MOVPE growth and characterization of AlG_1tnxGa_1tn_1tn-1tnxN/GaN heterostructures for HEMT application [Elektronische Ressource] / vorgelegt von Nicoleta Elena Kaluza

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• Physik

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• Publié par: rheinisch-westfalischen_technischen_hochschule_-rwth-_aachen
• Publié le: 01 janvier 2003
• Nombre de lectures: 19
• Langue: English
• Poids de l'ouvrage: 11 Mo

Extrait

MOVPE Growth and
Characterization of Al Ga N/GaNx
1−x
Heterostructures for HEMT
Application
VonderFakult¨atfur¨ Mathematik,InformatikundNaturwissenschaften
derRheinisch-Westf¨alischenTechnischenHochschuleAachenzur
ErlangungdesakademischenGradeseinesDoktorsder
NaturwissenschaftengenehmigteDissertation
vorgelegtvon
DiplomChemikerin
Nicoleta Elena Kaluza geb. Nastase
ausGalati,Rum¨anien
Berichter: Universit¨atsprofessorDr. H.Luth¨
Universit¨Dr. R.Dronskowski
Tag der mundlic¨ henPrufung:¨ 01.10.2003
DieseDissertationistaufdenInternetseitenderHochschulbibliothekonlineverfugbar.¨Contents
1 Introduction and motivation 1
2 Fundamental Properties of Group III Nitrides 7
2.1CrystalStructure................................ 7
2.2ChemicalBonding 9
2.3 SemiconductorHeterostructures . . . . . . . . . . . . . . . . . . . . . . . . 11
2.3.1 Polarizationeffects...........................12
2.3.2 Al Ga N/GaNHeterostructures...................13x 1−x
2.4RelevantPropertiesofNitrides........................14
3 MOVPE Growth of (Al)GaN 17
3.1BasicPrinciplesofMOVPE..........................17
3.1.1 MOVPEGrowthProcess:GeneralConsiderations. . . . . . . . . . 18
3.1.2 MOVPEGrowthIsuesforGroupIINitrides............36
3.2 SubstratesforEpitaxial(Al)GaNGrowth. . . . . . . . . . . . . . . . . . . 38
3.3 PrecursorsandChemistryof(Al)GaNGrowthinMOVPE......... 42
3.3.1 AluminumPrecursors..........................4
3.3.2 GalliumPrecursors . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.3.3 NitrogenPrecursors...........................59
3.3.4 AGrowthModelforGaN.......................62
4 Experimental and Characterization Methods 73
4.1 SetupoftheMOVPESystems.........................73
4.2in-situGrowthMonitoring.76
4.2.1 In-situNormalizedReflectometry...................7
4.3ex-situLayerCharacterization.79
4.3.1 DifferentialInterferenceContrastMicroscope . . . . . . . . . . . . . 79
4.3.2 SEM................................... 80
4.3.3 AFM...................................82
4.3.4 Photoluminescence...........................82
4.3.5 ThicknesMeasurements........................84
iii CONTENTS
4.3.6 RBS...................................84
4.3.7 X-RayDiffractionMeasurements...................86
4.3.8 HalEffectMeasurements ....................... 8
5 Growth of GaN in an AIX 200/4 RF-S System 89
5.1GaNtwo-stepgrowthprocess.........................90
5.2 InfluenceofNucleationParameters . . . . . . . . . . . . . . . . . . . . . . 94
5.2.1 InfluenceofNucleationTemperature . . . . . . . . . . . . . . . . . 95
5.2.2 IofNTime. . . . . . . . . . . . . . . . . . . . . . 98
5.2.3 InfluenceoftheNH Flow(V/IIratio)................101
3
5.3 InfluenceofTemperatureintheGaNEpilayerGrowth . . . . . . . . . . . 103
5.4 IofSubstrateMisorientation . . . . . . . . . . . . . . . . . . . . . 115
5.5 GaNMOVPEGrowth:ModellingandExperiments . . . . . . . . . . . . .126
5.6 GrowthReproducibilityInvestigations . . . . . . . . . . . . . . . . . . . . 142
6 Growth of (Al)GaN in an AIX 200 RF System 149
6.1 OptimizationofStructureswithAlNNucleationLayer . . . . . . . . . . . 150
6.2ofSwithGaNNLayer . . . . . . . . . . . 159
7 Summary and Conclusions 165Chapter 1
Introduction and motivation
Inthelastdecades,researchinthefieldofGaNandrelatedmaterials(AlN,InNandtheir
ternarycompounds)expandedexponentially.
The group-III nitrides with their special crystal structure and strong chemical bonds
coveralargerangeofbandgapsfrom1.9eV(InN)to6.2eV(AlN),formingacontinuous
anddirectbandgapalloy(Fig.1.1).
Figure 1.1: Bandgap and the lattice constant for the nitrides and other technologically
importantsemiconductors
These give group-III nitrides unique optical and electronic properties, different from
the ones of traditional semiconductors, such as Si, GaAs and InP making them suitable
foralargespectrumofapplicationsfromblue-greenlasers(LDs)andblue-greenorwhite
12 CHAPTER 1. INTRODUCTION AND MOTIVATION
light-emitting diodes (LEDs) to solar-blind detectors and high power, high frequency
and high temperature electronic devices. The next chapter presents some fundamental
propertiesofgroup-IIInitrides,especiallyforAl Ga Ncompounds(Chapter2).x 1−x
SincethefirstdemonstrationsofGaNbasedlight-emittingdiodesmadebyPankove et
al[1]andofstimulatedemissioninGaNunderopticalpumpingobtainedbyDingle et al
[2] in 1971, the interest of researchers was focused mainly on optoelectronic applications
forthegreentoUVspectralrange.
Further progress was quite slow in comparison with other III-V semiconductors due
tothelackofappropriatesubstrates. WhileforGaAsandInPtheexistenceoflargearea
substrates makes homoepitaxial growth possible, for GaN this option is not available.
AlthoughalotofeffortshavebeenmadewiththeaimtoobtainlargeareaGaNsubstrates
bygrowingGaNcrystalsatveryhighpressurethisstillprovidesatoosmallsizeofcrystals
for applications. Another promising approach is to obtain large area free standing GaN
substratesbymeansofHVPE(HydrideVaporPhaseEpitaxy).Sincealsothisapproachis
notwellestablishedyet,thegroupIIInitridesgrowthwasandstilliscommonlyachieved
heteroepitaxiallyonmismatchedsubstratessuchassapphire,SiCorSi.
WithalltheeffortsofcrystalgrowerstoimprovethepoorcrystallinequalityofGaN
and to control the thickness and the conductivity, it took many years until a new GaN-
based LED structure was grown by Kawabata et al[3]. HeemployedMOVPE(metal-
organicvaporphaseepitaxy),agrowthmethodintroducedin1971byManasevit et al[4],
usingtriethylgalliumandammoniaassourcegasesandobtainingGaNfilmsonsapphire
and SiC substrates. Despite the introduction of a new growth method for nitride based
LEDs, the performance was still limited by the poor quality of the material. Especially
the large number of N-vacancies had the effect of a very high n-type background carrier
19 20 −3concentration (∼10 −10 cm ), which made the conductivity control, necessary for
deviceapplications,impossible.
The problems related to the heteroepitaxial growth of GaN were first overcome by
AmanoandAkasakiin1986[5]. TheyintroducedathinAlNlayer,grownonsapphireat
lower temperature (LT) than the subsequent GaN layer, in a so-called two-step growth
technique. Later,NakamurausedGaNaslowtemperaturelayer[6]. Infact,thisLT-layer,
thesocallednucleationlayer,actslikeatemplatebetweenthesubstrateandtheepitaxial
layer,resultinginasignificantimprovementofthematerialqualityofthesubsequentGaN
17 −3layersandadramaticreductionofthebackgrounddopingconcentration(10 cm )[7].
Then, n-type doping of GaN was achieved in the late 80’s by using Si as the dopant
[8]. To achieve p-type doping proved to be more difficult. In earlier studies Zn was used
as a p-type dopant [9]. Since the control of the Zn incorporation was difficult, this was
later replaced by Mg [10]. Even though Mg can beorated more easily, still p-type
conductivity was not achieved, because the added Mg was mostly electrically inactive in
theas-grownmaterial. ThisisduetothepassivationoftheMgatomsbyhydrogen,which
isincorporatedduringthegrowthprocess. Whiletheexactreasonforthispassivationis
stillnotclear[11],AkasakiandcoworkersdiscoveredthattheMgatomscanbeactivated3
by a low energy electron beam irradiation treatment (LEEBI) [12]. Later Nakamura
found, that this can also be done by thermalt in a nitrogen atmosphere, which
isnowthemostcommonlyusedprocedure[6].
The establishment of a new route to obtain high quality nitride structures using a
LT layer as well as the conductivity control of GaN together with the growth of highly
luminescent InGaN [13] led to the fabrication and commercialization of blue and green
LEDs [14] as well as violet laser diodes (LDs) [15]. If for GaN based LED applications,
8 10 −2thehighdensityofdefects(dislocations)isstilltolerable(typically10 −10 cm ),for
LDsitmustbeminimizedinordertohavealongerlifetimeofthedevices. Aremarkable
step was the introduction of epitaxial lateral overgrowth (ELO) which can reduce the
7 −2dislocationdensityinGaNlayersgrownbyMOVPEbelow10 cm [16]. ELO uses a
dielectric mask such as SiO or SiN patterned on a GaN film with stripe windows inx x
themask. Theselectivelydepositedmaterialgrowsverticallytothetopofthemaskand
laterally over the mask and has a lower defect density. The contribution of this method
to the improvement of GaN material was proved by Nakamura [17] who succeeded to
fabricateaGaN-basedbluelaserdiodewithalifetimeover10000hours.
All these breakthroughs have driven the improvement of the quality of GaN-based
materialstoapointwherehighperformanceelectronicdevicesforhightemperature,high
frequency and high power applications can be produced. Of great technological interest
forapplicationssuchasradar,satellite,wirelessbasestationsandpowerapplicationsare
highelectronmobilitytransistors(HEMTs),whichhavebeenbyfarthemostinvestigated
group-III nitrides electronic devices. The nitrides are well-suited to high temperature
applicationsbecauseoftheirwidebandgapsandlowintrinsiccarrierconcentrations. The
electronmobilityinGaNisquitehighandisevenhigherinundopedorselectivelydoped
Al Ga N/GaNheterostructureswheretwo-dimensionalgases(2DEG)mayform,whichx 1−x
increase the carrier density without producing scattering due to ionized dopants. The
sheetconductivityisenhancedbythestrongpiezoelectricandpolarizationeffectspresent
intheAl Ga N/GaNh