NIR Sensors Based on Photolithographically Patterned PbS QD Photodiodes for CMOS Integration

The integration of infrared sensitive thin-film materials with solution processing capabilities on top of Si substrates is a significant step towards cost-efficient infrared imagers. Colloidal quantum dots based on lead sulfide are very attractive materials for the realization of novel image sensors combining low cost synthesis and processing with deposition over large area and on any substrate. The tunable band gap enables selective detection in wavelengths ranging from the visible up to the short-wave-infrared (SWIR). This work describes the first results of a roadmap that will enable the integration of quantum dot photodiodes (QDPD) on top of a Si based CMOS read-out circuit. Photodiodes using an n-p junction architecture are fabricated on Si substrates, showing low dark current of 30 nA/cm
²
at −1 V reverse bias, EQE above 20% and specific detectivity higher than
$\mathbf{10^{12}{cm}}\mathbf{Hz^{1/2}}\mathbf{W^{-1}}$
at the wavelength of 940 nm. Efficiency is improved by tuning the top contact transparency with optical modeling. Furthermore, photolithographic patterning of the thin-film stack is introduced for the first time, showing the feasibility of pixel pitches down to
$\mathbf{40}\mu$
, opening the way towards high resolution monolithic infrared imagers and the incorporation of infrared sensitive pixels next to visible ones.