纳米材料和纳米结构

ChemicalVaporDeposition

化学气相沉积纳米材料和纳米结构第六讲1IntroductionInchemicalvapordeposition(CVD),thevaporizedprecursorsareintroducedintoaCVDreactor,wheretheprecursormoleculesadsorbontoasubstrateheldatanelevatedtemperature.Theseadsorbedmoleculeswillbeeitherthermallydecomposedorreactedwithothergases/vaporstoformasolidfilmonthesubstrate.Suchagas-solidchemicalreactionatthesurfaceofasubstrateiscalledtheheterogeneousreaction〔多相反响〕.ABriefDescriptionAprocesswithpotentiallygreatcomplexityAprocessofgreatversatilityandflexibilityMetals,semiconductorsandceramicsAmorphous,polycrystallineorsinglecrystalline

Physicalproperties,

dependingupongrowthconditionsTechniqueFeaturesContentsinTheChapterPrinciplesofCVDmasstransportationreactionkineticsnucleationandgrowthCVDandmodifiedCVDsystemsExamplesinsynthesizingnanostructuredmaterials2

PrinciplesofChemicalVaporDepositionThermodynamicsisessentialtounderstandtheCVDprocessandtheunderlyingscienceforthekeyfactorsincludingvaportransport,reactionkinetics,nucleationandgrowthofdepositedmaterials.Formostofthesystems,CVDrequireshightemperatureandlowpressure.Undertheseconditions,achemicalsystemwillrapidlyfalltotheminimumGibbsfreeenergyandleadtotheformationofsolidreactionproducts.ThermodynamicsinCVDProcess

Masstransportofreactantstothegrowthsurfacethroughaboundarylayerbydiffusion;

Chemicalreactionsonthegrowthsurface,incorporatingthenewmaterialintothegrowthfront;

Removalofthegas-phasereactionby-productsfromthegrowthsurface.ThreeStepsConsistingtheCVDProcess反响物化学反响气态副产物边界层化学气相沉积过程的扩散模型〔1〕反响物输运通过边界层；〔2〕外表反响形成固态沉积物；〔3〕去处气相反响副产物.衬底Step1andStep3aredependent,bothofthemaffectthechemicalreactionrateandarecoupledbythestoichiometryofthereactionStep2isexceedinglycomplex,involvingsurfaceand/orgaseousreaction,simultaneouschemical/physicaladsorption-desorption,andnucleationprocessBoundarylayer:aspaceabovethesubstrateresistingdiffusion,varyingwithtime,thedistanceinthehorizontalreactoretc.KeyPointsDescriptiononCVDProcessThegrowthrateisdeterminedbythesloweststep

KineticcontrolofCVDprocess:occurredwhenmass-transfercoefficientisverylargecomparedtotheproductofthekineticrateconstantandthesystempressure

DiffusioncontrolCVDprocess:occurredwhentheoppositeistrue

Temperatureeffectongrowthrate

KeyPointsDescriptiononCVDProcess(2)CVD过程中温度对薄膜生长速度的影响直接沉淀扩散控制反响控制

Lowtemperature:limitedbystronglytemperature-dependentrateofheterogeneousnucleation,andbyadsorptionandkineticeffect

Highertemperature:limitedbyinterdiffusionofgaseousreactantsandproductsthroughtheboundarylayer

Stillhighertemperature:homogeneousnucleationresultinginprecipitationofsolidparticlesoccurringThreemajorgrowthmodes超晶格层状生长层+岛状生长岛状生长超晶格3ExperimentalApproachHowtodesignaCVDsystem?Geometry,Shape,CompositionofthesubstrateThetypeofthedepositionprocessusedThenatureofthedepositedmaterialsEconomicfactorsHowtointroduceprecursorsintoreactor?Solidorliquidprecursors:usingcarriergaspassingthroughabubble-bottlevaporizer〔bubbler，起泡器，扩散器〕，eitherdigitaloranalogmassflowcontrollerandpressure/massflowcontrollerarewidelyusedGaseousprecursors:directlyintroducedHeatingmethodadoptedInductionheating〔感应加热〕(cold-wallreactor)Internalresistanceheating〔内置电阻加热〕(cold-wallreactor)High-intensityvisibleorinfraredradiationheating〔高强度可见/红外光辐照加热〕(cold-wallreactor)Externalfurnaceheating〔外置炉加热〕(hot-wallreactor)3.1ChemicalVaporDeposition(CVD)Low-pressureCVD(LP-CVD)istypicallyusedtoreduceanygas-phasenucleation,andthustoproduceasolidfilmonasubstratewithoutincorporationofundesiredparticles.HorizontalMode:thereactorishorizontallyplaced,andthegasflowisparalleltothesurfaceofthesubstrateVerticalMode:thereactorisverticallyplaced,andthegasflowisverticaltothesurfaceofthesubstrateCVDforPreparingThinFilm(LP-CVD)

MetalorganicCVD(MOCVD):usingmatalorganicprecursorstoreducethegrowthtemperaturesandachievehigherfilmquality

Plasma-enhancedCVD(PECVD):usingplasmaasenergeticsource

Photo-CVD:usingultravioletasenergeticsourceNewlyDevelopedCVDTechniquesGas-phasenucleationandcontrolledgrowthoftheparticlesareofprimeconcernTheparticlesizeiscontrolledbythenumberofnucleiandconcentrationThroughincreasingthetemperatureandtotalpressureanddecreasingthetotalflowratetorealizeparticlegrowth

CVDforPreparingNanoparticles3.2ChemicalVaporCondensation(CVC)CVCisamodifiedCVDprocess,itsprincipleisalsobasedonthegas-phasenucleation(homogeneousnucleation均质成核).Metalorganicprecursorsaretypicallyusedbecauseoftheircommercialavailabilityandlowpyrolysistemperature.

典型CVC设备原理图载气质流计先驱物源针阀真空室加热炉纳米颗粒衬底控制阀抽气泵刮刀3.3Particle-Precipitation-AidedCVD(PP-CVD)InPP-CVD,anaerosol〔气悬体〕isformedatanelevatedtemperature,andparticlesareprecipitatedonasubstratebyintroducinganexternalforceforparticledeposition,suchasthermophoresis〔热迁移〕,electrophoresis〔电迁移〕,orforcedflow〔强制流〕.Thelooseparticledepositcanbesinteredtoformaporouslayeroraheterogeneousreaction〔复相反响〕occurssimultaneouslyontheparticlestointerconnecttheindividualparticles.PP-CVDcanbeusedtomakedensetoporousfilmswithnanosizedparticles,andthushasgreatpotentialinavarietyofapplicationssuchascatalystsupport,ceramicmembrane,orporouselectrode.Themicrostructureofthefinalstructureiscontrolledbyacombinationofparticledeposition,sintering,andheterogeneousreaction.ThreestepsinPP-CVDprocess:ParticleformationParticledepositionParticleinterconnectionorsinteringImprovementoftheuniformity:introducingtwoseparatereactionroute,oneforpowderformationandtheotherforheterogeneousreaction颗粒的形成颗粒沉积复相反响致密化烧结时间PP-CVD的三步过程：颗粒形成、颗粒沉积和颗粒的互相连接或烧结典型的PP-CVD原理图NH3,N2CCl4,H2喷嘴炉子3区1区2区喷嘴反响器冷却废气衬底喷嘴冷却喷嘴3.4CatalyticCVD(CCVD)InCCVD,transitionmetalparticles(Fe,Co,andNi)aretypicallyusedasthecatalystsforcatalyticdepositionofahydrocarbontoproducecarbonnanotubesandcarbonfibresThefirststepofCCVDprocessisdecompositionofhydrocarbonmoleculesonthesurfaceofcatalystparticles,followedbycarbondiffusionthroughthecatalystparticles.CarbonnanotubesthengrowbyprecipitationattheannularportionofthecatalystsurfaceThedrivingforceforcarbondiffusionisthetemperaturegradientcreatedintheparticlebytheexothermicdecompositionofthehydrocarbonattheexposedfrontfacesandendothermicdepositionofcarbonattherearsurface,whichisinitiallyincontactwiththesupportsurface.Twotypicalgrowthmodes:Tipgrowthmode:thecatalystparticlesweaklyadheretothesubstrate,theyremainonthetipsofthegrownnanotubesandresultinanopenendBasegrowthmode:thecatalystparticlesstronglybondtothesubstrate,theyremainattheinterfacesbetweensubstrateandthegrownnanotubesandresultinaclosedend顶生长模型底生长模型ExamplesofNanostructuredMaterials4.1SemiconductorQuantumDots(QDs)Two-orthree-dimensionquantum-confinedsemiconductorstructureshaveattractedattentionbecauseoftheirinterestingphysicalpropertiesandpotentialdeviceapplicationsAwidelyusedapproachistoexploitself-orderingprocessingonplanarsurfaces,asforstrain-inducedS-K(layerplusisland)growthofquantumdots(QDs),forovercominglimitationsinsizeandinterfacequalityoftraditionallithographyIssueofinterestinthegrowingfieldofQDnanostructuresincludetheabilitytotuneislandsizeandtheirsurfacedensities.Thisinturnmeanscontrolofthephysicalpropertiesofthenanosystems,suchasluminescenceThegrowthofIII-VQDstructuresandgrowthparameterswhichaffectstheQDssizeandsurfaceconcentrationwilldescribedbychoosingInGaAsasanexampleTechniqueAdopted:low-pressureMOCVD,15Torr(1Torr=133.3224Pa)Precursors:(CH3)3Ga,(CH3)3In,AsH3TemperatureRange:425-650°CFlowCondition:laminarSubstrate:GaAsandAlGaAssinglecrystalliteNominalLayerThickness:5monolayers(ML)GrowthRate:~0.03-0.05MLpersecondGeneralPreparationConditionofInGaAsQDsIslandaveragediameters,densities,andaverageheightsincreasesignificantlywithgrowthtemperature(30–130nm)LargediffusionlengthsactivatedthermallyathighergrowthtemperatureswillbemoreprobableforislandgrowththanfornewislandnucleationIncrementofcoarseningandcoalescencefrommobileclusterswillresultinthedeviationfromlinearityofdiameter-temperaturecurveTheformationofQDsisdrivenbyenergetics,whilethealignmentofQDs

isgovernedbykineticsTemperatureDependenceGaAs衬底上生长的InGaAs纳米岛的平均粒径对生长温度的依赖关系(衬底错切角依次为0.05、0.5和2°)ChangetheAsH3partialpressurecanproducelargevariationsindensityforequalislandsizeTheislandcoverageislowforthelowestAsH3partialpressure,risestoitsmaximumvalue,anddecreaseagaintoaverylowvalueathighAsH3partialpressureVariationwithAsH3PartialPressureInGaAs量子点面密度随AsH3分压的变化。〔a〕2.81010/cm2,110-6;(b)2.8109/cm2,210-5Themiscutangleθm

ofthesubstratecanbeusedtotuneself-organizedQDsAnincreaseθmwilldecreaseQDdiametersandincreasethesurfaceQDconcentrationsIslandscannucleatehomogeneouslyonterracesorheterogeneouslyonstepedges,thusstepedgenucleationenablesformationofsmallercriticalnucleiforsurfaceswithagreatersurfacedensityofsteps,resultinginsmalleraveragediametersandhigherislanddensitiesfornucleationonsteppedsurfacesQDsgrowselectivelyonmultiatomicstepedges,resultinginspontaneousalignmentofQDsVariationwithSurfaceStructure550°C生长的InGaAs量子点的直径、面密度与衬底错切角的关系曲线InGaAs量子点在外表台阶边缘的规那么排布〔a〕2°,570°C；〔b〕2.5°，550°C4.2CeramicNanostructuredMaterialsUsingasCatalysts:Uniformlydepositingthewelldispersed,nanosizedcatalyticparticlessuchasmetalormetaloxideonasupportingmatrixExhibitinghighercatalyticstability,betterandnovelcatalyticperformanceUsingasSensorMaterials:GreatlyenhancingthesensitivityandselectivityduetothenanosizeandquantumeffectBasedmainlyonhomogeneousnucleationandgrowth(gas-phasereaction)DopingatMolecularlevelDopantsareroutinelyaddedtocontroltheparticlephase,composition,morphologyandpowdersize,aswellasthephysicalpropertiesOxides1200°C制备的莫莱石粉末的颗粒尺寸分布Carbides(choosingSiCasanexample)andcarbide-basedcompositesarewidelyusedinstructural,thermal,andelectricalapplications.Useofnanocrystallinecarbidesandnanocompositescansignificantlyimprovetheirproperties.PreparingConditionsTypicallyinahot-wallreactortocarryoutCVDprocessTotalPressure:0.5atm(1atm=101325Pa)Precursors:SiH4,CH4+H2orC2H2(gas-phasecomposition:Si/C=1)Reactiontemperature:1200-1400°CTemperaturedependenceofSiCparticlesizeBothSiCparticlesizeandgrainsizeincreasingwithreactiontemperatureSolid,hollow,andcore-shellstructureparticlescouldbeobtainedwithdifferentreactiontemperaturesCarbide颗粒尺寸和晶粒尺寸随生长温度的变化不同硅烷浓度下反响温度对SiC生成的影响在(a)1400°C,(b)1350°C温度下不同气体流量生成的SiC纳米颗粒Nitrideshaveoutstandingcharacteristicsincludinglowdensity,highthermalstability,highstrength,andextremelylowthermalexpansioncoefficient.Thesepropertiesmakenitridesoneofthehardestandtoughestmaterialsbutinturn,difficulttosinter.HomogeneousCVDprocessisoneofthepromisingsyntheticroutesfornanosizedpowdersofnitrides.NitridesPreparedbycommonCVDtechniqueusingsilane(SiH4)andammonia(NH3)mixturesattemperaturesof~1000°C,withthepressurerangingbetween10and500TorrIncrementonthereactorpressurewillincreaseyieldoftheproducedpowdersAs-synthesizedpowdersareamorphous,withmeanparticlesizeof~200nmandspecificsurfaceareaof~19.2m2/g

SiliconNitride(Si3N4)TitaniumNitride(TiN)

Synthesizedbytheparticle-precipitation-aidedCVD(PP-CVD)process

TiNwasdepositedonporousAl2O3tubesinhorizontalreactorUsingdifferentnitrogencontainingreactants,bothTiNaerosolandlayerscouldbesynthesizedatthesamegrowthconditions:TiCl4+NH3+H2andTiCl4+H2+N2Temperaturegradienthasastrongeffectonthefinalmicrostructure:lowertemperatureleadingtoporouslayersandnon-uniformfilmthickness;hightemperatureleadingtolayerswithcolumnarstructure

NanostructuredSiCxNySynthesizedbychemicalvaporcondensation(CVC)inahorizontalreactor,usinghexamethydisilazane〔六甲基硅氮烷〕asprecursorReactiontemperature1100-1400°C,pressure1-10mbarThreekeyfactors:(a)lowprecursorconcentrationinthecarriergas;(b)rapidcoolingoftheparticlesattheexitofthereactor;(c)lowreactionpressures4.3CarbonNanotubesOnDenseSubstratesNi-particlecoveredquartssubstratesbythermalCVDFe-particlecoveredaluminasubstratesbymicrowaveplasma-enhancedCVDCo-implantedsiliconbythermalCVDOthercatalystmetalondifferentsubstratematerialsOnPorousSubstratesMesoporoussili