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 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 μm covers a wide part of the nightglow spectrum 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/cm² is considered mandatory. While the international 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 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-μm-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.
Short-wave infrared (SWIR) detection systems are increasingly demanded for surveillance, reconnaissance, and remote sensing applications. Passive SWIR cameras can benefit from an extended spectral range, compared to standard nightvision goggles, and the exploitation of the faint night-glow emission from the night sky. Furthermore, eye-safe SWIR lasers can improve the contrast and range of night-vision systems. High-performance SWIR photodetectors can be realized in the InGaAs material system, providing a typical cutoff wavelength of 1.7 μm, which covers a wide part of the night-glow spectrum as well the emission lines of available laser sources at typical telecom wavelengths around 1.55 μm. However, the low photon flux in night-vision applications demand for high responsivities and very low dark-current characteristics of the InGaAs photodetectors. We report on the current development activities of InGaAs SWIR photodetectors at Fraunhofer IAF. We have implemented a planar process technology of InGaAs/InP pin photodiodes for the fabrication of low-light-level SWIR cameras with 640× 512 pixels with 15 μm pixel pitch. Electro-optical characterization results of early-stage process runs exhibit darkcurrent densities below 10-7 A/cm2 at room temperature for 15-μm pitch detector elements. The current activities focus on further reducing the dark current to reach the international state of the art. Moreover, InGaAs-based avalanche photodiode (APD) have been developed for active SWIR imaging. Gain values of M ≈ 10 on camera level at a reverse bias voltage around 20 V have been achieved by a sophisticated vertical detector design. FPAs of such InGaAs-APD material have been successfully integrated into SWIR cameras with 640 × 512 pixels at 15 μm pixel pitch and operated in SWIR laser gated viewing mode.