Publications

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The thermal conductivity of amorphous TaOx memristive films having variable oxygen content is measured using time domain thermoreflectance. Thermal transport is described by a two-part model where the electrical contribution is quantified via the Wiedemann-Franz relation and the vibrational contribution by the minimum thermal conductivity limit for amorphous solids. The vibrational contribution remains constant near 0.9 W/mK regardless of oxygen concentration, while the electrical contribution varies from 0 to 3.3 W/mK. Thus, the dominant thermal carrier in TaOx switches between vibrations and charge carriers and is controllable either by oxygen content during deposition, or dynamically by field-induced charge state migration.

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In this paper, we provide an overview of the current knowledge of radiation effects in anion-based memristive devices. We will specifically look at the impact of high dose rate ionizing radiation, total ionizing dose (TID), and heavy ions on the electrical characteristics of tantalum oxide (TaOx), titanium dioxide (TiO2), and hafnium oxide (HfOx) memristors. The primary emphasis, however, will be placed on TaOx memristors. While there are several other anion-based memristive devices being fabricated by the semiconductor community for possible use in valence change memories, most of the present radiation work has focused on one of these types of devices. There have also been numerous studies on radiation effects in cation-based chalcogenides such as germanium sulfides and selenides. However, that will not be discussed in this paper.

In this paper, we provide an overview of the current knowledge of radiation effects in anion-based memristive devices. We will specifically look at the impact of high dose rate ionizing radiation, total ionizing dose (TID), and heavy ions on the electrical characteristics of tantalum oxide (TaOx), titanium dioxide (TiO2), and hafnium oxide (HfOx) memristors. The primary emphasis, however, will be placed on TaOx memristors. While there are several other anion-based memristive devices being fabricated by the semiconductor community for possible use in valence change memories, most of the present radiation work has focused on one of these types of devices. There have also been numerous studies on radiation effects in cation-based chalcogenides such as germanium sulfides and selenides. However, that will not be discussed in this paper.

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First-principles density-functional theory calculations are used to study the atomistic structure, structural energetics, and electron density near the O monovacancy (VOn; n = 0,1+,2+) in both bulk, amorphous tantalum pentoxide (a-Ta2O5), and also at vacuum and metallic Ta interfaces. We calculate multivariate vacancy formation energies to evaluate stability as a function of oxidation state, distance from interface plane, and Fermi energy. VOn of all oxidation states preferentially segregates at both Ta and vacuum interfaces, where the metallic interface exhibits global formation energy minima. In a-Ta2O5, VO0 is characterized by structural contraction and electron density localization, while VO2+ promotes structural expansion and is depleted of electron density. In contrast, interfacial VO0 and VO2+ show nearly indistinguishable ionic and electronic signatures indicative of a reduced VO center. Interfacial VO2+ extracts electron density from metallic Ta, indicating that VO2+ is spontaneously reduced at the expense of the metal. This oxidation/reduction behavior suggests careful selection and processing of both oxide layer and metal electrodes for engineering memristor device operation.

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