Large VLWIR Hg1-xCdxTe photovoltaic detectors

Very long wavelength infrared (VLWIR; 15 to 17 mu m) detectors are required for remote sensing sounding applications. Infrared sounders provide temper ature, pressure and moisture profiles of the atmosphere used in weather pre diction models that track storms, predict levels of precipitation etc. Trad itionally, photoconductive VLWIR (lambda(c) > 15 mu m) detectors have been used for sounding applications. However, photoconductive detectors suffer f rom performance issues, such as non-linearity that is 10X - 100X that of ph otovoltaic detectors. Radiometric calibration for remote sensing interferom etry requires detectors with low non-linearity. Photoconductive detectors a lso suffer from non-uniform spatial optical response. Advances in molecular beam epitaxy (MBE) growth of mercury cadmium telluride (HgCdTe) and detect or architectures have resulted in high performance detectors fabricated in the 15 mu m to 17 mu m spectral range. Recently, VLWIR (lambda(c) similar t o 17 mu m at 78 K) photovoltaic large (1000 mu m diameter) detectors have b een fabricated and measured at flux values targeting remote sensing interfe rometry applications. The operating temperature is near 78 K, permitting th e use of passive radiators in spacecraft to cool the detectors. Detector no n-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 mu m detectors. This excellent response uniformity, measured as a fun ction of spot size, implies that low frequency spatial response variations are absent. The 1000 mu m diameter, lambda(c) similar to 17 mu m at 78 K de tectors have dark currents similar to 160 mu A at a -100 mV bias and at 78 K. Interfacing with the low (comparable to the contact and series resistanc e) junction impedance detectors is not feasible. Therefore a custom pre-amp lifier was designed to interface with the large VLWIR detectors operating i n reverse bias. A breadboard was fabricated incorporating the custom design ed preamplifier interfacing with the 1000 mu m diameter VLWIR detectors. Re sponse 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 x 10(17) t o 3.5 x 10(17) ph-cm(-2)sec(-1) range. This non-linearity is an order of ma gnitude better than for photoconductive detectors.