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Functional Inorganic Materials and Devices

Reset Voltages-Dependent Multilevel Resistive Switching Behavior in CsPb1-xBixI3 Perovskite-Based Memory Device Shuaipeng Ge, Yuhang Wang, Zhongcheng Xiang, and Yimin Cui ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07079 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 3, 2018

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Reset Voltages-Dependent Multilevel Resistive Switching Behavior in CsPb1−xBixI3 PerovskiteBased Memory Device Shuaipeng Ge,† Yuhang Wang,‡ Zhongcheng Xiang,† Yimin Cui*,†



Key Laboratory of Micro-nano Measurement-Manipulation and Physics, Ministry of Education,

Department of Physics, Beihang University, Beijing 100191, China. ‡

State Key Laboratory of Low-Dimensional Quantum Physics, Collaborative Innovation Center

of Quantum Matter, Department of Physics, Tsinghua University, Beijing, 100084, China

KEYWORDS: CsPb1−xBixI3, perovskite, resistive switching, multilevel high-resistance states, memory device

ABSTRACT

All-inorganic CsPb1−xBixI3 perovskite film was successfully fabricated by incorporating Bi3+ in CsPbI3 to stabilize the cubic lattice. Furthermore, the perovskite film was applied to manufacture

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a simple Ag/CsPb1−xBixI3/ITO memory device with a bipolar resistive switching behavior. Nonvolatile, reliable and reproducible switching properties are demonstrated through retention and endurance test under fully open-air conditions, respectively. The memory device also presents the highly uniform and long-term stable characteristics. Importantly, by modulating the reset stop voltages, the multilevel high-resistance states are observed for the first time in lead halide perovskite memory device. The resistive switching behavior is proposed to explain with the formation and partial rupture of conductive multifilament that are dominated by the migration of iodine ions and their corresponding vacancies in perovskite film. This study suggests Ag/CsPb1−xBixI3/ITO device potential application for multilevel data storage in nonvolatile memory device.

INTRODUCTION Resistive switching random-access memory (ReRAM), as a typical nonvolatile memory device with excellent scalability, fast switching speed, low power consumption and high integration density, exhibits promising applications for information storage and processing in nextgeneration computing systems.1-5 Current studies of ReRAM mainly focus on the transition metal oxides and perovskite oxides as active materials, including ZnO, WO3−x, PrxCa1–xMnO3 and SrTiO3.6-12 Organic-inorganic lead halide perovskites as new type of active materials for ReRAM also have attracted a great deal of attention due to their unique current-voltage (I-V) hysteresis property originating from defects and fast ion migration,4,13-18 and these memory devices based on the lead halide perovskites usually show more specific and remarkable resistive switching behavior than the case of transition metal oxides and perovskite oxides. But, as we

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know, organic-inorganic lead halide perovskites are unstable because of the hygroscopicity and thermal instability of organic cations that will cause the limitations for applications in resistive switching memory devices.19-21 Interestingly, many reports of lead halide perovskite solar cells have proved that the organic cations can be substituted by inorganic cations such as Cs to improve the stability while maintaining their structural and electrical properties.22-24 It implies that more stable all-inorganic cesium lead halide perovskites (CsPbX3, X = Cl, Br and I) have huge potentials as active materials for use in resistive switching memory devices. Several CsPbBr3 perovskite-based memory devices with outstanding resistive switching performance have been successfully manufactured.25-27 However, few studies regarding the CsPbI3 perovskite-based resistive switching device have been reported to date because of the more unstable perovskite phase compared with CsPbBr3 and CsPbCl3. The perovskite structure of CsPbI3 in black phase tends to degrade rapidly to a yellow nonperovskite phase at room temperature and under open-air conditions, which has undesirable electrical properties for resistive switching due to the limited migration path for native defects or constituent ions in nonperovskite phase. To our knowledge, the CsPbI3 perovskite-based memory device was only reported by Han et al till now.28 Unfortunately, the electrical properties of their memory device were still measured in a vacuum chamber, and on the other hand, their device possesses a complicated structure causing a difficult fabrication process. Therefore, designing simple device structure based on stable CsPbI3 perovskite that can be operated under fully open-air conditions, is significant and urgent for their widespread applications in the field of information storage and processing. In this work, using a smaller Bi3+ cation to partially substitute Pb2+ into CsPbI3 perovskite, we prepared stable all-inorganic CsPb1−xBixI3 film with a typical cubic perovskite lattice by spin-

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coating and low-temperature annealing processes on ITO substrate. And silver was deposited as top electrode to manufacture a simple Ag/CsPb1−xBixI3/ITO memory device. The memory device showed a bipolar resistive switching behavior. A series of controlling tests under fully open-air conditions confirmed the nonvolatile, reliable, reproducible and long-term stable switching performance of the highly uniform memory device. In addition, the multilevel resistive switching was achieved by modulating the reset stop voltages. The Ag/CsPb1−xBixI3/ITO device with excellent resistive switching behavior exhibits a potential application in multilevel nonvolatile memory device.

EXPERIMENTAL SECTION CsPb1−xBixI3 Solution Preparation: The CsPb1−xBixI3 perovskite precursor solution was prepared by using a modified procedure.24,29,30 2 mmol of CsI and 2 mmol mixture of PbI2/BiI3 (4 mol % BiI3) were fully dissolved in 3 mL of N, N-dimethylformamide (DMF) under magnetic stirring. Afterwards, 132 µL of hydroiodic acid (HI) was added to improve the solubility of the precursors in DMF solution for stirring 10 min with ambient humidity below 30% at room temperature. Resistive Switching Memory Device Fabrication: First, an ITO substrate was ultrasonically cleaned by detergent, deionized water, ethanol for 15 min sequentially and then dried under a nitrogen gas flow. Subsequently, the CsPb1−xBixI3 perovskite precursor solution was spin-coated on the ITO substrate at 3000 rpm for 30 s. The precursor film was annealed at 80 °C for 1 min, then isopropanol (IPA) was immediately dispensed onto the perovskite film, followed by annealing for another about 1 min to form the black phase perovskite. Finally, 1 mm × 1 mm Ag top electrodes with ~80 nm thickness were deposited on the perovskite film by

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direct-current

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Ag/CsPb1−xBixI3/ITO structure. Device Characterization: The surface morphology and cross-sectional images of the device were observed by field-emission scanning electronic microscopy (FESEM, Hitachi S4800). The topography of CsPb1−xBixI3 film was measured using an atomic force microscope (AFM, Bruker-ICON2-SYS). The crystal structures and chemical compositions were recorded and analyzed by X-ray diffraction (XRD, X ’Pert Pro MPD system, Cu Kα radiation) and X-ray photoelectron spectroscopy (XPS, Thermo Fisher ESCALAB 250 system, Al Kα radiation), respectively. All of the I-V characteristics of Ag/CsPb1−xBixI3/ITO cells were measured at room temperature using a Keithley 2400 semiconductor parameter analyzer. Note that all the above procedures of solution preparation, device fabrication and characterization were performed under fully open-air conditions.

RESULTS AND DISCUSSION

Figure 1. Schematic diagrams of the Ag/CsPb1−xBixI3/ITO memory device and the measurement configuration.

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Figure 2. (a) Top-view SEM image of the CsPb1−xBixI3 perovskite film. The inset shows crosssectional SEM image of the device structure. (b) AFM image of the CsPb1−xBixI3 perovskite film. (c) XPS survey spectra of the CsPb1−xBixI3 perovskite film. The inset shows high-resolution spectrum of Bi 4f. And (d) XRD pattern of the CsPb1−xBixI3 perovskite film on ITO glass substrate. Figure 1 depicts a schematic illustration of the designed memory device and the measurement configuration. The device possesses a simple structure consisting of only a switching layer of CsPb1−xBixI3, a top Ag electrode and a bottom ITO electrode on a glass substrate. The crosssectional SEM image (inset in Figure 2a) also shows the device structure and reveals a perovskite film thickness of approximately 900 nm. Furthermore, top-view SEM and AFM

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images shown in Figure 2a,b confirm the uniform perovskite film with a root mean square roughness of 20.98 nm. The XPS survey spectra in Figure 2c present the elemental compositions of the CsPb1−xBixI3 film, especially the Bi 4f spectrum (inset in Figure 2c) with two peaks at 159.3 and 165.3 eV corresponding to the binding energies of Bi 4f7/2 and Bi 4f5/2 respectively, demonstrates the existence of the +3 valence state of Bi in the CsPb1−xBixI3 film. In Figure 2d, the peaks at 14.95, 20.88, 25.56, 29.63, 33.07, 36.07, and 41.54° in XRD pattern, assign to the (100), (110), (111), (200), (201), (211), and (220) planes of the cubic (Pm-3m) lattice, except for peaks of the ITO substrate marked with black dots. Compared with the reported peaks of the CsPbI3,24 the corresponding peaks of CsPb1−xBixI3 slightly shift to higher 2θ degree, suggesting a lattice shrinkage in the CsPb1−xBixI3 crystals due to the smaller Bi3+ (1.03 Å) cation partially substituted the Pb2+ (1.19 Å). Figure 3a shows the electroforming process of the Ag/CsPb1−xBixI3/ITO device in a semilogarithmic scale.The sharp increase in current according to the change of high resistance state (HRS, off state) to a low resistance state (LRS, on state), can be observed at the voltage about −3.6 V. By sweeping the voltage in the following order 0 V→ −5.0 V→ 0 V→ 5.0 V→ 0 V, the I V curve presents bipolar resistive switching behavior as shown in Figure 3b. The set process meaning the resistive switching from HRS to LRS, occurs during application of a negative voltage from 0 to −5.0 V. After this switching, the LRS remains until the applied positive voltage increases to ~4.0 V. Subsequently, the resistive state returns to the HRS corresponding to the reset process. In Figure 3c, retention characteristics of ON and OFF currents are obtained with a reading voltage (Vreadout) of 2.0 V for a time period of ~104 s to evaluate the nonvolatile properties of the memory device. Slight degradation of ON current can be observed, but the device still keeps a decent ON/OFF ratio. Furthermore, the switching

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endurance properties of the perovskite memory device are determined, which maintains a stable switching between HRS and LRS up to 500 cycles (Figure 3d), indicating a reliable and reproducible resistive switching behavior in Ag/CsPb1−xBixI3/ITO memory device.

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Figure 3. (a) Electroforming process of the Ag/CsPb1−xBixI3/ITO device. (b) The typical I-V curve of the device. (c) Retention test of ON/OFF current. (d) Switching endurance between HRS and LRS.

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Figure 4. (a) I-V characteristics for five random points at the same device. (b) I-V characteristics for five different devices at random points in each device. (c) Stability test of the memory device for five days. Additionally, the I-V characteristics for different points and devices are critically important measurement to evaluate reproducibility of the memory device. As shown in Figure 4a and b, both resistive switching behavior of five random points at the same device and five different devices are almost identical, confirming the high uniformity of the devices. It is known that the CsPbI3 perovskites are not stable and degrade rapidly to a nonperovskite phase at room temperature and under open-air conditions because the size of the cesium cation is too small to support the PbI6 polyhedral frame in the cubic perovskite structure.23,28 Cubic perovskite

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structure has more preferential migration path along the 1D-like channels for the native defects or constituent ions due to the linear alignment of corner-shared PbI6 polyhedral than the case of nonperovskite phase,28 and the fast ion migration will lead to excellent resistive switching performance in cubic perovskite-based memory devices. And partially substituting Pb2+ with a smaller Bi3+ cation has been proved that can cause the lattice microstrain to stabilize the CsPbI3 perovskite structure.29 Table 1 shows a comparison for the stability of CsPbI3 and the effects of Bi3+ cation incorporation. As seen, the air stability has a significant improvement over 144 h by incorporating Bi3+ in CsPbI3. Therefore, the measurement for long-term stability of the Ag/CsPb1−xBixI3/ITO memory device based on the Bi-incorporated CsPb1−xBixI3 film also should be performed. Figure 4c shows the characteristics of I-V curves for five days under fully open-air conditions. And with time prolonged, the resistive switching behavior of the memory device can be observed without any obvious degradation. All these resistive switching properties indicate that the Ag/CsPb1−xBixI3/ITO memory device exhibits potentials to be a good candidate for practical applications. Table1: A comparison of the stability of CsPbI3 films and the effects of Bi3+ incorporation. Preparation environment

Precursor solution

Air stability

Ref.

N2 atmosphere

PbI2, CsI, DMF

Rapid phase transition

31

N2 atmosphere

PbI2, CsI, DMSO

Less than 15 min

32

N2 atmosphere

PbI2, CsI, DMF, HI

Stable for several hours

23

Open-air

PbI2, CsI, DMF, HI (IPA treatment)

Stable for 72 h

24

Ar atmosphere

PbI2, CsI, HI, BiI3 DMSO, DMF

Stable for over 144 h

29

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Figure 5. The log I-log V characteristics with fitted conduction mechanism in positive voltage sweeps. In order to further understand the resistive switching behavior of Ag/CsPb1−xBixI3/ITO memory device, Figure 5 presents the log I-log V curve for the positive voltage sweeps after electroforming process. The fitting results in the LRS with a slope (S) of ~1.07 corresponding to the ohmic conduction (I∝V) that are always observed in conductive filament model, indicates that the switching mechanism is the bulk-limited conduction depended on the CsPb1−xBixI3 perovskite itself.15,33 Moreover, the sharp increase of the current in the electroforming process (Figure 3a) also implies the typical feature of conductive filament model. Especially, recent progress has proved that iodine ions and their corresponding vacancies are the dominant factor for the formation of the conductive filament in lead iodide perovskite memory device because of the minimum order of 106 for the migration rate constant of iodine vacancy and relatively low migration energy barrier of iodine ions among the other transferable compositions.4,15 The active electrode contributes less to the conductive filament due to the thicker film hindering the formation of metallic conductive filament.34 Although, the role of contact barrier at the interfaces between the electrode and perovskite film could not be ignored, but it can be tailored by the concentration of iodine ions and their corresponding vacancies.35

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Compared with the only ohmic conduction in the LRS, the HRS shows much more complicated behavior with three portions. They are dominated by trap-filled limited current (S2~4.02, I∝Vα, α > 2), space-charge-limited current (S3~2.26, I∝V2) and thermally generated free carriers (S4~1.40), respectively, indicating the trap-controlled charge transports behavior in the HRS.6,36-38 It should be noted that the current decreases smoothly and stepwise from LRS to HRS, which might be attributed to the partial rupture of multifilament. Afterwards, as the applied negative voltage sweeps again, resistance switches to LRS more easily with no sharp increase of the current during the set process due to the residual un-ruptured conductive filaments. And the mechanism for partial rupture of multifilament can be further explained when different reset stop voltages are applied.

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Figure 6. (a) The multilevel resistive switching of the memory device. (b) Endurance of the multilevel resistance states. It is known that the multilevel resistive switching can be achieved not only by modulating the current compliance in the set process but also by modulating the reset stop voltage in the reset process. Several research groups has reported that the multilevel low-resistance states is

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possible in lead halide perovskite memory device by modulating the current compliance,28,39 but multilevel high-resistance states are not observed to date. Here, Figure 6a shows the resistive switching with multilevel high-resistance states that is achieved by applying different reset stop voltage at 3.0 V, 4.0 V and 5.0 V respectively. Four distinguishable resistance states including one LRS (L1) and three HRS (L2, L3 and L4) are obtained, which correspond to the partial rupture of multifilament with different threshold voltages in perovskite film. The larger reset stop voltages are applied, the more filaments are ruptured resulting in the higher resistance states. And the endurance measurement in Figure 6b exhibits a stable switching and adequate separation among multilevel resistance states over to 200 cycles. The multilevel resistive switching of Ag/CsPb1−xBixI3/ITO memory device implies a potential for multilevel data storage by modulating the reset stop voltage in next-generation computing systems. As mentioned above, the resistive switching behavior may be attributed to conductive filament model dominating by the iodine ions and their corresponding vacancies in the CsPb1−xBixI3 perovskite film. When a negative voltage sweep is applied to the memory device in the HRS, iodine ions can easily migrate toward the ITO electrode with the accumulation of vacancies from the top to bottom electrode along the 1D-like channels, where corner-shared PbI6 polyhedral aligns linearly. Meanwhile, the injected carriers are captured by the traps. The increase of the concentration of iodine ions or their corresponding vacancies between the electrode and perovskite film reduces the contact barrier resulting in favorable conduction at the interfaces. Once the formation of vacancy multifilament along with the rising negative voltage, the electroforming process occurs with a sharp increase of current which induce a switch to LRS in the memory device. Sequentially, the LRS is maintained till a larger positive reset voltage is applied. It can cause the partial rupture of the vacancy filaments because of the different

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threshold voltage by the redistribution of the iodine ions aligned with vacancies which induce a recovery to HRS. By applying a larger reset stop voltage, the more vacancy filaments are ruptured resulting in a higher resistance state. As the applied negative voltage sweeps again, the resistance of the memory device keeps a lower resistance state than the untreated device due to the residual un-ruptured conductive filaments, causing the vacancy multifilament are formed easily. Moreover, because of the residual iodine ions or vacancies, the interfaces in HRS still keep a relatively favorable conduction. As a result, resistance switches to LRS with no sharp increase of the current during the set process. The CsPb1−xBixI3 perovskite with desirable migration path for iodine ions and their corresponding vacancies assembled into Ag/CsPb1−xBixI3/ITO device, exhibits excellent resistive switching behavior, especially the observation of multilevel high-resistance states. Moreover, the active layer of the CsPb1−xBixI3 perovskite film can be fabricated by simple precursor preparation, low cost spin-coating and lowtemperature treatment process compared with the many other active materials such as organic materials and inorganic oxides.40-45 All of them indicate that the Ag/CsPb1−xBixI3/ITO memory device shows a great potential for application in the field of data storage.

CONCLUSION In summary, based on the all-inorganic CsPb1−xBixI3 perovskite, an Ag/CsPb1−xBixI3/ITO device was fabricated with a bipolar resistive switching behavior. The designed device with high uniformity presents a nonvolatile, reliable, reproducible and long-term stable resistive switching behavior under fully open-air conditions. The formation and partial rupture of conductive multifilament caused by the migration of iodine ions and their corresponding vacancies were

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proposed to explain the resistive switching between the HRS and LRS. And the multilevel highresistance states were observed in lead halide perovskite memory device by modulating the reset stop voltages. These results suggest a good candidate for multilevel data storage memory device.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 51571006).

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22. Li, Z.; Yang, M.; Park, J.-S.; Wei, S.-H.; Berry, J. J.; Zhu, K. Stabilizing Perovskite Structures by Tuning Tolerance Factor: Formation of Formamidinium and Cesium Lead Iodide Solid-State Alloys. Chem. Mater. 2015, 28, 284-292. 23. Eperon, G. E.; Paterno, G. M.; Sutton, R. J.; Zampetti, A.; Haghighirad, A. A.; Cacialli, F.; Snaith, H. J. Inorganic Caesium Lead Iodide Perovskite Solar Cells. J. Mater. Chem. A 2015, 3, 19688-19695. 24. Luo, P.; Xia, W.; Zhou, S.; Sun, L.; Cheng, J.; Xu, C.; Lu, Y. Solvent Engineering for Ambient-Air-Processed, Phase-Stable CsPbI3 in Perovskite Solar Cells. J. Phys. Chem. Lett. 2016, 7, 3603-3608. 25. Liu, D.; Lin, Q.; Zang, Z.; Wang, M.; Wangyang, P.; Tang, X.; Zhou, M.; Hu, W. Flexible All-Inorganic Perovskite CsPbBr3 Nonvolatile Memory Device. ACS Appl. Mater. Interfaces 2017, 9, 6171-6176. 26. Wu, Y.; Wei, Y.; Huang, Y.; Cao, F.; Yu, D.; Li, X.; Zeng, H. Capping CsPbBr3 with ZnO to Improve Performance and Stability of Perovskite Memristors. Nano Res. 2017, 10, 15841594. 27. Liu, H.; Wu, Y.; Hu, Y. Reproducible Switching Effect of an All-inorganic Halide Perovskite CsPbBr3 for Memory Applications. Ceram. Int. 2017, 43, 7020-7025. 28. Han, J. S.; Le, Q. V.; Choi, J.; Hong, K.; Moon, C. W.; Kim, T. L.; Kim, H.; Kim, S. Y.; Jang, H. W. Air‐Stable Cesium Lead Iodide Perovskite for Ultra‐Low Operating Voltage Resistive Switching. Adv. Funct. Mater. 2018, 28, 1705783.

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