Organic resistive nonvolatile memory materials
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Introduction A memory device composed of organic materials sandwiched between two electrodes possesses at least two stable resistance states controlled by an external electrical stimulus.1–3 Organic materials provide many advantages, including simple device structures, low fabrication costs, printability, and flexibility. The device resistance states can be read nondestructively, and no electrical power is required to maintain the states, indicating a nonvolatile memory.1–3 Furthermore, essential requirements for high performance memory, such as a high ON/OFF ratio, long endurance and retention characteristics, and a fast switching speed, have been experimentally verified.4,5 Although it is difficult to identify the device operating mechanisms, in-depth studies of charge conduction mechanisms have contributed to the device physics that underlies the switching phenomena. Structural and electrical optimizations5,6 have also been applied successfully to improve device performance. The main focus of this review is to provide a general summary about the materials, structures, characteristics, and mechanisms of organic resistive memory devices. We also introduce several critical strategies for device integration and advanced circuit architectures.
Materials, structures, and characteristics of organic switching devices An organic resistive memory device consists of organic layers sandwiched between bottom and top electrodes. Various organic materials have shown switching resistances in response to
applied voltages.1,7–36 Organic materials that exhibit conductance switching include small molecules,17–20,36 polymers,1,21–27 and composites containing nanoparticles (NPs).8–10,28–35 Specifically, the conductance switching of single and bundled phenylene ethynylene oligomers isolated in matrices of alkanethiolate monolayers was caused by conformational changes in the molecules or bundles.17 A reliable nonvolatile memory device using a polyfluorene-derivative single-layer film has been characterized.24 Nonvolatile resistive memory effects of gold nanoparticles embedded in the conducting polymer poly(4-nhexylphenyldiphenylamine) also has been investigated using admittance spectroscopy.31 In particular, the following four device structures have been implemented ( Figure 1 ): (1) a single-layer structure containing only one type of organic material, (2) a bilayer structure containing two types of organic materials, (3) a trilayer structure in which nano-traps for charge carriers are sandwiched between two organic layers, and (4) spin-cast polymer-NP blends in which nano-traps are randomly distributed throughout the entire region of the host matrix. Among these device structures, the single-polymer-layer structure is more attractive due to the simplicity of device fabrication. Organic-based composite materials are commonly used to induce resistive switching.8–10,28–35,37 Polymer-blend systems containing organic or inorganic NPs mainly constitute the active materials of resistive memory devices. Resistance switching in these materials is
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