Atomic restructuring of ion beam sputtered TiO₂: GeO₂ thin films with annealing

Amorphous oxides thin films are extensively used in optical interference coatings, and as dielectric barrier films. Tunable properties accessible through the ion beam sputtered (IBS) codeposition of two or more cations in reactive oxygen atmospheres enables the synthesis of amorphous oxides mixtures with tailored optical and mechanical properties. This work reports on the study of the evolution of the atomic structure and bonding of an IBS mixture of 44% TiO₂ and 56% GeO₂ with annealing through Raman and Infrared spectroscopies. Characteristic vibrational signatures of a-GeO₂ are altered in the mixture, indicating that the Ti ions are introduced into the lattice of a-GeO₂ in substitutional mode, as evidenced by a higher distribution in the inter-tetrahedral Ge-O-Ge angles due to longer and weaker Ti-O bonds, and suggesting a more open and flexible network. Annealing causes structural reorganization on the TiO₂ : GeO₂ mixtures, as observed by the evolution in the Ge-O-X (X=Ge, Ti) vibrational signatures, more significantly at temperatures ≥ 500°C. This work brings out a new understanding of the modifications in the atomic structure of TiO₂ : GeO₂ mixtures upon thermal treatment, which is critical to understand the mechanisms involved in affecting internal friction of this promising high refractive index material for coatings of gravitational wave detectors [¹].

[1] G. Vajente et al., Phys. Rev. Lett. 127, 071101 (2021).