Nano Res
1
Wrinkle-Free Graphene with Spatially Uniform Electrical
Properties Grown on Hot-Pressed Copper
Jeong Hun Mun1, Joong Gun Oh1, Jae Hoon Bong1, Hai Xu2, Kian Ping Loh2, and Byung Jin Cho1()
Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0585-x
http://www.thenanoresearch.com on September 18, 2014
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DOI 10.1007/s12274-014-0585-x
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Wrinkle-Free Graphene with Spatially Uniform
Electrical Properties Grown on Hot-Pressed Copper
Jeong Hun Mun1, Joong Gun Oh1, Jae Hoon Bong1, Hai
Xu2, Kian Ping Loh2, and Byung Jin Cho1*
1 KAIST, Korea.
2 National University of Singapore, Singapore.
Page Numbers. The font is
ArialMT 16 (automatically
inserted by the publisher)
In this work, an approach utilizing hot-press to form wrinkle-free
monolayer graphene on Cu thin film using CVD process is introduced.
With this method, the extremely flat Cu thin film is obtained even after
the high temperature anneal for graphene growth, and it is also realized
that the formation of wrinkle-free monolayer graphene on top of the
flat Cu surface.
Provide the authors’ website if possible.
Author 1, website 1
Author 2, website 2
2
Wrinkle-Free Graphene with Spatially Uniform Electrical Properties Grown on Hot-Pressed Copper
Jeong Hun Mun1, Joong Gun Oh1, Jae Hoon Bong1, Hai Xu2, Kian Ping Loh2, and Byung Jin Cho1()
1 Department of Electrical Engineering, KAIST, Daejeon 305-701, Korea 2 Department of Chemistry, National University of Singapore, Singapore 117543, Singapore
Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher)
© Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011
ABSTRACT The chemical vapor deposition (CVD) of graphene on Cu substrates enables the implementation of large-area
monolayer graphene on desired substrates. However, during the transfer of synthesized graphene, topographic
defects unavoidably formed along the Cu grain boundaries, degrading the electrical properties of graphene
and increasing the device-to-device variability. Here, we introduce a method of hot pressing as a surface
pre-treatment to improve the thermal stability of Cu thin film for the suppression of grain boundary grooving.
The flattened Cu thin film maintains its smooth surface even after the subsequent high temperature CVD
process necessary for graphene growth, and the formation of graphene without wrinkles is realized. Graphene
field effect transistors (FETs) fabricated using the graphene synthesized on hot pressed Cu thin film exhibit
superior field effect mobility and significantly reduced device-to-device variation.
KEYWORDS CVD graphene, graphene synthesis, graphene wrinkle, graphene field effect transistor
Microscopic roughness on CVD graphene causes
undesirable spatial inhomogeniety in its electrical
properties.[1-12] While the problem is commonly
ignored by researchers who reported optimal
performance in graphene FETs fabricated from
micron-scale channel, it is of serious concern in
industrial scaling due to the issue of point-to-point
reproducibility on large scale graphene wafers.[3, 10]
The substrate induced wrinkles on graphene
originate from grain boundaries on the copper
substrate used in CVD, these wrinkles are inherited
by the graphene following the transfer of graphene
onto silicon substrate for device fabrication.[10, 11, 13-18]
To suppress the formation of grain boundary
grooving on the Cu substrate, herein, we introduce
a hot press method to recrystallize copper
substrates, which produces wrinkle-free graphene
with highly uniform Dirac voltages across area as
large as 2 × 2 cm2.
Nano Res DOI (automatically inserted by the publisher)
Research Article
————————————
Address correspondence to B. J. Cho, [email protected]
3
Figure 1. Illustrations showing the hot pressing of
copper-on-silicon oxide/silicon substrate used in graphene CVD.
a) The surface of Cu thin film is protected by a second oxidized
silicon sample, which prevents the reaction between pressing
unit and Cu thin film and also flattens the surface of Cu thin
film during the hot pressing. b) This sandwiched sample is
pressed and annealed by graphite heating module. c) After the
hot pressing, the oxidized silicon cover is carefully removed
from the Cu substrate. d) The graphene layers are then
synthesized by conventional CVD method on the hot pressed
Cu thin film substrate.
To characterize the effect of hot pressing, the
surface morphology and crystallinity of Cu thin
films were examined by atomic force microscopy
(AFM), scanning electron microscopy (SEM) and
electron back scattering diffraction (EBSD). When a
Cu thin film which has not been hot-pressed is
annealed at 900°C in Ar (1 atm), its surface shows
highly agglomerated grains and the typical depth of
the valley at the grain boundaries is ~40 nm (Figure
2a). When the Cu thin film is subjected to hot
pressing at 30 Mpa, the depth of the valley at the
grain boundary is only 1 ~ 2 nm (Figure 2b). The
surface flatness is improved as the pressure
increases (Figure S2, see Supporting Information).
Moreover, even after being subjected to the high
temperature CVD process in CH4 ambient (1 atm)
Figure 2. Surface morphologies of the hot pressed Cu film. a, b)
AFM images and corresponding line profiles of Cu thin film
after Ar (1 atm) annealing with (b) and without (a) hot pressing.
c, d) SEM images of Cu thin film after graphene growth with (d)
and without (c) hot pressing. e, f) EBSD maps of the Cu thin
film after graphene growth with (f) and without (e) hot
pressing.
for graphene growth, the Cu thin film maintains its
flat surface. The optical microscope (Figure S3, see
Supporting Information) and SEM images (Figure 2c,
d) also show clear differences of Cu surface
morphology after graphene growth with and
without hot pressing. From the EBSD maps,
however, it can be seen that both Cu substrates
show similar grain structures in spite of their
different surface morphologies. This indicates that
Cu grain growth occurs with or without hot
pressing, yet surface grooving of the grain
boundary is dramatically suppressed (Figure 2e, f)
on the hot-pressed substrate.
In order to understand the effect of hot pressing on
4
Cu thin film, the mechanical properties of Cu thin
films before and after hot pressing were also
investigated by nano indentation and X-ray
diffraction (XRD). The load and estimated hardness
for the as-deposited and hot pressed Cu thin film
(Figure 3a, b) show that the hot- pressed Cu thin
film has a 2 times higher hardness, indicating that
the hot pressing leads to densification of the Cu thin
film. In addition, the hot pressing changes the
residual stress of the Cu thin film from tensile to
compressive stress (Figure 3c, d). The relation
between the suppression of grain boundary
grooving and the change of mechanical properties
matches well with previously reported works on
the effect of hardness[19-21] and compressive residual
stress[22, 23] on the grain boundary grooving.
Figure 3. Mechanical properties of Cu thin film. a, b) Load and
calculated hardness for the as-deposited (a) and hot pressed (b)
Cu thin film. Nano indentation was performed with the
three-sided diamond pyramid (Berkovich) tip. After hot
pressing, the hardness of the Cu thin film is doubled. c, d)
Residual stress calculated using XRD (Omega method) for
as-deposited (c) and hot pressed (d) Cu thin films. Here, the
pressure was 30 MPa.
The ultra-flat surface of the Cu thin film after hot
pressing is expected to contribute to the removal of
the wrinkles of graphene. To confirm this, graphene
is synthesized using the CVD method (see
Experimental Section) on Cu thin films both with and
without hot pressing. Then, the CVD grown
graphene films are transferred onto an oxidized
silicon substrate and the surface morphology is
investigated. AFM images show that graphene
layers grown without hot pressing have closed-loop
shaped wrinkles (Figure 4a). However, with the hot
Figure 4. Characterization of synthesized graphene on hot
pressed Cu thin film. a, b) AFM images and corresponding line
profiles of graphene layers synthesized with (a) and without (b)
hot pressing. c-e) Two-dimensional Raman map of D (c), G (d),
and 2D band (e). f) The map of G/2D intensity ratio of the
synthesized graphene with hot pressing shows monolayer
coverage of 97.5%.
pressing, there is almost no wrinkle apart from
PMMA residues (Figure 4b). This result confirms
that the suppression of Cu grain boundary
grooving prevents the formation of curved
graphene along the grain boundary valley, thus
enabling the synthesis of graphene layers without
5
wrinkles. The optical microscope image of the
transferred graphene layer shows good uniformity
over a wide range (Figure S4a, see Supporting
Information). The Raman spectrum also shows a
negligible D-band and a high G- to 2D-band
intensity ratio, indicating the formation of high
quality monolayer graphene (Figure S4b, see
Supporting Information).[24,25] Two-dimensional
microRaman maps for the D-, G-, and 2D-bands are
shown in Figures 4c-e. The G-band intensity map
indicates that this graphene film is continuously
formed across the entire substrate. Occasionally,
small spots are found in the G-band map; these
spots imply the growth of multi-layer graphene at
certain points (Figure 4d). However, from the map
of the G- to 2D-band intensity ratio (Figure 4f), it
can be seen that 97.5% of the graphene area is
confirmed as monolayer (G- to 2D-band intensity
ratio < 0.5). The uniform D-band intensity map,
without any distinctive peaks, means that there are
no cracks or localized carbonaceous particles that
do not consist of sp2 hybridized carbons. The D- to
G- band intensity ratio is typically less than 0.2
across the film. In the TEM diffraction pattern,
(Figure S5a, b, see Supporting Information), a
hexagonal lattice can be clearly observed, indicating
that the synthesized graphene has good crystalline
quality.
Figure 5a shows the structure of a top-gated
graphene FET using graphene layers synthesized
with and without hot pressing of Cu thin film.
Details on FET fabrication can be found in
Experimental Section. The device-to-device
variability is evaluated by plotting the cumulative
distributions of Dirac voltage, channel resistance,
and carrier mobility measured on 100 FET devices
(Figure 5b-d). The control device, which uses
graphene synthesized by conventional way, has a
wide distribution of Dirac voltage, ranging from -2
to 5 V; such variation is typically found in graphene
FETs. On the contrary, the graphene synthesized by
hot pressing shows a much narrower distribution of
Dirac voltage. This indicates that the graphene
wrinkle affects the local doping concentration of
graphene layers and distorts the Dirac voltage. In
addition, the graphene FETs fabricated by hot
pressing also exhibit higher conductance and field
effect mobility, together with better uniformity,
compared to the control sample. The improvement
of device-to-device uniformity is extremely
important when we fabricate an integrated circuit
with hundreds or thousands of transistors. With
poor uniformity of the control device, it is
impossible to successfully operate an integrated
circuit with hundreds of transistors. It is worth
pointing out that the field effect mobility is also
very much improved in graphene grown on copper
substrate which has been restructured by the hot
presseing method. This indicates that the presence
of wrinkles in graphene is one of the key
performance killers in graphene channel devices.
Figure 5. Electrical properties of graphene FET array. a)
Optical microscope image of a graphene field effect device.
The length and width of each device are 4 and 10 μm,
respectively. b-d) Cumulative distributions of Dirac voltage (b),
Resistance at Vg=VDirac - 2 V (c) and top-gated field effect
mobility (d) for the graphene device arrays with and without
hot pressing. One hundred devices were measured for this
statistical analysis.
In summary, we have demonstrated a processing
step involving hot pressing of the catalytic metal
thin film to remove the wrinkles on CVD graphene
grown on Cu thin film. The hot-pressed Cu thin
film maintained its smooth, faceted crystalline
morphology even at the high temperature used in
graphene growth, enabling the formation of
wrinkle-free graphene. Graphene FETs derived
6
from this process exhibit substantially improved
device-to-device variation and superior electrical
performance. Incorporating hot-pressing as one of
the processing steps for the pre-treatment of the
copper substrate prior to CVD growth may help to
enable the large scale integration of graphene
devices.
Experimental
Graphene synthesis: CVD growth of graphene was
carried out in an induction furnace system.
300-nm-thick Cu thin film is deposited by thermal
evaporation on top of an oxidized silicon substrate
and used as the substrate for graphene growth.
After loading the Cu thin film substrate on a
tungsten susceptor, the chamber was pumped
down to a base pressure of ~1×10-7 torr. An Ar/H2
gas mixture (2700 sccm/300 sccm) was then
introduced into the chamber until the chamber
pressure became 1 atm. After this, the sample was
annealed at 900°C for 10 min to remove native
oxides and other residual impurities. Following,
CH4 (5 sccm) was flowed for 5 min for graphene
growth. Here, both the ramping and the cooling
rate were fixed at 10°C/s.
Fabrication and characterization of graphene FET devices:
The synthesized graphene on the hot-pressed Cu
thin film was transferred onto an oxidized silicon
substrate using PMMA and metal etching. The
transferred graphene layer was then annealed in an
1 atm of H2 ambient at 400C for 30 min to remove
polymer residues. Source and drain electrodes were
formed by the deposition of Au (50 nm)/Cr (2 nm)
and a lift-off process. The gate oxide was formed
using an oxidized Al layer, followed by the
deposition of a 20 nm Al2O3 film by atomic layer
deposition. The electrical characterizations were
carried out in a probe station at room temperature
under air ambient condition. The top-gated field
effect mobility of the devices were determined
using the relation µFE=(dG/dVTG)(L/WCTG), where
dG/dVTG is the differential change in conductance
(G) per differential change in top gate voltage (VTG),
CTG is the capacitance of the top gate dielectric, and
L and W are the length and width of the graphene
channel, respectively.
Acknowledgements
This work was supported by the Center for
Advanced Soft-Electronics Funded by Ministry of
Science, ICT and Future Planning as Global Frontier
Project (2011-0031638), and National Research
Foundation of Korea (NRF) Research Grants
(2008-2002744 and 2010-0029132).
Electronic Supplementary Material: Supplementary
material (further details of the hot pressing
procedures, AFM imaging and Raman spectroscopy
measurements) is available in the online version of
this article at
http://dx.doi.org/10.1007/s12274-***-****-*
(automatically inserted by the publisher). References
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Electronic Supplementary Material
Wrinkle-Free Graphene with Spatially Uniform Electrical Properties Grown on Hot-Pressed Copper
Jeong Hun Mun1, Joong Gun Oh1, Jae Hoon Bong1, Hai Xu2, Kian Ping Loh2, and Byung Jin Cho1()
1 Department of Electrical Engineering, KAIST, Daejeon 305-701, Korea 2 Department of Chemistry, National University of Singapore, Singapore 117543, Singapore
Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)
Figure S1 Hot pressing is carried out in 1 atm Ar ambient. The pressure reaches 30 MPa during the ramping
up of temperature and starts to decrease during the cooling. After annealing at 900°C for 1 hr, the chamber is
slowly cooled down with a cooling rate of ~5°C/s.
————————————
Address correspondence to B. J. Cho, [email protected]
9
Figure S2 a), b) AFM images and corresponding line profiles of Cu thin film after hot pressing with
pressures of 15 (a) and 30 (b) MPa.
10
Figure S3 a), b) Optical microscope images of Cu thin films after graphene growth with (b) and without (a)
hot pressing. Without hot pressing, the Cu surface is highly roughened due to thermal agglomeration. On
the contrary, with hot pressing, no grain boundary grooves are shown on the Cu surface.
11
Figure S4 a), b) Optical microscope image (a) and Raman spectra (b) of graphene layers transferred onto an
oxidized silicon substrate. Here, the thickness of SiO2 is 300 nm.
12
Figure S5 a-b) Low-magnification TEM image of synthesized graphene with hot pressing (a), and diffraction
pattern of the area marked as a blue dashed circle in a (b). Inset in a is high resolution TEM image of
synthesized graphene with hot pressing.