Progress on InGaAs-based SPAD fabrication for SWIR detection and imaging

Time-of-flight imaging techniques are utilized for 3D-scene reconstructions and non-line-of-sight imaging capable of revealing persons or objects hidden behind obstacles. Such sensor systems usually demand for high-performance photodetectors with single-photon detection capabilities due to the high reflection losses of the emitted laser pulses during multiple, usually diffuse reflections from a relay wall and the actual scene of interest. To further compensate the high signal loss, a laser wavelength in the eye-safe Short-Wave Infrared (SWIR) spectral range, typically around 1550 nm, allows for higher laser intensities. In turn, SWIR-matching Single-Photon Avalanche Diodes (SPADs) need to be fabricated from adequate semiconductor materials, e.g., the InGaAs/InP material system. We report on the progress of the InGaAs/InP-SPAD fabrication using a customized process technology. The key technology is the planar process technology via zinc diffusion to produce spatially confined p-type regions. For the zinc-diffusion process, a novel method of selective epitaxial overgrowth was developed. Following a prior recess-etch step, the single-step Zn-diffusion process results in the intended double-well-shaped doping profile. Experimental data of thus fabricated InGaAs/InP SPADs show the expected dark-current, photo-current, and multiplication-gain characteristics in linear-mode operating as well as dark count rates in Geiger-mode operation around 200 - 220 K, which corresponds to a typical operating temperature for InGaAs/InP SPADs achievable by thermoelectric cooling.