Electrodeposit Formation in Solid Electrolytes
Michael N. Kozicki, Cynthia Ratnakumar, and Maria Mitkova
Center for Applied Nanoionics
Arizona State University,
Tempe, AZ 85287-6206, USA
Abstract – Devices based on polarity-dependent switching in
solid electrolytes show great promise as next generation memory
and perhaps even logic devices. These elements operate by the
formation of robust but reversible electrodeposited conducting
pathways which can be grown and dissolved at low voltage and
current. Although such devices have been well characterized,
little has been presented on the exact growth mechanism and
nature of the conducting links themselves. In this paper we will
show and discuss examples of electrodeposition within ternary
silver-chalcogenide electrolyte device structures. The electrolyte
was sectioned using focused ion beam milling and imaged with an
in-situ scanning electron microscope to reveal the profile of the
structure. A variety of Ag electrodeposits were imaged in overwritten devices and it was clear that programming times in the
order of a few seconds will create multiple deposits on the inert
cathode, some of which appear to extend through to the anode.
The electron beam itself was also used to reduce silver ions
within the electrolyte to reveal how the electrodeposits might
nucleate on the Ag-rich phases within the film.
Index terms – non-volatile memory, solid electrolyte,
electrodeposition, focused ion beam, electron microscopy.
I. INTRODUCTION
There has been considerable interest in recent times in
switching devices based on reversible electrodeposition in
solid electrolyte films [1-3]. One approach, known by various
names including Programmable Metallization Cell (PMC) and
Conductive Bridging Random Access Memory (CBRAM),
shows great promise as a future low energy non-volatile solid
state memory [4-7]. The technology utilizes ternary solid
electrolytes, typically Ag-Ge-Se or Ag-Ge-S, sandwiched
between an inert (e.g., W) and an oxidizable (Ag) electrode.
A small bias, applied such that the oxidizable electrode is the
anode and the inert electrode the cathode, promotes the growth
of a conducting electrodeposit through the electrolyte and this
reduces the resistance of the device by many orders of
magnitude. A reverse bias dissolves the link and the device
returns to its high resistance state. Key attributes are low
voltage/low current operation, high speed, excellent
scalability, retention and endurance, and a simple fabrication
sequence within the back-end-of-line (BEOL) flow.
Although such devices have been well characterized with
respect to their electrical characteristics, little or no data has
been presented in the literature on the exact growth
This work was supported by Axon Technologies Corporation.
0-7803-9738-X/06/$20.00 (C)2006 IEEE
111
mechanism and nature of the conducting links that form
between the electrodes within the electrolyte film. In this
paper we will show examples of metal electrodeposition,
starting with a discussion of our previous work on surface
electrodeposits and moving to our recent analysis of growth
within ternary silver-chalcogenide electrolyte device
structures. In the case of the sub-surface growth, the
electrolyte was sectioned using a focused ion beam and
imaged with an in-situ scanning electron microscope to reveal
the profile of the electrodeposited features. A variety of Ag
electrodeposits were examined in devices that had been “overwritten”, i.e., an electrodeposition bias was applied for several
seconds to promote as much growth as possible for the
purposes of imaging. It was clear that programming times in
the order of a few seconds created multiple deposits on the
inert cathode, some of which appear to extend through to the
anode. We also used the electron beam itself to reduce silver
ions within the electrolyte to reveal how the electrodeposits
nucleate on the Ag-rich phases within the film.
II. ELECTRODEPOSIT GROWTH AND MORPHOLOGY
In order to rapidly grow a stable electrodeposit, it is
necessary to have an oxidizable anode to supply ions into the
electrolyte to maintain ion concentration and overall charge
neutrality, otherwise the electrodeposit will form at the
expense of the ions in the electrolyte and this process quickly
becomes self-limiting. In the case of a silver ion-containing
electrolyte, this oxidizable anode is silver or a compound or
alloy containing free silver. The inert electrode merely has to
supply electrons for ion reduction but should not be readily
oxidizable as this could allow growth to occur under reverse
bias which would make the turning-off of the device more
difficult to achieve. In a device that has an electrolyte
between two such electrodes, the anode will oxidize when a
bias is applied if the oxidation potential of the metal is greater
than that of the solution. Under steady state conditions, as
current flows in the cell, the metal ions will be reduced at the
cathode. For the case of silver, the reactions are:
Anode:
Ag → Ag+ + e-
(1)
Cathode:
Ag+ + e- → Ag
(2)
with the electrons being supplied by the external power
source. The silver ions migrate through the electrolyte by a
coordinated hopping mechanism toward the cathode under the
driving force of the applied field and the concentration
gradient [8]. At the boundary layer between the electrolyte
and the electrodes, a potential difference exists due to the
transfer of charge and change of state associated with the
electrode reactions.
This potential difference leads to
polarization in the region close to the phase boundary, known
as the double layer [9] and for the reduction-oxidation reaction
to proceed, the applied potential must overcome the potential
associated with this polarization. This leads to a threshold
voltage for electrodeposition, below which the small observed
steady state current is essentially electron leakage by tunneling
through the narrow double layer. Above the threshold, the ion
current flows and the ions are reduced to deposit on the
cathode, becoming part of its structure mechanically and
electrically.
In the most general case, the addition of atoms to the
growing electrodeposit occurs due to a diffusion-limited
aggregation (DLA) mechanism [10, 11]. In this process, a
seed is fixed on a plane which contains randomly moving
particles and those particles that move close enough to the
seed are attracted to it and attach and form the aggregate.
When the aggregate consists of a number of particles, growth
proceeds outwards and with greater speed as the new deposits
extend to harvest more moving particles so that the branches
grow faster than the interior regions. The precise shape of
these features depends on parameters such as the potential
difference and the concentration of ions in the electrolyte [12].
At low ion concentrations and low fields, the deposition
process is determined by the diffusion of metal ions in the
electrolyte and the resulting pattern is fractal in nature. For
high ion concentrations and high fields, conditions common in
our solid electrolyte devices, the moving ions have a stronger
directional component and dendrite formation occurs.
Dendrites also tend to have a branching shape but tend to be
more ordered than fractals and grow in a preferred axis
defined by the electric field.
The general models for electrodeposit evolution assume a
homogeneous electrolyte but since electrodeposit growth is
obviously related to the presence of available Ag ions in the
electrolyte surface, the morphology of the electrolyte will
have a profound effect on electrodeposit morphology. This is
of particular significance in ternary electrolytes such as AgGe-Se, which have been shown to be phase-separated into
metal ion-rich crystallites dispersed in an insulating glassy
material [13, 14]. The concentration of Ag in the electrolyte
and its physical distribution in the dispersed crystalline phase
depend on the composition of the base glass. The effect of
this is illustrated in the analysis of surface electrodeposition
on electrolytes formed from Se-rich and Ge-rich base glasses
[15]. In the case of an electrolyte based on a Ge0.30Se0.70 glass
which is over-stoichiometric in Se, we observe the growth of
relatively flat (around 20 nm tall) and continuous dendritic
deposits on surface of the films, as shown in Fig. 1(a). In the
case of the Ge-rich glasses (Ge0.40Se0.60), we see the growth of
isolated tall (>100 nm) electrodeposits as illustrated in Fig.
1(b). The Ge0.30Se0.70 material has the higher chalcogen
0-7803-9738-X/06/$20.00 (C)2006 IEEE
112
content of the two and therefore will possess greater and more
uniform quantities of the ion-supplying phase (in this case
Ag2Se) following the addition of Ag. This leads to dendritic
growth that is closer to that expected with a homogeneous
material. The isolated growth on the Ge0.40Se0.60 electrolyte is
a direct consequence of the greater degree of separation of the
dispersed Ag-containing phases in this material.
(a)
(b)
Figure 1. Atomic force microscope image (3D topographical scan) of (a) Ag
grown on Ag-saturated Ge0.30Se0.70. The maximum electrodeposit height is a
few tens of nm and the growth is continuous. (b) Ag grown on Ag-saturated
Ge0.40Se0.60. The maximum electrodeposit height is in the order of 100 nm and
growth is discontinuous (from [15]).
Whereas the growth of electrodeposits on the surface of an
electrolyte film has some useful applications [16] and allows
us to see the influence of the electrolyte morphology, it is the
formation of electrodeposits within an electrolyte that is of
more interest for applications in memory. The memory device
configuration has the electrodes on opposite sides of a thin
electrolyte film, so that the growth of the electrodeposit is
forced to occur through rather than on the electrolyte. Growth
within an electrolyte will tend to occur in the flexible channels
and nano-voids that exist in these materials and the confining
nature of the medium will distort the shape of the
electrodeposit, restricting its contours to conform with the
zones with the highest free volume. In addition, the nanoinhomogeneity of the electrolyte will have a significant effect
on local potential and ion supply. The net result is that the
electrodeposit will not appear to be dendritic in nature, instead
taking a form that is governed by the shape of the glassy voids
and crystalline regions in the electrolyte. In order to confirm
this, we performed a series of experiments on functioning
devices and these are described in the following section.
III. ELECTRODEPOSITION WITHIN SOLID
ELECTROLYTE FILMS
The PMC device structures fabricated for the sub-surface
electrodeposit characterization work consisted of a W bottom
electrode, a 60 nm thick Ag0.33Ge0.20Se0.47 electrolyte formed
by the photodissolution of 25 nm of Ag into a Ge0.30Se0.70 base
glass, and a Ag top electrode. These layers were deposited by
physical vapor deposition and the photodissolution was
performed using exposure to ultraviolet light, 405 nm, 1 J/cm2,
at room temperature. The electrodes were separated by a 100
nm thick silicon nitride dielectric layer and a window in this
insulating layer defined the electrolyte area that contacts the
bottom electrode. In this case the device area was 5 µm in
diameter to allow cross-sectioning of the active region.
To form sufficiently large and numerous electrodeposits
for analysis, the devices were programmed using a quasi-static
technique. In this, a slow voltage sweep from -1 V to +1 V
with a current compliance of 10 µA was performed using an
Agilent 4155C Semiconductor Parameter Analyzer. The
positive part of the sweep above the write threshold of 250
mV lasted approximately 3 seconds. Programmed and
unprogrammed devices were then transferred to a FEI Nova
200 Nanolab dual beam Focused Ion Beam (FIB) system with
integrated scanning electron imaging for sectioning and
analysis. A Ga+ ion beam current of 30 pA extracted from a
liquid metal ion source at 30 kV was used to mill 2.5 µm × 2.5
µm areas from the centers of the devices, removing the top
electrode and electrolyte down to the bottom electrode. The
milled holes in the devices were then imaged using the
system’s scanning electron microscope capability with a 5 kV,
98 pA beam.
Fig. 2 shows a section taken through the (rough) top silver
electrode and the Ag-Ge-Se electrolyte. The electrolyte
appears to be thicker than the 60 to 70 nm expected but this is
an artifact of the milling processes which produces a sloping
0-7803-9738-X/06/$20.00 (C)2006 IEEE
113
Ag
Ag-Ge-Se
Fig. 2. Electron micrograph of cross-section of a Ag electrode on a 60 nm
thick Ag-Ge-Se solid electrolyte with no connected W electrode.
sidewall. In this region, there was no connected tungsten
electrode so the structure would not support electrodeposition.
We see that the electrolyte in this case is essentially
featureless and there is no evidence of electrodeposit
formation. The programmed device cross-section shown in
Fig. 3 has a multitude of electrodeposits both on the tungsten
cathode and also in the electrolyte. We believe that most of
these deposits, particularly those on the cathode, were created
by the programming and not by the milling process as we do
not see any electrodeposition in the milled sample of Fig. 2.
We also performed ion imaging (no electron exposure) on the
programmed sample after sectioning to confirm that the ion
beam itself was not promoting growth. It should be noted, as
we will see later, that the electron beam is capable of
stimulating considerable growth and so the micrograph of Fig.
3 was therefore taken as quickly as possible to avoid this.
Ag
W
Ag deposits
in electrolyte
Ag-Ge-Se
Ag deposits
on cathode
Fig. 3. Electron micrograph of a cross-section of a Ag electrode on a 60 nm
thick Ag-Ge-Se solid electrolyte on a W bottom electrode. The device was
previously programmed to produce the electrodeposits (bright regions) on
both the cathode and in the electrolyte.
On close inspection of the deposits on the cathode in this and
other samples in our study, they generally appear to be thicker
at the base than at the tip and are frequently somewhat conical
in shape. This is not surprising as the cathode is the primary
supply of electrons and therefore the bulk of the
electrodeposition will occur there.
Fig. 4 gives an example of a particularly broad (around
250 nm diameter at its base) near-conical electrodeposit at the
edge of the ion milled region that had originally formed on the
cathode during device programming but continued to grow by
exposure to the electron beam during imaging. This region
received a higher electron dose than the material in Fig. 3
(hence the larger amount of growth) as the imaging of this
smaller feature took more time. Of course, only the
electrodeposits that were in physical contact with the intact
portion of the electrolyte could grow during electron-beam
exposure as electrodeposition requires both a supply of
electrons (from the beam) and a supply of ions (from the
electrolyte). The typical base diameter of the electrodeposits
which formed on the cathode away from the Ag-supplying
electrolyte edge, and therefore could not continue to grow
during electron-beam imaging, was less than 50 nm, as is
evident from Fig. 3. Fig. 5 is a magnified portion of the image
shown in Fig. 3, showing the growth of an electrodeposit in
the electrolyte film. The electrodeposited feature has an
apparent width of 20 to 40 nm but we believe that this has
grown slightly during exposure to the electron beam even
though the time taken to image the sample was short. There
appear to be three growth regions that make up this
electrodeposit; a small growth on the cathode topped with a
more prominent growth in the center of the electrolyte, and a
small deposit between the central growth and the bottom of
the silver electrode. This electrodeposit could very well be
continuous and thereby connect the tungsten cathode to the
silver anode, leading to the low on state resistance (around 25
kΩ) of the programmed device.
Ag-Ge-Se
Electrodeposit
40 nm
Fig. 5. Electron micrograph of a small region of the electrolyte shown in Fig.
3 showing what could be an electrodeposit that connects the cathode to the
anode.
Fig. 6 shows an interesting effect of prolonged electron
exposure of the edge of the Ag-Ge-Se electrolyte. At the start
of the imaging session, only a few small dispersed
electrodeposits could be seen but after several seconds
(following focusing), a number of Ag deposits, which appear
as light near-circular features in the micrograph, nucleated
away from the electrodes and independent of one another
within the electrolyte. Since electrodeposition will occur
where the supply of ions is the greatest (assuming that
electrons supplied by the beam are plentiful at all points), it is
most likely that these features have nucleated on the Ag-rich
phases within the film and grown out from these nuclei to
form the larger electrodeposits, of 30 nm average diameter,
shown in the micrograph.
Electrodeposits
Ag-Ge-Se
Ag-Ge-Se
Electrodeposit
30 nm
250 nm
Fig. 4. Conical electrodeposit on cathode at the edge of the milled region.
The feature continued to grow during imaging and is this much larger than the
original electrodeposit produced by the programming process.
0-7803-9738-X/06/$20.00 (C)2006 IEEE
114
Fig. 6. Electron micrograph of a section of a 60 nm thick Ag-Ge-Se solid
electrolyte film in which electron-beam stimulated electrodeposition has
occurred.
IV. CONCLUSIONS
Building on our earlier work on the characterization of
surface electrodeposits, we have studied aspects of subsurface
electrodeposition within solid electrolyte films.
Crosssections were taken through vertical Programmable
Metallization Cell devices based on Ag-Ge-Se electrolytes
using focused ion beam milling. The electrolyte sections in
regions which could not support growth were largely
featureless but the milled cross sections in over-programmed
devices showed a variety of electrodeposited structures.
Programming times in the order of a few seconds led to
multiple deposits on the cathode. These features were often
somewhat conical in shape, being broad and nearly circular at
the electron-supplying cathode and tapering off toward their
top. The base diameter of these cathode deposits was
typically less than 50 nm although larger electrodeposits were
observed to grow on the cathode under electron beam
exposure at the edge of the milled region where the electrolyte
could still supply Ag ions. All electrodeposits within the
electrolyte continued to grow with electron beam exposure
and appeared to be centered on and fed by Ag-rich regions in
the nano-phase-separated electrolyte.
We believe this
nucleated growth is significant as it suggests that the same
kind of electrodeposit evolution could take place during
electrical programming of the structure. Fed by electrons
from the cathode electrodeposits and via current leakage
through the electrolyte, the relatively high resistance regions
in the electrolyte are likely to be bridged by the
electrodeposits which nucleate on and grow from the Ag-rich
phases. The bridging electrodeposits will therefore consist of
a “chain” of joined sub-deposits, which in the initial stages
may even grow simultaneously rather than sequentially as
long as the supply of electrons via the leakage current is
sufficient.
ACKNOWLEDGEMENT
The authors gratefully acknowledge the help of the staff
and use of the facilities within the Center for Solid State
Electronics Research and the Center for Solid State Science at
Arizona State University.
REFERENCES
[1]
[2]
[3]
[4]
[5]
G. Müller, T. Happ, M. Kund, G.Y. Lee, N. Nagel, and R. Sezi,
“Status and outlook of emerging nonvolatile memory
technologies,” IEDM Technical Digest, 567-570 (2004).
T. Sakamoto, H. Sunamura, H. Kawaura, T. Hasegawa, T.
Nakayama, and M. Aono, “Nanometer-scale switches using copper
sulfide,” Appl. Phys. Lett., vol. 82, 3032-3034 (2003).
S. Kaeriyama, T. Sakamoto, H. Sunamura, M. Mizuno, H.
Kawaura, T. Hasegawa, K. Terabe, T. Nakayama, and M. Aono,
“A nonvolatile programmable solid-electrolyte nanometer switch,”
IEEE J. Solid-State Circuits, vol. 40, 168-176 (2005).
M. Kund, G.Beitel, C. Pinnow, T. Röhr, J. Schumann, R.
Symanczyk, K. Ufert, and G. Müller, “Conductive bridging RAM
(CBRAM): An emerging non-volatile memory technology scalable
to sub 20nm,” IEDM Tech. Dig., 31.5, 2005.
M.N. Kozicki, C. Gopalan, M. Balakrishnan, M. Park, and M.
Mitkova, “Non-volatile memory based on solid electrolytes,”
0-7803-9738-X/06/$20.00 (C)2006 IEEE
115
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
Proceedings of the 2004 Non-Volatile Memory Technology
Symposium (NVMTS), 10-17 (2004).
M.N. Kozicki, M. Park, and M. Mitkova, “Nanoscale memory
elements based on solid-state electrolytes,” IEEE Trans.
Nanotechnology, vol. 4, 331-338 (2005).
M.N. Kozicki, M. Balakrishnan, C. Gopalan, C. Ratnakumar, and
M. Mitkova, “Programmable Metallization Cell Memory Based on
Ag-Ge-S and Cu-Ge-S Solid Electrolytes,” Proceedings of the
2005 Non-Volatile Memory Technology Symposium (NVMTS), D5,
1-7 (2005).
P.L. Kirby, “Electrical Conduction in Glass,” Brit. J. of Appl.
Phys., vol. 1, 193-202 (1950).
C. Kotzeniewski, in The Electrochemical Double Layer, (Ed.: B.
E. Conway), The Electrochemical Society Inc., 1997.
T. A. Witten and L. M. Sander, “Diffusion-Limited Aggregation, a
Kinetic Critical Phenomenon,” Phys. Rev. Lett., vol. 47, 14001403 (1981).
P. Meakin, “Diffusion-controlled cluster formation in 2—6dimensional space,” Phys. Rev. A, vol. 27, 1495-1507 (1983).
Y. Sawada, A. Dougherty, and J. P. Gollub, “Dendritic and Fractal
Patterns in Electrolytic Metal Deposits,”Phys. Rev. Lett., vol. 56,
1260-1263 (1986).
M.N. Kozicki, M. Mitkova, J. Zhu, and M. Park, “Nanoscale phase
separation in Ag-Ge-Se glasses,” Microelectronic Engineering,
vol. 63, 155-159 (2002).
M. Mitkova, M. N. Kozicki, H. Kim, T. Alford, “Local Structure
Resulting From Photo- and Thermal Diffusion of Ag in Ge-Se
Thin Films,” J. Non-Cryst. Sol., vol. 338-340C, 552-556 (2004).
M. Mitkova, M.N. Kozicki, J.P. Aberouette, “Morphology of
Electrochemically Grown Silver Deposits on Silver-Saturated GeSe Thin Films,” J. Non-cryst. Solids, vol. 326/327, 425-429
(2003).
M.N. Kozicki and M. Mitkova, “Mass transport in chalcogenide
electrolyte films – materials and applications,” Journal of NonCrystalline Solids, vol. 352, 567–577 (2006).
Michael N. Kozicki (M ’84) was born in Scotland and received his B.Sc. and
Ph.D. degrees from the University of Edinburgh in 1980 and 1985
respectively. He was with Hughes Microelectronics Ltd. (now Raytheon
Systems Ltd.) before joining Arizona State University (ASU) in 1985, where
he is currently Professor of Electrical Engineering and Director of the Center
for Applied Nanoionics. He is holder of several key patents concerning
devices based on electrodeposition in solid electrolytes. He is also a founder
of Axon Technologies Corp., an ASU spin-out company formed to develop
and license solid-state ionic technologies, and has served as Director of
ASU’s Center for Solid State Electronics Research and Director of
Entrepreneurial Programs for the Fulton School of Engineering at ASU.
Cynthia Ratnakumar received her B.S. in Electrical Engineering from the
University of California, San Diego in 2003. She is currently pursuing an
M.S. degree at Arizona State University in the Center for Solid State
Electronics Research.
Her current research interests are fabrication,
characterization, and design integration of Programmable Metallization Cell
devices.
Maria Mitkova received her MS and PhD from the Technological University
in Sofia, Bulgaria. She was professor at the Technological University and
Bulgarian Academy of Sciences and has worked in the R&D of Ovonic
memory devices in the Institute for Microelectronics. Since 1997 she has
worked in the USA at the University of Cincinnati and Arizona State
University. Her interests are in the field of amorphous semiconductors,
including their characterization and application as optical and electronic
memory media. She has specialized in a number of Ag-containing
chalcogenide systems.