Recent developments in type-II superlattice-based infrared detectors

Much has been accomplished in the last few years in advancing the performance of type-II superlattice (T2SL) based infrared photodiodes, largely by focusing on device and heterostructure design. Quantum efficiency (QE) has increased to 50% and higher by using thicker absorbing layers and making use of internal reflections, and dark currents have been reduced by over a factor of ten by using bandstructure engineering to suppress tunneling and generation-recombination (G-R) currents associated with the junction. With performance levels of LWIR T2SL photodiodes now within an order of magnitude of that of HgCdTe (MCT) based technology, however, there is renewed interest in understanding fundamental materials issues. This is needed both to move performance toward the theoretical Auger limit, and to facilitate the task of transitioning T2SL growth from laboratories to commercial institutions. Here we discuss recent continuing efforts at NRL to develop new device structures for enhanced detector performance, and to further our understanding of this material system using advanced structural and electronic probes. Results from electron beam induced current (EBIC) imaging and analysis of point defects in T2SL photodiodes will be presented, showing differentiated behavior of bulk defect structures. We will also describe a study comparing intended vs. as-grown T2SL photodiode structures by crosssectional scanning microscopy (XSTM). Using parameters extracted from the XSTM images, we obtain detailed knowledge of the composition and layer structures through simulation of the x-ray diffraction spectra.