APPLIED PHYSICS LETTERS 97, 033108 ͑2010͒
Kenji Kisoda,1,a͒ Susumu Kamoi,2 Noriyuki Hasuike,2 Hiroshi Harima,2 Kouhei Morita,3
Satoru Tanaka,3 and Akihiro Hashimoto4
1Department of Physics, Wakayama University, Wakayama 640-8510, Japan
2Department of Electronics, Kyoto Institute of Technology, Kyoto 606-8585, Japan
3Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University,
Fukuoka 819-0395, Japan
4Department of Electronics and Electrical Engineering, University of Fukui, Fukui 910-8507, Japan
͑Received 28 February 2010; accepted 28 June 2010; published online 20 July 2010͒
Few layer epitaxial graphenes ͑1.8–3.0 layers͒ grown on vicinal 6H–SiC ͑0001͒ were characterized
by deep ultraviolet Raman spectroscopy. Shallow penetration depth of the probe laser enabled us to
observe G-peak of graphene without subtraction of the SiC substrate signal from observed spectra.
The G-peak was greatly shifted to higher frequency compared to that of graphite due to in-plane
compressive stress deriving from the substrate. The frequency shift decreased with the number of
graphene layers because of stress relaxation from layer to layer. Our experiment suggests that the
stress is completely relaxed within five to six graphene layers. © 2010 American Institute of
¯
Graphene is an allotrope of carbon arranged on a hex-
agonal honeycomb-structure sheet. Before its successful fab-
rication, it had merely been regarded as an ideal two-
dimensional ͑2D͒ system suitable for studying the electronic
first prepared by mechanical exfoliation of highly oriented
pyrolytic graphite ͑HOPG͒ on SiO2/Si substrate,3 has re-
vealed many remarkable physical and chemical properties
and is at present considered to be one of the most promising
building blocks for future electronic devices.4 However,
large graphene layers suitable for such applications are
hardly obtained by the exfoliation and other techniques have
to be developed.5 Among them, graphitization of SiC sub-
strates has shown high potential in producing large graphene
layers with good reproducibility.6,7 The samples, called here
as epitaxial graphene on SiC, are studied in this work by
deep ultraviolet ͑UV͒ Raman spectroscopy.
Raman spectroscopy is an indispensable characterization
tool for studying vibrational properties of various carbon-
based materials and is also employed to investigate physical
properties of graphene.8 The advantage of Raman spectros-
copy in studying graphene has been documented recently.9
Epitaxial graphenes on SiC have also been studied by Raman
spectroscopy using visible lasers for excitation.10–14 In the
reported spectra, however, two-phonon Raman bands of SiC
at 1500–1600 cm−1 strongly overlapped with the G-peak of
graphene, preventing precise evaluation of the G-peak fre-
quency and peak-shape. Therefore, subtraction of SiC signal
from the raw spectra was always necessary. Here we em-
ployed a deep UV laser at wavelength 266 nm instead of
visible lasers for excitation. As we will see later, the SiC
substrate signal is greatly suppressed because of small pen-
etration depth to the substrate and we can directly observe
the graphene signal.
substrate with the c-axis inclined to ͑112n͒ direction by 4°
was kept in ultrahigh vacuum chamber to be heated typically
to 1400 °C. The number of graphene layers was controlled
by adjusting the duration time. More details of the growth
process are described elsewhere.15 The surface morphology
of samples was carefully observed by atomic force micro-
scope and low energy electron microscope ͑LEEM͒. Accord-
ing to LEEM images,15 the epitaxial graphene layers con-
sisted of multiple domains with different numbers of
graphene layers. The value averaged by area was 1.8, 2.3,
and 3.0 layers for the three samples. The sample with 1.8
layer was clearly dominated by bigraphene layer regions but
tiny monolayer domains ͑typical dimension of 0.1 m or
less͒ were also randomly distributed. The other samples, 2.3
and 3.0 layers, showed similar morphology; they were domi-
nated by bilayer and triple-layer regions, respectively, with
inclusion of tiny domains with different layer numbers. The
probed area of our Raman microscope, which is a circular
region with diameter 1–2 m, covered a sample surface
with these multiple domains.
Microscopic Raman scattering measurements were con-
ducted at room temperature using deep UV laser at 266 nm
͑fourth harmonic of Nd:YVO4 laser͒ for excitation. For
comparison, we have also employed a visible laser at 488 nm
͑Ar-gas laser͒ for excitation. The scattered light was dis-
persed by a double monochromator of focal length 85 cm
and the spectra were recorded by a liquid-nitrogen-cooled
charge coupled device camera.
Figure 1 shows typical Raman spectra comparing be-
tween the deep UV and visible excitation. The upper figure
shows results of UV excitation for the graphene sample with
1.8 layers ͑a͒, the SiC substrate ͑b͒, and a HOPG reference
͑c͒. The G-peak of graphene is strongly peaked in ͑a͒ at
about 1600 cm−1. This peak position is about 20 cm−1
higher than that of HOPG ͑1582 cm−1͒. The lower figure
shows results of visible excitation for the same graphene
sample ͑d͒, the SiC substrate ͑e͒, and their difference ͑f͒. In
visible excitation, contrary to the deep UV excitation, there
Three epitaxial graphene samples were prepared for this
study by the solid state graphitization technique. A 6H–SiC
a͒
Electronic mail: kisoda@center.wakayama-u.ac.jp.
0003-6951/2010/97͑3͒/033108/3/$30.00
97, 033108-1
© 2010 American Institute of Physics
130.237.165.40 On: Wed, 12 Aug 2015 12:00:08