High-Speed Low-Voltage Silicon Microring Modulator Driven by Metal-Oxide-Semiconductor Capacitor

Introduction

Silicon microring modulators (Si-MRMs) are essential components in high-bandwidth optical interconnects. Traditional Si-MRMs rely on reversed PN junctions, offering ultra-high modulation bandwidth but requiring over 2 V driving voltage due to the weak plasma dispersion effect of silicon. This high driving voltage necessitates power-hungry amplifiers, resulting in pJ/bit energy consumption despite Si-MRMs only consuming <100 fJ/bit. To address this issue and reduce the driving voltage to sub-volt levels while maintaining high bandwidth, enhancing the electro-optic (E-O) efficiency of Si-MRMs is a promising solution.

Device Design

In this work, a highly efficient Si-MRM is demonstrated by heterogeneous integration with a high-mobility titanium-doped indium oxide (ITiO)/hafnium oxide (HfO2)/Si metal-oxide-semiconductor capacitor (MOSCAP) structure. The silicon microring waveguide is designed with a narrow 300 nm width to enhance E-O efficiency while preserving a reasonable quality factor (Q-factor).

The device fabrication involves post-processing steps on a silicon-on-insulator (SOI) wafer with a pre-fabricated passive silicon microring resonator. A 10 nm HfO2 insulator layer is deposited via atomic layer deposition (ALD), followed by RF sputtering a 14 nm ITiO layer as the top gate for the MOSCAP. Ni/Au electrodes establish Ohmic contacts with the ITiO gate and silicon substrate, deposited through thermal evaporation and lift-off techniques. The fabricated ITiO-gated MOSCAP Si-MRM is illustrated in Figure 1(a).

Fig. 1 (a) SEM image of the ITiO-gated MOSCAP Si-MRM, showing the active region in purple and contact electrodes in gold. (b) Normalized transmission spectra at various DC biases. (c) Transmission at λMOD (1317.54 nm) influenced by gate voltage, achieving an ER of 6 dB and IL of 3 dB at -1.9 V with a 0.8 Vpp swing.

Figure 1(b) shows the normalized transmission spectra of the ITiO-gated MOSCAP Si-MRM with different applied DC biases, exhibiting a blue shift when applying negative DC gate bias. This device achieves an E-O efficiency of 117 pm/V, corresponding to a low Vπ•L of 0.12 V•cm, and exhibits a Q-factor of approximately 4600 at 0 V, supporting an optical bandwidth of 50 GHz. Figure 1(c) presents the optical transmission under different DC gate biases at a modulation wavelength (λMOD) of 1317.54 nm. The device achieves a 3 dB insertion loss and 6 dB extinction ratio with a bias voltage of -1.9 V and a voltage swing of 0.8 Vpp (-1.5 V to -2.3 V).

High-Speed Characterization

The E-O response of the ITiO-gated MOSCAP Si-MRM is characterized by driving it with a sine wave of varying frequencies and detecting the resulting modulated optical signal with a photodetector before sending to a microwave spectrum analyzer. The received output RF power, normalized to the RF power at a low frequency of 500 MHz, is shown in Figure 2(a), indicating a measured 3-dB bandwidth of 11 GHz.

Fig. 2 (a) Normalized E-O response of the ITiO-gated MOSCAP Si-MRM, with normalization relative to 500 MHz. (b) Measured eye diagrams for NRZ modulation of the ITiO-gated MOSCAP Si-MRM. The modulation was achieved with a driving voltage of 0.8 Vpp at data rates of 20 Gb/s and 25 Gb/s, respectively.

To demonstrate high-speed operation, the ITiO-gated MOSCAP Si-MRM is driven by a 0.8 Vpp PRBS electrical signal generated by an arbitrary waveform generator, combined with a -1.9 V DC bias through a bias tee. The optical input comes from an O-band tunable laser, and the optical output from the MRM is amplified by an O-band amplifier before being sent to a digital communication analyzer. Remarkably, the eye remains open at a data rate of 25 Gb/s with only a 0.8 Vpp swing, as shown in Figure 2(b). Furthermore, the modulation energy consumption is estimated to be 53 fJ/bit, considering the 0.8 Vpp and the measured capacitance of 333 fF.

Conclusion

A high-speed ITiO-gated MOSCAP Si-MRM has been presented, achieving an E-O bandwidth of 11 GHz. Notably, it demonstrates an open eye at 25 Gb/s with a sub-volt driving voltage of 0.8 Vpp and an energy efficiency of 53 fJ/bit. Such a device addresses the challenges of power-hungry CMOS transmitter circuits and presents a promising solution for advancing energy-efficient optical communication and computational systems.

Reference

[1] W.-C. Hsu, N. Nujhat, B. Kupp, J. F. Conley, Jr., and A. X. Wang, "High-Speed Low-Voltage MOSCAP-Driven Silicon Microring Modulator," School of Electrical Engineering and Computer Science, Oregon State University, Corvallis, Oregon, USA, and Department of Electrical and Computer Engineering, Baylor University, Waco, Texas, USA, 2024.