Date of Award

2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Materials Science & Engineering

First Advisor

I-Wei Chen

Abstract

Nanometallic resistance switching devices based on amorphous insulator-metal thin films are developed to provide a novel non-volatile resistance-switching random-access memory (RRAM) that is CMOS-compatible and meeting technological demand. In these devices, data recording/converting is controlled by a bipolar voltage, which tunes electron localization lengths, hence resistivity, through electron trapping and detrapping. The low-resistance state is a metallic state while the high-resistance state is an insulating state, as established by conductivity studies from 2K to 300K.

The material is exemplified by a Si3N4 thin film with randomly dispersed Pt or Cr. It has been extended to other materials, spanning a large library of oxide and nitride insulator films, dispersed with transition and main-group metal atoms. Metallic nanoparticles, which form at metal levels greater than 10 atomic percent, are nonessential for resistance switching: nanometallicity and resistance switching in nanometer thin films start at levels well below the metal percolation threshold.

Nanometallic RRAMs have superior properties that set them apart from other RRAMs. The critical switching voltage is independent of the film thickness, device area, operating temperature and switching speed. Trapped electrons are relaxed by electron-phonon interaction, adding stability which enables long-term memory retention despite a low switching voltage. As electron-phonon interaction is mechanically altered, trapped electron can be destabilized, and sub-picosecond switching has been demonstrated using an electromagnetically generated stress pulse. The resistance state is finely tunable throughout the entire continuum between the fully metallic state and the fully insulating state, by voltage, thickness and composition. AC impedance spectroscopy confirms the resistance state is spatially uniform, providing a capacitance that linearly scales with area and inversely scales with thickness. The spatial uniformity is also manifested in outstanding uniformity of switching properties. Device degradation, due to moisture, electrode oxidation and dielectrophoresis, is minimal when dense thin films are used or when a hermetic seal is provided. The potential for low power operation, multi-bit storage and complementary stacking have been demonstrated in various RRAM configurations.

These studies furnish a firmer understanding of nanometallicity and nanometallic switching. They also establish nanometallic RRAM as a viable candidate for emerging memory.

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