Research On Device Physics Of PbS Colloidal Quantum Dots Based Short-Wavelength Infrared Photodiodes

Solution-processed thin-film photodetectors,which can be directly fabricated into a pixel stack on top of silicon-based CMOS readout circuits(ROICs),have attracted immense attention in recent years owing to their great potential in breaking the cost and resolution bottlenecks of compound semiconductor infrared focal plane arrays.In particular,PbS colloidal quantum dots(CQDs)have demonstrated extensive potential in emerging infrared thin-film photodetectors.Their tunable absorption spectra,high absorption coefficient,and variable doping types and concentrations have enabled siliconbased monolithic integrated PbS CQDs short-wavelength infrared(SWIR)imaging arrays.However,compared with commercial InGaAs SWIR photodetectors,their photodetection performances are still inferior,and meanwhile,the physical mechanism is not yet clear,which seriously hinders the commercialization process of PbS CQDs imaging arrays.Hence,this thesis focuses on the realization of high-performance PbS CQDs SWIR photodiodes by suppressing the reverse carrier injection from the electrodes through interfacial passivation and modulating the optical absorption,defect density and interfacial properties of the photodiodes through introducing PbS micro-nano structures.Finally,PbS CQDs SWIR photodiodes with both low reverse bias dark current and high photoresponsivity were obtained.The detailed research contents are as follows:1.The suppression effects of interfacial passivation on reverse bias dark current and the underlying mechanism were revealed.The suppression effects of dark reverse bias current were observed for both nip and pin structure photodiodes after exposure to dry air at room temperature.For nip photodiodes,surface oxidation of PbS CQDs produces a wide-bandgap lead-containing oxide layer,which passivates the PbS surface defects and provides a high reverse electron injection barrier.This effectively suppresses the reverse electron injection from the electrode and thus the reverse bias dark current is significantly decreased.For pin photodiodes,an aluminum oxide layer is introduced at the ZnO nanoparticles/Al interface after exposure to dry air at room temperature,which effectively passivates the ZnO nanoparticle surface defects and reduces the contribution of defectassisted reverse minority carrier injection to the reverse bias dark current.Ultimately,the reverse bias dark current densities of nip and pin photodiodes at a reverse bias of-0.1 V are as low as 2.08×10-8 A/cm2 and 1.25 ×10-7 A/cm2,respectively.2.PbS SWIR photodiodes with a low reverse bias dark current density,high photoresponsivity,and high air stability are obtained by introducing PbS micro-nano structures as the active layer.A randomly distributed micro-nano scaled sheet-like bump structure was constructed on the surface of PbS CQDs film by simply replacing the solvent from methanol to acetonitrile in ZnI2:MPA ligand exchange process.The micronano structure can significantly enhance the SWIR light absorption,passivate the defects in the CQD film bulk,and facilitate the passivation of the ZnO nanoparticles/Al interface,which simultaneously improves the photoresponsivity,reduces the reverse bias dark current,and improve air stability of the photodiodes.Ultimately,the optimized devices exhibit a low reverse bias dark current density(6.96×10-8 A/cm2 at-0.5 V),a high responsivity(0.982 A/W at 1413 nm)and a high specific detectivity(2.70×1012 Jones at 1413 nm).