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We have performed a numerical study of the carrier transport properties in gallium-free strained-balanced InAs/InxAs1-xSb type-II superlattices (T2SL) intended for infrared detector applications. The motivation was to understand whether the performance of T2SL detectors is ultimately limited by fundamental or technological processes. We have employed a rigorous one-dimensional quantum mechanical transport model based on the non-equilibrium Green's function (NEGF) formalism that includes a k.p description of the electronic structure. This approach has allowed us to avoid making any a priori assumptions on the physical mechanism (tunneling, sequential tunneling or hopping) dominating the transport. We find that regardless of its nature and cause, the presence of positional and compositional disorder, introduced inherently during materials growth by layer thickness fluctuations, nonuniform antimony composition and segregation throughout the superlattice stack, significantly affects the vertical carrier transport properties. In particular, the minority carrier hole mobility is fundamentally limited by the nonideal disorder. Furthermore, upon reducing the temperature, holes become fully localized and transport occurs by hopping, which explains published measured photodetector data that demonstrates the quantum efficiency exhibiting a very strong temperature dependence that degrades as the temperature is reduced. We also found that the minority carrier electron mobility is largely unaffected by disorder, indicating the p-type absorbing layer as the preferred option.
We have also performed simulations of typical nBn detectors with a simplified NEGF model based on the effective mass approximation and Büttiker-probe self-energies, which also allows us to explore the possibilities offered by quantum-corrected drift-diffusion approaches. We will present simulation examples of representative gallium-containing and gallium-free nBn structures intended for MWIR and LWIR photodetector applications. These examples will include calculations of I-V characteristics, SRH recombination rates and quantum efficiency.
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