Ultrafast Optical-Pump Terahertz-Probe Spectroscop

UltrafastOptical-PumpTerahertz-ProbeSpectroscopyoftheCarrierRelaxationandRecombinationDynamicsinEpitaxialGraphenePaulA.George,*JaredStrait,JahanDawlaty,ShriramShivaraman,MvsChandrashekhar,FarhanRana,andMichaelG.SpencerSchoolofElectricalandComputerEngineering,CornellUniVersity,Ithaca,NewYork14853ReceivedJuly2,2008;RevisedManuscriptReceivedSeptember7,2008ABSTRACTTheultrafastrelaxationandrecombinationdynamicsofphotogeneratedelectronsandholesinepitaxialgraphenearestudiedusingoptical-pumpterahertz-probespectroscopy.Theconductivityingrapheneatterahertzfrequenciesdependsonthecarrierconcentrationaswellasthecarrierdistributioninenergy.Time-resolvedstudiesoftheconductivitycanthereforebeusedtoprobethedynamicsassociatedwithcarrierintrabandrelaxationandinterbandrecombination.Wereporttheelectron-holerecombinationtimesinepitaxialgrapheneforthefirsttime.Ourresultsshowthatcarriercoolingoccursonsubpicosecondtimescalesandthatinterbandrecombinationtimesarecarrierdensitydependent.Grapheneisatwo-dimensionallatticeofcarbonatomsarrangedinahoneycombcrystalstructurewithazero(ornear-zero)bandgapandalinearenergy-momentumdisper-sionrelationforbothelectronsandholes.1,2Theuniqueelectronicandopticalpropertiesofgraphenemakeitapromisingmaterialforthedevelopmentofhigh-speedelectrondevices,includingfield-effecttransistors,pn-diodes,terahertzoscillators,andelectronicandopticalsensors.2-7Therealizationofgraphene-baseddevicesrequiresunder-standingthenonequilibriumcarrierdynamicsaswellastherateatwhichelectron-holerecombinationoccurs.Measurementsoftheultrafastintrabandrelaxationdynam-icsofphotogeneratedelectronsandholesinepitaxialgrapheneusingbothdegenerate8andnondegenerate9optical-pumpoptical-probespectroscopyhavebeenpreviouslyreported.Similarmeasurementsforexfoliatedgraphenemono-andmultilayershavealsobeencarriedout.10Thesemeasurementsweresensitivetotheinterbandconductivityofgrapheneandprobedthetimeevolutionofthecarrieroccupationatspecificenergiesinthebands.Consequently,theywerenotabletodirectlymeasurethetimescalesassociatedwithcarrierrecombination.Atroomtemperature,theopticalresponseofgrapheneintheterahertzfrequencyrangeisdescribedbytheintrabandconductivitysthefreecarrierresponseswhichdependsnotonlyonthetotalcarrierconcentrationbutalsoonthecarrierdistributioninenergy.11Therefore,terahertzradiationcanbeusedtostudythecarrierrelaxationandrecombinationdynamicsingraphene.Inthispaper,wepresentresultsobtainedfromoptical-pumptera-hertz-probespectroscopyofepitaxialgrapheneinwhichthetime-dependentconductivityofgraphenethathasbeenexcitedwithanopticalpumppulseisprobedwithafew-cycleterahertzpulse.Weobservecoolingofthephotoge-neratedcarrierdistributionaswellaselectron-holerecom-binationingrapheneinrealtime.Ourresultsindicatethattherecombinationtimesingraphenedependonthecarrierdensityandmaterialdisorder.Theepitaxialgraphenesamplesusedinthisworkweregrownonthecarbonfaceofsemi-insulating6H-SiCwafersusingtechniquesthathavebeenreportedpreviously.12Asdiscussedinrefs8and11,X-rayphotoemission,Raman,andoptical/IR/THztransmissionspectroscopywereusedtocharacterizeeachsampletodeterminethenumberofcarbonatomlayersandthecarriermomentumrelaxationtime.Inthiswork,sampleBconsistsof∼12carbonatomlayerswithamomentumrelaxationtimeτ∼20fsandsampleCconsistsof∼20layerswithτ∼4fs.11Ramanspectroscopy(at488nm)ofthesamplesrevealedaGpeakcloseto1580cm-1andaDpeakcloseto1350cm-1.TheDpeakisforbiddeninperfectgraphenelayerssinceitrequiresanelasticscatteringprocess,whichismadepossiblebydisorder,tosatisfymomentumconservation.13,14ThepresenceoftheDpeakthereforeindicatesthepresenceofdisorderinthesamples.TheaverageratiooftheintensityoftheRamanGandDpeaks,IG/ID,was17forsampleBand1.6forsample*Correspondingauthor,[e-mail]2008Vol.8,No.124248-425110.1021/nl8019399CCC:$40.752008AmericanChemicalSocietyPublishedonWeb11/07/2008C.Thisindicatessignificantlymoredisorderinthelattersample.13,14Resultsfromoptical-pumpoptical-probespec-troscopyofsampleBwerereportedearlier.8Ultrafast90fspulsesfroma81MHzTi:sapphirelaserwithacenterwavelengthof780nm(1.6eV)wereusedtosimultaneouslypumptheterahertztime-domainspectrometer(THz-TDS)andtheepitaxialgraphenesamples.TheTHz-TDSusedaphotoconductiveemitterandanelectro-opticdetector,15,16andgeneratedfew-cycleTHzpulseswitha0.3-3.0THzbandwidthandapowerSNRnear106.Theepitaxialgraphenesampleswereplacedatthefocusofa90°off-axisparabolicmirrorandexcitedwith1-16nJopticalpulsesfocusedtofwhmspotsizesof350-400µm.Thetime-dependenttransmissionoftheTHzprobepulsethroughtheopticallyexcitedsamplewasmeasuredbydelayingtheopticalpumppulsewithrespecttotheTHzprobe.Thepumpandprobesignalsweremechanicallychoppedat256and179.2Hz,respectively,andthechangeinthetransmissionwasmeasuredusingalock-inamplifierreferencedtothesumofthesefrequencies.ThecomplexamplitudetransmissionoftheTHzpulsethroughthegraphenelayersandtheSiCsubstrate(normal-izedtothetransmissionthroughtheSiCsubstrate)ist)11+Nηoσintra⁄(1+nSiC)(1)Nisthenumberofcarbonatomlayers,ηoisthefree-spaceimpedance,andnSiCistherefractiveindexofSiCandequal