Abstract
Thermal stability of highly ordered hafnium oxide (HfO2) nanotube arrays prepared through an electrochemical anodization method in the presence of ammonium fluoride is investigated in a temperature
range of room temperature to 900 ◦C in flowing argon atmosphere. The formation of the HfO2 nanotube
arrays was monitored by current density transient characteristics during anodization of hafnium metal
foil. Morphologies of the as-grown and post-annealed HfO2 nanotube arrays were analyzed by powder Xray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).
Although monoclinic HfO2 is thermally stable up to 2000K in bulk, the morphology of HfO2 nanotube
arrays degraded at 900 ◦C. A detailed X-ray photoelectron spectroscopy (XPS) study revealed that the
thermal treatment significantly impacted the composition and the chemical environment of the core
elements (Hf and O), as well as F content coming from the electrolyte. Possible reasons for the degradation of the nanotube at high temperature were discussed based on XPS study and possible future
improvements have also been suggested. Moreover, dielectric measurements were carried out on both
the as-grown amorphous film and 500 ◦C post-annealed crystalline film. This study will help us to understand the temperature impact on the morphology of nanotube arrays, which is important to its further applications at elevated temperatures.
