The Effect of Nb Incorporation on the Electronic Properties of Anodic HfO2

Hafnium oxide and Nb doped HfO2 were grown by anodizing sputtering-deposited Hf and Hf-4at.%Nb. Photoelectrochemical characterization was carried out in order to estimate solid state properties such as band gap, flat band potential and electrons internal photoemission threshold energy as a function of thickness and composition of anodic oxides. Optical transitions at energy lower than the band gap value of the investigated anodic films were evidenced, and they are attributed to optical transitions involving localized states inside the band gap. Such states were located at 3.6 eV and 3.9 eV below the conduction band edge for the Nb free and Nb containing hafnium oxide, respectively. Impedance measurements were performed in a wide range of electrode potentials under anodic and cathodic polarization with respect to the flat band potential, confirming the formation of insulating oxides with high dielectric constant. The latter increases from 20 to 41 due to incorporation of Nb into the oxide.