PbS and InAs Quantum-Dot\nThin Films for Short-Wave\nInfrared Detectors

Colloidal quantum dots (CQDs) are cutting-edge optoelectronic\nsemiconductor\nnanocrystals that enable short-wave infrared (SWIR) vision by a widely\ntunable SWIR light absorption. Thanks to the advances in CQD surface\nligand engineering, SWIR detectors and emitters will soon find their\nway into products. The CQD-based optoelectronic devices are being\noptimized by adapting the size of CQDs and selection of the ligands,\nand yet, the measurement schemes of energy band structure based on\ndifferent ligands and processes of ligand exchange are not systematically\nstudied. In this work, we systematically characterize the energy band\nstructure of PbS (absorbing at different SWIR wavelengths) and InAs\nwith various ligands for both solid-state and liquid-phase ligand\nexchange (LPLE) processes [solid-state ligand exchange (SSLE) and\nLPLE] by using ultraviolet photoelectron spectroscopy. The deduced\nenergy band structures reveal that the apparent energy difference\nbetween the Fermi and valence band maximum, |<i>E</i><sub>F</sub> – <i>E</i><sub>VBM</sub>|, largely depends\non the physical density and distribution of the CQDs within the probing\narea. Transmission electron microscopy images, X-ray photoelectron\nspectroscopy, atomic force microscopy, and variable angle spectroscopic\nellipsometry reveal details of the CQD distribution, surface elemental\nprofile, and topologies and how they affect the observed energy band\nstructure. We demonstrate that the multistep coating improves the\nCQD distribution and packing density, resulting in more reliable and\nreproducible results that represent the bulk CQD film energy band\nstructure. The comparison of solid and liquid phase ligand-exchanged\nPbS and InAs SWIR CDQs energetics indicates that the LPLE ensures\nmore uniform dispersion and a high packing density of CQDs regardless\nof the solution concentration. The photoemission-deduced energy band\nstructures are validated by fabricating thin-film photodiodes using\nSWIR SSLE PbS and LPLE In(As,P) CQDs. The Fermi-referenced band structures\nof the fabricated full photodiode stacks including band offsets and\nbending are discussed to improve our understanding of the device working\nprinciples and to further optimize the devices.