Theoretical and experimental studies of the 2-dimensional grating coupled structures for III-V quantum well infrared photodetectors

139 Pages·1994·5.2 MB·English

by  Wang, Yeong-Cheng, 1960-

THEORETICALANDEXPERIMENTALSTUDIESOFTHE
2-DIMENSIONALGRATINGCOUPLEDSTRUCTURESFOR
IH-VQUANTUMWELLINFRAREDPHOTODETECTORS
BY
YEONG-CHENGWANG
ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL
OFTHEUNIVERSITYOFFLORIDAIN
PARTIALFULFILLMENTOFTHEREQUIREMENTS
FORTHEDEGREEOFDOCTOROFPHILOSOPHY
UNIVERSITYOFFLORIDA
1994
ACKNOWLEDGEMENTS
Iwouldliketoexpressmysinceregratitudetomyadvisorandthechairmanofmy
committee,ProfessorShengS.Li,whogavemeanopportunitytoworkasoneofhis
graduatestudents. Thisresearchwascompletedunderhisguidance,encouragement,
andsupport. IalsowishtothankProfessorsDorotheaE.Burk,GysBosman,William
R.Eisenstadt,andChen-ChiHsuforservingonmysupervisorycommittee.
ThanksareextendedtoDr. LarryS.YuofNationalSemiconductorCorp. and
Dr. Ping-ChangYangofFan-ChiaUniversityfortheirvaluablediscussions,Yenhwa
Wangforhisgreathelpinsemiconductorprocessing,andmyfriendsJung-chiChi-
ang,Chih-HungWu,Ming-YehChuang,DanielWang,ZhiliangChenandYun-Shun
Changfortheirfriendship.
Iamgreatlyindebtedtomywifeandparentsfortheirlove,support, anden-
couragementduringthearduousprocedureofcompletingmyDoctorofPhilosophy
degree.
Finally,thefinancialsupportoftheDefenseAdvancedResearchProjectAgency
(DARPA)isgratefullyacknowledge.
n
TABLEOFCONTENTS
Page
ACKNOWLEDGEMENTS “
ABSTRACT
CHAPTER
1 INTRODUCTION 1
1.1QuantumWellDetectors 1
1.2QuantumWellIntersubbandAbsorption 2
1.3GratingCoupler ^
1.4SynopsisofChapters 6
2 ANUMERICALANALYSISOFREFLECTIONSQUARE
DOTMETALGRATINGFORMULTIQUANTUMWELL
INFRAREDPHOTODETECTORS 19
2.1Introduction 19
2.2TheTheoreticalFormalism 21
2.3ResultsandDiscussion 24
2.4Conclusion 28
3 DESIGNOFATWO-DIMENSIONALSQUAREAPERTURE
MESHMETALGRATINGCOUPLERFORAMINIBAND
TRANSPORTGaAsQUANTUMWELLINFRARED
PHOTODETECTOR 37
3.1Introduction 37
3.2BasicTheory 39
3.3ResultsandDiscussion 43
3.4Conclusion 46
4 GEOMETRYCONSIDERATIONSOFTWO-DIMENSIONAL
GRATINGCOUPLERFORGaAsQUANTUMWELL
111
INFRAREDPHOTODETECTOR 55
4.1Introduction 55
4.2TheoryandIllumination 57
4.3Conclusion 60
5 EXPERIMENTALIMPLEMENTATIONSOFTWODIMENSIONAL
SQUAREAPERTUREMESHMETALGRATINGCOUPLER 67
5.1Introduction 67
5.2SamplePreparation 68
5.3GratingFabrication 69
5.4Theory 76
5.5OpticalMeasurement 71
5.6ResultsandDiscussion 72
5.7Conclusion 74
6 DESIGNOFPLANAR2-DCIRCULARAPERTUREMETAL
GRATINGCOUPLERFORGaAsQUANTUMWELL
INFRAREDPHOTODETECTORS 78
6.1Introduction 78
6.2BasicTheory 80
6.3ResultsandDiscussion 83
6.3.1SquareSymmetryGratingStructure 83
6.3.2HexagonalSymmetryGratingStructure 84
6.3.3ComparisonbetweenExperimentandTheory 85
6.4Conclusion 86
7 DESIGNOFANOPTIMUMPERFORMANCEBTMQWIP 96
7.1Introduction 96
7.2GeneralConsiderationofaBTMQWIP 97
7.2.1BarrierHeightandAUGai-^AsComposition 98
7.2.2SuperlatticeBarrierWidth 98
7.2.3DopingConcentrationintheQuantumWell 99
7.3QuantumWellPeriod 99
7.4BlockingBarrierandDarkCurrent 102
7.5WaveguideStructure 103
7.6Conclusion 107
IV
SUMMARY 117
8
REFERENCES 119
APPENDIX 128
BIOGRAPHICALSKETCH 130
V
AbstractofDissertationPresentedtotheGraduateSchool
oftheUniversityofFloridainPartialFulfillmentofthe
RequirementsfortheDegreeofDoctorofPhilosophy
THEORETICALANDEXPERIMENTALSTUDIESOFTHE
2-DIMENSIONALGRATINGCOUPLEDSTRUCTURESFOR
HI-VQUANTUMWELLINFRAREDPHOTODETECTORS
By
Yeong-ChengWang
April1994
Chairman: ShengS.Li
MajorDepartment: ElectricalEngineering
ThisresearchmainlydealswiththequantumefficiencyenhancementofQWlPs
(QuantumWellInfrared Photodetectors) byusingthe2-dimensional (2-D) planar
metalgratingcouplers. Themotivationisoriginatedfromtheintersubbandselection
ruleofquantumwell,whichrequiresthepolarizationofincidentinfrared(IR)radi-
ationtohaveacomponentperpendiculartothequantumwelllayers. Theunique
featuresofthe2-Dplanarmetalgratingcouplerarethat(a)iteffectivelyscattersthe
normalincidentradiationindependentoflightpolarizationand(b)itcanbeeasily
fabricatedbyusingasimplemetallift-offtechnique. Threegrating structuresare
developedtocouplethenormalincidentIRlightintotheQWlPs. Thefirstgrating
structureisthe2-Dsquaredotreflectionmetalgratingwithacouplingefficiencyof
20%. Thecouplingefficiencycanbeincreasedbyemployingitscomplementarygeom-
etry,the2-Dsquareaperturemeshmetalgrating. About70%ofthenormalincident
lightisdiffractedintoTMwavesavailableforQWIPabsorption. Tosolvethecorner
VI
roundingeffectobservedinthesquareshapegrating,a2-Dcircularaperturemetal
gratingcouplerisdeveloped,whichhasthesamecouplingefficiencyof70%andwith
aefficiencyof2.7timeshigherthanthatofthe45°polishededgeQWIPwithcorre-
spondingquantumwellparameters.Inadditiontothesquaresymmetryarrangement
requiredinthethreegratingdesigns,onemorebenefitfromthehexagonalsymmetry
circularaperturegratingisawidercouplingbandwidth. Foreachgratingstructure,
severaluniversalplotsbasedontwonormalizedparameters\jgandajgwerepre-
sented,whereAisthewavelength,gisthegratingperiod,andaisthedimensionof
gratingshape. Theuniversalplotsprovideaconvenientwayforgratingdesign.
Modalexpansionandmethodofmomentsaretwobasicapproachesusedinthe
numericalanalysisofgratingcouplers. Thediffractedangleanddiffractedpowerare
drawn inuniversalplots, thediffractedangledetermines theabsorption constant,
combiningwiththediffractedpowerthespectralquantumefficiencyofQWIPcanbe
calculated.
Inadditiontocouplingenhancementbygrating,thewaveguidegeometryQWIP
showsafurtherimprovementinIRabsorption. ThedesignofQWIPwaveguideis
carriedoutnumerically. Togetherwiththeoptimumperiodofmultiquantumwells
and theblockinglayerstoreducethedark current, ahigh performanceQWIPis
proposed.
Vll
CHAPTER
1
INTRODUCTION
1.1QuantumWellDetectors
Thereisagreatneedforhighresolution,highsensitivityfocalplanearrayde-
tectorsinthelongwavelengthinfraredspectrumofthe8-14fimatmosphericwindow
forboththemilitaryandcivilianregimes. Themostwidelyusedinfrareddetector
materialsareintrinsicSiandHgCdTe(MCT).Evenwiththerapidgrowthofthe
silicon-basednewtechnologiesandmicroelectronicsindustry, it isgenerallyrecog-
nizedthatforlongwavelengthinfrareddetectorapplicationsIII-Vcompoundsemi-
conductorsaresuperiortoSi [1]. Thisisduetodifferentmaterialproperties,such
asawiderangeofavailablebandgaps,thedirectbandgapsthatfacilitatesefficient
conversionfromlighttoelectricity,andcarrierswithveryhighmobility. Thedif-
ficultiesinmaterialsgrowth,processing, andotherdetailsofthesetechnologiesof
HgCdTematerialhavemadeithighlydesirabletoexaminewhetherperformancecan
beimprovedbyothermaterialsystems[2]. Sincethematerialanddevicetechnolo-
giesinHI-VcompoundsemiconductorsarefarmoreadvancedthanHgCdTe,itis
naturaltoinvestigatetheseHI-Vcompoundsforpossibleapplicationsinlongwave-
lengthinfrareddetection. Recently,IH-Vquantumwell/superlatticestructureshave
beenusedtoconstructlongwavelengthinfraredphotodetectors [3]. Thequantum
wellisformedbyusingalayerofanarrowbandgapsemiconductor(suchasGaAs)
sandwichedbetweentwowider bandgap semiconductorlayers (such AlxGai_a;As).
Themotionoftheelectronperpendiculartothelayersbecomesquantizedsothat
localized(inthedirectiontransversetothelayers)two-dimensional(2-D)subbands
ofsize-quantizedstateswereformedinsidethequantumwell[4-6].
1
2
Thesemulti-quantumwellstructuresareusuallygrownbymolecularbeamepi-
taxy (MBE) technique, whichallowstheexact adjustmentofthemost important
deviceparameters. Bychangingthewellwidth andbarrierheight, detectors can
bedesignedwithresponsepeaksfromthemid-tothefar-infraredwavelengthre-
gions. Accordingtotheelectronstransportapproaches,twofundamentalstructures
ofquantumwellinfraredphotodetectors (QWlPs) areillustratedinFig. 1.1. For
thebound-to-continuous(BTC)QWlPs(Fig. 1(a)),theelectronsphotoexcitedfrom
theboundgroundstatebyinfrared(IR)radiationaretransportedthroughtheex-
tendedcontinuumbandabovethebarrierunderanappliedelectricfield [7],while
forabound-to-miniband(BTM)QWIP,asshowninFig. 1(b),theseelectronsare
transportedthroughaglobalminibandformedbysuperlatticebarriers.
1.2QuantumWellIntersubbandAbsorption
TheabsorptioninaQWIPisbasedonintersubbandtransitionofn-typedoped
quantumwellheterostructures. UponilluminationofIRradiation,electronsareex-
citedfromtheground stateintotheexcitedstate [8]. Onlyintheexcitedstate,
electronsmovinginthedirectionperpendiculartoquantumwellsbecomespossible,
whichproducetheelectricsignalunderanappliedelectricfield. Therefore,thebasic
operationofQWlPsisphotoconductivetype,andtheschematicdiagramofadetector
pixelisshowninFigure1.2. Carefulopticaldesignisneededsinceoneofthemajor
problemsencounteredintype-I(n-doped) QWlPsisthatthepolarizationselection
ruleforintersubbandtransitionrequiresonecomponentoftheelectricfieldvector
oftheIRradiationtobepolarizedperpendiculartothequantumwelllayerplanes
[9-11]. Thisresultsinthedetectorshavenophoto-responseforradiationincidentin
thedirectionnormaltothequantumwelllayers. Forfocalplanearrays(FPAs)ap-
plications,asshowninFig. 1.3,aresponsetonormalincidentradiationisrequisite.
Forthisreason,specialschemesofcouplingtoconvertthetransverseelectromagnetic
waveintoadirectionsuitableforabsorptionarenecessary. Avarietyoftechniques
3
havebeenemployed, suchasa45° anglepolishedfacet insubstrateofthedetec-
torsampleasshowninFig. 1.4(a),theilluminationatBrewster’sangleorientation
(Fig. 1.4(b)), aprismcoupler [12]inthedetectorsurface (Fig. 1.4(c)),alamellar
gratingcoupler(Fig. 1.5(a)), andadoubleperiodicmetalgratingonthedetector
surface(Figs. 1.5(b),(c)).Thecouplingoftheincidentintensitytotheintersubband
transitionsistooweektobeusedforsampleorientedatBrewster’sangleorcoupled
byprism. Thelightcouplingthrougha45°polishedfacetonlyallowsincidentlight
toaccessoneedgeofthedetector,whichispossibletoformonlyone-dimensional
(1-D)lineararrays. ForimagingapplicationinFPAs,itisnecessarytocouplelight
uniformlyintothe2-Darraysofsuchdetectors. A1-Dlineargratingeitherbyde-
positingmetallinesontothedetectorsurface(Figs. 1.2and 1.5(a))orbychemical
etchingthedetectorsurfaceintoperiodicprofile(Fig. 1.6(a))orfollowedbymetal
deposition(Fig. 1.6(b))arepotentiallyapplicabletothefabricationof2-Ddetector
arrays. However,theseapproachesarepolarizationdependent,onlycoupletransverse
magnetic(TM)componentoftheincidentlight.Tosolvethis,adoubleperiodiccross
gratingcouplerisusedtoeffectivelycouplethenormalincidentIRradiationintothe
dopedwellsindependentoflightpolarization.
The detector array consists ofpixelsofQWlPs arranged periodically on the
detectorsubstrate. Toallownormalincidenceillumination,thegratingcanbefabri-
catedeitheronthesubstrateside(Fig. 1.7(a))oronthetopsurface(Fig. 1.7(b))of
thedetectors. Itisworthyofnotingthatthelatterhcistheadvantageovertheformer
inthatiteliminatestheundesirablepixelcrosstalkasshowninFig. 1.7.
1.3GratingCoupler
Theabovediscussionssuggestthatdiffractiongratingisaneffectiveopticalcou-
plingmeansforfabricatinglargearean-typeQWIParrays. Thishasbeendemon-
stratedintheearlywork[13,14]ofintersubbandresonancesinelectroninversionlayer