Studying For Effects Of Defects In HfO₂-Based RRAM

In order to overcome the defects of traditional memories, people put forward many new devices, including phase change memory (PcRAM), ferroelectric random access memory (FeRAM), magnetic memory (MRAM) and resistive random access memory (RRAM). The RRAM memory, based on the binary metal oxide, has been widespread concern with its outstanding advantages such as:simple resistance based on metal oxide metal sandwich structure, high storage density, fast read and write speed, low power consumption, good compatibility with CMOS, due to its resistance change can only happen in a very small part of area leads to reduced the size of the device and the improvement of the integration.Although the study of resistive random access memory can be traced back to 50 years ago, explain to the mechanism for the resistive switching devices is not yet clear. However, in many of the resistive switching phenomenon theory, two kinds of physical mechanisms based on the phenomenon of the conductive filaments are accepted by most people. One of which is a metal conductive filaments formed by metal atom migration, some metals can be directly involved in the formation of conductive filaments. Another mechanism of resistance change phenomenon is the oxygen vacancy migration. And a lot of experiments proved to the doped metal elements in the system can greatly improve the performance parameters of resistance behavior. There is a vital role to visible defects in the resistive switching phenomenon. Therefore, in order to understand the resistance mechanism of device and improve the device performance,an in-depth study on the defects of system in the role of resistance is required.In order to study the micro mechanism of the device, excluding effects of the applied voltage, this paper employ the first principle to simulate and analyze the HfO2-based RRAM. Defects in cell can be divided into two kinds:dopants and intrinsic defects. This paper explores the effects of metal dopants and intrinsic oxygen vacancy defects respectively. Firstly, according to the formation energy of doped metal atoms exist in HfO2 cell, metal dopants will be divided into two types: interstitial and substitutional. In the charge density wave simulation of the doped system, we can observed the conductive filaments clearly in interstitial systems and in substitutional metal doped systems, filament system can not be found. The electron density of states (DOS) and the projected electron density of states (PDOS) are calculated and analysed, we found that interstitial metal dopants can directly participate in the formation of conductive filaments and substitutional metal dopants can not form the conductive filaments. Accordingly, we can classify the metal dopants preliminary and induce a macroscopical guideline for dopant classification and selection. Then we explore the effects of doped metal to oxygen vacancies when they exist at the same time, and based on the results the metal dopants are further classified. Finally, we explored the effects of oxygen vacancy intrinsic defects. The optimal value of crystal orientation [010] is identified by means of the calculated isosurface plots of partial charge density, formation energy, highest isosurface value, migration barrier, and energy band of oxygen vacancy in ten established orientation systems. It will effectively influence the SET voltage, forming voltage, and the ON/OFF ratio of the device. Based on the results of orientation dependence, different concentration models are established along crystal orientation [010]. The performance of proposed concentration models is evaluated and analyzed in this paper. The film is weakly conductive for the samples deposited in a mixture with less than 4.167at.% of Vo contents, and the resistive switching (RS) phenomenon cannot be observed in this case. The RS behavior improves with an increase in the Vo contents from 4.167at.% to 6.25at.%; nonetheless, it is found difficult to switch to a stable state. The results of the study play a guiding role on understanding and optimization of device macro characteristics.In a word, the effects of defects in HfO2-based RRAM are studied in this paper. The first chapter introduces the development and research status of memory and some faced problems, which illustrate the significance of the topic of this article. Chapter 2 introduces the micro structure and the work principle of RRAM, and introduces the research methods of this paper. Chapter 3 makes a concrete study on the effects of metal dopants in HfO2 system and classify the metal dopants preliminary. In addition, we study the effect of metal dopants on oxygen vacancy filament when they coexist in a system, and classify the metal dopants further based on the results. Chapter 4 makes a concrete study on the effects of the oxygen vacancy intrinsic defects in HfO2 system. Finally, in the fifth chapter, this work is summarized and prospected.