Strong Mid-Infrared Absorption and High Crystallinity of Microstructured Silicon Formed by Femtosecond Laser Irradiation in NF3 Atmosphere

Arrays of high-quality crystalline conical spikes were formed on silicon surface in the presence of NF3 with femtosecond laser irradiation. The surface of the structures is smooth, and Raman scattering spectroscopy shows that few silicon polymorphs such as a-Si, Si-XII, and Si-III were observed in the conical structures. In the mid-infrared wavelength range of 3–16 µm, the structured surface exhibits a high absorptance up to 0.8. Additionally, the absorptance of the sample remains almost unchanged after the annealing process. These properties of the structured silicon will make it a promising material for mid-infrared applications in optoelectronic fields.