A Comprehensive Study of Surface Defects in traditional Type II InAs_GaSb Superlattices and Ga_free Type II_Research Area 4_3_Electronic Sensing.

Invented by Nobel Laureate Leo Esaki, Type-II antimonide-based superlattices (T2SLs) have become an attractive material for infrared detection technology, due to the intrinsic advantages that they have over the mercury cadmium telluride (MCT) material system.1 Band alignment of T2SLs creates an effective energy gap that can be flexibly tuned across the entire infrared regime via precise control of the interface composition and layer thicknesses, without introducing large strain. Auger recombination, which is a limiting factor for high temperature operation of infrared detectors, can be suppressed by manipulating the superlattice to control the band structure.2 Compared to MCT and most of the small band gap semiconductors that have very small electron and hole effective mass, the effective mass in T2SLs is relatively large, due to its special design which involves the interaction of electrons and holes via tunneling through adjacent barriers. The larger effective mass reduces the tunneling current, which is a major contributor to the dark current of MCT detectors. Moreover, the capability of band structure engineering opens the horizon for exploring novel device architectures that are unthinkable using simple binary or ternary compound semiconductor band alignments like MCT. As an example, recent research has proposed a novel variant of T2SL, the M-structure SL,3 with large effective mass and large tunability of band edge energies.4 The structure has been shown to efficiently reduce the dark current in photovoltaic detectors.5 Due to all these fundamental properties, T2SL has experienced a rapid development over the past decade (Figure 1-a) and its performance has reached a level comparable to state of the art MCT detectors (Figure 1-b). Meanwhile, recent reports have shown that InAs/GaSb T2SLs are limited by a low minority carrier lifetime. With gallium being the suspected origin of defects in GaSb, another type-II superlattices material known as InAs/InAs1-xSbx T2SLs, has been proposed as another alternative. This material system, which does not contain gallium, has been proved to have a longer carrier lifetime. In addition, with two common elements (indium and arsenic) in superlattice layers, the InAs/InAs1-xSbx T2SLs has a relatively simple interface structure with only one changing element (antimony). The band structure engineering in InAs/InAs1-xSbx T2SLs relies on the layer thicknesses and the variation of arsenic/antimony composition. Therefore, it promises a better controllability in epitaxial growth and simpler manufacturability. These advantages are the driving force for InAs/InAs1-xSbx T2SL-based photodetectors to become the subject of extensive research recently. Despite this rapid development, there is still a large gap between the theoretical capabilities of this material system and the experimental performance of the detectors. Each Type-II structure consists of hundreds of alternating layers as thin as a few angstroms. Stacking of so many layers, along with thousands of MBE shutter actuations during each growth, introduces diverse imperfections in the material. Furthermore, this material needs to be etched as part of the fabrication process, which introduces surface defects. These defects and imperfections lead to deterioration of material quality evidenced by degradation of the carrier lifetime and mobility. The carrier lifetime and the mobility will be chosen as the ultimate figures of merit to evaluate the quality of the material because these key parameters are drastically affected by imperfections and defects