Electron Injection into Anodic Tantalum Oxide Assisted by Ionic Interface Polarization

Abstract

The investigation of electronic conduction processes in anodic tantalum oxide films with electrolytic and solid contacts is generally hampered by parallel ionic conductivity at high fields and the uncertain contributions of flaws, even in the case of high‐purity materials. We found that electronic conductivity could be induced reproducibly and with homogeneous current densities in the presence of thin layers of semiconducting oxides, such as Fe2O3 and MnO2 which were deposited by reactive sputtering. Experiments were carried out with electrolyte contact. The electronic current was found to follow logi normalprop.V½ in a wide range of current densities. A Poole‐Frenkel mechanism is proposed in which the current is controlled by thermionic emission at the Ta2O5/normalMeO ‐interface. Electronic conductivity increases as a time dependent activation process lowers the emission barrier through ionic interface polarization. The contribution of flaws to the electronic conductivity was investigated by means of the anodic printing technique. The electronic current resulting from the activation process was found to be of uniform density and unrelated to any physical flaws in the anodic tantalum oxide.

Electron Injection into Anodic Valve Metal Oxides

“…The electronic current was found to follow a thermionic emission characteristic /el : io.el exp (e~F'/2 -r [1] with e, charge of an electron; t~, field-lowering coefficient in agreement with Poole-Frenkel emission; F, applied field, r energy barrier at zero field; k, Boltzmann constant; T, absolute temperature. The process of activation was interpreted in terms of a reduction of the energy barrier r as a function of the ionic current resulting in a time-dependent increase of the electronic current at constant applied field r : 4o --const t/ion : 4o --const t A exp (BF) [2] with r energy barrier at zero field prior to activation; t, time.…”

“…This appears justified for the low current densities involved and at least for the linear regime of the activation curve, which is interpreted as current injection limited by coulombic centers at the valve metal oxide/Fe20~ interface. Galvani potential differences between the various phases were estimated as described previously (1) and were taken into account when calculating the true field in Fig. 5, 6, and 8.…”

“…In a recent publication (1) it has been shown that homogeneous electronic currents of the order of 10 mA/cm 2 could be injected into anodic tantalum oxide under high field conditions after inserting a thin layer of semiconducting transition metal oxide between the valve metal oxide and the electrolyte. The transition metal oxide layer restricted ionic migration while providing electronic equilibrium between the electrolyte and the surface of the valve metal oxide Figure 1 shows a schematic of the experimental system.…”

“…Combination of Eq. [1] and [2] led to an expression for the increase of the electronic current with time (activation) logl0iel : const --const t A exp (BF) [4] with a slope of s • const A exp (BF) [5] In this paper additional experimental results obtained for the activation process of A1/A1203/Fe203 and Nb/ Nb2OJFe203 electrodes, the electronic current-field characteristic of Nb/Nb2OJFe208 electrodes, as well as measurements of the temperature dependence of the activation process and the electronic conductivity of Ta/Ta2OdFe20~ electrodes will be presented. The results support our model for homogeneous electronic current injection into anodic tantalum oxide and its applicability to valve metal oxides other than Ta2Os.…”

Tantalum and Tantalum Compounds

“…Ta 2 O 5 has a resistivity of ca. 10 15 Ω · cm [56] and dielectric constant ε = 25. The anode is coated with a semiconducting layer of MnO 2 by impregnation with an aqueous solution of manganese nitrate, followed by pyrolysis.…”