In-Situ Gamma Radiation Testing of 2.4 Micron Wavelength Extended InGaAs Photodiodes at Dry Ice Temperatures

Multiple space applications require infra-red photodiodes, including spectroscopy, optical communication links, and rapid Doppler shift LIDAR. Extended InGaAs photodiodes with 2.4-micron cutoff wavelength have been recently shown to be resilient to irradiation with Protons, Alpha Particles, Carbon Ions, and Iron Ions for fluence levels corresponding to multi-year Low Earth Orbit, Geostationary, inter-planetary, and deep space missions. Our prior studies have shown that the radiation-induced displacement damage may lead to some elevation in photodiode's leakage current, without significant sign of ionization damage. To further confirm this finding, these devices were subjected to Gamma rays to explicitly measure the effect of ionization damage only. We have successfully tested 290 mu m diameter, 2.4-micron wavelength, Extended InGaAs photodiodes coupled with single mode fiber for gamma radiation. Three devices were cooled to dry ice temperatures (similar to-71 degrees C) and subjected to two rounds of 662 keV gamma rays from Cesium-137 for 15 krad (water) for a cumulative dose of 30 krad (water). The devices were reverse biased at 100 mV and their leakage current was monitored in-situ to simulate their function as exposed to radiation in space environment. The in-situ data showed slight increase in leakage current in the presence of gamma radiation, and returned to original value once the gamma rays were turned off, thus proving the resilience of Extended InGaAs Photodiodes to ionization damage. These results were corroborated with detailed pre- and post-radiation measurements, which also demonstrated unchanged quantum efficiency and bandwidth over a wide range of operating temperatures, from -71 degrees C to +20 degrees C.