Sub-volt high-speed silicon MOSCAP microring modulator driven by high-mobility conductive oxide (1).pdf
Article https://doi.org/10.1038/s41467-024-45130-4 Sub-volt high-speed silicon MOSCAP microring modulator driven by high-mobility conductive oxide Wei-Che Hsu1,2, Nabila Nujhat1, Benjamin Kupp 1, John F. Conley Jr 1, Haisheng Rong 3, Ranjeet Kumar 3 & Alan X. Wang 1,2 Silicon microring modulator plays a critical role in energy-efﬁcient optical interconnect and optical computing owing to its ultra-compact footprint and capability for on-chip wavelength-division multiplexing. However, existing silicon microring modulators usually require more than 2 V of driving voltage (Vpp), which is limited by both material properties and device structures. Here, we present a metal-oxide-semiconductor capacitor microring modulator through heterogeneous integration between silicon photonics and titanium- doped indium oxide, which is a high-mobility transparent conductive oxide (TCO) with a strong plasma dispersion effect. The device is co-fabricated by Intel’s photonics fab and our in-house TCO patterning processes, which exhibits a high modulation efﬁciency of 117 pm/V and consequently can be driven by a very low Vpp of 0.8 V. At a 11 GHz modulation bandwidth where the modulator is limited by the RC bandwidth, we obtained 25 Gb/s clear eye diagrams with energy efﬁciency of 53 fJ/bit. Optical microring resonators have emerged as a key building block of photonic integrated circuits (PICs) that can function as versatile optical devices, including modulators, wavelength ﬁlters and mul- tiplexers, in comb lasers, weight banks for neuromorphic comput- ing, and optical sensors1–3. They are playing increasingly critical roles in optical communication, optical interconnects, optical computing, and biomedical sensing due to their ultra-compact footprint and capability for on-chip wavelength-division multi- plexing (WDM)4. Although optical microring resonators have been implemented on various platforms such as thin-ﬁlm LiNbO3 5, silicon nitride6, and plasmonics7, active silicon microring modulators (Si- MRMs) that can perform high-speed electro-optic (E-O) modulation as the photonic engine for future PICs remains as the climax of research8–10. Silicon photonics provides a mature, cost-effective platform to integrate Si-MRMs with lasers, photodetectors, passive optical devices, and even microelectronic circuits through standard foundry fabrication, which is still the only feasible solution to build large-scale PICs with both active and passive devices for complex systems11,12. Existing Si-MRMs available in the foundry process are based on reversed PN junctions, which can achieve ultra-high modulation bandwidth but usually require more than 2 V of driving voltage (Vpp)13–16. Such high Vpp is induced by the relatively weak plasma dis- persion effect of silicon and the small capacitance per unit length of the reversed PN-junction, which is usually less than 2 fF/μm17,18. The high Vpp makes it unfeasible to drive Si-MRMs directly by CMOS logic circuits. For example, today’s 5 nm CMOS has a core supply of 0.65 V and input/output supply of 1 V19. Therefore, high voltage-swing CMOS transmitter (TX) circuits that consume hundreds of milliwatts of power must be used to drive the Si-MRMs, and the energy consumption of the Si-MRM is insigniﬁcant compared with that of its CMOS driver20. For instance, the 106 Gb/s 2.5 Vpp Si-MRM driver using 28 nm CMOS pro- cess consumes 1.33 pJ/bit energy with Pulse-amplitude modulation 4-level (PAM-4) while the Si-MRM itself usually consumes less than 100 fJ/bit energy21. Therefore, reducing the driving voltage of the mod- ulator not only lowers the power consumption of the modulator itself, but also enables energy-efﬁcient and high-speed modulator drivers using advanced CMOS nodes that cannot support the high driving Received: 29 August 2023 Accepted: 12 January 2024 Check for updates 1School of Electrical Engineering and Computer Science, Oregon State University, Corvallis, OR, USA. 2Department of Electrical and Computer Engineering, Baylor University, Waco, TX, USA. 3Intel Corporation, 3600 Juliette Ln, Santa Clara, CA, USA. e-mail: alan_wang@baylor.edu Nature Communications|         (2024) 15:826  1 1234567890():,; 1234567890():,; voltage of existing silicon photonic modulators22. As a comparison, MOSCAP-driven Si-MRMs can achieve a much larger capacitance density using ultra-thin high dielectric constant insulators such as HfO2. In addition, MOSCAP devices allow heterogeneous integration of more E-O efﬁcient gate materials with Si waveguide such as III-V compound semiconductors and graphene23–30. In the past two dec- ades, high-speed MOSCAP Si-MRMs, including heterogeneously inte- grated functional materials, have been demonstrated (Supplementary Information I)7,10,31–37. Nevertheless, there is still a strong desire to develop high-speed Si-MRMs with sub-volt Vpp and large E-O mod- ulation efﬁciency. In this article, we demonstrate a highly efﬁcient MOSCAP Si- MRM heterogeneously integrated with titanium-doped indium oxide (ITiO), a TCO with high carrier mobility. The focus of this study is to achieve sub-volt Vpp modulation with a high E-O bandwidth, which requires optimization of the quality factor (Q- factor) and the E-O efﬁciency of the MRM. A higher Q-factor allows for narrower resonant spectra, enabling lower Vpp. How- ever, it also leads to a longer photon lifetime, which imposes a limitation on the modulation bandwidth. Therefore, enhancing the E-O efﬁciency of the MRM through other mechanisms using more efﬁcient gate material and higher capacitance density becomes crucial. The ITiO-gated MOSCAP Si-MRM in this work improved the E-O efﬁciency by narrowing the microring wave- guide width, which effectively improved the overlapping factor between the accumulated carriers and the optical mode proﬁle38. The utilization of high-mobility ITiO reduces the optical wave- guide absorption, enabling a balanced Q-factor for sub-volt Vpp modulation while still supporting a large photon lifetime-limited bandwidth. Additionally, the high-mobility ITiO, along with opti- mized doping on the Si microring waveguide and metal electrode patterning, improved the RC bandwidth signiﬁcantly compared with all previous work39,40. As a result, the ITiO-gated MOSCAP Si- MRM can be driven by 0.8 Vpp with 11 GHz E-O bandwidth, demonstrating a clear eye diagram at 25 Gb/s with an energy efﬁciency of 53 fJ/bit. In addition, we would like to point out that TCOs have emerged as a new E-O modulation material for inte- grated photonics in recent years41. Although high bandwidth was claimed42,43, these devices only demonstrated small-signal RF modulation and no clear eye diagrams have been reported above 5 Gb/s. In this work, the ITiO-gated MOSCAP Si-MRM represents the ﬁrst TCO modulator capable of high-speed E-O modulation with clear eye diagrams exceeding 25 Gb/s, which marks a signiﬁcant milestone in the development of TCO modulators. Through further optimization of the device structure to enhance the RC bandwidth, which is the primary constraint on our current device, we propose a high-mobility TCO to enable a much higher bandwidth while operating at even lower driving voltages. This advancement holds the potential for high-speed E-O modulation with sub-volt driving voltages, thereby paving the way for future advancements in energy-efﬁcient optical communication and computation systems. Results Device design Figure 1 illustrates the design of the ITiO-gated MOSCAP Si-MRM, including a 3D schematic diagram and a cross-sectional view of the active region. The device comprises a 300 nm thick Si rib waveguide with a 100 nm slab thickness. A waveguide width of 300 nm is selected to enhance the E-O efﬁciency, while a radius of 8 μm is chosen to reduce the bending loss (Supplementary Information II). The Si doping proﬁle of p (1 × 1017cm−3), p+ (3 × 1018cm−3), and p ++ (1 × 1020cm−3) are designed to reduce the series resistance without compromising the optical absorption in the Si waveguide. The entire device has a back- ground Si p-doping, and the p+ region covers the top of the ring waveguide and part of the Si slab. The p ++ region is placed 600 nm away from the ring waveguide to maintain the Q-factor. Such a doping design allows the passive Si microring resonator to achieve a high Q-factor of 20,000 near the critical coupling condition. The active E-O modulation region, which represents approximately 62.5% of the microring circumference (L = 31.4 μm), consists of a 10 nm thick HfO2 insulator layer and a 14 nm ITiO layer on the top. The Ni/Au electrodes traverse the bus waveguide via the top SiO2 waveguide cladding, forming ohmic contacts with the ITiO gate and the Si substrate of the MOSCAP. Figure 2a shows the simulated carrier density distribution in the cross-sectional waveguide of the ITiO-gated MOSCAP Si-MRM in the active region. When a negative bias is applied, holes and electrons accumulate at the Si/HfO2 and HfO2/ITiO interfaces, respectively. Accumulated electrons within the ITiO layer is less than 1 nm thick, which is signiﬁcantly thinner than the hole accumulation layer in the Si waveguide due to the different Debye lengths caused by the varying carrier concentration and dielectric constant44. Figure 2b presents the optical mode proﬁle of the transverse-electric (TE) mode in the ITiO-gated MOSCAP Si-MRM. Due to the bending ring waveguide, the optical mode proﬁle shifts toward the outer sidewall of the waveguide. Upon a nega- tive bias, the optical mode interacts with the accumulation charges in both ITiO and Si, enabling E-O modulation and blue- shifting the resonant wavelength (Δλres). Our previous studies have discussed that phase modulation with moderate index change is majorly determined by the total charge variation, allowing us to simplify the simulation model to a uniform dis- tribution with 1 nm thick accumulation layers, aligning well with experimental results45,46. Therefore, Fig. 2b also provides a zoomed-in view of the optical mode proﬁle at the HfO2/ITiO interfaces, showcasing examples at 0 V and −1 V biases with a 1 nm thick uniform accumulation layer. Figure 2c shows the simulated transmission spectra under different gate biases. Our simulation assumes a dielectric constant of 12 for HfO2 and carrier mobility Si (P) Si (P+) Si (P++) HfO2 ITiO Ni Au 300 nm Top Cladding (SiO2) Bus Waveguide Fig. 1 | 3D schematic diagram of an ITiO-gated MOSCAP Si-MRM. The dashed line box shows the cross-sectional view in the active region of the ITiO-gated MOSCAP Si-MRM. Article https://doi.org/10.1038/s41467-024-45130-4 Nature Communications|         (2024) 15:826  2 Data availability All data generated or analyzed during this study are included in this published article and its supplementary information ﬁles. The experi- mental data generated in this study are provided in the Source Data ﬁles. Source data are provided in this paper. References 1. 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Besides, we would like to thank the DURIP grant FA9550-20-1-0151 and Baylor University Mearse Endowment for equipment support of high-speed optoelectronics characterization, Materials Synthesis and Characterization Facility (MaSC) and the Electron Microscopy Facility at Oregon State University for our device fabrication. MaSC is part of the Northwest Nanotechnol- ogy Infrastructure, a National Nanotechnology Coordinated Infra- structure site at Oregon State University supported in part by the National Science Foundation (grant NNCI-1542101). We acknowledge Meer Sakib for the helpful discussion in design and characterization, Duanni Huang, James Jaussi, and Pavan Kumar Hanumolu for technical discussions. Article https://doi.org/10.1038/s41467-024-45130-4 Nature Communications|         (2024) 15:826  8 Author contributions W.H. and A.X.W. were involved in the design, experiments, and manu- script writing. N.N. was involved in device fabrication. B.K. and J.F.C. were involved in atomic layer deposition. H.R. and R.K. were involved in the design. All authors reviewed the manuscript. Competing interests The authors declare no competing interests. Additional information Supplementary information The online version contains supplementary material available at https://doi.org/10.1038/s41467-024-45130-4. Correspondence and requests for materials should be addressed to Alan X. Wang. Peer review information Nature Communications thanks Manuel Ack- ermann, and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. A peer review ﬁle is available. Reprints and permissions information is available at http://www.nature.com/reprints Publisher’s note Springer Nature remains neutral with regard to jur- isdictional claims in published maps and institutional afﬁliations. 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Title: Sub-volt high-speed silicon MOSCAP microring modulator driven by high-mobility conductive oxide ​
Authors: Wei-Che Hsu, Nabila Nujhat, Benjamin Kupp, John F. Conley Jr, Haisheng Rong, Ranjeet Kumar, Alan X. Wang ​
Publisher: Nature Communications
Date: anuary 27, 2024
Abstract: Silicon microring modulators are essential for energy-efficient optical interconnects and optical computing due to their compact size and capability for on-chip wavelength-division multiplexing. ​ However, current silicon microring modulators typically require driving voltages exceeding 2 V, constrained by material properties and device structures. ​ This study introduces a metal-oxide-semiconductor capacitor microring modulator, integrating silicon photonics with titanium-doped indium oxide, a high-mobility transparent conductive oxide with a strong plasma dispersion effect. ​ Co-fabricated by Intel’s photonics fab and in-house TCO patterning processes, the device demonstrates a high modulation efficiency of 117 pm/V and operates with a low Vpp of 0.8 V. At an 11 GHz modulation bandwidth, limited by the RC bandwidth, the modulator achieves 25 Gb/s clear eye diagrams with an energy efficiency of 53 fJ/bit. ​ This advancement marks a significant milestone in the development of TCO modulators, paving the way for future high-speed, energy-efficient optical communication and computation systems. ​