Lithography-Free Dielectric Etalon on Silicon with Thin GST Semiabsorbing Boundary for Broadband Reflective Color Filtering

A reflective broadband color filter based on a bi-interface engineered all-dielectric Fabry–Pérot (FP) etalon structure has been demonstrated here. The filter was fabricated on a p-silicon substrate (p-Si) without needing lithography. First, a wet oxide (spacer) of variable thickness was grown on the p-Si. On the top of the spacer, a germanium–antimony–telluride (Ge₂Sb₂Te₅) semiabsorbing film was deposited which could couple input light to the FP cavity. Through a transfer matrix method (TMM)-based numerical simulation, we found that the underlying p-Si could behave as a back reflector of a FP etalon supporting cavity modes. Hence, no extra metal layer deposition and their etching for defining the reflector pairs of a FP etalon were needed, and a minimum number of dielectric layers were involved. Cavity resonance intensified absorption of the allowed modes, which led to obtaining reflection dips in the filter response as could be controlled by the thicknesses of spacer and the Ge₂Sb₂Te₅ layer and by the angle of incidence of the input excitation and its polarization state. Combined reflectance spectra from the FP etalon resulted in a broadband color gamut with respect to chromaticity, which was different from the spectroscopically pure colors. The all-dielectric FP etalon concept was further utilized to engrave a complex colorful image (World Map) over an approximate 5.2 mm² active area on silicon starting with direct oxidation of the substrate, followed by multiple levels of selective etching.