从最合适的方向合成金属薄膜.docx

从最合适的方向合成金属薄膜.docx

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从最合适的方向合成金属薄膜

SynthesisofFerroelectricThinFilmswithPreferredOrientation

INTRODUCTION

Thinfilmprocessingisquiteimportantforthedevelopmentofdeviceminiaturization,hybridizationandlowerworkingvoltage.Severalpropertiesoffunctionalmaterialsarerequiredforapplicationsatasub-micronlevelorlessthanthat.Thinfilmprocessingtechniquesalsohavebeenreceivinggreatattentionforapplicationsinsemiconductormemories,opto-electronicdevices,electroniccomponents,displaydevices,magneticdevices,sensorsandemergingarea.Thelowtemperaturethinfilmprocessingalsorequirestheprecisecontrolofchemicalcomposition,thedesireddirectionofcrystalgrowth,andthehighcrystallinity.Inthisarea,epitaxialsingle-crystalthinfilmswithepitaxyareusuallyexpected.Amongseveralfunctionalmaterialthinfilms,ferroelectricthinfilmswithpreferredorientationhavebeenmainlystudiedtoimproveferroelectricpropertiesandsatisfytherequirementsfordeviceapplicationsuptonow.Inaddition,highlyorientedelectro-conductiveincludingsuperconductivethinfilmshavealsobeeninvestigatedtoachievehighelectricalconductivityandtouseasabufferlayerofdielectricthinfilmswithpreferredorientation.Inotherexamples,thereareseveralmagneticandopticalmaterialsthinfilmswithpreferredorientation,etc.Recently,withtheprogressofthinfilmtechnology,intensiveeffortshavebeenfocusedonthepreparationofepitaxialthinfilmsmainlyonsinglecrystalsubstratesformakingclearthepropertiesofthinfilmswithseveralfunctionalities.

Varioustechniquesavailabletodayforthefabricationofthinfilmsarenoticeablymorevariedintypeandinsophisticationthancouplesofdecadesago.Betterequipmentandmoreadvancedtechniqueshas,undoubtedly,ledtohigherqualityfilms,andindeed,maybeaprimaryfactorinthenowroutineachievementofdesiredfunctionalitiesinthinfilms（50nmorgreater）preparedbyaselectionofdifferentmethods（Lee,1971）.

Thechemicalsolutiondeposition（CSD）,includingsol–gel,processisoneofthemostcommonprocessesasthemethodofthinfilmfabrication.Thisprocesscanbewidelyappliedforoptical,electrical,magnetical,mechanical,catalysis,etc.Themostimportantadvantagesofchemicalsolutionprocessarehighpurity,goodhomogeneity,lowerprocessingtemperature,precisecompositioncontrolforthepreparationofmulticomponentcompounds,versatileshapingandpreparingbysimpleandcheapapparatuscomparedwithothermethod.However,thelargerthenumberofelements,themorecomplicatedthesolutionchemistry,leadingtodifficultiesinachievingthedesiredcrystallinephases.Therefore,itisrequiredtodesignthemetal-alkoxideprecursorsthroughcontrollingthemetal–oxygen–carbonbondsincomponentsubstancesandtoinvestigatethesolutionofmulticomponentsystemindetail.Also,thecrystallizationbehavioriscomplicated,sotheinvestigationofcrystallizationprocessisakeyforfilmsynthesis.Thefilmsthatareusuallyproducedbychemicalsolutionprocessareofpolycrystallinenature,however,inmanyinstancesitisdesirabletoproduceepitaxialfilmgrowth.Theas-depositedgelfilmsontheproperlyselectedsubstratesundergotheatomicrearrangementduringcalcinationandcrystallizationyieldingepitaxialfilms.

Inthischapter,therefore,wedealswithvariousprocessingfactorsforpreparingthinfilmswithpreferredorientation,suchas（l）thecontrolofthestructureofmetal-organicprecursorswithconsideringthestoichiometryinsolutionforcoating,

（2）theselectionofthesubstratesforthinfilmfabrication,（3）theintermediatebufferlayers（includingaelectrodelayer）betweenthefilmandthesubstrate,（4）theheating（calcinationandcrystallization）conditionsforLiNbO3（LN）,K（Ta,Nb）O3（KTN）andtungstenbronze（Sr,Ba）Nb2O6（SBN）,whichareferroelectricsubstancesascasestudies.EpitaxialLN,KTNandtungstenbronzeSBN-basedthinfilmsaresuccessfullysynthesizedonsapphireC,MgO（100）andPt（100）/MgO（100）,etc.substrates.

CASESTUDY1:

PREPARATIONOFHIGHLYORIENTEDLiNbO3THINFILMS

LiNbO3FilmswithPreferredOrientation

Lithiumniobate（LiNbO3）hasaillumenitestructure,andhasvariousattractiveproperties,suchasacoustic,piezoelectric,pyroelectric,acoustoopticandelectroopticproperties.LiNbO3singlecrystalshavegenerallybeengrownmainlybytheCzochralskimethod.However,aLiNbO3singlecrystalhasbeengrownfromanonstoichometricmeltofharmoniccomposition（Li2O/Nb2O5=48/52）bytheCzochralskimethod.Recently,thedevelopmentofachemicalsolutionprocessingrouteforepitaxialstoichiometricLiNbO3thinfilmswithhighqualityonappropriatesubstrateshasbeenreceivinggreatattentions,andstronglyrequiredforminiaturizingandintegratingelectricalandopticalthinfilmdevices（Weis,1985;Hirano,1988a,1988b,1988c,1989,1991,1992a,1993;Nashimoto,1995）.ThestoichiometricLiNbO3filmscrystallizedonSisubstratesareusuallypolycrystalline,whilethefilmsonsapphiresubstratesarefoundtoshowpreferredorientationsdependinguponcrystallographicplanesofsapphire.Onlythe110,012and006reflectionsofLiNbO3wereobservedonsapphireR,AandCsubstrates,respectively（Hirano,1989,1991,1992a;Nashimoto,1996）.Also,themicro-patterningoforientedLiNbO3filmbythemodificationofligandsoftheLiNbO3precursorcombinedwithUVirradiationhavebeendemonstrated（Yogo,1995a）.

Thissectionfocusesontheorientationcontrolofalkoxy-derivedLiNbO3thinfilmsbycontrollingthestructure（doingthemoleculardesign）ofmetal-organicprecursorsinsolution.TheeffectofmodificationofLi[Nb（OEt）6]doublealkoxideisinvestigatedforthesynthesisofepitaxialLiNbO3thinfilms.LiNbO3filmsofexcellentpreferredorientationaresuccessfullysynthesizedonsubstratesfromthemodifiedLiNbO3precursors.

EffectsofModificationofLigandsofPrecursorsonthePreparationofEpitaxiallyGrownLiNbO3Films

PBD-ModifiedLiNbO3Precursor.Non-modifiedLiNbO3（STD-LiNbO3）precursorsolutionsarepreparedfromLiOEtandNb（OEt）5inethanol（Hirano,2002）.ThestructureoftheprecursorwasconfirmedtoconsistofacomplexalkoxideLi[Nb（OEt）6],whichwasreportedbyEichorstetal.（1990）indetail（Eichorstetal.,1990）.Ontheotherhand,threetypesofβ-diketonecompoundsareselectedbaseduponthenumberofbenzenerings（phenylgroups）initsstructure.Inthiscase,thecoordinationofl-phenyl-1,3-butanedione（PBD）totheLiNbO3precursorisconfirmedbyUVspectra.Thesimilarcoordinationisalsorealizedin2,4-pentanedione（PD）-and1,3-diphenyl-1,3-propanedione（DPPD）-modifiedLiNbO3precursors,whichwasconfirmedbyUV-spectra.Theproposedstructuresoftheseβ-diketonemodifiedLiNbO3precursorsareshowninFigure17-1.

SapphireCsubstratesareselectedinordertofabricatehighlyorientedLiNbO3thinfilms,becausethec-planeofLiNbO3hasthegoodcrystallographicmatchingwithcplaneofα-Al2O3.Figure17-2showstheXRDprofileoftheLiNbO3thinfilmfromthePBD-modifiedprecursoronasapphireCsubstratecrystallizedat550°C.Thisfilmshowsaremarkablec-axispreferredorientation.The006reflectionofLiNbO3appearsataslowas400°C,andincreasesinintensitywithincreasingheattreatmenttemperature.However,theXRDevaluationisnotproperforthejudgementofthedegreeofpreciseorientations.Therefore,furtherinvestigationisrequiredtoexaminethecrystallographicrelationbetweenfilmsandsubstratesasmentionedbelow.

Figure17-1.SupposedstructuresoftheSTD-LiNbO3andtheβ-diketonemodifiedLiNbO3（1equivof2,4-pentanedion（PD）,1-phenyl-1,3-butanedion（PBD）and1,3-diphenyl-1,3-propanedion（DPPD））precursors.

Figure17-2.XRDprofileoftheLiNbO3thinfilmpreparedonasapphireCsubstratesfromthe1equiv.PBDmodifiedLiNbO3precursorsolutionat550°C.

Figure17-3.（a）X-raypolefiguresoftheLiNbO3thinfilmspreparedonsapphireCsubstratesfromSTD-LiNbO3,（b）1equiv.PBDmodified-LiNbO3precursorsolutionsandheat-treatedat550°C[2θ=23.7°,for（012）].

X-raypolefiguremeasurementiscommonlyusedtostudythecrystallographicalignmentoforientedthinfilmsonsubstrates.Figures17-3（a）and（b）show（012）X-raypolefiguresoftheLiNbO3filmspreparedonsapphireCsubstratesfromSTD–LiNbO3and1equiv.PBDmodifiedLiNbO3precursorsolutions,respectively（Hirano,2002）.Thepolefiguresareconstructedfor{012}planes.Thetermβistherotationaxisperpendiculartothefilmplane,andaistherotationaxisperpendiculartoβand𝜃.TheX-raypolefiguresof（012）planefortheLiNbO3singlecrystalsandsapphireCshowthreespotsatevery120°alongβ.However,thepolefigureshowninFigure17-3（a）exhibitsadditionalspotsthanthatofLiNbO3singlecrystalwhennon-modifiedSTD-LiNbO3precursorsolutionsareused.SixspotsshowninFigure17-3（a）arecomposedoftwogroupsofrotatedthreespots,onegroupwithastrongerintensityandtheothergroupwithaweakerintensity.Thisratiosuggeststhatthestructuresappearatnucleationandgrewthroughcrystallization.SinceLiNbO3singlecrystalshavetriangularoxygenplanesthatarerotatedby60°aroundthec-axis,themisorientationshouldbeformedeasilyat60°alongthec-axisduringnucleationandgrowth.ThesimilartwinstructureofLiTaO3filmsonsapphiresubstrateisreportedtoformbypulsedlaserdeposition（Agostinelli,1993）.Ontheotherhand,thepolefigureoftheLiNbO3filmpreparedonsapphireCfromPBD-modifiedLiNbO3precursorsolution（Fig.17-3（b））isdifferentfromthatshowninFigure17-3（a）.Thefigureshowstheconcentratedspotswithathree-foldsymmetry,