納米材料和納米結(jié)構(gòu)

ChemicalVaporDeposition

化學(xué)氣相沉積納米材料和納米結(jié)構(gòu)第六講1IntroductionInchemicalvapordeposition(CVD),thevaporizedprecursorsareintroducedintoaCVDreactor,wheretheprecursormoleculesadsorbontoasubstrateheldatanelevatedtemperature.Theseadsorbedmoleculeswillbeeitherthermallydecomposedorreactedwithothergases/vaporstoformasolidfilmonthesubstrate.Suchagas-solidchemicalreactionatthesurfaceofasubstrateiscalledtheheterogeneousreaction〔多相反響〕.ABriefDescriptionAprocesswithpotentiallygreatcomplexityAprocessofgreatversatilityandflexibilityMetals,semiconductorsandceramicsAmorphous,polycrystallineorsinglecrystalline

Physicalproperties,

dependingupongrowthconditionsTechniqueFeaturesContentsinTheChapterPrinciplesofCVDmasstransportationreactionkineticsnucleationandgrowthCVDandmodifiedCVDsystemsExamplesinsynthesizingnanostructuredmaterials2

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

Masstransportofreactantstothegrowthsurfacethroughaboundarylayerbydiffusion;

Chemicalreactionsonthegrowthsurface,incorporatingthenewmaterialintothegrowthfront;

Removalofthegas-phasereactionby-productsfromthegrowthsurface.ThreeStepsConsistingtheCVDProcess反響物化學(xué)反響氣態(tài)副產(chǎn)物邊界層化學(xué)氣相沉積過程的擴散模型〔1〕反響物輸運通過邊界層；〔2〕外表反響形成固態(tài)沉積物；〔3〕去處氣相反響副產(chǎn)物.襯底Step1andStep3aredependent,bothofthemaffectthechemicalreactionrateandarecoupledbythestoichiometryofthereactionStep2isexceedinglycomplex,involvingsurfaceand/orgaseousreaction,simultaneouschemical/physicaladsorption-desorption,andnucleationprocessBoundarylayer:aspaceabovethesubstrateresistingdiffusion,varyingwithtime,thedistanceinthehorizontalreactoretc.KeyPointsDescriptiononCVDProcessThegrowthrateisdeterminedbythesloweststep

KineticcontrolofCVDprocess:occurredwhenmass-transfercoefficientisverylargecomparedtotheproductofthekineticrateconstantandthesystempressure

DiffusioncontrolCVDprocess:occurredwhentheoppositeistrue

Temperatureeffectongrowthrate

KeyPointsDescriptiononCVDProcess(2)CVD過程中溫度對薄膜生長速度的影響直接沉淀擴散控制反響控制

Lowtemperature:limitedbystronglytemperature-dependentrateofheterogeneousnucleation,andbyadsorptionandkineticeffect

Highertemperature:limitedbyinterdiffusionofgaseousreactantsandproductsthroughtheboundarylayer

Stillhighertemperature:homogeneousnucleationresultinginprecipitationofsolidparticlesoccurringThreemajorgrowthmodes超晶格層狀生長層+島狀生長島狀生長超晶格3ExperimentalApproachHowtodesignaCVDsystem?Geometry,Shape,CompositionofthesubstrateThetypeofthedepositionprocessusedThenatureofthedepositedmaterialsEconomicfactorsHowtointroduceprecursorsintoreactor?Solidorliquidprecursors:usingcarriergaspassingthroughabubble-bottlevaporizer〔bubbler，起泡器，擴散器〕，eitherdigitaloranalogmassflowcontrollerandpressure/massflowcontrollerarewidelyusedGaseousprecursors:directlyintroducedHeatingmethodadoptedInductionheating〔感應(yīng)加熱〕(cold-wallreactor)Internalresistanceheating〔內(nèi)置電阻加熱〕(cold-wallreactor)High-intensityvisibleorinfraredradiationheating〔高強度可見/紅外光輻照加熱〕(cold-wallreactor)Externalfurnaceheating〔外置爐加熱〕(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均質(zhì)成核).Metalorganicprecursorsaretypicallyusedbecauseoftheircommercialavailabilityandlowpyrolysistemperature.

典型CVC設(shè)備原理圖載氣質(zhì)流計先驅(qū)物源針閥真空室加熱爐納米顆粒襯底控制閥抽氣泵刮刀3.3Particle-Precipitation-AidedCVD(PP-CVD)InPP-CVD,anaerosol〔氣懸體〕isformedatanelevatedtemperature,andparticlesareprecipitatedonasubstratebyintroducinganexternalforceforparticledeposition,suchasthermophoresis〔熱遷移〕,electrophoresis〔電遷移〕,orforcedflow〔強制流〕.Thelooseparticledepositcanbesinteredtoformaporouslayeroraheterogeneousreaction〔復(fù)相反響〕occurssimultaneouslyontheparticlestointerconnecttheindividualparticles.PP-CVDcanbeusedtomakedensetoporousfilmswithnanosizedparticles,andthushasgreatpotentialinavarietyofapplicationssuchascatalystsupport,ceramicmembrane,orporouselectrode.Themicrostructureofthefinalstructureiscontrolledbyacombinationofparticledeposition,sintering,andheterogeneousreaction.ThreestepsinPP-CVDprocess:ParticleformationParticledepositionParticleinterconnectionorsinteringImprovementoftheuniformity:introducingtwoseparatereactionroute,oneforpowderformationandtheotherforheterogeneousreaction顆粒的形成顆粒沉積復(fù)相反響致密化燒結(jié)時間PP-CVD的三步過程：顆粒形成、顆粒沉積和顆粒的互相連接或燒結(jié)典型的PP-CVD原理圖NH3,N2CCl4,H2噴嘴爐子3區(qū)1區(qū)2區(qū)噴嘴反響器冷卻廢氣襯底噴嘴冷卻噴嘴3.4CatalyticCVD(CCVD)InCCVD,transitionmetalparticles(Fe,Co,andNi)aretypicallyusedasthecatalystsforcatalyticdepositionofahydrocarbontoproducecarbonnanotubesandcarbonfibresThefirststepofCCVDprocessisdecompositionofhydrocarbonmoleculesonthesurfaceofcatalystparticles,followedbycarbondiffusionthroughthecatalystparticles.CarbonnanotubesthengrowbyprecipitationattheannularportionofthecatalystsurfaceThedrivingforceforcarbondiffusionisthetemperaturegradientcreatedintheparticlebytheexothermicdecompositionofthehydrocarbonattheexposedfrontfacesandendothermicdepositionofcarbonattherearsurface,whichisinitiallyincontactwiththesupportsurface.Twotypicalgrowthmodes:Tipgrowthmode:thecatalystparticlesweaklyadheretothesubstrate,theyremainonthetipsofthegrownnanotubesandresultinanopenendBasegrowthmode:thecatalystparticlesstronglybondtothesubstrate,theyremainattheinterfacesbetweensubstrateandthegrownnanotubesandresultinaclosedend頂生長模型底生長模型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襯底上生長的InGaAs納米島的平均粒徑對生長溫度的依賴關(guān)系(襯底錯切角依次為0.05、0.5和2°)ChangetheAsH3partialpressurecanproducelargevariationsindensityforequalislandsizeTheislandcoverageislowforthelowestAsH3partialpressure,risestoitsmaximumvalue,anddecreaseagaintoaverylowvalueathighAsH3partialpressureVariationwithAsH3PartialPressureInGaAs量子點面密度隨AsH3分壓的變化?！瞐〕2.81010/cm2,110-6;(b)2.8109/cm2,210-5Themiscutangleθm

ofthesubstratecanbeusedtotuneself-organizedQDsAnincreaseθmwilldecreaseQDdiametersandincreasethesurfaceQDconcentrationsIslandscannucleatehomogeneouslyonterracesorheterogeneouslyonstepedges,thusstepedgenucleationenablesformationofsmallercriticalnucleiforsurfaceswithagreatersurfacedensityofsteps,resultinginsmalleraveragediametersandhigherislanddensitiesfornucleationonsteppedsurfacesQDsgrowselectivelyonmultiatomicstepedges,resultinginspontaneousalignmentofQDsVariationwithSurfaceStructure550°C生長的InGaAs量子點的直徑、面密度與襯底錯切角的關(guān)系曲線InGaAs量子點在外表臺階邊緣的規(guī)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制備的莫萊石粉末的顆粒尺寸分布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顆粒尺寸和晶粒尺寸隨生長溫度的變化不同硅烷濃度下反響溫度對SiC生成的影響在(a)1400°C,(b)1350°C溫度下不同氣體流量生成的SiC納米顆粒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