University of Groningen
Organic Nonvolatile Memory Devices Based on Ferroelectricity
Naber, Ronald C. G.; Asadi, Kamal; Blom, Paul W. M.; de Leeuw, Dago M.; de Boer, Bert
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Advanced materials
DOI:
10.1002/adma.200900759
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Citation for published version (APA):
Naber, R. C. G., Asadi, K., Blom, P. W. M., de Leeuw, D. M., & de Boer, B. (2010). Organic Nonvolatile
Memory Devices Based on Ferroelectricity.
Advanced materials
,
22
(9), 933-945.
https://doi.org/10.1002/adma.200900759
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Organic Nonvolatile Memory Devices Based on
Ferroelectricity
By Ronald C. G. Naber,* Kamal Asadi, Paul W. M. Blom, Dago M. de Leeuw,
and Bert de Boer
1. Introduction
The ability to store information is essential to many of the
envisioned applications of organic electronics. RFID tags, e.g.,
need to be able to send and receive stored information that is
communicated by means of a radio signal.
[1]
To store information,
memory devices make use of a physical property that displays
hysteresis in response to an applied electric field. The stored
information is retrieved by measuring the actual state of the
hysteretic property. Many different physical phenomena can be
used to obtain a memory effect. Today’s computer memories, for
example, are based on the hysteresis of charging and discharging
capacitors. A downside of this technology is that the stored charge
tends to leak away and needs to be restored at regular time
intervals. Volatile memories such as this are unsuitable for many
applications. RFID tags, for example, do not have a constantly
available power source as they derive power from the radio signal
that they receive,
[2]
which means that they cannot always perform
a memory refresh operation in case it would be necessary to
prevent loss of information. An organic
memory technology that can potentially be
used for RFID tags is the write-once
read-many (WORM) type of memory.
[3,4]
The operation mechanism of these devices
is similar to that of a safety fuse. Informa-
tion can be stored using the resistive
switching above a certain voltage level and
the actual state of the fuse can be read at a
lower voltage. However, once the resistive
switching has occurred there is no way to
return the device to its former state so this technology is not
suitable for applications that need to be able to adjust the stored
information, e.g., applications that require a book-keeping
capability. To obtain a more universally applicable memory
technology one therefore needs to combine nonvolatility and
rewritability.
Many efforts are ongoing toward a nonvolatile and rewritable
memory device that can enable a viable memory technology for a
wide range of applications. Some of these efforts are focused on
metal–organic semiconductor–metal junctions,
[5,6]
charge trap-
ping effects in field-effect transistors,
[7–9]
and electromechanical
switches,
[10]
to name but a few.
[11]
In this Review Article, we aim
to capture the developments for one particular approach, i.e.,
those based on ferroelectricity. In this section we introduce
ferroelectricity, explain why this physical mechanism has a great
potential for enabling an organic memory technology and we take
a look at the available organic ferroelectric materials. In Sections 2
to 4 we focus on three of the most important memory device
concepts based on ferroelectricity, namely ferroelectric capacitors,
field-effect transistors, and diodes. Section 5 looks at the
construction of multi-bit memories by the integration of discrete
memory elements. Finally, some challenges for future organic
ferroelectric memories are highlighted in Section 6.
1.1. Ferroelectricity
Ferroelectricity was discovered in 1921 in Rochelle salt
(KNa(C
4
H
4
O
6
) 4H
2
O).
[12]
It has been directly recognized that
the polarization response of ferroelectrics can potentially be used
for memory applications. However, applications for ferroelectric
materials have been found in many other areas as well, as
discussed recently by Scott.
[13]
The term ferroelectricity itself was
coined as such because of the analogy between the electrical
properties of ferroelectrics and the magnetic properties of
ferromagnets. The dielectric displacement D and polarization P
REVIEW
www.MaterialsViews.com
www.advmat.de
[*] Dr. R. C. G. Naber
ECN Solar Energy
P.O. Box 1, 1755 ZG Petten (The Netherlands)
E-mail: naber@ecn.nl
K. Asadi, Prof. P. W. M. Blom, Prof. D. M. de Leeuw, Prof. B. de Boer
Zernike Institute for Advanced Materials
University of Groningen
Nijenborgh 4, 9747 AG Groningen (The Netherlands)
Prof. P. W. M. Blom
Holst Centre
High Tech Campus 31, 5605 KN Eindhoven (The Netherlands)
Prof. D. M. de Leeuw
Philips Research Laboratories
High Tech Campus 4, 5656 AE Eindhoven (The Netherlands)
DOI: 10.1002/adma.200900759
A memory functionality is a prerequisite for many applications of electronic
devices. Organic nonvolatile memory devices based on ferroelectricity are a
promising approach toward the development of a low-cost mem ory tech-
nology. In this Review Article we discuss the latest developments in this area
with a focus on three of the most important device concepts: ferroelectric
capacitors, field-effect transistors, and diodes. Integration of these devices
into larger memory arrays is also discussed.
Adv. Mater. 2010, 22, 933–945 ß 2010 WILEY-VCH Verla g GmbH & Co. KGaA, Weinheim 933
REVIEW
www.advmat.de
www.MaterialsViews.com
of ferroelectrics vary with the electric field E in the same general
manner as the magnetic field B and magnetization M of
ferromagnets vary with the magnetizing field H. Ferroelectrics
show an electric hysteresis with loops distorted by an amount
corresponding to a remanent polarization P
r
.
For true ferroelectricity the remanent polarization has to
originate from the alignment of intrinsic dipole moments inside a
crystalline material.
[14]
Mobile charges inside some permeable
medium can achieve an electric hysteresis as well, but these
charges are not in thermal equilibrium and will return to an
equilibrium ground state with time when the applied field is
removed. The correct identification of ferroelectricity requires
close attention because of this.
[15]
Only crystalline materials can
give rise to ferroelectricity because intrinsic dipole moments in
amorphous materials will have a random orientation, yielding a
zero polarization overall. The crystal structure of ferroelectrics
also needs to be such that the dipole moments do not cancel each
other out completely. Consequently, the crystal symmetry should
not be too high. Many ferroelectrics have a phase transition at
elevated temperatures where the higher crystal symmetry negates
the ferroelectricity.
Another rudimentary requirement for ferroelectricity is that
polarization switching occurs below the dielectric breakdown
field of the material. Ferroelectrics have a coercive field (E
c
) which
is often defined as the minimum field that is required to switch
the full remanent polarization. Ferroelectric switching is an
activated process that depends both on temperature and the
applied field strength.
[16]
The switching mechanism generally
involves the nucleation and growth of ferroelectric domains in
which all the dipole moments have the same orientation.
Ferroelectric polarization gives rise to a depolarization field,
just like dielectric polarization.
[17]
In the absence of an externally
applied field, the depolarization field can negate any ferroelectric
polarization. To stabilize the polarization one therefore needs to
supply compensation charges at the surfaces of the material. For
ferroelectric thin films this can be done by placing a conductive
material on both surfaces and connecting the two (i.e.,
short-circuit conditions). Any additional polarization can then
be compensated for by charges that flow from one surface to the
other.
The ferroelectric properties of a material are evident from
charge displacement versus applied field measurements on
thin-film capacitors, as illustrated in Figure 1. At low fields the
applied field does not affect the ferroelectric polarization and the
measurement shows only a linear dielectric displacement. At field
strengths close to the coercive field the ferroelectric starts to
polarize and at high fields it saturates because the maximum
amount of polarization inside the material has been reached.
Under saturated conditions the intersections with the x-axis and
y-axis are usually identified as the coercive field and the remanent
polarization of the material, respectively. Memory devices make
use of the hysteresis by associating the polarization states þP
r
and
P
r
with a Boolean 1 and 0 that forms the basis for most logic
circuits in use today.
If the charge compensation is arranged in an ideal way then the
polarization states in a ferroelectric thin-film capacitor can in
principle be sustained for an infinite amount of time because the
electric fields in and outside the material are zero.
[17]
This aspect
of ferroelectricity is one of the main features that make it
Ronald Naber received an
MSc degree in Chemistry
(2001) and a PhD degree in
Physics (2006) from the Uni-
versity of Groningen. His PhD
thesis under the supervision
of Prof. Paul Blom and
co-supervision of Prof. Dago
de Leeuw was entitled
‘‘Ferroelectricity-
functionalized organic
field- effect transistors’’. He
was a post-doctoral research associate at the University of
Cambridge (2006–2008) under the supervision of Prof.
Henning Sirringhaus and he is now a research scientist in the
field of silicon photovoltaics at the energy research centre
ECN.
Paul W. M. Blom is since 2009
CTO at the Holst Centre,
Eindhoven, The Netherlands
and part-time professor at the
University of Groningen where
he is head of the group Physics
of Organic Semiconductors.
He received his Ph. D. Degree
in 1992 from the Technical
University Eindhoven. In 1992
he joined Philips Research
Laboratories, where he was
engaged in thin-film devices,
polymer light-emitting diodes, and rewritable optical
storageresearch. In May 2000 he was appointed as a
Professor of Physics at the University of Groningen, where
he is working in the field of electrical and optical properties
of organic semiconducting devices.
Dago de Leeuw is a research
fellow at Philips Research
Laboratories, Eindhoven, The
Netherlands, and professor in
molecular electronics at the
University of Groningen, The
Netherlands. His PhD degree
was obtained at the Free Uni-
versity of Amsterdam in 1979.
He has worked on materials
science and technology of
phosphors, high-T
c
superconductors, laser ablation, ferroelectrics, and polymer
electronics. His current research interests are molecular
electronics, nonvolatile data storage and biosensors.
934 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2010, 22, 933–945
REVIEW
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www.advmat.de
appealing for memory applications. In practice there can be
interfacial defect layers between the ferroelectric and the
compensation charges that can destabilize the polarization. This
depolarization effect becomes stronger as the ferroelectric film
thickness is reduced. The ferroelectricity can also be adversely
affected by other things such as conductivity inside the
ferroelectric or trapped charges at the interfacial defect layers.
However, modern inorganic ferroelectric materials perform well
enough for memory applications, as evidenced by their
commercial availability.
[14]
The retention time required by the
industry is about 10 years.
[19]
1.2. Organic Ferroelectric Materials
Most of the activity surrounding organic ferroelectric materials
is focused on PVDF [polyvinylidene fluoride; (CH
2
CF
2
)
n
] and
its copolymer P(VDF-TrFE) [poly(vinylidene fluoride-
trifluoroethylene); (CH
2
CF
2
)
n
-(CHFCF
2
)
m
]. The molecular struc-
ture of P(VDF-TrFE) is illustrated in Figure 1. A number of other
organic ferroelectric materials are known to exist,
[20,21]
but PVDF
has several advantageous properties including a relatively large
remanent polarization, a short switching time and a good thermal
stability. By contrast, ferroelectric nylons for example show a
switching time that is longer than that of PVDF by several orders
of magnitude at the same applied field.
[20]
The ferroelectricity of
PVDF stems from the dipole moments in the molecule that can
be aligned with the applied field by a rotation of the polymer
chain, as illustrated in Figure 2. The dipole moments originate
predominantly from the presence of the strongly electronegative
fluorine atoms. For a detailed discussion about the ferroelectric
properties of PVDF and their origins the reader is referred to a
number of excellent review articles.
[22,23]
PVDF thin films processed from the melt or from a solution
are not ferroelectric because the crystal structure and stereo-
chemical conformation are such that the dipole moments cancel
each other.
[23]
To make these films ferroelectric one needs to
perform additional steps such as stretching to force the polymer
into another conformation. These additional steps can be avoided
by using P(VDF-TrFE) instead of PVDF. Solution-processed films
of P(VDF-TrFE) are ferroelectric straight away due to the steric
hindrance from the additional fluorine atoms in polytrifluor-
oethylene (PTrFE) that induce an all trans stereochemical
conformation that aligns the direction of the dipole moments.
P(VDF-TrFE) thin films do require an annealing step at 140 8Cin
order to raise the crystallinity of this semicrystalline polymer and
concomitantly enhance its ferroelectric response.
[24]
This anneal-
ing step does not eliminate any applications for PVDF because
this temperature is compatible with a wide range of substrates
and organic electronic materials.
The durability of P(VDF-TrFE) and its general usability for
electronic device applications is evident from its widespread use
for piezoelectric sensors and actuators.
[25]
PVDF is produced on
an industrial scale and sold with brand names such as Solef and
Kynar. One of the main applications is as a protection coating due
to its abrasion resistance, stiffness, nonflammability, radiation
tolerance, and resistance to harsh chemicals.
[20]
PVDF and
P(VDF-TrFE) require no special precautions for handling and
storage.
The potential for creating a nonvolatile memory technology
based on P(VDF-TrFE) was already recognized over two decades
ago but this has not yet led to any commercial memory
products.
[26]
In contrast, memory technologies that use inorganic
ferroelectric materials have been available commercially for over a
decade.
[14]
However, the situation could well change in just a few
years because several business media reports and patent
applications indicate that a memory technology based on
ferroelectric polymer capacitors is at an advanced stage of
development.
[27,28]
This affirms that there is a large potential
for organic nonvolatile memory technologies based on ferroelec-
tricity.
2. Ferroelectric Capacitors
One of the simplest types of ferroelectric memory devices is the
thin-film capacitor. Information is stored by aligning the direction
of the internal polarization either up or down with an applied
field. To retrieve the information one applies a switching voltage
to obtain a high or a low charge displacement current response
depending on whether the internal polarization was aligned or
not with the direction of the applied field. Ferroelectric capacitors
Figure 1. Charge displacement D vs applied field E measurement of a
ferroelectric thin-film capacitor. The material used was poly(vinylidene
fluoride-trifluoroethylene) with a layer thickness of 1.7 mm. The upper
inset shows the chemical structure of this material. The lower inset
presents the Sawyer–Tower circuit used for the measurement. Adapted
from [18].
Figure 2. Artistic illustration of the dipole switching event in PVDF. On the
left, the larger fluorine atoms are on top and the dipole moment points
upwards. Toward the right, the molecule rotates around its axis, bringing
the dipole moment along with it.
Adv. Mater. 2010, 22, 933–945 ß 2010 WILEY-VCH Verla g GmbH & Co. KGaA, Weinheim 935
REVIEW
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www.MaterialsViews.com
therefore have a so-called destructive read-out since a read
operation can affect the stored information. If the polarization
direction was changed during the read operation then a reset
voltage needs to be applied afterwards.
Here, we will start with a brief look at the history of attempts to
attain low-voltage switching in P(VDF-TrFE). Due to the relatively
high coercive field of 50 MV m
1
of P(VDF-TrFE) sub-100 nm
thick layers are required in order to arrive at a switching voltage
below 10 V. Two seminal papers from 1986 and 1995 established
the fact that spin-coated films with a sub-100 nm thickness have
an extremely retarded ferroelectric response as compared to the
bulk material,
[29,30]
which implied that P(VDF-TrFE) is unsuitable
for low-voltage memory applications. In 1998 a report about thin
films made with a Langmuir–Blodgett deposition technique
appeared that showed that a ferroelectric response can be retained
down to layer thicknesses of 15 nm.
[31]
Unfortunately, these films
are probably even less suitable for applications than the
aforementioned spin-coated films because the switching times
are longer by orders of magnitude.
[32,33]
Two reports in 2001 and
2002 claimed some improvement for spin-coated films by
optimizing the annealing conditions,
[34,35]
but the improvements
were relatively minor and so the prospects for memory
applications of P(VDF-TrFE) remained bleak.
The situation came to a turnaround due to a serendipitous
discovery that was published in 2004.
[36]
Figure 3 shows charge
displacement measurements that established that the ferro-
electric response remains almost the same when the layer
thickness is reduced from 210 to 65 nm. These measurements
demonstrated that it is possible to have sub-10 V switching while
retaining the remanent polarization and switching times of
the bulk material. The low-voltage switching behavior was
obtained by using a bottom electrode stack that includes an
interfacial layer of the conductive polymer PEDOT:PSS [poly(3,4-
ethylenedioxythiophene):poly(styrene sulfonic acid)]. The top
electrode consisted of vapor-deposited gold. At the time when we
were attaining the results above with PEDOT:PSS others filed a
patent application on a similar idea.
[37]
The existence of this
application suggests also that conductive polymers can enable
memory applications based on the use of P(VDF-TrFE).
Xu et al. later published a report about capacitors with the
conductive polymer Ppy:PSS [polypyrrole-poly(styrene sulfonic
acid)] as a bottom and top interface.
[38]
Ferroelectric capacitors
with a layer thickness of 50 nm were shown to have the same
ferroelectric properties as the bulk material. The switching cycle
endurance properties of the capacitors were also investigated.
When a ferroelectric undergoes a large number of switching
cycles then so-called fatigue can occur, which manifests itself in a
lowering of the remanent polarization, higher coercive fields
and longer switching times. Cycle endurance is very important
because ferroelectric capacitors receive a large number of read
and write cycles during a normal product lifetime, due to the
destructive read-out operation mechanism of ferroelectric
capacitors. As presented in Figure 4, the polarization degradation
is limited to a value of 15% after 10
7
cycles at an elevated
temperature of 60 8C.
The reason why the observed low-voltage switching behavior as
obtained by inserting conductive polymers was not obtained
before lies in the fact that all previous investigations used
aluminum top and bottom electrodes.
[29–32,34,35]
X-ray photoelec-
tron spectroscopy (XPS) measurements have indicated that
aluminum reacts with P(VDF-TrFE) regardless of whether the
aluminum was deposited onto P(VDF-TrFE) or the other way
around.
[39]
The chemical reaction between aluminum and
P(VDF-TrFE) leads to the formation of nonferroelectric or ‘‘dead’’
layers near the electrode interfaces, the presence of which can be
derived from the retarded switching kinetics that are
induced.
[40,41]
One can also avoid the reaction by using the
chemically more inert element gold and, as expected, this yields
an improvement of the ferroelectric response of P(VDF-TrFE)
thin-film capacitors as well.
[42,43]
3. Ferroelectric Field-Effect Transistors
A disadvantage of using ferroelectric capacitors for memory
applications is that the charge displacement response scales with
Figure 3. Displacement charge D versus applied voltage V hysteresis loop
measurements on capacitors with a ferroelectric layer thickness of 210 and
65 nm. The capacitors have a PEDOT:PSS bottom interface, as shown in
the inset. Adapted from [36].
Figure 4. Remanent polarization P
r
versus number of switching cycles for
ferroelectric capacitors with a P(VDF-TrFE) layer thickness of 50 nm,
measured at room temperature (RT), 45 and 60 8C. The inset illustrates
the device structure with a Ppy:PSS top and bottom interface. Reprinted
with permission from [38]. Copyright 2007, American Institute of Physics.
936 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2010, 22, 933–945
![Figure 6. Capacitance C versus gate voltage VG measurement on a ferroelectric MIS diode with a P(VDF-TrFE) gate dielectric film thickness of 220 nm. The capacitance Cwas divided by the insulator capacitance Ci. The inset illustrates the device structure with P3HT as the semiconductor. Adapted from [62].](/figures/figure-6-capacitance-c-versus-gate-voltage-vg-measurement-on-2disi6f0.webp)
![Figure 7. Transfer measurement on an ambipolar polymer FeFET with an insulator layer thickness of 0.9mm. The inset illustrates the device structure withablendofMEH-PPVandPCBMasthesemiconductor.Adapted from[65].](/figures/figure-7-transfer-measurement-on-an-ambipolar-polymer-fefet-16ytf0js.webp)
![Figure 1. Charge displacement D vs applied field E measurement of a ferroelectric thin-film capacitor. The material used was poly(vinylidene fluoride-trifluoroethylene) with a layer thickness of 1.7mm. The upper inset shows the chemical structure of this material. The lower inset presents the Sawyer–Tower circuit used for the measurement. Adapted from [18].](/figures/figure-1-charge-displacement-d-vs-applied-field-e-sy1r4pu9.webp)
![Figure 4. Remanent polarization Pr versus number of switching cycles for ferroelectric capacitors with a P(VDF-TrFE) layer thickness of 50 nm, measured at room temperature (RT), 45 and 60 8C. The inset illustrates the device structure with a Ppy:PSS top and bottom interface. Reprinted with permission from [38]. Copyright 2007, American Institute of Physics.](/figures/figure-4-remanent-polarization-pr-versus-number-of-switching-em9kdspb.webp)