simulation of zincbl ende gallium nitride high …
TRANSCRIPT
SIMULATIONOFZINCBLENDEGALLIUMNITRIDEHIGHELECTRONMOBILITYTRANSISTORSFORNORMALLY-OFF
OPERATION
By
RyanGrady
SeniorThesisinElectricalEngineering
UniversityofIllinoisatUrbana-Champaign
Advisor:CanBayram
May2017
ii
Abstract
Forhigh-power,high-frequencyapplications,siliconisbeingpushedtoitsphysicallimits.Inordertomeetdemandfordevicesinthisarea,newmaterialsystemsneedtobeconsidered.Galliumnitride(GaN)andrelatedalloysincludingaluminumgalliumnitride(AlGaN)arehighlysuitedtotheseapplications,butareyettomature.TheleadingissueinthedesignofGaNhighelectronmobilitytransistors(HEMTs)isensuringnormally-offbehavior.Herewepresentanewmethodforcreatingnormally-offGaNHEMTs:theuseofpolarization-freezincblende(ZB-)GaN.Inordertocreate theconductive two-dimensionelectrongas (2DEG)channel, carriersare introducedthroughδ–dopingoftheAlGaNlayer.WiththeuseofSentaurusTechnologyComputerAidedDesign(TCAD),weareabletoshowaworkingdesignforZB-GaNHEMTsandgiveguidelinestoensurenormally-offbehavior.Thisincludestuningthealuminummolefraction,gatemetalworkfunction,δ-dopingdensity,andAlGaNlayerthicknesses.Itisfoundthataluminumcontentlessthan 35% and δ-doping below 4 x 1012 cm-2 result in normally-off behavior.We are able todemonstrateturn-onvoltages(VT)greaterthan1V,and2DEGsheetdensityexceeding1013cm-2.Additionally,thebreakdownvoltage(VBR)andon-stateresistanceisprovidedtocharacterizeanoptimizeddevice.Subject Keywords: cubic, zincblende, AlGaN, high electron mobility transistor, technologycomputeraideddesign,on-stateresistance,breakdownvoltage
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Acknowledgments
Iwouldliketothankmyadvisor,Prof.CanBayram,forhisguidancewhileworkingonthisproject,and
hisgeneralhelpincraftingavisionofmyacademicfuture.IdonotbelieveIwouldbeonthetrajectoryI
amtodaywithouthishelp.IwouldalsoliketothankmyfellowICORLABmembersfortheirassistance
withbothresearch-relatedquestionsandtheoccasionallaugh.IwouldalsoliketothankICORLAB
alumnusPhilTsaiforhishelpinlearningtouseSynopsysSentaurus.Finally,Iwouldliketothank
StephanieHoldingfordealingwithmeonadailybasisandbeingsupercute,andmyfamilyforalways
believinginme.
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Contents
1.Introduction.............................................................................................................................................1
2.LiteratureReview.....................................................................................................................................2
3.DescriptionofResearchApproach...........................................................................................................6
a.DeviceDescriptionandFabricationStrategy.......................................................................................6
b.DevicePhysicsandSimulationParameters..........................................................................................8
c.SimulationApproach............................................................................................................................9
4.Results....................................................................................................................................................11
a.EffectsofAl-contentintheAlxGa1-xNbarrier.....................................................................................11
b.EffectsofAlxGa1-xNbarrierlayerthicknessesandδ-doping...............................................................12
c.Effectsofgatemetal...........................................................................................................................15
5.Conclusion..............................................................................................................................................19
References..................................................................................................................................................23
1
1.IntroductionSilicon-basedtransistorsarereachingmaturityasincreasinglymoreapplications(suchas
DC/DC power conversion, 5G networks) demand high power and high speed operation,
motivating new material and device structure investigations [1]. Gallium nitride (GaN) high
electron mobility transistors (HEMTs) are ideal candidates to address such emerging needs,
particularlyinhighpower(>10W)highfrequency(>10GHz)applications.Figuresofmeritfor
highpowerandhighfrequencyapplicationsshowGaNhavingavalueof790forJohnson’sfigure
of merit [2,3] (indicating high speed and high power capabilities) and 100 for Baliga’s high-
frequencyfigureofmerit[3,4]whennormalizedtothevaluesforSi.Thewidebandgap(~3.4eV),
high critical breakdown field (~3.5 MV/cm), and high two-dimensional electron gas (2DEG)
mobility (> 1000 cm2/V·s) all contribute to the promise of GaN devices in such operation
conditions [5]. A key issue in GaN HEMTs is normally-on operation. Due to the inherent
polarizationfieldsinhexagonal(h-)phaseGaN,conventionalh-GaNHEMTsarenormally-on.This
means a conductive 2DEG channel is formed in the AlGaN/GaN hetero-interfacewithout an
externalbias[5].Forsafetyandenergysavingsinhighpower/frequencyapplications,normally-
offHEMTsaredesired[6].Severalmethodsofimplementingnormally-offGaNHEMTshavebeen
exploredincludingfluorineimplantationbelowthegate[7],arecessedgateapproach[8],andp-
GaNgateinsertion[9].However,normally-offdevicesenabledbytheseapproachesareyetto
mature.Recently,ZB-GaNmaterialshasemergedinphotonicdeviceapplications[10,11].Here
weproposethisemergingnaturallypolarization-freezincblende(ZB-)phaseforthecreationofa
new electrical device: normally-off ZB-phase GaN HEMT. Using Technology Computer Aided
Design(TCAD)software{SynopsysSentaurus[12]},weexploredesignparameters inZB-phase
AlGaN/GaNHEMTsandinvestigatedesignguidelinesallowingforanormally-offoperation.
2
2.LiteratureReviewToeffectivelyquantizetheperformanceenhancementpromisedbyGaN,severalfigures
ofmeritforhighpowerandhighfrequencyapplicationscanbeapplied.Johnson’sfigureofmerit
givestheproductofthemaximumappliedvoltageandtheswitchingfrequencyasproportional
totheproductofthecriticalelectricfieldandcarriersaturationvelocity[2].Thisfigureisbased
onmaterialparameters,andoffersanupper limit to thecapabilitiesofamaterial.Whenthe
valuesarenormalizedtothatofSi,wefindavalueof790forGaN[3].Foradditionalfiguresof
merit,GaNoffersan impressive910whennormalized toSi forBaliga’s figureofmeritwhich
indicateslow-frequencypowerlosses[3],andavalueof100onBaliga’shighfrequencyfigureof
meritwhich indicatesa reduction inswitching losses [3,4].Table1showsthevaluesof these
figuresGaNandtheclosestwideband-gapcompetitor,4H-SiC.Theexpressionforeachfigureis
givenaswell.
Table1:Figuresofmeritforpowersemiconductordevices[3]
Figure JFOM BFOM BHFFOM Expression 𝐸"𝑣$/2𝜋 𝜖𝜇𝐸*+ 𝑅-.𝐶0. 12 Si 1 1 1 GaN 790 910 100 4H-SiC 410 290 34
AkeyissueinGaNHEMTsisnormally-onoperation.Duetotheinherentpolarizationfields
in hexagonal (h-) phase GaN, conventional h-phase GaN HEMTs are normally-on,meaning a
conductive2DEGchannelisformedintheAlGaN/GaNhetero-interfaceevenundernoexternal
bias.Forsafetyandenergysavingsinhighpower/frequencyapplications,normally-offHEMTs
aredesired. Severalmethodsof implementingnormally-offGaNHEMTshavebeenexplored
includingfluorineimplantationbelowthegate[7],arecessedgateapproach[8],andp-GaNgate
insertion[9].However,normally-offdevicesenabledbytheseapproachesareyettomature.An
alternativemethodforcreationofanormally-offGaNHEMTdesigns isproposedhere,which
employstheuseofpolarization-freezincblende(ZB-)phaseGaN.
3
Thedesignofanormally-off,orenhancementmode,HEMTusingfluorineimplantation
wasdemonstratedin2005[7].Thisprocessissimilartothethresholdadjustmentimplantation
performed on silicon MOSFETs. The implantation is typically performed using a CF4 plasma
treatment followed by rapid thermal annealing (RTA) [7]. The resulting devices have a high
density of fluoride ions near the 2DEG channel, which prevent the 2DEG formation under
equilibriumconditions.Thistechniquerequirescare,however,asthedevicescreatedusingthis
techniquecansufferfromareducedon-state𝐼45[13].
RecessedgateHEMTsaremadebyetching aportionofAlGaNand thus reducing the
distancebetweenthegatecontactandthe2DEGinaselectiveregion[8].Thisincreasestheturn-
onvoltage𝑉; ,whilemaintainingcharacteristicsofadepletionmode,ornormally-on,device.In
particular,theonresistance𝑅-.andbreakdownvoltage𝑉<= showasimilartrade-offrelationship
astheirnormally-oncounterpart[8].Theissuewiththisdesignisthat𝑉; canonlybeshiftedby
asmallamount,andsoachievinghighturn-onvoltagesisproblematicandlowleakagecurrents
atoperatingvoltagescanbedifficult.
Employment of a p-doped GaN gate contact has also been studied recently. This
approachavoids thepitfallsof the fluorine-treateddevicesand the recessedgatestrategy to
achievehighturn-onvoltagesofupto3V[14].Thebiggestproblemistheactivationofthep-
type dopants. Magnesium is typically used to achieve a p-doped GaN layer, but the large
activation energy (approximately 170 meV) leads to a much lower hole concentration than
chemicalconcentration[9].
Combinationsoftheaboveapproacheshavebeenused,inparticularthecombinationof
a recessed gate and fluorine treatment [15]. This approach aims to use the benefits of both
fluorinetreatmentandarecessedapproachwhilelimitingthedetrimentaleffectson𝐼45.Devices
fabricatedthiswayareabletopush𝑉;toaround2.5V[15].
TheuseofZB-GaNhasshownpromiseinsimulations,achievingturn-onvoltagesinthe
1.1Vrange.Thesedevicesalsofeaturea2DEGsheetdensity𝑛$ontheorderof1013cm-2,aligned
withmanyh-GaNdevices.Thebiggestdrawback is in thegrowthofZB-GaN,althoughrecent
breakthroughshavegreatlyreducedthisproblem[11].
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In the design of a power transistor, there are several quantities of interest. These
include𝑅-.,𝑉;,and𝑛$aswellasbreakdownvoltageandswitchingspeed.Inordertoaccurately
extractthesequantitiesfromsimulationdataandoptimizeadevice,themechanismsgoverning
foreachquantitymustbeunderstood.
On-stateresistance,𝑅-.,istheresistancefoundbetweenthesourceanddraincontacts
intransistorwhenthedeviceison.Thisresistancecanbebrokendownintoseveralcomponents,
withcontributionsfromthesourcecontact𝑅?5,thesourcedoping𝑅AB,thechannelresistance
𝑅?C, and drain contact resistance𝑅45 [16]. Source contact resistance𝑅?5 is from themetal
contactmadetothe𝑛Bsourceregion.Thisresistanceisdeterminedbytheworkfunctionofthe
contactandthedopinginthe𝑛Bregion,aswellasgeometry.Thedopingresistance𝑅ABisdue
toohmiclossesascurrentflowsthroughthesource,andisinverselyproportionaltodopinglevels
andhasageometricfactor.Channelresistanceisafunctionofthedevicegeometry,aswellas
biasandmaterialparameters.ForaMOSFET,itisfoundtoobey𝑅?C =EFG
HIJK?LM NO1NPG[16].For
the HEMT devices discussed, the channel resistance depends on𝑛$. This parameter,𝑅-., is
typicallymeasuredempiricallyastheinverseslopeofthe𝐼45vs.𝑉45relationshipmeasuredin
thesaturationregion.
Turn-onvoltageintheHEMTislargelydeterminedbythefabricationtechniques,andwill
vary for the methods of making normally-off devices discussed so far. For h-GaN devices
fabricatedusingfluorineimplantation,thethresholdvoltagedependsonthedensityoffluorine
implanted[13].Fortherecessedgateapproach,theAlGaNthicknessbelowtherecessedgateis
responsibleforthepositiveshiftin𝑉; [8].WhenemployingaZB-GaNapproach,𝑉; wasfound
todependonaluminumcontent,𝛿-dopinglevels,andAlGaNlayerthickness.
Sheet density, 𝑛$, has been thoroughly studied. For h-GaN structures, it is given by
𝑛$(𝑥) =U VW− YZY V
[W\𝑒Φ< 𝑥 + 𝐸` 𝑥 − Δ𝐸? 𝑥 [17].Forthisexpression,𝜎 𝑥 referstothe
polarizationinducedchargedensity,Φ<tothesurfacebarrierheight,𝐸` totheFermilevel,and
Δ𝐸? to the conductionbanddiscontinuity at theAlGaN/GaNheterojunction. For the ZB-GaN
structure, there isnopolarizationfieldandsoallcarrier inthe2DEGmustbeaddedusing𝛿-
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dopingtechniques.Thevalueof𝑛$wasstillfoundtobestronglycorrelatedtotheconduction
bandoffset.
Breakdownvoltageisthemaximumvoltagethatbeappliedbetweendrainandsource
while the device is in the off-state without allowing current to flow. This value is typically
measuredbypickingathresholdcurrentontheorderof100nAandmeasuringthevoltage𝑉45
forwhich 𝐼45 equals the threshold voltagewhen thedevice is in theoff-state (𝑉*5 = 0for a
normallyoffdesign,𝑉*5 < 𝑉; foranormallyondesign).Thisvalueisofparticularimportancefor
GaNpowerdevices,andthecurrentrecordisover3.3kV[18].InthetraditionalpowerMOSFET,
thebreakdownvoltageisprimarilyduetoavalanchebreakdowncausedbythelargeelectricfield
inthechannel[16].Duringavalanchebreakdown,carriersareacceleratedtohighvelocitiesand
eventuallyreachasaturationvelocity𝑣$.Theseelectronshaveenoughkineticenergytoionize
latticefixedatomsonimpact,referredtoasimpactionization.InthecaseofaHEMT,thecauses
of breakdown are thought to be due to a combination of factors including punch through,
breakdownofthegate-drainjunctionduetosurfaceconduction,andimpactionizationdueto
hotelectrons[19].
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3.DescriptionofResearchApproach
a.DeviceDescriptionandFabricationStrategyThe zincblende AlxGa1-xN/GaN HEMT structure under consideration consists of an
unintentionally doped (UID-) ZB-GaN substrate on which high quality intrinsic ZB-GaN is
epitaxiallygrown. Tomakecontacts to thedevice,a layerofn-typeGaN is thengrown,and
etchedbacktocreateanopening.Insidetheopening,anAlxGa1-xNlayerisgrownontopofthe
epi-GaN(Figure1(a)).ThegrowthofhighqualityZB-GaNhasbeenrecentlydemonstratedusing
Sisubstrates,indicatingthatthisisatechnologicallyfeasiblestructure[11].TheAlxGa1-xNbarrier
layeriscomposedofthreeparts:undopedlayersofAlGaN1andAlGaN3(withthicknessof𝑡2and
𝑡+) and δ-doped layer of AlGaN2 (with thickness of 𝑡f). The δ-doped AlGaN2 layer provides
carrierstothe2DEGchannelformedattheAlGaN/GaNhetero-interface,andthecorresponding
thickness𝑡fdeterminesthenetnumberofdopants introducedandthustheamountofband
bendingintheAlGaNlayer.Theδ-dopingisachievedbycreatingafewnanometersofhighly-
doped AlGaN via MOCVD or MBE processes [20], and these carriers then diffuse to the
AlGaN/GaNheterojunctiontoformthe2DEG.TheionizedimpuritiesremainintheAlGaN2layer
afterthecarriersaredepleted,leadingtostrongbandbendinginthisregion.Becausethecarriers
are depleted, we instead find the peak electron concentration at the AlGaN/GaN junction
corresponding to the 2DEGas opposed to the doped layer. TheAlGaN3 layer of thickness 𝑡+
separatesδ-dopedAlGaN2layerfromthechannelandreducestheimpurityscatteringeffects.
TheAlGaN1 layerwith corresponding thickness 𝑡2 distances thegate contact from the2DEG,
controllingthegate-contact-inducedelectricfieldeffectsonthe2DEGchannel.Finally,0.25μm
longsourceanddraincontactsareformedwiththen-dopedGaN.
InFigure1(b),agenericbanddiagramforthestructureisgiven.Inthisplot,thedesign
parameters𝑡2,𝑡f,and𝑡+aremarked,aswellastheSchottkybarrierheight(𝜑h).TheSchottky
barrierensuresthatanelectricfield ispresentwhenthedevice isunbiased,preventing2DEG
formation,and𝜑hdeterminesthestrengthofthisfield.Thefinalparametersinconsiderationis
thealuminumcontent,𝑥ij,whichdeterminesthebandgapoffset,andthusdeterminesthedepth
ofthetriangularquantumwellintheconductionbandattheAlGaN/GaNjunction,aswellasδ-
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doping density (𝑁4) in the AlGaN2 layer, which both directly impact the number of carriers
presentinthe2DEG.
Figure1:(a)SketchoftheinvestigatedAlxGa1-xN/GaNHEMTstructure,consistingofanintrinsicunintentionallydopedZB-phaseGaNsubstrateonhighqualityintrinsicepi-GaNisgrown.OntopofthisisanAlxGa1-xNlayercomposedofthreeparts:undopedlayersofAlGaN1andAlGaN3(withthicknessoft1andt3,respectively)andδ-dopedlayerofAlGaN2(withthicknessoft2).Sourceanddrain contacts aremade ton-dopedGaNdirectly contacting the channel. (b)Ageneric band-diagram of Al0.25Ga0.75N/GaN HEMT (with Schottky barrier height 1.8 eVcorrespondingtoagatemetalof5.05eV;t1=15nm;t2=2nm;t3=3nm;δ-dopingof2E+12cm-2)throughthecross-sectionisshown.Themetal-semiconductorworkfunctiondifferenceisgivenby𝝋𝒃.Theregionst1,t2,andt3areseenintheirrespectivelocations.Thepositionx=0markstheAlxGa1-xN/GaNhetero-junction.
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b.DevicePhysicsandSimulationParametersFor simulating the band structure of the wide bandgap materials, the temperature
dependencegivenbytheVarshnimodel[21]𝐸* 𝑇 = 𝐸* 0 − o;\
pB;isused,withvaluesof𝛼, 𝛽
asgiveninTable2.Duringthesimulations,FermistatisticsareusedtoimproveaccuracyasFermi
energylevel(𝐸`)exceedsconductionbandenergylevel(𝐸?).Thermionicemissioncurrentsare
considered for electrons in order to accuratelymodel carrier flow along the heterojunction,
following the literature [22]. For carrier flowanddensities confinedalong the triangularwell
formed at the metallurgical junction {as in our AlxGa1-xN/GaN hetero-structure}, a quantum
potentialisconsidered.Thispotential,denoted𝛬.,addsquantizationtocarrierdensitiesinthe
quantum well according to 𝑛 = 𝑁?𝐹u\
vw,J1vF1xJy;J
without impacting simulation times
significantly[12]. Inthisequation,𝑁? istheeffectivedensityofstates,whileF1/2 istheFermi
integral of order 1/2. 𝛬.is obtained by solving 𝛬. = − zℏ\
2f|J 𝛻f 𝑙𝑛 𝑛 + 2
f𝛻 𝑙𝑛 𝑛 f =
− zℏ\
�|J
�\ ...Here𝛾isafitfactor,𝑛istheelectronconcentration,𝑚.istheelectronmassandℏ
isthereducedPlanckconstant.Theeffectsofbandgapnarrowingwithheavydopingisneglected,
basedonthedifficultyinactivatingdopantsinGaN.Carrierrecombinationiscomputedusinga
combination of Shockley-Hall-Read recombination, Auger recombination, and radiative
recombination models. While modeling the electrical behavior of the structure, the basic
transport equations are solved while the potential at various electrodes is swept. Transient
behaviorisignored,andonlyquasi-stationarysolutionsareused.Threeequationsaresolvedat
eachstepofthesimulation:Poisson’sequationandthecontinuityequationforbothelectrons
and holes. Poisson’s equation is given by𝛻 ⋅ 𝜀𝛻𝜙 = −𝑞(𝑝 − 𝑛 + 𝑁4B − 𝑁i1), where 𝜀 is the
materialpermittivity,𝜙referstotheelectrostaticpotential,𝑝and𝑛aretheholeandelectron
concentrations, and 𝑁4B and 𝑁i1 are ionized impurities. The continuity equation for both
electronsandholesisgivenby±𝛻 ⋅ 𝐽.,� = 𝑞𝑅��� + 𝑞�.,���
withthepositivetermcorresponding
to𝑛andthenegativeto𝑝,aswellasthequantumpotentialequations[12].Forthisequation,
𝑅��� denotes the net recombination rate, and 𝐽.,�is the respective electron or hole current
density.
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Table2:Thevaluesforsignificantmaterialparametersusedinthesimulations.TheVarshnimodelforthebandgapasafunctionoftemperatureisusedwithvaluesforbandgapat0K,𝜶,and𝜷asgivenforbothGaNandAlxGa1-xN.TherelativepermittivityforzincblendephaseAlNandGaNareusedtocalculatemole-fractiondependentpermittivitiesbylinearinterpolation.ElectronaffinitiesforAlNandGaNareusedinlinearinterpolationstodeterminebandoffsetsandtheSchottkybarrierheight.Elasticcoefficientshavebeenusedincalculatingthecriticalthickness. The doping in the n-GaN contact areas is provided, as well as the δ-dopingconcentration. Intrinsic concentrations in the substrateareextractedusing themass-actionlaw.
Parameter Symbol Value GaN bandgap at 0 K 𝐸*,*�A 3.299 eV [23] AlN bandgap at 0 K 𝐸*,ijA 6.00 [23] Varshni coefficients 𝛼*�A 5.93 × 10-4 eV/K [23] 𝛽*�A 6.00 × 102 K [23] 𝛼ijA 5.93 × 10-4 K [23] 𝛽ijA 6.00 × 102 K [23] Relative permittivity 𝜀�,*�A 9.7 [24] 𝜀�,ijA 9.14 [25] Electron affinity 𝜒ijA 0.6 V [24] 𝜒*�A 4.1 V [24] Elastic coefficients 𝑐22 25.3 × 1011 dyn/cm-2 [5] 𝑐2f 16.5 × 1011 dyn/cm-2 [5] n-GaN dopant concentration 𝑁4 1 × 1019 cm-3 i- GaN carrier concentration 𝑛0 6.23 × 10-9 cm-3
c.SimulationApproachTheZB-phaseAlxGa1-xN/GaNtransistorissimulatedasAl-contentintheAlxGa1-xNbarrier
(𝑥ij)(from0.10to0.40),gateSchottkybarrierheight(𝜑h)(from1.2to2.0eV),andAlGaN1,2,3
thicknesses(𝑡2 from10to35nm,𝑡ffrom1to5nm,and𝑡+from2to10nm)arevaried.Asthe
AlxGa1-xNelectronaffinity(𝜒5)changeswithaluminumcontentaccordingtoVegard’slaw,barrier
heightactsasabetterexperimentalcontrolandthuswevarythegateSchottkybarrierheight
(𝜑h){opposedtovaryingtheworkfunctionofthegatemetal(Φ�)}.Whilestudyingeffectsof
eachvariable{𝑥ij,𝜑h,𝑡2,𝑡f,𝑡+}ontheHEMT,eachvariableisvariedindependentlywhilethe
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otherparametersareheldatcontrolvalues(Table3).Thecriteriafornormally-offbehavioris
taken to be the formation of the 2DEGwhen the Fermi level exceeds the conduction band
minimumattheAlGaN/GaNhetero-interface.Thisdefinitionofnormally-offisjustifiedbythe
correspondingelectronconcentration.Theelectronconcentrationnear theheterointerface is
dictatedbythetriangularquantumwellformedalongtheinterface.As𝐸` and𝐸? coincide,the
probabilityoffindingelectronsintheavailablewellstatesbeginstoincreaseaccordingtothe
Fermi-Diracdistribution, leadingtoformationofthe2DEGandthusnormally-onbehavior. In
Figure 6, this phenomenon is shown as the electron density increases by several orders of
magnitudewhen𝐸` approaches𝐸? .
Table 3:While studying effects of each variable {Al-content in the AlxGa1-xN barrier (𝒙𝑨𝒍),Schottkybarrierheight(𝝋𝒃),AlGaN1layerthickness(𝒕𝟏),AlGaN2layerthickness(𝒕𝟐),AlGaN3layerthickness(𝒕𝟑)}ontheZB-phaseAlxGa1-xN/GaNHEMT,eachvariableisvariedoneatatimeastheotherparametersareheldatvaluesshownasinthistable.
Parameter Value 𝑥ij 0.25 𝑡2 15 nm 𝑡f 2 nm 𝑡+ 3 nm 𝜑h 1.8 eV δ-doping (in AlGaN2) 2× 1012 cm-2
Total AlGaN thickness (𝑡2 +𝑡f +𝑡+)
20 nm
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4.Results
a.EffectsofAl-contentintheAlxGa1-xNbarrierFigure2showsthebanddiagramacrosstheHEMTaswevary𝑥ij from0.10to0.40.This
range of Al-content can be attained despite latticemismatch, and doping of high Al-content
AlGaNhasbeendemonstrated [26,27].Additionally,deviceswithhighAl-contenthaveshown
promisingperformance,asthe2DEGisintheGaNlayerandunaffectedbypossibledislocations
intheAlGaNlayers.Inordertomaintainaconstant𝜑hof1.8eV,Φ�isallowedtochangewith
𝑥ij. CorrespondingvaluesofΦ� are calculatedusing𝜑h = 𝑞(Φ� −𝜒5) andwrittennext to
eachband.As𝑥ij increases,theconductionbandminimumapproachestheFermilevel.For𝑥ij
below 0.35, the HEMT is normally-off. This shows good agreement with existing literature,
indicatingnormally-offdevicesareattainablefor𝑥ij of0.33forsimilarrangesofcontrolvariables
inthisstudy[28].Itshouldbenotedthattoensureasufficientlyhighturnonvoltage(dependent
Figure2:Theequilibriumbandstructureisshownextendingfromthegatecontactto30nmbelowtheAlxGa1-xN/GaNjunction.Theinsetshowsthevaluesof𝒕𝟏,𝒕𝟐,and𝒕𝟑.Thevalueof𝝋𝒃hasbeensetto1.8eV,correspondingtochanging𝜱𝑴foreachvalueof𝒙𝑨𝒍.As𝒙𝑨𝒍increasesfrom0.10to0.40,thedeviceisseentoremainnormally-offforvaluesof𝒙𝑨𝒍lessthan0.35.
12
upontheapplication),itisdesirabletokeep𝑥ij small.However,as𝑥ij decreases,conduction
bandoffset(Δ𝐸?)alsodecreasesadverselyimpactingthe2DEGsheetdensity(𝑛$).Thisinturn
limits theoutputpower.Overall,byvarying𝑥ij,onecantrade-off turn-onvoltageand𝑛$ for
specific applications.Wehavechosen𝑥ij of0.25 for the restof this study to study trade-off
amongstotherdesignparameters.
b.EffectsofAlxGa1-xNbarrierlayerthicknessesandδ-dopingFigure3showsthebanddiagramacrosstheHEMTaswevary𝑡2 from10nmto25nm.
For a fixed 𝑥ij (0.25), as 𝑡2 is increased, the conduction band minimum decreases. This is
attributedtotheincreasedseparationbetweenthegatecontactandthe2DEG.Weobservethat
bylimitingthethicknessof𝑡2(inthiscasebelow25nm),normally-offHEMTsareenabled,which
isinagreementwithstudiesshowingnormally-offdevicesfor10nmofequivalentAlGaN3layers
[28].Thisapproachhastheadditionalbenefitofreducinglattice-mismatchrelateddislocation
Figure3:BanddiagramacrosstheHEMTisshownas𝒕𝟏 isvariedfrom10nmto25nm(forafixed𝒙𝑨𝒍(of0.25)).Theinsetshowsthevaluesof𝒙𝑨𝒍,𝒕𝟐,and𝒕𝟑.Arrowsindicatethedirectionofincreasing𝒕𝟏.Therelationshipbetween𝒕𝟏thicknessandtheminimumofconductionbandedgeindicatesthatathinner𝒕𝟏willleadtonormally-offbehavior.
13
formationintheAlGaNbarrier,asthecriticalthickness(ℎ")ofZB-phaseAl0.25Ga0.75NonZB-phase
GaNpredictedbytheMatthews-Blakesleeequationis9.18nm[29].Wehaveset𝑡2to15nmfor
theremainderofthisstudy.WhilethisvalueresultsinatotalAlGaNthicknessexceedingℎ",it
allowssufficientroomforfullinvestigationoftherelationshipbetweenthevariousparameters
andthenormally-offbehavior.
Figure4showsthebanddiagramacrosstheHEMTaswevary𝑡ffrom1nmto5nm.For
fixed𝑥ij (0.25)and𝑡2(15nm),whentheAlGaNbarrierthicknessiskeptat20nm,thedevice
remainsnormally-off for𝑡flessthan4nm,which isslightlysmallerthansimilarexperimental
workusing6nmdopedlayers.Thisisexplainedbytheincreaseddopinglevelinthepresent
Figure4:BanddiagramacrosstheHEMTisshownast2isvariedfrom1to5nm(forafixed𝒙𝑨𝒍(of0.25)and𝒕𝟏(of15nm)).Thedopingdensityofthe𝜹-dopedlayeriskeptconstant(as2×12cm-2).Theinsetshowsthevaluesof𝒙𝑨𝒍,𝒕𝟏,and𝒕𝟑.Arrowsindicatethedirectionofthebandshiftsforincreasing𝒕𝟐.TheundopedAlxGa1-xNbarrierthickness(𝒕𝟏 +𝒕𝟑)iskeptfixedas20nmbykeepingthe𝒕𝟑 barrierlayerconstantandadjusting𝒕𝟏appropriately.Minimizing𝒕𝟐 isshowntoimprovethenormallyoffbehavior.
14
work.Tomaximizethedensityofthe2DEG,maximizing𝑡fandsubsequentlythenumberof
carriersintroducedwhileretainingnormally-offbehaviorandpreventingadverseimpactsonthe
channelmobilityisideal.
Fixing𝑥ij (as0.25),𝑡2(as15nm),𝑡f(as2nm),andthedopingdensity(as12×102fcm-2),
wehavealsostudiedtheeffectof t3 (Figure5).It isshownthat t3hasminimal impactonthe
normally-offoperationof theHEMT.ThisAlGaN3 layerprimarily serves to keep theδ-doping
impuritiesfurtherawayfromthe2DEGinordertominimizeimpurityscatteringeffects.
Figure5:Effectsof𝒕𝟑barrierlayerthicknessontheconductionbandedgearereported.TotalAlxGa1-xNlayerthickness(𝒕𝟏 + 𝒕𝟐 + 𝒕𝟑)hasbeenkeptconstantas20nmwhere𝒕𝟐iskeptat2nmand𝒕𝟏and𝒕𝟑areadjustedaccordingly.ThebandminimumcanbeseentomoveclosertotheFermilevelwithincreasing𝒕𝟑.Duetoitsscatteringlimitedimpactonthebanddiagram,𝒕𝟑shouldbelargeenoughtominimizeimpurityandsmallenoughtodiffusecarriersfromdopedlayer𝒕𝟐totheAlxGa1-xN/GaNhetero-interface.
TheeffectofthedopingdensityisstudiedandshowninFigure6,asacomplementtothe
studyof𝑡f.Weshowthatforidenticalvaluesof𝑥ij andt1(0.25and15nm,respectively),with
𝑡ffixedto2nm,thedeviceremainsnormally-offwhenthedopingdensityis4×1012cm-2orless.
15
Thisvaluealignswithexistingexperimentalworkshowingnormally-offcubicGaNdeviceswith
equivalentδ-dopingdensityof2.4×1010cm-2[28].Theelectrondensityhasbeenincludedas
well,asthecarriersinthe2DEGareprovidedbytheδ-doping.Thisbringsanewdesigntrade-off
between𝑛$and𝑉;.
Figure6:BanddiagramacrosstheHEMT{forafixed𝒙𝑨𝒍(of0.25),𝒕𝟏(of15nm),𝒕𝟐(of2nm),𝒕𝟑(of3nm),and𝚽M(of5.05eV)}isshownaswevariedthe𝜹-dopingdensityfrom2E+12cm-2
to1×1013cm-2inuniformsteps.Itisshownthatforadopingdensityof4×1012cm-2andlessthedeviceremainsnormally-off. Additionally,theelectrondensity isplottedasthedopingdensitywillimpact𝒏𝒔inthe2DEG,indicatingadesigntrade-offbetween𝑽𝑻and𝒏𝒔.
c.EffectsofgatemetalFigure7showsthebanddiagramacrosstheHEMT{forafixed𝑥ij (of0.25),𝑡2(of15nm),𝑡f(of
2nm),and𝑡+(of3nm)}aswevariedΦ�torepresentcommongatemetalsandalloysolutes
(e.g.platinum,iridium,nickel,copper,tungstenandtitanium)enablingawiderangeof𝜑h.For
Φ�greaterthan4.55Vthedeviceexhibitsnormally-offbehavior.Bypickingagatemetalwitha
sufficientlylarge�thedevicewillexhibitnormally-offbehavior.Examplesofsuchmetalsare
16
platinum[30],gold-platinumalloys[31],andnickel[30].Experimentalresultsshowingnormally
offdeviceswithPd/Ni/Au{𝛷�(Pd)=5.22eV}gatecontactsexhibitingnormallyoffbehaviorfor
similarrangesofcontrolvaluescorroboratethisresult[28].
Figure7:BanddiagramacrosstheHEMT{forafixedxAl(of0.25),t1(of15nm),t2(of2nm),andt3(of3nm)}isshownaswevaried𝜱𝑴torepresentcommongatemetalsandalloysolutes(e.g.platinum,iridium,nickel,copper,tungstenandtitanium)enablingawiderangeof𝝋𝒃.TheinsetshowstheHEMTstructure.Thearrowindicatesthedirectionofdecreasinggateworkfunction.AlargeSchottkybarrierisdesirablefornormally-offbehavior.
Itisimportanttounderstandthebreakdownmechanismsinthedevicetomaximizethe
blockingvoltage.Forthisstudy,breakdownvoltageisfoundbymeasure𝑉45where𝐼45=100nA
while𝑉*5=0V.Thedominantbreakdownmechanismintheproposeddeviceispunch-through,
whichagreeswiththeliteratureonbreakdownintraditionalh-GaNHEMTs[19].InFigure8,the
currentdensityinsidethedeviceisshown.Forthissimulation,allparametersweresettovalues
indicatedinTable3.Thegatelength𝐿* wasfixedtobe1𝜇m,with250nmspacingbetweenthe
sourceelectrodeandthegateelectrodeandatotaldrain-sourcespacing𝐿45 of2𝜇m.Thepunch-
througheffectmanifests inthe𝐼𝑉characteristicsasaslowlyincreasingcurrent,basedonthe
17
highlyresistivenatureoftheintrinsicGaNlayerasopposedtothesharpincreaseincurrentseen
inavalancheorZenerbreakdowns.Thecorresponding𝐼𝑉curveisgiveninFigure9.
Figure8:CurrentdensityheatmapfortheZB-GaNHEMTduringelectricalbreakdown,with𝑽𝑮𝑺=0V,𝑽𝑫𝑺=69.68V.Thecurrentisseentobeisolatedtothe2DEGinareaswhicharenotgated,whilethecurrentbelowthegateextendsintotheepitaxiallygrownZB-GaNlayer,indicatingpunch-throughtobethedominantbreakdownmechanismfortheproposeddevice.
Figure9:Draincurrentvs.drainvoltageforthedeviceuptoelectricalbreakdownasseeninFigure8.Thedraincurrentreaches100nAat81.5V,indicatingbreakdown.
18
Inordertooptimizethebreakdownvoltageofthedevice,theimpactofxAlandsource-
drainseparationLSDwasstudied.Punchthroughrequiresthermionicemissioncurrentstoinject
carriers across the heterojunction and in to the substrate. Thermionic emission currents for
electronsaregivenby𝐽. = 2𝑞 𝑣.,*�A𝑛*�A −|O®|¯°O®
𝑣.,ij*�A𝑛ij*�A exp´vF
y;J,¯°O®,where𝑞is
the elementary charge, 𝑣.is the emission velocity given by 𝑣. = y;Jfµ|J
, 𝑛 is the electron
concentration,𝑚iseffectivemass,𝛥𝐸? istheconductionbanddiscontinuity,𝑘istheBoltzmann
constant, and𝑇is the carrier temperature [12]. This exponential dependence on𝛥𝐸? would
implythatlargeraluminumcontentwouldincreasebreakdownvoltage.However,theimpactof
𝑥ij on𝑉; offsetsthisimpact,asthelowerthresholdvoltagemeansalowerbreakdownvoltage.
TheseresultsareshowninFigure10.Theminimumwithrespectto𝐿54near3.5𝜇misnotyet
understood.
Figure10:Breakdownvoltageasafunctionof𝒙𝑨𝒍and𝑳𝑺𝑫.
19
For power applications, on-resistance 𝑅-. is another important parameter. Resistive
losseswhentheHEMTfunctionsforpowerswitchingareproportionalto𝑅-.,andsokeeping
𝑅-.assmallaspossibleisbeneficialtoperformance.InFigure11,𝑅-. isplottedagainst𝑉*5 for
adevicewithparametersasgiveninTable3,with𝐿54setto2𝜇mand𝑉45at10V.𝑅-.isseen
todropsignificantlyas𝑉*5beginstoexceed𝑉;.As𝑉*5increasesandthe2DEGbeginstosaturate,
𝑅-.reachesapproximately1𝛺/𝜇m.
Figure11:On-stateresistanceplottedagainst𝑽𝑮𝑺 forthenormally-offZB-GaNHEMT.
20
5.ConclusionBased on preceding design guidelines, a normally-off zincblende (ZB-) phase
AlxGa1-xN/GaNhighelectronmobilitytransistors(HEMTs)isproposed.Figure12showsthe𝐼4vs.
𝑉45responseoftheHEMTwith𝑥ij setto0.25,𝑡2setto15nm,𝑡fsetto2nm,𝑡+setto3nm,
and𝛷� set to 5.05 V, verifying the normally off behavior by showing that no current flows
throughthedevicewhenthegateisgroundedforvaluesof𝑉45upto10Vwhileincreasingthe
gatevoltageto4Vallowscurrentsof3mA/μm.Itisworthnotingthatthiscurrentexceedsmost
existentGaNHEMTdevicesbasedonwurtzicGaNbynearlythree-fold.Thisisduetoavarietyof
reasons,includingtheassumptionofperfectcontactstothesourceanddrainregions,aswellas
thelackoftrapsandotherinterfacestatesattheGaN/AlGaNheterojunctiontodiminishmobility
andthus𝐼45.
Figure12:𝑰𝑫vs.𝑽𝑫𝑺 responseissimulatedforgatebiasesbetween0and4V,demonstratingthattheproposeddeviceisnormally-off.Whenthegateisunbiased,nocurrentflowsfromthesourceasVDSrunsfrom0to10V.As𝑽𝑮𝑺increasesabovethresholdvoltage,𝑰𝑫vs.𝑽𝑫𝑺curvesfollowthosefromtypicallyfieldeffecttransistors.Thissimulationisrunusingthecontrolparameters(listedinTable2)andshowscurrentsontheorderof3mA/µmata4Vgatebias.Increasing𝑽𝑮𝑺willincrease𝒏𝒔andhencethecurrent.
21
Tailoring HEMT performance to specific applications requires investigation of key design
parameterssimultaneously.𝑥ij isanidealchoiceforexplorationduetoitspositivecorrelation
to𝑛$andinversecorrelationtoturn-onvoltage𝑉;.Todemonstratethedesignspaceavailablein
zincblendeAlxGa1-xN/GaNHEMTs,thedeviceissimulatedviavarying𝑥ij and𝑡2as𝑉; isextracted
fromthe𝐼4vs.𝑉* curves.Plottedasacontourmap,theresultingvaluesof𝑉; areseeninFigure
13(a).ThedashedlinemarksMatthews-Blakesleecriticalthickness(ℎ"),20witha20%margin
oferrorconsidered.FortheregionsofthedesignspacethatfeatureanAlxGa1-xNthicknessbelow
Figure13:(a)Turn-onvoltageasafunctionof𝒙𝑨𝒍and𝒕𝟏.Thecriticalthickness(𝒉𝒄)iscalculatedbyMatthews-Blakesleeformula[29]anddrawnafteraccountingforthelayers𝒕𝟐and𝒕𝟑(witha20%errormargin). (b) 2DEGdensity (𝒏𝒔) plottedas a functionof𝒙𝑨𝒍and𝑽𝑮𝑺.As𝑽𝑮𝑺 isincreasedfrom0Vto10V,𝒏𝒔increases.
22
ℎ", thethresholdvoltage isthe largestofferingstablenormally-offdesign.Asboth𝑡2and𝑥ij
increase,thevalueof𝑉; isshowntobecomenegativeindicatingthetransitionfromnormally-
offtonormally-onbehavioriscontrolledbytheproperlyselectingtheHEMTdeviceparameters.
Experimentaldataintheliteratureshowingnormally-offbehaviorwith𝑉; of0.6Vfor𝑥ij =0.33
and𝑡2=10nmshowexcellentagreementwithourresults[28].
Thevalueof𝑛$ isofparticularrelevance inHEMTdevicesforpowerapplications. It is
directlyrelatedtotheconductionbandoffset,whichisinturninfluencedby𝑥ij.Basedonthis
fact,𝑛$isplottedasafunctionofaluminumcontentand𝑉*5inFigure13(b).Thesheetdensity
iscalculatedbyintegratingthecarrierdensityalongthedepth.Thesheetdensityisseentoscale
withbothaluminumcontentandgatebias.Weseethatfor𝑉*5near10Vand𝑥ij near0.40,𝑛$
achievesvaluesnear2.75×1013cm-2.Previousstudiesshowing𝑛$=1.7×1012cm-2for𝑉*5=0.3
Vand𝑥ij =0.33areingoodagreementwithourdata[28].
In summary,we report the normally-off operation regime of ZB-phase AlxGa1-xN/GaN
HEMTsviavaryingkeydesignparameters(𝑥ij, 𝑡2, 𝑡f, 𝑡+,δ-dopingamount,and𝛷�).Particularly,
weconsiderandreportthetrade-offsbetween𝑥ij, 𝑡2,and𝛷�(viaitsimpactas𝜑h)tomaximize
𝑉; and𝑛$. ConsideringMatthews-Blakeslee critical thickness, we offer design guidelines for
maximizing 𝑉; while minimizing defectivity. Our results provide encouraging results for the
employment of ZB-phase GaN HEMTs, given the wide-range tenability in 𝑉; and 𝑛$, under
normally-offoperation.
23
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