Journal of Colloid and Interface Science 520 (2018) 19–24

Contents lists available at ScienceDirect

Journal of Colloid and Interface Science

journal homepage: www.elsevier.com/locate/jcis

Regular Article

A flexible nonvolatile resistive switching memory device based on ZnO

film fabricated on a foldable PET substrate
Bai Sun a,b,⇑,1, Xuejiao Zhang c,1, Guangdong Zhou e, Tian Yu d, Shuangsuo Mao a,b, Shouhui Zhu a,b,
Yong Zhao a,b, Yudong Xia a,b,⇑
a
School of Physical Science and Technology, Southwest Jiaotong University, Chengdu, Sichuan 610031, China
b
Key Laboratory of Magnetic Levitation Technologies and Maglev Trains, Ministry of Education of China, Superconductivity and New Energy R&D Center (SNERDC), Southwest Jiaotong
University, Chengdu, Sichuan 610031, China
c
College of Lab Medicine, Hebei North University, Zhangjiakou 075000, China
d
College of Physical Science and Technology, Sichuan University, Chengdu 610064, China
e
Institute for Clean Energy & Advanced Materials (ICEAM), Southwest University, Chongqing 400715, China

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o a b s t r a c t

Article history: In this work, a flexible resistive switching memory device based on ZnO film was fabricated using a fold-
Received 25 January 2018 able Polyethylene terephthalate (PET) film as substrate while Ag and Ti acts top and bottom electrode.
Revised 24 February 2018 Our as-prepared device represents an outstanding nonvolatile memory behavior with good ‘‘write–rea
Accepted 1 March 2018
d–erase–read” stability at room temperature. Finally, a physical model of Ag conductive filament is con-
Available online 2 March 2018
structed to understanding the observed memory characteristics. The work provides a new way for the
preparation of flexible memory devices based on ZnO films, and especially provides an experimental basis
Keywords:
for the exploration of high-performance and portable nonvolatile resistance random memory (RRAM).
ZnO film
PET substrate
Ó 2018 Elsevier Inc. All rights reserved.
Flexible
Nonvolatile
Memory device

⇑ Corresponding authors at: School of Physical Science and Technology, Southwest Jiaotong University, Chengdu, Sichuan 610031, China.
E-mail addresses: bsun@swjtu.edu.cn (B. Sun), ydxia@swjtu.edu.cn (Y. Xia).
1
These authors contributed equally to this work.

https://doi.org/10.1016/j.jcis.2018.03.001
0021-9797/Ó 2018 Elsevier Inc. All rights reserved.
20 B. Sun et al. / Journal of Colloid and Interface Science 520 (2018) 19–24

1. Introduction

In the past few years, the fabrication technology of electronic

device have been continuously improved, however, the size of
semiconductor electronic devices has being reduced continuously
in order to increase the integrated density of the electronic compo-
nent [1,2]. Especially, the integration of electronic devices has been
revolutionized owing to the emergence of nanotechnology. At the
same time, with the continuously development of information
technology and progress of semiconductor technology, the mem-
ory devices with higher storage density, ultra-faster read-write
speed and miniaturized storage unit are endlessly pursued [3].
Considering above problems, memory has become a research focus
in the last few decades [4–7].
In order to satisfy the widespread demand of memory applica-
tion, many memory devices based on new concept have constantly
being developed. Among the various new types of memory, the
most promising one is resistance random memory (RRAM) [8],
which is based on resistive switching effect. The resistive switching
(memristor) effect is that the resistance of a medium (semiconduc-
tor or insulator) can be reversible switched between a high resis- Fig. 1. The preparation process of resistive switching memory device.

tance state (HRS) and a low resistance state (LRS) under the
applied voltage [9,10]. If the HRS is defined as logic ‘‘0”, the LRS
holes. Cross section condition of device was characterized using
is defined as logic ‘‘1”, thus the switching phenomenon can be
SEM. The element composition of as-prepared films was observed
applied for the storage of information, the switching cell through
by energy dispersive X-ray spectroscopy (EDX). The memory char-
proper process can be used for preparing of memory device
acteristics of devices were measured using a workstation (CHI-
[11,12].
660E) at room temperature. The corresponding test circuit is
In many current reports, researchers have found that many
shown in Fig. 3b.
semiconductor and insulator materials exhibit resistive switching
memory behavior [13–15]. ZnO has many advantages, such as sim-
ple chemical composition, rich reserves, high transparency, non- 3. Results and discussion
toxicity, and so on. Therefore, ZnO has been widely used for
preparing many kinds of optoelectronic devices [16–18]. Recently, Fig. 2a shows the EDX spectra of as-prepared ZnO film on Ti bot-
the RRAM memories based on ZnO has also been reported [19–21]. tom electrode, confirming that as-prepared product is composed of
In particular, the fixable electronic devices are very useful because two elements (Zn and O) without any other impurities, which is
of the convenience of carrying and deformable [22]. In the prepa- pure ZnO according to the atomic ratio (1:1) of the elements from
ration of flexible electronic devices, PET is commonly used as sub- the inset to Fig. 2a. Among the Ti peaks are because of the used Ti
strate because of PET has a lot of advantages, such as excellent bottom electrode. Fig. 2b exhibits the cross-sectional FESEM image
physical and mechanical properties in a wide temperature range, of as-prepared device, we can see that a thickness of about 400 nm
high thermal stability, good electrical insulation, high creep resis- of ZnO film layer was deposited on the Ti bottom electrode. It can
tance and fatigue resistance, and good dimensional stability, and be observed that the multilayer films are relatively smooth, and
so on [23,24]. In addition, PET is nontoxic, tasteless and safety for there is an obvious interface between adjacent layers. The ideal
practical applications [25]. device structure is the key to further study its performance. The
In our work, in order to preparing a fixable resistive switching inset in Fig. 2b shows the optical photograph of the device. From
memory device, we use a fixable PET film as substrate. Ti film the above results, we can confirm that the as-prepared device is
was firstly sputtered on PET substrate by a magnetron sputtering qualified.
equipment, Ti as bottom electrode can further reduce the bending Fig. 3a display the photograph of the as-prepared memory
stiffness and strain, thus the as-prepared product can reduce the device, indicating an excellent bending stability and a great poten-
fatigue caused by the curling. The detail preparation process of tial for nonvolatile memory applications in flexible electronics. The
resistive switching memory device is shown in Fig. 1. Through fur- electrical performances is tested by the experimental test circuit
ther study, we found that the device with Ag/ZnO/Ti/PET structure shown in Fig. 3b under a scanning rate of 0.1 V ms1, resulting dis-
display an excellent resistive switching memory characteristic. play Ag/ZnO/Ti/PET device holds a resistive switching memory
Therefore, our device has the capability to work as a fixable mem- behavior. In the test process, we set up the compliance current
ory device, combination that opens the door to novel multifunc- (CC) of 10 mA for avoid electrical permanent breakdown during
tional fixable and wearable electronic devices. the test process. The sweeping directions of applied voltage are
from 0 ? 2.0 V ? 0 ? 2.0 V ? 0. The positive pole of the power
supply is connected to the top electrode of the device, the linear
2. Materials and methods I-V curves exhibiting an almost symmetrical hysteresis behavior
(Fig. 3c). This kind of I–V characteristic is termed resistive switch-
Fig. 1 schematically illustrates the fabrication process of Ag/ ing memory behavior, indicating the Ag/ZnO/Ti/PET memory
ZnO/Ti/PET structure device. Our device was fabricated by a mag- device exhibited a bipolar switching mode, which can provide a
netron sputtering equipment, the vacuum degree of the deposited most clear memory window when under read and write voltage.
film is 5
105 Pa and the working pressure of Ar is 0.5 Pa. the Ti The memory window is a key technical parameter for resistive
bottom electrode and ZnO thin film were orderly grown on a PET switch memory devices, which should provide different logic
substrate. Finally, the Ag top electrode (the thickness of 500 states for writing and reading data [26,27]. The corresponding log-
nm) was deposited by using a metal shadow mask with small arithmic I–V curves is exhibited in Fig. 3d. Application of a positive
 B. Sun et al. / Journal of Colloid and Interface Science 520 (2018) 19–24 21

Fig. 2. (a) EDX spectrum of ZnO film grown bottom electrode Ti, the inset shows Zn/O atomic ratios. (b) The cross-sectional FESEM image of our device.

Fig. 3. (a) The photograph of the as-prepared memory device. (b) The experimental test circuit. (c) The current-voltage (I-V) characteristics curve of Ag/ZnO/Ti/PET device
structure. (d) The corresponding logarithmic I–V curves.

voltage on Ag top electrode can switch the memory device from showing non-noticeable degradation after 200 switching cycles.
the HRS state to the LRS state at 1.45 V, which was defined as The resistance ratio is about 10 through appropriate calculations,
the ‘‘Set” process. Afterward, the device returned to the HRS state illustrating that the memory performance are outstanding with
when a reverse bias (1.5 V) was applied, which was called the larger memory window. According to the above observed results,
‘‘Reset” process, demonstrating the great potential of Ag/ZnO/Ti/ the nonvolatile memory behavior in Ag/ZnO/Ti/PET device pro-
PET device for nonvolatile memory applications. The above data vides the potential application value in fixable and wearable elec-
fully demonstrate the great non-volatile memory potential Ag/ tronic systems. Fig. 4b exhibits the bar graph of the HRS/LRS
ZnO/Ti/PET device in practical memory applications. resistance ratio and the thickness of the ZnO film. The dependence
Indeed, for the practical application of the resistive switching between the HRS/LRS resistance ratio and the thickness of the ZnO
memory, the HRS/LRS resistance ratio is an pivotal parameter film can be observed, in which the as-prepared devices display a
[28,29]. The resistance ratio of Ag/ZnO/Ti/PET devices under maximum HRS/LRS resistance ratio (10) when the thickness of
applied bias voltage of 0.4 V were depicted, as shown in Fig. 4a, the ZnO layer is reached to about 400 nm.
22 B. Sun et al. / Journal of Colloid and Interface Science 520 (2018) 19–24

rent, which further causes the increase of resistance before the

conductive filament is annihilated. [34].
 
DEae
I / Vexp ð2Þ
kT

where DEae representative the electron activation energy [35]. The

carrier transmission can be caused by the ohm current in LRS, which
would not be affected by the interface state, and the direction and
space charge are derived from the existence of the trap. [36]. Thus,
the current in the LRS might drift through a metallic conductive fil-
ament formed inside ZnO film layer under high internal field [37].
However, a great quantity of captured electrons can produce a
higher internal electric field inside ZnO film layer, which can hinder
the conductive filament formed inside ZnO film layer. In a word, the
above conclusions show that the redox of Ag atoms is relatively
active, and it is relatively easy to ionize into Ag ions under applied
voltage. Therefore, our device exhibits a perfect nonvolatile rewrita-
ble resistive switching memory effect.
Popularly speaking, in this work, the Ag atoms inside Ag elec-
trode are almost possibly ionized for Ag/ZnO/Ti/PET device, the
Ag atoms are easily ionized into Ag ions under the large enough
electric field because of the higher activity of the Ag atoms
[38,39], the reaction process can be described as:
þ
Ag ! Ag þe -

and, the Ag ions can be driven by an applied electric field and

move rapidly along the direction of the electric field. (Fig. 6a)
[19]. When Ag ions reach the bottom electrode, which will capture
electrons and be reduced to Ag atoms. With Ag atoms accumulate
to a certain, it will connect the top and bottom electrodes, thus it
Fig. 4. (a) The HRS/LRS resistance ratio under a bias voltage of 0.4 V. (b) The bar can play the role of conductive filament, which will greatly
graph of the HRS/LRS resistance ratio and the thickness of the ZnO film.
improve the conductivity of materials and can complete the ‘‘Set”
process (Fig. 6b), leading to the formation of Ag conductive paths
Now, in order to detailed study the transmission of interceptor near the Ti cathode region [40,41]. The Ag conductive filaments
and conduction mechanism for the Ag/ZnO/Ti/PET device, we plot- then begin to grow from Ti electrode with electric field extending
ted and fitted the I–V curve of the first sweeping cycle in the pos- to the Ag electrode, leading to the formation of Ag conductive fila-
itive voltage regions on a log–log scale for the Ag/ZnO/Ti/PET ments between top electrode Ag and bottom electrode Ti, which is
device, as shown in Fig. 5a. The slope of the linear fit for Ag/ZnO/ an indication that the device has switched to its LRS due to a short
Ti/PET devices under low scan voltage region (Region I), it could circuit happened between the top and the bottom electrodes by Ag
be the launching process of thermionic, which can be described filaments. Therefore, a large enough voltage (>VSet) on the Ag elec-
using the following equation. (Fig. 5b). trodes can generate a high electric field, which can drives Ag ions
" pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi # into the ZnO film layer to form Ag filaments inside the ZnO film
ð/  q qv =4dpeÞ
I / AT 2 exp ð1Þ layer for the device achieve the LRS, as shown in Fig. 6c. After
kT the ‘Set’ process, the device retains the LRS unless a sufficient volt-
age of opposite voltage (<VReset) is applied to annihilate the Ag fil-
where I show the current, A is the Richardson constant, T is the tem-
aments, corresponding to the ‘‘Reset” process of the device, and
perature, / is the Schottky energy barrier, q is the electron charge, V
then the HRS is reached [42–44], followed the Ag ions are driven
is the applied bias, d is the film thickness, e is the electrical constant,
drift back to the top electrode by the electric-field, as shown in
and k is the Boltzmann constant [30]. The curve (Region I) exhibits
Fig. 6d. Therefore, we observed an excellent Ag filaments inducted
the relation of I / V1/2, indicating the charges injected through Ag
resistive switching phenomenon in Ag/ZnO/Ti/PET device.
electrode into ZnO film layer dominates the main transmission
mechanism.
With the applied voltage increasing (Region Ⅱ), the relation 4. Conclusions
curve of In (I) versus In (V) exhibits linear relation with a large
slope (2.06) in the range of applied bias from 0.8 V to 1.5 V in In summary, a fixable resistive switching memory device based
Fig. 5c, which indicates the relation of I / Vm. The corresponding on ZnO film device on a fixable PET substrate were prepared. It is
conduction mechanism follows the classical trap-controlled space observed a preferential memory behavior under applied voltage
charge limited conduction (SCLC) and filamentary conduction with larger resistance ratio (memory window). This result indicates
mechanism [31,32]. The above results indicate the charge transfer that Ag is easily oxidized to form Ag filaments inside ZnO film. The
is primarily occurred between Ag electrode and ZnO film, which resistive switching memory behavior contribute to the formation
can be explained by the forming of Ag conductive filaments [33]. and rupture of Ag filament inside the ZnO layer of Ag/ZnO/Ti/PET
In the LRS (Region III), the I-V characteristics can be fitted well devices. Our results open a new way for the preparation of high-
with a slope of 0.98, as shown in Fig. 5d, which is conforming to Eq. performance and flexible nonvolatile memory devices based on
(2) with Ohmic conduction model. It can be observed that there is a ZnO films. In particular, this work provides an experimental basis
slight deviation between the slope and the initial part, this may be for the preparation of fixable and wearable electronic devices in
due to the Joule heat generated during the transmission of the cur- the future.
 B. Sun et al. / Journal of Colloid and Interface Science 520 (2018) 19–24 23

Fig. 5. (a) The I–V curves in log–log scale for Ag/ZnO/Ti/PET structure in the positive voltage case, the scatterer is the data obtained by the experiment, and the straight line is
the curve fitted by the theoretical model. (b) Region I in the HRS. (c) Region II in the HRS. (d) Region III in the LRS.

of Science and Technology of Hebei Colleges, China (QN2017010),

Population Health Information in Hebei Province Engineering
Technology Research Center.

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