Black silicon: using lasers to make novel materials

Irradiating the surface of a crystalline silicon wafer with intense laser pulses in a reactive gas environment changes the structure and properties of the wafer dramatically: the formerly smooth, highly reflective surface becomes covered with a forest of sharp microspikes. In addition to changing the surface morphology, this microstructuring process also dramatically alters the optical properties of the silicon. The microstructured surface is highly absorbing even at wavelengths to which the original wafer is transparent. We find that the laser structuring process incorporates high concentrations of the background gas constituents into the surface, producing a highly doped surface layer of silicon with significantly altered band structure.