22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
How remote r.f. plasmas can aid graphene technology?
M. Losurdo, M.M. Giangregorio, G.V. Bianco, A. Sacchetti, P. Capezzuto and G. Bruno
Institute of Inorganic Methodologies and of Plasmas, CNR-IMIP, via Orabona 4, 70126 Bari, Italy
Abstract: This paper reports on advantages and limits of plasmas in remote configuration
in graphene chemical vapor deposition and processing. The effect of plasma-CVD by
direct CH 4 -H 2 -Ar and remote CH 4 -H 2 -Ar is compared to conventional CVD by CH 4 -H 2 of
graphene on copper and nickel, investigating the quality of the grown graphene. We
demonstrate that although plasma activation presents some limits in the CVD growth of
graphene, the full advantages of H 2 remote plasmas can be exploited in the processing of
substrates to improve the subsequent growth and graphene quality, in tailoring graphene
optical and electrical properties by post growth plasma doping, as well as in nanopatterning
graphene, minimizing the defects in graphene basal plane.
Keywords: graphene, CVD, H 2 remote plasmas, plasma nanopatterning, ellipsometry
1. Introduction
Incorporation of graphene [1] into devices requires
large-scale fabrication of high-quality graphene, which
remains a significant challenge despite years of effort.
The method that shows the capability of large area
graphene is chemical vapour deposition (CVD), which,
however, requires high temperatures (generally over
1000 °C) and a metal catalyst (e.g., Cu [2], Ni [3], Ru) to
decompose and dehydrogenate C-precursors, e.g., CH 4 .
Plasma-enhanced CVD approach has been proposed by
various authors [4-6] to grow graphene at a low substrate
temperature (as low as 475 °C) [6]. Indeed, all previous
studies on plasma-CVD have shown the formation of
defects in graphene. Indeed there are other important
applications of plasmas in graphene processing, which
include etching and patterning graphene [7], Cl 2 , CF 4 ,
SF 6 plasma-assisted doping [8], O 2 and H 2 plasmaassisted surface cleaning [9, 10], plasma functionalization
and/or surface modification for bandgap engineering [11].
Noteworthy, there are potential benefits of remote
plasmas in graphene technology related to processing of
growth substrates as well as post-growth tailoring of
graphene properties that have not been duly considered.
This contribution aims at:
1) evaluating and discussing the effect of H 2 , H 2 -Ar and
H 2 -Ar-CH 4 remote plasmas on the graphene plasmaCVD growth. We discuss the limits of the plasma
activation during the growth on the graphene quality.
2) demonstrating the huge advantage of the H 2 remote
plasma in processing of the growth substrates to
improve graphene homogeneity and quality.
3) discussing the capability of H 2 remote plasmas in
tailoring the optical/electrical properties of graphene.
4) showing the potential of H 2 remote plasmas in
forming graphene nanoribbons which must be etched
laterally without damaging the graphene basal plane.
Specifically, we show significant improvement in the
control and reproducibility of CVD graphene growth and
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quality by a H 2 remote plasma processing of the main
catalytic metal substrates of nickel (Ni) and copper (Cu)
as thin films and foils. The main difference between the
Ni and Cu is in the growth of number of layers of
graphene, typically a monolayer on Cu, while multiple
layers can be grown on Ni depending on the growth time.
Implementation of CVD reactors with a simple remote
plasma configuration is presented, which is also
applicable to other 2D materials.
2. Processing and Deposition Details
We conducted CVD of graphene using a UHV (base
pressure 10-8 Torr) vertical reactor that can be fed with
Ar, H 2 and CH 4 flows [12]. This reactor has the unique
peculiarity of being equipped with a remote plasma
source for the in-situ cleaning and processing of
substrates and graphene, and with an in-situ spectroscopic
ellipsometer for the graphene deposition and processing
real time monitoring (see Fig. 1) [13].
We used as substrates for CVD graphene Cu foils and
Cu and Ni 300 nm films sputtered on a thermallyoxidized Si substrate. All those substrates were placed in
the 13.56 MHz remote plasma system, and the following
treatments were run and compared:
- Annealing in H 2 at 1000 °C (1 Torr) (conventional);
- H 2 plasma (5 W - 160 W, 300 - 800 °C, 1 Torr)
- H 2 -Ar plasmas (φ Ar /φ H2 = 1 - 10; 1 Torr, 60 Watt,
800 °C).
The CVD graphene was grown in a mixture of CH 4 -H 2
at 950 °C. The total gas pressure was fixed at 1 Torr.
After deposition, the sample was rapidly cooled to room
temperature at a cooling rate of about 10 °C/s by turning
off the heater and filling the chamber with a flow of H 2 to
promote carbon segregation and graphene formation.
This conventional CVD was compared with a:
- CH 4 -H 2 -Ar plasma CVD growth;
- CH 4 introduced in the downstream of H 2 -Ar plasma.
1
Fig. 1. Picture of the Remote Plasma CVD reactor with
an in situ spectroscopic ellipsometer for monitoring in
real time graphene growth kinetics and plasma surface
modifications.
The obtained graphene samples were characterized by
Raman spectroscopy, ellipsometry, van der Paw and Hall
measurements, and Kelvin probe microscopy.
3. Results and Discussion
3.1 Effect of plasma activation on CVD of graphene on Cu
Representative Raman spectra obtained for graphene on
Cu using a direct plasma of CH 4 -H 2 -Ar (at 800 °C) , a
plasma of H 2 -Ar for the afterglow activation of CH 4
(introduced in the plasma downstream), and thermal
CH 4 :H 2 (at 1000 °C) are shown in Fig. 2. The spectrum
of CVD graphene shows only the G and 2D peaks in a ratio
I 2D /I G ∼2.5 characteristic of single layer graphene [14].
Conversely, all samples grown in presence of plasmas
exhibit the additional distinct D peak, which is
characteristic for defective graphene [14].
The higher quality of the CVD sample reflects in the
lower sheet resistance, as also indicated in Fig. 2. The
sheet resistance strongly correlates to the defect level
(I D /I G ) of graphene. These characteristics have been
described previously and attributed to nanographene [15].
The diameter, L, of the nanographene grains can be
calculated from the D and G peak intensities as
L = [I G C(λ)]/I D , with C (λ) a variable scaling coefficient
that depends on the excitation wavelength [15]. This
yields a plasma grown nanographene with dimension of
10 - 20 nm.
Those results can be explained considering that, when
CH 4 is directly decomposed in the plasma, the formation
of C-atoms and methyl radicals are adsorbed on the Cu
rapidly, and such a fast graphene synthesis results in
defective graphene with amorphous and nanographitic
inclusions. Then, atomic H in the plasma acts as an
etchant for the C layer with creation of structural defects.
2
Fig. 2. Raman spectra of graphene on copper grown by
(a) conventional thermal CVD, (b) plasma of H 2 -Ar with
CH 4 introduced in its downstream, and (c) CH 4 -H 2 -Ar
plasma.
Inclusion of graphitic nanoclusters is avoided when the
CH 4 is introduced in the downstream of an Ar/H 2 plasma.
Indeed, the high density of plasma produced H-atoms
appear to be also a dominant factor in C etching by the H
atoms creating structural defects, such as re-hybridization
from C-sp2 to C-sp3 by C-H formation, as seen in Fig. 2.
Thus, although the plasma grown films have typically a
higher defect density compared to graphene obtained by
CVD, they can be suitable for applications such as
catalysis and composites not requiring high mobility.
3.2. Advantages of H 2 remote plasmas in graphene
substrates treatments
Various pre-treatments of Cu and Ni substrates are
being applied aimed at improving size and quality of
graphene domains. The substrate pre-treatments serve
several functions including removal of Cu and Ni native
oxide that reduces catalytic activity of CH 4
dehydrogenation. The conventional treatment is an
annealing in H 2 at 1000 °C for hours, which is also
important for increasing the Cu and Ni grain size and
rearranging surface morphology [16].
Indeed, the
annealing results in irreproducible graphene quality. In
this context, we demonstrate that H 2 remote plasmas of
Cu and Ni substrates are effective in improving graphene
quality. The role of the plasma produced hydrogen
species in the interaction with Cu and Ni surfaces is
discussed in detail at the conference. Here we provide
evidences of the technological advantages of the plasmas
in the graphene growth. Specifically, Fig. 3 shows that
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H 2 remote plasmas are effective in increasing the grain
size and smoothing of Cu and Ni surfaces, which are
necessary conditions to increase grain size of graphene.
This is because atomic hydrogen enhanced the reflow of
sputtered Cu films; an atomic hydrogen environment,
self-diffusion of Cu is enhanced due to the weaker Cu–Cu
interaction [17] reconstructing the Cu (or Ni) surface.
“as-received”
150µmx150µm
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H2 annealing
H2 remote plasma
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2 µm
2 µm
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H-plasma
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H2 annealing
Transverse Grain Size (µm)
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100
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0
Cu/SiO2/Si
volatile products [18], leaving surface sites available for
carbon nucleation.
Similarly, we performed H 2 plasma pre-treatments of
the Ni substrate, and we found a fast removal of residual
oxygen/contaminants and atomic hydrogen surface
chemisorption at Ni surface that passivates grain
boundaries [12] and anneal dangling bonds at elevated
temperatures. This atomic hydrogen passivation blocks
surface site for C-nucleation, so that a more homogeneous
graphene monolayer can be grown on Ni as shown in
Fig. 5a as compared to a more non-homogeneous
monolayer/bilayer (the dark spot in the micrograph in
Fig. 5b) obtained by CVD. Furthermore, Fig. 5c shows
that the deleterious addition of Ar to the H 2 plasma
introduces a more significant ion bombardment by the
long-living Ar metastable and ions that introduces
structural defects in the Ni substrates, increasing the random
nucleation of carbon.
(a) H2 remote plasma at 850°C
Cu-foil 900°C
Cu-foil_400°C
Fig. 3. 150 µm x 150 µm optical microscopy images of
copper substrates as received and after a conventional H 2
annealing and an improved H 2 remote plasma; the
effectiveness of the H 2 plasma in improving Cu grain
size, necessary for growing high-quality graphene, is
shown in the stick diagram.
(b) H2 Annealing at 850°C
50
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30
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Intensity (a.u.)
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15
10
5
Y (µm)
We also show through Raman spectra of Cu foils after
various treatments and of the graphene grown on them,
the effectiveness of the H 2 plasma in removing all carbon
and oxides impurities that inhibit graphene growth,
improving the quality of the graphene, as shown in Fig. 4.
55
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0
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2 µm
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(c) Ar/H2=100:10 plasma at 850°C
Graphene-on-Cu foils
Graphene on Cu foil
H2 annealed but still
with C-contamination
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Graphene on Cu foil H 2 annealed
Cu foils
Cu foil as received
Cu foil after H2 plasma
Y (µm)
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2D
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Contaminants
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Cu foil #A
2D
Intensity (a.u.)
Wavenumber (cm-1)
150µmx150µm
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Cu-oxides
Graphene on Cu foil treated by H 2 plasma
Cu foil #B
G
H2 Plasma
0
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250
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750 1000 1250 1500 1750 2000
Raman shift (cm-1)
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1500 2000 2500
Raman Shift (cm-1)
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Fig. 4. (a) Raman spectra of various Cu foils after common
annealing and after developed H 2 plasma treatment (blue
line); (b) Raman spectra of the graphene-on-Cu foils (same
as in (a)).
Therefore, hydrogen radical produced from the remote
plasma can penetrate into the Cu film and subsequently,
impurities inside the film such as carbon, oxygen and
sulphur that may cause local variations in the carbon
dissolvability, can be removed by the formation of
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Fig. 5.
Raman spectra and corresponding
150 µm x 150 µm optical micrographs of graphene grown
on Ni substrates pre-treated by (a) a remote H 2 plasma
(500 sccm, 60 W, 1Torr, 850 °C), (b) the conventional
annealing in 1 Torr of H 2 at 850 °C, (c) an Ar/H 2 remote
plasma (Ar/H 2 = 10, 60 W, 1 Torr, 850 °C).
This results in a more defective graphene as shown by
the D-peak in Fig. 5c and the higher density of
multilayers graphene flakes (dark spots in the micrograph
5c image).
Thus simple low power H 2 remote plasma is effective
in improving Cu and Ni substrates and the resulting
graphene quality.
3
22
20
18
π-Plasmon Photon Energy (eV)
4.75
4.70
C
4.65
4.60
4.55
4.50
B
D
16
Graphene on Various Substrates
14
B
D
Ä_i 12
A
10
8
C
6
4
2
1
2
3
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Photon Energy (eV)
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Fig. 6. Spectra of the imaginary part, ε i , of the dielectric
function of graphene on A) H 2 plasma treated Ni, B) H 2
annealed Ni, C) Ar/H 2 plasma treated Ni (the same
samples as in Fig. 5). For comparison sample D is the
reference exfoliated graphene on SiO 2 from ref. [20].
Therefore, H 2 plasma treatment of substrate is useful
for control the degree of isolation of graphene from the
substrate, and consequently graphene optical properties.
3.4 H 2 Plasma for graphene nanoribbons
Recent studies [21] indicate that graphene nanoribbons
can be achieved by using downstream H 2 plasmas, in
which H atoms attack preferentially the edges of the
ribbons. The remote H 2 plasma approach is of primary
interest because it could allow the patterning of various
4
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Si/SiO2
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0.4 µm
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2D
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D
Fig. 7. 70 µm x 70 µm optical micrography of
nanoribbons of graphene on SiO 2 ; the corresponding
Raman mapping in a selective area after the H 2 etching
process and a representative spectrum show that graphene
is not damaged by the H 2 remote plasma at low power
(60 W), as indicated by the small contribution of the
defects-related D-peak.
A
4.80
graphene nanostructures with desired placements,
controlled sizes/shapes. Among experimentalists, we
have recently explored various low temperature H 2
remote plasma activated reactions to selectively etch
graphene at edges without damaging the basal plane,
showing the potential of remote H 2 plasma in achieving
selective etching of graphene. An example is given in Fig. 7.
Graphene
3.3 Tailoring optical properties of graphene by H 2
plasma
In most cases, the strong hybridization of the electronic
states and the charge transfer between graphene and the
metal substrates modify the intrinsic hybridization
between the Ni-3d and C-2pz orbitals, leading to the
appearance of interface states at the K and M points of the
Brillouin zone and to the loss of the Dirac cones at K [19].
In this respect, the surface hydrogen atoms introduced
into the graphene/Ni(111) interface by the H 2 remote
plasma processing of the Ni surface offer an appealing
possibility to control the degree of hybridization between
the Ni-3d and C-2pz orbitals. We derived the optical
properties for the graphene films grown on the different
processed substrates as in Fig. 5. The main feature
observed in all spectra is the interband π-plasmon at
photon energies > 4.8 eV. However, we observe a clear
difference in the measured energies, being the π plasmon
peak shifted toward higher energy upon the atomic
hydrogen at the graphene/Ni interface, as shown in Fig. 6.
We assume this trend as an evidence of a transition from
an interacting to a quasi non-interacting regime.
4. Conclusions
We have been showing how “old” and apparently well
known H 2 remote plasmas can aid and implement the
“new” graphene technology. Although this manuscript
focused on providing various technological aspects of
surface cleaning, substrate pre-treatment, growth,
tailoring of optical and electrical properties and
nanopatterning of graphene to emphasize the renewed
interest in H 2 remote plasmas applications, aspects related
to the chemistry of the plasma phase in relation to the
various surface processes will be further elucidated at the
conference presentation.
5. Acknowledgements
The authors acknowledge funding by the European
Community’s 7th Framework Programme under grant
agreement no. 314578 MEM4WIN.
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