Intraband cascade electroluminescence with weakly n-doped HgTe colloidal quantum dots

Room temperature 6 μm intraband cascade electroluminescence (EL) is demonstrated with lightly n-doped HgTe colloidal quantum dots of ∼8 nm diameter deposited on interdigitated electrodes in a metal-insulator-metal device. With quantum dot films of ∼150 nm thickness made by solid-state-ligand-exchange, the devices emit at 1600 cm-1 (6.25 μm), with a spectral width of 200 cm-1, determined by the overlap of the 1Se-1Pe intraband transition of the quantum dots and the substrate photonic resonance. At the maximum current used of 20 mA, the bias was 30 V, the external quantum efficiency was 2.7%, and the power conversion efficiency was 0.025%. Adding gold nano-antennas between the electrodes broadened the emission and increased the quantum efficiency to 4.4% and the power efficiency to 0.036%. For these films, the doping was about 0.1 electron/dot, the electron mobility was 0.02 cm2 V-1 s-1, and the maximum current density was 0.04 kA cm-2. Higher mobility films made by solution ligand exchange show a 20-fold increase in current density and a 10-fold decrease in EL efficiencies. Electroluminescence with weak doping is interesting for eventually achieving electrically driven stimulated emission, and the requirements for population inversion and lasing are discussed.