Optical properties of LFZ grown β-Ga2O3:Eu3+ fibres

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AppliedSurfaceSciencexxx (2011) xxx–xxx

ContentslistsavailableatScienceDirect

Applied

Surface

Science

jo u rn a l h om epa g e :w w w . e l s e v i e r . c o m / l o ca t e / a p s u s c

Optical

properties

of

LFZ

grown

␤-Ga

2 O

3 :Eu

3+

ﬁbres

N.F.

Santos

a

_,

_J.

_Rodrigues

a

_,

_A.J.S.

_Fernandes

a

_,

_L.C.

_Alves

b

_,

_E.

_Alves

b

_,

_F.M.

_Costa

a

_,

_T.

_Monteiro

a,∗

a_Departamento_de_Física,_I3N,_Universidade_de_Aveiro,_Campus_{Universitário}_de_Santiago,_3810-193_Aveiro,_Portugal b_Instituto_Tecnológico_e_Nuclear,_2686-953_Sacavém,_Portugal

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received29April2011

Receivedinrevisedform1July2011 Accepted14July2011 Available online xxx Keywords: LFZ Ga2O3 Eu PL PLE

a

b

s

t

r

a

c

t

Duetotheirrelevanceforelectronicandoptoelectronicapplications,transparentconductiveoxides(TCO)

havebeenextensivelystudiedinthelastdecades.Amongthem,monoclinic␤-Ga2O3iswellknownbyits

largedirectbandgapof∼4.9eVbeingconsideredadeepUVTCOsuitableforoperationinshortwavelength

optoelectronicdevices.Thewidebandgapof␤-Ga2O3isalsoappropriatefortheincorporationofseveral

electronicenergylevelssuchasthoseassociatedwiththeintra-4fn_{conﬁguration}_of_rare_earth_ions._Among

these,Eu3+_ions_(4f6₎_are_widely_used_as_a_red_emitting_probes_both_in_organic_and_inorganic_compounds._In

thiswork,undopedandEu2O3doped(0.1and3.0mol%)Ga2O3crystallineﬁbresweregrownbythelaser

ﬂoatingzoneapproach.Allﬁbreswerefoundtostabilizeinthemonoclinic␤-Ga2O3structurewhilefor

theheavilydopedﬁbrestheX-raydiffractionpatternsshow,inadditionacubiceuropiumgalliumgarnet

phase,Eu3Ga5O12.ThespectroscopicpropertiesoftheundopedandEudopedﬁbreswereanalysedby

Ramanspectroscopy,lowtemperaturephotoluminescence(PL)andphotoluminescenceexcitation(PLE).

TheEu3+_luminescence_is_mainly_originated_in_the_garnet,_from_where_different_europium_site_locations

canbeinferred.ThespectralanalysisindicatesthatatleastoneofthecentrescorrespondstoEu3+_ions

indodecahedralsitesymmetry.Forthelightlydopedsamples,thespectralshapeandintensityratioof

the5_D

0→7FJtransitionsistotallydifferentfromthoseonEu3Ga5O12,suggestingthattheemittingions

areplacedinlowsymmetrysitesinthe␤-Ga2O3host.

1. Introduction

␤-Ga2O3isawellknownwidebandgapsemiconductorwith

reportedbandgapenergyatroomtemperature(RT)near4.9eV [1–4].TheRTthermodynamicallystablemonocliniccrystalhascell dimensionsa=1.223nm,b=0.304nm,c=0.580nmandˇ=103.7◦ [3,4].The materialbelongs toa C2/m space groupand theGa3+

ionsmayoccupytetrahedraloroctahedralcrystallographicsites [4,5].Thematerialexhibitsisolatorbehaviourorn-type conductiv-itygenerallyattributedtoanionoxygenvacancies,theresistivity beingstronglydependentontheatmospheregrowthconditions [6].However,thisbecame controversialsincerecently theoreti-cal studies[7] pointed that oxygenvacanciesgiverise todeep donors,withactivationenergiesaround1.0eV,andsocannotbe responsiblefortheunintentionaln-typeconductivityobservedin thisoxide.Duetotheabove-mentionedelectricalconductivityand hightransparency,␤-Ga2O3hasbeenfrequentlyexploitedfor

sev-eralelectronicandoptoelectronicapplicationsincludingelectrodes asatransparentconductiveoxide(TCO)[3,4].Forsuchpurposes, information aboutthe opticallyactivedefects and theirenergy

∗ Correspondingauthor.Tel.:+351234370824;fax:+351234378197. E-mailaddress:tita@ua.pt(T.Monteiro).

distributioninsidethewidebandgaphostconstitutesone impor-tantissue.Underabovebandgapexcitation,ultraviolet,blueand greenbroadluminescencebandspeakednear3.4,2.95and2.48eV havebeenreportedinundopedanddoped␤-Ga2O3 single

crys-tals[8–10].Theultravioletemissionhasbeenfoundtobesample independent,whiledeeplevelrecombinationhasbeenfoundto bedependentonthechemicalnatureoftheimpuritiesandtheir content[10].Particularly,theultravioletrecombinationhasbeen attributedtointrinsicluminescenceandthebluebandhasbeen assignedtoadonor–acceptorgallium–oxygenvacancypair recom-bination[9,10].

Furthermore,thewidebandgapof␤-Ga2O3makesthisTCOa

suitablehostforphosphorapplications[11–22].Asanexample, highluminancewasobtainedinthin-ﬁlmelectroluminescent dis-playswhengallium oxideis activatedwitheuropiumions[12]. However,due topoorcrystallization ofthegallium oxideﬁlms [13–15],therareearthionemissionlinesareverybroadand lit-tle isknown aboutthemechanisms behindthe intraionic Eu3+

luminescenceinthecrystallineGa2O3environment.Morerecently,

nanostructured␤-Ga2O3hasbeensynthesizedbydifferentroutes

andintentionallyactivatedwithrareearthionsfornanophosphor applications[17–22].Asforthethinﬁlms[13–15],largefullwidth athalfmaximumofthemain5_D

0,1→7FJtransitionsoftheEu3+ions

inthenanopowders[18,20]contrastwiththeexpectedsharplines

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Fig.1.Typicalvisualappearanceofthe(a)(top)undopedand(bottom)0.1mol%europiumdopedfibresgrownat30mmh−1_._SEM_micrographs_of_the_(b)_undoped_fibre_and europiumdopedfibreswith(c)0.1mol%and(d)3mol%grownat30mmh−1.

fortheemittingionsincrystallineenvironments,fromwherethe identiﬁcationofthenumberofemittingEucentresandsite symme-trycouldbeexploredviatheStarklevelsplittingduetothecrystal ﬁeldeffect.AlthoughtheEu3+_emission_has_been_reported_for_thin

ﬁlmsandnanopowders[11–22],informationontherareearthion spectroscopicpropertiesinbulk␤-Ga2O3remainsscarceand

con-stitutesanimportantissuetoclarifytheionpropertiesinsidethis TCOmaterial.

Inthepresentwork,undopedandEudoped␤-Ga2O3ﬁbreswere

grownbylaserﬂoatingzone(LFZ).Thesampleswerecharacterized inwhatconcernstheirmorphologyandstructure(SEM/EDS,PIXE, XRD,Ramanspectroscopy),andtheirresultinglowtemperature optical properties (photoluminescence and photoluminescence excitation).Theobservedopticalcentresarediscussedwithrespect totheirpossiblephaseoriginandsitesymmetry.

2. Experimentaldetails

The␤-Ga2O3crystallineﬁbresweregrownbytheLFZtechnique,

usingrodprecursorsfor bothfeedandseedmaterialsprepared bycoldextrusion.Theserodswereobtainedbyusinggallium(III) oxide(AlfaAesar)powdersfortheundopedsamplesandadding 0.1%and3.0mol%Eu2O3(Aldrich)forthedopedones.The

pow-dersweremixedwithpolyvinylalcohol(PVA0.1g/ml,Merck)in ordertoobtainaslurrythatwasfurtherextrudedintocylindrical rodswithdiametersof1.75mm.

TheLFZequipment comprises a 200W CO2 laser (Spectron)

coupledtoareflectiveopticalset-upproducingacircular crown-shapedlaserbeaminordertoobtainafloatingzoneconfiguration witha uniformradialheating.Crystallinefibres withdiameters of 1–2mm were grown at 10 and 30mmh−1 in air at atmo-spheric pressure. Fibre microstructure and phase development werecharacterizedbySEM(HitachiS4100)withenergy disper-sivespectroscopy(SEM/EDS)onpolishedsurfacesoflongitudinal fibresections.Additionally,inordertoobtaininformationonthe europiumdistributioninthefibres,particle-inducedX-ray

emis-sion(PIXE)wasperformedusinga2.0MeV1_H_micro_beam_and_a

10-mm2 _Si_(Li)_detector_with_a_resolution_of₁₄₅_eV_and_a_5-mm

Bewindow. Thestructuralcharacterization wasmadebyX-ray diffraction(XRD)experiments(PANalyticalX’PertPRO)andRaman spectroscopy.Thelatterwereperformedatroomtemperature(RT) inbackscatteringconﬁgurationwithultravioletexcitationbyusing the325nmlineofacwHe–Cdlaser(KimmonIKSeries)inaHoriba Jobin Yvon HR800systemand the532nmline froma Ventus-LP-50085(MaterialLaserQuantum)laserinaJobinYvonT64000 instrument.

SteadystatePLmeasurementswerecarriedoutat14Kusing a1000WXearclampcoupledtoamonochromatorasexcitation source.TheluminescencewasdispersedbyaSpex1704 monochro-mator(1m, 1200mm−1)and detectedby a cooledHamamatsu R928photomultiplier.For thePLexcitation(PLE)measurements theemissionmonochromatorwassetattheEu3+_emission_lines,

theexcitationwavelengthhavingbeenscannedupto240nm.The spectrawerecorrectedtothelampandoptics.

3. Resultsanddiscussion

The visual appearance of the undoped and Eu doped gal-lium oxidefibres are shown in Fig.1a. The undopedfibre was foundtobe transparentwhile the Eu dopedsamples exhibit a grey colour.The morphologicalanalysis ofthefibres longitudi-nalsections(Fig.1b–d)reveals,fortheundopedfibre,anuniform surfacewithoutgrainboundariesorsecondphases(Fig.1b) regard-less thepulling rate(10and 30mmh−1).In opposition,doping inducespolycrystallinenatureinbothlightly(0.1mol%)and heav-ily(3.0mol%)dopedsamples.However,theheavilydopedfibres showevidenceofasecondphaseplacedatthegrainboundaries (lightgreycontrast),whilethelightlydopedfibrespresent poly-crystallinemorphologywithgooduniformityevidencinghowever dispersedprecipitates.TheSEMimagesofdopedfibres(Fig.1d) showevidentgrainboundaryfeaturesalignedwiththefibreaxis asaresultofdirectionalsolidificationcharacteristicofthelaser

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Fig.2.SEMmicrographs(a,c)andEDSmaps(b,d)oftheﬁbresgrownat30mmh−1_lightly_doped_(a,_b)_and_heavily_doped_(c,_d).

floatingzonegrowthtechnique.Theadditionalcontrastedregions inheavilydopedfibresareduetolocalEurichercrystallinephases ascorroboratedbyPIXE(notshown),EDS(Fig.2),XRD(Fig.3)and Raman(Fig.4)analysis.ThepeakpositionsoftheXRDpatternsof undopedandlightlydopedfibresaftermilledintopowdersarein goodagreementwiththeexpectedreflectionsfromthemonoclinic ␤-Ga2O3crystallinestructure.Fortheheavilydopedfibres,besides

the_␤-Ga2O3crystallinestructure,anadditionalcrystallinephase

wasdetectedcorrespondingtothepresenceofeuropiumgallium garnet,Eu3Ga5O12.Thisadditionalcrystallinephasecorresponds

totheclearregionsinSEMwithhighereuropiumcontent.Since thisphasecouldnotbedetectedinthelightlydopedsamples,one caninferthattheamountofEuintheseﬁbresisstillscarceto pro-motethedevelopmentofthegarnetphase.Althougheuropiumrich precipitateswereobservedbySEM/EDS(Fig.2aandb)mostofthe europiumionsremaindispersedinthegalliumoxidematrix.

The Raman active modes were observed for the LFZ ﬁbres, eitherwithvisible(Fig.4a andb)and ultraviolet(Fig.4c) exci-tation.ThemeasuredvibrationalfrequenciesindicatedinTable1

Fig.3.XRDpatternsfortheundopedanddoped(0.1and3mol%Eu2O3)ﬁbres grownat30mmh−1_.

forboththeundopedandlightlydopedﬁbres,correspondwellto theonesof the␤-Ga2O3 host[23,24].TheobservedRaman

fre-quenciesareclassiﬁedinthreegroupsofvibrationsrelatedwith librationsandtranslations(lowfrequencymodesupto∼200cm−1) of tetrahedra–octahedra chains, vibrations of deformed Ga2O6

octahedra (mid frequency modes within 310–480cm−1) and stretching/bendingoftheGa2O4tethaedra(highfrequencymodes:

550–770cm−1)[23,24]. TheRamanspectraoftheundopedand lightlydoped fibresare comparable as shownin spectra1 and 2, respectively (Fig.4a).For theheavily dopedfibre (spectrum 3),additionalmodesat 261cm−1 and 279cm−1 wereidentified correspondingtotheeuropiumgalliumgarnetphaseEu3Ga5O12,

depictedintheexpandedregionobtainedwithvisibleexcitation (Fig.4b)[25].TheaboveinformationissummarizedinTable1.The visibleRamanspectrawereacquiredwiththesamplesoriented bothparallelyandnormallytothepolarizationplaneofthelaser

100 200 300 400 3) 2) 1) 346 320 199 168 144 111; 114 a) Raman shift (cm-1) λ = 532 nm; RT 400 600 800 3) 1) Intensity (arb. units) c) λ = 325 nm; RT 764 658 629 475 416 346 320 260 280 300 3) 2) 1) b) 279 261

Fig.4.(a)Ramanspectraobtainedwithvisibleexcitation(532nm)fortheundoped, lightlydopedandheavilydopedﬁbres,spectrum1,2and3,respectively.Thedashed lineinspectrum3correspondstodifferentsampleorientation(parallelandnormal totheplaneofpolarization).(b)ExpandedregionoftheRamanspectraobtained withvisibleexcitation.(c)UV(325nm)Ramanspectraforthesamesetofsamples.

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Table1

RamanfrequenciesofundopedandEu-doped␤-Ga2O3fibres.TheadditionalmodesdetectedfortheEu-dopedfibresareidentifiedbyanasterisk(*).

Presentwork(cm−1₎ _Ga₂_O₃ _Eu₃_Ga₅_O₁₂

Dohyetal.[23](cm−1₎ _Rao_et_al._[24]_(cm−1₎ _Mode_assignments_[23] _Middleton_et_al._[25]_(cm−1₎ _Mode_assignments_[25]

111 111 Ag 114(*) 114 Bg 113 Eg 144 147 144 Bg 144 Eg 159 T2g 168 169 169 Ag 170 T2g 180 T2g 199 199 200 Ag 233 T2g 239 T2g 261(*) 262 Eg 279(*) 273 T2g 291 Eg 309 T2g 320 318 317 Ag 346 346 344 Ag 346 A1g 353 Bg 367 T2g 407 T2g 416 415 416 Ag 415 Eg 475 475 472 Ag,Bg 507 T2g 523 A1g 580 T2g 595 T2g 629 628 629 Ag 651 Bg 658 657 654 Ag 674 Eg 736 A1g 764 763 767 Ag

incidence.Thechangesintheintensityratiosofthevibrational fre-quenciesduetolocalpolarizationeffects(dashedlinedspectra3) indicateastronganisotropy,conﬁrmingthetexturednatureofthe ﬁbres[23].

Fig.5showsthelowtemperatureluminescencespectraofthe undopedandEudopedﬁbrestakenat14K.Excitingtheundoped samplewithphotonsof260nm(4.77eV),theemissionis domi-natedbyabroadultravioletbandpeakedat377nmoverlapped byminorluminescencebandsnear438nmand526nm(Fig.5a). This luminescence spectrum is very similar to that previously reportedbyZhangetal.[26]in␤-Ga2O3singlecrystalsgrownby

thesameLFZtechnique.ForEudopedﬁbres(Fig.5b)the character-isticorange/redluminescenceduetothetransitionsbetweenthe

5_D

0and 7FJmultipletswasdetected.For heavilydopedsamples

grownat10mmh−1 (spectrum1)containingthetwocrystalline phases(_␤-Ga2O3andEu3Ga5O12),welldeﬁnedsharplines(∼1nm

offullwidthathalfmaximum)aredetectedat591nm,592nm and595nm(spectrum1)originatedfromthelevelofthe5_D

0state

andterminatingonthecrystalﬁeldsplitstatesofthe7_F

1manifold.

Thismagneticdipoleemissionisthemostintenseoneandforthe hypersensitive5_D

0→7F2transitiononlyonelinewasfoundpeaked

at∼610nm.Theheavilydopedﬁbresgrownatafasterpullingrate (30mmh−1)exhibitadistinctbehaviouroftheEu3+_luminescence

(spectrum2).Here,thesamelinespreviouslydetectedare over-lappedwiththeinhomogeneousbroaden5_D

0→7F1,2transitions

likelyduetoEu3+_ions_in_different_{environments.}_{Additionally,}_the

integratedintensityratiobetweenthe5_D

0→7F2 and 5D0→7F1

transitionsincreases,suggestingthattheeuropiumionsareplaced inlowersymmetrysitesasalsoindicatedbythepresenceofthe

5_D

0→7F0line.Theformerspectrumwellmatchestheoneobserved

fortheionsinsidethecubicEu3Ga5O12asreportedbyVanderZiel

andVanUitert[27].Inthiscase,thesplittingoftheJ=1stateinthree componentsandtheabsenceofforbidden5_D

0→7F0transitionis

inlinewiththeexpectedresultsfortheionsindodecahedralsites withD2localsymmetry[28,29].Moreover,rareearth(RE)ionsin

RE3Ga5O12hostsareknowntooccupymulti-siteswiththeREions

innon-regularsitesduetodistortionsoflocalsymmetry[27–30]. ThesimilarityoftheobtainedspectrafortheheavilydopedLFZ grownsampleseitherat10mmh−1 or30mmh−1(spectra1and 2)withthoseobtainedbyDaldossoetal.[29]fornanocrystalline Gd3Ga5O12alsodopedwitheuropiumions,suggestthatthemain

emittingeuropiumionsarelocatedintheeuropiumgalliumgarnet crystallinephase.Ontheotherhand,forthelightlydopedﬁbres,for whichnoEu3Ga5O12crystallinephasewasdetectedeitherbyXRD

andRamanspectroscopy,noticedspectralchangesoftheintraionic luminescence(spectrum3)were identiﬁed,suggesting thatthe mainemissionisoriginatedfromEu3+_ions_in_␤-Ga

2O3host.The

presenceofthe5_D

0→7F0,1,2transitionsandtheirintensityratios

indicatethat the europium ionsare placed in lower symmetry sites.Theinhomogeneousbroadeningofthelinesobservedforthis lightlydopedﬁbre,isconsistentwiththepresenceoftheEu3+_ions

inlowsymmetrysiteswithaninhomogeneousenvironment, prob-ablyduetotheﬁbrepolycrystallinestructure.

The identiﬁcation of the different Eu3+ _centres _was _further

assessedbylowtemperaturePLEandwavelengthdependentPL measurements.PLEallows identifyingtheexcitation population mechanismswhichgiverisetoeuropiumluminescenceindoped ﬁbres.ThenormalizedPLEspectramonitoredatthe5_D

0→7F2

tran-sitionareshownin Fig.5c.Thespectrawerenormalizedtothe broadultravioletexcitationband,fromwherealltheEu-centres arepreferentiallypopulated.Additionally,allthecentrescouldbe excitedviatheupperenergeticlevelsoftheEu3+_ions_which_exhibit

slightwavelengthdeviationsasexpectedfordifferenteuropium environments.Spectra2inFig.5ballowtotheidentiﬁcationthat distinctEucentrescouldbepreferentiallyexcitedwithphotons withdifferentwavelengths.

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700 650 600 550 500 450 400 350 300

Intensity (arb. units)

Wavelength (nm) a) 630 620 610 600 590 580 14K, PLE 3) c) b) 3) 2) 5 D₂ 7 F₀

→

7 F₂ 7 F₀ 7 F₁ 5 D₀

→

Normal iz e d intensi ty 14K, PL 450 400 350 300 250 2) 1) 5 H₃ 5 D₄ 5 G_J 5 L₆ 5 D 3 (x10) Wavelength (nm) 1)

Fig.5. 14KPLspectraexcitedwith260nmphotonsofthe(a)undopedﬁbregrownat 30mmh−1,(b)heavilydopedﬁbregrownat10mmh−1(spectrum1)and30mmh−1 (spectrum2,redline).Thebluelineinspectrum2wasobtainedunderresonant excitationat390nm,correspondingtoaPLEmaximumassignedtothe7_F

0→5L6 transition.Spectrum3correspondstothelightlydopedﬁbregrownat30mmh−1_. (c)14KPLEspectraofthesameﬁbresmonitoredonthe5_D

0→7F2transition.

4. Conclusions

Undoped␤-Ga2O3 LFZgrownﬁbres andintentionallydoped

with 0.1 and 3mol% of europium were produced at two dif-ferent pullingrates(10 and30mmh−1).Thestructuralanalysis using XRD and Raman measurements indicate that, regardless thepullingrate,onlythe_␤-Ga2O3crystallinephaseispresentin

theundopedandlightlydopedsamples.Forheavilydoped sam-ples, an additional Eu3Ga5O12 crystalline phase was identiﬁed.

SEM/EDSandPIXEanalysisshowthateuropiumrichregionsare presentinthelatterﬁbres,correspondingtotheeuropiumgallium garnetphase. LowtemperaturePL measurementsfor the heav-ilydopedsamplesevidencethatmostoftheemittingeuropium ions are in the garnet structure in different local site symme-tries and/or environments. Sharp Eu3+ _lines _were _assigned _to

europium ions in dodecahedral symmetry and other two cen-treswere visualizedwith theions in lowersite symmetry. On the other hand, for lightly doped samples,the intraionic lines areinhomogeneouslybroaderwithaspectralshapeofthe emit-tingionsdifferentthanthoseobservedfortheeuropiumgallium garnetphase. TheEu3+ _luminescence_from_the_ions_in _␤-Ga

2O3

hostis characterizedby the presenceof the 5_D

0→7F0,1,2

tran-sitions,suggesting that theions are placed in lower symmetry sites.

Acknowledgement

The authors acknowledge FCT for the ﬁnancial funding fromPTDC/CTM/66195/2006projectandPTDC/CTM/100756/2008 projects.

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