1. technology and computing

Single-crystalline CuO nanowires for resistive random access memory applications
Yi-Siang Hong, Jui-Yuan Chen, Chun-Wei Huang, Chung-Hua Chiu, Yu-Ting Huang, Ting Kai Huang, Ruo
Shiuan He, and Wen-Wei Wu
Citation: Applied Physics Letters 106, 173103 (2015); doi: 10.1063/1.4919102
View online: http://dx.doi.org/10.1063/1.4919102
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/106/17?ver=pdfcov
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APPLIED PHYSICS LETTERS 106, 173103 (2015)
Single-crystalline CuO nanowires for resistive random access memory
applications
Yi-Siang Hong, Jui-Yuan Chen, Chun-Wei Huang, Chung-Hua Chiu, Yu-Ting Huang,
Ting Kai Huang, Ruo Shiuan He, and Wen-Wei Wua)
Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan
(Received 16 January 2015; accepted 14 April 2015; published online 27 April 2015)
Recently, the mechanism of resistive random access memory (RRAM) has been partly clarified and
determined to be controlled by the forming and erasing of conducting filaments (CF). However, the
size of the CF may restrict the application and development as devices are scaled down. In this work,
we synthesized CuO nanowires (NW) (150 nm in diameter) to fabricate a CuO NW RRAM
nanodevice that was much smaller than the filament (2 lm) observed in a bulk CuO RRAM device
in a previous study. HRTEM indicated that the Cu2O phase was generated after operation, which
demonstrated that the filament could be minimize to as small as 3.8 nm when the device is scaled
down. In addition, energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy
(EELS) show the resistive switching of the dielectric layer resulted from the aggregated oxygen
vacancies, which also match with the I-V fitting results. Those results not only verify the switching
mechanism of CuO RRAM but also show RRAM has the potential to shrink in size, which will be
C 2015 AIP Publishing LLC.
beneficial to the practical application of RRAM devices. V
[http://dx.doi.org/10.1063/1.4919102]
Conventional charge-based flash memories, such as
floating-gate nonvolatile flash memory, are rapidly approaching their physical limits.1 To satisfy numerous data computing
and storage applications, non-volatile memory (NVM) has
attracted significant interest for extensive studies in recent
years. This type of memory consists of phase change random
access memory (PCRAM), magnetic RAM (MRAM), and
resistive RAM (RRAM). Among them, RRAM is the most
powerful candidate because of its excellent endurance, retention, low power consumption, fast write/speed read, and simple metal/insulator/metal (MIM) structure.2 To improve the
RRAM performance, a complete understanding of its switching properties is necessary. Recently, the mechanism of resistive switching behavior has been partly clarified and
determined that it relies on the generation of a conducting filament. When one of the electrodes is an active metal, such as
Ag or Cu, the active metal will be oxidized into ions and the
electrical field will then drive metal ions into the dielectric
layer. The conducting filaments (CF) will be generated from
the reduction of metal ions, called electro-chemical metallization (ECM).3 On the other hand, when both electrodes are
inert metals, such as Au or Pt, the anion migration in the insulator will lead to resistive switching.4 The conducting filament
will be composed of oxygen vacancies, called the valence
change mechanism (VCM),3 although the mechanism is still
unclear in many dielectric layers.
These different type of conducting filaments have been
observed in recent years; for example, the real time observation of the Zn structure filament in the ZnO dielectric layer5
and the Ti4O7 filament of in the TiO2 layer6 in a VCM system. The active metal type filament can generate a conducting bridge in various dielectric layers in an ECM system.7–11
a)
E-mail: [email protected]
0003-6951/2015/106(17)/173103/5/$30.00
By controlling the formation and rupture of the filament, the
state of resistance can be determined.
However, the factors impacting the generation and
dimension of filaments are still unknown, while the size of the
conducting filament may restrict the miniaturization of
RRAM.6 We chose CuO as our dielectric material because
there have been relatively few studies of CuO VCM systems
and our devices (150 nm) are smaller than the directly
observed CuO filament(2 lm) in the previous research. That
is, the diameter of the filament in CuO planar RRAM devices
has been observed to be larger than 1 lm several times,12–14
but there is no record of a nanoscale device. We want to determine whether the filament theory is still suitable for devices
with shrinking size. For the development of RRAM in both
industrial and academic communities, comprehension of the
knowledge of RRAM behavior at the nanoscale is required.
In this work, we synthesized CuO nanowires (NWs) that
are much smaller than the filament in the previous CuO planar RRAM devices. To be the building block for further
reduced RRAM devices, the resistive switching mechanism
of these CuO nanodevices was investigated. The results will
promote the realistic application of RRAM in nanoscale
array memory.
Single-crystalline CuO nanowires with a high aspect
ratio were synthesized through a vapor solid (VS) mechanism as the building block of RRAM nanodevices.15 Copper
foil was chosen as the substrate and cleaned in a 1.0M HCl
solution for 15 s, followed by rinsing with DI water. The
substrate was placed in a furnace and heated in air at 500 C
for 120 min. The temperature was ramped up at the rate of
20 C/s. After the growth process, the furnace was cooled
down to room temperature, and CuO nanowires were
synthesized on the surface of copper foil.
CuO NW RRAM devices were fabricated by electron
beam lithography (EBL). First, the CuO NWs were sonicated
106, 173103-1
C 2015 AIP Publishing LLC
V
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Hong et al.
in alcohol and then dispersed on a blank substrate. The center of the substrate was a Si3N4 membrane that was grown
by chemical vapor deposition with a thickness of 50 nm. The
Si3N4 membrane is transparent to the electron beams used
for TEM observation. Second, methyl methacrylate and
poly(methyl methacrylate) were coated as the photoresist
followed by baking at 150 C for 2 min. After e-beam lithography, the electron beam evaporation system was used to
deposit Ti (20 nm)/Au (180 nm) as the electrode; after the
lift-off process to remove the photoresist in acetone, the device was finished. The schematic diagram of device fabrication is shown in Fig. S1.
The typical unipolar resistive switching behavior of
CuO nanowires is shown in Fig. 1(a). The SET voltage and
RESET voltage were 2.4 and 2.2 V, respectively (the forming process is also included in the supplementary material,
Fig. S2).24 The current in the SET and RESET process was
three orders of magnitude less than that of CuO thin film
RRAM devices of micron size,16 resulting in CuO NW
RRAM devices with lower power consumption. The RESET
process operated in the positive bias, which was the same as
the SET process; therefore, the RESET process should result
from the joule-heating effect.17
Fig. 1(b) demonstrates the endurance test for a single
CuO NW RRAM device. The resistance of the original state
was close to that of an insulator. After the forming process,
the resistance decreased and the device transformed into a
low resistance state (LRS) with an ON/OFF ratio > 10. This
phenomenon may contribute to the generation of a conducting filament during the forming process.
To determine the resistive switching mechanism for a CuO
NW RRAM nanodevice, the device was analyzed by TEM
before/after operation. The energy dispersive spectroscopic
FIG. 1. Electrical measurement of a single CuO NW RRAM device. (a) The
typical unipolar resistive switching behavior and (b) the endurance test.
Appl. Phys. Lett. 106, 173103 (2015)
(EDS) mapping and electron energy loss spectroscopy
(EELS) analysis for CuO NWs at LRS are shown in Fig. 2.
Figs. 2(a)–2(d) are EDS mapping, showing the element distribution of Au, Ti, Cu, and O after the forming process. The
signal in the EDS mapping of oxygen was enhanced at
the anode and reduced at the cathode, while the signal of
copper was uniformly distributed in CuO NW. This result
indicates that the composition of oxygen in the nanowire
was decreased from anode to cathode; the ratio distribution
is shown in Fig. S3. The mapping results also demonstrated
that oxygen ions have higher mobility than metal ions, dominating the resistive switching behavior as the external electrical field was applied. The inhomogeneous radial distribution
resulted from the morphology of the nanowire; more details
are discussed in the supplementary material (Fig. S4).
Although the EDS mapping revealed that there was less oxygen in the cathode, the HRTEM image indicates that the
structure is still CuO. The lack of an oxygen signal resulted
from the accumulation of oxygen vacancies. A more precise
analysis was carried out using EELS. EELS is a multifunctional spectroscopy technique that can detect atomic
composition, chemical bonding, surface properties, and especially the valence state of ions.18 Because EELS is more sensitive to energy dispersion than EDS, it can distinguish the
valence number, i.e., whether the Cu was Cuþ or Cu2þ, by
analyzing the oxygen vacancies in detail. In Figs. 2(e) and
2(f), the Cu L-edge spectra for CuO NWs in their original
state and LRS after the forming process showed distinct
peaks, which matched with the Cu2þ and Cuþ of L23 edge,
respectively.19 The appearance of Cuþ revealed the high
concentration of oxygen vacancies generated during the
forming process.18 The EDS mapping indicated that the oxygen ions migration dominate the switching behavior, while
the EELS spectrum revealed Cu changes the valance state to
limit the amount of oxygen vacancies.
The oxygen vacancies can act as dopants, resulting in an
improvement in conductivity. However, the partial accumulation of oxygen vacancies at the cathode was not the crucial
reason for resistive switching. In fact, we obtained the same
mapping result in high resistance state (HRS); oxygen vacancies accumulated at the cathode, as shown in Fig. S5 (more
details were discussed in the supplementary material, Fig. S5).
To further understand the switching mechanism of the
CuO NW RRAM device, we analyzed the structure using
HRTEM and simulated the atomic positions. Fig. 3(a) is a
low-resolution TEM image of the CuO NW RRAM device,
and Figs. 3(b)–3(d) show a HRTEM image of a CuO nanowire at the left side, corresponding to the marks in Fig. 3(a).
The light contrast at the upper side of the NW revealed a different lattice structure, while the lower side remained CuO,
as shown in Fig. S6. The structure at the upper side was identified to be Cu2O by the HRTEM image and the fast Fourier
transformed (FFT) diffraction pattern. An enlarged HRTEM
image is shown in Fig. 3(e), and the inset is the corresponding FFT diffraction pattern, showing Cu2O with a [121] zone
axis. This result is consistent with the EELS analysis, which
demonstrated that the Cuþ signal was detected after the device was operated. The Cu2O structure appeared after the
forming step as a conducting bridge. However, the conductivity of Cu2O is only several times that of CuO.20 The phase
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Hong et al.
Appl. Phys. Lett. 106, 173103 (2015)
FIG. 2. EDS mapping and EELS analysis of the CuO NW RRAM device after the forming process. (a) Low-resolution TEM image of the CuO NW RRAM device. (b)–(d) EDS mapping shows the Au, Cu, and O distribution. The signal of copper in the mapping diagram was uniformly dispersed, while the oxygen
migrated from anode to cathode. (e) and (f) are EELS analyses that reveal a distinct peak indicating that Cuþ was created, which resulted from oxygen vacancies that were enhanced after an external bias applied.
transformation was not the main reason for resistive switching; we suggest that the phase transformation was a result of
the switching behavior. The resistive switching resulted from
the accumulation of oxygen vacancies, which act as a conducting path. It should be easier to release oxygen from the
surface and promote the accumulation of oxygen vacancies
because the CuO NW is a single crystal without a grain
boundary. Furthermore, the redox reaction was more intense
on the surface because of faster electron transport. As a
result, the conducting filament is generated at the surface, as
shown in Fig. 3. Additionally, the intensive oxygen vacancies not only served as a conducting path but also caused
FIG. 3. Observation of the conducting
filament by TEM. (a) Low-resolution
TEM image of the Cu NW RRAM device. (b)-(d) The conducting filament
was observed by HRTEM, which corresponded to the marks in (a). The diameter of the conducting filament was
smaller than 4 nm, and the diameter
was reduced from cathode to anode. (e)
HRTEM image of the interface of CuO
and Cu2O. The zone axis of Cu2O was
[12
1] and that of CuO was [2
11]. (f)
The simulated lattice structure of CuO
along the [11
1] zone axis and the structure with the oxygen ions at lattice sites
of (0, 7/12, 3/4) and (1/2, 1/12, 1/4)
removed. (g) The simulated lattice
1
1] zone
structure of Cu2O along the [1
axis.
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Hong et al.
mechanical stress, leading to the phase transformation. The
Cu2O phase, an oxygen-lacking structure, was formed to
reduce stress. Therefore, the CF region (oxygen vacancies)
overlapped with the Cu2O phase. We took Cu2O phase as CF
for convenience.
The HRTEM image also revealed that the epitaxial relationships across the interface were CuO(111)//Cu2O(11 1)
and [02
2]//[2
20]. A lattice simulation of the interface of
CuO and Cu2O structure is shown in Figs. 3(f) and 3(g),
respectively. The results illustrate that the structure of CuO
was similar to Cu2O when the oxygen ions at lattice sites of
(0, 7/12, 3/4) and (1/2, 1/12, 1/4) were replaced by vacancies
in Cu2O.21 As the conducting filament is generated at the
upper side of the NW because of the accumulation of numerous oxygen vacancies, the CuO will be transformed into
Cu2O. Additionally, the simulation demonstrated that the Cu
ions almost remained in the same atomic position when the
oxygen ions moved, coinciding with the results of the EDS
analysis, which suggested that oxygen ions dominated the
resistive switching behavior.
Figs. 3(b)–3(d) also indicate the morphology of the conducting filament in the CuO NW RRAM device. The diameter of the conducting filament was 3.8 nm near the cathode,
as shown in Fig. 3(b), and became smaller toward the anode,
as shown in Figs. 3(c) and 3(d). This result revealed the conducting filament was a conical pillar shape and generated
from the cathode to the anode.6
The ultra-small conducting filament may result from the
reduction of the device. Because the redox reaction occurs in
the forming process5 ðOxo ! Vo þ Oi00; Oi00 ! 12 O2 þ 2e Þ, the
current will influence the size of the filament.22 The current
through the device will decrease with the dimensions of the
device. Therefore, the filament we observed was much smaller
than that in bulk CuO RRAM devices in previous studies.12–14
Also, the tiny filament was destroyed with less intense jouleheating, resulting in the low power consumption.
The conducting mechanism of the CuO nanowire in
LRS was space charge limited current (SCLC), and the fitting I-V measurements are shown in Fig. S7. The SCLC
characteristics can explain the conducting method of VCM
RRAM devices, which resulted from accumulated oxygen
Appl. Phys. Lett. 106, 173103 (2015)
vacancies.23 Because the NW was a single crystal, the traps
may be related to oxygen vacancies, such as defects at the
CuO/Cu2O interface. When an external voltage was applied,
the electrons were driven into the dielectric layer, where
they would be captured by traps. As the voltage increased,
the inflow velocity of electrons was higher than the capture
speed of traps. The electrons pass through the dielectric layer
by the path of oxygen vacancies, and the conducting method
showed SCLC properties. The fitting results support the idea
that the conducting filament was composed of oxygen
vacancies.
Using the TEM results, we propose a resistive switching
mechanism for the CuO NW RRAM device. When an external voltage is applied, oxygen vacancies are generated by a
redox reaction and migrate toward the cathode while the Cu
ions stay in place, resulting in a huge number of oxygen
vacancies accumulated at the cathode. It is worth noting that
if we release the external voltage before the filament is generated, the concentration of oxygen vacancies will revert to
the previous distribution, shown as Figs. 4(a) and 4(b). More
details are given in the supplementary material (Fig. S8). As
the external voltage is increased, the concentration of oxygen
vacancies increases. After that, a part of the conducting filament composed of oxygen vacancies was generated at the
surface of the CuO nanowire as a result of faster electron
transport and an excited redox reaction. Once a part of the
conducting filament has been generated, the tip of the filament will increase the electric field because of the decreased
distance to the anode. Finally, the conducting filament connects both electrodes and the resistance decreases, as shown
in Figs. 4(c) and 4(d).
The oxygen vacancies are surrounded by a compressive
stress field, which drives the CuO structure transformation
into Cu2O, as observed by TEM.
In this work, we synthesized single-crystalline CuO
nanowires and fabricated a Au/Ti/CuO NW/Ti/Au RRAM
nanodevice. The electrical measurement showed a stable
switching behavior and outstanding low power consumption.
The EDS mapping revealed that the migration of oxygen
ions dominated the resistive switching behavior. In addition,
the EELS analysis indicated that a high concentration of
FIG. 4. Schematic diagrams of the resistive switching mechanism of the CuO NW RRAM device. (a) The CuO nanowire was in the original state and the oxygen vacancies were dispersed uniformly. (b) The oxygen vacancies were aggregated and driven to the cathode by an external voltage; the conducting filament
had not been generated. (c) Part of the conducting filament was generated as the external voltage was increased. The tips of the conducting filament increased
the electric field, accelerating the growth of the filament. (d) The conducting filament connected both electrodes, resulting in LRS.
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173103-5
Hong et al.
oxygen vacancies were generated during the forming process. Furthermore, the HRTEM images showed that a tiny
conducting filament with a diameter below 4 nm was generated at the surface of the CuO nanowire. The CuO structure
in the conducting region was transformed into Cu2O. The fitting of the I-V measurement revealed that the conducting
mechanism of the CuO NW was SCLC at LRS. This result
also demonstrated that the oxygen vacancies caused the
resistive switching. Compared with micron-size CuO thin
film RRAM devices, the diameter of the conducting filament
was minimized by three orders of magnitude, implying the
size of the filament can be reduced as the device is scaled
down to the nanoscale. Because the resistive switching was
controlled by generating and erasing the filament, the power
consumption also decreased as the filament was miniaturized. This study was the first to analyze a single CuO nanowire and shows its potential in nanodevice applications. The
results will push RRAM toward practical applications in
next-generation nonvolatile memory.
We acknowledge the support of the Ministry of Science
and Technology through Grant No. 103-2221-E-009-222MY3.
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