In-Situ Growth of Graphene on Hexagonal Boron Nitride for Electronic Transport Applications

4.1 MB·

by  Hadi Arjmandi-Tash

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.
(a) (b) to be made to evaluate the efficiency of other approaches
(e.g. bottom-up synthesis using polycyclic aromatic hydro-
carbons)ingrowinggrapheneonh-BN.
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