N-type Ga-free InAs / InAs1-xSbx T2SL strain-balanced on GaSb is conventionally considered as an anisotropic quantum structure with heavy holes highly confined in InAsSb and electrons distributed throughout the structure. It is then somewhat puzzling that Ga-free T2SL detectors have demonstrated optical responses together with modulation transfer function (MTF) at Nyquist frequency quite satisfactorily. To explain these non-intuitive trends, cross-sectional transmission electron microscopy (TEM) investigations have been carried out, highlighting antimony (Sb) segregation across InAs/InAsSb interfaces. As a result, these must be considered as graded rather than sharp, disrupting consequently, the schematic representation of abrupt quantum potentials. In this communication, the experimental Sb profiles coming from TEM images are used to reach a better understanding of important figures of merit such as absorption, MTF and QE.
Ga-free InAs/InAsSb T2SL XBn detector is now a reliable candidate for high-performance focal plane arrays in the MWIR (3-5μm) domain. However, this T2SL is a very anisotropic quantum structure having a type-IIb band offset alignment where electrons are rather delocalized all over the structure while holes are strongly confined in deep InAsSb quantum well. This configuration could penalize the absorption and the hole minority carrier transport but MWIR detector device without anti reflection coating shows quantum efficiency higher than 50%. Considering results of specific measurements and band structure calculation, possible carrier transport scenario is presented to explain such performance.
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