Flexible conductive-bridging random-access-memory cell vertically stacked with top Ag electrode, PEO, PVK, and bottom Pt electrode

Flexible conductive-bridging random-access-memory (RAM) cells were fabricated with a cross-bar memory cell stacked with a top Ag electrode, conductive polymer (poly(n-vinylcarbazole): PVK), electrolyte (polyethylene oxide: PEO), bottom Pt electrode, and flexible substrate (polyethersulfone: PES), exhibiting the bipolar switching behavior of resistive random access memory (ReRAM). The cell also exhibited bending-fatigue-free nonvolatile memory characteristics: i.e., a set voltage of 1.0 V, a reset voltage of –1.6 V, retention time of >1 × 105 s with a memory margin of 9.2 × 105, program/erase endurance cycles of >102 with a memory margin of 8.4 × 105, and bending-fatigue-free cycles of ∼1 × 103 with a memory margin (Ion/Ioff) of 3.3 × 105.


Introduction
Recently, extensive research on flexible nonvolatile memory cells has been conducted since information technology (IT) mobile devices have been rapidly transitioning from solid mobile devices to flexible mobile devices. Four types of flexible nonvolatile memory cells such as ReRAM [1,2], polymer random-access-memory (PoRAM) [3,4], conductive-bridging random-access-memory (CBRAM) [5,6], and nano-floatinggate-memory (NFGM) have been reported [7,8]. Among them, ReRAM, PoRAM, and CBRAM are fabricated with a 4F 2 cross-bar memory-cell structure, which enables the stacking of three-dimensional (3D) memory cells for achieving terra-bit-level integration. Although flexible ReRAMs using binary oxide material have demonstrated nonvolatile memory characteristics, their retention time and write/erase endurance cycles make it difficult to satisfy bending-fatigue-free criteria since they fundamentally use inorganic materials. Otherwise, PoRAM [9,10] or CBRAM [11] probably exhibits bendingfatigue-free nonvolatile memory characteristics since they use a polymer or polymer electrolyte. Although a CBRAM using a polymer electrolyte (PEO) fabricated on a flexible substrate (PES) exhibits the feasibility of 180 bending degrees, it does not exhibit flexible nonvolatile memory characteristics such as retention time, write/erase endurance cycles, and bending fatigue cycles [6]. Thus, we developed flexible CBRAM cells that use a double polymer layer (PEO/PVK) and have a crossbar memory-cell structure vertically sandwiched with a reactive Ag electrode, polymer electrolyte (PEO) layer, conductive polymer (PVK) layer, and inert Pt electrode. First, we investigated the dependency of bi-stable current-versus-voltage (I-V) curve on the concentration (weight percentage) of PEO and PVK for flexible CBRAM cells fabricated on a solid substrate (SiO 2 on Si substrate) to optimize the thickness ratio of the PEO to PVK layer. We also estimated the nonvolatile memory characteristics of flexible CBRAM cells, such as set voltage, reset voltage, retention time, program/erase endurance cycles, and bending-fatigue-free cycles, for optimizing the thickness ratio of the PEO to PVK layer. Finally, we reviewed the current conduction mechanism by which our flexible CBRAM cells exhibit nonvolatile memory characteristics.

Experimental procedure
To investigate the dependency of the bi-stable I-V curve on the weight percentage (wt%) of PEO and PVK, arrays of 20 × 20 CBRAM cells were fabricated on SiO 2 film grown on a Si wafer, as shown in figures 1(a) and 2(a). Arrays of 20 × 20 250 nm-diameter nano-holes were patterned on the bottom 50 nm thick Pt-electrode by using lithography and etching, where the thickness of the SiO 2 film isolating the holes was ∼50 nm. Then, PEO or PVK were spin-coated on the hole arrays at 2000 rpm for 120 s and baked at 120°C for 5 min and 60°C for 5 min, respectively. Note that the PEO was mixed with a co-solvent of acetonitrile and ethanol at 30°C for 1 h. Afterward, 300 μm-diameter 200 nm-thick top Ag electrodes were thermally evaporated at a rate of 1.0 Å s −1 under 10 −5 Pa by using a shadow mask. Thus, the CBRAM cells have a sandwich structure consisting of a bottom Pt electrode, PEO, and top Ag electrode on the SiO 2 film grown on a Si wafer. To investigate the bending fatigue of flexible CBREM cells, 2 × 6 cross-bar memory-cell arrays sandwiched with a bottom Pt electrode, PVK, PEO, and top Ag electrode were fabricated on a flexible substrate (PES), as shown in figure 3(a). The 150 μm wide bottom Pt electrode was patterned on the PES substrate by evaporation using a shadow mask. The PVK was spin-coated under 2000 rpm for 120 s and baked at 120°C for 5 min Then, PEO was spin-coated under 2000 rpm for 120 s and baked at 60°C for 5 min Finally, the 150 μm-wide top Ag electrode was patterned on the PEO film. Thus, these flexible CBRAM cells have a sandwich structure consisting of a bottom Pt electrode, PVK, PEO, and top Ag electrode on a flexible PES substrate.

Dependency of PEO-based CBRAM cell characteristics on electrolyte thickness
A cross-sectional transmission-electron-microscopy (x-TEM) image of a CBRAM cell using a solid electrolyte PEO layer showed that the thicknesses of the bottom electrode (Pt/TiN), polymer electrolyte (PEO 1.0 wt%), and top passivation (Al) layer were 92.6/50.0, 78.7, and 103.6 nm, respectively, and the hole diameter of the memory was 250 nm, as shown in figure 1(b). Note that the top Al layer was evaporated on the PEO layer to prepare an x-TEM sample and measure the PEO layer thickness. The thickness of the polymer electrolyte (PEO) layer was strongly dependent on the weight percentage (wt%) of the PEO solution, i.e., the PEO thickness linearly increased from 13 to 73 nm when the wt% of the PEO solution increased from 0.4 to 1.0 wt%, as shown in figure 1(c). In a CBRAM, the low resistance state (LRS) is achieved when conductive bridging filaments in the PEO layer are produced by Ag ion migration and stacking from the Ag to Pt electrodes in the polymer electrolyte (PEO) layer, which determines a set voltage (V set ). Otherwise, the highresistance state (HRS) is achieved when conductive bridging filaments in the polymer electrolyte (PEO) are broken by Ag ion migration and stacking from the Pt to Ag electrodes in the polymer electrolyte (PEO) layer, which determines the reset voltage (V reset ). In addition, the conical-shaped conductive-bridging filaments grow radially and vertically between the Ag and Pt electrodes so that the conical shape depends on the thickness of the PEO layer thickness, which determines V set or V reset . The nonvolatile memory characteristics of CBRAM cells depended on the PEO layer thickness (wt%), as shown in figure 1(c). All CBRAM cells showed a typical bi-stable I-V curve demonstrating the bipolar switching behavior of resistive RAM. When the PEO layer thickness increased from 13 to 73 nm, the set current (compliance level) of the CBRAM cells decreased from 10 -3 to 10 -6 A and the reset current decreased from 3.1 × 10 -6 to 5.6 × 10 -11 A. In addition, when the PEO thickness increased from 13 to 73 nm, V set increased from 0.3 to 1.7 V while V reset decreased from -3.4 to -0.85 V. The dependencies of the set current, reset current, V set , and V reset on the PEO layer thickness are related to the shape of the filaments in the PEO layer. The filaments in the PEO layer become longer when the PEO layer thickness increases, so that both set and reset current decrease with increasing PEO layer thickness and V set increases with the PEO layer thickness. The mechanism of set and reset current decreases with increasing PEO layer thickness, which was discovered by fitting the HRS of J-V curves with the current conduction mechanism of the Poole-Frenkel emission, as shown figure 1(d). The current mechanism of the Poole-Frenkel emission is defined by , k, T, and ΦB are elementary charge, PEO permittivity, PEO thickness, the applied voltage, Boltzmann's constant, temperature, and barrier height, respectively [12,13]. The slope of ln(J/V)-√V curves includes the PEO thickness so that a lower slope leads a greater PEO thickness. As expected, the thicker PEO layer showed a lower slope of ln(J/V)-√V, as shown figure 1(d). This result indicates that the Ag ion migration distance for forming the conductive-bridging filaments would increase with the PEO thickness. Otherwise, the diameter of the filaments in the PEO layer becomes smaller when the PEO layer thickness increases, so that V reset decreases with increasing PEO layer thickness. Furthermore, the PEO-based CBRAM (Ag/50 nm thick PEO/Pt) demonstrated at a retention time of ∼10 5 s at room temperature by sustaining a memory margin (I on /I off ) of 10 4 and program and erase cycles of 10 3 by sustaining a memory margin (I on /I off ) of 1.31 × 10 5 , as shown in figure 1(e).

Dependency of PVK-based ReRAM cell characteristics on conductive polymer thickness
Next, the dependency of the bi-stable I-V curves on conductive polymer (PVK) layer thickness was investigated in detail, as shown in figure 2. An x-TEM image of a ReRAM cell with a 250 nm diameter hole using a PVK layer showed that the thicknesses of the bottom electrode (Pt/TiN), polymer electrolyte (PVK 0.36 wt%), and top passivation (Al) layer were 100.6/50.5, 35.3, and 99.6 nm, respectively, as shown in figure 2(b). The thickness of the PVK layer was strongly dependent on the weight percentage (wt%) of the PVK solution, i.e., the PVK layer thickness linearly increased from 13 to 100 nm when the wt% of the PVK solution increased from 0.18 to 0.74 wt%, as shown in figure 2(c). All samples showed a typical bi-stable I-V curve demonstrating the bipolar switching behavior of ReRAM. However, there was no dependency of the nonvolatile characteristics on PVK layer thickness between 13 nm and 100 nm; i.e., the set current (10 −3 A), reset current (∼3.2 × 10 -8 A), V set (0.3 V), and V reset (-3.95 V) were independent of the PVK layer thickness. This result indicates that the current conduction for the ReRAM using a conductive polymer (PVK) layer is probably not associated with charged defects that exist in a solid electrolyte layer.

Flexible CBRAM fabricated with double polymer layers
To fabricate a flexible CBRAM cell, cross-bar (150 um long × 150 um wide) memory-cell arrays were fabricated with a cross-bar structure; top Ag electrode/polymer electrolyte layer (PEO)/conductive polymer (PVK) layer/bottom Pt electrode/flexible substrate (PES), as shown in figures 3(a) and (b). The CBRAM fabricated with the device structure of Ag/PEO/Pt performed with a quite low set voltage and ONstate current, as shown in figure 1(c). Thus, to overcome this issue, we developed a double-stacked polymer (PEO/PVK)based CBRAM since the PEO layer was used for a polymer electrolyte layer, while the PVK layer was used for a conductive polymer (or insulator), as shown in figure 3(a). The insertion of the thin PVK layer between the PEO layer and bottom Pt electrode may control the conduction current level and the shape of filaments in the PEO layer. A strain of ∼0.4% was applied to the CBRAM cells to estimate bending fatigue, where the diameter of the bended flexible CBRAM cells was 25 mm, as shown in figure 3(g) [14,15]. An x-TEM image of a flexible CBRAM cell using double polymer layers (PEO/PVK) showed that the thicknesses of the bottom Pt electrode, polymer electrolyte (PEO 0.8 wt%) layer, conductive polymer (PVK 0.3 wt%) layer, and top Ag electrode were 109.9, 20.8, 49.1, and 40.6 nm, respectively, as shown in figure 3(c). This flexible CBRAM cell showed a typical bistable I-V curve having a V set of 1.0 V, LRS of 9.5 × 10 -4 A, V reset of -1.6 V, and HRS of 8.8 × 10 -9 A when the bias was scanned from 0, 3, 0, -3, and 0 V, as shown in figure 3(d). In particular, the flexible CBRAM cell exhibited an asymmetrical I-V curve, where the absolute V set (1.0 V) was less than the absolute V reset (-1.6 V), implying that the shape of the conical conductive-bridging filament may be asymmetrical. A retention time of ∼105 s of the memory cell at room temperature was obtained by sustaining a memory margin (I on / I off ) of 9.2 × 10 5 , indicating a probable extension to 10 years, as shown in figure 3(e). In addition, program and erase cycles of more than 10 2 was obtained by sustaining a memory margin (I on /I off ) of 8.4 × 10 5 , as shown in figure 3(f). Furthermore, bending-fatigue-free cycles of ∼1 × 10 3 were obtained by sustaining a memory margin (I on /I off ) of 3.3 × 10 5 , as shown in figure 3(h). In addition, bending- fatigue-free time of ∼170 h was obtained by sustaining a memory margin (I on /I off ) of 2.34 × 10 6 , as shown in figure 3(i). These results indicate that a flexible CBRAM cell using a double polymer layer exhibits good bending-fatiguefree cycles, which suggests the possibility of commercially viable nonvolatile memory cells.

The current conduction mechanism of flexible CBRAM
The current conduction mechanism of a flexible CBRAM cell using a double polymer layer was reviewed by fitting the I-V curve in figure 3(e) when the applied bias was scanned from 0, 3, 0, -3, and 0 V, as shown in figure 4. The current conduction of the HRS between 0 V and V set (1.0 V) fitted well with the conduction mechanism of the Poole-Frenkel emission, having a slope of 0.02, as shown in figure 4(a), which is , where q, ε i , d, V, k, T, and Φ B are elementary charge, insulator permittivity, insulator thickness, the applied voltage, Boltzmann's constant, temperature, and barrier height, respectively. This result indicates that the conical-shaped filaments in the double polymer layer are formed due to the drift of Ag ions from the top Ag electrode to the bottom Pt electrode via the Poole--Frenkel emission process. After forming the conical-shaped filaments and sustaining the compliance level of the conduction current, the current conduction from V set to 0 V was fitted with the Ohmic conduction mechanism, as shown in figure 4(b), J/V ∝ e (−c/T) where c, V, and T are a constant, the applied voltage, and temperature, respectively [16]. In addition, the current conduction between 0 and V reset exhibited typical I-V behavior of negative-differential resistance (NDR), which implied the breaking process of the conical-shaped filaments in the double polymer layer, as shown in figure 4(c). After breaking the conical-shaped filaments, the current conduction from V reset to 0 V was fitted with the current conduction mechanism of the Poole-Frenkel emission, drifting back Ag ions from the Pt electrode to Ag electrode, as shown in figure 4(d). Therefore, the current conduction mechanism for a flexible CBRAM cell is well described by the forming and breaking process of the conical-shaped filaments in the double polymer layer between the Pt and Ag electrodes.

Conclusion
The nonvolatile memory characteristics of CBRAM cells are dependent on the cross-bar memory-cell size since nano-scale conical-shaped filaments are randomly distributed in the polymer electrolyte layer after the forming process; i.e., a larger cross-bar memory-cell size leads to a higher number of conical-shaped filaments. In general, a thinner polymer electrolyte layer in CBRAM cells results in a smaller distribution of nonvolatile memory characteristics since the length and diameter of such filament becomes smaller and more uniform as the polymer electrolyte layer thickness becomes thinner. However, a thinner polymer electrolyte layer in CBRAM cells produces a higher conduction current, which easily approaches the compliance current level. The insertion of a conductive polymer layer between the polymer electrolyte layer (PEO) and bottom Pt electrode in CBRAM cells could control the length and diameter of the conical-shaped filaments in the polymer electrolyte layer and adjust the conduction current level. Thus, the thickness ratio control of the polymer electrolyte layer to the inserted conductive polymer layer enable control of nonvolatile memory characteristics such as the  compliance current level, V set , V reset , memory margin (I on / I off ), retention time, write/erase endurance cycles, and bending fatigue behavior. It was confirmed that the thickness ratio of the electrolyte polymer layer to the conductive polymer layer strongly affects nonvolatile memory characteristics; i.e., there is a specific thickness ratio exhibiting the best nonvolatile memory characteristics since the drift velocity and solubility of Ag ions in the polymer electrolyte (PEO) layer is different from that of the conductive polymer (PVK) layer. In addition, since the interface between the conductive polymer layer and proper metal electrode is able to produce a Schottky barrier, investigating the design of a selecting diode using a Schottky barrier at the interface between the bottom inert electrode and inserted conductive polymer is key to engineering flexible 3D stacked cross-bar CBRAM-cell arrays for terra-bit-level nonvolatile flexible memory cells.