An Ellipsometric Study of Steady-State High Field Ionic Conduction in Anodic Oxide Films on Tantalum, Niobium, and Silicon

Abstract

The classical theory of ionic conduction in solids at high field strengths (10⁶ to 10⁷ v cm⁻¹) predicts that the relation between the ionic current density I and the field strength should be where the activation energy , is the magnitude of the charge on the ions, a is half the distance between successive sites occupied by the ions, and is a constant. Deviations from this supposed law have been reported in various forms. New experimental results for steady ‐state conditions are reported which were obtained by in situ ellipsometry. These confirm for tantalum and establish for the first time for niobium that the deviations may be simply and accurately described by taking the activation energy to be nonlinear in E in a way which may be represented over the experimental range of by . Data were also obtained for silicon, but were not sufficiently accurate to detect nonlinearity. Models are discussed which might give this effect. A model in which ions move fairly freely in channels with infrequent trapping by a coulombic potential leads to a law of the form , analogous to the Schottky and Poole‐Frenkel laws for electronic currents. Such a law fits the data well enough for the model to be considered as realistic.