Improving Pb-free Device Performance by Progress in InAs QDs
Short-wave infrared (SWIR) image sensors based on colloidal quantum dots (QDs) have progressed into commercial products over the past decade, with the first generation of SWIR cameras relying on PbS QDs. These early devices demonstrated that solution-processed QDs integrate well with monolithic CMOS readout integrated circuits (ROICs) and can deliver a high-performance alternative to incumbent InGaAs sensors.[1] However, the presence of lead remains a significant barrier to scaling this technology into high-volume markets, due both to regulatory constraints and customer demand for RoHS-compliant materials. Consequently, the industry has shifted its focus toward developing Pb-free QD materials capable of delivering comparable performance.
Among the various alternative candidates, III–V materials such as InAs have emerged as the leading Pb-free SWIR QD option.[2] InAs offers a bandgap comparable to PbS and provides excellent tunability across the SWIR range. In addition, its chemical similarity to incumbent InGaAs technology means that foundries are already familiar with the associated processing steps, without introducing new contamination risks.
At Nanoco, we have been at the forefront of InAs QD development. Using our proprietary molecular seeding process, we have achieved excellent optical performance across the SWIR, including well-defined first excitonic peaks at wavelengths up to 1800 nm, as detailed in our recent whitepaper.[3] The low half-width at half-maximum (HWHM) of the absorption spectrum for the 1500 nm InAs QDs shown below reflects the high uniformity of the nanocrystals. The narrow particle size distribution is further highlighted by the single narrow PL emission peak, as well as the uniformly shaped nanocrystals, exhibited in the TEM micrograph. A deeper discussion of the optical properties of InAs QDs – along with comparisons to other high-quality QD systems – will follow in a future post.
Characterisation of 1500 nm InAs QDs – left: absorption spectrum; middle: photoluminescence (PL) spectrum indicating the full-width at half-maximum (FWHM); right: TEM micrograph.
While narrow particle size distributions form the foundation of successful QD device development, careful management of the QD surface represents an equally critical challenge. High-performance devices require a balance between surface passivation and charge-transport characteristics to ensure efficient charge extraction from the active QD layer. To support these efforts, we have leveraged our in-house device facility, enabling rapid iteration and material optimisation across the full QD development workflow. Through this approach, we have refined surface ligands and ink formulations that yield uniform QD films with good electronic transport properties. As a result, we have recently achieved breakthrough performance levels in single-channel photodiode devices using QDs with a first absorption peak at around 1500 nm. These devices achieved dark currents of <50 nA/mm2 with EQE values above 20% at room temperature at -1 V; a significant milestone for solution-processed III–V materials.
While additional improvements are still required to match the performance of PbS QDs, achieving this level of device performance represents a significant milestone, clearly validating the quality and maturity of our materials. Looking ahead, we see substantial opportunity in further optimising device integration, and we expect that many of these gains will be unlocked through collaboration with experienced integration partners in the sensor space. These advancements will continue to narrow the remaining performance gap with PbS devices and drive wider adoption of InAs QDs in SWIR sensing applications. By enabling a unique combination of high performance, higher resolution, reduced SWaP (size, weight and power), and substantially lower costs, this Pb-free SWIR sensor platform is poised to unlock entirely new markets.
References:
- [1] J. S. Steckel et al., “1.62μm Global Shutter Quantum Dot Image Sensor Optimized for Near and Shortwave Infrared,” 2021 IEEE International Electron Devices Meeting (IEDM), San Francisco, CA, USA, 2021, pp. 23.4.1-23.4.4, doi: 10.1109/IEDM19574.2021.9720560.
- [2] O. Enoki et al., “Pb-Free Colloidal InAs Quantum Dot Image Sensor for Infrared,” 2024 IEEE International Electron Devices Meeting (IEDM), San Francisco, CA, USA, 2024, pp. 1-4, doi: 10.1109/IEDM50854.2024.10873373.
- [3] https://www.nanocotechnologies.com/swir-sensing-with-colloidal-quantum-dots/