Wang and Charles M Lieber Department of Chemistry and Chemical Biology and Di ision of Engineering and Applied Science Har ard Uni ersity Cambridge Massachusetts 02138 Received November 1 2002 ABSTRACT Silicon nanowires can be prepared with singlec ID: 24304
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HighPerformanceSiliconNanowireFieldEffectTransistorsYiCui,ZhaohuiZhong,DeliWang,WayneU.Wang,andCharlesM.Lieber*DepartmentofChemistryandChemicalBiology,andDiisionofEngineeringandAppliedScience,HarardUniersity,Cambridge,Massachusetts02138ReceivedNovember1,2002Siliconnanowirescanbepreparedwithsingle-crystalstructures,diametersassmallasseveralnanometersandcontrollableholeandelectrondoping,andthusrepresentpowerfulbuildingblocksfornanoelectronicsdevicessuchasfieldeffecttransistors.Toexplorethepotentiallimitsofsiliconnanowiretransistors,wehaveexaminedtheinfluenceofsource-draincontactthermalannealingandsurfacepassivationonkeytransistorproperties.Thermalannealingandpassivationofoxidedefectsusingchemicalmodificationwerefoundtoincreasetheaveragetransconductancefrom45to800nSandaveragemobilityfrom30to560cmswithpeakvaluesof2000nSand1350cms,respectively.Thecomparisonoftheseresultsandotherkeyparameterswithstate-of-the-artplanarsilicondevicesshowssubstantialadvantagesforsiliconnanowires.Theusesofnanowiresasbuildingblocksforfuturenanoelectronicsarediscussed.Semiconductornanowires(NWs)andcarbonnanotubesareattractivecomponentsforfuturenanoelectron-icssincetheycanexhibitarangeofdevicefunctionandatthesametimeserveasbridgingwiresthatconnecttolargerscalemetallization.Forexample,fieldeffecttransistors(FETs)havebeenconfiguredfromNWsandNTsdepositingthenanomaterialonaninsulatingsubstratesurface,makingsourceanddraincontactstotheNWorNTends,andthenconfiguringeitherabottomortopgateelectrode(Figure1).Thisbasicapproachmayserveasthebasisforhybridelectronicsystemsconsistingofnanoscalebuildingblocksintegratedwithmorecomplexplanarsiliconcircuitry,althoughanumberofissuesincludingdeviceperformance,reproducibility,andintegrationwillhavetobeaddressedinordertorealizesuchsystemsinthefuture.InthecaseofNTFETs,considerableefforthasbeenplacedonimprovingkeydeviceparameterssuchasthecarriermobilityandtransconductance.Forexample,recentstudiesreportedbyAvourisandothershaveshownthatthemeasuredcarriermobilitycanbeincreasedsignificantlybycontactthermalannealing.ThefactthatNTsamplesconsistofmixturesofsemiconductingandmetallicbuildingblockscould,however,representahurdletofuturedevelop-mentsinnanoelectronics.Ontheotherhand,siliconnanowires(SiNWs)arealwayssemiconducting,andthedopanttypeandconcentrationcanbecontrolledduringSiNWbuildingblocksmayalsobemorereadilyintegratedintosiliconindustryprocessingandfabricationthanNTs,andthusmightreducebarrierstothecreationofhybridstructures. *Correspondingauthor.E-mail:cml@cmliris.harvard.eduDepartmentofChemistryandChemicalBiology.DivisionofEngineeringandAppliedScience. Figure1.(A)SchematicofaSiNWFETshowingmetalsourceanddrainelectrodeswiththeNWandcontactsonthesurfaceof/Sisubstrate.(inset)High-resolutiontransmissionelectronmicrographofa5nmdiameterSiNW;thescalebaris5nm.(B)ScanningelectronmicrographofaSiNWFETdevice;thescalebaris500nm.Vol.3,No.210.1021/nl025875lCCC:$25.002003AmericanChemicalSocietyPublishedonWeb01/01/2003 InitialtransportstudiesofSiNWFETsshowedrelativelylowtransconductanceandcarriermobility(0.01cm/Vs).SubsequentinvestigationsofSiNWtransistorcharacteristicsinacrossedNWFETconfigurationindicatedthattheinitiallowtransconductanceandmobilityvaluesweredueinparttopoorcontactsbetweentheSiNWsandsource-drainelectrodesandarenotintrinsictothesingle-crystalNWbuildingblocks(inset,Figure1A).Inthisletter,weexplorethelimitsofSiNWFETsbyexaminingtheinfluenceofsource-draincontactannealingandsurfacepassivationonkeytransistorproperties.Thermalannealingandpassivationofoxidedefectsbychemicalmodificationwerefoundtoincreasetheaveragetransconductancefrom45to800nSandaveragemobilityfrom30to560cmswithpeakvaluesfrom2000nSand1350cms,respectively.Comparisonoftheseresultsandotherkeyparameterswithstate-of-the-artplanarsilicondevicesshowssubstantialadvantagesfortheSiNWsasbuildingblocks.TheSiNWsusedinthesestudiesareboron-doped(p-type),20nmdiametersingle-crystalstructures,preparedbyananocluster-mediatedgrowthmethoddescribedpreviously.TheSiNWsweredepositedontooxidizedsiliconsubstrates(600nmthermaloxide)fromethanolsuspension.ElectricalcontactstotheSiNWsweremadebydefiningsource-drainelectrodesseparatedby8002000nmwithelectron-beamlithographyandsubsequentevaporationof50nmTiand50nmAu.Rapidthermalannealingwascarriedoutat300Cfor3minintheforminggas(10%HinHe)toimprovethecontactandpassivateSiinterfacetraps.AtypicalSiNWdeviceisshowninFigure1B.Electricaltransportmeasurementswereperformedintheair.Source-draincontactstoSiNWFETsweremadeusingTimetal,andtransportcharacteristicswerestudiedasafunctionofannealing.Ingeneral,metalsiliconcontactshavebeenwellstudied,anditisknownthatTicanformastableconductingsilicidewithalowSchottckybarrierheightonp-typesilicon.Figure2Ashowsthecurrent(I)versussource-drainvoltage(V)behaviorofatypicalTi-contactedSiNWdevicebeforeandafterthermalannealing.TheIcurvesbecomemorelinearandsymmetric,theconductanceincreases3-fold,andthetransportbehaviorbecomesmorestableafterannealing.Tocharacterizethereproducibilityoftheseobservations,wehavemadesimilarmeasurementsonover50devices.Theseresultsaresummarizedinahistogramshowingthefrequencythatdifferentvaluesoftwo-terminalresistancewereobserved(Figure2B).Beforeannealing,theresistanceshowsalargedistributionrangingfromwithanaverageof160M.Incontrast,theresistanceafterannealinghasanarrowerdistributionof0.1to10Mwithanaverageof0.62M;thatis,a260inthetwo-terminalconductance.Theincreasedtwo-terminalconductanceandstabilitycanbeattributedinparttobetterSiNWcontacts,althoughpassivationofdefectsatSiinterface,whichcanoccurduringannealing,mayalsocontributetotheobservedenhancements.WehavealsoinvestigatedhowchemicalpassivationoftheSiOshellsurroundingthesingle-crystalSiNWcoresaffectstransportbehavior(Figure3A),sincetheSi/SiOinterfaceandSiOsurfacedefectscouldcompensatetheappliedgatevoltageandtrapandscattercarriers.modificationwascarriedoutbyreactionwith4-nitrophenyloctadecanoate.ThisspecificreagentwaschosensinceitwillleadtoastableandrelativelynonpolarSiCesterConductance(I/V)versusbackgatevoltage(Vmeasurementswerecarriedoutbeforeandaftermodificationtoassessclearlytheeffectofsurfacechemistryoncharac-teristicsofaspecificSiNWFET.Inatypicaldevice(Figure3B),theconductancerespondsweaklytoVbeforemodification.Incontrast,theconduc-tanceisextremelysensitivetoVaftermodificationandcanbeshutoffatV2.5Vwithanon/offratioover4ordersofmagnitude.Thetransconductanceaftermodificationisanorderofmagnitudelargerthanbeforemodification.Usingacylinderonaninfiniteplatemodel,weestimateaholemobilityof1000cms.ThismobilityissubstantiallylargerthanobtainedinconventionalSidevices.Toassessthereproducibilityofthissurfacechemistryandcorrespond-ingdramaticimprovementsindevicebehavior,wecarriedoutexperimentsonanumberofdistinctSiNWdevices.Thesummaryoftheseresults(Figure3CandD)showsthattransconductanceandmobilityincreaseanorderofmagni-tudeaftermodification.Significantly,thehighestandtheaverageholemobilityvaluesof1350and560cmsinp-SiNWsaremorethananorderofmagnitudelargerthanthevalueforbulkSi,40cms,atacomparableeffective Figure2.(A)IvsVmeasuredonthesameSiNWbefore(green)andafter(red)thermalannealing.(B)HistogramofTi-contactedSiNWresistancedeterminedfrommeasurementsbefore(opengreenbars)andafter(filledredbars)contactannealing.Thearrowsmarktheaverageresistancevaluesforthetwodistributions.NanoLett.,Vol.3,No.2, dopingconcentration.ThehighermobilitiesobservedinourSiNWdevicesmayarisefromimprovedstructuralperfectionandcylindricalmorphology,althoughadditionalstudiesareneedtoaddressunambiguouslythispoint.WealsonotethattheseSiNWvaluesarenotupperboundssincecontactsandsurfacepassivationhavenotbeenoptimized.Toobtainadditionalinformationabouttheoriginofthesepassivationresults,wehavecarriedoutotherexperiments.First,theeffectofchainlengthoncharacteristicsofSiNWFETswasinvestigated.Immediatelyaftermodification,18-carbonand6-carbonchainsgavesimilartransportresults,althoughthelifetimeoftheobservedimprovementwasweekand1day,respectively.Theseresultssuggestthataccessibilityofthesurfacetowaterhydrolysismaycontrolstabilityofthepassivation,andmoreover,thatthepassivationofpolarsurfacesites,suchasSiO,isimportanttotheobservedimprovedtransportbehavior.Second,andtotestthisidea,wehavemodifiedSiNWsurfaceswithtetraethyl-ammoniumbromidesolutions.Significantly,tetraethyl-ammonium-modifiedNWsalsoshowthesignificantin-creasesinthetransconductanceandmobility(Figure3E).Inaddition,itisworthnotingthatourSiNWFETsarestructurallyandchemicallyanalogoustothesilicon-on-insulator(SOI)structuresbeingdevelopedforfuturegenera-tionmicroelectronics.Toexplorethisanalogyinquanti-tativeterms,wehavecomparedkeycharacteristicsofSiNWFETswithstate-of-the-artplanarmetal-oxideFETs(MOSFETs)fabricatedusingSOI(Table1).First,theholemobility,whichisca.independentofdevicesize,isanordermagnitudelargerthanthatinplanarSideviceswithcomparabledopantconcentrations.Sincethemobilitydetermineshowfastchargecarriersmoveintheconductingchannel,itisonekeyparameteraffectingtherawdevicespeed.Ignoringotherfactors,ourmobilityimpliesthatterahertzoperationcouldbeachievedina2000nmSiNWFordirectcomparisonofotherkeyparameters,theSiNWFETresultshavebeenscaledusingtheSOIFETgatelengthof50nmandgateoxidethicknessof1.5nm.Significantly,thescaledon-statecurrent(I)fortheSiNWFETislargerthanstate-of-the-artSiFETs,andmoreovertheaveragesubthresholdslopeapproachesthetheoreticallimitandtheaveragetransconductanceisca.10timeslarger.Theseimprovementscouldleadtosubstantialbenefitsforhigh-speedandhigh-gaindevices.TheSiNWFETdevicesalsohavelargerleakagecurrents,butthisissuecouldbeaddressedbyimplementingpn-diodesatthesourceanddraincontactsasinconventionalMOSFETs.ThiscomparisonsuggeststhateffortstomakesmallerSiNWFETsandexplicitlytestscalingpredictionscouldhaveanimportantimpactinthefuture.Inconclusion,wehaveperformedstudiesaddressingthelimitsofp-typeSiNWFETs.Source-draincontactthermalannealingandsurfacepassivationwerefoundtoimprovesignificantlytheFETperformancewithincreasesintheaveragetransconductancefrom45to800nSandaveragemobilityfrom30to560cms,withpeakvaluesof2000nSand1350cms,respectively.Inaddition,comparisonofscaledSiNWFETtransportparameterswiththoseforstate-of-the-artplanarMOSFETsshowthatSiNWshavethepotentialtoexceedsubstantiallyconventionaldevices,andthuscouldbeidealbuildingblocksforfuturenanoelectronics.WethankH.ParkandL.Lauhonforhelpfuldiscussion.C.M.L.acknowledgestheDefenseAd-vancedResearchProjectsAgencyforgeneroussupportofthiswork.(1)Cui,Y.;Duan,X.;Hu,J.;Lieber,C.M.J.Phys.Chem.B,5213. Figure3.(A)Schematicillustratingsurfacedefectpassivation.(B)ConductancevsVmeasuredonthesameSiNWbeforeandafter4-nitrophenyloctadecanoatemodification.HistogramsofSiNWtransconductance(C)andmobility(D)beforeandafter4-nitrophenyloctadecanoatemodification.(E)HistogramofSiNWmobilityvaluesaftermodificationwithtetraethylammoniumbro-mide.Inplots(B)to(E),thegreen(red)colordesignatesbefore(after)surfacemodification. Table1.ComparisonoftheKeyDeviceParametersbetweenSiNanowireandSOIFET rawdatacoverteddataplanarSigatelength(nm)80020005050gateoxidethickness(nm)6001.51.5mobility(cm/Vs)2301350230m)5020020005600650m)2504459subthresholdslope609607010027007500650NanoLett.,Vol.3,No.2, (2)Cui,Y.;Lieber,C.M.,891.(3)Duan,X.;Huang,Y.;Cui,Y.;Wang,J.;Lieber,C.M.,66.(4)Huang,Y.;Duan,X.;Cui,Y.;Lauhon,L.J.;Kim,K.-H.;Lieber,C.,1313.(5)Huang,Y.;Duan,X.;Cui,Y.;Lieber,C.M.NanoLett.(6)Duan,X.;Huang,Y.;Lieber,C.M.NanoLett.,487.(7)Tans,S.;Verschueren,A.;Dekker,C.Nature(London)(8)Martel,R.;Schmidt,T.;Shea,H.R.;Hertel,T.;Avouris,Ph.Phys.Lett,2447.(9)Wind,S.J.;Appenzeller,J.;Martel,R.;Derycke,V.;Avouris,Ph.Appl.Phys.Lett,3817.(10)Derycke,V.;Martel,R.;Appenzeller,J.;Avouris,Ph.NanoLett.,453.(11)Martel,R.;Derycke,V.;Lavoie,C.;Appenzeller,J.;Chan,K.K.;Tersoff,J.;Avouris,Ph.Phys.Re.Lett.,256805.(12)Rosenblatt,S.;Yaish,Y.;Park,J.;Gore,J.;Sazonova,V.;McEuen,P.L.NanoLett.,869.(13)Lieber,C.M.Sci.Am.,50.(14)Wolf,S.;Tauber,R.N.SiliconProcessingfortheVLSIEra;LatticePress:SunsetBeach,CA,1986;Vols.1,2.(15)Cui,Y.;Lauhon,L.J.;Gudiksen,M.S.;Wang,J.;Lieber,C.M.Appl.Phys.Lett.,2214.(16)Sze,S.M.PhysicsofSemicondutorDe;JohnWiley&Sons:NewYork,1981.(17)Gambino,J.P.;Colgan,E.G.Mater.Chem.Phys.,99.(18)SiNWsurfaceswerecleanedinOplasma(20W,60s,0.4TorrOfollowedbyreactionin(a)thepyridinesolutionwith0.5mg/mL(dimethylamino)pyridine(Aldrich)and1mg/mL4-nitrophenyloctadecanoate(or4-nitrophenylhexanoate,Aldrich)for6h,or(b)1Mtetraethylammoniumbromide(Aldrich)aqueoussolutionfor30min.Thedeviceswerethenrinsedanddriedwithnitrogengas.(19)Iler,R.K.TheChemistryofSilica;Wiley:NewYork,1979.(20)Scriven,E.F.V.Chem.Soc.Re,129.(21)Chau,R.Proc.IEDM,621.(22)ThemeasuredSiNWFETvalues(column2)werescaledtothedimensionsofastate-of-the-artMOSFET(column4)assumingthecontactresistanceismuchsmallerthanthechannelresistanceandtheresistanceofchannelisproportionaltolength,andcalculatingthecapacitancebasedonthecylinderoninfiniteplatemodelthecurrentperunitwidthusingaNWdiameter20nm.Calculationsdonotaccountforinterfacetrapstates,andthusitshouldbepossibletoincreasethetransconductancefurther.NanoLett.,Vol.3,No.2,