Technology for diamond based electronics [Elektronische Ressource] / von Michal Kubovič

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TechnologyfordiamondbasedelectronicsDissertationzurErlangungdesakademischenGradeseinesDoktor Ingenieurs(Dr. Ing.)derFakultat¨ fur¨ IngenieurwissenschaftenderUniversitat¨ UlmvonIng. MichalKubovicˇausMyjavaGutachter: Prof. Dr. Ing. E.KohnProf. Dr. CarlE.KrillAmtierenderDekan: Prof. Dr. rer. nat. HelmuthPartschUlm,24. January2008iiContentsListoffigures viiListoftables ixSummary xi1 Introduction 12 Structureandpropertiesofdiamond 72.1 Structureofdiamond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.1.1 Crystalstructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.1.2 Bandstructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.2 Electricalpropertiesofdiamond . . . . . . . . . . . . . . . . . . . . . . . 82.3 Comparisontoothersemiconductormaterials . . . . . . . . . . . . . . . 93 Dopingofdiamond 133.1 p–typedoping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.1.1 Borondoping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.1.2 p typechannelinducedbyhydrogentermination . . . . . . . . . 173.2 n–typedoping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203.2.1 Nitrogendoping . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213.2.2 Phosphorousdoping . . . . . . . . . . . . . . . . . . . . . . . . . . 214 Growthofdiamond 234.1 MicrowaveplasmaCVD . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244.2 Principleofdiamondgrowth . . .
Publié le : mardi 1 janvier 2008
Lecture(s) : 62
Source : VTS.UNI-ULM.DE/DOCS/2009/6673/VTS_6673_9158.PDF
Nombre de pages : 127
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Technologyfordiamondbasedelectronics

Dissertation

zurErlangungdesakademischenGradeseines

-IngenieursDoktor

.-Ing.)(Dr

derFakult¨atf¨urIngenieurwissenschaften

at¨UniversitderUlm

von

cˇKuboviMichalIng.

Myjavaaus

Gutachter:Prof.Dr.-Ing.E.Kohn

Prof.Dr.CarlE.Krill

AmtierenderDekan:Prof.Dr.rer.nat.HelmuthPartsch

2008January24.Ulm,

ii

Contents

figuresofList

tablesofList

Summary

Introduction1

vii

ix

xi

1

2Structureandpropertiesofdiamond7
2.1Structureofdiamond..............................7
2.1.1Crystalstructure............................7
2.1.2Bandstructure.............................8
2.2Electricalpropertiesofdiamond.......................8
2.3Comparisontoothersemiconductormaterials...............9

13diamondofDoping33.1p–typedoping..................................14
3.1.1Borondoping..............................14
3.1.2p-typechannelinducedbyhydrogentermination.........17
3.2n–typedoping..................................20
3.2.1Nitrogendoping............................21
3.2.2Phosphorousdoping..........................21

23diamondofGrowth44.1MicrowaveplasmaCVD............................24
4.2Principleofdiamondgrowth.........................25
4.3Substrates....................................26
4.3.1SinglecrystallineCVDdiamondgrownonHTHPstones.....27
4.3.2SinglecrystallinediamondgrownonIr/SrTiO3substrate.....28
4.3.3NanocrystallinediamondgrownonSisubstrate..........30

iii

iv

ONTENTSC

5ConceptsfordiamondFETsanddiodes33
5.1SurfacechannelFET..............................35
5.2δ-dopedchannelFET..............................36
5.3Mergedp-i-nSchottkydiode.........................38

41FETchannelSurface66.1Surfacestabilityconcerns...........................41
6.2Technology....................................46
6.3DCcharacteristics................................48
6.4Smallsignalcharacteristics...........................50
6.4.1Cut–offfrequencymeasurements...................52
6.4.2Noisemeasurements..........................53
6.5Largesignalmeasurements..........................55
6.6FETfabricatedonNCD.............................58
6.7Conclusion....................................63

7Mergedp-i-nSchottkydiode65
7.1Mergeddiodeconcept.............................65
7.2Technology....................................66
7.3Characteristics..................................67
7.4Conclusion....................................73

75diodepnNanocrystalline88.1UNCDdoping..................................75
8.2Technology....................................76
8.3Characteristics..................................76
8.4Conclusion....................................78

Conclusions9

81

85parametersProcessAA.1ProcessparametersforsurfacechannelFET.................85
A.2Processparametersformergeddiode....................88

Bibliography

publicationsofList

Acknowledgments

107

109

113

ListFiguresof

1.1Powerandfrequencycapabilitiesofsemiconductors............
2.1Face-centeredcubiclatticeofdiamond....................
2.2Bandstructureofdiamond...........................
3.1Dopingconfigurationofdiamond......................
3.2Activationenergyofboronasfunctionofdopingconcentration.....
3.3ERDmeasurementofmultipleboronδ-dopedlayers............
3.4Hydrogen-terminateddiamondwithp-typechannel............
4.1Phasediagramofcarbon............................
4.2MicrowaveplasmaCVDsystem.......................
4.3SchematicsofprocessesoccurringduringCVDgrowth..........
4.4HTHPIbdiamondsubstrates.........................
4.5Diamondquasi-substratewithfabricatedFETstructures.........
4.6Diamondquasi-substratewithnearlysingle-crystalsurface........
4.7Non-epitaxialdefectindiamondquasi-substrate..............
4.8ThermalmismatchbetweendiamondonSrTiOanddiamondonSi...
34.9Differentsinglecrystallinediamondsubstrates...............
4.10NanocrystallinediamondwafercomparedtoHTHPsubstrate......
4.11AFMimageofNCDsurface..........................
5.1Diamondelectronicconcepts.........................
−+5.2HCdipoleondiamondsurface.......................
5.3Simplifiedschematicbanddiagramofhydrogen-terminateddiamond.
5.4Cross-sectionofMESFETwithsurfacechannel...............
5.5Cross-sectionofδ-dopedchannelFET....................
5.6Profileofδ-dopedspike............................
5.7Advantagesofδ-dopedchannelFETconcept................
5.8Requirementsonhighpowerswitchingdiodes...............
5.9SiCmergeddiodeconcept...........................

v

389141517202425262728292930303132343536363737383939

vi

FIGURESOFLIST

5.10Diamondmergeddiodeconcept.......................40
6.1Previouslyobservedcurrentinstabilityofungatedchanneldevice....42
6.2PreviouslyobserveddegradationofMESFEToutputcharacteristic...42
6.3Transfercharacteristicsmeasuredonvirgindeviceandafterbiasstress.43
6.4TransfercharacteristicsofvirginFETand6monthslater.........44
6.5SchematicofAFMKelvinprobemeasurementsetup............45
6.6Surfacepotentialanddraincurrenttransientmeasurement........45
6.7OutputcharacteristicsofFETstructure-longtimestability........46
6.8Pretreatmentofdiamondsurfaceinhydrogenplasma...........47
6.9FabricationsequenceofFETwithself-alignedgate.............48
6.10Etchingofgoldforself-alignedgate.....................48
6.11OutputcharacteristicofFETwithself-alignedgate.............49
6.12OutputcharacteristicofFETfabricatedonquasi-substrate........49
6.13FETdevicefabricatedonquasi-substrate..................50
6.14Smallsignalequivalentcircuit.........................51
6.15SEMimageofT-gatewith200nmgatelength................51
6.16RFgainplotsandextractedcut-offfrequenciesfordevicefabricatedon
quasi-substrate.................................52
6.17RFgainplotwithextractedcut-offfrequenciesfordevicefabricatedon
highqualityhomoepitaxiallayer.......................53
6.18Cut-offfrequenciesdependenceongatebias................54
6.19DependenceofintrinsicgmandCGSongatebias..............54
6.20Minimumnoisefiguredependenceonfrequency..............55
6.21Minimumnoisefiguredependenceondraincurrent............55
6.22FirstlargesignalpowermeasurementperformedondiamondFET...56
6.23Powerlevelsof0.9µmgatelengthdeviceatdifferentloads........57
6.24LargesignalpowermeasurementonFETdevicefabricatedonhighquality
homoepitaxiallayer...............................58
6.25Cross-sectionofsurfacechannelMESFETfabricatedonNCD.......59
6.26SEMimageofFETstructurewithdetailedviewofgateregion......59
6.27OutputcharacteristicofFETfabricatedonNCDwithcorresponding
timedomainmeasurement...........................60
6.28CarrierprofileobtainedfromCVmeasurement...............61
6.29Simulateddependenceofmaximumdraincurrentongatelengthwith
mobilityasfittingparameter..........................62
6.30RFgainplotof0.3µmgatelengthFETdevicefabricatedonNCD....62
6.31AFMimageofgateregionofFETfabricatedonNCD...........63

FIGURESOFLIST

7.1Inuenceofthicknessofnitrogendopedlayeronthresholdvoltage...
7.2Cross-sectionofmergeddiamonddiodestructure.............
7.3Imagesofetcheddiodestructure.......................
7.4IVcharacteristicsbeforeandafterSchottkycontactfabrication......
7.5TemperaturedependentIVcharacteristics..................
7.6Barrierheightextractionfromtemperaturedependentmeasurements..
7.7IVcharacteristicswithbreakdownoccurringat25V............
7.8TemperaturedependentIVcharacteristicsmeasuredinvacuum.....
7.9IVcharacteristicofmergeddiodemeasuredat1000°C..........
7.10Inuenceofdefectsondeviceperformance.................

8.18.28.38.4

9.1

Crosssectionofn-typeUNCD/p-typesingle-crystaldiamonddiode..
BarrierpotentialextractedfromIVandCVmeasurements........
IVcharacteristicofpndiodemeasuredat1050°C.............
Schematiccrosssectionofapndiodewithtwodifferentbarrierheights.

Advancesindiamondtechnologyopennewintegrationpossibilities..

vii

66676868707071727273

76777879

82

viii

LIST

FO

FIGURES

ofListablesT

2.1Propertiesofdiamond.............................
2.2Figuresofmerit.................................

4.1Diamondclasses.................................

A.1Parametersforcleaningofdiamondsubstrates...............
A.2MPCVDparametersforgrowthofdiamondlayers.............
A.3HFCVDparametersforgrowthofnanocrystallinediamond.......
A.4Lithographyparametersformesaetching..................
A.5Parametersusedforoxygentermination...................
A.6Lithographyparametersforohmicpads...................
A.7ElectronbeamlithographyparametersforT-gates.............
A.8Lithographyparametersforohmiccontacts.................
A.9MPCVDparametersforgrowthofintrinsicdiamondlayer........
A.10MPCVDparametersforgrowthofnitrogendopeddiamondlayer....
A.11MPCVDparametersforgrowthof5boronδ-dopeddiamondlayers...
A.12Lithographyparametersforimagereversalprocess............
A.13Parametersforreactiveionetchingofdiamond...............
A.14Sputteringparametersfortemperaturestableohmiccontacts.......
A.15Annealingparametersfortemperaturestableohmiccontacts.......

ix

1011

28

858586868686878788888990909090

x

LIST

OF

ABLEST

Summary

Overthepastyears,widebandgapsemiconductorsareconstantlypushingthelimits
ofhighperformanceelectronicsintermsofpower,frequencyandtemperature.There
aremanyapplicationswherethebetterpowerandfrequencyhandlingcapabilities,
andhigherthermalconductivityofwidebandgapsemiconductorswillgivethemsig-
nificantadvantageoverclassicsemiconductors.Thesuperiorelectrical,mechanical
andthermalpropertiesofdiamondpredestinethismaterialtobecomeanimportant
semiconductor.However,diamondisadifficultmaterialtocontrol,andlarge-area,
single-crystalwaferssizesubstratesarestillmissing.Furthermore,diamondislacking
fewshallowdevicedopingconceptsimpuritiesare(theavailableonlyrforelevantdiamondisboronelectrforonicsp-typeatprdoping),esent.andDuethustotheseonly
limitations,diamondelectronicdevicesarestillintheproof-of-conceptstage.How-
ever,avarietyofactiveandpassivediamonddeviceshasbeensimulated,developed,
fabricatedandanalyzedovertheseveralpastyears.Inthiswork,someofthefield
effecttransistoranddiodeconceptswererealizedandevaluated.Recentprogressin
thetechnologyenabledDC,smallandlargesignalmeasurementsonFETsemploy-
ingahydrogen-inducedsurfacechannel.Althoughthenatureofthischannelisstill
underdiscussion,thischannelconfigurationhasbeensuccessfullyusedtofabricate
FETdevices.Theiroperationatmicrowavefrequenciesshowedcut-offfrequencies
fTof25GHzandfmaxof80GHz.Theimprovedfabricationtechnologyenabledfirst
noiseandpowermeasurementsondiamondFETs.Thenoisemeasurementresulted
inminimumnoisefigureof0.72dBat3GHzandthepowermeasurementsat1GHz
ledtosaturatedoutputpowerof0.34W/mm.TheseMESFETsemployedap-type
channelincloseproximitytothesurface,whichwasobtainedbyhydrogentermination
ofthediamondsurfacewithoutextrinsicdopingimpurities.Ontheotherhand,this
devicedesignisrestrictedtoaplanardeviceconfigurationandthedevicesaresensitive
tosurfaceconditions.ForhighpowerFETs,anextendeddriftregionsupportedby
fieldplateisrequired.Thedesign(channeldopingprofileanddevicestructure)of
theseFETsisbasedonaboronδ-dopedchannel.Thestructurerequirementsand
expectedperformanceoftheseδ-dopedFETswillbediscussed.Duetothelackof
shallowdonors,thedevelopmentofbipolardiamonddeviceshasbeenlimitedso

xi

xii

Summary

far.However,ultra-nano-crystallinediamondcanben-typedopedwithoutnoticeable

activationenergy,andthisallowedthefabricationofanall-carbonpnjunctiondiode.

Thisdiodewashighlyrectifyingandsuccessfullytestedupto1050°Cinvacuum.In

ordertoobtainhighblockingvoltagesandlowforwardlossesinpowerdiodes,anovel

mergeddiodehasbeendeveloped.Inthisconfiguration,aSchottkycontactandapn

junctionaremergedtogether,resultinginadiodewithlowforwardthresholdvoltage,

lowreverseleakagecurrentandhighbreakdownvoltage.Thisdiodeconfiguration
showedcurrentrectificationratioof109,reversebreakdownoccurredat2.5MV/cm,

andwassuccessfullyoperatedat1000°Cinvacuum.Theprogressinfabrication

technologyshowsthecapabilitiesofdiamondandindicatesthatdiamondmayindeed

becomeanultimatesemiconductorforhighperformanceelectronics.

1Chapter

Introduction

Intodaysmodernsociety,wecouldonlyhardlyimagineeverydaylifewithoutthe
omnipresenceofelectronics.Semiconductordevicesfoundtheirwayinmanydomestic
andindustrialapplications.Theincreasingneedformicroelectronicdevicesoperating
athighpower,highfrequencyandhightemperaturepresentsacontinuouschallenge
forsemiconductormaterials.ClassicalsemiconductorssuchasSi,GeandGaAsare
fastapproachingtheirtheoreticallimits,andcannolongermeettheincreasingpower,
frequencyandtemperaturehandlingrequirementsoftheindustry.Furthermore,high
powerRF1transmittersandseveralotherapplicationsstillrelayonvacuumtube
systems.Modernindustryplacesnewdemandsoncontrolandregulationoftechno-
logicalprocesses.CommonlyusedSi-baseddevicescannotbehandletheincreasing
thermalandmechanicalburdenrequiredinsomeapplications(e.g.sensorsemployed
inharshconditions).Currentlymostusedsemiconductormaterialscannotachievethe
necessarypowerlevelsduetotheirphysicallimitations.Inthenearfuture,RFdevices
suchaspoweramplifiers,phasedarrayradars,etc.willrequireamorecompactdesign
withimprovedheathandlinganddissipatingcapabilities.Tomeetthesedemands,
anewgenerationofsemiconductormaterialsisneeded.Inrecentyears,widebandgap
semiconductorssuchasSiC,groupIII-Nitridesanddiamondhaveattractedalotof
attention.Duetotheirsuperiorelectricalproperties(biggerbandgap,highermobili-
ties,higherbreakdownvoltages,etc.),thermalandmechanicalstability,widebandgap
semiconductorsarethebestcandidateforhighpower,highfrequencyapplications.
Here,diamondhasbyfarthebestmaterialproperties.

Diamondisoneofthemostfascinatingmaterialsinnature.Diamondisanallotrope
ofcarbon,whichtogetherwithsiliconandgermaniumbelongstothefourthcolumn
oftheperiodictable,andrepresentstheultimatematerialforsingleelementsemicon-

1equencyFrRadio

1

2

Introduction

ductor.Itswiderangeofextraordinarypropertiesmakesdiamondanidealcandidate
formanyapplications,howeveritswiderusehasforalongtimebeenlimiteddue
toitsrarityandcost.Withrecentprogressinthediamondsynthesis,newfieldsof
applicationhaveemerged.Thecontinuousprogressofimprovingofthequalityof
HTHP2andCVD3diamondledtoasuccessfulapplicationofdiamondtechnology
inmanyindustrialfields,rangingfromcuttingtoolstoelectronics.Itsmechanical
properties,suchasextremehardnessandlowfrictioncoefficientcombinedwithchem-
icalinertnesspredetermineddiamondasamaterialforprotectivecoatingsforcom-
merciallyavailablecuttingandgrindingtools.Thehighestthermalconductivityofall
materialsmakesdiamondanidealheatsink,itsopticaltransparencyfromtheinfrared
totheultravioletregioncanbeusedforopticalwindows[1]anditsexcellentradiation
hardnessisusedinradiationdetectors[2,3].Inspiteofthesuccessinmechanical,
thermalandopticalapplications,theexploitationoftheuniqueelectronicproperties
(largebandgap,highcarriermobilities,highbreakdownfield,etc.)hasbeenslow.

Propermaterialdevelopmentisessentialforhighperformanceelectronicdevices.Un-
fortunately,diamondislackingastrongdrivingforce(suchasoptoelectronicsinthe
caseofGaN)orthemanyman-yearsofmaterialdevelopment(suchasinthecase
ofSi)sofar.Atthemoment,SiCandGaN-baseddeviceshaveantechnologicalad-
vantageduetodopingchallengesandwafersizelimitationofdiamondsubstrates.
However,diamondpossessesthenecessarypropertiesneededforcompetingandover-
performingotherwidebandgapsemiconductors.Inaddition,theextremechemi-
calstabilitygivesdiamondacompetitiveedgeinharshenvironments.Mechanical
stabilityandchemicalinertnessofdiamondcanbeusedinwiderangeofMEMS4
applications[4],suchasmicroactuatorsandswitches[5,6],resonators[7],micro
membranepumps[8]andinjetsformicrouidicsystems[9].Furthermore,diamondis
abiocompatiblematerialandtherecentboominbiomedicineopensnewopportunities
foritsapplicationinhealthcare[10–12].Becauseofthis,theresearchfocusingon
electronicapplicationsofdiamondisconstantlygrowinginrecentyears.

Oneofthemainareasofinterestfordiamondarehighpower,highfrequencyappli-
cations.Broadcastingstations,communicationsatellitesandradarsrequireRFoutput
powerdensities(kilowattsofpowerintheGHzfrequencyrange),whichonlyvacuum
tubescandeliveratthemoment.However,vacuumtubesrequirehighsupplyvoltages
andexhibitonlylowefficiency,whichleadstohighenergylossesandincreasedre-
quirementsforheatdissipation.DiamondFETsareexpectedtoreplacevacuumtubes
inthiskindofapplications.Theheatcandissipateveryrapidlythankstothehighest

2HighTemperatureHighPressure
3ChemicalVaporDeposition
4Micro-Electro-MechanicalSystem

3

thermalconductivityofdiamondamongallmaterialsof22W/mmKatR.T.5,and
duetolargebandgapof5.45eVandbreakdownelectricfieldof10MV/cm[13]these
devicescouldoperateatveryhighvoltages.Thehighcarriermobilitiesof4500cm2/Vs
forelectronsand3800cm2/Vsforholes[14],andsaturationvelocityofapproximately
1×107cm/sforbothelectronsandholesenablediamondFETsoperationinthehigh-
frequencyrange.Asanindicatorofthesuitabilityofasemiconductorforagivenappli-
cation,figuresofmeritcanbeused.TheJohnsonfigureofmeritisusedtotheoretically
assesshigh-frequency,high-powercapabilitiesofsemiconductors.Fordiamondthis
figureismanytimeshigherthaninthecaseofanyothersemiconductor[15].For
bettercomparison,differentsemiconductorsandtheirpower/frequencycapabilities
areschematicallyshowninfig.1.1,wherediamondhasthebestprerequisitetobecome
theultimatesemiconductorforhighpowerandhighfrequencyapplications.

Figure1.1:Powerandfrequencycapabilitiesofdifferentsemiconductors.

Althoughthepredictionsforapplicationofdiamondinelectronicsareverygood,
duetomaterialdifficultiesconcerningdopingandsubstratesizes,diamondhasnot
reacheditsfullpotentialyet.Properdopingcapabilityisoneofthemostimportant
prerequisitesforelectronicdevices.Diamond,duetoitswidebandgapisaninsulator,
butitcanbeturnedintoasemiconductorwhendoped.However,dopingpresents
aseriousobstacle,becausetheonlytechnicallyrelevantdopingisp-type.Although,
boronisadeepacceptor,itsactivationenergycanbedecreasedwithincreased20doping−3
concentration,whichatR.T.becomesnegligibleatconcentrationsabove10cm.
Then-typedopingbynitrogenorphosphorousinnoteffectiveenoughtoproducehigh
electrondensities,thusthistypeofdopingisnotsuitableforhighperformancedevices.
Thisleavesuswithtwodeviceapplications:p-typechannelFETsandSchottkydiodes.
InFETswithborondopedchannel,thesocalledδ-dopedchannelFETisbestsuited
forhighpowerdevices,howeverthegrowthprocesspresentsaserioustechnological
challenge[16–18].Asforpowerswitchingdiodes,theSchottkycontactwithhigh
5eemperaturTRoom

4

Introduction

purityactivelayerpresentsthemostpopularconceptatthemoment[19,20].The
challengehereliesinthegrowthoflayerswithbreakdownstrength,whichwillresult
inhighblockingvoltagesinthekV-range[21].

Thesetwodeviceconceptscanbefurtherextendedduetoanotheruniquepropertyof
diamond,ap-typesurfacechannelinducedbyhydrogenterminationofthediamond
surface.ThischannelisfullyactivatedatR.T.andisobtainedwithoutextrinsicdoping
[22],eventhoughthenatureoftheacceptorisnotfullyunderstoodyet.Therearetwo
differentgateconfigurationunderdevelopment,namelyMESFET[23]andMISFET
[24]structures.TheseFETstructuresaretheonlyonesthathavedemonstratedoper-
ationintheGHzfrequencyrange,andtheMESFETconfigurationhasbeenusedto
obtainfirstlargesignal[25,26]andnoisemeasurementdata[27].FETsinvestigated
inthisworkshowedsuccessfuloperationatmicrowavefrequencieswithcut-offfre-
quenciesfTof25GHzandfmaxof80GHz,andaminimumnoisefigureof0.72dB
at3GHz.Recently,diamondFETusinghigh-qualitypolycrystallinediamondwith
fTof45GHzandfmaxof120GHzhasbeenreported[28].Thesestructuresemploy
asimpletechnology,butarelackingmanyfeaturesofidealpowerFETstructures,thus
thedeliveredoutputpowerisstillbelowexpectation.Nevertheless,oneofthefirst
powermeasurementsondiamondFETsat1GHzledtoasaturatedoutputpowerof
0.34W/mm.AnimprovementoftheoutputpowerdensityofdiamondFETsfabri-
catedonadiamondlayergrownwithahigh-puritysourcegashasbeenreportedin
[29],andledtoamaximumoutputpowerdensityof2.1W/mm.

Asmentionedbefore,thelackofshallowdonorshindertherealizationofbipolar
devices.Recently,ithasbeenshownthatUNCD6canbedopedwithnitrogenwith-
outnoticeableactivationenergy.Thismaterialcanbedepositedontoborondoped
single-crystaldiamondformingapndiode.Suchaheterojunctionstructurecontains
onlycarbon,andthusisreliableevenatveryhightemperatures.Indeed,thisdiode
configurationwassuccessfullytestedupto1050°Cinvacuum[30].Theanalysisof
thediodesuggestedthepresenceoftwojunctionswithdifferentbarrierheightsin
parallel.Thisresemblesthemergeddiodeconcept(combinationofaSchottkycontact
withapnjunction)[31],whichwasfurtherexploredinthediamondmergeddiode
structure.Thisconceptwassuccessfullytransferredtodiamonddespiteitsdoping
limitations[32].Thisdiodeconfigurationshowedhighcurrentrectificationratio,and
wassuccessfullyoperatedat1000°Cinvacuum.

Afterdoping,thesecondmaintechnologicalchallengeisthediamondsubstrate.Up
tonowelectronicdeviceshavebeenrealizedmainlyonHTHPsingle-crystalswith
limitedsize.Homoepitaxialelectronic-gradediamondlayerscanbegrownonthese
stonesbymicrowaveplasmaCVD.Heteroepitaxialgrowthofdiamondresultsinpoly-
6DiamondeUltra-Nano-Crystallin

5

crystallinelayers,whicharewellsuitedforMEMSapplicationssuchasRFswitches
[5,33],butnotsowellforhighperformanceelectronics.Recently,heteroepitaxial
growthoniridium,resultedinsinglecrystallinequasi-substrate,whichwasusedfor
fabricationofFETdevices[26].Thisadvancementisveryencouragingandopens
newpossibilitiesforintegrationofactiveandpassivedevicesintohighpowerMMIC7
applications.

Thisthesisisorganizedasfollows:
Anoverviewofthestructureandpropertiesofdiamondisgiveninchapter2,where
differentfiguresofmeritwillbeusedtocomparetheelectricalpropertiesofdiamond
semiconductors.bandgapwideotherandThedopingissueswillbeaddressedinchapter3.Borondopingandhydrogen-induced
channelconceptswillbediscussedinmoredetail.
Chapter4willbefocusedonthegrowthofdiamondbymicrowaveplasmaCVD
systemandthesubstratesizelimitationwillbedebated.
Inchapter5thedifferentconceptsforactivediamonddeviceswillbepresented.The
conceptsoftwoFETstructures(surfacechannelandδ-dopedchannelFET)andone
diodestructure(mergedp-i-nSchottkydiode)willbediscussedinmoredetail.
SurfacechannelFETswillbeanalyzedinchapter6.Thehydrogenandoxygentermi-
nateddiamondsurfacewillbecompared,thefabricationtechnologyofthesedevices
willbedescribed,andDC,smallsignalandlargesignalresultswillbediscussed.
Chapter7willbedevotedtothemergedp-i-nSchottkydiode.Theconcept,technology
andmeasurementresultswillbediscussed.
Inchapter8thefabricationandcharacteristicsofananocrystallinepndiodewillbe
described.Thesummaryofthisthesisaswellastheoutlookcanbefoundinchapter9.

7MonolithicMicrowaveIntegratedCircuit

6

Introduction

2Chapter

diamondofpropertiesandStructure

Diamondpossessesmanyextraordinaryphysicalproperties.Itisexceptionallyhard,
opticallytransparent,chemicallyinert,biocompatiblematerialwithhighestthermal
conductivity,highestelasticmodulus,lowfrictioncoefficient,andwhendopeddia-
mondbecomessemiconducting.Theunderstandingofthesepropertiesofdiamond
liesinitscrystalstructure.

diamondofStructure2.1

structureCrystal2.1.1

Diamondisanallotropeofcarbon.Diamondpresentsthethermodynamicallymeta-
stable1phaseofcarbon,whilegraphitepresentsthestablephase.Thecarbonatom
hassixelectrons.Twoelectronsarelocatednearthenucleusoftheatomanddonot
participateinchemicalbonding.Thefouruppermostelectrons(valenceelectrons)
participateinchemicalbonding.Theelectronconfigurationofcarbonisasfollows:
1s22s22p2.Indiamond,thevalenceelectronsarespreadinonesandthreeporbitals
formingsp3hybridization.Inordertocreatecovalentbonds,eachcarbonatomis
boundtofournearestneighborsbyaσ–bond,formingatighttetrahedralstructure.All
thesebondare1.54A˚longwithabondangleof109◦,thelatticeconstantofdiamond
is3.566˚A[34].

1Undernormalconditions(R.T.,1atmosphere)isdiamondkineticallystableandcannot
phase.anothertoconvertspontaneously

7

8

Structureandpropertiesofdiamond

Thecrystalstructureofdiamondisshowninfigure2.1.Thediamondlatticeconsistsof
twoface-centeredcubic(fcc)latticesshiftedalongthediagonaloftheunitcellbyone
quarterofthelengthofthediagonal,andhastwoatomsperprimitiveunitcell[35].
Thetight,butsimpletetrahedralstructurewithhighlevelofsymmetryandstrong
covalentbondsisthekeyreasonfortheuniquepropertiesofdiamond.

Figure2.1:Face-centeredcubiclatticeofdiamondwithlatticeconstanta.

Band2.1.2structure

Electronically,diamondwithoutimpuritiesisaninsulatorwithalargeindirectband-
gapof5.45eV.Thebandstructureofdiamondisshowninfigure2.2andissimilarto
thatofSi.Ithassixequivalentelectronminimalocatedalongthe[100]k-axes,the
valencebandmaximumisinΓ-point(k=0),theconductancebandminimumcanbe
foundatapprox.80%ofX-pointposition.TheΓ-pointisthecenterofthefirstBrillouin
zoneandpossessesthehighestsymmetry.

Electronsindiamondhavetwodifferentmasses;longitudinalmassmle=1.4m0and
transversalmassmte=0.36m0[37].Holesindiamondhavethreedifferentmasses;
heavyholesmassmhh=0.57m0,lightholemassmlh=0.32m0andsplit-offholemass
mso=0.39m0[38].

diamondofpropertiesElectrical2.2

Owningtoitsatomicstructure,diamondpossessesmanyuniqueelectricalproperties.
Thereportedvaluesfordiamondareasfollows:largebandgapof5.45eV[1],high

2.3Comparisontoothersemiconductormaterials

Figure2.2:Thebandstructureofdiamond,after[36].

9

breakdownelectricfieldof10MV/cm[13],highcarriermobilitiesof4500cm2/Vsfor
electronsand3800cm2/Vsforholes[14],highsaturationvelocityof1.6×107cm/s
forelectronsand1.1×107cm/sforholes,andlowdielectricconstantof5.7[13].
Furthermore,diamondpossessesthehighestthermalconductivityamongallmaterials
atR.T.of22W/mmK[13].Thesignificanceofthesevaluescanbebestseenwhen
comparedtoothersemiconductormaterials.

2.3Comparisontoothersemiconductormaterials

Diamond,incomparisontoothersemiconductors,hasanumberofmaterialproperties
thataresuitableforhighperformanceelectronicapplications.Theelectricalproperties
ofmostcommonwidebandgapsemiconductoraresummarizedintable2.1.

Forevaluationofsemiconductormaterialsfordifferentapplicationsfiguresofmerit
canbeused.Thehighspeedpowerhandlingcapabilityofelectronicdevicescanbees-
timatedfromtheJohnsonfigureofmerit(JFM)[41].Thisisthemostinterestingfigure
fordiamond,sinceitsfocusedonpowerelectronicsoperatingathighfrequencies.

2JFM=Ebr∙vsat
π∙2

(2.1)

10

[eV]EBandgapGaconstantLattice[nm]0

Structureandpropertiesofdiamond

SiCGaNDiamondBandgapEG[eV]5.453.43.3
Latticeconstanta0[nm]0.37a:0.32a:0.31
1.51b:0.52b:Carriermobility(hole)µh[cm2/Vs]380012001140
Carriermobility(electron)µe[cm2/Vs]4500301600
Saturationvelocity(electron)vsate[107cm/s]1.62.52
Saturationvelocity(hole)vsath[107cm/s]1.120.3
BreakdownstrengthEbr[106V/cm]102.53
Dielectricconstantεr5.799.7
Thermalconductivityλ[W/cmK]221.34.9

Table2.1:Propertiesofdiamondcomparedtoothersemiconductorwidebandgap
materialssuchasGaNandSiC(4H)[1,13,14,39,40].

Theswitchingspeedandheatdissipatingcapabilitiesoftransistorscanbebeestimated
fromtheKeyesfigureofmerit(KFM)[42].

KFM=λ∙c∙vsat
4∙π∙r

(2.2)

ThecalculatedfiguresofmeritofdiamondintermsofhighpowerFETsandhighfre-
quencyintegratedcircuitsareextremelyhighduetoitsveryhighbreakdownelectric
fieldstrength(allowingoperationathighvoltages),itshighestthermalconductivity
meansthatthepowerdeviceshavetheirownintrinsicheatspreaderleadingtohigh
heatdissipationefficiencyduringoperation,thecarriersindiamondhavehighmo-
bilitiesandsaturationvelocities,whichallowshigh-speed,high-frequencyoperation,
andfinallythelowdielectricconstantissignificantlyimprovingthehigh-frequency
characteristics(thesedataarevalidforhighqualitysinglecrystallinediamondonly).
Both,JFMandKFMofdiamondarehighercomparedtootherwidebandgapsemi-
conductorsuchasGaNandSiC[15],asshownintable2.2.WhencomparedtoSi,
theJFMofdiamondisover1000xbiggerandKFMisabout20xbigger.Thus,dia-
mondseemstobethemostsuitablesemiconductorforhigh-frequency,high-power
electronics.Although,thepredictedcapabilitiesofdiamondareverygood,dueto
somematerialdifficultiesdiamondelectronicsisstillintheearlystageofdevelopment.

2.3Comparisontoothersemiconductormaterials

(4H)SiCGaNDiamond

11.13.3JFM

10.34.3KFM

11

Table2.2:Figuresofmeritforsomewidebandgapsemiconductors.ThevaluesforJFM
andKFMcorrespondtop-typediamondandn-typeGaNandSiC,andarenormalized
SiC.to

Diamondisthebestsemiconductormaterialforthefuture[43].Ithaslargebandgap,
largeelectronandholemobilities,andlargestthermalconductivityofallmaterials.
Thenearunitymobilityratiomakesdiamondidealmaterialforbipolardevices.How-
ever,therearesomeunresolvedissues.Diamondislackingreliablen-typedoping,
thesubstratesizeislimited,anddevicefabricationtechnologyislessmaturethanthat
ofGaNorSiC.Nevertheless,thereisasignificantefforttoovercometheseissuesand
diamondisonthewaytobecometheultimatefuturematerialforhighperformance
onics.electr

12

Structure

and

properties

of

iamondd

3Chapter

Dopingdiamondof

Probablythebiggestobstaclefordiamondelectronicsistheabsenceofshallowdonors
andacceptors.Becauseofthesmalllatticeofdiamondthedopingimpuritiesare
difficulttoincorporate,whichleadstoadistortionsofthelatticeandanincreased
stressinthefilms.Thus,themaingoalistofindimpuritiesthatcanbeintroducedinto
thecrystalstructureandwillhaveenergylevelsclosetotheconductionbandmini-
mumforn-typeorvalencebandmaximumforp-typedopingatroomtemperature.
Furthermore,thesizeofthedopingimpurityisimportantforthestraininthecrystal.
Smallatomscanoccupyaninterstitialsitewhileatomswithsizecomparabletothe
sizeofcarbonatomscanbyincorporatedinsubstitutionalsites.Unfortunately,most
atomsaretoobigandthissizerequirementofthedopantatomlimitsthechoiceto
lightatomsonly.Duetothelargebandgapandtightcrystalstructureofdiamond
theserequirementspresentsaserioushurdle.

Sofartheimpuritydopinghasbeenlimitedtothreemostsuitablecandidates:boron
forp-typedoping,andnitrogenandphosphorousforn-typedoping[44].Allthree
dopantsaredeep,thusnoneofthemisfullyactivatedatR.T.Theactivationenergyof
Bis0.37eV(althoughfullboronactivationatR.T.canbeobtainedathighconcentra-
tionsabove1020cm−3,whentheminibandstartstooverlapwiththevalencebandorat
lowconcentrations,whentheFermi-levelcrossestheacceptorlevel[45]),theactivation
energyofNis1.7eVandthatofPis0.6eV,asschematicallyshowninfig.3.1.Onthe
otherhand,nitrogencanbewellincorporatedintograinboundarynetworksofUNCD
withoutnoticeableactivationenergy[46].

Inaddition,thehydrogen-terminateddiamondsurfaceinducesap-typechannelin
closeproximitytothesurface.This2DHG1-likechannelisgeneratedwithoutimpurity

1gashole2-dimensional

13

14

diamondofgDopin

doping,buttheoriginoftheacceptorisstillunderdiscussion.Incontrast,oxygenter-
minationofthediamondsurfaceinduceselectronicsurfacestateswithinthebandgap,
pinningthesurfaceFermilevelandcausinganinsulatingdepletionlayer[47].

Finally,thereistheion-implantationtechnique[48],whichiswidelyusedinsemicon-
ductortechnologyforbothn-andp-typedoping.Ion-implantationisahighenergy
processandisaccompaniedbyseveredamagetodiamond.Thebrokensp3bondsmay
afterannealingreformtosp3bondsortheymaytransformtomorestablesp2graphitic
bonds[49].Duetothemetastablenatureofdiamondthisdopingtechniqueisrather
complicatedandwillnotbediscussedfurther.

Figure3.1:Dopingconfigurationofphosphorous,nitrogenandboronindiamond.

dopingp–type3.1

Duetothelackofshallown-typedoping,highperformanceelectronicsondiamond
islimitedtoap-typedoping.Thus,bipolardevicesaregenerallyoutofquestionand
unipolardevicesrelyoneitherborondopingoronhydrogen-inducedchannel.

dopingBoron3.1.1

Boronistheonlytechnicallyrelevantacceptorindiamond.Duetoitsactivationenergy
of0.37eV,boronisonlypartiallyactivatedatR.T.However,itcanbedopedhigh
enoughtoformaminiband,whichstartstooverlapwiththevalencebandandwill
lowertheactivationenergy,thusenablingtunnelingcontacts.Withincreaseddoping
beconcentrationachievedatofboronconcentrationstheactivationabove1020enercmgy−3[decr50].eases,SimilaranddecrfulleaseactivationoftheatR.Tactivation.can
energywithincreaseofborondopingconcentrationwasobservedinSiandGe[51],
by:givenisand

dopingp–type3.1

Eakt(NAeff)=E0−α∙(NAeff)1/3

15

(3.1)

NAeffistheeffectiveacceptorconcentrationandthisdependency,basedonexperi-
mentaldata,isshowninfig.3.2.Thediamondlayerisbecomingquasi-metallicat
concentrationabove2x1020cm−3.Ontheotherhand,themobilityisdecreasingwith
increasingdopingconcentration,duetothescatteringatthedopantatoms,andatvery
highdopinglevelsthemobilityisgivenbyhoppingconductionmechanism.Thishas
tobetakenintoaccountinFETswithboronδ-dopedchannel.Theδ-dopedFETdevice
conceptwillbedescribedinchapter5.2.

Figure3.2:Activationenergyofboronasafunctionoftheeffectivedoping
concentration(NA-ND),after[52].Thefitusesα=4.5×10−8eVcm.

Theboronactivationenergyisatlowerdopingconcentrationtemperaturedependent,
andafullactivationcanbealsoobtainedatR.T.atconcentrationsbelow1013cm−3[53].
Sincethebreakdownvoltagedependsontheconcentration,alowdopingconcentra-
tionisimportantfordiodesoperatingatveryhighvoltages,wherehighpuritylayers
arenecessarytoobtainhighbreakdownvoltages.

BorondopedlayerscanbegrownbymicrowaveplasmaCVDfromgasorsolidphase.
Diborane(B2H6)andlesstoxictrimethylboron(B(CH3)3orTMB)canbeusedasgas
phaseprecursors[54–57],whileB2O3[58]oraboronrod[16]canbeusedassolid
ce.sourphase

InaFETstructure,thechannelsheetchargeislinkedtotheacceptorconcentration.
Thechannelneedstobefullydepletedbythegatewithinthebreakdownstrengthof

16

diamondofgDopin

diamond,thusthesheetchargeinthechannelneedstobelimited.Ontheotherhand,
theneededfullactivationatR.T.canbeachievedathighdopingconcentrations.These
requirementsresultinachannelthatisonlyafewmonolayersthick(δ-dopedchan-
nel),whichinturnrequiresprocesswithamonolayergrowthprecision.Suchsharp
profilescannotbeeasilygrownusingagasphasedopingsource,duetolongertime
constantsinthegastransport.Systemswithagasdopingsourcearenormallyused
forgrowthofthicker,highlyborondopedlayers.Unfortunately,suchhighdoping
concentrationsdegradethecrystallineperfection,becausethetightdiamondstructure
isunabletoaccepthighimpurityconcentrations,andasaconsequencedefectsare
formed.Furthermore,thesurfacerougheningduetoheavyboronincorporationis
significant[59].Anotherimportantaspecttoconsideristheincorporationmechanism
ofboronintosubstitutionalsites.Increasedmethaneconcentrationinthegasphase
enhancesthegrowthrateandtheefficiencyofboronincorporationintothediamond
crystal.Atthesametime,thecrystallinequalityofsuchfilmsbecomesinferiorwith
increasingmethaneconcentration.Ontheotherhand,lowmethaneconcentrations
willresultsinaatomicatsurfacewithhighmobilityattheexpenseoflowdoping
concentrations,andthuslowcurrentlevels[60].Thesolutionofthisproblempresents
theδ-dopingconcept,whichcanprovidelargeamountofchargecarrierscombined
withhighmobility.Thiscanbeachievedbyspatialseparationoftheacceptorions
fromthefreecarriers.Athickerdopedlayercanbeobtainedbystackingofmultiple
δ-dopedlayers.Here,itisimportantthattheboronδ-dopedlayersareseparatedby
undopedintrinsicdiamondspacers.Thishastwoadvantages:
1.Thehighlydopedδ-layerwillmovetheFermi-leveltowardsthevalenceband,thus
decreasingtheactivationenergy.
2.Theexcitedholeswillmoveintotheintrinsiclayer,wheretheycanmovewithvery
mobilities.high

Theδ-dopingconcepthasbeenforthefirsttimeanalyticallystudiedin[61],andlater
ledtorealizationoffirstδ-dopedFETs[16,17].Theseproof-of-conceptdevicessuffered
fromtechnologicalchallenges,suchashighresidualdopingconcentrationinthein-
trinsiclayer,andthustheirabilitytomodulatethechannelwaslimited.Afterthefirst
devicesweredemonstrated,thetechnologicalrequirements(especiallytheextremely
narrowwindowfortheδ-profile)putthisconceptontheback20burner.−3Tolimitthe
spacechargeofthedopedlayerwithapeakconcentrationof10cm,thespike
whichcanbedepletedwithoutbreakdownofthediamondlayerneedstobeconfined
toafewmonolayersonly.Thiscanonlyhardlybeachievedbyagassource,thusasolid
phasesourceinformofaboronrodisnecessary.Suchboronrodcanbemovedvery
fastinandoutoftheplasmaofaMPCVDsystem,andthusverysharpprofilescan
begrownthisway.Thecontrolofthethicknessanddopingconcentrationofthespike
requirestightlycontrolledgrowthconditionsandawellthought-outgrowthsequence.

dopingp–type3.1

17

Thedopingconcentrationisextremelydifficulttocontrol,andpartialetchingofthe
channelmaybenecessarytoadjustthechannelsheetcharge.Despitethesechallenges
multipleboronδ-dopedlayerswithintrinsicspacershasbeensuccessfullygrown[18],
whichwasconfirmedbyERD2measurements,asshowninfig.3.3.

Figure3.3:ERDmeasurementshowing4boronδ-dopedlayerswithintrinsicspacer
layersgrownbetweenthem,after[18].The4δ-spikedwereconfinedwithin30nm.

Theserecentadvantagesinthegrowthofδ-dopedlayersopenthepossibilitytorevisit
oldconceptsandtofabricatehighpowerdiamondFETs(deviceswithfieldplatesup-
portingthedriftregionbetweensourceanddrainasdescribedinchapter5.2),orto
exploresomenewdeviceconceptssuchasstackedδ-dopeddiodessupportingvery
highbreakdownvoltages[62].

3.1.2p-typechannelinducedbyhydrogentermination

Thesurfaceconductivityofthehydrogen-terminateddiamondsurfaceisawellknown
phenomenon[63].Hydrogenterminationofdiamondresultsinap-typeconduc-
tivechannelincloseproximitytothesurface,withasheetcarrierdensityofabout
1013cm−2,whichisalmostconstantfrom70to400K[64,65].Thehydrogenated
diamondsurfaceexhibitsalowdensityofsurfacestates,whichmakestheSchottky
barrierheightofthemetal–diamondcontactstronglydependentonthemetalwork-
function[22,66].WithintheSchottky-limit(densityofsurfacestatesisnegligible)[67],
2nDetectioRecoilElastic

18

diamondofgDopin

theSchottkybarrierheightΦBisdeterminedbytheworkfunctionofthemetalΦM,
electron-affinityofthesemiconductorχandbandgapofthesemiconductorEG:

ΦB=EG+χ−ΦM

(3.2)

Furthermore,thebarrierheightdependsontheelectronegativityofthemetal[22].
Thus,aninterfacedipoldeterminedbythisdifferenceisinuencingtheSchottkybar-
rierheight[68].Thismeansthatdifferentmetalscanbeusedtofabricateohmicand
Schottkycontactsonahydrogen-terminateddiamondsurface.Ontheotherhand,the
oxygen-terminateddiamondsurfaceisduetothesurfacepinninghighlyinsolating
[22],andcanbeusedfordeviceisolation.Thesepropertiesofthediamondsurface
havebeenusedforthefabricationofMESFETs[27,69–74],MISFETs[75–78]andFET
structuresincontactwithliquids[79–81].

Hydrogenplaysanessentialroleintheformationoftheconductivechannel,and
becausehydrogenexhibitsacomplexbehaviorinsemiconductors(Hinuencesthe
surfacereconstruction,terminatesdanglingbonds,passivatesshallowanddeeplevels
[82]),anunderstandingofitsroleisofparticularimportance.Moreover,theoriginof
thesurfaceacceptorsofthe2DHG-likechannelandtheexactenergybandprofilenear
thediamondsurfaceisstillunderdiscussion.Atpresent,thereareseveralmodels
describingtheoriginoftheholeaccumulationlayer.Atfirst,itwasproposedthat
hydrogenpassivationofdeeptrapsisresponsiblefortheincreaseofconductivityin
thediamondfilm[83,84].Therectifyingpropertiesofmetalcontactsindicatedp-type
conductivity[66,85],whichwasconfirmedbySeebeckeffect[86]andHall[87,88]
measurements.Lateron,itwassuggestedthatthehighconductivityisrelatedto
hydrogenincorporationintothediamondsub-surfaceregion[89].Thiswasbasedon
theobservationofdiffusionofhydrogenintothediamondnear-surfaceregionafter
hydrogenplasmatreatment[65,87,90,91].Energybandsimulationsindicatedthat
thepresenceofdefectstatesandshallowacceptorsinthesub-surfaceregioncanplay
keyrolesintheconductivityofdiamond[92].Ontheotherhand,STM3observationof
thehomoepitaxialdiamondsurfaceindicatedthepresenceofsurfacehydrogenatoms
[93].Hydrogenmayinduceacceptor-typesurfacestates,which,whenlocatedbelow
theFermilevel,arenegativelycharged.Tosatisfysurfacechargeneutrality,anupward
bandbendingisneeded.Thiscausestheaccumulationofholesinthesub-surface
region,andconsequentlytheformationofaconductivechannel[93].Asimilarap-
proachsuggeststhatnegativelychargedadsorbatesreducethechargetrappedbythe
surfacestates,resultinginanupwardbandbending.Theincreaseoftheconductivity
waslinkedtochargedadsorbates,whichinessencebehavesimilarlytoaforward
3ScanningTunnelingMicroscope

p–type3.1doping

19

biasedSchottkyjunction[94].Anothermodeltakesintoaccountthedifferencein
electronegativitiesofcarbonandhydrogen[95].Thisdifferenceleadstotheformation
ofC–Hdipolesandtoaspontaneouspolarization[96](similarpolarizationiswell
knowninothermaterialsystems,suchasAlGaN/GaN[97]).Thepositivelycharged
hydrogen-terminatedsurfaceattractsnegativelychargedadsorbatesfromair,whichin
turnresultsinformationoftheholeaccumulationlayer[98].Finally,yetanothermodel
involvesanaqueousadsorbatelayerformedonthesurface.Inthiselectrochemical
model,thereductionofhydratedprotonsinanaqueoussurfacelayergivesriseto
aholeaccumulationlayer[99].Here,anacceptorisassociatedwithasurfaceadsorbate
withsufficientlylargeelectronaffinity,sothataholecanbeinducedintothevalence
bandofdiamondviachargetransferreactions[100].Theproposedcauseforthe
conductivityisatransferdopingmechanism[101],wheretheelectronsaretransferred
fromdiamondintoathinwaterlayer,whichformsnaturallyonsurfacesexposedto
atmosphere.Inthismodel,hydrogenationrisesthevalencebandmaximumofdia-
mondabovethechemicalpotentialofthewaterlayeratthesurface.Aredoxreaction
inanadsorbedwaterlayerprovidesanelectronsink,andthuselectrontransferfrom
diamondthenaccountsfortheholeaccumulationlayer[102].Overallchargeneutrality
ismaintainedbythenegativelychargedanionswhichremaininthewaterlayer[103].
Inanotherexperiment,C60moleculeswereevaporatedontothehydrogen-terminated
diamondsurface,andaholeconductivityhasbeenobserved.Thetransferdoping
mechanismproposesthattheelectronsaretransferredfromdiamondtotheC60layer
[104]andthatthisdopingmechanismdoesnotrequiretheintroductionofforeign
lattice.diamondtheintoatoms

Onestraightforwardconclusionofthetransferdopingmodelisthatthep-typechan-
nelislocateddirectlyatthediamondsurface.However,suchconfigurationwould
leadtoastrongtunnelingcurrentbetweentheH-terminateddiamondandthemetal
contactregardlessofthemetalwork-function.NorectificationofSchottkydiodesand
nooperationalMESFETdeviceswouldbepossible.Tobridgethiscontradiction,it
hasbeenproposedthatthehydrogen-inducedp-channelisseparatedfromdiamond
surfacebyafewnanometerthicklayer[105],asshownintheprincipalconfiguration
infig.3.4.Thislayerallowsthecompletedepletionofthehydrogen-inducedchannel
byaSchottkycontact,butpreventsthetunnelingthroughthebarrieratforwardbiases.
hydrIndeed,ogen-inducedelectricalcharacteristicsconductiveofchannelsurfaceischannelseparatedFETfrsomonthediamondgatemetalindicatecontactthattheby
aclearthinyet.Itinsulatingmaybelayer[associated106].withHowevera,highthedensityphysicaloforiginhydrofogenthisatomsdielectricdiffusedlayerintoisthenot
sub-surfaceregionofdiamondduringtheplasmatreatment,withthesurfaceadsor-
canbatesbeorafanfectedaluminiumbychangingdioxidethelayer.atmosphericThechargeofconditionsthe[105adsorbate].rWhenelatedinsurfacevacuum,statesthe

20

ofgDopindiamond

conductivechanneldisappears,whileitrecoverscompletelybysubsequentexposure
toair.Thesurfaceconductivityhasbeenfoundstronglydependentuponthechemical
environmentandatmosphericconditions,andacorrelationbetweenthechangesofthe
electricalpropertiesandchangesofthesurfaceadsorbateconfigurationwasobserved,
whichimpliesthatanacceptorisneeded.Morestudyisneededtoclarifytheexact
natureoftheconductivechannel.

Figure3.4:Hydrogen-terminateddiamondwithap-typechannelincloseproximityto
thesurface.Theoriginoftheacceptorisstillunclear.

dopingn–type3.2

Theinterestinn-typedopeddiamondhasincreasedinthepastyears,duetoitspos-
sibilityofuseinbipolardevices,LEDs,coldcathodeelectronemitters,etc.Thetwo
mostpromisingcandidates,phosphorousandnitrogen,havedeeptoverydeepdonor
levelsindiamond[107].Anothercandidateissulfur,whichshouldbeshallowerthan
phosphorous[108],howeverthereareonlyfewreportsaboutn-typeconductivityof
sulfur-dopeddiamond[109].Whileitseemstobeveryhardtoincorporateimpurities
oflargeatomradiusintosinglecrystallinediamond,ithasbeenfoundthatultra-nano-
crystallinediamondcanbedopedwithnitrogenwithoutnoticeableactivationenergy
[46].Nitrogenispreferentiallyincorporatedintothegrainboundariesnetwork,as
describedinmoredetailinchapter8.1.AlthoughUNCDmaterialisnotsuitedforhigh
performanceapplicationsitcanfinditsuseinbiomedical[110]orhightemperature
applications[30].Moreover,conversionoftheconductivityofboron-dopeddiamond
fromp-ton-typeupondeuterationhasbeenreported[111].Inthisprocess,atfirst
boronatomstrapdeuteriumandarepassivated,whileadditionaldeuterationcreates
excessofdeuteriumandanexcessbyfactorof2triggersthen–typeconductivity[112].
However,thethermalstabilityofsuchadonorislowerthan200°C[113],whichwill
applications.potentialitslimit

dopingn–type3.2

dopingNitrogen3.2.1

21

Nitrogenisthemostcommonimpurityinnaturaldiamond,andthusanaturalcandi-
dateforasubstitutionaldonorindiamond.ItcanbefoundinHTHPdiamondsand
isresponsiblefortheyellowcolorofthesestones.Nitrogencanbeincorporatedeither
insmallaggregatesordilutedinsubstitutionalsitesofthediamondlattice.Unfor-
tunately,duetothedeeplevelof1.7eVbelowtheconductionbandminimum[114],
suchdiamondsareelectricallyinsulatingatR.T.Nevertheless,ithasbeenreported
thatnitrogenincreasesthediamondgrowthrate[115],nitrogen-dopeddiamondlayers
havebetterfieldemission[116],anddespiteitsrelativelydeepdonorlevel,anitrogen
dopedsubstratecanalsocompensatetheacceptorstatesexistinginap-typesurface
channel[117,118].

Ontheotherhand,ithasbeenreportedthattheconductivityofUNCDfilmsincreases
byasmuchasfiveordersofmagnitudewhennitrogenisaddedtotheplasmaduring
diamondgrowth[46].UNCDiscomposedofdiamondcrystallites2–5nmindiameter
withalargenumberofgrainboundaries.Thesegrainboundariesarecomposedmainly
ofdisorderedcarbonwithamixtureofsp2andsp3bonds.Nitrogenisincorporated
intothegrainboundariesandnewelectronicstatesareintroducedintothebandgap
ofdiamond[119].Thisdoesnotrequirethesubstitutionalnitrogendonorandwill
occurbecausethenitrogenisincorporatedintothegrainboundaries.Withincreasing
concentrationofnitrogeninthegrainboundaries,semimetallicconductionstartsto
appear[46].Additionally,whenUNCDisdopedwithnitrogen,thegrainsarelarger
andgrainboundariesarewider,whichmaycreateadditionalconductionpathways
[120].Theincorporatednitrogenatomsinduceshallowdonorlevelsandthedensityof
theseshallowdonorsincreaseswithincreasingconcentrationofnitrogen.Filmsgrown
byMPCVDinargonrichplasmawith10%ofnitrogenexhibitashallowlevelpeakwith
anactivationenergyof0.05eV[121].Thegrainboundariesconductionisresponsible
forthehighelectricalconductivity,whichisthehighestn-typeconductivityreported
sofar.Thismaterialhasbeenusedincombinationwithborondopeddiamondto
fabricatehightemperaturestablepnjunctiondiodes[30,122],asdescribedinmore
.8chapterindetail

dopingPhosphorous3.2.2

Comparedtonitrogen,phosphorousisrelativelyshallowdopantwithanenergylevel
of0.6eV[123]belowconductionbandminimum.Phosphorouswasthoughttobe
themostsuitablen-typedopant,howeveritssmallsolubilityindiamondsignificantly
complicatesitsincorporationintothediamondlattice.Itcanberelativelywellin-

22

Dopindiamondofg

corporatedinto(111)-orienteddiamondusingphosphine(PH3)asdopantgassource

[124–126].However,the(111)-orientationpresentsabigdisadvantagefordevicefab-

rication.The(111)-orientedsurfaceisdifficulttopolish,thesubstratesareextremely

limitedinsizeandveryexpensive,whichmakesittechnicallyirrelevant.However,

recentlyphosphoroushasbeensuccessfullyincorporatedinto(001)-orienteddiamond

films[127,128].Theelectricalpropertiesarecomparabletovaluesreportedfor(111)-

orientation,

but

still

significantly

lower

than

that

of

on-dopedbor

diamond

films.

4Chapter

diamondofGrowth

Overthepasttwocentauries,manyattemptshavebeenundertakentosynthesize
diamondartificially.Thisprovedtobeverydifficult,becauseatroomtemperature
andatmosphericpressure,graphiteisthethermodynamicallystableformofcarbon.
Diamondandgraphiteareseparatedbyalargeactivationbarrier,whichispreventing
cantheirnotconversionspontaneouslytotheotherconvertphasetotheundermorenormalstablegraphiticconditions.phase.OnceThusformed,diamonddiamondis
metastable,whichmeansthatdiamondisstablekineticallybutnotthermodynami-
.cally

Basedonthephasediagramofcarbon(asshowninfig.4.1),therearetwopossibilities
howtofabricatesyntheticdiamond.Themajorityofsyntheticsingle-crystaldiamond
substratesisproducedbytheHTHPprocess,whichhasbeenusedtofabricateindus-
trialdiamondsforseveraldecades[129].Inthisprocess,temperatureandpressure
arehighenoughtoreachthethermodynamicallystableregion,underwhichcondition
naturaldiamondisformed.Here,carbonisheatedandcompressedinahydraulic
pressinthepresenceofasuitablemetalcatalystuntiladiamondcrystalisformed.
Suchcrystalsareusedinawiderangeofindustrialapplications,e.g.cuttingtools.
However,thismethodproducesverysmallcrystals(fewmillimetersinsize),which
presetsasignificantlimitationofthisfabricationprocess.

Theotherpossibilityisdiamondgrowthinthemetastablearea,wherebothcarbon
phasesarecreated,thusitisnecessarytoavoidtheformationofthegraphiticphase
duringthegrowth.Thiscanbeachievedinthechemicalvapordepositionprocess
[130],wherediamondisgrownfromagasphaseatmuchlowerpressureandtemper-
aturethanintheHTHPprocess.Furthermore,theCVDprocesshasanadvantagein
termsofenergyconsumption.Thisprocessisusedtofabricatehighqualitydiamond
crystals,whichcanbeusedindiamondelectronics.

23

24

4.1

Figure4.1:Phasediagramofcarbon.

MicrowaveCVDplasma

ofGrowthdiamond

TheCVDprocessinvolvesagasphasechemicalreactionoccurringabovethesubstrate
surface,whichcausesthedepositionofcarbonontothatsubstrate[131].TheCVD
techniquerequiresameansofactivatingofthecarbon-containinggasfollowedby
formationofactivehydrogenradicalsH*.Thiscanbeachievedforexamplebythermal
means(e.g.hotfilament)orbyelectricdischarge(e.g.microwaveplasma).

HotfilamentCVD(HFCVD)useselectricallyheatedtungstenfilaments.Thismethod
isrelativelysimpleandcheap,andproduceslargesizefilmswithgoodqualitypoly-
crystallinediamond.However,italsosuffersfromanumberofdisadvantages.The
hotfilamentsreactwiththeprocessinggasesandcarburize,reducingtheirlifetime
andhencethemaximumdepositiontime.Itisalsoimpossibletoavoidcontamination
ofthediamondfilmwiththefilamentmaterial.Thisisnotaproblemformechanical
applications,butitisunacceptableforelectronicapplications.

MicrowaveplasmaCVD(MPCVD)usesanelectricdischargetoactivatethegrowth
species.InMPCVD,themicrowavepoweriscoupledintothedepositionchambervia
aquartzwindowinordertocreateadischarge.Thechambersizeisdesignedsothat
onlyonemicrowavemodecanbesustainedinthechambercavity.Themicrowave
fieldcouplesenergyintoelectronsinthegasphase,whichinturntransfertheirenergy
tothegasthroughcollisions.Thisleadstotheformationofaplasma,heatingand
dissociationofthegasmoleculesintoreactivespecies,andfinallydiamonddeposition
ontoasubstrate.ThisdepositiontechniqueisamuchcleanerthanHFCVD,andthus
itisthedepositiontechniqueofchoiceforelectronicapplications.

growthdiamondofPrinciple4.2

25

AmodifiedmicrowaveplasmaCVDreactor(ASTeX)with2.45GHzRF-sourceused
inthisworkisschematicallyshowninfig.4.2.AsprocessgasesH,CH4andN2are
used.Forp-typedoping,aboronrodcanbemechanicallyinsertedintotheplasma
duringgrowth[16].Thisrodcanbeintroducedveryfastintoandremovedfromthe
plasma,andthusallowingthegrowthofverysharpborondopedprofiles.Considering
thatthesystemisbeingusedbothforgrowthofborondopedandundopedlayers,the
chamberiscontaminatedwithboronduetoamemoryeffect,whichpresentsaproblem
duringthegrowthofundopedlayers,wherethebackgroundboronconcentrationin
theintrinsiclayerisabout1016cm−3.Thisconcentrationcanbepartiallyreducedby
priorovergrowingofthechamberwallswithdiamondorbymechanicalcleaningof
thesystem.Thebackgroundconcentrationdoesnothaveanoticeableaffectonthe
FETswithhydrogen-inducedsurfacechannel,althoughitcanformaparallelconduc-
tivechannelandmaycausebufferleakage[132]whenthebackgroundconcentration
istoohigh.ThisbecomesarealdisadvantageinSchottkydiodes,wherehighpurity
intrinsiclayersarerequiredtoachievehighbreakdownvoltages.

Figure4.2:MicrowaveplasmaCVDsystem.

growthdiamondofPrinciple4.2

ThechemicalandphysicalprocesseswhichoccurduringdiamondCVDgrowthare
schematicallyillustratedinfig.4.3.Theprocessgases(H2withapprox.1%ofCH4)
aremixedbeforetheydiffusetowardthesubstrate.Enroute,theypassthroughan
activationbymicrowaveplasma,whichcausesthatthemoleculesfragmentintore-
activeradicalsoratoms(CH4→CHX→C),createsionsandelectrons,andheatsup

26

diamondofGrowth

thegas.Beyondtheactivationregion,thesereactivefragmentscontinuetomixand
undergoacomplexsetofchemicalreactionsuntiltheyreachthesubstratesurface.At
thispoint,thespeciescaneitheradsorbandreactwiththesurface,desorbbackintothe
gasphase,ordiffuseonthesurfaceuntilanappropriatereactionsiteisfound.Atomic
hydrogenplaysakeyroleintheprocessforthefollowingtworeasons:
1.Danglingbondsatthesurfaceneedtobeterminatedinordertoprohibitarecon-
structionofthesurfacetothegraphiticphase.Thissurfaceterminationisperformed
byhydrogen,whichkeepsthesp3diamondlatticestable.
2.Atomichydrogenisknowntoetchgraphiticsp2carbonmanytimesfasterthan
diamondsp3bondedcarbon.Both,sp2andsp3bondedcarbonisformedduring
growth,andthusitisnecessarytosuppressthegraphiticphase.
Theresultofthisprocessisthegrowthofadiamondlayer.TheCVDgrowthenviron-
mentisquitecomplexandstillnotcompletelyanalyzed.

Figure4.3:SchematicsofprocessesoccurringduringCVDgrowth.

Substrates4.3

Thechoiceofasuitablesubstrateisessentialforthequalityandpropertiesofthe
diamondfilm.Anidealsubstrateforhighperformance,highspeedelectronicshas
tocombineanactivetransistorstructurewithapassivenetwork.Both(FETsand
waveguide)havebeenalreadydemonstrated[133],thusdiamondwithitshighther-
malconductivityisanidealcandidateforhighpowerMMICs,ifwaferscale,single-
crystallinesubstratescanbedeveloped.Thereisasignificanteffortaimedtoachieve
wafers.diamondsinglecrystalline

Substrates4.3

27

SemiconductorgradelayersareusuallygrownbyMPCVDon(100)-orientedHTHP
chips.Suchlayerscanbeseparatedfromtheirsubstratesandhaveyieldedrecord
mobilities[14].Withhighlateralgrowthratethechipsizecanbeexpandedfrom
a4mmx4mmseedtoapprox.1cm2.

Anotherpromisingapproachistheheteroepitaxialdepositionofdiamondoniridium
[134],whichinturncanbedepositedontoSrTiO3[135–137],MgO[56,138,139],sap-
phire[140]orCaF/Si[141]substrates.Iridiumisaverygoodsubstrateforsingle-
crystallinediamondwafers,theatomicspacingoftheiridiumcrystalcloselymatches
thatofdiamond,butiridiumitselfmustbedepositedontoanothersubstrate.With
largewafersizeavailabilityandaverylowthermalmismatch,siliconmaybeaattrac-
tivesubstrateforiridiumgrowthaswell.

4.3.1SinglecrystallineCVDdiamondgrownonHTHPstones

Uptonow,vastmajorityofstate-of-artelectronicdeviceswasfabricatedonHTHP
stones.Although,theywillnotallowwaferscaledevicemanufacturing,thesestone
areanattractivewaytodemonstratediscretehighperformancediamonddevices.

Figure4.4:HTHPIbdiamondsubstrateswithsize4mmx4mm.Thestoneontheleft
sidecontainsfabricatedFETstructures.

AfewoftheseHTHPstoneswithnitrogenimpurities(whichareresponsibleforthe
yellowcolor)areshowninfig.4.4.Nitrogencanbeincorporatedeitherinsmall
aggregates(typeIa-highamountofN)ordilutedonsubstitutionalsitesinthedia-
mondlattice(typeIb-lowamountofN),andhasadirecteffectonthegrowthand
electricalresistivityofdiamond(seetable4.1forcompletetypeclassification).Inthis
work,mainlyIbHTHPstoneswereusedasasubstrateforhomoepitaxialdiamond
growth,whereFETsanddiodeswerefabricatedontopofthisepitaxiallayer.

28

diamondofGrowth

ColorConcentrationImpurityypeTIaN>100ppmbrightyellow
IbN1-100ppmyellow
IIaN<1ppmcolorless
IIbB>1ppmblue

Table4.1:Diamondclassesbasedonimpurityconcentration.

4.3.2SinglecrystallinediamondgrownonIr/SrTiO3substrate

HighperformanceMESFETshavebeenalsofabricatedondiamondquasi-substrate
grownonaSrTiO3ceramicsubstrateusinganiridiumbufferlayer[26]grownby
M.Schreck(UniversityofAugsburg,Germany).Onthisquasi-substrate,DC,small
signalandpowermeasurementcouldbeperformedforthefirsttime,asdiscussedin
chapter6.Thisquasi-substratewithfabricatedFETstructuresisshowninfig.4.5,the
sizeofthesubstrateisapprox.4timesbiggerthanthesizeoftheHTHPstones.

Figure4.5:Diamondquasi-substratewithfabricatedFETstructures,after[26].Dueto
highthermalstressitdelaminatedfromtheSrTiO3substrateafterthegrowth.

Thegrowthprocessofadiamondlayeroniridiumstartswithbiasenhancednucle-
ation(BEN)[135].Iftheindividualcrystallitesarewellalignedonthesubstrate,the
graincrystallineboundariesdiamond[between142]withthergrelativelyowingfewcrystallitesdefects(seeeventuallyfig.4.6).mergetoformsingle-

Togrowafreestandingdiamondlayerwithasufficientthickness,thissamplewas
grownatanincreasedrateofabout1µm/htoreducethegrowthtime.Duringthis
growthprocesstheα-parameter[143]waschanged,increasingthepossibilityofnon-
epitaxialdefects.Suchadefectisshowninfig.4.7.Thesedefects,whicharenotpresent
inthefilmsgrownatlowertemperatures[135]areattributedtotwins.

Substrates4.3

29

Figure4.6:Diamondquasi-substrateshowsnearlysingle-crystalsurfacewithisolated
defectareasofunorientedgrowth,after[26].

Figure4.7:Non-epitaxialdefectindiamondquasi-substrategrownonIr/SrTiO3buffer
layer,after[26].

Unfortunately,thedepositionofdiamondonIr/SrTiO3isplaguedbyathermalmis-
matchproblem.Whenthediamondfilmiscooledtoroomtemperature,thedifferent
thermalexpansioncoefficientsofiridiumanddiamondcreatethermalstress[144]that
mayleadtodelaminationofthediamondfilmfromthesubstrate.Andindeed,dueto
thelatticeandthermalmismatch,thisfilmdetachedfromthetemplateaftercooling.
Themismatchproblemhaslimitedthesizeofthisfreestandingquasi-substrategrown
onaSrTiO3ceramicsubstratetoapprox.0.6cm2andleadtocrackscausedbythe
uncontrollablelift-offprocess(seeveryleftsubstrateinfig.4.9).Theseverestress
problemmaypointtowardstheuseofamaterialssystembasedonSi,aspointedout
.4.8fig.in

Providedthattheepitaxialnucleationprocesswillbefurtherimproved,onecanex-
pectdiamondquasi-substrateswithpropertiessimilartotheseofsyntheticHTHP

30

Growthdiamondof

Figure4.8:ThermalmismatchbetweendiamondonSrTiO3anddiamondonSi,after
].72[

substrates.Thus,substratesoflargersizesmaybeavailableatlowercostinthenear
futureandmayallowwaferscaledevicedevelopment.Fig.4.9givesanimpressionof
substrates.singlecrystallineavailable

Figure4.9:Differentsinglecrystallinediamondsubstrates.Fromrighttoleft:(111)-
orientedHTHPsubstrate(3x3mm2)fromSumitomo,commerciallyavailable(100)-
orientedHTHPsubstratewithfabricateddevices(4x4mm2),CVDsubstratesupplied
byApolloDiamond,USAwithfabricateddevices(5x5mm2)andquasi-substrate
grownonIr/SrTiO3(0.6cm2)suppliedbyUniversityofAugsburg.

4.3.3NanocrystallinediamondgrownonSisubstrate

CVDdiamondfilmsgrownonSiwafersareduetotheirpropertiesandwafersizes
(seefig.4.10)consideredformanyapplicationsinmechanics,biochemistry,etc.The
combinationoftheseapplicationswithelectronicdeviceswouldenableintegratedin-
situsignalprocessingandthusthedevelopmentofsmartsensorsandactuators.In

Substrates4.3

31

ordertoinvestigatesuchintegrationconcepts,surfacechannelFETswerefabricated
(NCD).diamondnanocrystallineH-terminatedon

Figure4.10:4diameternanocrystallinediamondcoatedSi-wafer(intheback,
showingareectionofalamp)comparedtoapprox.2x2cm2waferpiececutfrom
suchasubstrateafterthegrowthofthediamondlayer(inthefrontright)andsmall
sized(4x4mm2)HTHPsingle-crystalsubstrate(inthefrontleft).Bothsubstrates
showninthefrontcontainfabricatedFETstructures.

AhotfilamentCVDprocesswithin-situBENpretreatmenthasbeenusedforthe
growthofNCDfilmson4Si(100)substrates.Thenanocrystallinelayersgrownat
ourlaboratoryusedaprocessdescribedin[145].Toavoidapolycrystallinegrowthof
largecrystallites,ahighrenucleationduringthegrowthprocesswasnecessary.This
resultedinacolumnarfilmstructure,wherethelateraldimensionsofthediamond
crystallitesweresmallerthan300nm,whereastheverticalextensioncanreachseveral
ometers.micr

Despitethesignificantamountofgrainboundariesinthenanocrystallinediamond
film(asobservedbyAFM1andshowninfig.4.11),fulloperationoftheFETscould
beobtained[146].DC,CVandsmallsignalmeasurementshavebeensuccessfullyper-
formedondeviceswithgatelengthscomparabletothegrainsizesofnanocrystalline
diamond(300nm),asdiscussedinchapter6.6.

1AtomicForceMicroscope

32

eFigur

4.11:

ofoughnessr

.topology

AFM

image

of

nanocrystalline

diamond

thesample,thehydrogen-inducedchannel

Fully

operational

sFET

ewer

successfully

Growth

surface.

followed

fabricated

on

of

Despite

diamond

the

high

surfacetheclosely

such

a

substrate.

5Chapter

ConceptsfordiamondFETsanddiodes

Duetoitsuniqueelectricalproperties,diamondseemstobeanidealmaterialfor
highperformanceelectronics.Althoughnoothermaterialcanmatchthepredicted
capabilitiesofdiamond,duetosometechnologicalrestrictionconcerningthelackof
largeareasubstratesandsuitablen-typedoping,diamondelectronicsisstillinanearly
developmentphase.Nevertheless,recentprogressinthediamondgrowthanddoping
openednewpossibilitiestoinvestigatedifferentdiamondelectronicconcepts.Upto
now,severalactiveandpassivedeviceconceptshavebeenexplored(seefig.5.1).
Consideringthedopinglimitation,therearetwomainareasofinterestintermsof
highpowerelectronics:highpower,highfrequencyfieldeffecttransistorsandhigh
diodes.voltage

WhenconsideringdiamondFETs,twodifferentapproachesareactivelyunderde-
velopment.Oneemploystheuniquepropertyofthehydrogen-terminateddiamond
surface,wherethesurfaceterminationinducesap-type2DHG-likeconductivesurface
channelwithoutextrinsicimpuritydoping.Thischannelwithasheetchargedensity
inthe1013cm−2rangeisfullyactivatedatR.T.Althoughthenatureofthischannel
isstillunderinvestigation,twodifferentFETgateconfigurationsarepursued:MES-
FETandMISFET.ThisworkfocusesonthesurfacechannelMESFETconfiguration
[26,27,71–73,106,146].TheotherFETapproachisbasedonanactivelayerdesign
andisfocusedonthin,highlyborondopedlayers.Inthisconcept,thechallenge
istofabricateachannelthatissimilartothehydrogen-inducedchannel,butwitha
carefullytailoreddopingprofile,whichwillallowtheimplementationofafieldplate
andgaterecess[23].AlthoughthisisnotpossibleintheplanarsurfacechannelFETs,
onlythisdeviceconfigurationhasdemonstratedcut-offfrequenciesinGHzrangeso
far.Ontheotherhand,theoreticalevaluationsshowlimitedpowerperformanceof
planardevicescomparedtoδ-dopedFETswithrecessedgateandfieldplate[62].The

33

34

ConceptsfordiamondFETsanddiodes

Figure5.1:Diamondelectronicconcepts.

technologicalchallengesandexpectedperformanceoftheseboronδ-dopeddevices
willbeaddressedaswell.

Intermsofpowerdiodes,Schottkydiodeswithhighpurityintrinsicactivelayers
havedemonstratedhighblockingvoltagesinthekVrange[21].However,thecar-
rierconcentrationintheselayerswaslow,andtheforwardcurrentwaslimitedby
thespacechargecurrentregime.Recentadvantagesinδ-dopingcouldsignificantly
improvetheon-resistancewhilepreservinghighbreakdownvoltagesofsuchdiodes
[62].Furthermore,withthepossibilityofanitrogendopedUNCDlayersall-carbon
pnjunctiondiodecanberealized.Thereplacementofarefractorymetalwithther-
mallystableUNCDmaterialhasallowedsuccessfuloperationofsuchdiodeat1000°C
withoutnoticeabledegradation[30,122].Thosetwodiodeconfigurationsdemonstrate
thatdiamonddiodescansuccessfullyoperateathighvoltagesandhightemperatures.
Basedontheseresultsanotherdiodeconfigurationemerged.Theconceptofadia-
mondmergeddiode(combinationofaSchottkycontactwithapnjunction)enables
tocombinelowforwardthresholdvoltagewithlowreverseleakagecurrentandhigh
breakdownvoltage.Furthermore,suchadiodecanalsooperateatveryhightempera-
tures[32].Thisdeviceconceptwillbediscussedinmoredetailaswell.

Surface5.1FETchannel

FETchannelSurface5.1

35

Uptonow,themostsuccessfuldiamondFETsapproachisbasedonthehydrogen-
terminatedsurface.Thisterminationinducesap-typechannelofhighcarrierdensity
incloseproximitytothesurface.Thehydrogen-terminateddiamondsurfacepossesses
astrongdipolemomentoftheC-Hbond[95],asillustratedinfig.5.2.Thesurfacehy-
drogenispositivelycharged,withmostofitselectronicchargebeingtransferredtothe
carbonatombelow.Hydrogenatomssaturatealldanglingbonds,whichreducesthe
surfacestatedensity,thustheH-terminatedsurfaceisunpinned.Ontheotherhand,
theoxygen-terminatedsurfaceishighlyisolatingduetosurfacepinningat∼1.7eV
abovevalenceband[147,148],thusO-terminationcanbeusedfordeviceisolation.

Figure5.2:H+C−dipoleondiamondsurface.

ThecharacteristicsofsuchFETscanwellbefittedwithHFETorMOSFETmodelswith
afewnanometerthickseparationdielectriclayer[70,105,106,146,149,150].Thislead
tothehypothesisofahydrogen-inducedchannelwithasurfacebarrierlayer.From
capacitance–voltagemeasurementsperformedonMESFETsfabricatedonhydrogen-
terminatedsingle-andnanocrystallinediamond,theextractedthicknessofthebarrier
layerwasonlyfewnanometersandthecapacitancewasaround1µF/cm2.Thepro-
posedbanddiagramofsuchachannelisshowninfig.5.3.

Duetotheunpinnednatureofthehydrogen-terminateddiamondsurface,thebarrier
heightofthemetal-diamondcontactsstronglydependsonthemetalworkfunction
[22].Thus,ohmiccontactscanbefabricatedusingmetalswithhighworkfunction
(suchasgold),whilealuminumcanbeusedforSchottkycontacts.Alwillcompletely
pinchoffthechannelat0Vgatebias,thustheFETdevicesoperateinenhancement
mode.Thecross-sectionofaMESFETwiththep-typechannelisshowninfig.5.4.

36

ConceptsfordiamondFETsanddiodes

Figure5.3:Simplifiedschematicbanddiagramofhydrogen-terminateddiamond
surfacewithap-typeconductivitychannel.

Figure5.4:Cross-sectionofaMESFETwithhydrogen-inducedchannel.

FETchannel-dopedδ5.2

BecauseofthesimpletechnologyofsurfacechannelFETs,allhighfrequencyFET
resultshavebeenobtainedonH-inducedchannelMESFETsandMISFETs.Thework
onδ-channelFETswasmainlyfocusedontheevaluationofcriticalpartsofthedevice
andonthedesignofanoptimumpowerFETconfiguration[62]basedonconventional
powerdesign.SuchapowerFETstructurebasedonarecessedgateandfieldplate
conceptisshowninfig.5.5.Firstδ-channelMESFETswithaSchottkygate[151]and
JFETswithaboron/nitrogenjunctionhavealreadybeenrealized[16].However,these
FETsaretechnologicallydemanding,andtheactualdeviceperformanceisbehindthe
expectations.

Highpowerstructuresneedanextendeddriftregionbetweengateanddrainsup-
portedbyafieldplate.Suchstructureshavebeenevaluatedbasedontheδ-channel
approach,whichprovideschemicallyandthermallystablechannel.Atconcentrations
above1020cm−3allboronacceptorsinthechannelarefullyactivatedandthechannel
conductivityistemperatureindependent.Inthiscase,thesheetchargedensityand
than3consequentlyx1013thecm−2,thicknesswhichofwillthetranslatechanneltoarealimitedchannelbythethicknessbrofeakdownlessfieldthanto3lessnm.
Masteringthediamondepitaxywillallowtogrowanarrowfewmonolayersthin

channel-dopedδ5.2FET

37

Figure5.5:Cross-sectionofaδ-dopedchannelFET.ThispowerFETconceptemploys
recessedgateandgatefieldplate,basedafter[23].

boronchannel,wherethecarriersdiffuseoutofthedopingspikeintotheadjacentun-
dopedarea,asindicatedinfig.5.6.Theholetransportwillthenoccurintheundoped
regions,wherethecarriermobilityissubstantiallyhigherthaninthehighlydoped
material[61]aslongastransitiontailsbetweenthedopingspikeandtheundoped
materialremainsteeperthantheDebyetail.

Figure5.6:Profileofaδ-dopedlayer.

InordertoeliminatefieldspikesinthegateregionofapowerFET,whicharerespon-
sibleforprematurebreakdown,agatefieldplatethatallowshigherdrainbiasand
higherpowerisnecessary.Thegoalistoseparatetheverticalfieldcausedbythegate
biasfromthelateralfieldcausedbythedrainbias.Extendingthegatefieldplate
towardsthedrainallowstomovethemaximumfieldawayfromgate(asshownin
fig.5.7),whichwilllowerthelateralfieldandallowhigherdrainbias.

Moreover,agaterecessisneededtoprovideaneffectivemodulationofthesheet
chargedensityinthechannelbymovingthegateclosertothechannelandawayfrom
surfaceeffectsanddepletioncausedbytheoxygenterminationofthesurface.These

38

ConceptsfordiamondFETsanddiodes

Figure5.7:Advantagesofaδ-dopedchannelFETconcept.Theseparationofthe
maximalelectricalfieldunderthegatecontactcanbeachievedbythefieldplate.The
depletionofthechannelcausedbyoxygenterminationofthesurfacecanbeavoided
byarecessedgate,basedafter[52].

conceptsweretheoreticallyevaluatedbyA.Denisenkoetal.[62]indicatingthatan
optimizeddevicestructurewithdraincurrentdensityof2.5A/mmcansustainup
toabout250Vdrainvoltage[53].Thiswouldrepresentamaximumoutputpower
densityof75W/mm,whichmeansthat150Wcouldbegeneratedwithadevicewith
2mmgatewidth.Thiswillofcourserequireanintegrateddiamondheatspreader
withoptimizedthermalmanagement.Theexperimentalverificationofthisprediction
outstanding.stillis

OneofthemostcrucialstepsinthefabricationprocessofsuchFETsisthegrowth
oftheδ-dopedlayer.Asearlierdiscussed,verytightchannelrequirementsplacehigh
demandsontheδ-channel,thussuccessfuloperationofsuchdeviceshasbeenhindered
duetothefact,thathighsheetcarrierconcentrationandratherbroaddopingprofile
resultinanineffectivemodulationofthechannel.Despitethissetback,multipleboron
δ-dopedlayerswithnominallyundopedspacershavebeensuccessfullygrownand
employedinadiamondmergeddiode,aswillbediscussedinchapter7.

diodeSchottkyp-i-nMerged5.3

Highvoltage,highpowerswitchingdiodesmustcombinetwocontradictingproperties,
namelyhighblockingvoltagesandlowforwardlosses.Asschematicallyshowninfig.
5.8,forsmallforwardlossesasmallthresholdvoltageisneeded,implyingalowbarrier
metal-semiconductorjunction.Atreversebiashowever,theSchottkybarrierlowering
effectwillincreasecurrentleakageandreducethebreakdownvoltage.Toobtainideal
reversecharacteristicsaburiedpnjunction,whichcanhandlehighforwardthreshold
employed.beshouldvoltage,

diodeSchottkyp-i-nMerged5.3

Figure5.8:Requirementsonhighpowerswitchingdiodes.

39

ThecombinationofaSchottkycontactwithapnjunctionwouldbenefitfromboth,low
forwardlossesandhighbreakdownstrength,ifthelowSchottkybarrierisshielded
athighreversebias.Inthemergeddiodeconcept,suchshieldingisprovidedby
thelateralfieldofthepnjunction[152,153].Inthesuperjunctionconcept,anMIS
junctionisusedtoreducelateralfieldspikes[154].Thus,toeliminatefringingeffects
intheseheterogeneousjunctionconcepts,Schottkycontactareasaresurroundedby
highbarriercontactareas.MergeddiodestructuresinSiC(seefig.5.9)usuallyuse
ion-implantationtodefineeitherthehighlydopedpnjunctionorthesemi-insulating
regions[31].Thus,theSchottkycontactsareplacedabovesuchjunctionprofiles,
resultsinaverticallyconfinedcurrentowthroughtheSchottkydiodeinterfacein
forwarddirectionandaverticalfieldprofileinreversedirection.

Figure5.9:SiCmergeddiodeconcept.TheSchottkycontactis−actuallyamerged
contactandconsistsofalargeSchottkycontact(fabricatedonthenepitaxiallayer)
andseveralsmallerohmiccontacts(onthep+epitaxiallayer)ofthepnjunction,after
].155[

40

ConceptsfordiamondFETsanddiodes

Inordertotransferthisconcepttodiamondfewadjustmentsarenecessary.Inthe
SiCconcept,thesmallareapnjunctionswerefabricatedbyion-implantation.This
allowseasycontrolofthedeviceparameters,suchasthedepthoftheimplantedarea,
thedistancebetweentheseareas,etc.Unfortunately,ion-implantationisnotareli-
abletechniqueindiamondandselectivegrowthofthedopedregionrequiresreliable
maskingduringthegrowthprocess.SiO2orSiNcanbeusedassuchmask,however
theformationofSiC(SiwillcreateaverystrongbondswithConthediamondsur-
face),presentsaseriousdifficultyduringthesubsequenttechnologicalsteps,especially
duringtheetchingofdiamondetching.Therefore,adifferentapproachwasnecessary.
Athinnitrogendopedlayerhasbeengrownontopoftheactivelayerandelectron
beam(e-beam)lithographyhasbeenusedtodefineapatternofsmallareaSchottky
contacts.Thesecontactshavetobesmallenoughtoenablelateraldepletionunder
thecontactatreversebiascondition.TheSchottkycontactswerethenrealizedby
dryetchingofthee-beamopeningsthroughthenitrogendopedtoplayer.Finally,
theentirediodesurfacewascoveredbytheSchottkymetallization.Thus,thefinal
structurewillcontainsmallareaswiththelowbarrierpotentialoftheSchottkydiode
andlargeregionswiththehighbarrierpotentialofthepndiode,asshowninfig.
5.10.Inforwarddirectionthecurrentispassingthroughthelowbarrierregions,but
atreversebiasthecurrentpaththroughthesesmallregionsislaterallypinchedoffand
theactivelayerisuniformlydepleted.Moredetailsaboutthisconcept,togetherwith
characterizationandevaluationofdiamondmergeddiodescanbefoundinchapter7.

Figure5.10:Diamondmergeddiodeconcept.

Chapter6

FETchannelSurface

Overthepastyears,surfacechannelFETsonH-terminatedsurfacewasthemainfocus
ofdiamondtransistorresearch.ThebasicstructurewaspioneeredbyKawarada[156],
andsincethentwomaingateconfigurationswerepursued:MESFET[25–29,69–74,
106,146,150,151,157–171]andMISFET[24,75–78,172–179].

concernsstabilitySurface6.1

Althoughnospontaneouspolarizationwasobservedinthediamondcrystal,thesur-
facebecomeshighlypolaruponhydrogentermination.TheH-terminatedsurfaceis
energeticallyunpinnedandwithoutnoticeablesurfacestateswithinthebandgap.This
isverysimilartootherpolarmaterials,suchashexagonalGaN[180,181].Therefore,
itisnotsurprisingthatsurfaceadsorbatescanchargethesurface,resultingineffects
similartothevirtualgateeffectinGaNbasedFETstructures.Thus,theH-terminated
diamondsurfaceiselectronicallyunstable,whichinturnwillaffectthechannelsheet
chargedensity.Thechangeofthesurfaceconditionsmaycausepermanentdegrada-
conductivitytheoftion.

Inpreviousexperiments[105],thehydrogen-terminateddiamondsurfacewasduring
patterningthecontactsincontactwithphotoresistanddeveloper.Thiswasgenerally
followedbyrinseinacetoneandisopropanol,whileavoidingoxidizingagents.The
characteristicsofthissurfacemaybesummarizedinfig.6.1,wheretheswitching
responseofanungatedchannelbetweensourceanddraincontacts(withspacingof
5ofµ40m)Visandshows.80V,rWhenespectivelyswitching,thefrcurromentthedecrquiescenteasedwithbiaspointtime.VDFullSr=0ecoveryVtoaofbiasthe
currentwasobservedafterswitchingthebiasoff.Themeasuredpointswereobtained

41

42

FETnnelchaSurface

byshortbiaspulsing,thebiaspulsemustbeshortenoughtonotdisturbtherecovery.
Itseemsthatthechannelbecomesslowlydepletedbysurfacechargeschangingthe
surfacepotentialoractingasvirtualgate.Theeffectwasreversibleandattributedto
anelectronicsurfaceinstability.

Figure6.1:Previouslyobservedcurrentinstabilityofanungatedchanneldeviceafter
applyingbiasbetweenthecontacts.Recoveryofthecurrentwasobservedwhenthe
biaswasswitchedoff,after[105].

Ontheotherhand,whenmeasuringtheFEToutputcharacteristicsathighgatebias,
theoutputcurrentdegradedwitheachmeasurementanddidnotrecovered,asshown
infig.6.2.Thispointedtowardsapermanentdegradationofthesurfacechargestate.
Scatteringparametermeasurementscharacterizingthehigh-speedperformancewere
thereforemostlytakeninthelowerpartoftheoutputcharacteristics.

Figure6.2:PreviouslyobservedirreversibledegradationofMESFEToutput
].150[aftercharacteristics,

Surface6.1concernsstability

43

Anewself-alignmentfabricationtechnologyledtoasignificantimprovementinterms
ofstabilityofthesurface.Inthenewexperiments,thesurfacewastreatedwithgold
etchingsolution(KI/I2)beforethedepositionoftheAlgateandwithoutexposureto
resist,developeroranyotherchemicalsolution.

IVoutputcharacteristicsofavirgindevicewith0.8µmgatelengthwasmeasured,as
showninfig.6.3.Afterwardsthedevicewasstressedfor10minutesinopenchannel
condition(VDS=-20VandVGS=-3.5V).Anincreaseofthemaximumdraincurrent
byapprox.20%wasobserved.ThismayberelatedtothestabilizationoftheSchottky
.barrier

Figure6.3:TransfercharacteristicsmeasuredatVDS=-20VandVGS=-3.5Von
a)virgindeviceb)afterbiasstressfor10min.

Thisresultindicates,thattheinstabilityanddegradationmechanismscanbeinuenced
byspecificsurfacetreatment.Thepresentunderstandingis,thatthesurfacedipole
layerisstabilizedbyanin-situpassivationofspecificionicadsorbates.However,this
pictureisstillhighlyspeculative.NeverthelessthestabilizationoftheFETcharac-
teristicshasbynowbeenobservedindevicesfabricatedonvarioussubstrates.

Aprerequisiteforcomprehensivemeasurementsisthelong-termstabilityofthedevice
parameters.Suchstudieshavenotbeenpossibleinthepastduetoaseriousdriftand
degradation.Withthenewfabricationprocess,whichwillbedescribedinthenext
chapter,stabilityofthesedeviceshasbeenessentiallyimproved.Infig.6.4areshown
thetransfercharacteristicsofa0.2µmgatelengthFETmeasuredafterfabricationand
6monthslater.Theshiftofthethresholdvoltagewasobservedfrequently,thisshift

44

FETnnelchaSurface

wassometimespositiveandsometimesnegative.Thismeansthatthecharacteristics
ofthedielectricgatebarrierandmetalinterfacearesubjecttosmallchanges.However,
theslopeofthetransfercharacteristicsremainedunchanged.Thisindicatesnochange
ofthechannelseriesresistanceandthereforenochangeofsurfacepotentialofthefree
surfacebetweenthegatemetalandthesourceanddrainmetalcontacts.

Figure6.4:TransfercharacteristicsofavirginFETandofthesamedevicemeasured
.latermonths6

ThiswasconfirmedbyAFMKelvinprobesurfacepotentialmeasurements.Thistype
ofmeasurementisgenerallydescribedin[182],theschematicmeasurementsetupis
showninfig.6.5.Theprobeispositionedclosetothegatemetalcontact(thegate-drain
distancewaslessthan0.5µm),inaregionwherechargeinjectionfromthegatemay
causeavirtualgateeffectsimilartotheoneobservedinGaN-basedFETs[183,184].
IntheswitchingexperimentperformedbyG.Koley(CornellUniversity,Ithaca,USA),
thedevicewasstressedinpinch-offconditionwithVD=-5Vfor2minutes.Such
soakingconditionisnormallyusedtochargeavirtualgatetowardsdepletion.Then,
thegatebiaslevelisswitchedintoaclassAbiaspointtoVG=-2V.Tomeasurethe
surfacepotentialbytheprobe,thedrainbiasneededtobereducedtoVD=-2V,to
avoidmaskingoftheprobesignalbythehighdrainbiaslevel.Ascanbeseeninfig.6.6
thesurfacepotentialremainedunchanged.Thedraincurrentshowedaslightincrease,
thesourceofwhichstillneedstobedetermined.Noparasiticchargeisleakingoffthe
gatemetalcontactorchargingthesurfacelaterally(thiswouldformavirtualgate),
whichingeneralactsasparasiticcurrentlimiterifchargedpositively.Thusnomajor
transientsmaybeexpected(itwouldrequiretheinjectionofH+)duringthelarge
signalmeasurements.Indeed,apowerslumpphenomenonhasnotbeenobserved
inthesedeviceswithintheprecisionofthemeasurement.

concernsstabilitySurface6.1

45

Figure6.5:SchematicofanAFMKelvinprobemeasurementsetup.Thetiptogate
distancewas0.1µmandthegatetodraindistancewas0.5µm.Thedistanceswere
extrapolatedfromtheAFMcontourmapbetweengateanddrain.

Figure6.6:Surfacepotentialanddraincurrenttransientmeasuredunderbiasstress
usingscanningKelvinprobemicroscopy,after[27].

Finally,intermsoflongtimestability,outputcharacteristicsofavirgindeviceandof
thesamedevicemeasuredfourandhalfyearslaterareshowninfig.6.7.A20%de-
creaseinthecurrentlevelaswellasdegradationoftheohmiccontactcanbeobserved.
Takingintoaccountthatthisresultwasobtainedonanunpassivateddevice,thelong
termstabilityisencouraging.Despiteofallofthis,thehydrogen-terminateddiamond
mainssurfaceanisimportantthermallyandtasktochemicallysolve.Theunstable,absencethusofafindingsurfaceaproperpassivationispassivationaseriousre-
obstacleinthedevelopmentofhighpowerFETdevices.

46

FETnnelchaSurface

Figure6.7:OutputcharacteristicofaFETstructurewithLG=0.2µmandWG=100µm
a)obtainedonavirgindeviceandb)theidenticaldevicemeasured56monthslater.

echnologyT6.2

ThemajorityofthesamplesusedinthisworkwereHTHPstonespredestinedtobe
usedincuttingtools,thusproperpretreatmentofthesubstratesurfacewasneces-
sary.Suchcrystalscontainalargenumberofdefectsandasmentionedbefore,the
mainimpurityofHTHPIbcrystalsissubstitutionalnitrogen.Thenitrogenrelated
impuritiescancompletelycompensatethehydrogen-inducedsurfaceacceptorandno
remainingconductivityisobservedinsuchcase[118].Itisthereforenecessarytogrow
anintrinsicbufferlayerontothesubstrate,followedbytreatmentinhydrogenplasma
toensureproperhydrogentermination.Duringthistreatment,thesurfaceroughness
willimprove,asshowninfig.6.8.

Afterthegrowthandsurfaceterminationthefabricationsequencewasasfollows(see
fig.6.9).First,angoldlayer(duetotheunpinnednatureofthehydrogen-terminated
diamondsurface,metalswithdifferentbarrierheightshavebeenemployedforohmic
andSchottkycontacts,namelyAuandAl)coveringtheentireactiveareaispatterned
bywetchemicaletching.Next,athree-layere-beamlithographyprocesshasbeenused
forthedefinitionofthesub-µmgatepatterns.Thefootprintofthegatehasthenbeen
openedbyetchingoftheAulayerinaKI/I2solutionandwasthereforeself-aligned
inrespecttotheAucontacts.Subsequentlythegatemetalhasbeendepositedand
f.lift-ofbypatterned

Inordertoincreasethedeviceperformance,thegatelengthandsource-gatedistance
needtobedecreased.Thereductionofgatelengthcontributesmostlytoanimproved
high-frequencyperformanceofatransistor(increaseofthecut-offfrequencyfT).Fur-
thermore,theparasiticsource-gateresistancedegradesthedeviceperformanceand

echnologyT6.2

47

Figure6.8:Pretreatmentofdiamondsurfaceinhydrogenplasmaandcorresponding
roughnessmeasurementbyAFM.Themeasurementsbeforeandafterthepretreatment
showsignificantreductioninroughnessbymorethan50%.

causespowerlossduringRFoperation.Thisresistancedependsonthesource-gate
spacing(thedistancebetweensourceanddrain)andisgivenbytheunderetchingthe
goldcontactthroughthegatefootprintopeningdefinedbye-beamlithography.The
timerequiredforacompleteremovalofthegoldfilmunderthegatedependsonthe
thicknessofthegoldlayer.Unfortunately,duetoinaccuraciesintheetchingprocess
intheKI/I2solution,theprecisionoftheunderetchingstillneedstobeimproved.As
showninfig.6.10,theetchingofgoldisinhomogeneousandleadstospikes,where
highelectricalfieldswillcauseprematurebreakdownathighervoltages.Thisprob-
lemcouldbeaddressedbyusingofaborondopedcontactlayerandbysubsequent
selectiveetchingoftheoverlappinggoldcontact.

Thesurfacehasbeenstabilizedbythechemicaltreatmentinthegoldetchingsolu-
tion,butwillneedfurtherrefinementbyimplementationofaproperpassivation.
Surfacebreakdownisstillthoughttobethelimitingfactorinthistypeofdevices.
Thepassivationlayermustthereforesustainahigherbreakdownfieldthandiamond.
Furthermore,tonotdisturbthehydrogentermination,thislayerhastobedepositedat
relativelylowtemperatures.Thesetworequirementsaremakingthesearchforasuited
task.ficultdifapassivation

48

FETnnelchaSurface

Figure6.9:FabricationsequenceofaFETwithH-inducedchannelandself-aligned
gate.

Figure6.10:Etchingofgoldforself-alignedgatewithgatelengthof200nm(left)and
0.8µm(right).Thejaggedfinishofthecontactswillleadtoelectricalfieldspikesand
eakdown.breematurpr

characteristicsDC6.3

AtypicalDCoutputcharacteristicofa0.2µmgatelengthFETisshowninfig.6.11.The
maximummeasureddraincurrentwasIDmax=275mA/mmatVGS=-3.5V(aparallel
conductivechannelinthebufferlayercausedbyaminorboroncontaminationwas

characteristicsDC6.3

49

responsiblefortheleakage).Themaximumtransconductance,whichdescribesthe
differentialdependenceofthedraincurrentonthegatebias,isgm(max)=100mS/mm
atVDS=-8V.

Figure6.11:OutputcharacteristicofaFETwithself-alignedgate(LG=0.2µmand
WG=100µm).

Figure6.12:OutputcharacteristicofaFETstructure(LG=0.24µmandWG=100µm)
quasi-substrate.onfabricated

Infig.6.12isshownatypicalDCoutputcharacteristicofaFETfabricatedonthe
quasi-substratedescribedinchapter4.3.2.Fordevicewiththegatelengthof0.24µm
amaximumdraincurrentofabout250mA/mmandamaximumtransconductanceof

50

FETnnelchaSurface

97mS/mmwasmeasured.Alldevicescouldbefullypinched-off,nogateleakagewas
observed.Themaximumsourcetodrainvoltageforthisdevicewas-90V.Thebreak-
downatthispointseemedtooriginatefromthedestructionofthecontactmetallization
ratherthanfrombreakdownofthediamondsemiconductor.Thebreakdownvoltage
forthedevicewith0.9µmgatelengthwasashighas-180V,howeverinthiscase
themaximumcurrentlevelwassignificantlylowercomparedtodevicewith0.24µm
gatelength.Despiteahighdensityofdefectsinthisquasi-substrate,manyelectrically
activedeviceshavebeensuccessfullyfabricatedbetweenthesedefect,asshowninfig.
.6.13

Figure6.13:TheFETdevicefabricatedonquasi-substratewillbenotaffectedbythe
defectvisibleontheleftside.Thegateregionitselfisdefectfree.

characteristicssignalSmall6.4

Toassesthehigh-frequencyperformanceoftheseFETdevices,cut-offfrequencies
canbeused.Fromthesmallsignalparameter(scatteringparameter)measurement
currentgainplotsandsmallsignalequivalentcircuitparameters(seefig.6.14)canbe
extracted.ToexpressestheintrinsicspeedofadevicethetransitfrequencyfTcanbe
used.Thecut-offfrequencyfTspecifiesthemaximumfrequencyatwhichthedevice
providessmallsignalcurrentgain.fTcanbeinterpretedastheinverseofthetotal
signaldelayinthedeviceandcanbeextractedfroms-parametermeasurements.fTis
thenthefrequencywheretheextrapolated-6dB/octaveslopeofthe|h21|parameter
(shortcircuitcurrentgain)reachesunityandisdefinedasfollows:

f=f|=gm∼vsat
T|h21|=12∙π∙(CGS+CGD)2∙π∙LG

(6.1)

characteristicssignalSmall6.4

51

Thisequationdemonstratestheimportanceofasmallgatelengthforachievingahigh
fT.However,asimplereductionofgatelengthinordertolowerthegate-source
capacitanceCGSandtoimprovethetransconductancegmresultsinagreatlyincreased
parasiticgateresistance.Toobtainboth,smalllengthandlowresistance,electronbeam
lithographyhasbeenusedforthedefinitionofaT-shapedgatestructure,wherethe
smallfootdefinesthelengthandthewidetopprovidesalowresistance.Asshownin
fig.6.15,T-gateswithgatelengthsassmallas0.2µmweresuccessfullyrealized.

Figure6.14:Smallsignalequivalentcircuit.

Figure6.15:SEMimageofaT-gatewith200nmgatelength.

ThedefinitionofthetransitfrequencyfTimpliesanidealcurrentsourceattheinput
andanidealshortcircuitattheoutput,andthussuppressesparasiticeffects(likegate
resistance),whichareveryimportantinpracticalcircuitdesign.Formicrowavecircuit
design,themaximumfrequencyofoscillationfmaxisamoresuitablefigureofmeritas
itdescribestheupperfrequencylimitofpowergain.fmaxcorrespondstothefrequency
wherethedevicespowergainreachesunity.Therearetwowaystomeasurefmax:
-themaximumavailablegainMAGisthegainofadevicewithconjugatelymatched
ports.outputandinput-theunilateralgainUassumesthattheadeviceisunilateralizedbyalosslesspassive
two-portandtheresultingtwo-portisthenconjugatelymatchedattheinputand
ports.output

52

FETnnelchaSurface

Cut-offfrequencyfTisthemoreoftenusedfigureofmeritfordigitalcircuitswhereno
interstagematchingisused.Ontheotherhand,fmaxisasuitablefigureofmeritfor
microwavecircuitswhereinterstageconjugatepowermatchingisused.

measurementsfrequencyfCut–of6.4.1

ThesmallsignalRFperformanceofFETsfabricatedonthequasi-substratewasmea-
suredinthefrequencyrangefrom50MHzto20GHz.Toobtaincut-offfrequencyfT
andmaximumfrequencyofoscillationfmaxformaximumavailablegain(MAG)and
unilateralgain(U),smallsignalparametermeasurementswereperformedwiththe
assistanceofU.SpitzbergandI.Kallfass.Cut-offfrequenciesfT,fmax(MAG)andfmax(U)
for0.24µmgatelengthdeviceof9.6GHz,16.3GHzand17.3GHzwereextractedfrom
thegainplots,asshowninfig.6.16.

Figure6.16:RFgainplotsandextractedcut-offfrequenciesforadevicewith
LG=0.24µmandWG=100µmfabricatedonquasi-substrate.

ThesmallsignalRFperformancewasalsoevaluatedondevicesfabricatedonCVD
diamondwithhighmobilitysuppliedbyM.Kasu(NTTBRLAtsugi,Japan)[27]inthe
frequencyrangefrom1to26GHz.Fig.6.17showstheRFgainplotofcut-offfrequency
andmaximumfrequenciesofoscillationatVGS=-0.3VandVDS=-20V.Theextracted
frequenciesforadevicewithLG=0.2µmandWG=200µmwerefT=24.6GHz,
fmax(MAG)=63GHzandfmax(U)=80GHz.Thesevalueswerethehighestvaluesfor
diamondFETsatthattime.Thehighratiooffmax/fTindicatedthathighamplifiergain
obtained.becan

characteristicssignalSmall6.4

53

Figure6.17:RFgainplotwithextractedcut-offfrequencyandmaximumfrequencies
ofoscillationfordevicewithLG=0.2µmfabricatedonahighqualityhomoepitaxial
layer.TheIVcharacteristicofthisdevicewiththecorrespondingbiaspointusedinthe
RFmeasurementsisintherightpicture.

Gatebiasscanofthesecut-offfrequenciesatthedrainbiasVDS=-10Visshownin
fig.6.18.Togainmoreinsightintothebehaviorastandardsmallsignalequivalent
circuit(asshowninfig.6.14)hasbeenextractedfrombiasdependents-parameter
measurements.Thevaluesfortheinputcapacitance(CGS)andintrinsictransconduc-
tance(gm),whicharetheessentialforhighfT,areshowninfig.6.19.Itcanbeseen
thatCGSisratherconstantacrossawidegatebiasrange,indicatingthattheFETmode
ofoperationissimilartothatofaMOSFETorheterojunctionFETwithaconstant
gatetochannelseparationbarrier.Nearpinch-offCGSdecreasesfast,whilegmisstill
high,thenalsodroppingoffquickly.Itmaybespeculatedthatnearpinch-offthe
holechannelchargeispushedintothesubstratebufferlayerandthusreachinginto
aregionwithhighmobility.Inthiscase,thebufferlayermobilitywasapprox.8times
largerthanthesurfacechannelmobility[132].ResidualDCcurrentinjectionintothe
bufferlayerandresidualbypassconductionwereindeedobserved.Nevertheless,in
notthesmallcontributesignalRFessentiallybehaviortothetheRFpinch-ofoutputfwasconductancesharpandasthemaybufbeferlayerevidencedfrleakageomthedid
highfmax/fTratio.

measurementsNoise6.4.2

Thehighfrequencyresultshaveencouragednoisemeasurements.Thesefirstandupto
nowonlynoisemeasurementshaveresultedinaminimumnoisefigureFmin=0.72dB
at3GHz,usingthesamebiaspointwherethehighestfTandfmaxwereobtained.
ThedependenceofFminonfrequencyofadevicewith0.2µmgatelengthisshown

54

FETnnelchaSurface

Figure6.18:Cut-offfrequenciesdependenceongatebias,themaximumfTandfmax
valueswereobtainednearthepinch-offregion.

Figure6.19:Dependenceofintrinsictransconductance(gm)andinputcapacitance
(CGS)vs.gatebiasatVDS=-10Vasextractedfromequivalentcircuitmodeling.

infig.6.20.Asexpected,thehighfrequencynoiselevelincreaseslinearlywiththefre-
quency.Thenoiseoor,whichisnormallymeasuredatmediumfrequency,wasnotyet
reachedat3GHzandthereforeitwasbelow1dB.Infig.6.21isshownthedependence
ofFminondraincurrent.ThelowestFminof0.72dB(measuredat3GHz)wasobtained
atID=45mA/mm,whichcorrespondsto15%ofthemaximumcurrent.Duetothe
highimpedancelevelatthisbiasconditiontheoptimumreectioncoefficientΓopthas
averyhighmagnitudeof0.951.ThenoiseresistanceRnwas151Ω.

measurementssignalLarge6.5

55

Figure6.20:Fmindependenceonfrequencyatdifferentgatebiaslevels.Fminofapprox.
1dBcanbeextractedat3GHzandlowgatevoltages(VDS=-10V).

Figure6.21:FmindependenceondraincurrentIDatVDS=-10Vand3GHz.The
minimumvalueofFminis0.72dBandwasobtainedatID=9mA(whichcorrespond
todraincurrentdensityof45mA/mm).

measurementssignalLarge6.5

Largesignalmeasurementsondiamondquasi-substratewereperformedat1GHz.
ThesepowermeasurementswereperformedinclassAoperation,wherethegatebias
pointislocatednearImax/2andtheoutputsignalexpandsaroundthispointacross

56

FETnnelchaSurface

theloadline.ThesaturatedpowerforclassAismeasured,whentheoutputsignal
amplitudebecomeslimitedbyforwardgateleakage(forwardhalfcycleoftheinput
signal)andbypinch-off(reversehalfcycle).Suchapowermeasurementisshownin
fig.6.22,whereatfirsttheoutputpowerincreaseslinearlywithincreasinginputpower
andthensaturatesathighinputlevels.Fromthemeasurementofthesaturatedoutput
poweratVDS=-40VandVGS=-2.2VinclassAbiaspoint,usingthebestavailable
loadofthetunersystem(approx.400Ω)thepowerplotshowninfig.6.22hasbeen
obtained.Alineargainof12dBandasaturatedpowerlevelof0.2W/mmcouldbe
obtained,asfirstreportedin[185].Thelargesignalmeasurementswereperformedby
I.Daumiller,M.NeuburgerandA.Aleksov.

Figure6.22:FirstlargesignalpowermeasurementperformedondiamondFETat
1GHzandclassAbiaspoint(VDS=-40VandVGS=-2.2V),showslineargain
of12dBandasaturatedpowerlevelof0.2W/mm.

ThepowerwhichcanbeobtainedinclassAoperationby3differentloadlinesisshown
infig.6.23.Atfirst,foraloadlineof50Ωanoutputpowerof20mW/mmwasobtained
atVDS=-30V.Atadrainbiasabove-30VtheRFcurrentenvelopesaturates,thus
shiftingoftheVDSbiaspointtowardhighervoltageswillnotresultinanincreaseof
thesaturatedoutputpowerusingthisloadline.Usingthebestavailableloadof400Ω
resultedinasaturatedoutputpowerof0.2W/mmatVDS=-40V.Again,further
increaseofVDSwillnotresultinahighersaturatedoutputpower.Asmentioned
previously,fora0.9µmgatelengthdevicebreakdownvoltageof-180Vhasbeen
measured.Insuchacase,theoptimumloadlinewouldbeapprox.2.5kΩ.Using
thisoptimumloadandanoptimumbiaspoint,asindicatedinfig.6.23,thesaturated
powercanbeestimatedto2.1W/mm,whichisstilloneorderofmagnitudebelowthe

measurementssignalLarge6.5

57

expectations.Thebreakdownresultedinthedestructionofthecontactmetallization,
nointrinsicbreakdownofthediamondsemiconductorwasobserved.Thisindicates
thatthemeasuredpowerwasnotlimitedbythediamondpropertiesbuteitherbythe
measuringsetuporbythelayoutoftheFET.

Figure6.23:Powerlevelsof0.9µmgatelengthdevicemeasuredatdifferentloads(50Ω
and400Ω).UsingthebestdataforVBRanda2.5kΩloadlinewithanoptimumbias
point,anoutputpowerof2.1W/mmcouldbeestimated(dashedline).

Furthermore,RFpowermeasurementshavebeenperformedondevicesfabricated
onCVDdiamondwithhighmobilities[27]onaFETdevicewithLG=0.4µmand
WG=550µminclassAoperation(VDS=-20VandVGS=-2V).Thelimitedload
matching(upto400Ω),wasnotsufficienttoobtaintheoptimumloadimpedance
duetothelimiteddraincurrentoftheFET.Thesmallergatelength(andthushigher
draincurrentlevel)allowedasaturatedoutputpowerof0.35W/mmat1GHzto
beextracted,asshowninfig.6.24.Theplotshowsalinearpowergainof14dB
andtheexaminationofthewaveformcharacteristicsconfirmedtheabsenceofdrain
currentcompressionandrelatedpowerslump.Unfortunately,theloadlimitationdid
notallowtoestimatethepowerhandlinglimitsofthedevicestructure.Aconductive
bufferlayerhasbeenobservedintheDCdevicecharacteristicsasalreadydiscussedin
conjunctionwiththecut-offfrequencybehavior.Inthelargesignalcharacteristicsthis
leadstodraincurrentleakageandprematurebreakdown.Thus,theidealmaterials
breakdownlimitcannotbereachedinthiscase,whichlimitedthepowerhandling
device.FETtheofcapability

58

FETnnelchaSurface

Figure6.24:LargesignalpowermeasurementonaFETdevicefabricatedonhigh
qualityhomoepitaxiallayerperformedat1GHzinclassAbiaspoint(VDS=-20Vand
VGS=-2V),showingsaturatedpowerlevelof0.35W/mmandlineargainof14dB.

NCDonfabricatedFET6.6

Inordertoinvestigatetheintegrationofactiveandpassivedevicesonasingledia-
mondsubstrate,surfacechannelFETswerefabricatedonH-terminatednanocrystalline
diamond.AHFCVDsystemwithbiasenhancednucleationBENpretreatmenthas
beenusedforthegrowthofNCDfilmson4Sisubstrate(seechapter4.3.3).Afterthe
growth,squareareasof2x2cm2werecutfromthe4Siwaferandusedforfabrication
ofFETstructures.ThenanocrystallinefilmsweregrownbyK.Janischowsky.The
cross-sectionofsuchaFETisshowninfig6.25.

AfabricatedFETdevicewithadetailedviewofthegateregionisshowninfig.6.26.
Despitetheroughnessofapprox.40nmrms1,theAlgateswith300nmgatelength
couldbewelldefined.Takingintoaccountthesurfaceroughnesstheactualgatelength
isapproximately20%largerthanthefootprint.Duetothesmallsizeofthediamond
crystallitesthegatecrossesmanygrainboundaries.

TheDCoutputcharacteristicofa0.3µmgatelengthdeviceisshowninfig.6.27.
Thecurrentcanbewellmodulatedandtheenhancementmodedevicecanbefully
pinched-off.Theohmiccontactsshowsomerectificationindicatingaresidualbarrier

1rootmeansquare

NCDonfabricatedFET6.6

Figure6.25:Cross-sectionofasurfacechannelMESFETfabricatedonNCD.

59

Figure6.26:SEMimageofaFETdevicewith0.3µmgatelength(left)anddetailed
viewofthegateregion(right).

betweenthesurfaceandthemetalcontact.Amaximumdraincurrentof25mA/mm
atVGS=-3.5Vandamaximumtransconductanceof12mS/mmhavebeenobtained.
ComparedtotheFETsfabricatedonsinglecrystallinediamondtheoutputcurrentis
essentiallylower.Thislowcurrentlevelmaybeattributedtoalowerholemobilityas
willbediscussedbelow.Intherightfig.6.27,theshorttermstabilityofthedeviceis
shown.contrary,Nothedraindegradationcurrentoftheshowsaoutputsmallcurrincrenteasewithtime(similarcouldtrendbehasobserved.beenOnobservedthe
previouslyondevicesfabricatedonsinglecrystallinediamondsubstrates).

x

60

FETnnelchaSurface

Figure6.27:OutputcharacteristicofasurfacechannelFET(LG=0.3µm,
WG=50µm)fabricatedonnanocrystallinediamond(left)andcorrespondingtime
domainmeasurementathighdrainandgatevoltagelevels(right).

Toobtainthecarrierprofileofthechannel,capacitance-voltagemeasurementshave
beenperformedat50kHz.Thedepthdependentdopingconcentrationisgivenby:

2NA(w)=d1
A2∙q∙ε0∙εr∙(dVC2)

(6.2)

Aistheareaofthecontact,ε0andεrarethedielectricconstantsofvacuumand
diamond,respectively.Thecorrespondingdepletionlayerwidthisgivenby:

w=A∙ε0∙εr
C

(6.3)

Inthisway,toeveryvoltagedependentcapacitanceadepletionlayerwidthwitha
correspondingdopingconcentrationcanbeattributed.TheCVprofilingperformed
ata5µmgatelengthdeviceresultedinaneffectivecarrierprofileofthe2Dchannel,
asshowninfig.6.28.Theextractedcapacitanceofthegate-channeldielectriclayer
was1µF/cm2,whichwasessentiallyidenticaltothebarriercapacitancepreviouslyex-
tractedfromMESFETsandH-terminateddiamond-liquidinterfaceonsinglecrystalline
diamond[106].Thus,thepresenceofabarrierlayerbetweentheholechanneland
thesurfacehasbeenconfirmed.Furthermore,aninterfaciallayerbetweenthemetal
contactanddiamondsurfacehasbeenobservedbycross-sectionalTEM2[186].Barrier
layerthicknessofapprox.4nmcouldbeextracted,whenusingthedielectricconstant
ofdiamond(εr=5.7).Thethicknesswasagainessentiallyinagreementwithresults
2TransmissionElectronMicroscopy

NCDonfabricatedFET6.6

61

obtainedfromFETsonsinglecrystallinediamond.However,thesurfacedielectriclayer
mayrepresentamodifiedmaterialsconfigurationwithadifferentdielectricconstant.

Figure6.28:CarrierprofileobtainedfromCVmeasurementusingthedielectric
constantofdiamond.Fromthismeasurementa4nmthickdielectricbarrierbetween
thegatemetalandthep-typechannel,andacapacitanceofthegate-channeldielectric
layerof1µF/cm2havebeenextracted.

Fromthisprofileasheetchargedensityof2x1013cm−2couldbeextracted.Usingthe
extractednsandCdiel,thedependenceofsaturatedcurrentID(sat)ongatelengthcan
besimulated.Thegradualchannelapproximationhasbeenusedforthesimulation,
]:52[after

22IDsat(LG)=Ci∙vsat∙LG∙1+2∙q∙ns∙µp−1(6.4)
2∙µpLG∙vsat∙Ci

Whenthemobilityisusedasafittingparameter,themeasuredpointscanbebestfitted
withamobilityof0.4cm2/Vs,asshowninfig.6.29.

ThesmallsignalRFperformancewasmeasuredinafrequencyrangefrom50MHzto
2GHz.Fig.6.30showstheRFgainplotsofthecurrentgainandmaximumpowergain
foranFETwith0.3µmgatelengthatVDS=-20VandVGS=-3V.Theextractedcut-off
frequenciesfTandfmaxwereslightlyabove1GHz,whichreectsthelowmobility.No
T-shapedgatehasbeenapplied,thusahighfmaxvaluecannotbeexpected.

62

FETnnelchaSurface

Figure6.29:Simulateddependence(dashedlines)ofmaximumdraincurrentongate
lengthwithmobilityusedasafittingparameter.Themeasuredcurrent(points)of
deviceswithdifferentgatewidthsandlengthscanbebestfittedwithmobilityof
2/Vs.cm0.4

Figure6.30:RFgainplotofa0.3µmgatelengthFETdevicefabricatedon
nanocrystallinediamondatdrainbiasVDS=-20VandgatebiasVGS=-3Vwith
extractedfTandfmaxslightlyabove1GHz.

Theseresultswereobtaineddespitethefactthatthegatecrossedmanygrainbound-
aries,asshowninfig.6.31.Thesegrainboundariesdidnotcauseanyshortcircuits,
otherwisenomodulationofthedraincurrentwithgatebiascouldbeobserved,and
ontheotherhandtheyalsodidnotblockcurrentow.Thisindicatesthatthechannel,
whichcanbefullymodulatedbytheelectricfield,isnotonlypresentinthegrains,

6.7Conclusion

63

butalsoacrossthegrainboundaries.Thisposesnewquestionsconcerningthephysi-
cal/chemicalnatureofthehydrogen-inducedp-typechannel.

Figure6.31:AFMimageofgateregionofanFETfabricatedonnanocrystalline
diamond,wheretheAlgateiscrossingmanygrainboundaries.Thesourcetodrain
spacingwasapprox.3µm,thegatelengthwas0.3µm.Thegrainboundarieslinesin
therightpictureswereaddedforbetterclarification.

6.7Conclusion

ThesurfacechannelFETdevicestructureusesap-typechannelinducedbythehy-
drogenterminationofthesurface.Thephysical/chemicalnatureofthischannelis
stillnotverywellcharacterized.Itseemsthereforethatthefrequencybehaviorisstill
dominatedbythelowmobility(holemobilityislowercomparedtothebulkmobility
ofhighqualitybufferlayers)anddespitethesmallgatelengthitsonlymarginally
inuencedbythesaturatedvelocity.Essentiallyhighercut-offfrequenciesareexpected
iflowthecurrchannelentdensities,mobilitywhercouldethebeimprlowestoved.minimumHighfTnoiseandffigurmaxeswervaluesewermeasureedobtainedaswell.at

FortheFETdevicesfabricatedonhighqualitydiamondbufferlayersthesmallsignal
measurementsresultedincut-offfrequenciesfT=24.6GHz,fmax(MAG)=63GHzand
fmax(U)=80GHzforadevicewithgatelengthof0.2µm.Inaddition,RFnoisemeasure-
mentsintheGHz-rangehavebeenperformedforthefirsttimeondiamondFETs,and
resultedinaminimumnoisefigureFminof0.72dBat3GHz.Powermeasurementsat
1GHzresultedinasaturatedoutputpowerdensityof0.35W/mm,whichwaslimited
bytheon-wafertuningrangeandthereforeessentiallylowerthantheexpectedlimit.

64

FETnnelchaSurface

Animportantaspectofthisworkwasthelongtimestability.Deviceshavebeen
testedoveralongperiodoftimewithoutmajordegradation.However,stabilityat
hightemperatureshasnotbeeninvestigated,allmeasurementshavebeenperformed
atroomtemperature.Thedevicestructureswerenotpassivated,howeversurface
passivationwillbeakeyfactorinthefurtherimprovementofthesedevices.The
breakdownwasstilllargelysurfacerelated.Duetothefactthatthesurfacechannel
followsthesurfacetopography,someconceptsfortherelaxationofthesurfacefield
distributioninpowerdevicestructureshavestilltobedeveloped.

Furthermore,FETdeviceshavebeenfabricatedonadiamondquasi-substrategrown
onaSrTiO3ceramicsubstrateusinganiridiumbufferlayer.Onthisquasi-substrate,
smallsignalandpowermeasurementcouldbeperformedforthefirsttimethanks
totheimprovedstability.Theobtaineddatawerebelowexpectationsandlatersur-
passedbymeasurementsperformedonhighqualitydiamond.However,provided
thattheepitaxialnucleationprocesswillbefurtherimproved,onecanexpectdiamond
quasi-substrateswithpropertiessimilartotheseofsyntheticHTHPsubstrates.Thus,
substratesoflargersizemaybeavailableatlowercostinthenearfutureandthismay
development.devicescalewaferallow

Finally,FETsfabricatedonnanocrystallinediamondhasbeenfabricatedandevaluated.
DCandsmallsignalperformancecouldbeconfirmed,althoughsignificantlylimited
bythelowholemobility,whichmaybeattributedtothehighgrainboundarycontent
inNCDfilms.Allotherelectricalparameterswereessentiallythesamecompared
totheelectricalbehaviorofdevicesfabricatedonsinglecrystallinediamond.Using
CVmeasurementsabarrierbetweenthegatecontactandthep-typechannelwith
thicknessoffewnanometerscouldbeconfirmed.

Recently,draincurrentdensityof550mA/mm,cut-offfrequenciesfTof45GHzand
fmaxof120GHz,andoutputpowerdensityof2.1W/mmat1GHzinclass-Aoperation
ofaFETfabricatedonhydrogen-terminated,highqualitypolycrystallinediamond
havebeenreported[171].ThesehighRFperformancevaluesmeettherequirements
forpresentwirelesscommunicationssystems,howevernoreliabilitydataareavailable
yet.

7Chapter

diodeSchottkyp-i-nMerged

Toobtainhighblockingvoltagesandlowforwardlossesinpowerdiodestructures,
aSchottkycontactcanbemergedwithaMISorpnjunction.Insuchaconfiguration
theSchottkycontactisresponsibleforalowforwardthresholdvoltageandtheMIS
orpnjunctionisresponsibleforalowreverseleakagecurrentandahighbreakdown
voltage.

conceptdiodeMerged7.1

Torealizesuchamergeddiodeindiamondonlyfewtechnologicallyrelevantbuilding
blocksareavailable.Theseareactivelayerdopingbyboronin-situduringepitaxy,
Schottkycontactontheoxygen-terminatedsurfacewithabarrierheightofapprox.
1.7eV,andnitrogen/boronpnjunctionwithabuilt-inpotentialof3.8eV.Inthediode
structuretheSchottkymetalwillbedepositedacrosstheentiresurfaceandwillthere-
forealsoserveascontactmetalforthepnjunction.Here,amergeddiodestructure
withsmallsizeSchottkycontactareasembeddedintoasurfacenearnitrogen/boron
junctionhasbeenrealized[32].Duetotheextremelydeepnitrogendonorlevel,thepn
junctionmayrepresentalossydielectricjunctionsimilartoaMIScontact(depending
onthethicknessofthenitrogendopedlayer).Foranitrogendopedlayerwithintun-
nelingthicknessthebarrierwillstillbedeterminedbytheSchottkybarrierpotential.
Wlowithseriesincrreasedesistance.thicknessOnlyofatthelayerextendedthecontactthicknesseswillthebecomenitrogenthatofdopedacamellayerdiodewillactwithas
lossydielectricwithMIS-likecharacteristics.Therefore,togainadvantageofthefull
pnSimulationjunctionresultsbarriershownpotentialinafig.7.1certainindicatethicknessthatofthisthenitrthicknessogenisardopedoundlayer30isnm.needed.

65

66

diodeySchottkp-i-nMerged

Figure7.1:Inuenceofthethicknessofthenitrogen(deepdonor)dopedlayeron
thresholdvoltagecomparedtoaSchottkycontactasobtainedbydevicesimulation
n-factor(Silvaco).incrWitheases)incrandeasingbeyondthicknessacertainofthenitrthicknessogenthedopedcurrentlayerowthestopsslopeentirchangesely.(the

echnologyT7.2

Commerciallyavailablesingle-crystaltypeIbsubstrateshavebeenusedforgrowthof
theverticaldiodestructuresbyMPCVD.Apre-growthH-plasmasurfacetreatment
hasresultedinasurfaceroughnessof0.5nmrms.Thiswasfollowedbyap+boron-
dopedcontactlayer,consistingofasequenceofmultipleboronδ-dopedlayersto
reducethestressinthislayergeneratedbythehighboronconcentration,whichis
neededtoobtainfullactivation.Theδ-dopingprocesswassimilartotheonedescribed
in[17].Inthecasetreatedhere,thelayerconsistsof5δ-dopedpeakswithatotal
thicknessof30nm.Intrinsiclayershavegrownbetweentheδ-dopedlayersinorder
toreducestress.Toassurecarriertunnelingfromonedopedlayertotheother,these
undopedlayersareonlyafewnanometersthick.Thisdopingprofileresemblesthe
profileshowninfig.3.3.TheMPCVDgrowthparameterswereasfollows:p=2kPa,
T=750°C,P=700Wandaspecificsequenceofgrowthsteps.Thiswasfollowedbythe
growthofa100nmthickintrinsiclayerinasecondMPCVDsystem(notcontaminated
withboron)byJ.E.Butler(NavalResearchLaboratoryWashington,USA).Ontopof
thisactive(nominallyundoped)layerathinnitrogendopedcaplayerofapprox.10nm
thicknesshasbeengrown(thiswassomewhatthinnerthanthetheoreticaloptimum
thickness).

Characteristics7.3

67

Figure7.2:Cross-sectionviewofthemergeddiamonddiodestructure.Thesmallarea
Schottkycontactsaremergedwithaverticalpnjunction.Thep+dopedlayerconsists
-dopedδ5oflayers.

Thediodestructureisschematicallyshowninfig.7.2.Circulardiodepatterns,with
diametersbetween20and160µm,havebeenmesaetchedbyO2/Ardryetchingto
obtainverticalcurrentow.Theetchinghasbeenterminatedonthep+dopedlayeron
whichohmiccontactswerefabricated.Thepnjunctionhasbeentestedbyemploying
Almetalpads.Later,theAl-layerwasremovedandthesurfacepatternedbye-beam
lithographytoobtainsmallsizedSchottkycontactareas,asillustratedinfig.7.3.The
diameteroftheSchottkycontactspotswasapprox.0.6µm.TheoverallSchottky
contactareawasapprox.10−2ofthetotaldiodesurface.TheSchottkycontactitself
wasthenrealizedbydryetchingofthee-beamopeningsinaresistbyO2/Ar-plasma
throughthenitrogendopedtoplayer.Finallytheentirediodesurfacewascovered
bytheSchottkymetallization,whichwasatfirstAlandlaterforhightemperature
measurementsW:Si/Ti/Au[187].

Characteristics7.3

TheIVcharacteristicsofthepndiodetakenbeforeandaftertheSchottkycontactareas
werepatternedareshowninfig.7.4.Ahighcurrentrectificationratioof109can
beseenforavoltageof±8V.Inforwarddirectiontheidealityfactorisstrongly
biasdependentandisbetweenn=2andn=7.Thehighidealityfactorindicatesthat
theIVcharacteristicdeviatesfromthethermionicemissiontheory,andthatitmaybe
dominatedbythecurrenttransportovernon-homogeneousSchottkybarrierscaused
inbyforwardefects.ddirThisectionmeanscurrthatentinjectionalthoughistheenhancedjunctionbyisdefectblockingstateswellatinthereverseinterface.direction,After

68

Schottkp-i-nMergeddiodey

Figure7.3:Patternformedbyelectronbeamlithographyforthedefinitionofthe
Schottkycontactareasonacirculardiodesurfacewith40µmdiameter.Intheleft
imageisshownanopticalimageofanetcheddiodestructurewithregulardistribution
ofsmallSchottkycontactareaswith0.6mdiameter.TherightAFMimageshowsthe
surfacetopologyaftertheetchingoftheSchottkycontactareas.

Figure7.4:IVcharacteristicsofthediodebefore(square)andaftertheSchottky
contactfabrication(circle).AfterthefabricationoftheSchottkycontacts,significant
improvementintheforwarddirectioncanbeobserved,whileinreversedirectionthe
lowleakagecurrentcanbepreserved.InbothmeasurementsAlcontactshavebeen
used.

thismeasurementthe(identical)pnjunctionwasconvertedintoamergeddiodeby
fabricatingoftheSchottkycontacts.Theforwardcurrentthresholdwasessentially
reducedandtheidealityfactorisnow2.1andbiasindependent.Thisstillrelatively

Characteristics7.3

69

highn-factormaybeattributedtotheinuenceofdefectrecombinationandthesmall
dimensionsoftheSchottkycontact,thustoa2D-fielddistribution[188].Thismeans
thatindeedtheforwardcharacteristicsarenowdominatedbythelowbarrierdiode
areas.Atthesametime,thereversecurrentdidnotincreaseandthecurrentrectifica-
tionratioof109hasbeenmaintained.

TheSchottkybarrierheightΦBcanbeestimatedfromthesaturationcurrentI0(thepn
junctioncurrentcanbeneglectedatthecorrespondingbiaslevel),wherethecurrent-
voltagecharacteristicofaSchottkydiodecanbedescribedby:

q∙(V−I∙Rs)
I=I0∙e(n∙k∙T)−1


(7.1)

RSistheseriesresistance,nistheidealityfactor,kisBoltzmannconstantandTisthe
temperature.ThesaturationcurrentI0isdeterminedbythermionicemissionoverthe
:ΦbarrierSchottkyB

Φ∙q−∗2k∙TB
I0=A∙AR∙T∙e

(7.2)

AistheSchottkycontactareaandA∗RistheeffectiveRichardsonconstant.Toidentify
thediodebarrierheight,theIVcharacteristicsweremeasuredattemperaturesupto
300°C,asshowninfig.7.5.Abarrierheightof1.5eVwasextractedfromtheslope,as
showninfig.7.6.ThisisclosetothevalueobservedinSchottkydiodesfabricatedon
O-terminateddiamond[148,189].

CVmeasurementsshowedessentiallyconstantcapacitance,indicatingthattheactive
layerisalreadydepletedatzerobias,thusadopingconcentrationprofilecannotbe
extracted.However,thisshowsalsothatthefieldacrosstheactiveregionisessentially
constantforhighreversebias.Thisallowstoestimatethefieldstrengthatbreakdown.
Thebreakdowncharacteristicsareshowninfig.7.7,whereirreversiblebreakdowncan
beseenatapprox.25V,correspondingtobreakdownstrengthofapprox.2.5MV/cm.
Thisrathercommonbreakdownstrengthisfarbelowthetheoreticalexpectation[13].
Thereversecharacteristicsshowcurrentenhancementsignaturespriortobreakdown
indicatingprematurecarriergenerationandtransportatdefects.Ontheotherhand,
highpurityandlowborondopedintrinsicactivelayerSchottkydiodeswithblocking
voltagesinthekVrangehavealreadybeendemonstrated[21].

IVcharacteristicsweremeasuredathightemperaturemeasurementsusingtempera-
turestableW:Sicontacts.Fig.7.8showstheIVcharacteristicsupto900°Cmeasuredin
vacuum.Theextrapolationoftheforwardcharacteristicsresultedinabuilt-inbarrier

70

diodeySchottkp-i-nMerged

Figure7.5:TemperaturedependentIVcharacteristicsshowmorethan9ordersof
magnituderectificationwithverysmallreversecurrentvariationupto300°C.The
idealityfactornslightlydecreaseswiththetemperaturefromn=2atR.T.ton=1.5at
°C.300

Figure7.6:1.5eVbarrierheightextractedfromtemperaturedependenceoftheforward
ent.currsaturation

heightofapprox.3.9eV,veryclosetowhatisexpectedforapnjunctionwithfully
activatedboronandsubstitutionalnitrogen(EG-(EC-ED)=5.45eV-1.7eV).Since
thenitrogendopedtoplayerisonly10nmthin,evenatR.T.theseriesresistanceisnot
limitingtheforwardcurrent.

Characteristics7.3

71

Figure7.7:IVcharacteristicsof100nmthickmergeddiodeshowingbreakdownat
∼25V,whichtranslatesinabreakdownfieldofapprox.2.5MV/cm.Intheleftis
semi-logarithmicrepresentationofthesamemeasurement.

Inthecaseshowninfig.7.8,thermallyactivatedleakageisobservedinreverseas
wellasinforwarddirection.Suchbehaviorhasbeenobservedpreviouslyandcould
beattributedtolowbarrierinjectionpaths[58].Non-epitaxialdefectscanbeanother
possiblesource[190].Thisleakageseverelylimitstheon/offratioathightemperatures
andmayalsobecorrelatedwiththeearlybreakdowncharacteristicatlowtempera-
tures.Nevertheless,stableIVcharacteristicscouldbeobservedforboththepnjunction
andthemergeddiodestructureinmeasurementsupto1000°Cinvacuum.SuchaIV
characteristicofamergeddiodestructureisshowninfig.7.9.Thiscomparativelyhigh
temperatureanalysisshowsthatdespitearectificationratioof109atR.T.,leakageand
breakdownaredominatedbydefectseveninthecaseofaburiedpnjunction.

Structuraldefectsrepresentachallengingproblemforhomoepitaxialdiamondgrowth.
Non-epitaxialcrystallitesareformedwhenthegrowthconditionsarenotchosenap-
propriatelyorwhencontaminationduetoforeignelementsoccurs.Thesenon-epitaxial
crystalsareformedduetoinstabilitiesinthegrowthconditions,theycangrowfaster
thanthesurroundingepitaxialmaterialandpyramidalhillockscanformaroundthem.
Themosteffectivewaytoavoidthesedefectsistouseverylowmethaneconcentra-
tionsinthegasphase.However,thisisattheexpenseofverylowgrowthrates,and
thusnotsuitedforthegrowthofthickdiamondlayers.Ontheotherhand,itcanbe
usedtoachieveanatomicallysmoothsurface,whichisanecessaryprerequisitefor
layers.-dopedδonbor

Defectscandegradethedeviceperformancesignificantly.Extendeddefectsactas
shortsandareresponsiblefortheohmicbehavior,asshowninfig.7.10.Itseems
thatsmallerdefectscanbepinchedoffatlowreversebias.Neverthelessasignificant
increaseintheleakagecurrentcanbeobservedathighbias.Diodeswithlargesurface

72

ySchottkp-i-nMergeddiode

Figure7.8:TemperaturedependentIVcharacteristicsmeasuredinvacuum.Thermally
activatedleakagecanbeobservedbothinreverseandforwarddirection.Abuilt-
inbarrierheightofthepnjunctionof∼3.9eVcanbeextractedfromtheforward
characteristics.

Figure7.9:IVcharacteristicofamergeddiodewith100µmdiametermeasuredat
vacuum.in°C1000

areasaremoreoftenaffectedbydefects,sometimesevenmaskingtheeffectofthe
Schottkycontacts.Thisonlyconfirmsthenecessityofhighquality,defectfreesub-
strates.Unfortunately,theinuenceoftheδ-dopedlayercouldnotbeobserveddueto
notoptimalgrowthconditionsforthegrowthoftheintrinsiclayer,whichledtoahigh
.densitydefect

Conclusion7.4

73

Figure7.10:Defectsrunningfromsubstratethroughtheactivelayertothesurfaceacts
asshorts.IntherightisIVcharacteristicsofadiodewith160µmdiametershowing
ohmicbehaviorandcompletelymaskingtheSchottkydiodecharacteristic.

Conclusion7.4

AnoveldiamondmergeddiodestructureemployingSchottkycontactscombinedwith
nitrogen/borondopedpnjunctionhasbeenfabricatedandevaluated.Inthisproof-
of-conceptexperimentonlyonedevicestructurehasbeeninvestigated.Thediode
characteristicsreectedinparttheSchottkybarriercharacteristicsandinpartthepn
junctionbehavior.Inthereversebiasregime,theleakagecurrentandbreakdown
characteristicsofthepnjunctioncouldbepreserved,althoughthesecharacteristicsare
stillinuencedbydefects.Inforwarddirectionthethresholdvoltagecouldbereduced
tothatoftheSchottkybarrierpotential.Thereforeitseemspossibletotransferthe
conceptofamergeddiodeontodiamonddespiteitsseriouslimitationsindoping
possibilities.Whenusingrefractorymetalsascontactsthisconfigurationhasalso
shownhighthermalstability,thediodehasbeensuccessfullytestedupto1000°C
vacuum.in

Themergeddiodebehaviorisverysimilartothatofpnjunctionsrealizedbynitrogen
dopedUNCDonborondopedsingle-crystaldiamond[30,122,191](seechapter8).
Furthermore,similarresultshavebeenobtainedonnanocrystallinediamondpndiode
structuregrownbyHFCVD[192].Furtherinvestigationofthemergeddiodestructure
maythereforealsoserveasamoredefinedmodel,andhelptoanalyzethedefect
behaviorofdiamondpndiodestructures.

74

Merged

p-i-n

ySchottk

diode

8Chapter

pnNanocrystallinediode

Thankstoadvancesinthen-typedopingofultra-nano-crystallinediamond,adiamond
basedpndiodeconsistingofap-typedopedsinglecrystallinediamondactivelayerand
ann-typedopedUNCDtoplayercouldbeinvestigatedindetail.Thecharacterization
ofthefabricatedpndiodes(withadeviceconfigurationverysimilartothatofthe
mergeddiode)waslargelyperformedbyT.Zimmermann[30,192].Theseresultswere
closelycorrelatedtothatofthemergeddiode.

dopingUNCD8.1

UNCDfilmdepositedviamicrowaveplasma3CVDinanAr-richplasma[193]isan
extremelyfine-grained(2–5nm),98%ofspbondscontainingdiamondfilmwith
atomicallyabruptgrainboundaries[194].NitrogendopedUNCDthinfilmscanalso
besynthesizedusingaMPCVDtechniquewithaCH4/Ar/N2gasmixture.These
growthtechniquesproduceveryconductiven-typediamonds,whichdemonstrate
reasonablemobilitiesandwhicharefullyactivatedatroomtemperature[194].Grain
boundaryconductionisresponsibleforthehighelectricalconductivitiesinthesefilms,
whichhavethehighestn-typeelectricalconductivityreportedfordiamondsofar[46].
ofNitraogeniscombinationpreferofspentially2andsp3incorporatedbonds,printotheomotinggraingrainboundariesboundary[119],conductivitywhich.consistThe
conductivityofnitrogendopedUNCDcanbecorrelatedtotheamountofnitrogenin
thefilm.Thefractionofsp2-bondedcarbontosp3-bondedcarbonincreasesfromabout
7%inundopedUNCDtoabout14%inUNCDdopedwith20%ofN2.Theamountof
nitrogeninthefilmsandthewidthofthegrainboundarieswasfoundtoincreaseas
thepercentageofnitrogenintheplasmaincreased[195].

75

76

echnologyT8.2

diodepnNanocrystalline

Thedesignoftheverticaldiodestructureisshowninfig.8.1.Asingle-crystalHTHP
diamond(typeIb)with(100)-orientationhasbeenusedasasubstrate.A0.5µmthick
p+borondopedcontactlayerhasbeengrownontothesubstratebyMPCVD,followed
bya0.5µmthickactivep−dopedlayer.Finally,a0.5µmthickhighlynitrogendoped
UNCDfilmhasbeengrownbyD.M.Gruen(ArgonneNationalLaboratory,USA).
Then-typeconductiveUNCDmaterialisrealizedusingagasphaseconsistingof
84%Ar,15%N2and1%CH4.An-typeconductivitywithacarrierconcentrationof
1019cm−3andanelectronmobilityofapproximately10cm2/Vshavebeenmeasured.
Theverticaldiodestructurehasbeenfabricatedinamesaconfiguration.Thecircular
activeareawith30µmdiameter1hasbeendefinedbymaskingusinga200µmthick
titaniummaskandbyRIEinanO2/Arplasma.Formeasurementsatveryhigh
temperaturestheTilayerhadbeenremovedtopreventcarbonization.

Figure8.1:Crosssectionofn-typeUNCD/p-typesingle-crystaldiamonddiode.

Characteristics8.3

AhighlyrectifyingpndiodehasbeenobservedintheIVcharacteristics[122].The
idealityfactorof3wasratherhigh,pointingtowardsparasiticeffectsorindicating
ahighlydisorderedinterfacebetweenthen-typeUNCDlayerandthep-typedia-
mond.However,thechangeoftemperatureshowedverylittlechangeinthen-factor
andaninterfacialbarrierpotentialofΦIV=0.72eVhasbeenextracted(seefig.8.2).
1EtchingIonReactive

Characteristics8.3

77

Thisbarrierpotentialisessentiallysmallerthanwhatisexpectedfromadiamond
pnjunction.Asmentionedearlier,apnjunctionformedbydopingwithboronand
nitrogenshouldresultinabarrierpotentialof3.8eV.Abarrierheightof0.72eV
indicatedthatthejunctionpotentialbetweentheborondopedactivediamondlayer
andthen-typeUNCDtoplayerisdeterminedbygrainboundarystates.Incontrastto
temperaturedependentIVmeasurement,CVmeasurementallowedustoextractthe
dielectricinterfacebarrierpotentialof3.9eV(extractedfromfig.8.2).Thedifference
inIV-barrierandCV-barrierheightsisnotunusual,sincebothvalueswereextracted
underdifferentconditions.Inthecaseoftheforwardcurrent,carrierinjectionacross
thelowestbarrierinthedisorderedsystemwilldeterminethecurrent.Inthecapacitive
measurement,thedepletionlayerwillbedeterminedbythelargebarrierandthelow
barriercontributionwillbenoticeableonlyintheleakagecurrent.Thus,thesystemis
highlydisordered,theforwardcurrentispassingthroughalowbarrierjunction,while
thedepletionisbeinggovernedbyadielectricinterfacewithalargebarrierheight.

Figure8.2:BarrierpotentialΦIV=0.72eVextractedfromtemperaturedependent
IVcharacteristic(left),andbarrierpotentialΦCV=3.9eVextractedfromCV
measurements(right),after[30].

Thisdiodeconfigurationrepresentsaheterogeneousjunctionsystem.Thetopcontact
seemstocontaintwophases:thatofthen-typeconductivegrainboundariesandthatof
thediamondnano-crystallites.Itwassuggestedthatthiscaserepresentstwojunctions
inparallel.TheratioofbothareaswasrelatedtothegrainsizeoftheUNCDmaterial.
Suchaconfigurationissimilartothemergeddiodeconfiguration,whereadistributed
pndiodeismergedwithaSchottkycontact,asdiscussedinchapter7.Suchadiode
containsasmallsurfaceareaofwithsmallbarrierpotential,howeverthelargerpart
ofthesurfaceiscoveredbythehighbarrierpotentialofthepnjunction.Inforward
operationthecurrentispassingthroughthelowbarrierregions,thusallowingcurrent
owatlowerbiasresultinginlowforwardlosses.

78

pnNanocrystallinediode

Athightemperaturethereverseleakagecurrentincreased,mostlikelyduetoahigh
interfacialdefectdensity.Itshouldbenotedthatonlyaverysmallpartofthesurface
isneededtocarrythisreverseleakagecurrent.Nevertheless,thejunctionseemedtobe
hightemperaturestable,sinceitdidnotshowdegradationduringseveralsuccessive
testsupto1050°Cinvacuum,asshowninfig.8.3.

Figure8.3:IVcharacteristicsofapndiodewith120µmdiametermeasuredat1050°C
invacuum,after[30].

Conclusion8.4

Noveldiamonddiodestructureconsistingofn-typeUNCDandp-typesingle-crystal
diamondhasbeengrownandanalyzed.TheIV-behaviorofthediodewasrather
complex.Theunusualbehaviorcanbeexplainedonthebasisofthemergeddiode
concept,whichwasbasedonthehypothesisofaheterogeneousinterfacerelatedtothe
UNCDgrainsandgrainboundarycharacteristics,asshowninfig.8.4.Itwasproposed
thattheindividualnitrogendopedgrainsformadiamondpnjunctionwithapprox.
3.8eVbuilt-inpotential.Inparallel,thegrainboundarynetworkactsasadistributed
Schottkydiodewithabuilt-inpotentialof0.7eV.Thus,inthismergedstructurelow
forwardlossescanbecombinedwithhighbreakdownstrength.

Thispndiodecontainscarbononly,andthusitshouldbeverystableandenable
reliablehightemperatureoperation,whichisdifficulttoobtainwithmetalSchottky
iscontactshighlyrbecauseectifyingofandinterfacialhighlycarbidetemperatureformation.stable,Andandhasindeed,beenthetesteddiodestrsuccessfullyucture

Conclusion8.4

eFigur8.4:

Schematiccrosssectionofpndiodewith

twodifferentbarrierheights.

therightsideisschematicdrawingofcorrespondingIV

upto1050°Cinvacuum.Thus,this

etemperatur

stable

onicelectr

devices.

diamond

diode

characteristics,

belongs

to

the

after

few

].122[

79

On

ultrahigh

80

Nanocrystalline

pn

diode

9Chapter

Conclusions

Diamondisanidealmaterialforhighperformanceelectronics.Withthedawnof
theCVDera,newareasofpossibleapplicationscameintosight.However,dueto
thedopinglimitations,onlyfewconceptsfordiamondelectronicsareavailable.De-
spitethisrestrictions,highpower,highfrequencyandhightemperaturecapabilitiesof
activediamondelectronicdeviceshavebeendemonstrated.

MESFETsfabricatedonhydrogen-terminateddiamondsurfaceweresuccessfullytested
intheDC,smallsignalandlargesignalregime.Theiroperationatmicrowavefre-
quenciesshowedcut-offfrequenciesfTof25GHzandfmaxof80GHz.Theimproved
fabricationtechnologyenabledfirstnoiseandpowermeasurementsondiamondFETs.
Thenoisemeasurementresultedinaminimumnoisefigureof0.72dBat3GHzand
thepowermeasurementsat1GHzledtosaturatedoutputpowerof0.34W/mm.
Here,themeasuredpowerwasnotlimitedbythediamondpropertiesbutbythe
on-wafertuningrangeofthemeasurementequipment.ForthereliabilityofFETs
withhydrogen-inducedchannelstabilizationandpassivationofthesurfacewillbe
essential.

DiamondmergeddiodestructuresemployingSchottkycontactscombinedwithani-
trogen/borondopedpnjunctionhavebeenfabricatedandevaluated.Inforwarddi-
rectionthethresholdvoltagecouldbereducedtothatoftheSchottkybarrierpotential.
Inthereversebiasregime,theleakagecurrentandbreakdowncharacteristicsofthepn
junctioncouldbepreserved,althoughthesecharacteristicswereinuencedbydefects.
Thisconfigurationhasalsoshownahighthermalstabilityandhavebeensuccessfully
testedupto1000°Cinvacuum.

Diamondbasedpndiodeconsistingofap-typedopedsinglecrystallinediamondactive
layerandann-typedopedultra-nano-crystallinediamondtoplayerhasbeeninvesti-

81

82

Conclusions

gatedaswell.Theanalysissuggeststhattheconfigurationofsuchadiodeissimilar
toamergeddiodeandcontainstwoareasofdifferentinterfacialbarrierpotentialin
parallel,whicharerelatedtotheultra-nano-crystallinegrainsandthegrainbound-
aries,respectively.Thebarrierbehaviorisrathercomplexandcanbedescribedbytwo
junctionsactinginparallel,reectingtheUNCDproperties.Thisnewmaterialsystem
displayedanextraordinarythermalstabilityandhasbeentestedsuccessfullyupto
vacuum.in°C1050

Figure9.1:Theadvancesindiamondtechnologyallowedthedevelopmentand
fabricationofwidevarietyofactiveandpassivedevicessuchasa)FETfabricated
onsinglecrystallinediamond,b)coplanardiamondmicroswitchfabricatedonSi
substrate,after[33],c)coplanarwaveguiderealizedonSisubstrate,after[133]and
d)FETsfabricatedondiamondquasi-substrate,after[26].Withtheincreasingsizeof
highqualitydiamondsubstratesnewintegrationpossibilitiesemerge.

Thesecondmaintechnologicalchallengeisrelatedtothediamondsubstrate.Upto
now,electronicdeviceshavebeenrealizedmainlyonHTHPsingle-crystalswithlim-
itedsize.Homoepitaxialelectronic-gradediamondlayerscanbegrownonthesestones
bymicrowaveplasmaCVD.Heteroepitaxialgrowthofdiamondresultsinpolycrys-

83

tallinelayers,whicharewellsuitedforMEMSapplicationssuchasRFswitches.Re-
centlyhighperformanceFETdeviceswerefabricatedonhighqualitypolycrystalline
films.Heteroepitaxialgrowthoniridiumresultedinsinglecrystallinequasi-substrate,
whichwasusedforfabricationofFETdevices.

Theseadvancementsareveryencouragingandopennewpossibilitiesfortheintegra-
tionofactiveandpassivedevicesintoMMICapplications.Inthenearfuturediamond
mayserveasasubstrateforhighpowerMMICintegratinghighspeedFETswithheavy
dutyMEMS(suchasmicroswitches)andcoplanarwaveguides(seefig.9.1).MEMS
elementsforadvancedcircuitapplicationshavebeenalreadydemonstrated.TheseRF-
switcheswerefabricatedonnanocrystallinediamondgrownonlargeareasubstrates.
Suchdeviceshavebeenoperatedathightemperaturesandmicrowavefrequencies.
Sincediamondisaninsulator,itisanidealmicrowavesubstratewithlowlossathigh
frequencies.Withconstantlyincreasingsizeofsinglecrystallinediamondsubstratethe
integrationofthesecomponentsmaybecomereality.

Theprogressinfabricationtechnologyshowedgreatcapabilitiesofdiamondandre-
centresultsindicatethatdiamondmayindeedbecometheultimatesemiconductorfor
onics.electrperformancehigh

84

Conclusions

AAppendix

parametersProcess

A.1ProcessparametersforsurfacechannelFET

[min]imeTSolution2(40%)HFH2O2+2H2SO410
H2O2+NH4OH+5H2O(75°C)10
H2O2+HCL+5H2O(75°C)10

TableA.1:Parametersforcleaningofdiamondsubstratesbeforethegrowth.After
eachstepthesubstrateswererinsedindeionizedwater.Attheend,thesampleswere
rinsedin1M2Pandacetoneandfinallydriedwithnitrogen.

StepH2[sccm]CH4[sccm]p[kPa]T[°C]P[W]Time[min]
Pretreatment200-26507005
Diamondgrowth2001.5265070015
H-termination200-265070010
Cooling+-→1atm→R.T.060

TableA.2:MPCVDparametersforgrowthofdiamondlayersusedforsurfacechannel
s.FET

85

86

parametersProcess

H2[sccm]CH4%p[kPa]Tsub[°C]Tfil[°C]Bias[V]Time[h]
BEN2002282021502001
Growth4000.21.56602200-15

TableA.3:HFCVDparametersforgrowthofnanocrystallinediamondlayersusedfor
s.FETchannelsurface

ProcessParameterTime[sec]
Spin-onAZ5214E(6000rpm)60
90°C100BackingExpositionMJB3(21mW/cm2)6.5
30MIF726AZDevelopment

TableA.4:Lithographyparametersformesaetching.

Gasp[Pa]P[W]Time[min]
310013.3Oxygen

TableA.5:ParametersusedforoxygenterminationinµEtchplasmasystem.

ProcessParameterTime[sec]
Spin-onPMGISF11(8000rpm)60
300°C180BackingSpin-onAZ5214E(8000rpm)60
90°C100BackingExpositionMJB3(21mW/cm2)6.5
30MIF726AZDevelopmentExpositionOAI(oodexposure)1000
105101PMGIDevelopment

TableA.6:Lithographyparametersforfabricationofohmicpads.

A.1ProcessparametersforsurfacechannelFET

ocessPr

Spin-on

Backing

Spin-on

Backing

Spin-on

Backing

Parameter

imeT[sec]

60rpm)(6000950kPMMA

300°C180

60rpm)(300033%PMMA/MA

300°C180

60rpm)(300050kPMMA

300°C180

lithographyE-beamvariedkV50

DevelopmentMIBK:Isopropanol=1:3195

87

TableA.7:ElectronbeamlithographyparametersforT-gates.Thethicknessesofthe
bottom,middleandtoplayerare∼180nm,600nmand110nm,respectively.

ProcessParameterTime[sec]

Spin-onLOR7B(4000rpm)35

Backing300°C180

Spin-onTI09XR(4000rpm)35

Backing60°C100ExpositionMJB3(17mW/cm2)2

AZDevelopment45MIF726

TableA.8:Lithographyparametersforohmiccontacts.

88

A.2

diodemergedforparametersProcess

parametersProcess

H2[sccm]CH4[sccm]p[kPa]T[°C]P[W]Time[sec]

200152311001500120

TableA.9:MPCVDparametersforgrowthofintrinsicdiamondlayer.

H2[sccm]CH4[sccm]N[sccm]p[kPa]T[°C]P[W]Time[sec]

200

200

-

3

4-300700750

5070075048

TableA.10:MPCVDparametersforgrowthofnitrogendopeddiamondlayer.

A.2Processparametersformergeddiode89
H2[sccm]CH4[sccm]Brodp[kPa]T[°C]P[W]Time[sec]
200--4750700300
2001.5-4750700600
200--4750700300
200-+475070010
20010+47507002
200--47507005
2003-475070035
200--475070060
200-+475070010
20010+47507002
200--47507005
2003-475070035
200--475070060
200-+475070010
20010+47507002
200--47507005
2003-475070035
200--475070060
200-+475070010
20010+47507002
200--47507005
2003-475070035
200--475070060
200-+475070010
20010+47507002
200--47507005
2003-475070035
TableA.11:MPCVDparametersforgrowthof5boronδ-dopeddiamondlayers.

90

parametersProcess

ProcessParameterTime[sec]
Spin-onAZ5214E(6000rpm)60
90°C100BackingExpositionMJB3(21mW/cm2)6.5
90°C100BackingExpositionMJB3(withoutmask)35
35MIF726AZDevelopment

TableA.12:Lithographyparametersforimagereversalprocess.

Gas[sccm]p[Pa]P[W]Etchrate[nm/min]
Ar17.5/O21.757252

TableA.13:Parametersforreactiveionetchingofdiamond.

MaterialGas[sccm]Time[min]Thickness[nm]
55,92Ar/1WSiWSi:NAr/1,92;N2/4,055
1010Ar/1,92iT10060Ar/1,92Au

TableA.14:Parametersfortemperaturestableohmiccontactsfabricatedbyion-beam
sputtering.

ProcessT[°C]Time[sec]
120.R.TgingPur1strampR.T.→65020
60650Heating2ndramp650→85020
60850AnnealingCooling850→R.T.900

TableA.15:Annealingparametersfortemperaturestableohmiccontacts.

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Listpublicationsof

A.Aleksov,M.Kubovic,N.Kaeb,U.Spitzberg,I.Daumiller,T.Bauer,M.Schreck,
B.StritzkerandE.KohnFirstdiamondFETRFpowermeasurementondiamond
quasi–substrate60thDeviceResearchConference(SantaBarbara,CA),(June2002)181.

M.Kubovic,A.Aleksov,A.DenisenkoandE.KohnAdvancesindiamondsurface
channelFETtechnologywithfocusonlargesignalpropertiesIEEELesterEastman
Conference(Newark,DE),(August2002)90.

M.Kubovic,A.Aleksov,M.Schreck,T.Bauer,B.StritzkerandE.KohnFieldeffect
transistorfabricatedonhydrogen–terminateddiamondgrownonSrTiO3substrate
andiridiumbufferlayerDiamondRelat.Mater.12(2003)403.

A.Aleksov,M.Kubovic,N.Kaeb,U.Spitzberg,A.Bergmaier,G.Dollinger,T.Bauer,
M.Schreck,B.StritzkerandE.KohnDiamondfieldeffecttransistors–conceptsand
challengesDiamondRelat.Mater.12(2003)391.

M.Kasu,M.Kubovic,A.Aleksov,I.Kallfass,U.Spitzberg,N.Kobayashi,H.Schu-
macherandE.KohnMicrowaveperformanceofdiamondsurface–channelFET61st
DeviceResearchConference(SaltLakeCity,Utah),(June2003)21.

E.Kohn,A.Aleksov,M.Kubovic,P.Schmid,J.Kusterer,M.SchreckandM.KasuDia-
mond–AnewmaterialsbaseforfutureultrahighpowerRFelectronicsCSManTech
2004)(MayFL),(Miami,2.5.

A.Aleksov,M.Kubovic,M.Kasu,P.Schmid,D.Grobe,S.Ertl,M.Schreck,B.Stritzker
andE.KohnDiamond–basedelectronicsforRFapplicationsDiamond.Relat.Mater.
233.(2004)13

109

110

ublicationspofList

M.Kubovic,M.Kasu,I.Kallfass,M.Neuburger,A.Aleksov,G.Koley,M.G.Spencer
andE.KohnMicrowaveperformanceevaluationofdiamondsurfacechannelFETs
Diamond.Relat.Mater.13(2004)802.

M.Kubovic,A.Denisenko,W.Ebert,M.Kasu,I.KallfassandE.KohnElectronic
surfacebarriercharacteristicsofH–terminatedandsurfaceconductivediamondDia-
mondRelat.Mater.13(2004)755.

M.Kasu,M.Kubovic,A.Aleksov,N.Teofilov,Y.Taniyasu,R.Sauer,E.Kohn,T.Maki-
motoandN.KobayashiInuenceofepitaxyonthesurfaceconductionofdiamond
filmDiamondRelat.Mater.13(2004)226.

E.Kohn,M.Kubovic,F.Hernandez-GuillenandA.DenisenkoDiamondforhigh
power/hightemperatureelectronics12thGAASSymposium(Amsterdam,Nether-
559.2004)(Octoberlands),

M.Kasu,M.Kubovic,A.Aleksov,N.Teofilov,R.Sauer,E.KohnandT.Makimoto
Propertiesof(111)diamondhomoepitaxiallayeranditsapplicationtofield-effect
transistorJpn.J.Appl.Phys.43(2004)L975.

M.Kasu,M.Kubovic,A.Aleksov,I.Kallfass,H.Schumacher,E.KohnandN.Kobayashi
MicrowaveperformanceofdiamondMESFETOYOBUTURI73(2004)363.

M.Kubovic,K.JanischowskyandE.KohnSurfacechannelMESFETsonnanocrys-
tallinediamondDiamondRelat.Mater.14(2005)514.

T.Zimmermann,M.Kubovic,A.Denisenko,K.Janischowsky,O.A.Williams,D.M.
GruenandE.KohnUltra–nano–crystalline/singlecrystaldiamondheterostructure
diodeDiamondRelat.Mater.14(2005)416.

O.A.Williams,T.Zimmermann,M.Kubovic,A.Denisenko,E.Kohn,R.B.Jackman
andD.M.GruenElectronicpropertiesandapplicationsofultrananocrystallinedia-
mondinD.M.Gruen,O.A.ShenderovaandA.Y.Vul(eds.),Synthesis,properties
andapplicationsofultrananocrystallinediamond(Springer,2005)373.

M.Schwitters,M.P.Dixon,A.Tajani,D.J.Twitchen,S.E.Coe,H.El-Hajj,M.Kubovic,
M.Neuburger,A.KaiserandE.KohnDiamond–MESFETs–Synthesisandintegra-
tionEuropeanRadarConference(Paris,France),(October2005)17.

111

T.Zimmermann,K.Janischowsky,A.Denisenko,F.J.HernandezGuillen,M.Kubovic,
D.M.GruenandE.KohnNanocrystallinediamondpn–structuregrownbyHotFila-
mentCVDDiamondRelat.Mater.15(2006)203.

E.Kohn,A.Denisenko,M.Kubovic,T.Zimmermann,O.A.WilliamsandD.M.Gruen
AnewdiamondbasedheterostructurediodeSemicond.Sci.Technol.21(2006)L32.

M.KubovicandE.KohnWidebandgapdiamonddiodeHETECH2006Proceedings
(Manchester,UK),(October2006)67.

M.Kubovic,H.El-Hajj,J.E.ButlerandE.KohnDiamondmergeddiodeDiamond
Relat.Mater.16(2007)1033.

H.El-Hajj,A.Denisenko,A.Bergmaier,G.Dollinger,M.KubovicandE.KohnCharac-
teristicsofborondelta-dopeddiamondforelectronicapplicationsDiamondRelat.
409.(2008)17.Mater

112

List

of

ublicationsp

Acknowledgments

IwouldliketoexpressmygratitudetoProf.ErhardKohnforgivingmetheoppor-
tunitytoworkinhisresearchgroup.Ithankhimforhisadviceandhelpfuldirection
throughthecourseofthisthesis.

IwouldliketothankmysupervisorAleksAleksov,Ihavelearnalotaboutdiamond
him.omfronicselectr

ManythankstoTomZimmermann,MartinNeuburger,MakotoKasu,UrsulaSpitzberg,
IngmarKallfass,AndrejDenisenko,KlemensJanischowsky,FaridMedjdoubandWolf-
gangEbertfortheirhelpwithmeasurements,technologyandfruitfuldiscussions.

Iwouldliketothankallthetechnicalstaff,inparticularYakivMenfore-beamlithog-
raphyandAlexSchreiberforthetechnicalsupport.

Iamverygratefultomycolleaguesandfriends.Fran,Hayssam,AlexandAndreas
thanksforthesupportandnecessarydistraction.

AspecialthanksgoestoˇSaˇnoKromka,whointroducedmeintotheworldofdiamond.

Finally,Iwouldliketothankparentsandbrother.Thiswouldnotbepossiblewithout
theirsupport.IwouldneverhavemadeitasfarasIhavewithoutthem.

113

114

ledgmentsAcknow

vitaeCurriculum

Name:xxxx.MichalKubovicˇ

Born:xxxxx.14.05.1977inMyjava,Slovakia

Education:

1991-1983

SlovakiaSenica,school,Elementaryx

1991-1995xSecondarytechnicalschool,Myjava,Slovakia

1995-2001xSlovakTechnicalUniversityBratislava,Facultyofelectrical
xxxxxxxxxxxengineeringandinformationtechnology,Slovakia
xxxxxxxxxxx(receivedIng.degreeinelectricalengineering)

2001-2006xUniversityofUlm,Departmentofelectrondevicesandcircuits
co-worker)(scientificxxxxxxxxxxx.

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