Mid-Infrared Photodiodes Using HgTe Colloidal Quantum Dots

HgTe colloidal quantum dot photodetectors offer an inexpensive alternative to the conventional, epitaxially grown, mid-infrared detectors. Recent work describing the thermodynamic limits of these materials suggest that their long Auger lifetime may allow for better performance over such conventional detectors in that region. Previous work has produced HgTe quantum dot photodiodes which are background limited at cryogenic temperatures. However, they exhibited a ~20-fold decrease in the quantum efficiency from 150K to 300K.<br/><br/>Modeling the IV curves has helped to better understand the mechanism of this limited performance. The temperature dependence, the origin of the shunt resistance (recombination mechanisms) and the series resistance (electrodes and the dots themselves) are rather well understood with a simple model. This model suggests that, in the regime where the shunt resistance is on the order of the series resistance, the quantum efficiency is expected to drop substantially. Modifications to the p and n-doping as well as microfabrication methods have been applied to increase the shunt resistance of the devices relative to the series resistance. Methods of producing low resistance, transparent electrodes for the infrared have also been explored. The diffusion length of the materials used requires that these devices remain thin, and so microfabrication is also employed to enhance to optical absorption of the diodes. Together, these efforts to improve single element detectors allow for higher temperature thermal imaging with this technology, closer to the thermodynamic limit of the material.