Date of Award

2021

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Materials Science & Engineering

First Advisor

I-Wei Chen

Abstract

Understanding resistance changes under a constant or set of bipolar-switching voltage(s) is important for thin-film devices, specifically multilayer capacitors and resistance-switching memory. However, identifying critical locations of changes and failures in thin films is difficult, so this work studies the same phenomena in single crystals of yttria-stabilized zirconia (YSZ) and iron-doped strontium titanate (STO) starting with highly accelerated lifetime tests (HALT) at higher temperatures. Although doped STO is a p-type semiconductor and YSZ a fast oxygen-ion conductor with little electronic conductivity, their DC resistance-degradation curves are remarkably indistinguishable. Yet different mechanisms were revealed by in-situ hot-stage photography and thermal imaging in two test environments—air and silicone oil. In YSZ, DC (electro)reduction does not appreciably alter oxygen stoichiometry; nevertheless, above a threshold voltage, it can raise the chemical potential of electrons to the conduction-band level, thereby triggering a metal-insulator (resistance) transition. In contrast, DC-stressed STO undergoes oxygen-vacancy demixing, forming a p-n junction with elevated electronic conductivity, albeit late-stage-demixing can be so sluggish that the steady state is difficult to reach in low-temperature HALT. In both oxides, an inherent instability in the governing field equation dictates degradation follows filament-like paths, which explains the strong field dependence and large variation of lifetimes. Upon further voltage reversals, degraded crystals exhibit different, large resistance changes. In YSZ, a change in DC voltage can already cause a resistance change, which is unipolar switching. But additional resistance degradation after voltage reversal can facilitate filament fragmentation, thus rendering the crystal bipolarly switchable due to a voltage-sensitive metal-insulator transition in a thin layer of barely metallic YSZ adjacent to the original anode. In STO, voltage reversals broaden/narrow a nanolayer of stoichiometric, ionic STO (called i-region) that straddles the p-n junction, by driving electromigration to act in-concert/against back-diffusion of oxygen ions. Thickening/thinning of such region leads to resistance increase/decrease, resulting in the so-called "eightwise” bipolar switching. (Interface-controlled, “counter-eightwise” switching was also observed in more severely degraded STO.) As these phenomena find analogies in thin-film devices, mechanisms revealed above have provided new insight that will help understand and improve the performance and reliability of engineering devices.

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