Prospects for large format IR astronomy FPAs using MBE-grown HgCdTe detectors with cutoff wavelength >4 μm

Rockwell Science Center has developed a double layer planar heterostructure (DLPH) detector array fabrication process with significant advantages over the PACE-1 process now being used to produce 256x256 and 1024x1024 FPAs for low background infrared astronomy. The DLPH detectors are p-on-n photodiodes fabricated in a double layer of wide and narrow bandgap HgCdTe grown by molecular beam epitaxy (MBE) on CdZnTe substrates. The double layer structure provides superior surface passivation while the lattice matched CdZnTe substrate reduces the defect density. DLPH FPAs have been fabricated in array sizes up to 640x480 and with cutoff wavelengths as long as 15 mu m. Quantum efficiencies are typically in the 0.5 to 0.8 range. For a 256x256 array DLPH detectors with 5.3 mu m cutoff wavelength at 50K, the median dark current was 0.39 e-/sec at 0.5V reverse bias. For 7 of 17 individual DLPH detectors with 10.6 mu m cutoff at 30K, the dark current was less than 10(4) e-/sec at 20 mV bias. For long cutoff wavelengths, the detector breakdown voltage is too low to permit signal integration directly on the reverse biased detector capacitance. Such detectors require a readout circuit that maintains the detector near zero bias and provides a separate capacitor to store the integrated signal.