蓝宝石衬底上使用氮化硅钝化的AlGaN_GaN高电子迁移率晶体管的研制与特性分析

FabricationandcharacterizationofhighperformanceAlGaN/GaNHEMTsonsapphirewithsiliconnitridepassivation

ZhangRenping(张仁平)1;󰂎,YanWei(颜伟)1,WangXiaoliang(王晓亮)2,andYangFuhua(杨富华)1

1State

KeyLaboratoryforSuperlatticesandMicrostructures,InstituteofSemiconductors,ChineseAcademyofSciences,Beijing100083,China

2KeyLaboratoryofSemiconductorMaterialsScience,InstituteofSemiconductors,ChineseAcademyofSciences,Beijing100083,China

Abstract:AlGaN/GaNhighelectronmobilitytransistors(HEMTs)withhighperformancewerefabricatedandcharacterized.Avarietyoftechniqueswereusedtoimprovedeviceperformance,suchasAlNinterlayer,siliconnitridepassivation,highaspectratioT-shapedgate,lowresistanceohmiccontactandshortdrain–sourcedistance.DCandRFperformancesofas-fabricatedHEMTswerecharacterizedbyutilizingasemiconductorcharacterizationsystemandavectornetworkanalyzer,respectively.As-fabricateddevicesexhibitedamaximumdraincurrentden-sityof1.41A/mmandamaximumpeakextrinsictransconductanceof317mS/mm.Theobtainedcurrentdensityislargerthanthosereportedintheliteraturetodate,implementedwithadomesticwaferandprocesses.Further-more,aunitycurrentgaincut-offfrequencyof74.3GHzandamaximumoscillationfrequencyof112.4GHzwereobtainedonadevicewithan80nmgatelength.

Keywords:GaN;HEMT;T-gate;AlNinterlayer;SiNpassivation;currentdensityDOI:10.1088/1674-4926/32/6/064001EEACC:2560

1.Introduction

Inrecentyears,wide-band-gapgalliumnitride(GaN)has

attractedmanyresearchers’interest,owingtoitshighbreak-downfields(3.3󰀂106V/cm),highsheetchargedensity(largerthan1󰀂1013cm󰀁2/,highpeakelectronvelocities(3󰀂107cm/s)andelectronsaturationvelocities(1.5󰀂107cm/s).AllofthesemakeGaN-basedhighelectronmobilitytransis-tors(HEMTs)potentialcandidatesforhigh-powerandhigh-frequencyapplications.

However,thereareseveralbarrierstoprogressinthefabri-cationofhighperformanceAlGaN/GaNHEMTdevices,suchasparasiticresistanceandcapacitance,thequalityofohmiccontact,surfacestatesandthecurrentcollapseeffect.Manyeffortshavebeenmadetoalleviatetheseissues,butnoneofthemalonecouldleadtotheoptimalperformanceofthede-vice,thereforeacombinationofthemisneeded.Inthiswork,wepresentthefabricationandcharacterizationofhighper-formanceAlGaN/GaNHEMTsonsapphiresubstrate.Avari-etyoftechniques,comprisinganAlNinterlayer,siliconnitride(SiN)passivation,highaspectratioT-shapedgate,lowresis-tanceohmiccontactandshortdrain–sourcedistance,arein-tegratedtoimprovedeviceDCandRFperformance.Devicesthatwefabricatedexhibitamaximumdraincurrentdensityof1.41A/mmandamaximumpeakextrinsictransconductanceof317mS/mm.Inaddition,aunitycurrentgaincut-offfre-quency(fT/of74.3GHzandamaximumoscillationfrequency(fMAX/of112.4GHzareobtainedonan80nmgatelengthde-vice.Asfarasweknow,2.1A/mmmaximumdraincurrentdensityŒ1󰂍and160GHzunitycurrentgaincutofffrequencyŒ2󰂍inAlGaN/GaNHEMTsonsapphirehavebeenreportedabroad,

andthebestdomesticresultsare1.07A/mmŒ3󰂍and77GHzŒ4󰂍,respectively.ThecurrentdensitythatweobtainedislargerthanthebestdomesticresultandthefTisclosetoit.

2.Devicefabrication

AlGaN/GaNheterostructuresweregrownbymetalor-ganicchemicalvapordeposition(MOCVD)ona2-inchsap-phiresubstrate.Theactivelayersofourdeviceconsistofa25nmundopedAlGaNlayer,a1nmAlNlayeranda3.5󰀁mundopedGaNlayer.Figure1showsaschematicdiagramof

Fig.1.AschematicmaterialstructureofAlGaN/GaNHEMTsonasapphiresubstrate.

󰂎Correspondingauthor.Email:zhangrenping@semi.ac.cn

Received8September2010,revisedmanuscriptreceived15February2011

c2011ChineseInstituteofElectronics󰀃

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Fig.2.SEMimageofaT-shapedgatewithan80nmgatelength.

Fig.3.I–VcharacteristicsofanAlGaN/GaNHEMTdevicewithan80nmgatelength.

thematerialstructure.

Thefabricationofthedevicebeganwithmesaisolation,whichwasrealizedbyinductivelycoupledplasmareactiveionetching(ICP-RIE)inaCl2/Ar-basedplasmaatmosphere.Theohmiccontactswereformedbye-beamevaporationofTi/Al/Ni/Au(30/90/50/20nm)ontodrain–sourceareas,fol-lowedbyannealingat835ıCfor35sinaN2ambient.Asource–drainspacingLSDof2.5󰀁mwasusedtoreducethesourceanddrainresistance.Thespecificcontactresistancewasevaluatedtobe4.8󰀂10󰀁5󰀏󰀁cm2byusingthetransmissionlinemodel(TLM)method.Subsequently,a50-nm-thicklayerofsiliconnitride,depositedbyplasmaenhancedchemicalva-pordeposition(PECVD),wasusedforsurfacepassivation.Two-stepe-beamlithographywasadoptedforthedefinitionofthegate.Thefirstexposuredefinedthefootofthegateonre-sist,whichwassubsequentlytransferredtothesiliconnitridelayerbydryetch,andthesecondexposuredefinedtheheadofthegate.Eventually,aNi/Au(20/200nm)gatewith80nmlengthwasformedbye-beamevaporationandalift-offpro-cess.Toreducegateresistanceandgatecapacitance,thelengthofthegate’sheadandtheaspectratiooftheT-shapedgateweredesignedtobe1󰀁mand10,respectively.Figure2showsaSEMimageofaT-shapedgatewithan80nmgatelengthfab-ricatedwiththismethod.

Fig.4.TransfercharacteristicsofanAlGaN/GaNHEMTwithan80nmgatelength.

3.Resultsanddiscussion

DCcharacteristicsweremeasuredbyusingaKeithley4200-SCS/Fsemiconductorcharacterizationsystem.Figure3showsthedrain–sourceI–Vcharacteristicsofadevicewitha10󰀁mgatewidth.Themaximumdraincurrentdensitywas1.41A/mmatVGSD2V.Figure4showsthetransferchar-acteristicatVDSD7V.Themaximumextrinsictransconduc-tancewas317mS/mm.Tothebestofourknowledge,thisisthelargestdrain–sourcecurrentdevicesofarimplementedwithadomesticwaferandprocesses.Tomaximizethesaturateddraincurrentdensity,theresistivityofthedevicechannelneedstobeminimized.Thismeansthatthesheetcarrierconcentra-tionandelectronmobilityinthechannelmustbemaximized.ThethinAlNinterlayerusedhereproducesalargeeffective󰂁ECbetweenAlGaNandGaNatbothsidesofAlN,whichincreasedtheelectronmobilityandthetwodimensionalelec-

trongas(2DEG)concentrationŒ5󰂍.Thisstructureoffersa2DEGwithatotalchargedensityof1.08󰀂1013cm󰀁2andanelec-tronmobilityof1941cm2/(V󰀁s),bothofwhicharemeasuredonunpassivatedsamplesbyusingthevanderPauwmethodatroomtemperature.Thecorresponding2DEGsheetresistancewas297󰀏/󰀁.TheSiNpassivationlayer,whichcompensatessurfacestatesanddefectsandpreventsthemfromtrappingcar-riers,furtherincreasestheelectronconcentrationin2DEGŒ6󰂍.Thedraincurrentdensityobtainednowisabout43.9%higherthantheresultthatweobtainedbeforeŒ7󰂍.Therefore,wecon-siderthatthisimprovementismainlyattributedtotheintroduc-tionoftheAlNinterlayerandtheSiNpassivationlayer,whichresultsinahigherelectricalconductivitychannel.Meanwhile,duetothelowaspectratiobetweenthegatelengthandtheAl-GaNbarrierlayer,thepinch-offcharacteristicofthedevicewasnotsatisfying,whichwascategorizedtoshort-channeleffect.Aspartofourongoingwork,wearecurrentlydevelopinggate-recesstechnologytosuppresstheshort-channeleffectsoastoimprovethepinch-offcharacteristic.

On-wafersmall-signalRFperformancesofdeviceswerecharacterizedwithanHP8720Dvectornetworkanalyzer,whichsweptfrom0.05to20GHzin0.05GHzsteps.Thecur-rentgainjh21jandthemaximumstablegain/maximumavail-ablegain(MSG/MAG)ofdeviceswerederivedfrommeasuredS-parametersasafunctionoffrequency.Thesmallsignalchar-

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addition,theshort-channeleffectlowersRds,resultinginhighfrequencyperformancedegradation.Briefly,thefTandfMAXcouldnotbegreatlyimprovedonlybyreducingthegatefrom180to80nm,andallfactorsmustbebalanced.

4.Conclusion

Insummary,wehavefabricatedandcharacterizedAl-GaN/GaNHEMTs(LGD80nm)withSiNpassivationonasapphiresubstrate.Byintegratingseveraltechniques,suchasAlNinterlayer,SiNpassivation,highaspectratioT-gate,lowresistanceohmiccontactandshortdrain–sourcedistance,improvedDCandRFperformanceshavebeenachieved.Themaximumdraincurrentdensityandmaximumpeakextrinsictransconductanceobtainedwere1.41A/mmand317mS/mm,respectively.Thisisthelargestdrain–sourcecurrentdevicetodateimplementedwithadomesticwaferandprocesses.Theshortchanneleffectcouldbeeliminatedbyemployinggatere-cesstechnologyinfuture.Onthesamewafer,aunitycurrentgaincut-offfrequencyof74.3GHzandamaximumoscillationfrequencyof112.4GHzwereobtained.Furtherexperimenta-tiononbalancingallofthefactorsaffectingthefTandfMAXcouldbehelpfultofurtherimprovedeviceRFperformances.

Fig.5.SmallsignalcharacteristicsofanAlGaN/GaNHEMTdevicewithan80nmgatelengthanda2󰀂40󰀁mgatewidth.

acteristicofthedevicewithan80nmgatelengthand2󰀂40󰀁mgatewidthisshowninFig.5.Acut-offfrequencyfTof74.3GHzwasobtainedbyextrapolatingjh21jwithaslopeof–20dB/decadeatVGSD–2VandVDSD8V.Similarly,amax-imumoscillationfrequencyfMAXof112.4GHzwasobtainedbyextrapolatingMSG/MAGwithaslopeof–20dB/decade.TheseresultsareslightlybetterthanourpreviousresultsŒ7󰂍,wherethegatelengthwas180nm.Obviously,theRFper-formanceofourdeviceislimitedbyotherfactors.ThefTismainlyaffectedbygatelength,sourceanddrainparasiticre-sistance,gate–drainparasiticcapacitance,etc.TheexpressionfortherelationshipbetweenfMAXandfTis

q.

fMAX=fTDRds=.RgCRch/2;(1)whereRdsistheoutputresistance,Rgisthegateresistance,

andRchisthechannelresistance.Ontheonehand,inordertoimprovefT,thehighaspect-ratioT-shapedgatewasusedtominimizethegatelengthto80nm,andlowresistanceohmiccontactandshortdrain–sourcedistancewereusedtolowerthesourceanddrainparasiticresistance.Meanwhile,Rgwaslow-eredbyusingahighaspect-ratioT-shapedgate,andRchwasloweredbyintroducinganAlNinterlayerandaSiNpassivationlayer.AhigherfMAXwasexpected.Ontheotherhand,theSiNpassivationlayergreatlyincreasesparasiticcapacitance,whichcouldsignificantlyreducethehighfrequencyperformanceofthedevice.Forinstance,an86GHzfTand115GHzfMAXde-vicewithSiNpassivationhasbeenreportedŒ8󰂍,andafteretch-ingawaytheSiNpassivationlayer,thefTandfMAXofthesamedevicebecame124GHzand230GHz,respectively.In

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