We demonstrate the concept of diagonal transitions for quantum cascade detectors (QCD). Different to standard, vertical QCDs, here the active transition takes place between two energy levels in adjacent wells. Such a scheme has versatile advantages. Diagonal transitions generally yield a higher extraction efficiency and a higher resistance than vertical transitions. This leads to an improved overall performance, although the absorption strength of the active transition is smaller. Since the extraction is not based on resonant tunneling, the design is more robust, with respect to deviations from the nominal structure. In a first approach, a peak responsivity of 16.9 mA/W could be achieved, which is an improvement to the highest shown responsivity of a QCD for a wavelength of 8 μm at room-temperature by almost an order of magnitude.

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With the increasing interest in low effective mass materials for intersubband devices, mixed As-Sb compounds, like GaAsxSb1−x or AlxIn1−xAsySb1−y, gain more and more attention. The growth of these materials, however, still provides significant challenges due to the complex interaction between As and Sb. In this work, we provide an in-depth study on the incorporation of Sb into the GaAsxSb1−x layers and compare our findings to the present literature on this topic. It is found that both the composition and the crystal quality of GaAsxSb1−x layers are strongly influenced by the growth rate due to the As-for-Sb exchange reaction which takes place at the growing surface, and that high crystal quality can be achieved when the growth is performed under Sb limited conditions.

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