Femtosecond laser doped silicon for photovoltaic applications

Doping silicon to concentrations above the metal-insulator transition threshold yields a novel material that has potential for photovoltaic applications. By focusing femtosecond laser pulses on the surface of a silicon wafer in a sulfur hexafluoride (SF6) environment, silicon is doped with 1% atomic sulfur. This material exhibits 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. We report on the femtosecond laser doping techniques and material properties. By changing the laser parameters and ambient environment we can control the dopant profiles, crystallinity, and surface morphology. We investigate mid-infrared absorption of femtosecond laser doped silicon. In addition, we use temperature-dependent Hall measurements to investigate electron transport and to identify the energy states of the sulfur donors. These two techniques could shed light on energy levels of dopant states or bands. We will also discuss potential applications for intermediate-band photovoltaics.