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 detectors’ dark current can increase by a factor of 10 – 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' 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.
The radiation-tolerant NIR detector technology developed in this program is broadly applicable to any mission or application requiring the ability to detect 1550nm photons in the presence of ionizing radiation. Examples of planned activities that would benefit from such free-space optical communications technology include the Laser Communication Relay Demonstration (LCRD), the Illuma -T Project, the EM-2 Optical to Orion (O2O) demonstration, and the DSOC Project technology demonstration hosted by the Psyche Mission spacecraft.
Amethyst predicts the technology will be disruptive in the NIR/SWIR imaging markets, where binary GaSb optical absorber material offers unparalleled pixel uniformity within a focal plane array. The practical outcomes are significant performance enhancements and relaxed manufacturing requirements which will result in a low-cost NIR/SWIR imaging technology with wide applicability.