Synthesis and Characterization of Indium Niobium Oxide Thin Films via Sol-Gel Spin Coating Method

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AvailableonlineatSciVerseScienceDirectJ.Mater.Sci.Technol.,2013,29(10),923e928SynthesisandCharacterizationofIndiumNiobiumOxideThinFilmsviaSoleGelSpinCoatingMethod1)1)*1,2)SaeedMohammadi,MohammadRezaGolobostanfard,HosseinAbdizadeh1)SchoolofMetallurgyandMaterialsEngineering,CollegeofEngineering,UniversityofTehran,P.O.Box11155-4563,Tehran,Iran2)CenterofExcellenceforHighPerformanceMaterials,UniversityofTehran,P.O.Box11155-4563,Tehran,Iran[ManuscriptreceivedJanuary29,2013,inrevisedformMay1,2013,Availableonline28June2013]Inthepresentstudy,niobium-dopedindiumoxidethinfilmswerepreparedbysolegelspincoatingtechnique.TheeffectsofdifferentNb-dopingcontentsonstructural,morphological,optical,andelectricalpropertiesofthefilmswerecharacterizedbymeansofX-raydiffraction(XRD),fieldemissionscanningelectronmicroscopy(FESEM),atomicforcemicroscopy(AFM),UVeVisspectroscopy,andfourpointprobemethods.XRDanalysisconfirmedtheformationofcubicbixbyitestructureofIn2O3withasmallshiftinmajorpeakpositiontowardlowerangleswithadditionofNb.FESEMmicrographsshowthatgrainsizedecreasedwithincreasingtheNb-dopingcontent.Opticalandelectricalstudiesrevealedthatoptimumopto-electronicproperties,includingminimumelectrical3resistivityof119.410Ucmandanaverageopticaltransmittanceof85%inthevisibleregionwithabandgapof3.37eVwereachievedforthefilmsdopedwithNb-dopingcontentof3at.%.AFMstudiesshowthatadditionofNbatoptimumcontentleadstotheformationofcompactfilmswithsmoothsurfaceandlessaverageroughnesscomparedwiththepreparedIn2O3films.KEYWORDS:Indiumoxide;Nb-doping;Solegelspincoating;Transparentconductiveoxide;Opto-electronicproperties1.Introductionwiththepreparationparameterssuchasannealingtemperatureand[10]thicknessofthefilms.Transparentconductiveoxide(TCOs)thinfilmswithlowre-Inthisregard,differenttypesofimpurities,includingtin[11][12][13][14]sistivityandhightransmissionareextensivelyusedbecauseof(Sn),gallium(Ga),copper(Cu),zirconium(Zr),[1]zinc(Zn)[15],titanium(Ti)[16],tungsten(W)[17,18],molybdenumtheirapplicationsindyesynthesizedsolarcells,liquidcrystal[2,3][4](Mo)[19e21],andniobium(Nb)[22e24]havebeendopedintothedisplays,organiclightemittingdiodes,andgassensingde-[5]InO-basedmatrix.vices.AmongthevariousavailableTCOssuchasCdO,ZnO,23[22,23]andSnO2,thebestknownisIn2O3duetoitshighconductivity,Currently,Guptaetal.successfullydevelopedNb-dopedgoodtransparencyinthevisibleregion,andexcellentadhesiontoIn2O3thinfilmswithhighconductivityandtransparencyusing[6e8]pulsedlaserdepositiontechniqueandreportedtheeffectofoxygensubstrate.Inrecentyears,thedevelopmentofnovelTCOshas[22][23]attractedagreatdealofattentionsbecauseoftheirwideapplica-partialpressureandsubstratetemperatureonconductivityin[25,26]tionsinvarietyofopto-electronicfields.MoreattentionismainlyconstantNb-dopingcontent.Furthermore,Lozanoetal.focusedonthebalancebetweenhighopticaltransmittanceandinvestigatedtheadditionofdifferentNb-dopingcontentsin[9]InOmatrixinconstantpreparationprocessusingmagnetrongoodelectricalconductivity.Basedontheliterature,thenature23andamountoftheaddedimpurityhavethemostimportanteffectsputteringmethod.Accordingtotheliterature,itisexpectedthatonimprovingtheopto-electronicpropertiesofTCOscomparedadditionofNbdopantatoptimumcontentsimprovebothelectricalandopticalpropertiesofIn2O3-basedthinfilms.Thevalence5þ3þdifferenceof2betweenNbionsandsubstitutedInionsin-creasesfreecarriers,andtherebyenhancestheconductivity.Itwas*reportedthatadditionofelementslikeMoandNbcausesanin-Correspondingauthor.Ph.D.;Tel.:þ989122300382;Fax:þ9882886076;[27]creaseinmobilityofthebasedmatrix.Asaresult,itisexpectedE-mailaddress:bostanfr@ut.ac.ir(M.R.Golobostanfard).1005-0302/$eseefrontmatterCopyrightÓ2013,TheeditorialofficeofthatadditionofNbdopantintotheIn2O3-basedmatrixincreasesJournalofMaterialsScience&Technology.PublishedbyElsevierthemobilityofthesampleandreducesthefreecarrierabsorptionLimited.Allrightsreserved.causinglowerimpurityscatteringandimprovementoftrans-http://dx.doi.org/10.1016/j.jmst.2013.06.012parencyforNb-dopedthinfilms. 924S.Mohammadietal.:J.Mater.Sci.Technol.,2013,29(10),923e928Thesolegelmethodhassomeadvantagesforfilmpreparationsuchastheeasycontrolofdopinglevelandsolutionconcen-tration,abilitytocoatonthedesiredshapeandlargearea,andratherinexpensivestartingmaterialsandequipmentcompared[28]withothertechniques.ConsideringthesignificanteffectofNbdopantandadvantagesofsolegeltechnique,itisworthinvestigatingtheadditionofdifferentNb-dopingcontentstoIn2O3-basedthinfilmsviasolegelspincoatingtechnique,inordertoobtainlowcostandhighqualityTCOforopto-electronicapplicationswithlowerresistivitythanpureIn2O3thinfilmssynthesizedunderthesameconditions.2.Experimental2.1.SolpreparationIndiumniobiumoxide(INO)thinfilmswerepreparedviasolegelspincoatingtechniqueusingInCl3(AlfaAesar,99.99%),andanhydrousNbCl5(Merck,>98%)asthesourceofFig.1FlowchartforINOthinfilmspreparation.indiumandniobium,respectively.Forthispurpose,firstInCl3wasdissolvedinacetylacetoneandtheresultantsol(0.2mol/L)washeatedat85Cfor1h.Then,therequiredamountofNbCl5wasdissolvedinethanolandaddedtothestartingsolutionto3.ResultsandDiscussionpreparethefinalsolution.Theresultantsolwasmixedandrefluxedat85Cforanother1huntiltransparentINOsolwas3.1.XRDstudiesachieved.TheamountofNbCl5wasvariedtoachievedifferentNb/(NbþIn)atomicratiosof0e5at.%.Finally,theobtainedsolFig.2showstheXRDpatternsoftheINOthinfilmsasawasagedfor1dayatroomtemperature.functionofNb-dopinglevelintherangeof28e36.AllofthepeakspositioninXRDpatternsareingoodagreementwiththe2.2.FilmpreparationvaluesgiveninJCPDSfileNo.06-0416forthecubicbixbyite[29]structureofIn2O3.InallsamplesnotraceofotherunwantedTheglasssubstrateswerewashedwithadetergentandcleanedphaseslikeNb2O5canbedetected.Accordingtothisfigure,ultrasonicallywithdeionizedwaterandethanol.Then,thepre-additionofNbdopantleadstopeakbroadening,decreaseinparedcoatingsolutionsweredepositedonsodalimesubstrateintensityofthepeaks,andslightshifttolowerdiffractionangles.usingspincoatingtechnique.Spinparameters,includingspinThepeakbroadeningandintensityreductionwithNb-dopingspeedandtime,werefixedat3000r/minand30s,respectively.maybeduetothedecreaseincrystallitessizeofINOfilmsandThecoatedsubstratesweredriedat200C.SpinninganddryingreduceincrystallinitywithadditionofNbdopant.Theobservedcycleswererepeated10times.Finally,as-depositedfilmswerepeakshiftwithincreasingNb-dopingcontentcanbedescribedasannealedat500Cinairatmosphereinanelectricfurnaceforfollows.WhenNb5þionsweresubstitutedwithIn3þions,one1h.Fig.1showstheoverallflowchartforINOthinfilmsoxygenvacancygeneratesinlatticeforchargecompensation.preparationviasolegelspincoatingtechnique.Thesevacanciescanattractneighboringatomsandthismaypresumablycauseshigherplanesdistanceinsomedirections2.3.Thinfilmcharacterizationdependingonwhichplanesthesesubstitutionsandvacanciesareplaced.ThishypothesisisstillunderdebateandneedsfurtherThecharacteristicsofnanostructuredthinfilmswereinvesti-investigations.gatedbyseveralanalysistechniques.Toidentifycrystallinephase,X-raydiffraction(XRD)patternswasused.XRDstudieswereperformedbyusinganX-raydiffractometer,modelPhilipsPW1730,withCuKaradiation(l¼0.15405nm,40kV,and30mA)intherangeof28e36withstepsizeof0.02.Themeanparticlesize,morphologies,andsurfaceroughnessofthefilmswereevaluatedbyfieldemissionscanningelectronmicroscopy(FESEM,modelHitachiS4160)andatomicforcemicroscopy(AFM,modelDMEC26operatedinanon-contactmode).ThetransmissionspectraandopticalbandgapintheUVeViseIRranges(300e1000nminwavelength)weredeterminedwithaUVeVisspectrophotometerPGinstrumentmodelT80þ.Thesubstratessignalsexcludedinordertoachievepreciseopticaltransmissionofthesynthesizedsamples.Theresistivitymea-surementswerecarriedoutbyfour-pointprobemethod(Keithleymodel196Sys-DMM2).TheelectricalresistivitywasmeasuredFig.2XRDpatternsofINOfilmswithdifferentNb-dopingcontents.atroomtemperature. S.Mohammadietal.:J.Mater.Sci.Technol.,2013,29(10),923e928925Table1XRDparametersofINOthinfilmswithdifferentNb-dopingcontentscalculatedformajor(222)peakIn2O3:Nb2q(deg.)d-spacingFWHMCrystallitedopant(at.%)(nm)(deg.)size(nm)030.590.291230.3922130.550.291970.4918330.520.292380.5117530.500.292380.5316Themeancrystallitesize(D)ofthefilmswascalculatedusingtheScherrer’sequation,D¼0.89l/(bcosq),wherelisthewavelengthoftheX-rays,bisthefullwidthathalfmaximum(FWHM)of(222)diffractionpeak,andqisthediffractionangleoftheXRDspectra[30].Theresultingd-spacing(nm)andcrys-Fig.4UVeVistransmittancespectraofpureIn2O3andINOthinfilmswithdifferentNb-dopingcontents.tallitesize(nm)areshowninTable1asafunctionofdifferentNb-dopingcontents.Accordingtothistable,thecrystallitesizeofthefilmsdecreasedwiththeincreaseoftheNb-dopinglevel.0e5at.%.Ascanbeseen,asharpincreaseintransmittancespectraoccurredinthevisibleregion,indicatingthatallofthe3.2.Microstructuralstudiespreparedthinfilmsaretransparentinthisregion.ThepureIn2O3thinfilmsexhibitanaverageopticaltransmittanceofabout75%.Fig.3showstheFESEMmicrographsofINOthinfilmswithHowever,thetransmittanceofthefilmsincreaseandreachtodifferentNb-dopingcontentsaswellascrosssectionimageofhighervaluesofabout85%withtheadditionofNb-dopinguptoINOfilmwithNb-dopingcontentof3at.%.FESEMimages3at.%.revealedtheformationofnanostructuredthinfilmswithhomo-RecentresearchindicatesthatNb-dopedIn2O3filmshavegeneous,nearlysphericalandfine-grainedmorphology.Thehighermobilityandlowerfreecarrierabsorptioncomparedwithaveragegrainsizewasfoundtodecreasefrom25to15nmbyundopedInOthinfilms[27].Inaddition,thereisalowdiffer-23increasingtheNb-dopingcontent.DecreaseofgrainsizewithencebetweenionicradiiofNb5þions(0.70nm)andsubstitutedincreasingthedopingcontentcanbeexplainedasfollows.WhenIn3þ(0.81nm)latticesites,resultinginlowimpurityscattering.newdopantisadded,thegrainsizeofthefilmdecreasesmainlyConsideringthese,itisexpectedthattheadditionofNbatop-duetoincreaseofnucleationratesandinterfacialenergy;asatimumcontentimprovestheopticalpropertiesofIn2O3-basedresult,furthercrystalgrowthisinhibited.Consequently,itisthinfilms.However,thedecreaseintheaverageopticaltrans-considerablymoredifficultfordopedcrystaltogrowincom-mittanceofthefilmsathigherdopinglevels(5at.%),canbe[31]parisonwithpurehostcrystalintheabsenceofimpurities.attributedtomoreelectronscatteringandionizedimpurityscatteringathigherdopingcontentbesidesthedecreaseinsol-3.3.OpticalstudiesubilitylimitofNbintheInOmatrix[10,19,27].23Fig.5showsthevariationofopticalbandgapforINOfilmsFig.4showstheopticaltransmittancespectraofINOthinpreparedwithdifferentNb-dopingcontents.TheopticaldirectfilmspreparedwithdifferentNb-dopingcontentsintherangeofbandgap(Eg)ofthefilmscanbedeterminedfromtherelationFig.3FESEMmicrographsof:(a)pureIn2O3andINOthinfilmswithNb-dopingcontentof(b)1,(c)3,(d)5at.%,and(e)crosssectionimageforINOfilmwithNb-dopingcontentof3at.%. 926S.Mohammadietal.:J.Mater.Sci.Technol.,2013,29(10),923e9283toaminimumresistivityof119.410UcmforaNb-dopingcontentof3at.%.FurtherincreaseofNbcontent(5at.%)causes3theresistivitytoincreaseto895.510Ucm.TheinitialdecreaseinelectricalresistivitywithincreasingNb-dopingcontentuntil5þ3at.%canberelatedtothevalencedifferenceof2betweenNb3þandInions,whichgeneratestwofreecarriersperatomicsubsti-tution,leadingtoincreasingfreecarrierconcentrations.Further-more,recentstudiesinchemicalcharacterofimpuritydopantelementinTCOmaterialsrevealedthatthemobilityofdopedIn2O3thinfilmsincreasesstronglywithincreaseintheLewisacidstrength[27]ofthedopingelement.TheLewisacidstrength(L)canbe[35]calculatedusingZhang’sformulaasfollows:2zFig.5Plotof(ahy)versushyforthepureIn2O3andINOfilmswithL¼7:7xzþ0:8(2)differentNb-dopingcontents.Insetshowsthecalculatedbandr2gap(Eg)forsynthesizedthinfilms.whereristheionicradiusoftheion,zischargenumberofatomiccore,andxzistheelectronegativityoftheelementintherespectivebetweenabsorptioncoefficient(a)andphotonenergy(hy)usingoxidationstate.Therefore,substitutionofIn3þionswithNb5þ[32]Taucequationasfollows:ionsincreasesbothfreecarrierconcentrationsandLewisacid

strengthofNb-dopedthinfilms,whichultimatelyenhances2ðahyÞ¼AhyEg(1)conductivityofthefilms.However,athigherNb-dopingcontentsNbatomsprobablydowhereAandEgareaconstantandopticalbandgapofthethinnotoccupyproperlatticesitesintheIn2O3crystallitesbecauseoffilms,respectively.Thedirectopticalbandgap(Eg)canbesolubilitylimitofNbinIn2O3lattice,resultinginanincreaseofcalculatedusingTaucequationbyextrapolationofthelinearlatticedistortion.Inaddition,theincreaseinfreecarriercon-regionoftheplotintothehy-axis.Egvalueof3.08eViscentrationathigherNb-dopingcontentincreasesbothelectronobtainedforpureIn2O3films.Themaximumbandgapofscatteringandionizedimpurityscattering,whichsignificantly3.37eVisachievedfor3at.%Nb-dopingcontent.Thereducetheconductivityofthefilms.Itwasobservedthatinthewideningofthebandgapcorrelateswellwiththeincreaseofcaseofdopedindiumoxidefilms,thescatteringfromtheim-3þpuritiesaredominantespeciallywhentheimpuritycontentiscarrierconcentrationsduetothesubstitutionofInionswith5þ[33]morethanoptimumlevel[27].Nbions,knownastheBursteineMossshift.Accordingtothisrelation,theabsorptionedgeshiftstowardhigherTherelativehighresistivityofthesolegelsynthesizedfilmsenergyasthecarrierconcentrationincreases.Athigherdopingcomparedwithotherdepositiontechniquescanbeduetothelevels,forexample5at.%,theEgdecreasedtothevalueofmoreporosity,discontinuitybetweenlayers,lowercrystallinity,the3.03eVwhichcanberelatedtotheincreaseofelectroneandalsomorepossibilityofenteringimpuritiesinthefilmselectronandelectroneionizeddonorinteractions(loweringofbecauseofabsenceofhighvacuumsystemsinsolegelmethod.conductionbandedge),holeeholeinteractionsandholeeInthisresearch,theoptimumopto-electronicproperties,acceptorinteractions(increaseofvalancebandedge),andalsoincludingminimumresistivityandthemaximumtransmittanceelectroneholeinteractions.TheseinteractionsconsiderablywereachievedthroughtheadditionofNb-dopingattheoptimumdecreasethebandgapofhighlydopedsemiconductors,knowncontentof3at.%.[34]asthebandgapnarrowingorbandgaprenormalization.3.5.AFMstudies3.4.ElectricalstudiesFig.7showsAFMimagesoftheundopedIn2O3thinfilmsFig.6showsthedependenceofelectricalresistivityofINOfilms(referencesample,S0)andNb-dopedIn2O3thinfilmswithop-depositedwithvariousNb-dopingcontents.Theresistivitytimumopto-electronicpropertiesat3at.%.AFMimagesindicate3decreasedfrom324.610UcmwiththeincreaseofNbcontenttheformationofcrystallinethinfilmswithsmoothsurfaceforundopedIn2O3thinfilmswithcalculatedaverageroughnessof26.4nmandroot-mean-square(RMS)roughnessof31.8nm.However,forNb-dopedfilms,theaverageroughnessandRMSroughnessofthefilmdecreasedto7.46,and9.44nm,respec-tively.Ascanbeseen,themorecompactsurfaceoftheNb-dopedfilmcanbeattributedtoitsfine-grainedmicrostructurecomparedwithundopedIn2O3film.Hence,themorecompactandfine-grainedsurfacewithlessaverageroughnesswasformedwithadditionofNbtoIn2O3latticeatoptimumcontent.Accordingtotheobtainedresults,solegelsynthesizedNb-dopedfilmatoptimumdopantcontenthastransparencyofmorethan85%invisiblerangewhichiscomparablewithotherdopedIn2O3-basedfilmssynthesizedwithdifferentdopantele-Fig.6SheetresistanceandelectricalresistivityforpureIn2O3andINOmentsordepositionmethods.Althoughtheelectricalresistivitythinfilms.ofthesefilmsareoneorderofmagnitudelargerthanmagnetron S.Mohammadietal.:J.Mater.Sci.Technol.,2013,29(10),923e928927Fig.7TwoandthreedimensionalAFMimagesfor:(a)pureIn2O3and(b)3at.%Nb-dopedIn2O3thinfilmswithoptimumopto-electronicproperties.3sputteredandpulsedlaserdepositedfilms(10Ucmcomparedhightransmittance,acceptableconductivity,andlowcostmight4with10Ucm),theabilitytosynthesizelowcosttransparentbesynthesizedwiththismethod.conductivefilmsmakesthismethodhighlybeneficial.Therefore,newlowcostINOthinfilmswithhighopticaltransmittanceinAcknowledgmentstheUVeVisibleregion,acceptableelectricalresistivity,andTheauthorsthankIranInitiativeNanotechnologyCouncilcomparableopto-electronicpropertiescanbesynthesizedbyforpartiallysupportingthisworkandMaharFanAbzarCo.forsolegelspincoatingtechnique.AFMspectroscopyresults.4.ConclusionREFERENCESInthepresentstudy,transparentandconductingniobium-[1]O.B.Regan,M.Gratzel,Nature353(1991)737e740.dopedindiumoxide(INO)thinfilmshavebeensynthesizedby[2]B.G.Lewis,D.C.Paine,Mater.Res.Bull.25(2000)22e27.solegelspincoatingtechnique.TheinfluenceofdifferentNb-[3]M.J.Chuang,J.Mater.Sci.Technol.26(2010)577e583.dopingcontentsoncrystalstructure,morphology,andopto-[4]H.Kim,A.Pique,J.S.Horwitz,H.Mattoussi,H.Murata,Z.H.electronicpropertiesofthefilmswereinvestigated.XRDpat-Kafafi,D.B.Chrisey,Appl.Phys.Lett.74(1999)3444e3446.ternsconfirmedtheformationofthecubicbixbyitestructureof[5]M.M.Rahman,M.K.R.Khan,M.R.Islam,M.A.Halim,In2O3withasmallshiftinmajorpeakpositiontoloweranglesM.Shahjahan,M.A.Hakim,D.K.Saha,J.U.Khan,J.Mater.withincreaseofNb-dopingcontent.FESEMmicrographsSci.Technol.28(2012)329e335.revealedthatadditionofNb-dopingdecreasedthegrainsizeof[6]A.Beaurain,D.Luxembourg,C.Dufour,V.Koncar,B.Capoen,thefilms.TheobtainedresultsrevealedthatthefilmsdepositedM.Bouazaoui,ThinSolidFilms516(2008)4102e4106.[7]T.M.Hammad,Phys.StatusSolidiA206(2009)2128e2132.withNb-dopingcontentof3at.%exhibitedahighestoptical[8]H.Cho,Y.H.Yun,Ceram.Int.37(2011)615e619.transmittanceofabout85%inthevisibleregionandthelowest[9]W.C.Chang,S.C.Lee,C.H.Yang,T.C.Lin,Mater.Sci.Eng.B1533electricalresistivityof119.410Ucmwithabandgapof(2008)57e61.3.37eV.AFMimagesshowedthatlowersurfaceroughnesswith[10]S.Kaleemulla,N.M.Rao,M.G.Joshi,A.S.Reddy,S.Uthanna,morecompactsurfacecouldbeachievedthroughadditionofNbP.S.Reddy,J.Alloy.Compd.504(2010)351e356.atoptimumcontenttotheIn2O3lattice.Inconclusion,itshould[11]Y.Chao,W.Tangy,X.Wang,J.Mater.Sci.Technol.28(2012)bementionedthattheINOtransparentconductivefilmswith325e328. 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