Demonstration of Inorganic Carbon Nanotube Enabled Vertical Field Effect Transistors

MISSING IMAGE

Material Information

Title:
Demonstration of Inorganic Carbon Nanotube Enabled Vertical Field Effect Transistors
Physical Description:
1 online resource (100 p.)
Language:
english
Creator:
Wang, Po-Hsiang
Publisher:
University of Florida
Place of Publication:
Gainesville, Fla.
Publication Date:

Thesis/Dissertation Information

Degree:
Doctorate ( Ph.D.)
Degree Grantor:
University of Florida
Degree Disciplines:
Physics
Committee Chair:
Rinzler, Andrew Gabriel
Committee Members:
Hershfield, Selman Philip
Hebard, Arthur F
Tanner, David B
So, Franky Fat Kei

Subjects

Subjects / Keywords:
nanotube -- vfet -- zno
Physics -- Dissertations, Academic -- UF
Genre:
Physics thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract:
Carbon nanotube enabled vertical field transistor (CN-VFET) was developed as a new class of transistors with novel architecture that enables high density circuitry as well as delivers industrially relevant performance. This new design took advantage of using carbon nanotube thin films as transparent electrodes, enabling low-cost processing and high transparency. These CN-VFETs and their relatives show great promise for the next generation of electronics. N-channel CN-VFETs based on a solution deposited ZnO nanoparticle thin film as the channel material were demonstrated. The devices exhibit on/off ratios approaching $10^4$ with output current densities exceeding 60 mA/cm$^2$. the modulation of the Schottky barrier formed at carbon nanotube/silicon heteronjunctions was explored using ionic liquid as gate dielectric. The influence of interface states at the interface were also studied via silicon surface oxidation as the means of surface passivation. The general results with proper surface passivation showed a modulation of 10$^3$--10$^4$ at gate voltages of $\pm$ 0.4V in either forward or reverse bias. Temperature dependence of the characteristics of a carbon nanotube enabled vertical field transistor (CN-VFET) had been investigated in detail. Channel layers with moderate and high mobility organic semiconductors have been fabricated and characterized at different temperatures. Results of high mobility semiconductors show that CN-VFETs operate with different principles from conventional field effect transistors (FETs) that the device operation is most likely dominated by carrier injection at the interface of the source and channel rather than the modulation of carrier transport inside the channel layer. We also demonstrated silicon based CN-VFETs. Unlike previous CN-VFETs, a single crystal silicon wafer is used as the channel layer and devices with a top gate dielectric were fabricated. The device exhibit a current on/off ratio of nearly 10$^5$ with an on current density exceeding 5 A/cm$^2$ over a drain voltage of 2 V.
General Note:
In the series University of Florida Digital Collections.
General Note:
Includes vita.
Bibliography:
Includes bibliographical references.
Source of Description:
Description based on online resource; title from PDF title page.
Source of Description:
This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility:
by Po-Hsiang Wang.
Thesis:
Thesis (Ph.D.)--University of Florida, 2013.
Local:
Adviser: Rinzler, Andrew Gabriel.
Electronic Access:
RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2014-05-31

Record Information

Source Institution:
UFRGP
Rights Management:
Applicable rights reserved.
Classification:
lcc - LD1780 2013
System ID:
UFE0045311:00001


This item is only available as the following downloads:


Full Text

PAGE 1

DEMONSTRATIONOFINORGANICCARBONNANOTUBEENABLEDVERTICALFIELDEFFECTTRANSISTORSByPO-HSIANGWANGADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOLOFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENTOFTHEREQUIREMENTSFORTHEDEGREEOFDOCTOROFPHILOSOPHYUNIVERSITYOFFLORIDA2013

PAGE 2

c2013Po-HsiangWang 2

PAGE 3

Towhoeverhelpedmewiththiswork 3

PAGE 4

ACKNOWLEDGMENTS IthasbeenalongjourneyobtainingaPhD,thatIbelieveI'velearnedthatbetterthananyoneelse.Therewereobstacles,frustrations,andthepathhadneverbeentrivial.Ihavedonethiswithmywholeheartandfulldeterminationandyetthatwasnotenoughforaccomplishingadegree.Thiscannotbedonewithoutthehelpfromothers.SoIwouldliketotakethisopportunitytothankDrRinzlerforhisfullfundingsupportandassistingmethroughoutmygraduatestudy.Ithankhimforintroducingmetotheworldofcarbonnanotubeandgivingmeanexcitingresearchproject.I'dalwaysappreciatedhisuniquewayofseeingthingsandpioneeringideas.Ialsowanttothankmyadvisorycommittee:Dr.DavidTanner,Dr.ArtHebard,Dr.SelmanHersheldandDr.FrankySo,forservingonmyadvisorycommitteeandfortheirhelpfulconversationsandadvices.Alloftheaboveteacherstaughtmealot,Iwascondentenoughtofeellikeadoctoralreadybeforemydefense,butfeltlikeastudentagainaftertheexam.Luckilytheywereeasyonme.AlsothankstoDrHebardandDrBiswasforprovidingknowledgeandhelpinoperationofcryostat.AlsothankstomycolleaguesintheRinzlergroup,Dr.SvetlanaVasilyeva,DrBoLiuandDrMitchellMcCarthy,DrRajibDas,DrMaxLemaitre,MaureenPetterson,RameshJayaraman,YuShen,XiaoChen,NanZhao,JieHou,MattGilbertandKyleDorothy.Manyofthemtaughtmeskillsorprovidinghelpinresearchandofferingmeinspiringconversations.Severalofthemshowedmethingsotherthanresearch,andsomeofthemlendmetheirnotes.Ithankoneofmyco-workerSanalBuvaev(whostillcouldn'tspellmylastname)fromtheHebardgroup.HehelpedmewiththecryostatsothatIcannishmylastworkinthisdissertation.IalsothankmanyotherfriendsinPhysicsforprovidingvaluablediscussionsandteachingmeseveralfundamentalprinciples.AndmysinceregratitudegoestoBoLiuandMitchellMcCarthy,withouttheirhelpandextensiveworksandexperiencesthisworkcannevergofasterandeasier. 4

PAGE 5

Also,IwanttothankpeopleinUFNRF,Dr.BrentGila,aswellastheengineeringstaff:BillLewis,DavidHaysandAlOgdenfortheirguidanceandsupportinprovidingtrainingandusingthefacility.Over80%ofmyworkwasdoneinNRF.Thesuccessreliesontheirgoodjobofmaintainingthefacilityingreatcondition.AndthankstoMarc,Ed,BillandtherestinPhysicsmachineshopfortheireffortinfacilitatingourresearch.IpersonallylearnedalotfromMarcLinkandEdStorchthroughtoutmymachinedesignanddrafting.IwouldliketothankthePhysicsDepartmentsupportstaff,PamMarlinDarleneLatimer,GregLabbeandtheelectronicsshopstaff,PeteAxsonandLarryPhelps.Finally,IwouldliketothankmyfamilyandallmyfriendsinGainesvilleforgivingmetheirsupport,andmyfriendsnotinGainesvillewhoconstantlymadelong-distancecallstome,givingmethestrengthtonishmywork. 5

PAGE 6

TABLEOFCONTENTS page ACKNOWLEDGMENTS .................................. 4 LISTOFFIGURES ..................................... 8 ABSTRACT ......................................... 10 CHAPTER 1INTRODUCTION ................................... 12 1.1OverviewofCarbonNanotubeThinFilm ................... 12 1.2CarbonNanotubeEnabledVerticalFieldTransistor ............. 15 1.3CarbonNanotube-SemiconductorHeterojunction .............. 19 2FUNDAMENTALSOFCARBONNANOTUBETHINFILM ............ 22 2.1OverviewofCarbonNanotube ........................ 22 2.2BandStructureofGraphene .......................... 25 2.3Single-WalledCarbonNanotubesasRolled-UpGraphene ......... 28 2.4ElectronicStructureofCarbonNanotubes .................. 30 2.5ElectronicPropertiesofCarbonNanotubeThinFilm ............ 34 3CN-VFETSWITHSOLUTIONDEPOSITEDZINCOXIDENANOPARTICLEBASEDCHANNELLAYERS ............................ 41 3.1Overview .................................... 41 3.2MaterialsandMethods ............................. 42 3.3ResultsandDiscussion ............................ 43 3.4Summary .................................... 48 4MODULATIONOFCARBONNANOTUBE/SILICONHETEROJUNCTIONSUSINGIONICLIQUID ................................ 50 4.1Overview .................................... 50 4.2MaterialsandMethods ............................. 53 4.3ResultsandDiscussion ............................ 55 4.4Summary .................................... 61 5CURRENTTRANSPORTCHARACTERISTICSINTHECN-VFET ....... 63 5.1Overview .................................... 63 5.2MaterialsandMethods ............................. 67 5.3ResultsandDiscussion ............................ 68 5.4Summary .................................... 71 6

PAGE 7

6SILICONBASEDCARBONNANOTUBEENABLEDVERTICALFIELDEFFECTTRANSISTORSWITHSOLIDSTATETOPGATE ................. 74 6.1Overview .................................... 74 6.2MaterialsandMethods ............................. 76 6.3ResultsandDiscussion ............................ 78 6.4Summary .................................... 87 7CONCLUSIONS ................................... 89 REFERENCES ....................................... 90 BIOGRAPHICALSKETCH ................................ 100 7

PAGE 8

LISTOFFIGURES Figure page 1-1ComparisonofconventionalTFTarchitecturetoCN-VFET ............ 17 1-2Simpliedenergy-banddiagramofametalp-typesemiconductor ........ 21 2-1Schematicdiagramofthegraphenelatticeandrollednanotubes ........ 24 2-2Graphenelatticeandbandstructure ........................ 27 2-3EnergydispersionofSWNT ............................. 28 2-4ThemapofchiralvectorsforSWNTs ........................ 30 2-5Nanotubedensityofstates ............................. 33 2-6CarbonNanotubethinlms ............................. 36 2-7SpectraltransmittanceoftheO-NFET ....................... 40 3-1TransfercurveoftheCN-VFETwithoutannealingandplasmatreatment .... 44 3-2TransfercurveoftheCN-VFETfollowingathermalannealandafterbothathermalannealandanoxygenplasmatreatment ................. 46 3-3TransfercurveoftheCN-VFETwithoutannealingandplasmatreatment .... 47 3-4On/OffratiofortheannealedCN-VFET ...................... 48 4-1DevicedesignforstudyingthemodulationofCNT/p-SiHeterojunction ..... 54 4-2CNT/p-SiJunction .................................. 56 4-3Current-VoltagecharacteristicsofaCNT/p-Siheterojunctiondevice ...... 57 4-4Schematicbanddiagramfordeviceswitchingmechanism ............ 58 4-5On/offratiosforCNT/p-Siheterojunctiondevices ................. 60 4-6Deviceperformancewithanadditionalprotectionlayer .............. 62 5-1SchematicofPPMScryostatchamber ....................... 69 5-2TemperaturedependentCN-VFETwithDNTTaschannellayer ......... 71 5-3TemperaturedependentCN-VFETwithTFBaschannellayer .......... 72 6-1SibasedCN-VFETdevicedesign ......................... 77 6-2CNT/p-SiJunction .................................. 79 6-3SibasedCN-VFEToutputcharacteristics ..................... 80 8

PAGE 9

6-4ModulationofSchottkybarrierheight ........................ 82 6-5On/offratiosforsiliconbasedCN-VFET ...................... 83 6-6EnergybandbendingdiagramforCNT/Siinterface ................ 85 6-7UniformityofsiliconbasedCN-VFETs ....................... 86 6-8TransfercharacteristicsforsiliconbasedCN-VFETatvarioustemperatures .. 87 9

PAGE 10

AbstractofDissertationPresentedtotheGraduateSchooloftheUniversityofFloridainPartialFulllmentoftheRequirementsfortheDegreeofDoctorofPhilosophyDEMONSTRATIONOFINORGANICCARBONNANOTUBEENABLEDVERTICALFIELDEFFECTTRANSISTORSByPo-HsiangWangMay2013Chair:AndrewG.RinzlerMajor:PhysicsCarbonnanotubeenabledverticaleldtransistor(CN-VFET)wasdevelopedasanewclassoftransistorswithnovelarchitecturethatenableshighdensitycircuitryaswellasdeliversindustriallyrelevantperformance.Thisnewdesigntookadvantageofusingcarbonnanotubethinlmsastransparentelectrodes,enablinglow-costprocessingandhightransparency.TheseCN-VFETsandtheirrelativesshowgreatpromiseforthenextgenerationofelectronics.N-channelCN-VFETsbasedonasolutiondepositedZnOnanoparticlethinlmasthechannelmaterialweredemonstrated.Thedevicesexhibiton/offratiosapproaching104withoutputcurrentdensitiesexceeding60mA/cm2.ThemodulationoftheSchottkybarrierformedatcarbonnanotube/siliconheteronjunctionswasexploredusingionicliquidasgatedielectric.Theinuenceofinterfacestatesattheinterfacewerealsostudiedviasiliconsurfaceoxidationasthemeansofsurfacepassivation.Thegeneralresultswithpropersurfacepassivationshowedamodulationof1034atgatevoltagesof0.4Vineitherforwardorreversebias.Temperaturedependenceofthecharacteristicsofacarbonnanotubeenabledverticaleldtransistor(CN-VFET)hadbeeninvestigatedindetail.Channellayerswithmoderateandhighmobilityorganicsemiconductorshavebeenfabricatedandcharacterizedatdifferenttemperatures.Resultsofhighmobilitysemiconductorsshow 10

PAGE 11

thatCN-VFETsoperatewithdifferentprinciplesfromconventionaleldeffecttransistors(FETs)thatthedeviceoperationismostlikelydominatedbycarrierinjectionattheinterfaceofthesourceandchannelratherthanthemodulationofcarriertransportinsidethechannellayer.WealsodemonstratedsiliconbasedCN-VFETs.UnlikepreviousCN-VFETs,asinglecrystalsiliconwaferisusedasthechannellayeranddeviceswithatopgatedielectricwerefabricated.Thedeviceexhibitacurrenton/offratioofnearly105withanoncurrentdensityexceeding5A/cm2overadrainvoltageof2V. 11

PAGE 12

CHAPTER1INTRODUCTIONThinlmsofpuriedcarbonnanotubes(CNTs)resultinanovelmaterialthatistransparentandconductiveandisemergingalongwithotherpotentialcandidatessuchasgraphene,metallicnanowires,andconductivepolymers,inreplacingthepresentlydominatingmaterial,indiumtinoxide(ITO)asakeymaterialinmodernelectronicdevices.Thesetransparentelectrodeshavenumerousapplicationsinmoderntechnologiesrangingfromvideodisplaytophotovoltaics.Amongalltheemergingmaterialshowever,CNTthinlmtechnologyisperhapsthemostpromisingandmaturetechnology. 1.1OverviewofCarbonNanotubeThinFilmWhilemostofthenoveltransparentconductorsmentionedabovepossessadvantagesovertraditionalmaterialsformanufacturingsimplicationandcostreduction;grapheneandCNTsaremorechemicallyandenvironmentallyrobustcomparedtoconductivepolymersormetallicnanowireswhichcandegradeunderheat,humidity,UVandotherconditions.Furthermore,grapheneandCNTthinlmscanbeeasilypatternedthroughstandardlithographyandsubtractiveetchingprocess[ 1 ].GrapheneandCNTthinlmswithathicknessintherangeof10to100nmshowhighopticaltransparencyandelectricalconductivitycomparabletoITO.[ 2 7 ]AlthoughCNTthinlmshavebeenstudiedlonger(sincemid-2000s),graphene,thesocalledwondermaterial,hasbeenbecomingmoreandmorepopulareversinceitsdiscoveryin2005[ 8 ]mainlyduetothefactthatasatransparentelectrode,graphenegenerallyperformsbettercomparedtoCNTthinlms[ 7 9 ].However,solutionbasedcoatingprocessesatambientconditionsofCNTthinlmscreatescriticalmanufacturingadvantagesoverpresentgraphenetransferprocess,whichoftenformscracksandwrinkles.Unlikegraphene,asingleatomiclayersheet,classiedasasemimetal;CNTthinlmismadeofmanyquasione-dimensionalCNTsexistingasatwodimensionalthin 12

PAGE 13

layer,inwhichCNTshavetwodifferentavors,metallicandsemiconducting,duetotheirelectronicproperties.FormetallicCNTs,theyarerealconductorswhereelectronstransportinaquasione-dimensionalspace;whileforsemiconductingCNTs,theyareactualsemiconductorsbecausetheyhaveanactualbandgapwhereasgrapheneisonlyasemi-metalwithoutabandgap.CNTthinlmsmighthaveyetanotheredgeovergraphenebybeingabletoeasilytunetheirsheetresistanceandopticaltransparencyatdifferentthicknesses.Thismayextendtheirusefulnessindifferentapplicationsthatrequirespecicopticalandelectricalproperties.Forexample,CNTthinlmswiththicknessbelow20nmareconsideredassub-monolayerandthelmsimplylooksmorelikearandomnetworkofCNTs.Atthenetworkdensityclosetothepercolationthreshold,CNTlmsshowsemiconductingbehaviorandcanbeusedastheactivelayerinthinlmtransistorsandsensors.[ 10 14 ]Ontheotherhand,lmsinthemicrometerthicknessrangearenano-porousandcanbeusedaselectrodesforsupercapacitors,fuelcells,andbatteries;[ 15 17 ]andcanalsobeevenfurtherengineeredforperformanceimprovements.[ 18 ]Forapplicationssuchasthese,grapheneneedstobefurtherengineeredforeachspecicpurpose,whichremainsachallengeforindustrialscaleapplications.[ 9 ]SeveralmethodsfortransparentCNTthinlmfabricationhavebeendeveloped.WhilemostofthefabricationtechniquesstartwithdispersedCNTsinsolution,asolvent-lessmethodwasexploredbyDr.Baughman'sgroupthatinvolvespullingasheetofCNTsdirectlyfromaverticallygrownMWNTforest.[ 19 ]Solventbasedmethodsincludespincoating,[ 20 ]dropcasting,[ 21 ]quasi-LangmuirBlodgettdeposition,[ 22 ]dip-coating[ 23 ]directCVDgrowth,[ 24 ]air-spraying,[ 4 ]ordraw-downsusingaMayerrodcoatingbar.[ 25 ]Andperhapsthesimplestwayofobtainingqualitythinlmamongallsolutionphasemethodisbyvacuumltration;[ 2 26 ]inwhichasolutionofdispersedCNTsislteredthroughaporousltermembrane(withvacuumassistance),followedby 13

PAGE 14

rinsingwithDIwater(orothersolvent)toremoveanysurfactantorothersolubilizationagent.TherearenumerousstudiesonCNTthinlmsandtheirdeviceapplicationswithsignicantcommercialopportunities.Asmentionedearlier,oneofthemostpromisingapplicationsofCNTthinlmsistransparentelectrodes,whichalreadyhavediverseapplicationsrangingfromtouchpanels,displays,andphotovoltaicstosmartwindows,andEMIshielding.[ 27 28 ]Inparticular,UnidymInc.,aCNTlmmanufacturingandapplicationdevelopmentcompany,wascommercializingCNTlmsforresistivetouchpanels,inwhichthemechanicalrobustnessofCNTlmsholdselectricalconductionatupto20%strainonPET.[ 29 ]Indisplays,reportshaveshowntheefcacyofimplementingCNTlmsinvariousLCDdisplaydevicesinreplacingofITO.[ 30 31 ]AnotherdisplayapplicationofCNTsfocusesonorganiclightemittingdiodes(OLED)whichholdsgreatpromisefornextgenerationcommercialelectronics.CNTlmsenabledOLEDdeviceshavebeendemonstratedwithcomparableperformancetoITO-basedones.[ 5 ]YetanotherimportantapplicationofCNTbasedtransparentelectrodesissolarcells.Photovoltaicdevicesonexibleorrigidsubstrateshavebeendemonstratedbyvariousgroups.[ 32 33 ]Duetoitshighworkfunctionof4.7.2eV,CNTlmsareparticularlysuitableforholeextractionfororganicsolarcells.[ 34 35 ]WuandCaoetal.[ 36 ]havedemonstratedtheuseofCNTthinlmsasthep-typematerialonn-typesiliconformingthep-njunctionofsolarcelldevices.TheCNT/siliconheterojuctionwasthenfurtherengineeredintoeldeffectsolarcellstogiveaperformanceboost.[ 37 39 ]Besidesreplacementofexistingmaterialsincurrenttechnologies,CNTthinlmshavealsobeenproductenablers,suchasnowadaysproductivedevelopmentsofexible/printedelectronicdevices.Devicesofsuchkindincludetransparentandexiblethinlmtransistors,[ 11 14 ]andarticialactuators;[ 40 ]whilepassivedevicesinclude 14

PAGE 15

microwaveshieldingscreens,[ 41 ]transparentloudspeakers,[ 42 ]andtransparentheaters.[ 43 ] 1.2CarbonNanotubeEnabledVerticalFieldTransistorBeginninginthelate-2000s,theapplicationofCNTelectrodesforanoveltransistorarchitecturewasdevelopedandoptimizedintheRinzlergroup.[ 44 46 ]ThisnewdesigninheritedtheadvantagesofusingCNTlmsastransparentelectrodes,whichhasthepromiseoflowcostsolutionprocessingandhightransparency;aswellashighdensitycircuitryandhighperformanceoftransistors.Thisemergingclassoftransistorshowspromiseforthenextgenerationelectronics,andwasnamedacarbonnanotubeenabledverticaleldtransistor(CN-VFET).TheCN-VFETiscategorizedasoneoftheverticaleldtransistors(VFETs).Figure 1-1 presentstheconceptofstructuraldifferenceofthisnewVFETdesignfromaconventionalthinlmtransistor(TFT).AconventionalTFTiscomprisedofasource,drainandchannellayer(asemiconductorinthinlmform)thatconnectsthesourceanddrain,wherethesethreecomponentsliehorizontallyonaplanefromwhichaplanargateelectrodeisisolatedbyagatedielectric.(Figure 1-1 A)Byapplyingapositiveornegativevoltagetothegatewithrespecttothesourceelectrode,theresultingelectriceldcausesaccumulationordepletionofchargecarriersinthesemiconductor,leadingtomodulationofthebandbendingofthesemiconductor.Throughapplicationofapositivegatevoltagetoann-channelTFT,theconductionbandedgeisloweredandbroughtclosertotheFermilevel,formingaconductivepathbetweenthesourceanddrainelectrodesswitchingthetransistoron.Onthecontrary,iftheelectriceldistakentheotherdirectionbyapplyinganegativegatevoltagetheconductionbandedgemovedawaytheFermilevelandcarriersareexpelledfromthechannel,switchingthetransistoroff.ThisconventionalTFTisanimportantbuildingblockinmoderndayelectronicssuchasTFTLCDsorthebackplanecircuitryforactivematrixorganiclight-emittingdiode(AMOLED)displays.Organiceldeffecttransistors(OFETs),whichusuallyrefer 15

PAGE 16

toTFTswithorganicchannellayers,havedrawnsignicantinterestoverthepasttenyearsduetotheirlowtemperature,largeareaandlowcostprocessingcompatibility.However,amajorissuepreventingtheiruseincommercializeddevicesisthatorganicsemiconductorsgenerallyhaveachargecarriermobilitythatistypicallyseveralordersofmagnitudelowerthanamorphoussiliconoratbest,comparable.Buthighmobilityorganicsemiconductorsaretypicallypolycrystallinewithpooruniformity,andstillnoneofthemiscomparabletothepolycrystallinesiliconorIndiumgalliumzincoxide(IGZO),thehighmobilitysemiconductorsusedinthemostrecentAMOLEDapplication.OneofthestrategiestocircumventthismobilitylimitationistomakethechannellengthCLofthetransistorsufcientlyshortsoastodeliverhighercurrentoutput.This,togetherwithsizingdown,causesanotherproblemofrequiringcomplexlithographypatterningthatbringssourceanddrainelectrodesclosertosub-microndistances;whichwouldnolongerbecosteffective.ThestructureoftheVFET,ontheotherhand,orientsthechannellayer(andsoasthecarrierconductivechannel)verticallywithrespecttothegateelectrodebystackingthesourceelectrode,channellayeranddrainelectrodeontopofthegatedielectric(Figure 1-1 B).Inthisway,thechannellength(CL)issimplythethicknessofthesemiconductingthinlmchannellayer,whichguaranteesinprincipleanarbitrarilythinlayerandthereforeaveryshortchannellength.TheshortchannelenabledbytheVFETarchitecturecompensatesforlowcarriermobilitieswithouthighresolutionpatterning.Neverthelessforthisnewarchitecture,theworkingmechanismoftheconventionalTFTnolongerappliesandnewconceptsfortheworkingprinciplewereintroduced.[ 44 47 ]SincefortheVFETarchitecture,acontinuousmetalsourceelectrodewouldcompletelyscreenthegateeldfrommodulatingthechannellayer,thesourceelectrodeisexpectedtorequirecertainengineeringforthisdesigntowork.Hence,either(1)aperforatedmetalsourceelectrodeor(2)anultrathinsourceelectrodewithultrahighgate 16

PAGE 17

Figure1-1. ComparisonofA)conventionalTFTarchitecturetoB)CN-VFETarchitecture,alsoshownisthewiringdiagramforCN-VFET.AnAFMimageofpercolatingnanotubenetworksourceelectrode(scale0.5m1m)isshownin(B).ThecurrentinconventionalTFT(redarrows)scaleswiththechannelwidthCW,whileinCN-VFETthisscaleswiththeoverlapareabetweenthesourceanddrainelectrodesCA.Imagesreproducedwithpermissionfrom[ 44 ]. capacitanceisrequiredforthegateeldtohaveaccesstothechannellayer.In2004YangandcoworkersdemonstratedatransistorwiththisVFETarchitectureusingexistingorganicsemiconductors.[ 48 ]InordertoswitchtheVFETonandoff,theirdevicereliedonacarefullycontrolledultrathin(<20nm)aluminumsourceelectrodethatrequiredpartialoxidationandanultrahighgatecapacitance.Theultra-highcapacitance(>1F/cm2)isenabledbyanelectrolytebasedsupercapacitordielectric,LiFthatreliedonconditionalhumidity.Further,duetothelowworkfunctionofthealuminumsourceelectrode,thedevicewasinitiallyrestrictedton-typesemiconductorslimitingthechoicescomparedtop-typeorganicmaterials.Fortheuseofmorecommonp-typeorganic 17

PAGE 18

semiconductorssuchaspentacene,thisdevicerequiresanadditionalinterfaciallayerofvanadiumoxidebetweenthesourceelectrodeandsemiconductor.[ 49 ]Asimilarapproachforimplementinganultrahighgatecapacitancewasrecentlydemonstratedusingtheenhancedelectricdoublelayergatedielectric;[ 50 ]andporousSiO2fabricatedbyplasmaenhancedchemicalvapordepositionundercertainconditionsresultsinacomparablesuper-capacitance.[ 51 ]However,allofthesedevicescanonlybeoperatedatlowfrequencybecausetheyarelimitedbytheslownessofionicmobilityonwhichtheyrely.Anotherapproachusesaperforatedsourceelectrodeandwasexploredbydifferentgroupswithwidelydivergentmeans.Forinstance,ametallicsourceelectrodepatternedintoametalgridstructuretolessensthescreeningeffectwasdemonstrated.[ 52 ]Inthisapproachasupercapacitorwasnotusedandtheoreticalfrequencylimitiseliminated,buttheon/offratiowaslowerandhighergatevoltageisrequired.Workingindependently,ourgrouphasachievedourporoussourceelectrodebyusingadilutenetworkofcarbonnanotubes.ThediluteCNTnetworkdoesnotfullycoverthedielectricsurfacebuthasasurfacedensitywellabovethepercolationthresholdsoastoretaingoodconductance.ThelevelofporosityandpercolationcanbesimplycontrolledbythenumberofCNTsonthesurfaceperunitarea;makingiteasytotunetheelectrodepropertywhilemanufacturing.Moreover,thenanotubespossessanumberofphysicalpropertiesthatbenetssuchpurpose.TherelevantpropertiesoftheCNTsare:(1)theyhaveahighaspectratio,(2)highcarriermobility,(3)lowelectronicdensityofstates,(4)arechemicallyinert,and(5)theyhavea-conjugatedsurface.First,thehighaspectratioofCNTsallowshighsurfacedensityofinterfacesbetweenCNTsandsemiconductoralongnanotubesidewalls,aswellaselectriceldenhancementthatfacilitatescarrierinjectionfromCNTstosemiconductor.ThehighmobilityofCNTsmakesthemgoodconductorscomparabletometals,whilethelowdensityofstates(DOS)meansthatandtheworkfunctioncanbemodulatedelectro-statically[ 2 ].Further,theFermilevelpinningthatiscommoninconventionalmetal-semiconductorinterface 18

PAGE 19

canbeeliminatedsincethechemicallyinertgraphene-likesurfaceofCNTssuppresstheformationofadditionalinterfacestateswithmaterialstheycomeintocontactwith.[ 44 53 ]Lastly,becauseCNTshavea-conjugatedsurface,whichinteractinapreferredmannerwith-conjugatedorganicmolecules;thatwhendepositedontopofCNTs,nucleatepolycrystallinegrainsoftheorganicmaterialwiththehighmobilitydirectionorientedvertically.[ 46 54 ]ThearchitectureoftheCN-VFETalsoaffordsimportantnewopportunitiesinelectronicallydrivingOLEDdisplays.BystackingaCN-VFETwithanOLEDaverticalorganiclightemittingtransistor(VOLET)results.[ 55 ]Thedevicewasmostrecentlyreportedtohaveacontrastratioapproach104andtheluminancesurpasses500cd/cm2atoperatingvoltagesaround5V.Plus,thisCN-VOLETemitslightacrossitsentirefacewithaneffectiveapertureratioof98%;andtheparasiticpowerdissipationisaround6.2%,whichisnearlyanorderofmagnitudelowerthanthatofmetalinsulatorsemiconductorOLETsorside-by-sidetransistor/OLEDcombinations.[ 56 ]ThesecharacteristicsofCN-VOLETmakesanimpactforfuturetechnologysinceanelectronicdevicewithhighenergyefciencygenerallyimpliesprolongedconsumerproductlifetime. 1.3CarbonNanotube-SemiconductorHeterojunctionTheCN-VFETfunctionsasaSchottkybarriertransistor[ 44 ]wherethemajormodulationofcurrentbygatingoccursattheinterfacebetweentheCNTsourceelectrodeandtheorganicsemiconductorlayer.Thisinterfaceconnedgatingisthereasonwhythechannelcanbeorientedintheverticaldirection.Withouttheinterfaceconnedgating,screeningeffectswouldnotallowthegateeldsufcientaccessthroughoutthemajorityofthechannellayertoeffectivelymodulatetheoutputcurrent.SinceaCNTthinlmcanbeseenasaconductingelectrodewithaworkfunction[ 2 ]thatistypicallydifferentfromthechemicalpotentialofasemiconductor;similartotheprototypicalmetal/semiconductorinterface,aSchottkybarrierisformedatthe 19

PAGE 20

nanotube/semiconductorjunctionduetochargetransferresultingfromthermalization.Inturn,thisgeneratesaspacechargeregioninthesemiconductorastheFermienergiesequilibrate.Thiselectrostaticbarrieryieldsanabruptpotentialdifference(theSchottkybarrierheight)atthejunctionandanelectricpotentialgradientpenetratingintothesemiconductorwhichhasacharacteristicdepletionwidth.Figure 1-2 illustratessuchapotentialgradientwhichisformedatametal/p-typesemiconductorjunction.TheSchottkybarrierheight,'bh,canbesimplyapproximatedbySchottkyMottmodel,'bh=+Eg)]TJ /F8 11.955 Tf 11.96 0 Td[(m, (1)wheremisthemetalworkfunction,Egistheenergybandgapandistheelectronafnityofthesemiconductor.Fortheorganicsemiconductor,+Egisreplacedbytheorganicmaterialshighestoccupiedmolecularorbital(HOMO)level.AkeydifferencebetweenCNT-semiconductorcontactsandconventionalmetal-semiconductorcontactsistheintrinsiclowDOSforthenanotubes.IncontrasttothehighDOSofmetals,theFermilevelofthelowDOSnanotubescanundergoanappreciableshiftinresponsetothegateeldwhichallowsmodulationoftheSchottkybarrierheight,'bh.ElectrostaticsimulationsoftheeffectonaCNT/p-typesemiconductorjunctionfordifferentgatevoltagesfurthershowsthatthebarrierthinning,resultingfromthegateinducedbandbending,andthebarrierheightloweringbothcontributetothemodulationoftheinterface.[ 44 ]Thecurrent-voltagecharacteristicofthedeviceisthereforeadirectresultofcurrenttransportthroughthistunableSchottkybarrier.Inprinciple,thecarriertransportthroughaplanarmetal/semiconductorjunctionisdominatedbythermionic-diffusionemissionandthequantummechanicaldirecttunneling.[ 57 ]Thermionicemissiondominatescurrentdensityinmoderatelydopedsemiconductorsathigh(room)temperatures,whiletunnelingusuallyappliestohighlydopedsemiconductorsorlowtemperatureoperations;andisresponsibleformostohmiccontactsinsemiconductorindustry.Mixedmodesof 20

PAGE 21

Figure1-2. Simpliedenergy-banddiagramofametalp-typesemiconductorcontact,wheremismetalworkfunction,istheelectronafnity,andEgistheenergybandgapofsemiconductor.ECandEVareconductionandvalencebandsforthesemiconductor.Inorganicsemiconductor,lowestun-occupiedmolecularorbital(LUMO)isanalogoustoconductionbandminimum,andhighestoccupiedmolecularorbital(HOMO)istovalencebandmaximum. chargeinjectionwherethermionicandeldemissionscoexistarealsopossible,andhavebeenobservedinnanotubedevices.[ 58 ]Therearetwoothercurrenttransportprocessesthatarecommonlydiscussedbutseldomdominatecurrentdensity.Recombinationofelectron-holeinthespacechargeregion(thedepletionlayer)isduemainlytominoritycarrierdriftsintothedepletionlayerandonlybecomessignicantforverylargeforwardbiasacrossthejunction.Ontheotherhand,whenapplyingveryhighreversebiasacrossthejunction,Fowler-Nordheim(FN)tunnelingbeginstocontributesignicantly.Theseprocesseswillbefurtherdiscussedindetailinthefollowingchapterswhereapplicable. 21

PAGE 22

CHAPTER2FUNDAMENTALSOFCARBONNANOTUBETHINFILMMorethantwodecadeshavepassedsincethediscoveryofcarbonnanotubesbyIjima.[ 59 ]Sincethen,numeroustheoreticalandexperimentalworkshavebeendevelopedandcarriedoutontheuniqueandintriguingpropertiesofcarbonnanotubes.Thischapterbrieydiscussesthetheoreticalbasisofthesinglewallcarbonnanotube(SWNT)withanintroductoryoverviewoftheSWNT,thebackgroundofSWNT/graphenebandstructure,andtheparticularelectronicstructureofSWNT.Attheendofthischapter,weturnourattentiontoaSWNTthinlmandseehowthesehighlightedcharacteristicsofSWNTdifferfromconventionalmetals. 2.1OverviewofCarbonNanotubeCarbonnanotubes(CNTs)arehollowcylindersmadeupofrollingupnanometer-widestripsofgraphenealongtheshortdirection.Grapheneisaone-atomicthickpurecarbonsheetwithsp2-bondedatomsdenselypackedintoahoneycomblattice.Graphenehasdrawntremendousattentionintherecentdecadeduetoitsuniquestructureandchemicalproperties,aswellasitspotentialapplicationsresultingfromitsexcellentelectricalconductivity,opticaltransparencyandmechanicalstrength.[ 60 ]Nanotubesarealsoofgreatinterestbecause,likegraphenetheyalsohaveexcellentelectrical,opticalandmechanicalproperties.Ontopofthat,theyaremoremechanicallyrobustagainstin-planecompressionortorsioncomparedtographene;andchemicallystrongersincetheydonthaveedgesfullofdanglingbondslikegraphene.IndividualCNTsarefoundtohaveacarriermobilityinexcessof100000cm2/Vs,[ 61 ]currentcarryingcapacityof109A/cm2;[ 62 ]andCNTeldeffecttransistorshavebeenshowntohaveanON/OFFcurrentratiohigherthan105.[ 63 ]Nevertheless,CNTshavebeenlosingtheirattractivenesstographenemainlybecauseoftheinabilitytoobtainsuitablepuritylevelsofCNTsinbulkquantities.CNTsynthesistypicallycontainsamixtureofvariousmaterialsincludingcatalystparticles,amorphouscarbonorfullerenes, 22

PAGE 23

andamixtureofnanotubesofvariouslengths,diameters,tubenumber,andchiralities.Theabilitytonelycontrolthenanotubechirality,sizeandtypedistributionatlargescaleremainsachallenge.WhenrollinggrapheneintoaSWNT,itispossibletohaveitsaxisofthetubehavingdifferentorientationsrelativetothehoneycomblatticeofgrahene,itisthenexpectedtohavedifferentavorsofatomicallystructurednanotubes,orchirality.Thechiralityofeachnanotubestructureisfurtherdenedbyasinglechiralvectorthatatomsofthegraphenelatticemeetatbothsidesofthestriptoformarolledtube.OncethechiralityofaSWNTisdened,itspropertiescanbefullydetermined.UnderstandingthechiralityofaSWNTisthuscrucialtoderiveitselectronicstructure.TheunrolledgraphenelatticeisshowninFigure 2-1 A,inwhichthechiralvector~ChtracesthecircumferenceoftheSWNTandisperpendiculartothenanotubeaxis(orthedirectionoftranslationalvector~T).IntheexampleofFigure 2-1 A,theshadedareawillberolledintoatubesothattheatomatpointsOandAwillbecoincide(andpointsBandB0aswell).TheatomicstructureoftheSWNTcanthusbefullydescribedbythechiralvector~Chasfollows.Usingtheunitvectorsofthehoneycomblattice~a1and~a2(wherej~aij=a=0.249nm),[ 64 ]thechiralvector~Chcanbeexpressedasasumofintegermultiplesoftheunitvectorsas~Ch=n~a1+m~a2(n,m),where0jmjn.Thesetwoparametersnandmaresufcienttouniquelydescribethechiralvector.OnecanfurtherclassifySWNTsintothreecategoriesduetotheirchirality:(1)n=mcorrespondstoarmchairnanotubes(2)m=0(orn=0)correspondstozigzagnanotubes,and(3)alltherestcorrespondtochiralnanotubes(Figure 2-1 B).Giventhechiralvector(n,m),thediameterofthenanotubedtcanbederivedasdt=ap n2+m2+nm=.TheseindiceswillappearindescribingandclassifyingthebandstructuresofSWNTs. 23

PAGE 24

A BFigure2-1. SchematicdiagramofthegraphenelatticeandrollednanotubesA)TheshadedareaiswherenanotubeisconstructedofrolledgraphenebymatchingOtoA,andBtoB0.~OAdenesthechiralvector~Chand~OBdenesthetranslationalvector~Tofthenanotube.B)Classicationofcarbonnanotubes.Fromtoptobottomarearmchair,zigzag,andchiralnanotubes,respectively.FigureB)isreproducedwithpermissionfrom[ 64 ]. 24

PAGE 25

2.2BandStructureofGrapheneBeforemovingontospecifytheelectronicpropertyofaSWNTwithauniquechiralvector(n,m),webrieydescribethetheoreticalresultsoftight-bingingcalculations[ 65 67 ]oftheelectronicstructureofgraphene.Thisresultshowsthatthetwodimensionalgraphenehasitsenergydispersioncurvesofanti-bonding(conduction)andbonding(valence)bandsjoiningattheunderlyingsix-foldsymmetriccornersofthehexagonalBrillouinzone.WewillthenderivethebandstructureoftheSWNTinthefollowingsectionsbyapplyingadditionalboundaryconditionsonthewavefunctionandenergydispersionofgraphene.Inthehoneycomblattice,eachcarbonatomhasthreevalenceelectronsinvolveinsp2in-planebonding,leavingthefourthelectroninthehybridized-orbitalthatformslobesextendingoutofthegrapheneplane.Itistheelectronsinthe-orbitalsthatconductingrapheneandSWNTs,sothatonlythe-energybandsareconsideredinthetight-bindingcalculation.InFigure 2-2 A,wedeneaunitcellofgraphenelatticecontainingtwonon-equivalentcarbonatomsatAandB.Thewavefunctionofthe-energybandisthusalinearcombinationoftwoBlochorbitalsas k(~r)=1 p N(XAexp(i~k~RA)k(~r)]TJ /F8 11.955 Tf 12.61 3.15 Td[(~RA)+XBexp(i~k~RB)k(~r)]TJ /F8 11.955 Tf 12.61 3.15 Td[(~RB)), (2)where~RAand~RBarethepositionsoftheAandBatoms,k(~r)]TJ /F8 11.955 Tf 12.72 3.16 Td[(~Ri)istheatomicorbitalofanisolatedcarbonatom,andexp(i~k~Ri)isthephasefactor.Wethenevaluatetheeigen-energies,Ek=h kjHj ki,ofthesystem.Intight-bindingcalculations,onlythenearestneighborinteractionisconsidered,sothatonlytheintegrationoverasingleatom,HAA,HBB;andBatomsrelativetoA,HAB(orHBA)areevaluated.ForHAA(orHBB),sincenootherA(orB)atomscontributetothenearestneighborinteraction,theenergyissimplyitsorbitalenergyofthe2plevel,thatisHAA=HBB=2p.Whilefortheoff-diagonalelement,HAB,threenearestneighborBatomscontribute,sothattheintegrationcomposedofatransferintegralbetweenA 25

PAGE 26

andBatoms,t,andasumofphasefactors,thatisHAB=tPjexp(i~k~Rj).DuetothesymmetrybetweenAandBatoms,wecaninferHBA=HAB.ReferringtothexycoordinatesinFigure 2-2 A,wecanevaluatethetotaleigen-energiesas:E(kx,ky)=2pts 1+4cosp 3kxa 2coskya 2+4cos2kya 2, (2)wherethetransferintegral,t,hasanexperimentalvaluet=)]TJ /F3 11.955 Tf 9.29 0 Td[(2.7eV;[ 68 ]anda=0.249nmisthelatticevectoringraphene.TheenergyrelationdescribedinEquation( 2 )isplottedinFigure 2-2 Busingtheparameters2p=0,jtj=2.7eV,anda=0.249nm.TheupperbranchofEquation(??)givestheanti-bonding(conduction)band,whilethelowerbranchgivesthebonding(valence)bandforgraphene.Thissimpliedapproach(Slater-KosterScheme)givessymmetricbandsaboveandbelow2pandwillbeusedtoobtainenergydispersionrelationsforSWNT.Ofparticularnote,Figure 2-2 Bshowsthatink-space,theconductionandvalencebandsaredegenerateatthesixhighsymmetriccornersoftheBrillouinzone,theKpoints,throughwhichtheFermienergypasses.AmongthesixK-points,onlytwoareirreducibleandarelabeledasKandK0.Thefactthattwoequivalentcarbonatomsinaunitcellofgraphenelatticeisthereasonthatmakesgrapheneaso-calledzero-gapsemiconductor(orsemi-metal).Furthermore,neartheseKpoints,theenergydispersionmimicstwoidenticalconicalsurfaceswithverticesmeetatFermilevelasshowinFigure 2-2 C.Thislineardispersionandelectron-holesymmetricbandstructureofgrapheneprovidesananalogtotwo-dimensionalmasslessDiracfermions,[ 8 67 69 70 ]withzeroeffectivemassforbothelectronsandholes,andalinearslopep 3ta 2withresultingFermivelocityvF8105m/s.Theselinearlydispersive,masslesscarriershavebeendescribedbytheDiracequationforlowFermienergies(<1eV)approximation,[ 69 ]whichhasleadtotheenergydispersionrelationbeencalledasDiracconeandtheK-pointsasDiracpoints.Formostofouranalysespresentedinthefollowsections,itis 26

PAGE 27

sufcienttouseonlythesetwonon-equivalentK-pointsandtheDiracconestoderivethebandstructureofSWNTs. A B CFigure2-2. GraphenelatticeandbandstructureA)TheUnitcellofgraphenelatticeisshownasdottedrhombuswithtwoinequivalentcarbonatomsAandB.Theunitvectorsofthelatticeisshownas~ai.B)Calculatedbandstructureofgrapheneusingtight-bindingcalculations.[ProducedwithMathWorksRMatlab.]C)DiracconeneartheK-pointsingraphenek-space. 27

PAGE 28

2.3Single-WalledCarbonNanotubesasRolled-UpGrapheneByrollinggrapheneintoacylinder,aperiodicboundaryconditioninthecircumferentialdirectionisimposed.Thewavevector,denedask?,associatedwiththechiralvector(~Ch)becomesquantized,whilethewavevector,kk,associatedwiththedirectionoftubeaxisremainscontinuousforaninnitelylongnanotube.Thisresultstoasetofone-dimensionalenergydispersionrelations(subbands)obtainedbysuperimposingthecrosssectionsoftheenergydispersionofgraphenebyasetofequallyspacedplanesinthedirectionofkk.ThewayofcuttingthoseDiracconesleadstotwodifferentavorsofSWNTsasshowninFigure 2-3 (onlyoneofsixconesisshown).Whenoneoftheplanescutsthroughthetipofthecone,theresultingsubbandislinearthroughout2pandthustheSWNTismetallicwithagaplessenergyband.Ontheotherhand,whennoneoftheplanescutsthroughthetipofthecone,theresultingSWNThasgappedenergysubbandsandiscalledsemiconducting. A BFigure2-3. EnergydispersionofSWNTA)AplanarcutthroughtheDiracconethatintersectstheK-pointresultsinenergydispersionforametallicSWNT.B)AplanarcutawayformtheK-pointinordertocompensatephasemismatchleadstoagappedenergydispersionforasemiconductingSWNT. Afullmathematictreatmentofthiszone-foldingmethodofSWNTbandstructurecanbefoundinSaitoandDresselhausses,thatleadstoanintriguingfactthat2/3ofnanotubesaresemiconductingwhiletheother1/3aremetallic.[ 64 ]Herepresents 28

PAGE 29

analternativeexplanationwithamoreintuitivephysicalpictureproposedbylaniandMcEuen.[ 71 ]Thewayofrollingupgrapheneintoatuberequiresthechiralvector(~Ch)goesfromoneAatomcarbontoanotherAatom(Figure 2-1 A),whileatthesametimerequiresthewavevectork?beenquantizedask?dt=2q,wheredtisthediameterofthenanotubeandqisaninteger.Figure 2-4 illustratesthemapofchiralvectorsofSWNTswithcolorcodedAatoms.OnlythechiralvectorsofredatomsdenotemetallicSWNTs,allothergreenandblueonesaresemiconducting.ConsideringaBlochwavetravelingfromtheAatomat(0,0),thecolorcoderepresentsthephaseoftheK-pointwavefunctiononeachoftheAatom.[ 71 ]Therearethreepossiblephases:0,2=3,and)]TJ /F3 11.955 Tf 9.3 0 Td[(2=3(or4=3),whicharecolorcodedasred,green,andblue,respectively.Ifthechiralvectorispointingfromtheredatomat(0,0)toanotherredatom,therewillbenophasedifferenceattheK-point,andthewavefunctionautomaticallysatisestheboundaryconditions;whichcorrespondstooneofthecrosssectioningplanecuttingthroughtheK-pointimplyingmetallicSWNT.Fortheothercases,wherethechiralvectorspointingtogreenorblueAatoms,aphasemismatchof2=3mustbecompensatedsoastomeettheboundaryconditions.Thusanadditionalenvelopeofik0?r?thatprovidesaslowmodulationofthephasealongthecircumferentialdirectionisintroduced.Inordertoexactlycompensatestheoriginalphasemismatchoveronerevolutionaroundthecircumference,thisslowmodulationsatisesk0?dt=2 3.Thisleadstoaleastdistanceofk0?forthesectioningplanesawayfromtheK-pointandtheSWNTissemiconducting(Figure 2-3 B).Theabovesimpleargumentleadstotwointerestingresults.First,sinceonlytheAatomswithzerophaseswillgeneratemetallic,andtheyonlyconstituteupto1/3ofallAatoms,indicatingroughly1/3ofmetallicSWNTsbeensynthesizedinarandomarrangement.Second,sincealltherest2/3semiconductingSWNTshavethesamemismatchedphaseof2=3,wecanderiveageneralresultthattheenergygapofall 29

PAGE 30

Figure2-4. ThewavefunctionontheAatomsiscolorcodedaccordingtorelativephaseaslabeled. semiconductingSWNTsequalstoEGAP=2~vFk0?=4~vF 3dt=2ta p 3dt, (2)wheret=)]TJ /F3 11.955 Tf 9.3 0 Td[(2.7eVistheoverlapenergy,a=0.249nmisthelatticeconstantofgraphene.Inotherwords,thebandgapofasemiconductingSWNTisinverselyproportionaltothediameterofthetube,afactthathasalsobeenexperimentallyveried.[ 68 ] 2.4ElectronicStructureofCarbonNanotubesTheexplicitexpressionsforthedispersionrelationsofametallicSWNTcanbefoundbysubstitutingthediscreteallowedvaluesofk?intothetwodimensionaldispersionrelationofgraphene,Equation( 2 ).[ 64 ]TheresultingenergybandscanalsobeseenintuitivelyfromFigure 2-3 AforalowFermienergy(<1eV)version.The 30

PAGE 31

one-dimensionalsubbandwiththelowestenergyofametallicSWNTisformedfromaplanarthatcutthroughthetipofDiraccones.Thenextplanesthatcutthroughtheconesatadistanceofk?=2=dtawayfromtheDiracpointformthesecondsubbandswiththesecondlowestenergies(Figure 2-3 A).ThisalsoimpliesthatatlowFermienergy,allmetallicSWNTsfollowthesamephysicalrulesregardlessoftheirchiralities.Animmediateexampleisthegeneralresultofelectronicdensityofstates(DOS)ofallmetallicSWNTs.Formetallictubes,theDOSisaconstantneartheFermienergy,andthenpeakstoinnityatthediscontinuouspointbetweenrstandsecondsubbandsandthentailsoff.Fromabove,therstsubbandofametallicSWNTisacrosssectioningattheDiracpointthatformsthedegeneratecaseoftwointersectinglines.Thenextsubbandswiththeallowedk?statesdonotpassthroughtheDiracpoint,inwhicheachoftheconicsectionisahyperbola.Nowthatweknowtheshapeofavailableenergiesasfunctionsofthewavevectorkk,wecanusethemtogettheDOS.Thedensityofstates,g(E),isasurfaceofathreedimensionalk-space,orthesurfaceintegraloftheinverseofk-gradientofenergy.Forquasi-onedimensionalnanotubes,itissimplythelinederivativeofthestateswithrespecttoenergyorsimplyberepresentedastheinverseofthederivativeoftheenergywithrespecttokk.Consideringthattherearetwosymmetriclinesinasubbandinvolvingleft-andright-movingcarriers,andthereissub-latticedegeneracyofgrapheneoftwo,thereisafour-folddegeneracyforeachderivativeofenergy.Anotherfactoroftwofortheelectronspindegeneracyalsoaccounts,whichgivesthedensityofstates(perunitlength)forametallicnanotubeas:g(E)=2(1 2)4 jdE=dkkj=4 ~vF, (2)wherethederivativeofametallicSWNTslowestenergysubbandistheuniversalslopeofDiraccones.Therefore,forallthemetallicSWNTsthereisaatDOSneartheFermienergy,regardlessoftheirchiralitiesandevendiameters. 31

PAGE 32

AswemoveawayfromtheFermienergy,morestatesbecomeavailablesincethelinesintersectinggraphenebandstructureathigherenergiesarehyperbolawhichgivesthederivativeofenergyasjdE=dkkj=~vFkk=q k2k+k2?.Followingtheargumentsgivenabove,thelinearDOSofthesubbandawayfromFermienergyisthengivenby:g(E)=4 ~vFq k2k+k2? kk=4 ~vFE p E2)]TJ /F3 11.955 Tf 11.95 0 Td[((~vFk?)2. (2)ThisexpressionleadstovanHovesingularitieswithinniteDOSatE=~vFk?,thentailsoffawayfromFermienergy.ThesameargumentappliestoasemiconductingSWNToutsideofitsbandgap,wheresimilarhyperbolaformsforthelowestenergysubband;andasimilarexpressioncanbegivenbyreplacingk?byk0?=2=3dt.TheDOSofasemiconductingSWNTthusbecomesg(E)=4 ~vFE p E2)]TJ /F3 11.955 Tf 11.95 0 Td[((~vFk0?)2=4 ~vFE p E2)]TJ /F3 11.955 Tf 11.95 0 Td[((EGAP=2)2. (2)InFigure 2-5 ,weplottedexamplesoftheDOSofbothmetallicandsemiconductingSWNTsasfunctionstotheircarrierenergy.TheseuniqueDOSsubbandstructureshavebeenexperimentallyprovedusingcapacitancemeasurementonaSWNT/oxide/metalstructure[ 72 ]andscanningtunnelingmicroscopyonanindividualSWNT.[ 68 ]SinceSWNTsarequasi-onedimensionalnano-materials,theyareexpectedtohavelowdimensionalchargetransportphenomena.[ 64 ]Consideringnovoltagedropacrossananotubeinwhichthetransportisballistic,theconductivityofsuchquasi-onedimensionalsystemcanbedescribedlinearlybyLandauerformula.Forametallicnanotube,thispredictsaquantizedresistanceofR=h 4e26.45k,where4isassociatedwitheachfour-folddegeneracyfromtwoorientationsoftheelectronspinandthetwosubbands(KandK0).Atroomtemperature,metallicnanotubescanapproachthisquantizedvalue,[ 73 74 ]andheavilydopedsemiconductingnanotubeswerealsoreportedtoapproachthislimit.[ 74 76 ]Nevertheless,becauseofdiffusivescattering,anotherfactorisoftenaddedandtheresistancebecomeslessperfectas 32

PAGE 33

A BFigure2-5. TheDOSofametallicandasemiconductingSWNTsareshowninA)andB),respectively.ItshowsvanHovesingularitiesateachcorrespondingsubband.InasemiconductingSWNT,thelowestenergysingularitiesoccurathalfofthebandgapenergy. 33

PAGE 34

R=h 4e2L Lm,whereListhenanotubelengthandLmistheelectronmeanfreepath.Ingeneralapplications,theresistivityofanindividualmetallicnanotubeofafewmicronisabout10)]TJ /F4 7.97 Tf 6.59 0 Td[(410)]TJ /F4 7.97 Tf 6.59 0 Td[(3-cmandamoderatelydopedsemiconductingnanotubeisabout101-cm.[ 64 ]AnotherkeydifferencebetweenmetallicandsemiconductingSWNTsoftheirconductanceisthewaytheirbehaveunderexternalgateelds.Withagatevoltageapplied,theFermilevelcanbetunedintothebandgapofanindividualsemiconductingSWNT,andnocurrentpassessincethethermalenergyisnotsufcienttoactivatecarrier.Ontheotherhand,ametallicSWNTcannotbeswitchedonandoffbuthasanearlyconstantconductancebecausetheDOSisaconstantneartheFermilevel.Veryofteninthecaseofarcdischargeorlaservaporizationgrownnanotubeswherealargequantityprocessiscarriedout,SWNTsbundledupasamixtureofmetallicandsemiconductingnanotubes.Thesizeofanindividualbundle(orcrystallinerope)dependsonthenumberofhexagonallyclosepackedSWNTs[ 77 ]andmayvaryfrom5nm.ForabundlemadeupofSWNTsthatwerecenteredaround1.4nm,[ 78 ]andthespacingbetweennanotubesbe0.34nm;sevenclosepackednanotubeswouldyieldabundlewithadiameteraround5nm.Whenconnectedtoanexternalelectrode,sinceeachSWNTofasmallbundlecanbeconnectedindependently,thebundlecanbeconsideredassevenparallelconductors.ThecarriertransportthroughsuchabundleisthusexpectedtodistributeamongalltheSWNTsinthebundleregardlessoftheirtypes,whichfurthersuggestslittleelectronicinteractionbetweenthem.Thiscurrentcarryingbehaviorisverydifferentfromthatofasinglemulti-walledcarbonnanotubewhereonlytheoutermostshellcarriescurrent.[ 79 ] 2.5ElectronicPropertiesofCarbonNanotubeThinFilmCarbonnanotubethinlmsisatwodimensionalnetworkofrandomlydistributednanotubesthatlaidhorizontallyonasurface.Thethinlmsusedinthisstudywerefabricatedbythevacuumltrationmethod[ 2 ]fromasolutionofpuriedCNTs.Inshort,laservaporizationgrownSWNTswererstacidpuried,thensuspendedin 34

PAGE 35

Triton-X100,asurfactantthatpreventsockingofnanotubesinaqueoussolution.TheCNTsuspensionisgenerallyfurtherpuriedbycrossowltrationtoremoveanyimpurities,andsonicatedtoreducedCNTbundlesizebeforethinlmdeposition.[ 78 ]Thenanotubeswerethendepositedonamixedcelluloseester(MCE)membranebyvacuumltrationofthenanotubesuspensionthroughthemembrane.De-ionizedwaterwasusedtowashawaysurfactantresidue.TheMCEbackedcarbonnanotubethinlmwasshapedbycuttingandthentransferringontothedesiredsubstrate.Generally,thenanotube/MCEsheetwasbroughtintocontactwiththedesiredsurfacenanotubesidedowninthepresenceofmoisture.Asthemoisturewasdrivenaway,thenanotube/MCEsheetwasbroughtintoclosecontactwiththesurfaceandendedupbondedtothesurfaceviavanderWaalforce.Finally,theMCEmembranewasdissolvedawayinanacetonevaporbath.Inthisway,thethicknessofthecarbonnanotubelmisdeterminedbytheconcentrationandvolumeofSWNTsuspensionliquidaddedtothevacuumltrationapparatus.Forinstance,aSWNTthinlmofabout50nmthickcorrespondstoacertainnumberofSWNTsdeposited,whichresultedinathicknesslargeenoughtobedeterminedbyAFMatstepedge.Whileamuchthinnerlmthatformsadilutenetwork,cannotbemeasuredbyAFM.Instead,itisestimatedbyarelativeamountofmaterialcomparedtothatusedinthe50nmlm.Forexample,1/25ofthematerialusedforthe50nmlmwouldyieldadilutenetworkthatcanbedesignatedasa2nmthicklmhavingacharacteristicsurfacedensityofCNTs.Figure 2-6 showssuchadiluteCNTnetwork(Figure 2-6 A)comparedtoa20nmthicklm(Figure 2-6 B).Thenanotubematerialusedhereconsistsmostlyofbundlednanotubescomposedofroughly1/3metallicand2/3semiconductingSWNTs.ThecarriertransportthroughaCNTnetworkcanthusbeconsideredasalargeensembleofdisorientedconnectorsrandomlydispersedintoatwodimensionalspacewheremanyparallelcurrentcarryingpathswereformedviatube-tubejunctionsandelectronicinteractionsbetweenthese 35

PAGE 36

A BFigure2-6. CarbonNanotubethinlmsA)AFMimagesofadiluteCNTnetworkofeffectively2nmthick,andB)a20nmthickCNTlm(scaleinm). pathswereinevitable.TheconductivityofCNTthinlmsislimitedlargelybythejunctionresistancebetweenCNTsratherthanbythenanotubesthemselves.[ 80 ]TheconductivityofasingleCNTcanapproach500000S/cm(calculatedusingthequantizedvaluewithanaspectratio3000),withamobilityinexcessof100000cm2/Vs;[ 61 ]whileade-doped,randomlyorientedCNTthinlmhasmanagedahighestconductivityofonly6600S/cm[ 2 ]andamobilityontheorderof1cm2/Vs.[ 10 14 ]Thetube-tubejunctionresistancesaretypicallyontheorderof200k20M.[ 81 ]Therefore,itcanbeanticipatedthatlongerCNTswillleadtomorehighlyconductivelmsbylimitingthenumberofCNTjunctionsperunitareaoflm.ThistheorywasveriedinworkconductbytheHechtgroupthatconcludedthelmconductivityvarieswiththetubelength(L)asapowerlaw(L1.46).[ 82 ]Moreover,thickerlmsmayalsoinferhigherconductivitysincemoreconductingpathsarecreated.HenceforverythinCNTlmsthatarejustabovepercolation,theconductivityincreasesdramaticallywithnetworkdensity,andsmallvariationsindensityleadtorelativelylargenon-uniformvariationsinsheetresistance.[ 83 84 ] 36

PAGE 37

SinceSWNTsusedinourgrouparefrequentlyaciddoped,everynanotubeinthenetworkshouldbeabletocarrycurrentandhavesimilarperformanceatroomtemperature.However,thesemiconductingCNTscanhavetheirresistivitysignicantlymodulatedbyanexternalgateeldandbeenswitchedonandoffaseldeffecttransistors,whereasthemetallicCNTscannot.SimilarphenomenonshouldalsoapplytobundlesinthenetworkwheretheinnersemiconductingSWNTsofabundlecanstillbemodulated,sinceSWNTsdonotshieldeachothereffectivelyinasmallbundle.[ 79 ]ThismakestheCNTnetworkitselfactslikeaeldeffecttransistor,whichhasmoreconductingpathswhenswitchedon,andonlypathsofmetallicnanotubesconductingwhenswitchedoff.However,forthethinlmthicknessesusedinthisstudywherethelmsarequitesubstantiallyabovepercolation,thisonlyresultsinapooroffcurrent(throughconductingpathsbridgedviametallicnanotubes)andtheon/offratio(3foldinRef.[ 2 ])ofsuchtransistorisnothigh.Hencetheswitchingpropertyofthelmsinthisstudyisnotamainfocusforthedeviceapplicationsinthisstudy.NowinsteadofputtingCNTnetworksswitchingabilityinapplicationbutfocusingonitsgoodconductingproperty,theCNTnetworkcanbeconsideredasaconductingelectrodewithanabilitytoshiftitsworkfunction.DuetothefactthatSWNTshavelowelectronicstatesnearFermilevel,excesschargesdrawntonanotubesfromcontactterminalsbybuildingapotentialdifferencebetweennanotubesandacounterelectrodewillmodulatetheircarrierconcentrationandshifttheirFermilevel.SuchFermilevelshiftingduetoelectrostaticdopinghasbeendemonstratedbeforebyvariousgroups.[ 75 85 86 ]ExactlyhowfartheFermilevelwillbeshiftedwasestimatedinpreviousworkbyWuetal.,[ 2 ]wheretheydemonstratedanopticalnanotube-basedeldeffecttransistor(OFET)bymodulatinglighttransmissionthroughaCNTlmatdifferentgatevoltages.ThedeviceconsistedoftwoCNTstrips(each150nm)depositedside-by-sideontoasapphiresubstrate,withoneendofbothstripseachconnectedtopalladiumelectrical 37

PAGE 38

contactsandtheotherendsbridgedbyionicliquid(IL).TheILwettedbothlmsandwasusedtoprovideanelectrolytegatepotential.OneoftheCNTstripswasheldatelectricalgroundthroughwhichasamplebeampassesfortransmittancemeasurement;whiletheotherstripactedasagatingcounterelectrode.Figure 2-7 showstheopticaltransmittancespectrumcurvesofthisdeviceasafunctionofgatevoltages.TherearethreemainvalleyseachassociatedwithS1(maximumat1676nm),S2(932nm)andM1(656nm)absorptionbandsinducedbyelectronictransitionsbetweenvanHovesingularitiesintherstandsecondsubbandsofsemiconductingandrstsubbandofmetallicSWNTs,respectively.ReferringtotheDOSdiagrams(Figure 2-7 C)ofmetallicandsemiconductingSWNTsofsimilardiameter,thelmwasinitiallyp-typedopedduetopuricationtreatmentwheretheFermilevelliessomewherebelowtherstsingularityinvalencebandofsemiconductingSWNTs.Whennegativegatepotentialwasapplied,electronsweredepletedfromthetestlmthusdepletingtheinitialgroundstatesingularityfortheelectronictransitioncorrespondingtotheS1absorption.Theabsorptionintensitiesdecreaseandtransmittanceincrease.Conversely,positivegatepotentialslltheinitialtransitionstatesingularityinthevalencebandandtheFermilevelwasshiftedintothegapatappliedpotentials,sothatallofS1,S2andM1absorptionbandsyieldtheirmaximumintensities(minimumtransmittance)attheverypositivelygatedlm.Figure 2-7 Bshowsthevoltagedependenttransmittanceofallthreeabsorptionbands.TheS1absorptionbandhasitsmosttransmittanceandS2hasappreciableincreaseintransmittanceatthemostnegativelygatedpotentialwhileM1absorptionbandisnearlyatthroughoutthevoltagescan.Thissuggeststhat-1.8Vonthegate,theFermileveloftheCNTlmisbetweenS2andM1valencevanHovesingularities.Armedwiththisdatadiscussedabove,wecannowestimatetheextentoftheFermilevelshiftoftheCNTlm.Figure 2-7 CillustratestheDOSofasemiconductingandametallicSWNTwithchiralvectors(12,8)and(10,10),respectively.Bothof 38

PAGE 39

themhavediametersofaround1.4nmthenominaldiameterofCNTmaterialusedinthestudy.[ 2 ]Asdiscussedpreviously,theenergygapbetweensingularitiesofeachsubbanddependsonthediameterofaSWNTonly,sothatthesetwochiralitiesarequitesufcientforprovidingrepresentativeexplanationtotheDOSforthenanotubesinthelm.BecausetheSWNTsusedinourgrouparehole-dopedvianitricacidpurication,theyareexpectedtohavetheirFermilevelabout0.3eVbelowtheirintrinsicworkfunction(0eVinFigure 2-7 C).[ 2 45 ]FillingelectronswillshifttheFermilevelinthepositivedirectionintotheenergygapbutnotreachtherstsingularityintheconductionband.ThereforetheshiftingoftheFermilevelinthisdirectionislessthan0.6eV.Intheotherdirection,depletingelectronsshiftstheFermilevelpastthesecondsingularityinvalencebandofsemiconductingSWNTbutdoesnotquitereachthesingularityofthemetallicSWNT.Henceitisalsolessthan0.6eV.ThetotalamountoftheFermilevelshiftoftheCNTlmintheOFETcanthenbeapproximatedto0.5eV(totalabout1eV).ThislevelofFermilevelmodulationisgreatlyassistedbytheILduetoitslargegatecapacitanceof5.9F/cm2.[ 87 ]ThisdemonstrationoftheabilitytomodulatethenanotubeFermilevelbylargeamounts(comparedtonegligiblemodulationinconventionalmetals)isputtouseinthecomingchaptersinCN-VFETarchitecturetransistorsbaseduponthedilutenanotubenetworksusedasaeldpermeablesourceelectrode. 39

PAGE 40

A B CFigure2-7. SpectraltransmittanceoftheO-NFETA)SpectraltransmittanceoftheO-NFETasafunctionofappliedcounterelectrodegatevoltage.B)Voltage-dependenttransmittanceattheS1(1676nm),S2(932nm),andM1(656nm)absorbances.C)TheDOSforarepresentative(12,8)semiconductingnanotube(solidcurve)andasuperimposed(10,10)metallicnanotube(dashedcurve)isshownusingtightbindingmodel.ArrowsbetweensingularitiesrepresentelectronictransitionsresponsiblefortheS1,S2,andM1absorptionbands.Depletionoftherstsingularity(lledelectronicstatesingray)resultsinthelossofthecorrespondingelectronictransition(dashedarrow)andlossoftheassociatedS1absorptionintensity.Figuresarereproducedwithpermissionfrom[ 2 ]. 40

PAGE 41

CHAPTER3CN-VFETSWITHSOLUTIONDEPOSITEDZINCOXIDENANOPARTICLEBASEDCHANNELLAYERS1Inthischapter,wedemonstratean-channelcarbonnanotubeenabledverticaleldeffecttransistors(CN-VFETs)withasolutiondepositedzincoxide(ZnO)nanoparticlethinlmasthechannelmaterial.Transistorperformancebenetsfromathermalannealfollowedbyanoxygenplasmatreatment.Thedevicesexhibiton/offratiosapproaching104withoutputcurrentdensitiesexceeding60mA/cm2.Combinedwithp-channelorganicCN-VFETsthesolutionbasedprocessingcouldallowforthedevelopmentoflow-costcomplementarycircuits. 3.1OverviewBymakingthethicknessofthechannellayerthechannellength,theverticaleldeffecttransistor(VFET)architectureallowsfornaturallyshortchannellengths,overcomingtherelativelylowmobilityoforganicsemiconductors,permittinghighon-currentsfromsmallfootprintdevices.[ 45 46 ]HerewedemonstrateVFEToperationusingasolutiondepositedinorganicchannellayerconsistingofZnOnanoparticles(NP).Nanoparticlebasedlmsalsosufferfromlowmobilityduetoimpedanceatthefrequentparticle-particlegrainboundariesandcanthusbenetfromtheVFETdesign.Moreover,sincemostairstableorganicsemiconductorstendtobep-type,airstablen-typeZnObasedVFETsshouldbeusefulinthefabricationofcomplimentarylogiccircuits,whichrequirebothp-andn-typesemiconductors.Conventional,lateralchannel,TFTsusingsolutiondepositedZnOnanoparticlechannellayershavepreviouslybeendemonstrated,withon/offratiosrangingfrom103105butrequiringcomparativelyhighsource-drainvoltagetoachievehighoncurrents.[ 89 91 ] 1Partofthematerialsinthischapterisreproducedwithpermissionfrom[ 88 ] 41

PAGE 42

TransconductanceinVFETsarisesfromthegateeldmodulationofacontactbarrieratthesource-electrode/channel-layerinterface.Acontinuoussourceelectrodelayertendstoscreenthegateeldfromthisinterface,sotominimizesuchscreeningthesourceelectrodeismadeporoustothegateeld.Differentgroupshaveattainedtheirsourceelectrodeporositybywidelydivergentmeans.[ 48 50 52 ]Wehaveexploiteddilutenetworksofsinglewallcarbonnanotubeswhichgivenecontrolovertheelectrodeporositybythenetworkdensityaswellasfurtheradvantagesdiscussedelsewhere.[ 2 ]DevicesofourdesignaredesignatedcarbonnanotubeenabledVFETs(CN-VFETs).TheconductionbandedgeofZnOat4.2eV,formsabarrierwiththecarbonnanotubeworkfunction.Singlewallcarbonnanotubeshaveanative(ungated)workfunctionof4.6eVbutthiscanbetunedbychargetransferdopingtoyieldtunableinitialbarriers.OurHNO3puriednanotubeshaveaworkfunctioncloseto4.9eV,butcontrolledbakingininertatmosphereisusedtotunethesetolieclosertothenative4.6eVworkfunction.[ 45 ] 3.2MaterialsandMethodsDetailsoftheCN-VFETdevicestructureandfabricationprocesshavebeendescribedelsewhere.[ 44 46 ]Briey(insetFigure 3-1 ),aheavilyp-typedopedsiliconwaferservesasthesubstrateandthebottomgateelectrode,witha200nmthermallygrownSiO2layerasthegatedielectric.Benzocyclobutene(BCB,Cyclotene,DowChemical)spincoatedontotheSiO2andcross-linkedbya1hourhardbakeat250CinanArgloveboxprovidesathin(5nm)hydrophobizinglayer,thatisunderstoodtoavoidatmosphericwaterinducedchargetrapsatthedielectric/channelinterface.[ 92 ]Adilutenanotubesourcelayer(sheetresistance10k=)fabricatedasdescribedpreviously[ 2 ]wastransferredontothegatedielectricwithoneendoverlappingapredepositedCr/Ausourcecontact.Followingthistransfer,thesubstratewasbaked 42

PAGE 43

inAronahotplateat225Cfor30minutestopartiallydedopethecarbonnanotubesreducingtheinitialbarrierbetweenthenanotubesandtheZnO.TheZnONPs,withaveragediameterof4nm,weresynthesizedusingasolutionprecipitationmethodasdescribedelsewhere.[ 93 94 ]TheNPsweredepositedfromanethanoldispersionacrossthenanotubeelectrodelayerbyspincoatingat2000rpm(30s),followedby5min.dryingat80Conahotplateinair.ToattainthickerNPlms,thiscyclewasrepeated7times.ThisyieldedauniformcoveringofNPswithalmthicknessaround400nm.Asdiscussedbelow,thebestperformancewasobtainedfromNPlmsbroughtto300ConahotplateinArfor30min.followedbyanoxygenplasmatreatmentinabarrelasher(AnatechBarrelSCE600,300sccmoxygen,300WRFpower,5min.)priortodepositionofthe40nmthickaluminumdrainelectrode.TheuseofanoxygenplasmatotunethecarrierdensityinZnOnanoparticlelmsusedinconventional,lateralchannel,TFTshasbeenreportedbyWaltheret.Al.[ 91 ]DrainelectrodesweredepositedthroughaTEMgridshadowmaskdeningdozensofhexagonallyshapeddeviceshavinganareaof0.035mm2.InconventionalTFTs,thecurrentscaleslinearlywiththechannelwidth;inVFETs,thecurrentscaleswiththearealoverlapofthesourceanddrainelectrodesandisthusbestreportedasacurrentdensitybynormalizingthedraincurrenttothedevicearea.Currentdensityversusvoltage(J-V)outputandtransfercurvesweremeasuredwithaKeithley2612AdualchannelSourcemetercontrolledbyaprogramwritteninLabVIEW. 3.3ResultsandDiscussionFigure 3-1 showsthetransfercurveofatypicalZnONPCN-VFETwithoutthermalannealingoroxygenplasmatreatment.Thedeviceshowstypicaln-channelcharacteristicswithhysteresisevidentbetweentheforwardandbackwardscan.Suchhysteresishasbeenshowntobecausedbychargetrapsatthenanotube/gatedielectricinterface[ 95 ]anditsmagnitudeinCN-VFETsisobservedtobedramaticallydecreasedwiththinner,high-kdielectricsinplaceoftheSiO2usedhere.[ 45 ]The 43

PAGE 44

maximumon-statecurrentsobtainedarelowfortheshortchannellengthsenabledbythearchitectureimplyingalowmobilityforthenanoparticlebasedlm.[ 90 ] Figure3-1. TransfercurveoftheCN-VFETwithoutannealingandplasmatreatmentattheindicateddrainvoltage.Theon-currentdensityislowforaCN-VFET.Inset:Schematicofthedeviceincludingthewiringdiagramforoperation.OncethegateturnstheVFETon,electronsareinjectedfromthenanotubesourceelectrode(heldatground)intotheZnOconductionbandandtravelverticallytobecollectedatthedrainelectrode. ThermalannealingcanhavebenecialeffectsontransportinNPlms.AtypicaltransfercurveforadeviceafterannealingtheZnOlmat300CisshowninFigure 3-2 (dash-dotline).Thishasseveralrelatedeffectsonthedeviceperformance:1)thethresholdvoltageisdecreased(shiftingtonegativevoltages)2)forgivendrainvoltage,theon-statecurrentsaremuchhigherthanthoseinthepre-annealedstate(notethedrainvoltagefortheannealedtransfercurveinFigure 3-2 isonly0.25V); 44

PAGE 45

and3)theoff-statecurrentshavealsoincreased.Whilealowerthresholdvoltageandincreasedon-currentaredesirable,anincreasedoff-currentisnot,particularlywhenitdegradestheon-offratiogoingherefrom103forthepre-annealeddevicestolessthan10postanneal.Henceatfacevaluethethermalannealappearsunproductive.However,alsoshowninFigure 3-2 isthetransfercurveofanannealedZnOCN-VFETsubsequentlytreatedwithanoxygenplasma.Thethresholdvoltageisshiftedbacktowardspositivevoltageandwhilethedevicenowexhibitsasmalldegradationintheon-currentcomparedtothesimplyannealeddevice,themorepronouncedeffectisadramaticreductionintheoff-current.ComparisonoftheoutputcurvesfortheannealedonlyandtheannealedplusplasmatreateddevicesisshowninFigure 3-3 ,whereonlythemostonandoffcurvesareshownforclarity.Figure 3-4 plotstheon-offratiofortheannealeddeviceasafunctionoftheon-currentdensityasthedrainvoltageisincreasedfromzeroto3V.Theon-offratiofortheannealedandplasmatreateddevicenowapproaches104overmuchofthisrange,duemainlytothereductionintheoff-currentinducedbytheoxygenplasmaexposure.Toexplaintheseresultswerefertosimilarbehaviorspreviouslyobservedinp-channelCN-VFETsthatusedpentaceneasthechannelmaterialandgoldasthedraincontacts.[ 46 ]Followingtheirfabricationinanargonglovebox,outputandtransferJ-Vmeasurementsofthepentacenechanneldeviceswereperformedintheglove-box,priortotheirairexposure.Measurementswereperformedagainafterthedeviceswereexposedtoairatincreasingtimesofexposure.Pentaceneiswellknowntoundergoanoxygendopingeffectthatincreasesthebulkholecarrierdensityofitslms.[ 96 ]Asanticipated,theincreasedcarrierdensitydramaticallyincreasedtheon-statecurrents(similartotheeffectofannealingtheZnOCN-VFETs).Simultaneously,however,theoff-statecurrentswerefoundtoincreaseandthethresholdvoltagesinthesep-typedevicesshiftedtowardpositivevoltages.Asthetimeofexposureprogressed,theriseintheon-currentssoonsaturatedbuttheoff-currentcontinued 45

PAGE 46

Figure3-2. TransfercurvesfortheCN-VFETfollowingathermalanneal(dashdotline,redonline)andafterbothathermalannealandanoxygenplasmatreatment(solidblackline).Forclarityonlythepositivetonegativegoingsweepsareshown(correspondingtothelowersweepinFigure 3-1 ). toclimb(degradingtheon/offratio),whilethethresholdvoltagecontinueditsshifttowardpositivevoltages.ThesebehaviorsndexplanationinthespecicoperationalmechanismoftheCN-VFETs.Sincethegatemodulatesthecontactbarrierbetweenthenanotubesandthesemiconductor,over-dopingthesemiconductoratthejunction(bythediffusingoxygen)thinsthebarrier,resultinginarelativelyhightunnelingcurrentatallgatevoltages,compromisingtheoff-statecurrentsanddegradingtheon/offratio.Theshiftinthethresholdvoltagearisesbecausethethinnedbarrierrequiresamorepositivegatevoltagetoraisethebarriersufcientlytoturnthedeviceasoffasitcannowgo. 46

PAGE 47

Figure3-3. Outputcurvesfortheannealedonly(dash-dotcurves,redonline)andtheannealed+O2plasmatreated(solidblackcurves)CN-VFETattheindicatedgatevoltages(displayingtheoutputcharacteristicsforthemostonandthemostoffstates). InZnOitisknownthatoxygendepletioncreatesoxygenvacanciesthatactasn-typedopants,[ 97 98 ]suggestingthatthethermalannealoftheZnOdevicesdopedtheZnOlayer,increasingtheon-currents.Byanalogywiththepentacenedeviceswecaninferthatover-dopinginthevicinityofthenanotubejunctionthinnedthebarrierresultinginthehigh(tunneling)off-statecurrentsandshiftofthethresholdtonegativegatevoltage(forthen-typematerial).Theoxygenplasmatreatmentrelledaportionofthedepletedoxygenvacanciesreducingthedopingdensitytoanintermediatelevel,shiftingthethresholdbacktopositivevoltageandreducingtheoff-statecurrenttoyieldthenetincreaseinthedeviceon/offratio.Sincetheon-statecurrentdoesnotdegrade 47

PAGE 48

Figure3-4. On/OffratiofortheannealedCN-VFETplottedagainsttheon-currentasthedrainvoltageissweptfrom0to3V,beforeandaftertheO2plasmatreatment. muchintheoxygenplasmastepthethermalannealingsteplikelyalsobenetsthedevicetransportbyimprovingthegraintograincontact,abenetnotlostduringtheoxygenplasmastep. 3.4SummaryWehavedemonstratedinitialn-channelCN-VFETsusingsolutiondeposited,inorganic,ZnOnanoparticlethinlmsforthechannelmaterial.Asdepositedthenanoparticlechannellayeryieldslowon-currentdensities.Thermalannealinggreatlyincreasestheon-currentsbutalsodisproportionatelyincreasestheoffcurrentsdegradingtheon/offratio.Thisdeciencyiscorrectedbyanoxygenplasmaexposure. 48

PAGE 49

TheseresultswereinterpretedonthebasisofdopingeffectsthroughoutthebulkZnONPchannellayerandatthenanotube/channelinterface. 49

PAGE 50

CHAPTER4MODULATIONOFCARBONNANOTUBE/SILICONHETEROJUNCTIONSUSINGIONICLIQUIDThischapterexploresthemodulationoftheSchottkybarrierformedatcarbonnanotube/siliconheteronjunctionsusingionicliquid.Carbonnanotube/siliconheteronjunctionsarethekeycomponentofsinglecrystalsiliconbasedcarbonnanotubeenabledverticaleldtransistors(CN-VFETs)whereaCNTthinlmservesasthesourceelectrodeintheCN-VFETandasinglecrystalsiliconwaferisusedasthechannellayer.ThefeatureoftheCNTthinlmisthatitsworkfunctioncanbetunedbyagatepotentialthatinturnmodulatestheSchottkybarrierheightoftheheterojunctionleadingtotransconductanceintheCN-VFET.Thecarriertransportthroughthisjunctionineitherforwardorreversebiasvaryaccordingtothegatevoltage,andcanbetunedbyafactorof1034atgatevoltagesof0.4V. 4.1OverviewMetal/semiconductorhetero-junctionsareimportantelementsinmodernmicroelectronicsoftheearly20thcenturyandareresponsibleforeitherreliableOhmiccontactsorrectifyingelectriccurrents.[ 57 ]LowresistanceandstableOhmicmetal/semiconductorcontactsarecriticalincircuitfabricationforhighperformanceandreliableintegratedcircuits.Whiletherectifyingmetal/semiconductorjunctionsareimplementedindiodesforradiosignalrecticationandphotovoltaics.Diodesmadeofmetal/semiconductorjunctionsareSchottkydiodeswithseveraladvantagesoverordinaryp-ndiodessuchaslowerturnonvoltagesandhavethecharacteristicsofhotcarrierinjection.However,Schottkydiodesfoundlesspracticalusethanp-ndiodesinmodernrectifyingdeviceapplicationsprimarilybecauseoftheirlimitedabilitytotunethepropertiesofthemetal/semiconductorjunctions.Secondarily,metal/semiconductorjunctionscannotbeeasilyintegratedintodevicessuchasbipolarjunctiontransistors.Also,theyarelesspopularinamplifyingandswitchingelectricsignalsinlogiccircuits,especiallywhenthedensityofintegratedcircuitsarefastapproachingtheirphysicallimits.[ 99 ] 50

PAGE 51

Rectifyingpropertiesofametal/semiconductorheterojunctiondependonthemetalsworkfunctionandthebandstructureofsemiconductor,whichdeterminestheSchottkybarrierheight(refertoFigure 1-2 ).FollowingtheSchottky-Mottmodel,onewouldexpectfullbarriertunabilitysimplybychoiceofthemetalusedinthejunction.Howeverinpracticeotherphysicalphenomenacomeintoplaythatlimitstherangeoftunabilityofthebarrierwithdifferentmetalsinthejunction[ 57 100 101 ]andcontributetowhatisreferredtoasFermilevelpinning.Asaresultanempiricalparameter,S,isintroducedastheindexofinterfacebehaviorofthesemiconductor,andthemodelisextendedtoexpresstheSchottkybarrierheight,'bh,ofajunctionwithametalofworkfunction,m,andann-typesemiconductorwithelectronafnity,,as,[ 102 ]'bh=S(m)]TJ /F8 11.955 Tf 11.96 0 Td[()+C, (4)whereCisaconstant.Fortheinterfacebehaviorofsilicon,forinstance,Shasavalueof0.27formostofthecommonmetals,[ 103 ]meaningtheSchottkybarrierheight'bhforsiliconismerelyweaklydependentonthemetalworkfunctionm.Fermilevelpinningrelatestoadditionalinterfacestatesattheimmediatesurfaceofthesemiconductorintheregionofcontactwiththemetal,butthephysicaloriginofthesestatesremainsundetermined.AccordingtoBardeensmodel,[ 104 ]eithertheabruptdiscontinuityoflatticeperiodicityordanglingbondsordefectsattheinterfaceofsemiconductorscreatessurfacestateswithintheforbiddenbandgap,whichaccommodateextracarriersfromdepletionregion,andcreateinterfacialdipoleswhichleadstolocalizedgapstates.Alternatively,amorechemicalpointofviewattributestheinterfacestatestothepolarizedcovalentbondsbetweenthemetalandsemiconductor,forminginterfacedipolesthatarecreatedduetochargetransferbetweenthejunctionmaterials.[ 102 105 106 ]ThelaterhypothesisalsoexplainstheFermilevelpinningobservedinmetal-organicsemiconductorinterfaces,[ 107 ]sincetheorganicsemiconductorsdonothavedanglingbondsanddonotformthreedimensional 51

PAGE 52

covalentlybondedcrystals.Nevertheless,theseargumentsprovidestrategiesforsuppressingsurfacestatessoastominimizetheFermilevelpinningasfollows:(1)apropersurfacepassivationofsemiconductorsisrequiredand(2)chemicalinteractionsbetweenmetalandsemiconductorsshouldtobeavoided.Thechemicallyinertandpassivatedgraphene/carbonnanotubesurfaceprovidesakeybreakthroughforsuppressingtheinterfacestatesbetweengraphene/nanotubesurfaceandacompletelysaturated(withoutdanglingbonds)siliconsurface,[ 53 ]thuseliminatingFermilevelpinning.Inadditiontothat,unlikeconventionalmetal/semiconductorjunctions,theworkfunctionofgraphene/nanotubecanbemodulatedbyshiftingtheFermilevelviaelectrostaticgating.[ 2 44 108 109 ]ThisprovidesanidealplatformformanipulatingtheSchottkybarrierheightinasinglematerialsystemasinSchottkymottlimit,wheretraditionallythebarrierheightmodulationhasbeendonebyselectingmetalswithdistinctworkfunctions.ThetruebarrierheightmodulationwasrstreportedwasbyLonerganin1997,[ 110 ]inwhichanelectrochemicallyinducedbarrierheightmodulationisdemonstratedinanair-sensitivepolymer/inorganic(poly(pyrrole)/n-indiumphosphide)contactbarrier(whereaconductingpolymertooktheroleofthemetalpolymerisused).Thechemicallyinertandpassivatedgraphene/carbonnanotubesurfaceprovidesakeybreakthroughforsuppressingtheinterfacestatesbetweengraphene/nanotubesurfaceandacompletelysaturated(withoutdanglingbonds)siliconsurface,[ 53 ]thuseliminatingFermilevelpinning.Inadditiontothat,unlikeconventionalmetal/semiconductorjunctions,theworkfunctionofgraphene/nanotubecanbemodulatedbyshiftingtheFermilevelviaelectrostaticgating.[ 2 44 108 109 ]ThisprovidesanidealplatformformanipulatingtheSchottkybarrierheightinasinglematerialsystemasinSchottkymottlimit,wheretraditionallythebarrierheightmodulationhasbeendonebyselectingmetalswithdistinctworkfunctions.ThetruebarrierheightmodulationwasrstreportedwasbyLonerganin1997,[ 110 ]inwhichanelectrochemicallyinducedbarrierheightmodulation 52

PAGE 53

isdemonstratedinanair-sensitivepolymer/inorganic(poly(pyrrole)/n-indiumphosphide)contactbarrier(whereaconductingpolymertooktheroleofthemetalpolymerisused).AsimilartransistorarchitectureusingCNT/siliconjunctionsinsteadofgraphene/siliconjunctionswasdemonstratedbyWuin2008,[ 112 ]inwhichanelectrolytegatingpotentialwasprovidedbyusingionicliquid.Although,thedevicedemonstratedevidencedanon/offratioofonly300andsufferedfrompossibleionicliquidinduceddegradation,italsoinspiredfollow-onresearchonnanotube/siliconsolarcells.[ 37 38 ]Here,wecontinueexploringthismodulationofnanotube/siliconheteronjunctionsfurtherusingdiluteCNTnetworkwithdifferenteffectivethicknessesofCNTthinlmsthatprovidepartialexposureofbareSisurface.TheexposedSisurfaceisslightlyoxidizedinambientairasameansforsurfacepassivation. 4.2MaterialsandMethodsThedeviceschematicisshowninFigure 4-1 ,wherethecircuitwiringisalsoillustrated.Moderatelydopedp-typesilicon(boron,resistivity2cm)witha200nmthermallygrownSiO2layerwaspurchasedfromSiliconQuestInternationalanddicedinto0.6inchby0.6inchchips.Agoldlayerwhichframesa2.52.5mm2windowwasthermallyevaporatedthroughashadowmaskontotheoxidetoprovideanelectricalcontactforthesourceelectrode.Anotherthicklayerofphotoresistwaspaintedaroundthewindowleavingasmallerwindowofabout22mm2,whichwaslateretchedthroughtothebareSiwithhydrouoricacid(HF).Afterthephotoresistwasremoved,athinSWNTlmwithacontrolledthicknessintherangeof2to20nmwastransferredtothewafercoveringacrossthewindowincontactwiththegoldelectrode,inwhichtheSWNTlmconformsitselfintothewindowcontactingbareSitoformtheCNT/Sijunction.Ohmiccontacttothep-Siside(drainelectrode)ismadebydirectlyevaporatingaluminumontothep-SiimmediatelyafterHFtreatmentontheoppositesideofthewaferandtransientlyannealedat350Cinaninertgasactivatingtheohmiccontact. 53

PAGE 54

Figure4-1. ThegoldcontactonSiO2surfaceframesa22mm2rectangularwindowinwhichtheoxidewasetchedtothebarep-Sisurface.ASWNTlmcontactsboththegoldelectrodeandthep-SiwithinthewindowformingtheSWNT/p-Sijunction.AlisevaporatedonthebacksidetoprovidetheOhmiccontact. ToobtaincontrolovertheFermi-leveloffsetsattheSWNT/Sijunction,anelectrolytegateisusedtoapplyagateeldatthejunction.AstripofSWNTsootwasdrapedaroundandhungfromaplatinumwireandservedasthegatingelectrodeoftheelectrolyte.ItwaspositionedovertheSWNT/Sijunctionincontactwiththeelectrolyte(Figure 4-1 ).TheSWNTlm(source)andtheSWNTsoot(gate)areonlyelectricallyconnectedthroughanionicliquid(IL),1-ethyl-3-methylimidazoliumbis(triuoromethylsulfonyl)imide(EMI-BTI),thatisdepositedbetweenandsaturatesbothSWNTsourcelmandsootgate.BeforethedepositionofIL,anynativeoxidegrownpostCNTdepositionontheSisurfacewascarefullyremovedbyanadditionalwetetchingprocessinHFfollowedbytwohoursofexposureinambientairinordertohaveawell-controlledoxidethicknessforthepassivationoftheSisurface.ApotentialwasappliedbetweenthesourceandgateelectrodesandelectrostaticallydopestheSWNTsatthejunctioninwhichtheultrahighcapacitanceoftheelectrolyte[ 87 ]providessubstantialmodication 54

PAGE 55

oftheirelectronicpopulationsforrelativelysmallvoltages.Bykeepingtheappliedgatepotentialswellbelowtheredoxpotentialforanychemicalreactionsnearthejunction,thegateleakageisnegligibleoncechargereorganizationiscomplete. 4.3ResultsandDiscussionThetypicalcurrentdensity-voltage(J-V)curvesinbothforwardandreversebiasfortheCNT/p-SiheterojunctionswereshowninFigure 4-2 fordeviceswith2nmthickCNTlms.Atroomtemperature,theidealityfactors(,accordingtoShockleydiodeequation)formostofthedeviceswerefoundtobearound1.55;exceptthedevicewith2nmthickCNTlmwhichhadanidealityfactorofabout1.97.Thecurrentdensitiesforthedevicewitha2nmthickCNTlmisalsomuchlessbymorethananorderofmagnitudeinbothforwardandreversebiascomparedtootherdeviceswheretypicallythecurrentdensitiesweremorethan102mA/cm2inforwardand10)]TJ /F4 7.97 Tf 6.58 0 Td[(1mA/cm2inreversebias,respectively.Thevariationandpoorperformanceofthe2nmlmcasemaybeduetothefactthatthesurfacedensityofnanotubesontheSisurfaceofa2nmlmismoredilutethantherest,suchthattheconditionsforthepassivationofSisurfacecommonlyusedforthickerlms1isnotapplicableto2nmlms.Figure 4-3 showsthecharacteristiccurrent-voltagecurvesofaCNT/p-Siheterojunctiondevicewith20nmthickCNTlmatvariousgatevoltages,wherebothforward(blue)andreverse(orange)biasareshown.DuetothepossibleredoxinteractionbetweenILandSisurface,andpotentialdegradationtothedeviceperformance,[ 112 ]thesource-drainvoltageandthegatevoltagewerelimitedto0.6Vand0.4V,respectively.BecausetheworkfunctionoftheCNTlmismodulatedelectro-staticallyviatheILanditsgate(counter)electrode,[ 2 ]theSchottkybarrierheight('bh)ofthejunctionvariesaccordinglyinadditiontogateinducedbarrierwidthmodulation.[ 44 ]Therefore,themagnitudeofcurrentpassingthroughthejunction(inbothdirections)can 1UnpublishedresultfromtheRinzlergroup 55

PAGE 56

Figure4-2. CurrentversusbiasvoltagecharacteristicsofCNT/p-SiheterojunctionswithdifferentCNTlmthicknesses,showingSchottkydiodelikeproperties.Theidealityfactorisabout1.55for20,14,08nmthickCNTlmdevicesand1.97for02nmthickCNTlmdevice. bedirectlycontrolledbygatevoltagesincetheinjectionofmajoritycarriersinSchottkybarrierismostlydeterminedbybarrierheight('bh),asdepictedintheenergyleveldiagramsinFigure 4-4 .Fortheconditionwhenthebarrierheight('bh)istunedtobeverysmall,anearOhmiccontactmaybecreatedattheCNT/p-Siinterfacesincethehotcarrierscouldsimplyhopacrossortunnelthroughthebarrier.InanorganicchannelCN-VFET,thedeviceisoperatedinreversebias(orangeregion),wherethecurrentisinjectedfromtheCNTsourcetothesemiconductor.[ 44 46 ]ThereasoncanbeseenfromFigure 4-3 ,whereinforwardbias(blue),thecurrentdensitymodulationismaximizedatlowbiasvoltage(0.2V),butisquickly 56

PAGE 57

Figure4-3. CurrentdensityversusbiasvoltagecharacteristicsofaCNT/p-Siheterojunctiondeviceusinga20nmthickCNTlmatvariousgatevoltages.Gatevoltagesrangefrom-0.4Vto0.4Vatastepsizeof0.2V. minimizedasthebiasvoltageincreasesandendsupgivingsimilarcurrentdensitiesatvariousgatevoltages.Ontheotherhandinreversebias(orange),thelargeamountofmodulationobservedatinitialbiasvoltagesremainsrelativelysteadyunderincreasedbiasvoltage,whichispreferred,andtypicalbehaviorforconventionaleldeffecttransistors.Theobservationsofcarrierinjectioncanbefurtherexplainedbythecurrenttransportmechanismofthermionicemissionwherecarriersrelyonthermalenergytobeexcitedoverthebarrier.Inthermionicemission,aninjectioncurrentdependsontheSchottkybarrierheight('bh),whichleadstoacurrentdensityexpressionas[ 57 ]J=AT2exp()]TJ /F6 11.955 Tf 10.49 8.09 Td[(q'bh kBT)[exp(qV kBT))]TJ /F3 11.955 Tf 11.96 0 Td[(1], (4) 57

PAGE 58

whereAistheRichardsonconstant,kBistheBoltzmannsconstant,istheidealityfactor,andVisthevoltagebiasacrossthejunction.Asnegativevoltagebiasisapplied(reversebias),exp(qV kBT))]TJ /F3 11.955 Tf 12.48 0 Td[(1inEquation( 4 )quicklyreducesto)]TJ /F3 11.955 Tf 9.3 0 Td[(1,leavingvoltageindependentandsaturatedcurrentdensity.SothattheoreticallythemodulationofcurrentdensityinreversebiasregimeissolelydependentonSchottkybarrierheight('bh).Intheforwardbiasregime(V>0),thetransitionofmodulationcanbesplitintothreesteps:thereis(1)aninitialvoltagerangewhereexp(qV kBT)1,followedby(2)anintermediaterangewhereexp(qV kBT)1inwhichtheJmodulationisdeterminedbycompetingV=and'bh,and(3)annalrangewhereV='bhinwhichthecurrentdensityissolelydeterminedbyvoltagebiasV. Figure4-4. EnergybanddiagramforaCNT/p-Siheterojunctionattwodistinctgatevoltages.WhenappliedgatevoltageinducesholesinCNTlm,reducingitsworkfunctionandincreasingtheSchottkybarrierheight.Asaresult,thecurrentinjectionfromCNTlmdecreases(left).Ontheotherdirection,theworkfunctionofCNTlmshiftedinawaythatreducedtheSchottkybarrierheightandthereforeincreasestheinjectioncurrent(right). Equation( 4 )doesnotsuccessfullypredicttheon/offratioofthesedevicessinceinpracticeothercarriertransportmechanisms(suchasseriesresistanceandleakage)mayplaysomeroles.Butifthermionicemissionwoulddominatethecurrentdensities,theon/offratioshouldbesimilarinbothdirections.Figure 4-5 AandBshowtheon/offratiosinforwardandreversebias,respectively,plottedagainstOn-currentdensities 58

PAGE 59

ofdeviceswithdifferentlmthicknesses.Althoughthereissomevariation,deviceswithsuccessfulSisurfacepassivation(and1.55)haveon/offratioscloseto104inforwardbias,and1034inreversebias.ThedevicewithoutproperSisurfacepassivation(2nm)hasthepoorestperformanceinbothon/offratio(<102)andoutputcurrentdensity.Exceptthedevicewith2nmCNTlm,similaron/offratiosinbothforwardandreversebiasfordeviceswithpropersurfacepassivationmayindicatethecurrenttransportmechanismcanbemostlikelyexplainedbythermionicemissionattheCNT/p-Sijunction,thatthemodulationofcurrentdensityisdominatedbySchottkybarrierheight.Thefactthatitisalittlehigherinforwardthanreversebiasmayduetotheunavoidablecurrentleakageatverylowcurrentregimewhichlimitsthelowerboundofoffcurrent.Suchleakagemayduetothesurfacerecombinationofelectron-holeatthesurfaceorinthespacechargeregionoftheSchottkybarrier,orsimplycircuitshortage.Inforwardbias,suchleakageproblemislessseverebecausethecurrentdensityinforwardbiasisgenerallylarger.Thatmayfurtherexplainsinreversebias,deviceswithhigheroncurrentdensitygenerallyhavebetteron/offratio(asthedevicewith14nmCNTlm).ItalsoseemsthattheCNTthinlmthicknessdoesnothaveadirectimpactonthedeviceperformance.FinallywestudiedtheambientenvironmentimpactonthecurrenttransportacrosstheCNT/Siheterojunction.Ourresultshows,thecurrentintensitywoulddegradeastimegoesonwhenthemeasurementsweretakeninambientair,anditwasmoresevereinthedevicewith2nmCNTlmthantherestdevices.Puttingthesamplesinaninertgasenvironmentwillgreatlyreducetherateofdegradation,ashavebeenobservedbefore.[ 112 ]Therefore,itcouldbetheoxygenandmoistureinairthatisaffectingthestabilityofthepassivatedsurface.Inordertosolvethisproblem,weemployathinlayerof7nmAl2O3grownbyatomiclayerdeposition(ALD)tofurtherenhancethesurfacepassivation.Wetestedthismethodonadevicewith2nmCNTlm,andtheresults 59

PAGE 60

A BFigure4-5. On/offratiosforCNT/p-Siheterojunctiondevicesplottedagainsttheon-currentasthedrainvoltageissweptfrom0to0.6VinbothA)forwardandB)reversebias. 60

PAGE 61

wereshowninFigure 4-6 AandB.WiththisadditionalALDgrownprotectionlayer,thedevicedemonstratedstrongerendurancethantheonewithoutALDlayerandboththegateanddrainvoltagescouldbesafelytakenupto1.6Vand-2V,respectively.Thedeviceof2nmCNTlmwithALDlayeralsoshowedcomparableperformancetootherdeviceswithdifferentthicknessesofCNTlm. 4.4SummaryModulationofcarbonnanotube/siliconheterojunctionswasdemonstratedasaproofofprincipleoftheFermilevelshiftingofCNTthinlmsviagatepotential.Suchaneffectivegatingeffectwasduetothe(1)lowDOSoftheSWNT;and(2)theporosityoftheSWNTlmallowingionstohaveaccesstothemajorityofthenanotubebundles.Inbothforwardandreversebias,thecurrentdensitycanbetunedbyafactorof1034atgatevoltagesof0.4V.Asthesourceelectrode,theCNTthinlmsofdifferentthicknessattherangeof820nmdoesnotprovidesignicantvariationstoeitheronstatecurrentdensitynoron/offratio.ForamuchmorediluteCNTnetwork(effectively2nmthick)thefailureoftheSisurfacepassivationwillreducethecarrierinjectionefciencyresultinginpoorperformance.Asforthetransistorperformance,thesuccessofSisurfacepassivationremainstobeakeypointforeffectivecarrierinjectionaswellasminimizingFermilevelpinning. 61

PAGE 62

A BFigure4-6. AnadditionalprotectionlayerofAl2O3isdepositedbetweenthe2nmCNTthinlmandSisurface.A)showstheJ-Vcurveswithenhancedvoltageendurnceinbothgateanddrainbias.B)showscomparableon/offratiotootherdeviceswithdifferentlmthicknesses. 62

PAGE 63

CHAPTER5CURRENTTRANSPORTCHARACTERISTICSINTHECN-VFETInthischapter,temperaturedependenceofthecharacteristicsofacarbonnanotubeenabledverticaleldtransistor(CN-VFET)hasbeeninvestigatedindetail.Channellayerswithmoderateandhighmobilityorganicsemiconductorshavebeenfabricatedandcharacterizedatdifferenttemperatures.ResultsshowthatCN-VFETsoperatewithdifferentprinciplesfromconventionaleldeffecttransistors(FETs).FromtheION/IOFFvariationwithtemperature,itisfoundthatthedeviceoperationismostlikelydominatedbycarrierinjectionattheinterfaceofthesourceandchannelratherthanthemodulationofcarriertransportinsidethechannellayer.Furthermore,thiscarrierinjectionattheinterfaceisdeterminedbythecompetitionbetweenthermionicemissionandtunneling.Therefore,themodulationofcontactbarrieratthesource-electrode/channel-layerinterfaceplaysacriticalroleindeviceoperation. 5.1OverviewVerticaleldeffecttransistor(VFET),apromisingclassoftransistorswithverticallystackedarchitecture,isgainingattentioninrecentyearsduetoitssimpleimplementationofscalingdownthechannellength,whichiscompletelydeterminedbythechannellayerthickness;suggestinglow-power,high-drivingcurrentdeviceswithoutcomplexlithographyprocessesinfabrication.[ 44 45 48 52 ]Benetingfromtheverticalstructurethatissimilartoorganiclightemittingdiodes(OLEDs),VFETsalsoaffordanotheropportunityofintegratingintoverticalorganiclight-emittingtransistors(VOLETs)byessentiallystackingOLEDsonVFETs.[ 44 113 ]AkeystructuralrequirementintheVFETisthesourceelectrodearchitecture.Sincethesourceelectrodeissandwichedbetweenthegateelectrodeandthechannellayer,itisrequiredtobesemi-transparenttothegateinducedelectriceldsoasnottoscreenthechannellayer.Recentexplorationofvarioussourceelectrodesincludepartiallyoxidizedultrathin(<20nm)aluminumwithasupercapacitorgatedielectric, 63

PAGE 64

[ 48 ],perforatedmetalgrid,[ 52 ]dilutedCNTnetwork[ 44 ],andgraphene[ 114 ].Amongall,sourceelectrodesinboththegrapheneVFETandCN-VFETaretheeasiesttofabricateanddeliversuperiorandindustriallyrelevantperformance.ThisisenabledbytheuniqueelectronicpropertiesofgrapheneandCNTspermittingatruemodulationofthecontactbarrierforcarrierinjectionthusallowinghighlevelsoftransconductanceatreasonablegatingelds.NeverthelessforthisnewarchitectureofVFET,theworkingmechanismoftheconventionalTFTnolongerappliesandnewconceptsofthetheoreticalbackgroundfortheworkingprinciplesarenecessary,andassuchhavebeenproposed.[ 44 47 ]ButthephysicalgroundrulesregardingspecicallytotheCN-VFET(orgrapheneVFET)haveneverbeenrigorouslycharacterized.Havingafullyironedouttheorywithfullexperimentalagreementmaynotonlyprovidefullunderstandingofthisnewtransistortechnologybutcouldalsoleadtocluestofurtherincreasetheperformanceofthedevices.Theswitchingmechanismdiscussedinref.[ 44 ]referstotheCN-VFETfunctioningasaSchottkybarriertransistor,andthekeymodulationofthesource-draincurrentisdeterminedbytheFermilevelshiftingviagatebias,whichimpliesthebarrierheightloweringandbarrierthinningattheCNT/semiconductorinterface.Foranunbiasedgatevoltage,thecontactbarrierwilllimitthecurrentinjection,wherethecontactlimitedinjectioncanbedescribedbythermionicemissionand/orquantummechanicaltunneling.[ 57 ]Asthegatebiasisapplied,thecontactbarrierislowered,facilitatingcurrentinjectionfromthesourceelectrode.FortheidealcasewherethebarrierheightislowenoughtocreateavirtualOhmiccontactbetweenthesourceelectrodeandthesemiconductor,thecontactlimitisliftedandthesource-draincurrentforthisfullyturned-ondeviceisonlylimitedbycarriertransportthroughthesemiconductor,whichthecurrenttransportcharacteristicsreliesonthesemiconductorbulkproperties.Forinorganicsemiconductors,thetransportisusuallyOhmic;whilefororganic 64

PAGE 65

semiconductors,thetransportmechanismismostlynon-linearandthecurrent-voltagedependencetendstofollowapowerlawwithanorderhigherthantwo.Inthermionicemission(TE),electronsinthemetalrelyonthethermalenergytobeexcitedoverthebarrier,leadingtoaninjectioncurrentthatsolelydependsontheSchottkybarrierheight(SBH).ThecurrentdensityacrosstheinterfacecanbeexpressedbyJ=Js[exp(qV kBT))]TJ /F3 11.955 Tf 11.96 0 Td[(1], (5)whereJs=AT2exp()]TJ /F7 7.97 Tf 10.5 5.26 Td[(q'bh kBT)isthesaturationcurrent,AistheRichardsonconstant,kBistheBoltzmannsconstant,istheidealityfactor,andVisthevoltagebiasacrossthejunction.Thethermionicemissioncanthenbeextendedwithdiffusiontheoryforsemiconductorswithlowmobilitysuchastheorganics.ThesaturationcurrentisthenreplacedbyJs=qNCEexp()]TJ /F7 7.97 Tf 10.49 5.25 Td[(q'bh kBT),whereNCistheeffectiveDOSintheconductionband,isthecarriermobility,andEistheelectriceldassociatedwiththebiasacrossthejunction.Theoverallcurrentdensityexpressionsaresimilar,exceptthesaturationcurrentfordiffusiontheoryislesssensitivetotemperaturebuttotheappliedvoltage.Fordirecttunneling,oreldemission(FE),electronsquantummechanicallytunnelthroughthebarrierthatusuallyleadstoanalyticallyuntractableexpressionsforthecurrentdensity,sincethequantumtransmissioncoefcientandoccupationprobabilitiesofthecontactingmaterialsvaryunderdifferentconditions.However,inheavilydopedsemiconductors,wheretunnelingcurrentscomparableto,orevenexceedthatofthermionicemissioncurrents,asimpleexpressionofcurrentdensitycanbeshownasJ=Jsexp(qV E00), (5)whereE00q~ 2r ND m (5) 65

PAGE 66

isanenergytermrelatedtobulksemiconductorproperty,NDisthecarrierdensity,isthepermittivityofthesemiconductor,misthecarriereffectivemass,andthesaturationcurrent,Js,hasanexpressionproportionaltoexp()]TJ /F7 7.97 Tf 10.49 5.25 Td[(q'bh E00).[ 115 116 ]Theenergycriterionterm,E00determinestherelativecontributionsfromTE,FE,andthermioniceldemission(TFE).TFE,orthermallyassistedtunneling,arisesasthecombinedmechanismofpartiallythermalactivationandtunnelingofcarriers.SincethetunnelingprobabilityincreasesdramaticallyasafunctionofcarrierenergyforaSchottkybarrier,whilethecarrierdensitydecreasesexponentiallywithenergy;thereisanarrowenergydistributionofcarriersthattunnelthroughthebarrier.TheTFEcanthereforebeseeneffectivelyasthermionicemissionwithareducedbarrierheight.[ 115 ]WhilethermionicemissionortunnelingdominatesrespectivelyattwoextremesatkBTE00andkBTE00,thermallyassistedtunnelingisthemainmechanismforcurrenttransportwhenkBTE00.Asageneralresultforcommonmetal/semiconductorjunctions,theeffectoftunnelingisnegligibleatroomtemperature(kBT25meV)forsemiconductorswithadopinglevellessthan1018cm3(siliconforexample,E005meV),whereastunnelingwillbesignicantforadopinglevelhigherthan1019cm3(E0017meV).InspiteofthefactthatbothTEandFEexpressionsdependexponentiallyonbiasvoltageacrossthejunction,tunnelingisinsensitivetotemperatureanddependsheavilyonsemiconductordopinglevel.TFEalsodependsondopinglevel,andhasatemperaturedependencethatisrelatedtopotentialbarrierprolebutlesssensitivethanTE.JVcharacteristicsonlyatroomtemperaturecannotleadustodifferentiatethesecurrenttransportmechanisms.Inthisstudy,thetemperaturedependenceoftheJVcharacteristicswereprobedatwidetemperaturerangeandresultswereanalyzedtounderstanddifferentaspectsofthecurrenttransportmechanism. 66

PAGE 67

5.2MaterialsandMethodsDetailsoftheCN-VFETdevicestructureandfabricationprocessissimilartothatdescribedelsewhere.[ 45 46 ]Insum,thegateelectrodewas40nmofaluminumthermallyevaporatedontoglasssubstrates.Analuminumoxidedielectriclayerof25nmwasformedonthealuminumgatebyatomiclayerdeposition(ALD)inacommerciallyavailableALDreactor(CambridgeNanoFiji200).Benzocyclobutene(BCB,Cyclotene,DowChemical)wasthenspincoatedtoathicknessof5nm,servingasathinhydrophobizinglayerthatavoidsanywatermoistureassistedchargetraps.[ 92 ]TheBCBlayerwassubsequentlyhardbakedat250Cforanhourinanargongloveboxwhereitcross-linksandbecomesinsolubletosolvents(AcetoneandIsopropanol).ContacttotheCNTsourcelayerwasmadebypredepositedCr/Au(5/30nmthick,respectively)electricalcontacts.Stripsof2mmwideCNTnetworks(sheetresistance10k=)werefabricatedandtransferredasdescribed[ 2 ],andsubsequentlypatternedintosmallstripsof200mwidevialithographyandsubtractiveetching[ 1 ]inabarrelasher(oxygenplasma).FollowingCNTdepositionandpatterning,thesubstrateswerebakedonahotplateinthegloveboxat225Cfor1h,priortothedepositionoforganicsemiconductors.ThepurposeofbakingCNTthinlmistoelevateitsworkfunctioncloseto-4.8eVsoastoincreasetheinitialbarrierheightatCNT/semiconductorjunction.[ 45 ]Thesubstrateswereloadedintoanargongloveboxwithseparateorganicsandmetalsvacuumthermalevaporationsystemsthatallowsconsecutivedepositionoforganicchannellayersandmetalcontactswithoutexposuretoambientair.[ 45 ]Inthecaseofamoderatemobilitychannellayer,poly(9,9dioctyluorenecoN(4butylphenyl)diphenylamine)(TFB,AmericanDyeSource,Inc.)wasusedbyspincoatingfromtoluenesolutiontoathicknessof350nm.Coatedsubstrateswerethensoftbakedinargonat100Cforahalfhour.Forhighmobilitychannellayer,dinaphtho-[2,3-b:20,30-f]thieno[3,2-b]-thiophene(DNTT)wasusedasreceivedfrom 67

PAGE 68

NipponKayakuCo.,Ltd.Itwasthermallyevaporatedfromaneffusioncellat210Catagrowthrateof4A/sinapressureof510)]TJ /F4 7.97 Tf 6.59 0 Td[(7Torrtoathicknessof500nm.Beforedrainelectrodedeposition,aninterfaciallayer,afewnmofMoOx,isdepositedtoensureOhmiccontactatsemiconductor-metalinterface.Substratesofeitherchannellayersweresubsequentlytransferredtothemetalsevaporator,where35nmofAuwasdepositedasthedrainelectrode.TheAudepositionwasthroughashadowmaskdeninga100mbaracrossthose200mCNTstrips,givingthecorrespondingsizeofeachpixelof0.02mm2.AQuantumDesigncryostat,QD-6000PhysicalPropertiesMeasurementSystem(PPMS),isusedforcharacterizethetransistorperformanceofCN-VFETsatvarioustemperatures.ThePPMSconsistsofatestingprobethatisinsertedintoadualdewarcontaininganinnerlayerofliquidheliumandanouterlayerofliquidnitrogen.[ 117 ]ThePPMSprobeisasealedsamplechamberthatisevacuatedtoapressureof1Torrbyanexternalmechanicalpump,andsurroundedbyacoolingannulusanda7Tsuperconductingmagnet.AschematicofthecryostatisprovidedinFigure 5-1 .Temperaturecontrolisachievedbythesystemoftwoheaters,threethermometers,andaowcontrolvalveseparatingtheliquidheliumandthemechanicalpumpthatdrawscoldheliumgasoverthecoolingannulus.Thesystemprovidesatemperaturesweeprangeof1.7K
PAGE 69

Figure5-1. SchematicofPPMSprobeinsideadualdewarandthecryostatchamberatthefrontendoftheprobe.Imagereproducedwithpermissionfrom[ 117 ]. eV,renderingitair-stable,[ 45 ]andtheeldeffectmobilityof2.9cm2/(Vs)reportedinconventionalTFTdevicesonSi/SiO2substrateswithanoctadecyltrichlorosilane(OTS)self-assembledmonolayer(SAM).[ 118 ]Figure 5-2 Bshowsthetemperaturevariationoftransfercurvesforthedeviceoverthetemperaturerangeof160Kto280K.Asap-channeltransistor,thedeviceisintheonstatewhennegativegatevoltage(-2V)isapplied,reachinganonstatecurrentdensityrangingfrom5.1to36.2mA/cm2atadrainvoltageof-1V.Intheoffstate,whenpositivegatevoltage(2V)isapplied,theoffstatecurrentdensityrangefrom7.310)]TJ /F4 7.97 Tf 6.58 0 Td[(5to1.610)]TJ /F4 7.97 Tf 6.59 0 Td[(2mA/cm2,morethanthreeordersofmagnitudeinvariation.Thedifferenceintemperaturecorrespondentvariationsincurrentdensityofonandoffstatesindicatesthatthecurrenttransportintheoffstateismoresensitivethanintheonstate.Figure 5-2 Cshowstheoutputcharacteristicsforthedeviceatvarioustemperatures,onlyfortheonandoffstates.SimilarbehaviorinagreementwithFigure 5-2 Bisshownasthelarger(smaller)variationatoff(on)statecurrent.Figure 5-2 Dplotstheon/offratioversustheon-statecurrentdensityasthe 69

PAGE 70

drainvoltagechangesfrom0to-1V.Theon/offratioincreasesasthetemperatureisloweredbyafactorapproaching100,meaningthetemperaturesensitivityoftheoffstatecurrentisnearly100thantheonstatecurrent.AnimmediateconclusioncanbedrawnthatthecarrierinjectionintheoffstateismorelikelydominatedbyTE,resultingfromthehighcontactbarrierthatiscreatedwhenthedeviceisinitsoffstate.Alternatively,thecarrierinjectionintheonstateisdominatedbytunneling,resultingfromtheloweredcontactbarrierwhereOhmiccontactispossible.ThevariationofcurrentdensityintheonstatemaythereforebecompletelyattributedtothetemperatureresponseofthebulkcarriertransportofDNTT.AnotherCN-VFETwithTFBastheholetransportchannellayerwasalsofabricated,andsimilarmeasurementswereperformedwithresultsshowninFigure 5-3 .TFBhastheHOMOlevelat-5.3eV,andtheeld-dependentmobility10)]TJ /F4 7.97 Tf 6.59 0 Td[(3cm2/(Vs)extractedbytime-of-ightmobilitymeasurements.[ 119 ]Figure 5-3 Bshowsthetemperaturevariationoftransfercurvesforthedeviceoverthetemperaturerangeof200Kto280K.Below200K,thecurrentwaslowerthantheSourcemeterandcannotbedetectedaccurately.Duetothelowmobilitycharacteristic,ahigherdrainvoltageof5Visappliedformoderatecurrentdensity.AtgatevoltageVG=)]TJ /F3 11.955 Tf 9.3 0 Td[(2V,thecurrentdensityofthedevicerangesfrom1.810)]TJ /F4 7.97 Tf 6.58 0 Td[(2to2.4mA/cm2overthetemperaturerange;whileatVG=2V,itrangesfrom1.210)]TJ /F4 7.97 Tf 6.58 0 Td[(4to3.210)]TJ /F4 7.97 Tf 6.58 0 Td[(3mA/cm2.OnthecontrarytotheDNTTdevice,thetemperaturecorrespondentvariationofcurrentdensityforonstateseemsmoresensitivethanoffstate.Ifwerefertotheplottedon/offratioversustheon-statecurrentdensityinFigure 5-3 D,theon/offratioisverycloseforthersttwohighesttemperaturesbutstartstodecreaseasthetemperaturedecreases.Thedecreaseofon/offratiocanbeexplainedbythedetectionlimitofourSourcemeter,whereattemperaturesbelow240Kanddrainvoltagebelow2V,thecurrentdensityisnearthelowerboundofdetectionlimit10)]TJ /F4 7.97 Tf 6.58 0 Td[(5(seeFigure 5-3 C).AlsobycomparingtheoutputcurvesofTFBandDNTTdevices,thetemperaturevariationsoftheoutput 70

PAGE 71

A B C DFigure5-2. TemperaturedependentCN-VFETwithDNTTaschannellayer.A)SchematicsoftheCN-VFETwithDNTTasthechannellayer.B)Temperaturedependenttransferscurvesofthedeviceoveragatevoltagerangeof4V.C)TemperaturedependentoutputcurvesforVG=2.D)On/offratiosplottedagainstoncurrentdensityfromT=180to300Kin20Ksteps. curvesinFigure 5-3 CdoesnothaveanobviousdifferencelikethatinFigure 5-2 C.Thissuggestsiftherewerenodetectionlimit,theon/offratiowouldverylikelybesimilaroverthetemperaturerangeinsteadoftemperaturedependent. 5.4SummaryWestudiedthetransistorcharacteristicsofCN-VFETsatvarioustemperatures.Accordingtoourobservation,thecurrenttransportmodulationforhighlyconductive 71

PAGE 72

A B C DFigure5-3. TemperaturedependentCN-VFETwithTFBaschannellayer.A)SchematicsoftheCN-VFETwithTFBasthechannellayer.B)Temperaturedependenttransferscurvesofthedeviceoveragatevoltagerangeof6V.C)TemperaturedependentoutputcurvesforVG=2.D)On/offratiosplottedagainstoncurrentdensityfromT=200to280Kin20Ksteps. organicsemiconductorsismostlydominatedattheCNT/semiconductorinterfaceratherthanchannelmodulation.Asthegateeldtunesthedevicefromoffstatetoonstate,thetemperaturedependenceofcurrenttransportismoresevereatoffstatecomparedtoonstate,suggestingthermionicemissionassociatedwithlargerbarrierheightcontributesatoffstateandtunnelingcontributesatonstatewherenearOhmiccontactiscreated.Forlessconductiveandmoderatemobilityorganicsemiconductors,thetemperaturedependenceiscomparableatbothonandoffstates,suggestingthe 72

PAGE 73

semiconductormobilitymaystillbealimitingfactorincurrenttransportmodulation.Ifweconsiderthecurrenttransportloopfromsourcetodrainasaseriesofresistancesfromthecontactbarrierandthechannel,theonewithhigherresistancewoulddominatetheresultingcurrentintheloop.Thereforesuchlimitingfactorfromchannellayermaypreventthequantitativeanalysisofcarriertransportfromgivingoutaccurateresults;sothatwecannotperformthemodelingonCN-VFETswithorganicchannellayeruntilmorestudyhasbeendoneonthecarriertransportoforganicsemiconductorsthemselves. 73

PAGE 74

CHAPTER6SILICONBASEDCARBONNANOTUBEENABLEDVERTICALFIELDEFFECTTRANSISTORSWITHSOLIDSTATETOPGATEThischapterdemonstratessinglecrystalsiliconbasedcarbonnanotubeenabledverticaleldeffecttransistors(CN-VFETs).UnlikeprevioustheCN-VFETs,asinglecrystalsiliconwaferisusedasthechannellayer,andthedeviceswithasolidstatetopgatedielectricwerefabricated.Thedemonstrateddevicesexhibitacurrenton/offratioofnearly105withanoncurrentdensityexceeding5A/cm2overadrainvoltageof2V.Suchdevicescouldprovideawiderangeofopportunitiesformicroelectronicpowerandlogicapplications. 6.1OverviewAeldeffecttransistor(FET)isathreeterminaldeviceforwhichthechannelconductionbetweenthesourceanddrainterminalsiscontrolledbyagateelectriceld.Thecarriersareelectronsinthecaseofann-channelFETorholesinap-channelFET.FETshaveplayedthemainpartinmodernsolidstateelectronicsforamplifyingorswitchingelectronicsignals.SiliconbasedFETssuchasmetal-oxide-semiconductoreldeffecttransistors(MOSFETs)havealsobeenthebasiselementsformoderndigitalintegratedcircuits(ICs).Verticaleldeffecttransistors(VFETs)areapromisingclassofeldeffecttransistorswithaverticallystackedarchitectureproposedinrecentyears.[ 44 48 52 ]Insteadoflayingoutthesourceelectrode,channel,anddrainelectrodeinthesameplane,thesethreecomponentswererotated90degreeswiththesourceelectrodestackedontopofgatedielectric.Duetothissimplemodication,VFETsaregainingmoreattentionbecauseoftheeasinessofscalingdownthechannellength,whichissimplydeterminedbythechannellayerthickness.Thechannellayerbeingathinlmthatcanbedepositednanometersinthicknesssuggeststhepossibilityforlow-power,high-drivingcurrentdeviceswithoutcomplexlithographyprocessesinfabrication. 74

PAGE 75

UnlikeconventionalFETs,inwhichthechannelconductivityismodulatedbygateelectriceld,thedeviceoperationofthisnewarchitecturereliesonthemodulationoftheSchottkybarrierformedbetweenthesourceelectrodeandthesemiconductingchannellayer.[ 44 47 ]AkeystructuralrequirementintheVFETisthereforethesourceelectrode,whichisrequiredtobesemi-transparenttothegateinducedelectriceldsoasnottoscreenitsaccesstothechannellayer.AmongtheseveralVFETsthatwereexploredrecentlywithwidelyvaryingsourceelectrodes[ 44 48 52 114 ],grapheneandcarbonnanotubethinlmenabledverticaleldtransistors(G-VFETsorCN-VFETs)havedemonstratedthebestperformancebytakingadvantageoftheuniqueelectronicpropertiesofthesequasi1and2dimensionalcarbonbasedmaterials.Thatis,onlyG-VFETsandCN-VFETsareoperatedbasedonthetruemodulationoftheSchottkybarrierheightthatisformedbetweengraphene-orCNT-semiconductorinterfaces.ThesameprincipleofelectronicallymodulatingtheSchottkyjunctioninaCNT-singlecrystalsiliconinterfacehasalsobeendemonstratedinaSchottkyjunctionsolarcellforwhichchangesinthebuilt-inpotentialresultedindramaticchangesinthepowerconversionefciency.[ 37 38 ]Alsorecentlythisphenomenonwasexploitedinadevicenamedagraphenebarristor.[ 111 ]Usingawaferscaletransferprocess,theywereabletocreateanatomicallycleaninterfacebetweengrapheneandahydrogenatedsiliconsurfacewithnegligibleFermilevelpinning(S1),wherethebarrierheightcouldbetunedupto0.2eV(asaconsequenceofthecorrespondingFermilevelshift)overtheappliedgatevoltagerangeof5V.Thedevicecouldbeoperatedasatransistorinboththeforwardandreversebiasregimesreferringheretotheconventionsforcurrentowinadiode.Inforwardbiasmode,theydemonstratedacurrentmodulationof105overalimitedbiasvoltagerange,withamaximumcurrentdensityabove1A/cm2.However,inthereversebiasmode,typicallywhereSchottkybarrierbasedtransistorsareoperated,theyshowedconventionalFETlikeoperationwithacurrentmodulationofonly300.Thelimitedbiasvoltagerangetoachievethehighon/offratioinforwardbiasrequiresthe 75

PAGE 76

devicebeoperatedataxedbiasvoltageof0.3V.Complementarycircuitsbasedonn-andp-channels(determinedbychoiceofdopantinthebulksilicon)suchasinvertersandhalf-addersweredemonstratedinthissetup.ThereportedgraphenebarristortookadvantageofthegateinducedFermilevelshiftinthegraphenelayertomodulatetheSchottkybarrierheightresultingintheobservedtransconductance.WorkonagraphenebasedorganicVFETpublishedataboutthesametimedemonstratedthatbypurposelycreatingholesintheotherwisecontinuousgraphenesourceelectrode,thegateeldwouldalsoenhancethetunnelingcurrentsacrossthegraphene-semiconductorinterface(duetothegateeldinducedbandbendingthatactstothintheSchottkybarrier).[ 114 ]CombinedwiththeFermilevelshiftinducedbarrierheightmodulation,theextracontributionoftunnelingmodulationallowedanadditionalseveralordersofmagnitudeimprovementinthedevicetransconductance.ThedilutenanotubesourceelectrodeusedinCN-VFETs[ 44 46 ],withitsnaturallyporousmorphology,alsoadmitsgateeldaccesstothesurroundingregionsofthenanotube-semiconductorinterfacepromotingtunnelingmodulation,whileatthesametimeallowingforbarrierheightmodulation.Followingthisrationalehere,weuseadilutenanotubenetworkasthesourceelectrodeinplaceofacontinuousgraphenelayertoformaSchottkybarrierwiththebaresiliconsurface,anddemonstratethatthisdevicecanbeoperatedthesamewayasconventionalsiliconbasedFETs. 6.2MaterialsandMethodsAschematicdiagramofthetop-gatedsiliconbasedCN-VFETisshowninFigure 6-1 .Aneffectively2nmthickdiluteCNTnetwork[ 44 ]servedasthesourceelectrodeandwasdepositedasshowninFigure 6-1 makingcontacttothebaresiliconsurface.FormationofaSchottkybarrierintheinterfacialregionbetweentheCNTsourceandsiliconservesasthefoundationforthedevicefunctioningmechanism.Thefabricationanddepositionofthenanotubethinlmhasbeendescribedelsewhere.[ 2 ]Moderatelydopedp-typesilicon(borondoped,resistivity2cm,SiliconQuestInternational) 76

PAGE 77

Figure6-1. ThegoldcontactingnmCNTdilutenetworkservesasthesourceelectrodethatformsaSchottkybarrierwithbarep-Sisurface.AtrenchofbaresiliconsurfaceiscreatedbyremovingtheSiO2layerusingBOE.AlisevaporatedonthebacksidetoprovidetheOhmiccontact. witha200nmthermallygrownSiO2layerservedasthechannellayer,wherethebacksideofthewaferismadetoformanOhmiccontactbydirectlyevaporatingaluminumontobaresiliconsurfacefollowedbyannealingat350Cinaninertgas.AthingoldlayerwasevaporatedontotheSiO2surfaceasasourcecontactpadpriortoCNTlmtransfer.ThedeviceareawasdenedastheoverlapbetweenastripofCNTdilutenetworkandatrenchopeningonSiO2layer.Stripsof400mwideCNTnetworks(sheetresistance6k=)weredenedviastandardlithographicprocessandsubtractiveetching[ 1 ]inanoxygenplasma.AtrenchintheSiO2layerof20mor200mwidewascreatedbyanetchingprocessusingbufferedoxygenetchant(BOE)withalithographicpatternedphotoresistastheetchingmask.Analuminumoxidelayerof50nmwasformedasthetopgatedielectricbyatomiclayerdeposition(ALD)inacommerciallyavailableALDreactor(CambridgeNanoFiji200).BeforeALDdeposition,nativeoxideontheexposedSisubstratewascarefullyremovedbyadditionalBOE 77

PAGE 78

etching.Ametallayerwasdepositedonthegatedielectriccoveringthedeviceareaandservedasthegateelectrode.ThecircuitwiringisalsoillustratedinFigure 6-1 .Thesourceelectrode(CNTdilutenetwork)washeldatground,whilethegateanddrainwerebiasrelativetogroundpotential.Thecurrentdensityversusvoltage(J-V)outputandtransfercurvesweremeasuredwithaKeithley2612AdualchannelSourcemetercontrolledbyLabviewprograms. 6.3ResultsandDiscussionFigure 6-2 showsatypicalcurrentdensity-voltage(J-V)curveforthedeviceoperatedasaSchottkydiode(ungated)inbothforward(biasvoltage,VB>0)andreversebias(VB<0)fortheCNT/p-Siheterojunction.Atroomtemperaturetheidealityfactor()formostofthedeviceswerefoundtobearound1.81;withcurrentdensitiesof1000mA/cm2inforwardbiasandlessthan1mA/cm2inreversebias.GatedJ-VcurvesareshowninFigure 6-3 ,inwhichthegatepotentialvariesfromVG=-11Vto1V.AtVG=1V,aclearlinearregioninthesemilogplotcanbeseencenteredaroundVB=0.2Vwhichgivessimilaridealityfactor(1.81)astheungateddiodecurve.Asthegatevoltageismademorenegative,suchlinearregionbecomesmoreobscure,anditisdifculttodenealinearregionontheJ-VcurveuntilatVG=-11Vsincethecurrent-voltagerelationbecomeslinear(seethelinear-linearplotininset).AlineartcenteredaroundVB=0.2Vonthiscurveonlygivesapooridealityfactor(>5)whichmeanstheidealShockleyequationnolongerapplies.ThisindicatesthattheSchottkybarrierattheCNT/p-Siinterfacehasdecreasedwithnegativegatevoltage,thusthediodebehaviorgoesfromrectifyingwithawelldenedturn-onbiasvoltagetonon-rectifying,whichimpliesbarrierheightloweringandbarrierthinningattheappliedgatevoltages.Figure 6-3 alsoshowsthatthevariationismuchstrongerinreversebiasthanforwardbias.Fortheminimumbarriercase,thecurrentdensityinreversebiasevensurpassesthatinforwardbias.Thisbehaviorcanalsobeascribedtothevariable 78

PAGE 79

FermileveloftheSWNTs,dependingontheirchargestate,buthereinresponsetothebiasvoltage,asisdiscussedfurtherbelow. Figure6-2. CurrentversusbiasvoltagecharacteristicsofCNT/p-Siheterojunction.Theidealityfactorisabout1.81. InordertoprobetheSchottkybarrierheightvariation,thethermonicemissionmodelisimplementedinanalyzingforwardbiasJ-Vcurvessincetheeffectofeldemissionisnotstronginthisregime.Traditionally,thethermionicemissionacrossaplanarmetal-semiconductorinterfaceisgivenby[ 57 ]:J=AT2exp()]TJ /F6 11.955 Tf 10.49 8.09 Td[(q'bh kBT)[exp(qVB kBT))]TJ /F3 11.955 Tf 11.96 0 Td[(1], (6)whereAistheRichardsonconstant,kBistheBoltzmannsconstant,'bhistheSchottkybarrierheight,andVBisthevoltagebiasacrosstheheterojunction.ForVB>3kBT=q, 79

PAGE 80

Figure6-3. CurrentversusbiasvoltagecharacteristicsofCNT/p-Siheterojunctionundervariousgatevoltagesfrom-11Vto+1V,instepsof+4V. Equation 6 canberepresentedbyJAT2exp()]TJ /F6 11.955 Tf 10.5 8.09 Td[(q'bh kBT)exp(qVB kBT)Jsexp(qVB kBT). (6)Alineartonforwardln(J))]TJ /F6 11.955 Tf 12.14 0 Td[(VBplotwillresulttoaninterceptatVB=0thatisequaltoln(Js).WethenperformtheactivationenergymeasurementindeterminingtheSchottkybarrierheight[ 57 ]overalimitedrangeoftemperatures(200K300K)wheretheRichardsonconstantandbarrierheightareassumedtobeessentiallytemperatureindependent.Thismethodrequiresnoassumptionofactualelectricallyactiveareaandisparticularlysuitablefornanotubedilutenetwork/siliconinterfacesincetheactiveareaisapparentlydifferentfromthedevicegeometricarea.Figure 6-4 AandBshowtheArrheniusplotofln(Js=T2)versus1000/Katvariousgatevoltagesandtheresulting 80

PAGE 81

Schottkybarrierheightversusappliedgatevoltages.Theestimatedungatedbarrieris0.25eV,whichisclosetopredictedinitialbarrierof0.27eVforouraciddopednanotube(workfunction-4.9eV[ 2 ]).Theresultshowsthatasthegatevoltagebecomesmorenegativethebarrierheightvariesfrom277meVto12meV,anestimatedtotalshiftof0.265eVforthetotalgatevoltagerangeof12V.Forlargeforwardbias,theexponentialtermsinEquation( 6 )reducestoexp(q(VB=)]TJ /F13 7.97 Tf 6.58 0 Td[('bh) kBT),inwhichthecurrentdensityiscompletelydominatedbybiasvoltageandirrelevantto'bhwhenVB='bh.ThisbehaviorisseeninFigure 6-3 ,wherethecurrentdensitiesatvariousgatevoltages(andthusvariousbarrierheights)havealargemodulationatVB0.1VbutstarttoconvergeatVB>1V.Therefore,inforwardbias,inordertohavethemaximumcontrastofcurrentdensitiesoveragivenrangeofappliedgatevoltages,thevariableShottkydiodeshouldonlybeoperatedatalowforwardbias[ 111 ]whichwilllimittheusefulnessmakingitapoorcandidatefordeviceapplications.Inreversebiasmode,ontheotherhand,thecurrent-voltagecurvesshowaconventionaleldeffecttransistorlikeoutputbehavior.Theoffstatecurrentdensity,Jo,wherethelargestSchottkybarrierheightexists,becomesinsensitivetolargereversebiasvoltage.ThiscanbeexplainedbyEquation( 6 ),whereatlargenegativeVB,exp(qVB kBT)1andJogoestothesaturationregime.Intheonstate,atthesmallestbarrierheight,thecurrenttransportisnotlimitedtothermionicemissionbuteldemissionalsocontributessignicantly,sothattheonstatecurrentdensity,Jon,doesnotsaturate.TheonlylimitingfactorinJonwouldbetheseriesresistanceexistinginthecircuit,includinge.g.thenanotube-metalcontactresistance,sheetresistanceofnanotubelmitself,and/orbulkresistanceofchannellayer.TheresultingJon=Joratiowillthereforeactuallyincreasewithincreasingreversebias.Figure 6-5 showstheon/offratio,denedasJon=JoorIon=Io,plottedagainstoncurrentdensity(Jon)inbothforwardandreversebias.Theforwardbiascanonlyreachamaximum(>104)atverylowJon,butreducesratherquicklytolessthen10.Thereversebiasstartswithalower 81

PAGE 82

A BFigure6-4. ModulationofSchottkybarrierheight.A)Arrheniusplotofln(Js=T2)versus1000/KatvariousgatevoltagesandB)SchottkybarrierheightversusappliedgatevoltagesextractedfromA). 82

PAGE 83

on/offratioatlowJonbutquicklyexceeds104withincreasingJonapproaching105withremarkablyhighoncurrentdensitiesupto5A/cm2.Forthepurposeofapplications,reversebiasistheproperworkingmodeforusingsuchSchottkybarrierbasedtriodesastransistors.Infact,alloftheexistingSchottkybarriereldeffecttransistors,includingorganicCN-VFETsandG-VFETsexhibittheirhighestperformanceoperatinginreversebiasmode. Figure6-5. On/offratiosforsiliconbasedCN-VFETplottedagainsttheon-currentasthebiasvoltageissweptfrom0to1Vinforward(black)andfrom0to-2Vinreverse(red)bias. ToexplainhowthereversebiasonstatecurrentdensityoftheSibasedCN-VFETcanactuallyexceedthatoftheforwardbiasedon-statecurrent(asseeninFigure 6-3 ),wenotethatforwardcurrentcorrespondstothermionicholeemissionfromthesilicontothenanotubesastheforwardbiasvoltage(siliconincreasinglypositive)unbendsthe 83

PAGE 84

bandasindicatedinFigure 6-6 .Simultaneously,sinceinforwardbiasthenanotubesaremadenegativeandtheirFermileveldependsontheirchargestate,theFermilevelofthenanotubesrisestowardthevacuumlevelwithincreasingforwardbias(thebiasvoltagedependenceofthenanotubeFermilevelalludedtoabove).SinceitisthedifferencebetweenthenanotubeFermilevelandthatofthesiliconthatsetsthedegreeofbandbending,therisingnanotubeFermilevel,whichactstointroducemorebandbending,servestocounteracttherateatwhichthebandisunbentbytheincreasingforwardbiasvoltage.I.e.theforwardbiascurrentgrowsmoreslowlythanitwouldwithoutthenanotubeFermilevelshift.Thiscircumstanceisreversedinreversebias,withtheonstategateeldbarrierfurtherreducedbytheincreasingreversebias.ThesephenomenacanbemodeledbyintroducingabiasvoltagedependencetothebarrierheightinEquation 6 havingthisform'bh='bh0CVB, (6)where'bh0isthenativebarrierheight,Cscaleschangeinthebarrierheightwiththebiasvoltageandtheplussignpertainstoforwardbiaswhilethenegativesigntoreversebias.Unfortunately,evenwiththismodicationEquation 6 stillignorestheimportanteffectsoftunnelingandeldemissionimportantinreversebiassowedonotperformsuchmodeling.Fortestingtheuniformityintermsofdeviceperformance,15deviceswerefabricatedonasingle0.6by0.6inchsiliconchip.Figure 6-7 Ashowsthetransfercurvesof15devicesonasinglechip.Exceptfor3devicesthathavedifferentthresholdvoltages,therestofthedeviceshaveathresholdvoltageof-4V;andallthedeviceshavesimilarsubthresholdswingofabout1V/dec.Thisrelativelylargesubthresholdswingcouldbereducedbythinningthegateinsulator.Theon/offratioofall15devicesisplottedinFigure 6-7 B,whereatadrainvoltageofVD=-1V,alldeviceshaveoutputcurrentdensities>1500mA/cm2andon/offratiowellabove104.Therefore,about80% 84

PAGE 85

Figure6-6. EnergybandbendingdiagramforCNT/Siinterfaceatforward(red)andreverse(blue)bias.Inforwardbias,therisingnanotubeFermilevelservestounbentthebandbytheincreasingforwardbiasvoltage;whileinreversebias,morebandbendingisintroducedinreversebias. ofthedevicesbehavesimilarly.Thevariationindevicetodeviceperformanceislikelyduetovariationinthebaresiliconsurfaceafterthewetetchingprocessingandpossiblythenanotubethinlmuniformity.Thetransfercurveofatypicaldevicewasalsotestedoverthetemperaturerangefrom300Kto200KasshowninFigure 6-8 .Intheonstate(at-11V)thecurrentdensityshowsasmallertemperaturedependencethanthatintheoffstate(at+1V);sothattheon/offratioofthedeviceis105atroomtemperature(300K)andincreasestomorethan106at200K.SuchimprovementofthedevicetransfercharacteristicsissimilartothatofaconventionalMOSFET,buttheoperatingprincipleisdifferent.InaMOSFET,themainreasonisanincreaseinthemobilityofsinglecrystalsiliconduetothesuppressionofphonons,sothattheon/offratioincreaseisbecauseofanincreaseintheoncurrent.InoursiliconbasedCN-VFET,bothonandoffstatecurrentdensitiesdecreaseasthetemperaturedecreases,butthedecreaseoftheoffcurrentdensityisgreaterthanthatoftheoncurrentdensity.ThisistypicalforaSchottkybarrierbasedtransistor,thatthecarrierinjectionismorelimitedbythermionicemissionintheoffstatewithalargebarrierbutnearlyunaffectedintheonstatewithanearlyOhmiccontact.IfthecontactbarrierattheCNT/siliconinterfaceiseliminated,thenthecurrent 85

PAGE 86

A BFigure6-7. UniformityofsiliconbasedCN-VFETsshowingA)transfercurvesandB)on/offratioversusoncurrentdensityof15devicesonasingle0.60.6inchsiliconchip. 86

PAGE 87

densityshouldbeaffectedbythebulktransportinsiliconwhichwouldimplyaslightlyhighercurrentdensityatlowertemperatureduetotheincreaseofitsmobility,however,theCNTthinlmsheetresistanceincreaseswithloweringofthetemperaturewhichevidentlydominates.[ 78 ] Figure6-8. TransfercharacteristicsforsiliconbasedCN-VFETfrom300Kto200K.Theinsetshowsthevariationofoncurrentdensities(at-11V)inlinearscaleofsource-drainresistance. 6.4SummaryInconclusion,wehavedemonstratedtherstsolidstatedielectricsiliconbasedCN-VFETsthatoperatewellinthereversebiasregimeofaSchottkydiodewithconventionalFETlikeoutputbehavior.Thecurrentdensityofthistopgatedevicecanbemodulatedbyafactorofnearly105withanoncurrentoutputexceeding5A/cm2overabarrierheightloweringof0.265eV.Thedevicedemonstratedsatisfactoryperformance 87

PAGE 88

formostofcurrentcircuitapplicationswithhighuniformityinabout80%ofdeviceswhichisindicativeofthesuccessofthesiliconsurfacetreatmentandtheuniformityofthenanotubedilutenetworksourceelectrode. 88

PAGE 89

CHAPTER7CONCLUSIONSWedemonstratedinorganicCN-VFETsusingchannelmaterialsbasedonbothsolutionprocessednanocrystalsandsinglecrystalwafer.ThesolutionprocessedZnOnanoparticlethinlmofferscosteffectivemethodfordevicefabricationbuttheasdepositedchannellayeryieldslowon-currentdensitiesduetotheexistinggrainboundariesbetweennanoparticles.Thermalannealingplusanoxygenplasmaexposuregreatlyincreasestheon-currentswhilemaintainssatisfactoryon/offratio.TheseresultswereinterpretedonthebasisofdopingeffectsduetothecreationofoxygenvacanciesthroughoutthebulkZnOnanoparticlechannellayerandatthenanotube/channelinterface.InitialsiliconbasedCN-VFETswerealsodemonstratedonasinglecrystalwaferwithatopgatedesign.ThedevicecanbeoperatedbymodulatingaSchottkybarrierheightinreversebiasregimeshowingconventionalFETlikebehavior.Thecurrentdensityofthetopgatedevicecanbemodulatedbyafactorofnearly105withanoncurrentoutputexceeding5A/cm2overabarrierheightloweringof0.265eV.Thedevicedemonstratedsatisfactoryperformanceformostofcurrentcircuitapplicationswithuniformityofabout80%.SuchdevicesnotjustprovideagoodplatformforstudyCN-VFETphysicsbutalsocouldprovideawiderangeofopportunitiesformicroelectronicpowerandlogicapplications. 89

PAGE 90

REFERENCES [1] A.Behnam,L.Noriega,Y.Choi,Z.Wu,A.G.Rinzler,andA.Ural,Resistivityscalinginsingle-walledcarbonnanotubelmspatternedtosubmicrondimensions,AppliedPhysicsLetters,vol.89,pp.093109,2006. [2] Z.Wu,Z.Chen,X.Du,J.M.Logan,J.Sippel,M.Nikolou,K.Kamaras,J.R.Reynolds,D.B.Tanner,A.F.Hebard,andA.G.Rinzler,Transparent,conductivecarbonnanotubelms,Science,vol.305,pp.1273,2004. [3] L.Hu,D.S.Hecht,andG.Gruner,Percolationintransparentandconductingcarbonnanotubenetworks,NanoLetters,vol.4,pp.2513,2004. [4] M.Kaempgen,G.S.Duesberg,andS.Roth,Transparentcarbonnanotubecoatings,AppliedSurfaceScience,vol.252,pp.425,2005. [5] J.Li,L.Hu,L.Wang,Y.Zhou,G.Gruner,andT.J.Marks,Organiclight-emittingdiodeshavingcarbonnanotubeanodes,NanoLetters,vol.6,pp.2472,2006. [6] D.Zhang,K.Ryu,X.Liu,E.Polikarpov,J.Ly,M.E.Tompson,andC.Zhou,Transparent,conductive,andexiblecarbonnanotubelmsandtheirapplicationinorganiclight-emittingdiodes,NanoLetters,vol.6,pp.1880,2006. [7] S.Bae,H.Kim,Y.Lee,X.Xu,J.-S.Park,Y.Zheng,J.Balakrishnan,T.Lei,H.R.Kim,Y.I.Song,Y.-J.Kim,K.S.Kim,B.ozyilmaz,J.-H.Ahn,B.H.Hong,andS.Iijima,Roll-to-rollproductionof30-inchgraphenelmsfortransparentelectrodes,NatureNanotechnology,vol.5,pp.574,2010. [8] K.S.Novoselov,A.K.Geim,S.V.Morozov,D.Jiang,M.I.Katsnelson,I.V.Grigorieva,S.V.Dubonos,andA.A.Firsov,Two-dimensionalgasofmasslessdiracfermionsingraphene,Nature,vol.438,pp.197,2005. [9] C.BiswasandY.H.Lee,Grapheneversuscarbonnanotubesinelectronicdevices,AdvancedFunctionalMaterials,vol.21,pp.3806,2011. [10] E.S.Snow,J.P.Novak,P.M.Campbell,andD.Park,Randomnetworksofcarbonnanotubesasanelectronicmaterial,AppliedPhysicalLetters,vol.82,pp.2145,2003. [11] E.Artukovic,M.Kaempgen,D.S.Hecht,S.Roth,andG.Gruner,Transparentandexiblecarbonnanotubetransistors,NanoLetters,vol.5,pp.757,2005. [12] A.Star,E.Tu,J.Niemann,J.C.P.Gabriel,C.S.Joiner,andC.Valcke,Label-freedetectionofdnahybridizationusingcarbonnanotubenetworkeld-effecttransistors,ProceedingsoftheNationalAcademyofSciencesoftheUnitedStatesofAmerica,vol.103,pp.921,2006. 90

PAGE 91

[13] H.E.Unalan,G.Fanchini,A.Kanwal,A.duPasquier,andM.Chhowalla,Designcriteriafortransparentsingle-wallcarbonnanotubethin-lmtransistors,NanoLetters,vol.6,pp.677,2006. [14] Q.Cao,H.S.Kim,N.Pimparkar,J.P.Kulkarni,C.Wang,M.Shim,K.Roy,M.A.Alam,andJ.A.Rogers,Medium-scalecarbonnanotubethin-lmintegratedcircuitsonexibleplasticsubstrates,Nature,vol.454,pp.495U4,2008. [15] P.L.Tabernaa,G.Chevalliera,D.P.P.Simona,andT.Aubertb,Activatedcarboncarbonnanotubecompositeporouslmforsupercapacitorapplications,MaterialsResearchBulletin,vol.41,pp.478,2006. [16] P.Ramesh,M.E.Itkis,J.M.Tangand,andR.C.Haddon,Swntmwnthybridarchitectureforprotonexchangemembranefuelcellcathodes,TheJournalofPhysicalChemistryC,vol.112,pp.9089,2008. [17] J.E.Trancik,S.C.Barton,andJ.Hone,Transparentandcatalyticcarbonnanotubelms,NanoLetters,vol.8,pp.982,2008. [18] R.K.Das,B.Liu,J.R.Reynolds,andA.G.Rinzler,Engineeredmacroporosityinsingle-wallcarbonnanotubelms,NanoLetters,vol.9,pp.677,2009. [19] M.Zhang,S.Fang,A.A.Zakhidov,S.B.Lee,A.E.Aliev,C.D.Williams,K.R.Atkinson,andR.H.Baughman,Strong,transparent,multifunctional,carbonnanotubesheets,Science,vol.309,pp.12151219,2005. [20] M.A.Meitl,Y.X.Zhou,A.Gaur,S.Jeon,M.L.Usrey,M.S.Strano,andJ.A.Rogers,Solutioncastingandtransferprintingsingle-walledcarbonnanotubelms,NanoLetters,vol.4,pp.1643,2004. [21] T.V.Sreekumar,T.Liu,S.Kumar,L.M.Ericson,R.H.Hauge,andR.E.Smalley,Single-wallcarbonnanotubelms,ChemistryofMaterials,vol.15,pp.175,2003. [22] N.P.Armitage,J.C.P.Gabriel,andG.Gruner,Quasi-langmuir-blodgettthinlmdepositionofcarbonnanotubes,JournalAppliedPhysics,vol.95,pp.3228,2004. [23] M.E.Spotnitz,D.Ryan,andH.A.Stone,Dipcoatingforthealignmentofcarbonnanotubesoncurvedsurfaces,JournalofMaterialsChemistry,vol.14,pp.1299,2004. [24] W.Ma,L.Song,R.Yang,T.Zhang,Y.Zhao,L.Sun,Y.Ren,D.Liu,L.Liu,andJ.Shen,Directlysynthesizedstrong,highlyconducting,transparentsingle-walledcarbonnanotubelms,NanoLetters,vol.7,pp.2307,2007. [25] B.Dan,G.C.Irvin,andM.Pasquali,Continuousandscalablefabricationoftransparentconductingcarbonnanotubelms,ACSNano,vol.3,pp.835,2009. 91

PAGE 92

[26] A.A.GreenandM.C.Hersam,Coloredsemitransparentconductivecoatingsconsistingofmonodispersemetallicsingle-walledcarbonnanotubes,NanoLetters,vol.8,pp.1417,2008. [27] G.Gruner,Carbonnanotubelmsfortransparentandplasticelectronics,JournalofMaterialsChemistry,vol.16,pp.3533,2006. [28] L.Hu,D.S.Hecht,andG.Gruner,Carbonnanotubethinlms:Fabrication,properties,andapplications,ChemicalReviews,vol.110,pp.5790,2010. [29] K.A.Sierros,D.S.Hecht,D.A.Banerjee,N.J.Morris,L.Hu,G.C.Irvin,R.S.Lee,andD.Cairns,Durabletransparentcarbonnanotubelmsforexibledevicecomponents,ThinSolidFilms,vol.518,pp.6977,2010. [30] A.Schindler,P.Schau,andN.Fruehauf,Active-matrixandexibleliquid-crystaldisplayswithcarbon-nanotubepixelelectrodes,JournaloftheSocietyforInformationDisplay,vol.17,pp.853,2009. [31] W.Q.Fu,L.Liu,K.L.Jiang,Q.Q.Li,andS.S.Fan,Super-alignedcarbonnanotubelmsasaligninglayersandtransparentelectrodesforliquidcrystaldisplays,Carbon,vol.48,pp.1876,2010. [32] X.Wang,L.J.Zhi,andK.Mullen,Transparent,conductivegrapheneelectrodesfordye-sensitizedsolarcells,NanoLetters,vol.8,pp.323,2008. [33] W.Xuan,Z.Linjie,andK.Mullen,Transparent,conductivegrapheneelectrodesfordye-sensitizedsolarcells,NanoLetters,vol.8,pp.323,2008. [34] M.W.Rowell,M.A.Topinka,M.D.McGehee,H.J.Prall,G.Dennler,N.S.Sariciftci,L.B.Hu,andG.Gruner,Organicsolarcellswithcarbonnanotubenetworkelectrodes,AppliedPhysicsLetters,vol.88,pp.233506,2006. [35] J.vandeLagemaat,T.M.Barnes,G.Rumbles,S.E.Shaheen,T.J.Coutts,C.Weeks,I.Levitsky,J.Peltola,andP.Glatkowski,Organicsolarcellswithcarbonnanotubesreplacingin2o3:Snasthetransparentelectrode,AppliedPhysicsLetters,vol.88,pp.233503,2006. [36] Y.Jia,J.Q.Wei,K.L.Wang,A.Y.Cao,Q.K.Shu,X.C.Gui,Y.Q.Zhu,D.M.Zhuang,G.Zhang,B.B.Ma,L.D.Wang,W.J.Liu,Z.C.Wang,J.B.Luo,andD.Wu,Nanotube-siliconheterojunctionsolarcells,AdvancedMaterials,vol.20,pp.4594,2008. [37] P.Wadhwa,B.Liu,M.A.McCarthy,Z.Wu,andA.G.Rinzler,Electronicjunctioncontrolinananotube-semiconductorschottkyjunctionsolarcell,NanoLetters,vol.10,pp.5001,2010. [38] P.Wadhwa,G.Seol,M.K.Petterson,J.Guo,andA.G.Rinzler,Electrolyte-inducedinversionlayerschottkyjunctionsolarcells,NanoLetters,vol.11,pp.2419,2011. 92

PAGE 93

[39] W.Regan,S.Byrnes,W.Gannett,O.Ergen,O.Vazquez-Mena,F.Wang,andA.Zettl,Screening-engineeredeld-effectsolarcells,NanoLetters,vol.12,pp.4300,2012. [40] W.Yuan,L.B.Hu,Z.B.Yu,T.L.Lam,J.Biggs,S.M.Ha,D.J.Xi,B.Chen,M.K.Senesky,G.Gruner,andQ.B.Pei,Fault-tolerantdielectricelastomeractuatorsusingsingle-walledcarbonnanotubeelectrodes,AdvancedMaterials,vol.20,pp.621,2008. [41] H.Xu,S.M.Anlage,L.B.Hu,andG.Gruner,Microwaveshieldingoftransparentandconductingsingle-walledcarbonnanotubelms,AppliedPhysicsLetters,vol.90,pp.183119,2007. [42] L.Xiao,Z.Chen,C.Feng,L.Liu,Z.Q.Bai,Y.Wang,L.Qian,Y.Y.Zhang,Q.Q.Li,K.L.Jiang,andS.S.Fan,Flexible,stretchable,transparentcarbonnanotubethinlmloudspeakers,NanoLetters,vol.8,pp.4539,2008. [43] Y.H.Yoon,J.W.Song,D.Kim,J.Kim,J.K.Park,S.K.Oh,andC.S.Han,Transparentlmheaterusingsingle-walledcarbonnanotubes,AdvancedMaterials,vol.19,pp.4284,2007. [44] B.Liu,M.A.McCarthy,Y.Yoon,D.Y.Kim,Z.Wu,F.So,P.H.Holloway,J.R.Reynolds,J.Guo,andA.G.Rinzler,Carbon-nanotube-enabledverticaleldeffectandlight-emittingtransistors,AdvancedMaterials,vol.20,pp.3605,2008. [45] M.A.McCarthy,B.Liu,andA.G.Rinzler,Highcurrent,lowvoltagecarbonnanotubeenabledverticalorganiceldeffecttransistors,NanoLetters,vol.10,pp.3467,2010. [46] M.A.McCarthy,B.Liu,R.Jayaraman,S.M.Gilbert,D.Y.Kim,F.So,andA.G.Rinzler,Reorientationofthehighmobilityplaneinpentacene-basedcarbonnanotubeenabledverticaleldeffecttransistors,ACSNano,vol.5,pp.291,2011. [47] A.J.Ben-SassonandN.Tessler,Patternedelectrodeverticaleldeffecttransistor:Theoryandexperiment,JournalofAppliedPhysics,vol.110,pp.044501,2011. [48] L.P.MaandY.Yang,Uniquearchitectureandconceptforhigh-performanceorganictransistors,AppliedPhysicsLetters,vol.85,pp.5084,2004. [49] S.H.Li,Z.Xu,L.P.Ma,C.W.Chu,andY.Yang,Achievingambipolarverticalorganictransistorsviananoscaleinterfacemodication,AppliedPhysicsLetters,vol.91,pp.083507,2007. [50] J.Jiang,J.Sun,B.Zhou,A.X.Lu,andQ.Wan,Verticallow-voltageoxidetransistorsgatedbymicroporoussio2=liclcompositesolidelectrolytewith 93

PAGE 94

enhancedelectric-double-layercapacitance,AppliedPhysicsLetters,vol.97,pp.052104,2010. [51] J.Sun,Q.Wan,A.X.Lu,andJ.Jiang,Low-voltageelectric-double-layerpapertransistorsgatedbymicroporoussio2processedatroomtemperature,AppliedPhysicsLetters,vol.95,pp.222108,2009. [52] A.J.Ben-Sasson,E.Avnon,E.Ploshnik,O.Globerman,R.Shenhar,G.L.Frey,andN.Tessler,Patternedelectrodeverticaleldeffecttransistorfabricatedusingblockcopolymernanotemplates,AppliedPhysicsLetters,vol.95,pp.213301,2009. [53] Y.Xu,K.T.He,S.W.Schmucker,Z.Guo,J.C.Koepke,J.D.Wood,J.W.Lyding,andN.R.Aluru,Inducingelectronicchangesingraphenethroughsilicon(100)substratemodication,NanoLetters,vol.11,pp.2735,2011. [54] C.-H.Chang,C.-H.Chien,andJ.-Y.Yang,Pentacene-basedthin-lmtransistorswithmultiwalledcarbonnanotubesourceanddrainelectrodes,AppliedPhysicsLetters,vol.91,pp.083502,2007. [55] M.A.McCarthy,B.Liu,E.P.Donoghue,I.Kravchenko,D.Y.Kim,F.So,andA.G.Rinzler,Low-voltage,low-power,organiclight-emittingtransistorsforactivematrixdisplays,Science,vol.332,pp.570,2011. [56] K.Nakamura,T.Hata,A.Yoshizawa,K.Obata,H.Endo,andK.Kudo,Improvementofmetal-insulator-semiconductor-typeorganiclight-emittingtransistors,JapaneseJournalofAppliedPhysics,vol.47,pp.1889,2008. [57] S.SzeandK.K.Ng,Physicsofsemiconductordevices,ImperialCollegePress,Hoboken,c2007. [58] J.Appenzeller,M.Radosavljevic,J.Knoch,andP.H.Avouris,Tunnelingversusthermionicemissioninone-dimensionalsemiconductors,PhysicalReviewLetters,vol.92,pp.048301,2004. [59] S.Ijima,Helicalmicrotubulesofgraphiticcarbon,Nature,vol.354,pp.56,1991. [60] A.K.GeimandA.H.MacDonald,Graphene:Exploringcarbonatland,PhysicsToday,vol.August,pp.35,2007. [61] T.Durkop,S.A.Getty,E.Cobas,andM.S.Fuhrer,Extraordinarymobilityinsemiconductingcarbonnanotubes,NanoLetters,vol.4,pp.35,2004. [62] Z.Yao,C.L.Kane,andC.Dekker,High-eldelectricaltransportinsingle-wallcarbonnanotubes,PhysicalReviewLetters,vol.84,pp.2941,2000. [63] A.Javey,H.Kim,M.Brink,Q.Wang,A.Ural,J.Guo,P.McIntyre,P.McEuen,M.Lundstrom,andH.J.Dai,High-kappadielectricsforadvanced 94

PAGE 95

carbon-nanotubetransistorsandlogicgates,NatureMaterials,vol.1,pp.241,2002. [64] R.Saito,G.Dresselhaus,andM.S.Dresselhaus,Physicalpropertiesofcarbonnanotubes,Wiley-Interscience,London,1998. [65] N.W.AshcroftandN.D.Mermin,SolidStatePhysics,Holt,RinehartandWinston,NewYork,1976. [66] J.W.McClure,Diamagnetismofgraphite,PhysicalReview,vol.104,pp.666,1956. [67] P.R.Wallace,Thebandtheoryofgraphite,PhysicalReview,vol.71,pp.622,1947. [68] J.W.G.Wilder,L.C.Venema,A.G.Rinzler,R.E.Smalley,andC.Dekker,Electronicstructureofatomicallyresolvedcarbonnanotubes,Nature,vol.391,pp.59,1998. [69] M.I.Katsnelson,K.S.Novoselov,andA.K.Geim,Chiraltunnellingandthekleinparadoxingraphene,NaturePhysics,vol.2,pp.620,2006. [70] L.A.Ponomarenko,F.Schedin,M.I.Katsnelson,R.Yang,E.W.Hill,K.S.Novoselov,andA.K.Geim,Chaoticdiracbilliardingraphenequantumdots,Science,vol.320,pp.356,2008. [71] S.IlaniandP.L.McEuen,Electrontransportincarbonnanotubes,TheAnnualReviewofCondensedMatterPhysics,vol.1,pp.1,2010. [72] S.Ilani,L.A.K.Donev,M.Kindermann,andP.L.McEuen,Measurementofthequantumcapacitanceofinteractingelectronsincarbonnanotubes,NaturePhysics,vol.2,pp.687,2006. [73] W.Liang,M.Bockrath,D.Bozovic,J.H.Hafner,M.Tinkham,andH.Park,Fabry-perotinterferenceinananotubeelectronwaveguide,Nature,vol.411,pp.665,2011. [74] J.Kong,E.Yenilmez,T.W.Tombler,W.Kim,H.J.Dai,R.B.Laughlin,L.Liu,C.S.Jayanthi,andS.Y.Wu,Quantuminterferenceandballistictransmissioninnanotubeelectronwaveguides,PhysicalReviewLetters,vol.87,pp.106801,2001. [75] A.Javey,J.Guo,Q.Wang,M.Lundstrom,andH.J.Dai,Ballisticcarbonnanotubeeld-effecttransistors,Nature,vol.424,pp.654,2003. [76] M.J.Biercuk,N.Mason,J.Martin,A.Yacoby,andC.M.Marcus,Anomalousconductancequantizationincarbonnanotubes,PhysicalReviewLetters,vol.94,pp.026801,2005. 95

PAGE 96

[77] A.Thess,R.Lee,P.Nikolaev,H.Dai,P.Petit,J.Robert,C.H.Xu,Y.H.Lee,S.G.Kim,A.G.Rinzler,D.T.Colbert,G.E.Scuseria,D.Tomanek,J.E.Fischer,,andR.E.Smalley,Crystallineropesofmetalliccarbonnanotubes,Science,vol.273,pp.483,1996. [78] A.G.Rinzler,J.Liu,H.Dai,P.Nikolaev,C.B.Huffman,F.J.Rodriguez-Macias,P.J.Boul,A.H.Lu,D.Heymann,D.T.Colbert,R.S.Lee,J.E.Fischer,A.M.Rao,P.C.Eklund,andR.E.Smalley,Large-scalepuricationofsingle-wallcarbonnanotubes:process,product,andcharacterization,AppliedPhysicsA-MaterialsScienceandProcessing,vol.67,pp.29,1998. [79] P.H.Collins,M.S.Arnold,andP.Avouris,Engineeringcarbonnanotubesandnanotubecircuitsusingelectricalbreakdown,Science,vol.292,pp.706,2001. [80] P.N.Nirmalraj,P.E.Lyons,S.De,J.N.Coleman,andJ.J.Boland,Electricalconnectivityinsingle-walledcarbonnanotubenetworks,NanoLetters,vol.9,pp.38903895,2009. [81] M.S.Fuhrer,J.Nygard,L.Shih,M.Forero,Y.G.Yoon,M.S.C.Mazzoni,H.J.Choi,J.Ihm,S.G.Louie,A.Zettl,andP.L.McEuen,Crossednanotubejunctions,Science,vol.288,pp.494,2000. [82] D.S.Hecht,L.Hu,andG.Gruner,Conductivityscalingwithbundlelengthanddiameterinsinglewalledcarbonnanotubenetworks,AppliedPhysicalLetters,vol.89,pp.133112,2006. [83] D.S.Hecht,L.Hu,andG.Irvin,Emergingtransparentelectrodesbasedonthinlmsofcarbonnanotubes,graphene,andmetallicnanostructures,AdvancedMaterials,vol.23,pp.1482,2011. [84] E.M.Doherty,S.De,P.E.Lyons,A.Shmeliov,P.N.Nirmalraj,V.Scardaci,J.Joimel,W.J.Blau,J.J.Boland,andJ.N.Coleman,Thespatialuniformityandelectromechanicalstabilityoftransparent,conductivelmsofsinglewallednanotubes,Carbon,vol.47,pp.24662473,2009. [85] S.J.Tans,A.R.M.Verschueren,andC.Dekker,Room-temperaturetransistorbasedonasinglecarbonnanotube,Nature,vol.393,pp.49,1998. [86] S.Rosenblatt,Y.Yaish,J.Park,J.Gore,V.Sazonova,andP.L.McEuen,Highperformanceelectrolytegatedcarbonnanotubetransistors,NanoLetters,vol.2,pp.869,2002. [87] R.Misra,M.McCarthy,andA.F.Hebard,Electriceldgatingwithionicliquids,AppliedPhysicsLetters,vol.90,pp.052905,2007. [88] P.-H.Wang,B.Liu,Y.Shen,Y.Zheng,M.McCarthy,P.Holloway,andA.Rinzler,N-channelcarbonnanotubeenabledverticaleldeffecttransistorswithsolution 96

PAGE 97

depositedZnonanoparticlebasedchannellayers,AppliedPhysicsLetters,vol.100,pp.173514,2012. [89] S.Y.Han,Y.J.Chang,D.H.Lee,S.O.Ryu,T.J.Lee,andC.H.Chang,Chemicalnanoparticledepositionoftransparentznothinlms,ElectrochemicalandSolidStateLetters,vol.10,pp.K1K5,2007. [90] H.Faber,M.Klamunzer,M.Voigt,D.Galli,B.F.Vieweg,W.Peukert,E.Spiecker,andM.Halik,Morphologicalimpactofzincoxidelayersonthedeviceperformanceinthin-lmtransistors,Nanoscale,vol.3,pp.897,2011. [91] S.Walther,S.Polster,M.P.M.Jank,H.Thiem,H.Ryssel,andL.Frey,Tuningofchargecarrierdensityofznonanoparticlelmsbyoxygenplasmatreatment,AdvancedPowderTechnology,vol.22,pp.253,2011. [92] L.Chua,J.Zaumseil,J.Chang,E.C.Ou,P.K.Ho,H.Sirringhaus,andR.H.Friend,Generalobservationofn-typeeld-effectbehaviourinorganicsemiconductors,Nature,vol.434,pp.194,2005. [93] L.Qian,Y.Zheng,K.R.Choudhury,D.Bera,F.So,J.Xue,andP.H.Holloway,Electroluminescencefromlight-emittingpolymer/znonanoparticleheterojunctionsatsub-bandgapvoltages,NanoToday,vol.5,pp.384,2010. [94] L.Qian,Y.Zheng,J.Xue,andP.H.Holloway,Stableandefcientquantum-dotlight-emittingdiodesbasedonsolution-processedmultilayerstructures,NaturePhotonics,vol.5,pp.543,2011. [95] B.Liu,M.A.McCarthy,andA.G.Rinzler,Non-volatileorganicmemoryelementsbasedoncarbon-nanotube-enabledverticaleld-effecttransistors,AdvancedFunctionalMaterials,vol.20,pp.3440,2010. [96] S.Ogawa,T.Naijo,Y.Kimura,H.Ishii,andM.Niwano,Photoinduceddopingeffectofpentaceneeldeffecttransistorinoxygenatmospherestudiedbydisplacementcurrentmeasurement,AppliedPhysicsLetters,vol.86,pp.252104,2005. [97] K.Vanheusden,C.H.Seager,W.L.Warren,D.R.Tallant,andJ.A.Voigt,Correlationbetweenphotoluminescenceandoxygenvacanciesinznophosphors,AppliedPhysicsLetters,vol.68,pp.403,1996. [98] Y.Ma,G.T.Du,T.P.Yang,D.L.Qiu,X.Zhang,H.J.Yang,Y.T.Zhang,B.J.Zhao,X.T.Yang,andD.L.Liu,EffectoftheoxygenpartialpressureonthepropertiesofZnOthinlmsgrownbymetalorganicvaporphaseepitaxy,JournalofCrystalGrowth,vol.225,pp.303,2003. [99] S.E.ThompsonandS.Parthasarathy,Moore'slaw:thefutureofSimicroelectronics,MaterialsToday,vol.9,pp.20,2006. 97

PAGE 98

[100] W.G.SpitzerandC.A.Mead,Barrierheightstudiesonmetal-semiconductorsystems,JournalofAppliedPhysics,vol.34,pp.3061,1963. [101] I.G.Hill,A.Rajagopal,A.Kahn,andY.Hu,Molecularlevelalignmentatorganicsemiconductor-metalinterfaces,AppliedPhysicsLetters,vol.73,pp.662,1998. [102] L.J.Brillson,Transitioninschottkybarrierformationwithchemicalreactivity,PhysicalReviewLetters,vol.40,pp.260,1978. [103] A.M.CowleyandS.M.Sze,Surfacestatesandbarrierheightofmetalsemiconductorsystems,JournalofAppliedPhysics,vol.36,pp.3212,1965. [104] J.Bardeen,Surfacestatesandrecticationatametalsemiconductorcontact,PhysicalReview,vol.71,pp.717,1947. [105] J.L.FreeoufandJ.M.Woodall,Schottkybarriers:Aneffectiveworkfunctionmodel,AppliedPhysicsLetters,vol.39,pp.727,1981. [106] R.T.Tung,Chemicalbondingandfermilevelpinningatmetal-semiconductorinterfaces,PhysicalReviewLetters,vol.84,pp.6078,2000. [107] C.ShenandA.Kahn,Theroleofinterfacestatesincontrollingtheelectronicstructureofalq3/reactivemetalcontacts,OrganicElectronics,vol.2,pp.89,2001. [108] Y.-J.Yu,S.Ryu,Y.Zhao,L.E.Brus,K.S.Kim,andP.Kim,Tuningthegrapheneworkfunctionbyelectriceldeffect,NanoLetters,vol.9,pp.3430,2009. [109] F.Xia,V.Perebeinos,Y.-M.Lin,Y.Wu,andP.Avouris,Theoriginsandlimitsofmetalgraphenejunctionresistance,NatureNanotechnology,vol.6,pp.179,2011. [110] M.C.Lonergan,Atunablediodebasedonaninorganicsemiconductorverticalbarconjugatedpolymerinterface,Science,vol.278,pp.2103,1997. [111] H.Yang,J.Heo,S.Park,H.J.Song,D.H.Seo,K.-E.Byun,P.Kim,I.Yoo,H.-J.Chung,andK.Kim,Graphenebarristor,atriodedevicewithagate-controlledSchottkybarrier,Science,vol.336,pp.1140,2012. [112] Z.Wu,Tunablecontactbarrierofsinglewallcarbonnanotubelmsforelectricalcontacttosemiconductorsandpolymers,Ph.D.dissertation,UniversityofFlorida,2008. [113] Z.Xu,S.H.Li,L.Ma,G.Li,andY.Yang,Verticalorganiclightemittingtransistor,AppliedPhysicsLetters,vol.91,pp.092911,2007. 98

PAGE 99

[114] M.G.Lemaitre,E.P.Donoghue,M.A.McCarthy,B.Liu,S.Tongay,B.Gila,P.Kumar,R.K.Singh,B.R.Appleton,andA.G.Rinzler,Improvedtransferofgrapheneforgatedschottky-junction,vertical,organic,eld-effecttransistors,ACSNano,vol.6,pp.9095,2012. [115] F.A.PadovaniandR.Stratton,Fieldandthermionic-eldemissionsinschottkybarriers,Solid-StateElectronics,vol.9,pp.695,1966. [116] C.CrowellandV.Rideout,Normalizedthermionic-eld(T-F)emissioninmetal-semiconductor(Schottky)barriers,Solid-StateElectronics,vol.12,pp.89,1969. [117] PPMS:Physicalpropertymeasurementsystem,QuantumDesign( http://www.qdusa.com/sitedocs/productBrochures/1070-002.pdf ),Febuary2013. [118] T.YamamotoandK.J.Takimiya,Facilesynthesisofhighly-extendedheteroarenes,dinaphtho[2,3-b:20,30-f]chalcogenopheno[3,2-b]chalcogenophenes,andtheirapplicationtoeld-effecttransistors,JournaloftheAmericanChemicalSociety,vol.129,pp.2224,2007. [119] M.Redecker,D.C.Bradley,M.Inbasekaran,W.W.Wu,andE.P.Woo,Highmobilityholetransportuorene-triarylaminecopolymers,AdvancedMaterials,vol.11,pp.241,1999. 99

PAGE 100

BIOGRAPHICALSKETCH Po-HsiangWangwasborninTaipei,Taiwan.Hewasthesecondofthree,withtwosisters.Po-HsianggrewupinsouthernpartofTaiwan,wherestudywasnotsocriticalandheplayedlikeeveryoneelseinhischildhood.Hebegantobeinterestedinscienceinhighschool,wherehefocusedonPhysicsandturnedouttobeoneofthetopstudents.HeendupmakinghisownwayintoNationalTaiwanUniversityandmajoredinphysics.Duringwhichisoneofthebesttimesofhislife,heworkedside-by-sidewithseveralinteligentandhardworkingphysicsstudents.Afterreceivinghisbachelordegree,hewentonwithtwoyearsmandatorymilitaryservice.HewentbacktoschoolandgainedtwomasterdegreesinPhysicsandBiomedicalEngineeringanddevelopedskillsofconfocalanduorescentmicroscopy,laserengineering,andgainedsomeknowledgeofbiophysics.Healsoholdajobinindustryfortwoyearsbeforeheeverhadachanceofstudyingabroad.In2007,hecametotheUniversityofFloridaforthedoctoralprograminPhysicsandjoinedDrPetkova'sgroupinspring2008.HelatertransferredtoDrRinzler'sgroupinsummer2009.DuringhisstayinGainesville,heenjoyedfarminginStudent'sAgriculturalGarden. 100