High-performance hydrogenated amorphous silicon deposited by ion-beam sputtering for gravitational-wave detectors

Amorphous silicon (a-Si) is a promising material with a high refractive index that shows great potential in the development of low-thermal-noise, highly reflective coatings for gravitational-wave detectors due to its low mechanical loss. However, its high optical absorption has hindered its practical use. The primary objective of this paper is to investigate the effects of hydrogen incorporation into a-Si on its optical properties and mechanical loss with ion-beam sputtering (IBS) technology, which is widely used to produce coatings with low absorption and scattering loss. It is demonstrated that hydrogenation can effectively reduce the optical absorption of a-Si, with the concentration of hydrogen proving to be a crucial factor. Optimal annealing temperature can further enhance the coating quality, in which silicon atom rearrangement also contributes to the improvement. Overall, using the process combination can reduce the extinction coefficient of a-Si coatings by 16-fold at 1064 nm and 85-fold at 1550 nm, while simultaneously reducing mechanical loss by a factor of 12. The degree of mechanical loss reduction is closely related to the order present in the atomic structure of the silicon network. To our knowledge, this study represents the first comprehensive analysis of coating mechanical loss in hydrogenated a-Si deposited via IBS.