Recent developments in very long wave and shortwave infrared detection for space applications

Abstract

There is considerable interest in sensors which are optimised for detecting infrared radiation outside the normal thermal bands (3-12μm). This paper presents the development of photodiode arrays in Hg_1-xCd_xTe (MCT) that are sensitive in the very long wave (VLW) band to 14μm or in the visible and SWIR band below 2.5μm wavelength. The VLW arrays are heterostructure diodes fabricated from MCT grown by Metal Organic Vapour Phase Epitaxy (MOVPE). These are staring, focal plane arrays of mesa-diodes bump bonded to silicon read-out circuits. Measurements are presented demonstrating state-of-the-art performance over the temperature range 55-80K, for detectors with a cut-off wavelength of up to 14μm (at 77K). The SWIR/Visible detectors consist of an array of loophole photodiodes fabricated using MCT grown by Liquid Phase Epitaxy (LPE). The technology is suited to imaging LIDAR, NIR/Visible imaging, spectroscopy or hyperspectral applications. The diodes operate as avalanche photodiodes (APDs) which provides near-ideal gain in the pixel. Measurements are presented demonstrating state-of-the-art performance in the range 80K-200K from arrays with a cut-off below 2.5μm. Supporting technologies are also discussed. Silicon circuitry must be implemented in the SWIR and VLW bands that is appropriate to avalanche operation or copes with the low photon flux or low photodiode impedance. Trade-offs between conventional direct injection (DI), buffered direct injection (BDI), pixel capacitive transimpedance amplifier (CTIA) and source-follower per detector (SFPD) are presented. Work is in progress to increase the MOVPE wafer size to 6" which will enable large area arrays to be produced in the SW, MW, LW and VLW bands.