Balancing the Source and Sink of Oxygen ... - ACS Publications

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Functional Inorganic Materials and Devices

Balancing the source and sink of oxygen vacancies for the resistive switching memory Taehyung Park, Young Jae Kwon, Hae Jin Kim, Hyo Cheon Woo, Gil Seop Kim, Cheol Hyun An, Yumin Kim, Dae Eun Kwon, and Cheol Seong Hwang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05031 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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ACS Applied Materials & Interfaces

Balancing the source and sink of oxygen vacancies for the resistive switching memory Tae Hyung ParkÁ, Young Jae KwonÁ, Hae Jin Kim, Hyo Cheon Woo, Gil Seop Kim, Cheol Hyun An, Yumin Kim, Dae Eun Kwon, and Cheol Seong Hwang* Department of Materials Science and Engineering, and Inter-University Semiconductor Research Center, Seoul National University, Seoul 08826, Republic of Korea *E-mail: [email protected] KEYWORDS Resistive switching; oxygen vacancy sink; oxygen vacancy source; negative set; uniformity; endurance

ABSTRACT

The high non-uniformity and low endurance of the resistive switching random access memory (RRAM) are the two major remaining hurdles at the device level for mass production. Incremental step pulse programming (ISPP) can be a viable solution to the former problem, but the latter problem requires a material-level innovation. In valence change RRAM, electrodes have usually been regarded as inert (e.g., Pt or TiN) or oxygen vacancy (VO) sources (e.g., Ta), but different electrode materials can serve as a sink of VO. In this work, an RRAM using a 1.5nm-thick Ta2O5 switching layer is presented, where one of the electrodes was VO-supplying Ta and the other was either inert TiN or VO-sinking RuO2. While the TiN could not remove the excessive VO in the memory cell, the RuO2 absorbed the unnecessary VO. By carefully tuning (balancing) the

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capabilities of VO-supplying Ta and VO-sinking RuO2 electrodes, an almost invariant ISPP voltage and a greatly enhanced endurance performance can be achieved.


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I. Introduction In oxide-based resistive switching random access memory (RRAM), the oxygen vacancy (VO) plays a crucial role in controlling the device performance.1,2 The VO¶V FDQ EH VXSSOLHG IURP WKH VO source (e.g., the Ta electrode) in the valence change mechanism (VCM) cell 3,4 or electrically produced within the switching oxide layer (e.g., TiO2)

1,5,6

during the electroforming process of

the thermochemical mechanism (TCM) cell. The adoption of the VO source in the memory cell is beneficial in achieving switching uniformity and reliability.7-9 The supplied VO¶V IRUP D FRQGXFWLQJ FKDQQHO FDOOHG ³FRQGXFWLQJ ILODPHQW &) ´ LQ WKH LQVXODWLQJ R[ide layer, which causes the memory cell to be switched on under the condition that positive bias was applied to the V O source electrode (e.g., Ta).4,8 In the VCM cell, the bias application with opposite polarity partly ruptures or contracts the CF, switching off the memory cell by driving back a certain portion of WKH 9R¶V IURP WKH &) WR WKH 9O source.2,10 While this is a straightforward switching mechanism, the involvement of adverse effects related with the lateral diffusion of the VO¶V LQWR WKH QRQ-CF region renders the actual operation complicated and less reliable.11 Ta2O5 is an appealing contender for the switching oxide layer in the VCM memory, where the switching performance has been significantly improved by adopting a TaO2 VO reservoir layer 7,12 or operated in the self-regulated scheme.9 Nonetheless, the device eventually fails after the H[WHQGHG RSHUDWLRQ RI WKH VZLWFKLQJ F\FOHV ZKLFK LV XVXDOO\ DFFRPSDQLHG E\ WKH ³QHJDWLYH VHW´ behavior.13-16 The negative set behavior indicates that the memory cell resistance decreases when the reset (switching from low-resistance state [LRS] to high-resistance state [HRS]) voltage increases. The detailed mechanism of such adverse effect is complicated, but it can be understood as follows. In many VCM cells, the CFs tend to have an hourglass shape, where two portions competitively extend or retract depending on the bias polarity (set [switching from HRS to LRS]


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or reset voltages).17-19 If the growing speed of one of the two portions exceeds the retraction speed of the other portion under a given bias condition, the cell sets, and if the opposite is the case, the cell resets.11 A negative set implies that the speed of the re-growing portion of the CF is faster than the retracting portion after the weakest portion of the connected hourglass-shaped CF is ruptured. Such adverse effect is the main failure mechanism of the Ta/Ta2O5/TiN VCM cell, where Ta is the VO source electrode and TiN is the inert electrode.20 Such adverse effect is much further aggravated when the non-CF region near the CF contains a higher concentration of VO¶V 7KLV LV EHFDXVH WKH higher VO concentration in the non-CF region, which constitutes a parallel resistance to the CF, increases the leakage current, which is accompanied by the increased set and reset voltages. The higher the voltage is, the higher the power consumption, which is accompanied by the further increase in the VO concentration in the non-CF region due to the higher lateral diffusion of the VO. Even when the closed loop pulse switching (CLPS) technique, which is similar to the industrystandard incremental step pulse programming (ISPP) in the flash memory technology, is adopted, such adverse effect is inevitable.16 For the CLPS technique, the average number of pulses required in each switching step must be minimized to save on the operation time and energy, and to increase the total endurance cycle number. More importantly, if the supply voltage range keeps changing during the repeated switching cycles, the voltage interface circuit becomes complicated, degrading the overall circuit performance. In this regard, the role of the counterelectrode of the VO-supplying Ta electrode, which was TiN in the previous work16, should be reconsidered. That is, can it be an appropriate sink of the detrimental excessive VO¶V" ,I WKLV LV WKH FDVH WKH DERYH-mentioned endurance-related reliability problem can be resolved. In this work, the VO-sinking (or O-supplying) RuOx

”[”

HOHFWURGH

is suggested to be used as the counterelectrode. By appropriately matching the capabilities of the


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VO-supplying Ta and VO-sinking RuOx electrodes, not only the resistance values of the LRS and HRS (RLRS, RHRS) but also the set and reset (Vset, Vreset) pulse voltages can be kept constant up to 107 cycles in the CLPS tests. It should be noted that the endurance cycles were conducted only up to 107 cycles due to the experimental limitations (107 cycles in the CLPS set-up with 2 s-long pulse takes ~50 hrs).

II. Experiment Section The TiN BE device was fabricated by depositing 40nm-thick Pt/20nm-thick TiN BE, a 1.5nmthick Ta2O5 switching layer, and 20nm-thick Ta/40nm-thick Pt TE, in sequence. The material stack was patterned into a cross-SRLQW VWUXFWXUH ZLWK D PLQLPXP ZLGWK RI

P 3W 7iN electrode was

deposited through the DC sputtering (Ar atmosphere) and reactive sputtering methods (using a Ti target under an N2 atmosphere), at room temperature. The Ta2O5 layer was formed via PEALD at a substrate temperature of 200oC. The RuOx BE device was fabricated by replacing the TiN sputtering step with the RuOx sputtering step using a Ru target and an O2 gas flow (0-4 sccm). The details of the device fabrication are included in the SI-Figure-S4-related section. The AES depth profiles were obtained on a Pt(40nm)/Ta(20nm)/Ta2O5 (10nm)/SiO2/Si sample (PHI-700, ULVAC, Japan). As the Ta working pressure increased, the O atomic percent increased significantly at the Ta2O5 and Ta interfaces. The upper-electrode Pt was etched with aqua regia (HNO3:HCl=3:1) at room temperature for 50 minutes for the XPS measurement. It was then insitu-etched for 60 seconds using an Ar+ ion etch gun with 5 kV energy just before the XPS measurement, to remove the top Ta layer. An XPS image was acquired on an Axis Supra (Kratos, UK) equipment using a Monochromatic Al-..

N9

:

P 7KH ELQGLQJ HQHUJLHV

were corrected relative to the C1s signal at 285.0 eV.


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I-V sweep measurement was performed with a Hewlett Packard 4155B semiconductor parameter analyzer with TE biased and BE grounded. Pulse measurement was performed with an in-house-built field-programmable gate array board, which allowed a closed-loop configuration, where the feedback of the output resistance value to the input pulse height was included. The details of the electrical measurement are reported in.20,21

III. Results and Discussion Crossbar-type resistive switching devices with the inert TiN bottom electrode (BE, 20 nm thick) or VO-sinking RuOx BE were fabricated according to the process steps described in the experimental section. The x value was varied by changing the O2 gas flow rate during the reactive sputtering (Ru target). The 1.5nm-thick Ta2O5 switching layer grown through the plasma-enhanced atomic layer deposition (PEALD) method was the switching layer. Sputter-deposited Ta was used as the top electrode (TE) for both device types, but the sputtering gas (Ar) pressure was varied, which varied the remaining oxygen concentration in the deposited Ta film. Through this method, the VO supply capability of the Ta TE could be varied. The cross-sectional structure of the device is shown in Figure S1 in the on-line Supporting Information (SI). The RuOx layer with an O2 flow rate of y standard cubic centimeters per min (sccm) during the reactive sputtering is referred to as ³5X \ ´ DQG WKH 7D OD\HU ZLWK DQ $U ZRUNLQJ JDV SUHVVXUH RI ]Â

-2

WRUU LV UHIHUUHG WR DV ³7D ] ´

After electroforming by applying a positive bias to the Ta TE, a positive (negative) bias was applied to the TE to set (reset) both types of the fabricated device. Figure S2(a) of SI shows the current-voltage (I-V) characteristics of the set and reset switching of the TiN BE device (TiN/Ta 1.5: TiN BE, and Ta TE with the Ta prepared with a working pressure of 1 Â

-2

torr) with

different reset stop voltages. When the reset stop voltage was -1.5 V, the usual bipolar resistive


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switching (BRS) operation was observed (black data points). When the reset stop voltage was decreased (absolute value increased) to -1.8 V, however, a sudden current jump occurred (red data points), which corresponds to the negative set phenomenon. Such a narrow margin for the reset voltage (only ~0.3 V) can be a significant problem for a high-density device because the switching voltages always have distribution. Similar I-V sweep results for the RuOx BE device (Ru 3.5/Ta 1.5: RuO2 BE prepared with an O2 flow rate of 3.5 sccm, and Ta TE prepared at a working pressure RI

Â

-2

torr) were included in Figure S2(b), where the negative set was not observed at -1.8V.

Figure 1(a) shows the variations in the LRS/HRS resistances as a function of the switching cycle in the direct current (DC) measurement mode (black and red symbols for the TiN BE and RuOx BE devices, respectively). DC switching was SHUIRUPHG ZLWK

$ FXUUHQW FRPSOLDQFH

during the set switching and -1.5 V stop voltage during the reset process with no current compliance condition. As the switching cycle increased, both the LRS and HRS resistance of the TiN BE device decreased, with the more obvious variation in HRS. After only ~90-100 sweeping cycles, the device showed failure, without the recovery of the HRS. The RuOx BE device, however, showed obvious increases in both its LRS and HRS resistance values. The variations in the HRS and LRS resistance values as a function of the cycle number were fitted through a best-linear-fit PHWKRG DQG WKH VORSH ZDV H[WUDFWHG ZKLFK LV FDOOHG ³UHVLVWDQFH-F\FOH VORSH´ YDOXH 7KH 7L1 %( and RuOx BE devices exhibited negative and positive resistance cycle slopes, respectively, in both LRS and HRS, meaning the defect concentration (VO) increased (TiN BE) and decreased (RuOx BE) with the increasing cycle number.16,22,23 While the fundamental assertion for oxide-based VCM memory cells is that the VO migrations are confined within the CF region, there is actually no guarantee for this. Indeed, the VO concentration varies gradually, as will be confirmed later by


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X-ray photoelectron spectroscopy (XPS) measurement, accompanied by the gradual variations in the HRS and LRS resistance values and eventual failure. CLPS measurements11 were performed to examine the switching performance variation while keeping the resistive switching window in the TiN BE and RuOx BE devices. For the CLPS measurements, the pulse height increased until the target resistance values were reached; as such, the set/reset switching voltage may be changed as the switching progresses, although the resistance ZLQGRZ LV UHWDLQHG XQWLO GHYLFH IDLOXUH LQ WKLV ZRUN WKH\ ZHUH VHW WR

DQG

N

respectively).

Figure S2(c) and (d) show the variations in the LRS and HRS resistance values of the TiN BE and RuOx BE devices, respectively, as a function of the switching cycles. The accompanying set and reset switching voltages of the two devices are shown in Figure 1(b) and (c), respectively. The TiN BE device failed after ~1.3x106 cycles, but the RuOx BE device did not show any indication of failure up to the end of the test (1x107 cycles). The reason for the failure of the TiN BE device may be the abruptly decreasing (absolutely increasing) reset switching voltage after ~1.2x10 6 cycles, as shown in Figure 1(b), due to the involvement of the negative set effect. More detailed discussions on the negative set effect are given in SI Figure S3. The positive resistance cycle slope of the RuOx BE device shown in Figure 1(a), however, implies that the VO-sinking effect was too severe to maintain a constant switching performance; the VO concentration in the switching layer gradually decreased with the increasing cycle number. This could have been supplemented by the gradual increase in the absolute values of both the set and reset voltages with the CLPS switching cycles (Figure 1 (c)). The LRS resistance is determined by the parallel resistances of the CF and non-CF regions. As the VO concentration decreases, the leakage current within the non-CF region also decreases. To reach the target LRS resistance at each CLPS cycle, the resistance of the CF must be further decreased to compensate for the increase


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in the resistance of the non-CF region. Therefore, the set voltage should become higher to make the CF stronger (lower resistance). A higher absolute reset voltage was thus needed to reset such strong formed CF. Even if the reset voltage increased, however, the negative set did not occur because the VO concentration near the RuO2 BE did not increase due to the VO-sinking capability of RuO2. The device still worked after 107 switching cycles, but the measurement was stopped because the measurement time was already too long. After 107 cycles, however, the switching voltage almost doubled from the initial value, which is a critical problem for the entire circuit operation, as mentioned previously. If sinking of only the unnecessary VO can be accomplished by RuO2, this problem can be mitigated. Similarly, the TiN BE device has the set and reset voltages that vary, but an endurance failure occurs due to the negative set effect before the change becomes noticeable.

The VO-sinking and VO-supplying capabilities are determined by the oxygen contents of the RuOx BE and Ta TE layers, respectively. For example, with the TE of Ta 1.5, all the RuO x BE devices have a positive resistance cycle slope value regardless of the O2 flow rate (y) during the sputtering, as shown in Figure 2(a). This means that the VO-supplying capability of the Ta 1.5 TE is too low to meet the VO-sinking capability of all the RuOx BE. There is an unusual trend, however, in Figure 2(a): it can be supposed that the RuOx layer sputtered with a higher O2 flow rate may have a higher x value in the film, which would have a higher VO-sinking capability and thus a higher positive resistance cycle slope. In the experiment, however, the maximum resistance cycle slope was achieved at the O2 flow rate of 2 sccm while the value was lowest at 0 sccm and was in between at 3.5 sccm. Here, the resistance cycle slope values were obtained under conditions identical to those shown in Figure 1(a). The low VO-sinking capability of Ru can be readily understood to be the result of the insufficient oxygen in it. RuO2 forms a stable stoichiometric compound, and as such, its VO-sinking capability is also relatively limited despite the fact that it has the highest oxygen content. RuOx (x~1), however, has an extremely expanded and seriously


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distorted lattice of Ru metal containing a huge amount of oxygen. Therefore, the oxygen atoms in that layer are highly unstable and show a radical-like behavior, making it an extremely active oxygen source24 (see Figure S4 and the related discussions in SI). As confirmed by Auger electron spectroscopy (AES) measurement, whose results are shown in Figure S4(c), (d), and (e), lowering the working pressure during the Ta deposition reduced the oxygen content in the Ta thin film because of the lower total oxygen content of the sputtering chamber. The lower the oxygen content in the Ta film is, the higher the V O-supplying capability. As shown in Figure 2(b), the resistance cycle slope value linearly increases with the increasing working pressure during the Ta deposition for the given O2 flow rate of the RuOx BE. This suggests that the VO content tends to increase as the switching proceeds when the working pressure is sufficiently low. Interestingly, even with the RuOx BE, the device with a negative resistance cycle slope value, such as the Ru 3.5/Ta 1.0 shown in Figure 2(c), showed a negative set behavior, as shown in the inset of Figure 2(c). Here, excessive VO means the VO retained in the Ta2O5 switching oxide layer after one cycle of switching (i.e., set/reset). XPS measurements were performed to chemically identify how the VO varies within the Ta2O5 switching layer in the different combinations of VO sink source electrodes. Towards this end, Ru 2.0/Ta 1.5, which represents the most positive resistance cycle slope device, and Ru 3.5/Ta 1.0, which represents the most negative resistance cycle slope device, were chosen and cycled 1 and 200 times in the DC sweep mode. The detailed experimental procedure for achieving reliable XPS data are included in Figure S5 in SI. Figure 3(a) shows the XPS data (data points; black for 1 cycle and red for 200 cycles) and the fitting results (lines; black for 1 cycle and red for 200 cycles) near the Ta4f region of the Ru 2.0/Ta 1.5 device. The detailed Ta4f spectra and their deconvoluted curves are shown in Figure S5. The


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absence of intensity at the Ta0 position of 21.5 eV25,26 suggested that the Ta TE layer was successfully removed by the in-situ plasma etching. The deconvoluted peaks at 23.0, 24.5, and 26.8 eV were assigned to the Ta1+, Ta2+, and Ta5+ peaks in Figure S526,27. The Ta5+ peak corresponds to the stoichiometric Ta2O5. Therefore, the variations in the Ta5+ and Ta2+ peaks by cycling were focused on. After 200 cycles, the relative intensity of the Ta5+ peak increased while the relative intensity of the Ta2+ peak decreased. The peak areas obtained by the fitting are included in Table S1. The peak area ratio of Ta2+ to (Ta2++Ta5+) was decreased from 0.142 to 0.125 (~12% decrease). This indicates that the relative portion of the stoichiometric Ta2O5 relatively increased, which can be ascribed to the more dominant VO-sinking action of the RuOx BE compared with the VO-supplying action of the Ta TE. In fact, the relative variation only in the switched cell area must be much higher (~78%; see Figure S5 and the discussion in SI). Figure 3(b) shows the similar XPS results of the Ru 3.5/Ta 1.0 device after 1 and 200 cycles of DC switching. The data and detailed deconvolution results (Figure S5) showed that the similar peak area ratio of Ta2+ to (Ta5++Ta2+) increased from 0.140 after 1 DC cycle to 0.146 after 200 DC cycles. This is only a ~4% increase, but it became ~29% after the detection area correction, as mentioned in SI. Therefore, the negative resistance cycle slope corresponds to the VO concentration increase in the memory cell area. Figure 2(b) shows that there are several experimental conditions where the resistance cycle slope is close to zero, which may involve very high reliability. Before presenting the experimental data with such conditions, the (semi) quantitative estimation of the relative VO-supplying and VOsinking capabilities of the RuOx and Ta electrodes was attempted. The parallel and linear behaviors in Figure 2(b) imply that the VO-sinking and VO-supplying capability of the RuOx and Ta electrodes are quite independent of each other. The change in the VO concentration in the Ta2O5 thin film could be attributed to the difference in the VO-supplying and VO-sinking capabilities.


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When the VO-sinking capability of Ru 1.0, Ru 2.0, and Ru 3.5 was a, b, and c, and the VO-supplying capability of Ta 1.0, Ta1.3, and Ta1.5 was i, j, and k, respectively, a Ru 3.5/Ta 1.0 device, for example, has a VO-decreasing tendency of c-i. This tendency must be coincident with the resistance cycle slope value of this device, which was -

Â

-3

cycles-1; thus, it was assumed that c-i=-1.5.

A set of equations can be similarly set, as shown in Table S2(a), based on the experimental data in Figure 2(b), which is an overdetermined set of equations; it has nine equations with six unknowns and no general solution. A solution set (a, b, c, i, j, and k values) with the smallest error in the experimental value can be obtained, however, as shown in Table S2(b), through appropriate tuning. With these a, b, c, i, j, and k values, the calculation showed a slight difference from the experiment values only for the Ru 2.0/Ta 1.0 case. These a, b, c, i, j, and k values can be taken as the relative capabilities of supplying and sinking VO¶V DV VKRZQ LQ )LJXUH 6

)URP 7DEOH 6

LW FDQ EH

anticipated that the devices with Ru 1.0/Ta 1.3 (resistance cycle slope 1.0), Ru 2.0/Ta 1.0 (slope 0.8), and Ru 3.5/Ta 1.3 (slope 1.6) will show high reliability due to the good match between the VO-supplying and VO-sinking capabilities. Figure 4 shows the CLPS results of the three devices mentioned above with (a)-(c) resistance variation and (d)-(f) switching voltage variation. As expected, all three devices showed constant switching voltages during all the cycles, but there are several notable findings. First, the Ru 2.0/Ta 1.0 device showed the smallest slope (0.8, calculated; 0.6, experimental), but its endurance cycle was only 5x105. This may have a close relationship with the very high VO-supplying (b=6.9) and VO-sinking (i=6.1) capabilities of these Ta TE and RuOx BE, which will run out of available VO¶V early. The comparison of the Ru 1.0/Ta 1.3 and Ru 3.5/Ta 1.3 devices showed another important factor for reliability despite the fact that the two devices commonly showed over 10 7 endurance cycles. The average numbers of pulses for reaching the target HRS/LRS values at each cycle are


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107/67 and 108/74 for the former and latter devices. It can also be noted that the relative variations in the set/reset voltages and LRS resistance are also small for the former case. Table S2(b) also reveals that the VO-sinking capability of Ru 1.0 (a=4.0) is lower than that of Ru 3.5 (c=4.6), which might have induced a lower variation in the overall VO concentration at each cycle. This provides another critical advantage with regard to the endurance and voltage control circuit for ISPP.

IV. Conclusions In conclusion, the reliability issue related to the endurance cycle of Ta2O5-based the VCM RRAM cell was addressed. The accumulation of unwanted VO¶V ZLWKLQ WKH VZLWFKLQJ R[LGH OD\HU supplied from the Ta electrode, was identified as the main reason for the undesirable negative set and eventual endurance failure mechanism. When the inert TiN electrode was replaced with RuOx, which acts as the sink of the excessively supplied VO¶V WKH GHYLFH UHOLDELOLW\ ZDV UHPDUNDEO\ improved. Furthermore, by carefully balancing the VO-supplying and VO-sinking capabilities of the Ta and RuOx electrodes, the uniformity of the set/reset voltages in the CLPS mode can also be greatly improved. Even under the conditions of balanced VO-supplying and VO-sinking conditions, the conditions involving the minimized VO supply from the Ta and VO sink of RuOx showed the highest reliability (endurance) and minimized the variations in the switching voltages. This is a good demonstration of careful materials research for nanoscale device engineering.


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FIGURES Figure 1. (a) LRS/HRS resistance by switching cycle in the DC measurements (set current FRPSOLDQFH

$ UHVHW VWRS YROWDJH -1.5 V) of the TiN and RuO2 BE devices. Switching

voltage according to the switching cycle of the CLPS measurement in the (b) TiN BE and (c) RuO2 BE devices. Figure 2. Resistance cycle slope in the DC measurements according to the (a) O2 flow rate during the RuO2 deposition and (b) working pressure of the Ta deposition. (c) Negative UHVLVWDQFH F\FOH VORSH VHW FXUUHQW FRPSOLDQFH

$ UHVHW VWRS YROWDJH -1.5 V) in the Ru

3.5/Ta 1.0 devices with high VO-sourcing capability. The inset shows the I-V sweep curve with a reset stop voltage of -1.8 V. Figure 3. XPS spectra of the Ta4f region and fitting results of the Ta5+, Ta2+, and Ta1+ peaks for the (a) Ru 2.0/Ta 1.5 device and (b) Ru 3.5/Ta 1.0 device. The black dot and lines are the experimental result and fitting results, respectively, after the 1st cycle (electroforming+reset), and the red dot and lines are the experimental result and fitting results after the 200 th cycle of DC VZLWFKLQJ VHW FXUUHQW FRPSOLDQFH

$ UHVHW VWRS YROWDJH -1.5 V).

Figure 4. Variations in the LRS/HRS resistances and switching voltages according to the number of switching cycles in the CLPS measurement of the [(a), (d)] Ru 1.0/ Ta 1.3, [(b), (e)] Ru 2.0/Ta 1.0, and [(c), (f)] Ru 3.5/Ta 1.3 devices.


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AUTHOR INFORMATION Corresponding Author * E-mail: (C.S.H) [email protected] Author Contributions T.H.P. (1st) designed and initiated the experiments and developed the theoretical model. Y.J.K. (1st) continued the experiments, and results analysis. T.H.P. and Y.J.K. contributed equally to this work. H.J.K., H.C.W., G.S.K., and C.H.A. prepared the sample. Y.M.K., and D.E.K. participated in the discussions and commented on the technical methods. C.S.H. provided guidance in all the steps, and wrote the manuscript. ACKNOWLEDGMENT This work was supported by SK Hynix Inc. and the Global Research Laboratory Program (2012040157) of the National Research Foundation (NRF) of the Republic of Korea. SUPPORTING INFORMATION Additional information about Vo properties such as sink capabilities using RuO2 and its electrical properties compared to TiN using XPS and AFM REFERENCES (1) Kim, K. M.; Jeong, D. S.; Hwang, C. S. Nanofilamentary Resistive Switching in Binary Oxide System; A Review on the Present Status and Outlook Nanotechnology 2011, 22, 254002. (2) Waser, R.; Dittmann, R.; Staikov, G.; Szot, K. Redox-Based Resistive Switching MemoriesNanoionic Mechanisms, Prospects, And Challenges Adv. Mater. 2009, 21, 2632-2663.


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(9) Kim, K. M.; Lee, S. R.; Kim, S.; Chang, M.; Hwang, C. S. Self-Limited Switching in Ta2O5/TaOx Memristors Exhibiting Uniform Multilevel Changes in Resistance Adv. Funct. Mater. 2015, 25, 1527-1534. (10) Lee, H. Y.; Chen, P. S.; Wu, T. Y.; Chen, Y. S.; Wang, C. C.; Tzeng, P. J.; Lin, C. H.; Chen, F.; Lien, C. H.; Tsai, M. J. Low Power and High Speed Bipolar Switching with a Thin Reactive Ti Buffer Layer in Robust HfO2 Based RRAM 2008 IEEE International Electron Devices Meeting, 2008,1. (11) Park, T. H.; Song, S. J.; Kim, H. J.; Kim, S. G.; Chung, S.; Kim, B. Y.; Lee, K. J.; Kim, K. M.; Choi, B. J.; Hwang, C. S. Thickness Effect of Ultra-Thin Ta2O5 Resistance Switching Layer in 28 Nm-Diameter Memory Cell Sci Rep 2015, 5, 15965. (12) Egorov, K. V.; Kuzmichev, D. S.; Chizhov, P. S.; Lebedinskii, Y. Y.; Hwang, C. S.; Markeev, A. M. In Situ Control of Oxygen Vacancies in TaOx Thin Films Via Plasma-Enhanced Atomic Layer Deposition for Resistive Switching Memory Applications ACS Applied Materials & Interfaces 2017, 9, 13286-13292. (13) Daniele, I. Resistive Switching Memories Based on Metal Oxides: Mechanisms, Reliability and Scaling Semicond. Sci. Technol. 2016, 31, 063002. (14) Balatti, S.; Ambrogio, S.; Wang, Z. Q.; Sills, S.; Calderoni, A.; Ramaswamy, N.; Ielmini,D. Pulsed Cycling Operation and Endurance Failure of Metal-Oxide Resistive (RRAM) in 2014 IEEE International Electron Devices Meeting, 2014, 14.3.1. (15) Liu, S.; Lu, N.; Zhao, X.; Xu, H.; Banerjee,W.; Lv, H.; Long, S.; Li, Q.; Liu, Q.; Liu, M. Eliminating Negative-SET Behavior by Suppressing Nanofilament Overgrowth in Cation-Based Memory Adv. Mater. 2016, 28, 10623-10629.


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(16) Kim, W.; Menzel, S.; Wouters, D. J.; Guo, Y.; Robertson, J.; Roesgen, B.; Waser, R.; Rana, V. Impact of Oxygen Exchange Reaction at the Ohmic Interface in Ta2O5-Based ReRAM Devices Nanoscale 2016, 8, 17774-17781. (17) Yoon, K. J.; Lee, M. H.; Kim, G. H.; Song, S. J.; Seok, J. Y.; Han. S.; Yoon, J. H.; Kim, K. M.; Hwang, C. S. Memristive Tri-Stable Resistive Switching at Ruptured Conducting Filaments of a Pt/TiO2/Pt Cell Nanotechnology 2012, 23, 185202. (18) Kim,G. H.; Lee, J. H.; Seok, J. Y.; Song, S. J.; Yoon, J. H.; Yoon, K. J.; Lee, M. H.; Kim, K. M.; Lee, H. D.; Ryu, S. W.; Park, T. J.; Hwang, C. S. Improved Endurance of Resistive Switching TiO2 Thin Film by Hourglass Shaped Magnéli Filaments Appl. Phys. Lett. 2011, 98, 262901. (19) Degraeve, R.; Fantini, A.; Clima, S.; Govoreanu, B.; Goux, L.; Chen, Y. Y.; Wouters, D. J.; Roussel, P.; Kar, G. S.; Pourtois, G.; Cosemans, S.; Kittl, J. A.; Groeseneken, G.; Jurczak, M.; Altimime, L. Dynamic µ+RXU *ODVV¶ 0RGHO IRU SET and RESET in HfO2 RRAM Symposium on VLSI Technology (VLSIT), 2012,75. (20) Park, T. H.; Kim, H. J.; Park, W. Y.; Kim, S. G.; Choi, B. J.; Hwang, C. S. Roles of Conducting Filament and Non-Filament Regions in the Ta2O5 and HfO2 Resistive Switching Memory for Switching Reliability Nanoscale 2017, 9, 6010-6019. (21) Park, T. H.; Song, S. J.; Kim, H. J.; Kim, S. G.; Chung, S.; Kim, B. Y.; Lee, K. J.; Kim, K. M.; Choi, B. J.; Hwang, C. S. Thickness-Dependent Electroforming Behavior of Ultra-Thin Ta2O5 Resistance Switching Layer Phys Status Solidi-R 2015, 9, 362-365. (22) Akbari, M.; Lee, J.-S. Control of Resistive Switching Behaviors of Solution-Processed HfOx-Based Resistive Switching Memory Devices by n-type Doping RSC Advances 2016, 6, 21917-21921.


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(23) Park, S. J.; Lee, J. P.; Jang, J. S.; Rhu, H.; Yu, H.; You, B. Y.; Kim, C. S.; Kim, K. J.; Cho, Y. J.; Baik, S.; Lee, W. In Situ Control of Oxygen Vacancies in Tio2 by Atomic Layer Deposition for Resistive Switching Devices Nanotechnology 2013, 24, 295202. (24) Jeon, W.; Lee, W.; Yoo, Y. W.; An, C. H.; Han, J. H.; Kim, S. K.; Hwang, C. S. Chemistry of Active Oxygen in RuOx and Its Influence on the Atomic Layer Deposition of TiO2 Films Journal of Materials Chemistry C 2014, 2, 9993-10001. (25) McGuire, G. E.; Schweitzer, G. K.; Carlson, T. A. Study of Core Electron Binding Energies in Some Group Iiia, Vb, and Vib Compounds Inorg. Chem. 1973, 12, 2450-2453. (26) Simpson, R.; White, R. G.; Watts, J. F.; Baker, M. A. XPS Investigation of Monatomic and Cluster Argon Ion Sputtering of Tantalum Pentoxide Appl. Surf. Sci. 2017, 405, 79-87. (27) Denny, Y. R.; Firmansyah, T.; Oh, S. K.; Kang, H. J.; Yang, D.-S.; Heo, S.; Chung, J.; Lee, J. C. Effect of Oxygen Deficiency on Electronic Properties and Local Structure of Amorphous Tantalum Oxide Thin Films Mater. Res. Bull. 2016, 82, 1-6.


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For Table of Contents Only


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Figure 1 171x57mm (300 x 300 DPI)



Figure 2 180x56mm (300 x 300 DPI)


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Figure 3 121x60mm (300 x 300 DPI)



Figure 4 177x114mm (300 x 300 DPI)


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Balancing the Source and Sink of Oxygen ... - ACS Publications

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