From front-side to back-side illumination of InGaAs/InP SPADs for photon detection efficiency enhancement

Low-noise front-side illuminated InGaAs/InP single photon avalanche diodes (SPADs) were converted into back-side illuminated ones to improve their photon detection efficiency (PDE). We introduced a mirror on the frontside and an antireflection-coated aperture on the backside, thus allowing photons to double-pass through the absorption InGaAs region. Such modifications did not affect the layer structure of the device and, consequently, its noise, while exploiting the double passage of photons to achieve higher PDE. In this work, we provide a direct comparison between the two structures, front-side illuminated (FSI) and back-side illuminated (BSI) SPADs, in terms of photon detection efficiency and dark count rate (DCR). Experimental results on devices with 10 μm and 25 μm active area diameters showcased a 24% increase in PDE, reaching up to 31% at 1550 nm and 5 V of excess bias, while maintaining the low DCR (a few kcps at 230 K) of the original FSI device.