Dependence of absorption spectrum and responsivity on the upper state position in quantum well intersubband photodetectors

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Abstract
Analytical results are derived for the intersubband absorption quantum efficiency, which take into account contributions from both bound‐to‐bound state and bound‐to‐continuum state transitions. Including the final state lifetime broadening, the absorption spectrum gradually decreases in peak strength and becomes broader from the pure bound‐to‐bound case to the pure bound‐to‐continuum situation. The physical reason for the divergence in absorption strength when the upper state is in resonance with the top of the barrier and in the absence of broadening is discussed. The detector current responsivity as a function of well width (and hence upper state position) is estimated. Calculated examples are given, covering the crossover region from the bound‐to‐bound case to the bound‐to‐continuum case. Our results compare well with experiments.