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In-Situ Growth of Graphene on Hexagonal Boron Nitride for Electronic Transport Applications 7 1 0 HadiArjmandi-Tash,∗a Zheng(Vitto)Han,b andVincentBouchiatc 2 Transferringgrapheneflakesontohexagonalboronnitride(h-BN)hasbeenthemostpopularapproachforthe n a fabricationofgraphne/h-BNheterostructuressofar. Theorientationbetweengrapheneandh-BNlattices,how- J 9 ever,arenotcontrollableandtheh-BN/grapheneinterfacesarepronetobecontaminatedduringthiselaborate 2 process. Direct synthesis of graphene on h-BN is an alternative and rapidly growing approach. Synthesized ] graphene via such approaches is personally tailored to conform to each specific h-BN flakes, hence the lim- i c itations of conventional fabrication approaches are overcome. Reported processes paved the initial steps to s - l improve the scalablity of the device fabrication for industrial applications. Reviewing the developments in the r t field, from the birth point to the current status is the focus of this letter. We show how the field has been m developedtoovercometheexistingchallengesoneaftertheotheranddiscusswherethefieldisheadingto. . t a m - d mean free paths and ballistic transport was reported even Introduction n o atroomtemperature67. Itsdielectricnatureandlatticepa- c [ Electronic mobility in supported graphene is highly limited rameter–whichisclosetothatofgraphene–aretheother 2 by the roughness and the charged impurities on the sur- remarkablepropertiesofh-BNasamatressforgraphene. v face ofthe substrate123. Inserting athick ((cid:38)10nm) buffer 2 In all these reports, chemical vapor deposition (CVD) or 6 layer of hexagonal boron nitride – with atomically flat and 0 exfoliatedgrapheneisfirstisolatedonanintermediatesub- 6 neutral surface – in between graphene and the supporting strate and then transferred onto the h-BN flakes. The pro- 0 . substrate helps to overcome those limitations. Heterostruc- 1 cess is of remarkable disadvantages: i) contaminating the 0 tureofgrapheneonmultilayerh-BNflakeswasfirstsuccess- 7 surface of h-BN and graphene is highly possible during this 1 fullyrealizedandstudiedbyC.Deanetal4;theirdeviceex- process as air or water molecules might be trapped at the : v hibitedchargecarriermobilitiesashighas140,000cm2/Vs i interface. ii)graphenecanbedamagedorwrinkledandiii) X whichwasnotreachableonsupporteddevicesbythattime. the process does not provide any control over the relative r a Next work from the same group5 also confirmed the ad- orientation of graphene and h-BN: the random orientation vantagesofh-BNasamatressforCVDgraphene. Latelyby ofthelatticesmayleadtoirreproducibleresults. iv)Macro- sandwiching graphene between two h-BN layers, very long scopic alignment of microscale graphene on h-BN flakes is another issue which is time-consuming and troublesome in ∗aLeidenInstituteofChemistry,FacultyofScience,LeidenUniversity,Leiden,Netherlands. practice. E-mail:[email protected] bInstituteofMetalResearchChineseAcademyofSciences,Shenyang,Liaoning,110016, Transfer-free, direct growth of graphene on h-BN tech- China cUniversityofGrenobleAlpes,CNRS,InstitutNéel,F-38000Grenoble,France. niques offer solutions to overcome the limitations listed 1 above. Particularly, in such techniques the graphene/h-BN layer graphene on h-BN powders. The importance of cata- interface is realized in-situ and thus no external contami- lystwas,however,overlooked: noremarkablearrangement nantmaygettrappedinbetween. was considered to compensate its absence. The size of the In this letter, we review the progress of the techniques graphenedomainsremainedunclear;even-thoughasanor- to grow graphene on thick h-BN flakes. Standard CVD mal CVD process with very short growth time (compared of graphene benefits from the presence of a catalyst (e.g. to the later reports) was used, domains larger than few copper) as the promoter of the growth; the absence of nanometers are hardly expected. Figure 1-a and b shows such a catalyst hinders direct growing of graphene on h- someoftheresults. Latelyandasthefieldstarteddevelop- BN. We discuss how different approaches tackled this lim- ing, fewapproacheshavebeenintroducedtoovercomethe itation. Different aspects of such growth methods are re- absenceofthecatalyst. viewed here. Note that there are some methods devel- opedforgrowingbothgrapheneandmono-(few-)layerh- 1.1 Elongatedgrowth BN together to make thin ((cid:46)10nm) graphene/h-BN stacks Elongating the growth course is the simplest approach to or patchworks891011; Interestingly, the growth of the first compensatetheabsenceofthecatalyst. Sonetal14mechan- sample of this type was reported even before the first real- ically exfoliated h-BN flakes on a silicon wafer and grew izationofgraphenebyexfoliation12. Thicknessofh-BNlay- graphene in an atmospheric pressure CVD chamber. Differ- ers achieved in such approaches, however, are not enough entgrowthtemperaturesrangingbetween900◦Cto1000◦C tosmoothentheroughnessofunderlyingsubstratesanddi- with a similar growth duration of 2 hours were tested. Do- minish the effect of the random potentials resting on the mains of ∼100nm in diameter achieved. They reported a wafer; such approached are out of the scope of this letter direct trend of increasing the density of the graphene pads then. uponincreasingthegrowthtemperature(Figure1-candd). 1 Yield issues in in-situ growth of graphene The grown graphene flakes were of rounded shapes with the thicknesses of the order of 0.5nm. AFM, Raman and Chemical growth of graphene relies based on the decom- XPS analysis have been performed to confirm the growth posing carbon-rich precursor molecules (e.g. methane) at andcharacterizethegraphene. the presence of a catalyst. The elevated temperature of the TheapproachwasfollowedlaterbyTangetal15. Likethe growthchamberprovidesenoughenergyforthereouultant Son’sexperiment,graphenewasgrownonhexagonalboron carbonatomstogetmobilizedandreachandbindtheother nitride flakes exfoliated on silicon wafer, albeit through a carbon atoms and form a graphene layer. Indeed the pres- lowpressureCVDprocess. Theynoticedthatscrewdisloca- enceofthecatalystplaysavitalroletospeed-upthegrowth: tions on the flakes are favorable nucleation sites. The slow skippingthecatalystisamajorlimitationforthegrowthof growth rate due to the missing of the catalyst was very ev- graphene on arbitrary (i.e. non-catalyst) substrates such as ident in the results reported; a growth duration of 6 hours onh-BN. only led to the formation of graphene grains of maximum The first paper about directly growing CVD graphene on 270nm in diameter (Figure 1-e to g). The graphene do- h-BN was submitted for publication just four months after the first realization of the graphene/h-BN stacking4. The mainsweremostlysinglelayer. rapidinceptionimpliestheimportanceofthein-situgrowth Elongatingthegrowth–althoughissimple–isofcertain approachesinthefirstplace. PublishedbyDingetal13,this drawbacks: The reported graphene domains – even after paper confirmed the possibility of chemically growing few- severalhoursofgrowth–couldhardlyreachfewhundreds 2 (a) (b) h‐BN hydrogen (H2 to CH4 ratio of 1:1) mainly achieved circu- u.) G yb(a. 2D lar poly-crystalline grains. Increasing the partial pressure nsit of hydrogen (H2 toCH4 ratio of 30:1), however, led to the e graphene/h‐BN int formationofvanderWaalsepitaxy(discussedinsection2) h‐BN 1000 2000 3000 withalignedhexagonalgrains. Indeedreducingthedensity Ramanbshiftb(cm‐1b) ofnucleationcentersviahydrogenetchingplaysavitalrole (c) 900b°C (d) 1000b°C inreportedresults. u.) h‐BN 1.3 Plasma-enhancedchemicalvapordeposition a. nsityb( D G 2D Inspiredbyearlierworksonthegrowthofgrapheneonnon- e 200bnm int1000 3000 200bnm catalystsubstrates1819,Yangetalemployedremoteplasma- Ramanbshiftb(cm‐1)b enhanced chemical vapor deposition (RPE-CVD) technique (e) 60bmin (f) 180bmin (g) 360bmin to grow graphene on exfoliated h-BN flakes20. Here a re- mote plasma source decomposes methane molecules into variousreactiveradicalsprierreachingthesubstrate;hence catalystcanbeomitted. Theapproachprovidesenoughcon- Fig. 1Earlyreportsonthedirectgrowthofgrapheneonthickh-BNflakes trol over the number of layers and uniformity of graphene. a)and(b)ResultsofDingetal: Graphenewasgrownonh-BNpowders throughaCVDprocess. TheRamanspectraon(b)aretakenfrombare In principal, the size of the graphene is only limited by the h-BNpowderandafterthegrowth. Reprintedfrom 13,Copyright(2011), withpermissionfromElsevier. sizeofunderlyingh-BNflakes. Growthtemperaturecontrols c)and(d)ResultsofSonetal: Theeffectofthegrowthtemperatureon thedensityoftheobtainedflakeswasreportedasAFMmappingsinthis boththeepitaxyandtherateofthegrowth: Even-thoughin- work.Thedensityraisedalotasthetemperatureincreasedfrom900◦Cto 1000◦C.Thisgrowthlastsfor2hours. Theinsetin(d)showsanexample creasingthegrowthtemperatureimprovesthegrowthrate, of the Raman spectrum reported on this sample. Adapted from14 with population of nucleation centers also increases at the same permissionofTheRoyalSocietyofChemistry. e),(f)and(g)ResultsofTangetal: AFMmeasurementsshowninthese timewhichmayleadtothree-dimensional–insteadoflayer figureswereperformedonthesampleswith1,3and6hoursofgrowth. Reprintedfrom15,Copyright(2012),withpermissionfromElsevier. by layer – growth and suppress the epitaxy. The growth temperature of ∼500◦C was the best compromise between ofnanometersandareincompatiblewithtypicaldevicefab- epitaxy and growth rate. In this condition, several growth ricationprocesses. Thelongoperationtimeandhighenergy periods, each of two to three hours are still required to ob- consumptionareunfavorableforindustrialapplications. tainthedesiredgraphenesizes. 1.2 Optimizedgrowthparameters 1.4 Gaseouscatalyst Fine-tuning the growth parameters is a wiser approach to While the presence of the h-BN as a background substrate improve the yield. Mishra et al16 showed that in a typi- does not leave any room for a solid catalyst, Tang et al17 cal CVD process, increasing the partial pressure of hydro- usedgaseoussilane(SiH )andgermane(GeH )catalyststo 4 4 gen and elevating the growth temperature can achieve an boost the growth. At a temperature of 1280◦C, the growth improved growth rate of up to 100nm/min (to be com- rate reached 50nm/min and 400nm/min, respectively in the pared with 5nm/min with standard CVD growth parame- presence of germane and silane: the yield was highly im- ters17)(Figure 2-a). They systematically investigated the proved comparing to the recorded 5nm/min in the absence effectofthepartialpressureofhydrogenonthecrystallinity ofanycatalyst. Theynoticedthatelevatingthegrowthtem- of the grown graphene domains: Low partial pressure of peratureupto1350◦Cfurtheracceleratethegrowthtoreach 3 ∼1µm/min (Figure 2-b). Importantly , the Auger electron spectroscopy of the domains did not exhibit any trace of silicon or germanium in the grown graphene crystal. Ap- parently,thoseatomsonlysticktotheedgeofthegraphene domains and lower the reaction barrier for carbon precur- (a) 20 mbar 90 mbar 150 mbar sors toform the honeycomb lattice duringthe growth (Fig- ure 2-c). AFM analyses confirmed that more than 93% of thegraphenedomainsarewellorientedwithrespecttothe 300 nm 800 nm 800 nm backgroundlatticeofh-BN. (b) (c) 16 CH SiH 2 2 1.5 Proximity-drivenovergrowth µm)12 silan e 4 A new approach in the growth of graphene on non-catalyst er ( 8 t e materials such as h-BN was introduced by our group re- am 4 germane cently21. Unlikethepreviousapproaches,thegrowthisper- di 0 no catalyst formedonh-BNflakeswhicharepre-exfoliatedonthecop- 0 10 20 30 40 h-BN SiO2 growth �me (min) perfoil(Figure2-d). Hence,thecarbon-richprecursorsstill (d) (e) op�cal image haveaccesstocatalyst,albeitindirectly. Thegrowthrateof graphene on h-BN was improved dramatically: a full cov- erage of graphene on millimeter sized h-BN flake is achiev- G mode posi�on ableinthesamerateofgrapheneonsurroundingcopperfoil (Figure2-e). Theobtaineddevicesexhibitedchargecarrier 2 µm 100 µm mobilitiesof20,000cm2/Vsandveryneutralgrapheneh-BN Fig. 2Variousapproachestoimprovethegrowthrate interfaces. a)Effectofincreasingthegrowthpressureonthegrinsize: SEMimages showing graphene grains synthesized on h-BN flakes at 1150◦C under 2 Growth mechanism variouschamberpressures.Reprintedfrom16,Copyright(2016),withper- missionfromElsevier. bandc)Gaseousphasecatalysttoimprovethegrowthrate: b)compar- In fabricating heterostructures by depositing a material on isonoftheexperimentallymeasuredgrainsizeofgrapheneflakesgrown a substrate at least two mechanisms are considerable: In at 1280◦C with and without gaseous catalysts. c) Schematic illustration showingthemechanismofgrowingmonolayergrapheneontoh-BN:sili- materials with terminating layers full of dangling bonds, conatoms(shownasredcolorspheres)achievedfromthedecomposition ofSiH4bindtotheedgeofthegrapheneandboostthegrowth.Reprinted atomsinonematerialcanestablishcovalentboundsonlyto withadaptationsfrom17. dande)Proximitydrivenovergrowthofgrapheneontoh-BNflakespre- theatomsofverysimilarlattice;indeedthecovalentbonds exfoliatedonthecopperfoil:d)AFMmappingshowinganh-BNflake(cov- are very sensitive to the length and the angle between the eredwithgraphene)onthecopperfoilattheendofthegrowthcourse.The colorcodeshowstheheight,rangingbetween0nmand500nm.e)Optical atoms. This is an important hindrance in epitaxial growth image(top)andRamanGmodepositionmapping(bottom)ofamillime- ter scale h-BN flake on the copper foil, fully covered by graphene. The between materials with different lattice parameters. Fig- colorcodeshowstheRamanfrequencyrangingbetween1584cm−1 and 1600cm−1.Reprintedwithadaptationsfrom21. ure3-aschematicallyillustratesthismechanism. Thesitua- tion is very different in between two materials with perfect terminating surfaces and no dangling bonds (Figure 3-b). Here, the absence of the dangling bonds can lead to the formationofverysharpinterfaceswithsmallamountofde- fects. ThisgrowthmechanismisreferredtoasvanderWaals 4 epitaxyandcanberealizedeveninthepresenceoflargelat- (a) film dangling tice mismatches2223. As graphene and h-BN are both free bounds film fromdanglingbonds,vanderWaalsepitaxyisnormallythe governinggrowthmechanism. Garcia et al24 first reported van der Waals epitaxy in in- situ growing graphene on h-BN. Unlike the previous exper- substrate substrate iments, graphene was grown by molecular beam epitaxy (b) (c) (MBE) using solid carbon sources. Combined Raman and AFM analysis revealed that the growth is independent of the flux of carbon atoms; instead, carbon atoms deposed 4 μm 4 μm on the surface, migrate freely and accumulate in selective spotsontheh-BNsurface(Figure3-bandc). Thisobserva- (d) monolayer tionpointedoutthatthecarbonatomsareofveryhighmo- graphene h-BN graphene bilities on the neutral h-BN, confirming that van der Waals cu epitaxy was achieved. Recently, van der Waals epitaxy was h-BN bilayer reportedinCVDgrowthofgrapheneonh-BNalso16. graphene 1 μm Asepratebutcomplementarygrowthmechanisminvolves extending graphene – already nucleated on the copper foil Fig. 3 Known mechanisms governing the growth of graphene on h-BN flakes – onto nearby h-BN flakes. This mechanism was first ob- a)Comparisonoftheepitaxialgrowthinbetweenmaterialswith(left)and without(right)danglingbonds,thelatterhasbeendubbedvanderWaals served in graphene grown on few-layer chemically grown epitaxy; inbothcasestheatoms, correspondingtothesubstrateandto thedepositedfilmareshowninblueandpinkrespectively. h-BNsheets11;ourrecentwork21,however,confirmedthat b)AFMandc)Raman(intensityof2Dpeakofgraphene)mappingofa the thickness of the h-BN is not any limitation as graphene h-BNflakewithdepositedgrapheneviamolecularbeamepitaxy;themap- pings show that carbon atoms are freely migrated and accumulated in canovergrowonhundreds-of-nanometers-thickh-BNflakes, somepreferentialspotsonthesurfaceofh-BN.Reprintedfrom24,Copy- right(2012),withpermissionfromElsevier. mechanicallypre-exfoliatedonthecopperfoil. InsettoFig- d)Schematicrepresentationofthemechanism(inset)andSEMimageof graphene(bothmono-andbi-layer)nucleatedonthecopperfoilandex- ure 3-d explains our hypothesized model for this growth. tendedovertheh-BNflakeviaproximitydrivengrowth. Precursorsarecrackedonthecopperfoil. Theachievedcar- bon radicals move randomly in different directions and are tialaffectsthepropagationofchargecarriersingrapheneby energeticenoughtocontinuouslyjumpovertheh-BNflake inducing new Dirac points, known as satellite Dirac points and bond as-growing graphene. The presence of the cop- (SDPs) in the band structure of graphene. The energy of percatalystandhighmobilityofcarbonatoms–asaresult SDPs(E )dependsonthewavevectorofthesuperlattice ofthevanderWaalsepitaxy–guaranteeshighgrowthrate SDP (λ ) which itself is a function of the misorientation angle atoph-BNflakes. ThemainpanelofFigure3-dshowsSEM SL (Φ)betweenthelattices:27 imageofah-Nflakecoveredwithgraphene. 3 Epitaxial Growth ESDP=±h¯vf2|G(cid:126)| =±√2π3h¯λvSFL,where: Once two similar patterns of crystalline lattices (e.g. λ = √ (1+δ)a SL 2(1+δ)(1−cosΦ)+δ2 grapheneandh-BNlattices)aresuperimposedwithasmall Inthisrelation,G(cid:126) representsthereciprocalsuperlatticevec- displacement or rotation in between, a secondary pattern tor and v ≈106m/s is the Fermi velocity of quasiparticles F known as moiré generates25 26. Moiré superlattice poten- in graphene. Additionally, a=2.46 and δ =1.8% are the 5 lattice parameter of graphene and the mismatch between thegrowthinawidetemperaturerange. grapheneandh-BNlattices,respectively. The size of the resultant graphene samples and growth Controlling and minimizing the misorientation angle in ratehavebeenimprovedgraduallyoverthelastyears. Sim- graphene/h-BN heterostructures is of great importance to ilar improvements in controlling the thickness (number of lower structural uncertainties. Indeed traditional method layers) of graphene is also detectable. While the orienta- of transferring exfoliated graphene on h-BN leads a ran- tionofgrapheneinearlyreportswereunclear,recentworks dom orientation of the latices. Although recent progresses reported a trend in graphene/h-BN lattice alignment. The in the field revealed that post-treatment of the samples at bestmobilityreportedforin-situgrownsamplesisstillmuch elevatedtemperaturescandrivegraphenetorotateandfol- inferior than what is achieved in transfer-fabricated sam- low h-BN lattices2829, such approaches are more efficient ples,evenwithCVDgraphene5 whichpointsouttheaffect in sub-micrometer flakes. In-situ growng graphene on h- of crystalline defects. Indeed the techniques employed to BN,however, isofprovencapabilitiestocontrolΦinmuch compensate the lack of catalysts – even-though successful largersamples. to preserve the growth rate – yet failed to yield crystalline Yang et al utilized plasma-enhanced CVD technique to qualities comparable to that of graphene grown on a cata- grow graphene on mechanically exfoliated h-BN flakes20. lyst. Large area, epitaxial and single crystal graphene domains 5 Conclusion and perspective directly grown on the h-BN flakes were obtained. Breaking In-situgrowthofgrapheneonh-BNprovidesthepossibility down the methane molecules with a remote plasma source for achieving hetero-structures with clean interfaces. The eliminatedtheneedforacatalystandenhancedthegrowth risk of damaging graphene during transfer-fabrication is rate and the domains size. The cleanness of the flakes was eliminated. Some growth approaches can even yield to the highenoughthattheymanagetoobservethemoirépattern alignmentofgraphenewithrespecttotheunderlyingh-BN associated to superposition of graphene and h-BN crystals lattice. While early attempts suffer from low growth rate by AFM analysis (Figure 4-a,b). This analysis showed that as a result of the skipping of the catalyst, the new devel- graphene’s lattice follows the orientation of the underlying opment in which the catalyst material indirectly promotes h-BN. The size of the graphene was limited by the size of the growth guaranteed full coverage of milimterscale h-BN the h-BN flake, large enough to fabricate devices for trans- flakeswitharateidenticalonthecatalyst. portexperiments. Thesignatureofthesuperpositionofthe The electronic transport properties of graphene achieved lattices as extra Dirac points in the resistivity and quan- by in-situ growth approaches are still inferior than the de- tumHalleffectmeasurementsrevealedatlowtemperatures vicesachievedintransfer-fabricationmethods. Indeedcrys- (Figure4-c,d). talline defects serving as charge carrier scattering centers WenotethatsimilaralignmentwasreportedlaterbyTang areabundant;certainmeasureshavetobetakentoimprove etal3017 (Figure4-e,f)andMishraetal16. thecrystallineorderofgrapheneinthefuture. 4 Discussion Sandwiching graphene in between two h-BN flakes pro- Table 1 summarizes the important reports of in-situ grow- vides the best device quality; no in-situ growth method has ing graphene on thick ((cid:29) monolayer) h-BN flakes in a yet succeeded to yield such structures. Additionally, di- chronological order. Chemical vapor deposition (including rect growth of van der Waals hetero-structures with mul- its derivatives) using methane precursor has been the most tiple two-dimensional materials is on the perspective. The frequently employed method. Such reports demonstrated choiceofCVDisbasedonthequalificationofthemethodsin 6 Table1Summaryofthereportsofin-situgrowinggrapheneonthickh-BNflakes,inchronologicalorder report process precursor temperature duration size thickness orientated‡ mobility Ding13 CVD CH4(50-90sccm) 1000◦C 3-8min notreported >6L notclear notreported Son14 CVD CH4(30-50sccm) 900-1000◦C 2hrs 100nm ≈0.5nm notclear notreported Tang15 CVD CH4(5sccm) 1200◦C 1-6hrs <270nm ML notclear notreported Garcia24 MBE solidcarbon 600-930◦C 40.6min nm-scale ML notclear notreported Yang20 PECVD CH4† ≈500◦C (cid:29)3hrs µm-scale ML&BL yes ∼5,000cm2/Vs(at1.5K) Tang30 CVD CH4(5sccm) 1200◦C 1-5hrs µm-scale ML&BL yes 20,000cm2/Vs(at300K) Tang17 CVD C2H2† 1280-1350◦C 5-40min µm-scale ML yes 20,000cm2/Vs(at300K) Mishra16 cold-wallCVD CH4† 1000-1150◦C 30min µm-scale ML yes notreported Arjmandi-Tash21 CVD CH4(5sccm) 1050◦C 90s mm-scale ML notclear 20,000cm2/Vs(at80K) ML:monolayer,BL:bilayer ‡ifgraphenefollowstheorientationofunderlyingh-BN †Theflowrateoftheprecursorwasnotreportedinthiswork. 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