Monolithic Integrated InGaAs/InAlAs WDM-APDs With Partially Depleted Absorption Region and Evanescently Coupled Waveguide Structure

In this work, theoretical optimization and experimental demonstration have been performed for high speed, highly sensitive, and P-down InGaAs/InAlAs avalanche photodiodes (APDs) with evanescently coupled (EC) waveguide structure. In consideration of the carrier transfer and lightwave propagation simultaneously, we adopt partially depleted absorption region (PDA) and tapered waveguide coupler, inspired by previously proposed APDs. The PDA is well known for improvement of responsivity-bandwidth product for APDs. However, it still remains undisclosed, for InGaAs/InAlAs APDs, to some extent, how the dark current, gain, and bandwidth systematically change with different thickness of PDA. The optimization of electric field distribution, dark current, photo-current, and bandwidth, have been performed. The dark current of APD is addressed to shed light on the underlying mechanism. The main origin of dark current for previously proposed APDs has been revealed to be the tunneling current from the multiplication layer, which can be alleviated by decreasing the doping concentration of the charge layer. At the same time, our results demonstrate that higher proportion of heavy doping absorption region is adopted, the wider bandwidth for the APD will be realized. Fortunately, it is also found that the heavy doping layer of absorption region brings limited deterioration to dark current, as well as gain for APDs. Combing our design with previously demonstrated results, we propose a methodology for the optimization of InGaAs/InAlAs APDs with PDA. And a series of EC-APDs, as well as monolithic integrated WDM InGaAs/InAlAs APD array, for exampling, have been experimentally demonstrated, which is hopeful for future potential applications.