1. Photodetectors for silicon photonic integrated circuits

Silicon-based photonic components are especially attractive for realizing low-cost photonic integrated circuits (PICs) using high-volume manufacturing processes (Heck et al., 2013). Due to its transparency in the telecommunications wavelength bands near 1310 and 1550 nm, silicon is an excellent material for realizing low-loss passive optical components. For the same reason, it is not a strong candidate for sources and detectors, and photodetector fabrication requires the integration of either III/V materials or germanium if high speed and high efficiency are required. Photodetectors used in phonic integrated circuits, like photodetectors used in most other applications, typically require large bandwidth, high efficiency, and low dark current. In addition, the devices must be waveguide-integrated (rather than surface-illuminated) and the process used to fabricate the photodiode must be compatible with the processes used to fabricate other components on the chip. For many applications where PICs are a promising solution, for example microwave frequency generation, coherent receivers, and optical interconnects relying on receiverless circuit designs (Assefa et al., 2010b), the maximum output power is also an important figure of merit. There are numerous design trade-offs between speed, efficiency, and output power. Designing for high bandwidth favors small devices for low capacitance. Small devices require abrupt absorption profiles for good efficiency, but design for high output power favors large devices with dilute absorption. Most of the work on silicon-based photodiodes to date has focused on PIN diodes. Both ultra-compact devices with abrupt absorption profiles and devices with larger active areas have been demonstrated. The results have been consistent with this trade-off: ultracompact devices have shown the highest bandwidth-efficiency products (up to 38 GHz (Virot et al., 2013)), while devices utilizing dilute absorption profiles had better power handling (up to 19 dBm output power at 1 GHz (Piels et al., 2013)). Recently, photodetectors with decoupled structures, the separate absorption charge and multiplication (SACM) avalanche structure and the uni-traveling carrier (UTC) structure, have been used in both germanium (Piels and Bowers, 2014; Dai et al., 2010, 2014; Duan et al., 2013) and hybrid III/V-silicon (Beling et al., 2013b) to push performance past the limits imposed by the PIN structure.