Characterization of Mid-Infrared HgTe Colloidal Quantum Dot Photodiodes

Top-illuminated mid-infrared HgTe colloidal quantum dot (CQD) photodiodes are compared with back-illuminated photodiodes. For top- and back-illuminated diodes, respectively, at 290 K, the external quantum efficiencies are 10% and 21%, the detectivities are 5.1 × 10⁸ and 9.4 × 10⁸ jones, and the cutoffs are 3.9 and 3.8 μm. The efficiencies peak around 110 K at 38% and 67%, respectively. The diodes are background limited below 130 and 140 K, respectively. Above 140 K, the IV curves fit well with a single diode model with a unity ideality factor, indicating the dominance of geminate recombination. The reduced activation energy of the dark current compared to the cutoff energy is explained by the finite edge width. Below 140 K, the ideality factor increases, indicating the growing importance of trap-like recombination. A photoconductive shunt resistance limits performance at low temperatures, which is proposed to originate from the hopping transport in CQDs, rather than device defects.