High Field Ionic Conduction in Tantalum Anodic Oxide Films with Incorporated Phosphate

Abstract

The dependence of the ionic current density J on the mean field trueE¯ in oxides grown on tantalum in H3PO4 electrolyte was studied using ellipsometry to estimate the thickness. The field in the outer layer which is believed to grow due to metal ion motion is higher because of phosphate incorporation. The field in this layer was estimated by assuming that the field in the inner undoped layer was the same as the mean field at the same current density and temperature in films made in dilute sulfuric acid, which are less affected by electrolyte incorporation. The results indicate that the effect of phosphate on the ionic conductivity may be due to its reducing the permittivity and hence the effective field. It is pointed out that electrolyte incorporation gives two effects which contribute to the nonlinearity of log J vs. trueE¯ plots. Thus, since the metal ion transport number increases with J so does the proportion of the film which contains phosphate and requires a higher field. Also the concentration of phosphate increases with J . The incorporation leads to history effects which are not removed by subsequent formation, since the profile of phosphate records the sequence of current densities used. The time dependence of the field on changing the electrolyte is consistent with the field locally determined by the local film material.

Preparation of Tantalum Anodic Oxide Film in Citric Acid Solution - Evidence and Effects of Citrate Anion Incorporation

“…It is practically well known that anion is readily incorporated into Ta anodic oxide and it is suggested that incorporated anions behave as defect sites in the oxide phase, resulting in alteration of dielectric properties of anodic film from the pure Ta oxide layer. [17][18][19] Recently, Di Franco et al reported the nitrogen incorporation into anodic Ta 2 O 5 layer from a bath containing ammonium ions. 25) Thus, it is expected that the electronic properties of the anodic Ta 2 O 5 film must be affected by anion incorporation, which might be dependent on the electrolyte concentrations.…”

“…It was reported that phosphoric acid is readily incorporated and the dielectric and ionic conduction properties of the outer layer of the anodic film is affected more strongly by incorporated phosphate anion than incorporated sulphate anion. [18][19][20] We have shown that nanosized patterns of tantalum oxide can be grown by applying a bias between a tip of current sensing AFM and Ta surface in an ambient condition based on electrochemical mechanism. 21) In this paper, tantalum anodic oxide film was grown in citric acid solution and the characterization of the prepared Ta anodic oxide film by various electrochemical techniques and X-ray photoelectron spectroscopy (XPS) proved the incorporation of citrate anion during the anodic oxide growth.…”

Structural engineering of nanoporous alumina by direct cooling the barrier layer during the aluminum hard anodization

“…These increments at 125 and 140 V are significant. As it is mentioned, by increasing the acid phosphoric concentration, the ionic mobility in barrier decreases [32,34] and the current needs more time to reach from point l to point k, therefore the response time is increased.…”

“…As we know during the anodization process the acid anions are injected to the outer layer of barrier layer in oxide/ electrolyte interface [32,33] and the electric field strength is greater across the inner layer with respect to the outer layer [32]. Dell'Oca and Young [34] have shown, the phosphate impurity reduces the permittivity and decreases the ionic mobility in barrier layer. The phosphate impurity in barrier layer increases by increasing the acid phosphoric ratio in electrolyte.…”

Galvanostatic formation of barrier‐type Anodic oxides. Simplified theory of anodization

“…Various relations exist which describe the dependence of i on the electric field strength H. Ellipsometric studies by Dell'oca and Young [15] have shown that the results on Ta follow:…”