Advancements in CMOS-Compatible Silicon Nitride Optical Modulators via Thin-Film Crystalline or Amorphous Silicon p–n Junctions

This paper proposes two types of electro-refractive optical modulator structures as a fully CMOS-compatible alternative solution. These modulators leverage the properties of amorphous (top) and crystalline (bottom) silicon films surrounding silicon nitride waveguides operating in the C-band communications range at a wavelength of 1550 nm. Various structures have been demonstrated and explored to compete with or surpass the current state-of-the-art performance of thermal tuners, the most widely used tuning mechanism in silicon nitride integrated photonics. Designs utilizing vertical and lateral p–n junctions with amorphous or crystalline films have been simulated and proposed. For the lateral p–n junctions, modulator lengths to achieve a π phase shift smaller than 287 μm have been demonstrated for the TE mode and that smaller than 1937 μm for the TM mode, reaching 168 μm in the case of a lateral p–n junction that is completely a p-doped region over or under the waveguide for TE, and 1107 μm for TM. Power consumption is higher for the TM modes than for the TE, being in the order of 100 mW for the former and lower than 23 mW for the latter. The modulators exhibit higher losses for amorphous material compared to crystalline, with losses smaller than 10.21 dB and 3.2 dB, respectively. The vertical p–n junctions present a larger footprint than the lateral ones, 5.03 mm for TE and 38.75 mm for TM, with losses lower than 3.16 dB and 3.95 dB, respectively, for the crystalline silicon. Also, their power consumption is on the order of 21 mW for TE and 164 mW for TM.