Deep insights into the local structure of amorphous Ta2 O5 thin films by x-ray pair distribution function analysis

Amorphous films of tantalum oxide (Ta2O5) are widely applied to build highly reflective mirrors used in interferometric gravitational wave detectors, such as the Laser Interferometer Gravitational Wave Observatory. Despite a large number of studies, the structural properties of amorphous Ta2O5 on the local scale still deserve several further investigations. Such information is essential to any attempt to understand the properties of this important material, and no property modeling efforts can be expected to yield reliable information until the local structure in the amorphous phase is better understood. In this paper we report the results obtained by analyzing the synchrotron x-ray pair distribution functions of pure and Ti-doped amorphous Ta2O5 films, deposited by ion-beam sputtering as prepared and after a thermal treatment. As a result, it is found that: (1) the arrangement of Ta atoms in amorphous Ta2O5 strongly resembles that characterizing the high-pressure Z-Ta2O5 polymorph, whereas the topological properties of O atoms resemble that observed in δ-Ta2O5; (2) structural correlations in amorphous Ta2O5 start to vanish above ∼5 Å and are completely suppressed for r > 10 Å on account of disorder; (3) Ti substitution retains the short-range topological ordering characterizing the as-prepared Ta2O5 amorphous film even after thermal treatment; conversely, pure Ta2O5 films undergo a significant rearrangement of the local structure after thermal treatment.