Date of Award

2017

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Materials Science & Engineering

First Advisor

I-Wei Chen

Abstract

This thesis describes conductivity in amorphous semiconductors and insulators—some doped with metals, in which elastic electrons can random walk across a transport length of ~10 nm. At low temperatures, back diffusion of coherent electrons causes constructive quantum interference that leads to reduced diffusivity/conductivity. Rich physics also arises in this so-called weak-localization (WL) regime from electron-phase mutilation by spin-orbit interaction (weak-antilocalization or WAL) and magnetic modulation, and from Friedel-oscillation-enhanced backscattering and Zeeman splitting (electron-electron-interaction or EEI). Conductivity is analyzed by a new tool to eliminate contact resistance without using the four-point-probe method.

The Aharonov-Bohm oscillation in magnetoresistance affords the cleanest evidence of interference, seen in amorphous HfO2 and Al2O3, each containing a single 6-7 nm conductive loop. The loop is atomically thin and has no in-loop path dispersion, which allows the oscillation to persist with uncharacteristically low attenuation. In amorphous Si, quantum conductivity correction and magnetoresistance are universally exhibited in multiple states and multiple samples. But doping amorphous Si3N4 with Pt creates a novel feature: a sharp resistance maximum at Tmax below which the WAL-mediated spin-orbit interaction completely dominates over the spin-insensitive WL. This interaction can further quench the coherent length to keep it commensurate with the magnetic length, thus permitting the EEI-directed magnetoresistance to develop to unprecedented strength. Reduced HfO2 and Al2O3 also feature the same characteristics, albeit in one-dimension conduction.

Ubiquitous to amorphous nano conductors is their strong electron-phonon interaction. It imparts electrons with up to 100x heavier mass, which is translated into up to 100x reduction in diffusivity. Yet at low enough temperatures, electron diffusion can fully saturate the 10 nm transport length in all our samples. This prevents weak localization from crossover to strong localization, so the nano amorphous devices remain conductive down to 0K. Yet under a critical voltage ~ 1 V, they can trap injected electrons at locally soft spots and stabilize them by bond relaxation; in doing so they promptly switch to the insulator state. Thanks to these attributes, which are reversible, our devices are all excellent resistance-switching non-volatile memory.

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