Role of the background gas in the morphology and optical properties of laser-microstructured silicon

We irradiated silicon with a train of femtosecond pulses in the presence of SF6, H2S, H-2, SiH4, and a mixture of Ar and SF6 in order to analyze the role of the background gas in determining the morphology and the optical properties of the resultant surfaces. We discuss factors that affect the surface morphology created during irradiation and show that the presence of sulfur in these gases is important in creating sharp microstructures. We also show that the presence of sulfur is necessary to create the near-unity absorptance for both above-band and below-band gap radiation (0.25-2.5 mu m) by silicon; only samples with sulfur concentrations higher than 0.6% absorb 95% for above-band gap radiation and have a flat, featureless absorptance of 90% for below-band gap radiation.