Urbach Tails and Optical Absorption of Amorphous Silicon Coatings

Due to its low mechanical loss and high refractive index, amorphous silicon is a very promising material to realize highly-reflective coatings with low thermal-noise, such as required for gravitational-wave detection. However, the optical absorption of amorphous silicon is too high, leading to heating of cryogenically cooled mirrors. Previous work on reducing the optical absorption has indicated correlations of the absorption with deposition parameters, post-deposition treatment and possibly the silica-substrate type the coatings were deposited on. In this article, we present a systematic study of the optical properties of amorphous silicon, showing a correlation of the optical absorption with the band-gap energy and with Urbach tails, and disproving previously suspected correlations with the silica-substrate type.