Recent developments in type-II superlattice detectors at IRnova AB

A mid wave infrared type-II superlattice focal plane array with 320x256 pixels, 30 μm pitch and 90 % fill factor was fabricated in house, using a conventional homojunction p-i-n photodiode design and the ISC9705 readout circuit. High-quality imaging up to 110 K is demonstrated with the substrate fully removed. The absorber is 2 μm thick, and no anti-reflection coating was used, so there is still room for significant improvement of the quantum efficiency, which is in the 40 % range. Studies of the dark current vs. temperature behavior indicate that the device is limited by Shockley-Read-Hall generation from the depletion region. The activation energy of this dark current component is 0.13 eV, suggesting an unidentified recombination center positioned halfway into the 0.24 eV bandgap. Furthermore, we report on detectors with 100 % cut-off at 13 μm. The dark current density at 60 K and -50 mV bias is 2x10-4 A/cm2. Quantum efficiency, NETD and BLIP temperature are also calculated. Position-sensitive photocurrent measurements on mesa-etched superlattice material were made at low temperatures using a focused laser spot. The lateral diffusion length for holes was extracted and is reported.