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GaAs/AlGaAs quantum well infrared photodetector arrays for thermal imaging applications

The performance of GaAs/AlGaAs multiple quantum well infrared detectors is studied theoretically and experimentally, with special emphasis on 8-12 μm thermal imaging applications. The dependence of detector performance on various factors like light coupling configurations (one and two dimensional reflection gratings or 45° polished edge), detector temperature, response wavelength and quantum well doping density is dealt with. An absorption quantum efficiency of 87% is demonstrated using a crossed grating and a waveguide (CGW). It is also found that an optimised 34 μm × 34 μm detector (a detector size suitable for large staring arrays, i.e. 256 × 256 or larger) with 9.0 μm cut-off wavelength, f# = 2 optics and 70% optical transmission reaches background limited operation at 74 K detector temperature. The potential of making highly uniform staring arrays utilising the mature GaAs material and processing technology is demonstrated by uniformity measurements of detector dark current. The experiments show that a metalorganic vapour phase epitaxy (MOVPE) grown structure can have a dark current standard deviation to mean value ratio over a 10mm long linear detector array of less than 2%. The staring array performance in terms of noise equivalent temperature difference (temporal NETD) is calculated to NETD < 20 mK at 77 K detector temperature and NETD < 10 mK at 70 K detector temperature.