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Effects of precursor parameters on the optical and electrical properties of AZO nano-composite films

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Optik

j ou rn a l h o m e p a g e :w w w . e l s e v i e r . d e / i j l e o

Effects of precursor parameters on the optical and electrical properties of AZO nano-composite films

ÜmitÖzlemAkkayaArıera,∗,BengüÖzu˘gurUysalb

aDepartmentofPhysics,MimarSinanFineArtsUniversity,Bes¸iktas¸,Istanbul34349,Turkey

bDepartmentofEnergySystemsEngineering,FacultyofEngineeringandNaturalSciences,KadirHasUniversity,Fatih,Istanbul34083,Turkey

a r t i c l e i n f o

Articlehistory:

Received5January2016 Accepted9February2016

Keywords:

Opticalproperties Nano-composites Thinfilms Surfaceproperties

a b s t r a c t

AldopedZnO(AZO)nano-compositefilmsweresynthesizedonglasssubstrateswiththesol–gelspin- coatingmethodattheroomtemperature. Theactivation energyofAZOnano-compositefilmswas calculatedtobe49kJ/molfortheparticlesgrowth.Theelectrical,structuralandopticalpropertiesof AZOfilmsweredeterminedbychangingZnO:waterandZnO:Alratios.ZnO:waterandZnO:Alratiosplay animportantroleincontrollingtheelectricalconductivityoftheAZOnano-compositefilms.Theopti- mumdopingratioofAlwasfoundtobe2%intermsofthelowestresistivity,andabove2%Al-doping concentration,thesurfaceresistivityofAZOnano-compositefilmsstartstoincrease.Theopticalhighest transmittanceofthefilmsof86%invisibleregion,andlowsurfaceresistivityof70/䊐canbeobtained fortheoptimumdopingratioofAl.

©2016ElsevierGmbH.Allrightsreserved.

1. Introduction

Al-dopedZnO(AZO)isaveryimportanttransparentconducting oxide(TCO)materialthathasgoodelectricalandopticalproper- ties.AZOstructureshaveanumber ofapplicationsinelectronic devicessuchasasensor,solarcellanddisplaydevices[1,2].AZO iscommonlyusedinvariousapplicationsduetoitshighlyvisi- bletransparencyandgoodelectricalconductivity.Manymethods havebeen usedtoprepareAZOfilmssuchassol–gel, chemical vapordeposition(CVD),pulsedlaserdeposition(PLD),andsputter- ing[3,4]generallyinresearches.Zincoxide(ZnO)whichisusedin variousresearchesisasemiconductormaterialthathasawideband gap,awiderangeresistivity,highmobilityandhightransparency [2,5].Itis acheap,non-toxicand chemicallystablemetaloxide material.Carriers,which actasdonors,areformedthroughthe ionizationofzincinterstitialsandoxygenvacanciestocontribute thenativeintrinsic n-typeconductivitybehavior. Awell-known wide-bandgap,wurtzitestructuredzincoxidewithfour-foldtetra- hedralcoordinationliesintheborderbetweenionicandcovalent semiconductors.Inourearlywork[6],wefoundtheproperties ofnanostructuredZnOnano-compositefilmscanbecontrolledby changingtheDea:waterratio.Inordertounderstandthesimilar influencesofchemicalratios,inthiscase,westudiedtheoptical

∗ Correspondingauthor.Tel.:+905323434202;fax:+902122611121.

E-mailaddress:oarier@gmail.com(Ü.Ö.A.Arıer).

andconductivitypropertiesoftheAZOfilms.Generally,Alatoms aredopedtoZnOfilmsfortheincrementconductivityofthefilms.

Inrecentyears,AZOfilmswerestudiedbymanyresearchers[7–9].

Especially,theactivationenergyfortheAZOdyeshavebeencal- culatedtobeintherangeof51–101kJ/mol[10–12],andatleast 298kJ/molfortheAldopedZnOpowders[13],buttheactivation energyofAZOasformofthinfilmpreparedbysol–geltechnique hasnotbeenreportedbefore.Inthisstudy,theimportantexper- imentalparameters, suchasheat-treatmenttemperatureof the nanostructured films or ZnO:Aland ZnO:water ratioof thesol beforeannealingwerevariedinordertounderstandtheinfluences oftheseparametersonelectrical,structuralandopticalproperties.

Thegrowthkineticsoffilmswereinvestigatedandtheactivation energywascalculated.Thisvaluewasfoundtobelowerthanthe valuesreportedintheliterature.Ontheotherhand,thecrystal- litesize,bandgapenergyandsheetresistanceofsingle-layerAZO filmswereinvestigated.TheresistivityoftheAZOthinfilmisalso variedbyotherparameters, suchascrystal orientation,defects, electronscatteringatthegrainboundaries,highannealingtem- perature,anddifferentmedium(pressure)asmentionedbeforeby otherauthors[14–17].Thesefactorsalsoaffecttheresistivity,but controllingtheparticlesizehasacrucialimportanceamongthem forfurtherapplicationsofsemiconductingthinfilms. Ourcalcu- latedresistivityvaluesarecomparableandenoughlowtothose findingsintheliterature.Asaresult,weexpectthattheincorpo- rationofAlatomsintheplaceofZnleadtothegenerationoffree electrons,todecreasethesheetresistivityofAZOfilms.

http://dx.doi.org/10.1016/j.ijleo.2016.02.020 0030-4026/©2016ElsevierGmbH.Allrightsreserved.

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geneousandstablesolwaspreparedbydissolvingthezincacetate inasolutionofisopropanoland diethanolamine.First,asapre- cursorsolutionofZnAc:2propanol:Dea:water:Al,avolumeratioof 0.4:4:0.1:0.2:(1,2,3,4%)wasused.Second,ZnAc,Al,2propanol, Dea,Al(%2)concentrationswereheldfixed,andcrystallitesize wascontrolledonlybychangingtheZnO:watervolumeratio,e.g., to5,4,2.5and2.AZOfilmswerepreparedonthecorning2947 glassbyaspincoater.Thesesampleswereheat-treatedat500C for1h,andinordertocalculatetheactivationenergy,thefilms wereannealedat550,600,and650C.

2.2. Samplecharacterization

Structural analysis and surface morphology of the resulting filmswerecarriedoutbyX-raydiffraction(XRD-GBC-MMA,Cu- Kradiation),atomicforcemicroscopy(AFM-Shimadzuscanning probemicroscopeSPM-9500J3),transmissionelectronmicroscopy (HR-TEM),and scanningelectron microscopy(SEM Shimadzu scanningprobemicroscopeSPM-9500J3).Theopticalanalysisof filmswasdeterminedbyaspectrophotometer(PerkinElmer).Four pointprobewasusedtodeterminethesheetresistivityoffilms.

3. Resultsanddiscussion 3.1. Structuralanalysis

XRD patterns of the nanostructured AZO films which were preparedfor differentZnO:Alratiosareshownin Fig.1A.Three well-defineddiffractionpeakswereidentifiedas{100},{002}, and{101}planesofhexagonalwurtzitestructured(JCPDS:36- 1451)zincoxide[18,19].TheAlphasewasnotobservedintheXRD

Bcos

whereDisthediameterofthenanocrystallites,Kisa constant (0.89),isthewavelengthoftheincidentlight(forCuKradi- ation=1.54056 ˚A),Bisthefullwidthathalf-maximum(FWHM) ofthediffractionlineandistheBraggangle.Thecrystallitesize increasesfrom1.78to7.9nm;2.1to9.1nm;1.95to8.5nm,forthe ZnO:Alvolumeratioof4,3,2,1%,respectively.Theresultssug- gestedthatthecrystallinityofAZOthin filmsisdecreased with ZnO:Alvolumeratio.TheXRDpatternsof2wt%aluminumdoped ZnOsynthesizedwithdifferentZnO:watervolumeratiosonaglass substrateareshowninFig.1B.ThediffractionpeaksofAZOnano- compositefilmswereobservedat{100},{002},and{101},which belongtohexagonalwurtzitestructure,again.Thecrystallitesizes arecalculatedforZnO:watervolumeratios:5;4;2.5;2from{100}, {002},and{101},andthepeaksincreasesfrom5.7to10.2nm, 6.8to15.2nm,and6.3to12.3nm,respectively.Theincrementof thewaterratiossupportedtheagglomerationandthenthecrys- tallitesizewasincreasedbydecreasingtheZnO:waterratio.The waterratioaffectsthehydrolysisandthenucleationreactionsin AZO solution.The hydrolysis rates are low for less water ratio andtoomuchalkoxideinthesolvent.ThecrystallitesizeofZnO nanoparticlesinAZOfilmsdecreasedasthevolumeofwateradded increased.Theseclearly demonstratethatwater cancontrolthe growthofZnOnanoparticlesinAZOfilms.Itwasalsonotedthat themorewaterratiopresentinthesolutioncouldfavorthehigh crystallinityoftheZnOnanoparticlesinAZOnano-compositefilms.

Fig.1CshowsthediffractionpatternsofAZOnano-compositefilms preparedforvariousheattreatmenttemperatures.Theinfluence oftheheattreatmenttemperatureonthecrystallinityoffilmwas investigated.WhileZnO:water at5andZnO:Alat2ratioswere heldfixed,crystallitesizewasincreasedbytheincrementofthe

Fig.1. X-raydiffractionpatternsofAZOnanofilmsfordifferent(A)ZnO:Alratiosannealedat500C,(B)ZnO:waterratiosheattreatedat500C,(C)heattreatment temperatures.(D)ThecalculatedaveragecrystallitesizeofthenanostructuredAZOthinfilmswithrespecttothevaryingvolumeratioofZnO:AlandZnO:waterincompositions atannealingtemperatureof500C.(Peakpositionsin2deg.:31.72,34.58,36.44).(E)Plotsoflndversus1000/TofAZOnanofilms(ZnO:water-5,ZnO:Al-2%).

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Fig.2.(A)TEM,(B)SEM,(C)AFMimagesofAZOnanofilmswithdifferentZnO:waterratios:(a)5,(b)2.5.

temperature. The increase in the heat treatment temperature encouragedthehighcrystallization,whichledtotheincreasein thecrystallitesizeoftheAZOnano-compositefilms.Theaverage crystallitesizescalculatedfromtheXRDdataofallfilmsweresum- marizedinFig.1D.TheactivationenergyofAZOnano-composite filmswasdeterminedbytheArrheniusequationasE=−RTln(d/a) whereTisthetemperature(Kelvin),Ristheuniversalgasconstant, distheaveragecrystallitesize,andaistheintercept.Theactivation energyofAZOnano-compositefilmswascalculatedas49kJ/molfor thenanoparticles’growthusingtheslopeofthelinesoftheplotin Fig.1E.

ThemicrostructureandmorphologyofAZOfilmsweredeter- minedwithTEM, SEMand AFMmeasurements.TEMimagesof AZOnano-compositefilmssynthesizedasZnO:Alratio%2at500C

wereshownforvariousZnO:waterratiosinFig.2A.Thesizesof nanoparticlesincreasedwiththedecreasingZnO:waterratio.

SEMimagesofAZOfilmswerepresentedinFig.2B.Thesurfaces ofthefilmswereobservedasauniformandnano-sizedstructure.

Thesizeofnanoparticleswasdecreasedwiththeincreaseinthe ZnO:watervolumeratios.TheseresultswerepromotedbyXRDand TEMmeasurements.ThesurfaceroughnessofAZOnano-composite filmswasobservedfor differentZnO:watervolumeratiosusing AFM.AFMimagesofAZOfilmsareshowedinFig.2C.Therough- nessof AZOfilmswasdeterminedtobeRms:2.96;3.31; 4.04;

6.54nmfor5;4;2.5;2,ZnO:waterratios. Theroughnessofthe filmsincreasedwithadecreaseintheZnO:waterratio.Thesurface roughnessofAZOfilmswasdeterminedtobeRms:3.06;4.19;5.37;

6.81nmfor4;3;2;1%ZnO:Alratios.

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Fig.3. SurfaceresistivityofAZOnanofilmsfordifferentZnO:AlandZnO:water ratios.

3.2. Electricalanalysis

ThesheetresistanceoftheAZOnano-compositefilmswasmea- suredbyafour-pointprobe.WhenZnO:waterratiowasheldfixed at5,thesurfaceresistivityofAZOfilmwasmeasuredfordiffer- entZnO:AlratiosasshowninFig.3.Theelectricalresistivityofthe filmsdecreasedfrom180to82/䊐astheZnO:Alratiodecreased from4to1%.Additionally,whenZnO:Alratioisfixedas2%,thesur- faceresistivityofAZOfilmwasdeterminedfordifferentZnO:water ratiosinFig.3.Theelectricalresistivityofthefilmsdecreasedfrom 70to46/䊐astheZnO:waterratiodecreasedto2%.

However, theresistivity of the film ZnO:Al at 3% increased greatlyto115/䊐.ZnO:waterandZnO:Alratiosareveryimportant

ofZnO:waterratiobecauseoftheadsorptionofwater.Aldoping with2%concentrationisfoundtobetheoptimalratiointermsof resistivity,andwithabove3%Aldopingconcentration,theresis- tivityofAZOnano-compositefilmsstartstoincrease.

3.3. Opticalanalysis

Absorbancevalues of AZO nano-composite filmswere mea- suredusingUV–vis spectrophotometerfor differentZnO:Aland ZnO:waterratiosinFig.4AandB.Itclearlyshowsthatthedecreased intheZnO:Alratioproducesablueshiftintheabsorbancespectra ofthefilmsinFig.4Aduetoquantumsizeeffect.Thediffrencein theabsorptionedgeisrelatedtoBurstein–Mosseffectbecauseof theincreasingwaterinFig.4B[21,22].Transmittancevaluesofthe filmsweredecreasedwiththeincreasingZnO:AlratiosinFig.4C.

Additionally,transmittanceofthefilmsweredecreasedwiththe decreasedZnO:waterratiosasshowninFig.4D.

Theopticaltransmittanceincreasedfrom91toabout93%by 1%andthendecreasedfor2%Al.Thetransmittancealsoincreased upto1%Al.Withafurtherincreaseinthedopedconcentration, Alatomsoccupyinterstitialsitesandincreasetheabsorption.The opticalbandgap(Eg)oftheAZOthinfilmwasdeterminedbythe Tauc’srelation[23]:

˛h

hEg1/2

wheretheabsorptioncoefficient(˛),opticalband-gapenergy(Eg), histhePlank’sconstant,isthefrequencyoftheincidentpho- ton;foradirecttransitionasillustratedinFig.5.Theeffectofthe ZnO:AlonAZOfilmsincreasedthebandgapvaluesfrom3.15to 3.32eVfor1and4%dopedfilms.ThebandgapvaluesofAZOnano- compositefilmsdecreaseafterloweringZnO:waterratiofrom5 to2,andZnO:Alratioat2%isfoundtobe3.24–3.28eV,respec- tively.Theshifttoashorterwavelengthintheabsorptionedgeis

Fig.4. (A)UV–visabsorptionspectraofAZOnanofilmsfordifferentZnO:Alratios.(B)UV–visabsorptionspectraofAZOnanofilmsfordifferentZnO:waterratios.(C) TransmittancespectraofAZOnanofilmsfordifferentZnO:Alratios.(D)TransmittancespectraofAZOnanofilmsfordifferentZnO:waterratios.

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Fig.5. (˛h)2–hgraphsofAZOnanofilmsfordifferent(A)ZnO:Alratios,(B) ZnO:waterratios.

associatedwiththeBurstein–MosseffectduetoFermilevelwhich entersintotheconduction band. AZOfilmsshowabsorptionin thelongerwavelengthregionbydecreasingZnO:water.Thus,the decreaseinthebandgapoffilmscancontributetowardtheelec- trical conductivity of the AZO films. Similar band gap energy calculationofZnO:Alsampleswasalsodonebeforefortheother transparentZnOnanostructuredthin filmsaredepositedonthe substrate,butusingpulsedlaserdeposition(PLD)andspraypyrol- ysistechniques[24,25].Thebandgapoffilmswasreportedtofound intherangefrom3.72to3.46eVinthesestudies.Calculatedband gapenergyvaluesofoursol–gelderivedAZOnano-compositefilms aremuchlowerincomparisontoreportedinotherstudies.This indicatesthatitispossibletoaltertransparencyandconductivity ofthefilmswithvaryingthecompositionofthefilms.

4. Conclusions

Inthisstudy,wereportonthepreparationandstructuralchar- acterization of AZO films. The structural, electrical and optical propertiesofAZOfilmsweredeterminedbychangingZnO:water andZnO:Alvolumeratio.ThisworkhasshownthatAZOthinfilms areobtainedashexagonalwurzitestructureat500Cheattreat- menttemperature. Thecrystallitesize values arecalculated for variousZnO:AlandZnO:waterratios.Thesurfaceresistivity,the activationenergyforthenanoparticlesgrowthandthebandgap valuesofAZOnano-compositefilmscanbecontrolledbychanging ZnO:waterandZnO:Alratios.Theresultsindicatethatadecrease intheZnO:waterratioleadstotheincreaseinthecrystallitesize ofAZOfilmsduetotheagglomeration.TheroughnessofAZOfilms

alsoincreaseswiththeincreasingcrystallitesize.AZOfilmsexhibit absorptionintheshorterwavelengthregionwiththedecreasing crystallitesize,which consequentlyincreasesthebandgapval- uesofthefilms.ThedecreaseintheZnO:waterratioresultsinthe increaseinthecrystallitesizeleadingtothelowerresistivityand higherconductivityduetothecarriers.Thefilmsurfaceresistivity decreasedfrom180to82/䊐,andtheaveragetransmittancein thewavelengthrangeof330and1030nmincreasedslightlyfrom 87%to93%fordifferentZnO:Alratios.Additionally,whenZnO:Al ratioisfixedas2%,thesurfaceresistivitydecreased from70 to 46/䊐,andtheaveragetransmittanceinthewavelengthrangeof 330and1030nmincreasedslightlyfrom82%to91%fordifferent ZnO:waterratios.Transmittancevaluesofthefilmsdecreasewith thedecreasingZnO:waterratio.Theoptimumvaluesforsurface resistivityandtheopticaltransmittanceofthefilmsweredeter- minedtobe70/䊐ofresistivityand86%oftransmittanceinvisible region.Asaresultofthis,AZOnano-compositefilmscanbeused easilyintheelectronicapplications.

Acknowledgments

TheauthorswouldliketothankProf.Dr.FatmaZ.Tepehan(ITU ThinFilm Laboratory).TheResearch Fundof Mimar SinanUni- versity(BAP Projectno:201206)hasgenerouslysupportedthis research.

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