S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Materials Science and Engineering (재료공학부) Theses (Ph.D. / Sc.D._재료공학부)
Developing the bi-layered Self-rectifying Resistive Switching Device and Improving Resistive Switching Parameters for ReRAM Applications
자가 정류 저항변화 메모리 소자 개발과 저항변화 특성 균일화에 관한 연구
- 공과대학 재료공학부
- Issue Date
- 서울대학교 대학원
- Resistive Switching Random Access Memory; Uniformity; Ru Nano-dots; Self-rectification; Electroforming-free; Multi level switching
- 학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2015. 2. 황철성.
- Resistance switching random access memory (ReRAM) is attracting a great deal of attention as one of the most promising next-generation non-volatile memory devices due to its exclusive properties like scalibility, low cost, fast opertion speed and simple structure. Although there have been huge improvements in the development of ReRAM during the past decade, there are still several key concerns remaining with RS materials. There are two most important problems. One is the non-uniformity problem of the switching performance and the other is the necessity of unprecedented concept ReRAM structure which is suitable for the 3D type planar or vertical CBA structure device.
Among the research part of ReRAM, the understanding of resistance switching (RS) mechanisms in many oxide materials and its application to ReRAM have been greatly improved over the last decade. This has mainly been attributed to the improvement in the device fabrication methods and application of various state-of-the-art analysis techniques. In ionic type RS switching phenomena, while there are detailed variations in the nature, shape, electrical properties, and distribution across the RS layer, the RS behaviors of many RS materials are closely related to the presence of nano-scale conducting filament (CF), where the repeated formation and rupture of CFs are controlled by the thermally-assisted electromigration of defects, mainly oxygen vacancies in oxides. The inappropriate repeatability of the RS parameters and low reliability issue are material-peroperty-related problems especially originated from the random formation of CFs. The non-uniform RS performance with the increasing number of RS cycles or memory cells is generally related to the non-uniform and uncontrolled formation and rupture of CFs over a large area of the memory cell, which are caused by their random nucleation and uncontrolled growth. This suggests that confining the location where the CFs form to a certain region of the memory cell would improve the repeatability problem. In this work, as for the solution of non-uniformity problem, Ru-nanodots (Ru-NDs) are embedded in TiO2 which is the most representative resistance switching film, which essentially eliminates concerns with non-uniformity. It was concluded that limiting the location where the electron injection occurs at the cathode interface to a narrower region was the key factor for achieving the highly improved RS performance, while the phases of TiO2 can hardly influence the RS performances. The position of the Ru-NDs relative to the cathode played an important role in limiting the field concentration area to a narrow region.
In the crossbar array (CBA) configuration, it currently has several obstacles to overcome, such as the high variability in electrical performances, the requirement of an electroforming step, and the necessary integration of a memory cell with selector devices to alleviate the sneak currents in CBA. These problems become even more serious when a three-dimensional (vertical) CBA structure is to be fabricated. In this respect, the development of RS memory cell which contains rectification functionality in itself, highly reproducible RS performance, multi-level functionality, and electroforming-free characteristics are the impending tasks for development of ReRAM. In this work, two-layered dielectric structure consisted with HfO2 and Ta2O5 layer which are in contact with the TiN, Ti and Pt electrode is presented for achieving these tasks simultaneously in one sample configuration. HfO2 layer works as the resistance switching layer by trapping or detrapping of electronic carriers, while Ta2O5 layer remained intact during the whole switching cycle, which provides the rectification. With the optimized structure and operation conditions for the given materials, excellent RS uniformity, electroforming-free and self-rectifying functionality could be simultaneously achieved from the Pt/Ta2O5/HfO2/TiN and Pt/Ta2O5/HfO2/TiN structure.