Magneto-transport experiments have been performed on Quantum Cascade Detectors. These experiments lead to the identification of the different electronic transitions from subbands in one cascade period to subbands in the following one. These transitions contribute to the total current flowing through the structure in the absence of illumination. This dark current is well described within a simple model based on the sum of diffusion events from one cascade to the next one through optical phonon mediated transitions. For the first time, the optical and electronic properties of such a complex heterostructure can be fully predicted without any other adjustable parameter than the doping density. This opens the way to a full quantum design of an infrared detector, in contrast with the phenomenological optimization of structures usually performed in this field.
A high responsivity GaAs/AlGaAs quantum cascade detector is demonstrated at a wavelength of 15 μm. The quantum design is optimized for negative bias operation, so that the capture of photoexcited electrons back to the fundamental level is minimized. The detectivity of the detector presented here reaches 1.1×1012 Jones at 25 K for an applied bias of −0.6 V.
The responsivity of polarization sensitive quantum well infrared photodetectors with small pixel size (down to 20μm) is investigated. It is shown that pixels suitable for integration into very large focal plane arrays (1000×1000) can discriminate the polarization of the incoming signal. A responsivity contrast higher than 60% is obtained although the grating size is reduced to only six periods. The quantum efficiency and polarimetric capabilities can be both optimized with the same grating parameters.
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