The structural and optoelectronic properties of Ti-doped ZnO thin films prepared by introducing a Cr buffer layer and post-annealing

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ContentslistsavailableatSciVerseScienceDirect

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

The

structural

and

optoelectronic

properties

of

Ti-doped

ZnO

thin

ﬁlms

prepared

by

introducing

a

Cr

buffer

layer

and

post-annealing

Y.C.

Lin

a

_,

_C.Y.

_Hsu

b

_,

_S.K.

_Hung

a

_,

_C.H.

_Chang

b

_,

_D.C.

_Wen

c,∗

a_Department_of_Mechanical_Engineering,_National_Chiao_Tung_University,_Taiwan,_ROC

b_Department_of_Mechanical_Engineering,_Lunghwa_University_of_Science_and_Technology,_Taiwan,_ROC

c_Department_of_Mechanical_Engineering,_China_University_of_Science_and_Technology,_Taiwan,_ROC

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received9May2012

Receivedinrevisedform11June2012

Accepted14June2012

Available online 23 June 2012 Keywords: Ti-dopedZnO Bufferlayer Post-annealing Magnetronsputtering Electricalresistivity Transmittance

a

b

s

t

r

a

c

t

ThisworkinvestigatestheeffectsofCrbufferlayersandpost-annealingonthepropertiesof titanium-dopedzincoxide(TZO)thinﬁlmspreparedbyradiofrequencymagnetronsputter.Allﬁlmshada(002) preferentialorientationalongthec-axisat2∼34◦_._The_{crystallinity,}_grain_size,_Hall_mobility_and

car-rierconcentrationofTZOﬁlmswereenhancedbyintroducingaCrbufferlayerandpost-annealing.The decreaseinresistivitywasmainlyattributedtotheincreaseinHallmobilityratherthancarrier concen-tration.AsaCrbufferlayerwasinserted,theﬁlmresistivitydecreasedby32%to5.41× 10−3_cm_while

theenergybandgapincreasedfrom3.252to3.291eVincomparisonwiththatofthefilmdeposited with-outthebufferlayer.WhentheCr-bufferedfilmswereannealedinavacuum,thestructural,electrical, andopticalpropertieswereimprovedwithincreasingannealingtemperature.Atanannealing temper-atureof500◦C,thegrainsize,resistivity,andenergybandgapattainedtheoptimalvaluesof28.12nm, 3.37×10−3_cm_and_3.357_eV,_{respectively.}_The_average_{transmittance}_of_TZO_films_in_the_visible_region

wasbetween75%and84%,anditdecreasedwithincreaseinthegrainsize.Thedecreaseintransmittance isattributedtoanincreaseinsurfaceroughnessduetothethree-dimensionalislandgraingrowthduring thermalannealing.

1. Introduction

Transparentconductingoxides(TCO)areextensivelyusedin liquidcrystaldisplays,energyefficientwindowsandtransparent electrodes,becauseoftheirexcellentelectricalandoptical proper-ties[1,2].Indiumtinoxide(ITO)isthemostpopularcommercial TCOmaterial,butindiumistoxic,rareandexpensive,sothe devel-opmentofalternativeTCOmaterialsisessential[3].Theadvantages ofzincoxide(ZnO)arelowmaterialcost,environmental friend-liness,wideenergybandgap(∼3.3eV)andhighcrystallinity,as comparedtoITOfilms[4].Inaddition,ZnOcanbedopedwithawide varietyofionsandthusmeetsthedemandsofseveralfieldsof appli-cation.AmongthevarioustypesofdopedZnOthinfilms,Ti-doped ZnO (TZO)films,in comparisonwiththeZnO filmsdopedwith GroupIIIelements,havemorethanonechargevalencestate.When titaniumatomsaredopedintoaZnOlattice,theyactasdonorsby providingtwofreeelectrons.Thisinturnincreasesthefreecarrier concentrationandhence,decreasestheresistivity.Several depo-sitiontechniqueshavebeenusedtogrowTCOfilms.Sputteringis consideredtobeasuitabletechniqueforthepreparationofTCO

∗ Correspondingauthor.Tel.:+886227867048;fax:+886227867253.

E-mailaddress:dcwen@cc.cust.edu.tw(D.C.Wen).

ﬁlms,becauseitisinexpensiveandoffersgooduniformityof depo-sitionoverlargeareas[5].

ThepropertiesofTZOfilmsaregenerallydependenton sputter-ingparameterssuchassubstratetemperature,workingpressure, and ambient gas. Any changes to the resulting resistivity are responsibleforchangesinboththecarrierconcentrationandthe mobility, whichare relatedtothecrystallinity ofthedeposited doped ZnO thin films [6,7]. Chunget al. [8] indicated that the crystallinity of TZOfilmscanbe improvedwitha low working pressure.Linet al.[9] investigatedtheeffects ofsubstrate tem-peraturesrangingfrom50to200◦Conthepropertiesofsputtered TZOfilmsandindicatedthatfilmresistivitydecreasedtoa min-imumof 9.69× 10−3_cm _at₁₀₀◦_C._Jiang_et _al._[10]_and _Chang etal.[11]studiedtheeffectsofannealingtreatmentonthe proper-tiesofTZOthinfilms.Theyreportedthatfilmresistivitydecreased andtheaverageopticaltransmittancein thevisiblewavelength rangechangedslightlyafterannealingtreatment.Itiswellknown thatdirectlydepositingTCOthinfilmsonglasssubstratesatlow growthtemperaturesisdifficultduetolargelatticemismatchand differencesin thermal expansioncoefficients betweenthefilms andthesubstrates[12,13],resultinginpoorfilm/substratebonding. InsertingabufferlayerbetweenTCOfilmsandglasssubstratesis helpfulforimprovingthebondingstrengthofTCOfilmstothe sub-strate,whichcanresultinstableperformanceofTCOfilms.Forthis

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Table1

DepositionparametersofRFsputteringforTZOﬁlmandDCsputteringforaCrbufferlayer.

Parameters RFsputtering DCsputtering

Target 98wt.%ZnO,2wt.%TiO2(99.995%purity) Cr(99.95%,purity)

Sputteringpower 130W,13.56MHz 50W,30kHzpulsefrequencyand3␮spulsetime

Basepressure 5.0×10−6_Torr _5.0_×₁₀−6_Torr

Workinggas Argon(99.995%) Argon(99.995%)

Depositionpressure 15×10−3_Torr ₁₅_×₁₀−3_Torr

Substrate-to-targetdistance 85mm 85mm

Substratetemperature 300◦C 100◦C

Substraterotateverticalaxis 10rpm 10rpm

Depositiontime 70min –

purpose,theCrlayercouldbeemployedforadhesionenhancement becauseCrhashigherchemicalreactivitythanTCOfilmswithglass surfaces.Moreover,Crhasahighdiffusioncoefficientanditcan migratequicklyintoTZOfilmduringthedepositionorannealing process.Thus,theexistenceofaCrbufferincreasesthecarrier con-centrationsandreducestheresistivityofthefilms.Severalresearch groupshavestudiedthegrowthofepitaxialGa-orAl-dopedZnO thinfilmsbyintroducingbufferlayers,suchasAl,ZnO,andSiO2,to improvethecrystallinequalityandstructureofthefilms[14–16]. However,theeffectofbufferlayersonthepropertiesofTZOthin filmshasrarelybeenreporteduntilnow.

Inthisstudy,TZOthinfilmsweredepositedonCr-bufferedsoda limeglass(SLG)substratesusingradiofrequency(RF)magnetron sputteringandthenannealedina vacuumtoinvestigatehow a bufferlayerandannealingchangethestructureandfurtheraffect thepropertiesofthefinalfilms.ATZOthinfilmwasalsodeposited onbareSLGsubstrateforcomparison.

2. Experimental

TheTZOtargetwaspreparedbythesolidstatereactionmethod. Atﬁrst,amixtureofZnO(99.995%inpurity)andTiO2(99.995%) powderswithacompositionratioof98:2inwt.%wasboll-milled for20handpressedintoacircularplatewithadiameterof50.8mm andthicknessof3mm,andthensinteredat1200◦Cinairfor8h.

TZO ﬁlms were deposited by RF magnetron sputtering on 25mm×25mm×1.1mmbareSLGandSLGwithaCrlayer, respec-tively. Here, the Cr layer was coated by direct current (DC) magnetronsputtering.Allsamplesweredepositedwithsubstrate rotationinordertoattaingoodsurfacemorphology.Before depo-sition,thetargetswerepre-sputteredfor5mininordertoremove anycontamination.Thesubstrateswereultrasonicallycleanedwith acetoneandde-ionizedwater,andthendriedunderblownnitrogen gas.TheRFandDCsputteringconditionsareshowninTable1.

Forhightransmittance,thethicknessofthemetallayerwas notallowedtoexceedacertainthreshold.Asthethicknessofthe metallayerincreases,thetransmittancedecreasesandreﬂection increases.Ourpreviousstudyshowedthattheoptimalthickness ofanAlbufferlayerisabout10nmforAZOﬁlms[17].Therefore, thedepositiontimeofDCsputteringwasadjustedtomakesure thatthethicknessoftheCrbufferlayerwas∼10nm.Studiesshow thattheresistivityofTZOandAZOwasimprovedaftervacuum post-annealingattemperaturesrangingfrom400to500◦Cbutwas decreasedwhenannealedat600◦C[10,18].Hence,thedeposited sampleswiththeCrlayerweresubsequentlyannealedinatube furnaceandevacuatedtoaprimaryvacuumlevel(5.0×10−6Torr) at450and500◦Cfor20min.

Afterthedepositionandannealing,filmstructureswere deter-minedby X-ray diffraction (Rigaku-2000 X-ray diffractometer), withCu-K␣ radiation (40kV, 30mA and =0.1541nm) and an angle incidence of 1◦. The elemental compositionsof TZO tar-get and thin films were determined by the energy dispersion spectrum (EDS) using a field emission scanning microscope

Table2

ElementalcompositionofTZOtargetandthinﬁlm.

Sample Ti(wt.%) Zn(wt.%) O(wt.%)

TZOtarget 2.02 79.16 18.82

TZOthinﬁlm 1.72 80.07 18.21

(FE-SEM,JEOL,JSM-6700F).Thesurfacemorphologicalproperties wereanalyzedusinganatomicforcemicroscope(AFM,SPA-400). Electricalresistivitywasmeasuredbythefour-pointprobemethod (MitsubishichemicalMCP-T600). CarrierconcentrationandHall mobilitywereobtainedfromHall-effectmeasurementbythevan derPauwmethod(Ecopia,HMS-3000).Opticaltransmittancewas measuredbyaUV/VIS/IRspectrophotometer(Jasco,V-570)inthe wavelengthrange from300to800nm.Allmeasurementswere performedatroomtemperatureinair.

3. Resultsanddiscussion 3.1. Structuralproperties

Theelemental compositions ofTZO target andthin filmare listedinTable2.Afterthedeposition,theTicontentdecreased from2.02wt.%intargetto1.72wt.%infilm,whereastheZn con-tentincreasedslightly.Thisisconsistentwiththeobservationsof Chungetal.[8].TheXRDpatternsoftheTZOfilmsdeposited with-outabufferlayer,withabufferlayer,andwithabufferlayerand post-annealingat450and500◦C(labeledassamplesA,B,CandD) areshowninFig.1Allfilmsexhibitedastrong(002)peak indicat-ingthattheTZOfilmswerehighlyorientedtothec-axisnormalto thesubstrate[19].ComparedtosampleA,the(002)peakintensity ofsampleBbecamemoreintenseandsharperafterusingtheCr thinfilmasabufferlayer.Itisindicatedthatthecrystalqualityof

2θ (degree)

Intensity (a.u.)

30 35 40 45 50

TZO/glass As-deposited TZO/Cr/glass Annealingat450C TZO/Cr/glass Annealingat500C

(0

0

2)

o

TZO/Cr/glass As-deposited

(1 0 2) SampleD SampleC SampleB SampleA 2θ=34.41 b=0.309 2 =34.40 b=0.321 2 =34.38 b=0.326 2 =34.28 b=0.365 θ θ θ o o o o

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Table3

PositionandFWHMofthe(002)peak,andgrainsizesofsamplesA,B,C,andD.

Samplecondition (002)peak position(◦) (002)FWHM Grain size(nm) A:TZO/glass,as-deposited 34.28 0.365 23.78 B:TZO/Cr/glass,as-deposited 34.38 0.326 26.65 C:TZO/Cr/glass,annealingat450◦_C _34.40 _0.321 _27.05 D:TZO/Cr/glass,annealingat500◦_C _34.41 _0.309 _28.12

theTZOthinfilmimproveswiththeadditionoftheCrbufferlayer. Afterannealing,theintensityofthe(002)peakfurtherincreases anditsfullwidthathalfmaximum(FWHM)alsobecomesmore narrowedthanthoseofsampleB,revealingthatbetterfilm crys-tallinityqualitywasobtainedbythermalannealing.Ascanbeseen, weakpeaksof(102)orientationareobservedinsamplesCandD. Thesemayhavedevelopedafterannealing.Itisassumedthat(102) orientationhashighersurfaceenergythan(002)orientationand needsmorethermalenergytodevelop[20].Thisresultindicates thatpreferredorientationchangeswithafewgrainsinthefilm andthestructureofTZOfilmbecomesmorerandom.

TheFWHMofthe(002)peakwasnarrowedfrom0.365 (sam-pleA)to0.326(sampleB),0.321(sampleC),and0.309(sample D).Thecorrespondingaveragegrainsizes(estimatedbyScherrer’s formula)were23.78,26.65,27.05,and28.12nm,respectively.The grainsizeofthefinalfilmwasincreasedbyusingabufferlayer andannealing.Thepositionofthe(002)peakwasshiftedfrom 34.28◦(sampleA)to34.38◦(sampleB)whenapplyingtheCrbuffer layer.EffectsofbothTiatomssubstitutedintoZnsitesandlattice mismatchbetweenthefilmsandthesubstrateledtocompressive stressinthelatticeofTZOfilmsinadirectionparalleltothesurface ofthesubstrate,whichcouldaffectthelatticespacing perpendic-ulartothesurfaceand,hencethe(002)peakpositionofsampleA changedtoalowerdiffractionanglethanthatoftheZnObulk posi-tion(_∼34.45◦).AstheCrbufferlayerwasinserted,thestraininthe TZOfilmwasrelaxed.Asaresult,thepositionofthe(002)peak wasshiftedtoahigherangle.However,thisshiftwasunobvious forsamplesCandD,ascomparedtosampleB.Itissupposedthat thedefectsinducedbythelatticemismatchandthermalexpansion coefficientcangetthermalenergyfromannealingtreatmentand movetothefilmsurface.Therefore,thestraininthefilmis effec-tivelyrelaxed.However,thebufferlayerinTZOfilmscanpreclude thiskindofmovementandconfinemostofthedefectsinitself. Thus,thepositionofthe(002)peakwasalmostunchangedforthe samplesCandD,afterthermalannealingat450and500◦C, respec-tively.ThepositionandFWHMofthe(002)peakandgrainsizeof samplesA,B,C,andDaresummarizedinTable3.Ascanbeseen, the(002)peakoftheTZOfilmsforsamplesB,C,andDbecomes sharperandthepeakpositionshiftsslightlytohighervalueswhen comparedwithsampleA,whichindicatesthatthegraingrowsand lessstressremainsinthefilmsafterannealing.Therefore, intro-ducingaCrbufferlayerandthermalannealingcanbeaneffective methodforimprovingthecrystallinityofTZOfilms.

Fig.2showsAFMimagesofTZOfilmforsamplesA,B,C,and D.Whenthefilmwasdepositedwithoutabufferlayer(sample A), somehillocks obviously occurred, as displayed in Fig.2(a), sincethefilmtendedtobeporousandloose.Ohring[21]has pro-posedthatlocalmassfluxdivergencesexistthroughoutfilmdue tovaryinggrainsizeanddistribution.Whenatomscomeintoa grainmorethanleaveit,thepileuporgrowthofthemasscanbe expected.Fig.2(b)showsthatthemorphologyofthefilmdeposited withbufferlayer(sampleB)wasintheshapeofcobblestoneand anisland. Thisindicatesthat growthhastakenplaceby nucle-ationandacoalescenceprocessduringthedepositionofTZOfilm bysputteringontoCr-bufferedsubstrates.Randomlydistributed nucleimayfirst formand thesenucleimay thengrowtoform

observableislands.WhentheTZOﬁlmswereannealedat450and 500◦C(samplesCandD),theislandscomeclosertoeachother. Thelargeronesappearedtogrowbycoalescenceofthesmaller ones.Thus,thesurfacestructureofﬁlmsbecamedenserandthe grainsgrewintolarger,asshowninFig.2(c)and(d).Comparing thesepictureswiththeresultscalculatedbyScherrer’sformula,it isnoteworthythatthecharacteristicsizesobtainedfromAFMand XRDanalysisareinagreement.

Surfaceroughness(Ra)wasalsocalculatedfromAFM.Itcanbe seenthatthesurfaceroughnessoftheTZOfilmswasrelatedto thatoftheunderlyingCrlayerandannealingtreatment.TheRa valueofTZOfilmsdepositedwithoutandwithaCrbufferlayer is4.224and12.110nm,respectively.ArelativelylargeRavalue appearedfortheTZOfilmdepositedontheCr-bufferedsubstrate. Pre-depositionofaCrbufferlayerincreasedthesurfaceroughness ofthesubstrate,resultingin theincreaseoftheRavalueofthe TZOfilms.TheRavalueincreasedto13.576nmwhentheTZOfilms wereannealedat450◦C.Asthetemperaturefurtherincreasedto 500◦C,thesurfaceoftheTZOfilmsbecamerougher.Ravaluesof theTZOfilmsincreasedwithincreasingtemperatureduetothe three-dimensionalislandgrowthduringthermalannealing. 3.2. Electricalproperties

Theresistivity(),Hallmobility()andcarrierconcentration (n) ofTZO filmfor samplesA, B,C, and Dare shownin Fig.3. AstheCrbufferlayerwasinserted,thefilmresistivitydecreased from7.98×10−3cminsampleAto5.41×10−3cminsample B(orareductionof32%).Meanwhile,theHallmobilityincreased from3.29to4.81cm2_/V_s_and_the_carrier_{concentration}_increased from1.59×1020_cm−3 _to_2.93_×₁₀20_cm−3_._According_to_the for-mulaofresistivity=1/(ne),theresistivityisacombinedresult ofboththeHallmobilityandthecarrierconcentration.Whenthe TZO filmswere preparedwith a Crbufferlayer, thegrainsize increasedfrom23.78nmto26.65nm.Largergrainsizecanreduce grainboundaryscatteringandincreasecarrierlifetime,thus lead-ingtoanincreaseinconductivityduetoanincreaseinHallmobility andcarrierconcentration[22];asaresult,theelectricalresistivity ofTZOfilmsdecreases.WhentheTZO/Cr/glassfilmswereannealed at450(sampleC)and500◦C(sampleD),thecarrierconcentration showedaslightincreasebuttheHallmobilityincreased contin-uously,andtheresistivityfurtherwasdecreasedby26and38% to3.98×10−3cmand3.37×10−3cm,respectively.Apossible reasonisthattheannealingprocesshastheeffectofincreasingthe grainsizeandcrystallinityofthefilm,asmentionedabove.This resultsinlessgrainboundaryscattering.Ontheotherhand,lossof oxygenfromthecrystalwillalsoproducetheinterstitialzincatoms whichresultsinZn1+xOinthematrixaccordingtothereaction:

ZnO+Oo=Zn∗_i+e+2−1O2(g) (1)

Thismeansthattheintrinsicdonorcanbeincreasedbythe vac-uumannealingprocessandthus,increasetheconductivity[23]. However,NaatomsinSLGareveryactiveathightemperatures andeasilydiffusedintofilmsduringannealing.IfaNaatom substi-tutesazincatom,itwillbecomeanacceptor,resultinginadecrease ofcarrierconcentration[24].AlthoughaCrbufferlayercould pre-ventthediffusionofNaatomsintofilm,ifannealingtemperature ishigh,someNaatomscouldbestilldiffusedintoTZOfilmthrough theCrlayer,loweringthecarrierconcentrationinTZOfilms.Thus, theHallmobilitycontinuestoincreasewhenthecarrier concen-trationexhibitsalittlevariationafterannealing.Thisimpliesthat thedecreaseinresistivitywasmainlyattributedtoanincreaseof Hallmobilityratherthancarrierconcentration,asthefilmswere annealed.

Asdescribedabove,thedecreaseofresistivityroughlyincreased withgrainsizeinourstudy.SinceTZOﬁlmpreparedwithaCrbuffer

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Fig.2. AFMmicrographsandsurfaceroughnessof(a)sampleA:as-depositedwithoutbuffered,(b)sampleB:as-depositedwithCr-buffered,(c)sampleC:depositedwith

Cr-bufferedandannealedat450◦_C_and_(d)_sample_D:_deposited_with_Cr-buffered_and_annealed_at₅₀₀◦_C.

layercangetlargergrainsand cansuppressthediffusionofNa atomsfromSLGsubstrateintofilms,therefore,itcanbeconcluded thattheimprovementofthecrystallinityatanearlystageofthefilm depositionisusefulforobtaininglowerresistivityintransparent conductingTZOthinfilms.

3.3. Opticalproperties

Thetransmittancespectraasafunctionofwavelengthsinthe range between300and 800nm for samplesA, B, C, andD are showninFig.4.Theaveragetransmittanceinthevisibleregionis morethan75%forallsamples,butthetransmissionintheUV–near visibleregiondecreasedabruptly.Whentheﬁlmsweredeposited withthebufferlayerandforthoseannealedat450and500◦C,the

optical absorptionedge shifted toward the shorter wavelength regionbuttheaveragetransmittancebecamelower,ascomparedto theas-depositedfilmwithoutabufferlayer.Inthisstudy,in addi-tiontothegraingrowthofTZOfilms,introducingabufferlayer andannealingalsoledtoanincreaseinsurfaceroughnessdueto thethree-dimensionalislandgrowth.Thisphenomenoncausesthe formationofmicrovoidsbetweenthegrainboundaries.Hencethe scatteringprobabilityofincidentlightbymicrovoidsexistingin thefilmsincreasesandwecanobservealowertransmittancein therange ofvisiblelight.Asaresult,theaveragetransmittance monotonicallydecreasedfrom83.95%to75.21%withtheincrease ofRavaluefrom4.224nmto14.002nm.

SinceZnOﬁlmsaredirecttransitiontypesemiconductors,the energybandgap(Eg)canbeestimatedbyplotting˛2 _versus_hv

Sample Re si st iv it y, ( 1 0 -c m ) 3 4 5 6 7 8 9 A ρ× -3Ω

(a)

D C B TZO/glass As-deposited TZO/Cr/glass As-deposited TZO/Cr/glass Annealingat 450C TZO/Cr/glass Annealingat 500C A: B: C: D: o o Sample H all mo bility, (c m /V -s ec) Carrie r conc en trat ion, n ( 10 cm ) 1 2 3 4 5 6 7 8 1 2 3 4 5 A μ 2

(b)

× 20 -3 Hallmobility Carrierconcentration

D C

B

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Wavelength (nm) Transmittance (%) 300 400 500 600 700 800 0 20 40 60 80 100 Glassbase

TZO/glass,as-deposited TZO/Cr/glass,as-deposited TZO/Cr/glass,annealingat 450C TZO/Cr/glass,annealingat 500C DCBA o o A: B: C: D:

Fig.4.OpticaltransmittancespectraofsamplesA,B,C,andD.

Photonenergy(eV)

2 2.5 3 3.5 0 1 2 3 4 α 2 /(a.u.)

A

B

C

D

TZO/glass,as-deposited, E =3.252eV TZO/Cr/glass,as-deposited, E =3.291eV TZO/Cr/glass,annealingat450C,E =3.331eV TZO/Cr/glass,annealingat500C,Eg=3.357eV g g g A: B: C: D: o o

Fig.5.Plotsof˛2_versus_hv_for_samples_A,_B,_C,_and_D.

(where˛istheopticalabsorptionandhvisthephotonenergy), andthenextrapolatingthestraight-linepartoftheplottothe pho-tonenergyaxis.Fig.5showsthevariationoftheenergybandgap forthefoursamples.Bycomparingwiththefilmdeposited with-outabufferlayer,insertingaCrbufferlayerandannealingat450 and500◦Cleadstoenergybandgapshiftsfrom3.252to3.291, 3.331,and3.357eV,respectively.TheenergybandgapofTZOfilms broadensowingtoanincreaseinthecarrierconcentration.This phenomenonisknownastheBurstein–Mossshift[25].BecauseaCr bufferlayercouldpreventthediffusionofNaatomsintoTZOfilms andprovidemorefreeelectronsbythediffusionofCratomsfrom itself,thecarrierconcentrationintheannealedsamplesareslight highercomparedwiththeas-depositedfilmasshowninFig.3(b).

Thustheenergybandgapincreasedwithincreasingannealing tem-perature.

4. Conclusions

TheinfluenceofCrbufferlayersandpost-annealingonthe struc-tural,electricalandopticalpropertiesofTZOfilmsdepositedon SLG substrateshasbeeninvestigated. Allfilmsexhibited strong (002)diffractionpeaksofhexagonalwurtzitestructure.AsaCr bufferlayerwasinserted,thecrystallinitywasimprovedandthe grainsizebecamelarger,Hallmobilityandcarrierconcentration werealsoenhancedduetosuppressionofdiffusionofNaatoms intofilms.Thefilmresistivitydecreasedby32%to5.41×10−3cm whiletheenergybandgapincreasedfrom3.252eVto3.291eVin comparisonwiththatofthefilmdepositedwithoutabufferlayer. Post-annealingfurtherimprovedthepropertiesofTZOfilmsdueto thedecreaseofgrainboundaryscattering.Theelectricalandoptical propertieswereimprovedwithincreasingannealingtemperature to500◦C,alowestresistivityof3.37× 10−3_cm_and_a_maximum energybandgapof3.357eVwereobtained.In addition,surface roughnessofTZOfilmsincreasedwithgrainsizeduetothe three-dimensionalislandgrowthduringthermalannealing.Thisresult ledtoaveragetransmittanceofannealedfilmsinthevisibleregion whichwerelowerthantheas-depositedfilmwithlowersurface roughness.

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