Jiebin Niu

Controllable Growth of Nanoscale Conductive Filaments in Solid-Electrolyte-Based ReRAM by Using a Metal Nanocrystal Covered Bottom Electrode

Resistive memory (ReRAM) based on a solid-electrolyte insulator is a promising nanoscale device and has great potentials in nonvolatile memory, analog circuits, and neuromorphic applications. The underlying resistive switching (RS) mechanism of ReRAM is suggested to be the formation and rupture of nanoscale conductive filament (CF) inside the solid-electrolyte layer. However, the random nature of the nucleation and growth of the CF makes their formation difficult to control, which is a major obstacle for ReRAM performance improvement. Here, we report a novel approach to resolve this challenge by adopting a metal nanocrystal (NC) covered bottom electrode (BE) to replace the conventional ReRAM BE. As a demonstration vehicle, a Ag/ZrO(2)/Cu NC/Pt structure is prepared and the Cu NC covered Pt BE can control CF nucleation and growth to provide superior uniformity of RS properties. The controllable growth of nanoscale CF bridges between Cu NC and Ag top electrode has been vividly observed by transmission electron microscopy (TEM). On the basis of energy-dispersive X-ray spectroscopy (EDS) and elemental mapping analyses, we further confirm that the chemical contents of the CF are mainly Ag atoms. These testing/metrology results are consistent with the simulation results of electric-field distribution, showing that the electric field will enhance and concentrate on the NC sites and control location and orientation of Ag CFs.

Confining Cation Injection to Enhance CBRAM Performance by Nanopore Graphene Layer

Conductive-bridge random access memory (CBRAM) is considered a strong contender of the next-generation nonvolatile memory technology. Resistive switching (RS) behavior in CBRAM is decided by the formation/dissolution of nanoscale conductive filament (CF) inside RS layer based on the cation injection from active electrode and their electrochemical reactions. Remarkably, RS is actually a localized behavior, however, cation injects from the whole area of active electrode into RS layer supplying excessive cation beyond the requirement of CF formation, leading to deterioration of device uniformity and reliability. Here, an effective method is proposed to localize cation injection into RS layer through the nanohole of inserted ion barrier between active electrode and RS layer. Taking an impermeable monolayer graphene as ion barrier, conductive atomic force microscopy results directly confirm that CF formation is confined through the nanohole of graphene due to the localized cation injection. Compared with the typical Cu/HfO2 /Pt CBRAM device, the novel Cu/nanohole-graphene/HfO2 /Pt device shows improvement of uniformity, endurance, and retention characteristics, because the cation injection is limited by the nanohole graphene. Scaling the nanohole of ion barrier down to several nanometers, the single-CF-based CBRAM device with high performance is expected to achieve by confining the cation injection at the atomic scale.

Overcoming the Dilemma Between RESET Current and Data Retention of RRAM by Lateral Dissolution of Conducting Filament

Resistive switching memory with low switching current is critical for low-power application. In this letter, we successfully demonstrated a four-terminal resistive RAM device with ultra-low switching current. The device is SET by one pair of electrodes and RESET by the other. The rupture process of conductive filament can be resulted from electrochemical reaction dominated by the lateral electric field. Therefore, during RESET process, no current flows through the filament, leading to an ultra-low switching current.

Breaking the Current‐Retention Dilemma in Cation‐Based Resistive Switching Devices Utilizing Graphene with Controlled Defects

Cation-based resistive switching (RS) devices, dominated by conductive filaments (CF) formation/dissolution, are widely considered for the ultrahigh density nonvolatile memory application. However, the current-retention dilemma that the CF stability deteriorates greatly with decreasing compliance current makes it hard to decrease operating current for memory application and increase driving current for selector application. By centralizing/decentralizing the CF distribution, this current-retention dilemma of cation-based RS devices is broken for the first time. Utilizing the graphene impermeability, the cation injecting path to the RS layer can be well modulated by structure-defective graphene, leading to control of the CF quantity and size. By graphene defect engineering, a low operating current (≈1 µA) memory and a high driving current (≈1 mA) selector are successfully realized in the same material system. Based on systematically materials analysis, the diameter of CF, modulated by graphene defect size, is the major factor for CF stability. Breakthrough in addressing the current-retention dilemma will instruct the future implementation of high-density 3D integration of RS memory immune to crosstalk issues.

Characterization of the photocurrents generated by the laser of atomic force microscopes

The conductive atomic force microscope (CAFM) has become an essential tool for the nanoscale electronic characterization of many materials and devices. When studying photoactive samples, the laser used by the CAFM to detect the deflection of the cantilever can generate photocurrents that perturb the current signals collected, leading to unreliable characterization. In metal-coated semiconductor samples, this problem is further aggravated, and large currents above the nanometer range can be observed even without the application of any bias. Here we present the first characterization of the photocurrents introduced by the laser of the CAFM, and we quantify the amount of light arriving to the surface of the sample. The mechanisms for current collection when placing the CAFM tip on metal-coated photoactive samples are also analyzed in-depth. Finally, we successfully avoided the laser-induced perturbations using a two pass technique: the first scan collects the topography (laser ON) and the second collects the current (laser OFF). We also demonstrate that CAFMs without a laser (using a tuning fork for detecting the deflection of the tip) do not have this problem.

Highly scalable resistive switching memory in metal nanowire crossbar arrays fabricated by electron beam lithography

Resistive switching memories (RRAMs) based on oxides are promising nonvolatile memories with numerous advantages in cell structure, switching speed, operation power, and fabrication process. To promote practical application, however, the scalability issue of RRAMs should be investigated. In this work, the maskless electron beam lithography (EBL) was used to fabricate very small structures in the electron beam resist layer, which were subsequently transferred to a metal and oxide film by the lift-off process. Metal nanowires with a minimum width of 9 nm and arrays with such nanowires were fabricated by optimizing the pattern design and by correcting the electron beam proximity effect. By combining three processes of EBL, film deposition, and lift-off, Ti/HfO2/Pt-based RRAM devices with a minimum size of 10 nm integrated into a crossbar array were successfully fabricated. The fabricated devices exhibit good memory performance, including low operation voltage, good endurance, retention, uniformity, and scala...

Ultra-low RESET current RRAM device by side-RESET operation method

Resistive switching memory with low switching current is critical for low power application. In this work, we successfully demonstrated a four-terminal RRAM device with ultra-low switching current. The device is SET by one pair of electrodes and RESET by the other. Therefore, during RESET process, no current flows through the filaments, leading to an ultra-low switching current.