Characterization of delafossite-CuCrO2 thin films prepared by post-annealing using an atmospheric pressure plasma torch

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AppliedSurfaceScience258 (2012) 8775–8779

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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

Characterization

of

delafossite-CuCrO

2

thin

ﬁlms

prepared

by

post-annealing

using

an

atmospheric

pressure

plasma

torch

Hong-Ying

Chen

∗

,

Wei-Jung

Yang,

Kuei-Ping

Chang

DepartmentofChemicalandMaterialsEngineering,NationalKaohsiungUniversityofAppliedSciences,415ChikenKuangRoad,Kaohsiung807,Taiwan,ROC

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received17November2011 Receivedinrevisedform23April2012 Accepted17May2012

Available online 24 May 2012 Keywords:

Transparentconductiveoxides Delafossite

CuCrO2 Thinﬁlms

Atmosphericpressureplasma Annealing

a

b

s

t

r

a

c

t

Thisstudyreportsthepreparationofdelafossite-CuCrO2thinﬁlmswerepreparedonquartzsubstrates

usingsol–gelprocessingandpost-annealingwithanatmosphericpressureplasmatorch.Theﬁlmswere ﬁrstdepositedonaquartzsubstratebyspincoating.Thespecimenswerethenannealedat500◦Cin airandpost-annealedwithanatmosphericpressureplasmatorchwithN2–5%O2at650◦Cfor20min.

ThespecimensannealedinairexhibitedCuOandCuCr2O4phases.Post-annealingusinganatmospheric

pressureplasmatorchobtainedthepureCuCrO2(delafossite,R3m)phase.Thebindingenergiesofthe

Cu-2p3/2andCr-2p3/2peaksoftheCuCrO2thinﬁlmswerecenteredat932.1±0.2eVand576.1±0.2eV,which

revealedthevalencestateofCu+_and_Cr3+_in_the_ﬁlms._The_chemical_composition_of_CuCrO

2thinﬁlmswas

closetothestoichiometry.AstheCuCrO2phaseformed,theﬁlmsurfacebegantoexhibitagglomerate

fea-turesandthecross-sectionalmorphologyshowedanequiaxedgrainfeature.Theaveragetransmittance ofCuCrO2thinﬁlmswasapproximately66%inthevisibleregion.ThedirectopticalbandgapofCuCrO2

thinfilmswas3.08eV,whichisconsistentwithreporteddataintheliterature.ThepositiveSeebeck coefficientsoftheCuCrO2thinfilmpreparedbypost-annealingusinganatmosphericpressureplasma

torchconfirmedthep-typecharacteristicsofthefilm.TheelectricalconductivityofCuCrO2thinfilmswas

2.7×10−2Scm−1withacarrierconcentrationof2.8×1013_cm−3_._Hence,_{post-annealing}_using_an

atmo-sphericpressureplasmatorchis aneffectivetooland afeasible methodforpreparingtransparent conductivedelafossitethinﬁlms.

1. Introduction

Wide-bandgapoxidesemiconductorshave becomeattractive inrecentyearsbecauseoftheiroptoelectronicproperties. Trans-parentconductingoxides(TCOs)combineelectricalconductivity andopticaltransparencyinasinglematerial[1].Therefore,TCOs have numerouspotential applications,includingsolarcells, ﬂat paneldisplays,electromagneticshieldingdevices, light-emitting diodes,andtransparentheatsources[2].Themostpopular wide-bandgapTCOscurrentlyavailable exhibitn-typecharacteristics. However,p-typewide-bandgapTCOshavenotbeeninvestigated thoroughly[1,2].Delafossitesarep-typewide-bandgapmaterials withthebasicformulaAI_MIII_O

2,where,Aisamonovalentcation, suchasCuorAg,andMisatrivalentmetalrangingfromAltoLa

[1].Cu-baseddelafossitesarethetypicallylayeredcrystal struc-tureswithCu2Olayersand MIII2O3 layers.AmongtheCu-based delafossites,CuCrO2hasahigherelectricalconductivitythanthe others[1].

∗ Correspondingauthor.Tel.:+88673814526x5130;fax:+88673830674. E-mailaddress:[email protected](H.-Y.Chen).

Based on the successfulpreparation of p-type CuAlO2 films [3],researcheshavedevelopeddifferentthin-filmdeposition tech-niquesfordepositingdelafossitematerials[2].CuCrO2thinfilms can bedeposited using pulsed laser deposition[4–8], chemical vapordeposition[9],andthesol–gelmethod[10–16].However, thesevacuum-basedprocessesarecostlyandtimeconsuming,and oftenrequirea highsubstratetemperatureorpost-annealingto formthedesirabledelafossitephaseorimprovefilmcrystallinity

[6,8].Thechemicalsolution-basedmethodhasrecentlybecomean effectiveandalternativetechniquefordepositingthedelafossite films.Thismethodhasmanyadvantages,includinglow-cost,easy setup,largeareacoating,andmassproduction.Thismethod gener-allyrequiresatwo-stepannealingprocess[10–16].Inprinciple,the first-stepannealingoxidizesthefilms,formingprecursorphases. Thesecond-stepannealingthenformsthedesireddelafossitephase andimprovesthecrystallinityoftheresultingfilms.

Fordelafossite-CuCrO2thinfilms,thefirst-stepannealinginair formsCuOandCuCr2O4phases[10–16].Thesecond-step anneal-inginflowingArorN2 convertstheseprecursorstotheCuCrO2 phase[14–16].However,thistypeofannealingprocessiscostly andtime-consuming.Therefore,itisnecessarytodevelop afast andeconomicalannealingprocess.Atmosphericpressureplasma sourceshaveattractedconsiderableattentioninthepastdecades.

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H.-Y.Chenetal./AppliedSurfaceScience258 (2012) 8775–8779 8779

Fig.5. TheopticaltransmittancespectrumoftheCuCrO2thinfilmpreparedby post-annealing(a)usinganatmosphericpressureplasmatorchor(b)inN2(insetfigures: directopticalbandgapofthefilm).

post-annealed using an atmospheric pressure plasma torch. The positive Seebeck coefficient of the CuCrO2 film (approxi-mately 230␮V/K) further confirmed its p-type characteristics. The electrical conductivity () of CuCrO2 thin films was 2.7×10−2Scm−1 and the corresponding carrier concentration was 2.8×1013_cm−3_. _The _electrical _conductivity _of _the _CuCrO₂ thin film prepared by post-annealing in N2 was 0.15Scm−1 with a carrier concentration of 1.7×1018_cm−3_. _The _positive Seebeckcoefficientofthefilmconfirmeditsp-type characteris-tics.TheelectricalconductivitiesoftheCuCrO2thinfilmsprepared bypost-annealingusinganatmosphericpressureplasmatorchor inN2areingoodagreementwiththosereportedintheliterature [6,9,14,16]forundopedCuCrO2thinfilms.

The electrical conductivityof delafossite compounds is con-trolledmainlybytwofactors.First,thestoichiometryofthefilms cansignificantlyinfluencetheelectricalconductivitythroughthe filmstheformationofdefects,suchascoppervacancies((VCu)) andantisitedefects((CuCr))[21].Second,grainboundariesplay acrucial role intheelectrical conductivityof thefilmsbecause ofthepolycrystallinenatureofthefilms[6].Inthisstudy,grain boundaryscatteringismostlikelytolimittheelectrical conductiv-ityoftheCuCrO2thinfilm.Thisisbecauseofthefine-grainsinthe

specimen post-annealedusing anatmospheric pressure plasma torch(cf.Fig.3(b)).

3. Conclusion

Delafossite-CuCrO2 thin filmswere preparedonquartz sub-strates bysol–gel processing followed by post-annealing using anatmosphericpressureplasmatorch.CuOandCuCr2O4phases weredetectedinthesol–gel-derivedfilmsannealedat500◦Cin air.Thespecimenpost-annealedusingan atmosphericpressure plasmatorchat650◦Cfor20min,singleandtransparentCuCrO2 thinfilmswithadelafossitestructurewereobtained.Thebinding energiesoftheCu-2p3/2 andCr-2p3/2 peakofCuCrO2 thinfilms preparedbypost-annealingusinganatmosphericpressureplasma torchwerecenteredat932.1±0.2eVand576.1±0.2eV, confirm-ingtheCu+_and_Cr3+_valence_state_in_the_films._The_{stoichiometric} CuCrO2thinfilmswerealsoobservedusingXPS.Thesurface mor-phology of CuCrO2 thin filmsexhibited anagglomerate feature andthecross-sectionalmorphologyhadanequiaxedgrainfeature. ThedirectopticalbandgapofCuCrO2thinfilmswas3.08eV,with anaveragetransmittanceof66%inthevisibleregion.Thep-type characteristicsofCuCrO2thinfilmswereconfirmedbypositive See-beckcoefficients.TheelectricalconductivityofCuCrO2thinfilms was2.7×10−2Scm−1 withthecorrespondingcarrier concentra-tionof2.8× 1013_cm−3_._These_results_show_that_sol–gel_processing andpost-annealingusinganatmosphericpressureplasmatorch formeddelafossite-CuCrO2thinfilms,makingthisapproacha fea-siblepreparationmethodwithashortpost-annealingtime.

Acknowledgments

TheauthorsthanktheNationalScienceCounciloftheR.O.C.for ﬁnancialassistanceundergrantnumbersNSC 98-2221-E-151-024-MY2andNSC100-2221-E-151-027.WearealsogratefultoDr. Ruei-SungYuatAsiaUniversity,Taiwan,forassistancewiththe Hall-effectmeasurements.

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