Flexible Graphene Devices with an Embedded Back-Gate

doi:10.4236/graphene.2013.21003

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Graphene, 2013, 2, 13-17

http://dx.doi.org/10.4236/graphene.2013.21003 Published Online January 2013 (http://www.scirp.org/journal/graphene)

Flexible Graphene Devices with an Embedded Back-Gate

Jasper van Veen, Andres Castellanos-Gomez*, Herre S. J. van der Zant, Gary A. Steele*

Kavli Institute of Nanoscience, Delft University of Technology, Delft, The Netherlands

Email: *[email protected] , *[email protected]

Received November 8, 2012; revised December 11, 2012; accepted January 9, 2013

ABSTRACT

We show the fabrication of flexible graphene devices with an embedded back-gate. The resistance of these devices can

be tuned by changing the strain through the bending of the substrate. These devices can be useful for applications re-

quiring a flexible graphene-based field effect transistor in where the graphene channel is not covered (such as biological

or chemical sensors and photo-detectors).

Keywords: Graphene Device; Flexible Electronics; Back-Gate; Strain Engineering

1. Introduction

Graphene is a two-dimensional material made of carbon

atoms arranged in a honeycomb lattice [1]. Since its iso-

lation by mechanical exfoliation, a great number of ex-

perimental and theoretical works have been carried out to

deeply study this novel material. Due to this effort a lot

of interesting properties and applications have been dis-

covered [2], such as the experimental observation of the

quantum Hall effect even at room temperature [3] and the

possibility to use graphene as a sensor of individual

molecules [4]. Additionally, due to its ambipolar field

effect graphene can be used as the channel material for a

Field Effect Transistor (FET) [5]. Graphene FETs could

have small channel lengths and high mobilities of the

charge carriers. These properties make graphene FETs

advantageous for high-speed applications [6-8].

Together with its electronic properties, graphene also

show exceptional mechanical properties that make this

material very promising for flexible electronic applica-

tions [9-12]. Flexible graphene-based devices will allow

one to modify the electronic properties of the device by

simply bending it [13-15].

A similar strategy, referred as strain engineering has

been successfully employed to modify the electronic pro-

perties of traditional semiconducting materials. In con-

ventional strain engineering, however, the amount of

strain is fixed and it is controlled by epitaxially growing

a certain semiconductor on top of an acceptor substrate

which has a different lattice parameter [16]. It could be

an advantage to be able to have control over the amount

of strain and therewith the electronic properties of a ma-

terial.

However, most of fabricated flexible graphene-devices

lack of a gate electrode which limits its usefulness [17,

18]. In fact, flexible graphene FETs with a local top gate

has been recently reported [19,20] and flexible devices

with a back-gate have not been reported yet. A flexible

back-gated graphene FET could be advantageous in

many applications requiring the graphene surface to be

uncovered such as graphene based chemical sensors or

graphene based photo-detectors.

In this paper we present a method to fabricate gra-

phene flexible FETs with an embedded back-gate. Subse-

quently, basic measurements of the strain dependent

electronic properties of graphene are presented.

2. Sample Fabrication

2.1. Flexible Substrates

The flexible graphene devices are fabricated onto a

commercial 500 µm thick polyimide substrate (Cirlex®).

Prior to the fabrication, the substrates have been polished

with Brasso® polishing solution to reduce the roughness

as much as possible. After that, two fabrication steps are

followed. In the first step a 100 nm thick layer of alumi-

num, which will be used as the back-gate, is evaporated

using electron beam evaporation. A shadow mask can be

used to pattern the back-gate in a strip geometry. In the

second step, a ~5 µm layer of polyimide is spin coated

and cured in vacuum to act as a dielectric layer between

the gate and the graphene. Figure 1 shows a schematic

representation cross-section of the final flexible sub-

strates.

2.2. Graphene Transfer

After the preparation of the flexible substrate with the

embedded gate, graphene is transfer to the substrate sur-

face. This process is schematically depicted in Figure 2

*Corresponding authors.

J. VAN VEEN ET AL.

Figure 1. Cross-section of the flexible substrate. Also, the

positions of the graphene and the source and drain elec-

trodes are shown. Polyimide is used as the bulk material of

the flexible substrate because of its mechanical properties.

The 100 nm thick Aluminium layer acts as the back-gate of

the FET. The upper layer of polyimide acts as dielectric

between the gate and the graphene.

Figure 2. The transfer of the graphene to the flexible sub-

strate. Panel (a) shows graphene op top of a 30 μm thick

layer of copper. Panel (b) depicts the ensemble after the

PMMA is spin-coated on top of the graphene and the layer

of PDMS is thereafter placed on top of the PMMA. In panel

ric-chloride in milli-Q water with a concentration of 1.0 g/L.

Panel (d) shows the process where the ensemble is stamped

on the flexible substrate. In panel (e) the PDMS is slowly

removed from the PMMA. Finally, panel (f) shows the re-

sult of the transfer process. The PMMA has been dissolved

in acetone. Thereafter, the sample has been heated to a

temperature of 150˚C to increase the sticking of the gra-

phene to the flexible substrate.

and it is described in detail in the literature [21]. Briefly,

the transfer process starts with a layer of graphene grown

by Chemical Vapor Deposition (CVD) on top of a copper

foil, see Figure 2(a). A polymethyl metracrylate (PMMA)

layer of 200 nm - 300 nm is spin coated on top of the

graphene and baked at 140˚C for 4 minutes. Thereafter, a

thick layer of polydimethylsiloxane (PDMS) is placed on

top of the PMMA, see Figure 2(b). This facilitates the

handling and transfer of the graphene layer to the sub-

strate. The next step is to remove the copper by etching it

in a solution of ferric-chloride in milli-Q water with a

concentration of 1.0 g/L, see Figure 2(c). Subsequently,

the ensemble is transferred to the flexible substrate. The

graphene is stamped on the desirable location of the sub-

strate, see Figure 2(d). Then, the PDMS is slowly pulled

of the PMMA, see Figure 2(e). It is important that this

step is executed slowly because otherwise the PMMA/

graphene ensemble would remain attach to the PDMS

layer. After this step, the PMMA is dissolved in acetone,

see Figure 2(f). Finally, the sample is placed on a hot

plate at a temperature of 150˚C to remove the traces of

acetone and to increase the sticking of the graphene to

the substrate.

After the transfer process two electrodes, a source and

a drain, are attached to the graphene using silver paint,

see Figure 1.

3. Device Characterization

Right after the device fabrication, we perform a rough

electrical characterization to check that the back-gate is

continuous, the graphene layer is electrically connected

and there is no leakage from the drain-source electrodes

to the back-gate. The electrodes (source, drain and gates)

are shown in Figures 3(a) and (b). The resistance be-

tween G1 and G2 was in the order of R ~ 0 - 50 Ω, which

indicates that the back-gate electrode is continuous. The

source-drain resistance was in the order of R ~ 200 - 500

kΩ. This is a typical resistance for a large area CVD gra-

phene sample. Finally, the drain-gate and the source-

gate resistances where measured giving an open circuit

(R > 10 GΩ). This indicates that there are no pinholes in

the dielectric layer.

The second characterization is done with Atomic

Force Microscopy (AFM). Figure 3(c) shows an AFM

image of the device. The image was obtained from the

region that is indicated with a red box in Figure 3(a).

The AFM image shows that the transferred CVD gra-

phene used in our devices presents some folds and cracks

and also continuous pieces of graphene. From the meas-

urement of the drain-source resistance we know that

there is a continuous path between drain and source elec-

trodes. However, the presence of folded and the cracked

regions may have an influence on the electronic proper-

ties of the device.

4. Experimental Setup

We have further studied the piezoresponse of the flexible

graphene-devices, that is the change in the resistance

with an externally applied strain. The strain is applied by

bending the device. With a simple mechanical model the

applied strain can be related to the radius of curvature of

the device. The mechanical model assumes that the ra-

dius of curvature due to bending the device is larger than

the thickness of the device. Also, the dominant deforma-

tion of the beam should be in the longitudinal direction.

This means that shear stresses and stresses normal to the

neutral axis are negligible. Both these conditions were

met in the performed expeiments. Based on these as r

J. VAN VEEN ET AL.

Figure 3. Top view and AFM image of the graphene-based FET. In panel (a) a schematic top view of the graphene FET is

shown. The source S, the drain D and the two electrodes of the continuous back-gate are indicated. The red dashed rectangle

indicates the region where the AFM image is obtained. Panel (b) shows a photograph of the device before the electrodes were

attached. The positions where the electrodes must be placed are shown with red dashed rectangles. Not surprisingly, the

atomically thick graphene is not visible in the photograph. Its position is indicated with a blue rectangle. Panel (c) shows a

AFM image of the device obtained from the region indicated with a red box in panel (a).

where “h” is the substrate thickness and “



” is the cur-

vature radius.

Figure 4 shows photographs of the device taken dur-

ing a measurement at different strain levels. By fitting

the photographs one can extract the radii of curvature and

thus to estimate the applied strain.

5. Experimental Results

Figure 5(a) shows the resistance of graphene plotted

against the applied strain at different gate voltages. From

the figure it appears that for small strains there is a linear

dependence between the resistance and strain. Also, from

measurement at zero gate voltage this behavior was ob-

served to be reversible over different strain apply/release/

apply cycles. More precisely, the slope dR/dε was roughly

constant for the different cycles. For larger strains we

have found that the silver paint electrodes tend to de-

laminate. Therefore, we also fabricated devices with the

source and drain electrodes evaporated using a shadow

mask. Furthermore, the resistance of graphene is an in-

creasing function of strain for all measured values of the

gate voltage. Also, the slope dR/dε is an increasing func-

tion of the gate voltage. This can be seen more clearly in

Figure 5(b) where this slope is plotted against the gate

voltage. This means that the response of the resistance of

graphene to applied strain can be tuned with an exter-

nally applied electric field.

Figure 4. Photographs of the device in the strain cell taken

at different strain levels. In panel (a) no strain is applied to

the sample. The base length L used as reference length is

indicated. In panel (b) about half of the maximum strain is

applied to the sample, here ε = 1.2%. In panel (b) maximum

strain is applied to the sample, in this case ε = 2.2%. In both

panel (b) and (c) the fitted circles used to obtain the radii of

curvature are indicated.

6. Conclusion

sumptions the following relation between the strain and

the radius of curvature can be derived: In this paper we have shown a method to fabricate gra-

phene-based devices with an embedded back-gate. In

these back-gated devices the graphene surface stays un-

covered which can be advantageous in many applications





 (1)

J. VAN VEEN ET AL.

Figure 5. Panel (a) shows the resistance of graphene against strain for different gate voltages. The dots are the measurement

points. The lines are linear fits included to guide the eye. The measurements were performed at room temperature. In panel

(b) the slope dR/dε is shown as function of the gate voltage. The points are the slopes from the linear fits shown in panel (a).

The line is a quadratic fit included to guide the eye. Note that the slope increases as function of the gate voltage. This indicates

that the piezoresponse can be tuned with an externally applied electric field.

such as chemical sensors and photo-detectors. We also

demonstrate the capabilities of these devices to measure

the piezoresponse of graphene. These experiments show

that the piezoresponse of graphene can be tuned with an

externally applied external field. This property of gra-

phene could be used to make gate-tunable strain sensors.

7. Acknowledgements

This work was supported by the European Union (FP7)

through the program RODIN. The authors would like to

thank E. Huisman for useful discussions.

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