Characterization of LWIR diodes on InAs/GaSb Type-II superlattice material

Long wavelength infrared (LWIR) focal plane arrays (FPAs) built on Type-II strained layer InAs/GaSb superlattice materials are emerging as an alternative to LWIR HgCdTe. We have made progress in the development of this technology in a collaborative effort between Raytheon Vision Systems and Jet Propulsion Laboratory, resulting in successful devices with LWIR cutoff wavelengths. We report here two investigations related to wafer processing and superlattice material characteristics. The critical interface between the superlattice and the silicon dioxide passivation was examined at the atomic scale by high resolution transmission electron microscopy (HRTEM), showing a conformal coating on an InAs/GaSb mesa sidewall, which undulates with the superlattice periodicity due to differential etching. Electron energy loss spectroscopy (EELS) showed that oxides of the superlattice elements were present but minimal, and some occasional arsenic precipitates were observed at the passivation interface. Our previous analysis of the current–voltage curves was extended further to reveal the minority carrier lifetimes responsible for producing the generation–recombination (GR) and the diffusion dark currents. Lifetimes at 78 K were found to be 6 and 20 ns in the GR and diffusion processes, respectively. Lifetimes from both mechanisms track together with temperature. A HgCdTe diode was analyzed in the same manner for comparison.