DTIC ADA512270: Dependence of Ohmic Contact Resistance on Barrier Thickness of AlN/GaN HEMT Structures

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Dependence of ohmic contact resistance on barrier thickness of AlN/GaN
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HEMT structures
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ContentslistsavailableatScienceDirect
Solid-State Electronics
journal homepage: www.elsevier.com/locate/sse
Letter
Dependence of ohmic contact resistance on barrier thickness of AlN/GaN
HEMT structures
D.A. Deen*, D.F. Storm, D.S. Katzer, D.J. Meyer, S.C. Binari
NavalResearchLaboratory,ElectronicsScienceandTechnologyDivision,Washington,DC20375-5347,USA
a r t i c l e i n f o a b s t r a c t
Articlehistory: Amulti-facetedstudyonthereductionofohmiccontactresistancetoAlN/GaN-basedheterostructuresis
Received1October2009 presented.Minimumcontactresistanceof0.5XmmhasbeenachievedbypartiallyetchingtheAlNbar-
Receivedinrevisedform20November2009 rierlayerusingachlorine-based plasma dry-etchprior toohmiccontact metallization.ForthinGaN-
Accepted24November2009
cappedAlN/GaNheterostructures,wefinditisnecessarytoremovetheGaNcapinthevicinityofthecon-
Availableonlinexxxx
tactmetalinordertoobtainalinearcurrent–voltagerelationship.Wecompareourresultsofthepre-
metallizationetchedcontactstothosewithoutanetchaswellastoresultsreportedintheliterature.
Thereviewofthispaperwasarrangedby
Prof.E.Calleja PublishedbyElsevierLtd.
Keywords:
AlN
GaN
HEMT
Ohmiccontact
Heterostructure
1.Introduction creaseswithanincreaseinAlNbarrierthicknessassuggestedby
Zimmermannetal.[4].However,alowR windowexistsforAlN
sh
Recently,theultra-thinbarrierAlN/GaNHEMThasbeengaining barriers with thicknesses between 3 and 5nm as shown by Cao
research momentum, due mostly to the relatively new achieve- [11].Therefore,inordertotakeadvantageofthelowsheetresis-
ments in extremely low sheet resistance from optimized growth tance window while simultaneously achieving sub-1 Xmm con-
[1–3]. Despite its technological immaturity, the AlN/GaN HEMT tact resistance a pre-metallization etch offers a viable solution.
has already set records for III-N HEMT technology in DC current ThismethodhasallowedustoobtaincontactresistancestoAlN/
density and transconductance [4]. Consequently, noteworthy RF GaN HEMTs with 5.5nm thick barriers as low as 0.5Xmm. Due
metricsforGaN-basedHEMTshavebeenobtained[5,6].However, to surface sensitivity of AlN/GaN structures, GaN capping layers
thereportedHEMTsarestilllargelylimitedinbothDCandRFper- were also explored. We find that for thin (3nm) GaN-capped
formance by very high ð>1XmmÞ contact resistance ðRÞ when AlN/GaNHEMTstheremovaloftheGaNcapbeforemetallization
c
fabricated using basic contact schemes [7]. Of the lowest R re- isnecessaryforcontactstobecomeohmic.
c
ported are those by Wang et al. showing R (cid:2)0:46Xmm for Ti/
c
Al/Mo/Aucontactstoan(cid:2)2.7nmAlNbarrierHEMTstructure[8]. 2.Experiment
Chabaketal.presentedsimilarfindingsforcontactsto2.7nmthick
barrierAlN/GaNMOSHEMTswithaCl-basedsurfacepre-treatment The AlN/GaN heterojunctions were grown by plasma-assisted
[9]demonstratingaRcvaluethesameasWang.Whilethisisarea- MBEon2-in.diametersemi-insulating4H-SiCsubstrates.Thepro-
sonablecontactresistancevaluetoaGaN-basedFET,itisstillhigh- ceduresforSiCsubstratepreparationaresimilartothosedescribed
erthandesiredfortheminimizationofparasiticaccessresistance. elsewhere[12].A60nmthickAlNnucleationlayerwasgrown,fol-
Theexploitationofashallowrecessetchpriortoohmicmetalliza- lowedbysubsequentunintentionallydoped(UID)GaNbufferand
tion provides a means to further lower Rc. Buttari et al. demon- AlN barrier layers with thicknesses of 1lm and 3–5nm, respec-
strated an optimumpre-ohmic Cl2 dry-etch that reduced contact tively. The AlN barrier thicknesses were chosen on the basis of
resistanceinAlGaN/GaNHEMTsto(cid:2)0:3Xmm[10].Inthisletter workbyCaoetal.whoshowedaminimumsheetresistanceinsin-
we show that contact resistance to AlN/GaN heterostructures in- gleheterojunctionAlN/GaNHEMTswithAlNthicknessesbetween
3and5nm[11].Forsomeofthesamplesdescribedherein,theAlN
* Correspondingauthor. barrier was followed by a 3nm GaN cap. Both the GaN and AlN
E-mailaddress:[email protected](D.A.Deen). growth rates were determined to be 1.2Å/s from optical
0038-1101/$-seefrontmatterPublishedbyElsevierLtd.
doi:10.1016/j.sse.2009.11.012
Pleasecitethisarticleinpressas:DeenDAetal.DependenceofohmiccontactresistanceonbarrierthicknessofAlN/GaNHEMTstructures.SolidState
Electron(2010),doi:10.1016/j.sse.2009.11.012
ARTICLE IN PRESS
2 D.A.Deenetal./Solid-StateElectronicsxxx(2010)xxx–xxx
reflectancemeasurements.Allepitaxiallayersweregrownwithout
interrupts.
Priortoanyprocessing,thepre-metallizationAr=BCl =Cl plas-
3 2
ma dry-etch procedure was developed in an Oxford Instruments
inductivelycoupledplasma(ICP)etchsystem.Theetchconditions
that were determined included an Ar=BCl =Cl (10/10/20 sccm)
3 2
chemistry, a chamber pressure of 5mTorr, and plasma power of
100 and 25W applied to the ICP coil and substrate electrode,
respectively. An etch rate for crystalline AlN of 0.5Å/s±0.1Å/s
was determined by Atomic Force Microscopy for the given etch
conditions.
Processingwasinitiatedwiththepatterningofthesource-drain
contacts using standard photolithographic techniques. The ex-
posedAlNwasthenetchedusingtheAr=BCl =Cl plasmatoreduce
3 2
the thickness in the contact region. A 47s etch time yielded an
(cid:2)2.5nm(±0.5nm)etchdepth,reducingthe5.5nmthickAlNbar-
rier to(cid:2)3nm in thecontact region.The 5.5nm barrier thickness
waschosenforthepre-metallizationetchasitbestdemonstrates Fig. 1. Two-point current–voltage characteristics comparing the 2.5nm pre-
thecapabilitytoreducecontactresistancefortheAlN/GaNhetero- metallization etched and non-etched contacts to AlN/GaN for as-deposited and
structure since we found that R increased with increasing AlN post-annealed metal. IV characteristics demonstrate the reduction in contact
c
resistanceduetothepre-metallizationetch.InsetshowingCTLMmeasurementand
thickness. The metal stack Ti/Al/Ni/Au (30/200/40/10nm) was
deposited by electron beam evaporation and then annealed at fitdataforthelowestRcvalueof0.5Xmmobtainedthroughthepre-metallization
etch.
800(cid:2)C for 30s in N . Lastly, a BCl =Cl plasma etch was utilized
2 3 2
for mesa isolation. On-wafer Hall effect measurements show for
athinGaNcapshowslittleresistancetometaldiffusionforohmic
the5.5nmthickAlNbarriersampleinparticular,anaveragesheet
contactformation.However,wehavefoundthisnottobethecase
resistanceof350X=(cid:2)witha2DEGsheetdensityof2:5(cid:3)1013cm(cid:4)2
when the combined GaN and AlN layers are more than 8.5nm
andmobilityof700cm2=Vs.
thick. The same measurement procedure as previously discussed
wasadoptedfortheanalysisofa3nmGaN-cappedAlN/GaNHEMT
witha5.5nmbarrierlayer.ShowninFig.2isthecomparativeevo-
3.Resultsanddiscussion
lutionforcontactswiththe3nmGaNcapremovedversusremain-
ing. It can be seen that the GaN-capped structure showed a
Immediatelyaftermetaldepositionandjustpriortoannealing,
reduction in resistance but remained slightly rectifying after the
two-pointprobecurrent–voltage(IV)characteristicsweretakenon
RTA.ThiscontrastswiththeteststructuresthathadtheGaNcap
circular transfer length measurement (CTLM) structures with a
removed by dry-etching which showed linear IV characteristics
4lmpadseparationandretakenfollowingarapid-thermal-anneal
after the anneal. This comparison suggests that it is necessary to
(RTA)at800(cid:2)Cfor30sinN .The30s,800(cid:2)CRTAprofileseemsto
2 removethe3nmGaNcapfortheproperformationofannealedoh-
beclosetooptimalforminimalchanneldegradationwhilesimul-
mic contacts. However, the precise GaN-cap and AlN thickness
taneously reducing R. Representative results are shown in Fig. 1
c combinationsthatcausethisnecessitywasnotthepurposeofthis
for a 5.5nm AlN barrier structure comparing characteristics be-
study and therefore was not explored in detail. R for the data
c
tween devices with and without the pre-metallization etch. It
shownoftheremovedGaNcapstructurewas1Xmm.
can be seen that for the pre-metallizationetched sample the an-
To date, little has been reported on low-resistive ð<1XmmÞ
nealedIVcharacteristicsshowalowerresistancethroughasteeper
ohmiccontacttechnologyforAlN/GaNheterostructures.However,
current–voltageslope(Fig.1).Drawingattentiontotheas-depos-
the extremely shallow etch reported in this letter offers a
ited IV characteristics in Fig. 1, it is interesting to note that the
pre-etched contact current is much lower than those of the con-
tacts without the pre-etch. We speculate this is due primarily to
the thinning of the AlN barrier causing the 2DEG density at the
AlN/GaNinterfacetoconsequentlybereducedfollowingthesame
2DEG-AlNthicknessrelationshipdiscussedbyCaoetal.[1].After
the800(cid:2)CRTAtheIVcharacteristicsimprovetolow-resistiveoh-
mic type. Chlorine surface treatment and/or enhanced tunneling
throughthethinnedbarriermayalsoplayaroletoreduceR for
c
the pre-metallization etched contacts. The RTA may also anneal
outanyplasmadamagefromthepre-metallizationICPetch.
CircularTLMmeasurementsweremadetoextractR.Gapdis-
c
tances of the CTLM annuli and contact radii were measured by
scanningelectronmicroscopy.Itfollowsthatthecontactresistance
for the pre-metallization etched ohmic contacts after annealing
was on average 0:7Xmm(cid:5)0:2Xmm with the lowest R being
c
0.5Xmm(insettoFig.1)comparedtocontactswithoutthepre-
metallization etch which showed R (cid:2)1:6Xmm on average. The
c
corresponding range of transfer lengths for the given range of
pre-metallizationetchedcontactR valueswas0.6–1.9lm.
c
Fig.2. Two-pointcurrent–voltagecharacteristicscomparingcontactsto3nmGaN-
Several groupshave reportedon the use ofthinGaN caps(1–
cappedandremovedGaNcapAlN/GaNforas-depositedandpost-annealedmetal.
3nm)toAlN/GaNHEMTstructurestotakeadvantageofthepro-
IVcharacteristicsdemonstratethenecessitytoremovethe3nmGaNcapinorder
tectivequalitiesofanMBE-growncap[2,8].Itisassumedthatsuch toachievealinearIVcharacteristicafterannealing.
Pleasecitethisarticleinpressas:DeenDAetal.DependenceofohmiccontactresistanceonbarrierthicknessofAlN/GaNHEMTstructures.SolidState
Electron(2010),doi:10.1016/j.sse.2009.11.012
ARTICLE IN PRESS
D.A.Deenetal./Solid-StateElectronicsxxx(2010)xxx–xxx 3
even lower values of R are obtainable for the AlN/GaN HEMT
Ti/Al/Ni/Au contact metallization c
structurewithadeeperpre-metallizationetch.However,asimilar
800oC, 30 second anneal
trendisexpectedasthatshownbyButtarietal.[10]whereamin-
imum R window exists for an optimum pre-metallization etch
without pre-metallization etch c
depthofthebarrier.
m)
m 4.Conclusions
Ω-
R( C ref. [7] WehaveshownforthefirsttimetheutilityofaCl-baseddry-
etchtoAlN/GaNHEMTsthatprovidesomeofthelowestreported
2.5nm BCl3/Cl2 contact resistances to AlN/GaN HEMTs to date. Through the pre-
ref. [8] pre-metallization etch
metallizationetchaminimumR of0.5Xmmhas beenachieved
c
for a 5.5nm thick AlN barrier HEMT. Additionally, the necessity
oftheremovaloftheGaNcaplayeronGaN-cappedAlNforthefor-
1 2 3 4 5 6 mationofohmiccontactshasbeenreported.Withthismethodfor
AlN thickness (nm) the reduction in parasitic contact resistance the extrinsic DC and
RFpotentialoftheAlN/GaNHEMTmaybemoreeasilyrealized.
Fig.3. ContactresistancecomparisonasafunctionofAlNbarrierthickness.Closed
circlesrepresentourcontactresistancestoAlN/GaNheterostructureswithoutthe
Acknowledgements
pre-metallizationetchdemonstratinganincreasingRcwithincreasedAlNthickness
(errorbarsdenoteonestandarddeviationfromthemean).Contactresistancesto
AlN/GaN heterostructures including the 2.5nm pre-metallization etch is plotted The authors gratefully acknowledge N. Green for device pro-
withitsrangeinstandarddeviationingreyshowingthesignificantreductionin cessing.ThisworkwassponsoredbytheOfficeofNavalResearch.
resistance.Opensquaresrepresentcontactresistancestakenfromthecorrespond-
ingreferences.
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Pleasecitethisarticleinpressas:DeenDAetal.DependenceofohmiccontactresistanceonbarrierthicknessofAlN/GaNHEMTstructures.SolidState
Electron(2010),doi:10.1016/j.sse.2009.11.012