High‐qualityInPonSiandconceptsformonolithicphotonicintegration

CARLJUNESAND

DoctoralThesisinMicroelectronicsandAppliedphysics

Stockholm,Sweden2013

MaterialsandNanoPhysics

SchoolofinformationandCommunicationTechnology

KTHRoyalInstituteofTechnology

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High‐qualityInPonSiandconceptsformonolithicphotonicintegration

AdissertationsubmittedtotheRoyalInstituteofTechnology,Stockholm,Sweden,inpartialfulfillmentoftherequirementsforthedegreeofDoctorofPhilosophy.

ISBN978‐91‐7501‐813‐3TRITA‐ICT/MAPAVHReport2013:05ISSN1653‐7610ISRNKTH/ICT‐MAP/AVH‐2013:05‐SE

©CarlJunesand,2013

PrintedbyUS‐AB,Stockholm,2013

Coverpicture:ArtisticdrawingofamicrodisklaserevanescentlycoupledtoaburiedSiwaveguide.

 

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AbstractAs the age of Moore’s law is drawing to a close, continuing increase in computingperformance is becoming increasingly hard‐earned, while demand for bandwidth isinsatiable.Onewayofdealingwith this challenge is the integrationof activephotonicmaterialwithSi,allowinghigh‐speedopticalinter‐andintra‐chipconnectsononehand,andtheeconomiesofscaleoftheCMOSindustryinopticalcommunicationsontheother.OneofthemostessentialactivephotonicmaterialsisInP,stemmingfromitscapabilityincombinationwithitsrelatedmaterialstoproducelasers,emittingatwavelengthsof1300and1550nm,thetwomostimportantwavelengthsindata‐andtelecom.

However, integrating InP with Si remains a challenging subject. Defects arise due todifferences in lattice constants, differences in thermal expansion coefficients, polarityand island‐like growth behavior. Approaches to counter these problems includeepitaxiallateralovergrowth(ELOG),whichinvolvesgrowingInPlaterallyfromopeningsinamaskdepositedonadefectiveInP/Sisubstrate.Thisapproachsolvessomeoftheseproblemsbyfilteringoutthepreviouslymentioneddefects.However,filteringmaynotbe complete and the ELOG and mask themselves may introduce new sources forformationofdefectssuchasdislocationsandstackingfaults.

Inthiswork,thevariouskindsofdefectspresentinInPELOGlayersgrownbyhydridevaporphase epitaxyon Si, and the reason for their presence, aswell as strategies forcounteracting them, are investigated. The findings reveal that whereas dislocationsappear in coalesced ELOG layers both on InP and InP/Si, albeit to varying extents,uncoalesced ELOG layers on both substrate types are completely free of threadingdislocations.Thus,coalescence isacriticalaspect in theformationofdislocations. It isshown that a rough surface of the InP/Si substrate is detrimental to defect‐freecoalescence.Chemical‐mechanicalpolishingofthissurfaceimprovesthecoalescenceinsubsequentELOGleadingtofewerdefects.

Furthermore,ELOGonInPsubstrateisconsistentlyfreeofstackingfaults.ThisisnotthecaseforELOGonInP/Si,wherestackingfaultsaretosomeextentpropagatingfromthedefectivesubstrate,andarepossiblyalsoformingduringELOG.Amodeldescribingtheconditions for their propagation is devised; it shows that under certain conditions, amask height to opening width aspect ratio of 3.9 should result in their completeblocking.Astothepotentialformationofnewstackingfaults,theformationmechanismis not entirely clear, but neither coalescence nor random deposition errors on lowenergy facets are the main reasons for their formation. It is hypothesized that thestackingfaultscanberemovedbythermalannealingoftheseedandELOGlayers.

Furthermore, concepts for integrating an active photonic device with passive SicomponentsareelucidatedbycombiningSi/SiO2waveguidesusedasthemaskinELOGandmulti‐quantumwell (MQW) lasers grownonELOG InP. Such a device is found tohave favorable thermal dissipation, which is an added advantage in an integratedphotonicCMOSdevice.

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TableofcontentsAbstract...................................................................................................................................................................3

Tableofcontents.................................................................................................................................................4

Acknowledgement..............................................................................................................................................6

Listofpublications.............................................................................................................................................8

Listofacronyms...............................................................................................................................................10

1 Introduction..............................................................................................................................................12

1.1 Historicaldevelopment...............................................................................................................12

1.2 Currenttrends.................................................................................................................................12

1.3 Integrationofphotonicswithelectronics...........................................................................13

1.3.1 Integrationbybonding.......................................................................................................14

1.3.2 Integrationbyheteroepitaxy...........................................................................................14

1.3.3 Currentstatusandscopeofthesis.................................................................................15

2 Theoreticalbackground.......................................................................................................................17

2.1 Semiconductors..............................................................................................................................17

2.1.1 Crystallinesemiconductors..............................................................................................17

2.1.2 Opticallyactivesemiconductors.....................................................................................17

2.1.3 InP................................................................................................................................................18

2.2 Crystalstrain....................................................................................................................................19

2.2.1 Latticemismatch...................................................................................................................20

2.2.2 Thermalmismatch................................................................................................................20

2.2.3 Effectofstrainonbandgap..............................................................................................21

2.3 Defects................................................................................................................................................21

2.3.1 Dislocations.............................................................................................................................22

2.3.2 Stackingfaults.........................................................................................................................24

2.3.3 Twinningdefects...................................................................................................................25

2.3.4 Inversiondomainboundaries.........................................................................................25

2.4 Crystalgrowthbyepitaxy..........................................................................................................26

2.4.1 Hydridevaporphaseepitaxy...........................................................................................27

2.4.2 Epitaxialgrowthmodes.....................................................................................................28

2.4.3 Heteroepitaxy.........................................................................................................................30

2.4.4 Epitaxiallateralovergrowth............................................................................................30

2.4.5 Polishingandplanarizationtechniques......................................................................31

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2.5 Characterization.............................................................................................................................32

2.5.1 Atomicforcemicroscopy...................................................................................................32

2.5.2 Photoluminescence..............................................................................................................33

2.5.3 Cathodoluminescence.........................................................................................................35

2.5.4 Transmissionelectronmicroscopy...............................................................................36

2.6 Photonicdevices.............................................................................................................................37

2.6.1 Semiconductorlasers..........................................................................................................37

2.6.2 Waveguidelaser....................................................................................................................39

2.6.3 Microdisklaser.......................................................................................................................39

3 Summaryofresultsanddiscussion................................................................................................41

3.1 ELOGofInPonSiforlargeareacoverage...........................................................................41

3.1.1 Layermorphology................................................................................................................42

3.1.2 DefectsinELOGlayers........................................................................................................45

3.2 ELOGofInPformonolithicallyintegrateddevices.........................................................53

3.2.1 MonolithicallyintegratedlaserbyELOG....................................................................53

3.2.2 Deviceconcept........................................................................................................................54

3.2.3 OptimizationofELOGonInP...........................................................................................55

3.2.4 OptimizationofELOGonInP/Si.....................................................................................56

3.2.5 Fabricationofdevices.........................................................................................................66

4 Summaryandconclusions..................................................................................................................69

5 Futurestudies...........................................................................................................................................71

6 Summaryofappendedpapers..........................................................................................................72

References...........................................................................................................................................................77

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AcknowledgementFirstandforemost,Iwouldliketoexpressmyutmostgratefulnesstomysupervisorandmentor,SebastianLourdudoss,forhisenduringguidance,supportandenthusiasm.Fewmanage to combine such a vast expertise with such a humble attitude and greatcharacterashedoes.FortheyearsunderhisleadershipIameternallygrateful.

Second,IwouldliketoextendmydeepestgratitudetoSvenValerio,whosetraininghasinmanywayscomplementedthatofmysupervisorextendingtosuchdiverseareasasvacuum systems, electronic circuits and peak oil. His resourcefulness and technicalaptitudehastimeaftertimeprovedinvaluabletotheoperationoftheHVPE,justashisstraight‐forward willingness to educate the lesser knowing has been to myunderstandingofthesame.

I also want to express my most heartfelt gratitude to Gunnar Andersson, the grandmasterofepitaxyandtheembodimentofatrueengineerinthebestpossiblesense;hisenormous knowledge, attention to detail, and technical cunning is rivaled only by hisincrediblekindheartedness.Notechnicalchallengeseemstoogreatforhim.

SpecialrecognitionisfurthermoreawardedJesperBerggrenforhistechnicalassistance,insightful views, and excellent music taste. His stoic calmness irrespective of thecircumstances,hisboundlesshelpfulnessandhisgreatsenseofhumorhavebeenahugereliefwhentimeshavegottentough.

IamalsodeeplygratefultoFredrikOlssonandMing‐HongGauforintroducingmetotheworldofscientificresearchinafriendlyway,aswellasgettingmestartedinrunningtheHVPE. Iwouldalso liketo thankmycolleaguesZhechaoWang,WondwosenMetaferia,HimanshuKataria, ChenHu andYan‐Ting Sun for a great collaboration and countlessdiscussions, some of themmore sprightful than others, duringmeetings, lunches andcoffee breaks. I have truly enjoyed your company. I have also enjoyed the assistance,friendshipandcompanyofAnandSrinivasan,whomIalsowanttothankespeciallyforhelpwiththeAFMandmanyenlighteningconversations,andMattiasHammar,whomIwanttothankformanyhelpfuladvicesandamusingchats,aswellasofLechWosinski,Henry Radamson, Hans Bergqvist, Mikael Östling, Carl‐Mikael Zetterling, Jan Linnros,GunnarMalm, Fredrik Lindberg,QinWang,OscarGustafsson (who is not only a goodfriend and colleague but also a formidable striker), Reza Sanatinia, ShaguftaNarueen,Naeem Shahid, ThomasAggerstam, RickardMarcks vonWürtemberg, AudreyBerrier,PriteshDagur,YuXiang,CarlReuterskiöld‐Hedlund,XingangYu,ThomasZabel,FeiLou,NagarajanMony,BikashDevChoudhury,ApurbaDev,MohammadSaad,AhmadAbedin,Luigia Lanni, Benedetto Buono, Terrance Burks, Katarina Smedfors, and ChristophHenkel.

Ihaveduringthisthesisworkbeencollaboratingwithseveralpeopleworthyofspecialacknowledgement: Dr. Galia Pozina and Prof. Lars Hultman of Linköping University,Prof.JuanJiménezofUniversityofValladolid,Dr.PabloPostigooftheMadridInstituteofMicroelectronics,Prof.JohnBowers,Dr.PhilMages,Dr.NickJulian,andChongZhangof

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UniversityofCalifornia,SantaBarbara,Dr.HyundaiPark,Dr.RichardJonesandDr.Hai‐FengLiuatIntelcorp.,Prof.IkaiLoofNationalSunYat‐SenUniversity,Kaohsiung,Prof.FransiscoMiguelMorales,JesusHernandezandProf.SergioMolinaofCadizUniversity,Dr. Aouni Abdessamad of FST Tanger, Prof. Pirouz Pirouz of Case Western ReserveUniversity,Cleveland,Dr.FabriceRaineri,Dr.GillesPatriarcheandAlexandreBazinattheNationalCenterforScientificResearch,Marcoussis,aswellasProf.S. J.BenYooofUniversityofCalifornia,Davis,.

I am furthermore grateful to all my thesis and intern workers: Yu Xiang, CharlesObikuro‐Adagbon,ChenHu,andSanitaZike.

In administrative issues, Marianne Widing and Madeleine Printzsköld have beenirreplaceableanddeservemydeepestgratefulnessfortheirhelpfulnesswithallsortsofthings.

ManythanksalsotoNilsNordell,AleksandarRadojcic,ChristerLindström,PerWehlin,TommySöderberg (who regretfully leftus all far tooearly),AndersLändin, andTimoSöderqvistformaintainingthelabingoodorderaswellasforhelpin‐andoutsidethelabwithvariousissues,bigorsmall.

Ialsowanttothankthepeopleworkinginthelabformakingitafriendlyandenjoyableworkingenvironmentwhereassistanceisneverfaraway:ArmanSikiric(towhomIamespeciallygratefulforhelpwiththeSEM,harmonicalessonsandenjoyablediscussions),Carl Asplund, Reza Nikpars, Smilja Becanovic, Maria Vornanen, Jan Borglind, WlodekKaplan, Susanne Almqvist, Sirpa Persson, Per‐Erik Hellström, Cecilia Aronsson, RogerWiklund,MagnusLindberg,PiaTinghag,BjörnSamel,HenryBleichner,NiclasRoxhed,ChristianRidder,GabrielRoupillardandAdolfSchöner.

Finally,Iwouldliketothankmyfriendsandfamily,andaboveallmyparentsforalwayssupportingandbelievinginme,evenattimeswhenImyselfdonot.

CarlJunesand

Stockholm,August2013

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Listofpublications

Listofpublicationsincludedinthethesis

A. C.Junesand,F.Olsson,Y.Xiang,M.‐H.Gau,andS.Lourdudoss,“Surfacemorphologyofindiumphosphidegrownonsiliconbynano‐epitaxiallateralovergrowth”,PhysicaStatusSolidi(C),vol.6,no.12,pp.2785–2788,2009.

B. C.Junesand,Z.Wang,L.Wosinski,andS.Lourdudoss,“InPOvergrowthonSiO2forActivePhotonicDevicesonSilicon”,ProceedingsofSPIE,PhotonicsWest,vol.7606,no.02,pp.1–10,2010.

C. W.Metaferia,C.Junesand,M.‐H.Gau,I.Lo,G.Pozina,L.Hultman,andS.Lourdudoss,“Morphologicalevolutionduringepitaxiallateralovergrowthofindiumphosphideonsilicon”,JournalofCrystalGrowth,vol.332,no.1,pp.27–33,2011.

D. Z.Wang,C.Junesand,W.Metaferia,C.Hu,L.Wosinski,andS.Lourdudoss,“III–VsonSiforphotonicapplications—Amonolithicapproach”,MaterialsScienceandEngineering:B,vol.177,no.17,pp.1551–1557,2012.

E. C.Junesand,C.Hu,Z.Wang,W.Metaferia,P.Dagur,G.Pozina,L.Hultman,andS.Lourdudoss,“EffectoftheSurfaceMorphologyofSeedandMaskLayersonInPGrownonSibyEpitaxialLateralOvergrowth”,JournalofElectronicMaterials,vol.41,no.9,pp.2345–2349,2012.

F. H.Kataria;C.Junesand;Z.Wang;WMetaferia;Y.T.Sun;S.Lourdudoss;G.Patriarche;A.Bazin;F.Raineri;P.Mages;N.JulianandJ.E.Bowers,“TowardsamonolithicallyintegratedIII‐Vlaseronsilicon:optimizationofmulti‐quantumwellgrowthonInPonSi”,SemiconductorScienceandTechnology,vol.28,no.9,pp.094008‐1–094008‐7,2013.

G. C.Junesand,M.‐H.Gau,Y.‐T.Sun,S.Lourdudoss,I.Lo,J.Jimenez,P.A.Postigo,F.M.M.Sánchez,J.Hernandez,S.Molina,A.Abdessamad,G.Pozina,L.Hultman,andP.Pirouz,“DefectreductioninheteroepitaxialInPonSibyepitaxiallateralovergrowth”,Manuscript,SubmittedtoMaterialsExpress,2013.

H. C.Junesand,H.Kataria,W.Metaferia,N.Julian,Z.Wang,Y.‐T.Sun,J.Bowers,G.Pozina,L.Hultman,andS.Lourdudoss,“StudyofplanardefectfilteringinInPgrownonSibyepitaxiallateralovergrowth”,Manuscript,2013.

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Relatedworknotincludedinthethesis

Journalpapers

1. W.Metaferia,J.Tommila,C.Junesand,H.Kataria,C.Hu,M.Guina,T.Niemi,andS.Lourdudoss,“SelectiveareaheteroepitaxythroughnanoimprintlithographyforlargeareaInPonSi”,PhysicaStatusSolidi(C),vol.9,no.7,pp.1610–1613,2012.

2. W.Metaferia,P.Dagur,C.Junesand,C.Hu,andS.Lourdudoss,“Polycrystallineindiumphosphideonsiliconusingasimplechemicalroute”,JournalofAppliedPhysics,vol.113,no.9,p.093504,2013.

Conferencecontributions

3. C.Junesand,F.Olsson,andS.Lourdudoss,“Heterogeneousintegrationofindiumphosphideonsiliconbynano‐epitaxiallateralovergrowth”,in2009IEEEInternationalConferenceonIndiumPhosphide&RelatedMaterials,2009,pp.59–62.

4. C.Junesand,W.Metaferia,F.Olsson,M.Avella,J.Jimenez,G.Pozina,L.Hultman,andS.Lourdudoss,“Hetero‐epitaxialindiumphosphideonsilicon”,inSPIEProceedingsVol.7719,SiliconPhotonicsandPhotonicIntegratedCircuitsII,2010,vol.7719,p.77190Q–77190Q–9.

5. C.Junesand,C.Hu,Z.Wang,W.Metaferia,andS.Lourdudoss,“Optimisationofseedandmasksurfacesinepitaxiallateralovergrowthofindiumphosphideonsiliconforsiliconphotonics”,inCompoundSemiconductorWeek(CSW/IPRM),2011and23rdInternationalConferenceonIndiumPhosphideandRelatedMaterials,2011,pp.3–6.

6. Z.Wang,C.Junesand,W.Metaferia,C.Hu,S.Lourdudoss,andL.Wosinski,“Amonolithicintegrationplatformforsiliconphotonics”,in2011ICOInternationalConferenceonInformationPhotonics,2011,pp.1–2.

Patents

1. C.JunesandandS.Lourdudoss,“Activephotonicdevice”,U.S.patent#US8290014B2,2012.

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Listofacronyms

AFMAtomicforcemicroscopy

ARAspectratio

AWGArrayedwaveguidegrating

CLCathodoluminescence

CMOSComplementarymetaloxidesemiconductor

CMPChemical‐mechanicalpolishing

CSCompressivestrain

DBRDistributedBraggreflector

DFBDistributedFeedback

DLDDarklinedefect

EDFAErbium‐dopedfiberamplifier

ELOGEpitaxiallateralovergrowth

FCCFace‐centeredcubic

FMFrank‐vanderMerwe

FPFabry‐Pérot

FWHMFullwidthathalfmaximum

HVPEHydridevaporphaseepitaxy

IDBInversiondomainboundary

LPELiquidphaseepitaxy

MBEMolecularbeamepitaxy

MDMicrodisk

MOVPEMetal‐organicvaporphaseepitaxy

MQWMulti‐quantumwell

PCLPanchromaticcathodoluminescence

PECVDPlasma‐enhancedchemicalvapordeposition

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PLPhotoluminescence

QWQuantumWell

RMSRootmeansquare

RTRoomtemperature

SAGSelectiveareagrowth

SEMScanningelectronmicroscopy

SFStackingfault

SKStranski‐Krastanov

SOGSpin‐onglass

SLSStrainedlayersuperlattice

TDThreadingdislocation

TEMTransmissionelectronmicroscopy

TSTensilestrain

VWVolmer‐Weber

WGWaveguide

ZBZincblende

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1 Introduction

1.1 HistoricaldevelopmentSilicon, first discovered by the Swedish chemist Jöns Jacob Berzelius in 1823, is anelementwhichhashadanimpactsoprofoundonsocietysincethebeginningofthe20thcentury that itssignificancerivals thatof iron in the19thcentury. Itwould thusbenomore than fair to call our time “the silicon age” in analogywith previous époques inhuman history revolutionized by a single material. Although not the first materialemployed in a semiconductor transistor, silicon was the material that madesemiconductor transistors practical, thus paving the way for the remarkabletechnologicalprogressoverthelast60years.

While silicon has played a crucial role in microelectronics leading to the highlycomputerized society we live in today, it lacks a fundamental property which is ascriticaltoopticalcommunicationsassemiconductivityistomicroelectronics:theabilitytoemitlightamplifiedbystimulatedemission.Inotherwords,siliconcannotbeusedtofabricate an electrically pumped laser of performance needed for opticalcommunication. Although amplification by stimulated emission was first discussedreferringtopotassiumvapor[1]andthefirstlaserwasachievedbyexcitingatomsinarubycrystal[2],itwasnotlongbeforeitwasrealizedthatanelectricallypumpedlasercould also be made by semiconductors [3], and in particular, by the combination ofsemiconductors with different band gaps in what is called heterostructures, andsubsequentlyinso‐calledquantumwells(QWs)[4],[5].Togetherwiththeinventionoftheopticalfiber,thispavedthewayforanotherrevolutionarytechnologywhichusheredin a new era, not of processing, but of transmitting information; fiber‐opticalcommunication.Two semiconductormaterialswereof particular interest due to theirabilitytoproduceactivelayersthatemittedlightatwavelengthsespeciallysuitablefortransmissioninsilicaopticalfibers;GaAsandInP.Thereasonforthisisthattheopticalfibermadeofsiliconandsilicondioxidehasattenuationminimaat1300nmand1550nmandzerodispersionat1300nm.Furthermore,awavelengthof1550nmallowstheuse of erbium‐doped fiber amplifiers (EDFAs), a crucial part in long haul opticalcommunications. GaAs and their related materials can provide active layers withwavelengths of 650 up to 1250 nm, while InP and their related materials allowwavelengths of 1300 nm and 1550 nm. Thus, although GaAs–based pump lasers forEDFAs, operating at 980 nm are of great importance, InP remains the single mostinteresting basic material in light sources for optical communications over longerdistances.

1.2 CurrenttrendsIn the recent decades, there has been an increasing tendency to incorporate newmaterialsthroughoutthesemiconductorindustry.Thisisperhapsthemostsalienttrendinthetraditionallyexclusivelysilicon‐basedchip‐makingindustry,whichnowincludesvarious metal oxides in gates and capacitors [6] as well as germanium‐containingtransistorsanddetectors [7]. Inphotonicapplications,opticallyactive III‐Vsarebeing

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accompaniedbySi/SiO2‐basedcomponentsformodulationandmultiplexing[8],[9]aswellasbymaterialssuchasGaNinlightsources[10].

Another trend is convergence, not only between short and long distance opticalcommunicationbutalsobetweencomputingandcommunicationaswellasofmaterials.With the limits of silicon having been reached in almost everywhere from gatedielectricstotransistorchannels,nottomentionitslackofabilitytofunctionasagainmaterialforactiveopticaldevices,itmightseemasiftheageofsiliconiscomingtoanend.Inrealityhowevernothingcouldbefartherfromthetruth;siliconhasemergedasthesinglematerialplatformofchoiceforintegratingallsortsofdifferentmaterials,sincesiliconisunmatchedinthesinglemostcrucialaspect:economy.Thereasonsforthisarethat silicon is an abundant and durablematerial with an intermediatemelting point,makingitnotonlyrelativelycheaptoproducebutalsopossibletomanufactureinlargesubstrate sizes with 450 mm wafers representing the next step. This allows hugeeconomiesof scale, a forcewhichhasbeendriving the entire semiconductor industryeversinceitsinfancy.

1.3 IntegrationofphotonicswithelectronicsConvergencebetweencomputingandcommunicationactsasadouble‐edgedsword indriving unification of optically activematerialswith silicon: the chip‐making industryneeds optically active material to continue increasing computing performance andrestrainpower consumption,whereas thedata‐ and telecom industryneeds silicon tokeep the cost down of its increasingly complex modules [11], [12]. An integratedphotonic CMOS chip, showing an example of integrated electronic and photonicfunctions,isshowninFigure1.1.However,achievingopticallyactivedevicesonsiliconrepresentsa tremendouschallengewithavarietyofapproacheshavingbeenorbeingattempted.These fall principally into two categories; thosebasedonbonding existingactive material to Si and those based on depositing or growing active materials onsilicon.

Figure1.1.ConceptualdrawingofaSichipwithCMOSelectronicsintegratedwithvariousphotonicelements.FigurereprintedwithkindpermissionofWondwosenMetaferia.

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1.3.1 IntegrationbybondingBothapproacheshavetheirownadvantagesanddrawbacks:bondinghastheadvantageofproducingworkingdevices[13],[14],[15]sincethematerialisgrowndefect‐freeonnativesubstratesandthentransferredtosiliconsubstrates,buthasthedisadvantageofpoor thermaldissipationdue to the low thermal conductanceof thebondingmedium,absenceof self‐alignmentproviding relatively low integrationdensityandan inherentunsuitability to large scale manufacturing depending on the bonding type; in waferbondingrelyingonbondingentirewafersofIII‐Vmaterialtosilicon,thereareproblemswithsizemismatchsinceIII‐Vwafersaremuchsmallerthansiliconwafersandwithasensitivebondinginterfacesusceptibletoadhesionfailure[16].Indie‐to‐waferbonding,individual dies of III‐Vmaterial are bonded to silicon,whichmakes batch processingmoredifficulteventhougharraysofdiescanbetransferredsimultaneouslybymountingseveraldiesonasinglestamp[17].Moreover, thetransferitselfmaysufferfromyieldproblems.

1.3.2 IntegrationbyheteroepitaxyDevicesfabricatedbygrowingactivematerialonsiliconontheotherhand,fromnowonreferred to as heteroepitaxy, enjoy good thermal dissipation since there is no low‐thermal conductivity medium in‐between the active layer and the substrate, self‐alignment provides high integration density and entire wafers can be processed in asingle lithography, etching or deposition step. Furthermore, epitaxy offers a higherversatilitywithe.g.theabilitytogrowlaserstructureswithdifferentwavelengthsinonestep[18].Thedisadvantageofheteroepitaxystemsfromfundamentaldifferencesinthephysical properties of active materials and silicon; they may have different latticeconstants,differentcoefficientsofthermalexpansionandmayconsistofmorethanoneelement making them polar. All of these aspects share the common property ofintroducing defects which are detrimental to the grown layer, making fabrication ofworkingactivedevicesimmenselydifficultalthoughexamplesofsuchdevicesexist[19].

Nevertheless,aplethoraofstrategieshavebeendevelopedtoovercometheseinherentproblems;growingnonpolarGeandalloysthereofonsiliconandstrain‐engineeringittoproduce lasing [20], [21], growing nanowires of III‐V material with such smalldimensionsthatnorelaxationtakesplace[22],[23],growthofGaSb‐basedcompoundsallowing fabrication of active devices on Si by complete relaxation through edgedislocations[24],growingIII‐Vmateriallattice‐matchedtosilicontoavoiddislocations[25]aswellasemployingmethodstofilteroutdefects.Aso‐calledstrainedsuperlattice(SLS) structure thatwasused toproduce InPmaterialand fabricationof laser thereof[26] is an example of the latter, where strain in the SLS cause dislocations to bendtowardssidewallssothattheydonotpropagatetotheactivelayer.Filteringofdefectscanalsobeachievedbyusingadielectricmaterialasa“mask”thatblocksdislocationsand stacking faults that propagate through the grown layer. A special case of this iscalledepitaxial lateralovergrowth(ELOGorsometimesELOorLEO)whichconsistsofcoveringadefective layerwithmaskmaterial,makingopenings in themaskand thengrowingmaterialfromtheopeningswhichthenextendslaterallyacrossthemask.This

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technique iswidely employed in fabrication of GaN layers on foreign substrateswithdevice‐gradequality[10],[27]andhasbeenusedtoaddresstheintegrationofGaAsandInPonsilicon [28], [29], [30], [31], [32], [33], [34] .Anoverviewof theseapproachescanbefoundin[35].

Although all approaches have resulted in more or less working devices, severalproblems remain unresolved. Firstly, although the material quality that has beenachievedishighenoughfordevicefabrication,thematerialsthemselvesarenotalwaysoptimal for the desired light sources; no device made from heteroepitaxy showsperformanceonparwiththebesthomoepitaxiallygrownInPlasers.Secondly,noviablescheme for integration of heteroepitaxially grown devices with passive silicon‐baseddevices has been proposed. Since InP and related materials, arguably constitute theoptimallasermaterialsintermsoftheapplicationsoutlinedhere,growingInPonsiliconusingELOGisapromisingapproachprovideditsparticularissuescanberesolved,andisthemainsubjectofinvestigationinthisthesis.

1.3.3 CurrentstatusandscopeofthesisThe ELOG technique has been previously used to grow InP on Si with low defectdensities. Improvementshavebeenmadebyoptimizing thepatterndesign to achievemaximum lateral overgrowth and complete filtering of threading dislocations incontinuousfilms.Despitetheseeffortsanumberofissuesremaintobeaddressed:

1. DislocationsarestillpresentinELOGlayers.Thereasonforthisisunclearsincetheoriginofthesedislocationsisnotknown.

2. Stackingfaultisanothertypeofdefectwhichhasnotbeenaddressedtoanylargerextent.

3. AconceptforintegratingactivephotonicInP‐deviceswithpassivesiliconcomponentsisstilllacking.

This work aims to address these pertinent issues and thus has the followingcorrespondingobjectives:

1. DeterminetheoriginandformationmechanismofdislocationsaswellasstackingfaultspresentintheELOGlayeranddevisestrategiesforeliminatingthemornullifyingtheireffectonactivedevices.

2. DeviseaconceptforintegrationofanInP‐basedactivephotonicdevicewithpassiveSi‐basedcomponents.

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Thisthesisisstructuredinthefollowingway.Chapter2providesatheoreticalbackgroundonthegrowthofInP,theuseofELOGinInPonSi,associateddefectsaswellascharacterizationtechniques.Chapter3containsanddiscussesthemostimportantfindingswhereaschapter4sumsuptheconclusionsthatcanbedrawnfromthesefindings.Chapter5offerssuggestionsofdirectionsforfutureresearchandchapter6finallycontainsasummaryofthepapersincludedinthisthesis.

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2 TheoreticalbackgroundThis chapter provides the theoretical background to the subject‐matter in this thesis.Fundamental aspects of semiconductor materials will be addressed with focus onoptically active materials used in optical communication. Techniques for epitaxialdeposition and processing of these materials will then be described with specialattentiongiventotheepitaxialgrowthandprocessingtechniquesappliedinthiswork.Furthermore, characterization methods relevant to this work will be presented andfinallytheconceptbehindphotonicdevicetypesconsideredwillbeoutlined.

2.1 SemiconductorsSemiconductorsareagroupofmaterialswith thecommonpropertyofhavingneitherdiscreteenergylevelsnorasinglecontinuousbandofenergystatesforchargecarriers,butsomethingin‐between:avalencebandandaconductionbandseparatedbyabandgap.Thisbandgapisthekeyfactorintheusefulnessofsemiconductorssinceitallowsnotonlythefabricationoftransistors,akeycomponentinelectronics,butitalsoallowsthefabricationofdevicesemittingaswellasabsorbinglight.

2.1.1 CrystallinesemiconductorsSemiconductorsthatareofparticularinterestandunderfocusinthisworkaretheonesmade up of crystals, so‐called single‐crystalline semiconductors. A crystal is simply amaterialwhereatomsarearrangedinapatternwhichisrepeatedperiodically,referredtoastheunitcell.Theunitcell isthesmallestunitacrystalcanbedividedintoandiscomprisedbyalatticeandabasis.Thelattice,definedbyanumberoflatticeconstants,describesthepositionsinspacemakinguptheunitcell,andthebasisdescribeshowtheatomic group at each position looks like. The simplest lattice is a simple cubic latticehavingonlyonelatticeconstantwithpositionsateachcornerofacube,andthesimplestbasisisaoneatombasiswhereeverypositioncorrespondstosimplyasingleatom.Themost relevant lattice in this work however is the so‐called face‐centered cubic (FCC)latticewithpositionsnotonlyat thecornersbutalso in themiddleof the facesof thecube. Of particular interest in this work is also so‐called compound semiconductorsconsisting of not only one but several elements, placed in two interpenetrating FCClatticesinastructurecalledzincblende(ZB).

2.1.2 OpticallyactivesemiconductorsIn principle all materials interact in various ways with light, or, more generallyexpressed, electromagnetic radiation. Semiconductors do this primarily in a waydeterminedbytheirbandgap;sincethebandgapcorrespondtoadifferenceinenergybetween the valence and conduction band, charge carriers in the valence band mayabsorb an amount of energy equal to or greater than the band gap energy, andconversely,carriers in theconductionbandmayemitenergy inthesameway.Thus,asemiconductormayabsorbandemit lightwitha frequencycorrespondingtothebandgapenergyorhigher.Moreover,asemiconductormaybemadetoemitlightbyhavingacurrentrunthrough itcausing injectionofelectronsandholes to theconductionbandand valence band, respectively, which subsequently recombine with each other by

18  

emittingtheenergyintheformofphotons.Amaterialwhichcanbemadetoemitlightinthis way is called an active material and is of crucial importance in many photonicapplications.

Whereasallsemiconductorshaveabandgap,notallofthemaresuitableaslightsourcesinphotonicapplications.Thereasonforthishastodowiththebandstructure; i.e. thevariationinenergylevelofthebandwithrespecttothewavevectork.Ifaminimumoftheconductionbandcoincideswithamaximumofthevalencebandink‐spacethennochangeinthechargecarriermomentumisnecessaryandthebandgapiscalleddirect.Ifhoweverthesedonotcoincide,thenachangeinchargecarriermomentumisrequiredandthebandgapiscalledindirect.Thishasprofoundeffectsonthelight‐emittingandabsorbing properties pertaining to the fact that the ratio between energy andmomentumismuchhigherforthephotonthanfortheelectron,henceitisnotpossibleforanelectronintheconductionbandtoemitaphotonthatfulfillsboththechangeinenergyandthechangeinmomentum;instead,transitionsalsorequirethesimultaneousabsorptionoremissionofaphononwhich isamuch less likelyevent.For thisreason,semiconductors with indirect band gaps are very poor light emitters as opposed todirect‐bandgapsemiconductors.

2.1.3 InP

2.1.3.1 GeneralpropertiesInP is a compound semiconductor consistingof the elements indiumandphosphorus,locatedinthegroupIIIandVoftheperiodictable,respectively.Sincethereareseveralsemiconductorsofimportancecontainingelementsfromthesespecificgroups,theyarereferredtoasIII‐VsemiconductorsorsimplyIII‐Vs.LikemostIII‐Vs,InPisadirectbandgapmaterialmakingitsuitableforlightemissionandabsorption.However,withabandgap of 1.34 eV at 300K (corresponding to awavelength of 925 nm), InP itself,whileappealinginsomephotonicapplicationssuchassolarcells,isnotthatinterestingforuseasa lightsource;asignificantlymoreinterestingpropertyofInPis its latticeconstant,5.87Å,whichmakes it lattice‐matched toactive layersGaxIn1‐xAsyP1‐yemittingatboth1550nmand1300nm, thetwomost importantwavelengths indata‐andtelecom.Ontheotherhand, InP isratherbrittleandnotasrobustassilicon,whichcomplicates itsmanufacturinginlargersubstratesizes.

2.1.3.2 CrystalstructureInPhasanFCClatticewithatwoatombasis,acrystalstructurewhichissimilartothatof diamond albeit with two types of atoms instead of one, called zinc blende (ZB)structure. Aswith other crystals with this structure, the stacking sequence of atomicplanes is ABABAB… in the case of the <001>, <110> and <1‐10> directions, andABCABC…inthecaseofthe<111>directions[36].Planesfacingdirectionsofthelattertype are of particular importance since they are the most close‐packed, whereas themore sparselypopulated {110} and {1‐10} planes arepreferred cleavageplanes sincethelowdensityofbondsalongthisplanescausethebindingenergybetweenthesetobesmaller.OwingtothepolarnatureofInPsinceInandPhaveslightlydifferentelectrical

19  

charge, the<110>and<1‐10>directionsarenotperfectly symmetrical, and the {111}planes terminated with an In atom (denoted {111}A) are not identical to planesterminatedwithaPatom(denoted{111}B).

2.2 CrystalstrainStrainoccurswhenamaterialiselasticallydeformed;i.e.whenremovingthedeformingforce is removed it will resume its natural shape and the strain is relieved. Once amaterialhasbecomeplasticizedhowever,itcannolongerreturntoitsoriginalnaturalstate by removal of the deforming force since its equilibrium state has becomepermanentlyaltered.Untilplastizationoccurs,thestraininthematerialisproportionalto theamountofdeformation [37]. Inhomoepitaxy, strain isnot that significant sinceeverylayerofatomshasthesamelatticeconstantandcoefficientofthermalexpansionas the substrate, but in heteroepitaxy it becomes an important source of defects. ThestrainεresultingfromagivenstressσisgivenbyHooke’slaw:

C (1)

whereεandσarethestrainandstress(3x3)matricesrespectively,andCisthestiffness(6x6)matrix.By taking thecubicsymmetry foranisotropicmaterials intoaccount, thestiffnesstensorsimplifiesto:

11 1111 12 12

22 2212 11 12

33 3312 12 11

4423 23

4431 31

4412 12

0 0 0

0 0 0

0 0 0

0 00 0 0

000 0 0

0 00 0 0

C C C

C C C

C C C

C

C

C

(2)

FromthisrelationtheYoung’smodulusE,thePoissonratioνandthesheermodulusGmaybederived:

11 12 11 12

11 12

2C C C CE

C C

(3)

12

11 12

C

C C

(4)

2 1

EG

(5)

ThebiaxialstrainmodulusY, i.e.theresistancewhenasubstanceissubjectedtostrainontwoaxes,isgivenby:

20  

1

EY

(6)

and thebiaxial relaxationconstantR, defining the relationshipbetween in‐andout‐ofplanstrain,by:

12

11

2CR

C

(7)

Finally,thebulkmodulusK,i.e.theresponsetouniformcompression,isgivenby:

3 1 2

EK

(8)

2.2.1 LatticemismatchOne important source of strain in heteroepitaxy is the difference in lattice constantsbetweendifferentmaterials,referredtoas latticemismatch.Thismismatchintroducesstrain in the epitaxial layer since it is forced to adapt to the lattice constant of thesubstrate it is being deposited on. At first, the layerwill be able to accommodate thestrain by deforming vertically, and, if possible, laterally. Eventually however, afterexceedingacriticalthickness,theenergystoredasstrainwillbecomesogreatthatthelayerwillrelaxbyformingdislocations.Thestraininanheteroepitaxiallayerresultingfrommismatchisgivenby[37]:

0

0

sm

a a

a

(9)

where ε is the elastic strain in the epilayer, δ is the plastic strain, i.e. strain that isrelieved by dislocations, and as and a0 are the substrate and overlayer latticeparameters,respectively.Inanepitaxiallayergrownonaforeignsubstrate,thelayerissubjected to biaxial strain in the plane of the substrate (normally the (001) plane),whichifunrelievedtranslatestoastrainintheverticaldirectionaccordingto:

11

12

1

2B

C

R C (10)

2.2.2 ThermalmismatchMostmaterials not only have specific lattice constants but also specific coefficients ofthermal expansion. This is highly relevant in heteroepitaxy since epitaxy is normallycarried out at a temperature several hundreds of degrees Celsius higher than roomtemperature,whichmeans that the lattices of two differentmaterialswill contract todifferent extents upon cool‐down. Going from growth temperature to roomtemperature, there will be an amount of strain introduced in the epitaxial layeraccordingto:

21  

0G

RT

th sTdT (11)

where αs and α00 are the thermal expansion coefficients of the substrate and theepitaxial overlayer, respectively, andTG the temperature atwhich growth takesplace.Since thegrown layer isnormallymoreor less relaxedduringgrowth, the introducedthermalstrainmayleadtocrackingandformationofdislocations.

2.2.3 EffectofstrainonbandgapStrain that isnot relievedaffects thebandgapof thematerial since itwill change thevolumeof theunit cell.Thevalenceband ismuchmore susceptible to strain than theconduction band. If the biaxial strain in the (001) plane (growth plane) isaccommodated without producing misfit dislocations, the epitaxial layer will distortvertically (i.e., in the growth direction) to an extent given by (10). Thus, due to theanisotropic deformation, different branches of valence band, degenerate underunstrainedcondition,willbedisplacedbydifferentamounts,affectinginparticulartheheavy and light hole band gaps. Thus, a compressive strain in the growth plane willincrease the band gap since the valence band will be displaced to lower energies,whereasatensilestrainwillhavetheoppositeeffect.Theshiftingoflightandheavyholebandsrespectivelybecome[38]:

11 12 11 12

11 11

22lh

C C C CE a b

C C

(12)

and

11 12 11 12

11 11

22hh

C C C CE a b

C C

(13)

whereaandbarethehydrostaticandsheardeformationpotentialsrespectively,Cijthecorrespondingelementsofthestiffnessmatrixandε∥isthebiaxialstrainintheplane.Since the latter is positive for tensile strain and negative for compressive strain it isobvious that a compressive strainwill increase the band gapwhereas a tensile strainwilldecreaseit.

2.3 DefectsAsinallcrystals,thereisavarietyofwaysinwhichtheperiodicstructureofthecrystalmaybebroken.Thesedisruptionsoftheorderaredefectswhichgenerallyfallintothreecategories: point defects, line defects and planar defects. Point defects are localperturbations and may consist of impurities, interstitials or vacancies. Of these, theformer two, linedefects,which are in effectdislocations, andplanardefects, inwhichcategorystackingfaultsandmicrotwinsfall,areofgreater importance, inparticular inheteroepitaxywheretheirnegativeeffectsareapersistingproblem[39].

22  

2.3.1 DislocationsDislocations are line defects representing a break of symmetry along a line, calleddislocationline,whichcanbedescribedatanypointbyalinevector.Theyaredefinedbya linevector,aBurgersvectordescribingthedistortionofthelatticealongtheline,and a glide plane onwhich the dislocationmoves. Although perhaps not immediatelyobvious, dislocations cannot terminate inside the crystal and must therefore eitherpropagate to one of its surfaces or form a closed loop. Dislocations can generally besubdivided into edge dislocations and screwdislocations. The fundamental differencebetweenthesetwodislocationtypesisthatwhereastheedgedislocationhasaBurgersvectorperpendicular tothedislocation linevector, thescrewdislocationhasaBurgersvectorparallel to the line vector. Screw dislocations are formed by shearingwhereasedgedislocationsareformedbyinsertinganextrahalf‐planeofatoms,asillustratedinFigure2.1.Inaddition,dislocationscanhavebothscrewandedgecharacter,sothattheanglebetweentheirBurgersandlinevectorscanbeanywherefrom0°to90°.InZB‐typecrystals, themost importantdislocations arepure edge, pure screwand so‐called60°dislocations,thenameofthelatterreferringtotheanglebetweentheirBurgersandlinevectors.

Figure2.1.Schematicillustrationofa)edgedislocationandb)screwdislocationwiththeirrespectiveBurgersvectorsindicatedbyagreenarrow.

Dislocationscanmoveeitherbyglideorbyclimb.GlideisthemovementofadislocationplanecontainingbothitsBurgersvectoranditslinevector,aso‐calledglideplane.Purescrew dislocations constitute a special case since their line vector is parallel to theirBurgersvectorsothatnouniqueglideplaneisdefined.Climbontheotherhandconsistsofmotionoutof theglideplaneandrequiresdiffusionofatomstooccur.Thisprocessrequires much more energy than glide motion and generally only occurs at highertemperatures.

Duetotheperturbationof thenatural lattice,dislocationsarealwayssurroundedbyacylindricallyshapedstrain field.Thisstrain increases theenergyof thecrystalsince itrequires work performed on the lattice to arise. By regarding the dislocation as acylinderwith a radial strain field symmetric around the line,where the height of thecylinder increases by bwith one revolution around the line, the elastic strain energyEel(screw)forascrewdislocationbecomes[36]:

23  

0

2 2

( )0

ln4 4

R

el screw

r

Gb dr Gb RE

r r

(14)

whereG istheshearmodulus,b themagnitudeoftheBurgersvectorandr0andR theinnerandouterradiiofthecylinder,respectively.Theinnerradiusdefinestheboundarywheretheelasticmodelbreaksdownasrgoestozero.

Inasimilarway,theelasticstrainenergyEel(edge)ofanedgedislocationcanbederived,where the radius of the cylinder instead of the height increases with b with onerevolution:

0

2 2

( )0

ln4 1 4 1

R

el edge

r

Gb dr Gb RE

r r

(15)

Finally,amixedtypedislocationwillhaveanelasticstrainenergyEel(mixed)describedby:

0

2 22 2 2 2

( )0

1 cossin cosln

4 1 4 4 1

R

el mixed

r

GbGb Gb dr RE

r r

(16)

whereθistheanglebetweenthedislocationlinevectoranditsBurgersvector.

Dislocationsarecommon inheteroepitaxyof lattice‐mismatchedmaterialswhere theycontributetorelievingstrain.However,whereasonlytheedgecomponentsinthe(001)plane of dislocations relieve strain, pure edge dislocations cannot glide to the (001)interface where they can relieve strain more efficiently because their glide plane is(001).SuchacaseisshowninFigure2.2,whereamaterialwithacertainlatticeconstanthas been grown on a substrate with smaller lattice constant. The grown layer hasrelaxedtoitsnaturallatticeconstant,producingmisfitdislocationsattheinterfaceintheprocessassomeatomshaveunsatisfied‘danglingbonds’.

Figure2.2.Misfitdislocationsataheterointerface.

The most common dislocation type formed in systems with small lattice mismatch(<2%) are instead 60° dislocations which can glide to the interface on {111} planes,whereaspureedgedislocationsdoforminsystemswithmismatchhigherthan2%[37],[40].The formerconstituteabiggerproblemsince they threadthrough the layeras itgrows and intersect the layer surface. This means that they will be present in e.g.

24  

subsequently grown QW structures leading to rapid device degradation. Pure edgedislocations do not thread upwards and are less of a problem though they providecharge traps that may impart unwanted effects when current is run through theinterface,asinthecaseofadevicewithtopandbottomcontacts.

Inadditiontocompletedislocations,so‐calledbecausetheirBurgersvectorsconstituteafulllatticevector,twoimportanttypesofdislocationsaretheShockleyandFrankpartialdislocations. These have Burgers vectors of the type 1/6 <112> and 1/3 <111>,respectively, and are usually found bounding a planar defect such as a stacking fault.Sincethesedonotconstitutefulllatticevectors,theyeitherappearinpairs(asShockleypartialsdo),ortheyformclosedloops(asFrankpartialsdo).

2.3.2 StackingfaultsStackingfaults(SFs)areplanardefectsrepresentingadisruptioninthecrystallographicstacking order. In crystalswith FCC‐type lattice they normally occur on {111} planessincethesehavethelowestSFenergy.SuchacaseisshowninFigure2.3.SFscanoccureither as an insertion or removal of a crystallographic plane. Thismay happen eitherduring deposition or by gliding of a plane from its natural position to another. Thus,instead of being stacked according to ABCABC…, the stacking sequence changesaccordingtoABCABA…incaseofextrinsicfault,andABCACBC…incaseofanintrinsicfault.

Figure2.3.Schematicofastackingfaultlyingona{111}plane.

AperfectdislocationmaydissociateintotwoShockleypartialsaccordingtothereaction[36]:

1126

1211

6

1110

2

1

(17)

The splitting reaction is favorable because the total magnitude of the Burgers vectordecreases according to Frank’s rule, which states that a dislocation dissociation isfavorable if the sum of the magnitudes of the Burgers vectors of the resultingdislocationsislessthanthemagnitudeoftheBurgersvectoroftheoriginaldislocation:

25  

2 2 22 2

2 2 2 2 2 22 2 2 2 2 2

110 1 12 4 2

211 12 1 2 1 1 1 2 ( 1)6 6 36 36 3 2

a a a

a a a a a a

Thetwodislocationswillrepeleachotherandastheymoveaparttheycreateastackingfault between them with an energy per area γ. Thus, the total force per dislocationlength unitF is made up of two counter‐acting forces as described by the followingexpression:

2

4

GbF

d

(18)

wheredistheseparationbetweenthetwodislocationsandtheothervariableshavethesamesignificanceaspreviouslydefined.Thetwoforcesbalanceoutattheequilibriumwidthwhichisequaltothestackingfaultwidth.

In the case of Frank stacking faults, interstitials and vacancies generally serve asnucleationsitesfromwhichthefaultgrows.Frank‐typefaultsare,unlikeShockley‐typefaults, bounded by a single dislocation forming a loop, a so‐called Frank loop. AnimportantdifferencebetweenFrank andShockleypartials is theglissilenatureof theShockley partial compared to the sessile nature of the Frank partial. Thismeans thatwhereas Shockleypartials are free tomoveby glide, Frankpartials canonlymovebyclimb,aprocessrequiringmuchmoreenergyaspreviouslydescribed.

There is still no consensus about the formation mechanism of stacking faults duringheteroepitaxial growth, though a number of causes have been identified, includingdissociationofdislocations[41],[42],reliefofstrainduetolatticemismatch[43],[44],[45], interstitialandvacancyformationduetosupersaturation[46],surfaceroughnessand interface defects [47], [48], random deposition errors on crystallographic planeswithlowstackingfaultenergy[49],[50],and,morerecently,todepositionerrorsduetostrain‐distorted bonds [51] and formation of dislocation loops during island growth[52]. It should however be noted that several of thesemechanisms rather than just asingleoneofthemmaywellbeoperating.

2.3.3 TwinningdefectsTwinning defects, or twins, is another kind of planar defect which occurs insemiconductor crystals. Twins arise when the direction of the crystal changes into amirror image in the composition plane, for example when the crystal is under shearstress. Twins are important in BCC and wurtzite systems and may occur insemiconductorswithFCC‐typelatticeaswell.

2.3.4 InversiondomainboundariesInversiondomainboundaries,orIDBs,areofimportanceinheteroepitaxy,inparticularinthecaseofgrowthofpolarsemiconductorssuchasInPonnon‐polarsemiconductors

26  

suchasSi. IDBsarisebecauseapolarsemiconductorcanend in twodifferentwaysatevery plane: with an A atom or a B atom. Thus, when a crystal grain starts with adifferent plane type than another grain, their merging will result in an inversionboundary.Thisisofparticularimportanceinheteroepitaxyofe.g.InPonSi,becauseInPis polar whereas Si is not. One way IDBs may form depends on the lack of naturalpreference of one of the elements over the other of the compound to stick to the Sisurface. Thus, in the case of a binary III‐V compound, in some places, the III‐speciesatomswillcovertheSisurface,whereasinotherplacestheV‐specieswillformthefirstmonolayer. Secondly, and normallymore importantly, the Si surface often contains amultitude ofmonoatomic steps, so that even if the InP layerwill startwith the sameatom,e.g. Inonbothsidesof thestep,onestepisonemonolayerhighersothatat thesameheight,oneplanewillbeanAplaneandtheotheraBplane.AnexampleofthisisshowninFigure2.4.

Figure2.4.Illustrationofaninversiondomainboundary(IDB).

Thefirstproblemcanbesolvedbyapre‐exposureofoneoftheatomicspeciesuntilacomplete surface coverage has occurred [53]. The second problem can be solved bymakingsurethatthesurfacestepsontheSisubstratearedoublemonolayersoranevennumberofmonolayers insteadofanoddnumber inheight.The InP layerwill thenbefreeof IDBs since the species after adoublemonolayer translation is the sameas theoriginal layer. It has been shown that by using Si(001)with 4° off‐cut towards (111),IDBscanbecompletelyremoved[53],[54].

2.4 CrystalgrowthbyepitaxyEpitaxy is the process of depositing crystalline material on an existing crystallinesubstrate, thereby extending the existing single crystal. Epitaxy can generally be sub‐divided into three different techniques: liquid‐phase epitaxy (LPE), molecular beamepitaxy(MBE)andvaporphaseepitaxy(VPE).LPEisoneoftheearliestepitaxialgrowthtechniquesandbasicallyconsistsofsubmergingthesubstrateintoameItofthematerialtobegrown.MBEisbasedonsublimationofsourcesconsistingofsolidmaterialwhichisheatedinanultra‐highvacuumenvironment.VPEfinally isbasedonlettinggaseousprecursors react on the substrate to deposit crystal material while by‐products aresweptawaybyacontinuousgasflow.

VPEinturnisgenerallydividedintometal‐organicvaporphaseepitaxy(MOVPE),orasit issometimescalled,metal‐organicchemicalvapordeposition(MOCVD),andhydride

27  

vapor phase epitaxy (HVPE). Although MOVPE and MOCVD are often usedinterchangeably, they are in fact not identical in meaning; MOCVD denotes chemicalvapordepositionusingmetal‐organicprecursorsingeneralwhichisawiderdefinitionsincethedepositiondoesnotnecessarilyhavetobeepitaxial[55],[56].Anexampleisatomiclayerdeposition(ALD)whichcanusemetal‐organicprecursorsandisgenerallyused to deposit amorphous layers. To avoid confusion, MOVPE is used exclusivelythroughoutthisthesiswiththemorenarrowdefinitionofanepitaxialprocess.

Furthermore,thenameHVPEmaybesomewhatmisleadingsincetheusageofhydridesof V‐species (e.g., PH3 and AsH3) as precursors is not exclusive to HVPE; it is in factcommon to use hydrides in MOVPE as well although organic V‐sources are alsoavailable. Conversely, metal‐organic sources are also frequently employed in HVPE,thoughnormally as dopant precursors. In general, the similarities betweenHVPE andMOVPEsystemsarethecommonrelianceongasflowstotransportprecursorsaswellasby‐products, the cracking of gaseousprecursors by heating, leading to similar growthtemperatures and the similarpressuresboth reactor typesoperate at, typically in therange of 10 – 100mbar, although operation at atmospheric pressure is also possible[57],[58].

2.4.1 HydridevaporphaseepitaxyInthiswork,HVPEisusedtoselectivelyand laterallygrowInPonmaskedsubstrates,whereasMOVPE is employed exclusively in the growthofQWs.The formerbeing themainfocusinthisthethesis,onlyHVPEwillbedescribedingreaterdetail.

Although MOVPE and HVPE both rely on gaseous precursors, there are a number ofimportant differences between them; first of all, HVPE reactors are hot‐wall reactorswhere the entire reactor wall is heated. This means that the precursors are heatedbefore they reach the substrate, in contrast to in an MOVPE reactor where only thesubstrateisheated,normallybyIRlamps.HotwallinHVPEisnecessarytomaintaintheIII‐chloridesintheirgaseousform,whichotherwisecannotbetransportedduetotheirhighboilingpoint.

Secondly, HVPE reactors operate close to equilibrium and as a result growth rate islimitedbyinputrateofreactants,unlikeMOVPEwhichisgenerallylimitedbydiffusionfrommaingasstreamtothesubstrate[59].Finally,HVPEreliesonchlorinecompoundsasprecursorsforthegrouplllelementssuchasInCl,GaClandsoon.Thesearenormallygeneratedin‐situthereactorbyflowingHClthroughmoltengrouplllmetal.AschematicillustrationofpartoftheHVPEreactorchamberisshowninFigure2.5.

28  

Figure2.5.SchematicdrawingoftheHVPEreactorchamberwheredepositiontakesplace.

Inahalide‐basedsystem,ready‐madeV‐chloridesareusedasprecursorswhichinturnalsoproduceIII‐precursorsbyreactingwithtypeIII‐elements,suchase.g.2AsCl3+6Ga→As2+6GaCl.ThisishoweverdisadvantageoussinceitcomplicatesV/IIIratiocontrol.[58].For this reason,usingHCl togenerate the type‐IIIprecursor ispreferable,and isthetypeofsystemusedinthiswork.

Chlorine‐metal compounds have rather low vapor pressures which combined withoperationclosetoequilibriumallowsmoreorlessperfectselectivity,apre‐requisiteforELOG.Selectiveepitaxyorselectiveareagrowth(SAG)ispossibleinMOVPEbutnottothesameextentas inHVPEanddemandsahighenoughratioofopen‐to‐maskedarea[60].Incontrast,theselectivityinHVPEisnotlimitedbysubstrateproperties.

Stemming from its operation close to equilibrium, HVPE enjoys significantly highergrowth rates than MOVPE, which however is a double‐sided coin; whereas highergrowthratesaredesirableinsomeapplicationssuchasplanarizingregrowth,itisalsooneofthereasonsgrowthofabruptinterfacesisinherentlydifficultinHVPE,makingitunsuitableforgrowthofQWsrequiringessentiallyatomicallyabruptinterfaces[61].

2.4.2 EpitaxialgrowthmodesThegrowthmodeisinfluencedpartlybythekineticsduringimpingement,diffusionandreactionofprecursoratomsandpartlybythestrainthatmayoccur in the layer itself.There are generally speaking three differentmodes inwhich growthmay take place:Frank‐van derMerwe (FM), referred to as layer‐by‐layer growth [62], Volmer‐Weber(VW),referredtoasislandgrowth[63],andStranski‐Krastanov(SK)[64],[65],[66].Inthefirstcase,growthtakesplaceoneatomiclayeratatime,inthesecond,growthtakesplaceonallexposedfacetsandinthelastcase,growthinitiallytakesplacelayer‐by‐layerup to a certain thickness after which islands form. The different growth modes areillustratedinFigure2.6.

29  

Figure2.6.Illustrationofa)Frank‐vanderMerwe,b)Stranski‐Krastanov,andc)Volmer‐Webergrowthmechanisms.

Whichgrowthmodeoccursdependsontheenergyofthefreesurfaceofthesubstrateγs,theenergyoftheinterfacebetweenthelayerandthesubstrateγi,andtheenergyofthefree surface of the epitaxial layer γe; for FM‐growth to occur, the epitaxial layermustfullywetthesubstratesurface.Theconditionforthis isthatthesubstratefreesurfaceenergymustbeequaltoorgreaterthanthesumofthefreeinterfaceandepitaxiallayersurfaceenergies:

s i e (19)

This ishoweveranecessarybutnotsufficientcriterion forFM‐growth; if thestrain inthelayeristoolarge,growthmodewillshifttoSK‐growthafterafewlayers.

Conversely,nowettingatall leadingtoVW‐growthwilloccuriftheinterfaceenergyislargerthanthesumofthefreelayerandsubstratesurfaceenergies:

i s e (20)

Wettingwillbepartialforallsituationstherein‐betweensothatthefreesurfaceenergyofthesubstratewillbeequaltothesumofthefreeinterfaceenergyandthedistortedfreesurfaceenergyoftheepitaxiallayeraccordingto:

coss i e (21)

whereθistheanglebetweenthesurfaceofthewettinglayerandthesubstratesurface.Inthiscase,eitherVWorSKgrowthwilloccurdependingonthestrainintheepitaxiallayer.ThedifferentcasesareillustratedinFigure2.7.Furthermore,growthkineticsalsoinfluencethegrowthmodesothatconsideringjusttheinfluenceofthermodynamicsongrowthmodemaybeinsufficient[67],[68],[69].

30  

Figure2.7.Conditionscorrespondingtoa)perfectnon‐wetting,b)partialwetting,andc)perfectwetting.

2.4.3 HeteroepitaxyHeteroepitaxyisdefinedasepitaxialgrowthofonematerialonasubstrateconsistingofadifferentmaterial.Thereareanumberoffactorscomplicatingthisprocesscomparedtohomoepitaxialgrowth:differenceinlatticeconstants,differenceinthermalexpansioncoefficient, polarity in case of compounds and differences in surface energy andconsequently in the wetting of the substrate surface by the precursors. There arebasicallytwowaysinwhichtheseaspectsinfluencetheresultinglayer;ingrowthmodeand in thegenerationofdefects [70].The latteraspecthasalreadybeencoveredwithboth defect types and the circumstances in heteroepitaxy resulting in their formationhavingbeendescribed.Anotheraspectisthesurfacekinetics;dependingontheenergiesofthesubstrateandtheepitaxiallayersurfacesandtheinterfacebetweenthetwo,thedegreeofwettingof thesubstratesurfacewillbedifferent.This in turnwillaffect thegrowthmodeoftheepitaxy.

Inheteroepitaxy,growthproceeds ingeneralaccordingtoVWorSKgrowthmodesaspreviouslyexplained.VW‐typegrowthgenerallyoccurswhenthereisalargemismatchinlatticeconstantbetweenthelayerandthesubstratewhichraisesthesurfaceenergy,or,tobeexact,thesurfacestress[55].SK‐typegrowthoccurswhenthesurfaceenergyislow enough to allow FM‐type growth initially. After a few atomic layer howeverdependingonthedifferenceinlatticeconstants,built‐upstraininthelayerwillleadto3Disland‐likegrowthoncethethicknessexceedsacriticalvalue.

Furthermore, the island‐like growth can in both cases (VW or SK) proceed in twodifferentways:itcanformstableislands,whichwilleitherstopgrowingcompletelyorgrowonlyinthevertical(001)direction,oritmaycontinuewithislandripening,whereislandscontinuetogrowinsizeuntiltheycoalesceandformacontinuouslayer.Whichwaygrowthwillproceeddependsonacomplexrelationbetweensurfaceandinterfaceenergies as well as strain [71]. In addition, epitaxy in general and heteroepitaxy inparticularoftentakesplaceinanon‐equilibriumsituation,wheregrowthkineticsmustalsobetakenintoaccountasstatedearlier.

2.4.4 EpitaxiallateralovergrowthEpitaxiallateralovergrowth(ELOG,ELOorLEO)isatechniquedevelopedtoovercomethedifficultieswithobtainingahigh‐qualityepitaxial layerona foreignsubstrate.Theidea is touseasubstrateofa firstmaterialwitha thin layerofasecondmaterialasastartingpoint.Thelayerofsecondmaterialwillbefullofdefectsduetothepreviously

31  

outlinedmechanisms.Ontopofthislayer,amask,normallyadielectricsuchasSiO2orSi3N4, is deposited and openings in the mask are defined by lithography and etched.Growth is then conducted selectively in the openingswithnonucleation on themask(shown in Figure 2.8 a)). Once the grownmaterial reaches the height of themask, itstartsgrowinglaterallyacrossthemaskwithoutnucleationonit,asshowninFigure2.8b).Inthelaterallygrownparts,propagatingdefectssuchasthreadingdislocationsandstackingfaultswillbeblockedbythemaskandconsequentlycannotpropagateintothelayer above the mask. ELOG has been used to grow GaN [10], [27] as well as GaAs,InGaAsandInPonSi[28],[29],[30],[31].

Figure2.8.a)PrincipleofSAGandb)ofELOGappliedtoheteroepitaxyofInPonSi.

Ithasbeenshownthat theanglebetweenthemaskopeningsandthecrystallographicdirection greatly influences the lateral and vertical growth rates as well as boundingfacetplane [72], and theboundaryplane in turnhasa significant influenceondopantincorporation [73]. Recently, it has also been shown that image forces acting ondislocations close to the mask sidewalls in the openings cause dislocations to bendtowards themask sidewalls, therebyenhancing the filteringeffect so that virtuallynodislocationpropagationthoughthemaskopeningsoccurs[74].

2.4.5 PolishingandplanarizationtechniquesAs a consequence on the mode in which heteroepitaxial growth proceeds, theheteroepitaxial layer normally develops a very unevenmorphology. This unevennesswillinthisworkbedescribedbytwoproperties:topography,referringtolong‐ranging(>1µm)variationsinlayerthickness,andmorphologyorsurfacequality,correspondingto smaller variations on a smaller scale (<1 µm). To describe the first property, stepheight,ordistancebetweenpeaktovalley, isusedasaparameter [75],whereasroot‐mean‐square (RMS) surface roughness, or Rq, is used as a parameter describing thelatter [76]. In the semiconductor industry, chemical‐mechanical polishing (CMP) haslongbeenused for thepurposesofplanarizingandpolishingsemiconductorwafersofvariousmaterials[77],[78],[79].TheconceptbehindCMPistousemechanicalabrasionas well as chemical etching in the same process, drawing on the advantages inplanarizingoftheformerandtheadvantagesinroughnessreductionofthelatter[80].Typically,aCMPequipmentconsistsofarotatingbaseplateonwhichapolishingpadisfixed.Oneorseveralwafersarethenmountedonchuckscounter‐rotatingtotheplateandplaced face‐downon thepolishingpad.Aslurrycontainingabrasiveparticlesand

32  

chemicalsiscontinuouslyflowingontothepolishingpadviaafeedingmechanism.Thematerial removal rate depends on the downward force exerted on the wafer, thepolishing pad material and naturally the rotating speed of the baseplate and waferchuck,aswellasontheslurrypropertiesintermsofabrasiveandetchingeffect.

TheremovalrateisgenerallydescribedbythePrestonequation[79]:

R KPV (22)

whereK is the so‐called Preston constant, P is the downward pressure and V is therelativelinearvelocitywithrespecttothepolishingpad.

Ithasbeenshownthatthestepheighthdecreasesexponentiallywithtimeaccordingtothefollowingfunction[75]:

0

pKV t

Hh h e

(23)

where h0 is the original step height, K is the wear coefficient,H the hardness of thepolishedmaterial, Vp, the relative velocity of the polishing pad, α the stiffness of thepolishingpadandtthepolishingtime.

CMP has successfully been employed to planarize dielectrics [76], [81] as well aspolishing III‐V semiconductors such as InP [78], [82], [83], both of which areadvantageousinheteroepitaxyandmonolithicintegration.

2.5 Characterization

2.5.1 AtomicforcemicroscopyAFM,oratomicforcemicroscopy,isoneofseveralcloselyrelatedtechniquesbasedonscanning probe microscopy (SPM). Their common feature is that they are non‐destructivecharacterizationmethodsbasedonanormallyverysmall(<100nm)probewhichismovedintoclosecontactwiththesurfaceofthespecimentobestudied.Inthecase of AFM, the probe is a very sharp tipmounted on a cantilever which in turn isattachedtoapiezocrystal.Thepiezocaneitherbekeptstaticor itcanbedrivenbyafeedbackloopwhichkeepsthecantileveroscillatingatitsresonancefrequency.Thetipisbroughtintoclose‐contactwiththesamplesurfacecausinginteractionbetweenthetipand the sample by van der Waals and ionic forces. A laser is being shined on thecantilever attached to the tip and then reflected into a photodetector, providing ameasureofthedeflectionwhichisthentranslatedtoasurfaceprofileofthesample.Incaseofresonanceoperation,theoscillationprovidesinputforthefeedbackmechanismin the formof amplitude shift or phase shift of the oscillation compared to the “free”cantileveroscillation.Thecantileveristhenscannedacrossthesurfaceofthesampleinagivenscanwindowproducingatopographic3Dmapofthesamplesurface.

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Becauseoftheweakforcesinvolvedandthesmallsizeoftheprobe(10nmtipradiusisnotuncommon),anAFMisextremelysensitiveandcanprovideaspatialresolutionofdowntosub‐nmscaledependingontheaspectratioofthesurfacefeatures;thegreatertheheightdifferences,thelowertheresolutionduetotheconicalshapeoftheprobetip.Partly for thesamereason,AFMisnotsuitable formeasuringsteep‐walled featuresofheights of greatermagnitudes (>1 µm), and at too large height differences (normallyseveralµm)thecantileverdeflectionbecomestoolargeforthepiezocrystaltorespondto, causing the resonance to break down. Nevertheless, for features of sub‐µm heightand small lateral dimensions, an AFM provides an unsurpassable way of studyingsurfacemorphology,includingdetermininge.g.stepheightsandsurfaceroughness.

2.5.2 PhotoluminescencePhotoluminescence (PL) is another non‐destructive characterization method fordetermining theopticalpropertiesof amaterial and therefromderivedproperties. Itsbasic principle stems from the intrinsic band gap that all semiconductors possess; byilluminatingthesamplewithalaserbeamofenergygreaterthanitsbandgap,electron‐holepairswillbegeneratedaselectrons in thevalencebandwill getphoto‐excited tothe conduction band. Depending on the mean‐free path of the carriers, these willrecombine with each other and by collecting the subsequently emitted light into aphotodetector,aluminescencespectrumofthematerialisattained.Fromthisspectrum,awealthofinformationcanbecollected,e.g.aboutdefects,impuritiesandstraininthelayersinceallofthesepropertiesaffectthebandgap.Figure2.9showsaschematicofaPLsetup.

Figure2.9.SchematicofatypicalPLsetup.

Incidentlight(whichisnotscatteredorreflectedatthesurface)isabsorbed,sothattheintensityIasafunctionofpenetrationdepthisdescribedby[84]:

0xI I e (24)

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whereI0istheinitialintensityofthebeam,xthedepthintothesampleandα,amaterial‐dependentabsorptionconstant.

Typically, lightpenetratesabout1µm(definedby1/α) into thesamplesof interest inthis study so that it is the bulk properties that are affecting the spectrum, althoughsurfaceeventsmayplayapartinsomecases.Therearebasicallytwowaysinwhichthespectrum may be influenced: by providing alternative recombination paths and bydistorting the band gap itself. Defects and impurities fall under the first categorywhereastemperatureandstrainfallunderthesecond.

As mentioned in 2.2.3, crystal strain affects the band gap in a way that depends onwhether the strain is compressive or tensile. The effect of temperature is somewhatmorecomplex,butingeneralahighertemperatureleadstoadecreasedbandgapduetothermalexpansionofthe latticeandinteractionsbetweenelectronsandphonons[85].ThiseffectyieldsthefollowingexpressionforthebandgapenergyofInP(ineV):

4 24.9 10 1.421

327BG

TE

T

(25)

whereEBG is thebandgap ineVandT is thetemperature inK. Ithoweveralsoallowsfillingofhigherenergystates intheconductionbandsothataBoltzmann‐tail towardshigher energy appears at sufficiently high temperature [86]. Finally, temperature alsoaffectsthespectrumbyprovidingthermalactivationofelectronsoutofstateswithintheband gap. Thus, a state inside the band gapmay be apparent in the spectrum at lowtemperature but at higher temperature any electron in that state will be instantlyexcitedtotheconductionbandduetophononabsorptionsothattransitionsfromthatstatetothevalencebandnolongeroccursandthecorrespondingpeakinthespectrumvanishes.

Defectsandimpuritiesprovideadditionalstateswithinthebandgapbutalsoaffectthespectruminmoreparticularways;dopants,supplyingorwithdrawingcarriersbyactingas donors or acceptors respectively, affect the band gap due to interactions betweencarriersandionizedatoms.Specifically,fordonorsithasbeenshownthatthebandgapcanbedescribedbythefollowingexpression[87]:

0 'BG F e cE E E E E (26)

WhereE0istheintrinsicbandgap,EF’thecontributionduetothemodifiedFermienergydue to band‐filling, Ee the contribution from electron‐electron interaction and Ec thecontributionfrominteractionsofelectronswithionizedimpurities.

Forn‐typeInP,thesearegivenbythefollowingexpressions,respectively[87]:

14 2/3 14 2/3' 4.733 10 (1 2.922 10 )FE n n (27)

8 1/32.25 10eE n (28)

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2/32 2 2/34/3 22/3

1/3

1.660 311

3 2ce e BG

nE n

m m E

(29)

wherenistheexcesselectronconcentrationincm‐3,me*istheeffectiveelectronmassinInP(=0.08m0kg,wherem0istheelectronrestmass),ħisthereducedPlanckconstantineV·sandtheenergiesaregivenineV.Whereasimpuritiesintroducestatesthatnormallyresultinradiativetransitions,defectstendtointroducestatesthatcausenon‐radiativetransitions.Therefore, defectsdecrease the intensity of the spectrum inproportion totheirdensity.ItwasfoundthatPLintensitydecreasedwithincreasingthicknessabovecritical thickness for InGaAs grown on GaAs, which the authors attributed to non‐radiative recombination at dislocations [88]. Furthermore, The linewidth of the PLspectrumprovidesanassessmentofthequalityofQW‐structuressinceabroaderpeakimpliesawiderdistributionofQWwidths[86].

2.5.3 CathodoluminescenceCathodoluminescence (CL) is a characterization method related to PL in that it alsoreliesonexcitationandsubsequentrecombinationofcarrierstogaininformationabouta semiconductor material. The fundamental difference between CL and PL is theexcitationsource–inthecaseofPL,theexcitationsourceisalaser,whereasinthecaseofCLit isanelectronbeam.Inpractice,CLiscarriedoutin‐situinascanningelectronmicroscope(SEM)simplybycollectinglightemittedinthespectrumofinterest.Thisisnormallydoneby introducing amirrorwith a tiny slit throughwhich the e‐beamcanpass.LightisthenreflectedandcollectedataphotodetectorjustlikeinthecaseofaPLsetup.

In the case of electrons, the penetration depth can be described by the followingfunction[89]:

12250 / /n

d A E Z (30)

whered is thedepth inÅngström,A is themassnumber,Z the atomicnumber,ρ thedensity,EtheelectronenergyineVandndependsonZinthefollowingway[89]:

1.2

1 0.29logn

Z

(31)

As these expressions suggest, the penetration depth of the electrons is directlydependent on the electron acceleration voltage, which makes it possible to probedifferent depths by varying this parameter, with limits in resolution due to electronscatteringandcarriermovements.

AnotheradvantageofCLstemsfromthefactthatitisperformedin‐situaSEM;byrasterscanning a given area, an imagewith contrast corresponding to the radiative to non‐

36  

radiativerecombinationratiocanbeobtainedinconjunctionwiththe imageproducedbysecondaryorbackscatteredelectrons.Thisimage,ormapasitisnormallyreferredtoas, can be eithermonochromatic, inwhich case only the contrast corresponding to asingle wavelength is used, or it can be panchromatic, in which case the contrastcorresponds to the total intensityof radiativerecombinationsover theentiredetectorspectrum. This is particularly useful for characterizing defects such as stacking faultsand threading dislocations since they affect the recombination in basically twoways:First, by increasing thenon‐radiative recombination, leading todarker contrast in theimage; it has been shown that there exists a one‐to‐one correspondence betweenthreadingdislocationsanddarkspots inCLimagesfordislocationsdensitiesuptotheorderof107 cm‐2 [90], [91], [92]. Second,by segregating impurities, leading to locallyhigher concentrations of impurities with corresponding brighter contrast due to ahigherrateofradiativerecombination[93],[94]

AlthoughaPLsetupcaninprinciplebeusedinthesameway[95], ithas inthisworkmostlybeenemployedtoacquirespectra,whereasCLhasbeenusedtostudyindividualdefectsanddeterminetheirdensity.

2.5.4 TransmissionelectronmicroscopyThe use of electron beams in the study of sampleswith small dimensions representsperhapsthebiggestparadigm‐shiftintheworldofcharacterizationsincetheinventionof the optical microscope. In terms of maximal magnification and resolution, fewcharacterization methods rival transmission electron microscopy (TEM), one of themethodsbasedontheuseofelectrons.Asthenamesuggests,inaTEMsetup,electronsaretransmittedthroughthesampleunderstudyandthencollected.Theinformationisprovidedbythe fact that thetransmittedelectronsarediffractedinthesample,whichfor this reasonmustbea crystal inorder toobtainanyuseful information.Obviously,sinceit isthetransmittedelectronsthatareof interest, ionization,back‐scatteringandabsorptionmustbekeptminimal,whichmeans that thesampleshouldnotbe thickerthanaround100nm,makingsamplepreparationcritical.

A TEM consists of an electron source, condenser lenses to ensure that the beam isparallel, a sampleholder, anobjective, intermediate andprojector lens for alternatingbetweenthediffractionpatternandthemagnifiedimageandfinallyanimagingscreenand/oracamera.

TheresolutionisjustasforopticalmicroscopesgivenbytheRayleighcriterion:

0.61

NA

(32)

whereNAisthenumericalapertureandλthewavelengthoftheelectrons,whichcanbeobtainedfromthefrequency,f,givenbythedeBroglie’sequations:

E hf (33)

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whereE is theelectronenergyandh isPlanck’sconstant.Thewavelengthλ is in turngivenby:

h

p

(34)

wherepisthemomentum.

Bywhich,disregardingrelativisticeffects,theelectronwavelengthλbecomes[96]:

1 2

1.22

E

(35)

where E is the electron kinetic energy in eV and λ the wavelength in nm. A TEM isoperatedinoneoftwodifferentmodes:diffractionmodeorimagemode.Indiffractionmode, the points of constructive interference due to refraction from the crystal areshown,thuscorrespondingtoBraggreflectionsfromthelatticeplanesinthecrystalandgive information about miller indices and lattice constants as well as size andorientation of grains in imperfect crystals. In image mode, the image is made up ofelectronspassingunperturbedthroughthesample.Thecontrast inthis imageisgivenbyperturbationsthatcausescattering,suchasdefects,andthosethatcauseabsorption,suchasthicknessvariation.Whereasthelattereffectisnotveryinteresting,theformerisextremelyusefulsinceitallowsnotonlydetectionofdefectssuchasdislocationsandstackingfaultsbutalsodeterminationofthetypeofdefectaswellasitsBurgersandlinevector.TEMthusconstitutesanexcellentcomplement tootherdefect‐characterizationmethodsoflowerresolutionbutgreatersamplingsizesuchasCLandSEM.

2.6 PhotonicdevicesThefinalgoaloftheintegrationofInPwithSiistheabilitytofabricateactivephotonicdevicesintegratedwithpassiveSiopticalcomponentsandinalongerperspectiveCMOSelectronics.Sincethemostcovetedphotonicdeviceisalaser,thisparticulardevicewillbedescribed in greaterdetail in this chapter. Semiconductor lasers tend to come in avarietyofformswiththeirparticularproperties,initselfafieldfartoolargetodealwithinthiswork.Instead,thispartwill focusononeofthemosttriedandtesteddesignaswellasamorerecentconceptwhichhasbeenemployedinthiswork.

2.6.1 SemiconductorlasersAsemiconductorlaserisnodifferentinitsbasicpropertiesandfunctionfromanyotherlaser – it consists of a gain medium which can be stimulated to emit light, and aresonance cavity where the positive optical feedback takes place. In principle, anymaterial with a direct band gap can be made to lase, provided that the gain is highenoughwithrespecttolosses.IndirectbandgapmaterialssuchasSihaveaspreviouslyexplainedusually too lowgainsince theradiativerecombinationofcarriersneeds theassistance of phonons, reducing the probability of recombination in general andradiativerecombinationinparticular,therebyreducingtherateofstimulatedemission.

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Theconditionforlasingistheachievementoftheso‐calledpopulationinversion,whichin its strictest sensemeans that the probability of a photon encountering an excitedelectron causing stimulated emission is higher than the probability of a photonencountering an electron in theground state andgetting absorbed. Itmay seem thatthisequateswithahigherpopulationinexcitedstatesthangroundstatesbutthisisnota sufficient requirement, since the probability of stimulated emission and absorptionmaynotbethesame.Thus,populationinversionrequiresthattheproductbetweenthenumberofelectroninexcitedstateandtheprobabilityforstimulatedemissionislargerthan the product between the number of electrons in the ground state and theprobabilityofabsorption.

Inordertomakealaserthatcanfunctioncontinuouslywhenacurrentisdriventhroughit,amechanismforenablingtransitionofchargecarriersthatcanrecombineisrequired.Adiode,initssimplestformap‐njunction,fulfillsthisneedwhenconnectedinforward‐bias mode since there is a continuous influx of electrons to the junction where theelectronsrecombinewithholes,emittinglightintheprocess.However,thep‐njunctionisnotverywellsuitedforthispurposesinceitdoesnotprovidemuchconfinementforeitherchargecarriersorlight,whichisarequirementforalasercavity.Moreover,thep‐n junction ismore or less limited to the band gap of thematerial in question,whichcorresponds to a wavelength normally not of interest in a laser. Instead, a so‐calleddoubleheterostructureisused.Thisdesignsharesthen‐andp‐dopedregionsofthep‐njunction but also has a layer of anothermaterial in‐betweenwith a band gap that issmaller than that of the p‐ and n‐doped regions. This structure acts as awell for theincoming electrons which recombine in the material in‐between the p‐ and n‐dopedlayers,thusconcentratingthelightemissiontoasmallervolume.Normally,aternaryorquaternarycompoundsemiconductorisusedastheactivematerial.

Theuseofternaryandquaternarymaterialsallowstailoringthebandgaptothedesiredwavelength by varying one or several constituents. Instead of having twoheterojunctions and one region of activematerial, a so‐calledmultiple quantumwell(MQW) design is often employed. This design consists of multiple layers of activematerialwithintercedingbarriermaterialthatarebothmadeverythin,intherangeofnanometers.Thisintroducesachangeindensityofstatesforelectronsandholesleadingto an enhanced probability of radiative recombination. These factors contribute to anincreased efficiency, reduced linewidth, higher modulation bandwidth and, partlybecause theamountofactivematerial ismuchsmaller,asignificantly lower thresholdvoltagethanregularheterostructureslasers.Finally,anadditionaladvantageofQWsisthat due to the small thickness, the material in them can be strained without theformationofdislocations.TheabilitytogrowstrainedQWsisadvantageoussincestrainwarpstheenergybandssothattheeffectivemassofinparticulartheheavyholecanbedecreased,greatlyincreasingefficiencyandreducingthresholdcurrent[97].

Thesecondpartofthelaseristhecavity,whichconfineslightandprovidesresonance.Since cavities canhaveverydifferentdesigns there isnogeneraldesign forobtaining

39  

confinementbutnormallylightisconfinedinthecavitybyacontrastinrefractiveindexbetween the active material and the surrounding material. It should be noted that‘material’hasaverybroaddefinitionheresince indexcontrastcanbeachievedbye.g.airsurroundingthecavity.Havingarefractiveindexof1,thisprovidesaratherstrongconfinementsinceactivematerialnormallyhasanindexofover3.Asmentioned,alargevarietyofopticalresonantcavitieshavebeenexploited for laserapplicationsofwhichonlytwoarediscussedingreaterdetailinthefollowingchapters.

2.6.2 WaveguidelaserInthecaseofawaveguidelaser,theresonancecavityhasbeenetchedintotheshapeofanelongatedridge,normallyatleast300µmlongandaround2µmwideandacoupleofµm high. A drawing showing the basic characteristics of this laser type is shown inFigure2.10a).Attheshortendsoftheridge, facets,oftenobtainedbysimplycleavingthematerialalongacleavageplaneresultinginanextremelyflatsurface,actasmirrorsreflectingthelightinsidethecavitycreatingalongitudinalmodewhilelettingoutpartofthe light. This design, with two facets acting as mirrors, is called a Fabry‐Pérot (FP)laser. Due to the long cavity, multiple modes may arise; however, instead of cleavedfacets,itcanalsohaveanetchedgratingprovidingreflectionattheshortends,allowingselection of a specific wavelength. This design is referred to as a distributed Braggreflector(DBR)lasertoseparateitfromtheFPdesign.Ifthegratingisintegratedtotheactive region as well, the design is referred to as a distributed feedback (DFB) laserinstead.

2.6.3 MicrodisklaserAmicrodisklaser,likethenamesuggests,hasaresonancecavityformedintheshapeofadisk.Inthiscase,therearenomirrors,buttheindexcontrastbetweenthediskandthesurrounding air gives rise to modes where light moves along the circumference, so‐calledwhisperinggallery‐modesaftertherelatedphenomenoninthewhisperinggalleryinStPaul’scathedralinLondon[98],[99].Thus,thelightcirculatesinsidethediskandisonlyemittedthroughthesidewallsbyscattering.Inthisdesign,lightisnotintendedtobeemitteddirectlyintoair,insteaditisnormallyemittinglightintoawaveguideinthecloseproximitybyso‐calledevanescentcoupling[100],asshowninFigure2.10b).Thisphenomenonstems from thewavenatureof lightwhichmeans it isnot localizedat asinglepointorvolumeinspace,butratherspreadsinanevanescentfieldinwhichtheintensityofthelight(or,tobeprecise,theintensityoftheelectromagneticfield)decayswith radial distance from the propagation vector. Thus, by placing the waveguidesufficiently close to the edge of the disc in a direction parallel to the sidewall, theevanescentfieldofthelightinthediskwilloverlapwiththewaveguidesothatpartofthelightwillbecoupledintothewaveguide.Thisconceptisnotlimitedtothedisklaserperse;inthesameway,lightcanbecoupledintoawaveguidefromtheafore‐mentionedridgelaser.However,inthecaseofadisklaser,cleavingfacetsforfree‐spaceemissionas in thecaseofa ridge laser isnotpossible so that lightmusteitherbecoupled toawaveguideorpossiblyby introducingagrating structure causing free‐spaceemission.Dueinlargeparttoapplicationsrequirements,couplingtoawaveguideratherthanfree‐

40  

spaceemissionisdesired,andthemostcommonwaytoextractlighthasbecometheuseofevanescentcouplingwithawaveguide,sinceitisapracticalwaytocouplelightouttoaSiwaveguide.

Figure2.10.Artisticdrawingofa)waveguidelaserandb)microdisklaserevanescentlycoupledtoaburiedwaveguide.

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3 SummaryofresultsanddiscussionThis work can be divided into two sub‐parts with different approaches and slightlydifferent focus;applicationofELOGof InPonSiwith thepurposeofcoveringa largerarea, and application of the same with the purpose of selectively growing integratedphotonicdevicesinaself‐alignedprocess.SincetheyshareincommonthefundamentalconceptsofSAGandELOGthereisobviouslyacertainoverlapinmanyaspects,notleastin the study of defects.paperA,paperCandpaperGdealwith growthof InP on SiaimedatcoveringalargerareawithInP,whereaspaperB,paperD,paperE,paperFandpaperHdealwithgrowthofInPonSiaimedatproducingmonolithicallyintegratedactive devices on Si. As previouslymentioned, the InP ELOG layers were in all casesgrownbyHVPE.

3.1 ELOGofInPonSiforlargeareacoverageInpaperAandpaperC,athinSiO2maskandvariouskindsoflineandmeshopeningswereusedtogrowELOGunderdifferentgrowthconditions.Themorphologyandopticalquality of these layers were studied by AFM and PL respectively, and a quantitativemeasureonthethreadingdislocationdensitywasextractedfromPCLmaps.InpaperG,theoriginand formationofdefects in layers frompaperA andpaperCwerestudiedwithCLandcross‐sectionaswellasplan‐viewTEM.

Inordertocoveranareaaslargeaspossibleataslowthicknessaspossible,highaspectratio in terms of lateral to vertical growth to achieve rapid coalescence is of primeimportance.Previousstudiesonthissubject[74],[101],[102]providedagoodstartingpoint for choosing mask patterns (shown in Figure 3.1.) which were then used toinvestigate the effect of different growth conditions in paper A. In the subsequentinvestigations, InP/Si samples with 40 nm SiO2 mask deposited by plasma‐enhancedchemical vapor deposition (PECVD) patterned with various line and mesh openingsaccordingtoFigure3.1wereused.Anopeningwidthof200nmandaseparationof3µmwas used in all cases, whereas α was 15°, and β was 105° or 120°, in case of meshopenings,whileαwas15°or30°incaseoflineopenings.Hence,twoopeningtypeswithtwodifferent specifications each for a total of four different varietieswere employed.Sulfur‐doped InP (InP:S) ELOG was then grown by HVPE under varying growthconditionsintermsofV/IIIratioandtemperature.

Figure3.1.Schematicsofa)lineopeningsandb)meshopenings.Thepatternswerepreparedwithe‐beamlithography(EBL)unlessotherwisestated.PicturefrompaperG.

42  

AsshowninFigure3.2a)andb),alowerV/IIIratioachievedbyincreasingtheInClflowresulted in higher growth rates whereas a higher temperature first lead to a weakincrease in growth rate followed by a sharp decline in growth rate with furtherincreasingtemperature.Furthermore,theRMSroughnesswasmeasuredandwasfoundto correlate inversely with thickness, with the lowest growth temperaturecorresponding to lower roughness for the same thickness as for higher growthtemperatures.Thiswas takenasan indication that lateralgrowthratewithrespect tovertical growth ratewas higher in the case of the lowest temperature, since a higherlateral‐to‐verticalgrowthrateisexpectedtoleadtohigherdegreeofplanarization.

Figure3.2.a)verticalgrowthrateandb)surfaceroughnessofInP:SELOGlayersgrownfrommeshopenings200nmwidewithseparation3µm,α=15°andβ=105°onInP/Sisubstratemaskedwith40nmSiO2for9minutes.Growthtemperatureina)was600°CandV/IIIratioinb)was10.PicturefrompaperA.

3.1.1 LayermorphologyAlso, ELOG layers exhibited lower roughness than the InP seed layer on Si,while InPgrowndirectlyon the InP seed layeronSi exhibited significantly larger roughness, asapparentfromFigure3.3a)–d).Sincedislocationsleadtoalocallyhighergrowthrate[46], [103], [104] thus exaggerating the roughness stemming from the island‐likemorphology of the InP seed layer [105], this reductionwas likely due to the filteringeffectof themaskcausingtheELOGlayerstopossessa lowerdislocationdensitythantheseedlayer.However,sincetheSiO2maskinheritsthemorphologyoftheseedlayer,roughnessoftheELOGlayerisstillonparwiththatoftheInPseedonSi.

43  

Figure3.3.AFMsurfaceimagesofa)InP(seed)/Sisubstrate,b)planarInPgrownonthesamesubstrate,c)InP:SELOGlayersgrownfrommeshopenings200nmwidewithseparation3µm,α=15°andβ=105°onInP/Sisubstratemaskedwith40nmSiO2for9minutes.Growthtemperaturewas600°CandV/IIIratiowas10.d)homoepitaxialInPgrownunderthesameconditionsasinc).PicturefrompaperA.

Thetime‐resolvedmorphologicalevolutionaswellasthethreadingdislocationdensitywas studied in paper C, where it was shown that depending on the opening type,differentboundaryplanesformedduringELOGpriortocoalescencewhichimpactedatwhatthicknesscoalescencetookplace.Coalescencewasfastestfrombothtypesofmeshopeningswithnoclear“typical”boundaryplanesformingbeforethelayershadbeguntocoalescewhereasincaseof lineopeningstherewasaconsiderabledifferencebetweenlineopeningswith15°off[110]openingsleadingtomuchslowercoalescencethan30°off openings (complete evolution shown in Figure 3.4). Previous studies have shownthatthelateralgrowthratefromopenings30°off[110]isconsiderablyhighercomparedto that of openings 15° off [110] [101]. The absence of growth‐retarding boundaryplanesinthecaseofmeshopeningswasthelikelycauseofthemuchfastercoalescenceofgrowthfromthese.

44  

Figure3.4.MorphologicalevolutionduringInP:SELOGgrowthonInP/Sisubstratemaskedwith40nmSiO2patternedwithdifferentpatterntypes.Growthtemperaturewas600°CandV/IIIratiowas10.Whitearrowsindicatesurfaceundulations,mostlikelycorrespondingtointersectionsbetweenplanardefectsandthelayersurface.PicturefrompaperC.

Interesting to note is thatwhereas the surface roughnesswas significantly higher forELOG layersgrown from lineopeningscompared to thosegrown frommeshopeningsshortly after coalescence, the surface morphology eventually becomes similar for allELOG layerswith almost identical roughness regardless of opening type and angle asshown in Table 1. This is consistent with the growthmodel where impinging atomsprefer to settle at kinks rather thanon flat surfaces [59], [103], [106], so that surfaceirregularitiestendtosmoothenoutsincegrowthlaterallywillbefasterthanverticallyasexplainedearlier.

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Table1.RMSroughnessfortwosampleswithdifferentgrowthtimesforrespectivepattern.

Patterntype

Growthtime

Mesh15°&105° Mesh15°&120° Lines15° Lines30°

9min 47nm 77nm 278nm 182nm

120min 18nm 16nm 23nm 25nm

3.1.2 DefectsinELOGlayersIn paperG, the defects present in the ELOG layers were studied with CL and TEM.Dislocationsthreadingtothelayerandterminatingatthelayersurfacehavebeenshowntoactascentersfornon‐radiativerecombinationandthereforeshowupasdarkspotsinPCL maps [90], an example of which is shown in Figure 3.5. Apart from threadingdislocations (TDs), there are also dark line defects (DLDs) present in the ELOG layer,most likely constituting planar defects whose intersections with the layer surfaceappears as bright lines with an adjacent dark shadow. This “dot‐and‐halo” effect hasbeendescribedearlierandisaresultofdopantsegregationalongthelineofintersectionbetweentheplanardefectandthelayersurface[94].

Figure3.5.a)PCLmapsofunprocessedInPseedlayeronSisubstrateandb)ofInP:SELOGlayergrownfrommeshopeningswithopeningwidth200nmandseparation3µm,α=15°andβ=105°onInP/Sisubstratemaskedwith40nmSiO2.Featurescorrespondingtothreadingdislocations(TDs)anddarklinedefects(DLDs)areindicated.

3.1.2.1 DislocationsItwasshownthatdislocationdensityintheELOGlayerswasdecreased1to2ordersofmagnitudecomparedtothedislocationdensityintheseedlayer.Thiswastrueevenifthe decrease in dislocation density due to increased annihilation with greater layerthicknesswastakenintoaccount,asevidentinFigure3.6.

The threadingdislocationdensityρwith respect to thickness canbedescribedby thefollowingfunction[107]:

46  

2

2

00

4ln

2

1

)1(

)1(2

h

(36)

whereε0andεaretheoriginalandresidualmisfits,respectively,ν isthePoissonratio,andhisthelayerthickness.Assumingthatε0>>ε,thefirsttermcanbeignoredandwithaInP=5.87ÅandaSi=5.43Åthemodelsimplifiestoρ≈4.0∙h‐2forInPonSi.Figure3.6shows this model (solid line) together with the experimental values extracted fromELOGlayersgrownfrommeshopeningsin40nmSiO2maskonInP/Si(diamonds).Alsoshownisthemodeldisplacedtofitthedislocationdensityoftheseedlayer(dottedline),aswellas the trend line for theexperimentaldatapoints(dashed line)and finally theexperimental trend line displaced to fit the seed layer dislocation density (dashed‐dottedline).Theseedlayerdislocationdensityismarkedbyasaltire.

Figure3.6.DislocationdensitywithrespecttolayerthicknessofInP:SELOGlayersgrownunderdifferentgrowthconditionsfrommeshopeningswithopeningwidth200nmandseparation3µm,α=15°andβ=105°onInP/Sisubstratemaskedwith40nmSiO2.Theoriginalandadaptedmodelfrom[107]arealsoshown,aswellasatrendlinefittedtotheexperimentaldatapointsandthesamelinedisplacedtofittheseedlayer.PicturefrompaperG.

Itwasalsonotedthatthemodeloverestimatesthereductionindislocationdensitydueto dislocation annihilation, at least for greater layer thickness. The reduction indislocationdensitywithrespecttothicknessintheELOGlayersinthespectrumof1μmto100μmthickness indeedfollowedapowerfunctionbutatamuchslowerratethantheoreticallypredicted[107].Assumingareductionwiththicknessattheratewhichwasactually observed (shown by the dashed‐dotted line), the reduction in dislocationdensity compared to that of the seed layer becomes two orders of magnitude sinceaccordingtothismodelthedislocationdensityat10µmisoftheorderof109cm‐2evencompared with the adopted model (shown by the dotted line), the ELOG dislocationdensityisatleastoneorderofmagnitudesmalleratthesamethickness.

Whereas different growth conditions in terms of V/III ratio and temperature did notappear tohavesignificant influenceondislocationdensitywhencompensating for thethicknesseffect, therewasacleartendencyforopeningsof linetypetoresult inELOG

47  

layerswithlowerdislocationdensitythanthosegrownfrommeshopenings,asshowninFigure3.7below.

Figure3.7.DislocationdensitythicknessofInP:SELOGlayersgrownunderdifferentgrowthconditionsonInP/Sisubstratemaskedwith40nmSiO2,withrespecttothicknessgroupedbypatterntypedenotedbytheanglesαandβ.PicturefrompaperG.

The origin of the dislocations was determined inpaperG to be both propagation ofexisting dislocations and creation of dislocations during coalescence of ELOG islands.Whichofthesemechanismsdominatedwashoweverdifficulttoascertain.However,inpaperEitwasshownthatpolishingtheInPseedlayerresultedinfewerdislocations,aresult which was attributed to improved coalescence, thus indicating formation as amore important mechanism than propagation. Also, in experiments with large areaELOGgrownfrommultiple lineopenings1µmwideseparatedby2µmpreparedwithoptical lithography,dislocationswerepresenteveninELOGonInPasshowninFigure3.8a),thoughinamuchlowerdensitycomparedtoELOGlayersgrownfromthesameline openings with the same SiO2 mask thickness on InP/Si grown under identicalconditions(showninFigure3.9b)).Thereasonforthisisnotclear, inparticularsincecoalesced layers grown from double line openings on InP substrate (discussed in3.2.4.1) consistently exhibited zero defects. As apparent in the corresponding SEMimage (Figure 3.8 a)), this layer showed a remarkably uneven surface, indicatingimperfectcoalescence.Thismayhavecontributedto the formationofdislocationsandmaybeaneffectofthewideropeningsorglitchesinopticallithographyandsubsequentetching of the SiO2 mask. It is also interesting to note that the PCL map fromhomoepitaxialInPELOGisfreefromDLDs,thusindicatingaverylowdensity(<105cm‐

2)ifanyofthiskindofdefect.

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Figure3.8.a)SEMimageandb)PCLmapofInP:SELOGlayergrownfrom1µmwidelineopeningswithseparation2µmandα=30°producedbyopticallithographyin2µmthickSiO2maskonInP.Theoildrop‐lookingspotsintheSEMimageemitbrighterluminescencethanthebulklayerandarethusnotassociatedwiththeblackspotsinthePCLmap,whicharetypicalofthreadingdislocations.

Figure3.9.a)SEMimageandb)PCLmapofInP:SELOGlayergrownfrom1µmwidelineopeningswithseparation2µmandα=30°producedbyopticallithographyin2µmthickSiO2maskonInP/Si.

3.1.2.2 StackingfaultsThe DLDs in the PCL maps were in paperG concluded to be intersections betweenstacking faults and the layer surface. From cross‐sectionTEM images such as the oneshowninFigure3.10a),itwasshownthatSFsweregenerallyblockedbythemask,butincertaincasesmanagedtopropagatethroughthemaskopeningsandincertainothercaseswerecreatedduringELOG.DuetothesmallELOGarea,preparationofplan‐viewTEM imagesweredifficult, but aplan‐viewTEM imageofdirect growthof InPon theInP/SisubstrateisshowninFigure3.10b).

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Figure3.10.a)cross‐sectionTEMimageofInP:SELOGlayergrownfrom200nmwidemeshopenings,with3µmseparation,α=15°andβ=105°,etchedin40nmSiO2maskonInP/Si.b)Plan‐viewTEMimageofdirectgrowthofInPonInP/Si.PicturefrompaperG.

Spectra taken fromdifferentpositions in thePCLmaptakenat80K(shown inFigure3.11) confirm that impurities are indeed segregated at the DLD; the spectrumcorresponding to position #3 has a profile similar to that of undoped InP, whereasspectra from the other positions are wider and blue‐shifted, indicating higherconcentrationsofsulfurcausingtheso‐calledBurstein‐Mossshift[87].

Aspreviouslymentioned,theadjustedbandgapenergyduetodopantsisequalto[87]:

0 'BG F e cE E E E E (26)

where E0is the intrinsic band gap energy, EF’ is the Fermi energy for non‐parabolicenergy bands due to band‐filling, Ee is the electron‐electron interaction, and Ec theelectron‐impurityinteraction,thelatterthreedependingonthecarrierconcentrationn,whichisassumedtobeequaltothecompensateddensity,ND–NA.At80K,theshiftedwavelengthsof850–860nmcorrespondtocarrierconcentrationsof8∙1018–1∙1019cm‐

3. Since this is slightly higher than the nominal sulfur concentration based onmeasurements on planar samples grown under the same conditions, it confirms thatdopant concentrationduringELOGbecomeshigher thanduringplanar growthdue toincreasedpreferentialsulfurincorporationoncertainfacets[73].

A similar effect was not observed for the threading dislocations, possibly because oftheir small size and their strongnon‐radiative recombination quenching the radiativerecombinationfrombandgaptransitions;DLDsdidnotappeartoexhibitnon‐radiativerecombinationofthesamemagnitude(atleastnotinRTmeasurements).Thedifferencein contrast between recombination at threading dislocations and at non‐defectivematerialbetweenlow‐TandRTmeasurementswaslikelyaneffectofincreaseddopantactivation at RT, providing more carriers leading to stronger luminescence and thusgreatercontrast.

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Figure3.11.a)PCLmapofInP:SELOGfrom200nmwidemeshopeningswith3µmseparation,α=15°andβ=105°,etchedin40nmSiO2maskonInP/Sisubstrate.b)CLspectratakenfromdifferentlocationsina).Thefiguresina)showthepositionswheretherespectivespectrashowninb)wereacquired.PicturefrompaperG.

Opening type did not have any significant impact on stacking fault density in ELOGlayers, thus indicating that formationof new faults, presumablyby othermechanismsthan coalescence, during ELOG was more important than propagation; otherwise asimilar trend as was observed for threading dislocations would have been expected.StackingfaultdensitydidshowsomecorrelationwithbothtemperatureandV/IIIratio,as shown in Figure 3.12; for a givenV/III ratio, stacking fault density decreasedwithtemperature.Foragiventemperatureontheotherhand,stackingfaultdensityreachedamaximumataV/IIIratioof10,anddecreasedforbothlowerandhigherV/IIIratios.The correlation with V/III ratio may be related to the formation of interstitials andvacancieswhich is affectedby theV/III ratio andwhich serves asnucleation sites forstackingfaults[37],or itmayberelatedtotheformationofdifferentboundaryplaneswith different stacking fault energies [72]. The correlation between temperature andstacking fault density however contrarily does not indicate interstitial/vacancyformationasanimportantsourceofstackingfaultformationsincethesetendtoincreaseindensitywithincreasingtemperature.Ontheotherhand,ahighergrowthtemperaturemayhaveanannealing‐likeeffect,leadingtolowerstackingfaultdensity[108].

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Figure3.12.DLDdensityextractedfromPCLmapstakenat80KofInP:SELOGonSifrom200nmwidemeshopeningswith3µmseparation,α=15°andβ=105°,etchedin40nmSiO2mask,shownwithrespecttothicknessgroupedbytemperaturewithV/IIIratioindicatedbyellipses.FrompaperG.

It was noted in paperG that the DLD width was distributed neither according to amonotonically decaying function, nor according to the bell‐shaped Gaussian of thenormaldistribution. Instead, thedistribution first increasedsharplyand thendecayedslowlywithincreasingwidth,asshowninFigure3.13below.ThisdistributionappliedtosampleswithELOGlayersfromacoupleofµmuptooverahundredµmthick,althoughthe widths had increased considerably with increasing thickness. This indicates thatstackingfaultsdidnotformcontinuouslyduringgrowth;ifthathadbeenthecase,thenthedensityof stacking faultswitha thickness less than10µmwouldbemoreor lessconstantregardlessofthickness.

Figure3.13.SFenergycalculatedfromSFwidthaswellasnormalizeddistributionofDLDlengthextractedfromInP:SELOGlayersgrownfrom200nmwidemeshopeningswith3µmseparation,α=15°andβ=105°,etchedin40nmSiO2maskonInP/Sisubstrate,grownfora)9minandb)120min.PicturefrompaperG.

Therearebasically twodifferent typesof faultswithdifferent formationmechanisms;oneboundedbyShockleypartialsandtheotheronebyaFrankloop[36].Thewidthofthe Shockley‐type fault can be estimated by considering the energy associated withcreatingthefault;thetotalforceoftheboundingpartialsisgivenby:

52  

d

GbF

4

2

(37)

whereG istheshearmodulus,b themagnitudeoftheBurgersvector,d theseparationbetween the partial dislocations andγ the stacking fault energy per area, providing aline force per unit length of dislocation. Thus, the repulsive force decreases withincreasing separation whereas the attractive force resulting from the stacking faultenergy increases with increasing separation. Integrating (37) yields the energyassociatedwiththestackingfault:

0

2

4

w

w

GbE dr

r

(38)

which,withthelowerboundaryw0settoa/√2,thesmallestinteratomicdistanceona{111}plane,becomes:

2 2

2

ln( ) ln( ) ln4 42 2

w

a

Gb a Gb aE r r w w

(39)

The function described by (39) is also plotted in Figure 3.13 andmore or less anti‐correlateswith theobservedDLD lengthdistribution, thoughonadifferent scale.Thereason for this is that the DLD width corresponds to the stacking fault width at theintersectionofthefaultwiththelayersurface,andasmentionedearlierthefaultwidthincreasesasthelayergrowsthicker.Sincetheboundingdislocationsextendalongmoreor less straight linesasobserved inFigure3.10a), it is reasonable toassumea linearincreaseinwidthwithincreasingthickness.Also,boththeaveragewidthandmaximumwidth seem to increase at the same (linear) rate, around 10 times, for a thicknessincreaseofthesamemagnitude(from10to100µm).WiththeaverageDLDlengthsof3and 30 µm for layers 10 and 101 µm thick respectively, this translates to an initialequilibriumwidthofaround30nmifthestackingfaultsformedatathicknessofaround100nm.

Theequilibriumwidth, calculatedbyequating (37) tozeroandsolving ford,becomesaround12nmwithb=2.4Å,G=46GPa[109]andγ=17mJ∙m‐2[110],almosthalfofthat estimated above but at least in the correct order of magnitude. However, theincorporation of sulfur has been shown to decrease the stacking fault energy therebyincreasing the equilibriumwidth [111], [112], so that the calculated valuemay be anunderestimation.

Another formation mechanism of stacking faults is by deposition errors. It has beenestablishedthattheformationenergyofstackingfaultsisconsiderablyloweron{111}facets in compound semiconductors of zinc‐blende type and fault formation on suchfacets has been observed during growth of e.g. GaP on Si [49], [50] where strain isnegligible due to almost perfect match in lattice constant. Although the ELOG

53  

investigatedhereisinessencehomoepitaxysinceInPisgrownonanInPseedlayer,theinitialELOGacrosstheSiO2maskprogresseswith formationof InP“islands”ontopofthemaskastheInPdoesnotgrowcontinuouslyfromtheopeningsasshowninFigure3.4. These islands are generally bounded by {221}, {311} and {331} planes [101]consistingof {111} stepswhere stacking faults thus formmore readily than on {001}facets.

The fact that no stacking faults were visible in homoepitaxial ELOG layers howeverindicate that random deposition errors is not a major reason for the formation ofstacking faults inELOG layersgrownon InP/Sieither, since thismechanismwouldbeequally prevalent in both cases. In another same study, it was observed that surfaceroughnessseemedtocorrelatewithanincreaseindensityofFrankpartialdislocations[47]. Surface roughnessmay introducepointdefects that serveasnucleation sites forstackingfaults,anditispossiblethatroughnessintheSiO2maskcontributedtothefaultformationduringELOG.

Thus,asregardsthestackingfaultsformedduringELOG,randomdepositionerrorsandimperfectcoalescenceappeartobeunlikelycausesfortheirformation,whileformationofShockley‐typefaultscreatedbydissociationofperfectdislocationsaswellasFrank‐type faultscreateddue to incorrectdepositionsonstrain‐distortedbonds [51]remainpossibleformationmechanisms.Aspreviouslymentioned,propagationofstackingfaultspre‐existingintheInPseedlayerisnotconsideredadominantfactorinthepresenceofstacking faults in theELOG layerssince therewasnosignificantdifference instackingfaultdensitybetweenELOGlayersgrownfromlineandmeshopenings.

3.2 ELOGofInPformonolithicallyintegrateddevicesAlthoughthere isgreat interest ingrowing InPonSi in itself, theusefulness inhavingInP in contact with Si is the possibility to fabricate truly monolithically integrateddevices, in contrast tohybriddevices fabricatedbybondingapproaches. Inparticular,activephotonicdevicesareofspecialinterestsincetheyarenotpossibletofabricateinSialoneduetothepoorgaininSistemmingfromitsindirectbandgap.TheconceptofsuchadeviceandoptimizationoftheELOGforthesameisdiscussedinpaperB,paperD,paperE,paperFandpaperH.InpaperB,optimizationofELOGoverstripesofSiO2mask on InP is undertaken, inpaperD, a device concept and fabrication route of anevanescentlycoupledInPlaserintegratedwithSiwaveguideispresentedtogetherwithsimulationsofsuchadeviceandstudiesofELOGonstripesofSiO2onInP/Si.InpaperE,the application of CMP to optimize the InP seedon Si is investigated. InpaperF, thegrowth of QWs on optimized ELOG material and the fabrication of MD lasers aredescribed. Finally, in paperH, planar defects in the ELOG layer and approaches toremovethemareinvestigated.

3.2.1 MonolithicallyintegratedlaserbyELOGFor an integrated device on Si to be useful, lightmust be coupled in and out of a Siwaveguidetoandfromtheactivelayer.SincetheQWsofinterestemitlightinthe1300

54  

nmand1550nmbands, InP is transparent to lightemittedby these.Thus, lightbeingemittedfromQWsgrownonanInPELOGlayercanbecoupledtoawaveguideintheSilayerbye.g.agratingcoupler.Anotherwayisbyevanescentcouplingexistingwhenthelight source isbrought close to thewaveguide, aspreviouslydemonstrated [13], [14],providedthattheELOGlayeristhinenoughtoallowenoughoverlapoftheevanescentfield. Furthermore, if the selective growth takesplace in anopening in thewaveguidecladding,thenthecladdingitselfcanbeusedasamasktoblockdefectpropagationfromthe InP seed layer, so that covering an entire Si wafer with defect‐free InP becomessuperfluous.AdrawingpicturingtheproposedconceptintheformofamicrodisklaserwhichiscoupledevanescentlytoaSiwaveguideisshowninFigure3.14.

Figure3.14.DrawingofamicrodisklaserevanescentlycoupledtoaSiwaveguide.

3.2.2 DeviceconceptInpaperD,aconceptforwaveguidealasergrownonELOGevanescentlycoupledwithaSiwaveguideispresented.Aschematicofthisstructureandasimulationoftheopticalmode are shown in Figure 3.15 a) and b) respectively. It was shown that sufficientcouplingtotheSiwaveguidecouldbeachievedwithreasonablethicknessoftheELOGlayer;60%opticalconfinementintheSiWGand4%intheQWscanbeachievedwithanELOGlayerthicknessof600nm.

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Figure3.15.a)SchematicofanintegratedlaserfabricatedonELOGandb)simulationofthesamestructure(frompaperD).

Furthermore,withtheselectivelygrownInPintheopeningsinthewaveguidecladdingfunctioningasthermalconductorstothesubstrate,thethermalresistivitywasshowntobeas lowas20K/W.This suggestsexcellentheatdissipationpropertiesof theactualdevice,oneofthemostimportantchallengesinaSi‐integratedlaser[17],[113].

3.2.3 OptimizationofELOGonInPIn order to optimize ELOG according to the requirements for fabricating the deviceoutlinedabove,anumberofexperimentswereundertaken.GrowthofathinandwideELOGlayerwasoptimizedinpaperB,wherea370nmthickSiO2maskpatternedwithmultiple openings was used to mimic an arrayed waveguide grating (AWG). Bothopening size and separationwere varied to test the effect on the planarization theseparameters would have had, and ELOG was performed under different growthconditions to find outwhatwould result inmaximumwidth andminimum thickness,measured as the aspect ratio (AR) which is defined as the ratio of lateral to verticalgrowth,aswellasareasonablysmoothsurfacemorphology.ItwasfoundthatwhereasELOG from openings varying in separation as well as in width were planarized (asshown in Figure 3.16), a higher ratio of opening‐to‐mask area was beneficial inachievingahighAR,andaV/IIIratioof10seemedtoresultinagoodcompromisewitha high AR combined with a reasonably low vertical growth rate, allowing the layerthicknesstobecontrolledefficiently.Moreover,highestaspectratiowasachievedatanangleof30°off[110].

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Figure3.16.a)SEMimageofInPELOGlayersgrownat610°CwithV/III=10fromthreesetsofopeningsangled30°off[110],eachsetwithvaryingopeningwidthandwithfixedopeningseparationsof(fromlefttoright)100,300and500nmonInPsubstrate.b)SEMimageofFIB‐cutthroughELOGlayerindicatedbytheblackrectangleina).Thewidthsoftheopeningsare(fromlefttoright):1000nm,500nmand300nm.

3.2.4 OptimizationofELOGonInP/SiExperimentsweresubsequentlyperformedonInP/Sisubstratesbutwithslightlydifferentdesigns;singleanddoublemaskopenings(theschematicsofwhichareshowninFigure3.17)wereusedtobettersimulateanactualdevice.

3.2.4.1 OptimizationofseedlayerTheInPseedlayeronSiwasfoundnotonlytobedefectivebutalsotohaveaveryroughandunevensurface,whichinturnleadtopoorgrowthmorphologyandwasdeemedtobe detrimental to coalescence. In order to improve the quality of the ELOG layer, anumber of approaches aimed at addressing this issue were investigated. For thispurpose,samplesusingtheopeningdesignsshowninFigure3.17wereutilized.

Figure3.17.Schematicsofa)doubleopeningsandb)singleopenings.

Inafisttrial,theSiO2maskwasdepositedasspin‐onglass(SOG)ratherthanbyplasma‐enhancedchemicalvapordeposition(PECVD).SincetheSOGisspunonthewaferasaliquid, it leavesaplanarsurfacesothatat leastthemaskmorphologybecamesmooth.Although the SOG did result in a fairly planarmask surface, it cracked easily and theproblemwith poor coalescence persisted. The reasonwhy coalescencewas still poorhadlikelytodowithfactthattheplanarmaskcreatedanewproblem;sincethemasksurfacewasflatwhereastheseedlayerwasnot,therewasavariationinthethicknessof

57  

themasksothatgrowthintheopeningsreachedthetopofthemaskatdifferentpointsin timeasevident inFigure3.18a). In thePCLmapshown inFigure3.18b), thehighdensityofdefectsintheregionofcoalescenceisapparent.

Figure3.18.a)SEMimageandb)correspondingPCLmapofInP:SELOGlayergrownfromdoubleopenings500nmwideseparatedby1µminSiO2maskdepositedasSOGonInPseedlayeronSi.

In paper E, a slightly different approach consisting of polishing the SiO2 mask wascompared to polishing the InP seed on Si instead prior tomask deposition. A sampleconsistingofInPwithsimilarSiO2maskandpatternswasincludedforcomparison.TheInPseedlayerwasfirstregrownwithHVPEtoprovidesomemarginforpolishing.Thepolishingwasthencarriedoutinatwo‐stepprocessdesignedtofirstplanarizethelayer,i.e. remove long‐ranging height variations, and then remove surface roughness by aprimarilychemicalprocess.Polishingofthemaskwascarriedout inasinglestep.TheresultingmorphologiesareshowninFigure3.19.

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Figure3.19.AFM3Drenderingsofa)unprocessedInP/Si,b)polishedSiO2maskonInP/Si,c)regrownInP/SibeforeCMP,andd)regrownInP/SiafterCMPPicturefrompaperE.

700nmSiO2wasdepositedonInP/SiandpolishedInP/Sisamples(reducedto450nminthecaseofthesamplewithpolishedmask),and900nmSiO2wasdepositedonanInPsample. InP:S ELOG layers were then grown on all three samples. SEM images andcorrespondingPCLmapsoftheselayersareshowninFigure3.20.

59  

Figure3.20.a)–c):SEMimagesofInP:SELOGlayersgrownfromdoubleopenings300nmwideseparatedby1µmona)InP,b)InP/SiwithpolishedSiO2mask,andc)polishedInP/Si.d)–f):PCLmapscorrespondingtotheSEMimagesina)–c).Fullrectanglesindicatedefects;dashedrectanglesindicatesitesofsurfacedamage.

Apparently,whereastheELOGlayergrownonInPwascompletelydefect‐free,thelayersgrown on InP/Si exhibited defects lying along a line in the center, thus implyingthreading dislocations formed during coalescence. However, the layer grown from apolishedseedlayerdisplayedalowerdefectdensitythanthelayergrownonapolishedmask instead. The lower defect density was attributed to improved coalescenceresultinginfewerdislocationsbeingformed.ThefactthathomoepitaxialELOGdidnotresult in coalescence defects contrastswith the results fromELOG frommultiple lineopeningsproducedbyopticallithography(chapter3.1.2.1).Possibly,thisisaneffectofsmaller openings or the e‐beam lithography producingmore uniformmask openingswithlowersidewallroughnessthanopticallithography.

3.2.4.2 ELOGoptimizationSince CMP was beneficial to ELOG crystal quality, experiments were henceforthundertakenwithpolishedInPseedonSi.InpaperH,patterndesignsconsistingofsingleopeningswith differentwidths aswell as double openingswith different separations,both of which angled at 30° and 60° off [110], were used to investigate the effect ofopeningdimensionandseparationondefectformationinELOGlayers.

In the first experiments, ELOG layers were grown from double openings on InP andInP/Si substrates to study in particular potential defect formation mechanisms anddiscernwhether theywerepresent inELOGonboth substrate types.The layerswere

60  

characterized with PCL and TEM to assess crystal quality. Whereas black spotsrepresenting threadingdislocationsareclearlyvisible inELOG layersgrownon InP/Sisubstrate30°offasevidentfromthePCLmapin Figure3.21c),nosuchdefectswerevisibleinthesimilarlayergrownfromopenings60°off,asshowninFigure3.21d).

Figure3.21.SEMimagesofInP:SELOGlayersgrownfromdoubleopenings300nmwideseparatedbya)1µm30°off[110]andb)300nm60°off[110]onpolishedInP/Sisubstratemaskedwith700nmSiO2.c)andd):PCLmapscorrespondingtotheSEMimagesina)andb)respectively.PicturefrompaperH.

Cross‐section TEM images of the ELOG layers grown from openings 30° and 60° off[110] are shown in Figure 3.22 and Figure 3.23, respectively. Evidently, threadingdislocationswerefilteredinthemaskopeningsasshowninFigure3.22b)andc),butalso formed at points of coalescence as shown in Figure 3.22 c) and Figure 3.23 c).Stackingfaultsontheotherhandwereblockedbythemaskatsomepoints(asshowninFigure 3.22 b)), propagated in some cases (as shown in Figure 3.22 a)). The stackingfaultSF1 inFigure3.22a)and the faults in theELOG layer inFigure3.22b) look likethey formedduringELOGwhich isonepossibility,or theycouldhavepropagatedatadistanceinthe[110]or[‐1‐10]directionfromthelamella.

61  

Figure3.22.cross‐sectionTEMimagesofInP:SELOGlayersgrownfrom300nmwidedoubleopenings30°off[110]witha)500nmseparationandb)–c)1000nmseparationonpolishedInP/Sisubstratemaskedwith700nmSiO2.Thevectorsuandvdenotedirectionsrotated~40°clockwisefrom[1‐10]and[110]respectively.PicturefrompaperH.

Figure3.23.cross‐sectionTEMimagesofInP:SELOGlayersgrownfrom300nmwidedoubleopenings60°off[110]witha)300nmseparationandb)–c)500nmseparationonpolishedInP/Sisubstratemaskedwith700nmSiO2.Thevectorsuandvdenotedirectionsrotated~70°clockwisefrom[1‐10]and[110]respectively.PicturefrompaperH.

Interestingly enough, coalescence seemed to generally result in fewer defects in thecase of growth fromopenings 60° off [110]. Stacking faultswere seen propagating insomecaseshereaswell,butdue to thesmalleraspect ratioof layersgrown from60°openings,stackingfaultsweregenerallyterminatedatasidefacetintheseratherthanintersecting the layer surface, as seen inFigure3.23 a).Thus, in some instances evenlayersgrownfromdoubleopeningsappearedtobecompletelyfreeofdefectsasshownin Figure 3.21 d). Furthermore, TDs can be seen bending towardsmask sidewalls inFigure3.22b)andc),mostlikelyduetoimageforcesaspredictedin[74].

ItwasalsoshownthatwhereasdislocationsandstackingfaultswerepresentinELOGonInP/Sisamples,nodefectswhatsoeverwereobservedinsimilarELOGlayersgrownonInP,eitherinthePCLmaps(Figure3.24)orthecross‐sectionTEMimages(Figure3.25).Nor were any stacking faults observed in coalesced ELOG frommultiple openings asmentionedinchapter3.1.2.1.Thus,ifstackingfaultsareformedduringELOGofInP/Si,the formation isnot inherent toELOG;rather, there issomeothermechanismcausingfaulting in case of ELOG on InP/Si. As previously mentioned, the faults are possiblyformedpartlyasaresultofunrelievedresidualstrainintheseedlayercausingtheearlyELOG layer to be strained as well, leading to incorrect depositions due to strain‐distortedbondsashasbeenpreviouslysuggested[51].Thus,amorethoroughthermal

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annealingoftheInPseedlayeronSipriortoELOGmayreleaseenoughofthisresidualstresstopreventfaultingduringsubsequentELOG.

Figure3.24.a)SEMimageandb)correspondingPCLmapofInP:SELOGlayersgrownonInPsubstratemaskedwith700nmSiO2fromopenings30°off[110]with(fromlefttoright)singleopeningsof300,500and100nmwidth(theleft‐mostthreelayers)and300nmwidedoubleopeningswithseparations300nm,500nmand1000nm(theright‐mostthreelayers).Theblackspotsonright‐mostlayerinthePCLmapcorrespondtothewhiteparticlesvisibleintheSEMimage.Theseparticlesweredepositedduringsamplehandlingpost‐growth.PicturefrompaperH.

Figure3.25.cross‐sectionTEMimagesofELOGgrownonInPsubstratemaskedwith700nmSiO2fromdoubleopenings30°off[110]witha)500nmseparationandb)1000nmseparation.Thecurvedlinescorrespondtothicknessfringesandarenotdefects.Thevectorsuandvdenotedirectionsrotated~40°clockwisefrom[1‐10]and[110]respectively.TheSiO2maskwasremovedpriortoTEMcharacterization.PicturefrompaperH.

In paperH, a model for propagation of stacking faults through mask openings waspresented. It was shown that the aspect ratio necessary to filter out stacking faultsdependsonthepropagationvectorofthepartialsboundingthefault,sinceifonlyapartof the fault propagates above themask sidewall, the faultwill continue to propagate.Such a case is shown in Figure 3.26 a), whereas Figure 3.26 b) demonstrates a casewherethefaultiscompletelyblocked.

Inthemodel,wistheopeningwidth,histhemaskthicknessandαistheanglebetweenthemaskopeningdirectionandthelinealongwhichtheSFintersectsthe(001)plane.TheangleθistheanglewhichtheplaneonwhichtheSFliesmakeswiththe(001)plane.TheboundingpartialcriticalforSFpropagationisdesignatedrandφistheanglewithwhichrdivergesfromthedirectionofpropagationoftheSFitself,inthisparticularcase[1‐12].Thus,iftheSFissymmetrical,φishalftheangleofdivergencebetweenthetwoboundingpartials.

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Figure3.26.Modeldisplayingastackingfaultpropagatingthroughanopeningangled30°off[110]showingthecaseofa)propagationintotheELOGlayerandb)ofthefaultbeingcompletelyblockedbythemasksidewall.PicturefrompaperH.

withthesedefinitions,therelationbetweenr,themagnitudeofr,andh,themaskheight,becomes:

sin cos

hr

(40)

wherehisthemaskthickness.Theprojectionr’ofronthe(001)planebecomes:

tansin 90 arctan

cos

wr

(41)

wherewistheopeningwidth.Finally,therelationbetweenh,randr’isdescribedby:

2 2 2r r h (42)

Bycombiningequations(40),(41),and(42),theexpressionforthemaskheightat

whichthestackingfaultiscompletelyblockedbecomes:

2

2

1

tan 1sin 90 arctan

cos sin cos

h w

(43)

Since the bounding partials’ direction of propagation need not be constant, r can bethoughtofasanaveragevalueduringgrowthintheopening.Sinceonlypartofthefaultwillbeexposedintheopening,itisnottrivialhowthefaultwillgrowinsidetheopening.Themodeldoesalsonottakeintoaccountinteractionsbetweenboundingpartialsandthemaskwhichmayoccurduetosurroundingstrainfields.

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Usingtheanglesfor{111}planesandopeningsangled30°off[110],α=30°,andθ≈55°.Assumingthatthepartialdislocationsboundingthestackingfaultpropagatealong[011]and [101], φ =30°, andh becomes≈3.9w.Thus, the ratioofmaskheight toopeningwidth should be > 3.9 to achieve complete blocking of stacking faults under theseconditions.

AlsoinpaperH,experimentswhereELOGlayersweregrownfromsingleopeningsonmaskedInP/Sisubstratewerecarriedoutandtheproposedmodelwasappliedtothese.Figure3.27andFigure3.28showSEMandPCLimagesofInPELOGlayersgrownfromsingleopeningsonInP/Simaskedwith400nmSiO2.AsthePCLimageinFigure3.27b)shows,no threadingdislocationswhatsoeverarepresent, confirming that theprimarysource of dislocations in ELOG layers is formation during coalescence. Also, the InP:SELOG layers show a somewhat lower stacking fault density than undoped InP ELOGlayers, implying a correlation between sulfur‐doping and decreased stacking faultformation.There are studies indicating that sulfur canprevent gliding of dislocations,whichcouldinhibitdissociation,byimpurityhardening[114].Thiswouldagreewiththecurrent observations, although the effect of sulfur differs depending on the type ofdislocation [115]. however, it has also been suggested that sulfur‐doping would beexpected to decrease stacking fault energy and thus increase the likelihood ofdissociation of perfect 60° dislocations into Shockley partials [111], [116], creatingstackingfaultsintheprocess.Theexacteffectofsulfur–dopingisthusnotclear.

Figure3.27.a)SEMimageandb)PCLmapofInP:SELOGlayergrownfrom400nmwideopeningonInP/Sisubstrate.PicturefrompaperH.

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Figure3.28.SEMimagesofundopedInPELOGgrownfroma)250nmwideopeningandb)400nmwideopening.PicturefrompaperH.

In Figure 3.29, cross‐section TEM images from the same ELOG layers are shown. Asshown in Figure 3.29 b), several SFs were seen propagating; although some faultsappear to propagate right through the mask, they are probably in fact propagatingthroughthemaskopeningatadistanceinoroutoftheplaneofthelamella.Withamaskthicknessof400nmandanopeningwidthof250nm,themaskheighttoopeningwidthratio becomes only 1.6, thus explaining why stacking faults are seen propagatingthroughtheopenings.EveninthepreviouscaseELOGgrownfromdoubleopeningsin700nmthickmaskandopeningwidthof300nm,theratiobecomesonly2.3,thusstilltoolowtoensurecompleteblocking.

InFigure3.29a),nofaultscanbeseentopropagatethroughthemaskopening;instead,several faults appear in theELOG layer but not in the seed layer, indicating that theyformedduringELOGratherthanpropagatedthroughthemaskopening.

Figure3.29.Cross‐sectionTEMimagefroma)sampleundopedInPELOGlayergrownonInP/Sisubstratefroma250nmwideopeningandb)InP:SELOGlayergrownonInP/Sisubstratefroma250nmwideopening.Stackingfaultsareindicatedwitharrows.Thecontrastcausestheopeningstoappearnarrowerthanitisinreality.PicturefrompaperH.

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Ifthestackingfaultsarearesultofincorrectdepositionsondistortedbondsasappearstobethecase,itispossiblethattheycanberemovedbythermalannealing[117],[118].Althoughannealingwouldmeanraisingthetemperatureabovethegrowthtemperature,thusintheoryleadingtoagreaterthermalstrainfromtheSisubstrate,itisunlikelythiswill introduce anydefects; thermal straindoesnot appear tobe amajor factor in theformationofstackingfaultssinceahighertemperatureduringELOGevidentlyleadtoalowerdensityofstackingfaults.

3.2.5 FabricationofdevicesAsaproofofconcept,inpaperFanexperimentwasdesignedinwhichanactivedevicewas fabricated on an ELOG layer grown on InP/Si substrate. A microdisk laser waschosensincethisstructuredoesnotinvolvecleavingoretchingoffacetsandisrelativelystraightforward to pump optically. Since a certain minimum diameter is required toachieveahighenoughQ‐factor, thisputsa lower limiton thedimensionsof theELOGlayer.Thus,apatterndesignconsistingoffiveadjacentlineopeningswaschosen,withopeningwidth300nmandopeningseparation1µm.SubsequentgrowthresultedinanInP:SELOG layer720nm thick and17µmwide.RMS roughnessmeasuredwithAFMwas10nm.OpticalmicroscopeimagesofthestructurebeforeandafterELOGisshowninFigure3.30a)andb)respectively.Onthislayer,aQWstructurewasgrowninMOVPE.Details of the structure are shown in Table 2, and the corresponding PL spectra areshowninFigure3.30c).

Table2.DescriptionofQWstructure.TS=tensilestrained,CS=compressivelystrained.

Layer# Description Material Thickness(nm)

Dopanttype

Dopantconcentration(cm‐3)

RefractiveIndex

11 Claddinglayer

InP 50 Zn 2e18 3.172

10 Spacer layer(Q1.2)

In0.78Ga0.22As0.479P0.521

120 Undoped 3.317

9 Barrier(0.9%TS)

In0.485Ga0.515As0.83P0.17

7 Undoped 3.4

4,6,8 QWs (1%CS)(x3)

In0.76Ga0.24As0.83P0.17 8 Undoped 3.5

3,5,7 Barrier(0.9%TS)(x3)

In0.485Ga0.515As0.83P0.17

7 Undoped 3.4

2 Spacer layer(Q1.2)

In0.78Ga0.22As0.479P0.521

120 Undoped 3.317

1 Claddinglayer

InP 50 Undoped 3.172

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Figure3.30.a)MicroscopeimagesofpatternfabricatedonInP/SipriortoELOG,b)afterELOGandc)PLspectraofQWgrowthonELOGandonplanarreference.

The quality of the QWswas characterized with PL showing significant differences inintensity,peakpositionandlinewidthbetweentheQWsonreferenceandQWsgrownonELOG; IntensityofQWsonELOGwasonly15%of thatof thereference,peakpositionwas1515nmand1552nm,andFWHM125nmand180nmforthereferenceandELOGrespectively.

Finally,microdisklaserswithdiameters10–15μmwereetchedtoadepthof700nm(Figure3.31).Themicrodisk laserswerealsocharacterizedwithPLandshowedsometendencies to lase but no single peakwas able to quench out the others as shown inFigure3.32b).PartofthereasonforthiswaslikelythequalityoftheQWs;asevidentinFigure 3.32 a), thickness varied considerably which explains the large FWHM of theQWs.ThereasonforthisvariationcouldberesidualstrainintheELOGlayeroritcouldsimplybetheQWgrowthitself.Also,theshallowetchdepthcontributedtolightleakingout from the disk and into the surrounding layer; according to simulations, an etchdepthof700nmwouldleadtoalossof100cm‐1,whereasthetotalgainfromtheMQWsis2500cm‐1whichwithaconfinementfactorof5%wouldyieldamodalgainofamere125 cm‐1. Taking into account the additional loss from scattering due to sidewallroughness,thetotallosswouldlikelybehigherthanthegain,thuseffectivelyimpedinglasing.

Figure3.31.a)SEMimageofmicrodisklasersfabricatedonELOGandb)cross‐sectionTEMimageofonemicrodisk.Theblackrectangleina)showswherethelamellainb)wascut.

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Figure3.32.a)Cross‐sectionTEMimageofQWsinonediskandb)PLspectraformicrodisksfabricatedonELOGandplanarreferencesample.

Interesting to note is that the intensity, peak position and linewidth of themicrodisklaseronInP/SisubstratearealmostidenticaltothatofthelaserfabricatedontheplanarInPreferencesample(peakpositionwas1547nmand1538nmfor thereferenceandELOGrespectivelyandFWHMwas146nm for the reference compared to158nm forELOG),whichalsodidnotlase.ThissuggeststhatthereasonforthelackoflasingwastheQWsandtheshallowetchdepthratherthanthequalityoftheELOG.Interestingly,the performance of the microdisk laser fabricated on ELOG showed a remarkableimprovement compared to the as‐grown QWs, whereas linewidth of the referenceincreased aftermicrodisk fabrication. Consequently, the ELOG approach remains verypromisinginrealizingactivephotonicdevicesonsiliconalthoughotherfactorssuchascarrierlifetimewerenotconsideredhere.

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4 Summaryandconclusions1. Dislocations arepresent in coalescedELOG layersbutnot inuncoalescedones.

Thus, although dislocations theoretically could propagate through maskopenings, this does not occur, at least not for sufficiently small openings.Dislocation formation is influenced by the coalescence process; if the masksurfaceonwhichthecoalescencetakesplaceisroughanduneven,theformationrate is higher. Planarizing and polishing the InP on Si seed layer prior todeposition of SiO2 mask results not only in a flatter and smoother surface onwhichthegrowthstarts,butitalsoresultsinaflatterandsmoothermasksincethis inherits themorphologyof theseed layer.Whereasonlypolishing theSiO2maskdepositeddirectlyontheseedlayerappearstohavesomebeneficialeffect,itdoesintroduceanotherprobleminthattherewillinfactbevariationsinmaskthicknesssincethereareconsiderableheightdifferencesintheseedlayer.Thisinturn causes the selectively grown InP to reach the topof themask at differentmoments, since at some points in the mask openings the mask thickness isthinner than in others. The result is that coalescence takes place at multiplepoints between ELOG islands varying greatly in height, which promotesdislocationformation.PolishingtheInPseedlayerhowevernotonlyresultsinaplanarandsmoothstartinglayerfortheselectiveareagrowth,butalsoinamaskwith similar properties thus reducing the probability of dislocation formationduringcoalescence.

2. Propagation isacertainwhile formation isapossiblecause for thepresenceofstackingfaults inELOGlayersonInP/Si.Asregardsthepropagationofstackingfaults, it ishypothesizedbasedongeometricalconsiderationsandthenatureofthestackingfaultsthattheymaybeblockediftheratiobetweenthemaskandtheopeningwidthexceeds3.9.Concerningthepotentialformationofnewfaults,itisestablished that if they do form, they do not do so during planar growth, butduringthephaseof lateralgrowth.Themechanismfortheformationprocessisnotentirelyclear,butsincenofaultsareformedduringELOGonInPsubstrate,the formation mechanism would not be inherent to ELOG, and neither simplerandom deposition errors on facets with low stacking fault energy norcoalescenceappeartobe involved.AmechanisminvolvingdepositionerrorsonELOGislandedgesduetostrain‐distortedbondsisconceivable.IfstackingfaultsareformedduringELOGbysuchdepositionerrorsandthusincreasetheenergyofthecrystal,itshouldbepossibletoremovethembythermalannealingwithoutintroducingadditionaldefects.

3. Anintegratedlaserstructurewherethewaveguidecladdingitselfactsamaskforblockingdefectshasbeen shown tobe apromising concept for achieving trulymonolithicallyintegratedactivedevicesonSisubstrates.Suchadevicehasbeensimulatedwithexcellentproperties in termsof couplingefficiencyand thermaldissipation,andasproof‐of‐conceptInPELOGonSihasbeengrownusingnarrow

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stripe openings. A disc laser on top of ELOG layer grown frommultiple stripeopenings without underlining waveguide was fabricated and characterizedshowingstrongluminescencebutdidnotshowanylasing,mostlikelyassociatedwith the quality of the QWs as well as a too shallow etching leading to highleakage to the substrate. With further optimization, it should be possible toachievelasingmakingELOGaninterestingapproachformonolithicintegrationofphotonicdevices.

 

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5 Futurestudies1. Recommended future studies include addressing the mask aspect ratio and

sidewallprofileaswellasthermalannealingofInPseedonSiandELOGlayers.ThesemeasuresarelikelyparamounttoachievingELOGlayersthatarefreenotonly of dislocations but also of planar defects like stacking faults. Also,characterizing the type of stacking fault and bounding partial dislocations (i.e.ShockleyorFrank)aswellastheirpropagationinsidemaskopeningswouldbevaluable inunderstanding theoriginandpotential formationmechanismof thestackingfaults,therebyfacilitatingtheirprevention.

2. Furthermore,theresultofarealizationoftheintegrationschemeoutlinedinthisworkwould be of great interest since itwould allow the fabrication of a trulymonolithically integrated laseronSi,somethingwhichremainstheelusiveholygrailofthesemiconductorindustry.Inthiseffort,utilizationofCMPtoplanarizethe upper waveguide cladding as well as to reduce the thickness of the ELOGlayerontopofthewaveguidewillbeanalternativewellworthstudying.

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6 SummaryofappendedpaperspaperA

Surfacemorphology of indium phosphide grown on silicon by nano‐epitaxiallateralovergrowthIn this paper, the growth of InP on Si by ELOG from mesh‐type nano‐sizeopeningsunderdifferentgrowthconditionswasstudied.Surfacemorphologywas characterized by AFM and profilometry while optical quality wascharacterizedbymicrophotoluminescence(µ‐PL). Itwas found thatgrowthconditions had some impact on both morphology and optical quality, andsurface morphology generally improved with thickness. In particular,morphologyofELOGlayerswassignificantlybetterthanthatofdirectgrowthonInP/Si.Contribution: Part of experiment planning, part of epitaxial growth, part ofcharacterization,dataanalysisandinterpretation,manuscriptwriting.

paperB

InPOvergrowthonSiO2forActivePhotonicDevicesonSiliconIn this paper, ELOG on SiO2 for device concepts was investigated. For thispurpose, different patterns resembling waveguide and arrayed waveguidegratingswereetchedinSiO2maskonInPsubstrate.Growthconditionswerealso varied in order to study their effect on the ELOG. It was found thatplanarizationwasachievedforELOGfrommultiple lineopeningsregardlessof whether opening width or separation was varied. Furthermore, it wasshown that by using appropriate growth conditions, ELOG layer thicknesscouldbecontrolledtobesufficientlythin(~200nm)forachievingevanescentcoupling between a laser structure on top of the ELOG and a waveguideburiedintheSiO2.Contribution:PartofExperimentplanning,epitaxialgrowth,characterization,dataanalysisandinterpretation,manuscriptwriting.

paperC

Morphological evolution during epitaxial lateral overgrowth of indiumphosphideonsiliconFollowing the investigation of morphology of ELOG layers grown underdifferentgrowthconditions,thetime‐resolvedmorphologyanddefectdensity

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ofELOGfromdifferentopeningtypeswerestudied.Meshopeningssimilartothose studied in a previous paper but with two different sets of openinganglesaswellaslineopeningswithtwodifferentsetsofopeningangleswereemployed. It was found that coalescence occurred much faster from meshopenings than from lineopenings.Thiswasattributed to the ratherunevengrowth from mesh openings preventing the formation of growth‐retardingboundary planes before coalescence could ensue. Just after coalescence,surfaceroughnesswasdramaticallyhigherforELOGfromlineopenings,butafter an extended period of growth surface roughness approached similarvaluesregardlessofopeningtype.Moreover,dislocationsdensitywas lowerfor ELOG from line openings, suggesting propagation or coalescence asimportantcausesforthreadingdislocationsintheELOGlayers.Contribution: Part of experiment planning, part of epitaxial growth, part ofcharacterization, part of data analysis and interpretation, manuscriptdiscussion.

paperD

III–VsonSiforphotonicapplications—Amonolithicapproach

In this paper, a concept of a laser on ELOG evanescently coupled to a Siwaveguide ispresented.Usingan InP/Sisubstrateasabase, theenvisionedstructureconsistsofaSiwaveguideburied100nmbelowthesurfacein2300nmSiO2actingbothaswaveguidecladdingandasamaskfortheELOG.Duallineopeningsalong thewaveguide coreactas trencheswhere InP is grownselectively.Usingafinitedifferencemodesolver,themodeconfinementintheSi waveguide was calculated for different waveguide geometries and ELOGlayerthickness.ItwasshownthatarelativelyhighmodeconfinementintheQWswhileachievingevanescentcouplingtotheSiwaveguidecanbeobtainedforafeasiblerangeofELOGlayerthicknesses.Furthermore,heatdissipationwasmodeledusingafiniteelementmethod,showingthatthetrenchesintheSiO2greatlyenhanceheatdissipationduetothehighthermalconductivityofInPcomparedtoSiO2.,leadingtoathermalresistivityof<20K/W.GrowthofInP ELOG from single and double line openings in 700 nm SiO2 mask onInP/Si substrate was also performed and analyzed withcathodoluminescence,demonstratingreasonablythinELOGlayerswithgoodopticalquality.Thus,theproposedconceptholdsgreatpromiseinachievingamonolithicallyintegratedInPlaseronSi.Contribution: Part of experiment planning, part of epitaxial growth,characterization, part of data analysis and interpretation, manuscriptdiscussion.

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paperE

EffectoftheSurfaceMorphologyofSeedandMaskLayersonInPGrownonSibyEpitaxialLateralOvergrowth

Drawingonthedeviceconceptpresentedinpreviouspapers,attentionwasinthis paper turned to optimizing the quality of the ELOG layer grown fromdoubleopeningsinSiO2.AstheInP/Sisubstratehasaveryunevenandroughsurface with detrimental effects on subsequently grown ELOG, differentapproaches to come to terms with this issue were investigated. Usingchemicalmechanicalpolishing,theeffectonELOGofpolishingtheseedlayerof InP/Si substrate as well as polishing the SiO2 mask on unpolished seedlayer of InP/Si substratewere compared.Whereas polishing the SiO2maskresultedinaplanarandsmoothmasksurface,theeffectonsubsequentELOGlayerwasnot as beneficial as the approach consistingof polishing the seedlayer of InP/Si substrate alone. Using a two‐step process aimed at firstmechanicallyplanarizingandthenchemicallypolishing,averysmoothInP/Sisurface was obtained. CL studies indicate that the density of coalescence‐induceddefects isdecreasedwhenpolishing the InP/Si substratecomparedtopolishingtheSiO2maskalone.

Contribution: Part of experiment planning, part of epitaxial growth,characterization,dataanalysisandinterpretation,manuscriptwriting.

paperF

TowardsamonolithicallyintegratedIII‐Vlaseronsilicon:optimizationofmulti‐quantumwellgrowthonInPonSiFollowing the conceptualization of a monolithically integrated laser andoptimizationofELOG,QWgrowthbyMOVPEonInPELOGfromlineopeningsinSiO2onInP/Siwasstudied.ELOGfromsingleanddoubleopeningsaswellasmultiple lineopeningswasusedas templates forQWgrowth. It is foundthat the SiO2mask gives rise to significant loading effect distorting theQWgrowth.AdesignconsistingofmultiplemaskopeningsinsuchawaythattheSiO2maskcaneasilybestrippedoffafterELOGwasthereforedevelopedandemployed. The inclusion of multiple openings also serves the purpose ofproviding large enough ELOG area for subsequent fabrication of microdisklasers. TEM studies indicate high quality InP ELOG, and microdisk lasersfabricatedfromsubsequentlygrownQWsshowPLemissiononparwiththatof identical structures fabricated fromQWs grown on planar InP substrate.

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However,itisfoundthatatooshallowetchleadtosignificantleakageoflightfromthemicrodisksintothesubstratepreventedlaseremission.Contribution: Part of experiment planning, part of epitaxial growth, part ofcharacterization, part of data analysis and interpretation, manuscriptdiscussion.

paperG

Defect reduction in heteroepitaxial InP on Si by epitaxial lateral overgrowth(manuscript,submittedtoMaterialsExpress)Returning toELOGon InP/Si substrategrown frommeshand lineopeningsfor large area coverage, the pertinent defects in these layerswere studied.UsingCLandTEM,threadingdislocationsandstackingfaultswereidentifiedasthemaintypesofdefects.Whereasdislocationdensitywas lowerfor lineopenings compared tomeshopenings, stacking faultdensity showsno suchcorrelation. Furthermore, whereas dislocation density showed negligiblecorrelationwithgrowthconditions,bothtemperatureandV/IIIratioseemedtoimpactstackingfaultdensity.Also,byanalyzingthewidthofstackingfaultsinlayersdifferinginthickness,itwasconcludedthatstackingfaultsgenerallyariseduringtheearlystagesofELOG,eitherbypropagationorformation,andremainmoreorlessconstantindensity.Threadingdislocationsontheotherhanddecreaseddrasticallywith layer thickness,probablydue to interactionandannihilation.AlthoughdislocationsoriginatingintheInPseedlayerwereblockedby themask, stacking faults alsooriginating in the seed layerwereshown to propagate in some cases. However, it was concluded that newstackingfaultsareprobablyalsoformed,thoughtheformationmechanismisnotclear.Contribution: Experiment planning, part of epitaxial growth, part ofcharacterization,dataanalysisandinterpretation,manuscriptwriting.

paperH

Study of planar defect filtering in InP grown on Si by epitaxial lateralovergrowth(manuscript)Defects in ELOG layers grown from the previously introduced single anddouble lineopeningswerestudied. Itwas foundthatELOG layersgrownonInP substrate were completely free of defects, regardless of opening type.ELOGlayersgrownonInP/Sisubstratefromdoubleopeningsexhibitedboth

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stacking faults and threading dislocations, while ELOG layers grown fromsingle openings exclusively contained stacking faults. It was clearlydemonstratedthatthreadingdislocationsarecompletelyblockedbytheSiO2mask, but new threading dislocations form at coalescence points in case ofELOGfromInP/Sisubstrate.Moreover,itwasfoundthattheoriginofstackingfaults in the ELOG layers is likely both formation and to some extentpropagation of faults pre‐existing in the seed layer. Amodel describing thepropagationofstackingfaultswaspresentedanditispostulatedthatstackingfaults canbeblockedusingahighenoughmask thickness‐to‐openingwidthratio. The formation mechanism of stacking faults is not entirely clear butformation due to coalescence as well as random deposition errors on low‐energy{111}facetsareruledoutduetoevidencetothecontrary.Contribution: Experiment planning, part of epitaxial growth, part ofcharacterization,dataanalysisandinterpretation,manuscriptwriting.

 

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