Epitaxial graphene prepared by chemical vapor deposition on single crystal thin iridium films on sapphire

Article abstract of DOI:10.1063/1.3585126

Uniform single layer graphene was grown on single-crystal Ir films a few nanometers thick which were prepared by pulsed laser deposition on sapphire wafers. These graphene layers have a single crystallographic orientation and a very low density of defects, as shown by diffraction, scanning tunnelling microscopy, and Raman spectroscopy. Their structural quality is as high as that of graphene produced on Ir bulk single crystals, i.e., much higher than on metal thin films used so far.

Full text of DOI:10.1063/1.3585126

Epitaxial graphene prepared by chemical vapor deposition on single crystal thin
iridium films on sapphire
Chi Vo-Van, Amina Kimouche, Antoine Reserbat-Plantey, Olivier Fruchart, Pascale Bayle-Guillemaud,
Nedjma Bendiab, and Johann Coraux
Citation: Applied Physics Letters 98, 181903 (2011); doi: 10.1063/1.3585126
View online: http://dx.doi.org/10.1063/1.3585126
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APPLIED PHYSICS LETTERS 98, 181903 ͑2011͒
Epitaxial graphene prepared by chemical vapor deposition on single
crystal thin iridium films on sapphire
Chi Vo-Van,1,a͒ Amina Kimouche,1 Antoine Reserbat-Plantey,1 Olivier Fruchart,1
Pascale Bayle-Guillemaud,2 Nedjma Bendiab,1 and Johann Coraux1,b͒
1Institut Néel, CNRS and Université Joseph Fourier, BP166, 38042 Grenoble Cedex 9, France
2INAC/SP2M/LEMMA, CEA-Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France
͑Received 18 February 2011; accepted 12 April 2011; published online 3 May 2011͒
Uniform single layer graphene was grown on single-crystal Ir films a few nanometers thick which
were prepared by pulsed laser deposition on sapphire wafers. These graphene layers have a single
crystallographic orientation and a very low density of defects, as shown by diffraction, scanning
tunnelling microscopy, and Raman spectroscopy. Their structural quality is as high as that of
graphene produced on Ir bulk single crystals, i.e., much higher than on metal thin films used so
far. © 2011 American Institute of Physics. ͓doi:10.1063/1.3585126͔
Since 2004, there has been an increasing effort in devel-
oping efficient methods for preparing graphene, mostly mo-
tivated by the prospect of applications. By chemical vapor
deposition ͑CVD͒ on transition metal surfaces large-area
few- layer of epitaxial graphene can be obtained and then
transferred in principle to any support. CVD of graphene has
been long performed on bulk transition metal single crystals
under ultrahigh vacuum ͑UHV͒.1 In 2008, a more versatile
method was introduced, using polycrystalline Ni films on Si
wafers as substrates for CVD, which was operated closer to
atmospheric pressure.2,3 However, due to the relatively high
solubility of carbon in Ni, the precise control of the graphene
layer thickness down to single layer turned out tedious.3 This
was partly circumvented by employing Cu films, for which C
solubility is much lower;4 yet, multilayer graphene could not
be avoided at the location where Cu grain boundaries cross
the surface. Intrinsic limitations to the quality of graphene
are also imposed by substrate roughening at the high tem-
perature of CVD.5 Moreover polycrystalline metal films im-
pose twinned domains in graphene, with length scale in the
range of the distance between graphene nuleation centers
͑typically a few 10 ␮m͒. Since graphene’s properties depend
on the number of layers, and charge carrier scattering is be-
lieved to take place at grain boundaries or substrate-induced
rippling both altering charge carrier’s mobility, so far limited
to a few 103 cm2 V−1 s−1 for CVD grown graphene, higher
and better defined performances are expected for higher
quality graphene.
Employing high quality thin metal films onto which a
continuous graphene sheet with single crystallographic ori-
entation could be achieved is thus a promising route, and
was reported only very recently on Co,6 Ru,7 Ni,8 and Cu.9 In
this Letter we report the preparation of single crystalline Ir
nanometer-thick films on sapphire, and of large-area, high-
quality single layer graphene on top. Iridium is one of the
low C solubility metals onto which pure single layer
graphene can be prepared, which is favorable for the
achievement of high quality graphene; it is also known to
weakly interact with graphene, which allows for the fine ma-
nipulation of graphene’s electronic band structure, for in-
stance the opening of a band gap at the Dirac point.10
Graphene on Ir thin films on sapphire is thus a model system
for multitechnique investigations, including ex situ ͑which is
not so convenient with bulk single crystals͒, of the properties
of graphene contacted to a metal.
Sapphire ͓␣-Al2O3͑0001͔͒ wafers ͑Roditi Ltd.͒, cut in
6.5ϫ8.5 mm2 pieces, were used as substrates. Iridium was
grown by pulsed laser deposition ͑0.1–1 J cm−2, 10 ns, and
10 Hz pulse fluence, duration, and repeat frequency͒ at a rate
of 0.1 nm min−1 in a UHV chamber ͑base pressure 5
ϫ10−11 mbar͒. As discussed later, we find that growth at
700 K and 30 min annealing to 1100 K yield the best quality.
CVD was performed in a second UHV chamber connected to
the first one ͑base pressure 10−10 mbar͒, with ethylene as a
carbon precursor, which was brought in the vicinity of the
sample via a dosing tube ensuring a local partial pressure
higher than that in the chamber ͑typically 10−8 mbar for the
latter͒. CVD during 10 min above 950 K ensured a graphene
coverage in excess of 95 %. Scanning tunneling microscopy
͑STM͒ and reflection high-energy electron diffraction
͑RHEED͒ were performed in situ, temperatures were mea-
sured with a pyrometer. X-ray diffraction, transmission elec-
tron microscopy ͑TEM, 400 kV͒, and Raman spectroscopy
͑WITec alpha500, 532 nm͒ were conducted ex situ.
We first address the crystalline structure of the Ir thin
films. Figure 1͑a͒ shows x-ray pole figures of sapphire and of
the Ir thin film ͑10 nm͒. The threefold symmetry and Bragg
diffraction angles prove that the Ir is ͑111͒-textured and has a
well-defined in-plane epitaxial relationship with sapphire,
¯
¯
ʈ
͗1120͘
͗121͘ ͓Fig. 1͑b͔͒, with a full-width at half-
sapphire
Ir
maximum ͑FWHM͒ spread of 1° as derived from azimuthal
angle scans. Before the 1100 K annealing step, x-ray pole
figures and STM ͑see later͒ evidence 180° twins along ͓111͔.
Consistently, TEM cross-sections ͓such as in Fig. 1͑c͔͒ con-
firm that a single orientation is obtained following the an-
nealing step, and that the Ir͑111͒ films are single-crystalline.
We then characterize the surface quality of Ir͑111͒.
Streaky RHEED patterns ͓Fig. 2͑a͒, inset͔͒ suggest that the
Ir͑111͒ surface is atomically smooth at the scale of a few
10 nm. Kikuchi lines are further evidences for the quality of
the surface. The patterns correspond to a single crystalline
surface, consistent with the volume characterization ͑x-ray
diffraction and TEM͒. STM after annealing reveals atomi-
a͒
Electronic mail: chi.vo-van@grenoble.cnrs.fr.
Electronic mail: johann.coraux@grenoble.cnrs.fr.
b͒
0003-6951/2011/98͑18͒/181903/3/$30.00
98, 181903-1
© 2011 American Institute of Physics
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181903-2
Vo-Van et al.
Appl. Phys. Lett. 98, 181903 ͑2011͒
(a)
1st moiré
1st
C
1st Ir
0th
-2nd
C
10 nm
120 nm
253
(c)
(d)
2D
G
10
FIG. 1. ͑Color online͒ ͑a͒ X-ray pole figures ͑logarithm of the scattered
intensity͒ along a ͓1014͔ direction of sapphire ͑black͒ and a ͓111͔ direction
1000 1500 2000 2500 3000
-1
13 μm
counts
¯
Raman shift (cm
)
of Ir ͑10 nm thick, grown at 700 K and annealed at 1100 K, gray͒ on the
same spherical coordinate system ͑azimuthal and scattering angles displayed
in gray and black, respectively͒. The Ir Bragg peaks are broader than the
sapphire ones, indicative of some spread in the epitaxial relationship. ͑b͒
Ball-model of graphene/Ir/sapphire ͑white: graphene, light gray: Ir, dark
gray: sapphire͒. ͑c͒ TEM cross section of a 9 nm Ir film on sapphire, along
FIG. 3. ͑Color online͒ Single graphene layer grown at 1300 K on the Ir͑111͒
film: ͑a͒ STM topographs evidencing the moiré ͑white rhombus͒, and show-
ing ͑inset͒ graphene wrinkles ͑black arrow͒ as well as reshaping of the
underneath Ir step edges ͑white arrow͒. ͑b͒ RHEED pattern of graphene/
¯
Ir͑111͒ ͑10 kV, ͓110͔ azimuth͒. First order Ir, graphene, and moiré streaks
¯
the ͓110͔ azimuth. Gray dots highlight the crystallographic structure of Ir.
are highlighted as well as the zeroth order streak and one second order moiré
streak. ͑c͒ Background corrected Raman spectrum showing G and 2D
speaks and ͑d͒ 30ϫ30 ␮m2 map of the G band intensity ͑laser wavelength:
532 nm, power: 1.7 m W ␮m−2͒.
cally smooth terraces, separated by atomic step edges, and
Ӎ300 nm wide ͓Fig. 2͑a͔͒, a width directly related to the
miscut of the wafer. Comparably smooth and wide terraces
are obtained on bulk single crystals only after ion bombard-
ment and high temperature flash ͑Ӎ1600 K for Ir͒ which
promotes the formation of metal step bunches, here absent.
The very thin metal films do not dewet upon annealing, prob-
ably thanks to the small lattice mismatch between Ir͑111͒
and sapphire͑0001͒ and to the high melting temperature of Ir.
The influence of the annealing step is obvious when compar-
ing Figs. 2͑a͒ and 2͑b͒. For too mild annealing ͑900 K͒, the
film surface exhibits curved features, which are atomic step
edges, as well as lines which are 120° rotated one with re-
spect to the other. We speculate that these are the surface
traces of grain boundaries between 180° in-plane twins
which are detected on such films with x-ray pole figures ͑not
shown͒. Iridium is fully relaxed to its bulk lattice parameter
as evidenced by the formation of a network of misfit dislo-
cations at the Ir/sapphire interface ͑STM data not shown͒.
Graphene growth on the Ir͑111͒ thin films proceeds like
on bulk single crystals.11 Small-angle twinned domains, hav-
ing an extension between a few 10 to a few 100 nm, are
found for growth performed at 1100 K, while a single crys-
tallographic orientation, corresponding to the typical
graphene/Ir͑111͒ moiré ͓Fig. 3͑a͔͒, is found at 1400 K. This
¯
¯
ʈ
orientation is ͗1100͘ ͗121͘Ir, as shown by RHEED.
C
¯
Graphene rods are only found along the ͓110͔ azimuth of
Ir͑111͒. In this orientation, also superstructure rods from the
moiré are observed ͓Fig. 3͑b͔͒. The distances from the Ir,
graphene, and moiré streaks to the central one give an esti-
mate for the in-plane lattice parameters of graphene and the
moiré, 0.246Ϯ0.001 nm and 2.634Ϯ0.014 nm, respec-
tively, which is similar to the situation on bulk single
crystals.12 STM topographs display the typical wrinkles3 and
FIG. 2. ͑Color online͒ STM topographs
of 10 nm-thick Ir͑111͒ films on sapphire,
after ͑a͒ a 1100 K and ͑b͒ a 900 K an-
nealing. Dashed curves in ͑b͒ highlight
two kinds of surface features ͑see text͒.
The inset in ͑a͒ is a RHEED pattern ͑20
¯
kV, ͓110͔ azimuth͒ showing the zeroth,
first, and second order streaks of Ir͑111͒
and Kikuchi lines ͑white arrow͒.
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181903-3
Vo-Van et al.
Appl. Phys. Lett. 98, 181903 ͑2011͒
Ir step edge reshaping11 ͓black and white arrows in the inset
of Fig. 3͑a͔͒. Neither STM nor RHEED detect multilayer
graphene. The Raman spectra presented in Fig. 3͑c͒ is aver-
aged over 20ϫ20 ␮m2. It shows the characteristic features
for single-layer graphene, i.e., the G and 2D peaks, whose
frequency shifts are 1594 cm−1 ͑FWMH: 7 cm−1͒ and
2688 cm−1 ͑FWMH: 17 cm−1͒, respectively. As compared
to graphene on other supports ͑epitaxially grown or exfoli-
ated͒, G and 2D band positions are both shifted to higher
frequency and their FWHM are extremely sharp, most
prominently for the G band. These observations indicate a
charge transfer between graphene and its Ir support,13 the
sign of the shift suggesting p-doping of graphene, which
agrees with photoemission spectroscopy.14 The absence of a
D band implies that graphene is quasi-defect-free, consistent
with the STM data. The narrow width of the G band rules out
the presence of amorphous carbon. The strong background
observed for all spectra is due to Ir luminescence. Previous
attempts in measuring Raman spectra for graphene on Ir bulk
single-crystals remained unsuccessful, which leads us to the
conclusion that the luminescence from bulk Ir, unlike in the
case of Ir thin films, overwhelms the graphene signature.
Figure 3͑d͒ shows a large-area spatial map of the Raman G
band intensity. This map is remarkably homogeneous with
no graphene-free region and is representative of maps ac-
quired at other locations on the sample. The observed fluc-
tuations are artifacts originating from low signal-to-noise ra-
tio due to the strong Ir background.
We thank Philippe David and Valérie Guisset for techni-
cal support, Luc Ortéga for x-ray pole figures measurement,
Violaine Salvador for the TEM sample preparation and mea-
surements, Zahid Ishaque for STM measurements. C.V.V.
and A.K. acknowledges financial support from the Nano-
sciences Fondation and EU contract No. NMP3-SL-2010-
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