Ionic Conductivity, Dielectric Constant, and Optical Properties of Anodic Oxide Films on Two Types of Sputtered Tantalum Films

The growth of anodic oxide films on sputtered tantalum films in dilute sulfuric acid has been studied by in situ ellipsometry. Two types of tantalum film were used which had been deposited onto substrates at different temperatures giving two structural forms of tantalum (``β'' and bcc). The optical properties of the metal films differed from each other and from those of the bulk metal but the refractive index and dielectric constant of the oxides were nevertheless very similar to those for the oxide on bulk tantalum. However, the field E in the oxide required to produce a given ionic current density I was about 4% higher than for the oxide on the bulk metal and ∂ logI/∂E was slightly less than for the bulk metal. This appears to show an effect of the metal/oxide interface as predicted by Mott and Cabrera.