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Band structure of the photodetector. The solid lines show the calculated , , and SO (blue, green, red) band edges at a bias of −8 (a) and (b) and a contour plot of the absolute squared wave functions at their eigenenergy values. The straight red lines indicating the quasi-Fermi-level at 77 K imply that the charge carriers are confined in W5 for negative and in W1 for positive applied bias. The black ellipses mark the quasibound final states of mid-IR absorption leading to the spectral responsivity characteristics presented in Fig. 2. Note that the excited level localized around W5 is mostly composed of states, while that around W1 is built up by mixed states. This results in the polarization dependence of the responsivity seen in Fig. 2(b).
Responsivity spectra. The solid and broken lines in (a) show the sample’s responsivity to TM polarized radiation as a function of the photon energy for an applied bias of −8 and , respectively. The responsivity peak can be switched from 220 to 320 meV by changing the applied voltage. Note that in comparison to the structure in Ref. 8, there is a very narrow linewidth of the responsivity peaks, which is due to the quasibound nature of the excited states. The curves in (b) highlight the strong polarization-dependence of the responsivity spectra at negative applied bias. The sharp peak at 220 meV originates from a transition between the ground state of W5 and an excited quasibound level mostly composed of states, which is only strong in TM polarization.
Dark-current and PC response. The broken line shows the sample’s dark-current density characteristics measured at 77 K in a liquid nitrogen vessel. The solid line represents the device’s response to black-body radiation measured in lock-in technique, which was used to calibrate the responsivity spectra. The inset shows the setup in which the spectroscopic measurements were performed.
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