S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Materials Science and Engineering (재료공학부) Theses (Ph.D. / Sc.D._재료공학부)
Threshold switching characteristic of Te-SbO films for the application to a selector in ReRAM devices
- 공과대학 재료공학부
- Issue Date
- 서울대학교 대학원
- ReRAM; crossbar array; selection device; chalcogenide; threshold switching; Te nano cluster
- 학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2015. 8. 김형준.
- Resistance-switching random access memory (ReRAM) is considered to be one of the most promising candidates for next-generation nonvolatile memory due to its scalability, non-volatility, fast switching speed, and low power consumption characteristics. Among the advantages of ReRAM, its scalability using the crossbar array is expected to be beneficial because of its small cell area (4F2, F is the minimum feature size) and the availability of three-dimensional stacking. However, there are several problems in using the crossbar array, such as the cross-talk problem between neighboring cells during the writing/reading processes because of sneak current. Thus, preventing sneak current in the crossbar array is an important issues, and there have been many reports about selection devices that suppress sneak current. Threshold-switching (TS) materials, such as chalcogenides, have recently been proposed as selection devices of ReRAM because of their high current density and fast switching speed, as well as their bi-directional property and two-terminal structure. However, common chalcogenide TS materials have serious problems in terms of thermal stability because they lose TS by thermal annealing and subsequent crystallization. Crystallization of chalcogenide materials removes some defects related to valence alternation pairs (VAPs) that cause TS. Thus, preventing crystallization during the device process is considered to be one of the most important issues in chalcogenide TS selection devices.
In this dissertation, the threshold switching behavior of Te?SbO films was investigated. The Te?SbO film showed stable TS behavior even when the majority of the Te in the film was crystallized, which means that the Te?SbO film is free from the crystallization issue. Significant differences between the Te?SbO film and general TS materials were verified in various aspects, and the TS mechanism of the Te?SbO film is proposed.
At first, electrical characteristics, such as general TS behavior and DC endurance, and pulse switching endurance were investigated. Stable TS behavior and endurance over 200 DC sweep cycles and over 10000 pulse switching cycles were observed. X-ray photoelectron spectroscopy and transmission electron microscopy revealed that Te-SbO consist of Te nano clusters and SbO amorphous matrix. Thermal stability of Te-SbO film was examined thorough annealing process and it showed stable TS behavior after 300 oC annealing. However, comparing with X-ray diffraction and electron diffraction patterns, Te-SbO showed TS behavior even when Te was crystallized, which is very unique phenomenon in chalcogenide TS materials. It was expected that electroforming process act as crucial role in TS of Te-SbO film, which was supported by electrical characterization of films which contained different Te content.
To verifying origin of TS behavior in Te-SbO film, various comparative experiments were carried out. Te was thought to occur the TS rather than SbO through the comparing Te / SbO and TiN / SbO stacked structure. To compare the Te with other metals, TS behavior of Te-SiO2 was investigated and Ni and Sb were incorporated in SiO2 matrix. In case of Ni and Sb, only unipolar resistive switching behavior was observed. Through annealing of electroformed Te-SbO cell, amorphous Te formed between Te nano clusters during electroforming was presumed to act as a conduction path, inducing TS. Formation of amorphous Te in the conduction path was explained by electric-field-driven drift of Te from the Te nanoclusters when a high field is applied during electroforming. Finally, Suppression of the leakage current by serial connection of Sb2O5 and TiO2 unipolar memories and Te-SbO threshold-switching thin film was observed, which indicates that Te-SbO threshold switch is a promising candidate for selector devices of crossbar array of resistive switching memory.