Date of Award
Doctor of Philosophy (PhD)
Materials Science & Engineering
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.
Alvarez, Ana Isabel, "An In Situ Study Of Resistance Degradation And Switching Of Bulk Yttria-Stabilized Zirconia And Strontium Titanate Single Crystals" (2021). Publicly Accessible Penn Dissertations. 4648.