Competition between Metallic and Vacancy Defect Conductive

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C: Physical Processes in Nanomaterials and Nanostructures

Competition between Metallic and Vacancy Defect Conductive Filaments in CHNHPbI-Based Memory Device 3

3

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Yiming Sun, Meiqian Tai, Cheng Song, Ziyu Wang, Jun Yin, Fan Li, Huaqiang Wu, Fei Zeng, Hong Lin, and Feng Pan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12817 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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The Journal of Physical Chemistry

Competition between Metallic and Vacancy Defect Conductive

Filaments

in

CH3NH3PbI3-Based

Memory Device Yiming Sun1,3, Meiqian Tai2, Cheng Song1,3,* , Ziyu Wang1, Jun Yin1, Fan Li1,3, Huaqiang Wu3, Fei Zeng1, Hong Lin2 and Feng Pan1, 3,* 1

Laboratory of Advanced Materials (MOE), School of Materials Science and

Engineering, Tsinghua University, Beijing 100084, China 2

State Key Laboratory of New Ceramics & Fine Processing, School of Materials

Science and Engineering, Tsinghua University, Beijing 100084, China 3

Beijing Innovation Center for Future Chip, Tsinghua University, Beijing 100084,

China

Abstract: Ion migration, which can be classified into cation migration and anion migration, is at the heart of redox-based resistive random access memory. However, the co-existence of these two types of ion migration and the resultant conductive filaments (CFs) have not been experimentally demonstrated in a single memory cell. Here we investigate the competition between metallic and vacancy defect CFs in a Ag/CH3NH3PbI3/Pt structure, where Ag and CH3NH3PbI3 serve as the top electrode and memory medium, respectively. When the medium layer thickness is hundreds of

*

E-mail: [email protected] (C.S), [email protected] (F.P) 1

ACS Paragon Plus Environment

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nanometers, the formation/diffusion of iodine vacancy (VI) CFs dominates the resistive switching behaviors. The VI-based CFs provide a unique opportunity for the electrical-write and optical-erase operation in a memory cell. The Ag CFs emerge and co-exist with VI ones as the medium layer thickness is reduced to ~90 nm. Our work not only enriches the mechanisms of the resistive switching, but also would advance the multi-functionalization of resistive random access memory.

1. Introduction In the last decade, redox-based resistive random access memory (ReRAM), based on ion migration in medium layers, has attracted extensive attention as one of the most

important

candidates

for

next-generation

nonvolatile

memories.1–5

Electrochemical metallization (ECM) memories and valence change memories (VCM), which depends on the formation/diffusion of metallic and vacancy defect conductive filaments (CFs) respectively, are two mainstream mechanisms of resistive switching behaviors in ReRAM.6–11 It is generally accepted that the formation of one type of CFs is an obstacle for the other, because the CFs serve as a short circuit, excluding the electric field effect.1,2 The formation of metallic or vacancy defect CFs are dependent on their activation and migration energies. Now the research interest is to seek an elegant approach to achieve the co-existence of different types of CFs in an appropriate memory structure, where the activation and migration energies of cation and anion are comparable. 2


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Inorganic-organic hybrid perovskite materials, such as methyl-ammonium lead iodide (CH3NH3PbI3, MAPbI3 for short), have become a star material for its remarkable performance in solar cells.12−14 It is noticed that ion migration and its resultant current−voltage (I−V) hysteretic phenomenon are notorious in battery cycles15−17, but it exactly provides great potential for application as memory devices18. If an electrochemically active electrode is used, such as Ag or Cu, the corresponding cation Ag+ and Cu2+ would be possible to migrate through the medium layer to form metallic CFs.2 Several studies have been reported to focus on the RS behaviors in MAPbI3 films, the thickness of which ranges from hundreds of nanometers to several micrometers similar to the solar cell structure.19−32 The huge gap between top and bottom electrodes hinders the formation of metallic CFs. Once the medium layer thickness is reduced, it is expected to obtain comparable ion migration for both of Ag ions and iodine ions. In the experiments below, the structure of Ag/MAPbI3/FTO is selected with a series of relatively small thicknesses of MAPbI3. The competition of Ag CFs and VI CFs dominates the conductive mechanism in MAPbI3-based memory devices. The co-existence of two types of CFs is observed in the 90 nm-thick cell.

2. Results and discussion A series of MAPbI3 perovskite films with the thicknesses of ~300 nm, ~240 nm, ~150 nm and ~90 nm were grown by solution processing (Figure S1). X-ray diffraction (XRD) pattern (Figure 1a) show the crystallinity of MAPbI3 in tetragonal structures. Intense main peaks of MAPbI3 appeared at 14.21°, 28.58° and 31.99°, 3



which respectively represent the (110), (220) and (310) lattice planes.33 The lattice constants can be calculated as a = 8.836 Å and c = 12.455 Å by Bragg equation.34 The uniform MAPbI3 layer synthesized on the FTO glass was composed of compact and dense grains with sizes of hundreds of nanometers (Figure S2a). The surface of MAPbI3 was characterized by an atomic force microscope (AFM) and the roughness Ra = 7.71 nm (Figure S2b). In the following, we focus primarily on data attained from the thickest (~300 nm) and the thinnest (~90 nm) MAPbI3 films and their memory devices as these results cover all the central feature found in the experiments. Subsequently, Ag arrays were sputtered on the MAPbI3 films as top electrodes (TE) by use of a stainless steel mask. The inset of Figure 1b shows a schematic of the sample layout and the measurement configuration. The direct current bias voltages were applied to Ag TE, and FTO bottom electrodes (BE) were grounded by use of a probe station on the Agilent B1500 analyser. The voltage was applied in a sequence of 0 → positive → 0 → negative → 0. A compliance current of 100 µA was set to prevent thermal-induced breakdown.35 Figure 1b presents typical I−V curves of Ag/MAPbI3/FTO memory cells with different thicknesses of MAPbI3. All the cells display stable bipolar resistive switching behaviors. The SET voltages get lower with decreasing of the thickness of MAPbI3. Taking the 300 nm cell as an example, the device shows a high resistance state (HRS) of ~1 GΩ, a low resistance state (LRS) of ~1 kΩ and the corresponding ON/OFF ratio of ~106. The endurance property was attained up to 103 cycles and the data retention time was measured up to 105 s (Figure 4


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S3). The situation changes dramatically when the thickness of MAPbI3 was reduced to 90 nm. Corresponding data for the dependence of RLRS on thickness are displayed in Figure 1c. Obviously, the RLRS of 90 nm cell was an order of magnitude lower than those of 150, 240 and 300 nm MAPbI3-based cells, suggesting the change of the conductivity of CFs. Hence, we measured the resistance−temperature (R−T) characteristics of LRS at different temperatures from 298 K to 326 K. The R−T curves in Figure 1d illustrate that the RLRS of 90 nm memory cell increases with the temperature, which is a typical metallic conductivity.36,37 In comparison, the LRS resistance of 300 nm cell exhibited a weak dependence on the temperature, which implies that defect conductive filaments are dominant in the cell.38,39 Meanwhile, another series of memory devices with Ni TE was fabricated and measured in an identical mode, no similar metallic conductive behavior was found (Figure S4). To investigate the conductive mechanism in MAPbI3-based memory cells, conductive atomic force microscope (C-AFM) was utilized to directly mapping the resistive switching behaviors in MAPbI3. A Pt-coated tip was used as TE, grounded, and FTO BE was used to apply bias voltages. When a low voltage of 0.1 V was applied, the MAPbI3 was uniformly insulating with current levels of tens of pA and the current mapping was totally different from the morphology images (Figure S5).20, 40

The schematic of C-AFM measurement is displayed in Figure 2a. We selected a 4

µm × 4 µm area and scanned the top half of it (~2 µm × 4 µm), applying a large 5



voltage bias (−8 V or 8 V) to the FTO BE. Subsequently, the whole area was scanned under a small voltage bias (0.5 V). Figures 2b and 2c display one area, and Figures 2d and 2e display the other. The top half of the two areas both become locally conductive with current levels of nA (Figures 2b and 2d), indicating that set processes take place. When a small voltage bias was applied, the two areas are obviously separated into the scanned and non-scanned parts in the current mappings, just as Figures 2c and 2e show. The current levels in the top half were significantly higher than those in the bottom half, suggesting that the MAPbI3 film can be activated to LRS without Ag TE in both directions, irrespective of from Pt to FTO and in reverse. Similar phenomena can be observed in the other MAPbI3 films. The results indicate that the CFs for the LRS state is localized and filamentary2, and the RS behaviors of MAPbI3 rely on its own defect. It should be noted that Figure 2e has a smaller of “completely” ON state than Figure 2c probably due to the different directions and asymmetry of bipolar resistive switching. One of the possibilities lies in that when the bias is −8 V, I− ions migrate to the Pt-coated tip whose area is so small that the conductive filaments can be formed more intensively and easily because of the concentrated electric field.41 When the bias is 8 V, I− ions migrate to the FTO BE, making it relatively harder to achieve the set process. Additionally, the radius of Pt-coated tip was ~50 nm, so the MAPbI3-based memory cell may realize ultra-low power consumption RS with the device miniaturization, because the current can be converted between nA and pA. 6


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Many types of point defects exist in MAPbI3 materials, such as vacancies (VPb, VMA, VI), interstitials (Pbi, MAi, Ii) and antisites (PbI, IPb).21, 23, 42, 43 Among them, VI and Ii can migrate with the lowest activation energies of 0.25 and 0.15 eV, respectively.20 The cations and anions in MAPbI3 include I−, Pb2+ and CH3NH3+, the migration activation energies of which are calculated to be 0.58 eV, 2.31 eV and 0.84 eV, respectively.15 Therefore, the migration of I− ions and their corresponding vacancy defect VI is supposed to be the major factor in the resistive switching mechanism. To ensure the exact composition of CFs in the memory devices, auger electron spectroscopy (AES) depth profile was carried out in the memory cells. Figure 3a demonstrates the schematic of the Ag/MAPbI3/FTO vertical device in the AES measurement setup. During the Ar+ ion etching process, the Ar pressure was steadily maintained and the etching rates of Ag and MAPbI3 were ~10 and 25 nm/min respectively. Each etching step lasted for 30 s between each two data collection of AES. In each depth location, the information of four elements (Ag, iodine, Pb and Sn) was obtained by AES in the ON-state Ag/MAPbI3 (300 nm)/FTO cell. When the atomic concentration of Sn reached a peak and remained steady, the depth profile measurement was stopped. We attained the atomic concentration as a function of the sputtering time, as shown in Figure 3b. The Ag/MAPbI3 interface was demarcated by the crossing of curves of Ag and iodine. The most eminent feature observed here is the shift of iodine composition. The shift has an apparent decent gradient through the MAPbI3 film, indicates that the dominant CFs should be originated from the 7



migration of I− and composed of VI. Differently, the concentration of Pb keeps relatively stable in the film, while the curve of Ag falls sharply after the Ag/MAPbI3 interface, indicating that the Ag atoms migrate into the MAPbI3 but cannot form complete CFs. When the thickness is small enough (such as 90 nm) to form an electric field which drives Ag+ to migrate through the medium layer, the Ag CFs tend to be formed, co-existing with the VI CFs. The identical measurement was performed on the as-fabricated MAPbI3 (~300 nm)/FTO (initial state), and the atomic concentration curves of three elements are shown in Figure 3c. The concentrations of both iodine and Pb keep relatively stable in the MAPbI3 film. It has been demonstrated that the Ag electrodes acted as I− reservoir during the SET process through the formation of stable AgIx.25, 44 This hypothesis was further supported by the atomic concentration ratio curves of iodine and Pb (Figure 3d). In accordance with the stoichiometric ratio of MAPbI3, the iodine/Pb ratio of the initial state fluctuates slightly around 3:1. The iodine/Pb ratio is close to 3 near the Ag/MAPbI3 interface, and decreased remarkably near the MAPbI3/FTO interface. These results suggested that in thick MAPbI3 memory cells, Ag+ can migrate into the MAPbI3 layer and store I− near the Ag/MAPbI3 interface and the formation/diffusion of VI filaments resulting from the I− migration was the principal factor of resistive switching. Based on the results above, we propose a physical model to describe the resistive switching process. In the structure of Ag/MAPbI3/FTO, RS behaviors depend on the 8


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competition between two kinds of conductive filaments, which consist of VIs and Ag atoms respectively. Figures 4a−d display the whole RS behaviors in thick MAPbI3-based memory cells. When a positive voltage is applied to the Ag TE, Ag atoms are oxidized into Ag+ ions (Ag − e− → Ag+), and negatively charged I− ions migrate towards the Ag TE, form a thin AgIx layer near the TE.25, 45 Meanwhile, the migration of I− leads to many VIs accumulating from the BE to TE, until throughout VI CFs are formed and grow from BE to TE, and the cell will be switched to LRS. Conventional Ag migration resistive switching memory devices typically required an electric field of at least higher than ~1 × 107 V m−1 [References 1,20], when the thickness is not small enough, the forming and SET voltage cannot offer enough electric fields to support Ag/Ag+ redox, migration and accumulation. When the thickness of medium layer is reduced significantly, the situation differs dramatically from above. Although the RS process is similar in the initial and forming states (Figures 4e and 4f), the distance between TE and BE greatly shrinks so that the Ag+ migration and reduction can form complete CFs in the SET process (Figure 4g). Because the conductivity of Ag CFs is higher than that of VI and they are shunt-wound in memory cells, the resistance of LRS mainly attributed to the Ag CFs which is an order of magnitude lower than the VI ones.46 When a negative voltage is applied to TE, the cell will also be switched back to HRS (Figure 4h). The VI CFs have been confirmed to be responsible for the RS mechanism of the Ag/MAPbI3/FTO memory structure. We now turn towards whether light illumination 9



can regulate RS behaviors based on the photoelectrical properties of MAPbI3. We used the visible light in the microscope equipped on the probe station as shown in Figure 5a. The 300 nm memory cell was selected which only possessed VI CFs in its RS behaviors. When the memory cell was in LRS, a DC voltage of 0.02 V was performed to read the resistance every two seconds. The light was turned on at 100 s, and it is easily observed that the current suddenly fell and the resistance increased to HRS (Figure 5b). This phenomenon suggested that light illumination led to the diffusion of CFs in the MAPbI3 film, because the VI CFs were unstable and can easily diffuse and combine with I− under light illumination.25 On account of the irreversibility of optical control, we used both applied voltage bias and light illumination to control the RS behaviors. Figure 5c showed the electrical SET and optical RESET in a single vertical memory cell. The ON/OFF ratio of the memory cell was close to the former at first, and gradually shrank to a stable constant. Similarly, the 240 nm and 150 nm can also achieve the electrical-write and optical-erase operations. Identical optical experiment was carried out in the 90-nm device, which involved the co-existence of VI and Ag CFs. However, the optical-erase unsurprisingly failed, as displayed in Figure 5d. These results suggested that the Ag CFs were formed through the medium layer and connect the Ag TE and FTO BE, whose work functions were 4.26 eV and 4.40 eV respectively2,47,48. When light illumination was applied, the cell exhibits no apparent response because the Ag CFs neutralize the effect of optical 10


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fields to the electrons and holes.49 Moreover, on account that the Ag CFs and VI CFs are difficult to encounter due to their random growth,50 the Ag-VI mixed CFs are difficult to form. Even though formed, they will be ruptured soon because of instability. It is also worthy noting that the possible reason for the shrinking ON/OFF ratio of electrical-optical operations is that the opacity of Ag TE hinders the light from reaching the central part of memory cells. On all accounts, this MAPbI3-based memory devices offer the opportunity to realize the multi-field coupling in RS behaviors.

3. Conclusions In summary, we fabricated a series of MAPbI3 perovskite films with thicknesses of ~300 nm, ~240 nm, ~150 nm and ~90 nm by solution processing and developed MAPbI3-based memory devices. The RS behaviors of Ag/MAPbI3/FTO originate from the formation/diffusion of Ag metallic CFs and iodine vacancy defect CFs and their competition. The conversion condition is pointed out that when the thickness of MAPbI3 comes to less than 90 nm, the dominant CF composition will turn from VIs to Ag atoms. A reasonable double-filament model is proposed to demonstrate the competition mechanism. Furthermore, an electrical-write and optical-erase operation in vertical memory cells is realized based on the results. We expect this work can help better understand the charge and ion transport in inorganic-organic hybrid perovskite materials and provide guidance to multi-functionalization of RRAM.

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4. Experimental Section Perovskite Deposition and Device Fabrication Before the device fabrication, the commercial FTO (~800 nm)/glass (2 mm) substrates were cleaned in deionized water, ethanol, acetone, and isopropyl alcohol in subsequence, and treated by oxygen plasma for 3 minutes to make them more hydrophilic. Then the perovskite precursor was prepared by mixing PbI2 (Sigma-Aldrich) and MAI (Dyesol) in 1:1 molar ratio in anhydrous DMF (N, N-dimethylformamide, Alfa Aesar) with different concentrations (0.5 M, 0.8 M, 1 M and 1.25 M). The precursor was then spin-coated on FTO/Glass substrates at 5000 rpm in a nitrogen-filled glovebox, and after 7 s 200 µL of chlorobenzene (Alfa Aesar) was dropped rapidly onto the substrates. Then the substrates were heated on a hot plate at 100 ℃ for 10 min. Finally, dot-shaped ~60 nm-thick Ag top electrodes were fabricated on the perovskite layer by sputtering through a shadow mask, whose diameter of dots were 100 µm. Device Characterization. The thickness of each layer in the Ag/MAPbI3/FTO sandwich structure was obtained based on a cross-sectional specimen that was examined by a scanning electron microscope (SEM, LEO1530). The crystal structure of the MAPbI3 films was examined in the θ−2θ mode by X-ray diffraction (XRD, Rigaku D/max-2500) with Cu Kα radiation (λ = 0.1542 nm). All electrical properties of the RRAM devices were characterized on a semiconductor device analyzer (B1500A, Agilent) in the atmospheric environment. To minimize the failure of the 12


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MAPbI3 and the possible diffusion of Ag and iodine at the Ag/MAPbI3 interface, all the electrical measurements and other characterizations were conducted within 48 h after the device fabrication. The visible light was launched by the lamp attached to an optical microscope. To explore the switching mechanism of the memory device, the conductance of MAPbI3/FTO glass was measured by Conducting Atomic Force Microscope (CAFM, Cypher S, OXFORD Instruments) with the use of a Pt-coated AFM-type tip as a counter electrodes. To observe the element distribution in the vertical memory device, Auger Electron Spectroscopy (AES, PHI710) was utilized to attain the element information within 5 nm on the surface and the variation tendency through the device by Ar+ ion sputtering.

Acknowledgements: This work was supported by the National Key R&D Program of China (Grant No. 2016YFA0203800) and National Natural Science foundation of China (Grant No. 51231004). The authors are grateful for the support of Beijing Innovation Center for Future Chip.

Content of Supporting Information: Basic characterization of CH3NH3PbI3 films, resistive switching properties of Ag/CH3NH3PbI3/FTO and Ni/CH3NH3PbI3/FTO as control experiments, background noise of conductive-AFM measurement. 13



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Figure Captions: Figure 1. (a) XRD pattern of MAPbI3 films of different thicknesses grown on the FTO glass substrates. (b) Typical I−V curves of the Ag/MAPbI3/FTO memory device of different thicknesses. The inset shows the schematic of the MAPbI3-based memory device. (c) Resistance-Thickness curve in LRS of Ag/MAPbI3 (x nm)/FTO, x = 90, 150, 240, 300. (d) Resistance-Temperature curves of the LRS in Ag/MAPbI3 (300 nm)/FTO and Ag/MAPbI3 (90 nm)/FTO devices. Figure 2. (a) Schematic of conductive-AFM measurement process. Current mapping images of a MAPbI3/FTO structure with a Pt-coated tip voltage bias of (b) −8 V, (c) 0.5 V, (d) 8 V and (e) 0.5 V. The positions of (b) and (c) are respectively the top half of the areas in (d) and (e). It should be noted that the current maps were obtained continuously in both two examination areas. Figure 3. (a) Schematic of AES depth profile measurement. The atomic concentration in different depths in (b) the ON-state of Ag/MAPbI3 (300 nm)/FTO device and (c) the initial state of MAPbI3 (300 nm)/FTO structure. (d) Ratio of iodine and Pb atomic concentration in two structures above. Figure 4. Double-filament model of resistive switching behaviors in the Ag/ MAPbI3/FTO memory device. (a) The initial state, (b) forming, (c) SET and (d) RESET process of the Ag/MAPbI3/FTO device with comparatively thick MAPbI3. (e) The initial state, (f) forming, (g) SET, (h) RESET process of the Ag/ MAPbI3/FTO device with comparatively thin MAPbI3. 22


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Figure 5. (a) Schematic of the semi-optical control equipment. (b) Evolution of the device current and resistance and (c) cycling of the LRS and HRS with electrical SET and optical RESET in Ag/MAPbI3 (300 nm)/FTO. (d) Evolution of the device current and resistance of Ag/ MAPbI3 (90 nm)/FTO.

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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TOC:

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Figure 1 140x122mm (300 x 300 DPI)


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Figure 2 140x145mm (300 x 300 DPI)



Figure 3 85x75mm (300 x 300 DPI)


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Figure 4 119x82mm (300 x 300 DPI)



Figure 5 119x100mm (300 x 300 DPI)


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ToC figure 85x39mm (300 x 300 DPI)


Competition between Metallic and Vacancy Defect Conductive

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