Revealing the role of hydrogen in reducing optical absorption and mechanical loss in magnetron-sputtered amorphous silicon for gravitational-wave detectors

Hydrogenated $a$-Si films grown by magnetron sputtering have greatly reduced subgap optical absorption, down to 122 ppm (9.0 ${\mathrm{cm}}^{\ensuremath{-}1}$) at 1064 nm, 6 ppm (0.4 ${\mathrm{cm}}^{\ensuremath{-}1}$) at 1550 nm, and 2 ppm (0.1 ${\mathrm{cm}}^{\ensuremath{-}1}$) at 2000 nm for quarter-wavelength-thick films, and they reduced room-temperature mechanical loss below $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$. The dangling bond density is reduced to ${10}¹⁷\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ and the refraction index remains above that of crystalline silicon for all wavelengths. The above wavelengths are well below the Urbach edge of $a$-Si, and thus absorption is associated with deep-trap states. Although reductions in absorption and dissipation increase with increasing hydrogen content in the working gas, the hydrogen concentration in the films remains low (below 1 at.%), and neither absorption nor mechanical loss correlate with dangling bond density. Raman-derived bond angle deviation measurements of $a$-Si show a reduction in structural disorder as the films are further processed by vacuum annealing and post-hydrogenation, while $a$-Si:H films show a lower disorder in the as-deposited state without further reductions after thermal processing. These results suggest that the presence of hydrogen during film growth, acting as a catalyst, facilitates local rearrangement of atoms and removes strained Si--Si bonds that are more responsible for sub-band-gap absorption and dissipative mechanisms than dangling bonds.