Recent interest in mid-infrared (MIR, 2-5 mm) light emitters and detectors is driven by the wide range of potential applications in this spectral region. However, losses by non-radiative Auger recombination paths result in low quantum efficiencies so that many operating devices have to be cooled. Auger rates are sensitive to the fundamental band gap energy, E₀ and the spin-orbit splitting energy, D₀. However, the band-structures of many narrow gap materials have not been very well characterised which makes engineering their properties very difficult. Part of the reason for this is that modulation spectroscopies such as photo-modulated reflectance (PR), the key non-destructive tool for optical characterisation of semiconductor band-structures, becomes increasingly difficult as one attempts to look further out into the IR. Here, we report on MIR PR of E₀ and E₀+D₀ features in high-quality, InAs-rich InAsSb and GaInAsPSb samples as functions of both temperature and antimony contents ≤22.5%, up to wavelengths as long as 4.75 mm. We find that the temperature dependence of E₀ is generally consistent with that accepted in the literature. We are able to measure E₀ up to 300K and E₀+D₀ over a wide temperature range, confirming that D₀ is temperature-independent. However, with regard to the behaviour as a function of Sb content, while our PR measurements confirm the literature value of the E₀ bowing parameter, we find a dramatically different, and oppositely signed, D₀ bowing as compared to the currently accepted value. Due to the importance of D₀ in the prediction and interpretation of the performance of InAsSb-based devices, our new result is expected to have an important impact.

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1. High-uniformity GaAs/AlGaAs quantum dots coupled with GaAs wetting layer embedded in a GaAs/AlAs superlattice