A study on Selector Devices for High-Density Memory Application

MasterEmerging non-volatile memories―such as resistive switching RAM, phase change memories, ferroelectric RAM, STT-MRAM―are studied extensively as a potential candidate for future high-density memory application owing to their simple 2 terminal structure, better scalability, and low power consumption. In order to achieve such high-density memory, a cross-point array structure, in which writing/erasing or reading of the desired (selected cell) can be done by applying a voltage at the edge of the array, has been introduced and investigated widely. In cross point array, it is critical to delivering the necessary power and current to the selected cell while avoiding any unnecessary perturbation to the unselected cell. To do that, a selector device is required in order to suppress sneak path current, undesired current flow through the unselected cell, during operation. Highly non-linear selectors are ideal to prevent such unwanted current from flowing through the unselected cell. The high resistance at low bias can effectively inhibit the sneak current while the low resistance at high bias can supply sufficient current to write/read or erase the adjacent selected memory cell. Insulator-to-metal transition (IMT) - and Ovonic threshold switching (OTS) based-selectors are the most promising selector devices for cross-point memory array owing to their fast, uniform and abrupt high resistance to low resistance transition. However, IMT based selectors are suffering from high off-state current which tackles their functionality for low power device application. Similarly, the realization of OTS based selector devices is hindered by their material complexity and thermal stability problem. This thesis explores the issues and solutions for the aforementioned two type of selector devices. In chapter three, the origin of the high off-state current of NbO2 investigated. Grain boundaries and local sub-stoichiometric (NbO2-x) regions are found to be mainly responsible for the high leakage current. By comparing sputtered NbO2 film on a substrate and thermally grown NbO2 film on polycrystalline Nb wire, it is found that the off-current of the thermally grown film was lower than the sputter deposited film. This off-current reduction could be explained by the less contribution of grain boundary leakage owing to the relatively large grain size of the polycrystalline Nb wire. Similarly, better stoichiometric NbO2 could be formed in the thermally grown film. On contrary, the sputter deposited film which has a small grain size and off-stoichiometry shows higher leakage current. In addition, the leakage conduction paths and grain size effects are studied by using Conductive Atomic Force Microscopy (CAFM) and supported by Simulations. Furthermore, by introducing a leakage passivation layer of 15nm HfO2, further reduction of the off-current is achieved. In chapter four, a new C-Te based binary Ovonic Threshold Switching (OTS) selector device is introduced. The proposed device is scalable down to Ø30nm and exhibits excellent performance and thermal stability. High selectivity (on/off ratio >105), extremely low off-current (~1nA), superfast switching speed of 212 word lines) in a fast read operation

Thermally Stable Te-based Binary OTS Device for Selector Application

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Excellent data retention characteristic of Te-based conductive-bridge RAM using semiconducting Te filament for storage class memory

In this work, we explore the electrical properties and data retention of Te-based conducive-bridge random-access memory (CBRAM) of ZrxTe1-x/Al2O3/Pt cells. The virgin resistance and forming voltage are decreased with increasing Zr composition (0.5 <= x <= 0.7) and decreasing electrolyte thickness. The resistance of the conducive filament (CF) formed in the Te-CBRAM shows semiconducting behavior that is decreased with increasing temperature, whereas a Cu-based CBRAM shows metallic behavior. Furthermore, the conductance change of Te-based CBRAM, during the filament dissolution step, occurs with lower conductance units than those of Cu/Ag-based CBRAM. The most differentiable characteristics of both devices are the data retention. Te-based CBRAM shows better data stability at high temperature (150 degrees C) than Cu-based device. Accelerated tests (250, 270, and 300 degrees C) were performed to understand the data retention of the Te-CBRAM, yielding excellent retention characteristics (10 years at 177 degrees C) despite its relatively low activation energy (E-a, 1.07 eV) than Cu/Ag- based devices. We believe that the excellent retention properties of Te-based devices are more influenced by the wide effective CF size than by E-a.11Nsciescopu

Self-Limited CBRAM With Threshold Selector for 1S1R Crossbar Array Applications

In this letter, we demonstrate a self-limited conductive-bridging random accessmemory (CBRAM) that removes the necessity for external current compliance in a one selector-one resistor (1S1R) architecture. The standard Ge2Sb2Te5 (GST) is used as a CBRAM switching layer. In addition, Te-rich GST is also considered. Their performance is then compared. Both samples exhibit self-limited on-current characteristics, and the on-currents of the standard GST and Te-rich GST are similar to 300 and similar to 20 mu A, respectively. The observed self-limited characteristics are caused by the Te in the GST layer because in the presence of Te, Cu tends to form a more stable CuTe phase that restrict Cu filament growth. Furthermore, to confirm the feasibility of crossbar array applications, the 1S1R device is evaluated using a Ag/TiO2-based threshold selector device reported in our previous work. Hence, we confirm leakage current reduction, a uniform resistance distribution, and stable retention characteristics in the 1S1R configuration with no external current compliance.113Nsciescopu

Effect of the Threshold Kinetics on the Filament Relaxation Behavior of Ag‐Based Diffusive Memristors

Owing to their unique features such as thresholding and self-relaxation behavior diffusive memristors built from volatile electrochemical metallization (v-ECM) devices are drawing attention in emerging memories and neuromorphic computing areas such as temporal coding. Unlike the switching kinetics of non-volatile ECM cells, the thresholding and relaxation dynamics of diffusive memristors are still under investigation. Comprehension of the kinetics and identification of the underlying physical processes during switching and relaxation are of utmost importance to optimize and modulate the performance of threshold devices. In this study, the switching dynamics of Ag/HfO2/Pt v-ECM devices are investigated. Depending on the amplitude and duration of applied voltage pulses, the threshold kinetics and the filament relaxation are analyzed in a comprehensive approach. This enables the identification of different mechanisms as the rate-limiting steps for filament formation and, consequently, to simulate the threshold kinetics using a physical model modified from non-volatile ECM. New insights gained from the combined threshold and relaxation kinetics study outline the significance of the filament formation and growth process on its relaxation time. This knowledge can be directly transferred into the optimization of the operation conditions of diffusive memristors in neuromorphic circuits

Communication-Reduced Off-Current of NbO2 by Thermal Oxidation of Polycrystalline NbWire

The origin of high leakage current in NbO2 is investigated on the basis of grain size and grain boundary distribution. We used thermally grown and sputtered NbO2 films on polycrystalline niobium microwires. The off-current of the thermally grown film was significantly decreased. This is attributed to the large size of grains in thermally grown film over sputtered one and better quality of oxide film could be grown in the thermal process than sputtering. Our assumptions are supported by Conductive Atomic Force Microscopy studies and simulations. In addition, by introducing 15 nm HfO2 dielectric layer further reduction of the off-current was achieved.111sciescopu