Experimental investigation and modelling of mid-infrared quantum cascade detectors operating at high temperature

The present thesis work concerns quantum cascade detectors (QCDs) for the mid-infrared spectral region. These devices are based on a multi-quantum well structure, where the infrared light detection occurs due to photo-stimulated electronic transitions between confined electronic states. In particular, the object of this thesis is an 8.6 µm QCD in a “patch antenna” architecture, where the active region of the device is embedded in a double-metal cavity. This geometry generates an antenna effect, increasing the photon collection area and thus improving the detector's responsivity and noise performance, especially at high temperatures. On such device, we first performed electrical measurements to assess its transport properties in absence of illumination. We extracted the activation energy from dark current measurements at different temperatures and validated our results by means of a simulation of the electronic wavefunctions in the active region. Then, we characterized the photo-detection response and its behavior as a function of temperature, comparing it to a QCD in a standard “mesa” architecture. We measured a ten-fold enhancement in the responsivity with respect to the mesa in the high temperature regime. Moreover, we observed that, thanks to the antenna effect of the double-metal cavities, the patch-antenna QCD displays good detectivity at high temperatures, easily accessible with a thermoelectric cooler. In the last part of this work, we focused on the design and simulation of a new patch-antenna QCD, operating at 4.4 µm. The higher transition energy with respect to the 8.6 µm device introduced challenges in its design and growth processes. To overcome them, we proposed a new QCD design, named “step-well”, where quantum wells of different heights are present in the structure. In this manuscript we describe how we were able to simulate such a system and we report the results of the simulations, along with the final QCD structure we designed.