Effects of post-annealing on the structural and nanomechanical properties of Ga-doped ZnO thin films deposited on glass substrate by rf-magnetron sputtering

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

Effects

of

post-annealing

on

the

structural

and

nanomechanical

properties

of

Ga-doped

ZnO

thin

films

deposited

on

glass

substrate

by

rf-magnetron

sputtering

Szu-Ko

Wang

_,

_Ting-Chun

_Lin

_,

_Sheng-Rui

_Jian

_,

_Jenh-Yih

_Juang

_,

_Jason

_S.-C.

_Jang

_,

_Jiun-Yi

_Tseng

a_{DepartmentofMaterialsScienceandEngineering,I-ShouUniversity,Kaohsiung840,Taiwan} b_{DepartmentofElectrophysics,NationalChiaoTungUniversity,Hsinchu300,Taiwan}

c_{DepartmentofMechanicalEngineering,InstituteofMaterialsScience&Engineering,NationalCentralUniversity,Chung-Li320,Taiwan} d_{InstituteofPhysics,AcademiaSinica,Taipei11529,Taiwan}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received12June2011

Receivedinrevisedform17August2011 Accepted20September2011

Available online 24 September 2011 Keywords: ZnO:Gathinfilms XRD AFM Nanoindentation Hardness

a

b

s

t

r

a

c

t

Inthisstudy,theeffectsofpost-annealingonthestructure,surfacemorphologyandnanomechanical propertiesofZnOthinfilmsdopedwithanominalconcentrationof3at.%Ga(ZnO:Ga)areinvestigated usingX-raydiffraction(XRD),atomicforcemicroscopy(AFM)andscanningelectronmicroscopy(SEM) andnanoindentationtechniques.TheZnO:Gathinfilmsweredepositedontheglasssubstratesatroom temperaturebyradiofrequencymagnetronsputtering.Resultsrevealedthattheas-depositedZnO:Ga thinfilmswerepolycrystallinealbeitthelowdepositiontemperature.Post-annealingcarriedoutat300, 400and500◦C,respectively,hasresultedinprogressiveincreaseinboththeaveragegrainsizeandthe surfaceroughnessoftheZnO:Gathinfilm,inadditiontotheimprovedthinfilmscrystallinity.Moreover, thehardnessandYoung’smodulusofZnO:GathinfilmsaremeasuredbyaBerkovichnanoindenter operatedwiththecontinuouscontactstiffnessmeasurements(CSM)option.ThehardnessandYoung’s modulusofZnO:Gathinfilmsincreasedastheannealingtemperatureincreasedfrom300to500◦C,with thebestresultsbeingobtainedat500◦_C.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Transparent conductiveoxides (TCOs) have become increas-ingly prominent in the fabrication of various devices such as heterojunction solar cells [1,2], gas sensors [3] and flat panel devices[4].Theefficiencyandperformanceofthesedevicesare largelydependentontheopticalandelectricalpropertiesofthe relevantTCOmaterials.Indiumtinoxide(ITO)isthemostwidely usedTCOduetoitshightransparencyinthevisiblerange(∼90% at550nm),lowresistivity(∼2×10−4cm)andlargework func-tion(∼4.8eV)[5].However,recentlyITOhasbecomeprohibitively expensive,inadditiontoitslackofthermalstability.Asaresult, alternativessuchastheimpurity-dopedZnOhavebeenactively investigated.GroupIIIelementsAl[6],Ga[7]andIn[8]havebeen demonstratedtoexhibitthecharacteristicsofn-typedopantsfor ZnO.Amongthem,Gaisconsideredasoneofthemost promis-ingdopants becausetheGa–O covalentbond length(1.92 ˚A) is veryclosetothatoftheZn–O(1.97 ˚A),which,inturn,isexpected to result in the possibility of obtaining wide range of doping concentrationwithminimumextentoflatticedeformation,even for highGa dopingconcentration[9]. Furthermore,it hasbeen

∗ Correspondingauthor.Tel.:+88676577711x3130;fax:+88676578444. E-mailaddress:srjian@gmail.com(S.-R.Jian).

shown that Ga is relatively oxidation resistive [10]. Previously, Ga-dopedZnOfilmshavebeenobtainedusingavarietyof depo-sition methods,namelysol–gel[11], chemical vapordeposition [12], molecularbeamepitaxy[13], pulsedlaser deposition[14] andradio-frequencymagnetronsputtering(rf-sputtering)[15,16]. Amongthem,therf-sputteringhasbeenwidelyusedforfabricating oxidethinfilmsbecauseofitsadvantagesofhighdepositionrates, lowcost,easycontrolandhighefficiencyforgrowingfilmswith goodquality.

In additiontomonitoringtheelectric and opticalproperties throughcarefulcontroloftheprocessingparameters, successful fabricationofdevicesbasedonGa-dopedZnOthinfilmsrequires betterunderstandingofthemechanicalcharacteristicsofthefilms, sincethecontactloadingduringprocessingorpackagingcan signif-icantlydegradetheperformanceofthesedevices.Therefore,there isagrowingdemandofinvestigatingthemechanical characteris-ticsofmaterials,inparticularinthenanoscaleregime.Nowadays, nanoindentationisprobablythemostprominenttechniqueused innanomechanicstoinvestigateandcharacterizethemechanical propertiesof thematerialsin thesub-micronscale.It hasbeen widelyusedtostudytheelastic–plasticandfracturepropertieson thesurfacesofbulksamples[17–19],thinfilms[20–23],aswell asforsmallstructures[24,25].Morerecently,itbecamepossible toperformcontrolledcompressionandsheartestsonsub-micron nanostructures,suchasnanopillars[26,27].

0169-4332/$–seefrontmatter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.09.088

1262 S.-K.Wangetal./AppliedSurfaceScience258 (2011) 1261–1266

Thisstudyisthereforefocusedonnanomechanical characteriza-tionsofGa-dopedZnOthinfilmsdepositedatroom-temperatureon theglasssubstratesusingrf-sputteringsystematvariousannealing temperaturesbymeansofnanoindentationtechnique.The struc-tureandsurfacemorphologyofthinfilmswerecharacterizedusing X-raydiffraction(XRD),atomicforcemicroscopy(AFM)and scan-ningelectronmicroscopy(SEM).Changesinmechanicalproperties fortheZnO:Gathinfilmsarediscussedinconjunctionwiththe vari-ationsincrystallinestructure,grainsizeandsurfacemorphology resultedfromannealing.

2. Experimentaldetails

TheZnO:Gathinfilmsinvestigatedinthepresentstudywere depositedonCorning1737Fglasssubstratesatroomtemperature usingrf-sputteringfromaZnOtargetwithanominalGa-dopingof 3at.%.Inthiswork,thethinfilmsare∼500nmthick.Thedetailed growthprocedurescanbefoundelsewhere[28].Theas-deposited filmsweresubsequentlypost-annealedatthetemperatures rang-ingfrom300◦Cto500◦Cfor1hinatmosphere.Theheatingrate wassetat20◦C/sandittookabout30minforthefurnacetocool downtoroomtemperatureafterannealing.

Thecrystalstructure of ZnO:Gathin filmswere analyzedby X-raydiffraction(PanalyticalX’PertXRD,CuK␣,=1.5406 ˚A).A scanningprobemicroscopy(SEM,HitachiS-2700)isusedtoanalyze thethinfilmscross-sectionalstructureand,thesurfacefeatures arecarriedoutusingatomicforcemicroscopy(AFM) (Topometrix-Accures-II).Thesurfaceroughnesscanberepresentedbycenter lineaverage(Ra)androot-mean-squareaverage(RRMS)[29]inthe

followingforms: Ra= 1 n n













Thecenterlineisthelinethatdividestheprofileinsuchaway suchthatthenetdeviationiszero.BothRaandRRMSmeasurethe

averageverticaldeviationofsurfaceprofilefromthecenterline.It shouldbenotedthattheseparameterscanonlybeusedtocompare samplesurfacesgeneratedbythesamemethod[30].

Nanoindentationwasconductedatroomtemperatureusingthe MTSNanoXP®_{system(MTSCorporation,NanoInstruments}

Inno-vationCenter,OakRidge,TN,USA)withforceanddisplacement resolutionsof50nNand0.1nm,respectively.ABerkovichdiamond indenterwaspressedintothefilmsupto80nmwithastrainrate varyingfrom0.01s−1to1s−1andadditionalharmonicmovements weresimultaneouslyperformedwiththeamplitudeandfrequency beingsetat 2nmand 45Hz, respectively,following the contin-uousstiffnessmeasurements(CSM)technique[31].Theindenter wasthenheldatthepeakloadfor10sbeforeitwascompletely withdrawnfromthespecimentoavoidtheinfluenceofcreepon unloadingcharacteristics,whichwereusedtocompute mechan-icalpropertiesofthespecimen.Eachofthetestswasperformed whenthethermaldriftdroppedtolessthan0.01nm/s.Atleast20 indents,eachtwoseparatedby10␮mtoavoidmutualinteraction, wereconductedoneachsample.Itisgenerallyacceptedthatthe indentationdepthshouldneverexceed30%ofthefilmsthicknessto avoidthesubstrateeffectonhardnessandmodulusmeasurements [24].Oursamplesandtestsmethodologywereconsideredto ade-quatebasedonthisconcept.Wealsofollowedtheanalyticmethod proposedbyOliverandPharr[32]todeterminethehardnessand Young’smodulusof as-depositedandtheannealedZnO:Gathin filmsfromtheload–displacementresults.Inthiswork,hardness

Fig.1.XRDpatternsof(a)as-depositedZnO:Gathinfilm,andannealedZnO:Gathin filmsatthedifferentannealingtemperaturesof(b)300◦_C,(c)400◦_Cand(d)500◦_C.

andYoung’smodulusareobtainedasacontinuousfunctionof pen-etrationdepth.

3. Resultsanddiscussion

ThecrystalstructuresandthelevelofcrystallinityforallZnO:Ga thin films were identified by XRD, as displayed in Fig. 1. The obtainedXRDpatternscorrespondtotheindexedsixdiffraction peaksofwurtzitestructuredcrystallineZnO(JCPDS36-1451).Itis evidentthatincreasingtheannealingtemperaturenotonlyleadsto significantincreaseintheintensityofmajordiffractionpeaks,such as(002),(101),and(103)peaks,butalsoinducestheevolvement ofnewdiffractionpeaks,suchas(110)and(112),thatare other-wiseabsentintheas-depositedZnO:Gathinfilms.Thisisindicative thatannealingdoeshavenoticeableinfluencesonthe microstruc-tureofthefilms,whichtendstodrivethefilmstobecomemore equiaxial.AssumingahomogenousstrainacrosstheZnO:Gathin films,theaveragegrainsizecanbeestimatedfromthefull-width athalf-maximum(FWHM)of(002)peakusingthefollowing Sher-rer’srelation[33]:

D= 0.9

Bcos  (3)

Here,BanddenotetheX-raywavelength,theFWHMof(002) peak,andthecorrespondingBraggdiffractionangle,respectively. Theestimatedmeangrainsizesoftheas-depositedthinfilmsis 23.5nmandthatforfilmsannealedat300◦C,400◦Cand500◦C are42.2,60.4and70.8nm,respectively.Itisinterestingto com-parethecurrentresultswiththatobservedforZnOfilmsdeposited using atomic layer deposition (ALD). In that, the as-deposited

Fig.2.AFMimagesofZnO:Gathinfilms:(a)as-deposited,andannealedatthedifferentannealingtemperaturesof(b)300◦_C,(c)400◦_Cand(d)500◦_{C.Theinsetofeach}

figureshowsthecorrespondingcross-sectionalSEMimage.

ZnO thin filmsdeposited at room-temperatureappeared tobe largelyamorphous andsubsequentannealing inthesame tem-peraturerangeresultedinsmallergrainsize,namelyfrom30nm to50nm[34].

Fig.2 displaysthesurfacemorphology ofthecorresponding ZnO:Gathin filmsrevealedbyAFM.Althoughitisimpossibleto giveadirectmeasureabouttheactualgrainsizefromtheAFM images, the resultsdo exhibit apparent evolution in film grain morphologieswithincreasingannealingtemperatures.The mor-phologystartswithanappearanceofpatchedgrainclustersfor theas-depositedfilm[Fig.2(a)].Aftersubjectedto1hannealing at300◦C,itappearstoevolveintoindividuallydistinctivegrains [Fig.2(b)].Grainagglomerationsareevidentforfilmsannealedat 400◦C [Fig.2(c)].For filmsannealed at500◦C, grains evidently start toalign along somespecific orientations[Fig.2(d)]. Since thefilmsweregrownonglasssubstratesandnoepitaxialrelation

wasexpected,thesephenomenamightbeunderstoodasadirect consequenceofsurfacediffusionenabledthree-dimensionalgrain growth[34].Inthisscenario,higherannealingtemperatureslend morethermalenergytoactivateatomdiffusionand,hence, facil-itate therepairing the dislocated atomic occupanciesand even promotethecoalescenceofadjacentgrains[34,35].Asshownin thecross-sectionalSEMimagesinsertedinFig.2,thefilmsareall withcolumnarstructures.Thus,themajorgraingrowthisexpected toresultinmarkedincreaseinboththeeffectivegrainsizeand thesurfaceroughnessof theresultantfilms[34–36].Indeed,as showninFig.3,theeffectivegrainsizeestimatedfromthe Sher-rer’srelationandthesurfaceroughnessobtainedfromtheAFM measurementsforallZnO:Gathinfilmsdogiverisetoconsistent resultsasexpected.

The typical load–displacement curves for the as-deposited and the annealed ZnO:Ga thin films are displayed in Fig. 4.

1264 S.-K.Wangetal./AppliedSurfaceScience258 (2011) 1261–1266

Fig.3. Grainsize(D),averagesurfaceroughness(Ra)androot-mean-squareaverage

surfaceroughness(RRMS)ofas-depositedandannealedZnO:Gathinfilms.

The load–displacement response obtained by nanoindentation containsinformationabouttheelasticandplasticdeformationof theindentedmaterials.Thus,itisoftenregardedasa“fingerprint” of the film properties under identification. Mechanical proper-ties,suchasthehardnessand Young’smodulus,canbereadily extractedfromtheload–displacementcurveslikethosedisplayed inFig.4.Forinstance,thehardness,H,beingdefinedasthemean pressureundertheindenter,canbecalculatedasthemaximum appliedloadduringindentationmeasurement,Pmax,dividedbythe

projectionarea,Ac,ofcontactbetweentheindenterandthesample

asexpressedbelow, H=Pmax

Ac (4)

Theprojectioncontactareaisafunctionoftheindenter’sshapeand thecontactdepth,hc.Forthecaseofanidealpyramidalindenter,

theareafunction(Ac)isgivenby[32]:

Ac≈24.5h2c (5)

Thecontactdepthpriortounloadingthuscanbedirectlyestimated fromtheload–displacementdatabythefollowingexpression[32]: hc=hmax−εPmax

S (6)

whereεisthegeometricconstantand,thevalueε=0.72isgenerally usedforaconicalorpyramidalindenter[32],Sisthe experimen-tallymeasuredstiffnessoftheupperportionoftheunloadingdata, whichisgivenby:

S=dP

dh (7)

Similarly,Young’smodulusisdeterminedbyassumingthatthearea incontactremainsconstantduringinitialunloading.The relation-shipbetweenloadanddisplacementoninitialunloadingisrelated tothestiffnessofthesampleandtheindenterand,totheconstant contactareabetweenthesampleandtheindenter,andisgivenby [37]: Er= 1 2ˇhc







1 Er = 1₋

v

v

v

Poisson’sratioforthesampleand,Eiand

vi

fortheindenter.Poisson’sratioforthesampleisassumedtobe0.25 [38]and,theelasticpropertiesofthediamondindenterusedinthis studyareEi=1141GPaand

vi

ThehardnessandYoung’smodulusfor theas-deposited and the annealed ZnO:Ga thin films as a function of penetration depthobtainedusingtheanalysesdescribedaboveareillustrated in Fig. 5(a) and (b), respectively. As shown in Fig. 5(a), all of thehardness–displacementplotscanbedividedintotwostages, namely, initial increase to a maximum value and subsequent decreasetoa constantvalue. Theincreasein hardnessatsmall penetrationdepthisusuallyattributedtothetransitionbetween purelyelastictoelastic/plasticcontactandatthisstagethe hard-nessisnotaccuratelymeasuredbythemeancontactpressure.Only undertheconditionofafullydevelopedplasticzonedoesthemean contactpressurerepresentthehardness.Whenthereisnoplastic zone,oronlypartiallyformedplasticzone,themeancontact pres-sureislessthanthenominalhardness[32].Afterthefirststage, thehardnessdecreasesandreachesaconstantvalue.Theconstant characteristicofhardnessisconsistentwiththatofasingle mate-rial;therefore,thehardnessvaluesatthisstagecouldberegarded asintrinsicpropertiesofthefilms.Theobtainedhardnessforthe as-depositedZnO:Gathinfilmsandthoseannealedat300,400and 500◦Care8.5,7.4,9.6,11.8GPa,respectively.Thehardness val-uesobtainedfromtheabovementionedCSMmeasurementsforall samplesunderstudyaresummarizedinFig.5.Theinitialdropof thehardnessfrom8.5GPato7.4GPafortheas-depositedZnO:Ga thinfilmandthatannealedat300◦Cisbelievedtoresultfromthe relaxationofresidualstressinthefilmbytheannealingprocess [39].

Ontheotherhand,asdisplayedinFigs.3and5,thereisaclear tendencyshowingthat,whileincreasingtheannealing tempera-turehasevidentlyincreasedthegrainsizeofZnO:Gathinfilms, theannealingtreatmenthasledtotheincreaseoffilmhardness, aswell.Theresultsappearedtofollowthenotionoftheinverse Hall–Petcheffect[40].It hasbeenpointedoutthat dislocations areplayingtheprimaryroleintheHall–Petcheffect,whileforthe inverseHall–Petcheffectgrainboundaryslidingisprominentfor thefilmhardness[41,42].Consequently,thebehaviorsobserved heremaybeindicativethatgrainboundarystructureismore rel-evanttotheprimarymechanicalresponsesduringindentationin theZnO:Gathinfilms.Thismightalsoexplaintheabsenceof appar-entpop-ineventintheload–displacementcurvesshowninFig.4, whereinthegrainboundariesacttheprimarystrain compensa-tionsitesandhencesuppressthenucleationandpropagationof threadingdislocationsneededforexhibitingpop-ins[43].

Fig.5(b)displaysaplotofYoung’smodulusofas-depositedand theannealedZnO:Gathinfilmsdeterminedusingthemethodof OliverandPharr[32].Thetendencyofvariationissimilartothe hardnessresultsillustratedinFig.5(a).ThevaluesofYoung’s mod-ulusforas-depositedfilmis113.4GPa,andthatforfilmsannealed at300,400,and500◦Care101.3,120.6,138.4GPa,respectively(see Fig.5(c)).Furthermore,thenanomechanicalpropertiesofZnOthin filmsbynanoindentationaresummarizedinTable1.

Asdiscussedabove,thenanoindentation-induceddeformation inthepresentfilmsbehavesverydifferentlyfromthoseexpected forbulkZnO [48].Namely,theinitiationofplasticdeformation, insteadofresultingfromthesuddenformationandrapidsliding ofalargenumberofdislocations,mayhavebeenmorerelevant tograinboundaryslidingand/orgrainrotations[41,42].Therefore, thegenerallylargergrains(∼40to70nm)obtainedinthecurrent

Fig.5. Nanoindentationresults:(a)thehardnessand(b)Young’smodulusvs. pen-etrationdepthcurvesofas-depositedandannealedZnO:Gathinfilms;(c)the hardnessandYoung’smodulusofas-depositedandannealedZnO:Gathinfilms.

studyascomparedtothatoftheatomic-layer-depositionZnOthin films,where grainsizeintherangeof30–50nmwereobtained [34],mayexplainthemechanicalresponsesobservedhere.Itis notedthatdopingmightalsomodifythegrainmorphologyand, hence,themechanicalbehaviorsofZnOthinfilms.For instance, Zhaoetal.[45]reportedthatborondopingcandecreasethe thick-nessofgrainboundariesinpulsedlaserdepositedZnOthinfilms, whicheventuallybroughtonimprovementoffilmshardness.

1266 S.-K.Wangetal./AppliedSurfaceScience258 (2011) 1261–1266

Table1

ThemechanicalpropertiesofZnOthinfilmsbynanoindentation.

ZnOfilms/a-SA ZnOfilms/c-SA ZnOfilms/6H-SiC ZnOfilms Al-dopedZnOfilms Ga-dopedZnOfilms

H

11.5±0.8GPa[38] 7.4±0.1GPa[38] 5.9±0.2GPa[38] 7.2–9.8GPa[34] 4.8±1.3–7.1±1.8GPa[47] 7.4–11.8GPaa

6.6±1.2GPa[44] 5.7±0.8GPa[44] 9.3–12.1GPa[45]

8.7±0.2GPa[46]

E

212.2±0.1GPa[38] 150.1±5.7GPa[38] 117.1±0.4GPa[38] 139.5–168.6GPa[34] 87.9±5.5–92.2±12.6GPa[47] 101.3–138.4GPaa

318.2±50GPa[44] 310.1±40GPa[44] 103.5–114.4GPa[45]

154±5GPa[46]

SA:sapphire.

a_{Thepresentstudy.}

4. Conclusion

Inconclusion,acombinationofXRD,AFM,cross-sectionalSEM andnanoindentationtechniqueshasbeencarriedouttoinvestigate themicrostructuralandnanomechanicalcharacteristicsofZnO:Ga thinfilmsannealedatthevarioustemperatures.

TheXRDanalysisshowedthatZnO:Gathinfilmswereequi-axial polycrystallineinnature,albeitthatpredominant(002) orienta-tionandaroughersurfacemorphologywasgraduallydeveloped withincreasingannealingtemperature.Nanoindentationresults indicatedthat,whilethegrainsizewasincreasedwithincreasing annealingtemperature,theZnO:Gathinfilmshavehardness rang-ingfrom7.4to11.8GPaandYoung’smodulusrangingfrom101.3 to138.4GPawithincreasingannealingtemperature.The appar-entinverseHall–Petcheffectisattributedtothegrainboundary dominatedmechanicalresponsesascomparedtothetraditional dislocationthreadingmechanism.

Acknowledgements

ThisworkwaspartiallysupportedbytheNationalScience Coun-cilofTaiwan,underGrantNo.:NSC100-2221-E-214-024.J.Y.J.is partiallysupportedbytheMOE-ATUprogramoperatedatNCTU. AuthorlikestothankDr.Y.-S.Lai,Dr.P.-F.Yang,Dr.G.-J.Chenand Dr.Y.-T.Chenfortheirtechnicalsupports.

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