Ferroelectric gating of graphene 9

Chapter 9
Summary, conclusion and outlook
9.1 Summary
Graphene is a very promising material for both fundamental physics studies and
many novel device applications. As an atomic layer thick two-dimensional crystal,
graphene’s various properties are strongly affected by its surrounding media and sub-
strate.
Utilizing ferroelectric dielectrics, we have experimentally demonstrated graphene-
based non-volatile memory devices, flexible transparent conductors and novel type
of transistors. The key difference between ferroelectric and other types of dielectrics
is that they guarantee the devices with low power consumption and high efficiency
behavior. Ferroelectric dielectrics allow allows the devices to function continuously
even after the power is turned off, making it quite desirable for the next generation
zero-power consumption electronics.
For the graphene-ferroelectric non-volatile memory devices proposed in this thesis,
writing and reading speed is determined by the switching time of the ferroelectric
materials, which can be as fast as 100 ps. In addition, this non-volatile memory has
109
110
an aggressive scaling ability. This makes the proposed graphene-ferroelectric non-
volatile memory devices a very promising candidate for flexible electronics and data
storage.
To further understand the charge transport of graphene on ferroelectric dielectrics,
we further introduced a quantitative way to determine and control the hysteresis
ferroelectric gating by using an independent linear dielectric gating for reference.
Moreover, this reference gating can also be used to control ferroelectric gating by
introducing a unidirectional shift in the hysteretic ferroelectric doping in graphene.
Using this idea, we further realized the symmetric bit writing in graphene which
directly uses remnant polarization with high speed and simplicity.
For all of its potential applications, graphene must b e both high quality and
producible on a large-scale. The technical breakthrough in synthesizing graphene

using chemical vapor deposition (CVD) method addressed the large-scale request
and many prototypes of graphene-based applications have been developed. However,
the quality of the CVD graphene is still generally lower than mechanically exfoliated
graphene. This implies that there are other unknown aspects of CVD graphene to
be further explored. Here, we show that the current growth and transfer methods
of CVD graphene lead to quasi-periodic nanoripple arrays in graphene. Such high
density NRAs partially suspend graphene giving rise to flexural phonon scattering.
This not only causes anisotropy in charge transport, but also sets limits on both the
sheet resistance and the charge mobility even in the absence of grain boundaries. At
room temperature, NRAs are likely to play a limiting role also for the mobility of
ultra-clean samples, in particular when the graphene sheets are transferred onto ultra-
flat BN substrates. Optimization and improvement of large-scale graphene synthesis
111
still remains a critical issue.
By introducing a ferroelectric thin film, we have demonstrated a new method to
simultaneously dop e and support large-scale CVD graphene without compromising
graphene’s high transparency. The proposed graphene-ferroelectric hybrid transpar-
ent conductor exhibits low sheet resistance (120 Ω/✷) at room temperature, high
optical transparency (> 95 %) in the visible range spectrum and excellent mechanical
flexibility. In addition, we show that low sheet resistance of 50 Ω/✷ at optical trans-
parency > 95 % in large-scale graphene-ferroelectric hybrid structure is achievable
through the optimization of existing graphene transfer processes.
Furthermore, we reported a new route to exploring graphene physics and function-
alities by transferring large-scale CVD graphene and bilayer graphene to functional
substrates. Using ferroelectric Pb(Zr
0.3
Ti
0.7
)O
3

(PZT), we demonstrated ultra-low-
voltage operation of graphene field effect transistors within ± 1 V with maximum
doping exceeding 10
13
cm
−2
and on-off ratios larger than 10 times. After polarizing
PZT, the switching of graphene field effect transistors is characterized by pronounced
resistance hysteresis, suitable for ultra-fast non-volatile electronics.
9.2 Unsolved questions
9.2.1 Can we reach the VHS regime?
Pushing the Fermi level to the VHS regime is one of the main goals in ferroelectric
inorganic substrate PZT gated GFET devices. Theoretically, large remnant polariza-
tion of ferroelectric inorganic substrate would provide enough electrostatic doping to
112
achieve this goal. However, up till now we have not reach the VHS regime experi-
mentally. The reasons are summarized in the following:
The interface between PZT and graphene is dirty. Currently, we are using CVD
graphene as the working media. This will inevitably involve other sources in graphene,
i.e., water, copper etching solvent residual, copper nanoparticles. These additional
materials will influence the charge transport properties of graphene and decrease
the gate efficiency, leading to a saturated resistivity character in graphene beyond a
certain doping level. Besides this, both CVD graphene and PZT substrate are poly-
crystal. For CVD graphene, this implies that vacancies, defects and, grain boundaries
reside within, influencing the electrical properties of graphene. For polycrystal PZT
substrate, this indicates that the remnant polarization is not as large as its single
crystal counterpart.
To reach the VHS regime, we might suggest the following. Utilize epitaxial or even
single crystal ferroelectric inorganic substrate, such as PZT or BFO. The ultra-high
remnant polarization will provide one of the highest levels of electrostatic doping

in graphene, up to 1.25×10
15
cm
−2
. Obtaining a clear interface is also critical for
the success this experiments. Directly grow a thin ferroelectric layer on graphene is
one possible method. However, a fine-tuned growth approach is required in order to
reduce the induced defect in graphene. One can also utilize the dry transfer method
as already demonstrated in several graphene and Boron Nitride experiments [116].
113
9.2.2 Can we completely remove the quasi-periodic nanorip-
ples?
In chapter 5, we proved that the origin of quasi-periodic nanoripples was the copper
step edges. Thus, in order to remove the quasi-periodic nanoripples, the starting
point should be the optimization of copper foils. In this regard, we would also like
to reiterate that the pre-growth annealing and the actual growth at high tempera-
ture growth (1000-1050
o
C) are both crucial for producing large-scale high quality
CVD graphene. The former removes the surface oxidation layer, while the latter
ensures that Cu catalyzes graphene growth instead of forming intermediate chemi-
cal compounds with incoming gases. However, the step edges will form even if one
leaves the pre-annealing step out. The high temperature process makes high density
single-crystal terraces and step edges a ubiquitous surface morphology in Cu. CVD
graphene growth follows the Cu surface morphology, but as the sample is cooled
the difference in thermal expansion coefficients leads to “excess” graphene and also
strain, which is accumulated at the step edges. The current wet transfer methods,
either polymer resist-based or thermal release tape-based, transfer this surface mor-
phology to the target substrates, leading to the formation of quasi-periodical NRAs
once the resist/polymer/tape is removed.

The large-scale transfer without any support should avoid the NRAs altogether
and be in principle scalable. However, such an approach introduces a very high density
of wrinkles and hence, would actually make the situation worse (not shown). The
Cu foil after graphene growth is etched in an Ammonium Persulfate (APS) solution
without any protective/supporting PMMA layer. After 24 hours, the graphene floats
freely on the surface. Note that the usual rinsing step in DI water had to be minimized
114
to ensure a greater chance of success. We can already observe a much higher density
of micron-size wrinkles of random orientation. These extra features are due to the
absence of a supporting (PMMA) layer, making CVD graphene very sensitive to the
flow and fluctuations of water and its surface tension. In light of the high wrinkle
density and the fact that in such samples residues from the etchant are ubiquitous,
we have not characterized these samples with AFM.
Avoiding the formation of NRAs is the more direct strategy to realize the ideal
properties of graphene. This can be realized by surface engineering of Cu, specifically
in terms of roughness and crystal orientations. A few possible strategies for future
studies are listed below. This is a long term goal for the larger community working
towards graphene-based applications, some of which require low sheet resistances, e.g.
displays and solar cell panels.
i) Low temperature CVD growth method
Currently, the high temperature CVD method provides the best quality CVD
graphene with unparalleled uniformity, grain size, and scalability. Unfortunately,
with this approach Cu step edges are unavoidable even if we skip the pre-annealing
steps because the Cu foils are self-annealed during the growth at T ∼ 1,000
o
C.
By lowering the growth temperature below ∼ 800
o
C, it is possible to reduce/minimize
the formation of Cu step edges, but in this case we introduce new challenges specific

to the low T growth, e.g. just to name a few: 1) insufficient Cu catalytic activity
limits the graphene growth from gaseous source 2) much enhanced D peak signal
present using solid carbon sources growth approach. We, as well as others, observe
more defects in Raman spectra. Therefore, in the long run one may potentially have
to find a compromise between ripple and defect control.
115
ii) Substrate engineering
Due to the direct relation between quasi periodic NRAs with Cu step edges, the
most useful and elegant way to eliminate NRAs in CVD graphene is by using ultra-flat
Cu foil for CVD graphene growth. Preliminary studies of graphene from flat areas
seem to indicate that they also have better mobility (not shown). Another possibility
is that the lower surface energy of Cu(111) surface can be utilized to prepare more
flat Cu surfaces. It is also reported that graphene grown on Cu(111) surfaces show
better quality because the Cu(111) lattice matches well with graphene. We note
that the electroplated Cu surface prefers the Cu(111) surface, which will be further
investigated in a separate study.
iii) Improvement of the way Cu foils are prepared
There can be also larger ripples (generally referred to as wrinkles and folds) formed
by micro-scale bumps on Cu foils prepared by a roll-press process. These also lead
to a partial suspension and hence, are a source of flexural phonon scattering. Thus,
controlling the quality of the rolls used in the roll-press process may at least help
avoid the partial suspension due to “Wrinkle” formation. The formation of these
macroscopic ripples can be suppressed by using electrochemically or mechanically
polished Cu surfaces.
9.2.3 Can we achieve less than 100 Ω/✷ sheet resistance in
CVD graphene?
In chapter 6, we showed that one of the lowest sheet resistance values achieved in
large-scale single layer CVD graphene (120 Ω/✷) without compromising its optical
transparency was found using ferroelectric polymer gating. Although it is already
116

useful for some of the applications in optoelectronics, it is still higher than the indus-
trial requirement of 100 Ω/✷. Consequently, the question of how to achieve less than
100 Ω/✷ sheet resistance in CVD graphene would becomes one of the most important
goals for subsequent work.
In order to do so, there are several aspects one needs to keep in mind. Firstly, the
optimization of CVD graphene growth and transfer technique is important because its
enhanced carrier mobility will dramatically reduce the sheet resistance value. Beyond
this, optimizing the synthesis of ferroelectric p olymer thin film is also critical. This
will not only reduce the non-ferroelectric phase present in our current specimens, but
also possibly enhance the electrostatic doping level in graphene. By doing so, it is
very likely that less than 100 Ω/✷ sheet resistance in CVD graphene will be achieved
in the future.
9.3 Future outlook
In this section, I will only focus on potential research topics involving graphene and
ferroelectric material.
9.3.1 Gate-tunable graphene-ferroelectric photonics
Over the past seven years, the main focus of graphene research has been its electronics
properites. However, graphene is also reported to possess intriguing optical properties.
Many proof-of-concept ideas and devices have emerged, ranging from graphene solar
cells, organic light emitting diodes and saturable absorbers in ultrafast laser systems.
However, research in this direction is still in an early stage.
117
Currently, graphene has been used as a saturable absorber in ultrafast laser sys-
tems. This is because graphene has obvious non-linear optical properties, which is
essential for its applications in photonics. Under strong light illumination, the optical
transparency of graphene tends to saturate due to the universal optical absorption
and zero band gap. However, all of the present studies are entirely focused on pristine
graphene with a fixed charge carrier density. A gate tunable graphene-based saturable
absorber would be more intriguing, not only for its applications, but also to explore its
underlying physics in terms of light-matter interactions. From the applications point

of view, the highly doped graphene would consume a much less power and provide
highly efficient saturable absorber devices.
9.3.2 Piezoelectric effect induced electrical nanogenerator
In our studies, we are mainly utilizing the ferroelectric prop erties of P(VDF-TrFE).
Meanwhile, P(VDF-TrFE) also has a pronounced piezoelectric effect [161]. This will
add one more degree of freedom in tuning the charge transport of graphene and holds
promise in fabricating new type of functional devices.
Currently, the prototype of electrical nanogenerators is utilizing metals such as
Au or Pt as electrodes. Although Au is an excellent conductor, it is not suitable
for repeated bending and stretching. On the other hand, graphene is an excellent
transparent conductor, and would greatly enhance its mechanical stability and dura-
bility. Consequently, the combination of graphene with P(VDF-TrFE) in new types
of electromechanical devices would be very promising.
118
9.3.3 Ultrahigh doping of graphene using single crystal fer-
roelectric thin film
Tuning the Fermi level approaching the VHS regime is expected to induce some pecu-
liar phenomena, such as superconductivity, ferromagnetism state or the observation
of charge density waves. In order to do so, a proper dielectric which can provide high
gate strength is critical.
Ferroelectric oxides have a high remnant polarization value, which essentially can
provide the desired charge carrier density in graphene through electrostatic doping.
For example, the remnant polarization of single crystal Bismuth Ferrite (BFO) thin
film is more than 100 µC/cm
2
, which approximately equals to more than 6×10
14
/cm
2
doping in graphene. One can also think about dual gated ferroelectric dielectric

to double the doping amount. Within this, it is expected that more than 10
15
/cm
2
doping in graphene can be achieved, which is already in the VHS regime.
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