Inter-relationships between ionic transport and composition in amorphous anodic oxides

The inter-relationships of alloy composition, film composition and ionic transport for formation of amorphous anodic oxide films are addressed quantitatively through systematic study of sputter-deposited Al-Ta alloys containing up to 39 at.% Ta. The work reveals the dependence of electric field, ionic transport number, incorporation of species into the anodic film at the alloy-film interface and mobility and distribution of species within the anodic film on alloy composition. Anodic oxidation, at high current efficiency, of alloys containing 2.8, 15, 32 and 39 at.% tantalum results in formation of two-layered anodic films by migration of cations outwards and by migration of anions inwards: an outer layer, 20% or less of the total film thickness, composed of relatively pure alumina and an inner layer containing units of Al2O3 and Ta2O5 distributed relatively homogeneously. Two-layered films develop due to the slower migration rate of Ta5+ ions relative to Al3+ ions in the inner layer of the growing anodic films, which changes progressively from about 0.6 for dilute alloys to about 0.9 for Al-39 at.% Ta. The average nm V-1 ratios, total transport numbers of cations and average Pilling-Bedworth ratios for the films change almost linearly with alloy composition between the values for anodic alumina and anodic tantala. A tantalum-enriched layer, about 1 nm thick, is formed in the Al-2.8 at.% Ta and Al-15 at.% Ta alloys just beneath the anodic film, indicating prior oxidation of aluminium in the initial stages of anodizing. In contrast, aluminium and tantalum in the alloys containing more than 30 at.% tantalum are immediately incorporated into anodic films in their alloy proportions, without development of a tantalum-enriched layer, at the available resolution. Boron species, incorporated from the electrolyte into the outer parts of the films, are immobile in films on alloys up to 15 at.% Ta but migrate outwards in other films, possibly due to the increased Lorentz field. Though the inter-relationships between film parameters and alloy composition are established for Al-Ta alloys specifically, the findings are considered to be equally relevant to amorphous anodic oxides formed on alloys and semiconductors generally.