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Growth of large area monolayer graphene on 3C-SiCand a comparison with other SiC polytypes

G. Reza Yazdi a,*, Remigijus Vasiliauskas a, Tihomir Iakimov a, Alexei Zakharov b,Mikael Syvajarvi a, Rositza Yakimova a

a Department of Physics, Chemistry and Biology, Linkoping University, SE-58183 Linkoping, Swedenb MaxLab, Lund University, S-22100 Lund, Sweden

A R T I C L E I N F O

Article history:

Received 26 October 2012

Accepted 7 February 2013

Available online 18 February 2013

0008-6223/$ - see front matter � 2013 Elsevihttp://dx.doi.org/10.1016/j.carbon.2013.02.022

* Corresponding author: Fax: +46 13 142337.E-mail address: yazdi@ifm.liu.se (G.R. Yaz

A B S T R A C T

Epitaxial graphene growth was performed on the Si-terminated face of 4H-, 6H-, and 3C-SiC

substrates by silicon sublimation from SiC in argon atmosphere at a temperature of

2000 �C. Graphene surface morphology, thickness and band structure have been assessed

by using atomic force microscopy, low-energy electron microscopy, and angle-resolved

photoemission spectroscopy, respectively. Differences in the morphology of the graphene

layers on different SiC polytypes is related mainly to the minimization of the terrace sur-

face energy during the step bunching process. The uniformity of silicon sublimation is a

decisive factor for obtaining large area homogenous graphene. It is also shown that a lower

substrate surface roughness results in more uniform step bunching with a lower distribu-

tion of step heights and consequently better quality of the grown graphene. Large homoge-

neous areas of graphene monolayers (over 50 · 50 lm2) have been grown on 3C-SiC (111)

substrates. The comparison with the other polytypes suggests a similarity in the surface

behaviour of 3C- and 6H-SiC.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Graphene is an exceptional material with a potential to

revolutionize materials physics and consequently electronic

technologies. It is a two-dimensional honeycomb lattice of

sp2-bonded carbon (C) atoms with outstanding properties

such as very high carrier mobility, long-range ballistic

transport at room temperature [1,2], quantum confinement

in nanoscale ribbons, and single-molecule gas detection sen-

sitivity, which make it suitable for a variety of applications in

condensed-matter physics, material science, microelectron-

ics, and sensing [3]. However, the control of the preparation

conditions for homogeneous large-area graphene monolayer

is a critical issue.

Different techniques have been developed to fabricate

monolayer (ML) or multilayer graphene. One of the methods

er Ltd. All rights reserveddi).

is exfoliation of graphite, unfortunately the largest flakes ob-

tained this way are about 10 · 10 lm2, which is not sufficient

for industrial purposes [1]. An alternative technique is chem-

ical vapor deposition (CVD) of graphene on metal substrates,

such as Cu, Ni or Ru [4,5]. Another method is chemical reduc-

tion of graphite oxide [3]. Graphene formation on hexagonal

silicon carbide (SiC) substrates by Si sublimation in vacuum

or argon atmosphere is becoming a technique of large interest

[2,6].

The main advantage of SiC decomposition in comparison

with other methods is that graphene layers are formed on a

semiconductor substrate; therefore no transfer is needed for

device processing. Furthermore, graphene grown by this

method shows promising electronic properties [7,8]. 4H-SiC

and 6H-SiC have a hexagonal structure and present an ideal

template for graphene growth. For this reason an enormous

.

478 C A R B O N 5 7 ( 2 0 1 3 ) 4 7 7 – 4 8 4

number of works has been done in this field. In comparison

with graphene growth on hexagonal SiC, limited attention

has been given to graphene growth on 3C-SiC, which has a cu-

bic structure. The (111) surface of this crystal is naturally

compatible with the sixfold symmetry of graphene. Published

works have been focused on graphene formation on 3C-SiC

deposited on Si. It was shown that graphene on 3C-SiC

(111) exhibits continuity on step edges with a single Si–C bi-

layer height, thus suggesting the feasibility of growing large

scale graphene layer on this polytype [9].

However 3C-SiC grown on Si is known to contain a lot of

defects, which prevent this material of extensive device

developments. In our study we use 3C-SiC (111) grown on

6H-SiC (0001) which eliminates thermal and lattice mis-

match, and opens a new possibility for applications. Recently

quasi-free standing graphene layer produced on 3C-SiC/4H-

SiC has been reported [10]. As an extension of graphene appli-

cations, the superior biocompatibility of 3C-SiC as compared

to Si can be used [11]. This may offer an attractive platform

for the growth of graphene that could lead to a new genera-

tion of advanced biomedical devices. Graphene may be used

for contact layers on solar cells made on 3C-SiC [12].

Surface restructuring after heating up SiC results in forma-

tion of steps and terraces and they may have an impact on the

doping uniformity (graphene conductance). The main effect

behind the surface restructuring is the phenomenon of step

bunching which is different in the different SiC polytypes

due to energetical reasons. Understanding this process may

facilitate the choice of optimal substrates. It has been shown

by scanning potentiometry that the graphene resistance in-

creases with step heights [13], step density [14], and step

bunching [15]. Therefore it is possible to speculate that spatial

control of step configurations can be used to concentrate cur-

rent into specific regions of a graphene sheet for a new device

design.

The steps with macro-terraces which appear during step

bunching are also responsible for the tighter thickness control

[16,17]. The new graphene layers start to grow from the step

edges due to a weaker bonding of C atoms there, and propa-

gate steadily to the center of the terrace. Therefore fewer

steps reduce the nucleation density of multilayer graphene.

Microscopy studies of vicinal SiC surfaces show that the

graphene growth morphology strongly depends on growth

conditions, vicinality, and SiC step heights [16,18]. For

example, growth on single-bilayer steps produces fingerlike

graphene morphology [19], while growth on three bilayer

(half-unit-cell of 6H-SiC) height steps produces long, straight

strips of graphene parallel to the steps edges. In this case

graphene width increases as growth proceeds [18]. The differ-

ence comes from the fact that due to the different densities of

SiC and graphene, three bilayers of SiC are needed to liberate

a sufficient number of carbon atoms to cover the sublimated

area with one layer of graphene.

In this work we report large area monolayer epitaxial

graphene on 3C-SiC (111). To better understand the material

quality pre-requisites 6H and 4H-SiC substrates have been

used, as well. Graphene formation has been analyzed in re-

spect to step bunching of SiC and taking into account the ini-

tial roughness of the substrate surface. The graphene surface

morphology and thickness uniformity study by using atomic

force microscopy (AFM) and low-energy electron microscopy

(LEEM), show that it is possible to achieve homogenous ML

graphene with area size �50 · 50 lm2 on SiC substrates.

2. Experimental section

Growth of epitaxial graphene was performed on the Si-termi-

nated face of SiC substrates in an inductively heated furnace

under isothermal conditions at a temperature of 2000 �C and

at an ambient argon pressure of 1 atm. 3C-SiC (111), 6H-SiC

(0001) and 4H-SiC (0001) polytypes were used as substrates.

Since 3C-SiC substrates are not available on the market they

were grown in house by sublimation epitaxy on 6H-SiC. Two

types of substrates were employed for the growth of 3C-SiC.

The first one was a nominally on-axis 6H-SiC (0001) wafer

and the second one was 6H-SiC with a thin (�1.5 lm) mono-

crystalline 3C-SiC (111) buffer layer produced by Vapor–

Liquid–Solid (VLS) growth mechanism [20]. The latter

substrate provides conditions for homoepitaxial SiC which

is expected to yield better material quality. Further growth

of thick cubic SiC was performed in a vertical radio frequency

induction heated sublimation reactor at 1775 �C [21]. The 3C-

SiC epilayers have a thickness of around 200 lm and they

were exploited as substrates for graphene growth. Some of

the 3C-SiC substrates were additionally polished in order to

examine the effect of the substrate surface roughness on

graphene quality.

The LEEM measurements were performed using the SPE-

LEEM instrument on beam line I311 at the MAX synchrotron

radiation laboratory (Lund in Sweden). For AFM characteriza-

tion a Digital Instruments Nanoscope IV was utilized. Graph-

ene energy band structure was measured by angle resolved

photoemission spectroscopy (ARPES).

3. Result and discussion

The graphene samples grown at identical conditions on 4H,

6H and 3C-SiC substrates were characterized by LEEM in order

to evaluate thickness distribution. The bright area in all LEEM

images (Fig. 1) represents a ML graphene, while the darker

areas represent bilayer graphene. Fig. 1(a) shows the LEEM im-

age for graphene grown on a 4H-SiC substrate. Graphene on

this sample consists of one and two MLs with one small area

of three MLs (the black spot). Large and homogeneous ML

graphene grown on 6H-SiC and 3C-SiC is shown in Fig. 1(b)

and (c). The areas of 1ML coverage as extracted from the LEEM

images are about 60%, 90%, and 98% for 4H, 6H, and 3C poly-

types, respectively.

During heating a SiC substrate above 1200 �C, SiC surface

undergoes microscopic restructuring by forming steps. This

process, called step bunching, is different from surface recon-

struction and refers to surface morphology. Step bunching,

which is governed by energy minimization on different ter-

races is a fundamental phenomenon in SiC. Several models

have been proposed for the step bunching mechanism during

SiC growth. Heine et al. considered that the energies of inter-

action for each SiC bilayer plane are different due to the un-

ique stacking sequence of the polytypes [22]. Kimoto et al.

used Heine’s calculation to discuss the formation of the unit

cell height steps from a viewpoint of the surface equilibrium

Fig. 1 – LEEM images of graphene on different SiC polytypes grown at identical conditions. (a) 4H-SiC with �60% coverage by 1

ML (bright area), darker areas represent bilayer while small black spot embodies three layers of graphene, (b) 6H-SiC with

�92% of 1 ML coverage (bright area), (c) 3C-SiC with �98% 1 ML coverage (bright area). The black areas in (b) and (c) represent

two MLs graphene.

C A R B O N 5 7 ( 2 0 1 3 ) 4 7 7 – 4 8 4 479

process in which the terrace with minimal stacking energy on

a stepped surface will take over during SiC epitaxial growth

[23]. In case of sublimation we have a reverse scenario which

is now related to decomposition/erosion of Si–C bilayers from

the lattice stack. In this act, since Si has the highest vapor

pressure, Si leaves the surface while C nominally rests and

migrates on the surface.

To study the surface restructuring during SiC substrate

sublimation at 2000 �C we examined around 300 steps for

each sample using AFM. Fig. 2(a–c) depicts histograms of

the step height probability for the graphenized surfaces of

4H-, 6H-, and 3C-SiC substrates, respectively. Starting from a

typical step height around 0.25 nm before heating, the steps

grouped in four major heights related to the polytype struc-

ture. Some dispersion of the step heights above 1 nm was ob-

served in all samples but with a very low probability (not

shown). Having a rather low step height distribution is one

advantage of our results, since it has been reported that the

resistance of epitaxial graphene on SiC increases linearly with

step height on the substrate [24].

The corresponding histogram of the step height for 4H-SiC

(Fig. 2a) indicates that two bilayer-height steps are the most

probable and four bilayer-height steps show a significant

probability. For the 6H-SiC sample (Fig. 2b) two and three bi-

layer-height steps dominate. On the 3C-SiC graphene sample

one Si–C bilayer height has the highest percentage (48%) of

appearance although some larger steps are present (Fig. 2c).

As illustrated in Fig. 2(d), the 4H-SiC polytype has two kinds

of decomposition energies, terraces 4H1 and 4H2, respec-

tively. The 6H-SiC (Fig. 2e) has three distinct terraces – 6H1,

6H2 and 6H3 while 3C-SiC (Fig. 2f) has only one kind of terrace

[25].

Based on the mentioned terrace energies for 4H-SiC, it will

cost less energy to remove a 4H1 terrace. Thus for the 4H1 ter-

race the step decomposition velocity will be faster (Fig. 2d),

and two different step heights of 0.5 nm, and 1 nm are prob-

able to form. A similar mechanism of energy minimization

is expected in the 6H-SiC polytype (Fig. 2e). As a result, first

the step 6H1 will catch step 6H2 and form two Si–C bilayers.

Then step 6H3 will advance and merge with the two bilayer

step. On 3C-SiC all terraces have the same decomposition

energy (Fig. 2f) and no energetically driven step bunching

should be expected. In fact, the most probable step height ob-

served (Fig. 2c) is 0.25 nm. On the image (Fig. 2c) one can see

some additional step heights which suggest that there are

additional factors governing the surface restructuring. In

3C-SiC a non uniformity of sublimation can be induced by

the presence of extended defects such as stacking faults

which are characteristic of this material.

A hypothetical model of graphene development on 4H-

and 3C-SiC is depicted in Fig. 3, without accounting for the

buffer layer formation. As shown in Fig. 3(a), from the edge

of the 4H1 terrace on the graphene free surface C atoms are

emitted onto the terrace as Si atoms leave the surface (stage

1). The C atoms coalesce and nucleate into graphene islands

(stages 1 and 2), which act as a sink for subsequently emitted

C atoms [18]. After the 4H1 terrace step catches the 4H2 step,

the newly formed two SiC bilayer height step provides more C

atoms as compared to the one bilayer height step and the

graphene layer extends along the step edge (stage 2). As

shown in Fig. 2a the dominant step heights correspond to half

and one unit cell height (1 nm). The large percentage of

bunched steps with four Si–C bilayers (Fig. 2a), i.e. an in-

creased source of carbon, will impose the formation of a sec-

ond layer graphene (Fig. 3a, stage 3) since some extra C will be

released. Therefore a full coverage of the 4H-SiC substrate

surface by just one ML graphene may be an issue.

Certainly, the thickness uniformity of graphene on SiC de-

pends on the uniformity of Si sublimation from the surface

and C availability. The decomposition rate of all 3C-SiC ter-

races is the same in a defect free crystal, thus providing a uni-

form source of C on the surface (Fig. 3b) which results in a

superior uniformity of the grown graphene layer (Fig. 1c).

Note the frequency of formation of one bi-layer step height

over the surface for 3C sample in Fig. 2c. However, the pres-

ence of defects, e.g. stacking faults, with typical density of

�5 · 103 cm�1 [21], may be a reason of step bunching on the

3C-SiC surface (Fig. 3c). Actually, step decomposition becomes

faster at the position of defects (stages 2 and 3 in Fig. 3c) on

the 3C-SiC surface, this resulting in non uniformity of terrace

removal and therefore defect driven step bunching on the 3C-

SiC surface (stage 4 in Fig. 3c).

Fig. 2 – Histograms of step heights as deduced from AFM images (not shown) for the three polytypes: (a) 4H, (b) 6H, and (c) 3C-

SiC. Stacking sequences and possible terraces on (d) 4H-SiC, (e) 6H-SiC, and (f) on 3C-SiC surfaces. Large (blue) and small (red)

circles represent Si and C atoms, respectively. Length of arrows indicates different step decomposition velocities. The surface

energies needed to remove a particular terrace are demarcated. (For interpretation of the references to colour in this figure

legend, the reader is referred to the web version of this article.)

480 C A R B O N 5 7 ( 2 0 1 3 ) 4 7 7 – 4 8 4

Another reason for a particular surface restructuring may

be related to the formation of the C buffer layer which lowers

the surface energy at the areas where it appears. This effect

may change the initial energy conditions and results in the

observed step height spreading since the C coverage is not

continuous. The corresponding histogram of the step heights

for 3C-SiC (Fig. 2c) shows that one bilayer height steps are

dominant, but steps bunched of three (unit cell) and more bi-

layer heights are also probable. Nevertheless, the graphene

homogeneity is advanced on 3C-SiC which may suggest that

the uniformity of Si sublimation has a prevailing role in the

graphene quality. A remarkable continuity on step edges of

3C-SiC/Si has been shown in a previous publication [9]. It is

worth noting in our results that the coverage of 1 ML graph-

ene is about six times larger in area.

The 3C- and 6H-SiC polytypes show similar excellence of

graphene, most probably because half of the unit cell of 6H-

SiC contains three Si–C bilayers, thus providing the right

amount of C for a monolayer formation. As evidenced in

Fig. 2(b) three bilayer step heights form and they appear in a

considerable percentage. Since the half unit-cell stacking in

the 6H-SiC polytype is 3-bilayers compared to 2-bilayers in

the 4H-SiC polytype, and the C in �3-bilayers is needed to

produce a single graphene layer, it seems that 6H-SiC would

be more favorable to a layer-by-layer growth mode and have

more uniform coverage and continuity of the graphene layer.

There is an evidence based on Auger electron spectroscopy

that graphitization on 4H-SiC starts at a 50 �C higher temper-

ature in comparison with 6H-SiC due to the difference in

chemical bond strength on these two polytypes [26].

Fig. 3 – Graphene growth evolution (buffer layer formation not shown): (a) On 4H-SiC, stage 0-steps with one Si–C bilayer

height and different terrace decomposition energies (red arrows). Stage 1-sublimation of Si atoms from the edge of steps and

a ML graphene formation. Stage 2-merging two Si–C bilayers and extending the graphene layer along the step edge (red

hexagonal network). Stage 3-source of C atoms increasing after four Si–C bilayer bunching, and a second graphene layer

formation on the first ML (black hexagonal network); (b) on a defect free 3C-SiC substrate. (c) On a 3C-SiC with defects (lines

indicate stacking faults), stage 0-steps with one Si–C bilayer height and the same terrace energy. Stage 1-sublimation of Si

atoms from the edge of steps and 1 ML graphene formation. V-shape pits show erosion on stacking faults and subsequent

step bunching (stage 2). Stage 3: after step bunching of three Si–C bilayers the source of C atoms increases and a larger

graphene layer starts to form. (For interpretation of the references to colour in this figure legend, the reader is referred to the

web version of this article.)

C A R B O N 5 7 ( 2 0 1 3 ) 4 7 7 – 4 8 4 481

According to this result, it seems that a higher growth tem-

perature is needed to have better thickness uniformity and

larger coverage by a ML graphene on 4H-SiC. Higher tempera-

ture results in an enhanced surface diffusion of C atoms and

consequently an increased probability of attaching carbons by

strong in-plane r bonds instead by week p bonds of the first

graphene monolayer. To support our consideration in general

we refer to Ming and Zangwill [27], who have found by mod-

eling the epitaxial growth of graphene on SiC that better sur-

face homogeneity can be achieved by increasing the substrate

temperature.

The effect of the substrate surface roughness on

graphene morphology was also studied in this work.

Fig. 4(a) and (b) show LEEM images of graphene grown on

unpolished (as-grown) and polished 3C-SiC (111) substrates,

respectively. Graphene on unpolished substrate constitutes

65% bilayer and some areas are covered by three MLs (dark

area). The graphene thickness on the polished substrate is

one ML over 93% of the total area (bright area), and some

minor areas of two MLs (Fig. 4b). AFM topography images

of graphene on the as grown 3C-SiC substrate and on the

polished sample along with their roughness parameters

(rms) are shown in Fig. 4(c) and (d), respectively. The an-

gle-resolved photoemission spectroscopy (ARPES) spectrum

of the pi band taken at the K point of the graphene Brillouin

zone for the latter sample demonstrates a perfect linear

dependence characteristic of 1 ML graphene. The surface

roughness of the unpolished and polished samples is

Fig. 4 – LEEM images of graphene layers on (a) as grown 3C-SiC with �65% coverage by 2 ML graphene (bright area) and the

rest is 3 ML, some of stacking faults (SF) are shown. (b) Polished 3C-SiC with �93% coverage by a ML graphene (bright area)

and�7% of 2 MLs. AFM images of graphene on (c) as grown 3C-SiC, some of stacking faults (SF) are shown; (d) polished 3C-SiC

substrate. Histograms of step heights for (e) graphene on as grown substrate showing a wide distribution of step heights; (f)

graphene on polished substrate. Inset image shows ARPES spectrum of the p band taken at the K point.

482 C A R B O N 5 7 ( 2 0 1 3 ) 4 7 7 – 4 8 4

2 nm and 0.6 nm, respectively. As illustrated in Fig. 4(e) and

(f) a lower surface roughness results in less pronounced

step bunching with less distribution of step heights and

consequently better quality of the grown graphene. Thus,

surface roughness should always be minimized before

growth of graphene.

We further compared graphene produced on cubic SiC di-

rectly grown on 6H-SiC (Fig. 4b) and with a seeding layer of the

3C polytype which mimics the homoepitaxial case and should

yield better material structure (Fig. 1c). On both polished sub-

strates more than 90% of an area over 50 · 50 lm2 is covered

by ML graphene. This result verifies that there is no substan-

tial difference in graphene thickness uniformity on these two

types of samples, which suggests that surface preparation is a

crucial step that determines the graphene quality indepen-

dently of the 3C-SiC growth procedure.

C A R B O N 5 7 ( 2 0 1 3 ) 4 7 7 – 4 8 4 483

4. Conclusions

We have studied thickness uniformity of graphene grown at

the same conditions on the Si face of nominally on-axis 4H-

SiC(0001), 6H-SiC(0001), and 3C-SiC (111) substrates by

means of AFM topology measurements and LEEM images.

Since Cubic SiC is not a common substrate, graphene band

structure was evidenced by ARPES. Graphene formation at

high temperature (2000 �C) and Ar atmosphere is influence

by the step bunching process and surface decomposition en-

ergy differences created by the SiC basal plane stacking se-

quence on different SiC polytypes. We have demonstrated a

ML graphene growth on all available SiC polytypes. We have

grown large area, over 50 · 50 lm2, monolayer epitaxial

graphene on cubic SiC (111). The sublimation rate of 3C-SiC

is the same over the whole defect-free substrate surface due

to the similar decomposition energy on all terraces, this pro-

viding a uniform source of C on the surface which results in a

superior uniformity of the grown graphene layer. It is worth

noting that C contained in one unit cell (three Si–C bilayers)

of 3C-SiC is sufficient to feed the formation of 1 ML graphene.

The 6H-SiC polytype shows close quality of graphene to that

on the 3C-SiC polytype, because half of the unit cell contains

three Si–C bilayers. The results for the 4H-SiC substrate cover-

age by graphene show that graphene formation process has

narrower window of growth parameters. As graphitization of

4H-SiC starts at a higher temperature in comparison with

other SiC polytypes, an increased growth temperature should

be used to have more uniform graphene thickness and larger

coverage by 1 ML graphene on 4H-SiC. An important conclu-

sion of the current experiments is that single Si–C bilayer steps

with the same decomposition energy in the beginning of the

graphene formation are the controlling factors for the unifor-

mity of Si subtraction. Although 3C-SiC substrates are not

commercially available, the present work may contribute to

understanding and quality control of graphene growth on SiC.

Acknowledgements

We greatly thank the financial support by the FP7 EU project

Concept Graphene and the Swedish Research Council (VR

contracts 2011-4447, 2010-3511 Grafic ESF). The authors would

like to acknowledge T. Balasubramanian for help with ARPES

measurements.

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