Influences of oxygen vacancy drifting on resistive switching behaviors in TiOx and TaOx based nonvolatile memories

Abstract

This dissertation is devoted to study the influence of oxygen vacancy on resistive switching behaviors in the valence change memories (VCM) and electrochemical metallization (ECM) memories The VCM and ECM memory devices were prepared with the device structures of Pt/TiOx/Pt/TiOx/Pt and Ag/TaOx/Pt respectively The current-voltage (I-V) characteristics including the electroforming unipolar resistive switching (URS) bipolar resistive switching (BRS) and reset current characters were investigated and understood in terms of the migration of oxygen vacancies in VCM and ECM system In the first part this dissertation the TiOx-based memory devices were fabricated for electrical and material characterization A distinct unipolar but single-polarity resistive switching behavior is observed in a TiOx/Pt/TiOx active layer formed by thermal oxidation of a Ti/Pt/Ti stack Introduction of the Pt mid-layer creates two additional Schottky barriers which mediate the band bending potential at each metal-oxide interface and attains a rectifying current conduction at the high resistance state (HRS) Experimental evidences proving the single-polarity switching behavior is a combination of bias-induced Schottky barrier modification and conduction filament construction both associated with the bias-driven migration of oxygen vacancies In addition the rectifying conduction behavior is also observed with an AFM-tip as the top electrode (TE) which implies the rectifying property is still valid when miniaturizing the device to nano-meter scale In the second part of this dissertation we explored the electroforming and resistive switching behaviors in the Ag/TaOx/Pt trilayer structure under a continual change of temperatures between 300 K and 100 K to distinguish the contributions of Ag ions and oxygen vacancies in developing of conducting filaments We found that either electroforming or resistive switching a significantly higher forming/set voltages is needed as the device is operated at 100 K as compared to that observed when operating at 300 K The temperature dependence of the pulsed switching time (tsw) measurement results indicates that larger activation energy (Ea) of Ag diffusion in TaOx (Ea??????eV) will lead to a faster decay of Ag ions mobility in TaOx with decreasing temperature Thus the disparity in electroforming/set voltages of Ag/TaOx/Pt operating at 300 K and 100 K can attribute to the mobility difference of Ag ions at 300 K and 100 K This mobility difference of Ag ions also leads a presence of metallic filament at 300 K while a filament composed of oxygen vacancy and Ag is formed at 100 K The presence of oxygen vacancy segment in the conducting filament also modifies the reset current from a gradually descending behavior (at 300 K) to a sharp drop (at 100 K) Furthermore the characteristic set voltage (Vset) and reset current are irreversible as the operation temperature is brought from 100 K back to 300 K indicating the critical role of filament constituents on the switching behaviors of Ag/oxide/Pt system

Date of Award	2015 Jan 22
Original language	English
Supervisor	Jen-Sue Chen (Supervisor)