Anodic oxidation and dielectric behaviour of aluminium-niobium alloys

The anodizing behaviour of sputtering-deposited Al-Nb alloys, containing 21, 31 and 44 at.% niobium, has been examined in 0.1 M ammonium pentaborate electrolyte with interest in the composition and the dielectric properties of the anodic oxides. RBS and TEM revealed amorphous oxides, containing units of Nb2O5 and Al2O3 in proportion to the alloy composition. Xenon marker experiments indicated their growth through migration of the Nb5+, Al3+ and O2- species, with cation transport numbers, in the range 0.31-0.35, and formation ratios, in the range 1.35-1.64 nm V-1, intermediate between those of anodic alumina and anodic niobia. Al3+ ions migrate slightly faster than Nb5+ ions, promoting a thin alumina layer at the film surface, although this layer is penetrated by fingers of the underlying niobium-containing oxide of relatively reduced ionic resistivity. The incorporation of units of Nb2O5 into anodic alumina increases the dielectric constant from about 9 to the range 11-22 for the investigated alloys. (c) 2005 Elsevier Ltd. All rights reserved.