Role of hydrogen in amorphous silicon – new mechanism of suppressing photon absorption*

Amorphous silicon (a-Si) is an attractive candidate for photovoltaics, photonics, and dielectric mirror coatings for gravitational-wave detectors. Its use however is often limited by optical absorption due to its low band gap.  Due to its disordered nature, carrier recombination and photon absorption are enabled by defects in the bonding network, which poses challenges to fabricating device-quality a-Si. Hydrogenation has been shown to be effective in improving the electrical performance of a-Si. In spite of many advances made, the mechanism by which introduction of hydrogen to a-Si suppresses recombination and light absorption remains elusive. It is widely accepted that the improvement is related to hydrogen passivation of dangling bonds. However, here we report that despite only a tiny amount of hydrogen in our hydrogenated a-Si films and almost no change in dangling bond density after hydrogenation, the optical absorption in the infrared wavelength range is reduced by over 70%. We found that instead of forming bonds with undercoordinated Si, hydrogen promotes structural relaxation of the a-Si network, which in turn reduces the optical absorption. Our work provides a conceptual framework to better understand the photon absorption mechanisms in a-Si and how to suppress them.