Physicochemical characterisation of thermally aged anodic films on magnetron sputtered niobium

The influence of thermal aging, at intermediate temperature (1h at 250°C) and in different environments, on the electronic and solid-state properties of stabilized 160 nm thick amorphous anodic niobia, grown on magnetron sputtered niobium metal, has been studied. A detailed physicochemical characterisation of the a-Nb2O5/0.5M H2SO4 electrolyte junction has been carried out by means of photocurrent and electrochemical impedance spectroscopy as well by differential admittance measurements. A change in the optical band gap (3.45 eV) of niobia film has been observed after aging (3.30 eV) at 250°C in air for 1 hour. A cathodic shift (0.15-0.2 Volt) in the flat band potential of the junction has been observed by the differential admittance (DA) measurements carried out in a large range of electrode potential values (6Volt) and ac frequencies (10 Hz-5 kHz). The frequency dependence of DA data, in absence of an appreciable contribution from surface states distribution, agrees with expectations of the theory of amorphous semiconductor Schottky barrier. The fitting of both components of DA allowed to get information on the distribution of electronic density of states (DOS) as a function of energy and distance from metal oxide interface. The DA measurements evidenced for vacuum treated niobia film an insulating to semiconductor transition with possible metallization of the inner metal/oxide interface. These findings can help to explain the large changes in the measured values of capacitance, after aging, and the larger leakage current observed in niobia electrolytic capacitors.