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REVIEWOpenAccessInterfaceengineeringforhighperformancegrapheneelectronicdevicesDaeYoolJung,SangYoonYang,HaminPark,WooCheolShin,JoongGunOh,ByungJinCho*andSung-YoolChoi*AbstractAdecadeafterthediscoveryofgrapheneflakes,exfoliatedfromgraphite,wehavenowsecuredlargescaleandhighqualitygraphenefilmgrowthtechnologyviaachemicalvapordeposition(CVD)method.
WiththeestablishmentofmassproductionofgrapheneusingCVD,practicalapplicationsofgraphenetoelectronicdeviceshavegainedanenormousamountofattention.
However,severalissuesarisefromtheinterfacesofgraphenesystems,suchasdamage/unintentionaldopingofgraphenebythetransferprocess,thesubstrateeffectsongraphene,andpoordielectricformationongrapheneduetoitsinertfeatures,whichresultindegradationofbothelectricalperformanceandreliabilityinactualdevices.
Thepresentpaperprovidesacomprehensivereviewoftherecentapproachestoresolvetheseissuesbyinterfaceengineeringofgrapheneforhighperformanceelectronicdevices.
Wedealwitheachinterfacethatisencounteredduringthefabricationstepsofgraphenedevices,fromthegraphene/metalgrowthsubstratetographene/high-kdielectrics,includingtheintermediategraphene/targetsubstrate.
Keywords:Graphene;Interfaceengineering;Transfer;Delamination;Mobility;Doping;Hysteresis;Substrateeffect;Dielectric;Transistor1IntroductionGraphenehasreceivedmassiveattentionasapromisingnewmaterialforapplicationtoelectronicandoptoelec-tronicdevicesbecauseofitssuperioranduniqueelec-trical,optical,andmechanicalproperties[1-10].
Intheearlystageofgrapheneresearch,highqualitygrapheneobtainedbymechanicalexfoliation[1-8]ofgraphitefa-cilitatedfundamentalstudiesontheoutstandingproper-tiesofgraphene,triggeringexplosiveresearchontheapplicationofgraphenetovariousfields[11-15].
How-ever,theapplicationofgraphenetoreal-worlddevicesrequiresascalablesynthesistechniquetoovercomethelimitedquantityandsizeofmechanicallyexfoliatedgra-phene.
Graphenesynthesisbychemicalvapordepos-ition(CVD)[16-25]iscurrentlythemostwidelyadoptedtechniqueforthescalableproductionofsinglelayergraphene,uptoasizeof30inches[19].
Althoughlarge-scale,high-qualitygrapheneisnowavailable,therealizationofhighperformancegraphenedevicesisstillchallenging.
Specifically,devicesfabricatedfromCVD-growngraphenehavenotyetshownthelevelofper-formancethatwasanticipatedupontheemergenceofgraphene[26-29].
Whilethedegradationoftheper-formanceofgraphenedevicescanbeattributedtomanyfactors,thesignificantissueoftheinterfaceswheregra-pheneinteractswiththeneighboringmaterialswarrantsextensiveconsideration[30-35].
Duetotheone-atom-thick,two-dimensional(2D)characteristicofgraphene,itselectricalpropertiesaredirectlyaffectedbytheinter-actionofthegraphenesurfacewithadjacentmaterials.
Thepurposeofthisreviewistoshedlightontheim-portanceofinterfaceengineeringthroughtheentirefab-ricationprocessofgraphenedevicesfromseveralrecentreportsongraphenetransferandgrapheneelectronicdevices.
Inthisreview,westartfromnoveltransfertech-niquesviadirectdelaminationofgraphenefromametalgrowthsubstrate,whichiscloselyrelevanttotheinter-facecontrolinagraphene/growth(orgraphene/target)substrate.
Afterthetransferprocess,grapheneformsaninterfacewithatargetsubstrate,whichalsoinfluences*Correspondence:sungyool.
choi@kaist.
ac.
kr;elebjcho81@kaist.
ac.
krEqualcontributorsGrapheneResearchCenter,DepartmentofElectricalEngineering,KoreaAdvancedInstituteofScienceandTechnology(KAIST),Daejeon305-701,RepublicofKorea2015Jungetal.
;licenseeSpringer.
ThisisanOpenAccessarticledistributedunderthetermsoftheCreativeCommonsAttributionLicense(http://creativecommons.
org/licenses/by/4.
0),whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedtheoriginalworkisproperlycited.
Jungetal.
NanoConvergence(2015)2:11DOI10.
1186/s40580-015-0042-xFigure1(Seelegendonnextpage.
)Jungetal.
NanoConvergence(2015)2:11Page2of17thechargecarriertransportofgraphene.
Hence,wedis-cussthesubstrateeffectsandthechoiceofanoptimumsubstrateforhighperformancegraphenedevices.
Finally,whengrapheneistransferredintactontothetargetsub-strate,itisnecessarytoconsiderotherinterfacesthatarecreatedbytheintegrationofgraphenewithotherelectroniccomponentssuchasgatedielectrics.
Thesein-terfacesalsogiverisetochallengingissuesrelatedtothechemicalinertnessofthegraphenesurfaceandthewet-tability/interfacialadhesion.
Thelasttopiccoversnovelstrategiestointegrateuniform,ultrathingatedielectricsonthegraphenesurfacetoguaranteehighperformanceofgraphenedevices.
2Review2.
1Transfertechniques:interfacesatgraphene/metalgrowthsubstrateandgraphene/targetsubstrateSincelarge-areagraphenefilmsaremainlysynthesizedoncatalyticmetalsubstrates[16-23],wefirstshouldconsidertheinterfacebetweengrapheneandthecata-lyticmetalsubstrate.
ToapplyCVD-growngraphenetoelectronicdevices,graphenemustbeisolatedfromthisinterfaceanddeliveredtoatargetdielectricsubstrate.
Themostcommonmethodforgraphenetransferhasbeenpoly(methylmethacrylate)(PMMA)film-assistedtransfer[36-39],whichinvolveswetetchingofthemetalsubstrateandwater-mediateddeliveryofgraphenetothetargetsubstrate.
Drawbacksofthismethodincludepossibleoxidationofgrapheneduetothestrongoxida-tionpowerofmetaletchants[19]andcontaminationofgraphenebyetchingresiduessuchasionicimpuritiesfromtheetchant[40-42]andmetallicresiduesfromin-completeetching[41].
Inaddition,polymericresidues[31-33]afterPMMAremovalareanothersourceofcon-taminationofgraphene.
Theseresiduesdirectlyaffecttheelectricalpropertiesofgraphene,resultinginsignifi-cantdegradationoftheperformanceofgraphenedevices[31-33,43].
Forthesereasons,metal-etching-freetransferbydelaminationofgraphenefromthemetalsubstratehasbeenpursuedasanalternative,non-destructivemeansofrealizingcleantransferofgraphene.
2.
1.
1Electrochemicaldelamination/transferofgraphene:physicalweakeningofthegraphene/metalinterfaceOnestrategyfordirectdelaminationofgrapheneisphysicalweakeningoftheinterfacialinteractionsbetweengrapheneandthemetalgrowthsubstrate.
Figure1aschematicallyil-lustratestheelectrochemicaldelamination(ECD)ofgra-pheneinwhichhydrogen(H2)bubblesgeneratedbytheelectrolysisofwaterareutilizedasatoolforphysicalweak-eningofthegraphene/metalinterface[44].
Theelectro-chemicalcellemployedforthedelaminationconsistsofaPMMA/graphene/metalsubstrate(cathode,biasednega-tively)andglassycarbonornoblemetalwithlowreactivity(anode,biasedpositively),whichareplacedinanaqueouselectrolytesolution,suchasK2S2O8[44],NaOH[45,46],Na2SO4[47]orNaCl[48,49].
Underappliedbias,electroly-sisofwateratthecathodeinducesthegenerationandpenetrationofH2bubblesalongtheinterfacebetweenPMMA/grapheneandthemetalsubstrate,resultingingradualseparationofPMMA/graphenefromthemetalsubstrate.
Notethatpartialetchingofacopper(Cu)ornickelsubstratecanoccurbytheelectrolyteduringtheECDprocess,andthiscanbesuppressedbythechoiceofanappropriateelectrolytesuchasNa2SO4[47].
Thismethodishighlyusefulforsystemswheregrapheneissyn-thesizedonchemicallyinert,noblemetalssuchasplatinum[46],ruthenium[50]oriridium[51],becauseetchantsforcorrespondingmetalsarerarelyavailable.
Wangetal.
re-portedtheapplicationoftheECDprocessforthedirecttransferofgraphenetoaflexiblepolyimidesubstratebyde-positingthetargetpolyimidesubstratedirectlyontothegraphene/metalgrowthsubstrate,insteadofPMMA,wherenowater-mediatedgraphenedeliverytoaforeigntargetsubstrateisrequired(Figure1b)[47].
EliminationoftheconventionaluseofasacrificialPMMAenablestheproduc-tionofnearlyresidue-freegraphenewithalowdensitylinedefects(ripplesandwrinkles),yieldingflexible,transparentconductingfilmswithlowsheetresistance(~459Ω/sqforsinglelayergraphene,~49Ω/sqformultilayergraphene)(Figure1c).
IntheECDbasedtransfermethod,onecriticalissueismechanicaldamageofgraphenebyH2bubbles.
These(Seefigureonpreviouspage.
)Figure1Electrochemicaldelaminationofgraphene.
a,IllustrationofelectrochemicaldelaminationofgraphenefromPtfoilwithPMMAsacrificiallayer.
Reproducedwithpermission[46].
Copyright2012,NaturePublishingGroup.
b,IllustrationofdirectdelaminationofgrapheneontopolyimidesubstratewithoutPMMAsacrificiallayer.
Reproducedwithpermission[47].
Copyright2014,JohnWileyandSons.
c,AFMimagesoftransferredgrapheneonpolyimidewithoutsacrificiallayer(left)andwithPMMAsacrificiallayer(middle)viaelectrochemicaldelamination.
SheetresistanceofmonolayergrapheneonpolyimidetransferredbythedirectdelaminationmethodandthePMMA-assisteddelaminationmethod(right).
Reproducedwithpermission[47].
Copyright2014,JohnWileyandSons.
d,'Bubble'(top)and'bubble-free'(bottom)delaminatedandtransferredPMMA/graphenestackonoxidizedsiliconwafer.
Reproducedwithpermission[48].
Copyright2014,JohnWileyandSons.
e,Histogramsofpercentageofexposedsubstratearea(toppanel)andsheetresistance(bottompanel)forfilmsdelaminatedusingthe'bubble-free'(green)and'bubble'(red)methods.
Insetofbottompanel:Sheetresistanceofsamplesobtainedvia'bubble-free'and'bubble'delamination.
Reproducedwithpermission[48].
Copyright2014,JohnWileyandSons.
f,Transfercharacteristics(Id-Vg)offabricatedgrapheneFET(toppanel)andFETmobility(bottompanel)ofwhichgraphenearetransferredvia'conventionalwettransfer'(yellow)processand'electrochemicaldelamination'(blue)process.
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NanoConvergence(2015)2:11Page3of17Figure2(Seelegendonnextpage.
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NanoConvergence(2015)2:11Page4of17bubblescandirectlydamagegrapheneduringthedelam-inationprocessbyconsiderableturbulenceorcanresultincracks(orvoids),ripples,andwrinklesduetothetrappedH2betweenthegrapheneandthetargetsub-strate(upperpanelinFigure1d)[45,48].
Suchtransfer-induceddamageordefectsseverelydegradetheelec-tricalpropertiesofgraphenedevicesintermsofmobilityandsheetresistance.
Toeliminateorreducethemech-anicaldamagecausedbyH2bubbles,Cherianetal.
de-velopeda'bubble-free'delamination/transfermethodbyexploitingtheelectrochemicalreduction(dissolution)ofadventitiouscuprousoxide(Cu2O)sandwichedbetweengrapheneandaCusubstrateasatoolforweakeningofthatinterface[48].
ReductionofinterfacialCu2Ooc-curredatanoptimumpotentiallowerthanthatrequiredforthegenerationofH2bubbles.
ThismethodresultedinuniformadherenceofPMMA/graphenetothetargetsubstrate(lowerpanelofFigure1d)andtheresultingtransferredgraphenefilmexhibitedanegligibleamountofvoids(upperpanelofFigure1e)andhighuniformityofelectricalproperties(lowerpanelofFigure1e).
Dam-agesingraphenebythegenerationofH2bubblescanbealsoalleviatedwithasimpleplastic-frame-assistedmethod[45].
InapreviousstudyweperformedECD-graphenetransferusingasimilarmethodtotheplastic-frame-assistedapproachtocomparethequalityofthetransferredgraphenebytheECDmethodwiththatbytheconventionalPMMA-assistedwetmethodintermsofthecharacteristicsofafield-effecttransis-tor(FET).
Experimentaldetailsaredescribedin[52].
WefoundthattheelectricalcharacteristicsofECD-grapheneFETsreflectedtheeffectivesuppressionofp-doping(reducedDiracpoint)andenhancedFETmobilitywithsymmetricalelectron–holeconduction(Figure1f).
2.
1.
2Drytransferofgraphene:differenceofadhesionenergybetweengraphene/metalandgraphene/targetsubstrateinterfacesGraphenetransferbasedondirectdelaminationcanalsobeachievedbyexploitingthedifferenceinadhesionenergiesbetweengraphene/metalandgraphene/targetsubstrateinterfaces[53,54].
Thebasicconceptofthisapproachisil-lustratedinFigure2a:Anadhesiveinterfaceisformedbe-tweenagraphene/metalsubstrateandatargetsubstrate,andthenthetwosubstratesareseparatedundertensileloadingbydoublecantileverbeamfracturetesting[53].
TheRamanspectrainFigure2bindicatethatapplica-tionofanappropriateadhesivelayerenablesmechan-icaldelaminationofgraphenefromtheCusubstrate.
AsshowninFigure2c,Shinetal.
measuredtheadhesionenergyofgraphenetovariousadhesivelayers(ortargetsubstrates)andfoundthatagraphene/poly(vinylphe-nol)(PVP)systemexhibitedthehighestadhesionenergy(2.
31±0.
11Jm2),higherthanthatofagraphene/Cusystem(0.
72±0.
07Jm2)[53].
ThisindicatesthatPVPcanactasanappropriateadhesivethatinducessuccess-fuldelaminationandtransferofgraphenefromCutoatargetsubstrate[54].
Becauseawetprocessisexcluded,thistransfermethodiscalled'drytransfer'.
Asmen-tionedinSection2.
1,advantagesofthedelamination/transfermethodincludetherestorationofchargeneu-tralityandsymmetricalelectron–holeconductionofgraphene,whichareusuallydegradedbyametaletchingprocessintheconventionalwettransferapproach.
Figures2dand2eshowp-dopingsuppressionanden-hancedelectroncurrentmodulationofdry-transferredgrapheneFETs,incomparisontographeneFETspre-paredusingeitherconventionalwettransferorECDtransfer(Figure1f)[54].
Theseenhancementsobtainedwithdrytransferareattributedtotheabsenceofoppor-tunityforgraphenetobecontaminatedbyionicimpur-ities(fromeithertheelectrolyteormetaletchant)andmetallicresidues(fromincompleteetchingofthemetalsubstrate).
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recentlyexploitedamechano-electro-thermal(MET)processtoinducedelaminationanddrytransferofgraphenefromaCusubstratedirectlytovarioussub-stratessuchasglass,PET,andPDMS(Figure2f)[55].
Thekeyaspectofthismethodistoformstrongandultra-conformalcontactbetweengraphene/Cuandatargetsubstratebyapplyinghightemperature,physicalpressure,andhighvoltagesimultaneouslytotheCu(Seefigureonpreviouspage.
)Figure2Drytransferusingdifferenceofadhesionenergybetweengraphene/metalandgraphene/targetsubstrate.
a,Illustrationofgraphenetransferusingthemechanicaldelaminationprocessandhigh-magnificationSEMimageofboundaryofdelamination.
Reproducedwithpermission[53].
Copyright2012,AmericanChemicalSociety.
b,Ramanspectraofthegraphene-delaminatedbarecopper(thelowerspectrum)andofthegraphene-coveredcopper(theupperspectrum).
Reproducedwithpermission[53].
Copyright2012,AmericanChemicalSociety.
c,Directlymeasuredadhesionenergyofgraphenetoneighboringmaterials(SiO2,PVP,andPMMA).
Reproducedwithpermission[54].
Copyright2013,AIPPublishingLLC.
d,Transfercharacteristics(IDS-VGS)ofthegrapheneFETsfabricatedusingconventionalwettransfer(black)methodanddrytransferwithPVPadhesivelayer(red).
Reproducedwithpermission[54].
Copyright2013,AIPPublishingLLC.
e,ChargedensityofthegrapheneFETsfabricatedusingconventionalwettransfer(black)methodanddrytransferwithPVPadhesivelayer(red).
Reproducedwithpermission[54].
Copyright2013,AIPPublishingLLC.
f,Schematicdescriptionofthemechano-electro-thermal(MET)delaminationprocessofgraphene.
Reproducedwithpermission[55].
Copyright2014,JohnWileyandSons.
g,StrongmechanicalstabilityofMETgrapheneviademonstrationofLEDelectricalcircuitbasedongraphene/PETfilmusingrepeateddetachingof3Mtape.
Reproducedwithpermission[55].
Copyright2014,JohnWileyandSons.
Jungetal.
NanoConvergence(2015)2:11Page5of17foil/graphene/targetsubstratestack.
GrapheneistransferredtothetargetsubstratesimplybypeelingtheCufoiloffaftertheMETprocess.
Nopolymericcarriersoradhesivesareusedinthisapproach.
Mostimportantly,graphenetransferredbytheMETprocessexhibitedout-standinginterfacialadhesionwiththetargetsubstrateasaresultoftheultra-conformalcontactformation:themech-anicaladhesionstabilityofgrapheneismaintainedevenafterseveralcyclesoftapedetachingtests,asshowninFigure2g.
2.
1.
3Directdelamination/transferofgraphenewithhighdegreeoffreedomRecently,Yangetal.
reportedthatthecombinationofpre-treatmentofgraphene/Cusubstratewiththewell-knowntransferprintingtechniqueallowscleandelaminationandtransferofgraphene,whichcanalsoendowthegraphenetransferprocesswithahighdegreeoffreedom(Figure3a)[56].
Delaminationofgrapheneisinducedbytheadsorp-tionofawatersolublepolymer(poly(vinylalcohol)inthiscase)onthegraphenegrowthsubstrate,followedbytheFigure3Directdelaminationandtransferofgrapheneusingtransferprintingmethod.
a,SchematicillustrationofdirectdelaminationandtransferofgraphenewithPVAcarrierlayer.
Reproducedwithpermission[56].
Copyright2014,JohnWileyandSons.
b,Three-layeredgraphenefabricatedbylayer-by-layerstackinginadeterministicmanneronoxidizedsiliconwafer.
Reproducedwithpermission[56].
Copyright2014,JohnWileyandSons.
c,False-colorscanningelectronmicroscopeimageoftransferredgrapheneongoldelectrodes.
Reproducedwithpermission[56].
Copyright2014,JohnWileyandSons.
d,Transfercharacteristicsofbottom-contactgrapheneFETwithtransferredgrapheneontopofgoldsource/drainelectrodes.
Reproducedwithpermission[56].
Copyright2014,JohnWileyandSons.
Jungetal.
NanoConvergence(2015)2:11Page6of17Table1SummaryofgraphenetransfermethodsGraphene/metalseparationGraphenedeliverytotargetsubstrateGrapheneisolation(itssupportinglayer)DegreeoffreedomRenewabilityofmetalsubstrateOriginofgraphenedefectscReferencesConventionalpolymer-assisted,wettransferMetaletchingWater-mediatedscoopingO(PMMA)HighXMetaletchingprocess[36-39]Electrochemicaldelamination/transferElectrochemicaldelaminationbyH2bubblesWater-mediatedscoopingO(PMMA)HighOH2bubbles[44-49,52]Adhesive-assisteddrytransferMechanicaldelaminationbyadhesiveDirecttransferaX(NA)bVerylowOIncompletedelamination[53,54]DrytransferwithMETprocessMechanicaldelaminationbyMETprocessDirecttransferaX(NA)bLowOIncompletedelamination[55]TransferprintingviadirectdelaminationMechanicaldelaminationbypre-treatmentStamp-mediatedprintingO(PVA)HighOIncompletedelamination[56]aDelaminationandtransferofgrapheneoccursimultaneously.
bSupportinglayerisnotapplicable.
cDefectsincludedamageorcontaminationoftransferredgraphene.
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NanoConvergence(2015)2:11Page7of17formationofacarrierlayerusingthesamepolymer.
Be-causethedelaminatedgraphenecanexistinanisolatedstateonanelastomericsupportduringthistransferprocess(step3inFigure3a),thistransfermethodallowsthegraphenetoformeffectivejunctionswithitself(layer-by-layerstacking,Figure3b)orwithotherelectroniccom-ponents(grapheneonsource/drainelectrodes,Figures3cand3d),indicatingthehighdegreeoffreedomandtheresultingversatilityofthedevelopedmethod.
Table1pro-videsasummaryofgraphenetransfermethods.
2.
2Interfaceengineeringofgraphene/targetsubstrate2.
2.
1Modificationofgraphene/targetsubstrateinterfaceAfterthetransferprocess,graphenemakescontactwithatargetsubstrate.
Thermallygrownsilicondioxide(SiO2)hasbeenwidelyusedasatargetsubstratefromtheearlystageofgrapheneresearchduetoitscommercialavailability,relativelysmallsurfaceroughness,andtheclearvisibilityofsinglelayergrapheneonitataspecificthicknessofSiO2(c.
a.
90or300nm)[57,58].
However,whengrapheneisplacedonaSiO2substrate,theperform-anceofgrapheneFETsisconsiderablydegradedbythesubstrateeffects[59-66].
Thesubstrateeffects,withrespecttothemobilitylimitationofgraphene,includethescatteringofcarriersingraphenebychargedimpurities[64]andsurfacephonons[60]:theFETmobilityofgraphene/SiO2isseveralordersofmagnitudelowerthanthatofsuspendedgraphenedevices[60,61].
Inaddition,theadsorbedwatermoleculesbysilanol(SiOH)groupsatthegraphene/SiO2interfaceresultinunintentionalp-dopingofgraphene[67-69]andhysteresisofgrapheneFETs[66,70,71].
Figure4Passivationoftargetsubstratefortransferredgraphene.
a,FieldeffectmeasurementatT=293KforgrapheneonHMDS(black)andforgrapheneonbareSiO2(red).
Reproducedwithpermission[73].
Copyright2010,AmericanChemicalSociety.
b,HistogramofmobilityandtheDiracpointofdifferentgrapheneFETsonbareSiO2/SiandonOTMS-modifiedSiO2/Sisubstratesatroomtemperatureunderambientconditions.
Reproducedwithpermission[77].
Copyright2011,JohnWileyandSons.
c,Transfercharacteristics(Ids-Vg)foratypicalparylenegatedFETinair,bakedat400Kinvacuum,andinair30minafterbaking(toppanel).
Drain-sourcecurrentversusback-gatevoltageforsiliconoxidedevicesinair,bakedat400Kinvacuum,andinair30minafterbaking(bottompanel).
Reproducedwithpermission[78].
Copyright2009,AIPPublishingLLC.
d,Thechangeincarrierdensityingrapheneondifferentsurface(fluoropolymerandSiO2)withelapseoftimeinanairambient.
Reproducedwithpermission[79].
Copyright2011,AIPPublishingLLC.
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NanoConvergence(2015)2:11Page8of17Figure5(Seelegendonnextpage.
)Jungetal.
NanoConvergence(2015)2:11Page9of17PassivationoftheSiO2surfacewithahydrophobicbufferlayerhasbeensuggestedasaneffectiveroutetoimprovetheinterfacepropertiesofSiO2.
Twotypesofbufferlayershavebeenstudiedforthispurpose:self-assembledmonolayers(SAMs)[72-77]andthinpolymerlayers[78,79].
Lafkiotietal.
reportedthattheintrinsicchargeneutralityofgraphenewasrecoveredandthehysteresisofgrapheneFETswasdramaticallyreducedbymodifyingthegraphene/SiO2interfacewithhexamethyl-disilazane(HMDS)(Figure4a)[73].
Wangetal.
alsoob-servedsimilarphenomenawithalkyl-terminatedSAM.
Inbothcases,themobilityofgrapheneincreasedseveral-fold,ascomparedtodevicesonbareSiO2.
Representativeresultsforalkyl-SAMareshowninFigure4b[77].
ThehydrophobictreatmentseliminateSiOHgroupsandtheadsorbedwatermoleculesatthegraphene/SiO2interface,resultinginsuppressionofp-dopingingraphene.
Inaddition,thescatteringinducedbychargedimpuritiesandsurfacepolarphononscanbescreenedduetothein-creaseofthegraphene-substratedistanceviainterfacemodificationwithHMDSororganosilaneSAMs.
Leeatal.
demonstratedthatchargedimpuritiesweremoreeffect-ivelyscreenedbySAMwithlongeralkylchainlengthsbyshowingthatoctadecyl-SAM(C18)resultedinsmallerDiracvoltageandhighermobilityofgraphenethanoctyl-SAM(C8)[72].
AccordingtoareportbySabrietal.
,depositionofathinpolymericlayer(a168nmfilmofParylene-C)onSiO2providedthesameeffectasSAMtreatmentintermsofre-ducedhysteresisandenhancedmobility(Figure4c)[78].
Recently,Shinetal.
foundthatelectricalreliabilityofgra-pheneFETsinanairambientcanbeachievedbyintrodu-cinganultrathinfluoropolymertothegraphene/SiO2interface[79].
Witha7-nm-thickCYTOPfluoropolymer,thecarrierdensityofgraphenewaschangednegligiblyevenafterexposureofthedevicetoanairambient(RH~45%)for3weeks(Figure4d).
Ahighlyhydrophobicbuffersuchasthefluorinatedbufferalsocontributestorecoveryofintrinsicchargeneutrality,suppresseshysteresis,andenhancesmobility,asmentionedabove.
Itisworthnotingthattheuniquewettingtransparencyofgraphene[80]pre-ventswatermoleculesfrombeingadsorbedonthegra-phenesurfaceintegratedonthefluoropolymer,resultinginexcellentambientstabilityofgrapheneFETs.
Modificationofthegraphene/targetsubstrateinterfacebySAMcanbealsousedtotunethecarriertypeordensity,namelydopingcontrol,withoutcompromisingtheintrinsicelectricalpropertiesofgraphene[81-84]:SAMsterminatedwithvariousfunctionalgroupsinducen-orp-typedopingofgraphenebychargetransferfromaspecificfunctionalgrouporthebuilt-inpotentialgen-eratedfromthedipolemomentoftheSAM.
Here,wedonotcoverthisindetail,butreaderswhoareinter-estedinthistopicmayrefertoarecentin-depthreview[85].
Inconnectiontothisreviewitshouldbemen-tionedthatthereporteddopinglevelshaveshownawiderangeofvariationintermsoftheDiracpointevenwhenthesameSAM(forexample,aminopropyltriethox-ysilane;APTES)wasused[84,86-89].
Thismightbeat-tributabletotheintegrityoftheSAMformedonSiO2,whichsometimesdependssensitivelyonthechemistryinvolvedinSAMformationsuchasthetreatmentmethods(phaseofSAMduringthetreatment)andcon-ditions(watercontentintheambientatmosphereorsolvent)[90].
Hence,morestudiesarerequiredtoex-ploitSAMdopingmethodsinpracticalapplicationsbe-yondacademicresearchbasedonSiO2.
Forinstance,giventhatAPTESSAMtreatmentcanprovideamoreeffectiveandstablen-dopingsource[87,88]overotherdopingmethods,itisworthwhiletoinvestigatehowtoachieveareliabledopinglevelandwhetherthisdopingmethodisalsocompatiblewithplasticsubstratesfortheapplicationofAPTESSAMtographeneflexible/transparentelectrodes.
Oneofthesourcesofthedeg-radationofthedopingstrengthisthedesorptionofdopantsinchemicaldopingmethodswhenthedopedgraphenesampleisexposedtotheairambient[91]whiletheformationofcovalentbondingbetweentheSAMandthetargetsubstratecanguaranteeenviron-mentaldopingstability.
2.
2.
2SubstrateswithhighsurfacephononenergyWhilethepassivationofSiO2withSAMsorfunctionalpolymersisusefultoconsiderablyreducethesubstrateeffectsontheelectricalcharacteristicsofgrapheneFETs,thecarriertransportingrapheneisstilllimitedbyther-mallyexcitedsurfacephononsofSAMsorpolymers,es-peciallyatroomtemperature.
Hence,thecombinationof(Seefigureonpreviouspage.
)Figure5h-BNandAlNassupportingsubstratesforgraphene.
a,Atomicstructureofgrapheneandhexagonalboronnitride.
b,Chargedensitymapofgraphene/BNandgraphene/SiO2.
Reproducedwithpermission[97].
Copyright2011,AmericanChemicalSociety.
c,Histogramoftheheightdistribution(surfaceroughness)measuredbyAFMforSiO2(blacktriangles),h-BN(redcircles)andgraphene-on-BN(bluesquares).
Reproducedwithpermission[92].
Copyright2010,NaturePublishingGroup.
d,TemperaturedependencesoftheresistivityatVg-VDirac=10VforCVD-growngraphene/h-BN,mechanicallytransferredgraphene/h-BN,andgrapheneonSiO2.
Reproducedwithpermission[98].
Copyright2013,JohnWileyandSons.
e,ResistanceversusappliedgatevoltageforCVD-growngraphene/h-BN,mechanicallytransferredgraphene/h-BN,andgrapheneonSiO2.
Reproducedwithpermission[98].
Copyright2013,JohnWileyandSons.
f,NormalizedchangeincarriermobilitywithtemperatureforgrapheneFETsonAlNandSiO2substrates.
Reproducedwithpermission[99].
Copyright2014,AIPPublishingLLC.
Jungetal.
NanoConvergence(2015)2:11Page10of17graphenewithsubstrateshavinghighsurfacephononen-ergyisthemostattractivewaytoachievehighperform-ancegraphenedevicesoperatedatroomtemperature.
Oneoutstandingmaterialforthispurposeishexagonalboronnitride(h-BN)[92-98].
h-BNisaninsulatingiso-morphofgraphite(Figure5a),alayereddielectricmaterialwithawidebandgapof~5.
97eVandadielectricconstantof~4[92].
Aplanar,hexagonallatticestructureoftheh-BNlayerisformedbystrongionicbondingbetweenboronandnitrogenatoms,whichprovidesachemicallyinert,dangling-bond-freeflatsurface[96].
AccordingtoDeckeretal.
,thesefeaturesoftheh-BNsurfaceinducelowerdensityofintroducedchargedimpuritiesingra-pheneandaconsiderablereductionofinhomogeneitiesofchargedensityingraphene/h-BN,ascomparedtoaSiO2substrate(Figure5b)[97].
Ripplesofgraphenearealsosuppressedonh-BNduetoitsatomicallyflatsur-face(Figure5c)and,evenmoreimportantly,thesurfacephononenergyofh-BNistwotimeslargerthanthatofSiO2[92].
Significantenhancementoftheelectricalchar-acteristicsofgraphenedevicescanhencebeexpectedbytheimprovedinterfaceofthegraphene/h-BNsystem.
ThehighestmobilityvaluereportedforCVD-graphene/h-BNis65500cm2/Vs,whichis~30timeshigherthanthatforCVD-graphene/SiO2[93].
Wangetal.
investigatedtheeffectsofgraphene/sub-strateinterfacesontheelectricalperformanceofgra-pheneFETswiththreedifferentgraphenesystems:CVD-graphenetransferredonSiO2,CVD-graphenetransferredonh-BNflakes,andgraphenedirectlygrownonaCVD-h-BNfilm[98].
Thetemperaturedependenceofthegrapheneresistivitywasnegligibleforgraphene/h-BN,indicatingthatnosurfacephononswereactivateduptoroomtemperatureinh-BNduetoitshighsurfacephononenergy(Figure5d).
Thegraphene/h-BNinterfacethusexhibitedsuperiormobility,anarrowerminimumconductivityplateau,andaDiracpointclosetozero,ascomparedtothegraphene/SiO2interface(Figure5e).
Su-perbperformanceofagraphenedevicewasobtainedwhenagraphene/h-BNinterfacewascreatedbysequentialCVDgrowthofgraphenedirectlyonCVD-grownh-BNonCuduetotheabsenceofresiduesandadsorbatesgeneratedfromthetransferprocess.
Whileh-BNisanidealsubstrateforhighperformancegraphenedevices,itisstillchallengingtosynthesizehighquality,largeareah-BNfilms.
Therefore,fromaprac-ticalpointofviewforgrapheneelectronics,itisneces-sarytodevelopalternative,cost-effectivesubstrateswithhighsurfacephononenergyandwithwhichitiseasytoobtainalargesizedfilmwithgoodreproducibilityanduniformity.
ArecentreportfromOhetal.
demonstratedthataluminumnitride(AlN)substratecanserveasanexcellentalternativetoh-BNwithseveraloftheadvan-tagesmentionedabove[99].
AnultrathinAlNfilmwithasmoothsurface(Rq~0.
5nm)wassimplyobtainedoveralargearea(4inchwafer)byaplasmaenhancedatomiclayerdeposition(PE-ALD)method.
TopgatedgrapheneFETsonanAlNsubstrateshowedhighermobilitythandevicesonSiO2,indicatingthesuppressionofsurfacepho-nonscattering.
ThehighsurfacephononenergyofAlN(Table2)resultedinweakertemperaturedependenceofmobility(Figure5f).
TheRFcut-offfrequencywastherebysignificantlyimprovedingrapheneFETsonAlN(115GHz),comparedtothoseonSiO2(55GHz).
Similaren-hancementofthecut-offfrequency(155GHz)wasalsoreportedbyWuetal.
foragrapheneFETondiamond-likecarbon(DLC)[100],whichshowshighsurfacephononenergy(Table2).
2.
3Interfaceengineeringofgraphene/gatedielectricIntegrationofgraphenewithpassivecomponentssuchasgatedielectricsisanotherimportantsteptoachievehighperformancegraphenedevices.
Oxidematerialswithahighdielectricconstant(k)havebeenusedasgatedielectricstofabricatetop-gatedgrapheneFETsbecausehigh-kdielectricsenablelowvoltageoperationofdevicesbytheirhighcapacitanceandalsoprovidescalingcap-ability[101-104].
Inconventionalelectronics,high-kdi-electricshavebeendepositedbyatomiclayerdeposition(ALD),becausethistechniquecanproduceultrathin,conformaloxidedielectricswithpreciselycontrolledthickness[105-109].
However,thebasalplaneofgra-phenehasfewdanglingbonds[110-113],whicharene-cessarytoinducethesurfacereactionofprecursorsintheALDprocess[114-116].
ThisuniquefeatureofthegraphenesurfaceresultsinirregularandpoorfilmformationofALD-dielectricsonpristinegraphene[111,113,117].
ALD-dielectricswitharoughsurfaceandmanypin-holescausehighleakagecurrent,lowbreak-downvoltage,andextrinsicscatteringofchargecarriersTable2MaterialpropertiesofvarioussubstratesusedingraphenedevicesSiO2AlNBNDLCSiC(6H)Bandgap(eV)8.
96.
285.
971.
43.
05Dielectricconstant3.
99.
145.
062.
5-69.
7CrystalstructureAmorphousWurtziteHexagonalAmorphousHexagonalSurfacephononenergy(meV)5983.
6101<165116Reproducedwithpermission[99].
Copyright2014,AIPPublishingLLC.
Jungetal.
NanoConvergence(2015)2:11Page11of17Figure6(Seelegendonnextpage.
)Jungetal.
NanoConvergence(2015)2:11Page12of17atthegraphene/high-kdielectricinterfaceingrapheneFETs.
Toobtainhighquality,uniformhigh-kdielec-tricsongraphene,theintroductionofseedsonthegraphenesurfacehasbeenproposedasaneffectivemeansoffabricatinggraphenedevicesusingtheALDtechnique.
2.
3.
1IntroductionofseedingmaterialsongrapheneVariousmaterialshavebeenusedasseedingmaterialsfortheformationofuniform,highqualityhigh-kdi-electricsongraphenebyALD[113,118-126].
Kimetal.
proposedtheuseofathinaluminumlayer(thickness1~2nm)todeposituniformAl2O3filmsongraphene[119].
AnativealuminumoxideisformedwhenthethinAllayerisexposedtoair,andthisoxideprovidesnucleationsitesforthesurfacereactionsduringALDofAl2O3.
Organicmoleculesorpolymershavebeenexploitedasseedinglayersfortheintegrationofdielec-tricsandgraphene[113,122-125].
Oneexampleistheuseofapoly(vinylphenol)(PVP)film[125].
Shinetal.
preparedanultrathin,cross-linkedPVPseedinglayer(thickness~5nm)onagraphenesurfacebyaspin-coatingmethod(Figure6a).
DuetoabundantfunctionalgroupsinPVPsuchashydroxylandhydrocarbon,theAl2O3filmdepositedonthePVPseedinglayerbyALDwassmooth(Rrms~0.
5nm)withoutpin-holes.
Theelectricalperformanceoftop-gatedgrapheneFETswasconsiderablyimprovedwithPVP-seededAl2O3,comparedtodeviceswithAl2O3depositedonbaregraphene.
Specifically,thedraincurrentandtranscon-ductancewereenhanced,resultinginamorethanfive-foldincreaseofmobility(Figure6b).
Inparticular,agrapheneFETwithPVP-seededAl2O3showedasup-pressedDiracpointshiftunderagatebiasstresscondition(Figure6c).
ArecentstudybyKimetal.
sug-gestedthatquantumdot(CdSe)arraysformedongra-phenealsocanserveasaseedinglayerfortheeffectiveALDofhigh-khafniumoxideongraphene[126].
2.
3.
2FunctionalizationofgrapheneTogenerateseedingsitesongraphene,functionalgroupscanbedirectlyintroducedtothegraphenesurfacebyoxidizingcarbonatomsofgraphene[112,127-129].
Leeetal.
reportedongraphenefunctionalizationusingozone(O3)duringALDofAl2O3[112,127].
Anultrathin(~1nm),smooth(Rrms~0.
1nm)seedlayerwasformedongraphenebyO3treatmentinthepresenceofatri-methylaluminumprecursor.
GentleO3treatmentcondi-tions(at25°Cfor20s)inducednegligibledefectsongraphenewhileitssurfacewaspartiallyfunctionalizedwithepoxidegroups.
A15-nm-thickAl2O3layerwasuniformlyformedbyO3basedALD,resultinginhighperformance,top-gatedgrapheneFETswithcarriermo-bilityof5000cm2/Vs,lowVdirachysteresis,andlowleakagecurrent.
Inthesamecontext,Nayfehetal.
dem-onstratedthataremoteO2plasma-assistedALDtech-niqueproduceda9-nm-thickAl2O3layerwithbetterconformalcoverageandlowerroughness,comparedtoAl2O3filmsdepositedbythermalALD[128].
Theyre-portedanincreaseofthedefectlevelingrapheneonthebasisofRamanmeasurements,indicatingthatgraphenewasfunctionalizedduringtheO2plasma-assistedALDprocess.
BothdraincurrentandmobilitywereenhancedingrapheneFETswith9-nm-thickAl2O3byplasma-assistedALD,comparedtothoseobtainedwitha9-nm-thick,e-beam-evaporatedSiO2interfaciallayerplusa15-nm-thickAl2O3layerbythermalALD(Figure6d).
InordertoavoiduncontrolleddamageofgrapheneinO2plasmaorO3-assistedALD,Shinetal.
proposedanovelapproachforreliablehigh-kdielectricformationongraphenewithALDbyintroducinganadditionalfunctionalizedgraphenesinglelayerasanultrathinseedlayeronthegraphenechannel(Figure6e)[129].
Pristinegraphenewastransferredtoatargetsubstrateandthenfunctionalized(O2plasmatreated)graphenewasstackedonthepristinegraphenepriortoconductingtheALDprocess.
Al2O3wasdepositedviaconventionalthermalALDonafunctionalizedgraphenelayerwherethesurface(Seefigureonpreviouspage.
)Figure6SeedingALDofhigh-kdielectricongraphene.
a,Schematicdiagramshowingtop-gategrapheneFETstructurewithPVP-seededAl2O3gatedielectric.
Reproducedwithpermission[125].
Copyright2012,AIPPublishingLLC.
b,Transfercharacteristics(Id-Vg)oftop-gategrapheneFETbefore(blacksquare)andafter(redcircle)thegraphenechannelisdepositedwithPVP.
Inset:transconductanceofthegrapheneFETswithdifferentgatedielectricsasafunctionofgatevoltage.
Reproducedwithpermission[125].
Copyright2012,AIPPublishingLLC.
c,Thetime-dependentVDiracshiftofgrapheneFETbefore(blacksquare)andafter(redcircle)thegraphenechannelisdepositedwithPVP.
Reproducedwithpermission[125].
Copyright2012,AIPPublishingLLC.
d,Outputcharacteristics(Id-Vd)oftransistorsbasedonCVDmonolayergraphenewith9nmAl2O3(redline)and24nmheterogeneousintegrateddielectrics(dashedblueline)viadifferentdepositingmethods.
Reproducedwithpermission[128].
Copyright2011,IEEE.
e,Schematicdiagrampresentingthefunctionalizedgraphene-seededAl2O3stackongraphene.
Reproducedwithpermission[129].
Copyright2013,AmericanChemicalSociety.
f,SurfacemorphologyoftheAl2O3filmsdepositedongraphene(top)andfunctionalizedgraphene(bottom).
Scansize:1*1μm2.
Reproducedwithpermission[129].
Copyright2013,AmericanChemicalSociety.
g,Leakagecurrentdensities(at+3MV/cm2)versusEOTfordielectricswithfunctionalizedgrapheneseedlayer(redtriangle)andAlseedlayer(blacksquare)ongraphene.
Reproducedwithpermission[129].
Copyright2013,AmericanChemicalSociety.
Jungetal.
NanoConvergence(2015)2:11Page13of17hasabundant,oxidizedcarbonmoieties.
Al2O3depositedonthefunctionalizedgraphene/pristinegraphenestackexhibitedexcellentuniformitywithlowdefectdensity(Figure6f;Rrms~0.
3nm).
Inaddition,capacitorswithfunctionalizedgrapheneseededAl2O3showedlowerleak-agecurrentdensityforthesameeffectiveoxidethickness(EOT),comparedtothosewithAl-seededAl2O3:thisisaconsiderableadvantageofthisapproachintermsoffur-therscalingofgateoxidethickness(Figure6g).
3ConclusionsThisreviewhighlightedtheimportanceofinterfaceen-gineeringforhighperformancegraphenedevicesbyconsideringeachinterfaceencounteredduringthefabri-cationofgraphenedevices,fromthegraphene/metalgrowthsubstratetographene/high-kgatedielectrics.
Foreffectivedelaminationandtransferofgraphene,adhe-sionattheinterfaceofthegraphene/metalgrowthsub-strateorgraphene/targetsubstrateshouldbeengineeredbyappropriateweakeningorstrengtheningmethodsforthoseinterfaces.
Intermsofgraphenedelaminationusingpolymeradhesivesoracarrierlayer,questionsremainsaboutwhichfunctionalgroupsinthepolymerplayacrit-icalroletoinducedelaminationofgraphene.
Thisshouldbeinvestigatedsystematicallybyapplyingpolymershavingvariousfunctionalgroupstographenedelaminationsys-tems,inconjunctionwithaninvestigationofdopingeffectsthatmightbeinducedfromthefunctionalgroupsofpolymers.
Aftergrapheneistransferredontoatargetsubstrate,interfacialissuesarisefromtheatom-thicknessofgra-pheneandthesurface-grapheneinteractions.
Sincethesurfacestatesofsubstratessignificantlyaffecttheoverallelectricalpropertiesofgraphenedevices,substrateswithachemicallyinert,dangling-bond-freeflatsurfaceaswellashighsurfacephononenergyarehighlydemanded.
Al-thoughh-BNisanidealsubstrateintermsofrealizinghighperformancegrapheneelectronics,obtainingreli-able,large-areasynthesismethodsforh-BNbeyondmechanicalexfoliationisstillachallengingissue.
Ontheotherhand,alternativesubstratematerials,suchasAlN,areattractive,ashighlightedinthisreview.
Todepositahigh-kdielectricusingALD,itisneces-sarytointroduceseedmaterialsontographeneduetothechemicallyinertsurfaceofgrapheneortogenerateseedingcentersongrapheneitself.
Theseapproachescauseheterogeneousdielectricstacks(orinterfaces)andgiverisetodifficultyincontrollingthefilmthickness,therebyconstrainingthescalingofgatedielectricthick-ness.
Anovelapproachforthedepositionofgatedielec-tricsthereforeshouldbeexploredtoachieveasinglecomponentgatedielectricthatformsahomogeneousinterfacewithouttheapplicationofadditiveseedlayers.
Oneexamplewouldbethedepositionofultrathin(lessthan10nm)polymerdielectricsbytheinitiatedCVDmethod,whichisunderinvestigationbyourgroup.
TheuseofultrathinpolymerdielectricsingrapheneFETswouldalsobedesirableforthedevelopmentofflexibleelectronicdevices.
Intensivestudiesinrecentdecadeshaveprovidedagreatdealofinsightintotheimportantroleofinterfaceengineeringingraphenesystems,andhaveopenedupopportunitiesfortherealizationofhighperformancegraphenedevices.
Weexpectthatknowledgeaccumu-latedfromgraphenewillbeextendedtoemerging2Dmaterialsfortheenhancementandoptimizationofdeviceperformance.
CompetinginterestsTheauthorsdeclarethattheyhavenocompetinginterests.
Authors'contributionsDYJandSYYequallycontributedtothisworkinthemanuscriptpreparation.
Allauthorsreadandapprovedthefinalmanuscript.
AcknowledgementsThisworkwassupportedbytheITR&Dprogram(10044412),theGlobalFrontierResearchCenterforAdvancedSoftElectronics(2011–0031640),theBasicScienceResearchProgram(2010–0029132)andNano-MaterialTechnologyDevelopmentProgram(2012M3A7B4049807).
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