JOURNAL OF APPLIED PHYSICS 105, 124516 共2009兲
Direct observation of localized conduction pathways in photocross-linkable
polymer memory
Wei Lek Kwan,1 Bao Lei,1 Yue Shao,1 Sergey V. Prikhodko,1 Noah Bodzin,2 and
Yang Yang1,a兲
1
Department of Materials Science and Engineering, The Henry Samueli School of Engineering
and Applied Science, University of California, Los Angeles, California 90095, USA
2
Nanoelectronics Research Facility, University of California, Los Angeles, California 90095, USA
共Received 14 April 2009; accepted 21 May 2009; published online 24 June 2009兲
Resistive switching in photocross-linkable polymer memory devices was found to occur in localized
areas of the device. In order to elucidate the reason behind the switching, we used focused ion-beam
to prepare a cross-section of the device. It was found that after the device was switched to the high
conductive state, in certain parts of the device, the electrodes were only about 5 nm apart. This was
probably caused by a combination of high electric field and metal injection into the polymer film.
Gold injection into the polymer film by locally enhanced electric field was confirmed by
transmission electron microscope-energy dispersive x-ray analysis. This model was in agreement
with both the temperature dependent and transient behavior of our device. We conclude that the
non-uniformities at the nanoscale interface of the electrode dominated the device characteristics
while the polymer played only a secondary role. © 2009 American Institute of Physics.
关DOI: 10.1063/1.3153980兴
I. INTRODUCTION
Polymer memory devices are identified as one of the
potential candidates for next generation memory storage.1,2
Ease of processing, flexibility,3,4 and stackability5–7 make it
ideal for both complementary metal-oxide semiconductor integration and low cost applications. However, despite many
years of development, the working mechanism is still highly
debatable.8,9 Various mechanisms, e.g., field induced charge
transfer,10–13 charge trapping,14–18 metal oxide switching,19,20
filament formation,21–24 and polymer conformation change25
have been proposed, but a comprehensive explanation with
direct experimental proof has been elusive.
Cölle et al.19 and others26–29 have shown that the switching phenomenon occurred in localized areas of the polymer
memory device. We have also observed similar effects in our
stackable memory devices using a photocross-linkable
polymer.7 Aluminum oxide switches,19 formation and rupture
of carbon,23 metallic,24 or other filamentary channels22,30
have been suggested to explain the filamentary nature of
switching for these polymeric devices. However, the precise
nature of these filaments and the formation mechanism are
still not clear. A similar mechanism was also proposed in
resistive memory using inorganic film.21 To clarify the working mechanism, efforts have been made to directly observe
the conducting filaments in these devices using electron
microscopy.31,32 Due to its accessibility, filaments are more
readily observed in a planar structure, although it does not
necessarily represent the actual conditions in a vertical stack
configuration. Filaments consisting of carbonaceous
materials,33 silver dendrites,34 and other materials32,35 were
observed in planar structures of devices over a wide range of
a兲
Electronic mail: yangy@ucla.edu.
0021-8979/2009/105共12兲/124516/5/$25.00
materials. Filament formations have also been detected in
vertical structures of chalcogenide36 and NiO 共Ref. 37兲 devices.
Even though filament formation have been observed in
various systems, the nature of the conductive pathways appeared to be very different in each system. Therefore, to fully
understand the memory mechanism, it is important to find
out how these filamentary pathways are formed in our polymer devices. Based on the results from thermal images of
polymer devices in operation19,26 and the model of Dearnaley
et al.,21 the conduction spots could range from 1 nm to
100 ␮m. Due to the small sizes and low density of these
spots, it has been very difficult to observe these localized
areas. In this paper, we describe a general method that can be
used to study these highly conductive pathways.
We found that for our lithographically patterned devices
using a photocross-linkable polymer, damages to the device
appeared near the bottom electrode edge after many cycles of
switching between high and low conductivity states.7 As the
current density flowing through localized conductive spots
can be very high, physical changes to the top electrode could
be observed after switching. In order to exclude the effects of
dust particles38 and other irregularities, the device area has to
be made small enough so that the whole device can be examined at high magnification using a scanning electron microscope 共SEM兲. Once the conductive spot is identified by
the changes to the top electrode, a cross-section of the spot
can be prepared by a focused-ion beam 共FIB兲. Transmission
electron microscope 共TEM兲 can then be used to examine the
local features which caused the switching.
II. EXPERIMENTAL
A. Device fabrication
4 ⫻ 4 ␮m2 devices were fabricated using a photocrosslinkable copolymer used previously.6,7 The chemical struc105, 124516-1
© 2009 American Institute of Physics
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124516-2
J. Appl. Phys. 105, 124516 共2009兲
Kwan et al.
O
O
(a)
O(
CH
2) O
6
N
O(
N
) 6O
2
CH
C8H17
0.25
C8H17
0.5
0.25
(b)
n
I-V sweep after forming
-3
-7
10
-5
10
Current [A]
Current [A]
10
-11
10
-10
10
B. Electrical characterization
-15
10
0
10
20
30
Voltage [V]
-15
10
0
2
4
6
8
10
Voltage [V]
FIG. 1. 共a兲 Chemical structure of photocross-linkable polymer, 共b兲 I-V curve
of the device during 共inset兲 and after the forming process.
ture of the polymer and device structure are shown in Figs.
1共a兲 and 2共a兲, respectively. The copolymer was synthesized
by Suzuki polymerization of 9,9-bis共4-共6-共共3-ethyloxetan-3yl兲methoxy兲hexyloxy兲phenyl兲-2,7-dibromofluorene, 9,9-bis
共4⬘-diphenylaminophenyl兲-2,7-dibromofluorene and 9,9dioctylfluorene-2,7-bis共trimethylborate兲. The copolymer was
then dissolved in 1,2 dichlorobenzene to form a 3 wt % solution. A photoacid generator 共PAG兲, PAG203 共Ciba Specialty Chemicals兲 was added to the polymer solution in the
weight ratio of 1:0.13 with respect to the copolymer.
A silicon wafer with 300 nm of thermally grown silicon
dioxide was used as the substrate. The bottom electrode, consisting of 3 nm of chromium 共as an adhesion layer兲 and 30
nm of gold, was patterned using standard lift-off techniques.
(a)
polymer
(b)
Al
Al
Au
(c)
Au
Pt
500nm
(d)
Al
Au
The polymer solution was then spin-coated onto the patterned electrodes and cured using a handheld UV lamp
共wavelength 254 nm兲. The substrate was then annealed at
90 ° C for 2 min and then 150 ° C on a hotplate. The average
polymer film thickness of the device, measured using a Dektak 3030, was about 50 nm at the uniform region. After the
curing process, the polymer became insoluble in organic solvent and was robust enough to withstand photolithography
processes. Finally, the top electrode of aluminum 共thickness
80 nm兲 was deposited and patterned using the lift-off
method. All metal deposition was done by thermal evaporation in a vacuum chamber with base pressure of ⬍10−5 Torr.
50nm
FIG. 2. 共a兲 Device schematic, 共b兲 top SEM view of device showing irregularities on the electrode, and 共c兲 TEM cross-section view of device of boxed
region in 共b兲. Near the edge of the gold electrode, the top electrode was
almost in contact with the bottom electrode. 共d兲 Closed up TEM image of
the boxed region in 共c兲.
Electrical dc measurements were carried out in a Janis
cryogenic vacuum probe station with a vacuum level of
⬍10−3 Torr using an Agilent 4155C parameter analyzer. The
aluminum electrode was biased as the reference electrode.
Temperature dependence studies were conducted from 235 to
295 K. A larger temperature range was not possible due to
the large difference in thermal expansion coefficient between
the metallic and polymer films, causing the delamination of
electrodes at lower temperatures. Transient current measurement was performed by applying a 500 ␮s pulse using the
“stress force” function of the Agilent 4155C. The slow rising
and falling time 共about 100 ␮s兲 of the pulse was chosen to
suppress the high transient charging and the discharging currents due to the capacitance of the device. The current of the
device was measured using a high speed transimpedance amplifier, Femto DHPCA-100, connected to a Tektronix
TDS3054 digital oscilloscope.
C. Preparation of device cross-section and TEM
imaging
As mentioned earlier, high current density flowing
through localized areas of the device could cause damage to
the top electrode. However, in order to observe the nature of
the filamentary conduction, it is important to limit the damage to the top electrode. For that reason, the polymer
memory device under test was switched between high and
low conductivity states for only five cycles by applying 4
and 10 V pulses alternatively. Finally, the device was
switched to the high conductivity state 共by applying a 4 V
pulse兲, and transferred to a FEI Nova 600 Nanolab Dualbeam™ SEM/FIB for examination and cross-section preparation. Figure 2共b兲 shows the top view of the device where
there was an irregular feature on the electrode. To protect the
underlying layers from ion-beam damage, a carbon-rich
platinum film was first deposited by decomposing a metalorganic precursor gas using electron beam. Ion-beam was
then used to deposit a thicker layer of platinum. The crosssection of the area was cut out by FIB and transferred to a
copper grid. The TEM study was performed using a JEOL
100CX and a FEI Titan at accelerating voltages 100 and 300
kV, respectively.
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J. Appl. Phys. 105, 124516 共2009兲
Kwan et al.
The device was first “formed” by increasing the voltage
until the current reached the compliance current of 10 ␮A.
After the device was formed, when the voltage bias was
swept from 0 to 10 V, the device switched from a low conductivity state 共off state兲 to high conductivity state 共on兲 at
around 3.5 V and exhibited a negative differential resistance
共NDR兲 behavior at higher voltages 关Fig. 1共b兲兴. It remained in
the high conductivity state when the voltage was swept back
to 0 V. The device could be switched back to the low conductivity state by applying a 10 V pulse to it. Similarly, it
could be switched to the high conductivity state by a 4 V
pulse. It is important to note here that devices with switching
characteristics can also be fabricated without PAG using
shadow masked electrodes. This proved that the memory
phenomenon was not due to the addition of the PAG.
To find the cause of the high conductivity state, we examined the area where high density of current flowed
through. Figures 2共c兲 and 2共d兲 show the TEM images of the
cross-section of the area with irregularities. Close to the edge
of the gold electrode, the bottom and top electrodes were
almost in contact with each other. The closest distance between the two electrodes was about 5 nm, while the average
distance in the uniform area was about 50 nm. Due to the
close proximity and the sharp edges of the two electrodes,
very high electric field could be generated between the electrode tips. The electric field at the tip was estimated to be
about ten times more than that at the uniform regions even
without considering the local field enhancement due to the
sharp tip. Therefore, most of the current would flow through
these localized spots even when a small voltage is applied to
the
device,
which
is
consistent
with
earlier
observations.7,19,26,27 This result, however, does not exclude
the possibility of the formation of metallic bridges in our
device. When the voltage bias was further increased, the current density flowing through these local spots could become
so large that the highly conductive paths were damaged by
joule heating. This explains how the device was switched
from the high to low conductivity state.
Next, we consider how these conductive spots are produced. We observed, from the TEM images, that the gold
electrode was not uniform throughout the device. The electrode was flat in the center, but was irregular near the edges.
This was due to the lift-off process that we used to pattern
the electrode. Figures 3共a兲 and 3共b兲 shows the scanning TEM
共STEM兲 images of two such regions after the device was
formed. We examined the differences between these two sections using TEM-energy dispersive x-ray spectroscopy
共TEM-EDS兲. Figure 3共c兲 shows the line scan data of the gold
L␣ signal. The results show that more gold was drifted
deeper into the polymer at places where the shape of the
electrode was not regular. This could be explained by the
much higher electric field at such locations.
During the forming process, gold ions were drifted into
the polymer film by the electrical field generated between the
two electrodes. At certain places where the electric field was
enhanced due to the nonuniformity of the electrode, more
gold ions were drifted deeper into the polymer film. The
(a)
Al
Gold
electrode
100
polymer
Au
30nm
(b)
Al
Counts [a.u.]
III. RESULTS AND DISCUSSION
Polymer
(c)
Au-Lα line for (a)
(b)
50
0
polymer
-10
0
10
20
Distance [nm]
Au
30nm
FIG. 3. Cross-section STEM images of device where the bottom gold electrode was flat 共a兲 and irregular 共b兲. The white lines denote the location of the
TEM-EDS line scan. 共c兲 TEM-EDS line scan data of 关共a兲 and 共b兲兴.
local conductivity of the polymer was thus increased by the
influx of gold ions. The combination of higher electric field
and increased conductivity resulted in very high current density at the local area. The forming process was stopped when
the current of the device reached the compliance current.
Subsequently, when a high enough voltage bias was applied to the device, local current density at these spots can
reach very high values due to the combination of enhanced
electrical field and high local conductivity. Local deformation may occur as a result of the joule heating and high
electric field, giving rise to the structures in Figs. 2共b兲 and
2共c兲. As mentioned earlier, these local conductive spots may
be destroyed when an even higher voltage is applied to the
device. New conductive spots could then be formed from the
irregular structures surrounding the damaged area.
Next, temperature dependence study of the high and low
conductivity states of the device was performed. Figure 4
showed the Arrhenius plot of the current at the on and off
states of the device. At the on state, the current showed a
very weak dependence on temperature 共Ea activation energy
⬃24 meV兲, indicating possibly a metallic or tunneling conduction mechanism. On the other hand, when the device was
at the off state, the current showed a much larger temperature
dependence 共Ea ⬃ 0.57 eV兲, implying that a thermally acti-12
-14
-16
on current @0.6V
off current @0.6V
-18
ln(I)
124516-3
-20
-22
-24
-26
3.4
3.6
3.8
4.0
4.2
4.4
1000/T
FIG. 4. Arrhenius plots of current measured at 0.6 V at on and off states of
the device.
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300
Current [µA]
J. Appl. Phys. 105, 124516 共2009兲
Kwan et al.
(a)
4
200
2
100
0
Voltage [V]
124516-4
0
12
(b)
10
8
200
6
4
100
2
0
Voltage [V]
Current [µA]
300
0
-2
0
200
400
600
Time [µs]
describe the formation of these highly conductive pathways
with evidences from STEM-EDS. During the forming process, metal ions are injected into the polymer film by enhanced localized electric field. When a sufficiently high voltage bias is applied to the formed device, highly conductive
spots are created by the locally enhanced electric field and
high current density. These conductive spots are destroyed
by joule heating when a higher voltage is applied to the
device. Temperature dependence studies were performed on
the device and the results were consistent with our model.
Transient current measurement further enhanced our understanding on the switching dynamics of the device.
We have shown that nonuniformities in the device, either
intentionally or unintentionally introduced during the fabrication process, are capable of dominating device characteristics. The polymer in our device played only a secondary
role in the switching mechanism. There are several reports in
the literature describing polymer memories with similar device characteristics but explained by different mechanisms.
While it is difficult to generalize our findings to all material
systems, we believe that it is necessary to carefully examine
the nonuniformities in both the electrode and film when
studying the device mechanism.
FIG. 5. transient current of device when a 4 V 共a兲 and a 10 V 共b兲 pulse were
applied to the device.
ACKNOWLEDGMENTS
vated mechanism was active. This could probably be related
to the transportation of charges through semiconducting
polymer. A more detailed study on the conduction mechanism was complicated by the nonuniformity of the device.
Nevertheless, these results are consistent with our model of
formation and destruction of conductive pathways.
Transient current measurements were performed on the
devices to observe the switching behavior when an electrical
pulse was applied to the device. Figures 5共a兲 and 5共b兲 show
the typical transient behaviors when a writing pulse 共4 V兲
and an erasing pulse 共10 V兲 were applied to the device. During the writing process, there were short durations when the
current dropped below a certain level, possibly indicating the
annihilation of weak filamentary paths. The current level was
restored when new pathways were formed by the electric
field. On the other hand, current spikes were observed during
the erasing pulse. This could be explained by the formation
and rapid destruction of the conductive pathways. Even
though conductive spots could be formed by the high electric
field during the erasing process, these pathways were not
stable under the high voltage and could be easily destroyed
by the large current density they needed to pass through. This
explains the relative instability of the I-V curve in the NDR
region, where there is a competition between formation and
destruction of weaker filaments. These transient measurements provided additional information on the switching dynamics of conductive pathways during the writing and erasing processes of the device.
In conclusion, we have described a FIB cross-sectioning
method to directly observe local conductive spots in our
polymer memory device. Based on the TEM results, we conclude that the memory mechanism is due to the formation
and annihilation of filaments. We also proposed a model to
The authors wish to thank Professor Qibing Pei and Dr.
Wei Wu from UCLA for providing the polymers, Dr. JinPing Zhang from UCSB for assistance in TEM-EDS, Mr.
Guanwen Yang and Dr. Rui Dong from UCLA for valuable
technical discussions, and Focus Center Research Program’s
共FCRP兲 Functional Engineered Nano Architectonics 共FENA兲
for funding and support.
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