Impact of graphene and single-layer BN insertion on bipolar resistive switching characteristics in tungsten oxide resistive memory

    Impact of graphene and single-layer BN insertion on bipolar resistive switching characteristics in tungsten oxide resistive memory Jongmin Kim, Duhwan Kim, Yongcheol Jo, Jaeseok Han, Hyeonseok Woo, Hyungsang Kim, K.K. Kim, J.P. Hong, Hyunsik Im PII: DOI: Reference:

S0040-6090(15)00524-6 doi: 10.1016/j.tsf.2015.05.002 TSF 34328

To appear in:

Thin Solid Films

Received date: Revised date: Accepted date:

10 January 2015 27 March 2015 1 May 2015

Please cite this article as: Jongmin Kim, Duhwan Kim, Yongcheol Jo, Jaeseok Han, Hyeonseok Woo, Hyungsang Kim, K.K. Kim, J.P. Hong, Hyunsik Im, Impact of graphene and single-layer BN insertion on bipolar resistive switching characteristics in tungsten oxide resistive memory, Thin Solid Films (2015), doi: 10.1016/j.tsf.2015.05.002

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Impact of graphene and single-layer BN insertion on bipolar resistive

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switching characteristics in tungsten oxide resistive memory

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Jongmin Kim a, Duhwan Kim a, Yongcheol Jo a, Jaeseok Han a, Hyeonseok Woo a, Hyungsang Kim a,, K. K. Kim b,, J. P. Hong c, Hyunsik Im a,

Division of Physics and Semiconductor Science, Dongguk University, Seoul 100-715, Korea

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Department of Energy and Materials Engineering, Dongguk University, Seoul 100-715, Korea

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Department of Physics, Hanyang University, Seoul 133-791, Korea

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ABSTRACT

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The role of the atomic interface in the resistive switching in Al-WO3-Al devices is

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investigated by inserting metallic graphene or insulating hexagonal BN sheet between the top

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Al electrode and WO3 film. Clear reversible bipolar-type resistive switching phenomena were observed, regardless of the interface modification. However, endurance and retention properties were affected by the nature of the interface. Whilst the device containing the

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graphene interface showed significantly improved performance, another device containing the hexagonal BN sheet showed degraded performance. These experimental findings suggest that atomic configuration of the electrode/oxide interface plays a key role in determining the resistive switching characteristics.

Keywords: - Resistive switching; Thin film; Graphene; Hexagonal BN; Interfacial property  Corresponding author.  Corresponding author.  Corresponding author. Tel.: +82 2 2260 3740. E-mail addresses: [email protected] (H. Kim), [email protected] (K. K. Kim), [email protected] (H. Im). 1

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1.

Introduction

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The continued strong demand for high-performance memory devices in the commercial

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electronics industry has led to huge efforts towards the development of novel non-volatile memory devices that are fabricated from various materials in a wide array of structures [1,2].

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Reversible resistive switching (RS) phenomena in metal oxide systems are not novel: they have been demonstrated since the 1970s [3,4] but there is renewed interest because of their

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potential use as an operating principle for non-volatile memory applications, namely resistive

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switching random access memory (ReRAM) [5]. There are two-types of RS mode, depending

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on the way reversible RS is controlled either by current magnitude (unipolar-type) [6,7,8,9],

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or by voltage bias polarity (bipolar-type) [10,11,12,13]. There are many mechanisms that are responsible for the observed RS characteristics [5,14]. For metal-RS oxide-metal capacitor

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structures that show bipolar RS characteristics, it is generally accepted that the nature of the interface between oxide medium and metal electrode plays an important role in determining the observed RS characteristics [15,16,17,18]. Given the rapid progress of memory device technology, a clear understanding of the role of the interface in switching properties is important for device physicists and engineers to undertake advanced research. In this work, we report a direct demonstration of how a modified interface at the atomic level affects bipolar RS characteristics using Al-WO3-Al ReRAM devices. For this, we inserted graphene or hexagonal-boron nitride (hBN) monolayer between the top Al and WO3 2

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layers, and compared their RS characteristics with a symmetric device without such interface

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modification. We found that the electronic nature of the interface plays a key role in

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determing bipolar RS properties. We propose a resistive switching model, stressing that atomic configuration at the electrode/oxide interface plays a key role in determining the

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resistive switching characteristics.

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2. Experimental

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A tungsten oxide film was deposited on a Al(bottom electrode)/Si substrate, using a

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conventional RF-magnetron sputtering method. Sputtering was done using a pure tungsten

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oxide target (WO3), under a working pressure of 1102 torr. During the growth, a working atmosphere was maintained, with an Ar to O2 gas flow rate ratio of 9:1. Three different

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ReRAM devices were prepared: symmetric Al-WO3-Al, Al(graphene)-WO3-Al, and Al(hBN)WO3-Al. 100 nm-thick Al top electrodes of 300 μm diameter were deposited at room temperature, using a dc sputtering method. We applied a bias volatage to the top Al electrode, with the bottom Al electrode grounded. Two-terminal resistance switching current-voltage (IV) measurements were carried out, using a parameter analyzer (Keithley 4200). Graphene synthesis and transfer: Graphene was synthesized by using low-pressure chemical vapor deposition on copper foil (25 μm, 99.8%, Alfa Aesar). The Cu foil was first annealed at 1000 °C for 30 min under hydrogen atmosphere with a flow rate of 10 sccm (~ 3

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350 mTorr), and graphene was synthesized at the same temperature for 40 min under methane

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and hydrogen atmosphere, with flow rates of 15 sccm and 10 sccm, respectively (~1.5 torr

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total pressure). After the growth, the temperature was cooled naturally, under hydrogen atmosphere. The graphene single sheet was transfered to the WO3 layer, using a conventional

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PMMA transfer method [19].

hBN synthesis and transfer: hBN was synthesized by using a chemical vapor deposition

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on Pt foil (PT000261, Good-fellow). To synthesize hBN, the furnace was then ramped up to

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1100 °C for 30 min under hydrogen atmosphere, and maintained there for 15 min, in order to

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stabilize the temperature. hBN was synthesized for 20 minutes under borazine and hydrogen

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atmosphere, at a flow rate of 0.2 and 10 sccm, respectively (total pressure was 0.1 Torr). After the growth, the furnace was cooled naturally, until it reached room temperature. For the hBN

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transfer onto the WO3 layer, a bubbling-based transfer method was performed [20].

3. Results and discussion Fig. 1(a) shows a schematic of the device structure. The real chemical composition of the tunsten oxide layer was found to be WO3+0.3, using an Rutherford backscattering spectroscopy (RBS) measurement. After being transferred onto SiO2/Si, the monolayer of graphene and hBN was characterized by Raman spectroscopy and atomic force microscopy (AFM). Fig. 1(b) and 1(c) show the measured Raman spectra of the graphene and hBN layers, 4

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respectively. The peak positions of the G-band and 2D-band for graphene on the SiO2/Si layer

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were 1594 and 2684 cm1, respectively. The peak of the 2D-band could be decomposed by a

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single component, indicating that the graphene is monolayer [21]. For hBN, the peak position

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of E2g was 1370 cm1, indicating that the hBN is monolayer [22]. In addition, the thickness of

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graphene and hBN was measured by using atomic force microscopy and found to be 0.50 (Fig. 2(a)) and 0.56 nm (Fig. 2(b)), respectively.

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Fig. 3(a), (b), and (c) show the measured RS current-voltage (I-V) curves of the

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fabricated symmetric Al-WO3-Al, Al(graphene)-WO3-Al, and Al(hBN)-WO3-Al devices

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under consecutive voltage bias sweeping. The forming-free RS characteristics of the devices

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are presumably due to the oxygen deficiency of the tungsten oxide films [23,24]. The RBS measurement confirmed that our films are nonstoichiometric [25]. All three devices show

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clockwise bipolar RS characteristics, as indicated by the dotted black arrows (1231). When a negative voltage bias is applied to the top electrode, the high resistance state (HRS) switches to the low resistance state (LRS). This HRSLRS transition is called the Set process. In contrast, in the positive voltage regime, the opposite transition from LRS into HRS occurs, which is called the Reset process. As the bias-voltage sweeps repeatedly, the graphene-inserted device exhibits more stable switching I-V characteristics, while the hBNinserted device shows a worse switching I-V behavior. The I-V stability of the symmetric device is in between. 5

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Fig. 3(d), (e), and (f) show the cumulative probability (or distribution) of the HRS

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current at reading voltages of  0.5 V. While the HRS current for the Al(graphene)-WO3-Al

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is quite stable in both bias voltage polarities, that for the other devices either fluctuates or deviates significantly from its initial value. For practical nonvolatile memory applications,

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the current in each switching cycle must be stable within a tolerable range; the current distribution should be narrower.

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Fig. 4(a), (b), and (c) display the endurance characteristics (current of the HRS and LRS

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as a function of switching cycle at 0.1 V) of the devices achieved by using a fast dc sweep

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mode. While the endurance of the graphene-inserted sample for both the HRS and LRS is

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stable up to 500 cycles with a constant on/off ratio of ~10, that of the other samples is unstable showing a dramatic change in the current level. In particular, the h-BN inserted

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sample survived only up to ~100 switching cycles. Data retention tests were also performed. Fig. 4(d), (e), and (f) show the retention characteristics of the devices, reading the current at the same voltage of 0.1 V. All the samples showed good data retention properties, without noticeable degradation in the initial on/off ratio. From these results, one can conjecture that the inserted graphene or hBN plays an important role in determining the switching reproducibility of the resistive switching characteristics. Because the bipolar resistive switching in the Al-WO3-Al device is due to the oxygen ion-diffusion induced redox chemical process at the junction interface where a bias 6

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voltage is applied [26,27,28], the inserted graphene contributes to stabilizing the chemical

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process under consecutive switching operations.

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In order to investigate how the inserted graphene or hBN affects the corresponding transport mechanisms in the initial insulating state (IS), LRS, and HRS states, we carried out

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their temperature (T)-dependent transport measurements from 300 K to 50 K. Fig. 5(a), (b), and (c) show the variation of the current in each resistance state, measured at 0.1 V with

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decreasing T. In order to keep conduction paths unchanged during the measurement, the T-

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dependent current values were taken in the low voltage region, without switching. As the

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temperature decreased, the electrical conduction in each resistance state showed similar

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behaviors, which are often observed in metal-insulator-metal structures. No noticeable differences were detected among the samples. The analyses of the temperature-dependent

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transport in each resistance state (IS, LRS, HRS) indicate that it is associated with the electronic and structural nature of the WO3 layer, rather than a potential discontinuity at the Al/WO3 interface. The electrical transport in the Al-WO3-Al devices can be described by a complex combination of several conduction mechanisms, such as the Poole-Frenkel emission, the space-charge-limited current, and Schottky emission, etc. The thermal activation energies of the IS, HRS, and LRS in the low temperature region are estimated to be ~ 40 meV, regardless of the insertion of graphene or hBN. Because the amorphous WO3 layers have similar defect configurations that have similar statistical thermal activation energies, the 7

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observed T-dependent transport in the IS, HRS, and LRS is due to the microscopic nature of

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defects in WO3.

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The electrical transport in the Al-WO3-Al devices can be described by a complex combination of several conduction mechanisms [29]. Fig. 5(d), (e), and (f) show the

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logarithmic I-V curves in the positive voltage region. The red and blue lines indicate the 1st and 30th resistive switching I-V curves, respectively. The slopes of the LRS and HRS are

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about ~ 1 in the low voltage region, following the Ohm’s law [30]. However, the current

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increases nonlinearly in the high voltage region, and this behavior is consistent with the trap-

voltage-sweeping

transport

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described

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process:

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the

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controlled space charge limited conduction (SCLC) [31]. Thus, the conduction mechanism of

OhmicSCLCOhmic. The measured slopes in the high voltage region are different in the

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LRS and HRS. The slopes (1.5 ~ 1.6) of the SCLC process in the LRS are much smaller than those (2.2 ~ 3.5) in the HRS. This is because the trapped carriers are released in the HRS leading to a steeper slope of the SCLC process [32]. The above experimental findings and subsequent analysis suggest a meaningful conclusion that the inserted atomic monolayer does not change the electrical conduction but plays an important role in determining the resistive switching stability. A general model for the observed resistive switching is schematically proposed in Fig. 6. We investigated RS in the same structure changing the oxygen contents of the WO3 layer 8

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and found that the observed RS is strongly affected by the oxygen stoichiometric of the film

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[25]. This implies that the observed RS is characterized by oxygen vacancies near the

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interface rather than other factors including electrode’s size. The resistive switching mechanism is mainly based on oxygen ions migration near the top electrode/oxide interface

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[33,34], generating oxygen vacancies which are related to defects-induced paths through the WO3 layer. When a negative bias voltage is applied on the top electrode, oxygen ions are

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pushed away from the electrode/oxide interface and creating oxygen vacancies near the top

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interface and defects-induced paths are formed across the WO3 film (Fig. 6(a)). This process

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corresponds to the SET process (HRSLRS). On the other hand, a positive bias voltage

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leads to the opposite process (RESET process: LRSHRS) blocking the defects-induced paths (Fig. 6(b)). The role of the interfacial layer, graphene or hBN, on the observed RS

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properties is understood as follows. For the symmetric device, as the switching cycle increases the redox chemical reaction of the reactive Al electrode becomes unsteady in a reversible manner causing the observed unstable RS current. However, for the grapheneinserted device, graphene acts as “oxygen barrier” preventing the oxygen ions from diffusing into the reactive Al electrode [35,36,37] and the redox process near the interface becomes stable with increasing switching cycle [38]. Finally, for the hBN-inserted device the same redox-reaction for the RESET process is not fully recovered due to its electrical insulating nature and low permittivity (ɛ = 3 ~ 4) [39], leading to the increase in current with increasing 9

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RS cycle.

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4. Conclusions

In summary, in order to investigate the effect of the interfacial nature between the

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electrode and insulating oxide medium on resistive switching properties, we fabricated three different ReRAM devices, symmetric Al-WO3-Al, Al(graphene)-WO3-Al, and Al(hBN)-WO3-

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Al. These devices showed bipolar-type resistive switching characteristics, with a clockwise

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direction. While the Al(graphene)-WO3-Al device showed considerably improved resistive

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switching stability and endurance properties, the hBN inserted device showed degraded

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resistive switching performance. We propose that the inserted graphene and hBN modify of atoms near the junction interface. The changed chemical properties at the interface result in

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the modification of the redox chemical reaction at the interface leading to improved or degraded RS properties. Our experimental findings would provide device engineers a useful method of improving resistive switching properties in bipolar resistive switching memories, based on the redox chemical reaction at the interface.

Acknowledgements This project was supported by the National Research Foundation of Korea (Grant No. 2013-044975). 10

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Figure Captions

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Fig. 1. (a) Schematic layout of an Al-WO3-Al resistive switching memory cell. In order to study the role of the interface on resistive switching, graphene or one-monolayer hBN is

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inserted between the top Al electrode and the WO3 layer. Raman spectroscopy data of (b)

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graphene and (c) hBN on SiO2/Si.

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Fig. 2. AFM images of (a) graphene and (b) hBN on SiO2/Si and the height profiles along the

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black dashed line.

Fig. 3. Resistance switching I-V characteristics of (a) symmetric Al-WO3-Al, (b) Al(graphene)-WO3-Al, and (c) Al(hBN)-WO3-Al devices. Cumulative probability of the HRS current at  0.5 V for (d) symmetric Al-WO3-Al, (e) Al(graphene)-WO3-Al, and (f) Al(hBN)-WO3-Al devices. Note different x-axis ranges.

Fig. 4. (a), (b), and (c) Endurance, and (d), (e), and (f) retention properties, of symmetric AlWO3-Al, Al(graphene)-WO3-Al, and Al(hBN)-WO3-Al devices. The endurance and retention tests are performed by measuring the LRS and HRS currents at 0.1 V.

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Fig. 5. Temperature dependences of the IS, LRS, and HRS currents at 0.1 V for (a)

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symmetric Al-WO3-Al, (b) Al(graphene)-WO3-Al, and (c) Al(hBN)-WO3-Al devices. Log-

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Al(graphene)-WO3-Al, and (f) Al(hBN)-WO3-Al devices.

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Fig. 6. Schematic resistive switching model for Al(graphene)-WO3-Al. (a) SET process and

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reversibility of the redox-chemical process.

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