Design of Materials Configuration for Optimizing Redox‐Based Resistive Switching Memories

Abstract

random access memory (DRAM) and flash memory are reaching the physical scaling limits. To tackle this problem, emerging memory technologies have been proposed during last years. [8][9][10] Among them, redoxbased resistive random access memories (ReRAMs) have received special attention for its CMOS-compatible fabrication, performance, multi-functionality and scaling potential. [1,11,12] It is recognized important building block for next generation storage memories, computation-in-memory architecture and artificial intelligence. [3,8,[10][11][12] ReRAM is a two-terminal metal-insulatormetal cell. The electrical conductivity of the insulating layer, typically a transition metal oxide, can be tuned by ionic modulation, caused by external electrical stimulation. [11] The oxide film has the ability to conduct ions such as metal cation, protonic charge and oxygen ions/vacancies, therefore it is often referred to as solid electrolyte. [13][14][15] Depending on the functional principles, two type ReRAMs are particularly promising-electrochemical metallization memory (ECM) and valencechange memory (VCM). [11,16,17] The resistive switching in ECM cells relies on a metallic filament that forms, and respectively dissolves, between the active electrode and counter electrode. [16] The formation of filament corresponds to a SET process, during which the cell switches from high resistive state (HRS) to low resistive state (LRS). The SET process is accompanied by multiple individual electrochemical processes, i.e., ionization (oxidation) of active electrode, diffusion of metal cation in the oxide electrolyte and nucleation/growth at counter electrode. The application of reverse potential switches the cell back to HRS by oxidizing/dissolving the filament, resulting in a RESET process. Electrochemically active metals, such as Ag, Cu, or their alloys/ compounds are typically employed as active electrodes. [13,18,19] The counter electrode is made by inert materials like Pt, Ir or compounds such as TiN. [18][19][20] VCM cells consist of bottom electrode with high work function (e.g., Pt, TiN) which forms a Schottky interface with the oxide. The top electrode is electrochemically active, typically metal with high oxygen affinity (e.g., Ta, Ti, Hf) which allows redox reactions/ion exchange and forms an ohmic contact to the oxide, favoring the oxygen vacancy defect formation. [21,22] It is widely accepted that the resistive switching for VCM cell is achieved by modulating the electrostatic barrier at Schottky interface by migration and redistribution of oxygen vacancy defects. [11,23] Redox-based resistive random access memories (ReRAMs) are based on electrochemical processes of oxidation and reduction within the devices. The selection of materials and material combinations strongly influence the related nanoscale processes, playing a crucial role in resistive switching properties and functionalities. To date, however, comprehensive studies on device design accounting for a combination of factors such as electrodes, electrolytes, and capp...

Forming‐Free Resistive Switching Memory Crosspoint Arrays for In‐Memory Machine Learning

“…Figure 1d shows the measured initial leakage current before forming and the average forming characteristics are collected. As pointed out in previous works, [ 17–20 ] the forming voltage decreases for decreasing oxide thickness, with the thinnest layers showing forming‐free characteristics. We explain this behavior as the combination of a substochiometric composition of HfO 2 and a high concentration of defects in the thin film.…”

“…[1][2][3][4][5][6][7][8][9][10] The RRAM switching mechanism can be explained by the oxide layer being capable of locally changing the oxygen vacancy concentration. [20,21] Metals with high work function (such as Pt or TiN) are usually adopted as Schottky-type bottom electrode materials as they are inert with respect to the oxide interface. [18,20] On the other hand, an active metal with good oxygen affinity, such as Ta, Ti, or Hf, [20][21][22] can act as top electrode material for oxygen scavenging, thus leading to the formation of a thin vacancy-rich oxygen exchange layer.…”

“…[20,21] Metals with high work function (such as Pt or TiN) are usually adopted as Schottky-type bottom electrode materials as they are inert with respect to the oxide interface. [18,20] On the other hand, an active metal with good oxygen affinity, such as Ta, Ti, or Hf, [20][21][22] can act as top electrode material for oxygen scavenging, thus leading to the formation of a thin vacancy-rich oxygen exchange layer. By applying a positive voltage to the top electrode, the oxygen vacancies can migrate and reallocate inside the oxide layer with a consequent change of the electrical properties, where the formed oxygen vacancy-based conductive channel dictates a low-resistance state (LRS).…”

“…[23,24] The forming voltage is generally proportional to the oxide thickness, [9,22] but also depends on the stoichiometry and the interface with the metallic electrodes. [13,20] Within a crosspoint array, a high forming voltage may cause disturbs to the RRAM [25] devices sharing the same row or column of the selected device. As a result, many efforts have been devoted to reducing the forming voltage, [17][18][19] mainly tuning the oxidized interface between oxide and scavenger metal [20] or changing the oxide stoichiometry.…”

“…[ 13,20 ] Within a crosspoint array, a high forming voltage may cause disturbs to the RRAM [ 25 ] devices sharing the same row or column of the selected device. As a result, many efforts have been devoted to reducing the forming voltage, [ 17–19 ] mainly tuning the oxidized interface between oxide and scavenger metal [ 20 ] or changing the oxide stoichiometry. [ 26 ] Ideally, one should aim at the elimination of the forming operation, by fabricating the device in the LRS instead of the more common HRS.…”