Surface-driven, one-step chemical vapor deposition of γ-Al4Cu9 complex metallic alloy film

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Prud’homme, Nathalie and Duguet, Thomas and Samélor, Diane

and Senocq, François and Vahlas, Constantin Surface-driven,

one-step chemical vapor deposition of γ-Al4Cu9 complex metallic alloy

film. (2013) Applied Surface Science, vol. 283 . pp. 788-793. ISSN

0169-4332

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Surface-driven,

one-step

chemical

vapor

deposition

of

g-Al

4

Cu

9

complex

metallic

alloy

film

Nathalie

Prud’homme

_,

_Thomas

_Duguet

_,

_Diane

_Samélor

_,

Franc¸

ois

Senocq

_,

_Constantin

_Vahlas

a_{CIRIMAT,UniversitédeToulouse-CNRS,4alléeEmileMonso,BP-44362,31432ToulouseCedex4,France} b_{UniversitéParis-Sud11,LEMHE/ICMMO,Bat410,91405OrsayCedex,France}

a

b

s

t

r

a

c

t

Thepresentpaperisaparadigmfortheone-stepformationofcomplexintermetalliccoatingsby chem-icalvapordeposition.Itgenuinelyaddressesthechallengeofdepositinganintermetalliccoatingwith comparablecontentsofCuandAl.Dependingonprocessingconditions,apureg-Al4Cu9andmulti-phase

Al-Cufilmsaregrownwithwettingpropertiesoftheformerbeingsimilartoitsbulkcounterpart.The depositionprocessanditsparametricinvestigationaredetailed.Twometalorganicprecursorsareused takingintoaccounttheirtransportandchemicalproperties,anddepositiontemperatureranges.Online andexsitucharacterizationsenlightenthecompetitionwhichoccursatthegrowingsurfacebetween molecularfragments,andwhichlimitsgrowthrates.Notably,introducingapartialpressureofhydrogen gasduringdepositionreducesAlgrowthratefromdimethylethylaminealane(DMEAA),bydisplacingthe hydrogendesorptionequilibrium.ThisAlpartialgrowthratedecreaseisnotsufficienttoachieveaCu/Al atomicratiothatishighenoughfortheformationofintermetallicswithcloseAlandCucompositions. Afivefoldincreaseofthefluxofthegaseouscopper(I)cyclopentadienyltriethylphosphineCpCuPEt3,

whereastheDMEAAfluxremainsconstant,resultsinthetargetedAl/Cuatomicratioequalto44/56. Nevertheless,theglobalgrowthrateisrenderedextremelylowbythedepositioninhibitioncausedby amassivephosphineadsorption(-PEt3).Despitetheselimitations,theresultspavethewaytowardsthe

conformalcoatingofcomplexsurfacegeometriesbysuchintermetalliccompounds.

1. Introduction

Coating of complex 3-dimensional surfaces with complex metallic alloys (CMAs) including quasicrystals provides added valuematerialsforinnovativeapplications.Examplesarelow stick-ingandlowfrictionglassmoldsthatsustainhightemperatures, andporouspreformsinfiltrationfortheproductionofsupported catalysts([1–3],andreferencestherein).CMAcoatingsshow excel-lentmechanicalpropertiessuchaslowadhesion,anti-fretting,or wearresistance,butthedrawbackoflowadhesionisthatitalmost alwaysresultsintoautodelaminationthatcanonlybeovercome byanefficientincreaseofinterfacialaccommodationandadhesion. Severalsolutionshavebeenproposedtofacethisproblem;one beingtheuseofabufferlayerofamaterialthatshowsintermediate electronicandstructuralpropertiesbetweenthoseofthesubstrate andthecoating[4,5].TheAl-Cusystempresentssuch intermedi-atephaseswhichhaveproventheirefficiencyforaccommodating icosahedralAl-Pd-MnandAl-Cu-Fequasicrystals[5,6].

∗ Correspondingauthor.

E-mailaddresses:[email protected],[email protected](T.Duguet).

Actually,thereisnosatisfactorytechniquefortheprocessing ofsuchcoatingsoncomplexsurfaces.Byoperatinginthesurface reactionlimitedregime,chemicalvapordepositioncaninprinciple meetthisrequirement.Inthispaperwepresentforthefirsttime thedirect,one-stepprocessingofapureCMA-containingcoating bymetalorganicchemicalvapordeposition(MOCVD).Specifically, wereportontheMOCVDofAl-Cualloyedcoatings,includingthe g-Al4Cu9 approximantphasewithsimilarpropertiestothebulk

crystal.

PreviousCVDco-depositionexperimentsofAl-Cufilms were performedintheearlyninetiesinordertoreplacepureAl intercon-nectionsinverylargescaleintegrationtechnologybya-(Al)solid solutionswhoseelectromigrationandresistivityarelower. Tomo-haruetal.[7]andEiichietal.[8]useddimethylaluminumhydride, DMAH,andcoppercyclopendadienyltriethylphosphine,CpCuPEt3

asprecursorsfortheco-depositionofAlandCu,respectively.They obtainedpurefilmswithhomogeneousdistributionofCuintheAl matrix,containingAl2CuprecipitatestypicalofhypoeutecticAl-Cu

alloys.Themajordifferencebetweentheseworksandthepresent oneliesinthelowconcentrationofCuintheformer,whichisin theorder1wt%.Processingoffilmswithcomparablecontentof AlandCuismoreconstrainingbecauseofpotentialsignificantgas

phaseand/orsurfaceinteractionsbetweenthetwoprecursorsor theirfragments.InafirstapproachtotheMOCVDofAl-Cucoatings, werecentlypublishedresultsonthesequentialdepositionofAl andCufollowedbyannealing.Intermetallicsarealsoobtainedbut whilethemethodissimplerthantheoneproposedhere,itpresents additionalstepsthataresourceoffilmscontaminationandinvolve higherthermalload[9].

Inaco-depositionprocess,cautionhastobetakenregarding compatibilitybetweentheselectedmetalorganicprecursors. Com-patibilityconcernsdepositiontemperatures,chemicalcomposition (O, F, and C contaminants release), chemical interactions, and transportinthegasphase.Forthisreason,thepresentworkhas beenprecededbytheinvestigationoftheMOCVDofunaryfilms of Aland Cu. Cuis obtained by decomposition of CpCuPEt3, a

solidprecursor,thesaturatedvaporpressureofwhichis0.2Torr at 60◦_{C [10]. Its thermal decomposition in the gas phase at}

150◦_C<T<270◦_{C results in theformation of PEt3gas and a}

sur-face intermediate {CuCp}surf that rapidlyconvertsinto metallic

Cuand(Cp2)gas.Above270◦C,themechanismispresentedas

fol-lows: CpCuPEt3→Cusurf+HCpgas+other organic fragments [10].

In vacuum, mass spectrometry experiments show an intensity decrease of themolecular peak at temperaturehigher than ca. 150◦_{C, whereasdecompositionwithhydrogenissomehow}

sta-bilized and shifted at approx. T>170◦_{C. CuCpPEt}

3 is therefore

suitableforMOCVDinthesurfacetemperaturerange150–290◦_C.

In this range, growth rate increases with increasing precursor feedingrateindicatingthatdepositiontakesplaceinadiffusion controlledregime.Nonetheless,massspectrometryexperiments show that the growing surface saturates with adsorbed phos-phine leading to a continuous decrease of the Cu growthrate withdepositiontime.Thiscanbecounterbalancedbyincreasing deposition temperature above 240◦_{C in order to desorb}

by-products andofferenoughfreesurfaceforprecursoradsorption

[11].

Ontheotherhand,Alfilmsareobtainedbydecompositionof dimethylethylaminealane,DMEAA–aliquidprecursor.The mech-anismattributedtoitsdecompositionatsurfacescanbestressed asfollows:adsorption,breakingoftheAl-Nbond,desorptionof thedimethylethylamine,cracking ofthealaneAlH3 into

metal-licAlandadsorbedHatomsthatrecombinetoformH2gas[12].

Aboveasubstratetemperatureof150◦_{C,Fouriertransforminfrared}

spectroscopyrevealedthatDMEAAdecomposesinthegasphase

[13],andchemicalkineticmodelingallowedtoestimatethatonly ca. 5% of theprecursor participates in the depositionreaction, with a stickingcoefficient onAl(111)closeto unity [14].Jang et al. [15] showed that the Al films with the lowest electrical resistivity,highestdensity,andhighestpurity,areobtainedata substrate temperature of 160◦_{C which alsocorresponds tothe}

highestAlgrowthrateintheinvestigated100–220◦_Ctemperature

range.

Obviously, a compromise must be found for co-deposition involving thetwo aforementionedprecursors. Specifically, sub-stratetemperaturemustbechoseninsuchawaythatgrowthrates arecompatiblewiththetargetedAl-Cuphases’compositionswhile minimizinghomogeneousdecompositionofprecursors.Itis reas-suring thoughthatpreliminarymassspectrometryexperiments during co-deposition show nointeractions betweenprecursors

[11].

Thearticleispresentedasfollows.Theexperimentalprotocol involvingMOCVDofAl-Cufilmsispresentedindetails,first.Then, thecomposition,theobtainedAl-Cuphasesandthemorphologyof thefilmsarepresentedanddiscussed.Finally,preliminaryresults ofthewettingbywateroftheg-Al4Cu9bulkandMOCVDprocessed

phasearepresented,priortoprovidingconcludingremarks.

2. Materialsandmethods

Depositionsareperformedintheexperimentalsetupdescribed indetailsandmodeledinRef. [16].Thesetupiscomposed ofa stagnantflow,cylindrical,stainlesssteelreactor.Thedeposition chamberfeaturesadoubleenvelopeallowingthemonitoringof wallstemperaturethroughthecirculationofthermallyregulated silicon oil. A turbomolecular pumpensures a base pressure of 1.3×10−4_{Pa.Thepumpinggroupisprotectedfromthecorrosive}

by-productsbyaliquidnitrogentrap.Gasisdistributedthrough ashowerheadsystem,describedandmodeledinRef.[17].Gases arefedthroughelectropolishedstainlesssteelgaslineswithVCR fittingsandtheirflowrateiscontrolledbycomputerdrivenmass flowcontrollers.HFcleaned10×5mm2_{siliconcouponsareused}

assubstrates. For each experiment,fivesamplesare positioned horizontallyona58mmdiametersusceptorandheatedbya resis-tancecoilgyredjustbelowthesurfaceofthesusceptor.DMEAA (Epichem)(99%pure)isusedasreceivedandCpCuPEt3(Strem)is

purifiedbyvacuumsublimationat70◦_{C.N2(99.9992%pure)andH2}

(99.995%pure)(airproducts)arefedthroughelectropolished stain-lesssteelgaslineswithVCRfittingswithatotalfluxmaintainedat 325standardcubiccentimetersperminute,sccm.Allexperiments areperformedat10Torr;i.e.areduced,althoughtechnologically convenientpressure.

Table1isanoverviewofexperimentalconditionsandofresults thatwillbediscussedinthefollowingsection.Thevaporization ves-selofDMEAAismaintainedat8◦_{C,correspondingtoasaturated}

vaporpressureof0.5Torr.TransportofDMEAAinvolvesaflowrate ofN2 throughtheprecursorFN2,DMEAA,equalto5sccm.Inthese

conditions,themaximumflowrateofDMEAA,FDMEAAiscalculated

asbeingequalto0.26sccminallexperiments[18].Thesublimation vesselofCuCpPEt3ismaintainedat75±5◦C.Twosetsoftransport

conditionsofCuCpPEt3areinvestigated,correspondingtotwo

dif-ferentflowratesofN2throughtheprecursor(30and150sccm).

FCpCuPEt3isestimatedfromweightlossoftheprecursorsublimator

andusingtheanalyticalformulaofRefs.[18,19].Additionally,two substratetemperatures(200and260◦_{C)areinvestigatedaswellas}

depositionwithandwithoutH2intheinputgas.

Electronprobemicroanalysis(EPMA)isusedfordetermining theatomiccompositionsofthefilms,andX-Raydiffraction(XRD) isusedforstructuralinvestigation.Growthrate(GR)iscalculated by weightgain of each sample prior and after depositionover thedepositionduration. Table1presentsGRsandatomic com-positions (at±1%) averagedover the 5 samplesplaced in each experiment.Thiswaytoquantifythegrowthratedoesnotprovide directaccesstothethicknessof thefilm.However,it has been adoptedsincethemeasurementisstraightforwardanditismore accuratethaninvestigation ofcrosssections bySEMduetothe roughnessandporosityofsomesamplesaswillbeshowninthe nextsections.GRsoftheindividualelementsduringco-deposition areestimatedwiththefollowingformulas:GR(Al)+GR(Cu)=GR

Table1

Investigatedoperatingconditionsandresultingaveragegrowthrates(inmg/cm2/h),averageelementalcompositionsandstructureofthefilms. Experiment# t(h) T(◦_{C) H2(sccm) FN}

2DMEAA/FN2CpCuPEt3 FCpCuPEt3(sccm) GR EstimatedGR(Al) EstimatedGR(Cu) Al/Cu(at.%) Phases

1 2 200 25 5/30 0.07 80 78.1 1.9 99/1 Al+Al2Cu

2 2 260 25 5/30 0.04 30 24.8 5.2 33/67 Al2Cu+Al4Cu9

3 2 260 0 5/30 0.06 50 42.5 7.5 93/7 Al+Al2Cu

and[GR(Al)/MAl]/[GR(Cu)/MCu]=[Al/Cu](withMEmolecularmass

ofelementE), usingEPMAresults.Oxygen has beensubtracted fromelementalcompositionsonpurposebecausesystematic mon-itoringofthegasphaseduringallexperimentsbyonlinemass spectrometrydoes notshowsignificantamountofoxygenorof watervapors,thusensuringthatallexperimentswereconducted intheabsenceofdetectableleaksordesorptionfromthereactor walls.

Wettabilitybywaterisperformedbythesessiledropmethod withaGBXDigidropinstrument.

3. Resultsanddiscussion

3.1. Averagecompositionsandgrowthrates,andcrystallographic phases

Itiswellknownthatsublimation ofafixedbedisneithera robustnorareproduciblemeanforfeedingthedeposition cham-berwithvaporsofa solid precursor.Indeed,statistical analysis over10experimentsofCuprecursorsublimationwithourreactor demonstratesthatitisnotpossibletodiscriminatetheresults#1–3 attheFN₂,DMEAA/FN₂,CpCuPEt₃ fluxratioof5/30inTable1.Hardly,

weareabletoconfirmthatthe5/150fluxratiodoescorrespond toahigherfeedingrateofCuprecursor(0.12sccmcomparedto 0.04–0.07sccm).Anadditionalreasonforthisdiscrepancyisthe observeddegradationoftheprecursoronthereactorwallsand insidethesublimatorbecauseofitsreducedstabilityaboveroom temperature.Moreover,itappearsthattheelementalcomposition ofthefilmsdoesnot followthatofthegasphasebasedonthe estimationofFprecursor.Let’sconsiderexperiments#2and#3at

260◦_{C inTable1,forwhichtheonlydifferenceliesinthe}

pres-enceofhydrogenornot.TheestimationsofFCpCuPEt3,at0.04and

0.06sccm,canbeconsideredclosevalues.With(without) hydro-gen,thefilmscontain33at.%Al(93at.%Al)andtheglobalgrowth rateis30mg/cm2/h(50mg/cm2/h).Amoredirectcomparisoncan bedonethroughtheindividualgrowthrates.SuppressionofH2

intheinputgasleadstodoublingGR(Al),whereas GR(Cu)only increasesfrom5.2to7.5mg/cm2/h.Werecallthatthemechanism ofAldepositionfromDMEAAinvolvesdesorptionofH2gasfrom

thegrowingsurface,afterthealanedecomposition.Asa rule-of-thumb,wepostulatethattheincreaseofthepartialpressureof H2inthereactordisplacesthereactionequilibrium,andtherefore

decreasesthereactionrateofDMEAAatthesurface.

Thedifferencesobservedforexperiments#1and#2inTable1

aremorestraightforwardandmainlyrelyonthedeposition tem-perature. At 200–220◦_{C, the deposition conditions of Al from}

DMEAAareoptimal.Abovethistemperature,gasphasereactions consumemorethan97%oftheprecursorandgrowthratedecreases rapidly[14].Inversely,increasingTfrom200to260◦_Cleadstoa

highergrowthrateofCufromCuCpPEt3[10].Consecutively,the

individualgrowthratesofCuandAlvaryinoppositedirections fromT=200◦_CtoT=260◦_{C;GR(Cu)beingmultipliedby≈2.5while}

GR(Al)isdividedby≈3.However,increasingtemperatureto260◦_C

isnotanappropriateoptionforincreasingtheCucontentofthefilm sinceitdrasticallyaffectstheglobalgrowthratethatdecreasesfrom 80to30mg/cm2/h.ThesolutionforgettingCuenrichmentmust comefromthegasphaseandnotfromahighersurface tempera-ture.

Therefore, an attempt has been made (experiment #4 in

Table 1) with a substrate temperature of 200◦_{C and a lower}

FN2,DMEAA/FN2,CpCuPEt3 ratio,equalto5/150.H2isalsointroduced

inordertofavorCudepositionattheexpenseofAl.Inthese condi-tions,theestimatedCuprecursorfluxhasincreasedto0.12sccm. Theobtainedglobalgrowthrateof25mg/cm2/histhelowestin thisseriesofexperiments,mainlybecauseitisgovernedbyGR(Al) whichislow.DuringCuunarydeposition,ithasbeendemonstrated

bymass spectrometrythat PEt3 isextensively adsorbedonthe

growingsurfacewithsubsequenthinderingofprecursorsupplyto thedepositionzone,resultinginself-inhibitionofCudeposition andgradualdecreaseofthegrowthrate[11].Here,thissituationis validinalltheexperimentsbutweprobablyattainsucha precur-sorconcentrationinexperiment#4,thatinhibitingeffectsbecome predominantandaffectthedepositionmechanismsofAl,aswell. Consequently,inadditiontothedisadvantageofusingH2,Al

depo-sitionrateisfurtherreducedbythesaturationofthesurfaceby adsorbedPEt3.

Tofurthersupporttheabove-mentionedassumptions,wenow considerexperimentally-determinedgrowthratesofunaryAl depo-sitions.ThesegrowthratesaredifferentfromtheestimatedGR(Al) reported in Table1. They result fromdedicated Al depositions performed without H2(g) witha flow rate of 0.26sccm. In

co-depositionconditionsbeneficialtotheDMEAAprecursor,i.e.either 200◦_{CorwithoutH2,growthratesofexperiment#1(80mg/cm}2_/h)

and#3(50mg/cm2/h)aresimilartoexperimentally-determinedAl depositionsgrowthrates(128and40mg/cm2/h,respectively).In co-depositionconditionsbeneficialtotheCpCuPEt3precursor,i.e.

either260◦_{C(experiment#2)orhighF}

CpCuPEt3 at200◦C

(exper-iment#4), thepollution ofthesurface byphosphine resultsin (i) a low globalgrowthrate,and (ii) a low GR(Al),to be com-paredwiththeexperimentally-determinedAlgrowthrateat200◦_C

(128mg/cm2/h). Therefore, theformation of Cu-rich samplesis more due totheinefficient depositionof Alrather than tothe increaseofthegrowthrateofCu.

Fromtheresultsreportedaboveweconcludethatwecan suc-cessfully obtain intermetallic compounds of interest. Despite a majordrawbackreferringtothereducedgrowthrateandprobably theprocessyield,solutionsareavailabletocircumventthose prob-lemswhichareattributedtothecontroloftheco-depositionby synergeticphenomena.Fromtheanalysisoftheresultsreported inTable1itappearsthatbetterconditionsforthegrowthof Al-CucompoundsfromCuCpPEt3andDMEAAshouldinclude:(i)the

useofbothprecursors–butmoreimportantly ofCpCuPEt3–in

liquidsolutionincombinationwitharegulated deliverysystem (e.g.directliquidinjection,pulsedinjection,...),(ii)avoidingthe useofH2formaximizingAlgrowth,(iii)determiningacoupleof

[T;FCpCuPEt₃]parametersthatallowtheproperdesorptionofPEt3

fromthesurface,andpreventthethermaldegradationofDMEAA inthegasphase,and(iv)elucidatesurfacemechanismsthatare

Fig.1.XRDspectraofsamplesfromexperiments#1–4withgivenidentificationsof phases.Phasesreportedontheright-handsideofthefigurewhereidentifiedwith aidofJCPDS#04-0787forfcc-Al,#25-0012for-Al2Cu,and#24-003forg-Al4Cu9.

responsibleforthosesynergeticeffectsandtunethesubstrate reac-tivity,accordingly.

Fig.1showstheXRDpatternforasprocessedsamplesof exper-iments#1–4.Thefilmsarecomposedof3differentphases:fcc-Al,

-Al2Cu,andg-Al4Cu9.ComparisonofthesephaseswiththeAl/Cu

ratios shown inTable1,allows concludingthatAl-rich compo-sitions(experiments#1and#3)correspondtotheformationof

eitherpureAlorofana-(Al)solidsolutionplus-Al2Cu,

eventu-ally.Indeed,whencompositiongetsricherinCu(fromexperiments #1–3)thenthecontributionofthe-Al2CuphasetotheXRD

dia-grambecomes larger. If thecompositionin Cu is large enough thenthetargetedg-Al4Cu9isformed,eitherpureorwith-Al2Cu.

Hence,phaseformation qualitativelyfollowsthesimplemixing rulethatcanbededucedfromtheAl-Cuphasediagram.Itisworth

notinghowever,thatthesampleofexperiment#2whichshows theelementalcompositiontheclosesttothetheoreticalAl4Cu9

composition(Al33Cu67andAl31Cu69,respectively)isnottheone

giving rise to the formation of a single-phase g-Al4Cu9

coat-ing. This is observed for the sample of experiment #4 whose elementalcompositionissomehowshiftedfromthisideal com-position.Thisresultisincontrastwithreports,followingwhich fora given coatingcompositionthecorrespondingpoint in the phase diagram can beused to predict the final phase fraction

[6,20]. It is rather attributed to partial alloying giving rise to transient phases, mainly-Al2Cu and g-Al4Cu9 because of the

relativelylow activationenergy required fortheir formation in thinfilms [9,21].Moreover,it hasbeenreportedthat, indepen-dentlyoftheAl/Curatio,thermalannealingofAl-Cubilayersfirst yields-Al2Cu at temperatures aslow as 130◦C followed by a

smallamount ofa secondphase, AlCu3. Thesubsequent phase

formation wasfoundtodepend ontheCu:Al atomic ratio. For Cuconcentrationshigherthan50at.%,thereactionsubsequently proceedstowardthephasesg-Al4Cu9 anda-Cu.ForCu concen-trationlowerthan50at.%,AlCu3transformstoanewhexagonal

phaseAlxCuwhichsubsequentlytransformsbacktoAlCu3before

the reaction proceeds toward the end phases AlCu and Al2Cu

[22].

EPMAandRutherfordbackscatteringexperimentsrevealthat oxygen concentration in the films is high, ranging from 13 to 61%. Taking into account that on line mass spectrometry unambiguouslyshowsbackgroundlevel forall oxygen contain-ing species, it is concluded that oxidation of the films occurs

ex situ.Based onSEM observations illustratedin the next sec-tion,thehighsurfacetovolumeratiois mostlikelyresponsible for surface oxidation in addition to O2 and water

adsorp-tion.

3.2. Morphology

Fig.2regroupssurfaceandcross-sectionSEMimages.Thefirst obvious observation is that rough morphologies correspond to Al-richcompositions(experiment#1and#3)whereassmoother morphologiesarefoundonCu-richsamples(experiment#2and #4).However,sinceAl-richfilmsaremuchthickerbasedonweight gainandonSEMcross-sectionsanalysis,itisunclearwhetherhigh thicknessorAlgrowthorthecombinationofbothisresponsible forroughnessandporosity.Agoodindicationcomesfrom deposi-tiontemperatures.Thefilmofexperiment#1wasgrownat200◦_C

whereasthefilmofexperiment#3wasgrownat260◦_C.At200◦_C,

growthrateishigherbecauselessprecursorisconsumedin homo-geneousreactions.Fromthesurfacepointofview,itcorrespondsto ahigherfluxofprecursor.Inversely,ahighertemperatureresultsin thecombinationofalowerprecursorfluxandhigherbulkand sur-facediffusionrates;bothmechanismsarebeneficialtothesurface smoothness.

The surface of the sample of experiment #2 is composed of a dense film with equiaxed grains on top of which quasi-unidirectionalfilamentsarepointingawayfromthesurfaceplane. In Fig. 3, similar filaments are found for experiments #2–4. It was not possible to determine their structure and compo-sition due to their size which is not compatible with SEM, XRD or EPMA. Experiment #4 is the most promising applica-tion wise:the film is single-phase g-Al4Cu9 (within thelimits

of XRD characterization) and it shows large surface regions like the one illustrated in Fig. 3 with no parasite filament grains.

Thissampleprovides thefirstexperimental proofofconcept accordingto which MOCVD co-deposition canform a complex metallicalloycoatinginonestep.

3.3. Wettingof -Al4Cu9surfacesbywater

The useof MOCVD films containing approximant phases as buffer layers for the interfacial accommodation and adhesion betweenametallicsubstrateandaquasicrystallinecoatingis sub-jectedtotheirsurfaceenergy,whichinturnimpactstheirwetting byliquids,e.g.water.Itiswellknownthatinambientair,typical valuesforaquasicrystalofhighlatticeperfectionlikeanannealed singlegrainicosahedralAl-Pd-Mnareintherange90◦_<<100◦_,

whereaspurealuminummetalshowsvaluesaround70◦ _[23].In

thiscontext,itisexpectedthatag-Al4Cu9 single-phasedsurface

shouldpresentacontactanglewithwaterbetween70◦ _and90◦_.

Inordertoverifythispointaseriesofwettingexperimentswere performedwithultrapurewater(resistivity18.2MOhm/cm)onthe surfaceofsample#4(pureg-Al4Cu9)andonthesurfaceofapure

g-Al4Cu9bulksample.Beforewettingexperiments,thesurfaceof

thebulksamplewaspolishedwithSiCdisks.Threesurfacefinishes wereobtainedwith600,2400and4000grit,providingsurfaces withmeanroughnessRaof246nm,16nmand7nm,respectively. Fourmeasurementsofcontactanglewithwaterwereperformedon eachsurfacefinish.Theobtainedvaluesare82◦_,84◦_,and87(±2◦₎

fromtheroughertothesmoothersurface.Theseresultsconfirmthe intermediatevalueofthecontactanglewithwateronthesurface oftheg-Al4Cu9approximantbetweenthoseobtainedonthe

sur-faceofaluminummetalandoficosahedralcrystals.Theyarealso consistentwithWenzel’stheory,followingwhichanintrinsically hydrophilicsurfacebecomesmorehydrophilicwhenitsroughness increases[24].Followingthesameprotocol,fourmeasurements ofcontactanglewithwaterwerealsoperformedonthesurface oftheMOCVDprocessedpureg-Al4Cu9film.Theobtainedmean

valueis89(±2◦_{).TakingintoaccountthatRaofthelatter}

sam-plewasmeasuredtobe5nm,thisresultistobecomparedwith theoneof87◦_{obtainedonabulkg-Al4Cu9surfacewith}

compara-bleroughness.ItisconcludedthatMOCVDprocessedapproximant phasespresentsimilarsurfacecharacteristicstothose of equiv-alentbulkcrystals.Inamore prospectiveview,itispossibleto engineerthesurfaceenergyofMOCVDAl-Cufilmbyproper selec-tionofdepositionconditionsleadingtoamixtureofintermetallic phasesincludingapproximantones.

4. Conclusions

We present a one-step process for the deposition of Al-Cu intermetallic coatings by metalorganic chemical vapor deposi-tion.Dimethylethylaminealane(DMEAA),aliquidprecursor,and copper(I)cyclopentadienyltriethylphosphine(CpCuPEt3),asolid

precursor,arechosenasprecursorsfortheco-depositionofAland Cubecausetheycontainnooxygen,theyarechemicallycompatible, andtheirdepositiontemperaturesaresimilar.

Theelementalcompositionofthefilmsdoesnotfollowthatof thegasphase.Thepresenceofhydrogengasplaysanimportantrole intheco-depositionprocessbydisplacingthereactionequilibrium ofDMEAAatthesurface,thereforedecreasingtheAlgrowthrate. Additionally,whentheCpCuPEt3gasfluxishighenoughfor

achiev-ingAl/Cucompositionsofinterest,theexperimentalgrowthrateof Al-Cufilmsislimitedbytheadsorbedphosphineoriginatingfrom CuCpPEt3decompositiononthegrowingsurface.Encouragingly,

solutionsareavailabletocircumventthoseproblemswhich are attributedtothecontroloftheco-depositionbysynergeticsurface phenomena.

Thesingle-phaseg-Al4Cu9isformedwhentheCucomposition

intheAl-Cufilmsishighenough.Thesurfaceroughnessofsuch filmsisverylow,withaRavalueequalto5nm.Wetting

measure-mentswithwateronthissurfaceprovideacontactangleof89±2◦_;

i.e.roughlythesameastheoneonabulkpolycrystallineg-Al4Cu9of

approximantphasepresentsimilarsurfacepropertieswiththose ofequivalentbulkcrystalsandcan,thereforebeusedas interfa-ciallayerbetweenmetallicsubstratesandquasicrystallinecoatings accommodatingthemismatchofsurfaceenergybetweenthem.

ThepresentworkdemonstratesthatitispossiblebyMOCVD toprocessinasinglestepandatrelativelylowtemperature,films containingintermetallicphasesincludingapproximantones.This workmaybeconsideredasapreliminaryapproachpavingtheway totheconformalcoatingofcomplexsurfacegeometriesbysuch intermetalliccompounds.

Acknowledgments

WeareindebtedtoLyacineAlouiandtoMaëlennAufray,both atCIRIMAT,Toulouseforperformingthe,andforadviceon, con-tact angle measurements,respectively, to Philippe de Parseval, Observatoire Midi-Pyrénées,Toulouse,forEPMAanalysisandto Marie-CéciledeWeerd,InstitutJeanLamour,Nancy,forproviding theg-Al4Cu9crystal.Thisworkwassupportedbythe6th

Frame-workEUNetworkofExcellence‘ComplexMetallicAlloys’(Contract No.NMP3-CT-2005-500140),andbytheFrenchAgenceNationale delaRecherche(ANR)undercontractNo.NT05-341834.Itwould neverhavebeencompletedwithoutthepersistentsupportof Jean-MarieDubois,CNRS,Nancy.

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