Extended short wavelength infrared FPA technology: status and trends

This manuscript presents a survey of the extended short wavelength infrared (SWIR) FPA technologies with the emphasis on In_xGa_1-xAs imagers. We discuss the limitations of 2.5 μm cut-off extended In_xGa_1-xAs photodetector technology arising from the lattice mismatch between the absorber material and the InP substrate, as well as presenting the characteristics of molecular beam epitaxy grown In_0.83Ga_0.17As photodetectors on InP substrates with different buffer layers. It is possible to achieve desirable pixel and large format focal plane array (FPA) characteristics with In_0.83Ga_0.17As epilayers on linearly graded Al_xIn_1-xAs buffer layer. No considerable difference was detected in the G-R components of the dark currents of the detectors with graded AlInAs and InGaAs buffers. Therefore, the utilization of these materials as the buffer layer seems to generate similar trapping centers in the In_0.83Ga_0.17As absorber layer. While no considerable 1/f noise was observed in the extended SWIR In_0.83Ga_0.17As detectors operating under small reverse bias at 200 K, the signal to noise ratio of the detectors was limited by 1/f noise for higher reverse bias voltages. The 1/f noise was found to be related with the shunt leakage and the generation-recombination processes in the depletion region with a relatively larger shunt leakage noise coefficient in the order of 10^-3. We also present a comparison of extended SWIR InGaAs and HgCdTe detector technologies and discuss the potential of these technologies for future developments. While 2.5 μm cut-off mesa type In_0.83Ga_0.17As FPA pixels exhibit dark current densities in the same order with those of the best planar-type HgCdTe sensors on CdZnTe substrates near room temperature, HgCdTe on CdZnTe sensors provide more than an order of magnitude smaller dark current density at the typical operation temperature of these detectors. On the other hand, In_0.83Ga_0.17As sensors on InP substrates and HgCdTe detectors on Si substrates display dark current densities in the same order with different dominancy levels of shunt and G-R leakage currents. High noise coefficient of the shunt leakage is an important limitation for the lattice mismatched detectors narrowing the operation range of these sensors.