Synthesis, Transport, and Passivation of Mercury Telluride Colloidal Quantum Dots for Infrared Detection

Colloidal Quantum Dots (CQDs) are a nanoscale material exhibiting strong quantum confinement that alters the electronic structure of the corresponding bulk material. Mercury telluride is a semi-metal and in CQDs quantum confinement opens up the bandgap. The resulting ability to tune the band gap by adjusting the size of the CQDs makes HgTe a compelling material for infrared applications. However, CQDs present many challenges, including a large surface to volume ratio that complicates stability, doping, and transport. We demonstrate an improved synthesis to obtain CQDs for operation in short-wave (SWIR) to long-wave infrared (LWIR) bands (2.25 – 9 μm), with narrower absorption energy levels and higher signal to noise ratio (SNR). We find the CQD bandgap is dependent on the size and on the temperature. As expected the bandgap of the CQD decreases with increasing size, and similar to bulk mercury-cadmium-telluride (MCT), the bandgap increases with increasing temperature. To evaluate material parameters such as mobility, photoconductivity, spectral response, and carrier concentration, these CQDs are used to fabricate field effect transistor (FET) and photoconductor (PC) devices. We find that conduction is temperature activated which is consistent with hopping transport in a disordered solid. At higher temperature the hopping transport appears to transition to band transport. We further find CQDs are close to intrinsic when prepared but unintentionally p-doped with exposure to standard atmospheric conditions. To stabilize the p-doping, the films are encapsulated with aluminum oxide (Al2O3) and polymethyl methacrylate (PMMA). Al2O3 encapsulated films are found to be n-doped likely because of Al doping in the CQD. The PMMA did not successfully prevent the time-dependent p-doping.