Non-alloyed Cr/Au Ohmic contacts to N-face and Ga-face n-GaN

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JournalofAlloysandCompounds516 (2012) 38–40

ContentslistsavailableatSciVerseScienceDirect

Journal

of

Alloys

and

Compounds

jo u r n al h om ep a ge :w w w . e l s e v i e r . c o m / l o c a t e / j a l l c o m

Non-alloyed

Cr/Au

Ohmic

contacts

to

N-face

and

Ga-face

n-GaN

Liang-Jyi

Yan

a

_{, Cheng}

_Huang

_Kuo

b

_{, Jinn-Kong}

_Sheu

a,c,∗

_{, Ming-Lun}

_Lee

c,d

_,

_Wei-Chun

_Tseng

a

a_Department_of_Photonics_and_Advanced_{Optoelectronic}_Technology_Center,_National_Cheng_Kung_University,_Tainan_70101,_Taiwan b_Institute_of_Lighting_and_Energy_Photonics,_National_Chiao_Tung_University,_Tainan_County_71150,_Taiwan

c_Center_for_Micro/Nano_Science_and_Technology,_National_Cheng_Kung_University,_Tainan_70101,_Taiwan d_Department_of_{Electro-Optical}_Engineering,_Southern_Taiwan_University_Tainan_County_71001,_Taiwan

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received16August2011 Receivedinrevisedform 21November2011 Accepted24November2011 Available online 2 December 2011 Keywords: GaN Cr–Au Nonalloyed Ohmic

a

b

s

t

r

a

c

t

Non-alloyedCr/AuOhmiccontactsonN-faceandGa-facen-GaNwerestudied.Thespeciﬁccontact resis-tances(c)ofCr/AucontactsontotheN-faceandGa-facen-GaNwereaslowas2.4×10−4cm2and

2.4×10−5_cm2_,_{respectively.}_Native_oxide_formed_on_the_n-GaN_surface_was_believed_to_be_the_key

factorforhigherc.TheresultsofX-rayphotoelectronspectroscopyconﬁrmedthatn-GaNsampleswith

differentsurfacepolaritiesortreatedbydifferentchemicalsolutionsexhibitedsigniﬁcantdifferencesin galliumoxidecontentonthesurface,whichledtoamarkeddifferenceinthecofnon-alloyedCr/Au

OhmiccontactstoGaNﬁlms.

1. Introduction

Recently, InGaN/GaNlight-emittingdiodes(LEDs) havebeen widelyappliedtotrafficsigns,backlightingofliquidcrystal dis-plays,andgenerallighting. WhiteLEDsusedasgenerallighting sourcesareoperatedatrelativelyhighpowercomparedwithLEDs usedinbacklightingortrafficsigns.Atpresent,theefficiencyof GaNLEDsisstillnotperfect.High-poweroperationmeansmass heatgenerationinLEDs.Therefore,thermalmanagementbecomes acriticalpointinhigh-powerwhiteLEDs.However,conventional GaN/sapphire-basedLEDsmaybenotsuitablefornext-generation generallightingduetothepoorthermalconductivityofthe sap-phiresubstrate.VerticalGaN-basedLEDs,whosesapphiresubstrate wasremoved bylaser lift-off (LLO)methodand whose epitax-iallayerwasbondedtoagoodthermalandelectricalconductive substratesuchasSiandCu,havedemonstratedhigh-power oper-ation characteristics[1–3].Generally,III-nitride semiconductors grownbymetalorganicvapor-phaseepitaxyonsapphiresubstrate withlow-temperature-grownGaNorAlNbufferlayersexhibit Ga-facesurfacepolarity[4].Therefore,forverticalLEDs,GaNsurface withN-facepolarityiscreatedaftertheremovalofthesapphire

∗ Correspondingauthorat:DepartmentofPhotonicsandAdvanced Optoelec-tronicTechnologyCenter,NationalChengKungUniversity,Tainan70101,Taiwan. Tel.:+88662758021;fax:+88662758021.

E-mailaddresses:[email protected](J.-K.Sheu), [email protected](M.-L.Lee).

substrateusingtheLLOprocess.Thismeansthen-typeelectrode contactsintheverticalGaNLEDsareformedontheN-faceGaN surface.MetalcontactsonGa-facen-GaNsurfacehavebeenwidely studiedin thepastdecade.Different metals, suchas Ti/Al-and Cr/Au-basedmetals,havebeenproventoexhibitgoodOhmic con-tactpropertyandstabilitywhendepositedontheGa-facen-GaN surface[5].TheTi/Al-basedcontactsarethemostpopularmetal schemes for n-GaN due to their low work function. However, theydo notalleviatetheneedfor highannealing temperatures toformintermetallicalloysfurtherwithlowworkfunctionatthe metal/semiconductorinterface[5].Theeffectofthermalannealing ontheelectricalpropertyofmetalcontactswasdifferentbetween theTi/Al- and Cr/Au-basedcontactson theGa-face n-GaN sur-face[5].SeveralreportshaverecentlyrevealedthatAl-basedmetal contactscouldformlow-resistivityOhmiccontactsontheN-face n-GaNsurface[6,7].However,thereisstilladearthofstudieson Cr/Au-basedmetalsontheN-facen-GaNsurface.Asthechemical activityofN-faceGaNsurfacewasconsideredtoberelativelyhigher thanthatofGa-faceGaNsurface,thecharacteristicsofmetal con-tactsdepositedontheGa-faceGaNsurfacewouldbesubstantially differentfromthecontactsontheN-facen-GaNsurface.Inthis study,weperformedthenon-alloyedCr/Au-basedOhmiccontacts onGa-faceandN-faceGaNsurfacestoelucidatethepropertiesof bothcontactschemes.

2. Experiment

Inthisstudy,Si-dopedn-typeGaN(n-GaN)epitaxialﬁlmswithcarrier concen-trationof1× 1019_/cm3_were_grown_by_MOVPE_on_sapphire_substrates._The_surface

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L.-J.Yanetal./JournalofAlloysandCompounds516 (2012) 38–40 39 ofn-GaNepitaxialﬁlmsexhibitedGa-facepolarity(0001).Thegrowthprocedure

oftheGa-facen-GaNlayersstartedwithalow-temperatureGaNnucleationlayer depositedonthesapphiresubstrate.High-temperatureunintentionallydopedGaN (u-GaN)andSi-dopedn-GaNwerethensequentiallygrown.Incontrast,n-GaNﬁlms withN-facepolarity(000 ¯1)wereachievedusingLLOanddryetchingprocessesto removethesapphiresubstrateandtheu-GaNlayer,respectively[1–3].Inthisstudy, Indium(2.5␮m)andTi/Pt/Au(30nm/200nm/100nm)weredepositedonthe sur-faceofGaNepitaxialwaferbeforethewaferbondingprocess.Asiliconsubstrate depositedwithTi/Au(50nm/600nm)bilayermetalsservedasthetemplateforthe waferbondingprocess.Duringthewaferbondingprocess,waferswerebrought togetherunder8kg/cm2_pressure_and_an_elevated_temperature_of₂₁₀◦_C._A₂₄₈_nm

excimerlaserservedasthelightsourcetoconducttheLLOprocessforremoving thesapphiresubstratefromthebondedwafers.Then-GaNfilmsweretransferred totheSisubstrates.Toinvestigatetheeffectofsurfacetreatmentontheproperty ofOhmiccontacts,bilayermetalsofCr/Au(50nm/1.5␮m)weredepositedonthe Ga-faceandN-facen-GaNfilms.Hydrochloricacid(HCl)-basedsolutionswereused totreattheGaNsurfacebeforemetaldepositionontotheGaNfilmstoformOhmic orSchottkycontactsbecausethenativeoxidelayerinherentlyformsastheGaN filmsareexposedtotheatmosphere[8,9].Inthisstudy,twodifferentHCl-based solutionswereusedtotreattheN-facen-GaNfilmsfor4minbeforethedeposition ofCr/Aumetals:HCl:H2O=1:2solutionmaintainedatanelevatedtemperatureof

50◦CandHCl:H2O=1:10maintainedatroomtemperature.Samplestreatedbythe

formersolutionwerelabeledsampleA,whereassamplestreatedbythelatterwere labeledsampleB.Ga-facen-GaNﬁlmstreatedbyHCl:H2O=1:10solutionfor4min

werealsopreparedforcomparisonandwerelabeledsampleC.

3. Resultsanddiscussion

Fig.1showsthetypicalI–Vcharacteristicsoftheas-deposited Cr/Au contacts on samples A, B, and C. All samples exhibited Ohmicproperty.Thespeciﬁccontactresistances(c),whichwere

determined by the transmission line model of the Cr/Au con-tacts on samples A, B, and C, were 2.4×10−4, 2.4×10−5, and 6.7_×10−5cm2_,_{respectively.}_For_the_formation_of_Ohmic_contact

ton-typesemiconductors,twomainmechanismsgoverncarrier transportacrossthemetal/semiconductorinterface,i.e.,thermionic emissionand tunneling. For Au/Cr/n-GaN contacts,tunneling is consideredtobetheprimarymechanismforlow-resistanceOhmic contact,especiallyforsemiconductorswithheavydoping.As Ga-and N-face n-GaN ﬁlmshave thesame bulk carrier concentra-tion,whichisashighas1×1019_/cm3_,_carrier_transport_through

theCr/n–GaNinterfaceisinevitablyconductedthroughthe tun-neling mechanism [5]. The Ga- and N-face n-GaN ﬁlms used in this study were degenerated semiconductors. Therefore, the non-alloyedOhmiccharacteristicswereexpectedandreasonable. However,the remarkabledifference in c betweenthe

experi-mentalsamplesimpliedthatn-GaNﬁlmswithdifferentsurface polaritiesortreated bydifferentchemicalsolutionspossessed a substantialdifferenceinsurfacestatesbetweentheGaNﬁlms.

Fig.1. TypicalI–Vcharacteristicsoftheas-depositedCr/Aucontactsonthen-GaN samplesA,B,andC.Thespacingbetweencontactpads(100␮m×100␮m)was 40␮m.

Fig.2. DisplaystheXPSspectraofO1sphotoelectronsfor(a)sampleA(b)sample (c)sampleC.

Karrer et al.proposed thatsurface band bendingin Ga-face n-GaNisfarlargerthanthatinN-facen-GaNduetothehigher spontaneouspolarizationintheGa-facen-GaNlayer[10]. There-fore,metalcontactsonGa-facen-GaNhavehigherSchottkybarrier heightscompared withthose ontheN-face n-GaN.In thelight oftheaforesaidcontention,metalcontactsonN-facen-GaNcan generatelowerOhmiccontactresistancecomparedwiththoseon Ga-facen-GaN.Inthisstudy,theas-depositedsamplesCexhibited amarkedlylowerccomparedwiththeas-depositedsamplesA

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40 L.-J.Yanetal./JournalofAlloysandCompounds516 (2012) 38–40 Table1

TheatomiccontentsofGa,N,andOtakenfromthen-GaNsamplesA,B,andCby XPS.

Samples Ga(at.%) N(at.%) O(at.%)

C O+O H Ga O

SampleA 40.6 39.9 16.4 3.1

SampleB 42.8 40.0 11.5 5.7

SampleC 51.8 40.2 6.3 1.7

andB. Ti/AlcontactsonGa-facen-GaNhave demonstratedthat

AlN interfacelayers formedafter thermal annealing can create

polarization-induced two-dimensional electron gases(2DEG) at

theAlN/GaNinterfacetofacilitatecarriertransportandreducec

[11].However,signiﬁcantinterfacialalloycompounds,suchasCrN, shouldbeabsentintheas-depositedAu/Cr/n-GaNsamples.In addi-tion,thebandgapenergyofCrNislowerthanthatofGaN[12]. Therefore,thelowercobtainedfromtheas-depositedsamplesC

isnotattributedtothepolarization-induced2DEGattheCr/GaN interfacetofacilitatecarriertransport.Zywietzetal.reportedthat N-faceGaNsurfacegenerallyexhibitshigheractivitytowards oxy-genadsorption[13].Consideringtheas-depositedsamplesBand C,thedifferenceincvaluesisattributedtotheeasierformation

ofnativegalliumoxidesontheN-faceGaNsurfacecomparedwith theGa-faceGaNsurface.Fig.1showsthattheas-deposited sam-plesAhavealowercvaluecomparedwithsamplesB.Thisresult

isduetothefactthatnativeoxidecanberemovedfurtherfrom theN-faceGaNsurfacewhenthesamplesweretreatedinheated HClsolution.Toclarifythispoint,X-rayphotoelectronspectroscopy (XPS)wasusedtoanalyzethesurfacesofthen-GaNsamples.Fig.2 displaystheXPSspectraofO1sphotoelectronsforsamplesA,B, andC,respectively.Toseparatethechemicalbondingstates,the spectrallineshapewassimulatedusingasuitablecombinationof Gaussianfunctions.FittingtheO1sspectrawith3components, O H,C O,andGa Obonds,theintegratedintensityofGa Opeak intheGa-facesampleswasmarkedlylowerthanthoseintheN-face samples.Incontrast,themagnitudeofGa Obondsobtainedfrom samplesAwaslowerthanthosefromsamplesB.Table1showsthe atomiccontentsofGa,N,andOinsamplesA,B,andC.Accordingto Table1,therearetwofactswheresamplesCexhibitedGa-rich sur-faceandhadloweroxygencontentoriginatingfromnativeoxide. Theseresultsareconsistentwiththoseinthepreviousreports stat-ingthattheN-faceGaNsurfacehashigheractivitytowardsoxygen adsorption.Theformer fact inducedhigher surfacecarrier con-centrationduetotherelativelyhighernitrogenvacancies,namely, Ga-richsurface.Thelatterfactledtoalowerpotentialbarrieratthe Cr/n–GaNinterface.Asaresult,theCr/AucontactsontheGa-face n-GaN(samplesC)exhibitedlowercontactresistance.Althoughthe totaloxygencontentinsamplesAwashigherthanthatinsamples B,thecomponentofGa ObondsinsamplesBwashigherthanthat

insamplesA.Therefore,heatedHClsolutionisbelievedtoremove nativegalliumoxideeffectivelyfromtheN-facen-GaNsurface. Evi-dently,nativegalliumoxideisthekeyfactorinﬂuencingthecontact property,nottheC Oand/orO Hbond-containingcompounds.

4. Conclusion

Insummary,non-alloyedCr/AuOhmiccontactsonGa-faceand N-face n-GaN were studied. Although the N-face GaN surface, which haslowerbandbending,wasmorebeneficialtothe for-mationofSchottkycontactswithlowerbarrierheightsthanthe contactsonGa-faceGaN,thepropertyofhigheractivitytowards oxygenadsorptionontheN-faceGaNsurfacewasharmfultothe formationoflow-resistivityOhmiccontacts.Itwasconfirmedthat heatedHClsolutioncouldfurtherremovethenativegalliumoxide fromtheN-faceGaNsurface,therebyreducingthespecificcontact resistanceofnon-alloyedCr/AucontactsontheN-faceGaN.

Acknowledgments

Theauthorswouldliketoacknowledgethesupportfromthe BureauofEnergy,MinistryofEconomicAffairsofTaiwan,R.O.C. throughgrantno.99-D0204-6isappreciated.Theauthorswould alsoliketoacknowledgetheNationalScienceCouncilforthe ﬁnan-cialsupportoftheresearchgrantnos.100-2112-M-006-011-MY3, 98-2221-E-218-005-MY3and100-3113-E-006-015.

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