Advancing All Silicon MOSCAP Ring Modulators With Ultra-Thin Sub-5 nm Insulator

We demonstrate silicon/SiO $_{2}$ /polysilicon lateral MOS-Capacitor (MOSCAP) RRM operating above 50 GHz with modulation amplitude enhanced by a large plasma absorption within the MOS junction. A MOSCAP ring resonator modulator (RRM) model has been built using Lumerical software, in which the plasma effect is defined by adopting a reported superlinear rather than linear plasma absorption equation, which aligns well with our experimental results. The performance of the MOSCAP RRMs has been analyzed with different thicknesses of insulator oxide ( $t_{\text{ox}}$ ). The modulation performance is enhanced with thinner $t_{\text{ox}}$ down to 3 nm, giving a lower insertion loss and larger optical modulation amplitude (OMA) when benchmarked with a conventional depletion type RRM with a low $V_{\pi }L$ of 2.6–4.0 V $\cdot$ mm under a bias voltage $V_{\text{b}}$ 0–3 V. High-speed operation of the MOSCAP RRM with radius 15 μm demonstrated an average power insertion loss (IL $_{\text{ave}}$ ) of 3.5dB and one level insertion loss (IL $_{\text{one}}$ ) of 2dB for achieving a 3dB dynamic ER at a data rate of 30 Gb/s and bit-error-rate (BER) $<\! 1 \times 10^{-12}$ . The same performance is possible at 50 Gb/s when feed-forward-equalization is enabled on the detection side. We also show the possibility of operating at 224 Gb/s using 4-level pulse amplitude modulation (PAM-4) for a MOSCAP RRM incorporating two active segments. The MOSCAP RRM provides an attractive solution to surpass the performance of the conventional depletion-type RRM, for which future performance scaling is limited with increased doping density towards $1 \times 10^{19}$ cm $^{-3}$ .