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The Anistropy of Field Effect Mobility of CVD Graphene Grown on Copper Foil

Dayong Zhang , Zhi Jin , * Jingyuan Shi , Peng Ma , Songang Peng , Xinyu Liu , and Tianchun Ye

surface after CVD process reported by others. [ 3b , 9 ] Through

adjusting the focal plane of the optical microscope, we have

verifi ed that these trenches on the copper foil have a con-

cave cross section. It is similar with the surface topography of

copper foil, whose optical image is shown in the inset panel of

Figure 1 a. The surface of copper foil shows a directional texture

consisting of many parallel trenches with spacing on the order

of micrometers, these trenches are considered to be produced

during the foil rolling process used to fabricate the Cu foil, with

the trenches running parallel to the shear/drawing direction. [ 10 ]

After CVD process, the high temperature treatment makes the

adjacent small trenches melt together and increases the space

between adjacent wrenches. The AFM image (Figure 1 b) col-

lected at the region enclosed by the white dotted square in

Figure 1 a shows the representative morphology of copper foil

surface. The height profi le taken from the white dash line indi-

cated the height of trench is about 120 nm.

After CVD process, graphene was transferred onto

300 nm thick SiO 2 /Si substrate using a PMMA-assisted

DOI: 10.1002/smll.201303195

Graphene

Graphene has attracted enormous interest due to its extraor-

dinary properties. [ 1 ] In particular, the extremely high carrier

mobility of graphene at room temperature has made it a

promising candidate for electronic devices with higher oper-

ating frequencies, and ultimately superior performance. [ 2 ]

Chemical vapor deposition (CVD) on copper substrate as

a low cost and scalable technique for obtaining large-area,

high-quality, and uniform graphene fi lms have recently

attracted great attention. [ 3 ] Wrinkles are commonly observed

in CVD graphene, which formed for two reasons. Firstly, the

compressive stress between graphene and the metal substrate

due to the differential thermal expansion coeffi cient induced

the formation of wrinkles upon cooling process. [ 4 ] Secondly,

the surface morphology difference between growth substrate

and transfer substrate could produce morphology-induced

wrinkles during graphene transfer process. [ 5 ] Theoretical and

experimental results have shown that wrinkles in graphene

lead to the formation of electron-hole puddles, [ 6 ] the sup-

pression of weak localization, [ 7 ] and the change of electrical

resistivity. [ 8 ] In this work, graphene with oriented wrinkles

was skillfully prepared by using copper foil with special sur-

face morphology as the growth substrate. Furthermore, we

experimentally studied the relationship between fi eld effect

carrier mobility and the wrinkles in graphene and found that

the carrier mobility of graphene is anisotropic both parallel

and perpendicular to the wrinkles.

Figure 1 a shows the optical image of the copper foil surface

after graphene growth by CVD at temperature up to 1050 °C

using a gas mixture of methane and hydrogen. There are some

parallel trenches on copper foil surface whose edge is smooth,

and which is similar with the morphology of copper foils

small 2014, DOI: 10.1002/smll.201303195

Figure 1. a) Optical image of copper foil after growing a graphene fi lm by CVD. The inset panel in Figure 1 a is the optical image of copper foil before graphene growth. b) AFM image of the region enclosed by the white dotted square in panel (a). A height profi le was taken from the white dashed line. c) Optical image of graphene transferred on 300 nm SiO 2 substrate. The white arrows in Figure 2 c show wrinkles in graphene. d) A representative Raman spectrum of graphene on a SiO 2 /Si substrate.

Prof. D. Zhang, Prof. Z. Jin, Dr. J. Shi, Dr. P. Ma, Dr. S. Peng, Prof. X. Liu Department of Microwave Device & IC Institute of Microelectronics of Chinese Academy of Sciences Beijing 100029 , P. R. China Tel.: +86 10 82995597E-mail: jinzhi@ime.ac.cn

Prof. T. Ye Department of Micro-fabrication and Nano Technology Institute of Microelectronics of Chinese Academy of Sciences Beijing 100029 , P. R. China

D. Zhang et al.

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communications

© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

transfer technique. [ 11 ] Figure 1 c shows

the typical optical microscope image of

transferred graphene. The color contrast

of image indicates the thickness of gra-

phene is uniform, and the white arrows

in Figure 1 c show the wrinkles in trans-

ferred graphene which run parallelly with

the direction of trench in copper foil. The

layers and quality of graphene were con-

fi rmed by Raman spectroscopy, which has

been regularly employed for identifying

layers and evaluating the quality of gra-

phene. [ 12 ] Figure 1 d shows a typical Raman

spectrum of our graphene sample. The D

band (∼1363 cm −1 ), which correspond to

defect levels in graphene, showed very low

intensity, indicated the high quality of our

graphene fi lm. The 2D band (∼2715 cm −1 )

is symmetric, the full width at half max-

imum (FWHM) is ∼30 cm −1 , and the inten-

sity ratio of G band (∼1594 cm −1 ) to 2D

band ( I G /I 2D ) is ∼0.3. All these informa-

tion confi rm that our graphene fi lm is a

monolayer. [ 13 ]

As illustrated in Scheme 1 , the sur-

face morphology of copper foil was accu-

rately replicated by CVD graphene. After

etching the copper substrate and suc-

cessively transferring the graphene onto

smooth SiO 2 /Si substrate, the graphene in

the fl uctuant region collapses into wrinkles that frequently

run along the direction of original trench on the copper foil.

A similar phenomenon has been reported by Liu and co-

workers to produce graphene nanoribbons. [ 5c ]

The electronic transport properties of graphene along

the two directions of parallel and perpendicular to wrin-

kles are evaluated with back-gated fi eld effect transistor

(FET) devices. Figure 2 a shows the optical image of the

device, and the white arrow in which shows the direction of

wrinkle in graphene. The graphene channel of FET device

is cross-shaped and the channel width and length are 6 µm

and 16 µm, respectively. We intentionally make one pair of

electrode parallel with the wrinkles and the other pair of

electrode perpendicular with the wrinkles. The procedure of

devices fabrication was shown in Figure S1. Firstly, the gra-

phene transferred on substrate was patterned with optical

lithography and oxygen plasma etching. Following this, the

source and drain electrodes were fabricated by optical lithog-

raphy and successively metallic electron beam evaporation

of Ti and Au, the thickness of which is 10 nm and 200 nm,

respectively. We must point out that the contacts between

graphene and electrodes have relations with the device

preparation process and will infl uence the transport proper-

ties of graphene. [ 14 ] Here, the special structure of our device

ensures that the effect from contacts between graphene and

electrodes are same along these two directions and would not

infl uence the comparison of graphene carrier mobility. The

devices were measured at room temperature under ambient

condition. To measure the electronic properties of graphene

along the direction of wrinkle, the source 1 electrode was

grounded and the drain 1 bias was kept constant at V D =

50 mV, the other two electrodes (source 2 and drain 2) were

fl oating (Figure S2). The gate voltage ( V G ) has been swept

from V G = –40V to V G = 40V. Similarly, the electronic prop-

erties of graphene along the other direction were measured

by the same method. Typical curves of drain current ( I D )

versus the gated voltage ( I D –V G ) for one FET device along

small 2014, DOI: 10.1002/smll.201303195

Figure 2. a) Optical image of the cross-shaped graphene back-gated FET device atops a 300 nm SiO 2 /Si wafer with 10 nm/200 nm Ti/Au as the source and drain electrodes. The white arrow in Figure 2 a shows the direction of wrinkles. b) The representative I D - V G curves along two directions of parallel and perpendicular to wrinkles measured from one cross-shaped graphene FET device at source-drain voltage ( V ds ) = 10 mV. Histogram distributions of the hole mobility of 15 FET devices along the direction of c) perpendicular, and d) parallel to the wrinkles, respectively.

Scheme 1. Schematic illustration of the oriented wrinkles formation in graphene during the transfer process.

The Anistropy of Field Effect Mobility of CVD Graphene Grown on Copper Foil

3www.small-journal.com© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

the two directions are shown in Figure 2 b. Although the gra-

phene was pristine without any doping atoms, the neutrality

point moved the positive gate voltage beyond 40 V shown

p-type behavior. It probably arose from the physical sorption

of small molecules, such as H 2 O and O 2 in air or chemical

doping induced by etching agents and polymer resists. [ 15 ]

Figure 2 c,d show the histogram distributions of hole

mobility values obtained from 15 graphene FET devices.

The hole fi eld effect mobility µ is extracted using the fol-

lowing formula of µ = (Δ I D /Δ V G )( L/W )/( CV DS ) from the

I D –V G curve. [ 16 ] Where L/W is the length-to-width ratio of

the channel and C is the gate capacitance. The gate capaci-

tance per unit area is give by C = εε 0 / t , where ε is the dielec-

tric constant (3.9 for SiO 2 ) and t is the thickness of the SiO 2

dielectric layer (300 nm for our device), Thus C is about

11.5 nF/cm 2 for our device confi guration. [ 17 ]

Table 1 shows the statistical results of carrier mobility

along the two directions. The average hole mobility

value along the direction of parallel with the wrinkle is

2294 cm 2 V −1 s −1 . Correspondingly, the average value along

the direction perpendicular to the wrinkles decreased to

2191 cm 2 V −1 s −1 . It is important to ascertain whether any

differences are statistically signifi cant when comparing the

values of carrier mobility along the two directions. In this

study, a paired-sample two-tailed t -test was performed to

determine signifi cant differences between the two groups of

value. A confi dence level greater than or equal to 95% was

taken to indicate that these two groups of value of carrier

mobility are signifi cantly different from each other. The sta-

tistical result of P value is 0.012 shown that the hole mobility

along the two directions was signifi cant different. When the

carrier transport along the wrinkles, The folded wrinkle can

be viewed as a strip of trilayer graphene. [ 8b ] Due to non-

linear charge screening, [ 18 ] the holes are almost all confi ned

to the bottom layer when the device was biased. The carrier

mobility is commonly seen to improve with decreasing carrier

density. [ 19 ] Therefore, the charge distribution in the trilayer

structure would improve its effective carrier mobility. When

the carrier transport along the direction perpendicular with

the wrinkle, for the joule heat cannot escape effi ciently at the

wrinkle segment, the increased phonon scattering because

of the local temperature elevation degrade the carrier trans-

port. [ 20 ] Moreover, the refl ection of the electron wave at the

wrinkle may also reduce the transmission probability of the

carrier. [ 8b , 21 ] In summary, we prepared graphene with oriented

wrinkles taking advantage of the special surface morphology

of copper foil substrate. Furthermore, using cross-shaped

FET devices we studied the electronic transport proper-

ties along two directions of paralle and perpendicular to the

wrinkles and found that the carrier mobility of graphene is

anisotropy along these two directions. This result will help us

to prepare electronic device with higher performance.

Experimental Section

Graphene Transfer : Graphene was transferred from copper foil onto silicon substrates with 300 nm layer of thermally grown silicon dioxide using a nondestructive polymer-mediated transfer technique. Briefl y, as-grown graphene fi lms on copper foil were spin-coated with poly-methyl methacrylate (PMMA) at 500 rpm for 5 s and 4000 rpm for 60 s. The PMMA coated Cu foils were baked at 180 °C for 2 min and etched with 0.5 M FeCl 3 solution. After com-plete removal of the copper substrate, the PMMA/graphene fi lm was rinsed with deionized water several times and then was trans-ferred on target substrate, with the PMMA side up. Another layer of PMMA was applied to the sample surface, followed by baking at 180 °C for 2 min. Finally, the PMMA protective layers were removed by immersing the sample in a large volume of acetone for 30 min at room temperature and then dried under nitrogen fl ow.

Raman Measurements : Raman spectra were acquired under ambient conditions with a LabRAM micro-Raman spectrometer equipped with a 433 nm wavelength excitation laser. A 100× objective lens was used to focus the excitation laser light spot of about 2 microns on the graphene samples with an on-sample inci-dent power of less than 2 mW to avoid local heating effects.

Device Fabrication : Graphene FET device was fabricated on heavily doped silicon substrates with a thermally grown 300 nm silicon dioxide dielectric layer. First, the graphene transferred on substrate was patterned with optical lithography and oxygen plasma etching. Following this, the source and drain contacts were fabricated by optical lithography and electron beam evaporation, and the contact metal is Ti/Au (10 nm/200 nm).

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements

The authors are grateful for the fi nancial support of the National Science and Technology Major Project (2011ZX02707.3), and the National Natural Science Foundation of China (61136005, 50972162, 51072223 and 61006063). We also thank the help of all the members of the IMECAS compound semiconductor device department in our device preparation.

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small 2014, DOI: 10.1002/smll.201303195

Table 1. The statistical results of carrier mobilities along the two directions.

CM // a) (cm 2 V −1 s −1 ) CM ⊥ b) (cm 2 V −1 s −1 ) P value of paired-sample t test

2294 ± 72 2191 ± 129 0.012

The value of carrier mobility are presented as Mean ± Standard Deviation; a) ( CM // is the carrier

mobility along the wrinkles); b) ( CM ⊥ is the carrier mobility along the direction perpendicular to

the wrinkles).

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Received: October 8, 2013 Revised: December 25, 2013 Published online:

small 2014, DOI: 10.1002/smll.201303195