Metasurface Cointegration with Colloidal Quantum Dot Photodiodes Enabling Facile Spectral Sensors

This paper presents the cointegration of metasurfaces with colloidal Quantum Dot Photodiodes (QDPDs) for the development of compact, high-resolution short-wave infrared (SWIR) spectral sensors. By combining these two technologies, the authors enable the creation of a CMOS-compatible platform for miniaturized, high-throughput spectral detectors. This innovation paves the way for applications in areas such as environmental monitoring, biomedical imaging, and food analysis.

Key Contributions:

1. SWIR Spectral Imaging and the Challenges:

• SWIR spectral imaging is crucial for various applications, but it has been traditionally limited by the high cost, size, and complexity of existing technologies, such as thin-film Fabry-Pérot (FP) filters. These systems are not compatible with CMOS technology, limiting their integration into low-cost, compact devices.

• Colloidal quantum dot-based (QDB) sensors have emerged as a cost-effective solution for infrared spectral imaging, as they can be directly deposited onto CMOS readout integrated circuits (ROICs). However, QD sensors suffer from high spectral crosstalk and limited scalability due to their broad absorption spectrum and the need for separate processing steps for each pixel.

2. Metasurfaces for Spectral Resolution:

• The paper introduces metasurfaces as a solution to overcome these limitations. Metasurfaces are nanoscale optical structures that allow for tunable light-matter interactions.

• The metasurfaces are integrated into the microcavity structure of colloidal QD photodiodes (QDPDs), allowing for narrowband resonance tuning and enabling the spectral absorption of QDPDs to be precisely controlled.

3. Design and Fabrication:

• The authors design meta-optic structures with Si pillars embedded in SiO₂ to control the refractive index seen by the light. By adjusting the pillar diameter, they can tune the absorption resonance of the QDPDs across the SWIR range.

• The fabrication process uses DUV lithography to create the metasurface and integrates it with the colloidal QD layers on a 300mm wafer using CMOS-compatible techniques. This approach ensures high-resolution, scalable fabrication.

• The vertical cavity design is realized using a combination of Cu vias and ITO current spreading layers, enabling efficient electrical access to the QDPDs.

4. Experimental Results:

• Resonance Tuning: The absorption resonance of the microcavity-QDPDs is successfully tuned across the range of 1230–1550 nm by controlling the QD film thickness and adjusting the metasurface structure.

• Efficiency: The external quantum efficiency (EQE) for the microcavity-QDPDs is demonstrated to be up to 40%, with normalized EQE measurements showing strong performance in the SWIR range.

• Dark Current and Leakage: The dark current density of the devices is measured at 90 μA/cm² at -3V, indicating a good diode response.

5. Applications and Future Directions:

• The developed meta-optic architecture offers the potential for high-resolution SWIR imaging with low-cost fabrication and scalable integration.

• The next steps outlined include improving the dark current performance (aiming for values below 1 μA/cm²) and increasing the device efficiency (targeting EQE > 50%). Additionally, efforts are underway to extend the spectral range to the mid-wave infrared (MWIR) by using lead-free alternatives like InAs QDs.

Conclusion:

This work demonstrates a breakthrough in spectral imaging technology, combining metasurfaces and colloidal QD photodiodes to create a compact, CMOS-compatible spectral sensing platform. The integrated approach provides tunable SWIR absorption, enabling applications in wearable sensors, environmental monitoring, and biomedical imaging. The ability to scale this technology could revolutionize the cost and performance of spectral imaging systems for various industrial and healthcare applications.