Low-light-level SWIR photodetectors based on the InGaAs material system

The short-wavelength infrared (SWIR) regime between 1 and 3 mm is of high interest especially for surveillance, reconnaissance, and remote sensing applications. The availability of high-power, yet eye-safe SWIR laser sources is an important asset enabling scene illumination and implementation of advanced active imaging concepts like gated viewing (GV) or light detection and ranging (LIDAR). With atmospheric nightglow also a natural, but faint source for scene illumination is available for passive low-light-level imaging in the SWIR region. The most commonly employed material system for realizing SWIR photodetectors is InGaAs with an indium content of 53%. The spectral sensitivity of In0.53Ga0.47As with its cut-off wavelength of 1.7 mm covers a wide part of the nightglow spectrum as well the emission lines of available laser sources at typical telecom wavelengths around 1.55 mm. However, for low-light-level passive SWIR imaging a dark-current density around 10-9 A/cm² is considered mandatory. While the international state-of-the-art has already achieved this performance at room-temperature operation, today's European state of- the-art is still lagging behind. The development of InGaAs-based SWIR detectors at Fraunhofer IAF aims at pin as well as avalanche photodiodes (APDs) for imaging applications with 640×512 pixels. While InGaAs APDs play to their strength in GV applications with typically rather short integration times, planar processed InGaAs/InP pin photodiodes with lowest possible dark-current and noise characteristics are the detector devices of choice for passive low-light-level detection. Within a few planar-process batches, we approached the European state-of-the-art for the dark-current density of 15-mm-pitch InGaAs pin detectors by a remaining factor of five. The most recent process run yielded further slightly improved dark-current characteristics on test devices. Recently, we have started with in-house characterization of such focal plane detector arrays hybridized with suitable SWIR read-out integrated circuits.