Effect of Patterned Sapphire Substrate Shape on Light Output

944IEEEPHOTONICSTECHNOLOGYLETTERS,VOL.23,NO.14,JULY15,2011

EffectofPatternedSapphireSubstrateShapeonLight

OutputPowerofGaN-BasedLEDs

Xiao-HuiHuang,Jian-PingLiu,Ya-YingFan,Jun-JieKong,HuiYang,andHuai-BingWang

Abstract—Anopticalsimulationincludingre ectionandrefrac-tionisusedtosimulatethelightilluminationintensityofgalliumnitride(GaN)-basedlight-emittingdiodes(LEDs)onvariedpat-ternedsapphiresubstrates(PSS)withdifferentslantedanglesand llfactors().Itisfoundthatamicropyramidarraywithaslantedanglefrom25to60isabletoeffectivelyimproveilluminationin-tensitywhichreachestosummitastheslantedangleisaround33.Andilluminationintensityenhancesmonotonouslyasincreases.Inadditiontothementionedwork,epitaxialgrowthofGaN-LEDsonPSSisinvestigatedforcomparison.TheoutputpowerofGaN-LEDsonPSSincreaseswithsimulatedlightilluminationintensityincreasing.Theexperimentalresultsprovethesimulation.IndexTerms—GaN,illuminationintensity,light-emittingdiode(LED),patternedsapphiresubstrate(PSS).

hadbeenstudiedandsuccessfullyappliedtothefabricationofhigh-ef ciencyLEDs[9],[10].AndtherewerealotofreportsfocusingonthecomparisonsoftheLEDsEQEbetweenunpat-ternedsapphiresubstrate(un-PSS)andPSS[11],[12].How-ever,illuminationintensityenhancementbyusingPSSisstilllackoftheoreticanalysisandexperimentalproofs.

Inthiswork,periodicpyramidalarrayPSSwithvariousslantedanglesand llfactors()wereusedassubstrates.IlluminationintensityofGaN-basedLEDsonthesePSSwassystematicallysimulated.Epi-GrowthofGaN-basedLEDsonthesePSSwascarriedoutbymetal-organicchemicalvapordeposition.Theoutputpowerwastestedtocomparewithsimulatedilluminationintensity.

II.EXPERIMENT

FourtypesofPSSswerepreparedwiththeirdiam-eter/spacing/heightbeing3.75/1.25/1.85,3.0/2.0/1.5,2.4/0.6/1.5and2.0/1.0/1.5mmm,namedsampleA,B,CandD,respectively.Theslantedangles()ofthesesidewallsarecal-culatedfromheightdiameter,whichare44.6,45,51.3and56.3forthefourkindsofsubstrates.And, llfactors(),de nedastheequationof(PSSarea/totalareas),are0.51,0.33,0.58and0.4,respectively.AplanarsapphirewasalsopreparedforcomparisonanditwastitledassampleE.

TheLEDstructurescompriseda25nmnucleationlayeronthePSS,a1.5mundoped-GaNlayer lm,a2.0mn-GaNlayer,an veperiodsInGaN/GaNmultiplequantumwell(MQW)withemissionwavelengthintheblueregion(460nm),a20nmAlGaNelectronblockinglayeranda150nmp-GaNlayer.Trimethylgallium(TMGa)andammoniagas(NH3)wereusedasprecursorsforGaandNsource,respectively.Thedevicemesawithachipsizeof350350mwasthende nedbyinductivelycoupledplasma.Theindiumtinoxide(ITO)layerwasdepositedtoformap-sidecontactlayerandacurrentspreadinglayer.TheCr/AulayerwasdepositedontotheITOlayertoformthep-sideandn-sideelectrodes.

Asimulationincludingre ectionandrefractionisusedtoevaluateilluminationintensityofGaN-basedLEDsonPSS.AndtheopticalandelectricalpropertiesofLEDsweretestedbyLEDchip/waferproberandtester(IPT-6000)withaninte-gratespherefromFitTechCo.,Ltd.

III.RESULTSANDDISCUSSION

TheuseofPSSforLEDsgrowthisbelievedtobeverypromisingasitimprovestheinternalquantumef ciency(IQE)byreducingthreadingdislocations(TDs)densityandenhancesLEEbyenhancinglightescapeprobability[13].Itisaverycrit-icalparameterofPSSshapethataffectsilluminationintensityconsiderably.InordertostudyilluminationintensityofPSSindetails,lightpathofthatemittingfromactivelayerispresented

I.INTRODUCTION

ITHemissioninthenearultraviolet(UV),blue,andgreenspectralrange[1],groupIII–nitride-basedsemi-conductorshavebeenusedrecentlyashigh-brightnessLEDsinlarge-areafull-coloroutdoordisplays,signallightsandhighper-formancebacklightingunitsinliquidcrystaldisplays.Thoughcommerciallyavailable,theexternalquantumef ciency(EQE)ofGaN-basedLEDsstillfallsshortofwhatisexpected.WiththedisparaterefractiveindicesofGaN()andair(),theinternallighthasdif cultyinescapingintotheairfromthesemiconductor.Thelightreachingthesurfacewithincidentanglelargerthanthecriticalangle(24.6)willnotemittotheairbutwillexperiencetotalinternalre ectionandcontinuestobere ectedwithintheLEDuntilitgetabsorbed[2],[3].Im-provinglightextractionef ciency(LEE)hasthusbecomethefocusofresearches.Thetechniquesofsurfaceroughening[4],nanoimprinting[5],havebeenusedtoimproveLEEbutnotef-fectively.Currently,GaNbased-LEDsgrownonpatternedsap-phiresubstrates(PSS)havebeenstudiedbymoreandmorere-searchers[6],[7].ThePSS,whichisamask-freeandthere-forecontamination-freemethod,reducesthethreadingdisloca-tiondensityinGaNepilayersaswellasenhancesLEEofLEDsduetoincreasedlightescapeprobability[8].ManytypesofPSS

ManuscriptreceivedJanuary19,2011;revisedMarch23,2011;acceptedApril09,2011.DateofpublicationApril15,2011;dateofcurrentversionJune22,2011.ThisworkwassupportedbytheNationalNatureScienceFoundationofChina(Grant61006084andGrant61076119),andbyTechnicalCorporationInnovationFoundationofSuzhouIndustrialPark(GrantSG0962).

X.-H.HuangiswithSuzhouInstituteofNano-TechandNano-Bionics,CAS,DepartmentofNanoDevice,Suzhou,215125,China,andalsowiththeGraduateUniversityofChineseAcademyofSciences,Beijing,100049,China.

J.-P.Liu,Y.-Y.Fan,J.-J.Kong,H.Yang,andH.-B.WangarewithSuzhouInstituteofNano-TechandNano-Bionics,CAS,DepartmentofNanoDevice,Suzhou,215125,China(e-mail:hbwang2006@).

Colorversionsofoneormoreofthe guresinthisletterareavailableonlineat.

DigitalObjectIdenti er10.1109/LPT.2011.2142397

W

1041-1135/$26.00©2011IEEE

HUANGetal.:EFFECTOFPATTERNEDSAPPHIRESUBSTRATESHAPEONLIGHTOUTPUTPOWEROFGaN-BASEDLEDs

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Fig.1.SchemeoflightpathwithintheLEDs.

inFig.1.Forsimplicity,wedonotconsidertheeffectsoftheelectricpadaswellascurrentspreadinglayer.ThepatternedarrayofPSSisintroducedasre ectorofaGaN-basedLEDtoenhanceLEEbylightre ection.Re ectionandrefractiononPSSarrayaredeterminedbyFresnelequations.ItisbelievedthatscatteringlossesandmaterialabsorptionareequalforallPSS,andthereforeareneglectedinthesimulation.AndrefractiveindicesforGaN(nGaN,MQWandpGaN),sapphire,ITOandairare2.4,1.78,2.14and1inthiswork.

There ectioncoef cientcanbecalculatedfromtheequationasbelow,

(1)

Where

isincidenceangleand

isrefractionangle:

(2)

Thetotalilluminationintensitycomprisesfourparts(,,,)asshowninFig.1.isthepartwithinthecriticalangle()withoutanyre ectionasdepictedin(3):

(3)

Whereisinitialilluminationintensityoflightemittingfromactivelayerandisincidenceangle,respectively.TheincidenceangleoftherayisdenotedasescapeanglewhentherayescapesfrompGaNtoair.Sincecriticalangle()is24.6,iscalculatedtobefrom(3).isnotaffectedbyPSSshapeever.Beside,andarethepartthatre ectedfromtheslantedareaandisthepartthatre ectedfromplanararea,whicharedescribedin(4):

(4)

Therayisre ectedbypatternedarrayforonetimebeforeitescapestotheairwhenislargerthanslantedangle(),thepartofwhichisdenotedas.However,whenislessthan,therayis rstre ectedtoplanarareaortheslantedareaaroundforsecondre ectionbeforeitescapestotheair.Thepartwhichisre ectedfortowtimesisdenotedas.Multiplyingwithre ectioncoef cient,andcanbewrittenin

(5):

(5)

Fig.2.Simulatedilluminationintensityversusslantedanglewithis033,0.4,0.51,and0.58,respectively,andtheSEMimagesforsampleA,B,C,andD.

Where,andarere ectioncoef cientasthein-cidenceangleis,and,respectively.,andcanbededucedfromlightpathinFig.1as,and,respectively.ItiscalculatedthatincidenceangleshouldbelargerthanfortoescapefrompGaNtoair.Andtheescapeangleforis.Thesimulatedvalueofisontheorderofmagnitudeof,whichisneglectedinthiswork.Therefore,illuminationinten-sityismainlydeterminedbythesumof,and.

Thesimulatedrelativeilluminationintensityisplottedasafunctionofasis0.33,0.4,0.51and0.58inFig.2.Andscanningelectronmicroscope(SEM)imagesofsampleA,B,CandDareinsertedinFig.2,theofwhichis44.6,45,51.3and56.3respectively.Itisseenthatilluminationin-tensityincreaseastheslantedangleincreasefrom25untilto33.Thenilluminationintensitydecreasesasslantedanglein-creases.Andilluminationintensitycomestobeaninvariablewhichisequaltothatonplanarsapphirewhenslantedangleislargerthan60.LightextractionanalysiswithMonteCarloraytracingpresentedbeforealsoconcludedasimilarresult[14].ThesimulatedilluminationintensityforsampleA,B,CandDinourworkcanbefoundonthecorrespondingcurves.Illumi-nationintensityenhancementcanbeattributedtothere ectionofPSSarray,resultinginawiderrangeofcriticalangleforin-ternalre ection.TheescapeprobabilityinLEDsgrownonPSSissigni cantlyhigherthanthatobtainedfroma atsurface.Be-causeofthelimitsofcriticalangle,increasesastheslantedangledecreasesuntilto33,anddecreasesastheslantedangledecreasefrom33.Alsoincreasesastheslantedangledecreases.However,fewofcanescapetheinterfacewhenslantedangleislessthan33.Inanotherword,thereisawidestrangeofescapeangleastheslantedangleisaround33,whichresultsinthehighestilluminationintensity.Thisincreaseofil-luminationintensityarisesfromthereducedamountoftotalin-ternalre ectionofphotonsattheinterface.

ThemotivationofPSSLEDsistoutilizeslantedplanestoscatterthelightandreduceinternalre ection,andthereforetoimprovelightextraction.Webelieveslantedangleisthedomi-natefactor.However,lightextractioncanbeenhancedasin-creasesfortheLEDsgrownonPSSwiththesameasslantedangle.Fig.3showsthatilluminationintensityincreasesmonot-onouslyasincreases.There ectedfractionoflightemittingfromactivelayerincreaseswiththeincreasingofPSSarea.Itis

946Fig.3.Simulatedilluminationintensity

versuswith

is44.6,45,51.3,

and56.3,respectively.

Fig.4.PlotofoutputpowerofLEDsgrownonsampleA,B,C,D,andEatinjectioncurrentof20mAasthefunctionofsimulatedilluminationintensity.

concludedthatilluminationintensityofaLEDgrownonPSSisdeterminedbyandslantedangleofPSSsimultaneously.Inordertoprovethesimulationresultsexperimentally,LEDsonsampleA,B,C,DandEhavebeenproducedandrenamedasLED-A,-B,-C,-Dand-E.Theopticalandelectricalpropertiesofthesebarechipsweretestedwithasampleratioof50:1.Theforwardvoltagesforthe vechipsare3.05V,3.02V,3.10V,3.15Vand3.0V,respectively.Andtheleakagecurrents(V)for vesamplesareallbelow0.05A.Theoutputpowerof vesamplesisplottedasafunctionofsimulatedrelativeintensityasshowninFig.4.TheoutputpoweroftheLEDsgrownon vesamplesincreaseslinearlyassimulatedilluminationintensityincreases.Theoutputpowerimprovementpercentageof26%fortheLEDgrownonPSSwithslantedangle44.6incom-parisonwiththatgrownonPSSwithslantedangleof56.3isingoodcorrelationwithsimulationresultof29%.TheoutputpoweroftheLEDsprovesthesimulatedresultsexperimentally.Also,itisbelievedthatscatteringlossesandmaterialabsorptionareimportantparameterswhichdeterioratetheoutputpowerofaLEDevidently.Asweknow,EQEistheproductofLEEandIQE.TheoutputpowerisdeterminednotjustbyLEEbutalsoIQE.TheIQEalsohassomethingtodowithslantedangleandwhichwillbediscussedinnextwork.Since,thedislocationsactasthenonradiativecenterswhichdeterioratetheIQE,itisreasonableforthedifferencebetweensimulatedilluminationin-tensityandtheoutputpower.

IV.CONCLUSION

Insummary,asimulationincludingre ectionandrefractionisusedtoevaluatethelightextractionofGaN-basedLEDson

IEEEPHOTONICSTECHNOLOGYLETTERS,VOL.23,NO.14,JULY15,

2011

PSSwithdifferentslantedanglesand.Micropyramidarraywithaslantedanglefrom25to60isshowntoeffectivelyimproveilluminationintensity,whichreachessummitastheslantedangleisaround33.AlsoLEDsweregrownonvar-iousPSSwithdifferentslantedanglesand.ThetestedoutputpoweroftheseLEDsprovesthesimulationexperimentally.AnditisconcludedthatilluminationoutputpowerofaLEDisde-terminedbyslantedangleandincombination.

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