The cryogenic etching of the black silicon (BSI) has been demonstrated as a superior absorber in par with other ultraabsorbers such as carbon nanotubes in the visible and near-infrared spectrum. In this work, we discuss the fabrication, modeling, and characterization of the BSI targeting the 2.5-5 microns range. We investigated a series of cryogenic parameters such as temperature, pressure, oxygen flow rate, power, and etching duration and fabricated a series of uniformly etched wafers. Additionally, we established a three-dimensional mathematical model of a unit cell and manipulated the silicon needle geometry and shape. Our preliminary results show five orders of magnitude specular reflectance in the infrared region. The technique employed here could be used to scale the etching process and enhance the absorption in the far-infrared and submillimeter range.
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