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

Doping silicon to concentrations above the metal-insulator transition threshold yields a novel material that has potential for photovoltaic applications. By focusing femtosecond laser (fs-laser) pulses on the surface of a silicon wafer in a sulfur hexafluoride (SF6) environment, silicon is doped with 1% atomic sulfur. Similar concentration of heavy chalcogen dopants (Se and Te) is achieved by fs-laser doping with solid-phase dopant precursors. Fs-laser doped Si:chalcogen systems exhibit near-unity, broadband absorption from the visible to the near infrared (< 0.5 eV, deep below the silicon bandgap), and metallic-like conduction. These unusual optical and electronic properties suggest the formation of an intermediate band. In this work, we investigate mid-infrared absorption of fs-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.