Producing amorphous Si with high index of refraction, low thermal noise and extremely low optical absorption at infrared wavelengths

Amorphous Si has properties that make it attractive as a coating material for gravitational-wave detectors: high index of refraction and low mechanical loss hence low thermal noise. Its use however is currently precluded by high optical absorption in the infrared. We have shown that annealing and hydrogenation of amorphous Si (a-Si) by e-beam evaporation yields large reductions in the optical absorption of amorphous silicon films at infrared wavelengths (1064, 1550 and 2000 nm), which are of interest for gravitational-wave detectors. We have grown hydrogenated a-Si (a-Si:H) by magnetron sputtering using Ar:H mixtures from 5 to 20% H₂, which leads to films with hydrogen contents below 1.5% at. H. The absorption coefficient of these films reaches values below 1 cm^-1, which is a target material design for the development of gravitational-wave detectors. H reduces the dangling bond density but the reduction in absorption does not correlate with the films’ dangling bond density. Instead, H acts by reducing energy barriers between weak Si—Si bonds, eliminating structural defects and lowering the systems’ energy, which reduces the density of deep trap states and hence absorption at infrared wavelengths and the mechanical noise at all temperatures.