L
Rareearthphosphors:fundamentalsandapplications
¨C.R.Ronda*,T.Justel,H.Nikol
PhilipsGmbHForschungslaboratorienAachen,P.O.Box500145,D-52085Aachen,Germany
Abstract
Inthispaperanoverviewisgivenofluminescentmaterials,basedonrareearthions.Fundamentalaspectsandclassicalapplicationsof
luminescentmaterialsappliedinfluorescentlampsanddisplaysarediscussedwithregardtoopticalproperties.TheapplicationpotentialofrareearthphosphorswithregardtonewdevelopmentssuchasphosphorsforLEDsandluminescentmaterialswithquantumyieldlargerthanunityisalsodiscussed.©1998ElsevierScienceS.A.
Keywords:Rareearthphosphors;Quantumcutter
1.Introduction
Luminescentmaterials,alsocalledphosphors,aremost-lysolidinorganicmaterialsconsistingofahostlattice,usuallyintentionallydopedwithimpurities.Theabsorptionofenergytakesplaceviaeitherthehostlatticeoronimpurities.Inaddition,transferofenergythroughthelatticecantakeplace.Inalmostallcases,theemissionoriginatesfromimpurities.
Luminescentmaterialsarewidelyappliedtoday.Majorapplicationsareinemissivedisplaysandfluorescentlamps.Inaddition,someX-raydetectorsystemsarebasedonluminescentmaterialsaswell.
Inthelastdecades,alargenumberofluminescentmaterialsbasedonrareearthionsorrareearthhostlatticeshasbeeninvented.Quiteafewofthesematerialsalsofoundtheirwayintoapplications.Inmanycases,rareearthphosphorsdramaticallyimprovedtheperformanceofthedevicesinwhichtheyareapplied.
Inthefirstpartofthispaper,theemissionpropertiesofrareearthionsarediscussedandcomparedtotheemissionpropertiesofotherluminescentionsormoleculargroups.Thesecondpartdealswithapplicationsofluminescentmaterialswithemphasisonrareearthphosphors.Thepaperendswithadiscussionofnewdevelopmentsinthefieldofrareearthphosphorsinthethirdpart.
2.Emissionpropertiesofrareearthionsincomparisontootheremittingcenters
Inmostcases,theemissionofrareearthionsisduetoopticaltransitionswithinthef-manifold(e.g.Tb31(4f8),Gd31(4f7)andEu31(4f7).Thef-electronsarewellshieldedfromthechemicalenvironmentandthereforehavealmostretainedtheiratomiccharacter.Asaconse-quence,thef–femissionspectraconsistofsharplines.Inaddition,thef–ftransitionsarepartiallyforbiddenandmanyofthemarealsospinforbidden.Therefore,theseopticaltransitionsaregenerallyveryslow,viz.intherangeofmicrosecondstomilliseconds.
Foranumberofrareearthions,however,alsobroademissionbandsareknown.ProminentexamplesareEu21(4f7)andCe31(4f1).Inthiscase,emissionisdueto5d–4fopticaltransitions.Asdelectronsparticipateinthechemi-calbonding,thed–femissionspectraconsistofbroadbands.Furthermore,thed–fopticaltransitionsareallowedandareconsequentlyveryfast(afewmsorless).Exam-plesoff–fandd–femissionspectraaregiveninFig.1andFig.2respectively.
Itisinterestingtocomparetheemissionpropertiesofrareearthphosphorswithotherluminescentcentres.Manytransitionmetalionsareknownasluminescentcentres,e.g.Mn21inZn2SiO4:Mn(Fig.3).The4T14132(t2geg)→6A1(t2geg)transition,leadingtoagreenemissioninthiscompound,involvesachangeinthechemicalbondingduetothetransitionofanelectronfromthet2gorbitalstotheegorbitals.Therefore,theemissionbandisbroad.TheemissionspectrumofMn41in
*Correspondingauthor.fax:ronda@pfa.research.philips.com
1492416003442;e-mail:
0925-8388/98/$19.00©1998ElsevierScienceS.A.Allrightsreserved.PII:S0925-8388(98)00416-2
670C.R.Rondaetal./JournalofAlloysandCompounds275–277(1998)669–676Fig.1.EmissionspectrumofY2O3:Eu.
Fig.2.EmissionspectrumofBaMgAl10O17:Eu.
Fig.3.EmissionspectrumofZn2SiO4:Mn.Fig.4.EmissionspectrumofMg4GeO5.5F:Mn.
Mg4GeO5.5F:Mn(Fig.4)consistsofarelativelynarrowemissionlinewithsomestructureduetovibronicinterac-tions.Here,theemissionisduetoaspinfliptransitionwithinthet32gmanifold,i.e.thechemicalbondingishardlychanged.Thislatterexampleshowsthatalsoincasesofopticaltransitionsinvolvingstateswhichparticipateinthechemicalbondingsharpemissionbandscanbeobservedwhenthechemicalbondstrengthinground-andexcitedstatedoesnotdiffersignificantly.
Emission,originatingfromcomplexanions(likeinCaWO4,Fig.5),usuallyresultsfromachargetransfertransition:atransitioninvolvingelectronicstatesofdiffer-entions.Here,thechemicalbondingwillvarystrongly,leadingtobroademissionbands.
Thetreatmentgivenaboveshowsthatlineemissionisnotaspecificpropertyofrareearthionsandinadditiontothat,inthecaseofrareearthions,broademissionspectracanbeobtainedaswell,dependingontheopticaltransi-
Fig.5.EmissionspectrumofCaWO4.
C.R.Rondaetal./JournalofAlloysandCompounds275–277(1998)669–676671
tionsinvolved.Inthisrespect,rareearthionsarenotunique.Theuniquepropertiesofrareearthionsoriginatefromthefactthatthespectralpositionoftheemissionlinesisalmostindependentofthehostlattice,incontrasttolineemissiongeneratedbytransitionmetal1ions.Inaddition,someofthelineemittingions(Tb3andEu31)emitatspectralpositions,enablinghighlumenefficaciesandaverygoodqualityofwhitelight(seebelow):theemissionspectraoftheseionsareadaptedtothehumaneyeinaverygoodway.
3.Excitationandemissionpropertiesofappliedrareearthphosphors
Inthissection,wediscussapplicationalaspectsofphosphorsforfluorescentlampsanddisplays.
3.1.Phosphorsforfluorescentlamps
Wewillconsiderfluorescentlampsforgeneralillumina-tionpurposesonly,i.e.fluorescentlampswhichemitwhitelight.Apartfromtheefficiencywithwhichwhitelightisgeneratedanotherimportantpropertyoflampsistheirabilitytoreproduceallcoloursinanaturalwayandalsohavingthecolourtemperatureofthelightgenerated.Thesefactorsaredeterminedtoalargeextentbythephosphorsapplied.Thecolourtemperatureequalsthetemperatureofablackbodyradiatorwhoseemissionspectrumhasthesamecolourcoordinates.Thesocalledcolourrenderingindex(CRI)isdeterminedbycomparingtheemissionspectrumofthelampwiththatofablackbodyradiatorhavingthesamecolourtemperature.Ablackbodyradiator(andalsoanincandescentlamp)hasaCRIof100bydefinition.Untilabout20yearsago,mainlyCa5(PO4)3(F,Cl):(Sb,Mn)(halophosphate,CHP)wasusedinfluorescentlampsforgenerallightingpurposes.Inthiscompound,theUVradiation,generatedbytheHgdis-charge,isabsorbedbySb31andpartlytransferredtoMn21.Thecombinationofablue(Sb31)andyellow(Mn21)emissionbandresultsinwhitelight.LampsbasedonCHPhaveamoderateenergyefficacy(75lm/W)andarelativelylowCRI(50–60).
Rareearthphosphorshavechangedthesituationdramatically.BoththeCRIandtheenergyefficacyoffluorescentlampsbasedonrareearthemittersarecon-siderablyhigherthaninthecaseoflampsbasedonCHP,seee.g.[1].Insuchlamps,CRIslargerthan80canbeobtainedatefficaciesof100lm/W,see[2].Examplesforphosphorsusedinthesehighqualityfluorescentlampsaregivenbelow.
BaMgAl110O17dopedwithEu2(BAM)isusedasblueemittingphosphorinhighqualityfluorescentlamps.21InthismaterialthephotonsareabsorbedbyEuions,theabsorptionbeingduetoa4f→5dtransition.Theemission
takesplaceonEu21aswell,theemissionoriginatesfromthe5d→4fopticaltransition.
TheredemissionintheselampsisgeneratedbyY2O3:Eu.Here,excitationinvolvesachargetransfertransitionfromO22ionstoEu31.Emissiontakesplacewithinthef-levelsoftheEu31ions.
Itispossiblethattheopticalabsorptiononacentreshowingthedesiredemissionisnotstrongenough.Insuchcases,absorptionandemissiontakeplaceondifferentopticalcentres.Afteropticalabsorption,theenergyistransportedthroughthelatticetowardstheemittingopticalcentre.AllgreenemittingTb31phosphors,appliedinhighquality1fluorescentlamps,relyonopticalabsorptiononCe31ions,followedbyenergytransfertotheemittingTb3ions.Anexampleis(Ce,Tb)MgAl11O19,wherethe
energyistransferredfromCe31toTb31.Itsemissionspectrumisgivenin1Fig.6.ApartfromthelineemissionoriginatingfromTb3,onealsoobservesatraceofUVemissionoriginatingfromCe31,indicatingthattheenergytransferprobabilityisslightlybelowunity.AsmalleramountofUVradiationisgeneratedbye.g.(Ce,Gd,Tb)MgB315O10,whereenergytransferfromCe31toTbproceedsviatheGd31ionsublattice.
3.2.PhosphorsforCRTs
InstandardCRTs,theonlyrareearthphosphorappliedisthered-emittingY2O2S:Eu.Forblueandgreen,ZnSdopedwithAgandCu,Au,1respectively,areapplied.ThelineemissionoftheEu3ionsenablesahighluminousefficiencyofthismaterialdespitetherelativelylowenergyefficiency.Especiallyforredemittingphosphorsbeinglineemitters,thisisimportantinviewoftherapiddecreaseineyesensitivityforlightwithwavelengthlargerthan600nm.
InCRTsappliedinprojectiontelevision,Y2O3:Euis
Fig.6.Emissionspectrumof(Ce,Tb)MgAl11O19.
672C.R.Rondaetal./JournalofAlloysandCompounds275–277(1998)669–676appliedasredemittingphosphorbecauseitshowsabettersaturationbehaviourthanY2O2S:Eu.Here,thegreenemittingphosphorappliedalsoisarareearthphosphor;inmostcasesY2SiO5:Tbisused.ContrarytotheZnSphosphors,therareearthactivatorionconcentrationscanbechosenrelativelyhigh(intheorderof5%,whereastheactivatorconcentrationsinZnSphosphorsareintheorderof100–1000ppm).Forblue,noalternativestoZnS:Aghavebeenfound.
3.3.PhosphorsforvacuumultravioletexcitationInPlasmaDisplayPanels(PDPs)andapossiblenewgenerationofHg-freefluorescentlamps,aXedischargeisusedtogenerateUVphotons.Inthiscase,thephotonenergiesareintheVacuumUltraviolet(VUV)partofthespectrum.Therefore,excitationoftheemissiongenerallytakesplaceviathephosphorhostlattice,followedbytransferoftheenergytoactivatororsensitiserions.
Itisofinteresttocomparethisexcitationmechanismwiththeexcitationmechanismunderlyingcathode-rayexcitation.Inthecaseofcathode-ray(CR)excitation,thefinalstepinthemechanismleadingtoemissionisthegenerationofbandgapexcitations[3],asinthecaseofPDPphosphors.Thereforeonewouldexpect,basedon
ThegreenprimaryforPDPsisbasedonMn21;withtheemissionspectrumofMn21ine.g.Zn2SiO4:Mnor(BaO)x.6Al2O3:Mnbeinglocatedatthetopofthecolourtriangleaverylargecolourgamutcanbereproduced.Adisadvantageofthesematerialsisthefactthattheemissionisspinforbidden,resultingintoolongadecaytimefordisplayapplications.Thespinselectionrulecanbeliftedbymagneticinteractionbetweenmagneticions[8].Inthisway,theradiativelifetimeisshortened,enablingfasterdecaynotoriginatingfromlossprocesses.Forlamps,Tb31emissionispreferredinviewofthehighcolourqualityrequired.
RedlightisgeneratedeitherbyY2O3:Euorby(Y,Gd)BO3:Eu.TheVUVabsorptionofY2O3:Euisweakerthanthatof(Y,Gd)BO313:Eu,however,theemis-sionspectrumofY2O3:EubettermatchestheEBUre-quirementsfordisplayapplications.Theemissionspec-trumofY2O3:Euhasitsmaximumat611nm,whereastheemissionspectrumofthemixedboratehasitsmaximumat59531nm.Thisnicelyillustratesthat,thoughtheemissiononEuisduetoopticaltransitionswithinthef-manifold,neverthelessthedetailedshapeoftheemissionspectrumdependsonthecrystalhost.Inthisparticularcase,theemissionat611nminY2O3:Euoriginatesfromanelectricdipoletransition,whereastheemissionat595nmin
C.R.Rondaetal./JournalofAlloysandCompounds275–277(1998)669–676673
Fig.7.ComparisonofemissionspectrumofEu31inLuBO3(continuousline)andLu0.5In0.5BO3:Eu(dashedline).
tochangesofthelocalenvironment.Weassigntheincreasedabsorptiontoaliftingofthedegeneracyoftheoxygenpstates,originatingfromaloweringofsymmetryintheternaryborates.Asaconsequence,thepmanifoldisbroadenedwhichresultsinenhancedabsorptionoflightof254nminawayelucidatedinFig.9.
4.2.ApplicationofrareearthphosphorsinsemiconductorLEDs
Theinventionofthebluelightemittingdiode(LED)basedonGaNcanberegardedasatriumphofmaterialchemistry.ElectroluminescencebasedonIII–VnitridesemiconductorshadalreadybeenabandonedwhenS.NakamurafromNICHIAChemicalspickedupthetopicagaininthelate1980sanddevelopeditwithinonly5yearstoanimpressivematuritylevel.ExcellentreviewsabouttheunderlyingsemiconductorphotophysicsaswellastheMOCVDtechniqueusedforLEDchipproductioncanbefoundelsewhere[9–11].Thebluediodeiscapableofchallengingthelightingmarket.Forthefirsttimeitis
Fig.8.ReflectionofLuxIn12xBO3:Euat254nmasafunctionofx.
Fig.9.Broadeningofoxygenp-bandduetolowsymmetrydistortion,causingincreasedabsorptionat254nm.
possibletogeneratewhitelightfromLEDswithefficien-ciesequaltohalogenlamps.
Twowaysofwhitelightgenerationappearlikely.First,blue,green,andreddiodescanbecombinedtoyieldwhitelight.ThisisattractivesincebyextendingtheGaNtechniquebrightgreendiodesbasedonAlInGaNareavailableaswellwithhighexternalquantumyields.So-calledclusterlampsconsistingofblue,green,andredLEDsarealreadyonthemarket.Second,inaverycheapwayasinglewhiteLEDcanbeobtainedbycombiningablueLEDwithgreen,yellow,and/orredemittingphos-phormaterialswhichabsorbandconvertpartoftheincidentbluelight.IncomparisontothethreeLEDdevicethiscausesadecreaseintheoverallefficiencybecausethequantumdeficitofthelightconversionfrombluetoyellow/redhastobetakenintoaccount.Inthefollowing,thisoptionwillbediscussedwithregardtolightqualityandluminousefficiency.
Inprinciple,itispossibletovarytheemissionwave-lengthofblueGaNbasedLEDsbetween370nm,whichmarksthebandgapofpureGaN,and470nmbyincreasingtheIncontentintheInGaNdevices.Assumingaconver-sionfromtheincidentlightbyaphosphormaterialemittingat555nm,thealreadymentionedquantumdeficitdecreasesinthesamedirectionfromabout33%to15%.Atafirstglance,theobviousoptionwouldthereforebeacombinationofablueLEDemittingat470nmandabroad-bandphosphorwhichcanbeexcitedatthatwave-lengthandre-emittingatabout555nm,creatingwhitelightcomposedoftheblueandyellow–greenemissions.Suchadevicehas(Y31beenrealizedusing
12xGdx)3(Al12yGay)5O12:Ce(YAG:Ce)asabroadbandyellowphosphor,seeFig.10.ThevariationofxandycausesaYAG:Ceemissionshiftbetween510and570
674C.R.Rondaetal./JournalofAlloysandCompounds275–277(1998)669–676Fig.10.EmissionspectrumofawhiteLEDconsistingofablueLEDandyellowemittingYAG:Ce.
nm.Thisallowstheadjustmentofwhitecolourtempera-turesfrom8000downto3000K.Theformer,veryhighcolourtemperaturehasbeenchosenforcommercializationintheJapanesemarketwhichdemandsthiscolourquality.DuetothebroadYAG:CeemissiontheCRIishigh,reachingCRI585.Finally,theluminousefficiencyis5lm/W9.ThisisalreadyhalfofthatofanincandescentlampandshowsclearlythepotentialoftheblueLEDanditspossibleimportanceforthefuturelightingmarket.Alllow-powerwhitelightsourceswhichdemandlonglifetimecouldpossiblybereplacedbywhiteLEDsinthemediumterm.However,itshouldbepointedoutthattherearestillabout80suchLEDsnecessarytoreplacea10Wlightbulb,assumingamediumpowerdrivingschemeof50mAat5V.Themajordrawbackofthiscombinationisthestronglydecreasingoverallefficiencyuponloweringthecolourtemperature.ThisiscausedbythebroadYAG:Ceemissionwhich,whenshiftedtowardslongerwavelength,showslessandlessintegraloverlapwiththeeyesensitivitycurve,hencedecreasinginluminousefficacy.Thiscallsforotheroptionswhichmeansotherphosphormaterialsorcombinationsofthem.Thebasicrequirements,however,areratherstringent.Thephosphormaterialhastohaveasufficientabsorptionattheemissionwavelengthofthebluediode,thequantumyieldshouldbehighunderUV/VISexcitationandthefullwidthathalfmaximum(FWHM)oftheemissionbandshouldbeassmallaspossibleinordertoachievehighluminousoutput.31SelectedrareearthphosphorsbasedonEu31andTbfulfilthelattertwooptions;however,itcanbeshownthatthereisatheoreticallowenergylimitfortheexcitationpeakmaxi-mumwavelength.Eveniftightsensitizationschemescanbefound,i.e.31sensitizerswithenergylevelsclosetothe5DtermsofEuandTb31,theseionscannotbeexcitedwithpeakwavelengthlongerthanapproximately420and370nm,respectively.Thisisduetothedownwardenergycascadewhichisnecessarytoeffectivelysensitisetherareearthionwithoutbackenergytransfer[12],takingalsothe
StokesShiftonthesensitiserintoaccount.InorganicEu31phosphorswithhighquantumefficiencieswhichabsorbsatisfactorily3at1370nmareY2O2S:Eu31andY(V,P,B)O4:Eu.Tb31dopedmaterialcanhardlybefound,(Ce,Tb,Gd)MgB5O10(CBT),mainlyusedintubu-larfluorescentlamps,absorbsonlyto1about10%at370nm.OrganicphosphorsemployingEu3andTb31coordi-natedtolight-absorbingorganicligandssuchasacetylacetonateandphenanthrolinecanbedesignedtoabsorbefficientlyat370–400nm.However,stabilityandsaturationissueshaveyettobeimprovedforthesematerials.
DespitethesedifficultiesandthehigherquantumdeficitaLEDemittingUVlightat370nmcoveredwitharareearthphosphormixturewouldbeanattractiveoption.IncontrasttoaphosphordepositedonablueemittingLEDchipat470nmwhichonlypartlyabsorbsthebluelightinordertogeneratewhitelight,amixtureofblue,greenandredphosphorsonaUVLEDabsorbsalloftheincidentlightemittedbythechip.Thewhitelightcolourcoordi-natesaredeterminedonlybythephosphorportionsinthemixturewhichcanbeadjustedratheraccurately.Conse-quently,whiteLEDscanbeproducedwithcolourcoordi-natesvaryingonlyverylittlefromLEDtoLED.ThisisconsiderablymoredifficultforanLEDfromwhichwhitelightiscomposedfromincidentblueemissionfromthediodeandyellow/redlightemittedbythephosphor.Depositionofawell-defined2phosphorlayerontheLEDchipofsome1mminordertoachievecontrolledabsorptionmeansarealmanufacturingchallenge(Fig.11).Further,theadditionalquantumdeficitofabout15%ofacolourconversionfrom370nmvs.470nmcanbecompensatedconsiderablybytheveryhighlumenequiva-lentsofEu312andTb31-activatedphosphors.Inanalogy
Fig.11.Sketchofaphosphor-LEDlamp.
C.R.Rondaetal./JournalofAlloysandCompounds275–277(1998)669–676675
tofluorescentlamps(seeabove),highcolourrenderingscanbeobtainedatthesametime.ThesearchforstableinorganicrareearthphosphorswithhighabsorptionsinthenearUV/bluespectralregionisthereforeanattractiveresearchtask.
4.3.Luminescentmaterialswithquantumyieldlargerthanunity
In1974,twopapersappearedinthesameeditionoftheJournalofLuminescence,onthegenerationofmorethanonevisiblephotonoutofoneUVphoton[13,14].Therareearthionperforming31thequantumcuttingorquantumsplittingwasPrinYF3ashostlattice.ItisquitesurprisingthatwithUVphotonswithwavelengthofabout200nm,blueandredemission,originatingfromPr31,canbegeneratedwithaquantumefficiencyofabout140%.Theopticalabsorptionisduetoa4f→5dtransitiononPr31(inthislatticelocatedatabout185nm).Thoughabreakthrough,thismaterialhasneverbeenappliedinfluorescentlamps.Animportantreasonisthefactthatthebluelineemission,generatedbyPr31,islocatedatabout410nmandthereforenotsuitableforlampswithgoodcolour1rendering.Theredemission,alsogeneratedbythePr3inthecascadeleadingtoquantumcuttinghasverygoodspectralpropertiesforhighqualityfluorescentlamps;itislocatedatabout620nm.
Quiterecently,Srivastavaetal.reportedtheobservationofthebluelineemissionofPr31inoxidicmaterialsinwhichtherareearthioniscoordinatedbyalargenumberofoxygenions(likeinSrAl3112O19,wherethePriscoordinatedby12oxygenions)[15,16].Aprerequisitefortheobservationofblueemissionisthattheenergyofthe5dlevelishigherthanthatofthehighest4flevel,fromwhichtheblueemissionoriginates.Bychoosingahighrareearthioncoordinationnumber,themeanmetal–liganddistancebecomeslarge.Thecrystalfieldsplittingde-creasesonincreasingthemetal–ligand1distance.Asaconsequence,inSrAl12O19thePr3ionexperiencessuchaweakcrystalfieldthatindeedtheenergypositionofthelowest5dlevelishigherthanthatofthehighest4flevel,asituationuntilthenonlyobservedinfluorides.Again,thiscanberegardedasabreakthrough,howeverthequestionconcerningthebluelineemissionat410nmisnotsolved.Apossiblesolutionistheuseofadditionalphosphorsshowingastrongabsorptionintheblue-UVpartoftheopticalspectrumandwhichemitlightwithlongerwave-length,inthesamemannerasdiscussedabove.Inthisway,alsotheverydeepbluePr31emissioncouldbeutilized.
In1997,Meijerinketal.publishedaluminescentmaterial(LiGdF4:Eu)withaquantumefficiencyofalmost200%;31theemissionoriginatingalmostexclusivelyfromEu[17].ExcitationwithVUVradiationisontheGd31ion,usingradiationwithawavelengthofabout160nm.Inthefirststep,Eu31ionsareexcitedviacrossrelaxation
Fig.12.ProcessesleadingtoquantumcuttinginLiGdF4:Eu.
betweenGd31andEu31.(Weremarkthattheauthorsalsoobservedorange–redemission,originatingfromGd31inthiscompoundwhennotdopedwithEu31[18]).Inthesecond31step,Eu31ionsareexcitedviaenergytransferfromGd→Eu31,seeFig.12.Thisisthefirstmaterialshowingsuchahighquantumefficiencyonexcitationwithradiationwithwavelengthlongerthan100nm.
5.Conclusions
Inmanyapplications,theperformanceofrareearthphosphorsisalmostidealandconsequentlytheuseofrareearthphosphorshaspushedtheperformanceofdevicesbasedonthemtotheirphysicallimits.NewimpetusforresearchonphosphorsisexpectedfromnewapplicationslikeluminescentmaterialsappliedinLEDsorphosphorsshowingnewfeaturese.g.exhibitingquantumefficiencieslargerthanunity.
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