THE GROWTH AND PROPERTIES OF THIN OXIDE LAYERS ON TANTALUM ELECTRODES

The dependence of the growth of thin oxide films on tantalum electrodes on the electrode potential (0 v less than E less than 10 v) and pH (0 less than pH less than 14) was investigated by coulometric and simultaneous impedance measurements. After chemical polishing, there is a thin film of about 2 nm thickness. At anodic polarization, the film grows by 1. 76 nm/v. The dielectric constant D is about 25. Various anions have no influence, but the pH shifts the oxide growth by minus 60 mv per pH unit. Thin films behave as n-type semiconductors, and show a Schottky-Mott behavior at negative potentials E less than 0 v. The donor density decreases with increasing thickness so that films thicker than 10 nm behave as pure insulators. Cathodic hydrogen evolution takes place at large overvoltages of about less than 1 v. The characteristic potentials of passive tantalum are compared and discussed.