Real-Time Observation on Dynamic Growth/Dissolution of Conductive Filaments in Oxide-Electrolyte-Based ReRAM

Transcription

1 Real-Time Observation on Dynamic Growth/Dissolution of Conductive Filaments in Oxide-Electrolyte-Based ReRAM Qi Liu, Jun Sun, Hangbing Lv, Shibing Long, Kuibo Yin, Neng Wan, Yingtao Li, Litao Sun, and Ming Liu * The efforts to find alternative approaches to replace conventional charge-based nonvolatile memories (NVM) is one of the most attractive topics for satisfying the requirements of ultradensity storage, high-speed and low-power consumption in the current information era. [ 1 7 ] Solid-electrolyte-based resistance random access memory (ReRAM), also known as conductive bridge random access memory (CBRAM) or programmable metallization cell (PMC), has attracted extensive attention as a candidate for the next-generation NVM due to its simple structure and outstanding device performances. [ 8 16 ] Generally, the device consists of a solid-electrolyte layer sandwiched between an oxidizable electrode (e.g., Cu or Ag) and an inert electrode (e.g., Pt or W), and can be switched between a high resistance state (HRS or OFF-state) and a low resistance state (LRS or ON-state) under electrical stimulations due to the formation/annihilation of conductive filaments (CFs) inside the solid-electrolyte layer. [ 8 14 ] The so-called electrochemical metallization model is widely accepted to explain the dynamic process of CF formation/dissolution. [ 12 ] When a positive voltage is applied on the oxidizable electrode (e.g., Ag), Ag + ions are generated and will migrate onto the inert cathode, then, will be reduced to Ag atoms at the inner cathode. CFs will be resultingly grown from the cathode toward the anode. Vice versa, the CF will be dissolved via oxidation when a reversed voltage is applied. Recently, the morphologies and chemical compositions of the CFs have been directly observed and characterized using scanning electron microscope (SEM) [ ] or transmission electron microscopy (TEM). [ ] However, the most important questions critical to the device operation such as how the filaments are formed and the dynamics of the CF formation and dissolution processes still remain unaddressed. Dr. Q. Liu, [+] Dr. H. Lv, Prof. S. Long, Dr. Y. Li, Prof. M. Liu Lab of Nanofabrication and Novel Device Integration Institute of Microelectronics Chinese Academy of Sciences Beijing , China Tel: , Fax: liuming@ime.ac.cn J. Sun, [+] Dr. K. Yin, Dr. N. Wan, Prof. L. Sun SEU-FEI Nano-Pico Center Key Laboratory of MEMS of Ministry of Education Southeast University Nanjing , China [+] These authors contributed equally to this work. DOI: /adma In this work, we performed real-time characterizations of the CF formation and dissolution processes of a technologically important oxide-electrolyte-based ReRAM device, which attracted significant interests due to their CMOS compatibility and excellent performance. [ 11, 13, 24, 25, ] Using in situ TEM studies, our approach allows us to directly address several unresolved fundamental issues related to the resistive switching (RS) effect, including the starting point of CF formation/dissolution, the direction of CF growth/dissolution, the number of filaments formed under the SET process, and the degree of CF dissolution under the RESET process. [ 6 ] Obtaining these pieces of information could help elucidate the underlying nature of the RS phenomenon and guide the design of ReRAM devices with desirable properties. The device is based on a vertical Ag(or Cu)/ZrO 2 /Pt structure with both small lateral sizes and ZrO 2 thickness to facilitate direct TEM studies. Figure 1 a depicts the schematic diagram of the specimen fabrication process. Specimens with complete Ag(or Cu)/ZrO 2 /Pt structures were directly fabricated on the platform of a W probe using a dual-beam focused ion beam (FIB) system. The detailed fabrication process is provided in Experimental Section. By reducing both the device thickness and lateral size, this approach allows the whole active layer to be monitored at high magnifications to observe any structural changes in the device. Figure 1 b shows the SEM images of the sample at each step of the fabrication process. Multiple ReRAM devices can be fabricated on one W probe, which not only reduces the lateral size as discussed above but also greatly enhances the fabricating efficiency of TEM specimens. This method can also be used to fabricate other two-terminal electronic devices for in situ TEM experiment. Figure 2 a shows the schematic sketch of the in situ TEM experimental set-up. The W probe with the TEM specimen and another W probe with a sharp-tip were inserted into the fixed and movable contacting terminals of TEM holder, respectively. During the electrical testing, the TEM specimen was connected by the two W probes, and the voltage was applied to the fixed contacting terminal while the movable terminal was grounded. Figure 2 b shows a typical cross-section TEM image of a fresh Ag/ZrO 2 /Pt device. The freshly prepared TEM specimens are generally in OFF-state and the distribution of the initial resistance is from to Ω at 0.1 V read voltage. The variation initial characteristics of the specimens may be caused by the roughness of the W platform surface, which can be seen from the images 2 and 3 in Figure 1 b. No change was observed in the structure of stacked ReRAM films after suffering the electron irradiation with a high electron beam current density (300 fa/nm 2 ) even for one hour time. The in situ TEM 1844

2 Figure 1. In situ fabrication technique used to produce ReRAM TEM specimen. (a) Schematic of the methods used to fabricate the Ag(or Cu)/ZrO 2 / Pt TEM specimen. (b) Corresponding SEM images in each of the TEM specimen fabrication processes. Images (1)-(2) show the cross-sectional of a W platform and the top-view of the platform with ReRAM multi-layer, indicating the platform has a smooth surface to meet the requirement for fabricating the ReRAM device. Images (3)-(5) confi rm that multiple TEM specimens with small size ReRAM structure can be achieved through this method. experiments were carried out at a much lower beam current density ( <100 fa/nm2 ). Therefore, the influence of electron irradiation is negligible. The insets of Figures 2 c and d show the TEM images of the same Ag/ZrO 2 /Pt device after being switched to the ON-state and erased again to the OFF-state, respectively. As shown in Figure 2 c, during the negative voltage sweep, the device current rapidly increased at a threshold voltage (denoted as SET voltage, V SET ) and saturated to the 110 μ A compliance current (I CC ), indicating the fresh device has been switched from the HRS to the LRS. In the meantime, a dark nano-bridge region across the ZrO 2 layer suddenly appeared between the Ag and the Pt electrodes, verifying that the resistance switching is caused by the formation of a CF, as shown in the inset of Figure 2 c. When the voltage was swept in the positive direction (0 2.5 V), the device suddenly switched back to the HRS at another threshold voltage (denoted as RESET voltage, V RESET ) and the dark nanobridge region disappeared at the same time (Figure 2 d). Careful examining of the TEM image in Figure 2 c shows that the shape of the nanobridge region is asymmetric. It has a cylindrical base with a 30 nm diameter near the Ag electrode, and has an oblique cone shape with a much smaller 5 nm diameter at the narrowest point near the Pt electrode. Remarkably, the observed CF shape contradicts to the widely accepted electrochemical metallization theory. [ 12 ] In this theory, the growth process of the metallic CF is depicted as follows: when a positive voltage is applied to the active electrode (Ag or Cu), active metal ions (M z + ) are formed from the oxidation of the anode metal atoms (M) and migrate onto the inert cathode (Pt), where they are reduced back to metal atoms. As this process continues, the metallic precipitates on the cathode accumulate and eventually come into contact with the anode, resulting in a conical or a nearly cylindrical shape with the narrowest cross sectional area at the active electrode terminal instead. [ 10, 12 ] The chemical compositions of the CF region in the initial-, ON- and OFF-states were analyzed using energy-dispersive X-ray spectroscopy (EDX) analysis for further understanding the nature of the CF. Figure 2 e shows that, no Ag element signal was found in the initial-state in the ZrO 2 films, but a high Ag signal was present in the ON-state. Detailed EDX elemental mapping studies on a similar device show that the Ag signal intensity raised sharply in the CF region (Supporting information, Figure S1). These results clearly indicate that Ag is the primary chemical component of the CF in the present Ag/ZrO 2 /Pt memory device, which is consistent with both the electrochemical metallization theory, [ 12 ] and the previous results obtained by ex situ TEM. [ 24, 25 ] When the device was reset back to the OFF-state, a weak Ag signal was still detected in the region where the CF previously existed, showing that the Ag atoms in the CF cannot be completely re-collected into the Ag electrode. The residual Ag atoms in the ZrO 2 film may increase the leakage current of the device in the OFF-state, which explains why the resistance of the device did not return to the initialstate level after the RESET process. The density of residual Ag 1845

3 Figure 2. RS behavior and corresponding micro-structure changes during the in situ electrical test. (a) Schematic of the in situ TEM experimental setup. (b) Cross-sectional TEM image corresponding to a fresh Ag/ZrO 2 /Pt memory device. (c) I V curve of the Ag/ZrO 2 /Pt TEM specimen during SET operation. The inset shows the TEM image of the specimen after the SET process. (d) I V curve of the TEM specimen during RESET operation. The inset shows the TEM image of the specimen after RESET process. (e) EDX analysis of the TEM specimen conducted at the initial-, ON- and OFF-states. The EDX spectra were sequentially obtained at the CF region with 30 s acquisition time with a 20 nm electron beam. atoms will increase with the number of switching cycles, which will deteriorate the reliability of the device. [ 32 ] Therefore, how to effectively remove the residual metal atoms in oxide-electrolyte films is a critical subject in future research. During the SET process, the dynamic process of CF growth was simultaneously recorded by a video camera at a speed of 33 ms per frame. However, the CF growth rate appears considerably faster than the camera speed as we failed to capture the continuous images that reflect the CF growth process in the Ag/ZrO 2 /Pt device. To fully capture the real-time, dynamic process during CF growth, Ag was replaced with Cu as the top electrode to form a TEM specimen with a Cu/ZrO 2 /Pt structure, given that Cu would have slower diffusion than Ag in the ZrO 2 fi lm. [ 33 ] Figures 3 a e show the growth dynamics of CFs between the Cu and Pt electrodes under a 4 V constant voltage stress, with a 1.1 μ A compliance current (I CC ) applied to the Pt electrode. These images were extracted from a 140 s video, with images a-e representing frame times at 0, 60, 110, 120, and 130 s, respectively. In the video images, a first CF suddenly emerged across the ZrO 2 film between the two electrodes at 60 s. The sudden appearance of the first CF was accompanied by an equally sudden increase in the electrical current through the device ( Figure 3 k). However, the low resistance state was only maintained for a short time, probably because of the instability of this particular CF, which led to a spontaneous rupture. [ 19 ] When the stress time reached 110 s, the second CF began to form inside the ZrO 2 fi lm and appeared to be connected only to the Cu electrode. Similar partially formed CF has been observed in other Cu/ZrO 2 /Pt devices. An example is shown in Supporting Figure S2, which more clearly demonstrated that the initial CF was nucleated on the Cu electrode with a conical structure and a wide base at the Cu electrode side. From 110 to 130 s, the second CF continued to grow until reaching the Pt electrode (Figure 3 c e). During this process, the electrical current continued to increase and finally jumped to the value of I CC (Figure 3 k). The chemical compositions of the CF in Cu/ZrO 2 /Pt device were analyzed through EDX analysis. As can be seen from Figure 3 l, no Cu element signal was found in the initial-state in the ZrO 2 films, while a high Cu signal was found in the ON-state, which confirmed that the main composition of CF in the Cu/ZrO 2 /Pt device is Cu. It is noted that the minimum size of the focused electron beam spot is about 20 nm when we carry out the EDX analysis by using T20 TEM system. To avoid collecting the signal from the Ag (or Cu) electrode, we placed the electron beam spot away from the Ag (or Cu) electrode when we carried out the EDX analysis, resulting in that some Pt electrode area may be covered in the electron beam. Therefore, Pt signals appeared in all EDX spectrums in Figure 2e and 3l. These dynamic images prove that the growth direction of the CF in the ZrO 2 films is from anode (Cu or Ag) to cathode (Pt), which again contradicts to the theoretical prediction that the metallic precipitates should begin to form on the cathode surface and subsequently grow towards the anode. [ 12 ] Based on the electrochemical metallization theory, M is oxidized to M Z + from 1846

4 Figure 3. Dynamics of CF growth. (a) (e) A series of TEM images capturing the dynamic CF growth processes in the Cu/ZrO 2 /Pt TEM device. (f) (j) Black-and-white images converted from the raw TEM images of the (a)-(e) to highlight the fi laments. (k) I-t curve of the TEM specimen under a 4 V constant stress. (l) EDS spectrum of the TEM specimen tested in the initial- and ON-states. The EDS spectrum of ON-state was obtained in the CF region whereas initial-state was obtained in the ZrO 2 fi lms. (m) Schematic of the CF growth mechanism. the anode (Ag or Cu), then these M Z + ions migrate onto the inert cathode (Pt) and are deoxidized back to metallic atoms. In this theory, a key hypothesis is that the M Z + will not capture electrons when transfers through the RS film. This hypothesis is reasonable for traditional solid-state electrolyte materials, such as AgS, CuS, AgGeS and CuGeS, because these materials have high solubilities and diffusion coefficients of the M Z + (Ag + or Cu 2 + ). The large amount of M Z + can only obtain enough electrons at the cathode for deoxidation to M atoms. The fact is also confirmed by visual evidences from other previous studies. For example, an Ag filament grows from the Pt cathode to the Ag anode in the Ag/H 2 O/Pt structure, [ 18 ] whereas a Cu filament grows from the Pt-Ir cathode to the Pt-Ir probe anode in a CuGeS-based ReRAM. [ 29 ] However, ZrO 2 as a oxide-electrolyte, the solubility and diffusion coefficients of metal ions are far lower than those of traditional solid-state electrolytes. [ 31 ] Banno et al. demonstrated that the Cu ions flux (product of the diffusion coefficient and the concentration) in oxide-electrolyte is 10 orders of magnitude lower than that in traditional electrolyte. [ 31 ] The ionic conductivity is dependent on the metal ions flux. Hence, the ion conductivity is very low in oxide-electrolyte and the current through our device mainly consists of electron current. Therefore, the impact of electron current should be taken into consideration, and the M Z + cations can easily capture the incoming electrons and be reduced back to M atoms inside ZrO 2 film without having to reaching the cathode. Based on the above analysis, the CF growth process in the ZrO 2 -electrolyte-based ReRAM in the present study can be depicted as follow. When a positive voltage is applied to the active electrode (M = Ag or Cu), the metal atoms (M) are oxidized to the active metal ions (M Z + ). The mobile M Z + drifts to the anode surface under the electrical filed. Given the low migration speed of M Z + in ZrO 2 materials, they will be reduced back to M after traveling a short distance. With the continuation of the oxidation/reduction processes, the successive accumulation of M atoms at the anode/oxide interface leads to the nucleation of the M CF at some local region of the anode surface. The CF nucleus then serves as an extension of the anode, which provides a fast-track for M Z + ions migrate to the front of the nuclei due to the enhanced electric field. As this process continues, CF will gradually grow until it reaches the cathode. The scenes of CF growth are visually described by a cartoon (Figure 3 m). In this case, a highly conductive path is formed across the oxide layer and the device switches to the ON-state. Finally, the dynamics of the CF dissolution process was also investigated by in situ TEM. Here, a Ag/ZrO 2 /Pt device was first set to the ON-state, and then reset to the OFF-state with a voltage sweep (0 1.5 V, repeated for several times). The TEM 1847

5 Figure 4. Dynamics of CF dissolution. (a) (e), A series of high-magnifi cation TEM images showing the dynamic dissolution process of the CF. (a) Highresolution TEM image of the CF region in a Ag/ZrO 2 /Pt TEM specimen after SET process. (b)-(e) Continuous high-resolution TEM images showing that the CF dissolves from the anode (Pt) to the cathode (Ag) terminal during RESET voltage sweeps (0 1.5 V). (f) I V curves of the TEM specimen under successive RESET operations. (g) Schematic of the CF dissolution mechanism. observation window was positioned in the CF region (dark region in Figure 4a) at magnification. After the first RESET voltage sweep, the electrical current passing through the device drastically decreased from 1 μ A to 40 na at 300 mv read voltage (Figure 4 f). Meanwhile, the CF region near the Pt electrode was substantially weakened after the first RESET process as indicated by the increase of the brightness of the dark region in TEM image (Figure 4 b). Additional voltage sweeps eventually led to the gradual decrease of the dark region along the direction from the anode (Pt) surface to the cathode (Ag) terminal (Figures 4 c 4 e). During these repeated RESET processes, the current across the device remained almost constant (Figure 4 f), implying that the conductive path in the CF was already disrupted after the first RESET process, whereas further disruptions of the CF by the subsequent RESET actions did not lead to appreciable changes of current that can be detected by the instrument. These results indicate that the CF dissolution direction was from the anode (Pt) to cathode (Ag) terminals, which can be explained by thermal-assistant electrochemical reaction model. Because the bottleneck of the CF is nearby the Pt electrode (Figure 2 c, 4 a and Supporting Figure S3), the bottleneck region has the highest current density when RESET voltage is applied to the device, leading to high temperature in there. The oxidation reaction will be accelerated by high temperature, resulting in the CF first breaks near the anode (Pt). In this case, the device is switched back to the OFF-state. By continuously applying negative voltage to the active electrode, the residual protrusion will be dissolved and shrunk toward the cathode terminal. The scenes of CF dissolution are visually described by a cartoon (Figure 4 g). It is worth mentioning that the CF did not completely dissolve under repeated RESET operations. The CF residues may serve as the nuclei in the subsequent SET process, which explains why the CF formed, ruptured and re-formed in the same region in the ZrO 2 film during repetitive switching cycles. In summary, real-time observations of the CF formation and dissolution processes in ZrO 2 -electrotlye-based ReRAM devices were conducted using in situ TEM. The observed phenomena are different from the predictive model based on the electrochemical metallization theory. A modified microscopic mechanism based on the local redox reaction inside the ZrO 2 -electrolyte system was suggested to account for observed RS effect. The methodology reported in the present work can be used to investigate the real-time nanostructure evolution process of ReRAM systems at atomic resolutions and lead to a more complete understanding of the underlying nature of the RS behavior, which may serve as a guide to improve the performances and reliability of ReRAM. Although the work was carried out using Ag(or Cu)/ZrO 2 /Pt structures, the experimental method can be easily extended to other RS materials systems and further promote the rapid development in universal nonvolatile memory system. Experimental Section Specimen Fabrication : The in situ TEM specimens of the Ag(or Cu)/ ZrO 2 /Pt memory device were directly fabricated on the platform of a W probe using a dual-beam FIB system (FEI Nov 200 nanolab) to simplify the preparation process. The fabrication process is depicted in Figure 1 a. First, the tip of the W probe was cut to form a smooth platform using 1848

6 the FIB. Second, Ti/Pt/ZrO 2 /Ag(or Cu) fi lm stack were successively deposited on the W platform using e-beam evaporation. The thicknesses of the fi lms were approximately 30, 70, 40 and 80 nm, respectively. Third, a 2- μ m-thick Pt layer was deposited by FIB on the fi lm stack. Fourth, the width of the stack fi lms was reduced to 100 nm by FIB cutting and milling. Finally, multiple Ag(or Cu)/ZrO 2 /Pt/Ti ReRAM TEM specimens were fabricated on the W probe with FIB cutting. During the FIB cutting and milling processes the Pt layer also serves as a protection layer for the rest of the ReRAM fi lm stack. In Situ TEM Characterization : The in situ TEM experiments were carried out in two different microscopes (FEI Tecnai T-20 and Titan ) located at the Southeast University. A STM equipped TEM holder (Nanofactory Instruments) was used for in situ manipulation and electrical measurement. The holder contains two electrical contacting terminals, one is movable and the other is fi xed. The movable terminal movement is controlled at sub-nanometer resolutions by a piezotube. In the in situ TEM observation, the W probe with the TEM specimens was directly inserted into the fi xed terminal of the TEM sample holder, and the other W probe with a sharp tip was inserted into the movable terminal and brought into contact with the device (Figure 2 a). During the electrical measurement, the movable terminal was grounded and an electrical bias was applied to the fi xed terminal. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements The authors thank K. W. Peng from NCNST for help with TEM sample preparation and thank Prof. W. Lu from University of Michigan, Prof. M. N. Kozicki from Arizona State University, Dr. J. J. Yang from HP Labs, Prof. J. Shen from Fudan University and Dr. Y. H. He from Osaka University for discussions. This work is funded by the Ministry of Science and Technology of China under grant Nos. 2011CBA00602, 2010CB934200, 2011CB921804, 2009CB930803, 2011CB707600, 2009CB623702, 2008AA031403, 2011AA010401, 2011AA and 2009AA03Z306 and NSFC under grant Nos , , , , , and Received: October 26, 2011 Revised: December 1, 2011 Published online: March 7, 2012 [1 ] G. I. Meijer, Science 2008, 319, [2 ] R. Waser, M. Aono, Nat. Mater. 2007, 6, 833. [3 ] A. Sawa, Mater. Today 2008, 11, 28. [4 ] M.-J. Lee, C. B. Lee, D. Lee, S. R. Lee, M. Chang, J. H. Hur, Y.-B. Kim, C.-J. Kim, D. H. Seo, S. Seo, U.-I. Chung, I.-K. Yoo, K. Kim, Nat. Mater. 2011, 10, 625. [5 ] J. J. Yang, M. D. Pickett, X. Li, D. A. A. Ohlberg, D. R. Stewart, R. S. Williams, Nat. Nanotechnol. 2008, 3, 429. [6 ] H. Schroeder, R. Pandian, J. Miao, Phys. Status Solidi A 2011, 208, 300. [7 ] K. Nagashima, T. Yanagida, K. Oka, M. Kanai, A. Klamchuen, J.-S. Kim, B. H. Park, T. Kawai, Nano Lett. 2011, 11, [8 ] S. Dietrich, M. Angerbauer, M. 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