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 ${}^{\circ }\mathrm{C}$ is shown to decrease for elevated growth temperature. Films grown at 310 ${}^{\circ }\mathrm{C}$ and annealed yield a mechanical loss of $3.1\times {10}^{-4}$ 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.

• Received 2 November 2023
• Accepted 12 February 2024

DOI:https://doi.org/10.1103/PhysRevMaterials.8.035603