Quantum Cascade Detector
the different temperature between two glasses can be seen
one being white is filled with a hot liquid, the other being black is filled with a cold liquid.
In the field of infrared detection [3-12 μm], quantum wells detectors (QWIPs) possess strengths that allowed them to become an advanced technology for thermal imaging. One of the key operating parameters of quantum well detectors is the integration time: it can be short due to some saturation of the reading circuit capacity. In QWIPs, because the dark current is large, the integration time is approximately 10 times smaller than the video time-frame associated with the thermic image. Therefore only a fraction of the time is actually useful and a large portion of the signal is lost.
In this context a photovoltaic version of QWIP detector has been proposed: the quantum cascade detector (QCD) (patent Thales 2001). Unlike its photovoltaic predecessors, QCD has a high quantum efficiency. It operates at zero voltage and show no dark current. This would make a good candidate for large arrays of small pixels for which the saturation of reading capacities is a limiting problem.
Il fonctionne à tension nulle c’est pourquoi il ne transporte aucun courant noir indésirable. Il serait donc très intéressant pour des grandes matrices de petits pixels pour lesquelles la saturation des capacités de lecture est un problème limitant.
The principle of operation of a QCD is the following: an electron occupying the fundamental level of the structure is transfered to the exited state by absorption of infrared photon. The electron on the excited level is captured by a cascade of levels and is taken to a nearby well. This generates a charging space potential: it is indeed a photovoltaic detector. This structure can be replicated periodically so that the neighbouring well in which the electron ends eventually is itself an absorbing well, identical to the first one. Therefore, we get a photocurrent that flows through the entire structure, and for that reason higher efficiency is expected than in other photovoltaics devices.
Initial QCDs structures were studied during the PhD thesis of Laure Gendron. The principle of the detector has been validated and published in Applied Physics Letters and the first measurements have served to highlight the great potential of these detectors. It now remains to optimize the band structure and compare the performance of these cascade detectors to those of conventional QWIPs.
The results of measurements made on the first QCDs at 8 μm, studied by Laure Gendron in her thesis, are very promising. The QCDs present the highest performance that can be found in the literature about photovoltaic quantum well detectors. A 44 mA/W at 50 K response has been measured. The specific detectivity limited by the Johnson noise is 4.5 × 10 11 Jones at 50 K and 2.1 × 10 10 Jones at 77 K. The R0 A of 1000 W.cm 2 et 77K is of the same order of magnitude as that of MCT detectors. However, the first structures at 8 μm have not been yet optimized due to the large number of degrees of freedom that exist in the quantum design. A systematic study of the design of structures in conjunction with the modeling should be conducted to propose an optimal structure. A simulation program, based entirely on the electron-phonon interaction, already allows to model, ab initio, the dark current in QCDs. To validate this model, a more thorough study of the mechanisms responsible for the dark current is necessary in particular through the magnetic field.
we see the area that allow heat to pass through: the stained-glass windows
and the areas that allow less heat to pass: the walls