UC Davis

Over the last five decades, there has been continuous development in the field of optical communication and sensing applications. Timely development of cost-effective, power and absorption efficient, low-noise and ultra-fast optical interconnects/sensors is crucial to meet the high demand for data transfer in the era of the Internet of Things (IoT), augmented reality (AR), virtual reality (VR), light detection and ranging (LIDAR), quantum communication, biomedical imaging and emerging applications that are expected to connect billions of devices/sensors with different functionalities. Datacenters are envisioned to scale up to meet the high connectivity demand as big data and cloud computing continues to grow exponentially. Intra- and inter-datacenter communications require optical links for reach gap (500 m–2 km), long-reach (∼10 km), and extended-reach communications (up to 40 km), which requires optical transceivers/PDs that work in a wide range of the optical spectrum. In a surface-illuminated PD, high speed and high efficiency are often a tradeoff since a high-speed device needs a thin absorption layer to reduce the carrier transit time. In contrast, a high-efficiency device needs a thick absorption layer to compensate for the low absorption coefficient of some semiconductors such as Si Germanium (Ge), GaAs, and InGaAs at wavelengths near the bandgap. This thesis presents the recent development in enhancing the photon–material interactions by utilizing photon-trapping (PT) nanostructures that can control light for more interaction with the photoabsorbing materials, slow down the propagation group velocity and reduce surface reflection. Since ultra-fast PDs suffer from low optical absorption, photon-trapping nanostructures can be utilized to enhance their efficiency. We demonstrated that a perpendicular light beam could be bent to allow guiding parallel to the surface of the PDs, greatly enhancing the interaction of light with the absorption material, which allows for improving broadband absorption by photon manipulation. Consequently, the speed of carrier collection can be increased by designing a thin absorbing layer with a reduced transient time without losing the sensitivity of the PD. Another advantage of developing PT nano-designs is to reduce the junction capacitance by decreasing the junction area. That helps reduce RC time, which is one factor limiting a photodetector's speed. The capacitance reduction in designed PT PDs results in faster response compared to its counterpart without PT PDs. Additionally, thinner absorbing material with integrated PT nano-designs could also help to reduce the bulk dark current, which is one of the noise components in the PDs. Different passivation methods were applied to improve the surface damages/traps to achieve low leakage of less than 1 nA. Moreover, photon-trapping designs add another parameter to guide the light to a specific preferential depth to maximize the gain bandwidth and absorption efficiency in PDs.

This thesis presents the modeling, fabrication, and characterization of various photon-trapping designs in Si, Ge, III-V, and quantum-well PDs. Si photon-trapping PDs enable the development of efficient ultra-fast PDs suitable for monolithic integration with CMOS electronics for the short-reach (850 nm) multimode optical data links used in datacom and computer networks. Such an all-Si optical receiver offers great potential to reduce the cost of short reach, <300 m optical data links in data centers. Additionally, Ge-on-Si PT PDs have the potential to be monolithically integrated with CMOS/ BiCMOS ASICs. Si and Ge-on-Si photon-trapping per pixel designs are presented, which show high absorption efficiency and enable high-performance CMOS image sensors. The unique response of the Si photontrapping PDs paves the way for computational imaging development and spectroscopy on chips utilized for biomedical applications. In addition, highly sensitive photon-trapping Avalanche Photodetectors (APDs) and Single Photon Avalanche Photodetectors (SPADs) are designed with low noise, high gain, and ultra-fast characteristics. The monolithic integration of Si and Ge-on-Si offers low-cost packaging solutions and allows low parasitics, resulting in high-performance on-chip detection. Ge-on-Si, InGaAs, III-V quantum-well PT PDs can be utilized for short- and long-reach communication at intra- and inter-datacenters, passive optical networks, LIDAR, and quantum communication systems, as well as enhancing the capacity of long-haul DWDM systems beyond the L band. III-V PT PD modeling is presented to enhance their bandwidth to meet future THz optical detection and communication demand in the C and L bands and other emerging applications.