiveoxidewithhigh[10-19]electronic,ionicconductivityandcatalystactivities,whereasCeY2Fe5O12(CeYIG)isamagneticinsulatorwithstrongmagneto-opticalpropertiesandwidelyappliedinnon-reciprocal[20-23]photonicdevices.ByapplyinganelectricfieldacrosstheLSCO/CeYIGinterface,we70observedreversibletuningofthestructure’sresistance,capacitanceandreflectivityatroomtemperature.Oxygenionmigrationacrosstheinterfaceisbelievedtobethemechanismforthereflectivitytuning.1ExperimentalsectionTheY3Fe5O12(YIG)thinfilmwasdepositedonsiliconsubstratebypulsedlaserdeposition75(PLD)asaseedlayerforCeYIGgrowth.AKrFexcimerlaseroperatingatλ=248nmwasusedto2ablatetheYIGpolycrystallinetargetwithrepetitionrate10Hzandenergydensity~2.5J/cm.Thedistancebetweenthetargetandsubstratewasfixedat5.5cm.Thesubstratetemperaturewas-4400°Candoxygenpressurewas0.67Pawithabasevacuumof5×10Pa.TheYIGfilmwascooleddowntoroomtemperaturein0.67PaO2attherateof5°C/min.Subsequently,thefilm80wasrapidlyannealedat850°Cfor180swithoxygenpressurepO2=266Paandcoolednaturally[24]downtoroomtemperatureinaRTA(rapidthermalannealing)furnace.TheCeYIGfilmwasdepositedonYIG/SibyPLD.Thelaserwavelength,repetitionrate,laserenergydensity,distancebetweenCeYIGtargetandsubstrate,andbasevacuumarethesameastheprocessparametersofYIG.ThesubstratetemperatureandoxygenpressureduringCeYIGdepositionwere650°Cand-4851.33Parespectively.CeYIGfilmwascooleddowntoroomtemperatureinavacuumof5×10Paattherateof5°C/min.TheLSCOfilmwasdepositedonCeYIG/YIG/SibyPLD.AmetalshadowmaskwasplacedonCeYIG/YIG/SitofabricateLSCOpads.Thelaserwavelength,laserenergy-2- 中国科技论文在线http://www.paper.edu.cndensity,distancebetweenLSCOtargetandsubstrate,andbasevacuumarethesameastheprocessparametersofYIG.Therepetitionrate,substratetemperatureandoxygenpressureduring90depositingLSCOwere4Hz,500°Cand1.33Parespectively.Thesamplewascooleddownto-4roomtemperatureinavacuumof5×10Paattherateof5°C/min.Toformagoodelectricalcontactbetweenthebottomelectrodeandmicroprobe,theTifilmwasdepositedonSibyRFsputteringwithargonpressurepAr=0.5PaandRFpower100W.TheAufilmwassubsequentlydepositedonTi/SibyDCsputteringwithargonpressurepAr=1PaandDCpower50W.The95thicknessesofYIG,CeYIG,LSCO,Ti,Aufilmsare60nm,45nm,80nm,10nm,100nmrespectively,whicharemeasuredbycross-sectionalSEMimagesoffilms.PhaseidentificationwasperformedbyX-raydiffraction(XRD)usinganX-raydiffractometer(ShimatzuXRD-7000).I-VandC-VcharacterizationswerecarriedoutonaKeysightB1500Asemiconductorparameteranalyzer.ThereflectivityandLMOKE(longitudinal100magneto-opticalKerreffect)spectrumweremeasuredonahomemadeMOKEspectrumcharacterizationsystemequippedwithasemiconductorlaserat635nmwavelength.ThereflectivityandLMOKEcharacterizationwerecarriedoutwithanincidentangleof35degwithrespecttothesamplenormalatroomtemperature.2Resultsanddiscussion105ThefabricateddevicestructureisshowninFig.1(a).ThedevicewithacapacitorstructureconsistsofLSCO(80nm),CeYIG(45nm),YIG(60nm),andAu/Ti(100/10nm)onSisubstrate.+LSCOandAu/Ti/pSisubstratefunctionasthegateandbottomelectrodesrespectively.CeYIGandYIGarebothinsulatorssothatanelectricfieldcanbegeneratedinthemafteravoltagewasapplied.TheopticalmicroscopyimageofthedeviceisshowninFig.1(b).ThediameterofLSCO110padwasabout370μm.OnemicroprobewasplacedontheLSCOpad.BecausethevolumeofLSCOpadwastoosmallforXRDcharacterizations.AcontinuousLSCOfilmwasdepositedonCeYIG/YIG/SiunderthesameprocessparameterasLSCOpadwasusedforXRDcharacterizationsthatisshowninFig.1(c).ThediffractionpeaksofCeY2Fe5O12crystallinethinfilmarelocatedatloweranglesthanthepeaksofYIG.ThehighLSCO(110)diffractionpeaklies115nearthe(420)peaksofYIGandCeYIG,whichresultsinthatthethreepeaksmergeintoonebroaderpeak.TheMOKEhysteresisloopsofLSCO/CeYIGisshowninFig.1(d).ThelinearsignalbackgroundfromtheLSCOpadhasbeensubtracted.Thesaturationlongitudinalmagneto-opticalKerrrotationangleofLSCO/CeYIGisabout0.01°.120Fig.1(a)Schematicsofdevicestructureandexperimentalsetup;(b)OpticalmicrographofLSCOpad;(c)XRD-3- 中国科技论文在线http://www.paper.edu.cnpatternofcontinuousLSCOfilmonCeYIG/YIG/Si;(d)Thepristinelongitudinalmagneto-opticalKerrrotationcurveofLSCOpad/CeYIG/YIG/SiTheelectricalcharacterizationofthedeviceisshowninFig.2.I-VcharacterizationofLSCOpadsshowninFig.2(a)indicatesthatthecontactandLSCOelectroderesistanceis~2MΩunder125anappliedvoltageof2V,whereaswhenapplyingavoltageof5Vacrosstheinterface,thedeviceresistanceis~250MΩ,indicatingthecurrentisdominatedbythetunnelingcurrentofCeYIG/YIGinsulatinglayers.TheI-VcurveofthedeviceisshowninFig.2(b).Thevoltagewasfirstlyappliedfrom0Vto+5V,thendecreasedfrom+5Vto0V,followedbyrampingfrom0Vto-5Vandthenreturningfrom-5Vto0V.I-Vcurveisnotsymmetrictotheorigin,which130suggestsatleastonenon-ohmiccontactexistsinthedevice.Theunipolarresistiveswitchingshouldnotbecausedbytheheatgeneratedbyleakagecurrentbecausetheinsulator’sresistivityshoulddecreasewiththerisingofthetemperature.TheI-Vhysteresisloopislikelytoindicatetheexistenceofoxygenion’smigrationthatcanchangetheresistanceofthedevice.What’smore,thereisnorelationshipbetweenpolarityofvoltagebiasandHIGH/LOWresistancestates.The135relationsofthecapacitanceofthedeviceversusDCvoltagebiasisshowninFig.2(c).It’smeasuredbyaddinga30mVAC(1kHz)signalondifferentDCvoltagesignalssweepingfrom-1Vto1Vandreturningto-1V.Thecapacitanceofthedeviceisaround220pF.Fig.2Electricalpropertiesofthedevice.(a)I-VcurveofLSCOpad;(b)I-VcurveofLSCO140pad/CeYIG/YIG/Si;(c)C-VcurveofLSCOpad/CeYIG/YIG/Si;(d)Time-resolvedresistanceofthedeviceunderdifferentDCvoltagebias.Time-resolvedresistance(R-t)wasmonitoredduringtheprocessofelectricfieldgatingofthedeviceasshowninFig.2(d).WhenVg=+1V,theresistanceincreasedfrom0.205GΩto2.39GΩin10sandsubsequentlyitgraduallyconvergedtothestablestate(~2.75GΩ)at100s.When145Vg=+2V,theresistanceincreasedfrom0.0170GΩto0.0618GΩin5sandconvergedto0.0679GΩat100s.Withtheenhancementofgatevoltage(Vg),thetimespentreachingastableresistancestatebecameshorter.Nomatterwhatpolarityofvoltagewas,theresistancebothincreasedwiththetime.However,theresistancevaluescorrespondingthevoltagewiththesameabsolutevaluebutthereversepolaritywerenotequal,whichisconsistentwiththeIn2O3/YSZ-4- 中国科技论文在线http://www.paper.edu.cn[7]150systemandattributedtooxygenionmigrationunderelectricfields.Basedonionhoppingmodel,[25]themobilityofoxygenionincreaseswithenhancementofelectricfield.Therefore,itis2-speculatedthatahigherelectricfieldleadstoalargermigrationvelocityofOandashortertimetoreachthestablestate.Resistancereflectingthecharacteristicsofchargecarrierstransportislikelytobeinfluencedbyoxygenionmigrationthatmightenhancethescatteringsufferedfrom155electrons.Therefore,thechangeofresistanceinR-tcurvecanprovidesomeevidencesforoxygenionmigration.Fig.3Electricfieldcontrolofinterfacialreflectance.(a)Thehysteresisloopsofphotocurrentsunderdifferentvoltagebiases.(b)Thedependenceofsaturationphotocurrentsongatevoltages.(c)Themagneto-opticalKerr160rotationcurvesunderdifferentvoltagebiases.(d)Thedependenceofsaturationmagneto-opticalKerrrotationanglesongatevoltages.Fig.3showsthereflectivityandLMOKEcharacterizationofthedevice.TheintensityoflightreflectedfromtheLSCOpadwasin-situmonitoredwiththeelectricfieldapplyingonthedevice.Toavoidscatteringcausedbymicroprobes,thelaserspotwasfocusedfarawayfrom165microprobesasshowninFig.1(b).Fig.3(a)showstheLMOKEhysteresisloopswithdifferentappliedvoltages.Interestingly,thewholeloopmovedupwardsafterVg=-5Vapplyingonthedevice.Onthecontrary,theloopmoveddownwardsafterVg=+5Vwasapplied.Themovementoftheloopisrelatedtothepolarityofappliedvoltage.Therefore,theheatingeffectcausedbytheleakagecurrentshouldn’tbethefactorthatresultsintheaforementionedphenomenon.The170intensityofreflectedlightcanbequantitativelydescribedbyphotocurrent(I)collectedbyaphotodiode.ThesaturationphotocurrentmeasuredatH=-300Oe(I-sat.)correspondingtoVg=-5Vwas9.545nAandtoVg=+5V,I+sat=9.534nA.Thus,thereflectanceofthedevicecanbemodulatedby0.12%byreversingthepolarityof5Vvoltage.TheintensityofthereflectedlightasafunctionofgatevoltageisshowninFig.3(b).Althoughtherewasanaturalupwardsdriftof175loops,whichwaspossiblycausedbydriftingoftheopticaltestsetupduringthelongtimetest,thetwocontinuousmeasurementsweresoquickthattheslowdriftdidn’tsurpassthemovingamplitudeofloopscausedbytheappliedvoltage.TheregularfluctuationsofI-sat.correspondingtodifferentpolarvoltagesappliedcontinuouslycanbeobviouslyobservedfromFig.3(b).Asweswitchthevoltagepolarity,thereflectivityshowsclearcyclingbehaviorontopofaslowdrifting-5- 中国科技论文在线http://www.paper.edu.cn180ofthecurve.Moreover,theamplitudeofthereflectivitymodulationincreaseswithvoltageamplitudes,indicatingafield-drivenreflectivitychangeinthestructure.TheLMOKErotationangle(θK)isshowninFig.3(c),whichiscalculatedfromthephotocurrentbyusingtheformula:θK=σ·(I-sat-I+sat)/2I0,whereσistherotationangleofthepolarizer(inthemeasurement,σ=2°);I-satisthesaturationphotocurrentmeasuredatH=-300Oe;185I+sat.isthesaturationphotocurrentmeasuredatH=+300Oe;I0istheaveragevalueofthephotocurrentsmeasuredatH=0Oe.Althoughreflectivitychangeswithappliedelectricfield,thechangeoftheKerrrotationwastoosmalltobedetected,whichisbeyondtheaccuracylimitofourmagneto-opticalKerreffectmeasurementsystem.ThesaturationθKmeasuredunderdifferentgatevoltagesisshowninFig.3(d)thatsuggeststhereisnoobviouscorrelationsbetweenθKandVg.190WeconsiderthemechanismofobservedreflectivitymodulationbyelectricfieldisoxygenionmigrationacrossLSCOandCeYIGinterface.TheproposedmechanismisshowninFig.4.WhenVg=0V,theoxygenvacanciesweredistributedinLSCOfilmsatrandomsitesasshowninFig.4(a).WhenVgwassettoapositivebias,anelectricfieldwasappliedinCeYIG/YIGfilms.Duetothecontinuityofelectricdisplacementvector,theelectricfieldcanalsobegeneratedina195thinregioninLSCOneartheLSCO/CeYIGinterface.UnderthedrivingofelectricfieldthatpointstoLSCO/CeYIGinterface,thepositivelychargedoxygenvacancieswouldmigratetowardstheinterface,whichcanmodulatetheopticalconstantsandreflectivityofLSCOandCeYIGasisshowninFig.4(b).Conversely,whenVgwassettoanegativebias,anelectricfieldthatpointedawayfromLSCO/CeYIGinterfacecanbegeneratedinathinregioninLSCO.Theelectricfield200candrivetherandomlydistributedoxygenvacanciestomigrateawayfromtheinterface.Consequently,theopticalconstantsofLSCOandCeYIGaremodulatedreversely,whichenhancesthereflectanceofthedevice.Sofar,theoxygenionmigrationeffectisstilltooweaktocauseobviousmodulationonthemagneto-opticalpropertiesofCeYIG,consideringthereflectivitymodulationamplitudeisonly0.12%.Optimizationoftheoxygenstoichiometry,205CeYIGandYIGthicknessanddevicestructuretopromotestrongerfieldmodulationeffectsiscurrentlyunderinvestigation.Fig.4Mechanismexplainingelectricalcontrolofreflectancebasedonoxygenionmigration.(a)Randomly2-2-2-distributedOwhenVg=0V;(b)OdepletedneartheLSCO/CeYIGinterfacewhenVg>0V;(c)Oaccumulated210neartheLSCO/CeYIGinterfacewhenVg<0V.3ConclusionAprototypedevicewasfabricatedforelectricfieldcontrolofmaterials’opticalpropertiesbasedonelectrochemicalgatingatoxygenlevel.Theelectricalproperties,magneto-opticaleffectanddependenceofinterfacialreflectanceongatevoltageofthedeviceweresystematically215investigated.TheinterfacialreflectanceofLSCO/CeYIGcanbemodulatedby0.12%atroomtemperatureviareversingthepolarityofa5Vappliedvoltage.Anoxygenionmigrationmodelhasbeenproposedtoexplainthechangeofthereflectance.Ourworkprovidesanewwayto-6- 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中国科技论文在线http://www.paper.edu.cn280电调控La0.5Sr0.5CoO3/CeY2Fe5O12氧化物异质结的反射性能1,2,31,2,3朱银龙，毕磊（1.国家电磁辐射控制材料工程技术研究中心,成都610054；2.电子薄膜与集成器件国家重点实验室,成都610054；2853.电子科技大学微电子与固体电子学院,成都610054）摘要：引入可被外界操控的电场自由度实现材料性质的调控有助于拓展电子器件的功能与应用。本文从实验角度研究了La0.5Sr0.5CoO3/CeY2Fe5O12氧化物异质结的电学性能，磁光效应与可电调控的反射性能。在室温下施加±5V电压于异质结，其反射率能被可逆调制达0.12%。同时，器件电阻与电容均被可逆的调控。位于La0.5Sr0.5CoO3/CeY2Fe5O12界面附近的氧离子290迁移被认为是电场控制反射率的机制。此工作基于氧离子级别的电化学栅控方法，实现了磁光器件光学性质的电调控，为设计可重构的集成光学器件提供了一种新的可能性。关键词：电控反射；电化学栅控；钇铁石榴石薄膜；集成光学器件中图分类号：O469-8-'