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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
Effects
of
substrate
temperature
on
properties
of
NbN
x
films
grown
on
Nb
by
pulsed
laser
deposition
Ashraf
Hassan
Farha
a,
Ali
Oguz
Er
b,
Yüksel
Ufuktepe
c,
Ganapati
Myneni
d,
Hani
E.
Elsayed-Ali
a,∗aDepartmentofElectricalandComputerEngineeringandtheAppliedResearchCenter,OldDominionUniversity,Norfolk,VA23529,USA
bDepartmentofPhysics,OldDominionUniversity,Norfolk,VA23529,USA
cDepartmentofPhysics,CukurovaUniversity,Adana01330,Turkey
dThomasJeffersonNationalAcceleratorFacility,NewportNews,VA23606,USA
a
r
t
i
c
l
e
i
n
f
o
Articlehistory:
Received28June2011
Receivedinrevisedform
19September2011
Accepted3October2011
Available online 10 October 2011 PACS: 74.70.Ad 74.78.Db 74.62.Bf 79.20.Eb 61.05.cp 68.37.Ps 68.55.-a Keywords: NbNx
Pulsedlaserdeposition
Thinfilms
Surfacemorphology
a
b
s
t
r
a
c
t
NbNxfilmsweredepositedonNbsubstrateusingpulsedlaserdeposition.Theeffectsofsubstrate
depo-sitiontemperature,fromroomtemperatureto950◦C,onthepreferredorientation,phase,andsurface
propertiesofNbNxfilmswerestudiedbyX-raydiffraction,atomicforcemicroscopy,andelectronprobe
microanalyzer.Wefindthatthesubstratetemperatureisacriticalfactorindeterminingthephaseofthe NbNxfilms.Forasubstratetemperatureupto450◦Cthefilmshowedpoorcrystallinequality.With
tem-peratureincreasethefilmbecametexturedandforasubstratetemperatureof650−850◦C,mixofcubic
␦-NbNandhexagonalphases(-Nb2N+␦-NbN)wereformed.Filmswithamainly-Nb2Nhexagonal
phasewereobtainedatdepositiontemperatureabove850◦C.Thec/aratioof-Nb2Nhexagonalshows
anincreasewithincreasednitrogencontent.ThesurfaceroughnessoftheNbNxfilmsincreasedasthe
temperaturewasraisedfrom450to850◦C.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Transitionmetalnitridesarewellknownforpossessinga num-berofextremeproperties[1].Amongthenitridecoatingsthatareof increasinginterestisniobiumnitride(NbNx),becauseofits
excel-lentphysicalproperties,suchashighhardness[2],wearresistance
[3],andsuperconductivity[4].Cubic␦-NbNhasasuperconducting transitiontemperatureof17.8K,comparedwith9.2KforpureNb
[5].Thephaseandpreferredorientationofthefilmstronglydepend onpreparationparametersaswellasthedepositionmethods.The preferredorientationintransitionmetalnitridesaffectsmany pro-cesses,suchascarrierdiffusioninmicroelectronicdevices,thewear resistanceoftoolcoatings,thecorrosionresistanceofthe coat-ing,andinthecaseofNbNx,itssuperconductingproperties.The
superconductingpropertiesofNbNxarestronglyaffectedbyNb–N
∗ Correspondingauthor.
E-mailaddress:[email protected](H.E.Elsayed-Ali).
compositionandcrystalstructure.Hence,controllingthephaseand crystalstructureofNbNxisimportantformanyapplications.
NbNx crystallizes in several phases and in most cases the
NbNxfilmsarepresentinamixedphasecomposition[6,7].NbNx
existsindifferentcrystalstructures,including-Nb2N(hexagonal),
-Nb4N3(tetragonal),␦-NbN(face-centredcubic),␦-NbN
(hexago-nal),-NbN(hexagonal),and-NbN(hexagonal).Itisreportedthat thehexagonalphases(-Nb2N,␦-NbN)exhibitsignificantlyhigher
hardnessvaluescomparedtothatofthecubicphases[8]. There-fore,theinvestigationsofthestructuralpropertiesofthehexagonal phasesareofparticularinterest.
DifferentdepositiontechniqueshavebeenusedtoobtainNbNx
thinfilms,suchasreactivemagnetronsputtering[9,10],ionbeam assisteddeposition[11,12],filteredarcdeposition[13],andpulsed laserdeposition(PLD)[14,15].Pulsedlaserdepositionmethodisa highlyflexiblethin-filmgrowthtechnique.ForPLD,the composi-tionandstructuralpropertiesofNbNxfilmsaremainlyinfluenced
bynitrogenpressure,laserpower,andsubstratedeposition tem-perature.TherehavebeenmanyreportsaboutNbNxthinfilmson
0169-4332/$–seefrontmatter © 2011 Elsevier B.V. All rights reserved.
formationofNbNxonNbinthetemperaturerangefromroom
tem-peratureto950◦Cby PLD.Theformationof differentphasesat variousdepositiontemperaturesandtheirinfluenceonthe mor-phologyandcrystalstructurewereinvestigated.
2. Experimental
NbNxfilmsweregrownonNbsingle-crystalsubstrateby
ablat-ingNbtarget.Thechamberwasequippedwithaturbo-molecular andionpumpoperatedatabasepressureof∼1× 10−9Torratroom
temperature.ApulsedNd:YAGlaser(wavelength=1064nm, rep-etitionrate10Hz,pulseduration∼40ns)wasfocusedwitha50cm focallength lensat 45◦ onto a rotating(25rpm)Nb metal tar-get(99.995%pure).The inputpressureoftheambientnitrogen gaswascontrolledbyahigh-precisionleakvalveandmeasured witha convectron gauge. The nitrogenoperating pressure was achievedbyfillingthevacuumchamberwiththerightamountof gas(99.999%pure)andkeepingitinstaticcondition.Thesamples (8.6mm×6.6mm×0.7mm)were cutby wireelectro-discharge machiningfromanNbsliceofingot“H”fromthecompany(CBMM), Brazil.Theresidualresistivityratioofthisingotisabout200.Before cuttingthesamples,thesurfaceoftheNbslicewasmechanically polishedwithAl2O3 abrasivedisks.TheNbslice fromtheingot
haslargegrains(severalcm2 areas);therefore,thesampleswere
eithersinglecrystalsorhadasinglegrainboundary.Thesamples wereetchedbythebufferedchemicalpolishing(BCP)method[21]. TheBCPsolutions(1:1:1and2:1:1HPO3:HNO3:HF)werecooled
downto10◦C duringthecleaningprocess.Approximatelya 50 micronlayerofmaterialwasremovedbyBCPthatresultedinNb substrateswithroot-meansquare(RMS)surfaceroughnessabout 0.6nm for 2m×2m scan area, determined by atomic force microscope(AFM).Theetchedsampleswererinsedindeionized water,driedundernitrogenflow,andthendegassedforseveral hoursat∼800◦Cintheultrahighvacuum(UHV)chamberundera
basepressureof∼1×10−9TorrbeforestartingPLD.X-ray diffrac-tion(XRD)ofcleanedNbsubstrateindicatesthatmostcrystalline grainsofNbsubstrateexhibitcrystallographicNb(110),(200), (211),and (310)orientations[22]. AfterBCPand rinsingwith deionizedwater, the substrateswere attachedtothe substrate heaterwithtungstenwireandheld∼6cmfromthetarget.
Differentsubstratetemperatureswereusedfordeposition.The substrate temperature was measured using a chromel–alumel (K-type)thermocouplethatwasmechanicallyattachedtothe sub-stratesurface.Thethermocouplecalibrationwascheckedbefore installingitintotheUHVchamber,usingtheboilingpoint tem-peratureofwater.Inordertoobtainaccuratetemperaturevalues, firstwecarriedoutrelatingthecurrent–voltagecalibrationcurve fromtheheatertothesubstratesurfacetemperature.Wecan mea-suretemperaturereproduciblyandwithanestimatedaccuracyof ±20◦Cat250◦Cand±50◦Cat950◦C;thisaccuracyislimitedbythe
Fig.1. XRDpatternsofNbNx thinfilmsdepositedonNbsubstrateatdifferent
temperatures.TheXRDpatternoftheNbsubstrateisalsoshown.Forasubstrate
temperatureupto450◦C,thefilmshowspoorcrystallinequality.Withtemperature
increase,thefilmbecomestexturedandforasubstratetemperatureof650−850◦C,
mixofcubic␦-NbNandhexagonalphases(-Nb2N+␦-NbN)areformed.
samplecontacttotheheatersurfaceandthetemperaturegradient acrosstheheater.ThethicknessofthePLDgrownfilmsisabout 45nmascalibratedbyconductingsimilardepositionconditions onsiliconsubstratefollowedbycross-sectionalscanningelectron microscopy(SEM)measurements.AJEOLJSM-6060LVSEMwas usedforthesecross-sectionalmeasurements.Assoonasthe depo-sitionwascompleted,theheaterwasturnedoffandthesample waslefttocooldownforseveralhoursunderUHV.Thedeposition wasperformedfor1h.
XRDmeasurementsweremadeusinggraphite-monochromated CuK␣ radiation on a Bruker-AXS three-circle diffractometer, equippedwithaSMARTApexIICCDdetector.Thefilmswere iden-tifiedinthesymmetric/2 geometry,rangingfrom2=30◦ to 100◦.Thechemicalcomposition(N/Nb=x)oftheNbNxsamples
wasdeterminedbyaCamecaSX100electronprobemicroanalyzer (EPMA).
TheconcentrationoftheAphaseinanA+Bmixedphasefilm wascalculatedbythefollowingformula:
A [%] =
IA IA+ IB (1) whereIA andIB arethesumoftheintensitiesof Aphase
andBphasepeaksinthemeasuredXRDpatterns.The morphol-ogyoffilmswasobservedusingaDigitalInstrumentsDimension 3100AFM.AllAFMimagesweretakeninairusingtappingmode. SEMimagesweretakenatanacceleratingvoltageof30kV.The filmswere deposited at differentsubstrate deposition temper-aturesfromroomtemperature to950◦C underconstant values ofnitrogenpressure andlaserenergy densityof100mTorrand ∼15J/cm2,respectively.
3. Resultsanddiscussion
Fig.1showsXRDpatternsofNbNxfilmsonNbpreparedat
dif-ferentsubstratedepositiontemperatures.TheXRDpatternofthe NbsubstrateisalsoincludedinFig.1asareference.Diffractionpeak assignmentsareindicated.Thefilmsgrownatroomtemperature and250◦Cshowedpoorcrystallinequalities,whileincreasingthe substratetemperatureresultedinbettercrystallization.Asthe tem-peraturewasincreasedfrom450to650◦C,thefilmsbecamehighly texturedandcanbeindexedwithamixtureofcubicand hexago-nalphases.PreviousworkshowedthatNbNxfilmsformedincubic
Fig.2.XRDpatternsofNbNxthinfilmsdepositedat250,750and950◦C.The
pat-ternsrecordedfromthefilmsgrownatdifferenttemperaturesshowdiffraction
peaksofthe-Nb2N,␦-NbN,and␦-NbNphases.
andhexagonalphasesforfilmsdepositedinthesametemperature range[9,23].FromFig.1,wecanseethatsubstratetemperatures above750◦Cresultintheformationofmainly-Nb2Nfilms.
Fig.3.(a)StoichiometriccoefficientxinNbNxfilmsasafunctionofdeposition
sub-stratetemperature.Theatomicnitrogenratiointhefilmincreaseswithtemperature
upto850◦Candthendropsto0.43duetothephasechangeofNbNx.(b)
Depen-denceofthelatticeparameter(c/aratio)ofthehexagonal-NbN2andN/Nbratio.
Thec/aratioisinitiallyreducedasthecompositionchangesfromNbN0.43toNbN0.57.
However,afterthenitrogencontentincreasestoNbN0.57,thec/aratioincreaseswith
increasingN/Nbratio.Thedrawnlinesserveonlytoguidetheeye.
Fig.4. (a)Influenceofthedepositiontemperatureonconcentrationofthe-Nb2N
hexagonalphase.Whenthedepositiontemperatureisincreasedto950◦C,thefilm
changesfrommixedphasetohexagonalphasewithmainly-Nb2N.(b)Themean
hexagonalcrystallitesizeasafunctionofsubstratedepositiontemperatures.The
drawnlinesserveonlytoguidetheeye.
Furtherincreaseofthesubstratetemperaturecanleadtothe for-mationofasublayerofNbNxbyreactionwiththeNbsubstrateand
highernucleationdensitydevelopsonthesurface.Athigher tem-peratures,adatomsonthesubstrate’ssurfacegainthermalenergy andsurfacediffusionisenhanced,thuspromotingcrystalgrowth. Thetransitionfromatexturedmicrostructuretoadenselypacked crystalstructureoccursoverarangeoftemperatures.Inorderto showthephasecontentfortheNbNxfilmsgrownatdifferent
sub-stratetemperatures,threeoftheXRDpatternsinFig.1areredrawn inexpandedscale,asshowninFig.2.Thepatternsrecordedfrom thefilmsgrownatdifferenttemperaturesshowdiffractionpeaks of the-Nb2N,␦-NbN,and ␦-NbNphases butwithdifferences
in therelative intensities. In particular,the ␦-NbNcubic phase appeared at650, 750, and 850◦C withcorresponding planesof (111),(200),and(220),aswellas-Nb2N,␦-NbNandNbpeaks
fromthesubstrate.ThecompositionofNbNxfilmisalsoconsistent
withthechemicalformulasof␦-NbN(PCPDFfile00-038-1155), ␦-NbN(PCPDFfile00-020-0801),and-Nb2N(PCPDFfile
00-040-1274) [23–25].Thepossibility of nitridegrowthbyheating the substratein100mTorrnitrogenfor1hwascheckedandfoundnot toaffectthereportedresults.Thisisattributedtothelownitrogen backgroundpressureused.ThereactivePLDprocessinvolvesthe formationofhighlyreactiveatomicnitrogenspeciesintheplasma whichinteractwiththeablatedNbplumeandthesurface.
Fig.5. TopographicAFMimagesoffilmsgrownat(a)450,(b)650,(c)750,and(d)850◦C.
Thesurfacechemicalcompositionofthefilmswasdetermined byEPMA.Fig.3(a)showsthestoichiometriccoefficientxinNbNx
asafunctionofdepositiontemperature.TheerrorbarsonEPMA valuesrepresentthestandarddeviationforanalysisoffivepoints doneoneachsample.Theatomicnitrogenratiointhefilmincreases withtemperatureupto850◦C andthensuddenlydropsto0.43 duetothephasechangeofNbNx.Itisnoteworthythatıcubic
nio-biumnitridecanbeobtainedwithintherangeofx:(0.57<x<0.99)
[1].Regardingthehexagonalphase,wefoundawiderangeof x=0.43–0.93,whichis inagreementwiththephasediagram of theNb–Nsystem[1,26].Fig.3(b)showsaplotofc/aratioofthe hexagonalphase latticeparametersversusatomicpercentage ofnitrogeninthefilm.Theerrorofcalculatedlatticeconstantis <1%.Thec/aratioisinitiallyreducedasthecompositionchanges fromNbN0.43 toNbN0.57,which isconsistentwiththeliterature
[1,27].However,afterthenitrogencontentincreasestoNbN0.57,
the c/a ratio increases with increasing N/Nb ratio. The lattice parameterstronglydependsonthecrystalimperfection.The lat-ticeconstantaisindependentofcomposition,butcincreaseswith nitrogencontent.Uponthedeviationofxfrom0.5,thec/aratio increases.
Increasingthetemperatureofthesubstratenotonlychanges thecrystal orientation of thefilm but alsochanges the phases frommixed(cubic+hexagonal)tohexagonal(-Nb2Nand␦-NbN).
Whenthesubstrate depositiontemperaturereaches650◦C,the cubic␦-NbNphasecouldbedetectedwith(111),(200),and(220) reflectionsattheinitialgrowthofthecrystallizedNbNx.Treece
etal.[16,17]pointedoutthatNbNxfilmsgrownonMgO(100)by
PLDacquireametastableprimitivecubic(PC)NbNxstructurewhen
depositedatsubstratetemperatureintherangeof400–700◦C.The metastablePCNbNxphaseisstabilizedonMgO(100)andcanbe
transformedto␦-NbNcubicphasebyannealingofthesample.We haveobserved␦-NbNcubicphaseintherangeof450–850◦C sub-stratetemperature.Inourcase,thecubicphasewasformedinthe higherthanthesuggestedtemperaturerangeofPLDdeposition conditionsduetothedifferentsubstratematerial.
Thetexturefactorofthe␦-NbNphaseisdefinedasthe inte-gratedintensityofanXRDpeakrelativetotheintegratedintensity ofallpeaks.Calculatedtexturefactorswerelistedforthethree peaksofthe␦-NbNphaseinTable1.Itcanbeseenthatthe tex-turechangesfromastrong(200)orientationtoadominant(220) orientation,followedby(111)orientationasthesubstrate deposi-tiontemperaturewasincreasedfrom450to750◦C.Forthis␦-NbN cubicphase,thefilmdepositedat650◦Chasastrongpreferential (220)orientation,whilethe750◦Csampleshowsmainly(111) preferredcrystallographicorientation.
Thehexagonalphaseconcentrationwascalculatedastheratio ofthesumoftheintensitiesofthehexagonalpeaksdividedby thetotalintensitiesofall(+␦+␦)peaksofFig.1.Theeffectof thedepositiontemperature ontheconcentrationof the hexag-onalphase in the filmis shown in Fig. 4(a).The error bars in
Fig.4(a)representthestandarddeviationsofthedata.Whenthe depositiontemperatureisincreasedto850and950◦C,thepeaks fromthe␦-NbNphaseintheXRDdiffractionpatternsdisappear. Phase composition of thefilm waschanged from mixed phase to hexagonal phase with mainly -Nb2N structure due to the
decreaseofnitrogencontentoftheNbNx films.Mainly
hexago-nal-Nb2Nphasewithapredominant(110)textureand some
traceof␦-NbNphaseexistsintheNbNxfilmdepositedat950◦C.
Forcrystallinesamplesathightemperature,the␦-NbNbecomes unstable[7].WhensomeofthedepositedNbNxfilmswere
sub-sequentlyannealed at about1000◦C in UHV,a decrease ofthe
Table1
TexturefactorofthecubicphaseinNbNxfilms.
Tsubstrate(◦C) Texturefactor
I111/Itotal I200/Itotal I220/Itotal
450 NA 0.14 NA
650 0.20 0.21 0.28
750 0.24 0.19 0.14
Fig.6.SurfaceroughnessofNbNx grownat450,650,750,and850◦Cshows increasedroughnesswithincreasedsubstratetemperature.
nitrogencontentintheNbNxfilmswasobserved(notshownhere).
Thisisingoodagreementwithresultsobtainedbyotherstudieson
theeffectofannealingtemperatureonnitrogencontentinNbNx
films[28].
Themeanhexagonalcrystallitesizesaredeterminedfromthe XRD patterns for two crystallographic orientations of -Nb2N
phase((100)and(110)planes)asafunctionofsubstrate depo-sition temperatures and plotted in Fig.4(b). The error bars in
Fig.4(b)representthestandarddeviationincalculatingthe hexag-onalcrystallitesize fromtheXRDpeaks ofthesame phase for differentdiffractionpeaks.Thesizeof thedeposited crystallites rangesfrom3to18nm.Itshouldbenotedthatthehexagonal crys-tallitesizesinitiallyincreasewithtemperatureandthendecrease at 650◦C asthe cubicphase appears,then increase again with furtherincreasein thedepositiontemperature.Thisincrease of crystallitesizeisdue toincreasedsolid-stateand surface diffu-sioncoefficients withtemperature[29].WhentheNbNx filmis
depositedathighersubstratetemperatures, theincreased solid-stateandsurfacediffusionresultintheformationoflargergrains. A small decrease of crystallite size was observed in the film depositedat950◦C duetophasechangeandlossofnitrogenin thefilm.
The influence of substrate deposition temperature on the microstructures and other surface properties of the films are investigated by AFM. Fig. 5 shows topographic AFM images of NbNxfilmsdepositedatdifferentsubstratetemperatures.TheAFM
imagesconsistofirregularsubmicron-sizedfeaturesforthe sam-pledepositedat450◦C,asshowninFig.5(a).Whenthesubstrate temperatureis increasedto650◦C, very denseand almost uni-forminheightislandsarevisibleinFig.5(b).Theseislandsshow an increase in their averageheight and decrease in their den-sity.
A regulararray ofislands is observed asthetemperature is increasedto750◦C,asshowninFig.5(c).Oncethetemperature wasincreasedto850◦C,we observedlowdensity micron-sized islands.TheAFMimageforthisconditionrevealsbettercrystalline structureandmoreuniformsizedistribution.Therootmeansquare values(RMS)ofsurfaceroughnesswereobtainedfromeachAFM imagewiththeNanoscopeV5.31r1imagingsoftware.Fig.6shows theroughnessofNbNxfilmsasafunctionofsubstrate
tempera-ture.TheerrorbarsrepresentvariationsinRMSroughnessinline scansfromthreedifferentAFMimageswithsamearea.Wehave foundthattheRMSroughnessof thefilmwasincreasedaswe increasedthesubstratetemperature.Whenthesubstrate temper-atureis450◦C,theRMSvalueis∼7nm.Whenthetemperaturewas increasedto650◦C,thento750◦C,andfinallyto850◦C,RMSvalues
increasedto10,21,and23nm,respectively.Thisisconsistentwith theresultsforthecrystallitesizesthatwereobtainedfromXRD. Thesmallerthecrystallitesizesthesmootherthesurface.
4. Conclusions
Niobium nitride thin films weregrown by PLD onniobium substrate at differenttemperatures. The substrate temperature duringdepositionofNbNxthin filmsonNb significantlyaffects
thephase,morphology,andcrystallinityof thefilms. NbNxfilm
with highest concentration of -Nb2N hexagonal phase was
obtainedfor substratetemperaturesabove850◦C. Asystematic increaseinlatticeparameterswithsubstratedeposition temper-aturealongwithanincreaseinthesizeofthegrainswasobserved. The cubic phase of NbNx was formed for deposition at 650◦C
and 750◦C, although it was mixed with the hexagonal phase. The atomic ratio of nitrogen in the film depends on the sub-strate temperature. Thehighest N/Nb=0.93 ratiowas obtained at deposition temperature of 850◦C at which the highest lat-ticeconstantratioof phase(c/a)wasobtained.XRDandAFM resultsshowed that thecrystallite sizesand surface roughness of the NbNx films increased as thesubstrate temperature was
increased.
Acknowledgements
We would like to thank S. Herman for his great support during the EPMA measurements, Dr. G. Ciovati for providing the niobium samples, and Dr. R. Pike for giving us access to XRD. This work was partially supported by U.S. DOE Contract Nos.DE-AC05-06OR23177 and DE-FG02-97ER45625and by the NationalScienceFoundationGrantNos.DMR-9988669and MRI-0821180.
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