Silicon photodetector integrated silicon nitride-on-SOI platform for communication and sensor applications

Abstract

Driven by the exponential growth of data tra c, current infrastructure and standards are evolved to meeting the requirements. In long haul communication, 1550/1310 nm based singlemode ber technology is a commercially viable platform. For short-reach optical interconnects for rack-to-rack communication and within buildings, the matured 850 nm VCSEL based multimode ber (MMF) technology is an industry-standard. IEEE has recently proposed a 400 Gbps roadmap for data centers to scale up short-reach infrastructure. However, the current shortreach datacom infrastructure is not scalable to support a 400 Gbps data rate. Integration of all the functional components of an optical interconnect on a single platform can meet the requirement of a scalable, energy-e cient, and a ordable system. Additionally, CMOS compatibility can leverage electronic and photonic circuits' co-existence on a single platform and low-cost mass manufacturing. Integrating optical functionalities on a single-chip also o ers application in sensing as well. Due to the low absorption in water, the 850 nm wavelength window is also attractive for realizing Lab-on-a-chip biosensors. Integrated photonic circuits at 850 nm band can therefore be useful for a lab-on-a-chip biosensor platform as well. This thesis presents an integrated photonic platform comprising silicon nitride (SiN) waveguide, SiN surface grating coupler, silicon photodetector, and wavelength lters integrated monolithically on the SiN-on-SOI platform at 850 nm wavelength. Our primary focus is to overcome the limitation of lower responsivity and bandwidth of silicon photodetector. We extensively study various techniques to integrate silicon photodetector with passive SiN waveguides e ciently suitable for future short-reach datacom and lab-on-a-chip biosensors. In the rst part, we realize a single-mode SiN waveguide along with high-e ciency surface grating couplers. We have demonstrated a uniform and apodized grating coupler with a bottom Bragg re ector. Apodized gratings provide higher coupling e ciency than uniform gratings due to better mode pro le matching between Gaussian-shaped ber mode and the apodized grating eld pro le. Distributed Bragg re ector (DBR) reduces the optical loss due to high order di racted light directed towards the bottom substrate. SIN apodized grating coupler with DBR as the bottom re ector achieves the highest ever coupling e ciency of 2.19 dB/coupler and 3dB bandwidth of 40 nm at 876nm wavelength. In the second part, we demonstrate various architectures to integrate high-speed silicon photodetector with SiN waveguide. First, we demonstrate the integration of SiN waveguide with high-speed, lateral silicon pin photodetector. Compared to the silicon photodetector realized on bulk silicon, photodetector on an SOI has higher bandwidth due to the lower cross-section. We use silicon inverse taper to improve the coupling from SiN to silicon, which results in better responsivity of silicon photodetector. We have achieved the highest responsivity of 0.44 A/W and bandwidth of 15 GHz for the integrated silicon pin. Bandwidth improvement without degradation of responsivity is attributed to the lateral collection of photocarriers transverse to the propagation direction, and low RC time-limited bandwidth due to the thin silicon. To enhance the photodetector responsivity further, we propose a SiN ring resonator enhanced silicon metal-semiconductor-metal (MSM) photodetector. Compact, cavity-enhanced silicon- MSM photodetector responsivity is estimated to be 0.81 A/W at 5 V, which is 100 times higher than the conventional waveguide photodetector. Moreover, the photodetector's compact size (6X6 m2) can o er high bandwidth due to reduced RC time-limited bandwidth. In this section, we also discuss the integration of SiN waveguide with a thin silicon-MSM photodetector (70 nm thick). In this con guration, the SiN waveguide is placed on top of the silicon-MSM. Since the silicon's thickness is low SiN, the waveguide does not su er from mode mismatch losses between silicon and SiN. Such con guration is attractive due to its high responsivity and bandwidth, along with ease of fabrication. We have shown the DC measurements with a maximum responsivity of 0.56 A/W at 10 V bias. Finally, we have demonstrated the integration of wavelength division multiplexer with silicon photodetector since the shortwave wavelength division multiplexing (SWDM) at 850 nm wavelength band is considered one of the viable solutions to attain 400 Gbps roadmap. We have realized the WDM using SiN Echelle gratings and integrated the output channel waveguides with a silicon-MSM photodetector. Experimentally, we have shown that the Echelle grating has the insertion loss of 4.3 dB and adjacent channel cross talk of 22 dB for the channels having wavelength separation of 10 nm. Future exploration of the demonstrated device can lead to precise wavelength ltering with on-chip detection useful for both high-speed short-reach datacom and lab-on-a-chip biosensors. In summary, we have demonstrated the capability of realizing a scalable, energy-e cient, and cost-e ective silicon nitride based integrated photonic receiver in the 850 nm wavelength band.