High photoconductive gain in a GaAs/PbS heterojunction based SWIR detector

An internal quantum efficiency (IQE), defined as the electron-hole pair to photon conversion ratio, of ∼10 was obtained for an n-GaAs/p-PbS heterojunction when illuminating the device with short wavelength infrared (SWIR) light in the wavelength range of 1300 nm-1500 nm. The PbS layer, which was comprised of nano-scale domains (NDs) grown by Chemical Bath Deposition (CBD), was quantum confined to absorb SWIR light. The heterojunction showed tunneling characteristics with a soft breakdown at a relatively low reverse bias (∼-1 V) and a strong photoconductive response at a negative bias above -1.8 V. The voltage dependent behavior is explained using the band structure of the heterojunction. The high IQE observed in the photoconductive response at -2 V is attributed to a high photoconductive gain of more than 40. This assumption was confirmed by mixed conduction behavior observed in a magnetic field dependence Hall effect measurement. These measurements enabled extracting concentrations and mobilities of both electrons and holes. It was found that the CBD grown p-type PbS NDs layer has a mixed conduction nature due to the high electron-to-hole mobility ratio of more than one order of magnitude. This explains the high photoconductive gain achieved and, thus, the high IQE measured for these devices.