Self-Ordered Hexagonal Nanoporous Hafnium Oxide and Transition to Aligned HfO2 Nanotube Layers

We show that hexagonally ordered HfO2 structures can either be grown to form a self-ordered pore or tube array, depending only on the electrochemical conditions during anodization of Hf in fluoride containing electrolytes. The water content in the electrolyte is the main factor affecting the transition from pores to tubes and the regularity of the structures. In the self-ordered nanoporous anodic hafnia layers, the cell diameters can be adjusted from 40 to 120 nm controlled by the applied voltage and a tube/pore length of similar to 20 mu m can be reached.