PLD脉冲激光沉积简介

英文原文：CHAPTER1PulsedLaserDepositionofComplexMaterials:ProgressTowardsApplicationsDAVIDP.NORTONUniversityofFlorida,DepartmentofMaterialsScienceandEngineering,Gainesville,Florida1.1INTRODUCTIONInexperimentalscience,itisararethingforanewlydiscovered(orrediscovered)synthesistechniquetoimmediatelydeliverbothenhancedperformanceandsimplicityinuseinafieldofacceleratinginterest.Nevertheless,suchwasthecasewiththerediscoveryofpulsedlaserdeposition(PLD)inthelate1980s.Theuseofapulsedlaserasadirectedenergysourceforevaporativefilmgrowthhasbeenexploredsincethediscoveryoflasers[HassandRamsey,1969;SmithandTurner,1965].Initialactivitieswerelimitedinscopeandinvolvedbothcontinuous-wave(cw)andpulsedlasers.Thefirstexperimentsinpulsedlaserdepositionwerecarriedoutinthe1960s;limitedeffortscontinuedintothe1970sand1980s.Then,inthelate1980s,pulsedlaserdepositionwaspopularizedasafastandreproducibleoxidefilmgrowthtechniquethroughitssuccessingrowinginsituepitaxialhigh-temperaturesuperconductingfilms[Inametal.,1988].Thechallengesforinsitugrowthofhigh-temperaturesuperconductingoxidethinfilmswereobvious.Thecompoundsrequiredmultiplecationswithdiverseevaporativepropertiesthathadtobedeliveredinthecorrectstoichiometryinordertorealizeasuperconductingfilm.Simultaneously,thematerialwasanoxide,requiringanoxidizingambientduringgrowth.Pulsedlaserdepositionhadseveralcharacteristicsthatmadeitremarkablycompetitiveinthecomplexoxidethin-filmresearcharenaascomparedtootherfilmgrowthtechniques.Theseprincipleattractivefeatureswerestoichiometrictransfer,excitedoxidizingspecies,andsimplicityininitialsetupandintheinvestigationofarbitratryoxidecompounds.Onecouldrapidlyinvestigatethin-filmdepositionofnearlyanyoxidecompoundregardlessofthecomplexityofthecrystalchemistry.Significantdevelopmentofpulsedlaserdepositionhascontinuedandoverthepast15years,PLDhasevolvedfromanacademiccuriousityintoabroadlyapplicabletechniqueforthin-filmdepositionresearch[Saenger,1993;Kaczmarek,1997;WillmottandHuber,2000;Dubowski,1988;Dielemanetal.,1992].Today,PLDisusedinthedepositionofinsulators,semiconductors,metals,polymers,andevenbiologicalmaterials.Fewmaterialsynthesistechniqueshaveenjoyedsuchrapidandwidespreadpenetrationintoresearchandapplicationvenues.PulsedLaserDepositionofThinFilms:Applications-LedGrowthofFunctionalMaterialsEditedbyRobertEasonCopyright#2007JohnWiley&Sons,Inc.34PULSEDLASERDEPOSITIONOFCOMPLEXMATERIALS1.2WHATISPLD?Theapplicabilityandacceptanceofpulsedlaserdepositioninthin-filmresearchrestslargelyinitssimplicityinimplementation.Pulsedlaserdepositionisaphysicalvapordepositionprocess,carriedoutinavacuumsystem,thatsharessomeprocesscharacteristicscommonwithmolecularbeamepitaxyandsomewithsputterdeposition.InPLD,shownschematicallyinFigure1.1,apulsedlaserisfocusedontoatargetofthematerialtobedeposited.Forsufficientlyhighlaserenergydensity,eachlaserpulsevaporizesorablatesasmallamountofthematerialcreatingaplasmaplume.Theablatedmaterialisejectedfromthetargetinahighlyforward-directedplume.Theablationplumeprovidesthematerialfluxforfilmgrowth.Formulticomponentinorganics,PLDhasprovenremarkablyeffectiveatyieldingepitaxialfilms.Inthiscase,ablationconditionsarechosensuchthattheablationplumeconsistsprimarilyofatomic,diatomic,andotherlow-massspecies.Thisistypicallyachievedbyselectinganultraviolet(UV)laserwavelengthandnanosecondpulsewidththatisstronglyabsorbedbyasmallvolumeofthetargetmaterial.Laserabsorptionbytheejectedmaterialcreatesaplasma.Forthedepositionofmacromolecularorganicmaterials,conditionscanbechosenwherebyabsorptionisoveralargervolumewithlittlelaserabsorptionintheplume.Thispermitsalargefractionofthemolecularmaterialtobeablatedintact.Forpolymericmaterials,transferofintactpolymerchainshasbeendemonstrated.Foreven‘‘softer’’materialsinwhichthedirectabsorptionbythelaserwouldbedestructivetomolecularfunctionality,theformationofcompositeablationtargetsconsistingofthesoftcomponentembeddedinanopticallyabsorbingmatrixhasbeeninvestigated(see,e.g.,Chapter3).SeveralfeaturesmakePLDparticularlyattractiveforcomplexmaterialfilmgrowth.Theseincludestoichiometrictransferofmaterialfromthetarget,generationofenergeticspecies,hyperthermalreactionbetweentheablatedcationsandthebackgroundgasintheablationplasma,andcompatibilitywithbackgroundpressuresrangingfromultrahighvacuum(UHV)to1Torr.MulticationfilmscanbedepositedwithPLDusingsingle,stoichiometrictargetsofthematerialofinterest,orwithmultipletargetsforeachelement.WithPLD,thethicknessdistributionfromaFigure1.1SchematicofthePLDprocess.WHATISPLD?55stationaryplumeisquitenonuniformduetothehighlyforward-directednatureoftheablationplume.Tofirstorder,thedistributionofmaterialdepositedfromtheablationplumeissymmetricwithrespecttothetargetsurfacenormalandcanbedescribedintermsofacosnðyÞdistribution,wherencanvaryfrom～4–30.However,rasterscanningoftheablationbeamoverthetargetand/orrotatingthesubstratecanproduceuniformfilmcoverageoverlargeareas,andthistopiciscoveredinChapter9.OneofthemostimportantandenablingcharacteristicsinPLDistheabilitytorealizestoichiometrictransferofabla