Nucleation and growth of the nanostructured anodic oxides on tantalum and niobium under the porous alumina film

Abstract

Anodic oxidation of Ta–Al (aluminium deposited on tantalum) and Nb–Al (aluminium deposited on niobium) has been performed in organic and inorganic acid electrolytes for porous alumina formation. Arrays of tantalum nanoscale oxide ‘hillocks’ and niobium oxide ‘goblets’ derived from the anodised Ta–Al and Nb–Al bilayer samples have been investigated by scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy depth profiling. Anodising proceeds in the sequence of growth of porous anodic alumina and, when the aluminium layer is consumed up to the underlying metal, growth of anodic tantalum (niobium) oxide under the bottoms of the alumina pores. The oxidation of the underlying metal results from metal ions migrating outward and oxygen ions transported through, and released from, the alumina barrier layer, which dissolves at the tantala/alumina interface, i.e. without tantalum being in direct contact with the electrolyte. The shape and mutual arrangement of the anodic oxide nanostructures depend on the nature of the underlying metal, anodising solution, and are consistent with the difference between the resistivities of the tantalum (niobium) oxide formed and the barrier layer of the overlying alumina cells, which is influenced by incorporated electrolyte-derived species.