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Effect of Electronegativity on Bipolar Resistive Switching in a WO3‑Based Asymmetric Capacitor Structure Jongmin Kim, Akbar I. Inamdar, Yongcheol Jo, Hyeonseok Woo, Sangeun Cho, Sambhaji M. Pawar, Hyungsang Kim,* and Hyunsik Im* Division of Physics and Semiconductor Science, Dongguk University, Seoul 100-715, Korea S Supporting Information *
ABSTRACT: This study investigates the transport and switching time of nonvolatile tungsten oxide based resistiveswitching (RS) memory devices. These devices consist of a highly resistive tungsten oxide film sandwiched between metal electrodes, and their RS characteristics are bipolar in the counterclockwise direction. The switching voltage, retention, endurance, and switching time are strongly dependent on the type of electrodes used, and we also find quantitative and qualitative evidence that the electronegativity (χ) of the electrodes plays a key role in determining the RS properties and switching time. We also propose an RS model based on the role of the electronegativity at the interface. KEYWORDS: resistive switching, electronegativity, oxygen ion migration, tungsten oxide, switching time
1. INTRODUCTION Nonvolatile memory (NVM) technologies have been widely implemented for data storage of mobile electronics due to their low power consumption, and resistive random access memory (ReRAM) has recently attracted a considerable amount of attention for use as next-generation NVM. ReRAM offers several potential advantages, including low power consumption, simple metal-insulating metal oxide−metal (MIM) structure, nonvolatility, faster access speed, higher density, and easier fabrication processes.1−4 Numerous ReRAM materials, including SrZiO3,5 ZnO,6,7 NiO,8,9 Ni−Ti−O,10 Al2O3,11 and TiO2,12,13 exhibit resistive switching memory characteristics that make them suitable for use in next-generation memory devices. Although many of these materials have been studied, tungsten oxide (WO3)-based resistive switching memory (ReRAM) has attracted a considerable amount of attention because it exhibits promising properties in terms of scalability, switching speed, endurance, retention, CMOS compatibility, etc.14−17 Many microscopic factors determine the resistive switching characteristics of MIM-based ReRAM devices. In particular, oxygen vacancies in the resistive switching oxide materials and near the metal-oxide interface play an important role because these are associated with the oxygen ion drifts under a bias.18,19 The formation of the oxygen vacancies depends on a number of factors, including the stability of the crystal lattice, relative bond strengths, and electronegativities of the constituting cations. O vacancies have been found to be favorable for use as highly electronegative cations, and their formation energy increases monotonously as the electronegativity of the cations increases.20−22 Thus, the electronegativity (χ) might play an © XXXX American Chemical Society
important role in current conduction and carrier transport at the interface between the electrode (M) and the insulator layer (I) in the MIM structures.23 For example, rectifying behavior or symmetric I−V curves may be shown depending on the electronegativity of the metal in a Pt/TiO2/metal device.24 However, the complex interplay of electronegativity and other material parameters is thought to play an important role in the resistive switching properties,25−28 but to the best of our knowledge, the effect of the electronegativity of a metal electrode on resistive-switching memory properties has not been satisfactorily elucidated yet. A metal electrode with a low electronegativity may form an interfacial suboxide layer due to the interdiffusion of oxygen from the oxide layer to the metal electrode. In this study, we demonstrate the tuning resistive switching properties in a TE/WO3/Pt device (TE, top electrode; Al, Cu, and Pt with electronegativity of 1.5, 1.9, and 2.2, respectively).24
2. EXPERIMENTAL SECTION Amorphous tungsten oxide WO3 films are grown on Pt/SiO2/Si substrates using conventional magnetron sputtering. First, a pure tungsten oxide target (WO3) is attached to a copper plate for sputtering, and the sputtering chamber is initially evacuated to up to 3.2 × 10−6 Torr and is further maintained at 10 mTorr with Ar and oxygen gas flow. The O2 to Ar gas flow rate ratio was kept at 1:9 during the tungsten oxide layer growth, and 100 W of power were applied for 1 h to grown the samples. Then, Al, Cu, and Pt were Received: December 3, 2015 Accepted: March 23, 2016
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DOI: 10.1021/acsami.5b11781 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 1. (a) Schematic diagram of the device structure (metal/WO3/Pt). (b) Cross-sectional SEM images of the Al/WO3/Pt, Cu/WO3/Pt, and Pt/ WO3/Pt devices. (c) Cross-sectional TEM images of the Al/WO3/Pt, Cu/WO3/Pt, and Pt/WO3/Pt devices.
Figure 2. Resistance-switching I−V characteristics of the (a) Al/WO3/Pt, (b) Cu/WO3/Pt, and (c) Pt/WO3/Pt devices. The switching voltage distribution of the (d) Al/WO3/Pt, (e) Cu/WO3/Pt, and (f) Pt/WO3/Pt devices. The inset in (d) shows the positions of the SET and RESET voltages. deposited at room temperature via magnetron sputtering with a shadow mask to form the top electrodes (TE) with a thickness of 100 nm and a diameter of 300 μm. The middle WO3 insulator layer has a thickness of ∼170 nm, and the TE and bottom Pt electrode have a thickness in the range from 100 to 120 nm. Rutherford backscattering spectroscopy (RBS) was carried out to determine the chemical composition of the WO3+δ films (see Figure S1). The δ value was determined to be 0.3, and the excess oxygen is a result of the effect of the negative ions during sputtering. A parameter analyzer was used to obtain the two-terminal resistance switching current−voltage (I−V) measurements. We applied a bias voltage to the TE while the bottom Pt electrode remained grounded. The cross-sectional configuration of the metal/WO3/Pt devices was
then verified using a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The switching time measurements were obtained using a pulse pattern generator and a digital oscilloscope.
3. RESULTS AND DISCUSSION Figure 1 shows a schematic of the WO3-based RS structure together with the cross-sectional SEM and TEM images of the RS devices. Sharp boundaries can be seen between the WO3 and metal electrodes. A thin interfacial layer can be seen at the TE/WO3 interface region in the TEM images for the Al/WO3/ B
DOI: 10.1021/acsami.5b11781 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces Pt and Cu/WO3/Pt RS devices, but such a layer is not present in the Pt/WO3/Pt devices because only Al and Cu react chemically with oxygen ions to form suboxides. For the same reason, such an interfacial suboxide layer is not detected at the Pt/TiOx interface.29 Figure 2a−c shows the typical current−voltage (I−V) characteristics of the fabricated Al/WO3/Pt, Cu/WO3/Pt, and Pt/WO3/Pt devices under consecutive voltage bias sweeping. The switching direction is denoted with arrows. When a large positive-voltage is applied to the pristine RS devices, a process called “forming” (which changes the insulating high-resistance phase into a bistable reversible switching phase occurs; see Figure S2). Afterward, when a positive voltage bias is applied to an Al/WO3/Pt device (Figure 2a), the relative amount of current measured at ∼1 V increases sharply, indicating a decrease in the resistance state. This sharp switching to a lowresistance state (LRS) is referred to as a set process and is observed at ∼1 V for the Al/WO3/Pt device. When the voltage is applied in the negative range, a sharp decrease can be observed in the current at ∼ −1.2 V. This sharp change from a low- to a high-resistance state (HRS) is known as a reset process, and these SET and RESET processes are quite reproducible for consecutive cycles under negative and positive voltage sweeps. The SET and RESET voltages for the Cu/ WO3/Pt and Pt/WO3/Pt devices (Figure 2b,c) are ∼1.2, ∼2 (SET) and ∼−1.5, ∼−2 V (RESET), respectively. All three devices exhibit stable bipolar RS characteristics for the switching cycles measured in the counterclockwise direction, as denoted with numbers (1 → 2 → 3). The memory window and the switching voltage have no significant degradation after repeated sweepings of each device. The different LRS and HRS resistance (or current level) between the devices is presumably because of the nonuniformity of the WO3 film rather than the TEs used. Although we fabricated the RS devices using the same WO3/Pt wafer, defect configurations that affect the leakage-type current in metal−WO3−metal devices may differ at the microscopic level. The different (although small) temperature- and bias-voltagedependent I−V characteristics of the devices (see Figure 6 and Figures S3 and S6) support the idea that the bulk WO3 film has nonuniform defect configurations. Figure 2d−f shows the SET and RESET voltages for both samples. As the electronegativity of the TEs increases, the SET and RESET voltages also tend to increase. The Al/WO3/Pt device (Figure 2d) exhibits uniform SET and RESET voltages when compared with the other two devices (Figure 2e,f). The different SET and RESET voltages that were observed for all three devices may be due to the different localized conduction channels at the interface between WO3 and TE. This could be quite possible because all three TEs have a different electronegativity. The main RS mechanism is attributed to the aggregation of the oxygen vacancy at the interface.30 Figure 3 summarizes the SET and RESET voltages versus the electronegativity for the TEs. As the electronegativity value of the TEs is decreased, for the Reset process, the electrochemical reaction occurs at a lower voltage because the low-electronegativity (chemically more active) TE can interact more effectively and strongly with oxygen ions nearby the interface. The reverse process can be similarly understood. Thus, the difference between the Set and Reset voltages becomes larger with increasing electronegativity of the TEs. The switching voltage values of W/WO3/Pt and Au/WO3/Au devices are taken from the results in refs 31 and 39, respectively. Even
Figure 3. Graph of the SET and RESET voltages vs top electrode electronegativity for the Al/WO3/Pt, Cu/WO3/Pt, Pt/WO3/Pt (solid circles: results in the present work), W/WO3/Pt, and Au/WO3/Au devices (open circles: refs 31 and 39). Electronegativity values are taken from ref 24.
when the different device configurations (i.e., thickness of WO3) are considered, these values are qualitatively consistent with the dependence of switching voltage on electronegativity of TE. Panels b and c of Figure 4 show the cumulative probabilities (or distributions) of the LRS and HRS currents at reading voltages of ±0.5 V (as illustrated in Figure 4a) for the RS devices. As the voltage-bias sweeping operation continues, the LRS and HRS currents for the Al/WO3/Pt and Cu/WO3/Pt devices are quite stable, and those for the Pt/WO3/Pt device fluctuate significantly from their initial values. The LRS and HRS currents in both bias voltage polarities should be uniform within a tolerable range for the devices to be used in nonvolatile memory applications, and these results suggest that TEs with a low electronegativity are suitable in this aspect. Panels a−c of Figure 5 show the endurance characteristics of the three Al/WO3/Pt, Cu/WO3/Pt, and Pt/WO3/Pt devices measured at a 0.1 V read voltage. The plot shows the SET and RESET currents as a function of the switching cycles. The SET and RESET currents in the LRS and HRS states for the Al/ WO3/Pt (Figure 5a) are quite stable. The difference in the switching current between the LRS and HRS states (i.e., the on−off switching ratio) is found to be greater than 10 over 200 cycles. For a practical NVM device, both of the current states must be stable, and the current dispersion should be narrower. In the case of the Cu/WO3/Pt and Pt/WO3/Pt devices, the current values for the HRS states fluctuate while those for the LRS states remain stable (Figure 5b,c). As the switching cycle progresses in the LRS state of the Cu/WO3/Pt device (Figure 5b), the current values start to decrease gradually after the initial 100 cycles while the Pt/WO3/Pt device (Figure 5c) maintains an on−off switching ratio on the order of 10. Thus, the above discussion reveals that the Al/WO3/Pt device exhibits excellent endurance characteristics at over 200 cycles when compared to the other two devices. The retention properties of the Al/WO3/Pt, Cu/WO3/Pt, and Pt/WO3/Pt devices are measured in their LRS and HRS states under a constant readout voltage of 0.1 V. Figure 5d−f shows the time-dependent retention properties in the LRS and HRS states for all three devices measured at room temperature. The retention properties in the LRS state for all three devices are more stable relative to those in HRS, and all three samples maintain sufficiently large resistance ratios (stable retention) for over 30 000 s, resulting in nonvolatile behavior for the resistive switching. The current level in the LRS state of Pt/WO3/Pt is much higher than that of the other two devices, and this may be C
DOI: 10.1021/acsami.5b11781 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 4. Cumulative probabilities of the (a) LRS (circles) and HRS (squares) currents at ±0.5 V. (b) Probability plots of LRS and (c) HRS currents for the Al/WO3/Pt, Cu/WO3/Pt, and Pt/WO3/Pt devices. The solid and open symbols represent the positive- and negative-bias voltage regions, respectively.
due to the similarity in the work function of the top and bottom electrodes. This indicates that all three devices have stable retention with no obvious degradation and also have excellent durability and reliability. Temperature-dependent resistive switching current measurements are carried out to understand the transport mechanism of the Al/WO3/Pt, Cu/WO3/Pt, and Pt/WO3/Pt devices. Figure 6a−c show the normalized current values (I/Imax) measured at a fixed voltage of 0.1 V as a function of the temperature in the initial state (IS), LRS, and HRS, respectively. Regardless of the resistance states, the electrical conductivity of all three devices decreases as the temperature decreases, indicating typical thermally activated transport characteristics.32 The estimated thermal activation energies are small, typically below 10 meV, and the thermally activated transport in these samples at various resistance states seems to be independent of the TE that is used (see Figure S6). Thus, the thermally activated transport behavior that is observed is
Figure 5. Endurance properties of the (a) Al/WO3/Pt, (b) Cu/WO3/ Pt, and (c) Pt/WO3/Pt devices. Retention properties of the (d) Al/ WO3/Pt, (e) Cu/WO3/Pt, and (f) Pt/WO3/Pt devices. Endurance and retention tests are performed by measuring the LRS and HRS currents at 0.1 V.
Figure 6. Temperature dependences of the normalized current in each resistance state (IS, LRS, and HRS) for the (a) Al/WO3/Pt, (b) Cu/WO3/Pt, and (c) Pt/WO3/Pt devices. Log−log scale I−V curves in a positive bias region for the (d) Al/WO3/Pt, (e) Cu/WO3/Pt, and (f) Pt/WO3/Pt devices. D
DOI: 10.1021/acsami.5b11781 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 7. Measured switching times of the SET (HRS → LRS) and RESET (LRS → HRS) processes for the (a) Al/WO3/Pt, (b) Cu/WO3/Pt, and (c) Pt/WO3/Pt devices. (d) Switching time displayed as a function of the electronegativity of the top electrode.
SET and RESET processes. The switching time is defined as the pulse-duration time when the voltage-pulse-induced current of one resistive state reaches the preset current value of the other resistive state. The Al/WO3/Pt device has faster switching times of 100 (SET) and 140 ns (RESET) (Figure 7a). In contrast, the other devices, Cu/WO3/Pt (Figure 7b) and Pt/WO3/Pt (Figure 7c) exhibit values of 150 and 400 ns (SET) and 400 and 550 ns (RESET), respectively. Figure 7d presents a summary of the switching time measurements with respect to the electronegativity of the TEs. The switching time clearly increases as the electronegativity of the TEs increases, and the faster switching time of the Al/WO3/Pt device is presumably due to the fast oxygen redox reaction at the Al− WO3 interface resulting from a low electronegativity (high attractability of oxygen ions) of Al TE relative to that of the other two TEs. On the basis of the results above, we proposed a model for resistance switching for Al/WO3/Pt, Cu/WO3/Pt, and Pt/ WO3/Pt devices. Figure 8 shows the schematics of the proposed switching model. The defect-induced path is created during the forming process, and this is considered to be a result of the soft dielectric breakdown in the metal−insulator−metal ReRAM structures. It is noted that, as the electronegativity of the TEs increases (Al < Cu < Pt), the forming voltage tends to increase (see Figure S2). Evolution of conducting channels and its effect on RS were observed in TiO2- and WO3-based bipolar RS devices.38,39 The resistive switching mechanism is mainly based on oxygen ion migration at the interface, and it should be noted that an oxygen diffusion layer can form at the interface between the top electrode and the WO3 layer. The interfacial oxygen diffusion layer forms as a result of the interdiffusion of oxygen from WO3 to TE during the deposition of the TE layer, and this oxygen interdiffusion creates oxygen vacancies in the WO3 layer. When a positive voltage bias is applied, the oxygen ions from WO3 migrate toward the low-electronegativity electrode (Al or Cu), causing the formation of an oxygendeficient layer (that is, with oxygen vacancies) in the WO3 layer, which thus results in a decrease in the resistance of the device and causes switching from HRS to LRS (the so-called
presumably due to electron hopping through the defects (mainly oxygen vacancies). A similar type of hopping effect is observed in Al-embedded TiO2 films, but in this case, conduction at lower voltages is due to electron-hopping in the oxygen vacancies and to the tunneling effect at higher voltages. In addition, the hopping rate of the oxygen vacancies was studied according to the temperature on the TiO2 surface.33,34 The electrical transport of the measured switching current can be explained by several conduction mechanisms.35 Figure 6d−f shows the first HRS and LRS I−V curves replotted in the log−log scale in the positive voltage region. The slopes of the LRS and HRS are about ∼1 in the low voltage region, following Ohm’s law (I∝∼V1). The current increases as the voltage increases, following the square dependence on the voltage. In this regime, the transport mechanism changes from Ohmic conduction to trap-controlled space charge limited conduction (SCLC) by shallow traps.36 In the higher voltage region, the HRS current rises rapidly because all traps have been filled. High voltages need to be applied to the excess charges in the traps, and this is referred to as the trap-filled limit (TFL). The TFL region is gradually reduced with a slope of the current of about ∼2, which means that the trap-free SCLC is present following Child’s law (I∝∼V2). However, the 30th slope of the Pt/WO3/Pt device is lower than first slope in a high-voltage region, which is related to the trapped carriers that are released in the HRS due to a steeper slope of the SCLC process (see Figure S3). Thus, the following process describes the conduction mechanisms for voltage-sweeping transport: Ohmic → SCLC (controlled by a single shallow trap) → TFL → SCLC (controlled by Child’s law) → Ohmic. The switching time is an important parameter for ReRAM devices, and we measured the SET (HRS → LRS) and RESET (LRS → HRS) switching times for all three Al/WO3/Pt, Cu/ WO3/Pt, and Pt/WO3/Pt devices by measuring the voltagepulse induced RS I−V characteristics as a function of the pulse duration (see Figures S4 and S5). The details of the experiment are described in ref 37. Figure 7 shows the variation in the current as a function of the voltage-pulse duration for both the E
DOI: 10.1021/acsami.5b11781 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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at the TE−WO3 interface that takes into account the effect of the electronegativity of the TE in resistive switching.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b11781. Figures showing RBS of the WOx film, forming voltage properties of the different electronegativities of the top electrodes, log−log scale I−V characteristics of the 1st and 30th resistive switching I−V curves, applied pulsed voltage patterns, I−V curves after voltage pulse operation, temperature dependence of the measured current at 0.1 V and corresponding I−V curves, and thermal activation energy of the RS devices. (PDF)
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Figure 8. Schematic of the proposed switching model for the SET and RESET processes. During the RESET process, an interfacial charge blocking layer is formed. The defects-induced transport path (dotted lines) includes charge hopping via defects states, as described in the text.
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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SET process). When the polarity of the applied voltage is reversed, the oxygen ions from the top electrode migrate toward the WO3 layer. The drift in the oxygen ions toward the WO3 make the device switch back to HRS (RESET process).40 The electrical nature of the interfacial layer controls a flow of electric charge, and one of crucial factors to determine the electrical nature is the electronegativity of the metal electrode. The results for the switching time indicate that the lowelectronegativity electrode (Al or Cu) devices have a fast switching behavior due to the fast electrochemical reaction at the interface. For the Pt/WO3/Pt device, the same oxygen ion migration for the switching process is slower than in the other two devices due to the higher electronegativity of Pt, leading to a slower electrochemical reaction. The lower electronegativity of Al and Cu relative to that of Pt result in an electrochemical reaction at the Al−WO3 and Cu−WO3 interfaces that is somewhat faster than that at the Pt−WO3 interface, which in turn causes shorter resistive switching times for the Al/WO3/Pt and Cu/WO3/Pt devices. This is consistent with the model that shows that the bipolar resistive switching properties of these devices are a result of the redox reaction at the interface between the TE and WO3 layer.
ACKNOWLEDGMENTS This project was supported by the National Research Foundation (NRF) of Korea (grant nos. 2015R1A2A2A01004782, 2015M2A2A6A02045252, 2015R1D1A1A01058851, and 2015R1D1A1A01060743).
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4. CONCLUSIONS The effect that the electronegativity of the metal top-electrode (TE) has on the resistive-switching properties in a TE/WO3/Pt capacitor structure is investigated by using three different TE electrodes: Al (χ = 1.5), Cu (χ = 1.9), and Pt (χ = 2.2). The results of the experiment and a subsequent analysis indicate that the electronegativity (χ) of the TEs plays an important role in determining their resistive switching properties. As the electronegativity of TE decreases, the SET and RESET processes occur in a more stable manner around well-defined switching voltages. Thus, we obtain a relative improvement in the endurance and retention characteristics with less current dispersion for the device with a low-χ TE (Al/WO3/Pt device). The Al/WO3/Pt device also shows faster switching times, 100 (SET: HRS → LRS) and 140 ns (RESET: LRS → HRS) relative to the other two devices. We therefore propose a resistive switching model based on the redox chemical reaction F
DOI: 10.1021/acsami.5b11781 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsami.5b11781 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
