Publication date: 15th May 2025
Quasi-spherical HgTe CQDs of 12.8 nm diameter are studied for long-wave infrared photodetection at 85 K using their interband absorption. The work highlights one limitation of the interband absorption spectrum which is that the band-edge is rather soft even for a good size distribution. This is because the absorption is made of at least two excitonic transitions to the lowest electronic state 1Se, with the weaker one on the red side, and this reduces the absorption near the edge.
Using a polar ligand exchange and an HgCl2 film treatment that gives a relatively high electron mobility of ~ 3 cm2/(V·s), minimizes the doping of the dots and slightly improves the PL, we found that the detector is also strongly limited by the short carrier lifetime, which is determined to be ~ 30 ns from an analysis of the photocurrent magnitude. From absorption and PL data, the photoluminescence radiative lifetime is estimated to be 2 µs, the external photoluminescence quantum yield of the films is about 10-5, and the non-radiative excitonic lifetime is estimated to be about 50 ps. The carrier density was around 0.05% per dot, and the geminate recombination lifetime is then found to be roughly consistent with the lifetime determination from the photocurrent. The analysis suggests that the 85 K photoconductivity of our HgTe LWIR CQD is mainly limited by the weak absorption edge and the short nonradiative exciton lifetime.
With these limitations, the performance is dramatically improved with a nanoantenna substrate designed to increase the 8.5 microns absorption of a 100 nm thick film of CQDs. At 85 K, the responsivity increases 24-fold under TE polarization reaching an external quantum efficiency of 12.5%. At 25 kHz, with the optimum bias of 0.8 V, the 8.5 microns detectivity is 1.4×1010 Jones. This value is qualitatively validated by a 10-fold superior signal to noise in a side-by-side comparison of FTIR spectra taken with a standard DTGS pyroelectric detector, and with a simple scanning imager that resolves body heat. Further improvement will be possible with stronger optical enhancement, and chemical advances that increases the exciton strength and lifetime.
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