Large VLWIR Hg1?xCdxTe photovoltaic detectors

Very long wavelength infrared (VLWIR; 15 to 17 μm) detectors are required for remote sensing sounding applications. Infrared sounders provide temperature, pressure and moisture profiles of the atmosphere used in weather prediction models that track storms, predict levels of precipitation etc. Traditionally, photoconductive VLWIR (λ_c >15 μm) detectors have been used for sounding applications. However, photoconductive detectors suffer from performance issues, such as non-linearity that is 10X – 100X that of photovoltaic detectors. Radiometric calibration for remote sensing interferometry requires detectors with low non-linearity. Photoconductive detectors also suffer from non-uniform spatial optical response. Advances in molecular beam epitaxy (MBE) growth of mercury cadmium telluride (HgCdTe) and detector architectures have resulted in high performance detectors fabricated in the 15 μm to 17 μmm spectral range. Recently, VLWIR (λ_c ∼ 17 μm at 78 K) photovoltaic large (1000 μm diameter) detectors have been fabricated and measured at flux values targeting remote sensing interferometry applications. The operating temperature is near 78 K, permitting the use of passive radiators in spacecraft to cool the detectors. Detector non-AR coated quantum efficiency >60% was measured in these large detectors. A linear response was measured, while varying the spot size incident on the 1000 μm detectors. This excellent response uniformity, measured as a function of spot size, implies that low frequency spatial response variations are absent. The 1000 μm diameter, λ_c ∼ 17 μm at 78 K detectors have dark currents ∼160 μA at a −100 mV bias and at 78 K. Interfacing with the low (comparable to the contact and series resistance) junction impedance detectors is not feasible. Therefore a custom pre-amplifier was designed to interface with the large VLWIR detectors operating in reverse bias. A breadboard was fabricated incorporating the custom designed preamplifier interfacing with the 1000 μm diameter VLWIR detectors. Response versus flux measurements were made on the large VLWIR detectors and non-linearity <0.15% was measured at high flux values in the 2.5×10¹⁷ to 3.5×10¹⁷ ph-cm⁻²sec⁻¹ range. This non-linearity is an order of magnitude better than for photoconductive detectors.