MOVPE of GaSb-based materials and solar cell structures

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(10) :USAMMENFASSUNG DEUTSCH. "AUELEMENTE ENTWICKELT %INE GRUNDLEGENDE :ELLSTRUKTUR WURDE AUF "ASIS DER -ATERIALDATEN ENTWORFEN UND ABGESCHIEDEN %INE GUTE /BERFLCHENPASSIVIERUNG DES -ATERIALS ERWIES SICH ALS UNERLLICH UND KONNTE DURCH EINE ÌL'A ÌS3B À&ENSTERSCHICHTÁ ERREICHT WERDEN )NFOLGE DER VIELVERSPRECHENDEN GRUNDLEGENDEN %RGEBNISSE ZUM 3ILIZIUMÌKZEPTOR WURDE DIESES %LEMENT AUCH IN DEN 3OLARZELLEN 3TRUKTURENALS$OTIERSTOFFEINGESETZT$IEINTERNEN1UANTENEFFIZIENZENDER"AUELEMENTE DIE ZU TEILS SEHR GUTEN 7ERTEN VON MEHR ALS ¬BER EINEN GROEN 3PEKTRALBEREICH ABGESCHTZT WERDEN K¦NNEN BELEGEN DIE HOHE -ATERIALQUALITT 3IDOTIERTEN 'ALLIUM ANTIMONIDS ¬BER WEITE $OTIERBEREICHE ÌUF DER 'RUNDLAGE DER UMFASSENDEN MATERIAL WISSENSCHAFTLICHEN 5NTERSUCHUNGEN ZUM 7ACHSTUM ALUMINIUMHALTIGER +RISTALLE KONNTE AUCHEINE ÌL'A ÌS3B 3OLARZELLE % ^E6 REALISIERTWERDEN$IESE3OLARZELLEISTDAS ERSTE-/60%GEWACHSENE-INORITTEN"AUELEMENTMIT ÌL'A ÌS3B IMAKTIVEN"EREICH $ARAUF AUFBAUEND KONNTE AUCH ERSTMALIG EINE 'A3BBASIERENDE MONOLITHISCHE ÌL'A ÌS3B 'A3B4ANDEMZELLE MITTELS DER -/60% ABGESCHIEDEN WERDEN DIE JEDOCH NOCHWEITERVERBESSERTWERDENMU(IERSOLLTESICHDIEZUK¬NFTIGEÌRBEITINERSTER,INIE AUF DIE IN DER 3TRUKTUR ENTHALTENE 4UNNELDIODE KONZENTRIEREN F¬R DIE DER 4YP ))" BERGANGZWISCHEN)NÌSUND'A3BVON)NTERESSEIST3CHLIELICHWURDEEINENEUE+LASSE 'A3BBASIERENDER 3OLARZELLEN ABGESCHIEDEN UND UNTERSUCHT DIE AUF EINEM PN )FUFSP¬BERGANG BASIEREN "EI ERH¦HTER ,EERLAUFSPANNUNG LIEFERTE EINE P'A3B N ÌL'A ÌS3B 3TRUKTURDIEGLEICHE1UANTENEFFIZIENZWIEEINEANALOGE3OLARZELLE DIEAUF EINEMKONVENTIONELLENHOMOEPITAKTISCHENPNBERGANGIN'A3BBERUHT$IEVERBESSERTE 3PANNUNG WIRD EINEM VERRINGERTEN 3TTIGUNGSSTROM ZUGESCHRIEBEN $IE WEITERE %NTWICKLUNG VON 3OLARZELLEN MIT E6ÌBSORBERMATERIALIEN SOLLTE SICH DAHER AUF DIESEN 3TRUKTURTYP KONZENTRIEREN DER AUCH IN ZUK¬NFTIGE 4ANDEMZELLEN EINGEBAUT WERDENKANN GAP. )NSGESAMT KONNTEN DIE AUF DEN 'EBIETEN DES +RISTALLWACHSTUMS +APITEL UND DER 0HOTOLUMINESZENZ#HARAKTERISIERUNG VON 3IDOTIERTEM 'A3B +APITEL GEWONNENEN GRUNDLEGENDEN WISSENSCHAFTLICHEN %RKENNTNISSE ERFOLGREICH F¬R DIE %NTWICKLUNG PHOTOVOLTAISCHER "AUELEMENTE +APITEL EINGESETZT WERDEN 7EITERGEHENDE ÌSPEKTE F¬RANKN¬PFENDEÌRBEITENAUFSOWOHLGRUNDLEGENDENALSAUCH ANGEWANDTEN 'EBIETEN WURDENAUFGEZEIGT.

(11) -OTIVATIONANDINTRODUCTION. . .PUJWBUJPOBOEJOUSPEVDUJPO. 0HOTOVOLTAIC CELLS BASED ON )))6 SEMICONDUCTORS CAN ACHIEVE MUCH HIGHER EFFICIENCIES THAN THEIR SILICON BASED COUNTERPARTS $UE TO THE HIGH COSTS OF )))6 SUBSTRATES ECONOMICALLY COMPETITIVE TERRESTRIAL APPLICATIONS OF )))6 BASED SOLAR CELLS INCLUDE CONCENTRATING SYSTEMS WHERE THE AREA OF CELL REQUIRED FOR A GIVEN POWER OUTPUT IS NOTABLYREDUCED(ERE THECOSTSOFTHEWHOLESYSTEMAREDISPLACEDFROMTHECELLSTOTHE CONCENTRATINGELEMENTS INCLUDINGTRACKINGSYSTEMS SOTHATSOPHISTICATED)))6CELLSWITH MAXIMISEDEFFICIENCIESCANBEEMPLOYED /NEOFTHEMAINLOSSMECHANISMSINSOLARCELLSISTHETHERMALISATIONOFHOTPHOTOEXCITED CARRIERS TO THE BAND EDGES (IGHER EFFICIENCIES CAN BE ACHIEVED WITH MULTIBAND GAP SYSTEMS (ERE HIGHENERGY PHOTONS ARE ABSORBED IN A WIDEBAND GAP SOLAR CELL WHILE LOWENERGY PHOTONS ARE TRANSMITTED THROUGH THE MATERIAL Ì SECOND CELL BASED ON A LOWERBAND GAP MATERIAL IS PLACED BEHIND THE FIRST ONE 4HIS SECOND CELL CONVERTS THE LOWERENERGY PHOTONS INTO ELECTRICITY 3UCH A PHOTOVOLTAIC SYSTEM IS CALLED A ÀTANDEM CELLÁ Ì CORRESPONDING DEVICE THE MONOLITHIC 'A)N 0'AÌSTANDEM CELL IS ALREADY IN COMMERCIALUSE4HEEFFICIENCYOFSUCHASYSTEMCANBEENHANCEDEVENMOREBYFURTHER INCREASING THE NUMBER OF CELLS BASED ON DIFFERENT BAND GAPS 4HE EFFICIENCY GAIN PER ADDITIONAL CELL IS REDUCED WITH EVERY ADDED DEVICE Ì REASONABLE TRADEOFF BETWEEN EFFICIENCYANDTECHNOLOGICALEFFORTISGIVENFORASYSTEMBASEDONFOURCELLSÌPOSSIBLE REALISATIONCOULDBEA 'A)N 0'AÌSTANDEMCELLWHICHISMECHANICALLYSTACKEDONTOPOF A SECOND MONOLITHIC TANDEM CELL -ODELLING SHOWS THAT THE LATTER DEVICE SHOULD BE BASED ON THE BAND GAPS E6 AND E6 4HESE VALUES SUGGEST THE MATERIAL COMBINATION ÌL'A ÌS3B 'A3B AS PROMISING CANDIDATE (OWEVER A MONOLITHIC ÌL'A ÌS3B 'A3BTANDEMCELLSTILLREMAINSTOBESHOWN 4HE PRODUCTION OF SOPHISTICATED LARGE AREA )))6 DEVICES E G PHOTOVOLTAIC CELLS MAKES THE EMPLOYMENT OF INDUSTRIAL SIZE MULTIWAFER -/60% METALORGANIC VAPOUR PHASE EPITAXY REACTORS DESIRABLE (OWEVER THE -/60% GROWTH OF ANTIMONIDES IS A RATHER RECENTFIELD$UETOTHELOWVOLATILITYOFELEMENTALANTIMONYANDTHEINSTABILITYOFSTIBINE THE GROWTH OF 3BCONTAINING )))6 SEMICONDUCTORS IS NOTABLY DIFFERENT AND MUCH LESS STUDIED COMPARED TO ARSENIDES AND PHOSPHIDES ÌS A RESULT A 'A3BBASED -/60% GROWN LASER STRUCTURE WAS NOT REPORTED UNTIL )N ADDITION T HE COMPLEX GROWTH PROCESSESININDUSTRIALSIZEMULTIWAFER-/60%REACTORSREPRESENTANADDITONALCHALLENGE FORTHEDEPOSITIONOFHIGHQUALITY3BCONTAINING)))6COMPOUNDSANDDEVICE STRUCTURES "ASEDONTHEABOVECONSIDERATIONS THISTHESISCOVERSAWIDERANGEOFBOTHFUNDAMENTAL AND APPLIED ASPECTS 4HESE INCLUDE THE COMPLEX -/60% GROWTH OF VARIOUS ))) ANTIMONIDESASWELLASTHEELECTRONICANDOPTICALPROPERTIESOFSILICONDOPED'A3B4HE CORRESPONDING RESULTS MADE THE DEVELOPMENT OF NOVEL PHOTOVOLTAIC DEVICE STRUCTURES POSSIBLE EGMONOLITHIC ÌL'A ÌS3B 'A3BTANDEMCELLS 4HEPROPERTIESOFTHERELATED SOLARCELLSAREANALYSEDWITHREGARDTOTHERESULTSONGROWTHANDMATERIALPROPERTIES #HAPTERGIVESABRIEFSUMMARYOFFUNDAMENTALASPECTSOF)))6SEMICONDUCTORS RELATED CRYSTAL GROWTH TECHNIQUES AND THE PECULIARITIES OF THE ANTIMONIDE MATERIAL SYSTEM -OREOVER THEBASICDEVICEPROPERTIESOFSOLARCELLSAREDESCRIBEDÌSIMPLIFIEDMODELFOR PNJUNCTIONSWITHANINCIDENTLIGHTSPECTRUMISDERIVED3UBSEQUENTLY THEKEYPROPERTIES.

(12) -OTIVATIONANDINTRODUCTION OF REAL PHOTOVOLTAIC DEVICES ARE INTRODUCED AND CHARACTERISTIC )6CURVE PARAMETERS ARE PRESENTED %VENTUALLY MULTIJUNCTION DEVICES ARE DESCRIBED AND THE RESULTS OF A RELATED BANDGAPVERSUSEFFICIENCYMODELLINGAREDEMONSTRATED #HAPTER PRESENTS THE MAIN EXPERIMENTAL TECHNIQUES THAT HAVE BEEN USED FOR THIS THESIS)TSTARTSWITHASECTIONONTHEFUNDAMENTALSOF-/60%4HEEMPLOYEDREACTORIS DESCRIBED AND A FIRST OVERVIEW IS GIVEN CONCERNING THE PECULIARITIES OF THE -/60% OF ANTIMONIDES 3UBSEQUENTLY THE MAIN CHARACTERISATION METHODS ARE INTRODUCED (IGH RESOLUTION8RAYDIFFRACTION (282$ PHOTOLUMINESCENCE 0, (ALLEFFECT SURFACEPHOTO ABSORPTION 30Ì ANDSECONDARYIONMASSSPECTROSCOPY 3)-3 #HAPTERSTOAREDEDICATEDTOTHERESULTSOFTHISTHESIS7HILECHAPTERDEALSWITHTHE ACTUAL CRYSTAL GROWTH CHAPTER DESCRIBES THE RESULTS CONCERNING SILICON AS NOVEL ACCEPTORDOPANTINTHE-/60%OFANTIMONIDES ANDCHAPTERFOCUSESONPHOTOVOLTAIC DEVICES #HAPTERBEGINSWITHAGENERALINTRODUCTIONRELATEDTOTHE-/60%OFANTIMONIDES)N SECTION CURRENT POINTS AT ISSUE CONCERNING THE -/60% OF ÌLFREE ANTIMONIDES ARE DISCUSSED SUCH AS SUBSTRATE PREPARATION AND SUBSTRATE ORIENTATION 4HE ELECTRICAL AND OPTICALDATAOFTHERELATEDMATERIALARESHOWNTOBEWELLINLINEWITHTHEBESTVALUESIN THELITERATURE3ECTIONDEALSWITHTHE-/60%OFALUMINIUMCONTAININGMATERIALSÌS THISFIELDISSTILLONEOFTHEMOSTDEMANDINGCHALLENGESFOR-/60%GROWTHNOWADAYS A COMPREHENSIVEINTRODUCTIONISGIVENFIRST3UBSEQUENTLY THESUCCESSFULGROWTHOFHIGH QUALITY P AND NTYPE ÌL'A ÌS3B IN A MULTIWAFER -/60% REACTOR IS REPORTED FOR THE FIRST TIME 4WO PROMISING NOVEL ÌLPRECURSORS TRITERTIARYBUTYLALUMINIUM AND DIMETHYL ETHYLAMINEALANEADDUCT ARE COMPARED TO EACH OTHER 6ARIOUS ASPECTS OF THEGROWTH PROCESS AND THE RELATED PROPERTIES OF THE CRYSTALS ARE DISCUSSED IN DETAIL &INALLY A SUMMARY OF THE GROWTH OF ANTIMONIDES IN AN INDUSTRIAL SCALE MULTIWAFER PLANETARY -/60%REACTORISGIVEN #HAPTER DEALS WITH SILICON AS NOVEL ACCEPTOR DOPANT IN THE -/60% OF 'A3B 4HE CHAPTER PRESENTS THE INCORPORATION BEHAVIOUR AND THE ELECTRICAL PROPERTIES 4HE COMPENSATION RATIO IS DETERMINED FOR VARIOUS DOPING LEVELS 4HE 0, PROPERTIES OF NON INTENTIONALLYDOPEDANDSILICONDOPED'A3BARESTUDIEDÌLL0,PEAKSINTHEHIGHENERGY PARTOFTHESPECTRUMCANBEEXPLAINEDWITHINTHECONCEPTOFPOTENTIALFLUCTUATIONSDUE TOTHECHARGEDIMPURITYDISTRIBUTION4HEOBSERVEDPROPERTIESMAKETHESILICONACCEPTOR ANINTERESTINGNOVELCANDIDATEFOR'A3BBASEDDEVICEAPPLICATIONS #HAPTER PRESENTS DEVICE RELATED RESULTS 4HE FABRICATION TECHNOLOGY IS DESCRIBED IN DETAIL3EVERALDEVICESTRUCTURESAREPRESENTED NAMELYPNHOMOJUNCTIONSFORBOTH'A3B AND ÌL'A ÌS3B PNHETEROJUNCTIONS AND TANDEM CELLS 4HE IMPORTANCE OF SURFACE PASSIVATION BY HIGH BAND GAP WINDOW LAYERS IS DEMONSTRATED 4HE SILICON ACCEPTOR PROVESTOBEBENEFICIALFORTHEDEVICEPERFORMANCE #HAPTER GIVES AN OVERALL SUMMARY AND A RELATED OUTLOOK )N ADDITION A COLLECTION OF Ì&-PICTURESISGIVENINTHEANNEXCONCERNINGSUBSTRATESANDEPILAYERMORPHOLOGIES 0ARTSOFTHISWORKHAVEALREADYBEENPUBLISHEDELSEWHERE#ORRESPONDINGREFERENCESARE GIVENINTHEPUBLICATIONLISTATTHEENDOFTHISWORK.

(13) &UNDAMENTALS. . 'VOEBNFOUBMT. ***7TFNJDPOEVDUPST *OUSPEVDUJPO )))6 SEMICONDUCTORS ARE CRYSTALLINE MATERIALS WITH THE :N3STRUCTURE FIGURE 4HIS STRUCTURE CONSISTS OF TWO INTERPENETRATING FACE CENTRED CUBIC SUB LATTICES WHICH ARE DISPLACEDFROMEACHOTHERBYATRANSLATIONALONG 1 4 1 4 1 4 . 'JHVSF ;O4DSZTUBM TUSVDUVSF 5XP JOUFSQFOFUSBUJOH GDD MBUUJDFT BSF EJTQMBDFE GSPN FBDIPUIFSCZBUSBOTMBUJPOBMPOH 1 4 1 4 1 4 0OFMBUUJDFDPOTJTUTPGHSPVQ*** UIFPUIFSPGHSPVQ7FMFNFOUT /NE LATTICE CONSISTS OF GROUP ))) THE OTHER OF GROUP 6ELEMENTS )N CONTRAST TO THEIR ELEMENTAL GROUP )6COUNTERPARTS THESE SOCALLED ÀCOMPOUND SEMICONDUCTORSÁ HAVE PARTLY IONIC BONDS DUE TO TRANSFER OF ELECTRONIC CHARGE FROM THE GROUP ))) ATOM TO THE GROUP6ATOM4HEYCANBEBINARYCRYSTALS SUCHASGALLIUMARSENIDE ORMIXTURESONTHE RESPECTIVE SUBLATTICES SUCH AS GALLIUMINDIUMPHOSPHIDEARSENIDE ÌS THEY WILL ALWAYS CONSIST OF GROUP ))) AND GROUP 6ATOMS THE USUAL NOTATION GIVES ONLY THE COMPOSITION WITH REGARD TO THE RESPECTIVE SUBLATTICE EG ÌL 'A ÌS 4HE BAND STRUCTUREOF)))6SEMICONDUCTORSCANBEÀENGINEEREDÁBYCHANGINGTHECOMPOSITIONOF THE CRYSTAL &OR EXAMPLE ADDING ALUMINIUM TO 'A3B LEADS TO AN INCREASE OF THE DIRECT BANDGAP WHILEATRANSITIONTOANINDIRECTBANDGAPOCCURSATAROUND ÌL 'A 3B4HE BAND GAPS OF BINARY )))6 SEMICONDUCTORS AND THEIR TERNARY MIXTURES ARE SHOWN AS A FUNCTIONOFTHECOMPOSITIONINFIGURE7HILETHELATTICECONSTANTISALINEARFUNCTION OFTHECOMPOSITION À6EGARD¾SLAWÁ THISDOESNOTHOLDFORTHEBANDGAPENERGY . . . .

(14) "ANDGAPENERGY;E6= Bandlücken Energie (eV). &UNDAMENTALS. 2,5. AlP GaP. direkt indirekt. AlAs. 2,0. AlSb. 1,5. GaAs Si. 1,0. InP. Ge. 0,5. GaSb InAs InSb. 0,0. 5,4. 5,6 5,8 6,0 6,2 6,4 6,6 Gitterkonstante (Å) ,ATTICECONSTANT;ÌNGSTROM=. 'JHVSF #BOE HBQT PG CJOBSZ ***7 TFNJDPOEVDUPST BOE UIFJS UFSOBSZ NJYUVSFT BT B GVODUJPO PG UIF DPNQPTJUJPO %JSFDU NBUFSJBMT BSF SFQSFTFOUFE CZ TPMJE JOEJSFDUNBUFSJBMTCZEPUUFEMJOFT -OST DEVICES BASED ON )))6 SEMICONDUCTORS CONSIST OF EPITAXIAL STRUCTURES GROWN ON BINARY SUBSTRATE CRYSTALS SEE AS WELL SECTION 4HE MOST COMMON SUBSTRATE MATERIALSARE'AÌSAND)N0&ORSOMEAPPLICATIONSMOREEXOTICSUBSTRATESAREUSEDSUCH AS'E 'A0 'A3B )NÌSOR)N3BÌSLATTICEMATCHEDGROWTHISIMPORTANTINORDERTOAVOID STRUCTURAL DEFECTS ONLY CRYSTAL COMPOSITIONS WITH IDENTICAL LATTICECONSTANTS ARE USUALLY GROWN ONTO EACH OTHER 7HILE TERNARY CRYSTALS ARE LIMITED TO ONE PARTICULAR LATTICE MATCHED COMPOSITION E G 'A )N ÌS)N0 QUATERNARY MIXTURES PROVIDE AN EXTRA DEGREE OF FREEDOM E G 'A)N 0ÌS )N0 (ERE A WHOLE RANGE OF DIFFERENT BAND GAPS CAN BE REALISED FOR A FIXED LATTICE CONSTANT ÌN EXCEPTION IS THE MATERIAL SYSTEM ÌLÌS'AÌS WHERE THE LATTICE CONSTANTS ARE SO SIMILAR TO EACH OTHER THAT ALL TERNARY COMPOSITIONSOF ÌL'A ÌSCANBEGROWNON'AÌSASSUBSTRATEMATERIAL . . -ODERN DEVICE STRUCTURES CONSIST OF MANY DIFFERENT LATTICEMATCHED MATERIALS GROWN ONTO EACHOTHER 4HEIR PARTICULAR PROPERTIES CAN VARY IN MANY RESPECTS E G BAND GAP ANDDOPING)NADDITION INTENTIONALSTRAINCANBEUSEDTOALTERTHEBANDSTRUCTUREINTHE VALENCE BAND THE BAND ALIGNMENT FOR HETEROSTRUCTURES CAN VARY CONSIDERABLY AND VERY THINFILMSCANBEGROWNTHATEVENSHOWQUANTUMEFFECTS4HESEFEATURESFORMTHEBASIS OF ALL MODERN OPTOELECTRONIC DEVICES &OR FURTHER INFORMATION THERE ARE VARIOUS EXCELLENTTEXTBOOKSONTHISSUBJECTSUCHAS ; = 7ITH RESPECT TO THE OPTOELECTRONIC DEVICE ÀSOLAR CELLÁ THE MAJOR DIFFERENCE BETWEEN ELEMENTAL GROUP )6SEMICONDUCTORS E G SILICON AND )))6 SEMICONDUCTORS IS THAT THE FIRSTAREINDIRECTWHILETHELATTERAREOFTENDIRECTMATERIALS(ENCE THEABSORPTIONDEPTH FORLIGHTISNOTABLYDIFFERENT)))6DEVICES HAVE THE EXTRA ADVANTAGE THAT COMPLEX HIGH EFFICIENCYMULTIJUNCTIONSTRUCTURESCANBEREALISED SEESECTION /NTHEOTHERHAND SILICONHASAMUCHLOWERPRICEANDBETTERAVAILABILITY4HEPRESENTTHESISWILLFOCUSON3B CONTAINING)))6SEMICONDUCTORSFORSOLARCELLAPPLICATIONS.

(15) &UNDAMENTALS. $SZTUBMHSPXUI 4HE CRYSTAL GROWTH OF )))6 COMPOUND SEMICONDUCTORS CAN BE CATEGORISED INTO ÀBULK CRYSTAL GROWTHÁ AND ÀEPITAXIAL CRYSTAL GROWTHÁTECHNIQUES 6ERY GOOD SUMMARIES ON THISSUBJECTCANBEFOUNDIN ;=AND ;=ÌCORRESPONDINGOVERVIEWWILLBEGIVENIN THEFOLLOWING 4HE AIM OF THE BULK GROWTH IS TO PRODUCE LARGE INGOTS WHICH CAN THEN BE SLICED AND PROCESSED INTO SUBSTRATES 4HE TWO MAIN BULK GROWTH TECHNIQUES ARE THE À#ZOCHRALSKI METHODÁ AND THE À"RIDGMAN METHODÁ 4HE #ZOCHRALSKI METHOD INVOLVES MELTING THE RAWMATERIALINACRUCIBLEANDTHENPLACINGASEEDCRYSTALINCONTACTWITHTHEMELT7HILE THE SEED IS SLOWLY ROTATED THE MELT SURFACE COOLS AND ADDITIONAL MATERIAL IS SOLIDIFIED FROM THE MELT ONTO THE SEED ÌS ARSENIC AND PHOSPHORUS HAVE VERY HIGH VAPOUR PRESSURES AT THE MELTING TEMPERATURES OF THE COMPOUNDS THEY TEND TO LEAVE THE MELT ANDCONDENSEONTHESIDEWALLS4HEREFORETHE#ZOCHRALSKIMETHODISUSUALLYMODIFIEDFOR )))6 GROWTH BY ISOLATING THE MELT FROM THE AIR BY A LAYER OF MOLTEN BORON OXIDE 4HIS MATERIAL FLOATS ON THE SURFACE AND PREVENTS THE VOLATILE ELEMENTS FROM ESCAPING 4HIS MODIFICATION OF THE #ZOCHRALSKI METHOD IS CALLED À,%# METHODÁ ,IQUID%NCAPSULATED #ZOCHRALSKIMETHOD )NTHE"RIDGMANMETHODATEMPERATUREGRADIENTISCREATEDALONG THE LENGTH OF A STATIONARY BOAT WHICH CONTAINS THE MOLTEN MATERIAL 4HE TEMPERATURE AROUND THE SEED CRYSTAL IS HELD BELOW THE MELTING POINT TO FORM THE CRYSTAL ÌLL THESE BULKCRYSTALGROWTHTECHNIQUESAREREASONABLYMATUREDFOR'AÌSAND)N0WHILEITISSTILL DIFFICULTTOOBTAINHIGHQUALITYMATERIALFORMOSTOTHERCOMPOUNDSEMICONDUCTORS(ERE ESPECIALLY THE SURFACE TREATMENT FOR SUBSEQUENT EPITAXIAL PROCESSES IS PROBLEMATIC 2ECENTEFFORTSALSOINCLUDETHEGROWTHOFTERNARYMATERIALSFORSUBSTRATEPURPOSES ;= )NEPITAXIALTECHNIQUES ANEPITAXIALLAYER ORÀEPILAYERÁ ISSUBSEQUENTLYGROWNONTOTHE SUBSTRATE4HEWORDÀEPITAXYÁCOMESFROMTHE'REEKWORDSÀEPIÁ UPON ANDÀTAXISÁ ORDERED AND DESCRIBES THE ORDERED CONTINUATION OF THE UNDERLYING SUBSTRATE CRYSTAL STRUCTURE 4HE MAIN EPITAXIAL TECHNIQUES ARE À,IQUID 0HASE %PITAXY ,0% Á À-OLECULAR "EAM%PITAXY -"% ÁANDÀ6APOUR0HASE%PITAXY 60% Á)N,0% USUALLYAGROUP)))METAL IS USED AS SOLVENT FOR THE GROUP 6 ELEMENT 7HEN A SUBSTRATE IS BROUGHT INTO CONTACT WITHTHELIQUIDPHASEANDTHESOLVENTGETSSUPERSATURATEDWITHTHEGROUP6ELEMENTBY COOLING THENUCLEATIONOFTHECRYSTALSTARTSONTHESUBSTRATESURFACE#ONSEQUENTLY THIS IS A TECHNIQUE CLOSE TO EQUILIBRIUM WHERE IT IS DIFFICULT TO GROW HETEROSTRUCTURES WITH ABRUPTINTERFACESÌSITISQUITEINEXPENSIVE MANYRELATIVELYUNCOMPLICATEDDEVICESSUCH AS ÌL'A ÌS DOUBLE HETEROSTRUCTURE LASERS AND ,%$S ARE STILL GROWN WITH ,0% )N -"% CRUCIBLESCONTAININGELEMENTALCHARGESAREHEATEDINANULTRAHIGHVACUUMENVIRONMENT 5(6 ^ TORR 4HIS CREATES A MOLECULAR OR ATOMIC BEAM OF THE ELEMENTS FROM THE CRUCIBLESTOTHEHEATEDSUBSTRATESURFACEWHEREEPITAXIALGROWTHTAKESPLACE3INCETHE MOLECULESCANTRAVEL FOR METERS WITHOUT COLLISION AND NO CHEMICAL REACTIONS OCCUR THIS METHODISCONCEPTUALLYSIMPLEANDVERYPOWERFULWITHREGARDTOHETEROSTRUCTUREPHYSICS (OWEVER THEREARESTILLPROBLEMSWITHCERTAINMATERIALS EGPHOSPHIDESAND NITRIDES )N60%TECHNIQUESTHEPRESSURESAREMUCHHIGHER(ERE GASESTHATCONTAINTHENECESSARY CHEMICAL ELEMENTS ARE DELIVERED FROM A GASEOUS ENVIRONMENT AND MADE TO REACT CHEMICALLY IN THE VICINITY OF THE SUBSTRATE Ì TYPICAL EXAMPLE FOR THIS TECHNIQUE IS THE GROWTHOFSILICONACCORDINGTO SiH 4 → Si + 2H 2 7HILETHISTECHNIQUEISMAINLYUSEDFOR HOMOEPITAXIAL GROWTH OF HIGH PURITY MATERIALS A RELATED BUT MUCH MORE SOPHISTICATED METHOD IS USED FOR )))6 HETEROSTRUCTURES THE À-ETALORGANIC 6APOUR 0HASE %PITAXY .

(16) &UNDAMENTALS -/60% Á(ERE THE60%PROCESSISBASEDONGASEOUSMETALORGANICCOMPOUNDSÌSTHIS TECHNIQUE WAS EMPLOYED FOR THE GROWTH OF )))ANTIMONIDES IN THE PRESENT THESIS A DETAILEDDESCRIPTIONWILLBEGIVENINSECTION .BUFSJBMTBOEEFWJDFTCBTFEPO(B4C )))ANTIMONIDES HAVE LARGER LATTICE CONSTANTS AND SMALLER BAND GAPS THAN THEIR ARSENIC AND PHOSPHORUSBASED COUNTERPARTS FIGURE ÌPART FROM THESE OBVIOUS DIFFERENCES THEY ALSO HAVE MANY MORE INTERESTING PECULIARITIES &OR EXAMPLE THE ,VALLEY IN THE CONDUCTIONBAND OF 'A3B ISONLY APPROXIMATELY E6 ABOVE THE Γ VALLEY ;= NOT INTENTIONALLYDOPED'A3BISALWAYSHIGHLYPTYPEDUETOANATIVESTRUCTURALDEFECT ÌL3B IS OPU LATTICEMATCHED TO 'A3B IN CONTRAST TO THE 'AÌSÌLÌSSYSTEM MIXTURES ON THE GROUP6 SUBLATTICE HAVE LARGE MISCIBILITY GAPS AND ÌUGER RECOMBINATION IS SIGNIFICANT #OMPAREDTOARSENIDESANDPHOSPHIDES GROWTHANDDEVICEPROCESSINGTECHNOLOGYISIN ITSINFANCYÌTPRESENT THEONLYANTIMONIDEBASEDDEVICESINLARGESCALECOMMERCIALUSE ARE(ALLANDMAGNETORESISTIVESENSORSBASEDON)N3B ; =WITHITSEXTRAORDINARILY HIGHELECTRONMOBILITY 4HE GROWTH OF ANTIMONIDES IS DIFFICULT DUE TO THE LOW VAPOUR PRESSURE OF ANTIMONY SPECIES4HISISILLUSTRATEDBYTHEFOLLOWINGCONSIDERATIONS)FTHEARSENICPRESSUREDURING THEGROWTHOF'AÌSGETSVERYHIGH ASECONDCONDENSEDPHASE GaAs(s ) + As(s ) CANIN THEORYBEFORMED ;= FIG 4HECORRESPONDINGARSENICPRESSUREFORTHEFORMATION OFTHESECONDPHASEISAPPROXIMATELYEQUALTOTHEARSENICPRESSUREOVERPUREELEMENTAL ARSENICANDCLEARLYEXCEEDSBARATTYPICALGROWTHTEMPERATURES$UETOTHISHIGHVALUE THE GaAs(s ) + As(s ) PHASE WILL NEVER OCCUR IN TYPICAL CONDITIONS OF EPITAXIAL GROWTH TECHNIQUES )N CONTRAST FOR 'A3B THE CORRESPONDING PRESSURE OF THE LESS VOLATILE ANTIMONY OVER PURE ELEMENTAL ANTIMONY WILL BE BELOW MBAR ;= FIG AND A GaSb(s ) + Sb(s ) PHASE MAY WELL BE EXPERIMENTALLY OBSERVED 4HE RANGE OF SUITABLE GROWTHPARAMETERSFORSTOICHIOMETRICGROWTHOFANTIMONIDESISTHEREFORERATHERNARROW $ESPITE THE ABOVE DIFFICULTIES THE ANTIMONIDES ARE SUBJECT TO RENEWED INTEREST DUE TO THEIR MANY POTENTIAL APPLICATIONS IN NOVEL DEVICES 4HESE INCLUDE LOWBAND GAP PHOTOVOLTAICCELLSASWELLASINFRAREDEMITTERSANDDETECTORS SEESECTION 7HILEA WIDE VARIETY OF DIRECT AND INDIRECT MATERIALS CAN BE GROWN ON 'A3B AND )NÌS AS SUBSTRATES THE MOST PROMISING SUGGESTIONS WITH RESPECT TO COMMERCIAL APPLICATION ARE BASEDON)N0ÌSBOTH'A ÌS3B ANDÌL ÌS3B CANBEGROWNLATTICEMATCHEDTO)N0 THE LARGEDIFFERENCEBETWEENTHEIRREFRACTIVEINDICES ^FOR'A3BÌL3B REFIN ;=. CANBEUSEDFORTHEGROWTHOFHIGHREFLECTIVITY"RAGGMIRRORSWITHAREDUCEDNUMBEROF PERIODS COMPARED TO THE 'A)N 0ÌS SYSTEM -OREOVER ÌS3B COMPOUNDS HAVE BEEN SUGGESTED FOR VERY FAST TRANSISTORS (OWEVER THE CORRESPONDING ÌS3BRATIOS ARE IN THE CENTRE OF RELATED MISCIBILITY GAPS FOR THE MATERIALS #ONSEQUENTLY THEY CAN ONLY BE GROWNWITHEPITAXIALTECHNIQUESTHATWORKFARFROMTHERMODYNAMICALEQUILIBRIUM EG -"% -/60% %VENHERE PHASESEPARATIONEFFECTSMUSTBEEXPECTEDANDTHERANGEOF SUITABLEGROWTHPARAMETERSISNARROW EG ;= &ORFURTHERREADINGVERYGOODREVIEWARTICLESEXISTINTHELITERATURE SUCHAS ;=ONTHE PHYSICS OF 'A3B ;= ON DEVICE RELATED PROPERTIES OF 'A3B AND ;= ON THE -/60% GROWTHOFANTIMONIDES.

(17) &UNDAMENTALS. 4PMBSDFMMT. 5IFJEFBMEFWJDF Ì PHOTOVOLTAIC CELL IS BASICALLY A LARGE AREA PNJUNCTION )N ORDER TO UNDERSTAND THE INTERNAL OPERATION OF SUCH A DEVICE THE SPATIAL DISTRIBUTION OF THE IMPORTANT ELECTRICAL QUANTITIESMUSTBEDESCRIBEDBYASETOFFIVEEQUATIONS ;= 0OISSON¾SEQUATION ONEOF-AXWELL¾SEQUATIONS ISTHEDIFFERENTIATEDFORMOF'AUSS¾S LAWANDRELATESTHEDIVERGENCEOFTHEELECTRICFIELDTOTHESPACECHARGEDENSITY 4WOCURRENTDENSITYEQUATIONS ONEFORELECTRONSANDONEFORHOLES DESCRIBETHETOTAL CURRENT DENSITY RELATED TO ELECTRIC FIELDS IE DRIFT CURRENTS AND CONCENTRATION GRADIENTS IEDIFFUSIONCURRENTS 5NDERCERTAINCONDITIONS ;= P MOBILITIES ANDDIFFUSIONCONSTANTSARERELATEDTHROUGHTHE%INSTEINRELATIONSHIPS 4HE TWO CONTINUITY EQUATIONS ARE ÀBOOKKEEPINGÁ EQUATIONS THAT CONSIDER THE GENERATIONANDRECOMBINATIONOFELECTRONSANDHOLES 4HESE EXPRESSIONS FORM A COUPLED SET OF NONLINEAR DIFFERENTIAL EQUATIONS )T IS NOT POSSIBLE TO FIND A RELATED GENERAL ANALYTICAL SOLUTION (OWEVER REASONABLE APPROXIMATIONS CAN GIVE VALUABLE SOLUTIONS )N THIS CONTEXT A WIDELY USED MODEL TO DESCRIBEPNJUNCTIONSISTHE3CHOTTKYMODELWHICHWILLBEDESCRIBEDINTHEFOLLOWING 7HENAPTYPESEMICONDUCTORISINDIRECTCONTACTWITHACORRESPONDINGNTYPEMATERIAL THECONSTANT&ERMILEVELTHROUGHOUTTHEWHOLESTRUCTUREINTHERMODYNAMICEQUILIBRIUM MEANSTHATTHEREMUSTBEAPOTENTIALCHANGEINTHETRANSITIONREGIONNEARTHEJUNCTION 0OISSON¾S EQUATION SAYS THAT THIS POTENTIAL CHANGE IS RELATED TO A SPACE CHARGE 4HIS SPACE CHARGE ARISES FROM A DIFFUSION OF MAJORITY CARRIERS INTO THEIR MINORITY REGIONS 4HERE THEY RECOMBINE WITH THE OTHER CARRIER TYPE AND THE RESULTING CONCENTRATIONS OF MOBILE CARRIERS ARE NEGLIGIBLE THE REGION AROUND THE JUNCTION BECOMES A SOCALLED ÀDEPLETIONREGIONÁ4HEMAINCONTRIBUTIONTOTHESPACECHARGEDENSITYCOMESFROMTHE IONIZED DOPANTS 4HE ELECTRIC FIELD ASSOCIATED WITH THE SPACE CHARGE COUNTERACTS THE DIFFUSIONANDASTABLE SITUATION ARISES 4HEPOTENTIAL CHANGE IS USUALLY CALLED ÀDIFFUSION VOLTAGE 6 Á AND IS DETERMINED DIRECTLY THROUGH THE POSITION OF THE &ERMI LEVEL IN THE QUASINEUTRALREGIONSFARFROMTHEJUNCTION $. 4HE USUAL APPROXIMATION TO DESCRIBE A PNJUNCTION IS THE SOCALLED ÀDEPLETION APPROXIMATIONÁORÀ3CHOTTKYMODELÁ WHERE ACONSTANTSPACECHARGEISASSUMEDWITHINTHEDEPLETIONREGION THECORRESPONDINGSPACECHARGEISGIVENBYTHEDENSITYOFIONIZEDIMPURITIES AND THETRANSITIONBETWEENTHEDEPLETIONREGIONANDTHEQUASINEUTRALREGIONSISABRUPT 4HEELECTRICFIELDANDTHEPOTENTIALDISTRIBUTIONACROSSTHEJUNCTIONCANNOWBECALCULATED ;=)MPORTANTRESULTSARETHEMAGNITUDEOFTHEBUILTINPOTENTIAL6 ANDTHE WIDTH OF THEDEPLETIONREGION7 $. $. N N e ⋅ V D = kT ⋅ ln A 2 D  ni.    .  2εε 0 N A + N D  W D =  ⋅ ⋅ V D  NA ⋅ ND  e . 1. 2. )TISNOWIMPORTANTTODERIVETHE)6CURVEOFTHEBIASEDJUNCTIONÌQUALITATIVEOVERVIEW CAN PROVIDE A FIRST INSIGHT ÌT ALL TIMES THERE WILL BE A THERMAL GENERATION OF MINORITY.

(18) &UNDAMENTALS CARRIERS)FTHESEDIFFUSETOTHEJUNCTIONTHEYWILLBESWEPTINTOTHEOTHERREGIONBYTHE ELECTRIC FIELD 4HIS IS THE SOCALLED ÀGENERATION CURRENTÁ OR ÁDRIFT CURRENTÁ )N THERMAL EQUILIBRIUM THIS CURRENT WILL BE COMPENSATED BY A ÀRECOMBINATION CURRENTÁ OR ÀDIFFUSIONCURRENTÁ OFHIGHENERGETICMAJORITYCARRIERSTHATCANOVERCOMETHEPOTENTIAL BARRIER AND RECOMBINE AS MINORITY CARRIERS WITH MAJORITY CARRIERS IN THE OTHER REGION 7HILETHEGENERATIONCURRENTWILLNOTBEINFLUENCEDBYANAPPLIEDEXTERNALVOLTAGE ;= P THERECOMBINATIONCURRENTDEPENDSSTRONGLYONTHEMAGNITUDEOFTHEPOTENTIAL BARRIERANDWILLBEANEXPONENTIALFUNCTIONOFTHEAPPLIEDVOLTAGE ÌN APPLIED VOLTAGE IS DEFINED AS POSITIVE WHEN THE POSITIVE CONTACT OF THE SOURCE IS CONNECTED TO THE PREGION OF THE DIODE 3UCH A POSITIVE VOLTAGE ÀFORWARD BIASÁ WILL LOWER THE POTENTIAL BARRIER AND ALLOW A CORRESPONDING INCREASE OF THE RECOMBINATION CURRENT 4HE RELATED CARRIERS WILL RECOMBINE WITH MAJORITY CARRIERS IN THE OTHER REGION 4HEMINORITYCURRENTSHAVETHEIRMAXIMUMVALUESATTHEEDGESOFTHEDEPLETION REGION BUT CAN SPREAD WIDELY INTO THE QUASINEUTRAL REGIONS THIS IS AN IMPORTANT DESIGN ISSUE FOR EXAMPLE FOR ,%$S SEE P AND FIG IN ;= )F A NEGATIVE VOLTAGE IS APPLIED ÀREVERSE BIASÁ THE POTENTIAL BARRIER INCREASES AND EVENTUALLY THE TOTAL CURRENT WILL BE DETERMINED BY THE GENERATION CURRENT ÌS THE GENERATION CURRENT IS THE MAXIMUM POSSIBLECURRENTINREVERSEBIAS ITISFREQUENTLYASWELLDENOTEDASSATURATIONCURRENT 4HE ABOVE QUALITATIVE DESCRIPTION CAN BE EXPRESSED MATHEMATICALLY BY THE 3HOCKLEY EQUATION (ERE AN IDEAL SITUATION IS DESCRIBED WHICH NEGLECTS GENERATION AND RECOMBINATIONEFFECTSINTHEDEPLETIONREGION)NPHOTOVOLTAICS THE3HOCKLEYEQUATIONIS OFTENREFERREDTOASTHEONEDIODEMODEL   e ⋅V   I d (V ) = I 0 ⋅ exp  − 1   kT   4HE INDEX ÀDÁ STANDS FOR ÀDARKÁ BECAUSE NO PHOTOGENERATED CURRENT HAS BEEN CONSIDEREDUPTOTHISPOINT4HECONSTANT *ISTHESATURATION GENERATIONDRIFT. CURRENTMENTIONEDABOVEWHICHISOFTENASWELLTERMEDÀDARKÀCURRENT.ATURALLY BOTH CARRIER TYPES HAVE TO BE CONSIDERED FOR A COMPLETE ANALYSIS ;= P I 0 = I ngen + I pgen. (. ). 4HE MINORITY CARRIER CONCENTRATIONS AT THE EDGES OF THE DEPLETION REGION AND THE RESULTING MINORITY CARRIER DIFFUSION CURRENTS CAN BE CALCULATED ÌS DIFFUSION RECOMBINATION CURRENTS DETERMINE ALL CURRENT FLOW ASSOCIATED WITH AN APPLIED EXTERNAL VOLTAGE THE )6CURVE OF THE PNJUNCTION CAN AS WELL BE EXPRESSED IN TERMS OF THE DIFFUSION CURRENTS Ì COMPARISON OF THE RESULTING EXPRESSION WITH THE ABOVE EQUATION GIVESTHESATURATION GENERATIONDRIFT CURRENTINTERMSOFMATERIALPARAMETERS ;= P 1  De Dh + I 0 = I ngen + I pgen = A ⋅ e ⋅ ni2   N A Le N D L h. (. ).   . . .OTETHAT ni2 = N A ⋅ n p = N D ⋅ p n , (D τ ). 1. 2. = D L = L τ , D = kT ⋅ µ q ..

(19) &UNDAMENTALS. WHERE"ISCROSSSECTIONALAREAOFTHEDIODE %FAND%IARETHEDIFFUSIONCOEFFICIENTSOF THEMINORITYCARRIERS -FAND-IARETHEDIFFUSIONLENGTHSOFTHEMINORITYCARRIERS /"AND /% ARE THE ACCEPTOR AND DONOR CONCENTRATIONS AND OJ IS THE INTRINSIC CARRIER CONCENTRATION 4HE FACTOR ni2 MEANS THAT THE SATURATION CURRENT WILL INCREASE EXPONENTIALLYWITHDECREASINGBANDGAPENERGYOFTHE MATERIAL 7ITHOUTPROOF THE)6CURVEOFAPNDIODEWHEN JMMVNJOBUFECANBEWRITTENAS   e ⋅V I (V ) = I 0 ⋅ exp   kT.    − 1 − I L  . (ERE *ISAGAINTHEABOVESATURATIONCURRENT4HEPHOTOGENERATEDCURRENT *-ISGIVENBY I L = e ⋅ A ⋅ G ⋅ (Le + Lh + W D ) WHERE(ISTHERATEOFGENERATIONOFELECTRONHOLEPAIRSBYLIGHTAND 8%ISTHEWIDTHOF THEDEPLETION REGION ;= EQU 4HEEQUATIONSHOWSTHATBASICALLYALLTHECARRIERS GENERATEDBYLIGHTINTHEDEPLETIONREGIONANDWITHINAMINORITYCARRIERDIFFUSIONLENGTH ON EITHER SIDE CONTRIBUTE TO THE PHOTOCURRENT .OTE THAT THE PHOTOCURRENT IS IN THE DIRECTIONOFAREVERSECURRENTÌSTHEPHOTOCURRENTISLINEARLYSUPERIMPOSEDONTHEUSUAL FORWARDCURRENTINTHE)6CURVE THEILLUMINATEDDIODEHAS A FORWARD CURRENT WHEN AN EXTERNALVOLTAGE BIGGER THAN THEOPENCIRCUIT VOLTAGE 7PD DEFINITION OF 7PD SEE SECTION BELOW IS APPLIED &OR VOLTAGES CFMPX 7PD THE CELL ACTS AS A CURRENT SOURCE IN THE REVERSEDIRECTION 7HEN THE )6CURVE OF A DEVICE IS KNOWN THE MAXIMUM POWER OF THE CELL AND THE P EFFICIENCY η = max CANBECALCULATEDÌTHEORETICALMAXIMUMFOR η CANBEDERIVEDWITH Plight THEFOLLOWINGASSUMPTIONS ÌLLINCIDENTPHOTONSWITHENERGIESABOVETHEBANDGAPAREABSORBEDA NDCONTRIBUTE TOTHEPHOTOCURRENT*- ÌNINFINITENUMBEROFPHOTOVOLTAICCONVERTERSWITHALLPOSSIBLEBANDGAPSISASSUMED TO BE IN THERMODYNAMIC EQUILIBRIUM WITH THE SUN 4HE NECESSARY LUMINESCENCE OF A CORRESPONDINGCONVERTERCANBETRANSLATEDINTOASATURATIONCURRENT *P $ETAILSCANBE FOUND IN ; = &OR A SINGLE PNJUNCTION A THERMODYNAMIC ÀRADIATIVEÁ SATURATION CURRENTMINIMUMISOBTAINED I 0 = e0.  Eg  2π 2 2 2 2 2 3 3 −  kT E k T k T E k T ⋅ ⋅ − − + ⋅ 2 2 2 exp g g  kT  h3c 2  . (. ). 7ITHTHESEASSUMPTIONSALLTHEPARAMETERSAREKNOWNFORTHECALCULATIONOFANIDEALISED )6CURVE&ORAGIVENINCIDENTSPECTRUMTHEEFFICIENCYCANTHEREFOREBECALCULATED AS A FUNCTION OF THE BAND GAP OF THE UNDERLYING MATERIAL &OR EXAMPLE THE IDEALISED EFFICIENCY OF A 'AÌS SOLAR CELL IN DIRECT SUNLIGHT Ì- D AT A CONCENTRATION RATIO OF ANDACELLTEMPERATUREOF+IS ;=4HISVALUEEMPHASISESTHATEVENIN THEIDEALISEDCASETHEPHOTOVOLTAICCONVERSIONEFFICIENCYISFARFROM4HEMAINLOSS MECHANISMS DEPEND DIRECTLY ON THE BAND GAP 0HOTONS WITH ENERGIES FAR ABOVE THE.

(20) &UNDAMENTALS BAND GAP CREATE HOT CARRIERS WHOSE EXCESS ENERGY IS LOST IN THERMALISATION TO NEAR THE BAND EDGE 0HOTONS WITH ENERGIES BELOW THE BAND GAP ARE NOT ABSORBED AND LOST FOR THECONVERSIONPROCESS4HESETWOEFFECTSALONELEADTOAREDUCTIONOFTHEEFFICIENCYIN THE ORDER OF ;= P 4HE THIRD UNAVOIDABLE LOSS MECHANISM IS THE THERMALISATION OF CARRIERS OVER THE POTENTIAL BARRIER Ì DIODE CAN NEVER GIVE A VOLTAGE OUTPUTCORRESPONDINGTOTHEBANDGAPENERGY AND e ⋅ V < E g WILLALWAYSAPPLY ;= P 5IFSFBMEFWJDF )NSECTIONANIDEALPNJUNCTIONWASDESCRIBED&ORAMOREREALISTICMODELADDITIONAL POINTSHAVETOBECONSIDERED4HETHERMODYNAMICSATURATIONCURRENTMINIMUMMUSTBE CONSIDERED UNREALISTIC AS THE VALUE BASED ON THE 3HOCKLEY MODEL IS GENERALLY MUCH HIGHER"ESIDES SURFACERECOMBINATIONCANINFLUENCETHESATURATIONCURRENT)FASURFACE IS CLOSER TO THE DEPLETION REGION THAN APPROXIMATELY ONE MINORITY CARRIER DIFFUSION LENGTH THESURFACERECOMBINATIONVELOCITY3MUSTBECOMPAREDTOTHERATIO D INORDER L TODETERMINEWHETHERTHEPROXIMITYOFTHESURFACEWILLLEADTOANINCREASEORADECREASE OFTHESATURATIONCURRENT* [38] FIG )F3 D THEMINORITYCARRIERCURRENTFLOWINTOTHESURFACEWILLBE UIFDOMINATING L EFFECT TRANSPORTISCONTROLLEDBYTHEMINORITYMOBILITYOFTHELAYERAND *WILLBEMUCH HIGHERTHANINTHEINFINITELAYERCASE )F 3 ≈D WITHIN APPROXIMATELY TWO ORDERS OF MAGNITUDE THE MINORITY CARRIER L CURRENTFLOWINTOTHESURFACEWILLBE BDOMINATINGEFFECT*WILLBEHIGHERTHANINTHE INIFINTELAYERCASEFOR3 D ANDLOWERFOR3 D L L )F3 D THEMINORITYCARRIERCURRENTFLOWINTOTHESURFACEWILLBE OPDOMINATING L EFFECT VERYFEWCARRIERSWILLBELOSTATSUCHANEXCELLENTSURFACE TRANSPORTISNOLONGER CONTROLLEDBY3BUTBYTHELIFETIMEOFTHELAYER AND *WILLBEMUCHLOWERTHANINTHE INFINITELAYERCASE (OWEVER THEEFFECTOFSURFACERECOMBINATIONISNOTONLYLIMITEDTOTHECHANGESINTHE ÀDARKÁSATURATIONCURRENT.ATURALLY AHIGHRECOMBINATIONVELOCITYSURFACEWILLALSOLEAD TO A LOSS OF PHOTOEXCITED MINORITY CARRIERS WHICH HAVE BEEN CREATED WITHIN A DIFFUSION LENGTHFROMTHESURFACE "OTH GENERATIONRECOMBINATION IN THE DEPLETION REGION AND /HMIC RESISTANCES MUST AS WELLBECONSIDERED4HESEEFFECTSAREINCLUDEDINTHESOCALLEDTWODIODEMODELWHICHIS BEYONDTHESCOPEOFTHISINTRODUCTION3IMILARLY LOSSESASSOCIATEDWITHREFLECTIONOFTHE MATERIALANDTHEMETALGRIDARENOTDISCUSSED %XAMPLESFORMEASURED)6CURVESOFAREALDEVICEUNDERDIFFERENTLEVELSOFILLUMINATION ARE SHOWN IN FIGURE 4HESE )6CURVES ARE USUALLY DESCRIBED BY A SET OF TYPICAL PARAMETERSWHICHWILLBEPRESENTEDINTHEFOLLOWING 4HESHORTCIRCUITCURRENT*TDISDEFINEDFOR77ÌTTHISPOINTTHE)6CURVEGIVES I sc = I (0 ) = − I L.

(21) &UNDAMENTALS. Voltage [V] 0.0 0.0. 0.2. 0.4. 0.6. 0.8. 1.0. 1.2. Voc. Current [A]. -0.2. -0.4 MPP -0.6 Isc -0.8. 'JHVSF *7DVSWFT PG BO -1&HSPXO DPODFOUSBUPS (B"TTPMBS DFMM EJBNFUFS NN. VOEFS EJGGFSFOU MFWFMT PG JMMVNJOBUJPO 5ZQJDBM QBSBNFUFST GPS UIF DIBSBDUFSJTBUJPOPG*7DVSWFTBSFQSFTFOUFE0QFODJSDVJUWPMUBHF6 TIPSU DJSDVJUDVSSFOU) BOENBYJNVNQPXFSQPJOU-00 OC. SC. ÌS *- IS PROPORTIONAL TO THE GENERATION RATE ' THE SHORTCIRCUIT CURRENT OF THE DEVICE IS PROPORTIONALTOTHEINTENSITYOFTHELIGHT4HEOPENCIRCUITVOLTAGE 7PDISDEFINEDFOR * ÌTTHISPOINTTHE)6CURVEGIVES Voc =.  kT  I sc ln + 1 e  I0 . 4HEEQUATION SHOWS THAT7PDINCREASESLOGARITHMICALLY WITH *- I E WITH THE INTENSITY OF THELIGHT/NTHEOTHERHAND 7PDDECREASESWITHINCREASINGSATURATION CURRENT* ÌS*IS PROPORTIONALTO ni2 SEEPAGE 6 ISDIRECTLYRELATEDTOTHEBANDGAPOFTHEMATERIAL OC. 4HEDEVICE GIVES THE MAXIMUM POWER AT THE POINT MARKED AS -00 ÀMAXIMUM POWER POINTÁ INFIGURE"YDEFININGTHESOCALLEDFILLFACTOR ''ASTHERATIO FF :=. I MPP ⋅ V MPP ÌREADEFINEDBYDOTTEDRECTANGLEINFIG = I sc ⋅ Voc ÌREADEFINEDBYDASHEDRECTANGLEINFIG. THEEFFICIENCYOFACELLCANBEDESCRIBEDINTERMSOF 7PDAND*TDAS. η=. Pmax I sc ⋅ Voc ⋅ FF = Plight Plight. 4HISEXPRESSIONSHOWSTHEDEPENDENCEOFTHEEFFICIENCYONTHEBANDGAPVERYCLEARLY/N THEONEHANDAHIGHERBANDGAPWILLREDUCEBOTH OJAND*SOTHAT7PDWILLBESUBJECTTOA.

(22) &UNDAMENTALS CORRESPONDINGINCREASEÌTTHESAMETIME *TDWILLDECREASEDUETOTHEREDUCEDABSORPTION INWIDEBANDGAPMATERIALS4HEOPTIMUMBANDGAPFORAGIVENINCIDENTSPECTRUMCAN BEOBTAINEDFROMSIMULATION EG ;= 3IMILARLYTHEEFFECTOFCONCENTRATIONOFTHEINCIDENTLIGHTCANBESEEN7HILE *TDAND1MJHIU WILLINCREASEPROPORTIONALTOEACHOTHERWITHOUTCHANGINGTHEOVERALLEFFICIENCY 7PDWILL ALSOGROWWITHMO *TD ANDGIVEANINCREASEOFTHEEFFICIENCYUNDERCONCENTRATEDLIGHT 4HEEVALUATIONOFTHEDEVICESDEVELOPEDINCHAPTERWILLBASICALLYFOCUSON 7PDAND *TD )NSTEADOFTHEMEREVALUEFOR *TD THEEXTERNALSPECTRALQUANTUMEFFICIENCYWILLBEGIVEN THERE4HISQUANTITYISDEFINEDBY I L (λ ) EQE (λ ) := (1 − R(λ )) ⋅ e ⋅ Γ(λ ) WHERE3ISTHEREFLECTION Γ ISTHEPHOTONFLUX AND*-ISTHEPHOTOGENERATEDCURRENT4HE %1% IS DETERMINED BY MEASURING THE SHORTCIRCUIT CURRENT USING MONOCHROMATIC LIGHT WITHKNOWNPHOTONFLUX4HE%1%ISMAINLYUSEFULINTWORESPECTS ÌS DIFFERENT WAVELENGTHS ARE ABSORBED IN DIFFERENT PARTS OF THE DEVICE THE SPECTRAL INFORMATIONOFTHE%1%GIVESGUIDELINESFORFURTHEROPTIMISATIONOFADEVICESTRUCTURE 7HENTHE%1%ISKNOWN INABSOLUTETERMS THETOTALLIGHTGENERATEDCURRENTCAN BE CALCULATEDFORANYDESIREDINCIDENTSPECTRUM dΓ(λ ) dλ EG;= EQUIN;= ∞. I L = I sc = e ⋅ ∫ EQE (λ ) ⋅ 0. dΓ(λ ) dλ dλ. 5IFNVMUJKVODUJPOEFWJDF ÌTTHEENDOFSECTIONTHETWOMAINLOSSMECHANISMSFORTHEPHOTOVOLTAICENERGY CONVERSIONWEREIDENTIFIED&IRSTLY PHOTONSWITHENERGIESFARABOVETHEBANDGAPCREATE HOT CARRIERS WHOSE EXCESS ENERGY IS LOST IN THERMALISATION 3ECONDLY PHOTONS WITH ENERGIES BELOW THE BAND GAP ARE NOT ABSORBED AND LOST FOR THE CONVERSION PROCESS 3ELECTING THE OPTIMUM BAND GAP FOR THE MATERIAL OF A SINGLEJUNCTION SOLAR CELL CAN BE REGARDEDASATRADEOFFBETWEENTHESETWOLOSSMECHANISMS (IGHEREFFICIENCIESCANBEACHIEVEDWITHMULTIBANDGAPSYSTEMS(IGHENERGYPHOTONS ARE ABSORBED IN A WIDEBAND GAP SOLAR CELL WHILE LOWENERGY PHOTONS ARE TRANSMITTED THROUGHTHEMATERIALÌSECONDCELLBASEDONALOWERBANDGAPMATERIALISTHENPLACED BEHIND THE FIRST ONE 4HIS SECOND CELL CAN NOW CONVERT THE LOWERENERGY PHOTONS INTO ELECTRICITY3UCHAPHOTOVOLTAICSYSTEMISCALLEDAÀTANDEMCELLÁ4HETWOCELLSCANEITHER BE REALISED SEPARATE FROM EACH OTHER AND CONNECTED INDEPENDENTLY FOUR TERMINAL APPROACH OR GROWN MONOLITHICALLY ON TOP OF EACH OTHER TWO TERMINAL APPROACH 4HE LATTERAPPROACHISDIFFICULTWITHRESPECTTOTHREEPOINTS 4HE/HMICCONNECTIONBETWEENTHETWOCELLSMUSTBEREALISED WITH A TUNNEL DIODE 4HE DEPOSITION OF SUCH A STRUCTURE IS DIFFICULT BECAUSE VERY HIGH DOPING LEVELS AND SHARPINTERFACESMUSTBEACHIEVED 4HE TWO TERMINAL APPROACH MEANS THAT THE SAME CURRENT FLOWS IN BOTH CELLS &OR OPTIMUM EFFICIENCIES THIS CURRENT MUST BE EQUAL TO THE CURRENT IN THE MAXIMUM POWER POINT OF CPUI CELLS /THERWISE THE CELL WITH THE LOWER CURRENT WILL LIMIT THE.

(23) &UNDAMENTALS PERFORMANCEOFTHEOTHERCELL5SUALLYTHEEMPLOYEDBANDGAPSINCOMBINATIONWITHA GIVENSPECTRUMDONOTAUTOMATICALLYFULFILTHISCONDITIONÌPARTIALTRANSPARENCYOFTHE TOPCELLACHIEVEDBYTHINNINGTHEABSORBERLAYERSCANOFTENBEEMPLOYEDASASOLUTION TOTHISPROBLEM $IFFERENT BAND GAP MATERIALS HAVE TO BE REALISED THAT ARE LATTICEMATCHED TO EACH OTHER4HECHOICEOFSUITABLEMATERIALCOMBINATIONSISVERYLIMITED 4HE EFFICIENCY OF SUCH A SYSTEM CAN BE ENHANCED EVEN MORE BY FURTHER INCREASING THE NUMBEROFCELLSBASEDONDIFFERENTBANDGAPS4HEEFFICIENCYGAINPERADDITIONALCELLIS REDUCED WITH EVERY ADDED DEVICE FIG IN ;= Ì REASONABLE TRADEOFF BETWEEN EFFICIENCYANDTECHNOLOGICALEFFORTISGIVENFORASYSTEMWITHFOURCELLSÌSASUCCESSFUL MONOLITHICFOURJUNCTIONDEVICEHASNOTBEENSHOWNUPTONOW AMECHANICALSTACKOF TWO MONOLITHIC TANDEM CELLS IS A PROMISING ALTERNATIVE APPROACH (ERE THE TECHNOLOGICALLY MATURE MONOLITHIC 'A)N 0'AÌS TANDEM CELL COULD SERVE AS A FIRST TANDEM CELL FOR THE IJHIENERGY PHOTONS 4HIS DEVICE WILL BE MECHANICALLY STACKED ON TOP OF A SECOND MONOLITHIC TANDEM CELL WHICH ABSORBS MPXENERGY PHOTONS 4HE OPTIMUM BAND GAP COMBINATION OF THE SECOND MONOLITHIC TANDEM CELL HAS TO BE CALCULATED BY SIMULATION )N ORDER TO DESIGN A CORRESPONDING SIMULATION SOFTWARE THE FOLLOWINGASSUMPTIONSWEREMADE . ÌSOLARSPECTRUMBELOWA'AÌSWAFERISUSEDASINCIDENTSPECTRUM 4HETOPCELLISCOMPLETELYTRANSPARENTFORPHOTONSWITHENERGIESBELOWTHEBANDGAP 4HE%1%ISFORALLPHOTONSABOVETHEBANDGAPINBOTHCELLS #URRENTMATCHINGISACHIEVEDBYAPARTIALLYTRANSPARENTTOPCELLWHEREAPPLICABLE 4HE SATURATION CURRENT IS ASSUMED TO BE THE THERMODYNAMIC ÀRADIATIVEÁ MINIMUM GIVENINSECTION. 4HE3HOCKLEYEQUATIONCANNOWBEAPPLIEDINORDERTODEVELOPAMODELFORANÀIDEALÁ TANDEMCELL   e ⋅V    − 1 − I L I (V ) = I 0, ⋅ exp   kT   TOP CELL. TOP CELL. I. BOTTOM CELL. TOP CELL. (V ) = I 0,. TOP CELL.   e ⋅V ⋅ exp kT  . BOTTOM CELL. BOTTOM CELL. V. TANDEM CELL. I (V ). TANDEM CELL. =V. TOP CELL. = I (V ).    − 1 − I  . , BOTTOM CELL. +V. BOTTOM CELL. TOP CELL. = I (V ). BOTTOM CELL. ÌSF7L5 THEÂÀCANBENEGLECTEDANDTHERESULTING)6CURVEISGIVENBY V. TANDEM CELL. =. kT e.  I ln  . TOP CELL. (V ) + I I. TOP CELL. , TOP CELL.  I +    . BOTTOM CELL. I. (V ) + I. , BOTTOM CELL. BOTTOM CELL.    .

(24) Band gap of top cell [eV]. &UNDAMENTALS. efficiency [%]. 1.0. 54 53 52 51 50 49 48 47 46 45. 0.9. 0.8. 0.3. 0.4. 0.5. 0.6. 0.7. - 55 - 54 - 53 - 52 - 51 - 50 - 49 - 48 - 47 - 46. 0.8. Band gap of bottom cell [eV] 'JHVSF *EFBM FGGJDJFODZ PG B NPOPMJUIJD UBOEFN DFMM BT GVODUJPO PG UIF VOEFSMZJOH CBOE HBQT DFMM UFNQFSBUVSF , 5IF JODJEFOU TQFDUSVN JT UJNFT ".E 8N VOEFS B (B"TXBGFS UFNQFSBUVSF , 5IF SFTVMUJOHJODJEFOUQPXFSJTUJNFT8N5XPBSFBTXJUIPQUJNJTFE FGGJDJFODJFT DBO CF DMFBSMZ JEFOUJGJFE <4PGUXBSF BOE DBMDVMBUJPOT ( -ÀUBZ 'SBVOIPGFS*4&> 4HE )6CURVE CAN NOW BE CALCULATED FOR VARIOUS BAND GAP COMBINATIONS 4HE RESULTING POWER AT THE -00 CAN BE TRANSLATED INTO AN EFFICIENCY 4HIS INFORMATION CAN BE SUMMARISEDINAMATRIXOFEFFICIENCIESASAFUNCTIONOFTHEBANDGAPS FIGURE 4WOAREASWITHOPTIMISEDEFFICIENCIESCANBEIDENTIFIEDINTHEFIGURE4HECORRESPONDING BANDGAPCOMBINATIONSAREAPPROXIMATELY F7F7ANDF7F74HEFIRST COMBINATION GIVES SLIGHTLY HIGHER VALUES FOR THE EFFICIENCY BUT THE ONLY SUITABLE LATTICE MATCHED MATERIAL COMBINATION IS ÌL'A ÌS3B 'A)N ÌS3B I E TWO QUATERNARIES WITH 'A3B OR )NÌS AS SUBSTRATE MATERIAL SECTION /N THE OTHER HAND THERE ARE THREE POSSIBLECANDIDATESFORALATTICEMATCHEDBANDGAPCOMBINATIONCLOSETO F7F7 A 'A)N .ÌS 'E ÌBANDGAPOFF7CANBEREALISEDIN 'A)N .ÌS WHICHISGROWNLATTICEMATCHEDTO GERMANIUM SUBSTRATES ÌS THE BAND GAP OF GERMANIUM AT + IS E6 A CORRESPONDINGTANDEMDEVICESEEMSPOSSIBLE(OWEVER 'A)N .ÌS HASNOTYETBEEN REPORTEDINSUFFICIENTQUALITYUPTONOW B 'A)N 0ÌS 'A)N ÌSON)N0SUBSTRATES 4HIS MATERIAL COMBINATION IS TECHNOLOGICALLY MATURE "OTH MATERIALS ARE ÌLFREE AND CAN BE GROWN IN HIGH QUALITIES ON )N0 AS SUBSTRATE MATERIAL (OWEVER INDIUM IS A RATHERRAREELEMENTAND)N0ISQUITEEXPENSIVE4HESEFACTORSMIGHTEVENTUALLYPROHIBIT A LARGESCALE DISTRIBUTION OF CORRESPONDING PHOTOVOLTAIC DEVICES 7HILE A MECHANICAL STACKOFA'AÌS 'A)N ÌSTANDEMCELL ;=ANDA#"%GROWN 'A)N ÌSTUNNELJUNCTION HAVEALREADYBEENSHOWN ARELATEDTANDEMDEVICEHASNOTBEENREPORTEDYET.

(25) &UNDAMENTALS C ÌL'A ÌS3B 'A3B 7ITH 'A3B % + E6 AS BOTTOM CELL FIGURE SUGGESTS A THEORETICAL EFFICIENCY OF APPROXIMATELY FOR THE TANDEM CELL WHEN A CONCENTRATED SOLAR SPECTRUMBELOWA'AÌSWAFERISUSEDASREFERENCEPOWER IETIMES7M Ì'A3BSINGLEJUNCTIONCELLALONEGIVESINEQUIVALENTCALCULATIONS&ORAREFERENCE POWER OF 7M I E THE COMPLETE SOLAR SPECTRUM WITHOUT THE 'AÌS FILTER. THESEVALUESCORRESPONDTOFORTHETANDEMCELLANDFORTHESINGLEJUNCTION CELL (ENCE THE IDEAL JUNCTION DEVICE WILL GIVE ONLY MORE THAN A JUNCTION DEVICE WITH REGARD TO THE ABSOLUTE EFFICIENCY OF THE WHOLE SYSTEM #ONSEQUENTLY THEE6CELL IETHE JUNCTIONINAJUNCTIONSYSTEM MUSTBEVERY GOODINORDERTOLEADTOAREALGAINOFEFFICIENCY ÌSDESCRIBEDINCHAPTER THE-/60%OF'A3BBASEDMATERIALSISARATHERRECENTAND DIFFICULTFIELD ANDTHEMATERIALQUALITYISOFTEN UNSATISFACTORY(ENCE THEDEVELOPMENT OF A SUCCESSFUL ÌL'A ÌS3B 'A3B TANDEM CELL MUST BE CONSIDERED AN AMBITIOUS CHALLENGE (OWEVER IT WAS DECIDED TO CHOOSE THE ANTIMONIDEBASED APPROACH FOR THISWORKDUETOTHECONSIDERABLEPOTENTIALOFTHESEMATERIALSWITHREGARDTOAWIDE RANGEOFINTERESTINGPHYSICSANDNOVELDEVICES EGE6THERMOPHOTOVOLTAICCELLS GAP. . . RD. ÌSSOLAR CELLS BASEDON )))6 SEMICONDUCTORS ARE SIGNIFICANTLY MORE EXPENSIVE THAN THEIR SILICON BASED COUNTERPARTS AN ECONOMICALLY COMPETITIVE TERRESTRIAL APPLICATION CAN ONLY BEINACONCENTRATINGSYSTEMWHERETHEAREAOFCELLREQUIREDFORAGIVENPOWEROUTPUTIS NOTABLYREDUCEDÌNEXAMPLEFORSUCHASYSTEMBASEDONTHEABOVEJUNCTIONAPPROACH ISSHOWNINFIGURE. &RESNEL LENS. 'A)N 0 SOLAR CELL. #U. 'AÌS SOLAR CELL. ÌL'A ÌS3B SOLAR CELL. #U. 'A3B SOLAR CELL. 'JHVSF 'PVSKVODUJPOQIPUPWPMUBJDTZTUFNJOBDPODFOUSBUJOHBQQMJDBUJPO 5IF NPOPMJUIJD UBOEFN DFMM GPS UIF IJHIFOFSHZ QIPUPOT DPOTJTUT PG UIF NBUFSJBM DPNCJOBUJPO (B*O 1(B"T XIJMF UIF TFDPOE UBOEFN DFMM GPS UIF MPXFOFSHZQIPUPOTJTCBTFEPOUIFDPNCJOBUJPO "M(B "T4C (B4C.

(26) %XPERIMENT. . &YQFSJNFOU. (SPXUI.FUBMPSHBOJD7BQPVS1IBTF&QJUBYZ .071&. gas stream diffusion & activation desorption adsorption. surface diffusion. substrate. 'JHVSF 1SJODJQMF PG NFUBMPSHBOJD WBQPVS QIBTF FQJUBYZ 5IF TPVSDF TQFDJFT BSF DBSSJFEUPUIFTVCTUSBUFJOBO) HBTTUSFBN"GUFSUIFSNBMBDUJWBUJPOUIFZ HFUBETPSCFEPOUIFTVSGBDFXIFSFUIFZEJGGVTFUPUIFJSJODPSQPSBUJPOTJUFT 'JOBMMZ PSHBOJDSFTUHSPVQTEFTPSCBOEBSFDBSSJFEXBZXJUIUIFHBTTUSFBN -ETALORGANICVAPOURPHASEEPITAXY -/60% WASALREADYINTRODUCEDINSECTIONAS VAPOURPHASE EPITAXIAL PROCESS BASED ON GASEOUS METALORGANIC COMPOUNDS #OMMON SYNONYMSAREORGANOMETALLICVAPOURPHASEEPITAXY /-60% ANDMETALORGANICCHEMICAL VAPOUR DEPOSITION -/#6$ 4HEY ALL DESCRIBE A VERY POWERFUL TECHNIQUE FOR THE EPITAXIALGROWTHOF)))6SEMICONDUCTORS )N -/60% A COMBINATION OF LIQUID OR SOLID METALORGANICS AND HYDRIDE GASES IS USUALLY EMPLOYED AS SOURCE MATERIALS THE SOCALLED ÀPRECURSORSÁ 4HE FOLLOWING DESCRIPTION OF THETECHNIQUEWILLFOCUSONTHEGROWTHOF'AÌSASMODELSYSTEM(YDROGENISBUBBLED THROUGHMETALORGANICCOMPOUNDS EGTRIMETHYLGALLIUMORÀ4-'AÁASGROUP)))SOURCE. WHICH ARE STORED IN STEEL CYLINDERS IN TEMPERATURESTABILISED BATHS 4HE METALORGANIC EVAPORATES AND A SATURATED MIXTURE WITH HYDROGEN IS FORMED 4HIS MIXTURE IS THEN FURTHER DILUTED WITH ( AND TRANSPORTED TO THE ACTUAL GROWTH CHAMBER 3IMILARLY THE GASEOUSHYDRIDEARSINE AS GROUP 6 SOURCE IS TRANSPORTED TO THE REACTOR IN HYDROGEN AS CARRIERGAS ÌNINCREASINGTENDENCYTOREPLACETHISSTABLEANDHIGHLYDANGEROUSHYDRIDE GASBYMETALORGANICLIQUIDSSUCHASTERTIARYBUTYLARSINE 4"ÌS CANCURRENTLYBEOBSERVED. ÌSARESULT THEREISACONTINUOUS FLOWOF HYDROGEN CARRIER GAS IN THE GROWTH CHAMBER WITHEMBEDDEDGROUP)))AND6SOURCEMOLECULESOVERAHEATEDSUSCEPTORONTOWHICHA SUBSTRATEISPLACED#LOSETOTHESUBSTRATESURFACEABOUNDARYLAYERISFORMEDWHERETHE GAS VELOCITY IS ALMOST ZERO 4HERMALLY ACTIVATED PRECURSOR SPECIES DIFFUSE THROUGH THIS.