Design and Analysis of Metal-Oxide-Semiconductor–Capacitor Microring Optical Modulator With Solid-Phase-Crystallization Poly-Silicon Gate

We present a rigorous electrical and optical analysis for a metal-oxide-semiconductor (MOS)-capacitor microring optical modulator on silicon-on-insulator (SOI) with a low-mobility and high optical loss polysilicon gate that could be fabricated by the complementary metal-oxide-semiconductor (CMOS)-compatible solid-phase-crystallization (SPC) process. Critical coupling was designed for the 25.5 mum radius rib-waveguide microring. Modulation speed, operating power at 3.3 V operating voltage, and insertion loss (IL) were analyzed with respect to the doping level of the SPC p-polysilicon gate and the n-crystalline silicon channel. 4.6 times10^-2 pJ/mum² operating power per switch can be achieved with 74 GHz modulation speed for 3 times 10¹⁸ cm^-3 doping level in both the SPC p-polysilicon gate and the n-crystalline silicon channel. For 40 GHz operation, 10-12 dB IL is achievable with the SPC polysilicon, and 9 dB IL is achievable with the well-annealed polysilicon that is lossless. Tolerance of the critical coupling to the gap width variation, temperature drifting of the microring, and wavelength drifting of the light source were analyzed and discussed extensively, and the extinction ratio (ER) was estimated for various situations. The 90 nm CMOS fabrication specification, if applied to the gap width variation of the microring, would leave a large margin in the ER to other sources of the critical coupling deviation. We found that ~ plusmn0.2degC temperature stability for the microring and the light source is required for a minimum ER of 5 dB if temperatures of both elements are controlled independently. Comparison between the MOS-capacitor modulators with microring and with Mach-Zehnder types was analyzed and discussed. In contrast to the Mach-Zehnder modulator, the modulation speed of the microring can be pushed up by increasing the doping level up to ~ 1times10¹⁸ cm^-3 without significantly increasin- - g the IL.