The influence of composition on band gap and dielectric constant of anodic Al-Ta mixed oxides

Al-Ta mixed oxides were grown by anodizing sputter-deposited Al-Ta alloys of different composition. Photocurrent spectra revealed a band gap, E<inf>g</inf>, slightly independent on Ta content and very close to that of anodic Ta<inf>2</inf>O<inf>5</inf> (∼4.3 eV) with the exception of the anodic film on Al-10at% Ta, which resulted to be not photoactive under strong anodic polarization. The photoelectrochemical characterization allowed to estimate also the oxides flat band potential and to get the necessary information to sketch the energetic of the metal/oxide/electrolyte interfaces. Impedance measurements allowed to confirm the formation of insulating material and to estimate the dielectric constant of the oxides, which resulted to be monotonically increasing with increasing Ta content (from 9 for pure Al<inf>2</inf>O<inf>3</inf> to 30 for pure Ta<inf>2</inf>O<inf>5</inf>).