Radiation-tolerant C-band Detectors for Free Space Optical Communications

InGaAs photodiodes are widely available and are the standard detectors for optical telecommunication systems operating at 1300nm and 1550nm wavelengths. They are high performance detectors in ground-based applications, however InGaAs detectors are not suitable for space-based applications because their dark current performance significantly deteriorates during exposure to ionizing radiation. Literature reports indicate that over a several year space mission, InGaAs detectorsrsquo; dark current can increase by a factor of 10 ndash; 25 x, which increases power consumption and produces excess noise. An alternative to InGaAs is the semiconductor material GaSb. The two materials have similar bandgap energies leading to similar nominal performance characteristics, however GaSb has more favorably located fundamental defect energies than InGaAs, which will produce superior radiation tolerance through reduced defect-related dark current generation in the presence of space-based radiation.This program will develop GaSb detectors with nominal performance equal to conventional InGaAs telecom detectors, but with greatly improved radiation hardness. The key innovation in this proposal is to assess radiation tolerance by considering defect-related energy levels in the detectors#39; semiconductor materials. The main defect level in InGaAs is at an energy of 0.46 eV above the valence band. This is near the middle of the bandgap, which from a dark current perspective is the worst possible scenario. In contrast, the main defect level in GaSb is at an energy of 0.26 eV above the valence band, which is sufficiently far away from midgap to enable significant reduction in defect-related dark current generation. This can be seen from the well-known equation for Shockley-Read-Hall generation, which shows that the electron-hole generation rate, and thus also the radiation-induced dark current, of InGaAs defects are more than 40 x times greater than those of GaSb defects.