Resistive switching characteristics and mechanism in binary oxide materials for nonvolatile memory applications

Abstract

ABSTRACT

Resistive switching characteristics and mechanism in binary oxide materials for nonvolatile memory applications

Young Ho Do Dept. of Physics The Graduate School Hanyang University

This dissertation describes an experimental studied on resistive switching characteristics and resistive switching mechanism of the TiO2 thin films and multi layer homojunctions.

At first, polycrystalline TiO2 thin films for nonvolatile memory applications were successfully prepared on Pt metal electrodes at various crystallization phases by utilizing a conventional rf magnetron sputtering system. All the samples clearly exhibited unipolar resistive switching phenomenon and nonvolatile memory switching behavior. The resistance ratio between the two stable states of the film constituted approximately 103. We also investigated possible conduction mechanism of the high resistance state and the low resistance state, which typically depended on the temperature and compliance current. The influence of the crystal structure and the cell size on the reversible resistance switching properties, were investigated. The stability of the ON/OFF voltage was found to strongly depend on the crystallinity of the oxide materials. The results suggest that the resistive switching behavior is related to the formation and rupture of the conducting path which exist due to oxygen vacancies in the TiO2 thin film.

Second, the TiO2 thin film involving two different electrodes (Pt and Al) clearly revealed the resistive switching transition from unipolar to bipolar switching behaviors, depending on the degree of redox properties at TiO2 layer-electrode interfaces. With an Al electrode, the transition to bipolar resistive switching was clearly observed, together with counter-clockwise and clockwise switching directions, which depended on the position of the Al electrode (top or bottom electrode). The current level analysis of devices presented that the forming process method basically leads to the formation of conducting paths inside the TiO2 layers. More detailed electrical properties analysis (compliance currents, operation voltages, current level) and the Auger electron spectroscopy measurements of on/off switching states in Pt/TiO2/Pt and Al/TiO2/Pt structures demonstrated their implications about the evolution of the oxygen ion migration-induced chemical reaction at the Al -TiO2 interfaces. Therefore, the transition from unipolar to bipolar resistive switching seems to be attributable to the redox reaction and trap/detrap at the interfaces between the Al electrode and TiO2 layer due to the migration of oxygen ions and electrons. Moreover, highly stable bipolar resistive switching characteristics of Pt/TiO2/TiN structure were demonstrated. Excellent bipolar resistive switching characteristics, including a highly uniform operation voltages (set/reset) and resistance states (LRS/HRS), as well as long retention time were achieved. The improved switching behavior of Pt/TiO2/TiN structure could be attributed to the oxygen reservoir effect of TiN bottom electrode on the formation and rupture of the conducting paths by modifying the concentration distributions of the oxygen ions in TiO2 thin films.

Finally, TiO2 (oxygen rich)/TiO2-x (oxygen poor) multi-layer homojunctions were studied as alternative resistive switching structures for both high and low resistance transitions. Stable bipolar resistive switching characteristics, including stable switching speeds (μs) and endurance behaviors, as well as long retention times (> 104 s) were demonstrated. The nature of the resistive switching phenomenon in multi-layer structures seems to be a combination of the conduction path and the redox reaction, resulting from the oxygen ions drifting between the oxygen rich and poor regions of the multi-layer structures. A possible conduction sketch for unipolar and bipolar switching behaviors in multi-homo junctions is also discussed, respectively.