Laterally configured resistive switching device based on transition-metal nano-gap
electrode on Gd oxide
Masatoshi Kawakita, Kyota Okabe, and Takashi Kimura
Citation: Applied Physics Letters 108, 023101 (2016); doi: 10.1063/1.4939683
View online: http://dx.doi.org/10.1063/1.4939683
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APPLIED PHYSICS LETTERS 108, 023101 (2016)

Laterally configured resistive switching device based on transition-metal
nano-gap electrode on Gd oxide
Masatoshi Kawakita,1 Kyota Okabe,1 and Takashi Kimura1,2,a)
1

Department of Physics, Kyushu University, 6-10-1 Hakozaki, Fukuoka 812-8581, Japan
Research Center for Quantum Nano-Spin Sciences, Kyushu University, 6-10-1 Hakozaki, Fukuoka 812-8581,
Japan
2(Received 13 October 2015; accepted 26 December 2015; published online 11 January 2016)
We have developed a fabrication process for a laterally configured resistive switching device basedon a Gd oxide. A nano-gap electrode connected by a Gd oxide with the ideal interfaces has been created by adapting the electro-migration method in a metal/GdOx bilayer system. Bipolar set and reset
operations have been clearly observed in the Pt/GdOx system similarly in the vertical device basedon GdOx. Interestingly, we were able to observe a clear bipolar switching also in a ferromagnetic
CoFeB nano-gap electrode with better stability compared to the Pt/GdOx device. The superior performance of the CoFeB/GdOx device implies the importance of the spin on the resistive switching.
C 2016 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4939683]
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Resistive switching device consisting of a metallic oxide
layer sandwiched by the metallic electrodes has paid considerable attention as a next-generation nonvolatile memories
owing to its simple device structure with low power consumption.1–3 Moreover, the nonlinear transport property withmemorizing the applied voltage history is expected to contribute the development of the innovative logic circuits suchas neuromorphic and learning-type devices.4–9 As the origins
for the transition between the low-resistive and highresistive states, two major mechanisms—filament-type andinterface-type models—are proposed.1 However, the microscopic mechanism for the resistance change is still a controversial issue under the irreconcilable conflicts between theabove two models, and the dominant mechanism seems todepend on the preparation condition of the devices. Since theestablishment of the device fabrication with the operation
under the mechanism control is an important milestone forthe reliable and low-power-consumption operations, proper
understanding of the mechanism of the resistance change is a
key for the development of the resistive switching device. In
addition, the development of more simple and flexible fabrication processes is also important from the view point of thecost-competitive device compared to the conventional flash
memory based on the floating-gate transistor.
Apart from the device fabrication, it should be notedthat most of the metallic oxides showing the resistive switching include the transition metals with finite magnetic
moments.10–18 This implies that the resistance switching isrelated to the spin configuration in the metallic oxide.
Especially, the interface between magnetic metal and magnetic oxide show various functional properties through theexchange interaction.19 Moreover, the resistive switching
devices with the ferromagnetic electrode have been reported
in the vertical stuck structures although the switching mechanisms do not seem to be related to the spin.20,21 If one candemonstrate the resistive switching operation with additional
spin functionalities, we may have greater control based ona)t-kimu@phys.kyushu-u.ac.jp0003-6951/2016/108(2)/023101/4/$30.00

the direction of the spin in the magnetic oxide. A laterally
configured structure is suitable for demonstrating such attractive properties because of high flexibility of the device geometry. However, most of the resistive switching devices
reported so far consist of vertical stuck structures because of
the regulation of the short channel length in the metallic oxide. This is because the operation voltage is believed to be
proportional to the channel length of the oxide layer.
Moreover, the difficulty for the preparation of the clean
metal/oxide interface without using vertical stack structures
is another important reason. From these view points, in thepresent study, we developed an original method for the fabrication of the resistive switching device in lateral configuration and investigated the influence of the ferromagnetic
electrode and spin orientation on the switching property.
To fabricate laterally configured switching devices with
ideal clean interfaces, we used a break junction technique,22–24 whose process is schematically shown in Fig. 1(a).
First, we prepared a patterned metal/oxide bilayer with ananoconstruction on a SiO2/Si substrate. In the present study,
we adapted GdOx to a metallic oxide layer. For the metallic
layer, we used Pt and CoFeB. Here, the metal/oxide bilayer
film have been continuously deposited on the resist template
patterned by electron beam lithography by the magnetron
sputtering under the base pressure of 106 Pa. Gd oxide(GdOx) has been grown by the magnetron reactive sputteringof the Gd in a mixed gas of Ar and O2 with the O2 flow ratioof 1/7. The chemical composition for GdOx is unclear, but its
electrical property is highly insulating. To prevent the distribution of the composition, we made all of the devices reported
here almost same time. Here, the thicknesses for Gd oxideand metallic electrodes are 100 nm and 30 nm, respectively.
The electrical resistivities for Pt and CoFeB are, respectively,
32 lX cm and 52 lX cm. Figure 1(c) shows a scanning electron microscope (SEM) image of the fabricated electrode pattern consisting of the Pt/GdOx bilayer after the lift-off
process. A nano-gap electrode was formed by using anelectro-migration technique with the real-time monitoring of
the current. In the bilayer system, the current almost perfectly

108, 023101-1

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Kawakita, Okabe, and Kimura

Appl. Phys. Lett. 108, 023101 (2016)

FIG. 1. (a) Schematic illustrations of alaterally configured resistive switching
device and formation process of nanogap electrode. (b) I-V characteristic
measurement during nano-gap formation process SEM images. (c) before
the nano-gap formation and (d) after
the nano-gap formation.

flows in the metallic layer because of the large difference ofthe electrical resistivity. The wire pattern was an asymmetricstructure formed by the connection between triangular andrectangular electrodes. Since the current density becomesmaximum at the boundary between the triangular and rectangular electrode in this structure, the nano-gap will be formed
around the boundary. We performed a break-junction experiment at low temperature in order to further increase the electrical insulation of the oxide layer and to minimize the gap
length. As shown in Fig. 1(b), the nanogap formation was
clearly distinguished by monitoring the flowing current.
Figure 1(d) shows a SEM image of the typically fabricatednano-gap electrode. A nanogap whose gap length is less than
10 nm has been obtained. Finally, we were able to fabricate alaterally configured metallic/oxide/metallic junction with theclean interfaces as schematically shown in Fig. 1(a). After the
nano-gap formation, the resistance switching properties were
evaluated by measuring the I-V characteristic. To clarify thecorrelation between the interface condition and device performance, we also fabricated the similar planar device with
non-ideal interface, where the metal/oxide interface has been
formed by ex-situ sputtering process with breaking thevacuum.
First, we evaluated the lateral Pt/GdOx device, which isa typical material combination in the vertical devices basedon Gd-oxide.25 We measured I-V characteristics after the formation of the nanogap. Figure 2(a) shows a representative
result of the I-V characteristic observed in the laterally configured Pt/GdOx switching device. Clear resistance switchings with set and reset transitions were observed. Here, theresistance for the low resistive state RLRS and that for thehigh resistive state RHRS were approximately 3 kX and
50 kX, corresponding to 1500% resistance change, which isdefined by ðRHRS  RLRS Þ=RLRS . This value was slightly
smaller than those reported in the vertical device with thesuperior performance.25,26 We believe that this is because
the resistance of the metallic electrode was much higher thanthe vertical situation. Since RLRS can be reduced by increasing the width of thickness for the patterned electrode, a resistance change in the lateral device can be comparable tothe vertical devices. Interestingly, the switching voltages for
set and reset transitions were, respectively, 1.1 V and1.2 V, which were relatively low compared to those in thevertical devices. Since the low voltage operation leads to the

reduction of the power consumption,26 this is the strong
advantage of the laterally configure device. We also emphasize that the transition curve shown in Fig. 2(a) has beenobtained from the virgin curve without performing the filament forming process.27 Although the voltage of 1.5 V,
which is slightly higher than that in the I-V measurements,

FIG. 2. (a) Representative I-V characteristic for a lateral Pt/GdOx device
exhibiting clear set and reset processes. The inset shows the switching endurance property of the fabricated device. (b) Dispersion of I-V characteristic for the lateral Pt/GdOx device with the intermediate resistance state. The
inset shows the distribution of the switching voltages for set and reset
processes.

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023101-3

Kawakita, Okabe, and Kimura

Appl. Phys. Lett. 108, 023101 (2016)

FIG. 3. I-V characteristic for a Pt-nano-gap device fabricated on GdOx filmwith unstable transport property.

had been applied during the nano-gap forming process, it
had been performed at low temperature. Therefore, we
expected that the nano-gap forming process could not play
an important role for the filament forming process.
Moreover, we did not observe unipolar-type switching in thevoltage range from 4 V to 4 V.28 In addition, the I-V curve
at a high resistance state is well reproduced by a Schottky
characteristics. These facts imply that the main mechanismof the resistance switching is interface type.
We also demonstrated a high endurance of the switchingproperty from the application of the alternative voltage asshown in the inset of Fig. 2(a). Thus, the validity of the fabrication technique for the resistive switching device with a lateral configuration has been confirmed. We found that 70%
devices among 20 devices showed clear bipolar switching.
Although the device dependence of the switching voltages
was small, the resistance at low resistive state strongly
depends on sample to sample. This is probably because theeffective junction size for the switching shows the strong
sample dependence. The control of the junction size is an important milestone for this fabrication method. Moreover, we
often observed intermediate resistive states under the highbias condition just after the transition as shown in Fig. 2(b)in some of the devices. In addition, the switching voltage for
set and reset processes showed large dispersions, as shown in
the inset of Fig. 2(b). At the moment, the detailed mechanism of the switching and the intermediate state are still
unclear but may be related to the interface condition.29
To clarify the influence of the interface condition on theswitching more clearly, we also evaluated the resistive
switching property of the Pt/GdOx sample with the non-idealinterfaces. As schematically shown in the inset of Fig. 3, thepatterned asymmetric Pt electrode has been fabricated on a
uniform GdOx film by using a conventional lift-off technique. Here, we did not perform any interface treatment suchas Ar ion milling and wet-etching processes. Therefore, theresidual resist, other organic components and/or natural oxidation layer may exist at interface between Pt and GdOx.30
Figure 3 shows the I-V characteristics after the nano-gap formation. Although the switching behaviors have been
observed, the resistance change due to the transition is quite
small. Especially, in lower resistance state, the resistance

FIG. 4. (a) Representative I-V characteristic for a lateral CoFeB/GdOx device together with the SEM image in the vicinity of the junction. (b)Dispersion of I-V characteristic for the lateral CoFeB/GdOx device with theintermediate resistance state. The inset shows the distribution of the switching voltages for set and reset processes. (c) Representative HRS and LRS
I-V curves fitted to the equations for the Schottky barrier.

value was much higher than that in the previous device. Thisis probably due to the non-clean condition for the interface
between Pt and GdOx. Thus, the interface condition was confirmed to play an important role for obtaining the effective
resistance switching.
We then investigate the material dependence of metallic
electrode on the switching property. We fabricate the resistive switching device using ferromagnetic CoFeB electrode.
Here, because of its large resistivity and low heat conductivity for CoFeB electrode, it was difficult to obtain the nanogap less than 10 nm. The size of the nanogap isapproximately 20 nm, but it is difficult to estimate precise
electrode distance only from the SEM image. As discussed
earlier, the direct connection between CoFeB and GdOx

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023101-4

Kawakita, Okabe, and Kimura

induces the exchange interaction at the interface.19 In addition, the electric current from the ferromagnetic electrodegenerates the spin-polarized current, which exerts the torquein the magnetic moment via spin transfer torque.31 These
additional effects originating from the ferromagnetic metal
may affect the switching property, leading to the additional
controllability utilizing the spins. Figure 4(a) shows the I-V
characteristics after the formation of CoFeB nanogap.
Interestingly, we clearly observed bipolar switching behavior
also in CoFeB/GdOx device at room temperature. We quantitatively evaluated the I-V characteristics in CoFeB/GdOx
devices. The resistance change due to the switching was
smaller than that in the Pt/GdOx device. This is mainly
because the transport at the high resistive state is more conductive than that in the Pt/GdOx device. The switching voltages both for set and reset transitions were also higher thanthose in the Pt/GdOx device. Thus, the device performance
for CoFeB/GdOx device seems to be worthy than that for thePt/GdOx device. However, there are several advantages in
CoFeB/GdOx devices compared to the Pt/GdOx device, as
follows. Figure 4(b) shows the distribution of the I-V curves
in another CoFeB device. Although a variation of RHRS
seems to be not so small, the distribution of the switchingvoltage is quite small compared to the Pt/GdOx device, as
shown in the inset of Fig. 4(b). Moreover, in the CoFeBbased devices, we have never observed the intermediate state
between the high and low resistance observed in Fig. 2(c). In
addition, it should be noted that the I-V curves both for the
low and high resistive states are well reproduced by the
Schottky characteristics, as shown in Fig. 4(c), strongly suggesting the interface-type switching.32 Since our CoFeB film
was confirmed to have an efficient spin polarization and saturation magnetization, these superior stabilities obtained in
CoFeB-based devices may be related to the spin-polarized
CoFeB electrodes and/or the exchange interaction play animportant role for the resistance switching. However, it
should be noted that the switching endurance for the CoFeBbased device is worse than that for Pt-based device. Thismay be related to the higher switching voltage in CoFeB
electrode with the Schottky interface and relatively wide
nanogap electrode.
In short, by performing the electro-migration techniquein a transition metal/Gd oxide bilayer, we have fabricated anano-gap electrode on a Gd oxide. The I-V characteristics forthe nano-gap electrode showed bipolar switching behaviors,
which are consistent with the results in the vertical resistive
switching device based on Gd oxide. We have also investigated the influence of the ferromagnetic electrode on theresistive switching and observed more stable bipolar switching property than that in the Pt/GdOx system. The results
imply that the spins both in metal and oxide play an important role in the resistive switching process.

Appl. Phys. Lett. 108, 023101 (2016)

This work was partially supported by Grant-in-Aid for
Scientific Research on Innovative Area, “Nano Spin
Conversion Science” (26103002) and that for Scientific
Research (S)(25220605).
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