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 cm⁻¹) at 1064 nm, 6 ppm (0.4 cm⁻¹) at 1550 nm, and 2 ppm (0.1 cm⁻¹) at 2000 nm for quarter-wavelength-thick films, and they reduced room-temperature mechanical loss below 2×10⁻⁵. The dangling bond density is reduced to 10¹⁷cm⁻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.