Researchers from ICFO and ICFO computer vision spinout Qurv have fabricated a high-performance shortwave-infrared (SWIR) image sensor based on nontoxic colloidal quantum dots (CQDs). In their study, the team reports on a method for synthesizing functional high-quality nontoxic CQDs integrable with CMOS technology.

Invisible to our eyes, SWIR light can enable reliability, functionality, and performance in high-volume computer vision first applications in service robotics, automotive, and consumer electronics markets. CQD-based image sensor technology offers a promising technology platform to enable high-volume compatible image sensors in the SWIR. CQDs — nanometric semiconductor crystals — are a solution-processed material platform that can be integrated with CMOS and enables access to the SWIR range.

However, a fundamental roadblock exists in translating SWIR-sensitive quantum dots into key enabling technology for mass-market applications because they often contain heavy metals, such as lead or mercury. These materials are subject to regulations by the Restriction of Hazardous Substances (RoHS), a European directive that regulates their use in commercial consumer electronic applications.

ICFO researchers led by Gerasimos Konstantatos, in collaboration with researchers from Qurv, have reported on the development of high-performance infrared photodetectors and a SWIR image sensor operating at room temperature based on nontoxic CQDs. The study describes a new method for synthesizing size-tunable, phosphine-free silver telluride (Ag₂Te) quantum dots while preserving the advantageous properties of traditional heavy-metal counterparts paving the way to the introduction of SWIR CQD technology in high-volume markets.

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ICFO researcher Yongjie Wang (left) and Qurv researcher Julien Schreier. The researchers collaborated to develop a high-performance shortwave-infrared (SWIR) image sensor based on nontoxic colloidal quantum dots (CQDs). The researchers specifically integrated a photodiode with a CMOS-based read-out integrated circuit focal plane array to demonstrate a proof-of-concept, nontoxic, room temperature-operating SWIR sensor. Courtesy of ICFO.

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In their new synthetic method, the team used different phosphine-free complexes, such as a tellurium and silver precursors, that led them to obtain quantum dots with a well-controlled size distribution and excitonic peaks over a very broad range of the spectrum. After fabricating and characterizing them, the newly synthesized quantum dots exhibited remarkable performances, with distinct excitonic peaks at >1500 nm.

The researchers decided to implement the phosphine-free quantum dots to fabricate a simple laboratory-scale photodetector on the common standard indium tin oxide-coated glass substrate to characterize the device and measure its properties.

“Those lab-scale devices are operated with shining light from the bottom. For CMOS integrated CQD stacks, light comes from the top, whereas the bottom part of the device is taken by the CMOS electronics,” postdoctoral researcher and first author Yongjie Wang said. “So, the first challenge we had to overcome was reverting the device setup. A process that in theory sounds simple, but in reality proved to be a challenging task.”

Initially, the photodiode exhibited a low performance in sensing SWIR light, prompting a redesign that incorporated a buffer layer. This adjustment significantly enhanced the photodetector performance, resulting in a SWIR photodiode exhibiting a spectral range from 350 to 1600 nm, a linear dynamic range exceeding 118 dB, a −3-dB bandwidth surpassing 110 kHz, and a room temperature detectivity of the order 10¹² Jones.

“To the best of our knowledge, the photodiodes reported here have, for the first time, realized solution processed, nontoxic shortwave-infrared photodiodes with figures of merit on par with other heavy-metal containing counterparts,” Konstantatos said. “These results further support the fact that Ag₂Te quantum dots emerge as a promising RoHS-compliant material for low-cost, high-performance SWIR photodetectors applications.”

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Postdoctoral researcher Yongjie Wang, first author on a study detailing the use of colloidal quantum dots (CQDs) to develop shortwave-infrared (SWIR) CMOS sensors. Courtesy of ICFO.

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With the successful development of this heavy-metal-free quantum dot-based photodetector, the researchers went further and teamed up with Qurv to demonstrate its potential by constructing a SWIR image sensor as a case study. The team integrated the new photodiode with a CMOS-based read-out integrated circuit focal plane array demonstrating a proof-of-concept, nontoxic, room temperature-operating SWIR quantum dot-based image sensor for the first time, the researchers said. The team tested the imager's operation in the SWIR range by taking several pictures of a target object. Specifically, images were taken of the transmission of silicon wafers under the SWIR light, and they were able to visualize the content of plastic bottles that were opaque in the visible light range.

“Accessing the SWIR with a low-cost technology for consumer electronics will unleash the potential of this spectral range with a huge range of applications, including improved vision systems for automotive industry-enabling vision and driving under adverse weather conditions,” Konstantatos said. “[The] SWIR band around 1.35 to 1.40 µm can provide an eye-safe window free of background light under day/night condition, thus, [it can] further enable [lidar], three-dimensional imaging for automotive, augmented reality, and virtual reality applications.”

The researchers want to increase the performance of photodiodes by engineering the stack of layers that comprise the photodetector device. They also want to explore new surface chemistries for the Ag₂Te quantum dots to improve the performance and the thermal and environmental stability of the material on its way to the market.

The research was published in Nature Photonics (www.doi.org/10.1038/s41566-023-01345-3).