Broadband antireflective structures applied to high resistive float zone silicon in the THz spectral range

The optimal structural parameters for an antireflective structure in high resistive float zone silicon are deduced for a rectangular and a hexagonal structure. For this the dependence of the effective index from the filling factor was calculated for both grating types. The structures were manufactured by the Bosch (R)-process. The required structural parameters for a continuous profile require an adaption of the fabrication process. Challenges are the depth and the slight positive slope of the structures. Starting point for the realization of the antireflective structures was the manufacturing of deep binary gratings. A rectangular structure and a hexagonal structure with period 50 mu m and depth 500 mu m were realized. Measurements with a THz time domain spectroscopy setup show an increase of the electric field amplitude of 15.2% for the rectangular grating and 21.76% for the hexagonal grating. The spectral analysis shows that the bandwidth of the hexagonal grating reaches from 0.1 to 2 THz.