thin ﬁlms ZnO Co doping structural induced and optical properties of sol–gelprepared Applied Surface Science

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ContentslistsavailableatScienceDirect

Applied Surface Science

j o ur na l ho me pa g e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Co doping induced structural and optical properties of sol–gel prepared ZnO thin ﬁlms

Ebru Gungor

^a,∗

, Tayyar Gungor

^a

, Deniz Caliskan

^b

, Abdullah Ceylan

^c

, Ekmel Ozbay

^b

aEnergySystemsEngineeringDepartment,MehmetAkifErsoyUniversity,Burdur15030,Turkey

bNanotechnologyResearchCenter,BilkentUniversity,Ankara06800,Turkey

cSNTGLaboratory,PhysicsEngineeringDepartment,HacettepeUniversity,Ankara06800,Turkey

a r t i c l e i n f o

Articlehistory:

Received15November2013 Receivedinrevisedform14June2014 Accepted20June2014

Availableonline27June2014

Keywords:

ZnO Co:ZnO Thinﬁlm

Ultrasonicspraypyrolysis

a b s t r a c t

ThepreparationconditionsforCodopingprocessintotheZnOstructurewerestudiedbytheultrasonic spraypyrolysistechnique.StructuralandopticalpropertiesoftheCo:ZnOthinfilmsasafunctionofCo concentrationswereexamined.ItwasobservedthathexagonalwurtzitestructureofZnOisdominantup tothecriticalvalue,andafterthevalue,thecubicstructuralphaseofthecobaltoxideappearsintheX-ray diffractionpatterns.Everyband-edgeofCo:ZnOfilmsshiftstothelowerenergiesandallareconfirmed withthePLmeasurements.CosubstitutioninZnOlatticehasbeenprovedbytheopticaltransmittance measurementwhichisobservedasthelossoftransmissionappearinginspecificregionduetoCo²⁺

characteristictransitions.

1. Introduction

Zincoxide(ZnO)is a II–VIcompound semiconductor witha widedirectbandgapof3.37eVatroomtemperature.Inaddition totheelectricalandopticalpropertiesofundopedZnO,thetran- sitionmetaldopedZnOformsarepromisingcandidatematerials intheﬁeldofspintronics(spin-electronics).Variousmethodssuch aspulsedlaserdeposition[1],chemicalvaportransport[2],elec- trodeposition[3],co-precipitation[4]andsolid-statereaction[5], andspraypyrolysis[6]canbeusedtosynthesizeZnO.Amongthese methods,spraypyrolysistechniquecanbeapplicablewithoutvac- uumenvironment;ofcourse,thistechniqueischeapanddisplaying comparablepropertiesandcompetitivefunctionalitywiththatpro- ducedbyothertechniques.Theresearchhasbeenincreasedon theternarysemiconductorssuchastransitionmetaldopedZnO owingtoitshighCurietemperaturefortheferromagnetictran- sitioncalculatedinbulkmaterialsandfoundtobearound300K [7–10].

CodopingcreatesaconsiderablechangeinthebandgapofZnO [11–14],butthisvariationhasbeenreportedasanincreaseinsome otherresearchandasadecreaseinthebandgapforZnOinother research.Thiscaseindicatestheuncertainty.Areasonforthiscan bestructuraldefectsintheZnOcrystallatticeaswellasbecause

∗ Correspondingauthor.

E-mailaddress:egungor@mehmetakif.edu.tr(E.Gungor).

ofthevacanciesinthecrystalstructureorinterstitials[15].The uncontrolledcasesasthestructuraldefectsand/orimpuritiesthat ariseasgrowingthefilmaffectsthebondingnature,chargetransfer andthebandstructureinthematerial.Thismakesitverydifficult toobtainreproducibledeviceperformanceand reliability.Some authorsreportedintheliteraturethatred-shiftisattributedtothe sp–dexchangeandsomeotherauthorsobservedthatblue-shiftis attributedtotheBurnstein–MosseffectconsideringtheCocon- centration.Whenthevolumesolubilitylimit intwo-component andmulticomponentalloyshasreachedacertainconcentration, thefirstphaseremainsconstantandthentheextraphasesappear.

Inordertodeterminethesolubilitylimit, onehastofollow the changeofthelatticespacingandconcentrationobtainedfromthe X-raydiffraction(XRD)andSEM-EDSspectrum,respectively.There isuncertaintyaboutsolubilitylimitforCodopedZnO. Leeetal.

[16]reportedthatthedopedCoionwasfullysubstitutedintoa ZnOlatticeat5mol%,butthesecondaryphaseoftheCo₃O₄ was formedabove5mol%ofCodoping.However,otherreports[17,18]

haveindicatedthatCocanbeincorporatedinthematrixofZnOup to7–10at%withoutforminganysecondphase.Allthesereports thusindicatethatCohasalimitedsolubilityinZnOupto10at%.In contrasttothis,Rath[19]observedthatallthepeaksmatchwell withthewurtzitestructureofZnOinbothpureandCodopedZnO samplesuptoacobaltconcentrationof20%.

Inthisstudy,weaimedtocontributetoclarifytheuncertainty forbandgapshift.Thatiswhy,weinvestigatethestructuraland opticalpropertiesofZnOandCo:ZnO(CZO)thinﬁlmsthatwere http://dx.doi.org/10.1016/j.apsusc.2014.06.132

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ZnOandCo:ZnO(CZO)thinfilmsweredepositedontoultrason- icallycleanedglasssubstratesusingtheultrasonicspraypyrolysis (USP)method.Forconventional USPmethod,thesubstratesare fixedandprecursorsolutionsprayedoverahotsubstrate.Thesub- stratetemperaturewaskeptat400^◦C. Thesaltsofzinc acetate dehydrate(Zn(CH3COO)2·2H2O,99.9%-Merck)andcobaltacetate tetrahydrate(Co(CH₃COO)₂·4H2O,99.9%-Merck)wereusedasthe metalsourceswhich weresolvedinmethanol.In ordertopro- duceaclearandhomogeneoussolutionmonoethanolamine(MEA) andaceticacidwereaddedintotheprecursorsolutionwhichwas stirredat60^◦Catamoderatespeedfor1h.Inthestartingsolu- tions,Cocontentswerechangedfrom0.01Mto0.05Mandsamples werelabeledasCZO1–CZO5(Table1).Zncontent(0.05M)washeld atconstant.Thesolutionflowratewasheldconstantat5ml/min.

Nozzle,100kHzoscillatorfrequency,usedinthisstudywasina downwardverticalconfigurationandthenozzletosubstratedis- tancewas12cm.Compressedairwasusedasthecarriergas.The filmsweredepositedforabout10min.Amoredetaileddescription ofthemethodtoobtainthethinfilmsandthecharacteristicsof thespraypyrolysisdeviceusedwerereportedinpreviouspaper [6].X-raydiffraction(XRD)spectrawerecollectedwithaD-MaxX- raydiffractometer(RigakuInternationalCorp.,Japan)withCuK␣ (=1.5405 ´˚A)toobtainthestructuralinformationofthefilms.The chemicalcompositionofthethinfilmswasmeasuredusingenergy dispersivespectroscopy(EDS)withaJeolJSM-7000F-EDSelectron microscope.TheopticalmeasurementsoftheCo:ZnOthinfilms werecarriedoutatroomtemperatureusingT70ModelSpectropho- tometer(PG Instrument)in thewavelength range 300–900nm.

Photoluminescence(PL)spectraweremeasuredusinga100mW He-Cdlaser (=325nm)astheexcitationsourceanda HORIBA Jobin-Yvon1mmonochromator.

3. Resultsanddiscussion

3.1. Structuralproperties

TheX-raypatternsforCo:ZnOthinﬁlmsatroomtemperature andreferencepeakpositionsarepresentedinFig.1a.Theresults pointedoutshowedthatthere isnoimpurityand/orunreacted phaseofZnandCoconsideringthereferencepeakpositions,(100), (002),(101),and(103)peaksofZnOwereobserved.However, (111)and(200)peaksat36.9^◦ and42.7^◦ Braggangleofcobalt oxide,respectively, wereobserved. The starting molarity upto 0.03MofprecursorsolutionincludedCo,(002)peakofhexagonal wurtzitestructurebecomesmoreintensivecomparingwithother peaks(Fig.1b).Intheseﬁlms,upto0.03M,thehexagonalwurtzite structureofZnOseemstobeprotected.Inthedopingprocess,itis observedthatthereisalimitationofCodopingintotheZnOstruc- ture.Themolarityisthengreaterand/orequaltothe0.03Mvalue,

Fig.1. X-raydiffractionpatternsofCo:ZnOthinﬁlms(a),andintensitydifferences of(002)and(200)peaksfortheﬁlms(b).“|”and“”symbolsindicatethereference forZnO(JCPDS36-1451)andforCoO(JCPDS43-1004),respectively.Thevariation oftheCoconcentrationobtainedfromEDSwithstartingsolutionmolarity(c).

(200)peakwhichbelongstothecubicstructureofcobaltoxide whichthenstartstooccur.ThislimitvalueisconﬁrmedusingEDS measurements.Table1summarizestherelativechemicalcontent oftheoxygen,zincandcobaltpresentintheﬁlmsasafunctionof contentofthecobaltacetatetetrahydrateinsertedinthestarting solution.WeobservedthattheCosubstitutedZnsiteupto12%

(Fig.1c)whichcorrespondedto0.03Mwithoutshowinganyextra phaseinXRDspectra.Peakscorrespondingtotheglasssubstrate elementssuchasSiandCawerealsodetected.The(002)peakindi- catingastrongorientationalongthec-axisofZnOwithhexagonal wurtzitestructureisreplacedby(200)orientationwiththecubic structureasComolarityincreases.Thepeakpositionof(002)shifts

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Fig.2.StructuralparametersofCo:ZnOthinﬁlmsaccordingtotheXRDresultsvs Comolarity;forc-latticeparameter(“䊉”symbol)anddiffractionangle2for(002) peak(“”symbol).

Fig.3.NormalizedexperimentalopticaltransmissionspectraforCo:ZnOthinﬁlms accordingtotransmissionvalueat800nm.

tothehigherBraggangle,andalsothecalculatedc-latticeparame- terdecreasesduetotheincreasingofCoconcentrationintheZnO structure(Fig.2).Buttheintensityof(200)peakdecreaseswhen CoisdopedintotheZnOstructurewiththemolarityvalueofmore than0.04M.

3.2. Opticalproperties

TheopticaltransmissionspectraofZnOandCodopedZnOthin ﬁlmsamplesareshowninFig.3.TheeffectsofCodopingintothe ZnOlatticeareclearlyobservedintheopticaltransmissionspectra.

TheincreaseofCoconcentrationintheZnOstructuredecreasesthe opticaltransmittanceandimpartsdeepgreencolortothesamples (Fig.4).Inaddition,increasingCoconcentrationalsomodifiesopti- caltransmittanceforthespecificregionduetoCo²⁺characteristic transitions.Thesetransitionsalsosuppresstheinterferencefringes inthisregionofCo:ZnOfilmsifthefilmthicknessissufficientto createinterferencefringe.In ordertoeliminatetheinfluenceof differencesinsamplethickness,normalizedtransmittancetothe valueat800nmweretakenintoaccount.Theabsorptionpeaks, indicated with arrows in Fig. 3, centered at 571nm (2.18eV), 619nm(2.01eV)and 662nm(1.88eV),arerelatedtocharacter- isticfeaturesofd–dtransitionofCo²⁺ions.Theyareassignedto transition from⁴A₂(F)state to²E(G), ⁴T₁(P) and ²A₁(G) states, respectively[15].Thisisaclearevidencetoprovetheexistenceof

Fig.4.Experimentalopticaltransmissionspectra(solidline)of0.02MCocontent instartingsolutionforCo:ZnOthinﬁlm:Alsothetheoreticalopticaltransmission spectra(“o”symbol)areshownforcomparison.

Table2

Calculatedﬁlmthicknesst(nm),refractiveindexnfor532nmwavelengthandEg

(eV)opticalbandgapvaluesoftheCo:ZnO(CZO)thinﬁlmsvsthemolarityofCoin thestartingsolutions.

Samplename MolarityofCo(M) t(nm) n(532nm) Eg(eV)

ZnO 0.00 75±2 1.73 3.330

CZO1 0.01 105±2 2.23 3.072

CZO2 0.02 95±2 2.42 3.061

CZO3 0.03 160±2 2.73 2.985

CZO4 0.04 170±2 2.63 3.020

CZO5 0.05 180±2 2.59 3.030

CoatthetetrahedralsitesoftheZnOhexagonalwurtzitestructure asCo²⁺.ComparingtheionicradiiofCo²⁺(0.058nm)whichisvery closetoionicradii ofZn²⁺(0.060nm)andtheabsorptionpeaks wecanconcludethattheCoatomicallysubstitutesonZnsites.

Thisisalsoconﬁrmedinmanygroups,whichincludedavariety ofmethodsandopticalabsorption[15,18,20–22].

Theopticaltransmissionspectrumcanbeusedinthedetermi- nationoftheopticalconstantsofthethinfilmdepositedontothe transparentsubstrate.Whentheproductoftherefractiveindex andfilmthicknessofthefilmhaveallowedtheformationofinter- ference fringes,classicalmethods suchas theenvelope method developedbySwanepoel [23]canbeused.Aswellasthenum- beroftheinterferencefringesanddepthofthefringesarecrucial toperformthismethod.However,PointwiseUnconstrainedMini- mizationAlgorithm(PUMA)[24]andmanyotheriterativemethods [25]canbeusedwhentheinterferencefringesobservedornot observedin thetransmission spectrum.Consideringthenormal dispersionrelation,therefractiveindexdecreaseswiththeincreas- ingwavelength.ThisisnotvalidfortheCo:ZnOthinfilmsforthe region where theloss of transmissiondue to Co²⁺ characteris- tictransitionswhichmodulatethetransmittancespectrumofour samplesisobserved.Therefore,weusedPUMAtechniqueforthis limitedregionwhichstartsfromtheinflexionwavelengthtothe wavelengthcorrespondingfirstcharacteristictransitionwhichis centeredat571nm(2.18eV).Inflexionwavelengthisdefinedfrom secondderivativeofopticaltransmissioncurve[6,26].Thereisan excellentagreementbetweentheexperimentalspectraandtheo- reticalspectraforallthesamplesandoneoftheexperimentaland computedopticaltransmissionspectrafortheCo:ZnOthinfilmare showninFig.4.Calculatedfilmthicknessandrefractiveindexfor 532nmaregiveninTable2.Thevalueofrefractiveindexispass- ingthroughamaximumconsideringtheCoconcentrationsuchas

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Fig.5. OpticalabsorptionspectraoftheCo:ZnOandZnOthinﬁlmsas(˛h)²−h

behavior.

0.03M.Then,therefractiveindexstartstodecreaseagain.Having suchaturningpointisinagreementwiththeotherobservations suchasXRDandPLmeasurements.

Notonlythefilmthicknessandrefractiveindexbutalsooptical absorptionspectrumcanbeobtainedfromtheopticaltransmission spectrumusingsuitablemethods.Absorptionedgesforthesemi- conductorsinformthethresholdofchargetransitionbetweenthe highestfilledbandandthelowestemptyband.Theopticalbandgap ofthefilmscanbecalculatedusingthefollowingequation[27]:

˛hv₌_A(hv₋_Eg)ⁿ (1)

whereAistheprobabilityparameterforthetransition,Eg isthe bandgapofthematerial,histheincidentphoton energy,and nisthetransitioncoefﬁcient.Thevalueofnisknown:,2forthe measurementofanindirectbandgapand1/2foradirectbandgap.

Fig.5a–fshowstheplotof(˛h)²vsthephotonenergy(h)of theCo:ZnOthinﬁlms.Thedirectbandgapoftheﬁlmswasdeter- minedbytakingtheintersectionoftheextrapolatedlinesfromthe linearverticalandhorizontalregionsneartheband-edgeofthe (˛h)²=0curve.AstheCoconcentrationisincreasedintheZnO

Fig.6. PLspectraofCo:ZnOandZnOthinﬁlmsatroomtemperature.

Table3

Photoluminescence(PL)analysisfromGaussianﬁttingprocesswithtwoGaussian peakscenteredat1and2forsamples,andcomputedopticalbandgap(Eg).

MolarityofCo(M) 1(nm) 2(nm) Eg(eV)

0.00 379.65 – 3.26

0.01 390.95 407.41 3.17

0.02 397.64 415.20 3.12

0.03 401.87 417.93 3.09

0.04 389.01 406.93 3.18

0.05 396.18 412.80 3.13

structure,redshiftisobservedintheabsorptionedgeduetothe sp-dexchangeinteractions betweenthebandelectronsandthe localizedd-electronsoftheCo²⁺ionssubstitutingZn²⁺ions.Cal- culatedvalueoftheopticalbandgapwithincreasingComolarity from0to0.05MisgiveninTable1.

Fig. 6 shows the PL spectra of as-grown samples. Photolu- minescencespectraofthesampleshavebeenrecordedatroom temperature.ThePLemissionintheUVbandswasobserved.Band- edgetransitionsaswellasdirect-bandtransitionsforZnOataround 380nm(3.26eV)areobserved.Gaussianﬁttingwasperformedon thePLspectraofthesamplescontainingCo.Amongthetwopeaks oneiscenteredaround400nm(1)assignedtothebandgaptran- sitionandtheotherpeaksassignedtothenear-band-edge(NBE) emission are centered at 407nm, 415nm, 418nm,407nm and 413nmforthesamplesCZ01,CZ02,CZ03,CZ04andCZ05,respectively(Table3).Bandgapvaluesaredecreasinguptoathreshold valueofCoconcentration.Thisbehaviorisalsoobservedinthe refractiveindexvariation.Inaddition,theobservedPLintensity begantodecreasewhenthecobaltwasintroducedintotheZnO structure.

4. Conclusion

ThestructureandopticalpropertiesofCodopedZnOﬁlmswere studiedwithrespecttocobaltconcentrationinthestartingsolution inwhichthecobaltacetatetetrahydratewasusedasaCosource.

OurstudiesshowthatCodopingaffectsZnOlatticeimmediately.

WhentheComolarityisgreaterthanthe0.03MorCoconcentra- tionobtainedfromEDSanalysisisabout12%,theZnO(002)peak intensitydecreasesandCoO(200)peakintensityincreaseswith increasingCoconcentration.CosubstitutioninZnOlatticehasbeen provedbytheopticaltransmittancemeasurement,whereasitis notclearlyseenintheXRDdiffractogramfortheCZO1andCZO2 sample.The opticaltransmittance decreaseswithincreasing Co concentration.ZnOiswhitecoloredbecauseitdoesnotabsorbany

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visiblelight.WhentheZn²⁺ionsarereplacedwithCo²⁺ionsinthe ZnOlattice,thefilmsabsorbvisiblelight,andthecolorofthefilms turntodeepgreen.ThecharacterofthebandgapoftheCZOfilmsis directtypewithcobaltdoping.Whenthecobaltdopingisincreased, Taucplotsarebecomingamoreroundedinshapeandtheinflexion pointmovestothelongerwavelengths.Thebandgapobtainedfrom Tauc’splotshiftingtowardtothelongerwavelengthswasverified withroomtemperaturePLmeasurement.Thebandgapnarrowing canbeattributedthattheconductionbandandthevalenceband shifteddownwardandupward,respectively.Weconcludedthat thered-shiftistypicallyattributedtothesp–dexchangebetween theZnObandelectronsandlocalizedd-electronsassociatedwith thedopedCo²⁺cations.

Acknowledgement

ThisstudywassupportedbyThe ScientiﬁcResearch Unitof MehmetAkifErsoyUniversitywithprojectnumbers110-NAP-10, 0172-NAP-13,and0173-NAP-13.

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