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

摘要

The short-wavelength infrared (SWIR) regime between 1 and 3 μm is of high interest especially for surveillance,reconnaissance, and remote sensing applications. The availability of high-power, yet eye-safe SWIR laser sources is animportant 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 sceneillumination 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 of53%. The spectral sensitivity of In0.53Ga0.47As with its cut-off wavelength of 1.7 μm covers a wide part of the nightglowspectrum as well the emission lines of available laser sources at typical telecom wavelengths around 1.55 μm. However,for low-light-level passive SWIR imaging a dark-current density around 10~(-9) A/cm~2 is considered mandatory. While theinternational state-of-the-art has already achieved this performance at room-temperature operation, today’s European stateof-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 typicallyrather short integration times, planar processed InGaAs/InP pin photodiodes with lowest possible dark-current and noisecharacteristics 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-μm-pitch InGaAs pin detectors by aremaining factor of five. The most recent process run yielded further slightly improved dark-current characteristics on testdevices. Recently, we have started with in-house characterization of such focal plane detector arrays hybridized withsuitable SWIR read-out integrated circuits.