Controlling light penetration depth to amplify the gain in ultra-fast silicon APDs and SPADs using photon-trapping nanostructures

Abstract

The gain in Avalanche Photodiodes (APDs) and Single Photon Avalanche Diodes (SPADs) is dependent on the probability of photo-generated carriers to trigger an avalanche process, which is correlated to the depth where a photon is absorbed by the photodiode. For silicon photodiodes, most of the photons with wavelengths in the visible spectrum are absorbed near the surface in the highly doped contact regions where the recombination rate is high. Thus, they do not contribute significantly to the avalanche multiplication process. By integrating photon-trapping nanostructures, we facilitate deeper penetration of photons into the devices, enhancing light absorption to generate more carriers that can trigger the avalanche process. This improves the gain-bandwidth of silicon APDs and SPADs significantly. Photon-trapping nanoholes can reduce the thickness of silicon without compromising its quantum efficiency, while a perforated surface reduces the device capacitance improving the bandwidth. Therefore, the manipulation of light penetration depth using photon-trapping nanoholes leads to ultrafast high-gain photodetectors capable of detecting faint light signals particularly useful for low light applications such as fluorescent lifetime imaging microscopy and time-of-flight positron emission tomography.