薄膜材料科学PPT演示课件

MaterialsScienceofThinFilms,1,Chapter1AReviewofMaterialsScience,2,1.1INTRODUCTION,Solidsubstances:revealsoutwardlyappearstobeanendlessmultitudeofexternalformsandstructurespossessingabewilderingvarietyofproperties.MaterialsScienceevolvedinparttoclassifythosefeaturesthatarecommonamongthestructureandpropertiesofdifferentmaterials.,3,Categoriesofsolids-electronicstructure,SolidscanbeclassifiedasbelongingtypicallytooneofonlyfourdifferentcategoriesmetallicioniccovalentvanderWaals)Dependingonthenatureoftheelectronicstructureandresultinginter-atomicbondingforces.,4,Categoriesofsolids-engineering,Anotherschemebasedonengineeringusewouldagainarguablylimitmaterialstofourchiefclasses,namely,metalssemiconductorspolymersceramics.,5,Categoriesofsolids-Crystalline,Solidsareeitherinternallycrystallineornoncrystalline.Thosethatarecrystallinecanbefurthersubdividedaccordingtooneof14differentgeometricarraysorlatticesdependingontheplacementoftheatoms.,6,1.2STRUCTURE,7,1.2.1CRYSTALLINESOLIDS,Actualcrystalstructurescanbeimaginedtoarisefromathree-dimensionalarrayofpointsgeometricallyandrepetitivelydistributedinspacesuchthateachpointhasidenticalsurroundings.ThereareonlyfourteenwaystoarrangepointsinspacehavingthispropertyandtheresultingpointarraysareknownasBravaislattices.,8,9,Positions,Planes&Directions,Positions:(u,v,w)Planes:(l,m,n)Directions:,10,Specificcrystalplanesanddirections,Specificcrystalplanesanddirectionsarefrequentlynoteworthybecausephenomenasuchascrystalgrowth,chemicalreactivity,defectincorporation,deformation,andassortedpropertiesarenotisotropicorthesameonallplanesandinalldirections.Therefore,theimportantneedarisestobeabletoaccuratelyidentifyanddistinguishcrystallographicplanesanddirections.,11,Identifyingagivenplane-Millerindices,Asimplerecipeforidentifyingagivenplaneinthecubicsystemis:Determinetheinterceptsoftheplaneonthethreecrystalaxesinnumberofunitcelldimensions.Takereciprocalsofthosenumbers.Reducethesetosmallestintegersbyclearingfractions.,12,13,Directions,Perpendiculartoaspecificplane.Foraspecificplane(l,m,n),directionisrepresentedas,orl,m,n.,14,Theangleabetweenanytwodirectionsorplanes,可通过软件求得。,15,Thespacingbetweensuccessive(hkl)planes,wherea0isthelatticeparameter.,16,example,1,1,1-typedirectionstobe-1,-1,1,17,1.2.3AMORPHOUSSOLIDS,Insomematerialsthepredictablelong-rangegeometricordercharacteristicofcrystallinesolidsbreakdown.Thesearethenoncrystallineamorphousorglassysolids.Silicaglass,inorganicoxidemixtures,andpolymers.,18,19,20,1.3DEFECTSINSOLIDS,vacancies,dislocations,andgrainboundaries,21,structuraldefects,Perfectcrystalissomewhatofanexaggeration.carefullygrownsiliconsinglecrystalshavevirtuallyperfectcrystallographicstructuresextendingovermacroscopicdimensions.Inthincrystallinefilmsthepresenceofdefectsnotonlyservestodisruptthegeometricregularityofthelatticeonamicroscopiclevel,butsignificantlyinfluencesmanyfilmpropertiessuchaschemicalreactivity,electricalconduction,andmechanicalbehavior.Thestructuraldefectsbrieflyconsideredinthissectionarevacancies,dislocations,andgrainboundaries.,22,1.3.1VACANCIES,Themostelementaryofcrystallinedefectsarevacancies,andtheyarisewhenlatticesitesareunoccupiedbyatoms.Thefraction/oftotalsitesthatwillbeunoccupiedasafunctionoftemperatureTispredictedtobeapproximately.NotingthatkBistheBoltzmannconstantandEfistheformationenergy,whichistypically1eVperatomgivesf=10-5at1000K.,23,Rolesofvacancies,Vacanciesplayanimportantroleinallprocessesrelatedtosolid-statediffusion,includingrecrystallization,graingrowth,sintering,andphasetransformations.Insemiconductors,vacanciesareelectricallyneutralaswellaschargedandcanbeassociatedwithdopantatoms.Thisleadstoavarietyofnormalaswellasanomalousdiffusional-dopingeffects.,24,1.3.2DISLOCATIONS,Thesearelinedefectsthatbearadefinitecrystallographicrelationshiptothelattice.Thetwofundamentaltypesofdislocationstheedgeandthescrew.Anedgedislocationcanbegeneratedbywedginganextrarowofatomsintoaperfectcrystallattice,Ascrewdislocationrequirescuttingfollowedbyshearingoftheresultanthalveswithrespecttoeachother.,25,26,Burgersvector,Thegeometryofacrystalcontainingadislocationissuchthatwhenattemptingasimpleclosedtraverseaboutitsaxisinthesurroundinglattice,thereisaclosurefailure.Onefinallyarrivesatalatticesitedisplacedfromthestartingpositionbyalatticevector,theso-calledBurgersvectorb.Thisvectorliesperpendiculartotheedgedislocationlineandparalleltothescrewdislocationline.,27,Straincausedbydislocations,Crystallatticearenowdistortedsomewhatinthepresenceofdislocations.Therefore,evenwithoutapplicationofexternalforcesonthecrystal,astateofinternalstrain(stress)existsaroundeachdislocation.Furthermore,thestrains(stresses)differaroundedgeandscrewdislocationsbecausethelatticedistortionsdiffer.Closetothedislocationaxisorcorethestressesarehighbuttheyfalloffwithdistance(r)accordingtoa1/rdependence.,28,Vacancies&dislocations,Incontrasttovacancies,dislocationsarenotthermodynamicdefects.Coupledwithahighformationenergyduetothemanyatomsinvolved,thermodynamicswouldpredictadislocationcontentoflessthanonepercrystal.Thus,whileitispossibletocreateasoliddevoidofdislocations,itisimpossibletoeliminatevacancies.,29,Effectsonmechanicalproperties,30,small-anglegrainboundaries,31,Dislocationsplayrolesinthinfilms,Ifthelatticeparameterinthefilmandsubstratediffer,thensomegeometricaccommodationinbondingmayberequiredattheinterfaceresultingintheformationofinterfacialdislocations.agoodmatchoflatticeparametersissoughtforepitaxialgrowth.Ifthesubstratehasscrewdislocationsemergingnormaltothesurface,depositingatomsmayperpetuatetheextensionofthedislocationspiralintothegrowingfilm.dislocationscanbesitesofchargerecombinationdanglingbonds.Filmstress,thermallyinducedmechanicalrelaxationprocesses,anddiffusionofatomsinfilmsareallinfluencedbydislocations,32,1.3.3GRAINBOUNDARIES,Grainboundariesaresurfaceorareadefectsthatconstitutetheinterfacebetweentwosingle-crystalgrainsofdifferentcrystallographicorientation.,33,Roles,Thenormalatomicbondingingrainsterminatesatthegrainboundarywheremorelooselyboundatomsprevail.Likeatomsonsurfaces,theyarenecessarilymoreenergeticthanthosewithinthegraininterior.Thiscausesthegrainboundarytobeaheterogeneousregionwherevariousatomicreactionsandprocessessuchassolid-statediffusionandphasetransformations,precipitation,corrosion,impuritysegregation,andmechanicalrelaxationarefavoredoraccelerated.,34,Roles,electronictransportinmetalsisimpededthroughincreasedscatteringatgrainboundaries.serveaschargerecombinationcentersinsemiconductors,35,Controllinggrainmorphology,Controllinggrainmorphology,orientation,andsizearenotonlyimportantobjectivesinbulkmaterialsbutarequiteimportantinthin-filmtechnology.Indeedamajorgoalinmicroelectronicapplicationsistoeliminategrainboundariesaltogetherthroughepitaxialgrowthofsinglecrystalsemiconductorfilmsonorientedsingle-crystalsubstrates.,36,1.4BONDSANDBANDSINMATERIALS,37,1.4.1BONDINGATTHEATOMICLEVEL,Thereasonthatwidelyspacedisolatedatomscondensetoformsolidsistheenergyreductionaccompanyingbondformation.Thus,ifNatomsoftypeAinthegasphase(g)condensetoformasolid(s),thebindingenergyEbisreleasedaccordingtotheequation,38,1.4.2BONDINGINSOLIDS,39,1.4.3THEFOURCLASSESOFSOLIDS:BONDINGANDPROPERTIES,MatellicIonicCovalentvanderWaals,40,Metallic,Theso-calledmetallicbondoccursinmetalsandalloys.Inmetalstheoutervalenceelectronsofeachatomformpartofacollectivefree-electroncloudorgasthatpermeatestheentirelattice.Eventhoughindividualelectron-electroninteractionsarerepulsive,thereissufficientelectrostaticattractionbetweenthefree-electrongasandthepositiveioncorestocausebonding.,41,properties,Electronsreadilyrespondtoappliedelectricfields,thermalgradients,andincidentlight.Highelectricalandthermalconductivitiesaswellashighopticalreflectivities.Comparablepropertiesareobservedinliquidmetals,absenceofacrystalstructure.Electricalresistivitiestypicallyrangingfrom10-5to10-6ohm-cm.,42,mechanical,Bondingelectronsinmetalsarenotlocalizedbetweenatomsandnondirectionalbondsaresaidtoexist.Thiscausesatomstoslidebyeachotherandplasticallydeformmorereadily.,43,Applications,contactsandinterconnectionsinintegratedcircuits.ferromagneticalloysfordatastoragemirrorsdecorativeandprotectivecoatings.,44,Ionic,Ionicbondingoccursincompoundscomposedofstronglyelectropositiveelements(metals)andstronglyelectronegativeelements(nonmetals).Thealkalihalides(NaCl,LiF,etc.)arethemostunambiguousexamplesofionicallybondedsolids.Inothercompoundssuchasoxidesandsulfidesaswellasmanyofthemorecomplexsaltsofinorganicchemistry.,45,Electrical,Ionicsolidsarecharacterizedbystrongelectrostaticbondingforcesandthusrelativelyhighbindingenergiesandmeltingpoints.Theyarepoorconductorsofelectricitybecausetheenergyrequiredtotransferelectronsfromanionstocationsisprohibitivelylarge.Athightemperatures,however,thechargedionsthemselvescanmigrateinanelectricfieldresultinginlimitedelectricalconduction.Typicalresistivitiesforsuchmaterialscanrangefrom106to1015ohm-cm.,46,Covalent,Elementalaswellascompoundsolidsexhibitcovalentbonding.TheoutstandingexamplesaretheelementalsemiconductorsSi,Ge,anddiamond,aswellasIII-VcompoundsemiconductorssuchasGaAsandInP.,47,hybridization,ThestrongdirectionalbondscharacteristicofthegroupIVelementsisduetothehybridizationormixingofthe2sand2pelectronwavefunctionsintoasetoforbitalsthathavehighelectrondensitiesemanatingfromtheatomintetrahedralfashion.Apairofelectronscontributedbyneighboringatomscomprisesacovalentbond,andfoursuchsharedelectronpairscompletethebondingrequirements.,48,Electrical,CovalentsolidsarestronglybondedhardmaterialswithrelativelyhighmeltingpointsDespitethegreatstructuralstabilityofsemiconductors,relativelymodestthermalstimulationissufficienttoreleaseelectronsfromfilledvalencebondingstatesintounfilledconduction-electronstates.Asthenameimplies,semiconductorsliebetweenmetalsandinsulatorsinsofarastheirabilitytoconductelectricityisconcerned.Typicalsemiconductorresistivitiesrangefrom10-3to106ohm-cm.,49,vanderWaals,Alargegroupofsolidmaterialsareheldtogetherbyweakmolecularforces.Thisso-calledvanderWaalsbondingisduetodipole-dipolechargeinteractionsbetweenmoleculesthat,thoughelectricallyneutral,haveregionspossessinganetpositiveornegativechargedistribution.,50,Suchsolidshavelowmeltingpointsandaremechanicallyweak.ThinpolymerfilmsusedasphotoresistsorforsealingandencapsulationpurposescontainmoleculesthataretypicallybondedbyvanderWaalsforces.,51,1.4.4ENERGYBANDDIAGRAMS,52,Workfunctionofsemicondutors,53,1.5THERMODYNAMICSOFMATERIALS,54,1.6KINETICS,55,1.6.1MACROSCOPICTRANSPORT,Wheneveramaterialsystemisnotinthermodynamicequilibrium,drivingforcesarisenaturallytopushittowardequilibrium.Suchasituationcanoccur,forexample,whenthefreeenergyofamicroscopicsystemvariesfrompointtopointbecauseofcompositionalinhomogeneities.Theresultingatomicconcentrationgradientstriggertime-dependent,masstransporteffectsthatreducefree-energyvariationsinthesystem.,56,diffusion,Masstransportisaccomplishedbydiffusionofanatomicormolecularspeciesundertheinfluenceofaconcentrationgradient.Fickestablishedthephenomenologicalconnectionbetweenconcentrationgradientsandtheresultantdiffusionaltransportthroughtheequation,57,diffusioncoefficient,DincreasesinexponentialfashionwithtemperatureaccordingtoaMaxwell-BoltzmannrelationwhereD0isaconstant,EDistheactivationenergyfordiffusion,andkBThastheusualmeaning.,58,Timedependentdiffusionequation,59,1.6.2ATOMMOVEMENTS,60,1.6,2.1DiffusionalTransport,Macroscopicchangesincompositionduringdiffusionaretheresultoftherandommotionofcountlessindividualatomsunawareoftheconcentrationgradienttheyhaveestablished.macroscopicchangesaremanifestedbycountlessrepetitionsofsuchunitjumps.,61,62,Diffusionflux,Thefractionofvacantlatticesiteswasgivenbyexp(-Ef/kT).Thenumberoftimespersecondthatanatomsuccessfullyreachestheactivatedstateisexp(-Em/kT).whereEmisthevacancyjumpormigrationenergyperatom,vislatticefrequency.,63,nonlatticesites,Althoughtheabovemodelisintendedforatomisticdiffusioninthebulklattice,asimilarexpressionforDwouldholdfortransportthroughgrainboundariesoralongsurfacesandinterfacesoffilms.Atsuchnonlatticesites,energiesfordefectformationandmotionareexpectedtobelower,leadingtohighdiffusivities.,64,Arrheniusplot,InsuchcasesthekineticscanbedescribedgraphicallybyanArrheniusplotinwhichthelogarithmoftherateisplottedontheordinate,andthereciprocaloftheabsolutetemperatureontheabscissa.Theslopeoftheresultinglineisthenequalto-ED/kTfromwhichthecharacteristicactivationenergycanbeextracted.,65,AtomTransportinaForceField,Formatrixatomstherearedrivingforcesotherthanconcentrationgradientsthatoftencausethemtomigrate.Examplesareforcesduetostressfields,electricfields,andinterfacialenergygradients.,66,67,AtomTransportinaForceField,68,1.7NUCLEATION,69,nucleation,Whenthecriticalboundariesseparatingstablephasefieldsonequilibriumphasediagramsarecrossed,newphasesappear.Mostfrequentlyadecreaseintemperatureisinvolvedandthismay,forexample,triggervapor-phasecondensation,solidification,orsolid-statephasetransformationsfromnowunstablegases,melts,orsolidmatrices.,70,nucleation,Whensuchatransformationoccurs,anewphaseofgenerallydifferentstructureandcompositionemergesfromthepriorparentphaseorphases.Theprocessknownasnucleationoccursduringtheveryearlystagesofphasechange.Itisimportantinthinfilmsbecausethegrainstructurethatultimatelydevelopsinagivendepositionprocessisofteninfluencedbywhathappensduringthefilmnucleationstep.,71,example,Homogeneousnucleationofasphericalsolidphaseofradiusrfromapriorsupersaturatedvapor.Gas-to-solidtransformationresultsinareductionofthechemicalfreeenergyofthesystemgivenby4pr3DGv.whereDGvcorrespondstothechangeinchemicalfreeenergyperunitvolume.,72,newsurfacesandinterfaces,Simultaneously,newsurfacesandinterfacesform.Thisresultsinanincreaseinthesurfacefree-energyofthesystemgivenby4pr2g,wheregisthesolid-vaporinterfacialenergyperunitarea.Thetotalfree-energychangeinformingthenucleusisthusgivenby,73,MinimizationofDGwithrespecttor,ord(AG)/dr=0,yieldstheequilibriumsizeofrequaltor*=-2y/DGv.SubstitutioninEq.1-40givesDG*=16pr3/3(DGv)2.DG*representsanenergybarriertothenu