Chemical vapor deposition of low reflective cobalt (II) oxide films

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Chemical vapor deposition of low reflective cobalt (II)

oxide films

Eliane Amin-Chalhoub, Thomas Duguet, Diane Samélor, Olivier Debieu,

Elisabeta Ungureanu, Constantin Vahlas

To cite this version:

Eliane Amin-Chalhoub, Thomas Duguet, Diane Samélor, Olivier Debieu, Elisabeta Ungureanu, et al..

Chemical vapor deposition of low reflective cobalt (II) oxide films. Applied Surface Science, Elsevier,

2016, 360, pp.540-546. �10.1016/j.apsusc.2015.10.188�. �hal-01264368�

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To link to this article : DOI :

10.1016/j.apsusc.2015.10.188

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To cite this version : Amin-Chalhoub, Eliane and Duguet, Thomas and

Samélor, Diane and Debieu, Olivier and Ungureanu, Elisabeta and

Vahlas, Constantin Chemical vapor deposition of low reflective cobalt

(II) oxide films. (2016) Applied Surface Science, vol.360. pp.540-546.

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Chemical

vapor

deposition

of

low

reflective

cobalt

(II)

oxide

films

Eliane

Amin-Chalhoub,

Thomas

Duguet

∗

_,

_Diane

_Samélor,

_Olivier

_Debieu,

Elisabeta

Ungureanu,

Constantin

Vahlas

CIRIMAT,CNRS–UniversitédeToulouse,4alléeEmileMonso,BP-44362,31030ToulouseCedex4,France

Keywords:

Opticalreflectivity

MOCVD

Refractiveindexgradient

CoO

Cobaltoxide

a

b

s

t

r

a

c

t

LowreflectiveCoOcoatingsareprocessedbychemicalvapordepositionfromCo2(CO)8attemperatures

between120◦_C_and₁₉₀◦_C_without_additional_oxygen_source._The_optical_reflectivity_in_the_visible_and

nearinfraredregionsstemsfrom2to35%dependingondepositiontemperature.Thecombinationof specificmicrostructuralfeaturesofthecoatings,namelyafractal“cauliflower”morphologyandagrain sizedistributionmoreorlesscoveringthenearUVandIRwavelengthrangesenhancelightscattering andgivesrisetoalowreflectivity.Inaddition,thecolumnarmorphologyresultsinadensitygradientin theverticaldirectionthatweinterpretasarefractiveindexgradientloweringreflectivityfurtherdown. Thecoatingformedat180◦_C_shows_the_lowest_average_reflectivity_(2.9%),_and_presents_an_interesting

deepblackdiffuseaspect.

1. Introduction

Lowreflectivityfilmsareusedinopticalinstrumentsand sen-sorswiththeaimtoattenuatenoise,namelytoreducetheeffect ofstrayandscatteredlightindifferentspectraldomains[1].Such filmsfindapplicationsinmilitarynightsightforopticalguidance systems[2],in space applications[3] and canalso beused for heatdissipationbetweendifferentcomponentsinmicroelectronic devices[4].Moreover,lightabsorberfilmsareusedinthe renew-ableenergydomaininphotothermalenergyconversion[5–8],in photoelectricenergy conversion[9], and insolar thermophoto-voltaicconversionsystems[10–12].

Blackcoatingsarespread overalargevarietyofmaterialsin thefamilies of metals, alloys, ceramics, and polymers[5,6,13]. Amongthem,thecobalt(II,III)oxideCo3O4 exhibitshighoptical

absorbancewhichisappropriateforsolarabsorbersfor photother-malconversion[14].Indeed,Co3O4 presentsspectrallyselective

surfacesexhibitinghighvaluesofsolarabsorbance(˛)inthe visi-bleandthenearinfra-red(NIR)spectrumandlowvaluesofthermal emittance(ε)intheIRspectrumwhichimprovestheirthermal per-formancebyreducingtheradiativeheatlosscomponent.Moreover, incontrasttochromiumbasedblackcoatings,Co-basedones main-tainacceptablevaluesof˛_andε_even_after_exposure_to_air₍₁₀₀₀_h at650◦_C_[15]_).

∗ Correspondingauthor.Tel.:+33534323439.

E-mailaddresses:thomas.duguet@ensiacet.fr,doug181@gmail.com(T.Duguet).

Cooxidefilmsareobtainedbywetprocessessuchasthermal decomposition,electrodeposition[14,16,17]andsol–gel[18].Dry processesarealsoemployedsuchasPVD[19,20],ALD[21–24]and

metalorganicCVD(MOCVD)[25–28].SeveralauthorsuseMOCVD

togrowcobalt oxidefilms. Fujiet al.formedCooxidefilmsby

plasmaenhancedMOCVD(PEMOCVD).TheyobtainCoOfilmsat

lowoxygenflowratewithasurfacemorphologyintheformof

packedcolumnargrains.Increasingtheoxygenflowrateleadsto theformationofcolumnarCo3O4filmswithinthesame

temper-aturerange(400◦_C)_[29]_._Similar_observations_are_mentioned_by

Tyezkowskietal.whoshowthatfilmcompositionishighly corre-latedtotheO2molarfraction.Filmsarecomposedessentiallyof

Co3O4 withthepresenceofcarbonandamorphousCoOxatlow

O2 flowrate[25].ManeandShivashankar similarlyobtainedby

MOCVDdenseandcontinuousCo3O4 filmswithapackedgrains

morphologywhenusingO2,butporousfilmscomposedofa

mix-tureofCo3O4andCoOwhileusingN2O[30].Co3O4filmsaregrown

aswellbyDirectLiquidInjection(DLI-)MOCVDunderaworking pressureof5Torrbutattemperatureshigherthanours:350–600◦_C

[26]. Authors notice that while increasing deposition tempera-ture,thefilmmorphologyvariesdependingonthesubstratetype. FilmroughnessincreasesinthecaseofLaAlO3substratewhileit

decreasesinthecaseofsapphireandMgO;aninterestingwayfor tailoringthemicrostructureofCooxidefilms.

Finally,differentMOCVD precursorscanbeusedin orderto

obtain Co oxide films, suchas tricarbonyl nitrosyl [27], dicar-bonylcyclopentadienylcobalt[31],Cobalt(II)b-diketonateadducts

[26], andCobalt(II) acetylacetonate[30]. In particular,dicobalt octacarbonyl(Co2(CO)8)isusedtoprocessfilmsatlowtemperature

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Table1

MOCVDprocessparametersusedforthedepositionofcobaltoxidefilms.

Depositiontemperature(◦_C)_–_T_d _120–190

Precursortemperature(◦_C) ₂₇

Totalpressure(Torr) 5

SaturationvaporpressureofCo2(CO)8(Torr) 0.0041

N2carriergasflowrate(sccm) 30

N2dilutiongasflowrate(sccm) 170

Co2(CO)8flowrate(sccm) 24.6×10−3

Co2(CO)8molarfraction* 1.23×10−4

Depositionduration(min) 80

*_Upper_limit_assuming_full_efficiency_of_precursor_vaporization_and_transport.

(<200◦_C),_ultimately_allowing_processing_thermally_sensitive

sub-strates[32–36].ThepresentworkrevisitstheuseofCo2(CO)8for

thelowtemperatureMOCVDofcobaltoxides,withfollowingthree peculiarities:(a)itinvestigatesthethermolysisoftheprecursorin theabsenceofreactivegas,e.g.oxygen,(b)itaimsatprocessing pureCoOfilms,(c)itfocusesonthemicrostructureofthefilms.Itis worthnotingthatsuchanapproachhasneverbeeninvestigatedfor theMOCVDofcobaltoxides.Therationaleoftheworkisto

elab-orateaprocessoperatinginaparametricwindowwhichallows

processingfilmswithstablephysicochemicalandmicrostructural characteristicsandthushighperformanceopticalproperties.

The paper is organized as follows. After the experimental

details section, we present and discuss the results

concern-ing the process–structure–properties relationship. Growth rate,

crystallography, composition, and morphology of the coatings

arecorrelatedwithdepositiontemperature(Td).Then, the

opti-cal reflectivity of the films is correlated with chemical and

microstructural characteristics,namelysurfaceroughness, grain sizedistributionandfractaldimension,beforeproviding conclud-ingremarks.

2. Experimentaldetails

CVDexperiments areperformedinaverticalcold-wall

reac-torcomposedofa46mmdiameterquartztube.Siliconsamples

(20×10×1mm3₎_are_cut_from₄′′_Si(1₀₀₎_wafers_and_sonicated

inacetoneandethanolbathsfor5min,driedinanArflow,and

baked at 60◦_C _for ₂₀_min._They_are _weighted _and _immediately

positionedflatonaninductivelyheatedstainlesssteelsusceptor. DepositiontemperatureTdiscontrolledbyaK-typethermocouple

insertedintothecoreofthesusceptor.Surfacetemperatureis cali-bratedwithanadditionalthermocoupleattachedtothesurfaceofa dummySisample.PureN2(99.9999%,AirProducts)isusedasboth

carrieranddilutiongasandisdeliveredthroughtwogaslines, ther-mallyregulatedat35◦_C,_and_equipped_with_mass_flow_controllers.

Avanepumpandpressuregaugesconnectedtotheoutputofthe

quartztubeisusedtoregulatethereactionpressureat5Torr.Tdis

variedfrom120to190◦_C_in_order_to_investigate_its_effect_on_the

filmsmorphology,compositionandopticalabsorptivity.Co2(CO)8

powder(AlfaAesar,stabilizedin5%hexane)isfilledintoaUshaped tube,inagloveboxunder99.9997%pureAr(AirProducts)before beingconnectedunderN2flowwiththeCVDset-up.Theprecursor

isthermallyregulatedat27◦_C_resulting_in_a_saturated_vapor

pres-sureof0.0041Torr.ItsvaporiscarriedwithN2leachingthesurface

ofthepowder,andthenmixedwiththedilutiongasN2.Thisgas

mixtureisintroducedfromtheupperpartofthereactorthroughan 8mmtubefacingthesubstrateholder.Theresultinginputprocess conditionsarelistedinTable1.

Surfacemorphologyandroughnessaremeasuredusing

scan-ning electron microscopy (SEM) and atomic force microscopy

(AFM).SEMisperformedonaLEO435VPmicroscope.AFMisused

inambientconditionsonanAgilentTechnologies5500instrument. Scanningisperformedincontactmodewithtipsofspringconstant

Fig.1.Arrhenius-typeplotofthegrowthrateofCoOfilmsfromCo2(CO)8onSi.

ofabout0.292N/m(AppNano). Scanningrateis2mm/s.Images

areprocessedwiththesoftwarePicoImage(AgilentTechnologies). FilmsthicknessesaremeasuredbySEMoncrosssectional micro-graphsatthecenterofeachsample.Crystallographicstructuresare determinedbyX-raydiffraction(XRD)onaSEIFERT-3000TT instru-mentusingaCuKa(1.5418 ˚A)X-raytubeoperatedat40kVand

40mA,aNifilterandsolid-stateLynxeyedetector.EPMAisusedto determinetheelementalcompositionofthefilms(CamecaSXFive instrument).Calibrationisperformedusinghighpuritystandards.

Each measurement is repeatedfive times atdifferent locations

todeterminespatialhomogeneity.Finally,theUV–vis–NIRtotal

reflectivity spectra are measured by a Perkin-Elmer

Lambda-19spectrophotometerequippedwithanintegrationsphere.The

reflectedlightiscollectedinthedirectional-hemispherical geom-etrywithanincidenceangleof8◦_.

3. Resultsanddiscussions 3.1. Growthkinetics

Processedfilmssystematicallypresentamataspect.Thecolor changeisvisuallyobservedonthesiliconsubstratesduringthefirst minutesofdeposition,correspondingtotheupperlimitof incuba-tiontime.Visualinspectionofthesurfacefacingthefeedingtube revealsthatfilmsdepositedat160◦_C_and₁₈₀◦_C_are_black,_while

thoseobtainedat120◦_C,₁₃₀◦_C,₁₄₀◦_C_and₁₉₀◦_C_are_grey._Fig.₁

presentsanArrhenius-typeplotoftheglobaldepositionreaction. Linesconnectingpointsareguidetotheeye.Growthratefluctuates asafunctionofTd,increasingfrom120to130◦C,thendecreasing

downto160◦_C_and_finally_increasing_up_again_at_higher_T_d_._Growth

ratevaluesvarybetween110nm/minatTd=120◦Cand160◦C,and

225nm/minatTd=130◦C,resultinginfilmswhosethicknessvaries

between9and18mm,asmeasuredoncrosssectionsbySEM.

Then,thegrowthratedoesnotshowanArrhenius-type behav-ior.Itevolvesbetweentwomaximaat130◦_C_and₁₉₀◦_C_and_one

minimumat160◦_C._This_behavior_has_already_been_reported_for

processesinvolvingCo2(CO)8.Whilescanningarangeof

tempera-turefrom50◦_C_to₂₅₀◦_C_at_a_working_pressure_of_0.01_Torr_(which

istwoordersofmagnitudelowerthanours),Yeetal.noticetwo growthratemaximaat120◦_C_and₂₂₀◦_C._They_attribute_the_sharp

increaseofthedepositionratearound120◦_C_to_a_possible_increase

of the stickingcoefficient of the precursor at low temperature

[33,37].Then,thegrowthrateislikelytodecreaseasTdisfurther

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Fig.2.XRDpatternsoffilmsprocessedatdifferentTd.StarscorrespondtotheSi

substratepeaks.

thesamepressurerange(0.05Torr)andatasubstratetemperature

varyingfrom160◦_C_to₂₈₀◦_C,_Rhee_and_Ahn_obtain_a_maximum

growthrateat200◦_C_[38]_,_in_good_accordance_with_the

tempera-turesdeterminedinthepresentstudy(190◦_C)_and_reported_by_Ye

etal.’s(220◦_C)._Similarly,_Ko_et_al._observed_a_maximum_at₁₄₀◦_C

(P=0.6Torr)[35],ingoodaccordancewithourlowestmaximum (130◦_C)_and_Ye_et_al.’s₍₁₂₀◦_C).

3.2. StructureandmorphologyoftheCoOfilms

Fig.2presentsX-raydiffractogramsofthefilmsprocessedat varioustemperatures. TheX-raypeaksat2 equal36.4◦_,_42.4◦_,

61.5◦_,_73.7◦_and _77.6◦ _correspond_to₍₁₁₁₎₍₂₀_0),₍₂₂_0),₍₃₃₁₎

and(222)planesofthecubiccobaltoxide(II)CoOphase.Noother cobaltoxide,namelyCo3O4orCo2O3isrevealedfromthe

diffrac-tograms.Filmspresentnopreferentialorientationandtheaverage crystallitediametercalculatedfromtheSherrerequationis esti-matedbetweenca.100nmand150nm.Itisconcludedthatallfilms arecomposedofCoOwithnoothercrystallinephase.Thisresultis corroboratedwiththeelementalcompositionofthefilms, deter-minedas50±1at.%Co,50±1at.%O(exceptat140◦_C_where_the

compositionis47%at.%Co,53at.%O)withoutcarbon contamina-tion,withinthedetectionlimitofEPMA.Itisinterestingtorecall thatweobtainpureCoOfilmswithoutintroductionofanyreactive gas.Thisprecursorisgenerallyusedforthedepositionofsmooth metalliccobaltfilmsatlow pressure(0.03–0.2Torr)[33–36,39]. Theappliedworkingpressureof5Torrinthisworkleadstothe formationofcobalt(II)oxide.Asimilarresultismentionedinref

[28],whereauthorsobtainedcobaltoxidefilmsunderN2working

pressuresof2.2and45Torr.Whereastheyintentionallyadda par-tialpressureofhydrogenfordecreasingtheamountofCoOinthe films,theyobservedanincreaseoftheamountofCoOfrom43to 76at.%!Thispinpointsthecomplexchemistrytakingplaceatthe surfaceofagrowingtransitionmetaloxidefilminthepresenceof

Fig.3.Topview(left)andcrosssection(right)SEMimagesoffilmsdepositedunder

differenttemperatures.Filmthickness(mm)isshownatcorrespondinglocations.

acarbonylprecursor,bothbeingcatalysts.Thoroughinvestigation ofsuchphenomenonhasbeenattemptedintheliterature(seefor example[40])butitisoutsideofthescopeofthepresentwork.

Fig.3showsSEMimagesoftheCoOfilmsdepositedat

differ-enttemperatures, both in surface viewand cross-section view.

Filmsaredenseandpresentacompactcolumnarstructurewith

acauliflowermorphologywhichisreproducedatdifferentlength scalesfrom0.05mmatthesurfaceofthegrainsto5mmforthe larger features diameter. Films processed at 120◦_C _and ₁₆₀◦_C

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Fig.4.AFMimageofaCoOcoatinggrownat160◦_C.

present asurface thatis more accidentaland rougherthanthe

otherfilms,andtheircolumnsarewiderwithalargerconicalangle. Interestingly,theycorrespondtothetwominimumgrowthratesas showninFig.1,andtheyaretheonlysampleswithnovisiblecracks. CrosssectionSEMimagesshowthatfilmthicknessvariesbetween

9mmand19mminagreementwiththoseestimated

experimen-tallybysamplemassdifferencebeforeandaftereachdeposition;a goodhintforalimitedporositywithinthefilms.

Fig.4showsanAFMimageofthe160◦_C_CoO_film,_with_the

typ-icalcauliflowermorphology.Itexhibitsaroot-meansquare(RMS)

roughnessofca.350nm,whereasfilmsgrownat190◦_C_show_a

RMSroughnessof120nm,only.Thedifferencebetweenthe

pro-jectedsurface(400mm2for160◦Cand300mm2for190◦C)andthe scannedsurface(225mm2forboth)showsahigherspecificarea forthesampleprocessedat160◦_C._{Interestingly,}_fractal

dimen-sionsdeterminedbytheboxpartitioningmethodare2.35and2.31, at160◦_C_and₁₉₀◦_C, _{respectively;}_values _that_equal_the_natural

cauliflowerfractaldimensionof2.33.

Therefore,thepresentCoOfilmsexhibitafractalmorphology. Thecauliflowershape isrepeatedidenticallyat differentscales, fromnano-tomicro-.Intheliterature,onlyafewgroupsreported ontheformationofCoOfilmswithacauliflowerstructure[41,42].

Guptaetal.obtainedthecauliflower morphologyon

electrode-positedfilmsofcobalt–nickeloxide(intheformsofNiOandCo3O4)

withatypicalthicknessofabout10mm[42].Alternatively, differ-entfractalstructuresofcobaltoxide(CoOandCo3O4)thinfilms

havebeensynthesizedbyusinglaserCVDatroomtemperatureand atmosphericpressure.Sizeandmorphologyofthestructurescan betailoredbythelaserirradiationtimeandthegasflowratio[43]. Atwo-stepprocesscanalsobeusedinordertoobtainaporousand fractalsurfacemorphology,wheresmoothmetalliccobaltsurface arefirstformedbyPEMOCVDandthenpost-treatedinanO2plasma

[44].Theabove-mentioneddepositiontechniquesprovideCooxide filmswithtargetedcauliflower microstructure.However,unlike abovewhereonlysomedegreesoffreedomaretechnically avail-able,MOCVDallowstoobtainthefractalcauliflowermorphology

withauniqueprecursorandnoreactivegaswhateverthe

depo-sitiontemperatureis.ButwewillshowbelowthatTdaffectsthe

microstructure,whichinturnimpactsthereflectivityperformance ofthefilms.

Inanattempttoperformastatisticalanalysisofthegrainsize distributionweusethefollowingprocedure.SEMImagesata mag-nificationof×10,000areprocessedateachdepositiontemperature (Gwyddionfreeware[45]).First,amaskisappliedoneachimageby adjustingaslopethreshold,takingadvantageofthefactthatgrains visibleonthesurfaceareseparatedbyvalleys.Thereforethemask

floodsdepressionsbetweengrainsand underlinessurface grain

boundaries.Inordertoassesstheconsistencyofthisanalysis,four slopethresholds(7.5%,10%,12.5%and15%)areused;thetwo max-imabeingdeterminedvisuallybytheoperator.Itshowsthatthe

Fig.5. Grainsizedistribution(Countsvs.Diameter)forsixdifferentdeposition

temperatures.

trenddoesnotvary,meaningthatthetemperaturedependenceof thegrainsizedistributionisconstantwhatevertheslopecriterion.

Oncethemaskhasbeeninverted,anequivalentradiusis

auto-maticallycomputedforeachgrain.Then,weperformafrequency analysistoextractthegrainsizedistributions.

Fig.5showstheresultswithaslopethreshold fixedat10%. Weobserveaclassificationofgrainsizedistributions.The distri-butionforfilmsgrownat160◦_C_is_larger_than_that_at₁₉₀◦_C,₁₈₀◦_C

and120◦_C,_themselves_being_larger_than_that₁₃₀◦_C_and₁₄₀◦_C.

Therefore,withinstatisticalandSEMresolutionlimits,twomain featurescanbeextractedfromthegrainsizedistributionanalysis. First,eachT_dleadstotheformationofsurfacegrainswith

diam-etersintherange50nmtoca.200nm(ingoodagreementwith

thebulkSherreranalysisofmeancrystallitessizeof100–150nm). Secondly,thedispersionofthedistributiondependsonTd,andthe

threehighestdepositiontemperaturesproducethelargest distri-butions.Hence,surfacesgrownat160◦_C,₁₉₀◦_C_and₁₈₀◦_C_exhibit

arelativelylargersizedistributionoflighttraps,inthe subwave-lengthlightscale,inadditiontocauliflowerswithdiametersupto severalmicrometers.

3.3. Opticalproperties

Fig.6showsthereflectivityspectraoftheCoOfilmsprocessed at different Td. Each spectrum shows a low reflectivity

broad-bandwhichpresentsasignificantevolutionwithvaryingTd.Its

positionisshiftedtolongerwavelengthsanditswidthgradually increaseswithincreasingTduntilitcoversthewholevisible

spec-traldomainat160◦_C_and₁₈₀◦_C._The₁₆₀◦_C_and₁₈₀◦_C-CoO_films

showariseoftheirreflectivityat1050nmwhichcorrespondstothe band-gapenergyoftheSisubstrate.Theyexhibitinterestinglylow [400–800nm]averagereflectivityof4.2and2.9%,respectively.This observationiscomparablewithliteratureresultsbyseveralgroups whodepositedeitherCoO[41]orCo3O4[46]filmsby

electrodepo-sitiontechniquesforsolarabsorberapplications.Interestingly,the electrodepositedCoO(OH)films,thatresultfromsurfacehydration, showaverysimilarsurfacemicrostructure.Thereflectivitytrendof thefilmprocessedat190◦_C_is_rather_offset_compared_to_the_other

fivefilms.Whileitpresentsalowreflectivitybandaswell,itis nar-rowandlocatedatlowwavelengths(i.e.300nm);similartothatof thefilmprocessedat120◦_C._The_mean_value_of_the_reflectivity_of

thisfilminthe400–800nmdomainislargelyhigherthanthoseof theotherfilms.However,itdecreaseswithincreasingwavelength, reachingca.8%at1200nm.Whereasitcouldrepresentthemost

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Fig.6. ReflectivityspectraofCoOfilmsonSiwafersforvariousTd.

appropriatesampleforapplicationsintheinfraredpartofthe elec-tromagneticspectrum,weconsideritasanexperimentalartifact regardingthetrendvs.T_d.

Inthefollowingwediscusspossiblemechanismsresponsible

forthelowreflectivity,anditsevolutionwithTd.

SincetheCoOfilmsthicknessismuchhigherthanthevisible wavelengths,andsincethesefilmsarenotcomposedofnanoscale multilayersstack,weassumethatinterferencesdonotaffect reflec-tivity.Besides,spectralfeaturescannotbeattributedtointerference fringes.Distinctinterferencefringescouldbeobservedbutonlyfor thinnerfilms(notshownhere).

Althoughseveralstudies onCoO show a bandgapof about

5–6eV (200–250nm) [47,48], othergroups showthat the

indi-rect band gap of CoO ranges from 2.3 to 2.8eV (440–550nm)

[30,49,50].Asa result,interbandtransitions cannot explainthe reflectivityofallthespectra.Especiallythetwolowerreflective samplesdepositedat160◦_C_and₁₈₀◦_C_show_a_rise_of_their

reflec-tivityatnoticeablylowerenergies of1.2–1.3eV(950–1000nm). However,onecannottotallyexcludethattransitionsatenergies

lowerthantheCoObandgapcanoccurinourfilmsasreported

elsewhere[47,49].AndCoOiseffectivelystudiedforitsabsorption propertiesforsolarthermalabsorbers[51],showingthatitisable toabsorbefficientlyinthevisiblerange.Nevertheless,thedistinct riseofthereflectivityattheSiband-gapat1050nmofthe160◦_C

and180◦_C-CoO_films_demonstrates_that_the_absorption_decreases

towardsNIRwavelengths.Theabruptincreaseofreflectivityisdue tothethresholdwheretheextinctioncoefficientofSibecomesnull, andwherethemetallicsampleholdercontributionincreases.

AscanbeennoticedfromSEMobservationinFig.3,thelow reflectivefilmdepositedat160◦_C_shows_a_surface_with_the_highest

densityofmicrometriccauliflowergrains.Butasimilarfilmsurface isobtainedat120◦_C_whereas_it_shows_the_worse_visible

reflectiv-ityperformance.Then,weassumethattheTd-dependenceofthe

reflectivityisnotduetoscatteringcausedbymm-wide cauliflow-ers:reflectivityismanifestlynotcorrelatedtosurfacedensityand shapeofmm-sizedcauliflowers.However,lightscatteringplaysa significantroleinthereflectivitysincethediffuselightreflectivity spectrumoverlapalmostperfectlywiththetotalreflectivity spec-trum(notshown).Therefore,intheabsenceofastrongcorrelation betweenmicrometricfeaturessurfacedensityandreflectivity,we focusontheevolutionofthefilmstructureatthenanoscale.

Takingintoaccountthecomplexsurfacemicrostructure,wecan consideramodelsurfacelayermadeofsubwavelengthCoOgrains

andvoids(air).Thesurfacecanthenbemacroscopicallyconsidered usingtheeffectivemediumapproximationbyalayerwhose refrac-tiveindexisavolumedensity-weightedaverageofthetwomedia. Althoughtherefractiveindex(n)ofCoOsingle-crystalliesbetween 1.8and2.6(imaginarypart0.03<k<1.37)inthewavelengthrange [200–1200nm][47],therefractiveindexoftheeffectivemediumat thesurfacecanbesignificantlyloweredbecauseofthecontribution ofair(n=1.0).Moreover,thelikelyoccurrenceofmicrostructural changesalongthecolumnarstructurecouldresultinariseofthe densityfromfreesurfacetowardsthebulk.Asaconsequence,the refractiveindexoftheCoOfilmsincreaseswithdepth.Suchfilms havebeenfirstanalyzedbyLordRayleigh[52]whonoticedthat reflectioncanbeconsiderablyreducedwhenthetransitionofthe densitybetweentwomediaisgradual.Foraninfinitelythick non-absorbinglayer,asmoothtransitionbetweentherefractiveindex oftheincidentmediumandtherefractiveindexinthebulkofthe layerresultsinreflectivityclosetozeroaccordingtotheFresnel coefficient[53,54].Foranabsorbinglayerhavingafinitethickness, thatisgreaterthantheskindepthoftheeffectivemedium,the reflectivityisreducedprovidedthattherefractiveindexatthefree surfaceofthelayerisclosefromtheoneoftheairandalsoifno sharpmodificationoftheeffectiverefractiveindexoccursalongthe skindepthofthelayer.

Accordingtothismodel,weconcludethatTdappearsto

influ-encetheeffectiverefractiveindexofthelayerprobablythrough thefilmdensity.Severalauthorsdemonstratethatthewideningof therefractiveindexgradientresultsinasignificantincreaseofthe lowreflectivebroadbandwidth[54,55]similartoourexperimental spectra.Otherparameterscouldalsoinfluencetherefractiveindex suchasthestructuralconfigurationofCoOwhichvaries accord-ingtothedepositionparametersand/orimpuritiesinclusioninthe film.Whateverthecauseleadingtothemodificationofthe effec-tiveindex,itresultsinasignificantreflectanceresponsevariation withTd.

4. Conclusions

We formblackCoO films(ca.50at.% Co,50at.%O) onSiby MOCVDfromCo2(CO)8ataworkingpressureof5Torr.Wefocus

ontheinfluenceofgrowthtemperature(120–190◦_C)_on_the_film

microstructureandtheresultingopticalreflectivity.Weformthick

(9–18mm)columnarfilmswhichexhibitafractalmorphologyof

cauliflowers,aspecificityofferedbyMOCVDascomparedtoother

depositiontechniques.ThefractaldimensiondeterminedonAFM

surfaceimagesis2.31–2.35;i.e.veryclosetothatofthenatural cauliflower(2.33).Opticswise,filmsareverydiffuseandturnblack fortwopeculiardepositiontemperatures,160and180◦_C,_for_which

reflectivityisthelowestonthewholevisiblerange(2–5%). Thelow reflectivityarises fromthe peculiar filmstructures.

Sinceweuseaspectrophotometerequippedwithanintegrating

spherethetotalreflectedlightistakenintoaccountintheanalysis. Hence,thedecreaseoftheintegratedlightintensityisdueto(i) specularreflectivity(arelativelylowrefractiveindexofthe effec-tivesurfacelayermaterial(CoO+voids(air)),(ii)lightdiffusion,(iii) absorptionwithinthelayer,andmostlikely(iv)acombinationof

theabove-mentionedmechanisms.

Thetrendofthereflectivitywithincreasingdeposition temper-atureisassumedtocomefromarisingrefractiveindexgradient fromtheair/surfaceinterfacetowardsthebulkoftheCoOcoating.

Thecoatingsshowacolumnarstructurewithaconicalgeometry

whosesurfaceisassumedtobeaneffectivemediumofCoO+voids (air)(withn«2.2).Hence,weassumethatthereisarefractiveindex gradientfromthesurfacelayertowardsdeepercoordinateswhere thefilmgetsdenser,andconsequentlyclosertonCoO=2.2.This

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onthespectra.Itissomehowlinkedtothemicrostructureofthe filmsbutwearenotsuccessfulintheclassificationofthe struc-tureswithT_d(mainlybecause190◦_C_shows_an_unclear_reflectivity

behaviormakingthisclassificationtoospeculative). Acknowledgments

TheDIRECCTEMidi-Pyrénéesisacknowledgedforfinancial

sup-port in the framework of the AEROSAT 2012 program, under

contractn◦_43186.

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