Infrared absorption limits of femtosecond laser doped silicon – effect of dopant types and thermal treatments

Silicon doped with non-equilibrium concentrations of chalcogen exhibits strong sub-bandgap photon absorption. In this work, we investigate mid-infrared absorption of femtosecond laser doped silicon. Fourier transform infrared spectroscopy could shed light on energy levels of dopant states or bands. We study samples doped with sulfur, selenium and tellurium. In addition, we also investigate the effect of annealing temperature. Preliminary results suggest that near-unity absorption of sulfur doped silicon extends to mid-infrared. However, absorption decreases for photons with energy less than about 300 meV. We will take into accounts of possible absorption contributions from surface morphology, free carrier absorption, oxide and other types of defects and compare the effects of different dopant types and thermal treatments.