Nano Res

1

Filament growth dynamics in solid electrolyte-based

resistive memories revealed by in situ TEM

Xuezeng Tian, Lifen Wang, Jiake Wei, Shize Yang, Wenlong Wang, Zhi Xu() and Xuedong Bai()

Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0469-0

http://www.thenanoresearch.com on April 4, 2014

© Tsinghua University Press 2014

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Nano Research

DOI 10.1007/s12274-014-0469-0

1

TABLE OF CONTENTS (TOC)

Filament growth dynamics in solid electrolyte-based

resistive memories revealed by in situ TEM

Xuezeng Tian, Lifen Wang, Jiake Wei, Shize Yang,

Wenlong Wang, Zhi Xu* and Xuedong Bai*

Beijing National Laboratory for Condensed Matter

Physics, Institute of Physics, Chinese Academy of

Sciences, Beijing 100190, China

Page Numbers. The font is

ArialMT 16 (automatically

inserted by the publisher)

The real-time imaging of filament growth process of a resistive

memory was achieved using a specially designed sample. By

combining the in situ TEM technique and finite element method

simulation, the correlation of conductive filament growth with the

electric field distribution and several other affecting factors was

demonstrated.

2

Filament growth dynamics in solid electrolyte-based resistive memories revealed by in situ TEM

Xuezeng Tian, Lifen Wang, Jiake Wei, Shize Yang, Wenlong Wang, Zhi Xu() and Xuedong Bai()

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190,

China

Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher)

© Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011

ABSTRACT Solid electrolyte based-resistive memories have been considered to be a potential candidate for future

information technology with applications for non-volatile memory, logic circuits and neuromorphic computing.

A conductive filament model has been generally accepted to be the underlying mechanism for the resistive

switching. However, the growth dynamics of those conductive filaments is still not fully understood. Here, we

explore the controllability of filament growth by correlating the filament growth observation with the electric

field distribution and several other factors. The filament growth behaviors have been recorded using in situ

transmission electron microscope. With the real-time recorded filament growth behavior and morphologies, we

have been able to simulate the electric field distribution in accordance with our observations. Other factors

affecting the filament growth have also been testified, such as Joule heating and electrolyte infrastructures. This

work provides insight in exploring the controllable growth of conductive filaments and help guide research

into further functionalities of nanoionic resistive memories.

KEYWORDS Resistive switching, Conductive filaments, In situ transmission electron microscope, Real-time observation,

Computer simulation

1 Introduction

Resistive switching memories are devices based on

the resistive switching between a low resistance state

(LRS) and a high resistance state (HRS). These

devices exhibit multi advantages such as fast

switching speed, long retention time, ultrahigh

scalability, multi-level switching, spike-timing

dependent plasticity, etc. [1-5]. Thus, they are

considered to be promising in next generation

non-volatile memories [6,7], reconfigurable logic

systems [8,9], and neuromorphic computing [10,11].

3

Resistive memories based on solid electrolytes are

usually termed electrochemical metallization

memories (ECM). The ECM cells consist of a solid

electrolyte layer sandwiched between an active

electrode (e.g., Ag or Cu) and an inert electrode (e.g.,

Pt or heavily doped Si). Generally, the resistive

switching mechanism of ECM cells is related to the

formation and rupture of conductive filament (CF) in

the solid electrolyte layer [1,12]. Upon applying a

positive voltage to the active electrode, it will inject

ions into the solid electrolyte and the ions will

migrate under the electric field until they are

reduced to atoms and accumulate into a conductive

filament. The solid electrolytes can be a variety of

oxides, chalcogenides and organics [13-15]. The

above mentioned CF model has been testified by

various experiments and theoretical works [16-19].

But the random occurrence of CF formation limits

the device scalability and optimization. In order to

achieve the controllability of CF formation, many

research groups have developed various methods,

such as ion irradiation, electrode patterning and seed

embedment [20-22]. Those reported works have

shown great improvement in device performance.

But how the electric field affect the CF growth is still

on the theoretical stage. Some reported simulation

works were based on assumptions without direct

linkage to the observed CFs, which limits their

reliability. To effectively control the switching

characteristics with excellent memory performance, a

thorough understanding of the determining factors

on CF growth is essential.

In this work, we directly correlate the filament

growth behavior with the simulated electric field

distribution. By taking advantage of in situ

transmission electron microscope (TEM), we

observed the CF growth in real time. Then the

electric field distribution was simulated in

accordance with the observed CF morphologies. We

find that the mass transfer was directly driven by the

electric field, while simultaneously affected by other

factors. The real-time observed atomic-scale images

give insight into the factors affecting the

electrochemical behavior of silver in the SiO2 matrix,

such as Joule heating and electrolyte infrastructures.

This work should be desirable in understanding the

switching dynamics and suggestive in device

functionalities and optimization.

Figure 1 Experimental setup. (a) Schematic of sample structure (top panel) and in situ TEM experiment setup (bottom

panel). (b) High resolution TEM image showing the freshly made sample. The lattice fringes are indexed into Ag (111) and

Si (111).

2 Results and discussion

The schematic sample structure and the in situ

experiment setup are shown in Fig. 1a. The device

structure was deliberately designed as Au/SiO2/Ag

nano clusters/p-Si, where the Ag nano clusters acted

as the active electrode and Au acted as the inert

electrode. The Ag nano clusters were directly

deposited to the highly conductive Si substrate and

then capped by the SiO2 layer (see Experimental

Section). During the experiment, electric voltage was

applied across the Au tip and the Si substrate. As the

4

Ag clusters were connected to the Si substrate, they

shared an equipotential during measurements. This

structure ensures that the Ag clusters are all

encapsulated by the SiO2, thus we ensured that the

formed filament is in the SiO2 matrix rather than on

the surface. The electron beam was also dispersed

during the experiment to minimize the irradiation

influence. In this way, our in situ experiments

resemble the real devices as far as possible. The

original Ag clusters were all in their elementary

cubic phase, as can be seen in the freshly made

sample in Fig. 1b.

Figure 2 Forming and erasing of SiO2-encapsulated Ag system and potential profile for corresponding stages. (a) The

pristine sample. The numbers and bars in this and following graphs are markers for the embedded clusters. (b) Result of the

first forming with a constant voltage of 8V to the substrate. Device is in LRS. Inset is the current-time plot (same in c and d).

(c) Result of the first erasing operation by applying a 6V voltage to the tip. Device is in HRS. (d) Result of the second

forming operation by applying a 8V voltage to the substrate. Device is in LRS again. The yellow dotted lines sketch the

silver cluster formation path. (e-f) Simulated electric potential distribution corresponding to (a-d). The circles in the figures

denote the location of silver clusters. We assumed a voltage of 10 V across the top and bottom electrodes. The width of

dielectric layer is 60 nm.

Figure 2a shows the pristine sample. By applying a

constant voltage of -8V to the tip, the embedded

sliver clusters would be oxidized and transport into

the electrolyte to form a CF. We have recorded the

electrical current and the dynamic process

simultaneously. A current compliance of 10nA was

set to the source meter. At the beginning the current

fluctuate severely. This feature accompanies with the

filament growth process. After stressing for 72

seconds, the current increased abruptly to the

compliance. Figure 2b shows the result of the formed

device. We found that the CF was formed with series

of silver clusters in the SiO2 electrolyte to bridge the

silicon substrate and the gold tip. (A high resolution

image of this CF and EDX characterization are

shown in Fig. S-1 and S-2 in the Electronic

Supplementary Material (ESM)). We term those silver

clusters in the CF as CF clusters, in contrast to the

originally embedded clusters. The CF clusters are

originated from the embedded clusters. As the

electron beam was dispersed during experiment to

minimize irradiation influence, the video quality was

also reduced for this device. It’s interesting to notice

that most of the CF clusters were originated from site

3 and site 4. This feature may imply the driving force

for CF growth.

By reversing the voltage, the device was switched

back to HRS, as shown in Fig. 2c. The reversed

voltage was intentionally applied for constantly 30

seconds rather than a pulse, in order to see the

erasing effect by the reversed voltage. As we have

expected, the reversed electric field resulted in the

5

shrinking and disappearing of some of the CF

clusters. Then we performed the switching on

operation by applying a constant -8V to the tip again,

as shown in Fig. 2d. After stressing for 43 seconds

the device was switched to LRS. The device

underwent a forming process the second time. The

Ag clusters connected the gold tip again, as the white

arrow indicates. And the CF clusters became denser

and larger than those in Fig. 2b. Moreover, the

original silver clusters were further decomposed

during the second forming process, especially site 3,

which is directly facing the gold tip. We believe this

feature is related with the larger electric field

strength at site 3.

Another two features directly related with the

electric field strength were also found. One is shown

in Fig. 2d by the dotted yellow lines. Those yellow

lines sketch the mass transfer path, each of which

originates from an original embedded silver cluster.

The whole sketch is in a volcano shape. Another

interesting feature to notice is that the breaking and

coalescing point of the filament were all near the tip

end, as the white arrows indicate in Fig. 2b-2d. This

phenomenon is suggestive of the hot spot in a

filament during switching off. All those features are

issues to be addressed.

For ECM cells, the CFs are formed by the

electrochemical mass transfer of active metal (in this

case, silver). The driving force for electrochemical

mass transfer is the external electric potential energy.

We have simulated the electric potential profiles for

the four situations of Fig. 2a-2d in one to one

correspondence. The electric potential profiles are

shown in Fig. 2e-2h. From the simulated potential

profiles, the first feature we noticed was shown in

Fig. 2e. The dotted lines demonstrate the cross lines

perpendicular to the potential profile, thus can be

regarded as the electric field lines. The distribution

of the electric field lines is in a volcano shape, exactly

resembling that of the mass transfer paths in Fig. 2d.

Most of the serial clusters forming the conductive

filament are arranged along the electric field lines.

This is a direct exhibition of the electric field effect to

the mass transfer.

By comparing the potential profiles in the four

situations, we noticed that the potential profiles for

regions away from the conductive filament were not

changed evidently. But in the CF region, the potential

profiles were significantly changed. This is because

the CF silver clusters are much more conductive than

the SiO2 electrolyte, thus most of the potential

difference falls into the gaps between neighboring

silver clusters, as can be seen from Fig. 2f, 2g and 2h.

This feature results in two direct consequences. The

one is that potential difference in Fig. 2g mostly fall

to region between the tip and the remaining CF,

other than a uniform distribution. Thus reversing the

voltage will cause filament growth at the broken site

preferentially, resulting in a similar filament

morphology as the first one, as shown in Fig. 2d.

Another direct result as the potential difference

localized in the gaps is the greatly enhanced electric

field in between the gaps. To better illustrate the

electric field enhancing effect, we have simulated the

electric field distribution, as shown in Fig. 3. This

simulation reproduces the electric field distribution

in the real devices. The color in the graphs denotes

the absolute strength of the electric field.

Figure 3 Simulations of electric field distribution. The

images are corresponding to the 4 device images shown in

Fig. 2a to 2d, respectively. The green arrow in the color bar

denotes the middle point of the electric field strength.

Figure 3a shows the very beginning when forming a

device. It can be found that the cluster directly facing

6

the gold tip experiences the largest electric field. This

explains why most of the CF clusters were originated

from site 3 and site 4, as shown in Fig. 2b. And we

can make a conclusion that larger fields induce faster

electrochemical mass transfer. These results further

proved that we can control the filament formation by

properly control the electric driving force. The

controlled filament formation could also be extended

to directional electrical contact formation, even to

form a directional circuit pattern [23,24].

In Fig. 3b, 3c and 3d, the electric field between

neighboring CF clusters is enhanced magnificently,

which is consistent with the potential profile results.

And the field strength in the non-filament region

was not obviously changed. As the gaps between CF

clusters are mostly in a few nanometers, the

enhanced electric field will significantly enhance the

electronic conduction between the CF clusters. The

overall effect is that the high field regions and the CF

clusters together constitute a percolation conduction

path. The percolation paths can be directly perceived

in Fig. 3b and 3d. Based on our experiments and the

simulation results, we safely propose that the

percolation conduction mechanism accounts for the

electronic conduction in nanoionic resistive

memories [25,26]. When a resistive memory was

formed, one or more percolation paths were formed

for electronic carriers to flow, thus circuiting the

device. OFF and ON states correspond to the

annihilation and reunion of the percolation paths.

The higher electric field between neighboring CF

clusters also facilitates the electrochemical mass

transfer. The breaking point of the filament during

switching off should experience a very strong electric

field, such as that we observed in Fig. 2c. The electric

field effect on the mass transfer behavior can be

further understood by looking into the

electrochemical reaction principles. The CF

formation process is usually termed the

electrochemical metallization process, featuring in

the electrochemical mass transfer of the active

electrode atoms into the solid electrolyte matrix.

Take Ag as the active electrode for example. The

overall electrochemical reaction involves three steps:

(i) oxidation of Ag atoms into Ag+. (ii) electric field

driven migration of Ag+. (iii) reduction of Ag+ into

Ag atoms. Step (i) and (iii) are usually termed the

electrode process in electrochemistry. Both the

electrode process and the field driven migration are

related with the electric field. In general, the electric

field driven migration of cations in the amorphous

solid electrolyte can be described by Mott-Gurney

model [1]. The dependence of the ionic current

density i on E is given by:

0

2 exp sinh2

aW aqEi qcav

kT kT

(1)

where c is the concentration of mobile cations (in

this case Ag+), a is the jump distance of the ions

and v is a frequency factor, aW is the energy

barrier for ion hopping, E is electric field, k is

Boltzmann’s constant, and T is the absolute

temperature. The Mott and Gurney model describes

a positive correlation between the ionic current and

the external electric field.

The electrode process is also the charge transfer

process across the electrode/electrolyte interfaces,

which follows the well-known Butler-Volmer

equation [27]. It describes how the electrical current

on an electrode depends on the electrode potentials:

0 exp expa cq qi i

kT kT

(2)

Here i is the current density; 0i is the exchange

current density; a is the transfer coefficient at the

anode and c at the cathode; n is the number of

exchanged electrons; is the activation

overpotential; k is Boltzmann’s constant, and T is

the absolute temperature. The Butler-Volmer

equation also describes a positive correlation

between the electrode current and the electric

potential (in the term of activation overpotential).

Based on the above discussions, we proved that the

electric field distribution directly leads to the

morphologies of the CFs. The CF formation would

also change the electric field resulting in percolation

conduction paths along the CFs.

7

Figure 4 High-resolution images of the mass transfer process. (a-h) the serial images extracted from the real-time recorded

video. The time is labeled in the images. The white arrows denote the nanogap between clusters. Scale bar is 5nm.

To better understand the electric field induced mass

transfer behavior, we have recorded the

atomic-resolution mass transfer process, as shown in

Fig. 4. The serial images were extracted from a

real-time recorded video. As the white arrows

indicate, a nanogap always existed between the

original cluster and the new cluster. This nanogap

behaved as the electrolyte for an electrochemical

system. As the reaction continued, the original

cluster was consumed and the new cluster grew

larger and larger. At last, the original cluster was

totally consumed. But the nanogap remained along

with the whole mass transfer process. For any

electrochemical system, the electrolyte is

indispensable. The persistent existence of the

nanogap also proved that mass transfer was induced

by the electrochemical reactions from another side.

Due to the essential role the nanogaps have played,

the filament growth should have been greatly

affected by the sturdiness of the electrolyte.

We have used a brighter electron beam in this device

in order to get the atomic-scale images. Thus the

bombardment-induced temperature increase was not

negligible. And the Joule heating effect is also very

large for this device due to the small size of the

active region. Both electron dose and Joule heat

would cause temperature increase in the device. A

direct result of the temperature increase was the

higher reaction activity, as the mass transfer became

faster along with the reaction continuing. The

temperature increase also changes the mechanical

properties of the local electrolyte, which further

modulate the mass transfer behavior.

Another atomic-scale experiment is shown in Fig.

S-3 in the ESM. In contrast to the former experiments,

the mass transfer direction in Fig. S-3 was not always

complied with the electric field direction. It deviated

a little from the electric field direction, as the arrows

indicate. We believe this phenomenon is related with

the local infrastructure of the electrolyte. The PECVD

grown SiO2 film is known to be less dense than the

thermal oxidation silicon [28]. It’s reasonable to

8

assume that the mass transfer will take a path with

the least resistance in the electrolyte, thus resulting

in the deviation from the electric field direction. But

in a whole the mass transfer direction was complied

with the electric field, as the mass transfer direction

deviate again towards to the electric field direction in

Fig. S-3c. In summary, the electric field provides the

driving force for mas transfer, but the mass transfer

behavior would be modulated by electron dose,

Joule heating and electrolyte infrastructure.

3 Conclusions

This work has provided direct correlation of filament

growth behaviors with electric field strength and

several other factors. This correlation is based on the

real-time recorded filament growth experiments and

the simulation in one-to-one correspondence with

the experiment results. A direct influence of electric

field on electrochemical mass transfer was clarified.

Many other factors affecting the filament growth

were also testified. Our results also support a

percolation conduction mechanism to explain the

electronic conduction in nanoionic resistive

memories. This work can be helpful in

understanding the resistive switching dynamics and

suggestive in the controllability of the filament

formation. Further optimization could be achieved

by taking into account of the factors disclosed in this

work. In particular, novel architectures can be made

to manipulate the electric field distribution in order

to be readily compatible with the current

semiconductor production line.

Experimental section

The Ag nano clusters were directly deposited to the

highly conductive p-Si substrate by electron beam

evaporation at room temperature. The deposited Ag

would naturally be clustered due to surface energy

limitation. Then the SiO2 layer was deposited by

plasma-enhanced chemical vapor deposition

(PECVD) at temperature of 180 C , using dinitrogen

oxide and silicane as precursors. All the cross-section

TEM samples were made by traditional manual

grinding and then milled by a RES 101 ion miller to

achieve nanoscale thickness. The gold tips for in situ

measurements were made using a homemade

electrochemical corrosion cell using 1M KOH

solution electrolyte.

During all measurements, the silicon substrate was

grounded and current compliance was applied to

protect the devices. The in situ measurements were

conducted in a JEOL 2010F TEM combined with an

Agilent B2900 Precision Source/Measure Unit (SMU).

An accelerating voltage of 200 kV was used for all

the characterizations. The TEM holders in our

experiments were all homemade and dedicatedly

designed for in situ TEM experiments. The gold tip

was driven by a nanomanipulator as a moveable

electrode.

For computer simulation, we have used the finite

element method (FEM). We assumed a voltage of 6 V

across the top electrode and the substrate. The width

of dielectric layer is 40 nm. Thus the average electric

field strength is 1.5MV/cm. For simplicity, we have

neglected the electronic barriers across the interfaces.

Acknowledgements

This work is supported by National 973 projects

(Grant Nos. 2012CB933003, 2013CB932601 and

2013CB934500) from Ministry of Science and

Technology and NSF (Grant No. 51172273) of China.

Electronic Supplementary Material: Supplementary

material (High-resolution image of CF, EDX of CF

and another real-time recorded filament formation

process) is available in the online version of this

article at http://dx.doi.org/10.1007/s12274-***-****-*

(automatically inserted by the publisher). References

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Electronic Supplementary Material

Filament growth dynamics in solid electrolyte-based resistive memories revealed by in situ TEM

Xuezeng Tian, Lifen Wang, Jiake Wei, Shize Yang, Wenlong Wang, Zhi Xu() and Xuedong Bai()

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, 100190,

China

Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)

12

Figure S1 High-resolution image of the CF in Figure 2.

Figure S2 EDX characterization of the CF in Figure 2.

13

Figure S3 Serial images of another real-time observed mass transfer process. The gold tip was large in this experiment, thus the

Au/SiO2/Si structure could be regarded as a parallel capacitor. The dotted lines in (a) sketch the electric field distribution. The arrows

in the following images denote the mass transfer directions.