Femtosecond laser doping of silicon

Silicon is an abundant, stable, and efficient material for use in photovoltaic devices. However, it is costly to process, and is transparent at wavelengths longer than 1100nm, a spectral region containing 25% of solar energy. The limitations of silicon have spurred significant research into complex heterostructures that capture a greater fraction of sun’s energy. Engineering silicon to extend its effective spectral range, however, might offer a simpler way to increase the efficiency and decrease the cost of silicon-based photovoltaics. We report the creation of a thin, highly absorbing layer on silicon surfaces using the intense conditions at the focus of a high-intensity, femtosecond laser pulse. Irradiation of a silicon surface with 100-femtosecond laser pulses in the presence of a sulfur-containing gas creates a highly doped layer of nanocrystalline silicon in the top several hundred nanometers. The incorporation of this thin layer results in near unity absorption of light from the near-ultraviolet (250 nm) to the near-infrared (2500 nm), including wavelengths normally invisible to silicon (> 1100 nm). After thermal annealing, a photovoltaic responsive junction is formed between the nanocrystalline layer and the underlying silicon. We have previously shown that this process can be used to make viable silicon-based visible and infrared photodetectors. Here we report details on the photovoltaic properties of femtosecond-laser microstructured silicon using dopants in addition to sulfur, such as tellurium and selenium. We discuss the observed changes in optical and electrical properties under different annealing conditions and correlate these changes with dopant diffusion in the nanocrystalline layer.