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Design and Characterization of Quantum Cascade Detector
Quantum Cascade Detector (QCD) works in intersubband absorption mechanism of a cascade structure. These devices are made of very thin layers of two alternating materials so that the electrons become confined at discrete energy levels within the quantum wells. The quantized energy values of the electrons are functions of the thicknesses of the wells and barriers. Therefore, the transition energy can be engineered merely by changing the layer thicknesses.
In these design, two alternating materials are GaN/AlGaN. Design was made such a way for the relaxation of the absorbed electron by LO phonon scattering through the ladder type energy states. In this structure the electrons are absorbed from E1 to E5 when light with energy that equal to the energy separation falls on the detector. The excited electrons then relaxes through the ladder to the ground level of the adjacent period that ensures a steady current flow. The absorption spectrum of the detector is shown in Fig. 2 that also indicates the performance degradation with the increase of the temperature. This behavior can be explained by the increased scattering of carrier with increased temperature. Figure 3 shows the variation of responsivity of the detector with the change of temperature.
Fig 1: Simulated Band profile and wavefunction of a GaN based QCD.
Fig 2: Absorption coefficient of a GaN based QCD designed to absorb at 6.3 µm.
Fig 3: Responsivity variation of a GaN based QCD over temperature designed to absorb at 6.3 µm.
Mid-IR region has a variation of application in terms of environmental and healthcare point of view. Nitride based structure allows a wide range of window in the spectrum which was unexplored for detector design. I have proposed two design of nitride based QCD in the Mid-IR region (4-7 µm).
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