A Study on Carrier Transport Mechanisms of Organic Memory Devices by Using a Numerical Method

Abstract

The electronic memory devices require the larger memory storage. The memory devices have developed to scale-down a unit cell size of devices to satisfy these demands. However, the problem of current technology development is limitations of the scaling down a unit cell size. Among various emerging memory devices, organic memory devices have been particularly attractive due to their potential applications that offer the excellent advantages of low driving voltage, low power consumption, high contrast, wide viewing angle, high flexibility, low cost, and fast response. This research has investigated the electrical characteristics of the organic memory devices sandwiched between two electrodes and established the model for the carrier transport mechanisms in organic materials, the space charges, the field dependent mobility, memory state transition, and leakage current of organic memory devices. The device structure used in this research consists of one organic layer sandwiched between two electrodes. The Pöisson equation and the transport equation are calculated to investigate the electric properties of organic memory devices. The trap distribution in the organic layer containing the nanoparticles is assumed to be a double Gaussian distribution consisting of shallow and deep traps. The trapped carrier density is determined by multiplying the double Gaussian trap distribution by the Fermi-Dirac occupation probability function. This thesis is divided into six chapters including introduction and conclusion. As an introductory part, chapter 1 provides the category information and brief introduction of current memory devices. The modern technical issues of current memory devices are investigated to overcome the limitation of memory devices with international technical roadmap for semiconductor. In Chapter 2, the next generation nonvolatile memory devices are described, such as phase-change random access memory, resistive random access memory, ferroelectric random access memory, magnetoresistive random access memory (MRAM), and spin-transfer torque MRAM. The overview of organic memory devices are described with figures. The charge transport simulation methods in organic materials are introduced and explained to simulate the organic electronic devices. In chapter 3, the electrical properties of the organic memory devices were investigated by using space charge limited current (SCLC) model with field dependent trap occupancy. The single level trap and the Gaussian trap distributions were utilized to research the electrical bistability of two states with different conductivities. The electrical bistability of the modified SCLC model was compared with organic memory devices, which based on nanocomposites containing C60 embedded in the Poly(methyl methacrylate) (PMMA) layer and SnO2 nanoparticles in the PMMA layer were fabricated by using a spin coating method. In chapter 4, the organic memory devices containing the blocking layer were fabricated to control the leakage current. The effect of the blocking layer between the electrode and the active layer was investigated to prevent the leakage current. To decrease of leakage current in organic materials, various structures of organic memory devices were fabricated and investigated. In chapter 5, the SCLC and trap assisted tunneling (TAT) model was used to simulate the leakage current in an organic layer at low voltages. To compare with experimental results, Alq3 and mCP layers with a thickness of 100 nm sandwiched between two electrodes were fabricated by thermal evaporation. The leakage current of an organic layer containing enough traps was clearly explained by using the TAT mechanism. The simulation models in future works will be unified to the single model based on drift-diffusion model. The unified simulation model will have to consider various physical effects, such as TAT current to simulate the leakage current, generalized Einstein relation affected by the Gaussian density of state of disorder organic materials, and the field dependent mobility calculated by hopping transport in traps. When the unified simulation model based on drift-diffusion model will be developed, the researches for the trap distribution in organic materials and the leakage current control offer the solutions to the weak point of previous organic simulation models.