Intraband Transition of HgTe Nanocrystals for Long-Wave Infrared Detection at 12 μm

The synthesis of n-doped HgTe colloidal quantum dots was optimized to produce samples with a 1S_e–1P_e intraband transition in the long-wave infrared (8–12 μm). The spin–orbit splitting of 1P_e states places the 1S_e–1P_e1/2 transition around 10 μm. The narrow line width of 130 cm^–1 at 300 K is limited by the size distribution. This narrowing leads to an absorption coefficient about 5 times stronger than is possible with the HgTe CQD interband transition at similar energies. From 300 to 80 K, the intraband transition blueshifts by 90 cm^–1, while the interband transition redshifts by 350 cm^–1. These shifts are assigned to the temperature dependence of the band structure. With ∼2 electrons/dot doping at 80 K, a photoconductive film of 80 nm thickness on a quarter wave reflector substrate showed a detectivity (D*) of ∼10⁷ Jones at 500 Hz in the 8–12 μm range.