Band gap narrowing and dielectric constant enhancement of (NbxTa(1-x))2O5 by electrochemical nitrogen doping

Anodic films were grown to 5 V and 50 V on Nb, Ta and Ta-Nb sputtering deposited alloys in 0.1 M ammonium biborate solutions in order to induce N incorporation. Their properties were compared to those of N free anodic films grown to the same formation voltages in 0.1 M NaOH. Photoelectrochemical measurements evidenced the presence of optical transitions at energy lower than the band gap of the oxides, attributed to localized states located close to the valence band mobility edge of the films, generated by N 2p orbitals, with consequent narrowing of the band gap. Since N incorporation occurs in the outer 70% of the anodic films, the dependence of the measured photocurrent as a function of photon energy was modelled considering a bi-layered film with an outer N doped layer and an inner N free layer. The mobility of the photogenerated carriers involving states due to N 2p orbitals is lower than the mobility of extended states due to O2−. Differential capacitance measurements allow to prove an enhancement of the dielectric constant of at least 20% due to the electrochemical N doping. For anodic films on Ta-66 at.%Nb a dielectric constant even higher (εox = 58.8) than that for pure Nb2O5 was measured.