Structural transitions and optical behavior of sputter deposited hafnia-titanium nanolaminates

HfO2-TiO2 nanolaminates are grown by sputter deposition with six different architectures (HfO2 and TiO2 layer thickness) on unheated fused SiO2 and Si substrates. The films are sequentially annealed in air at 573, 673, 773, 973, 1173, and 1273 K for 1h, and 1273 K for 24h. X-ray diffraction and x-ray photoelectron spectroscopy are used to study the crystallography and phase composition of the nanolaminates. Results show the presence of the standard-temperature-and-pressure phases monoclinic HfO2 and orthorhombic HfTiO4 in as-grown films and films annealed to moderate temperature (573 K-1173 K). In addition, the results definitively show the presence of a non-equilibrium interfacial structure, monoclinic Hf1-xTixO2. This structure is formed to accommodate a change in average cation size with increasing Hf concentration in a bilayer in a growth regime of kinetic constraint. The Hf1-xTixO2 structure further develops upon annealing.;When annealed at higher temperature (1273 K), all nanolaminates develop two different monoclinic Hf1-xTixO2 solid solutions, with x ≅ 0.05 and ≅ 0.3. Hf0.95Ti0.05 O2 contains the equilibrium amount of Ti whereas Hf 0.7Ti0.3O2 is clearly metastable. After 24h at 1273 K, Hf0.95Ti0.05O2 crystallites grow, but Hf0.7Ti0.3O2 crystallites remain stable. These different compositions are associated with formation of Hf1-xTi xO2 at different positions within a bilayer.;The fundamental optical absorption edge position and shape is analyzed from transmission and reflection spectrophotometry measurements. Results show that the values of the optical band gap lie between the values for the end-member oxides. However, the nanolaminates' fundamental absorption edge shapes are combinations of five characteristic features belonging to the end-member oxides. These results are consistent with a persistence model for optical absorption.