Surface morphology and infrared absorption of silicon irradiated by picosecond laser pulses in SF6

The microstructured samples were prepared by irradiating silicon surface with picosecond laser pulses in SF6. The surface morphology of microstructured samples irradiated at different laser fluence was characterized by SEM. The samples exhibited high optical absorptance over a wide wavelength range between 300 and 2700 nm. The absorptance of samples irradiated with the fluence of 1.0 J/cm2 was measured to be up to 95% between 1100 and 2700 nm. The infared absorptance of the surface-structured samples increased with increasing fluence. Whereas, as the annealing temperature was increased, the infared absorptance of the samples irradiated at the same fluence decreased. A tentative explanation for the effects of laser fluence and annealing temperature on the infared absorptance has been proposed based on the formation of mid-band gap impurity bands and the multiple reflections of light between microstructures.