InGaAs SWIR photodetectors for night vision

Abstract

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.