Strong sub-band-gap infrared absorption in silicon supersaturated with sulfur

Silicon supersaturated with up to 0.6at.% sulfur in solid solution was fabricated by ion implantation and pulsed-laser-melting-induced rapid solidification. The optical absorption coefficient over the range of 1200–2500nm is uniformly high at ∼2.5×104∕cm, which is at least an order of magnitude greater than the maximum value attributable to free carriers. High crystal quality was confirmed by transmission electron microscopy and ion channeling. The absorption coefficient decreases markedly with subsequent furnace annealing over the range of 200–600°C. We propose that the high absorptivity is due to a broad distribution of sulfur-related localized states within the band gap.