Structure, morphology, and dielectric properties of nanocomposite oxide films formed by anodizing of sputter-deposited Ta-Al bilayers

Anodizing of Ta-Al bilayers (aluminum deposited on tantalum) was performed in 0.2 M H2C2O4 solution to transform the aluminum metal into its nanoporous oxide followed by pore widening and reanodizing to different voltages in the range of 100-600 V. The anodic films consist of an upper layer of nano-sized tantala columns penetrating into the pores and a lower layer of continuous tantalum oxide under the porous alumina film. The columns are mainly composed of tantalum pentoxide and tantalum sub-oxides TaO2, Ta2O3, and TaO while the lower film layer is tantalum monoxide. At the boundary between the columns and alumina cells, a region of mixed (composite) Ta2O5 and Al2O3 is formed due to channeling of the ionic current through the outer part of the alumina cell walls. The relationship between the layers and the ionic transport during oxide growth depend on pore size and formation voltage. The dielectric properties of the anodic films are close to those of an ideal capacitor. Voltage-independent apparent dielectric constant of 12.6 was determined for the films formed by normal reanodizing. The relatively higher dielectric constant for the films formed by reanodizing through the widened pores rises from 17.6 to 24.0 in the voltage range of 270-400 V, which is due to the change in morphology, relative amount and chemical composition of anodic tantala in the complex film structure. The nanocomposite anodic films can be used as dielectrics for high-voltage low-leakage current electrolytic capacitors.