黑磷场效应晶体管

1BlackPhosphorusField-effectTransistorsLikaiLi1,YijunYu1,GuoJunYe2,QingqinGe1,XuedongOu1,HuaWu1,DonglaiFeng1,XianHuiChen2*andYuanboZhang1*1StateKeyLaboratoryofSurfacePhysicsandDepartmentofPhysics,FudanUniversity,Shanghai200438,China2HefeiNationalLaboratoryforPhysicalScienceatMicroscaleandDepartmentofPhysics,UniversityofScienceandTechnologyofChina,Hefei,Anhui230026,China*Email:chenxh@ustc.edu.cn,zhyb@fudan.edu.cn2Two-dimensionalcrystalshaveemergedasanewclassofmaterialswithnovelpropertiesthatmayimpactfuturetechnologies.Experimentallyidentifyingandcharacterizingnewfunctionaltwo-dimensionalmaterialsinthevastmaterialpoolisatremendouschallenge,andatthesametimepotentiallyrewarding.Inthiswork,wesucceedinfabricatingfield-effecttransistorsbasedonfew-layerblackphosphoruscrystalswiththicknessdowntoafewnanometers.Draincurrentmodulationontheorderof105isachievedinsamplesthinnerthan7.5nmatroomtemperature,withwell-developedcurrentsaturationintheI-Vcharacteristics–bothareimportantforreliabletransistorperformanceofthedevice.Samplemobilityisalsofoundtobethicknessdependent,withthehighestvalueupto~1000cm2/Vsobtainedatthickness~10nm.Ourresultsdemonstratethepotentialofblackphosphorusthincrystalasanewtwo-dimensionalmaterialforfutureapplicationsinnano-electronicdevices.BlackphosphorusisalayeredmaterialinwhichindividualatomiclayersarestackedtogetherbyVanderWaalsinteractions,muchlikebulkgraphite1.Insideasinglelayer,eachphosphorusatomiscovalentlybondedwiththreeadjacentphosphorusatomstoformapuckeredhoneycombstructure2–4(Fig.1a).Thethreebondstakeupallthreevalenceelectronsofphosphorus,sounlikegraphene5,6amonolayerblackphosphorus(termed“phosphorene”)isasemiconductorwithapredicteddirectbandgapof~2eVattheΓpointofthefirstBrillouinzone7.Forfew-layerphosphorene,interlayerinteractionsreducethebandgapforeachlayer3added,andeventuallyreach~0.3eV(refs8–11)forbulkblackphosphorus.ThedirectgapalsomovestotheZpointasaconsequence7,12.Suchabandstructureprovidesamuchneededgapforthefield-effecttransistor(FET)applicationoftwodimensional(2D)materialssuchasgraphene,andthethickness-dependentdirectbandgapmayleadtopotentialapplicationsinoptoelectronics,especiallyintheinfraredregime.Inaddition,observationsofphasetransitionfromsemiconductortometal13,14andsuperconductorunderhighpressure15,16indicatecorrelatedphenomenaplayanimportantroleinblackphosphorusunderextremeconditions.Herewefabricatefew-layerphosphorenedevicesandstudytheirelectronicpropertiesmodulatedbyelectricfieldeffect.Excellenttransistorperformancesareachievedatroomtemperature.Inparticular,importantmetricsofourdevicessuchasdraincurrentmodulationandmobilityareeitherbetterorcomparabletoFETsbasedonotherlayeredmaterials17,18.Westartwithbulkblackphosphoruscrystalsthatwegrowunderhighpressureandathightemperatures(seeMethods).Thebandstructureofbulkblackphosphorusisverifiedbyangle-resolvedphotoemissionspectroscopy(ARPES)measurementsaswellasabinitiocalculations.ThefilledbandsoffreshlycleavedbulkcrystalasmeasuredbyARPESareshowninFig.1b,whichforthemostpartagreewithscreenedhybridfunctionalcalculationswithnomaterial-dependentempiricalparameters(dashedandsolidlinesinFig.1bforfilledandemptybandsrespectively).Thecalculatedbandgap(~0.2eV)alsoagreesreasonablywellwithprevious4measurements8–11,consideringscreenedhybridfunctionalcalculationstendtounderestimateslightlythesizeofthebandgapinsemiconductors19–21.Wenextfabricatefew-layerphosphoreneFETswithaback-gateelectrode(seeFig.2a).Ascotchtapebasedmechanicalexfoliationmethodisemployedtopeelthinflakesfrombulkcrystalontodegeneratelydopedsiliconwafercoveredwithalayerofthermallygrownsilicondioxide.Opticalmicroscopyandatomicforcemicroscopy(AFM)areusedtohuntthinflakesamplesanddeterminetheirthickness(Fig.2a).Metalcontactsarethendepositedonblackphosphorusthinflakesbysequentialelectron-beamevaporationofCrandAu(typically5nmand60nm,respectively)throughastencilmaskthatisalignedwiththesample.StandardelectronbeamlithographyprocessandothercontactmetalssuchasTi/Auarealsousedtofabricatefew-layerphosphoreneFETs,andsimilarresultsareobtainedintermsofdeviceperformance.Theswitchingbehaviourofourfew-layerphosphorenetransistoratroomtemperatureischaracterizedinvacuum(~10-6mBar),inaconfigurationdepictedinFig.2a.Wesweeptheback-gatevoltage,gV,appliedtothedegenerateddopedsilicon,asthesource-drainbias,dsV,acrosstheblackphosphorusconductivechannelisheldatfixedvalues.Resultsobtainedfromadevicewitha5nmchannelontopof90nmSiO2gatedielectricareshowninFig.2b.Whenthegatevoltageisvariedfrom-30Vto0V,thechannelswitchesfrom“on”stateto“off”state,andadropindraincurrentbyafactorof~105isobserved.Themeasureddraincurrentmodulationis4ordersofmagnitudelargerthanthatingraphene(duetoitslackofbandgap),and5approachesthevaluerecentlyreportedinMoS2devices17.Suchahighdraincurrentmodulationmakesblackphosphorusthinfilmapromisingmaterialforapplicationsindigitalelectronics22.Similarswitchingbehaviour(withvaryingdraincurrentmodulation)isobservedonallblackphosphorousthinfilmtransistorswiththicknessesupto50nm.Wenotethatthe“on”statecurrentofourdeviceshasnotyetreachedsaturation,duetothefactthatthedopinglevelislimitedbythebreak-downelectricfieldoftheSiO2back-gatedielectric.Itisthereforepossibletoachieveevenhigherdraincurrentmodulationbyusinghigh-kmaterialsasgatedielectricsforhigherdoping.Meanwhile,asubthresholdswing(SS)of~5V/decadeisobserved,whichismuchlargerthantheSSincommercialSi-baseddevices(~70mV/decade).WenotethattheSSinourdevicesvariesfromsampletosample(from~3.7V/decadeto~13.3V/decade),andisonthesameorderofmagnitudeasreportedinmultilayerMoS2deviceswithasimilarback-gateconfiguration23,24.TheratherbigSSismainlyattributedtothelargethicknessoftheSiO2back-gatedielectricthatweuse,andmultiplefactorssuchasinsulatorlayerthickness25,Schottkybarrieratsubthresholdregion24,andsample-substrateinterfacestatemaybealsoatplay.Theswitching-offatthenegativesideofgVsweepisaccompaniedbyaslightturn-onatpositivegatevoltagesasshowninFig.2b.Tofurtherexplorethisambipolarbehaviour,wefabricatefew-layerphosphorenedeviceswithmultipleelectricalcontacts(Fig.2cinset),andperformHallmeasurementusingtwoopposingcontacts(V2andV4forexample)perpendiculartothedrain-sourcecurrentpathtomeasurethetransverseresistance,xyR.TheHallcoefficientHR,definedastheslope6ofxyRasafunctionofexternalmagneticfieldB,reflectsboththesignandthedensityofthechargecarriersinthesample.AsshowninFig.2c,acarriersigninversionisclearlyobservedinthe“on”states,withpositiveandnegativegatevoltagecorrespondingtoholeandelectronconductionrespectively.ThisunambiguouslyshowsthattheambipolarswitchingofthedevicesiscausedbytheFermilevelshiftingfromthevalencebandintotheconductionband.ThenatureoftheelectricalconductionisfurtherprobedbyperformingI-Vmeasurementinatwo-terminalconfiguration(Fig.2a).Asshowninfig.2d,thesource-draincurrent,dsI,varieslinearlywithdsVinthe“on”stateoftheholeside,indicatinganohmiccontactinthisregion.MeanwhiledsIv.s.dsVisstronglynon-linearontheelectronside(Fig.2dinset),typicalforsemiconductingchannelswithSchottkybarriersatthecontacts.TheobservedI-Vcharacteristicscanbereadilyexplainedbyworkfunctionmismatchbetweenthemetalcontactsandfew-layerphosphorene:thehighworkfunctionofthemetalelectrodescausesholeaccumulationatthemetal-semiconductorinterface,whichformsalowresistanceohmiccontactforthep-dopedsample;whileforn-dopedsampleadepletionregionisformedattheinterface,leadingtoSchottkybarriersandthusthenon-linearconduction.Thismodelalsoexplainstheobserveddisparitybetweentheconductionatelectronandtheholesideinalloursamples(Fig.2b),andiswidelyacceptedtodescribethecontactbehaviourinMoS2devices26.Forpotentialapplicationsindigitalandradio-frequencydevices,thesaturationofthedraincurrentiscrucialtoreachthemaximumpossibleoperationspeeds22.By7carefullychoosingtheratiobetweenchannellengthandsilicondioxidelayerthickness,awell-definedcurrentsaturationcanbeachievedinthehighdrain-sourcebiasregion(Fig.3a).Meanwhiletheelectricalcontactsremainohmicinthelinearregionatlowdrain-sourcebiases.ResultsshowninFig.3aareobtainedinthe“on”stateoftheholesideoftheconductionina5nmsample,witha4.51-µm-longchannelon90nmSiO2gatedielectric.Suchawell-developedsaturationbehaviour,whichisabsentingraphenebasedFETdevices22,iscrucialforreachinghighpowergains.Coupledwiththefactthatourchannelthicknessisontheorderofnm,andthusrobustagainstshort-channeleffectswhenthechannellengthisshrunktonmscale,ourresultssuggestthehighpotentialofblackphosphorusinhighspeedfield-effectdevicesapplication.Wenotethatthe“on”stateconductanceofourdeviceisrelativelylowandthethresholdsource-drainbiasisrelativelyhighcomparedtotypicalSi-baseddevices.Bothofthesetwofactorsareattributedtothelongchannellengthinourcurrentdevice.Betterdeviceperformance,i.e.largersaturationcurrentandlowerthresholdbias,isexpectedifthechannellengthandthegateoxidethicknessarereduced.FurtherinvestigationsareneededtotestthelimitofthedeviceperformancesofblackphosphorusFETs.Wenowturntothecharacterizationoffield-effectmobilityinfew-layerphosphorenedevices.Conductance,G,wasmeasuredasafunctionofgV,andweextractthefield-effectmobility,FE,inthelinearregionofthetransfercharacteristics27:)(1thggFEVVddGCWL(1)8whereLandWarethelengthandwidthofthechannelrespectively,gCisthecapacitanceperunitarea,andthVthethresholdgatevoltage.Holemobilityashighas984-1-12sVcmisobtainedona10nmsampleasshowninFig.3b,andisfoundtobestronglythickness-dependent.Transfercharacteristicsoftwoothertypicalsamplesofdifferentthicknesses(8nmand5nm,withthe5nmsamplethesameonemeasuredinFig.3a)areshowninFig.3b.Theconductanceismeasuredinafour-terminalconfigurationtoavoidcomplicationsfromelectricalcontacts25.Two-terminalconductancemeasurementsetupisalsousedonsomeofourdevices.Itisfoundtoover-estimatetheholemobility,butstillyieldsvaluesonthesameorderofmagnitude(Fig.3binset).Suchmobilityvalues,thoughstillmuchlowerthanthatingraphene28–30,comparefavorablywithMoS2samples17,18,24,andarealreadymuchhigherthanvaluesfoundintypicalsilicon-baseddevicescommerciallyavailable(~500/Vscm2)22.Thethicknessdependenceofthetwokeymetricsofmaterialperformance–draincurrentmodulationandmobility–isfurtherexploredtoelucidatethetransportmechanismoffew-layerphosphoreneFETs.TheexperimentalresultsaresummarizedinFig.3binset.Thedraincurrentmodulationdecreasesmonotonicallyasthesamplethicknessisincreased,whilethemobilitypeaksat~10nmandcomesdownslightlybeyondthat.Similarthickness-dependenceofcarriermobilityhasbeenreportedinother2DFETssuchasfew-layergrapheneandMoS2,wheremodelstakingintoaccountthescreeningofgateelectricfieldwereinvokedtoaccountfortheobservedbehaviour32,33.Simplyspeaking,thegateelectricfieldinducesfreecarriersonlyinthe9bottomlayersduetochargescreening.Sothetoplayersstillgivefiniteconductioninthe“off”state,reducingthedraincurrentmodulation.Meanwhilefield-effectmobilityisalsodominatedbythecontributionfromlayersatthebottom.Thinnersamplesaremoresusceptibletothechargeimpuritiesattheinterface(thustheirlowermobilities)whichareotherwisescreenedbytheinducedchargeinthickersamples.Thisexplainsthesharpincreaseofthefield-effectmobilitybelow~10nm.Asthesamplesgetthicker,however,anotherfactorhastobetakenintoaccount:sincethecurrentisinjectedfromelectricalcontactsonthetopsurface,thefiniteinter-layerresistanceforcesthecurrenttoflowinthetoplayerswhicharenotgatedbytheback-gate.Thiseffectdepressesthefield-effectmobilityforsamplesthickerthan~10nm.Basedonabovearguments,wemodeltheelectricalconductioninoursamplesusingaself-consistentlyobtainedcarrierdistribution(formoredetailsseeSupplementaryInformation)andourcalculationfitswellwiththeexperimentaldataasshowninFig.3binset.Themodelalsosuggestsawaytoachievehighermobilitywithoutsacrificingthedraincurrentmodulation:usingatop-gatedevicestructurewithalayerofhigh-kdielectricmaterialasthegatedielectric,onecouldeffectivelyscreenthechargeimpurities,butleavethedraincurrentmodulationintact.Inaddition,sincethetoplayerswherethecurrentflowsarenowgatedbythetop-gate,themobilityisnolongeraffectedbytheinter-layerresistance.SuchamethodhasbeenproventoworkinMoS2FETs17,18.FinallyweexaminethetemperaturedependenceofthecarriermobilitytouncovervariousfactorsthatlimitthemobilityinourFETs.Heretwotypesofcarrier10mobilityaremeasuredonthesamedeviceforcomparison:oneistheFEextractedfromthelinearpartofthegate-dependentconductance(Fig.4a)accordingtoEq.(1);andtheotherisHallmobilityHobtainedbyneGWLH(2)whereeisthechargeofanelectronandnisthe2Dchargedensitydeterminedfromgatecapacitance)(thggVVCn,whichequalstothedensityextractedfromHallmeasurementHeRn/1ifthesamplegeometrypermitsanaccuratedeterminationofHallcoefficientHR(seeSupplementaryInformationfordetails).Thetwomobilitiesinan8nmsampleasfunctionsoftemperatureareshowninFig.4b.Theyfallinthevicinityofeachother,andshowsimilartrendasthetemperatureisvaried:bothdecreaseattemperaturehigherthan~100K,andsaturate(ordecreaseslightlyforlowcarrierdensities)atlowertemperatures.Thebehaviourofthemobilityasthetemperatureisloweredto2Kisconsistentwithscatteringfromchargedimpurities31.WenotethatinthistemperaturerangetheHallmobilityincreasesasthegateinducedcarrierdensitygetslarger(Fig.4d).Thereducedscatteringinthesamplepointstothediminisheddisorderpotentialasaresultofscreeningbyfreechargecarriers.Thisfurthercorroboratesourmodelthatthechargedimpurityatthesample/substrateinterfaceisalimitingfactorforthecarriermobility.Ontheotherhandthedropinmobilityfrom~100Kupto300Kcanbeattributedtoelectron-phononscatteringthatdominatesathightemperatures31,andthetemperaturedependenceroughlyfollowsapowerlawTasseeninFig.4b.Theexponentdependsontheelectron-phononcouplinginthesample,andisfoundcloseto~0.5in11our8nmdevice(asaguidetotheeye,5.0~TisplottedinFig.4basadashedline).Thisvalueforfew-layerphosphoreneisnotablysmallerthanthevalueinother2Dmaterials32andbulkblackphosphorus11,butagreeswiththatinmonolayerMoS2coveredbyalayerofhigh-dielectric18.Theexactmechanismofthesuppressionofphononscatteringinfew-layerphosphoreneisnotclearatthismomentanddeservesfurtherstudy.Inconclusion,wehavesucceededinfabricatingp-typeFETsbasedonfew-layerphosphorene.Oursamplesexhibitambipolarbehaviorwithdraincurrentmodulationupto~105,andafieldeffectmobilityvalueupto~1000-1-12sVcmatroomtemperature.Thecarriermobilityislimitedbychargeimpurityscatteringatlowtemperaturesandelectron-phononscatteringathightemperatures.Theoncurrentislowandsubthresholdswingishigh,butoptimizationofthegatedielectricshouldimprovethesecharacteristics.Theabilitytomaketransistorscombinedwiththefactthatfew-layerphosphorenehasadirectbandgapintheinfraredregime,makeblackphosphorusacandidateforfuturenano-electronicandopto-electronicapplications.12MethodsSamplegrowthBlackphosphoruswassynthesizedunderaconstantpressureof10kbarbyheatingredphosphorusto1000°Candslowlycoolingto600°Catacoolingrateof100°Cperhour.RedphosphoruswaspurchasedfromAladdinIndustrialCorporationwith99.999%metalsbasis.Highpressureenvironmentwasprovidedbyacubic-anvil-typeapparatus(RikenCAP-07).X-raydiffraction(XRD)wasperformedonaSmartlab-9diffractometer(Rikagu)usingCuKαradiation(Fig.S1,SupplementaryInformation).MeasurementsTheARPESmeasurementswereperformedatBaDElPhbeamlineattheElettrasynchrotronradiationfacilitywithanMBSA-1electronanalyzer.Theoverallenergyresolutionwassetto20meVorbetterandthetypicalangularresolutionis0.5Deg.Duringthemeasurementthetemperaturewaskeptat60Ktoavoidtheonsetofcharging.DatashowninFig.1bweretakenwiths-polarized20eVphotonsandnoobviouspolarizationdependentbutnoticeableintensityvariationwereobservedforobservedbands.TransportmeasurementsaremainlyperformedinanOxfordInstrumentsOptistatAC-V12cryostatwithsamplesinvacuum(~10-5mBar).PartofmeasurementsisdoneintheOxfordInstrumentsIntegraTMACcryostatandQuantumDesignPPMSwhenmagneticfieldisneeded.DataarecollectedinaDCsetupusingDL121113currentpreamplifierwithavoltagesource,orKeithley6220currentsourcecombinedwithKeithley2182nanovoltmeter.SomeoftheHallmeasurementsaredoneusinganSRS830lock-inamplifier.BandstructurecalculationOurabinitiobandstructurecalculations,basedondensityfunctionaltheory,wereperformedusingtheprojectoraugmentedwavemethod33,34,asimplementedintheViennaabinitioSimulationPackage(VASP)code35.ThecrystalstructuredataofblackphosphorusweretakenfromReferences2,7.Fortheexchange-correlationenergy,weusedthescreenedhybriddensityfunctionaloftheHeyd-Scuseria-Ernzerhoftype(HSE06)36.DetailsofthecalculationaredescribedintheSupplementaryInformation.References:1.Delhaès,P.Graphiteandprecursors.(GordonandBreachSciencePublishers,2001).2.Brown,A.&Rundqvist,S.Refinementofthecrystalstructureofblackphosphorus.ActaCrystallographica19,684–685(1965).3.Slater,J.C.,Koster,G.F.&Wood,J.H.SymmetryandFreeElectronPropertiesoftheGalliumEnergyBands.Phys.Rev.126,1307–1317(1962).4.Cartz,L.,Srinivasa,S.R.,Riedner,R.J.,Jorgensen,J.D.&Worlton,T.G.Effectofpressureonbondinginblackphosphorus.TheJournalofChemicalPhysics71,1718–1721(1979).5.Novoselov,K.S.etal.ElectricFieldEffectinAtomicallyThinCarbonFilms.Science306,666–669(2004).6.Berger,C.etal.UltrathinEpitaxialGraphite: 2DElectronGasPropertiesandaRoutetowardGraphene-basedNanoelectronics.J.Phys.Chem.B108,19912–19916(2004).7.Takao,Y.&Morita,A.Electronicstructureofblackphosphorus:Tightbindingapproach.PhysicaB+C105,93–98(1981).148.Keyes,R.W.TheElectricalPropertiesofBlackPhosphorus.Phys.Rev.92,580–584(1953).9.Warschauer,D.ElectricalandOpticalPropertiesofCrystallineBlackPhosphorus.JournalofAppliedPhysics34,1853–1860(1963).10.Maruyama,Y.,Suzuki,S.,Kobayashi,K.&Tanuma,S.Synthesisandsomepropertiesofblackphosphorussinglecrystals.PhysicaB+C105,99–102(1981).11.Akahama,Y.,Endo,S.&Narita,S.ElectricalPropertiesofBlackPhosphorusSingleCrystals.JournalofthePhysicalSocietyofJapan52,2148–2155(1983).12.Asahina,H.,Shindo,K.&Morita,A.ElectronicStructureofBlackPhosphorusinSelf-ConsistentPseudopotentialApproach.JournalofthePhysicalSocietyofJapan51,1193–1199(1982).13.Jamieson,J.C.CrystalStructuresAdoptedbyBlackPhosphorusatHighPressures.Science139,1291–1292(1963).14.Vanderborgh,C.A.&Schiferl,D.Ramanstudiesofblackphosphorusfrom0.25to7.7GPaat15K.Phys.Rev.B40,9595–9599(1989).15.Kawamura,H.,Shirotani,I.&Tachikawa,K.Anomaloussuperconductivityinblackphosphorusunderhighpressures.SolidStateCommunications49,879–881(1984).16.Wittig,J.&Matthias,B.T.Superconductingphosphorus.Science160,994–995(1968).17.Radisavljevic,B.,Radenovic,A.,Brivio,J.,Giacometti,V.&Kis,A.Single-layerMoS2transistors.NatNano6,147–150(2011).18.Radisavljevic,B.&Kis,A.Mobilityengineeringandametal–insulatortransitioninmonolayerMoS2.NatMateradvanceonlinepublication,(2013).19.Heyd,J.,Scuseria,G.E.&Ernzerhof,M.HybridfunctionalsbasedonascreenedCoulombpotential.TheJournalofChemicalPhysics118,8207–8215(2003).20.Heyd,J.,Peralta,J.E.,Scuseria,G.E.&Martin,R.L.EnergybandgapsandlatticeparametersevaluatedwiththeHeyd-Scuseria-Ernzerhofscreenedhybridfunctional.TheJournalofChemicalPhysics123,174101–174101–8(2005).21.Marsman,M.,Paier,J.,Stroppa,A.&Kresse,G.Hybridfunctionalsappliedtoextendedsystems.J.Phys.:Condens.Matter20,064201(2008).22.Schwierz,F.Graphenetransistors.NatNano5,487–496(2010).23.Liu,H.,Neal,A.T.&Ye,P.D.ChannelLengthScalingofMoS2MOSFETs.ACSNano6,8563–8569(2012).24.Das,S.,Chen,H.-Y.,Penumatcha,A.V.&Appenzeller,J.HighPerformanceMultilayerMoS2TransistorswithScandiumContacts.NanoLett.13,100–105(2013).25.Knoch,J.,Zhang,M.,Appenzeller,J.&Mantl,S.Physicsofultrathin-bodysilicon-on-insulatorSchottky-barrierfield-effecttransistors.Appl.Phys.A87,351–357(2007).26.Fontana,M.etal.Electron-holetransportandphotovoltaiceffectingatedMoS2Schottkyjunctions.Sci.Rep.3,(2013).27.Schroder,D.K.SemiconductorMaterialandDeviceCharacterization.(JohnWiley&Sons,2006).1528.Chen,J.-H.,Jang,C.,Xiao,S.,Ishigami,M.&Fuhrer,M.S.IntrinsicandextrinsicperformancelimitsofgraphenedevicesonSiO2.NatNano3,206–209