Highly Flexible Resistive Switching Memory Based on the Electronic

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Highly flexible resistive switching memory based on the electronic switching mechanism in the Al/TiO2/Al/polyimide structure Jinshi Zhao, Ming Zhang, Shangfei Wan, Zheng-Chun Yang, and Cheol Seong Hwang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16214 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017

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ACS Applied Materials & Interfaces

Highly flexible resistive switching memory based on the electronic switching mechanism in the Al/TiO2/Al/polyimide structure Jinshi Zhao1, Ming Zhang1, Shangfei Wan1, Zhengchun Yang*1, and Cheol Seong Hwang*2 1

School of Electrical & Electronic Engineering, Tianjin Key Laboratory of Film

Electronic & Communication Devices, Tianjin University of Technology, Tianjin 300384, China. E-mail: [email protected] 2

Department of Materials Science and Engineering and Inter-University

Semiconductor Research Center, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-744, Republic of Korea. E-mail: [email protected]

ACS Paragon Plus Environment

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Abstract A highly flexible resistive switching (RS) memory was fabricated in the Al/TiO2/Al/polyimide structure using a simple and cost-effective method. An electronic-resistive-switching-based flexible memory with high performance that can withstand a bending strain of up to 3.6% was obtained. The RS properties showed no obvious degradation even after the bending tests that were conducted up to 10,000 times, and over 4,000 writing/erasing cycles were confirmed at the maximally bent state. The superior electrical properties against the mechanical stress of the device can be ascribed to the electronic RS mechanism related to electron trapping/detrapping, which can prevent the inevitable degradation in the case of the RS related with the ionic defects.

Keywords: electronic resistive switching, highly flexible memory, mechanical endurance, titanium dioxide, electron trapping/detrapping, high performance

Introduction Electronic devices have been going through extensive innovations to meet the ever-increasing technological demands of multiple functionalities, such as transparency, portability, and flexibility.1-3 Among the newly required functionalities,


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flexibility is gaining enormous importance as wearable electronics, flexible displays, electronic skins, and implantable elements are rapidly being developed.4-9 These devices require high-density random access memory embedded in the system, which may greatly decrease the power consumption necessary for the communication between the edge device and the server/data center. Thanks to its simple structure, fast speed, low power consumption, long retention time, excellent scalability, and nondestructive readout method, the resistance switching random access memory (RRAM) has been actively researched and developed as one of the most promising contenders for information storage in flexible electronic devices.9-15 The resistive switching (RS) mechanism in many oxide materials is induced by the formation and subsequent repeated rupture and rejuvenation of conducting filaments (CFs), which are generally composed of ionic defects.16-21 These CFs are typically composed of oxygen vacancy (VO), which is the crucial factor of memory operation and responsible for the unipolar RS (URS).20,21 In the bipolar RS (BRS) devices, however, the set and reset switching occur in the opposite bias polarity. Such a switching mechanism is usually ascribed to the accumulation and depletion of VO’s at the actively switching (Schottky) interface whereas the opposite interface remained the (quasi-) Ohmic contact state.18,19 Fabricated on soft substrates, such flexible RRAM offers the advantages of high flexibility, low cost, and lightweight.8,9,14 According to the earlier reports on the ionic RS in the flexible memory system14,15, however, the advantages of Si-based RRAM are not always valid for the flexible memory. The most flexible memories reported recently adopted the ionic RS mechanism, but they show insufficient


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performance under the bending condition.9,14,15,22-25 Table 1 summarizes the performances according to the bending strain of the flexible resistive memories. As can be expected, the configuration of the ionic defects (Vo) and CF may be influenced by the repeated bending stress during RS. As a result, the resistance ratio and the switching voltages drifted with the switching time and cycles. There have been several reports on the impacts of the bending stress on the rupture and rejuvenation of CFs, resulting in the poor reliability and low mechanical flexibility of the device.9,24 Moreover, the possible large strain caused by the repeated bending can induce mechanical cracks, which constitute a critical failure.8,14,26 Although much effort has been exerted to address these issues by controlling the materials, geometrics, process method, etc., most of the problems remain. This difficulty might be unavoidable with the adoption of ionic-defect/CF-mediated RS mechanisms, requiring a different switching mechanism. The more recently emerged electronic bipolar resistance switching (eBRS) mechanism could be a feasible RS mechanism for the flexible RRAM. In eBRS, the trapping (corresponding to the set) and detrapping (corresponding to the reset) of carriers (electrons) constitute the main RS mechanism, which does not involve ionic motions.27-31 The electronic RS mechanism must have an inherently bipolar nature, which can be achieved when the carrier injection/ejection at one electrode interface is highly fluent whereas the carrier exchange at the opposite interface is sufficiently prohibited by the presence of an asymmetric potential barrier.31-34 Such electronic RS mechanism can have several merits over the ionic RS mechanisms, especially for


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flexible RRAM application. Although the eBRS mechanism also requires ionic defects, which will trap/detrap the electrons, they do not need to be percolated, as in the ionic RS case.27,29 Therefore, the adverse influence of the bending strain on the ionic defects may be less significant for the electronic-RS-based flexible memory. As such, eBRS may be a viable mechanism for the flexible RRAM to achieve the goal of high performance with a much higher mechanical threshold. There has been barely any report, however, on the electronic-RS-based flexible memory. In this work, stable eBRS characteristics were demonstrated in the Al/TiO2/Al RRAM structures, which fabricated on the polyimide (PI) substrate. The electricfield-induced trapping and detrapping of the electrons according to the bias polarity lead to the reversible switching between the low resistance state (LRS) and the high resistance state (HRS). The properties of the Al/TiO2/Al RRAM cell fabricated on a standard Si wafer have been extensively studied by the authors29, and an almost identical performance was achieved even when they were fabricated on a much more irregular and softer PI substrate. The Al/TiO2/Al RRAM cell on a PI substrate was found to be a highly flexible and cost-effective electronic RS memory; the present device shows a uniform and long-term stability RS effect within the wide strain range of 0-3.6%. The simple structure and highly ductile property of the aluminum electrode also contributed to the high flexibility, large endurance to mechanical cycling, and feasible durability of the fabricated RRAM devices.

Results and Discussion


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The basic BRS I-V curves of the flexible Al/TiO2/Al/PI sample at the flat state are shown in Figure 1. For such BRS type I-V curves, the voltage was swept according to: 0 → −3.0 V (1) → 0 V (2) →3.0 V (3) → 0 V (4), and the corresponding current was recorded. The pristine sample showed the insulating state, and it was switched to LRS at ∼-2.3 V [the black square symbol in Figure 1(a)]. The LRS was kept up to ~2.2 V and gradually switched back to HRS by the voltage sweep back to the positive voltage region at the higher voltages. The sample showed an electroforming-free behavior, i.e., the second voltage sweep curve (the red-circle symbol) almost overlapped with the curve of the first sweep (the black-square symbol). Also, the sample showed the gradual switching characteristics when the set/reset I-V curves were measured under no compliance current (ICC) condition. These are the similar features to the authors’ previous report on the similar RRAM cell fabricated on a Si substrate29, and the eBRS mechanism previously reported explains the device’s performance well. Therefore, the RS mechanism in this flexible RRAM is regarded as the same eBRS presented in the authors’ previous works. The absence of a sudden current increase or decrease during the set and reset steps indicates that the VO-mediated CF mechanism is not relevant to this work. In addition, the samples with different top electrode area (S) are also fabricated to study the size-effect of the cell (inset of Figure 1(d)). The slope 1 in the log R vs. log S graphs of both HRS and LRS indicates that the current flows uniformly across the entire electrode area. The result is inconsistent with the CFrelated ionic RS behavior, but is coincident with eBRS reported elsewhere.29 The flexible operation of the ReRAM device was examined under various


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bending conditions. In this study, all the RS measurements related to mechanical deformation were implemented through the flexible samples bent on a series of convex shape molds with different radii (see the inset figure in Figure 1(c)). The flexible operation of the Al/TiO2/Al/PI sample was examined to evaluate the HRS resistance (RHRS) and the LRS resistance (RLRS) values measured at 0.1 V under various bending radii. The respective strains (inset in Figure 1(b)) was estimated by using the equation, ε=t/2R, where ε is the strain, t is the thickness of the substrate (145 µm for the Al/PI substrate), and R is the bending radius.35 It can be noted in Figure 1(b) that the performance of the devices was almost unaffected until the bending radius was reduced to 2 mm (ε =3.62%). Figure 1(c) shows the measured I-V curves of the device after being bent to a 2 mm radius for the first and 10,000th times, respectively. The I-V curves, including the set voltage (Vset) and reset voltage (Vreset), showed no significant changes before and after the repeated bending testes as well as when they were compared with those of the flat state. Figure 1(d) shows the RHRS and RLRS uniformity of the Al/TiO2/Al/PI structure under different bending radii, wherein 20 cells were tested for each condition. To check the device yield, 111 devices were tested, and the 100 cells worked (Figure 1(d)), meaning that the device yield is ~90%. It was confirmed that all the cells had high uniformity and consistency at each bending state. Consequently, the Al/TiO2/Al/PI structure showed a stable RS and maintained high mechanical flexibility even at a high bending stain of 3.62%.


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Figure 1. (a) The typical BRS I-V curves of the Al/TiO2/Al/PI sample at the flat state. The arrows and numbers indicate the switching sequences. The inset shows the schematic illustration of the device structure. (b) The statistical distribution of the HRS and LRS resistances as a function of the bending radius. The plot of the mechanical strain vs. the bending radius for the device is shown in the inset in Figure 1(b). (c) The measured I-V curves of the device after being bent to a 2 mm radius for the first (the red-diamond symbol) and 10,000th times (the blue-star symbol). The insets show how the bending test was performed as well as the schematic diagram of the series of test fixtures. (d) Cumulative probability of the resistance states in both the RHRS and RLRS under different bending radii in 20 working cells. The inset in (d) shows the electrode area-dependency of LRS (red circle plots) and HRS (black square plots).

In most of the reported flexible RRAMs based on the ionic RS, the RS performances are insufficient under a high bending strain. This is due to the failure of the repeated formation and rupture of the CFs under the severe mechanical strain condition.14,36 Shang et al. reported that the CFs aligned along the mechanical stress direction rupture easily, failing in the LRS. Other works reported that a portion of the VO’s in the film is tied up at the location where the bending stress was most significant, and they thus cannot participate in the RS.37 In this system, by contrast,


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the RS mechanism is not based on the ionic motions; thus, such undesirable effect of the repeated strain on the ionic motion can be mitigated. Based on the retained high performance of the Al/TiO2/Al/PI sample even after the repeated bending tests, it can be understood that the electronic switching through the trapping and detrapping of carriers was not severely interfered with. Although a more in-depth (theoretical) investigation is needed, it can be conjectured that the electronic states of the dispersed defects are less influenced by the imposed strain compared with the percolating paths composed of VO’s. Figure 2 shows the endurance test results of the Al/TiO2/Al/PI structure achieved by the repeated I-V sweeps for the flat cell, the cell with 2 mm bending, and the cell after being bent to a 2 mm radius 10,000 times, respectively. Here, the resistance values were estimated at 0.1 V. The three cells showed high reliability up to 4,000 switching cycles, and the RHRS/RLRS ratios of such samples were always higher than 10. The actual maximum endurance cycle number might be much higher, but the ultimate endurance could not be tested by the time-limitation when such a timeconsuming I-V sweep method was adopted. Nonetheless, such I-V sweep test imposes a much higher stress on the sample than the pulse-switching-type test. Therefore, it is believed that the reliability of the device in this work could be even higher than demonstrated presently. Meanwhile, Lin et al. reported the low endurance property of the bent Al/TiO2/Al structure with an ionic RS mechanism.37 The authors have elucidated in the previous study that the endurance degradation in the present Al/TiO2/Al structure is caused by


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the unbalanced migration of electrons and not by the ionic defect29, making it less prone to degradation from the bending tests. The unbalanced migration meant that some of the trapped electrons during the set operation did not detrap during the reset operation, making the overall trapped electron density increases with the increasing switching cycle number. No significant endurance degradation according to the bending strain occurred in this work, suggesting that the bending stress hardly impacts the electronic RS mechanism.

Figure 2. (a) The endurance test result achieved from the I-V sweeps when the device was flat. The endurance cycles of the memory device after being bent to a 2 mm radius for the first (b) and 10,000th times (c). There was no significant endurance degradation according to the bending strain.

The retention data for the flat cell and the cell with 2 mm bending, respectively, measured at room temperature and 85℃ for up to 106 s are shown in Figure 3. It was found that the two cells had similar retention results at room temperature and that the resistance ratio (>10) was maintained for up to 106 s. The resistance values of the LRS and HRS slightly increased with time at the two temperatures. The LRS showed a higher relative increase in resistance relative to the value at 100 s compared with the HRS, and such phenomenon became more evident as the temperature increased. The increasing resistance with the time indicated that the detrapping of the electrons occurred, of which degree increased with increasing temperature due to the thermal


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activation. The increase in RLRS was more significant because there is a higher chance of the occurrence of carrier detrapping in LRS as it has a trap-filled electronic configuration, where the trap depth is not deep enough to prevent thermal detrapping. This study result well coincides with the general model of the electronic RS.27,29,32 Nevertheless, the resistance ratio is still slightly over 10 up to 106 s even at 85℃, demonstrating that the Al/TiO2/Al/PI structure has useful non-volatile and nondestructive readout properties.

Figure 3. (a) Retention characteristics of HRS and LRS measured at room temperature (RT) and 85℃ at a flat state. (b) Retention property of the memory under a bending state at its maximum radius (R=2 mm) at room temperature.

Nonetheless, the possible introduction of micro- or nano-cracks in the oxide films, which may adversely affect the device integrity in the integrated structure through the repeated bending tests, cannot be completely ruled out. To investigate the mechanical bending effect on the microstructure (micro- or nano-cracking) of the Al and TiO2 film, atomic force microscopy (AFM) was used to observe the surface morphology of the films before and after the bending tests. Figure 4 shows the AFM images of Al and TiO2 that were investigated on the flat cell, the cell with a 2 mm bending radius, and


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the cell after being bent to a 2 mm radius 10,000 times. The similar AFM images indicate that the bending strain did not change the microstructure of the Al/PI and TiO2/Al/PI and that the two sets of AFM results consisting of three samples each showed similar root-mean-squared roughness values. These findings suggest that no significant cracks were induced by the bending tests. The elastic modulus of the 70nm-thick TiO2 film was estimated by a nanoindentation method (Agilent Nanoindentation, G200), and it was estimated to be 10.7 GPa. This value is lower than ~10% of reported bulk values of TiO2 (116 to 209 GPa),38 which appears to be reasonable considering the porous structure of the film (See Figure 2 of Ref. 29). Such a low mechanical property might have degraded the resistance to the bending test. However, as shown in this work, the device did not show any significant degradation up to 104 cycles of repeated bending. The voids within the TiO2 film might absorb the bending stress, which contributed to the high reliability. The high adhesion between TiO2 and Al and the high ductility of Al and TiO2 may be the main reasons that the structure showed a high performance and that the damage from the repeated bending was minimized.


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Figure 4. (a)-(c) AFM surface morphologies of Al/PI for the flat cell, the cell with 2mm bending, and the cell after being bent to a 2 mm radius 10,000 times, respectively. (d)-(f) AFM surface morphologies of TiOx/Al/PI under different bend states, respectively, corresponding to (a)-(c).

The possible change in the chemical state of the TiO2 film after the repeated bending test was examined via X-ray photoelectron spectroscopy (XPS). The C 1s photoemission signal was used to calibrate the observed peak positions. The Ti2p XPS spectrum of TiO2/Al/PI sample under flat state is shown in Figure 5(a). The main peaks of Ti2p1/2 and Ti2p3/2 were located at 464.6 and 458.8 eV, respectively, which are associated with the TiO2 component.39 The Ti2p1/2 and Ti2p3/2 signals with the binding energies of 462.7 and 456.9 eV originating from the Ti2O3 are negligible. Also, the Ti metallic peak was not found. The results showed that the film was almost fully oxidized to TiO2.40 The O1s XPS peak showed an asymmetric shape which was deconvoluted using the two peaks shown in Figure 5(b). The peak with the binding energy of 530.1 eV and 531.6 eV can be attributed to the oxygen bound to the TiO2 layer lattice, and the oxygen ions near the VO in the TiO2 layer, respectively.41 It has


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been reported, in the eBRS mechanism, that the presence of VO plays a crucial role in inducing the electronic RS performance in this material system.29 Figure 5(c) and (d) show the Ti2p and O1s XPS spectra after the 10,000-time bending tests. A negligible change in the detailed spectra was induced by the repeated bending tests, suggesting that the chemical structure of the TiO2 film was not changed largely by the repeated bending tests, which corroborates the invariant RS mechanism after repeated bending tests shown in Figure 1-3. An electrical conduction mechanism analysis was further performed to confirm the invariant RS performance in the repeated bending tests, as shown in Figure 6. Figures 6(a) and (b) show the log I-log V curves of the flat cell and the cell after being bent to a 2 mm radius 10,000 times, respectively, of the LRS. During these tests, the measurement was performed at temperatures ranging from 300 to 350 K while the TE Al was negatively biased. In the low-voltage region, the best-linear-fitted graphs from both samples showed the slopes close to 1, and the current increased slightly with the increasing temperature. This behavior coincides with the hopping conduction mechanism with a relatively high activation energy (Ea). The Ea values are shown in the inset figure. As the voltage increased, the slope became close to 2, with the temperature dependence eventually disappearing, which is the characteristic feature of the space-charge-limited current (SCLC). The Ohimc current density (Johm) by the hopping mechanism and that of the trap free SCLC (JSCLC) are given as follows:

Johm = qn0µV/D

(1)


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JSCLC = 9µε0εrV2/8D3

(2)

, where q is the elementary charge, n0 is the thermally produced free carrier density, ε0 is the permittivity of vacuum, εr is the relative dielectric constant of media materials, µ is the electron mobility, V is the applied voltage, D is the thickness of the film. The activation energies were calculated from the Arrhenius-type plot at each voltage as shown in the inset Figure 6. The conductive mechanism and the variation of Ea for both cells were similar. The electrical conduction mechanisms of the HRS of the two cells above were analyzed, as shown in Figure 6(c) and (d), respectively. A hopping conduction mechanism was observed for the HRS of both cells at a low (absolute) voltage, and the SCLC mechanism was the feasible conduction mechanism in the high-(absolute)-voltage region, where the slope of 3.8 was extracted from the bestlinear-fitted graphs. In this case, the trap-limited SCLC can be expressed as follows: JSCLC = 9θµε0εrV2/8(1+θ)D3

(3)

, where θ is the ratio of free electrons and trapped-electrons. The results show that the conduction mechanism of the repeatedly bent cell is almost identical to that of the fresh cell. These findings suggest that the electrical properties of the eBRS Al/TiO2/Al/PI structure are not significantly influenced by severe mechanical stressing and bending, which is also consistent with the XPS result. This must be a reasonable consequence considering that the RS is governed by the trapping and detrapping of the electrons, meaning that electronic RS plays a positive role in the flexible memory devices.


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Figure 5. (a) Ti2p and (b) O1s XPS spectra of the TiO2/Al/PI sample under a flat state. (c) Ti2p and (d) O1s XPS spectra of the sample after the 10,000-time bending tests.


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Figure 6. The log I-log V curves in the negative bias region measured from 300 to 350 K. (a) LRS of the sample under a flat state; (b) LRS of the sample after the 10,000-time bending tests; (c) HRS of the sample under a flat state; (d) HRS of the sample after the 10,000-time bending tests. The inset figure shows the variation in the activation energy (Ea) as a function of the voltage.

Conclusions A highly flexible resistive switching (RS) memory device using the Al/TiO2/Al/PI structure was fabricated using a simple and cost-effective method. With the electronic bipolar resistive switching mechanism of the device, the flexible resistive switching random access memory device showed a highly uniform and reproducible performance that could withstand the mechanical tensile stress. The device worked appropriately when it was bent to a 2 mm radius, and maintained 4,000 writing/erasing cycles under the bent configuration with a radius of 2 mm, which corresponds to a tensile strain of 3.6%. These properties were also confirmed even


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after the device was bent to a 2 mm radius for 10,000 times. The detailed electrical analysis indicated that the RS could be attributed to the electronic switching mechanism mediated by the electron trapping/detrapping. Such mechanism is much less prone to mechanical stress than the ionic switching mechanism mediated by the VO CF. The structural and electrical conduction mechanism analysis also confirmed that the chemical and electrical states of the RS TiO2 film hardly varied after the repeated bending up to 10,000 cycles. These superior performances could be achieved by the high adhesion between the TiO2 and Al electrodes, and the high ductility of the Al layer.

Experimental methods First, a 200nm Al thin film was deposited on a 145-µm-thick PI flexible substrate as a bottom electrode (BE) through the thermal evaporation method. Then, a 70nmthick TiO2 film was reactively sputter-deposited on the blanket Al BE at room temperature using a 99.995% Ti target and a 2% O2/98% Ar mixture gas with a sputtering power of 100W for 12 minutes. During the oxide film growth, 3- to 5-nmthick AlOx layer was formed at the interface between the TiO2 and Al layers, which plays a critical role as an asymmetric carrier injection barrier from the BE during RS operation. Finally, the top electrodes (TEs) were evaporated on the TiO2 through a metal shadow mask with circular holes (~400 µm diameter). To change the TE area, a blanket Al layer was deposited through the electron-beam-evaporation process on the TiO2 film, and photolithographic, and lift-off processes were performed to achieve a


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TE area in the range of 6,500–350,000µm2. The detailed experimental conditions and device performance on the Si wafer are described elsewhere.29 The analysis of the chemical bonding in the TiO2 thin film was done through XPS (Thermo VG, Sigma Probe), and the morphologies of the ﬁlms were observed through AFM (Agilent 5600LS). The I-V characteristics of the sample were measured in the voltagesweeping mode with an Agilent B1500A at room temperature. For the retention property measurement, the resistance values of LRS and HRS were measured as a function of time at room temperature and 85℃. For all the I-V tests, the TE was biased, and the BE was grounded, and no ICC was adopted. The electroforming-free BRS behavior was observed in the fabricated sample, where the set and reset occurred in the negative and positive bias regions, respectively. A switching endurance test of the sample was also performed in the voltage sweep mode. The sample was clamped by a series of convex shape molds with different radii.

Table 1. Comparison of this work with those on other flexible resistive memories Reference

Ji et al. 9

Shang et al. 14

Ji et al. 15

Kim et al. 22

Hong et al. 23

Hu et al. 24

Kim et al. 25

This work

Material

WO3-x

HfOx

PI: PCBM

TiOx

G-O film

G-O film

NiO

TiO2

Substrate

PET

PET

PET

Plastic

PET

PES

Plastic

PI

1000

108

50

100

100

100

100

4000

5×105

105

104

104

107

105

104

106

5.53

3

9

8.4

4

7

7.5

2

Strain

1.58%

2.12%

----

0.15%

----

----

----

3.62%

Bending cycles

103

1200

140

103

103

103

103

104

Endurance (cycle) Retention (s) Radius (mm)


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AUTHOR INFORMATION Corresponding Author #

E-mail: [email protected]

*E-mail: [email protected]

Author Contributions J.Z. and M.Z. conceived of the study and designed the experiments. M.Z. and S.W. fabricated the devices. J.Z., M.Z., and S.W. performed electrical characterizations of the samples. J.Z., Z.Y., and C.S.H. guided the conduct of the entire experiment and wrote the manuscript. All the authors have given their approval of the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT This work is supported by the National Natural Science Foundation of China (Grant No. 51502203), Tianjin Young Overseas High-level Talent Plans (Grant No. 01001502) and Special Program of Talent Development for High-Level Innovation and Entrepreneurship Team in Tianjin. Zhao also acknowledges the support of the Tianjin Natural Science Foundation (Grant No. 14JCZDJC31500), The Thousand Talents Plan of Tianjin, and the Tianjin Distinguished Chair Professor Foundation.


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