Synthesis of graphene-based conductive thin films by plasma-enhanced chemical
vapor deposition in a CO/H2 microwave discharge system
Youhei Anekawa1, Shinsuke Mori1and Masaaki Suzuki1
1
Department of Chemical Engineering, Tokyo Institute of Technology
,2-12-1 O-okayama, Meguro-ku, Tokyo, Japan
Abstract: Synthesis of graphene-based conductive thin films was performed by
plasma-enhanced chemical vapor deposition in a CO/H2 microwave discharge system.
Considered from viewpoint of practical application, we tried to synthesize these films
without catalysts. By optimizing input power and CO/H2 flow ratio, graphene-based
conductive thin films with relatively low resistance were successfully synthesized.
Keywords: plasma, graphene, chemical vapor deposition, conductive film, etc
1. Introduction
CNW has high conductivity and can be synthesized
Recently graphene, one layer of graphite consisting of
without catalyst but is not transparent because of stacking.
sp2 bonds has been studied eagerly in the world because
In this study, we tried to synthesize graphene-based
of outstanding characteristics.[1] Graphene has a two
conductive thin films without any catalyst by decreasing
dimension structure, high transmittance ranging from
the thickness of CNW layers.
visible to infrared light and very high conductivity in the
room temperature, so these properties are attractive from
2. Experiments
the view point of various applications[2-4]. For example,
The schematic of experimental apparatus is shown in
it is expected that graphene is used for the transparent
Fig.1. This CVD apparatus is assembled with a 2.45GHz
conducting films of solar cells and touch panels as
microwave generator (ASTeX DPA25) with maximum
substitutes of indium tin oxide including rare metal.
power of 250W. The quartz tube of a 15mm inner
Several methods to synthesize graphene have been
diameter as discharge tube is utilized. Quartz glass
reported: 1.micromechanical cleavage of bulk graphite
substrates are cut with size of 9×9mm2. CO and H2 are
with tape, 2.chemically reduction of graphite oxide,
used for CVD and Ar and O2 are used for cleaning of
3.vacuum annealing of single crystal SiC, 4.CVD on
sample stage and discharge chamber. These gases were
metal substrates. Among them, CVD is the most
fed from the top of chamber and evacuated from the
promising method to obtain high quality and large area
bottom of
graphene. However, the CVD method has a significant
experimental condition is as follows: total flow rate,
problem. Graphene is synthesized on metal catalyst like
50sccm; total pressure, 250Pa. Input microwave power,
Cu or Ni in this case, so transfer process is
CO/H2 flow ratio and deposition time are changed by
indispensable[2,5-7]. On the other hand, in our group, the
each experiment. Although substrate temperature which
carbon nanowall (CNW) which is called three dimension
was measured from the bottom of experimental apparatus
graphene is successfully synthesized by the PECVD in
by radiation thermometer was different with input
CO/H2 microwave discharge systems. CNW is formed by
microwave power, it was about 700 to 950℃ in steady
stacking graphene and growing vertically to substrates.[8]
condition. Note that deposition time is so short as
chamber
by oil
rotary pump.
Other
expressed below that actual substrate temperature during
short wavelength. Therefore these spectra were able to be
deposition was much lower.
regarded as the peculiar spectra to graphene-based thin
films. In addition, high transmittance films were able to
be synthesized by decreasing deposition time. On the
other hand, Fig.3 show relationship between transmittance
at wavelength of 550nm and sheet resistance of each
sample. Square symbols are the measured values. As you
can see, sheet resistance is proportional to transmittance
reasonably. These measured values were compared with
reported values of graphene synthesized by PE-CVD with
Fig.1 Schematic of experimental apparatus
3. Results and discussion
Firstly, typical CNW was selected as the base sample
transmittance(%)
80
because CNW was considered as starting point of
19s
20s
60
21s
40
22s
23s
20
30s
60s
0
experiments. Typical CNW was synthesized with input
400
microwave power of 80W and CO / H2 flow ratio of
92%CO where a%CO was defined as following.
600
800
wavelength(nm)
1000
Fig.2 transmittance spectra of synthesized films
where F is flow rate. As the thickness of CNW layers was
decreasing, it is investigated that how the characteristics
of synthesized films was changed. The thickness of CNW
layers
was
controlled
by
deposition
time.
For
characterization of synthesized films, transmittance and
sheet resistance were measured because conductive thin
films were requested to have both high transmittance and
conductivity.
The
spectrophotometer
measured
was
ranging
wavelength
from
by
400cm-1
sheet resistance(kΩ/□)
100
measured value without catalyst
reference value with catalyst
10
1
0.1
0
10 20 30 40 50 60 70 80 90 100
transmittance(%)
the
Fig.3 transmittance-sheet resistance curve
to
metal catalyst [2]. Triangle symbols are the reference
-1
1100cm .
values. The measured values have lower transmittance
Figures 2 and 3 show transmittance spectra of
and higher sheet resistance than reported values.
synthesized films and transmittance-sheet resistance curve
Accordingly, we had considered the reduction of sheet
respectively. As shown in Fig. 2, the all transmittance
resistance. To do this, the crystalline of films need to be
were almost constant ranging from visible to infrared light
improved. In our group, it was confirmed by Raman
in each sample and were decreasing gradually toward
spectroscopy that the crystalline of CNW had been
improved when input power increased or CO/H2 flow
changing CO/H2 flow ratio (92%CO, 70%CO and
ratio decreased. Therefore the relationship between
30%CO). Input power was fixed for 200W because of the
transmittance and sheet resistance was investigated
above results. As you can see, sheet resistance is
regarding these parameters as manipulated variable.
decreased at low transmittance when CO/H2 flow ratio is
Figure 4 shows transmittance-sheet resistance curve
decreased. This reason is that H2 etching effect for
changing input power (60W, 80W, 140W and 200W).
amorphous carbon removal is enhanced when CO/H2 flow
CO/H2 flow ratio was fixed for 92%CO. As you can see,
ratio decreases. However, sheet resistance is not
sheet resistance is decreased at high transmittance when
dependent on CO/H2 flow ratio at high transmittance and
input power is increased while sheet resistance is not
this reason is described below.
dependent on input power at low transmittance. This
In summary, the synthesized films in optimized
reason is that the increment of substrates temperature
experimental condition (200W, 30%) showed better
becomes large when input power increases. So the
results than that of the initial CNW condition as shown in
crystalline of films was improved only near the substrate
Fig.3. The crystalline analysis of films was performed by
surface.
Raman spectroscopy because the conductivity of the films
Figure 5 shows transmittance-sheet resistance curve
structure.
100
sheet resistance(kΩ/□)
is strongly correlated with the crystalline of graphite
60W
80W
140W
200W
10
Raman spectra of CNW have three main peaks: the G
peak at 1590cm-1 which indicates the presence of sp2
carbon, the D peak at 1360cm-1 which indicates defects
and edges of graphite structure and the 2D peak at
1
2720cm-1 which generated by double-resonant Raman
scattering. The intensity ratio of D peak to G peak, ID/IG,
0.1
0
20
40
60
transmittance(%)
80
100
is used commonly as an index of crystalline for graphite
films.
Figure
6
shows
the
relationship
between
Fig.4 transmittance-sheet resistance curve (changing
transmittance and ID/IG. As transmittance was increasing,
input power)
the ID/IG increased and reached constant. It means that the
deterioration of films was proportional to transmittance
100
sheet resistance(kΩ/□)
92%CO
but was independent at high transmittance region. In order
70%CO
10
to find out the reason of the independence of film quality
30%CO
at high transmittance condition, early growth stage of
CNW was observed by TEM.
1
Figure 7 and 8 show sample’s picture and TEM images
of CNW at early growth stage and thickness of each layer.
0.1
0
20
40
60
transmittance(%)
80
100
Observation points were number from p1 to p5. Top part
of each TEM image is Pt/Pd coating films for the
Fig.5 transmittance-sheet resistance curve (changing
protection in FIB milling process and bottom part is Si
CO/H2 flow ratio)
substrate. Three layers between top and bottom is
presence. The layer with red line is amorphous carbon
[8] S. Mori, , T. Ueno1, M. Suzuki, Diamond and Related
layer since structure is not observed and the layer with
Materials, 20(2011) p.1129
blue line is graphite layer since lattice fringes of graphite
0.8
structure is observed in a horizontal direction. The layer
disappeared but amorphous carbon layer was remaining
ID/IG
with green line is oxide layer of Si. Graphite layer was
ID/IG
0.6
0.4
when the thickness of films decreased. This shows that
0.2
initial amorphous layer cannot be etched by H2 plasma.
0
0
This would be the reason why the peak ratio is constant at
20
high transmittance region in Fig.6. Therefore the
crystallization of amorphous layer at early growth stage is
40
60
transmittance(%)
80
100
Fig.6 relationship between transmittance and ID/IG
needed to synthesize the higher performance transparent
conductive films.
3nm
4. Conclusion
Graphene-based conductive thin films with relatively
low resistance were successfully synthesized without
catalyst by optimizing input power and CO/H2 flow ratio.
Further high performance films need to promote the
crystallization of amorphous layer at early growth stage.
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0
p1
p2
p3
p4
sumple number
p5
Fig.7 photograph of the sample for TEM(p1,p2,p3,p4,p5
from left to right)(top), TEM images of CNW at early
growth stage(center), thickness of each layer(bottom)