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

© Tsinghua University Press 2014

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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, elebjcho81@kaist.ac.kr

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, elebjcho81@kaist.ac.kr

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.