Chapter 3
Fabrication and experimental
setups
This chapter outlines the experimental techniques used in subsequent chapters. Sec-
tion 3.1 describes two different graphene film synthesis approaches which are used
in this dissertation; section 3.2 presents the process of fabricating GFET devices,
which will be the starting point for experiments covered from chapter 3 to chapter 7;
Section 3.3 presents the preparation of ferroelectric thin films, which will be used in
experiments from chapter 3 to chapter 6; Section 3.4 is devoted to charge transport
measurement schemes used throughout the rest of the thesis.
3.1 Graphene fabrication
Since its first discovery, the synthesis and growth of graphene films has been central
to graphene research and its potential applications. Here we introduce two different
approaches to graphene synthesis.
25
26
Single layer
SiO
2
substrate
Multi-layer
Bilayer
Figure 3.1: Optical image of single layer, bilayer and multilayer graphene on Si/SiO
2
substrate. The scale-bar is 10 μm.
3.1.1 Mechanical exfoliated graphene
Graphene is obtained by the mechanical exfoliation method from Kish graphite sources.
Adhesive tape (Scotch tape) is used as a holder of the graphite source and repeti-
tive folding and peeling processes are required in order to split graphite crystals into
increasingly thinner pieces. When these graphite pieces are thin enough (optically
transparent), they are directly transferred onto Si/SiO

2
thickness (reproduced using equals in [79]). (b) Extracted single contrast curve as a
function of SiO
2
thickness, the corresponding wavelength is 550 nm.
The reason why one can see monolayer graphene with the naked eye is due to the
interference effect induced by the SiO
2
layer. Indeed, the thickness of the SiO
2
layer
significantly affects the visibility of monolayer graphene, as shown in Fig. 3.2. As
we can see clearly, 285 nm and 90 nm thickness SiO
2
layer provide the best contrast
28
for the detection of graphene flakes with bare eyes. Using this model, one can also
identify the visibility of graphene on other substrates [79].
3.1.2 Chemical vapor deposition of graphene
Temperature ( C)
O
Time (min)
b
a
c
Figure 3.3: (a) Growth parameters of large-scale graphene using chemical vapor de-
position method; (b) Optical image of thermal furnace and copper foils for graphene
growing. (c) 30-inch large-scale graphene after transferred onto transparent substrates
[44, 50].
Although the mechanical exfoliation method can provide single crystal graphene

is determined with respect to pre-defined alignment marks. A single layer of 495 K
molecular weight Polymethyl Methacrylate (PMMA) is spin-coated at 4000 rpm onto
the samples, then baked at 180
o
C for 2 minutes. A subsequent standard electron
beam lithography (EBL, 30 KeV) process locates the graphene samples and defines
thermally evaporated electrodes (Cr/Au = 5/30 nm) on top, followed by a liftoff
process in acetone. Depending on the needs of specific experiments, the contact pads
are designed to either Hall-bar geometry or four-terminal configuration (Fig. 3.4).
Sometimes, a second EBL step is performed to pattern graphene into a specific shape
followed by oxygen plasma etching. The doped silicon is used as the gate electrode
(back gate) and the SiO
2
as the gate dielectric in transport measurements.
Although the back gate provided by heavily doped silicon yields many interesting
transport phenomena [17, 81], it represents only the first step towards more complex
graphene devices. Thus, fabrication of dual gated structures such as lateral graphene
p-n or p-n-p junction devices is sorely needed for further studies [19, 39, 82]. Figure
3.5 shows the ferroelectric top gated device structure using the metal mask approach.
The top dielectric that we are using is ferroelectric copolymer P(VDF-TrFE).
31
a
b
Figure 3.5: (a) Schematics of a graphene-ferroelectric p-n-p junctions; (b) Optical
image of a graphene-ferroelectric p-n-p junction device fabricated using metal mask
method. Scale bar is 5 μm.
3.2.2 GFET devices made out of chemical vapor deposition
graphene
Unlike the mechanically exfoliated graphene, the chemical vapor deposition method
makes monolayer graphene much more easily accessible. After CVD graphene growth,

thickness of P(VDF-TrFE) thin film. In our experiments, we utilized 5:95 ratio
33
70 nm
b
c
a
Topography
Topography
Phase
y
y
180 nm
0.0 nm
β-phase
d
Topography
Phase
100 nm
0.0 nm
0.0 nm
Figure 3.7: (a) Optical image and AFM scanning of spin-coated P(VDF-TrFE) on
graphene flakes. Scale bar is 15 μm. (b), (c) Comparison of P(VDF-TrFE) mor-
phology and phase diagram at short/long annealing time. Scale bars are 400 nm. (d)
Comparison of XRD results of both sufficient and insufficient annealed P(VDF-TrFE)
film.
34
(500 nm) and 10:90 ratio (1 μm).
• Completely dissolve the P(VDF-TrFE) solution at 50
o
C for 2 hours.

is used to provide a gate voltage to tune the carrier density of graphene. The typical
values of the resistors used are 10 MΩ and a typical AC frequency used is 13.373 Hz.
The measurement configuration technique for the ferroelectric top gated devices
is illustrated in Fig. 3.8b. The poling of ferroelectric thin film is realized by the
application of a DC voltage to the top contact. In this architecture, graphene serves
as the bottom contact for the polarization of ferroelectric thin film. The resistance
change of graphene as a function of the ferroelectric polarization is recorded by the
lock-in amplifier.
The experimental setup for electronic transport measurements in variable tem-
perature insert is shown in Fig. 3.9. Leads from the GFET device are bonded onto
the chip carrier using a wire bonder. The chip carrier is then inserted to the sample
holder mounted on the VTI probe. The VTI insert is designed such that the sample
can be heated up to 400 K for high vacuum thermal annealing, which is critical to
removing resist residues and water contaminants.
36
I
a
Lock-in
Amplifier
Rs
Input A
Rg
Keithley Source
Meter 6430
Computer
Digital
Singal
Input B
b
Figure 3.8: (a) Schematics of a quasi-DC measurement of the electronic properties of