IN THE CORRESPONDING #OULOMB POTENTIAL - MOVPE of GaSb-based materials and solar cell structure

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"OHRRADIIOFTHESHALLOW DONORS AND ACCEPTORS IN 'A3B AS A_"^E^ z AND A_"^H ^ z RESPECTIVELY

4HESE "OHR RADII MUST NOW BE COMPARED TO THE SCREENING RADIUS IN SAMPLE ÌCCEPTINGTHEFREEHOLECONCENTRATIONASMEASUREDBY(ALLATROOMTEMPERATUREASTHE CONCENTRATION OF UNCOMPENSATED MAJORITY CARRIERS p#550 =N_A −N_D =5 E.3 17cm^-3 ONE OBTAINSASCREENINGRADIUSOFRz

ÌS MENTIONED ABOVE BOUND STATES IN A SCREENED POTENTIAL EXIST ONLY IF THE SCREENING RADIUSSISMUCHGREATERTHANTHERELATED"OHRRADIUSB_#4HECOMPARISONBETWEENTHE CALCULATEDSANDTHEABOVE"OHRRADIISHOWSTHATBOUNDDONORSTATESCANBEEXCLUDED FORSAMPLEANDBOUNDACCEPTORSTATESAREHIGHLYUNLIKELYÌSARESULTTHEBROAD0, OFSAMPLEISTHEREFORECONSIDEREDASDUETOBANDBANDLUMINESCENCEINAHIGHLY DOPEDANDPARTIALLYCOMPENSATEDSEMICONDUCTOR;=

)NPTYPEDEGENERATEDIRECTGAP)))6SEMICONDUCTORSTHEOPTICALBANDGAPSHRINKSWITH THECONCENTRATIONINCREASESINCETHE"URSTEIN-OSSEFFECTFORHOLESISLESSEFFECTIVETHAN THE BANDGAP NARROWING &OR NTYPE 'A3B THE OPPOSITE IS TRUE AND A CORRESPONDING BLUESHIFTOFTHE0,WITHINCREASING4EDOPINGHASBEENOBSERVED;=)NREF;*AIN=

OF;=THEBANDGAPNARROWINGOFPTYPE'A3BATROOMTEMPERATUREASAFUNCTIONOF THEDOPINGLEVELISAPPROXIMATEDAS∆%_GAP =Ì.+". +#.WITHTHEFOLLOWING PARAMETERS FOR P'A3B ÌX E6 CM "X E6 CM #X

E6 CM 4HIS FUNCTION IS PLOTTED IN THE FOLLOWING FIGURE TAKEN FROM ;=

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ÌSANOVERALLRESULTTHEBROADLINEINTHE0,SPECTRUMOFSAMPLECANBECONSIDERED AS DUE TO BANDBAND LUMINESCENCE IN HIGHLY DOPED AND PARTIALLY COMPENSATED 'A3B WITHABANDGAPREDUCEDBYABOUTME6

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4HISSITUATIONRESULTSINCHARGESWHICHARERANDOMLYLOCATEDINTHESEMICONDUCTOR4HE PRESENCE OF THESE CHARGES MEANS AN ADDITIONAL POTENTIAL ENERGY FOR MOBILE CHARGE CARRIERS IN THE CRYSTAL Ì MOBILE CHARGE CARRIER AT THE BAND EDGE WILL HAVE A POTENTIAL ENERGYDEPENDINGONTHEDISTRIBUTIONOFLOCATEDCHARGECLUSTERSNEARBY)NOTHERWORDS THEBANDEDGEENERGYCOINCIDESWITHTHECARRIER¾SPOTENTIALENERGYANDCONSEQUENTLYTHE BANDEDGEITSELFWILLBESUBJECTTOAMODULATIONINREALSPACE ;=CHAPTER4HIS MODULATIONISCHARACTERISEDBYTHERELATEDMEANSQUAREPOTENTIALENERGY EQUIN

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ÌSACONSEQUENCEOFTHEFLUCTUATIONSDESCRIBEDBYAPERCOLATIONLEVEL&_DFORELECTRONS IS FORMED BELOW THE NONPERTURBED CONDUCTION BAND EDGE EC⁰ ÌBOVE THIS LEVEL ELECTRONSHAVEANONVANISHINGMOBILITYANDMAYBEREGARDEDASFREE;=4HEENERGETIC DISTANCEγeBETWEENTHENONPERTURBEDBAND GAPENERGY AND THEPERCOLATION LEVEL IS

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ÌNOTHER VERY IMPORTANT CONCLUSION CAN BE DRAWN FROM THE DATA IN THE ABOVE TABLE

#ONSIDERINGTHATBOUNDSTATESINASCREENEDPOTENTIALEXISTONLYIFTHESCREENINGRADIUSS IS MUCH GREATER THAN THE RELATED "OHR RADIUSB_# A_"^E^ z A_"^H ^ z SEE SECTION THEABOVESCREENINGRADIIINSAMPLESANDIMPLYTHAT

NOBOUNDSTATESASSOCIATEDWITHISOLATEDDONORSCANBEEXPECTEDINTHEMATERIALWHILE WELLDEFINEDBOUNDSTATESASSOCIATEDWITHISOLATEDACCEPTORSSHOULDBEPRESENT

ÌS A CONSEQUENCE TWO MAIN KINDS OF OPTICAL TRANSITIONS INVOLVING SHALLOW IMPURITIES HAVE TO BE DISCUSSED NAMELY TAILIMPURITY 4) TRANSITIONS I E A RECOMBINATION OF AN ELECTRONCAPTUREDINALOCALIZEDSTATEINTHECONDUCTIONBANDTAILWITHAHOLEATONEOF THENEIGHBOURINGACCEPTORSANDBANDIMPURITY ")TRANSITIONS IEARECOMBINATIONOFA FREE ELECTRON WITH A HOLE CAPTURED BY AN ACCEPTOR 4HE SITUATION IS ILLUSTRATED IN THE FOLLOWINGFIGURE

%LECTRONS ARE NOT LOCALISED IN A TYPICAL POTENTIAL WELL OF DEPTH AND SIZES_P IN THE CONDUCTIONBANDBECAUSETHEIREFFECTIVEMASSESARESMALLANDTHETYPICALWELLISRATHER SHALLOW;=#ONSEQUENTLYMOSTELECTRONSWILLBEFREEONTHEIRPERCOLATIONLEVEL/NLY POINTCLUSTERSOFLARGENUMBERSOFDONORSWILLLEADTOLOCALISEDSTATESINTHECONDUCTION BAND TAIL /N THE OTHER HAND HOLES WILL USUALLY BE LOCALISED IN THE VALENCE BAND TAILS BECAUSEOFTHEIRHIGHERMASSES3IMILARLYITISMUCHEASIERFORELECTRONSTHANFORHOLESTO TUNNEL INTO THE BARRIERS 4HIS WILL BE CONSIDERED IN THE DISCUSSION OF THE POSSIBLE TRANSITIONS BELOW Ì THEORETICAL TREATMENT AND NUMERICAL SIMULATION OF THE 0, OF THIS KINDOFMATERIALISGIVENIN;=4WOIMPORTANTPOINTSSHALLBEEMPHASIZED

ÌSA4)TRANSITIONINVOLVESALOCALISEDELECTRONITCANONLYOCCURINTHEVICINITYOFADEEP POTENTIALWELLÌTLOWTEMPERATURESHOLESCANNOTAPPROACHSUCHADEEPDONORWELLAND CAN THEREFORE NOT BE CAPTURED BY AN ACCEPTOR CLOSE TO THE WELL ÌS A RESULT A 4) TRANSITIONATLOWTEMPERATURESCANNOTBEVERTICALINREALSPACEANDINVOLVESTUNNELINGOF THEELECTRONINTOTHEBARRIER$UETOTHERESULTINGSMALLOVERLAPOFTHEWAVEFUNCTIONSTHE PROBABILITYOFA4)TRANSITIONATLOWTEMPERATURESISLOW;=ÌTELEVATEDTEMPERATURES 4)TRANSITIONSAREQUENCHEDASWELLDUETOTHETHERMALRELEASEOFHOLESFROMTHEACCEPTOR LEVELSÌSARESULTTHECALCULATEDINTENSITYOFTHE4)LUMINESCENCEINFIGUREOFREFERENCE

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ÌSSTATEDINTHEINTRODUCTIONINTHEREGIONOFTHEOBSERVED$LINEENERGIESAFREE EXCITON &% TRANSITION IN 'A3B IS REPORTED AT ME6 ÌT FIRST SIGHT ONE MIGHT THEREFORE EXPECT THE $LINE TO REPRESENT FREE EXCITON RECOMBINATION (OWEVER THE OVERALLEXPERIENCEWITH-/60%GROWN'A3BMAKESTHEAPPEARANCEOFAWELLRESOLVED&%

UNLIKELY"ESIDESTHE$LINEBECOMESDOMINANTINTHE0,SPECTRUMOFNOTINTENTIONALLY DOPED 'A3B FOR TEMPERATURES ABOVE + IT MOVES TO LOWER ENERGIES WITH THE TEMPERATURERISEANDTOHIGHERENERGIESWITHINCREASINGEXCITATIONDENSITY

4HEREFOREAMUCHMORELIKELYINTERPRETATIONOFTHE$LINEISARECOMBINATIONOFAFREE ELECTRON FROM THE ELECTRONIC PERCOLATION LEVEL WITH A FREE HOLE FROM THE VALENCE BAND PERCOLATIONLEVEL#ONSEQUENTLYTHE$LINEPEAKPOSITIONWILLREFLECTTHEBANDGAPENERGY IN THE PERTURBED SYSTEM 7ITH INCREASING TEMPERATURE THE BAND GAP DECREASES AND A REDSHIFT OF THE 0, IS OBSERVED WHILE AT INCREASING EXCITATION DENSITY THE QUASI&ERMI LEVELS MOVE TO HIGHER ENERGIES AND CAUSE A BLUESHIFT OF THE 0, LINE ÌS A RESULT THE OBSERVEDEXPERIMENTALBEHAVIOURCANBEWELLUNDERSTOOD"ESIDESTAKINGTHEPOSITIONOF THE$LINEAT+TOBE⁸⁰⁸^.⁵±⁰^.⁴ME6 SEEFIGUREANDACCEPTINGABANDGAPOF ME6FORNONPERTURBED'A3BATTHISTEMPERATURETHEDIFFERENCEOF³^.⁵±⁰^.⁴ME6 CAN SERVE AS A GOOD ESTIMATE OF THE OVERALL BAND GAP REDUCTIONγ_E γ_H DUE TO THE PERTURBATIONS

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ÌSEXPECTEDFROMTHELITERATUREDATAONTHISLINE SEESECTIONTHE"%LINECANBE CONSIDEREDAS0,RECOMBINATIONOFANEXCITONBOUNDTOANUNIDENTIFIEDNEUTRALACCEPTOR 4HELINEISTHERMALLYQUENCHEDABOVE+ANDKEEPSCONSTANTPOSITIONWITHINCREASING TEMPERATURE

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ÌSREPORTEDINSECTIONTHE %LINE APPEARS AT APPROXIMATELY ME6 AND IS THE DOMINANT LINE IN THE INTENTIONALLY SILICON DOPED SAMPLE ÌS DESCRIBED IN THE INTRODUCTIONALINEATME6WASPREVIOUSLYIDENTIFIEDASRECOMBINATIONOFEXCITONS BOUNDTOANUNSPECIFIEDACCEPTORIN,0%GROWNCRYSTALSANDCONSEQUENTLYDENOTED"%

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4HE%LINEINSAMPLEREMAINSOBSERVABLETOTEMPERATURESABOVE+ANDSHIFTSTO HIGHERENERGIESWITHTEMPERATURERISE APPROXME6BETWEEN+#OMPARING THE THERMAL QUENCHING OF THE %LINE TO THE CORRESPONDING BEHAVIOUR OF THE EXCITONIC

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ÌSANEXCITONICANDA$Ì0CHARACTERCANNOWBOTHBERULEDOUT4)AND")TRANSITIONSARE LIKELY CANDIDATES FOR THE %LINE NATURE 4HE RELATED IMPURITY IS ALMOST CERTAINLY SILICON BECAUSEOFTHEEVIDENTINCREASEOFTHE%LINEINTENSITYINTHEINTENTIONALLYSILICONDOPED SAMPLE )N THE NOT INTENTIONALLY DOPED SAMPLE A CERTAIN BACKGROUND OF SILICONISBELIEVEDTOBEPRESENTDUETOIMPURITIESINTHEEMPLOYEDMETALORGANICSÌSNO INDICATIONFORSILICONWASFOUNDIN'AÌSLAYERSGROWNWITHTHESAMEBATCHOF4%'ATHE 4-3BISCONSIDEREDTOBETHESOURCEOFBACKGROUNDSILICONINSAMPLE

)NORDERTOESTIMATETHEBINDINGENERGYOFTHESILICONACCEPTORIN'A3BTHEEXACTNATURE OF THE SILICONRELATED %LINE MUST BE IDENTIFIED 4HE RELATIVE %LINE INTENSITY SHOWS A MONOTONOUSDECREASEWITHINCREASINGTEMPERATUREINTHEWHOLERANGEBETWEENAND +.OTHERMALQUENCHINGATLOWTEMPERATURESCOULDBEOBSERVEDÌSALREADYSTATED ATTHEENDOFSECTIONTHISBEHAVIOURRULESOUTA4)TRANSITION/NTHEOTHERHAND BOTH THE %LINE¾S STRONG BLUE SHIFT WITH INCREASING TEMPERATURE AND ITS THERMAL QUENCHINGATELEVATEDTEMPERATURESAREWELLINLINEWITHTHEPROPERTIESOF")TRANSITIONS 7HILETHESHIFTFROMLOWENERGY")TOHIGHENERGY")TRANSITIONSEXPLAINSTHEOBSERVED BLUESHIFT OF THE %LINE PEAK THE THERMAL RELEASE OF HOLES FROM THE ACCEPTOR LEVELS EXPLAINSTHETHERMALQUENCHINGATELEVATEDTEMPERATURES)NSUMMARYA")CHARACTEROF THE%LINECANBEASSUMED

)TISIMPORTANTTOEMPHASIZETHEFOLLOWINGPOINT4HESTANDARDWAYTODESCRIBE EÌ OR À")Á TRANSITIONS IN NONPERTURBED BAND STRUCTURES IS THE %AGLES MODEL ;= 4HIS MODELPREDICTSTHETHERMALDEPENDENCEOFTHEPEAKPOSITIONAS

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= 0 ² WITH THE 6ARSHNI COEFFICIENTSA%E6+ANDB+FOR'A3B;=4HISMODELPREDICTSAMAXIMUM BLUESHIFTOFME6AT+WHEREASSAMPLESHOWSAMUCHMORESIGNIFICANTSHIFT OF APPROX ME6 BETWEEN + SEE FIGURE #ONSEQUENTLY THE %AGLES MODELISNOTSATISFACTORYWITHRESPECTTOSAMPLEANDITISESSENTIALTOCONSIDERTHE INFLUENCEOFTHEFLUCTUATINGPOTENTIAL

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&ROMTHE$LINEPOSITION AT+ SEESECTIONFIGUREAVALUEOF³^.⁵±⁰^.⁴WASDERIVEDFORγ_Eγ_H ÌCCEPTING&_H , E6 AND !ω _%_LINE₊ ≈ME6FROM THE MEASUREMENT AT +ONEGETSANACTIVATIONENERGYOF&_"⁸^.⁸±⁰^.⁴ME6&ORCOMPARISONTHELITERATURE DATAONTHEACTIVATIONENERGYOFTHESILICONACCEPTORAREBETWEENANDME6 SEE SECTION WHILE THE EFFECTIVE MASS ACCEPTOR IN 'A3B HAS BEEN SUGGESTED TO HAVE ACTIVATIONENERGIESBETWEENME6 ;=CHPANDME6;=

ÌFTERHAVINGSHOWNTHATTHE%LINEMUSTBEINTERPRETEDASBANDIMPURITYTRANSITIONTHE NATURE OF THE ÌLINE NEEDS SOME FURTHER COMMENTS TOO ÌS SAID IN THE INTRODUCTION SECTIONTHELINEHASBEENSUGGESTEDTOBEADONORACCEPTORPAIRTRANSITION $Ì0 FOR EXAMPLE IN ;= BECAUSE OF A SIGNIFICANT SHIFT TO HIGHER ENERGIES WITH INCREASING EXCITATIONDENSITY/THERAUTHORSHAVESUGGESTEDTHEÌLINETOBEACONDUCTION BAND NEUTRAL NATIVE ACCEPTOR TRANSITION E .Ì ; = 4HIS OBVIOUS CONTRADICTION CAN BE SOLVEDBYACCEPTINGTHATTHE%LINEANDÌLINEAREOFTHESAMENATUREIEBOTHAREDUE TO EÌ ORÀ")ÁTRANSITIONSWITHTHEPARTICIPATIONOFTWODIFFERENTTYPESOFACCEPTORS NAMELYSILICONANDTHE.ÌFOR%ANDÌLINERESPECTIVELY

7HILETHEÌLINEANDTHE%LINEINSAMPLEBOTHSHOWABLUESHIFTOFAPPROXIMATELY ME6BETWEENAND+THEABOVEBLUESHIFTINSAMPLEISSMALLER4HISCAN BEUNDERSTOODBYCONSIDERINGTHATTHERMSFLUCTUATIONSOFTHEPOTENTIALINSAMPLE ARE SMALLER COMPARED TO SAMPLE DUE TO THE REDUCED DEGREE OF COMPENSATION

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LINE SUGGEST THAT THE ORIGIN OF THIS LINE IS MOST PROBABLY A RADIATIVE RECOMBINATION OF EXCITONSBOUNDTO3I_3B

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&OR PARTIALLY COMPENSATED SAMPLES AT LOW TEMPERATURES TRANSPORT CAN ONLY OCCUR BY CARRIERSJUMPINGFROMNEUTRALMAJORITYIMPURITIESTOCHARGEDONESWITHOUTANYEXCURSION TOTHEBANDS ÀHOPPINGÁ)NTHISCASEQSTANDSFORQ/_"/_% ;=P)NTHECASE OF THE ABOVE SAMPLE THE RESULTING SCREENING RADIUS WAS Å. ÌS HOPPING TRANSPORTNATURALLYDEPENDSONTHEPRESENCEOFIMPURITYSTATESASPHEREWITHARADIUSOF ÅMIGHTWELLBETOSMALLTOCONTAINANADEQUATENUMBEROFACCEPTOR MAJORITYSTATES WHICHCANHOSTSCREENINGCHARGESÌSARESULTTHETRUESCREENINGRADIUSINTHEMATERIAL MIGHT WELL BE LARGER THAN IN THE ABOVE CALCULATIONS ANDBOUND STATES ASSOCIATED WITH ISOLATEDDONORSMIGHTBECOMEPOSSIBLEFROMTHISPOINTOFVIEW

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4HE0,PROPERTIESOFNONINTENTIONALLYDOPEDANDLIGHTLYSILICONDOPED'A3BHAVEBEEN STUDIEDÌFORMERLYCONTROVERSIAL0,LINEATAPPROXIMATELYME6COULDBEASCRIBEDTO SILICONACCEPTORSINTHEMATERIAL"YCONSIDERINGTHEPOTENTIALFLUCTUATIONSINDOPEDAND PARTIALLYCOMPENSATEDSEMICONDUCTORSTHELINECOULDBEIDENTIFIEDASDUETOATRANSITION OFAFREEELECTRONTONEUTRALACCEPTOR

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4HECOINCIDENCEOFTHISSOCALLED%LINE WITH THE "% LINE WHICH WAS PREVIOUSLY REPORTED AT THE SAME ENERGY MUST BE CONSIDERED ACCIDENTAL 3IMILARLY THE NATURE OF THE ÌLINE WAS SHOWN TO BE DUE TO A TRANSITIONOFAFREEELECTRONTONEUTRALOBUJWFACCEPTOR E.ÌÌLLOTHER0,PEAKSINTHE HIGHENERGY PART OF THE SPECTRUM COULD BE EXPLAINED WITHIN THE CONCEPT OF POTENTIAL FLUCTUATIONSDUETOTHECHARGEDIMPURITYDISTRIBUTION

4HEACTIVATIONENERGYOFTHESILICONACCEPTORCOULDBEESTIMATEDASME6'ENERALLY LIGHTDOPINGWITHSILICONRESULTEDINAHIGHERMOBILITYAREDUCEDCOMPENSATIONRATIOAND ANIMPROVED0,INTENSITYCOMPAREDTONOTINTENTIONALLYDOPEDSAMPLES3ILICONDOPING SEEMSTORESULTINASUPPRESSIONOFTHENATIVEDEFECTIN'A3BANDLEADTOAREDUCTIONOF THE NONRADIATIVE RECOMBINATION RATE IN THE MATERIAL 4HESE FEATURES MAKE THE SILICON ACCEPTORANINTERESTINGNOVELCANDIDATEFORVARIOUS'A3BBASEDDEVICE APPLICATIONS )N THIS CONTEXT THE FIRST EVER REPORTED 3IDOPED 'A3BBASED DEVICES WILL BE PRESENTED IN THEFOLLOWINGSECTION

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Im Dokument MOVPE of GaSb-based materials and solar cell structures (Seite 93-103)