Thin absorber AlInAsSb SACM APDs with photon-trapping gratings for eSWIR applications

High-sensitivity photodetectors with a high signal-to-noise ratio are essential for numerous infrared applications. While avalanche photodiodes (APDs) offer high internal gain, their performance is limited by high dark current in the narrow-bandgap materials used for infrared absorption. This results in operation at cryogenic temperatures, which in turn increases system complexity and additional cost. The dark current can be reduced by decreasing the absorption layer thickness, but this also lowers the quantum efficiency. Here, we report separate absorption, charge, and multiplication (SACM) Al x In 1−x As y Sb 1−y APDs with thin absorber thicknesses, ranging from 50 to 400 nm. These devices exhibit low dark current at room temperature. A 2D metal grating structure, consisting of a periodic array of parallel Au pillars, on top of these devices, can compensate for the decrease in external quantum efficiency (EQE) in thin absorbers. This is due to lateral coupling into the absorber waveguide modes and plasmon-enhanced transmission. Non-vertical sidewalls of the Au pillars will limit the input coupling efficiency. This can be significantly mitigated by filling the gap between the metal pillars with a material having an optimized refractive index. This was successfully demonstrated using BK-7 index-matching liquid (n∼1.5). Due to the metal grating configuration, the photoresponse is polarization-dependent. By combining index matching and optimizing the input polarization, we have achieved EQE of 8.2%, 16%, 30.6%, and 35% for 50, 100, 200, and 400 nm, respectively.