InAs/(In,Ga)Sb type-II strained-layer superlattices (T2SLSs) were first proposed as an infrared (IR) sensing material in the 1980s by Sai-Halasz, Tsu, and Esaki. IR detectors based on InAs/(In,Ga)Sb T2SLSs have been under investigation ever since they were suggested several decades ago by Smith and Mailhiot. In 1990, Chow and coworkers reported the first Ga1-xInxSb/InAs T2SL material with high structural quality, longwave IR (LWIR) photoresponse, and LWIR photoluminescence. While theoretical predictions of detector performance seem to favor the InAs/InGaSb system due to the additional strain provided by the InGaSb layer, the majority of the research in the past five years has been focused on the binary InAs/GaSb system. This is attributed to the complexity of structures grown with the large mole fraction of In.
The InAs/GaSb T2SLS consists of alternating layers of nanoscale materials whose thicknesses vary from 4 to 20 monolayers (MLs). The overlap of electron (hole) wave functions between adjacent InAs (GaSb) layers results in the formation of electron (hole) minibands in the conduction (valence) band. Optical transitions between holes localized in GaSb layers and electrons confined in InAs layers are employed in the IR detection process. Thus, using two mid-bandgap semiconductors, devices can be fabricated with operating wavelengths anywhere between 3 and 32 μm.
Tunneling currents in T2SLS are lower than in mercury-cadmium-telluride (MCT) detectors of the same bandgap due to larger electron effective mass Auger recombination is supressed due to the large splitting between heavy-hole and light-hole valence subbands. Moreover, the T2SLSs are less sensitive to the bandgap variations due to compositional nonuniformities than the MCT alloys with the same bandgap. In contrast with quantum well infrared photodetectors (QWIPs), normal incidence absorption is permitted in T2SLSs, contributing to high conversion quantum efficiency. Two factors provide T2SLSs with technological advantages over competing technologies: commercially available low defect density substrates and a high degree of growth and processing uniformity over a large area. Focal plane arrays (FPAs) of sizes up to 1024 × 1024 have been demonstrated using the T2SLS material system, highlighting the potential of this technology. Thorough comparisons between MCT, InSb, QWIP, and T2SLS technologies can be found in the literature.
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