Effects of vacuum annealing on surface and optical constants of hafnium oxide thin films

Effect of annealing on optical constants for hafnium oxide (HfO2) thin films has been reported. HfO2 films deposited by electron beam evaporation were thermally annealed at 500 °C for an hour in vacuum. These films were characterized through XRD, AFM and optical spectrophotometry before and after thermal annealing. It was observed that the as-deposited HfO2 films were amorphous and transformed to polycrystalline monoclinic phase after annealing at 500 °C. Optical constants such as refractive index, extinction coefficient, band gap, and Urbach energy have been determined by analyzing experimentally recorded absorption, transmittance, and reflection data in the wavelength range 200–1500 nm. AFM micrographs indicate smooth surface with low values of RMS roughness. Furthermore, optical properties such as Urbach energy, refractive index, and extinction coefficient depict an increase on annealing whereas optical band gap energy shows opposite trend caused by crystallization and crystallite orientations. RMS surface roughness also increases on annealing. Present HfO2 films annealed at 500 °C show better reflectivity (~10%) in the NIR region which can further be improved by adding a metallic layer for its applications as a heat mirror.