Epitaxial graphene on single domain 3C-SiC(100) thin films grown on off-axis Si(100)A. Ouerghi, A. Balan, C. Castelli, M. Picher, R. Belkhou et al. Citation: Appl. Phys. Lett. 101, 021603 (2012); doi: 10.1063/1.4734396 View online: http://dx.doi.org/10.1063/1.4734396 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v101/i2 Published by the American Institute of Physics. Related ArticlesAnisotropic surface properties of micro/nanostructured a-C:H:F thin films with self-assembly applications J. Appl. Phys. 111, 124323 (2012) Silicon layer intercalation of centimeter-scale, epitaxially grown monolayer graphene on Ru(0001) Appl. Phys. Lett. 100, 093101 (2012) Synthesis of zinc fulleride (ZnxC60) thin films with ultra-low thermal conductivity J. Appl. Phys. 110, 124320 (2011) Amorphous interface layer in thin graphite films grown on the carbon face of SiC Appl. Phys. Lett. 99, 101904 (2011) Structure of few-layer epitaxial graphene on 6H-SiC(0001) at atomic resolution Appl. Phys. Lett. 97, 201905 (2010) Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors

Epitaxial graphene on single domain 3C-SiC(100) thin films grown on off-axisSi(100)

A. Ouerghi,1 A. Balan,2 C. Castelli,2 M. Picher,1 R. Belkhou,3 M. Eddrief,4 M. G. Silly,3

M. Marangolo,4 A. Shukla,2 and F. Sirotti31Laboratoire de Photonique et de Nanostructures (LPN-CNRS), Route de Nozay, 91460 Marcoussis, France2Universite Pierre et Marie Curie (CNRS–IMPMC), 4 Pl. Jussieu, 75005 Paris, France3Synchrotron-SOLEIL, Saint-Aubin, BP48, F91192 Gif sur Yvette Cedex, France4Institut des NanoSciences de Paris, UPMC, CNRS UMR 7588, 4 Place Jussieu, 75252 Paris Cedex 5, France

(Received 21 May 2012; accepted 22 June 2012; published online 11 July 2012)

The current process of growing graphene by thermal decomposition of 3C-SiC(100) on silicon is

technologically attractive. Here, we study epitaxial graphene on single domain 3C-SiC films on

off-axis Si(100). The structural and electronic properties of such graphene layers are explored by

atomic force microscopy, x-ray photoelectron spectroscopy, and Raman spectroscopy. Using low

energy electron diffraction, we show that graphene exhibits single planar domains. Near-edge x-ray

absorption fine structure is used to characterize the sample, which confirms that the graphene

layers present sp2 hybridization and are homogeneously parallel to the substrate surface. VC 2012American Institute of Physics. [http://dx.doi.org/10.1063/1.4734396]

Graphene ranks highly as a promising material for future

nanoelectronics devices because of its exceptional electron

transport properties. It appears as a material of choice for

future electronic and optical applications, including conven-

tional components such as high frequency analog devices, as

well as devices in emerging fields such as spintronics, tera-

hertz oscillators, and single-molecule gas sensors.1,2

Conventional techniques, such as the micromechanical exfo-

liation of isolated graphene from bulk graphite2 or alterna-

tively the graphitization of bulk SiC have been developed.

This method is up to now the most promising but still suffers

very high production costs limiting industrial approaches.3,4

Concurrently, chemical vapor deposition (CVD) on metal

substrates has lately received much attention, due to low cost

and easiness of development aspects.5 Unfortunately, growth

on metals suffers from the disadvantage that electronic appli-

cations require graphene on an insulating substrate, and

although wafer-scale transfer is possible, it is a difficult pro-

cess.6 Developing graphene synthesis methods on silicon

substrates, compatible with silicon mass production indus-

tries, and conventional electronics could enable drastic

reduction of production cost and make graphene more rele-

vant nanoelectronics technologies. In order to address this

issue, the heteroepitaxy of cubic polytype (3C-SiC) on larger

diameter (600) silicon wafers was proposed.7–9 Nevertheless

high-quality growth of 3C-SiC epilayers remains delicate,

especially for (111) and (001) oriented films with high resid-

ual stress generating considerable bowing and possible film

cracking.10 Efforts are on to overcome these problems and

get a better insight in the defect generation during the growth

mechanism, making 3C-SiC(111)/Si heteroepilayers a suita-

ble template of great interest for graphene elaboration. Epi-

taxial graphene on 3C-SiC(100) has been recently

demonstrated11–13 with electronic properties similar to those

of epitaxial graphene on 6H-SiC bulk, including the massless

Dirac like energy dispersion relation for electrons. The

investigation of graphene on 3C-SiC thin films on silicon

wafers is especially important in order to decrease the cost

and to go towards large-scale applications. However, the 3C-

SiC(100) heteroepitaxial on-axis Si substrates exhibit a high

density of interfacial twins and antiphase domain (APD)

boundaries.12,13 In this context, the ability to obtain mono

domains of 3C-SiC is of prime importance for producing

large area graphene on 3C-SiC/Si. In our study we have

developed an original recipe for graphene growth based on

the use of 3C-SiC thin films directly grown on off-axis

Si(100). The APD boundaries are suppressed in the heteroe-

pitaxial off-axis substrate (vicinal), showing the single-

crystal nature of the 3C-SiC thin films.14

In this letter, we have studied the epitaxial graphene on

heteroepitaxial 3C-SiC(100) on off-axis Si(100) substrates

(henceforward noted 3C-SiC(100)/off-axis Si, with the axis

4� off toward h110i). The SiC thin films developed in this

study have no APD boundaries, i.e., the crystalline domains

have the same stacking on large scales. The graphene layer

is grown by thermal annealing of the sample at furnace pres-

sure, which is important for controlling the quality of the

layer. A detailed analysis of the synthesized graphene layers

was performed, including atomic force microscopy (AFM),

x-ray photoelectron spectroscopy (XPS), micro-Raman spec-

troscopy, and micro-Raman mapping. Additionally elec-

tronic characterization of graphene layers was performed

using near-edge x-ray absorption fine structure spectroscopy

(NEXAFS).

The 3C-SiC(100)/off-axis Si substrates were first studied

using dark-field low-energy electron microscopy (DF-

LEEM). In the LEEM instrument (Elmitec GmBH - LEEM

III), the electron gun and sample were biased at 20 kV. The

bias difference between the electron gun and sample is the

start voltage VST and eVST is roughly equal to the primary

electron-beam energy. The DF-LEEM images have been

obtained by selecting the corresponding low energy electron

diffraction (LEED) spots via an aperture (contrast aperture)

placed on the optical path of the microscope column.

Epitaxial graphene on unpolished 3C-SiC(001)/off-axis

Si substrates was grown in semi vacuum (10�5 Torr) by

0003-6951/2012/101(2)/021603/5/$30.00 VC 2012 American Institute of Physics101, 021603-1

APPLIED PHYSICS LETTERS 101, 021603 (2012)

electron-bombardment heating at 1300 �C. Substrates were

first degassed for several hours at 600 �C under UHV condi-

tions and then annealed under a (1 ML/min) Si flux to remove

the native oxide. Though the annealing temperatures are gen-

erally higher on 3C-SiC(111) (Refs. 7 and 8) similar prepara-

tion procedures were applied. The sequence of surface

reconstructions in the two cases however is completely differ-

ent. XPS and NEXAFS experiments were performed in ultra

high vacuum conditions at TEMPO beamline at the SOLEIL

Synchrotron facility (France).8 Raman spectroscopy was per-

formed at room temperature with a Renishaw spectrometer

using 514 nm laser light focused on the sample by a DMLM

Leica microscope with a 50� (NA¼ 0.75) objective.

The most important prerequisite for graphene growth is

the preparation of a single-crystal SiC film without twins,

defect, and ADP boundaries. However, in a traditional CVD

experiment, the SiC film grown on Si(100) presents APD

boundaries (Fig. 1(a)).12,13 Epitaxial graphene formed on

these APD boundaries consists of many small domains with

different layer numbers, which deteriorate the intrinsic prop-

erties of ideal graphene. APD boundaries can be suppressed

by using off-axis silicon substrates.14 Figure 1(d) illustrates

our approach of growing epitaxial graphene on single crys-

talline SiC films.

The 3C-SiC(100) were deposited on off-axis Si(100) in

a resistively heated hot wall reactor. The process involves (i)

a nucleation step (“carbonization step”) performed at

1100 �C which aims to develop a buffer SiC layer by decom-

posing propane of the gas phase on the silicon substrate.

Then (ii) the substrate is exposed to a mixture of propane

and silane diluted in hydrogen (around 0.01%-0.02%) at

high temperature (circa 1350 �C) in order to develop the epi-

taxial film.15 One way to answer questions about the struc-

ture of heteroepitaxied 3C-SiC on off-axis Si is through DF-

LEEM, which is a probe of the crystal symmetry. A typical

DF LEEM image of two SiC LEED spots “(01)SiC” and

“(10)SiC” showing the spatial distribution of stacking

domains on a nominal and off-axis Si substrate is presented

in Fig. 1. The two DF-LEEM images of 3C-SiC on on-axis

Si clearly show that for some regions of the sample the inten-

sity is reversed depending on which LEED spot has been

considered (Figs. 1(b) and 1(c)). The presence of both

domains indicates that the films present two distinct epitaxial

orientations with respect to the Si(100). On the contrary,

APD boundaries are suppressed in the 3C-SiC(001)/off-axis

Si substrate, showing the single-crystal nature of the 3C-SiC

thin films (Figs. 1(e) and 1(f)). This is an ideal substrate for

producing large areas of epitaxial graphene.

Graphene growth on ideal APD-free large terraces was

optimized in order to prevent carbon atoms from the growth

environment to diffuse into the film. The 3C-SiC surface was

cleaned by a Si deposition and annealing step.6,16 Subse-

quently, monolayer graphene was prepared by sublimating

Si from SiC heated to high temperatures under Ar gas. At

high pressure conditions (>10�5 Torr), the homogeneity is

improved by lowering the sublimation rate and consequently

by allowing a better control of the graphene layer formation.

LEED provides evidence for long-range crystallinity and the

absence of rotational disorder in both the 3C-SiC film and

the graphene. Figure 2(a) shows LEED patterns of a probed

area of approximately 1 mm2 obtained after graphene

growth. These images exhibit a well defined six fold (1� 1)

diffraction pattern rotated from the SiC(1� 1) pattern with

uniform spot shape. This finding indicates the existence of

oriented single-domain graphene patches forming a layer

and the absence of rotational disorder and twinning domains.

From the LEED image, we determine that the orientation of

the graphene mainly consists of one hexagonal lattice rotated

by þ15� (62�) with respect to the square SiC lattice

(Fig. 2(b)).

FIG. 1. Schematics of the graphene synthesis on 3C-SiC(100) epilayers on silicon substrates. (a) APD boundaries 3C-SiC film is formed on the Si(100) sur-

face, which gives disordered graphene with broad layer distribution and randomly orientated small domains. (b) Dark-field image (VST¼ 7.97 eV) for the

(01)SiC LEED spot; (c) dark-field image (VST¼ 7.97 eV) for the (10)SiC LEED spot. The microscope field of view was 10 lm. (d) On the other hand, crystalline

3C-SiC film is formed on Si(100) and assists the growth of uniform, well-defined graphene with controlled orientation. DF-LEEM images, taken from the 3C-

SiC(100) on off-axis Si(100); (e) DF image (VST¼ 7.97 eV) for the (01)SiC LEED spot; (f) DF image (VST¼ 7.97 eV) for the (10)SiC LEED spot. The micro-

scope field of view was 10 lm.

021603-2 Ouerghi et al. Appl. Phys. Lett. 101, 021603 (2012)

In order to assess the morphology of the graphene layer,

AFM was performed in the tapping mode. Figures 2(c) and

2(d) show typical AFM images of this graphene. No pits and

anti-phase domain boundaries are seen. A characteristic fea-

ture of this sample was the appearance of terraces along the

h110i silicon direction. The surface consists of atomically

flat terraces with 600 nm widths separated by steps with mul-

tiple unit cell heights (1–2 nm). Terrace widening probably

resulted from the step formation induced by the step bunch-

ing of the epilayers in 3C-SiC(100)/off-axis Si. For compari-

son, growth of graphene on on-axis 3C-SiC(100) was

attempted using the method described in Ref. 13. The result-

ing surface morphology is shown in Fig. 2(d). This is charac-

terized by large terraces presenting height density of the

boundaries. The boundaries between the terraces (shown by

a few white arrows) of the sample can be correlated with the

ADP boundaries of the twofold symmetry of the 3C-

SiC(100).13 We thus establish that the graphene terraces are

bordered by the domains of the substrate. In our case, the

presence of argon molecules hinders the sublimation of

silicon atoms away from the 3C-SiC surface, reducing the

overall sublimation rate and allowing an increase in graphiti-

zation temperature and provides a smooth decomposition of

the SiC.3 Another advantage of the high pressure growth

conditions is to allow higher growth temperatures, which are

favorable to the carbon atoms diffusion rate and thus to the

rearrangement processes occurring during graphene forma-

tion. This leads to a better crystalline quality of the layer.

To further characterize the graphene layer, we used

scanning Micro-Raman spectroscopy (Fig. 3). We used a

laser with 514 nm wavelength as excitation. The Raman

spectra present three peaks at 1350, 1594, and 2718 cm�1,

which are attributed to the D, G, and 2D bands, respectively.

The presence of a single G and 2D bands indicates that our

graphene consists of a sp2 reorganization. The 2D peak

(overtone of the zone-boundary A1g phonon) at 2718 cm�1

originates from a two-phonon resonant process. In some

areas of the sample the 2D peak has a full-width-half-max-

ima (FWHM) of 60 cm�1 (Fig. 3(a), blue curve), but in the

majority areas of the sample it has a width around 90 cm�1

(Fig. 3(b), red curve) and a symmetrical shape, correspond-

ing to a bilayer.16 The observed D band is mostly the result

of a structural disorder and a high density of defects such as

domain boundaries, vacancies, and distortions.13,15 Both the

large width of the Raman lines and the D line intensity sug-

gest that defects are present in the graphene layer.

Compared with that on exfoliated graphene, the signifi-

cant blueshifts of the G-band (14 cm�1) and 2D-band

(38 cm�1) indicate that the epitaxial graphene is either com-

pressively strain during the cool down procedure or n-doped

by the substrate,16,17 as in graphene grown on single-

crystalline hexagonal 6H-SiC and 3C-SiC(111) on sili-

con.16,18,19 The influence of the charge has been studied by

Das et al.17 They observe an upshift of the G-peak frequency

by as much as 20 cm�1 when the graphene layer is doped

with electrons with a density of 4� 1013 cm�2. It is shown

that the dependence of doping on shift in the 2D-band is

very weak. It is roughly 10%-30% compared to that of G-

band (3 and 5 cm�1).17 Therefore, the 38 cm�1 2D-band shift

is too large to be achieved by electron/hole doping. If we

assume that the mismatch between the graphene and the sub-

strate is completely relaxed at the growth temperature a

FIG. 2. (a) LEED image (E¼ 130 eV) of epitaxial graphene on

3C-SiC(100)/off-axis Si showing the diffraction spots due to the SiC(100)

substrate and the graphene lattice; (b) the crystallographic axes of the cubic

substrate and graphene layer are determined from the LEED; (c) AFM image

of epitaxial graphene on 3C-SiC(100)/off-axis Si; (d) AFM image of gra-

phene on 3C-SiC(100)/on-axis Si (the white arrows show the ADP).

FIG. 3. Micro-Raman characterization of graphene and Raman mapping

images of 2D band. (a) Raman spectra taken on the epitaxial graphene

layers—red: bilayer, blue: monolayer; (b) false color 2D peak position car-

tography; (c) false color 2D peak FWHM cartography; (d) distribution of the

2D peak position; (e) 2D peak FWHM; the bar height indicates the percent-

age of the total points which had the position (respectively FWHM) in the

1 cm�1 (respectively 2 cm�1) range around the bar.

021603-3 Ouerghi et al. Appl. Phys. Lett. 101, 021603 (2012)

residual compressive strain should arise during sample cool-

ing to room temperature because of the large difference in

the coefficients of linear thermal expansion between gra-

phene and SiC. The strain in our layer can be deduced

(though we cannot exclude doping effects) at room tempera-

ture from the Raman shift of the 2D bands (38 cm�1). It is

close to 0.25%, which is in good agreement with the previ-

ously reported results of epitaxial graphene on SiC bulk.18

Since our aim is to investigate the homogeneity of this layer

we have performed large-area Raman-mapping. Figure 3(c)

maps in false color the position of the 2D peak collected in

points uniformly spaced over an area of 20� 20 lm2. Figure

3(d) presents the FWHM of the 2D peak as computed from

Lorentzian fits of the measured signal on the same surface.

Figure 3(b) presents the distribution of the positions (in red)

and FWHM (in blue) of the measured 2D peaks over the sur-

face. We can see that the position of the 2D peak has a very

tight distribution around 2718 cm�1, with 95% of the data

collected between 2714 and 2718 cm�1. The same for the

FWHM which is centered around 87 cm�1, as 95% of the

points have a width between 80 and 93 cm�1. Our Raman

and LEED results suggest that the considerably higher

annealing temperature and Ar flux appear to be the key fac-

tors for obtaining a large homogenous graphene layer. Ho-

mogenous sample desorption of Si, high diffusion of carbon,

and low nucleation rate may thus be of importance for

obtaining high quality graphene layers on 3C-SiC(100).

Synchrotron radiation based core-level spectroscopy

shows the dependence of the synthesis process on the three

annealing temperatures (1150 �C, 1200 �C, and 1250 �C).

The evolution of the C 1s spectra upon graphitization taken

at a photon energy of 340 eV is shown in Fig. 4(a). The C 1speak at 284.5 eV attributed to graphene appears clearly at

1200 �C. For higher temperature and longer annealing time,

this graphene peak intensity increases. At the C 1 core-level,

we can determine the average graphene film thicknesses by

measuring the attenuation of the SiC contribution (282.8 eV)

with respect to the graphene signal (284.5 eV).13 The synthe-

sis of an epitaxial graphene layer is supported by the follow-

ing observations: (i) the attenuation of the SiC component,

(ii) the C 1s peak position at 284.5 eV which confirms the

exclusive presence of sp2 hybridized C–C bonds, (iii) the

pronounced asymmetrical shape of the peak which confirms

the conductive state of carbon.

Typical C 1s spectra collected from the 1.5 monolayer

graphene sample at different photon energies (340 and

510 eV) are displayed in the inset of Fig. 4(a). It is interest-

ing to point out that after a single layer of graphene has

developed the bulk SiC signal disappears in the C 1s spec-

trum collected at the photon energy of 340 eV. When

increasing the incident energy, the lineshape and the position

of the graphene peak remain unaffected. This confirms the

absence of an interfacial graphitic layer covalently bound to

the 3C-SiC(100).11,12

In contrast to epitaxial graphene on SiC(111), where

three components can be observed (graphene, SiC, and inter-

face layer contributions), only two components can be

resolved on 3C-SiC(100).11 The SiC bulk component

appears at 282.8 eV binding energy and the graphene related

component at 284.5 eV. In the case of graphene on 3C-

SiC(111), covalent bonding gives rise to one surface related

component in the buffer layer, which remain unperturbed

during the graphene growth.

To determine the electronic properties of the carbon

overlayer on SiC, we studied its unoccupied electronic states

using NEXAFS spectroscopy. Figure 4(b) shows the C 1sNEXAFS spectra taken on the epitaxial graphene as a func-

tion of the angle h between the wavevector of the photoelec-

tron and the normal surface. The two sharp peaks at

285.5 eV and at 292 eV correspond to the 1s-p* excitations

at the K and M points and to the 1s-r* orbitals at the C point

of the Brillouin zone, respectively. Their in-plane and out-

of-plane characters are confirmed by the dependence from

the polarization of the incident photons. More specifically,

when the light polarization is in-plane, only the in-plane r*

states at 292 eV contribute to the C 1s edge, while when the

out of plane polarization component increases, the intensity

of the p* feature at 285 eV strongly increases.12,20 Note that,

in addition to these main features, other peaks are observed,

likely due to the substrate, as observed from a direct

FIG. 4. (a) Evolution of the C1s spectra upon

graphitization taken at a photon energy of

340 eV. Insert, C1s XPS spectra of graphene on

3C-SiC(100) films at different photon energies,

(b) carbon K-edge NEXAFS spectra of the gra-

phene layers on 3C-SiC(100) films, measured at

various incidence angles of x-rays. Peaks at

285.5 eV and 291.4 eV are assigned to 1s!p*

and 1s! r* transitions, respectively.

021603-4 Ouerghi et al. Appl. Phys. Lett. 101, 021603 (2012)

comparison with the absorption spectra of SiC.21 These

NEXAFS spectra confirm that the graphene layers present

sp2 hybridization and are homogeneously parallel to the sub-

strate surface. We focus on the peak detected at 286.5 eV

that can be clearly distinguished in the NEXAFS spectra. In

the past, different mechanisms have been cited as the origin

of this peak: (i) interaction with the underlying substrate, (ii)

disorder and edge states arising from the finite size domains

of graphene,22 (iii) predicted interlayer state,23 (iv) oxygen

contamination.24 However, our surface treatment has

removed oxygen contamination, as shown on our XPS O-1s

spectrum, which excludes the last hypothesis.

In conclusion, we report growth of epitaxial graphene

on 3C-SiC(100)/off-axis Si substrates. We have used investi-

gative techniques like LEED, NEXAFS, and Raman map-

ping to gauge the quality of graphene on 3C-SiC(100)/off-

axis Si substrates. These provide interesting information like

the modification in morphology due to the change in sub-

strate and the corresponding disappearance of anti-phase

boundaries. The Raman data indicate that problems remain,

like considerable intensity in the defect peak and broad

peak-width. Polarized NEXAFS provides proof of planar

growth and sp2 hybridization and the important fact that no

interface layer exists, excluding strong interaction with the

substrate. Further optimization of the growth process to

obtain more graphene monolayer coverage and better quality

should be possible by among other possibilities, controlling

the H2 etching treatments and the surface quality of the 3C-

SiC substrates.

We are grateful to A. Michon and M. Portail (CRHEA-

CNRS, Sophia Antipolis, France) for fruitful discussions and

providing 3C-SiC(001) on off-axis silicon, D. Martinotti for

his outstanding efforts and the technical assistance during

the LEEM experiments at the CEA/IRAMIS/SPCSI labora-

tory, and Olivier Beyssac for access to the Renishaw Raman

setup.

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