RHEED原理及说明

RHEED(reflectionhigh-energyelectrondiffraction)YiLiBackground•ThefirstRHEEDexperimentwasconductedbyNishikawaandKikuchiin1928.•Theoriginofpatterns,especiallythestreaks,wereexplainedindetailbyusingkinematicsdiffractiontheory(KirchnerandRaether,1932;Raether,1932).•ArthurandLePoreincorporatedaRHEEDmeasurementintoanapparatusdevelopedbyArthurforthemolecularbeamepitaxial(MBE)growthofGaAs.•Harrisetal.(1981),Wood(1981),Neaveetal.(1984),VanHoveetal.(1983b)andSakamotoetal.(1986)sawintensityoscillations,RHEEDbecameessentialformeasuringgrowthrate.ElectronDiffractionTechniques1LowEnergyElectronDiffraction(LEED)(E:10~500eV)Electronwavelength:0.5~4ÅPenetrationlength:5~10Å2ReflectionHigh-EnergyElectronDiffraction(RHEED)(E:5~100keV)Electronwavelength:0.02~0.2ÅPenetrationlength:20~100Å-Ahighenergyelectrongun-AfluorescentscreenRHEEDInstrumentsPrinciplesofRHEED•BriefoftheDiffractionTheory(1)EwaldSphereandLaueequation(Laueequation)Weknow(2)Braggdiffraction2sinhkldnk=2/2sinhklkG2221/2hklG()/hklhkldThefigureshowstheEwaldsphereoftheincomingelectronwavesandthereciprocalspacemapofaperiodicstructure.WheretheLaueequationismet,theEwaldsphereoftheincomingelectronwaveswillintersectthereciprocallatticerodsofthesurfaceofthesample.Thiswillresultinstreaksofspacingsontheflorescentscreen.AstheradiusoftheEwaldsphereisabouttwoordersofmagnitudegreaterthanthemodulusofthereciprocallatticevector,wefindthattheanglebetweenpointsintersectingthespherewillbealmostthesame220.5150/2kVVaak=50Å-1for10keVelectronsa*=2Å-1fora=3ÅtheEwaldsphereisverylargecomparedtoreciprocalvector,soitcutsthereciprocalrodsalmostalongitslength.RHEEDpatternFlat-streaks:IncidentanglewidthEnergywidthThermalvibrationRough-spots:manyelectronswillbetransmittedthroughsurfaceasperitiesandscatteredindifferentdirections,resultinginaRHEEDpatternconstitutedbymanyspottyfeatures.Amorphous:-nopatternitisevidentthatdiffractionfromAnamorphoussurface(suchasanoxideontopofasemiconductor)givesnodiffractionpatternatall,andonlyadiffusebackgroundRHEEDTechnique•Changing•ChangingChangingIncreasemeansmorespots,butlesssurfacesensitiveChangingfcc(110)DifferentatomicrowsDiffractiongratingsDifferentRHEEDpatternsDiffractionpatternobserveddependsstronglyonincidentbeamdirection!RHEEDAnalysis:Whenthelocationofthebulk-generateddiffractionspotsisknown,informationaboutsurfacereconstructioncanbeobtainedduetothepositionsofreconstruction-generateddiffractionspotssuperimposedonthe11reciprocallatticemappedoutbythebulkspots.•PatternrecognitionSi(111)Si(111)-77(HT,UHV)Thebulkspotsarestillpresent,buttheadditionalspotsalongthearrowsshowthatinthesedirectionsthesurfacereciprocallatticespacingis1/7ofthatofthebulkreciprocallattice.Surfacelatticeconstant2sindtan2/SL221222hkadGhakahkBraggdiffractioncondition:Forsquaremesh:2sind221222hkadGhakahkThentakeanapproximation:Surfacelatticeconstantacanbedetermined:Surfacelatticeconstantacanbedeterminedbymeasuringthedistancesbetweenspotsatthephosphorscreen.sintan22LhkaS22LhksaTogetmeasurablesonthephosphorescentscreen,Lshouldbeaslongaspossible.Forexample:=0.1Å,a=3Å,h=1,k=0s=1mmL=30mmCharacteristicofRHEED•Adescriptionofthevariousintensitiesofdiffractionspotsinvolvesthecomplexproblemofdescribingmultiplescatteringprocesseswithinthetopmostatomiclayersofacrystal.•Fortunately,simpleinspectionofaRHEEDpatternandmeasurementofthegeometricalspotpositionsisenoughtogainagreatdealofusefulinformationaboutasurface.•Thefirststepofanysurfaceexperimentusingacrystallinesubstrateistochecktheroughnessofthesurface.RHEEDIntensityOscillation1.Roughness-Reflection2.Өrepresentsthecoverageofthesurfaceatoms3.Amaximumintheintensityindicatesthecompletionofanewlayer4.Controlofthegrowthrate5.ThicknessmonitorRHEEDintensityoscillationduringtheMBEgrowthofGaAsonaGaAs(001)(2X4)reconstructedsurfacePS:Thisisnottrueinsomecase,becauseofthemuchlowersurfacemobilityoftheatomsGaAsMBE外延生长动力学研究1.Sameoscillatingperiod2.Differentphase3.(00)streakoscillatesmoreobviouslyVariousbeamsoscillationsofRHEEDduringGaAsMBEgrowthComparingwithLEED:•AgrazingincidencegeometryisusedinRHEED,wheretheelectrongunandfluorescentscreencanbeplacedwellawayfromthesamplesurfacesothatthediffractionapparatusdoesnotimpedethegrowthprocess.ByusingRHEEDitispossibletomeasureandcontrol,atomiclayergrowthratesinfilmgrowth.ThisisbyfarthemostimportantapplicationofRHEEDtechnique.LEEDisnotwellsuitedtothistaskbecausetheelectrongunaswellasthefluorescentscreenneedstobeplaceddirectlyinfrontofthesample.NofilteringoftheenergyinfrontofthefluorescentscreeninRHEED.•StrengthsofRHEED:–Robust:Allowsonetoobservechangesinsurfacestructureasfunctionoftime.–CostlessthanLEED•Weaknesses:–DiffractionpatternhardertointerpretthanLEED–Quantitativeanalysisofsurfacestructuredifficult.–Maydestroythesample.OverviewoftheRHEEDTechniqueThankYou!