Influence of post deposition thermal annealing on the optical characteristics of hafnia nanofilms

Hafnia nanofilms of 60 nm thickness were deposited on Corning glass substrates at different substrate temperatures (from 25 to 120 °C) by applying electron-beam evaporation technique. After deposition, all the nanofilms were thermally annealed at temperatures between 300 to 500 °C for about half an hour in vacuum conditions. All the post thermally annealed nanofilms were characterized through x-ray diffraction and spectrophotometry. Structural characterizations showed that all the as-deposited hafnia nanofilms and the films annealed up to a temperature of 400 °C were amorphous in nature and the films annealed at 450 °C and 500 °C were transformed to polycrystalline phase possessing monoclinic structure. Hafnia nanofilms possessing different morphologies and crystallite sizes demonstrate variations in the optical properties including refractive index (1.88-2.94), optical band gap energy (3.29-3.85 eV), extinction coefficient (0.023-0.063) etc. Almost all the optical parameters portray oscillatory trend as a function of substrate temperature, which may be attributed to the removal of any residual stresses as well as to the transformation of amorphous phase to polycrystalline one and crystallite growth at high annealing temperatures. In addition, both un-annealed and annealed hafnia nanofilms depicted high reflectance (50-70%) in the visible region. Further improvement in reflectance can be made by inserting a metallic layer to form the oxide-metal-oxide stack morphology, which might have possible applications as heat mirrors. On the basis of above mentioned facts, a noteworthy conclusion is that the substrate temperature and post thermal annealing possesses greater tendency of changing the structural and optical properties of hafnia nanofilms.