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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
a,b,
Thomas
Duguet
a,∗,
Diane
Samélor
a,
Franc¸
ois
Senocq
a,
Constantin
Vahlas
aaCIRIMAT,UniversitédeToulouse-CNRS,4alléeEmileMonso,BP-44362,31432ToulouseCedex4,France bUniversitéParis-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:thomas.duguet@ensiacet.fr,doug181@gmail.com(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 1.3×10−4Pa.Thepumpinggroupisprotectedfromthecorrosive
by-productsbyaliquidnitrogentrap.Gasisdistributedthrough ashowerheadsystem,describedandmodeledinRef.[17].Gases arefedthroughelectropolishedstainlesssteelgaslineswithVCR fittingsandtheirflowrateiscontrolledbycomputerdrivenmass flowcontrollers.HFcleaned10×5mm2siliconcouponsareused
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/cm2/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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