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Ionic conduction at high fields in anodic oxide films on tantalum Dell'Oca, Conrad Joseph

Abstract

The technique of ellipsometry was applied to the study of nonuniform anodic oxide films resulting from electrolyte incorporation into the oxide on growth. Ellipsometry results obtained in air and in situ on oxides formed on Ta in phosphoric acid are consistent with the results of tracer studies in which the oxide consists of two layers which grow simultaneously due to metal and oxygen ion transport during anodization, and further that electrolyte incorporation into the outer layer on growth modifies its properties with respect to the inner layer. The ellipsometry results were not consistent with a single homogeneous layer film or with a film possessing an index of refraction changing linearly with thickness. Thus ellipsometry provides a new, nondestructive method of determining ion transport numbers. The index of refraction and thickness of each layer were obtained by curve fitting ellipsometry results obtained as a function of increasing oxide thickness. Computer methods for solving the ellipsometry equation and curve fitting are given. Computed results are given and discussed for cases of one and two layer films growing on a metal. Finally, an error analysis of ellipsometry is made. Ellipsometry results were obtained and curve-fitted for oxides grown in various solutions, at different rates and for anodization in a sequence of electrolytes. The major findings of this part of the study are as follows: Electrolyte incorporation decreases ionic conductivity, dielectric constant and index of refraction. At constant current formation, the fraction of oxide made up by the outer layer increases with current density, and electrolyte concentration, and depends on previous formation of the oxide. The log J-E characteristics at constant voltage in dilute phosphoric acid are curved and occur at higher fields than those for dilute sulphuric acid. Analysis of the above results indicates that: a) the conduction process is bulk controlled b) ionic conduction and dielectric properties arise from the same process and c) that electrolyte incorporation is responsible for part if not all the curvature in the logJ-E characteristics of ionic conduction. Photo-stimulated growth at low electric fields was investigated by ellipsometry. The effect of radiation is to first modify the properties of the existing oxide after which photo-stimulated growth occurs accompanied by a build up of secondary current. The secondary photocurrent is ionic in nature and the radiation rather than the applied field is responsible for the generation of ions to sustain this current. The photo-grown oxide consists of two layer with the outer layer having a much lower index of refraction than normally grown oxide. The thermal recrystallization of stripped anodic oxide films was studied using transmission electron microscopy. Various diffraction patterns were obtained and analyzed. The major result in terms of ionic conduction is that electrolyte incorporation inhibits recrystallization, again, consistent with a decreased ionic mobility with incroporation. A critical test has been devised and applied to a recently proposed theory of ionic conduction, the dielectric polarization theory. This theory postulates that the autocatalytic build up of ionic current on applying a constant high field to the oxide is due to an internal field controlled process and that the rate of build up of polarization (P) towards its equilibrium value (P₀) is enhanced by the passage of current, J, given by dp/dt = AJ(P₀-P). It is shown that this theory predicts an increase in small signal capacitance during the passage of the transient. However, measurements indicate that the capacitance decreases.

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