Effects of doping concentration and annealing temperature on properties of highlyoriented Aldoped.docx
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Effects of doping concentration and annealing temperature on properties of highlyoriented Aldoped.docx
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EffectsofdopingconcentrationandannealingtemperatureonpropertiesofhighlyorientedAldoped
Effectsofdopingconcentrationandannealingtemperatureonpropertiesofhighly-orientedAl-dopedZnOfilms
Shou-YiKuoa,
Wei-ChunChena,b,Fang-ILaic,Chin-PaoChengb,Hao-ChungKuod,Shing-ChungWangdandWen-FengHsiehd
aInstrumentTechnologyResearchCenter,NationalAppliedResearchLaboratories,20R&DRoadVI,HsinchuSciencePark,Hsinchu300,Taiwan
bDepartmentofMechatronicTechnology,NationalTaiwanNormalUniversity,No.162,HopingE.Road,Section1Taipei106,Taiwan
cDepartmentofElectronicEngineering,ChingYunUniversity,229JianshingRd.,JungliCity,Taoyuan320,Taiwan
dDepartmentofPhotonicsandInstituteofElectro-OpticalEngineering,NationalChiao-TungUniversity,1001TaHsuehRoad,Hsinchu300,Taiwan
Availableonline1December2005.
Abstract
Transparentandconductivehigh-preferentialc-axis-orientedAl-dopedzincoxide(ZnO:
Al,AZO)thinfilmshavebeenpreparedbythesol–gelroute.Filmdepositionwasperformedbyspin-coatingtechniqueonSi(1 0 0)andglasssubstrate.Structural,electricalandopticalpropertieswereperformedbyXRD,SEM,four-pointprobe,photoluminescence(PL)andUV-VISspectrummeasurements.Theeffectsofannealingtemperatureanddopantconcentrationonthestructuralandopticalpropertiesarewelldiscussed.ItwasfoundthatbothannealingtemperatureanddopingconcentrationalterthemicrostructuresofAZOfilms.Also,PLspectrashowtwomainpeakscenteredatabout380 nm(UV)and520 nm(green).ThevariationofUV-to-greenbandemissionwasgreatlyinfluencedbyannealingtemperaturesanddopingconcentration.ReductioninintensityratioofUV-to-greenmightpossiblyoriginatefromtheformationofAl–ObondsandlocalizedAl-impuritystates.Theminimumsheetresistanceof104 Ω/□wasobtainedforthefilmdopedwith1.6 mol%Al,annealedat750 °C.Meanwhile,allAZOfilmsdepositedonglassareverytransparent,between80%and95%transmittance,withinthevisiblewavelengthregion.Theseresultsimplythatthedopingconcentrationdidnothavesignificantinfluenceontransparentproperties,butimprovetheelectricalconductivityanddiversifyemissionfeatures.
Keywords:
B1.ZnO:
Al;B2.semiconductingII–VImaterials
PACS:
70.;73.61.Ga;73.90+f;68.60.−p
ArticleOutline
1.
Introduction
2.
Experimentalprocedure
3.
Resultsanddiscussions
4.
Conclusion
References
1.Introduction
Zincoxide(ZnO)isawide-bandgapII–VIsemiconductorwithveryattractivepropertiessuchashightransparencyinthe0.4–2 μmopticalwavelengthrange,highpiezoelectricconstant,largeelectro-opticcoefficient,andlargeexcitonbindingenergyof
60 meVatroomtemperature[1],[2],[3]and[4].Inrecentyears,thefabricationofZnO-basedfilmshasattractedaconsiderableamountofinterestduetotheirpotentialapplicationinsolarcells,gassensors,opticalwaveguides,surfaceacousticdevices,piezoelectrictransducersandvaristors[5],[6],[7],[8]and[9].Inaddition,dopingofZnOwithvariouselementshasbeenreportedtoimprovetheirelectricalconductivityforuseinoptoelectricdevices.Therefore,dopedZnOfilmsarealternativematerialstotinoxideandindiumtinoxide,whichhavebeenwidelyusedastransparentconductingoxide(TCO)becauseoftheirsuperiorelectricalandopticalproperties,abundanceinnature,nontoxicity,andtheexcellentstabilityinhydrogenplasma,anunavoidableprocessingambientinsilicon-relatedfields[10].
Itiswellknownthatthestructuralpropertiesanddopantsmaydeterminetheelectronicandphotoluminescence(PL)propertiesofthematerials.ThetypicaldopantsthathavebeenusedtoenhancetheconductivitiesofZnOarethegroupIII(B,Al,In,Ga)oftheperiodictable.Amongthem,Al-dopedZnOfilmshavebeenwidelystudiedandareconsideredascandidatematerialsfororganicelectroluminescencedisplays[11].SeveraltechniqueshavebeenusedforthepreparationofZnOthinfilmssuchasspraypyrolysis[12],chemicalvapordeposition[13],sputtering[14],reactiveevaporation[15],pulsedlaserablation,filteredcathodicvacuumarctechnique[16]and[17],andhydrothermalmethod[18]and[19].Anumberofreportshavealsobeenpublishedonsol–gel-derivedundopedandimpurity-dopedzincoxidefilms[20],[21]and[22].Despitethecrystallinequalitybeinginferiortoothervacuumdepositiontechniques,thesol–geltechniquestillhasdistinctadvantagessuchascosteffectiveness,simplicity,excellentcompositionalcontrol,homogeneityandlowercrystallizationtemperature.Moreover,incorporationofdopantsiseasierinthistechnique.Hence,thistechniqueissuitableforfrontierresearches.
Inthepresentstudy,Al-dopedZnO(AZO)thinfilmswith0–5%molarconcentrationswerepreparedbysol–gelmethod.Previousworksmostlyfocusedontheelectricalandopticalpropertiesinfluencedbyeitherdopantsorthermaltreatments.Differentfromtheirinvestigation,theinfluencesofdopantconcentrationandthermaltreatmentonthestructural,electricalandopticalpropertieswereinvestigated.
2.Experimentalprocedure
Zincacetate-2-hydrate[Zn(CH3COO)2·H2O]andaluminumnitratenonahydrate[Al(NO3)3·9H2O]werechosenasthestartingmaterials,andalcoholicsolutionwereusedassolvent.Thedetailsofthepreparationmethodaresimilartothosedescribedinearlierliteratures.Theconcentrationofzincacetatewaschosentobe1 mol l−1,andtheprecursorsolutionhasbeenmixedthoroughlybyamagneticstirrer.Appropriateamountsofaluminumdopingwereachievedbyaddingaluminumnitratetotheprecursorsolution.Inourexperiment,thedopantlevel,determinedby100×[Al]/[Al+Zn],wasvariedfrom0to5 mol%.Theresultantsolutionwasstirredat80 °Cfor2 htoyieldaclearandhomogeneoussolution,whichservedasthecoatingsolutionaftercoolingdowntoroomtemperature.Thecoatingwasusuallymade3daysafterthesolutionwasprepared.
Thesolutionwasspinning-coatingontofusedquartzandSi(1 0 0)wafersubstrates,whichwererotatedat3000 rpmfor30 s.Aftereachlayerdeposition,thegellayerswerepre-heatedat350 °Cfor20 minoverahotplatetoevaporatethesolventandremoveorganicresiduals.Theprocesswasrepeatedseveraltimestoobtainadesiredthickness.Afterthedepositionofthelastlayer,thefilmswereinsertedintoafurnaceandannealedinambientatmosphereatseveraltemperaturesfrom450–850 °Cfor1 h.
X-raydiffraction(XRD,SiemensD5000)analyseswereconductedusingCu-KαradiationtodeterminethecrystallinityoftheAZOfilms.Surfaceandcross-sectionmorphologywereexaminedbyaHitachiS-4300scanningelectronmicroscope(SEM).Theelectricalresistancewasmeasuredbyafour-pointprobemethod.OpticaltransmittancemeasurementswerecarriedoutusingaUV–VIS–NIRspectrophotometer.ThePLspectrawererecordedatroomtemperatureusingaHe–Cdlaseratthewavelengthof325 nmasexcitationsource.
3.Resultsanddiscussions
TypicalXRDpatternsofthe3 mol%AZOthinfilmsannealedatvarioustemperaturesareshowninFig.1(a).XRDspectrainFig.1(a)showthatthesol–gel-derivedZnO:
AlfilmsdevelopedwithouttheformationofsecondaryphasesandclusterssuchasAl2O3andamorphousZnO.Allfilmsexhibitonlythe(0 0 2)peak,indicatingthattheyhavec-axispreferredorientationduetoself-texturingphenomenon.FromFig.1(a),itisreadilyobservedthattheintensityofthe(0 0 2)diffractionpeakincreaseswhentheannealingtemperaturerisesupto850 °C,andthecrystallinityofthefilmsdeterminedfromthefull-widthathalf-maximum(FWHM)valuesshowsthesametrend.Bothtendenciesindicateacommonrepresentativeoftheimprovementincrystallinequalitywithraisingannealingtemperature.Usually,itissuggestedthatthecrystallinitycanbeenhancedwhileoneincreasestheannealingtemperatures,consistentwiththeXRDresultsofFig.1(a).Tocalculatetheaveragegrainsizeofthesamples,theScherrer'sequation[23],
(1)
isemployed.Whereλisthex-raywavelengthof1.54 Å,θBisBraggdiffractionangle,andBistheFWHMofθB.ThecalculatedaveragecrystallitesizesofpureZnOthinfilmsannealedat450,550,650and750 °Care10,11,11.5and13 nm,respectively.Theresultsimplythatthegrainsizeofsol–gel-derivedfilmsenlargewiththeincreasingannealingtemperatures,whichcanbeunderstoodbyconsideringthemergingprocessinducedfromthermaltreatment.ForZnOnanoparticles,therearemanydanglingbondsrelatedtothezincofoxygendefectsatthegrainboundaries.Asaresult,thesedefectsarefavorabletothemergingprocesstoformlargergrainswhileincreasingtheannealingtemperature.Moreover,the(0 0 2)diffractionpeakintensitiesofAZOfilmsdecreasedwithincreaseddopingconcentrationsmorethan1.6 mol%asshowninFig.1(b).Thisindicatesthatanexcessincreaseindopingconcentrationdeterioratesthecrystallinityoffilms,whichmaybeduetotheformationofstressesbythedifferenceinionsizebetweenzincandthedopantandthesegregationofdopantsingrainboundariesforhighdopingconcentrations.
Fig.1. (a)XRDpatternsof3 mol%Al-dopedZnOfilmsasafunctionoftheannealingtemperature.(b)XRDpatternsof750 °CAZOfilmswith1,1.6and3 mol%aluminum-dopingconcentration.
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AshightransparencyisthemostimportantfactorintheapplicationofZnO:
AlfilmstoTCOs,theopticaltransmittancewasdeterminedbyaspectrophotometerwithinthewavelengthfrom350to850 nm.RegardlessoftheAlcontentsinthefilms,AZOfilmsannealedabove450 °Cdemonstratetransmittanceofabove80%intherangeofthevisiblespectrum.Bandgapenergieses
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