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

(1)

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

a_Energy_Systems_Engineering_Department,_Mehmet_Akif_Ersoy_University,_Burdur_15030,_Turkey b_{Nanotechnology}_Research_Center,_Bilkent_University,_Ankara_06800,_Turkey

c_SNTG_Laboratory,_Physics_Engineering_Department,_Hacettepe_University,_Ankara_06800,_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

Zinc oxide(ZnO)isa II–VIcompound semiconductor witha

widedirectbandgapof3.37eVatroomtemperature.Inaddition

totheelectricalandopticalpropertiesofundopedZnO,the

tran-sitionmetaldopedZnOformsarepromisingcandidatematerials

intheﬁeldofspintronics(spin-electronics).Variousmethodssuch

aspulsedlaserdeposition[1],chemicalvaportransport[2],

elec-trodeposition[3],co-precipitation[4]andsolid-statereaction[5],

andspraypyrolysis[6]canbeusedtosynthesizeZnO.Amongthese

methods,spraypyrolysistechniquecanbeapplicablewithout

vac-uumenvironment;ofcourse,thistechniqueischeapanddisplaying

comparablepropertiesandcompetitivefunctionalitywiththat

pro-ducedbyothertechniques.Theresearchhasbeenincreasedon

theternarysemiconductorssuchastransitionmetaldopedZnO

owingtoitshighCurietemperaturefortheferromagnetic

tran-sitioncalculatedinbulkmaterialsandfoundtobearound300K

[7–10].

CodopingcreatesaconsiderablechangeinthebandgapofZnO

[11–14],butthisvariationhasbeenreportedasanincreaseinsome

otherresearchandasadecreaseinthebandgapforZnOinother

research.Thiscaseindicatestheuncertainty.Areasonforthiscan

bestructuraldefectsintheZnOcrystallatticeaswellasbecause

∗ Correspondingauthor.

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

ofthevacanciesinthecrystalstructure orinterstitials[15].The

uncontrolledcasesasthestructuraldefectsand/orimpuritiesthat

ariseasgrowingtheﬁlmaffectsthebondingnature,chargetransfer

andthebandstructureinthematerial.Thismakesitverydifﬁcult

toobtainreproducible deviceperformance andreliability.Some

authorsreportedintheliteraturethatred-shiftisattributedtothe

sp–dexchangeandsomeotherauthorsobservedthatblue-shiftis

attributedtotheBurnstein–MosseffectconsideringtheCo

con-centration.Whenthevolumesolubilitylimitintwo-component

andmulticomponentalloyshasreacheda certainconcentration,

theﬁrstphaseremainsconstantandthentheextraphasesappear.

In ordertodeterminethesolubilitylimit,onehastofollowthe

changeofthelatticespacingandconcentrationobtainedfromthe

X-raydiffraction(XRD)andSEM-EDSspectrum,respectively.There

isuncertaintyaboutsolubilitylimitforCodopedZnO.Leeetal.

[16]reportedthatthedopedCoionwasfullysubstitutedintoa

ZnOlatticeat5mol%,butthesecondaryphaseoftheCo3O4was

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

(2)

ZnOandCo:ZnO(CZO)thinﬁlmsweredepositedonto

ultrason-icallycleanedglasssubstratesusingtheultrasonicspraypyrolysis

(USP)method.Forconventional USPmethod,thesubstratesare

ﬁxedandprecursorsolutionsprayedoverahotsubstrate.The

sub-stratetemperaturewaskeptat 400◦C. Thesaltsofzinc acetate

dehydrate(Zn(CH3COO)2·2H2O,99.9%-Merck)andcobaltacetate

tetrahydrate(Co(CH3COO)2·4H2O,99.9%-Merck)wereusedasthe

metalsourceswhich weresolvedin methanol.Inorder to

pro-duceaclearandhomogeneoussolutionmonoethanolamine(MEA)

andaceticacidwereaddedintotheprecursorsolutionwhichwas

stirredat60◦Catamoderatespeedfor1h.Inthestarting

solu-tions,Cocontentswerechangedfrom0.01Mto0.05Mandsamples

werelabeledasCZO1–CZO5(Table1).Zncontent(0.05M)washeld

atconstant.Thesolutionﬂowratewasheldconstantat5ml/min.

Nozzle,100kHzoscillatorfrequency,usedinthisstudywasina

downwardverticalconﬁgurationandthenozzletosubstrate

dis-tancewas12cm.Compressedairwasusedasthecarriergas.The

ﬁlmsweredepositedforabout10min.Amoredetaileddescription

ofthemethodtoobtainthethinﬁlmsandthecharacteristicsof

thespraypyrolysisdeviceusedwerereportedinpreviouspaper

[6].X-raydiffraction(XRD)spectrawerecollectedwithaD-Max

X-raydiffractometer(RigakuInternationalCorp.,Japan)withCuK␣

(=1.5405 ´˚A)toobtainthestructuralinformationoftheﬁlms.The

chemicalcompositionofthethinﬁlmswasmeasuredusingenergy

dispersivespectroscopy(EDS)withaJeolJSM-7000F-EDSelectron

microscope.TheopticalmeasurementsoftheCo:ZnOthin ﬁlms

werecarriedoutatroomtemperatureusingT70Model

Spectropho-tometer(PG Instrument) inthe wavelengthrange 300–900nm.

Photoluminescence(PL)spectraweremeasuredusinga100mW

He-Cdlaser(=325nm)astheexcitationsourceanda HORIBA

Jobin-Yvon1mmonochromator.

3. Resultsanddiscussion

3.1. Structuralproperties

TheX-raypatternsforCo:ZnOthinﬁlmsatroomtemperature

andreferencepeakpositionsarepresentedinFig.1a.Theresults

pointedout showedthat thereisnoimpurity and/orunreacted

phaseofZnandCoconsideringthereferencepeakpositions,(100),

(002),(101),and(103)peaksofZnOwereobserved.However,

(111)and(200)peaksat36.9◦ and42.7◦ Braggangleofcobalt

oxide,respectively, were observed.The starting molarity up to

0.03MofprecursorsolutionincludedCo,(002)peakofhexagonal

wurtzitestructurebecomesmoreintensivecomparingwithother

peaks(Fig.1b).Intheseﬁlms,upto0.03M,thehexagonalwurtzite

structureofZnOseemstobeprotected.Inthedopingprocess,itis

observedthatthereisalimitationofCodopingintotheZnO

struc-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.We observedthattheCosubstitutedZnsiteupto12%

(Fig.1c)whichcorrespondedto0.03Mwithoutshowinganyextra

phaseinXRDspectra.Peakscorrespondingtotheglasssubstrate

elementssuchasSiandCawerealsodetected.The(002)peak

indi-catingastrongorientationalongthec-axisofZnOwithhexagonal

wurtzitestructureisreplacedby(200)orientationwiththecubic

(3)

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

Fig.3. NormalizedexperimentalopticaltransmissionspectraforCo:ZnOthinﬁlms accordingtotransmissionvalueat800nm.

tothehigherBraggangle,andalsothecalculatedc-lattice

parame-terdecreasesduetotheincreasingofCoconcentrationintheZnO

structure(Fig.2).Buttheintensityof(200)peakdecreaseswhen

CoisdopedintotheZnOstructurewiththemolarityvalueofmore

than0.04M.

3.2. Opticalproperties

TheopticaltransmissionspectraofZnOandCodopedZnOthin

ﬁlmsamplesareshowninFig.3.TheeffectsofCodopingintothe

ZnOlatticeareclearlyobservedintheopticaltransmissionspectra.

TheincreaseofCoconcentrationintheZnOstructuredecreasesthe

opticaltransmittanceandimpartsdeepgreencolortothesamples

(Fig.4).Inaddition,increasingCoconcentrationalsomodiﬁes

opti-caltransmittanceforthespeciﬁcregionduetoCo2+_{characteristic}

transitions.Thesetransitionsalsosuppresstheinterferencefringes

inthisregionofCo:ZnOfilmsifthefilmthicknessissufficientto

createinterferencefringe.Inordertoeliminate theinﬂuenceof

differencesinsamplethickness,normalizedtransmittancetothe

valueat800nmweretakenintoaccount.Theabsorptionpeaks,

indicated with arrows in Fig. 3, centered at 571nm (2.18eV),

619nm(2.01eV)and662nm(1.88eV),arerelatedto

character-isticfeaturesofd–dtransitionofCo2+_ions._They_are_assigned_to

transition from 4_A

2(F)state to 2E(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+_._Comparing_the_ionic_radii_of_Co2+_(0.058_nm)_which_is_very

closetoionicradii ofZn2+ _(0.060_nm)_and _the_absorption_peaks

we canconcludethat theCoatomicallysubstitutesonZnsites.

Thisisalsoconﬁrmedinmanygroups,whichincludedavariety

ofmethodsandopticalabsorption[15,18,20–22].

Theopticaltransmissionspectrumcanbeusedinthe

determi-nationoftheopticalconstantsofthethinﬁlmdepositedontothe

transparentsubstrate.Whentheproductoftherefractiveindex

andﬁlmthicknessoftheﬁlmhaveallowedtheformationof

inter-ference fringes,classicalmethods suchastheenvelopemethod

developed bySwanepoel [23]canbeused.Aswellasthe

num-beroftheinterferencefringesanddepthofthefringesarecrucial

toperformthismethod.However,PointwiseUnconstrained

Mini-mizationAlgorithm(PUMA)[24]andmanyotheriterativemethods

[25] canbeusedwhentheinterferencefringesobservedornot

observed inthetransmissionspectrum.Consideringthenormal

dispersionrelation,therefractiveindexdecreaseswiththe

increas-ingwavelength.ThisisnotvalidfortheCo:ZnOthinﬁlmsforthe

region where theloss of transmissiondue toCo2+

characteris-tictransitionswhichmodulatethetransmittancespectrumofour

samplesisobserved.Therefore,weusedPUMAtechniqueforthis

limitedregionwhichstartsfromtheinﬂexionwavelengthtothe

wavelengthcorrespondingﬁrstcharacteristictransitionwhichis

centeredat571nm(2.18eV).Inﬂexionwavelengthisdeﬁnedfrom

secondderivativeofopticaltransmissioncurve[6,26].Thereisan

excellentagreementbetweentheexperimentalspectraand

theo-reticalspectraforallthesamplesandoneoftheexperimentaland

computedopticaltransmissionspectrafortheCo:ZnOthinﬁlmare

showninFig.4.Calculatedﬁlmthicknessandrefractiveindexfor

532nmaregiveninTable2.Thevalueofrefractiveindexis

(4)

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

behavior.

0.03M.Then,therefractiveindexstartstodecreaseagain.Having

suchaturningpointisinagreementwiththeotherobservations

suchasXRDandPLmeasurements.

Notonlytheﬁlmthicknessandrefractiveindexbutalsooptical

absorptionspectrumcanbeobtainedfromtheopticaltransmission

spectrumusingsuitablemethods.Absorptionedgesforthe

semi-conductorsinformthethresholdofchargetransitionbetweenthe

highestﬁlledbandandthelowestemptyband.Theopticalbandgap

oftheﬁlmscanbecalculatedusingthefollowingequation[27]:

˛h

v

=A(h

v

−Eg)n (1)

whereAistheprobabilityparameterforthetransition,Egisthe

bandgapofthematerial,histheincidentphotonenergy,and

nisthetransitioncoefﬁcient.Thevalueofnisknown:,2forthe

measurementofanindirectbandgapand1/2foradirectbandgap.

Fig.5a–fshowstheplotof(˛h)2_vs_the_photon_energy_(h₎_of

theCo:ZnOthinﬁlms.Thedirectbandgapoftheﬁlmswas

deter-minedbytakingtheintersectionoftheextrapolatedlinesfromthe

linearverticalandhorizontalregionsneartheband-edgeofthe

(˛h)2₌₀_curve._As_the_Co_{concentration}_is_increased_in_the_ZnO

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-dexchange interactionsbetweenthebandelectronsand the

localizedd-electronsoftheCo2+_ions_substituting_Zn2+_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)assignedtothebandgap

tran-sitionandtheotherpeaksassignedtothenear-band-edge(NBE)

emission are centered at 407nm, 415nm,418nm, 407nm and

413nmforthesamplesCZ01,CZ02,CZ03,CZ04andCZ05,

respec-tively(Table3).Bandgapvaluesaredecreasinguptoathreshold

valueofCoconcentration.Thisbehavioris alsoobservedin the

refractiveindexvariation.Inaddition,theobservedPLintensity

begantodecreasewhenthecobaltwasintroducedintotheZnO

structure.

4. Conclusion

ThestructureandopticalpropertiesofCodopedZnOﬁlmswere

studiedwithrespecttocobaltconcentrationinthestartingsolution

inwhichthecobaltacetatetetrahydratewasusedasaCosource.

OurstudiesshowthatCodopingaffectsZnOlatticeimmediately.

WhentheComolarityisgreaterthanthe0.03MorCo

concentra-tionobtainedfromEDSanalysisisabout12%,theZnO(002)peak

intensitydecreasesandCoO(200)peakintensityincreaseswith

increasingCoconcentration.CosubstitutioninZnOlatticehasbeen

provedbytheopticaltransmittancemeasurement,whereasitis

notclearlyseenintheXRDdiffractogramfortheCZO1andCZO2

sample. Theopticaltransmittance decreaseswithincreasingCo

(5)

visiblelight.WhentheZn2+_ions_are_replaced_with_Co2+_ions_in_the

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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