Revelation on the Interrelated Mechanism of Polarity-Dependent and

Feb 22, 2013 - Revelation on the Interrelated Mechanism of Polarity-Dependent and Multilevel Resistive Switching in TaOx‑Based Memory Devices...
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Revelation on the Interrelated Mechanism of Polarity-Dependent and Multilevel Resistive Switching in TaOx‑Based Memory Devices Ying-Chuan Chen,† Yu-Lung Chung,† Bo-Tao Chen, Wei-Chih Chen, and Jen-Sue Chen* Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan S Supporting Information *

ABSTRACT: In this study, polarity dependent and multilevel resistive switching characteristics of the Ta/TaOx/Pt device are investigated. The resistive switching polarity is decisively associated with the location of the Ta/ TaOx interface while at a specific switching polarity, and multiple (four) resistance states in the Ta/TaOx/Pt device are achieved by controlling the stop voltage during the reset process and are stable for more than 104 s under continuous readout testing. X-ray photoelectron spectroscopy reveals that the Ta/TaOx interface is rich in oxygen vacancies. A schematic on the configurations of oxygen-vacancy aggregated conducting paths is proposed, where the Ta/TaOx interface acts as an oxygen vacancy reservoir, and the migration of vacancies can be driven by the applied electric field as well as the vacancy concentration gradient emanated from the Ta/TaOx interface. The polarity-dependent and multilevel resistive switching behaviors are then discussed in terms of filament configuration variation, and their interrelationship is visibly unveiled.

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INTRODUCTION Among the numerous nonvolatile memory devices, resistance random access memory (RRAM), which utilizes the resistance change to store cell information, is widely investigated for nextgeneration memory. RRAM research to date has mainly focused on developing high-density storage capability,1 fast switching speed,2 and low voltage/current switching.3 Additionally, the multilevel resistive switching behaviors had been investigated for high-density data storage application by controlling either set/reset voltage or set/reset current compliance.4−7 Regarding the multilevel resistive switching mechanism, it was reported that the resistivity of a conductive filament may be varied due to a change of the chemical composition of the filament by Joule heating.8 However, the multilevel resistive switching mechanism of RRAM is still under investigation. Recently, the TaOxbased memory devices are frequently investigated because of its excellent switching endurance and fast switching speed characteristics.9,10 In this work, we report the electrical characteristics of a Ta/TaOx/Pt device, which demonstrates bias polarity-dependent (both bipolar and unipolar) resistive switching characteristics and a multilevel resistive switching behavior achieved by controlling the sweeping-stop-voltage during the reset process (Vreset‑stop) in bipolar mode. Resistive switching behaviors in the Ta/TaOx/Pt device are referred to the role of a thin interface layer between the Ta electrode and the TaOx layer (denoted as the “Ta/TaOx interface” hereafter). Attainment of multiple resistance states suggests that configuration of conducting paths can be modified by the magnitude of the reset voltage. The polarity-dependent and multilevel resistive switching behaviors are discussed in terms of configuration of oxygen-vacancy aggregated conducting path © 2013 American Chemical Society

(filament), while the Ta/TaOx interface acts as an oxygen vacancy reservoir and the supply of vacancies can be regulated by the polarity and magnitude of applied voltage. As a comparison, the memory devices of Ta/TaOx/Ta (with two Ta/TaOx interfaces) and Pt/TaOx/Pt (without any Ta/TaOx interface) are also fabricated, but they fail to exhibit resistive switching behaviors, which confirm the role of Ta/TaOx interface as an oxygen vacancy reservoir. This research provides the experimental evidence on the correlation between the metal/oxide interface and multilevel switching properties, as well as offer an opportunity to fabricate high-density data storage memory devices.

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EXPERIMENTAL SECTION First, an 150 nm Pt was deposited on the Ti/SiO2/Si substrate as the bottom electrode (BE). Then, the TaOx thin film with thickness of ∼65 nm was deposited on the Pt BE under an Ar/ O2 (1:1) ambient mixture by reactive sputtering from a Ta target. After that, a 100 nm-thick Ta, top electrode (TE) was patterned through a shadow mask with an area of 4 × 104 μm2. For comparison, two additional metal−insulator−metal (MIM) structures of Ta/TaOx/Ta and Pt/TaOx/Pt were fabricated for electrical measurement. I−V characteristics of all devices were measured using an Agilent 4156C precision semiconductor parameter analyzer. Chemical bonding states of Ta/TaOx/Pt and Pt/TaOx/Pt samples were characterized by X-ray photoelectron spectroscopy (XPS, JEOL, JAMP-9500F). The probe Received: December 5, 2012 Revised: January 23, 2013 Published: February 22, 2013 5758

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size and depth resolution of XPS are 6 mm in diameter and ∼5 nm in depth, respectively. In order to differentiate the chemical information between the top interface and the bulk film, the preparation of Ta/TaOx/Pt and Pt/TaOx/Pt samples for XPS analysis is slightly different from the device sample for electrical measurement. For XPS samples, the Si substrates are about 1 × 1 cm2. The layered structure is almost the same as that of the device samples except that the top Ta (or Pt) film (to simulate TE) is only ∼10 nm and not patterned. Ar ion milling (done in XPS vacuum chamber) was employed to obtain the depth profiling spectra.

Table 1. Calculated Area Ratio of Deconvoluted Ta5+, Ta4+, and Ta2+ Subpeaks in Ta 4f XPS Spectra Obtained at the Top Electrode (Ta)/TaOx Interface and in the Bulk TaOx of the Ta/TaOx/Pt Sample

RESULTS AND DISCUSSION Figure 1a shows the Ta 4f XPS spectra (Ta 4f5/2 and Ta 4f7/2 doublet peaks) at the TE/TaOx interface and in TaOx bulk of

Pt sample at the TE/TaOx interface and in TaOx bulk are also investigated, as shown in Figure 1b. The sum of intensity ratio of Ta4+and Ta2+ subpeaks at interface layer (50.55%) and in the bulk (49.03%) are almost the same (data shown in Table 2). The result indicates that oxygen vacancy concentration at the Pt/TaOx interface and in the bulk TaOx layer are about identical.

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position 1 (near Ta/TaOx interface)

Ta5+

Ta4+

Ta2+

binding energy (eV) area (%) Position 2 (in the TaOx bulk)

26.71 33.49 Ta5+

24.49 19.61 Ta4+

22.85 46.90 Ta2+

binding energy (eV) area (%)

26.78 51.38

24.41 21.19

22.93 27.43

Table 2. Calculated Area Ratio of Deconvoluted Ta5+, Ta4+, and Ta2+ Subpeaks in Ta 4f XPS Spectra Obtained at the Top Electrode (Pt)/TaOx Interface and in the Bulk TaOx of the Pt/TaOx/Pt Sample position 1 (near Pt/TaOx interface)

Ta5+

Ta4+

Ta2+

binding energy (eV) area (%) position 2 (in the TaOx bulk)

26.82 49.45 Ta5+

24.4 22.34 Ta4+

22.86 28.21 Ta2+

binding energy (eV) area (%)

26.84 50.97

24.40 21.85

22.90 27.18

The resistive switching characteristics of the Ta/TaOx/Pt memory devices are executed under a direct current (dc) bias sweeping mode, in which the bias voltage was applied on the TE while the BE was grounded. The as-fabricated Ta/TaOx/Pt devices exhibit a high resistance state (HRS). Additionally, no initial electroforming process is needed for our fresh devices to complete the resistive switching (see Figure S1 in the Supporting Information). Figure 2a,b shows the I−V characteristics of Ta/TaOx/Pt devices operating in bipolar resistive switching mode by applying the opposite sweeping bias polarity. In Figure 2a, as the voltage sweeps from zero to a positive value, a sharp increase in current appears at ∼3.5 V, indicating that the device switches from HRS to low resistance state (LRS). In LRS, a current compliance value of 1 mA is applied to prevent the device breakdown. During negative voltage sweeping, the current compliance is released to 100 mA, and the current first increases rapidly from V = 0 to −1 V, but then gradually deceases from V = −1 to −4 V. At this moment, the device is switching from LRS to HRS. Thus, the bipolar resistive switching is achieved. On the other hand, the bipolar switching is also executed with the bias polarity opposite to that of Figure 2a. As shown in Figure 2b, the device can be switched to LRS with negative bias sweeping but fails to be reset to HRS with positive bias sweeping. By contrast, Figure 2c,d shows the unipolar resistive switching behavior. Figure 2c shows the unipolar resistive switching by applying a positive bias sweeping. The switching-on process is reached, but the reset process is failed. In Figure 2d, the Ta/TaOx/Pt device is switched to an LRS when a set voltage of about −3.5 V is reached. Once the transition is achieved, the device remains at the LRS even when the applied voltage is removed. When the

Figure 1. Ta 4f XPS spectra taken near the top electrode/TaOx interface (upper) and in the bulk of TaOx (lower) of (a) Ta/TaOx/Pt, and (b) Pt/TaOx/Pt samples.

the Ta/TaOx/Pt sample. The surface of the Ta top layer was oxidized by exposing the XPS samples to air (2−5 days before carrying out XPS analysis). Therefore, Ar ion milling was employed to remove the oxidized surface and obtain spectra at various depths. The spectra associated with the TE/TaOx interface and bulk TaOx were obtained after Ar ion milling of ∼30 s and ∼70 s, respectively. The spectra are deconvoluted to three pairs of Ta 4f doublet peaks pertaining to three oxidation states: Ta5+ (Ta2O5), Ta4+ (TaO2), and Ta2+ (TaO). The Ta 4f7/2 peaks associated with Ta5+, Ta4+, and Ta2+ are located at 26.78 eV,11 24.45 eV,12 22.93 eV,13 respectively. On the basis of the Ta 4f7/2 spectra, the concentration of suboxide can be represented by the sum of intensity for Ta4+ and Ta2+ (TaO2, TaO) signals. The area ratio of Ta4+ + Ta2+ subpeaks is about 66.51% at the interface layer, which is greater than that (48.62%) in the bulk (data shown in Table 1). The result indicates that oxygen vacancy concentration at the Ta/TaOx interface is much greater than that in the bulk TaOx layer. This vacancy-rich Ta/TaOx interface should be generated by the oxidation of Ta TE or interdiffusion between the Ta TE and the TaOx layer. In addition, Ta 4f XPS spectra of the Pt/TaOx/ 5759

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Figure 2. I−V characteristics of Ta/TaOx/Pt device. (a,b) Bipolar resistive switching by applying opposite bias polarities. (c,d) Unipolar resistive switching by applying identical bias polarities.

Figure 3. I−V characteristics of Ta/TaOx/Ta device. (a,b) Bipolar resistive switching by applying opposite bias polarities. (c,d) Unipolar resistive switching by applying identical bias polarities.

asymmetrical TE and BE interfaces. To find out the role of electrode/oxide interfaces in resistive switching, two more MIM structures, Ta/TaOx/Ta and Pt/TaOx/Pt, are investigated. Figure 3a−d shows the I−V characteristics of the Ta/ TaOx/Ta device. This device can only be switched from the initial HRS to LRS by sweeping the voltage to about +4 or −4 V. However, the device cannot be switched back to HRS. On the other hand, a significantly large voltage (over 10 V) is needed to switch the Pt/TaOx/Pt device from the initial HRS to LRS at either polarity, as shown in Figure 4a−d. Again, the

current compliance is released to 100 mA, the device can be switched back to the HRS by applying again a negative voltage sweeping to about −1 V. Thus, the unipolar resistive switching is achieved. On the basis of the above results, we found that the resistive switching behaviors in Ta/TaOx/Pt are critically dependent on the bias polarity. This bias polarity-dependent resistive switching phenomenon suggests that there exists an electric field dependent mechanism, and it shall also associate with the electrode/oxide interfaces because the Ta/TaOx/Pt device has 5760

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Figure 4. I−V characteristics of Pt/TaOx/Pt device. (a,b) Bipolar resistive switching by applying opposite bias polarities. (c,d) Unipolar resistive switching by applying identical bias polarities.

force and concentration gradient. The concentration gradient will drive massive oxygen vacancies out from the Ta/TaOx reservoir, which will develop an inverted conical shape filament and incorporate with conical shape filament formed by oxygen vacancies originally in TaOx. Consequently, an hourglass sharp filament is established, as depicted in Figure 5b. Thus, the Ta/ TaOx/Pt device is switched from HRS to LRS. In the reset process, oxygen vacancies in the filament are mostly driven back to the reservoir at the Ta/TaOx interface under reversing the bias polarity, and a minor part of them are dispersed in the bulk TaOx region via Joule heating. Hence, the device returns to its initial HRS state, as shown in Figure 5c. Figure 5d shows that the device is switched to LRS under a negative bias (corresponding to Figure 2b). Because the oxygen vacancies in Ta/TaOx interface reservoir will be constrained by the negative bias, only a small amount of oxygen vacancies diffuse out of the reservoir due to concentration gradient to form a part of the filament. The rest of the filament is formed via the aggregation of oxygen vacancies originally in the TaOx layer. Thus, a thin filament is established. When applying a positive bias on Ta TE, the oxygen vacancies will migrate in a way similar to the case of Figure 5b; the filament will be thicker, as shown in Figure 5e. Thus, the filament cannot be ruptured in this bias polarity, and the device will stay at the LRS permanently, as revealed in Figure 2b. In contrast to bipolar resistive switching, the unipolar resistive switching mechanism is shown in Figure 5f,g and h,i. The principle of building a filament is still the same: a positive bias on the Ta TE builds an hourglass shape thick filament, while a negative bias on the Ta TE can only form a thin filament. Accordingly, with double positive bias sweepings, a thick filament is built up at the first positive voltage sweeping, and it will become even thicker at the second sweeping, as shown in Figure 5f and 5g. Therefore, the device can be switched to LRS but cannot be switched back to HRS, as revealed in Figure 2c. Conversely, with double negative bias sweepings, a filament is set up, but it will be easily ruptured at

Pt/TaOx/Pt cannot be switched back to HRS, possibly due to the permanent breakdown under high electric field. Thus, the switching behavior of the Pt/TaOx/Pt device is not observed either. These experimental results demonstrate that the electrode materials will critically decide the resistive switching characteristics. Accordingly, we establish the following models to explain the switching mechanism of TaOx-based memory devices. Since the Ta/TaOx interface contains a larger amount of oxygen vacancies than the bulk TaOx layer (according to XPS analysis), the oxygen vacancies are delineated into two categories: the high-density group at the Ta/TaOx interface, and the low-density group in the TaOx layer. The density of oxygen vacancies at the Pt/TaOx interface is the same as that in the TaOx layer since there is no oxidation of Pt at the Pt/TaOx interface. Figure 5a schematically shows the initial HRS of a Ta/TaOx/ Pt device characterized with the high-density oxygen vacancy group at the top Ta/TaOx interface and the low-density oxygen vacancy group within the TaOx layer. According to XPS analysis (Figure 1), we treat the Ta/TaOx interface as an oxygen vacancy reservoir to supply and store oxygen vacancies, similar to the proposed models in the literature.14,15 Note that the vacancies being driven from the Ta/TaOx reservoir into the TaOx layer correspond to the migration of oxygen ions from the TaOx layer to the reservoir. When a positive bias is applied on Ta TE, the positively charged oxygen vacancies are driven toward Pt BE. Kim et al.16 reported that oxygen vacancies will pile up locally at the cathode (negatively biased electrode) due to the electrostatic force of the applied electrical field as well as the concentrated electron injection at local regions. The pile-up vacancies will then propagate to the anode to develop conical shape filaments. In our case, the exceedingly high oxygen vacancy concentration at the Ta/TaOx interface will produce a concentration gradient; therefore, when a positive bias is applied on Ta TE (corresponding to Figure 2a), the oxygen vacancies are driven toward the Pt BE via both electrostatic 5761

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the second negative bias sweeping because of the short supply of oxygen vacancies and Joule heating effect,17,18 as shown in Figure 5h,i. Hence, the resistive switching behavior can be attained, as showed in Figure 2d. The above analysis indicates that the existence of the Ta/ TaOx interface, which acts as an oxygen vacancy reservoir, will dominate the resistive switching behavior under different bias polarities. This proposition is confirmed by the switching characteristics of the Ta/TaOx/Ta device (see Figure 3). This device can be switched from initial HRS to LRS at 3−4 V but fails to be switched back because the two Ta/TaOx interfaces in the Ta/TaOx/Ta device will provide a large quantity of oxygen vacancies and lead to development of a thick filament at either positive bias or negative bias polarity (see Figure S2 in the Supporting Information). Therefore, the Ta/TaOx/Ta device fails in resistive switching. In addition, the failure of resistive switching is also observed in the Pt/TaOx/Pt device (see Figure 4). Because the two Pt/TaOx interfaces cannot supply oxygen vacancies, the filament is difficult to form in the TaOx layer. Therefore, a large set voltage (more than 10 V) is needed to switch on the Pt/TaOx/Pt device at either polarity, but the device cannot be switched off. This high set voltage possibly brings a permanent breakdown. Thus, the Pt/TaOx/Pt device does not exhibit switching behavior. Accordingly, only the Ta/TaOx/Pt device can be successfully operated for resistive switching because it contains one, and only one, critical Ta/TaOx interface. Although the Ta/TaOx/Pt device can be switched either by positive-set/negative-reset or negative-set/negative-reset operations, the reset current undergoes gradual changes in Figure 2a, while the reset current exhibits a sudden drop in Figure 2d. The gradual changes in reset current indicate the opportunity for attaining multilevel switching (MLS), which has been reported in literature.4,19,20 The MLS characteristics of Ta/TaOx/Pt device are carried out in bipolar operation mode, as depicted in Figure 6a. The I−V curve of MLS is obtained by controlling the Vreset‑stop during reset process, where Vreset‑stop are −2 V, −3 V, and −4 V for

Figure 5. Schematics of the resistive switching mechanism in a Ta/ TaOx/Pt device under four different bias polarities. (a) At initial HRS, a thin layer marked at the Ta/TaOx interface denotes the high-density oxygen vacancy region (i.e., the vacancy reservoir). The dark blue and light blue circular dots represent oxygen vacancies of the TaOx layer and Ta/TaOx interface, respectively. (b) An hourglass sharp filament is formed under application of a positive bias on Ta TE. (Red arrow indicates the migration direction of oxygen vacancies owing to the bias field.) (c) Filament is disassembled via back drifting of oxygen vacancies and Joule heating effect by applying negative bias on TE. (d) A thin filament is formed via the oxygen vacancies only from the TaOx layer under negative bias, and (e) the thin filament becomes a thicker filament under positive bias. (f) An hourglass shape filament is formed when applying a positive bias. (g) An even thicker filament is developed at the second positive voltage sweeping. (h) A thin filament is set up under negative bias, and (i) the thin filament is ruptured by the high current Joule heating during a second negative bias sweeping.

Figure 6. (a) Demonstration of MLS by varying the spans of voltage during the reset process. (b) Retention tests show no significant change of four different resistance states at a read voltage of 0.05 V. 5762

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Figure 7. Schematic illustration for the mechanism of MLS in a Ta/TaOx/Pt device. (a) An hourglass shape filament formed under application of positive voltage on the Ta TE in the set process to reach LRS3. (b) The filament changes from hourglass shape to an inverted conical shape by applying −2 V on the Ta TE, and the resistance state switches from LRS3 to LRS2. (c) By further increase of the negative bias to −3 V, the filament changes from the inverted conical sharp to a thin cylinder, and the resistance state switches from LRS2 to LRS1. (d) Vacancies are dispersed, and the filament is disassembled at −4 V. The resistance state switches from LRS1 to HRS.

LRS2, LRS1, and HRS, respectively. Figure 6b demonstrates the nondestructive readout properties of the Ta/TaOx/Pt device, corresponding to LRS3, LRS2, LRS1, and HRS. LRS3 refers to the LRS in the previous discussion (Figures 2 and 5) in order to distinguish it from the other two less conductive states (LRS2 and LRS1). All memory states are distinguishable under constant readout voltage at 0.05 V, and there is no significant degradation for each memory state over 104 s. This result indicates that that Ta/TaOx/Pt device has good potential for MLS applications. Regarding to the mechanism of MLS, we suggest that the variation of resistance states is originated from the change of filament configuration. Therefore, there will be two intermediate steps between the schemes shown in Figure 5b,c, and the switching mechanism is redepicted in Figure 7a−d. Figure 7a shows the filament formed in the TaOx layer at LRS3 under application of a critical voltage on the Ta TE in the set process. As discussed above, the filament is in hourglass shape and composed of vacancies from the Ta/TaOx reservoir and within the TaOx. In the reset process, the oxygen vacancies of the Ta/ TaOx interface are gradually driven back to the Ta/TaOx interface, the filament changes its configuration from hourglass shape to an inverted conical shape, as shown in Figure 7b, and the resistance state at this moment corresponds to LRS2. As the negative bias is further increased, the filament changes from the inverted conical sharp to a rather thin cylinder, as shown in Figure 7c (the resistance state is corresponding to LRS1). Finally, the vacancies are dispersed, and the filament is disassembled, as shown in Figure 7d. Thus, the device changes from LRS1 to the HRS. The above analysis reveals that the MLS is attained if the filament configuration can be gradually modified. This modification is realized when an oxygen vacancy reservoir, the Ta/TaOx interface in this study, will supply and “gradually” take up oxygen vacancies in cooperation with an adequate bias polarity. Note that, although the device can be switched with an unipolar operation (negative-set/negative-reset), MLS is not attainable for this switching polarity because the filament built at the negative bias is too thin and will be ruptured instantly (as illustrated in Figure 5h,i). Therefore, our proposed scheme evidently reveals that the MLS of the Ta/TaOx/Pt device is correlated with its bias polarity-dependent switching behavior, which fundamentally arises from the regulation of oxygen vacancy migration.

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CONCLUSIONS

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ASSOCIATED CONTENT

Bias polarity-dependent resistive switching and multilevel (four levels at least) resistive switching are demonstrated for data storage in a Ta/TaOx/Pt device. The polarity dependent and multilevel switching behaviors are interrelated and decisively associated with the Ta/TaOx interface. According to XPS analysis, the Ta/TaOx interface is rich in oxygen vacancies; therefore, the Ta/TaOx interface acts as an oxygen vacancy reservoir that may supply and store oxygen vacancies. Simultaneously, an oxygen vacancy concentration gradient is created due to the presence of a vacancy reservoir. Our proposed scheme visibly demonstrates that the polaritydependent and multilevel switching behaviors are interrelated, and both depend on the filament configuration arisen from the migration of oxygen vacancies, which are driven jointly by the vacancy concentration gradient emanated from the Ta/TaOx interface and bias polarity-induced electric field. As a result, the Ta/TaOx/Pt device performs a distinguishable and nondestructive multilevel readout property that gives this device promising potential for nonvolatile memories, particularly for the fabrication of high-density data storage applications.

S Supporting Information *

Additional results and graphs are available in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone number: +886-6-2757575, ext. 62948. Fax number: +886-6-2762541. E-mail address: jenschen@mail. ncku.edu.tw. Author Contributions †

The first two authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully appreciate the financial support from the National Science Council of Taiwan (Grant No. NSC-1002628-E-006-026-MY3). 5763

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(20) Wang, S. Y.; Huang, C. W.; Lee, D. Y.; Tseng, T. Y.; Chang, T. C. Multilevel Resistive Switching in Ti/CuxO/Pt Memory Devices. J. Appl. Phys. 2010, 108, 114110.

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