Sub-bandgap absorptance in chalcogen-hyperdoped silicon

~It has been shown that optical doping with pulsed lasers~can achieve non-equilibrium concentrations up to one atomic {\%} of heavy chalcogens in silicon.~~Compared to intrinsic silicon, this material exhibits near-unity absorption of sub-bandgap photons and has potential use in silicon infrared photodetectors and high-efficiency photovoltaics.~~Successful application of this material, however, requires better understanding of the exact mechanism responsible for sub-bandgap absorptance.~~Using a variety of techniques, we probe the chemical structure of this material system.~~We find that the short range structure of the dopant atom is correlated to the amount of sub-bandgap absorptance.~~We also compare the structure of different dopant species (S and Se) as well as different hyperdoping mechanisms (fs-laser doping vs. ion implantation followed by pulsed laser melting).~~In conjunction with theoretical modeling of expected chalcogen defect states, we identify dominant structural characteristics related to the observation of sub-bandgap absorptance. ~ Expanding on previous results, we demonstrate control of sub-bandgap absorptance through thermal processing. ~~In addition to suggesting a method to engineer the optical properties of the material, this result provides further insight into the thermodynamics of formation of a possible dopant-related defect state.~~We compare the thermodynamics measurements to the dopant structural measurements and posit a model of sub-bandgap absorptance and defect dynamics. ~These results provide a better understanding of the phenomena of sub-bandgap absorptance in chalcogen-hyperdoped silicon and a pathway to explore other hyperdoped semiconductors. ~~