A monolithically integrated HgCdTe short-wavelength infrared photodetector and micro-electro-mechanical systems-based optical filter

A monolithically integrated low-temperature micro-electro-mechanical systems (MEMS) and HgCdTe infrared (IR) detector technology is introduced, implemented, and characterized. The ultimate aim of this project is to develop a MEMS-based optical filter, integrated with an IR detector, that selects narrow wavelength bands within the short-wavelength IR (SWIR) region of the spectrum. The entire fabrication process is compatible with two-dimensional IR focal plane array technology, and needs to be compatible with a proposed electrically tunable MEMS filter based on a Fabry-Perot optical cavity. The fabricated device consists of an HgCdTe SWIR photoconductor, distributed Bragg mirrors formed of Ge-SiO-Ge, a sacrificial spacer layer within the cavity, and a silicon nitride membrane for structural support. Mirror stacks fabricated on silicon, identical to the structures that will form the optical cavity, have been characterized to determine the optimum filter characteristics. The measured full-width at half-maximum (FWHM) was 34 nm at the center wavelength of 1,780 nm with an extinction ratio of 36.6. Fully integrated filters on HgCdTe photoconductors with a center wavelength of approximately 1,950 nm give a FWHM of approximately 100 nm, and a peak responsivity of approximately 8 × 10⁴ V/W. The experimental results are in good agreement with the optical model, which takes into account mirror reflectivity, absorption within the cavity by the spacer material, and absorption by the silicon nitride support structure.