Low mechanical loss and high refractive index in amorphous Ta2O5 films grown by magnetron sputtering

The ability to observe astronomical events through the detection of gravitational waves relies on the quality of multilayer coatings used on the optical mirrors of interferometers. Amorphous ${\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$ (including ${\mathrm{TiO}}_{2}:{\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$) currently limits detector sensitivity due to high mechanical loss. In this paper, mechanical loss measured at both cryogenic and room temperatures of amorphous ${\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$ films grown by magnetron sputtering and annealed in air at 500 ${}^{\ensuremath{\circ}}\mathrm{C}$ is shown to decrease for elevated growth temperature. Films grown at 310 ${}^{\ensuremath{\circ}}\mathrm{C}$ and annealed yield a mechanical loss of $3.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ at room temperature, the lowest value reported for pure amorphous ${\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$ grown by magnetron sputtering to date, and comparable to the lowest values obtained for films grown by ion beam sputtering. Additionally, the refractive index $n$ increases 6% for elevated growth temperature, which could lead to improved sensitivity of gravitational-wave detectors by allowing a thickness reduction in the mirrors' coatings. Structural characterization suggests that the observed mechanical loss reduction in amorphous ${\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$ films with increasing growth temperature correlates with a reduction in the coordination number between oxygen and tantalum atoms, consistent with ${\mathrm{TaO}}_{\text{x}}$ polyhedra with increased corner-sharing and reduced edge- and face-sharing structures.