Flexible Nanoporous WO3−x Nonvolatile Memory Device Yongsung Ji,†,△ Yang Yang,⊥,∥,△ Seoung-Ki Lee,† Gedeng Ruan,† Tae-Wook Kim,# Huilong Fei,† Seung-Hoon Lee,¶ Dong-Yu Kim,¶ Jongwon Yoon,† and James M. Tour*,†,‡,§ †
Department of Chemistry, ‡NanoCarbon Center, and §Department of Materials Science and NanoEngineering, Rice University, 6100 Main Street, Houston, Texas 77005, United States ⊥ Nanoscience Technology Center and ∥Department of Materials Science and Engineering, University of Central Florida, 12424 Research Parkway, Orlando, Florida 32826, United States # Soft Innovative Materials Research Center, Institute of Advanced Composite Materials, Korea Institute of Science and Technology, Joellabuk-do 565-905, Republic of Korea ¶ School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea S Supporting Information *
ABSTRACT: Flexible resistive random access memory (RRAM) devices have attracted great interest for future nonvolatile memories. However, making active layer films at high temperature can be a hindrance to RRAM device fabrication on flexible substrates. Here, we introduced a flexible nanoporous (NP) WO3−x RRAM device using anodic treatment in a room-temperature process. The flexible NP WO3−x RRAM device showed bipolar switching characteristics and a high ION/IOFF ratio of ∼105. The device also showed stable retention time over 5 × 105 s, outstanding cell-to-cell uniformity, and bending endurance over 103 cycles when measured in both the flat and the maximum bending conditions. KEYWORDS: resistive random access memory, flexible memory, WO3−x memory, nanoporous
R
RRAM device, a low-temperature process is preferred without the additional plasma treatment or thermal annealing process steps. Recently, an electrochemical anodic treatment was introduced to convert metal thin films to metal oxides. Such anodic treatment produces a nanoporous (NP) structure with defects such as oxygen vacancies in the anodized thin films.25 The NP structure has high surface area and is electrochemically active. Because of these properties, the anodized NP metal oxide layers have been used as active materials in supercapacitors, catalysts, and water electrolyzers.26−29 The NP structures have the potential to be used in RRAM devices as mobile ion or charge trap sites and vacancy-assisted switching materials.30 To date, there have been only a small number of reports on RRAM devices that have a NP structure for active materials.7,30 In this work, we introduce a NP WO3−x layer produced after anodic treatment as the active layer in a RRAM device and demonstrate a flexible Cu/NP WO3−x/indium tin oxide (ITO)
esistive random access memory (RRAM) devices have been studied for use as next-generation memory devices in order to solve the limitations of commercial transistor-based memory devices, such as minimum cell sizes, high fabrication costs, complicated fabrication processes, and high power consumption.1−3 RRAM can have excellent scalability with a simple structure, two-terminal fabrication process, high switching speed, low power consumption, high endurance cycles, and large material variety.1,4−10 As the active materials for RRAM, transition metal oxides (TMOs) have been extensively studied because of their excellent resistive switching ability, flexible stoichiometry, and compatibility with the complementary metal oxide semiconductor (CMOS) process flow.1,11−15 Among TMO materials, WO3−x is considered to be a particularly favorable material for back-end-of-line (BEOL) processing in CMOSintegrated circuits.16,17,20,21 In order to achieve good quality WO3−x layers for memory devices, additional oxygen plasma treatment, or thermal annealing processes are required after WO3−x layer depositions.16,17,22−24 However, such additional oxygen plasma treatment and thermal annealing processes are not easily compatible processes for producing flexible electronics on plastic substrates. In order to develop a flexible © 2016 American Chemical Society
Received: April 22, 2016 Accepted: August 2, 2016 Published: August 2, 2016 7598
DOI: 10.1021/acsnano.6b02711 ACS Nano 2016, 10, 7598−7603
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Figure 1. (a) Schematic of a fabricated flexible Cu/NP WO3−x/ITO memory device. (b) Top view SEM image of the flexible Cu/NP WO3−x/ ITO memory device. (c) Cross-sectional TEM image of the flexible Cu/NP WO3−x/ITO memory device. (d) TEM image of NP WO3−x film showing pores and the rough surface. (e) XPS spectrum of the W 4f peaks and O 1s peak (inset) of the W surface after anodic treatment.
Figure 2. (a) I−V characteristics of a flexible Cu/NP WO3−x/ITO memory device. The blue open circles show the I−V curve when the device was flat, and the red open circles show when the device was bent. The inset shows the optical image of a flexible and bent Cu/NP WO3−x/ITO memory device. Scale bar: 15.5 mm. (b) Log−log plot analysis of I−V characteristics of the flexible Cu/NP WO3−x/ITO memory device, with four points of interest labeled 1−4 and discussed in the text.
thermal annealing process is required to achieve a good-quality oxide layer.16,17,22−24 However, these additional treatments could damage flexible substrates and cause poor device performance. In order to solve this problem, we introduced anodic treatment which converts a metal layer to a metal oxide layer in a room-temperature process.25 Figure 1c shows a crosssectional transmission electron microscope (TEM) image of the flexible Cu/NP WO3−x/ITO memory device. The NP WO3−x layer was prepared by anodic treatment on a W (40 nm)/ITO (50 nm)/PET substrate. Because of the limited oxidation depth during anodic treatment, the anodized W layer showed different contrast, as shown in Figure 1c. Also, an X-ray photoelectron spectroscopy (XPS) depth profile analysis of the W layer showed continuously decreasing oxygen in the WO3−x layer as the depth increased (see Figure S1, Supporting Information). A TEM micrograph shows that nanoscale pores in the anodized W layer are randomly distributed with diameters less than 5 nm (Figure 1d). The pores created in the WO3−x create a pathway that could possibly be used by the top Cu electrode to form a bridge with the bottom ITO
memory device on a poly(ethylene terephthalate) (PET) substrate. The flexible Cu/NP WO3−x/ITO memory device showed typical bipolar switching behavior. The switching mechanism of the memory device is strongly related to the trapped and formed filamentary path of Cuz+ (where z is 1 or 2) cations in the NP structure. When Pt was used as a top electrode, the device did not switch, indicating that W does not form filaments. The memory device showed a ∼105 ION/IOFF ratio, a stable retention time over 5 × 105 s, with endurance over 103 cycles, and uniform ON and OFF current distribution even under maximum bending conditions. In addition, the flexible Cu/NP WO3−x/ITO memory device showed robust switching behavior after 1000 bending cycles.
RESULTS AND DISCUSSION Figure 1a,b shows a schematic image and a top view of the scanning electron microscope (SEM) image of a fabricated Cu/ NP WO3−x/ITO memory device on a flexible PET substrate, respectively. Typically, an additional plasma treatment or 7599
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Figure 3. Schematic diagram of the proposed mechanism for the operation of the Cu/NP WO3−x/ITO memory device. (a) Oxidation of Cu at the top electrode. (b) Migration of the Cuz+ ions to the bottom electrode. (c) Formation of the Cu metal filament path between the top and bottom electrodes. (d) Electrochemical dissolution of the Cu filamentary path. CAFM images of the Cu/NP WO3−x/ITO memory device (e) at the HRS and (f) at the LRS.
bending (r ∼ 5.53 mm) condition, as shown in the Figure 2a (red open circles). In order to study the resistive switching of the Cu/NP WO3−x/ITO memory device, a log−log plot of the I−V analysis was obtained, as shown in Figure 2b. The labeled points are as follows: (1) At a low voltage regime below 0.1 V, the current was almost linearly proportional to the applied voltage (α1 ∼ 1.1), which can be explained by thermally generated free carriers.23,33−37 (2) At a voltage regime between 0.1 and 0.7 V, the slope of the I−V plot increased (α2 ∼ 2.0), indicating that the switching was related to the space-charge-limited current.21,33−37 In this regime, the injected carriers are captured by trap states in the NP WO3−x layer. (3) At a voltage regime between 0.7 and 1.0 V, the slope of the I−V plot was ∼4.2 (α3). In this regime, the trap states in the switching region (NP WO3−x) were filled with injected carriers; the current increased rapidly due to trap-charge-limited current, and the device turned ON.37,38 (4) After the turn ON process, the slope of the I−V plot was ∼1.0 (α4), indicating ohmic behavior.21,33−37 Based on the above experimental results and theoretical analysis, a schematic diagram for the switching mechanism is illustrated in Figure 3a−d. Generally, a Cu electrode is used for the active electrodes in cation-based RRAM due to its high ion mobility, ease of oxidation, and reduction.21,39−44 When a positive voltage is applied to the Cu top electrode, oxidation occurs and Cuz+ (where z is 1 or 2) cations are generated, which could be described as Cu → Cuz+ + ze− (Figure 3a). The generated Cuz+ cations that migrate into the NP WO3−x layer are captured in the NP WO3−x layer’s defects or vacancies and are reduced by electrons from the cathode (bottom ITO electrode), that is, Cuz+ + ze− → Cu (Figure 3b).42 This migration of the oxidized Cuz+ into the active layer is verified by XPS analysis that is shown in the Supporting Information Figure S3.17,21 Once the applied voltage reaches a certain value, a conductive filamentary path is formed between the anode (Cu top electrode) and cathode (ITO bottom electrode) through the NP WO3−x layer. The memory device is in the ON state (Figure 3c). When the polarity of the applied voltage is reversed, an electrochemical dissolution occurs in the Cu
electrode, producing memory switching effects. However, the top to bottom size gradient that is created during the anodic treatment lessens the possibility of Cu shorts. In addition, this rough pore surface could induce a highly localized electric field and subsequent migration of the material from the anode (Cu top electrode) to form a conductive filamentary path.17−19 The valence states of W in the anodized layer were studied by XPS analysis. As shown in Figure 1e, two XPS peaks, W 4f7/2 at 35.8 eV and W 4f5/2 at 38.0 eV, are observed that show formation of WO3 after anodic treatment of the W layer.17,31 The XPS O 1s peak is also observed, as shown in the inset of Figure 1e. Anodization converts W metal compact films to defective WO3−x porous layers rapidly at room temperature, which reduces the cost and time to produce flexible memory. The memory switching behavior is characterized as shown in Figure 2. Figure 2a shows the current−voltage (I−V) graph of a flexible Cu/NP WO3−x/ITO memory device before (flat, blue open circles) and after maximum bending (radius of curvature ≈ 5.53 mm, strain of 1.58%, red open circles). The detailed bending methods, estimation of the radius of curvature, and strain are explained in Figure S2, Supporting Information.32 The memory device showed an initial high-resistance state (HRS, OFF state). To turn ON the memory device, an external bias was applied from 0 to 1.5 V in dual sweep mode (0 V → 1.5 V → 0 V). When the applied bias exceeded 1.0 V, the HRS was changed to a low-resistance state (LRS, ON state). Once the memory device turned ON (LRS), the ON current state was kept even in the low voltage region (V < 1.0 V), showing the nonvolatile memory property. To turn OFF the memory device, an external bias was applied at 0 V to −1.5 V in dual sweep mode (0 V → −1.5 V → 0 V). Unlike the positive polarity sweep, the memory device turned OFF when the applied bias exceeded −1.1 V, showing typical bipolar switching behavior. To verify that the memory device can show reliable operation even under a bent condition, the I−V was recorded under bending conditions. In the bending test, ∼5.53 mm was the minimum radius of curvature (i.e., maximum bending condition). The memory device on the flexible substrate showed the same switching behavior even under the maximum 7600
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Figure 4. (a) Statistical ION/IOFF ratio distribution of the flexible Cu/NP WO3−x/ITO memory devices under gradual bending conditions and (b) for repeated bending cycles in 30 working cells. The error bars for each graph are the standard deviation of 30 working cells.
Figure 5. (a) Retention time of the flexible Cu/NP WO3−x/ITO memory device when the device is flat or bent at its maximum bending condition. (b) Cumulative probability of the resistance states in both the HRS and LRS when it is flat or bent at its maximum bending condition in 30 working cells. Endurance cycles of the memory device when the device is (c) flat or (d) bent at its maximum bending condition.
filamentary path and the memory device is turned OFF (Figure 3d). To support the memory switching induced by filamentary path formation, conductive atomic force microscopy (CAFM) was performed to analyze both the HRS (OFF) and LRS (ON). A conductive cantilever coated with Pt was used to scan the top electrode in the CAFM experiment. The conductive cantilever was scanned over an area of 2 μm × 2 μm in both the HRS and LRS, where the read voltage of each state was made to be 1.0 V. As shown in Figure 3e, when the memory device was in the HRS, the current value of the HRS was very low and no remarkable localized current paths were observed. However, when the memory device was in the LRS, several localized current paths were observed, as shown in Figure 3f. This highly localized current shows the formation of the filamentary path in the LRS. In order to investigate how the flexible Cu/NP WO3−x/ITO memory device behaved under bending conditions, an investigation of ION/IOFF ratios was performed while the
memory device was bent using 30 working cells. The PET substrate (15.5 × 15.5 mm2) holding the memory device was bent from flat to radius of curvature (r) of ∼5.53 mm (strain of 1.58%). As shown in Figure 4a, the memory device was perfectly functional, even after the strain of 1.58% bending showed a ∼105 ION/IOFF ratio. Also, the memory device was bent more to assess the operational failure during bending conditions. When the memory device was bent up to a strain of 1.95% (r ∼ 4.48 mm), the memory device was broken and both ON and OFF currents were reached at the compliance current (0.1 A) of the Agilent B1500 and showed no ION/IOFF ratio, as shown in Figure 4a. The memory device does not show any switching behavior at a strain of 1.95%. To examine the operational uniformity of the memory device, ION/IOFF ratios were statically analyzed during repetitive bending conditions using 30 working cells. The memory device showed a stable ION/IOFF ratio (∼105) distribution without any significant electrical degradation during 1000 bending cycles. One bending 7601
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ACS Nano cycle is defined as moving from a flat device to the maximum bending condition (r ∼ 5.53 mm, strain of 1.58%) and back to flat. It is notable that, in spite of using the inorganic oxide, flexible operation was well maintained. To evaluate operational reliability of the flexible Cu/NP WO3−x/ITO memory device, the memory performance characteristics such as retention, cumulative probability, and endurance cycles were analyzed in both the flat and the maximum bending conditions. As shown in Figure 5a, the retention characteristics of the flexible Cu/NP WO3−x/ITO memory device were tested when the memory device was flat (blue circles) and bent at its maximum bending condition (red circles). The memory device showed stable retention properties, showing ∼105 ION/IOFF ratio over 5 × 105 s, and no remarkable degradation was observed, even in the maximum bending condition. In order to check the memory device operational uniformity, the resistance of ON and OFF current values was statistically analyzed on 30 working cells in both the flat and the maximum bending condition, as shown in Figure 5b. The flexible Cu/NP WO3−x/ITO memory device showed a narrow distribution in both ON and OFF current states even under its maximum bent radius of ∼5.53 mm (strain of 1.58%), exhibiting good operational uniformity. The endurance cycle test of the flexible Cu/NP WO3−x/ITO memory device was carried out before and after maximum bending to verify switching reproducibility (Figure 5c,d). The memory device showed good endurance properties up to 103 cycles without significant variation in current states, thereby exhibiting reliable switching reproducibility. The memory states were programmed to turn ON by a writing pulse of 1.5 V and turn OFF by an erasing pulse of −1.5 V during 500 μs, followed by the reading process at 0.5 V.
solution was prepared by dissolving 0.2 M NH4F (98% Sigma-Aldrich, USA) in 2 M deionized (DI) water in ethylene glycol (Fisher Scientific, USA). A two-electrode system was used in the anodic treatments with the W as the anode and Pt foil as the cathode. For anodic treatment, the W/ITO/PET substrate was wrapped with Al foil in order to make conductive contact with the Cu plate (anode). Note that the area reacting with NH4F/ethylene glycol solution should not be wrapped with Al foil. To form the NP WO3−x layer from the W layer, 20 V was applied for 20 s. After this anodic treatment, the sample was rinsed with DI water and dried under a nitrogen flow. Then, 50 nm thick Cu was deposited through a square patterned shadow mask (200 × 200 μm2) on the anodic-treated W/ITO/PET substrate (the detailed fabrication methods and scheme are explained in Supporting Information Figure S4). All electrical characterizations were performed using Agilent B1500 semiconductor parameter analyzers equipped with a pulse generator.
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b02711. Oxygen ratio of NP WO3−x (Figure S1); bending method for the flexible Cu/NP WO3−x/ITO memory device (Figure S2); XPS spectra for the flexible Cu/NP WO3−x/ITO memory device (Figure S3); fabrication of NP WO3−x memory device on a flexible substrate (Figure S4); optical and electrical characteristics of films after oxygen plasma treatment (Figure S5); ION and IOFF current with respect to top Cu electrode diameter (Figure S6); estimated radius of curvature and strain of the flexible Cu/NP WO3−x/ITO memory device (Table S1) (PDF)
AUTHOR INFORMATION CONCLUSIONS In summary, we demonstrated a flexible Cu/NP WO3−x/ITO memory device using an anodic treatment. The anodic treatment enables low-temperature materials processing (to convert the W layer to a NP WO3−x layer), which allows device fabrication on a flexible PET substrate. The flexible Cu/NP WO3−x/ITO memory device showed good electronic and switching behavior even under maximum bending conditions. The flexible Cu/NP WO3−x/ITO memory device exhibited stable retention time (over 5 × 105 s), good endurance cycles up to 103 times without current overlap, and narrow operational uniformity in both the ON and OFF current states even under maximum bending conditions. Furthermore, the flexible Cu/NP WO3−x/ITO memory device showed robust memory operation up to 1000 bending cycles without any serious switching failure. The memory switching process was induced by space-charge-limited current and trapped-chargelimited current mechanisms, and the current flow was related to a Cu filamentary path.
Corresponding Author
*E-mail:
[email protected]. Author Contributions △
Y.J. and Y.Y. contributed equally.
Notes
The authors declare no competing financial interest.
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EXPERIMENTAL METHODS A 50 nm thick ITO-coated PET substrate (175 μm thick, 15.5 × 15.5 mm2, MTI Corporation, ITO-PF-14K-300300) was cleaned by an ultrasonic bath (Crest, CP1100HT) using acetone and isopropyl alcohol. In order to synthesize the NP WO3−x layer for use as a memory switching layer, a 40 nm thick W layer was deposited on the ITO-coated PET substrate using a DC sputter (Desk V, Denton Vacuum). To electrochemically form a NP WO3−x layer from the W layer, the sample was treated by an anodization in an ammonium fluoride (NH4F)/ethylene glycol solution. The NH4F/ethylene glycol 7602
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DOI: 10.1021/acsnano.6b02711 ACS Nano 2016, 10, 7598−7603
