Electrical/Dielectric Properties Of Metal-Oxide Nanofilms Via Anodizing Al/Hf Metal Layers

Hafnium oxide (HfO2) is a high-temperature ceramic with excellent electrical, dielectric and optical properties, which may be substantially enhanced in the nanostructured material. Here, we have developed self-organized arrays of hafnium-oxide nanorods and examined their properties by electrochemical impedance spectroscopy (EIS). For sample preparation, Al/Hf layers are magnetron sputtered onto SiO2/Si substrates, anodized and then re-anodized to a more anodic potential. This results in the growth of a porous alumina film, followed by pore-assisted oxidation of the Hf underlayer. The films consist of discrete HfOx protrusions, penetrating the alumina pores and anchored to a uniform oxide layer that forms under the pores. Post-anodizing treatments include annealing at 600 degrees C in air or vacuum and selective dissolution of the alumina overlayer. The electrical/dielectric behavior of the hafnium oxide nanorod arrays, embedded in or free from alumina, was EISinvestigated in a borate buffer solution. In the re-anodized (not annealed) state the bottom oxide layer behaves as a good dielectric whereas the nanorods are semiconducting in nature. This situation does not change substantially by the annealing in air, still resulting in a dielectric bottom layer and semiconducting nanorods. However, after the annealing in vacuum, the whole film becomes an n-type semiconductor. Further investigation is in progress to understand the formation-structure-morphology relationship, aiming at exploring the functional properties of the films.