MolecularBeamEpitaxy

(MBE)Contents:1.Whatisthemoleculebeamepitaxy2.ThemechanismofMBE3.Application

1.Whatisthemoleculebeamepitaxy

AtypicalMBEsystem*MolecularBeamEpitaxy**MBESourceSourceisinheremolecularbeamcomesouthererrInventedinlate1960’satBellLaboratoriesbyJ.R.ArthurandA.Y.Cho.分子束外延(MBE)超高真空10-8Pa，分子束流直接噴射到襯底。到達(dá)襯底的分子僅由蒸發(fā)系統(tǒng)的幾何形狀和蒸發(fā)源溫度決定。所以可以精確控制晶體生長(zhǎng)速率、雜質(zhì)濃度、多元化合物成分比。AlfredCho–thefatherofMBE卓以和，美籍華裔科學(xué)家。美國(guó)科學(xué)院院士,因?yàn)樵诜肿邮庋渝兡ぜ夹g(shù)方面具有卓越的貢獻(xiàn)被譽(yù)為“分子束外延之父”卓以和是國(guó)際公認(rèn)的分子束外延、人工微結(jié)構(gòu)材料生長(zhǎng)和新型器件研究領(lǐng)域的奠基人與開拓者。他用新材料成功研制出10多種極為重要、性能優(yōu)異的新型微波高速電子儀器及光電子儀器，多次領(lǐng)先同儕。最近10年間，分子束外延研究獲得長(zhǎng)足發(fā)展，在激光、醫(yī)學(xué)檢測(cè)、環(huán)境分析、半導(dǎo)體制造等方面得到廣泛應(yīng)用，令卓以和在2007年再次獲頒美國(guó)國(guó)家科學(xué)獎(jiǎng)?wù)录皣?guó)家技術(shù)獎(jiǎng)?wù)耇hreegrowthmodes

2.ThemechanismofMBEIslanding(Volmer-Weber)Intheislandingmode,smallclustersarenucleateddirectlyonthesubstrateandthengrowintoislandsofthecondensedphase凝固相.Thisresultsfrom

astrongerbindingofthegrowingconstituentstoeachotherthantothesubstrate.LayerbyLayer(Frank-vanderMerve)Thelayerbylayermodeshowsoppositecharacteristics.Sincetheatomsaremorestronglyboundtothesubstratethantoeach

other,acompletemonolayer(ML)isformedfirst,beforegrowthofthenextMLbegins,whichissomewhatlessstronglybound.Layergrowthisobtainedprovided

thatthedecreaseinbindingenergyismonotonicandreachesthevalueofthesuppliedmaterialinbulkphase.Layer+Island(Stranski-Krastanov)Thisintermediatecasecanariseifaftergrowthofthe

first(few)ML,whicharestronglyboundtothesubstrate,astrongdecreaseinthe

bindingenergymakessubsequentlayergrowthunfavourable.Islandsareformed

ontopofthiswettinglayer.Theremaybemanydifferentoriginsforthisgrowthmode.Anythingthatdisturbsthemonotonicdecreaseinbindingenergymaybe

theorigin.

ApopularapplicationoftheStranski-Krastanovgrowthmode,wheretheaccumulationofstrainleadstoislandformationistheself-organizedgrowthof

quantumdots.Stranski-KrastanovgrowthofGeonSi(001)3Dislandsformation~3.5MLGe,475°C,(110nm)2hutspyramidsWettinglayer~2.5MLGe,475°C,(44nm)24%latticemismatchbetweenGe&SiAtomicProcessesinNucleation&Growth

Adsorption,diffusion,incorporation,nucleation,desorption,coarsening

wheretheycanundergodifferentprocesses(Fig.8b).Atomscandiffuseonaflatsurface(a),wheretheycanre-evaporate(b),meetotheratomstoformtwo-dimensionalclusters(c),reachastepwheretheycanbeincorporated(d),orfurthermigratealongthestep(e)tobeincorporatedatakink(f).KineticsExampleAtomscanbondtoflatsurface,steps,orkinks.OnsurfacerequiressomecriticalradiusEasieratstepsEasiestatkinksAs-richGaAssurfaceAsonlyformstwobondstounderlyingGaVeryhighenergyReconstructsbyformingAsdimersLowersenergyCauseskinksandstepsonsurfaceResultsinmotionofstepsonsurfaceIfstartwithflatsurface,createsteponcefirstgrouphasbondedGrowthcontinuesinsameway/nsls2/sciOps/chemSci/growth.asp17BenefitsandDrawbacksofMBEAdvantagesDisadvantages

Cleansurfaces,freeofanoxidelayer

Expensive(106$perMBEchamber)

In-situdepositionofmetalseeds,semiconductormaterials,anddopants

Verycomplicatedsystem

Lowgrowthrate(1μm/h)

Epitaxialgrowthunderultra-highvacuumconditions

Preciselycontrollablethermalevaporation

Seperateevaporationofeachcomponent

Substratetemperatureisnothigh550℃forGaAsPrecisecontrolofsurfacecompositionandmorphology3.ApplicationMBEandSolarCellsCanmakeastackwithdifferentbandgapsandreachhighersolarenergycaptureefficiencyMolecularbeamepitaxy(MBE)isperformedwithdifferenttypesofsemiconductingmaterialslike:i)GroupIVelementalsemiconductorslikeSi,Ge,andCii)III-V-semiconductors:arsenides(GaAs,AlAs,InAs),antimonideslikeGaSbandphosphideslikeInPiii)II-VI-semiconductors:ZnSe,CdS,andHgTeNovelstructuresasquantumdevicesSuperlatticesMicroelectronicDevices

Self-catalysedgrowthofGaAsnanowiresbySelf-catalysedgrowthofGaAsnanowiresbyMBEGenerationofSinanowiresby

MolecularBeamEpitaxy

semiconductorvaporreachestheliquiddropletsurfaceandformsaSi-Au-alloy

nanowhiskersgrownbythesupersaturationofSiintheSi-Au-alloyatthemetal/semiconductorInterfacesemiconductoratomsdiffuseonthedropletsurfacetothemetal/semiconductorinterfacesemiconductoratomsimpingeonthewholewafersurfaceanddiffusealongthewhiskertotheliquid/alloyinterfaceThesapphiresubstrateswerenitridatedat870°Cfor1h,followedbydepositionofa300-nm-thickAlNfilmgrownunderAl-richconditionsatthesametemperature.Thesubstratetemperaturewasthenreducedto770°CfortheGaNQDSLgrowth.TheGaNQDsweregrownusingthemodifiedSK（layerplusisland）growthmode.Inthismode,severalMLsofGaNweredepositedunderGa-richconditionsGa/N1.1,followedbyevaporationofexcessGainvacuum,andsubsequentrearrangementofthe2DGaNlayersinto3DGaNdots.The2Dto3Dtransitionwasconfirmedbytransformationofthereflectionhigh-energyelectrondiffractionRHEEDpatternfromstreakytospottyduringgrowthinterruption.AfterdepositionofthelastGaNQDlayer,thesamplewasleftinvacuumfor1minbeforethesubstratetemperaturewas

rampeddownto200°Cat20°C/min.GaNQuantumDotSuperlatticeGrownbyMolecularBeamEpitaxyatHigh30TemperatureSelective-areacatalyst-freeMBEgrowthofGaNnanowiresusingapatternedoxidelayerGrowthofZnOandZnMgOnanowiresbyAu-catalysed

molecular-beamepitaxyAlGaNbaseddeepultravioletlightemittingdiodesUV-LEDsbymolecularbeamepitaxy.BygrowingtheAlGaNwelllayerunderGa-richconditionstoproducestrongpotentialfluctuations,internalquantumefficiencyofaquantumwellstructureemittingat300nmwasfoundtobe32%.BycombiningsuchGa-richgrowthconditionintheactiveregionwithpolarizationfieldenhancedcarrierinjectionlayers,deepUV-LEDsemittingat273nmwereobtainedwithoutputpowerof0.35mWand1.3mWat20mAcontinuouswaveand100mApulseddrivecurrent,respectively.Themaximumexternalquantumefficiencywas0.4%.參考文獻(xiàn)1./profile/Theodore_Moustakas/publication/234