8-Ultrathin-epitaxial-graphite.-2D-electron-gas-pr

UltrathinEpitaxialGraphite:2DElectronGasPropertiesandaRoutetowardGraphene-basedNanoelectronicsClaireBerger,†ZhiminSong,TianboLi,XuebinLi,AsmeromY.Ogbazghi,RuiFeng,ZhentingDai,AlexeiN.Marchenkov,EdwardH.Conrad,PhillipN.First,andWaltA.deHeer*SchoolofPhysics,GeorgiaInstituteofTechnology,Atlanta,Georgia30332-0430ReceiVed:October7,2004Wehaveproducedultrathinepitaxialgraphitefilmswhichshowremarkable2Delectrongas(2DEG)behavior.Thefilms,composedoftypicallythreegraphenesheets,weregrownbythermaldecompositiononthe(0001)surfaceof6H-SiC,andcharacterizedbysurfacesciencetechniques.Thelow-temperatureconductancespansarangeoflocalizationregimesaccordingtothestructuralstate(squareresistance1.5k¿to225k¿at4K,withpositivemagnetoconductance).Low-resistancesamplesshowcharacteristicsofweaklocalizationintwodimensions,fromwhichweestimateelasticandinelasticmeanfreepaths.Atlowfield,theHallresistanceislinearupto4.5T,whichiswell-explainedbyn-typecarriersofdensity1012cm-2pergraphenesheet.ThemosthighlyorderedsampleexhibitsShubnikov-deHaasoscillationsthatcorrespondtononlinearitiesobservedintheHallresistance,indicatingapotentialnewquantumHallsystem.Weshowthatthehigh-mobilityfilmscanbepatternedviaconventionallithographictechniques,andwedemonstratemodulationofthefilmconductanceusingatop-gateelectrode.Thesekeyelementssuggestelectronicdeviceapplicationsbasedonnanopatternedepitaxialgraphene(NPEG),withthepotentialforlarge-scaleintegration.1.IntroductionTheexceptionalelectronictransportpropertiesoflow-dimensionalgraphiticstructureshavebeenamplydemonstratedincarbonnanotubesandnanotube-basedtransistors.Ballistictransporthasbeenobserveduptoroomtemperature,1-3andquantuminterferenceeffectsatcryogenictemperatures.4-6Simplenanotubetransistors,7,8andinterconnectedlogicgates9havebeendemonstrated,whichrelyontheabilitytocontrolthenanotubeconductanceviaanelectrostaticgate.Thebasictransportparametersofthesedevicesaresocompellingthatnanotubesareconsideredtobeacandidatematerialsystemtoeventuallysupplantsiliconinmanyelectronicdevices.Anunder-appreciatedfactisthatmostelectronicpropertiesofcarbonnanotubesaresharedbyotherlow-dimensionalgraphiticstructures.Forexample,planarnanoscopicgrapheneribbons(i.e.,ribbonsofasinglesheetofgraphite)havebeenstudiedtheoretically,10,11andtheyexhibitpropertiesthataresimilartonanotubes.Grapheneribbonswitheithermetallicorsemiconductingelectronicstructurearepossible,dependingonthecrystallographicdirectionoftheribbonaxis.10Thus,ifsuitablemethodsweredevelopedtosupportandaligngraphenesheets,itwouldbepossibletocombinetheadvantagesofnanotube-likeelectronicpropertieswithhigh-resolutionplanarlithographytoachievelarge-scaleintegrationofballisticdevices.Anessentialdifferencebetweennanotubesandplanargrapheneribbonsisthepresenceofdanglingbondsattheedges.Normallythesewouldbehydrogen-terminated,withlittleinfluenceonthevalenceelectronicproperties.However,edgeatomscouldbepassivatedwithdonororacceptormolecules,thustuningtheelectronicpropertieswithoutaffectingthegraphiticbackboneofthedevice.Thisletterpresentsrecentresults12thatshowthetwo-dimensionalnatureofelectricaltransportinultrathingraphite(multilayeredgraphene)grownepitaxiallyonSiC(0001).6H-SiCisalargebandgap(3eV)semiconductor,whichprovidesaninsulatingsubstrateattemperaturesbelow50Kforthen-type(nitrogen)dopingemployedhere.Weusemagnetoconductancemeasurementsandthephysicsofweaklocalizationtodeterminetransportparametersofthegraphite2Delectrongas(2DEG),andweshowthatthecharacterofthemagnetotransport/localizationspansawiderangeofbehaviors,dependingontheamountofdisorderinthefilmorsubstrate.QuantumoscillationsinthemagnetoconductanceandintheHallresistancearefoundforthemostorderedsample.ThecharacterofthesefeaturessuggeststhatthequantumHalleffectcouldbeobservedatlowertemperatures,higherfields,orinultrathingraphitefilmsofonlyslightlyhighermobility.Toourknowledge,thesearethefirsttransportmeasurementsonorientedandpatternedgraphitefilmsofonlyafewmonolayersthickness(hence“graphene”films),althoughrelatedtransportexperimentshavebeendoneonthicker(65-100graphenelayers)free-standinggraphitemicrodisks,whichwerenanopatternedbyfocused-ion-beamlithography.13Giventhelargemeanfreepathsmeasuredinhigh-qualitygraphites,14theunusualelectronicdispersionofgraphene,andthefactthatthecarrierslienearanair-exposedsurface,thisunique2DEGsystemholdsgreatscientificpotential.Further-more,withsufficientlyhigh-qualitymaterial,ballisticandcoherentdevicesanalogoustonanotubedesigns15wouldbepossible.Thisgoalrequiresthattheepitaxialgraphenecansurvivetheprocessingnecessaryforcreationofsubmicronribbons,10,11andthatthe2DEGcanbegatedelectrostatically.Belowwealsodemonstratethesecriticalelementsforthe*Correspondingauthor.E-mail:[e-mail]†Permanentaddress:CNRS-LEPES,BP166,38042GrenobleCedex,France.19912J.Phys.Chem.B2004,108,19912-1991610.1021/jp040650fCCC:$27.50©2004AmericanChemicalSocietyPublishedonWeb12/03/2004realizationofelectronicdevicesbasedonnanopatternedepitaxialgraphene(NPEG).2.ResultsandDiscussionUltrathinepitaxialgraphitef