New High-Performance SWIR Image Sensor Based on Non-Toxic Colloidal Quantum Dots

A novel high-performance shortwave infrared (SWIR) image sensor built with non-toxic colloidal quantum dots has been created by ICFO and Qurv researchers.

They describe a novel approach to producing high-quality, non-toxic, functional colloidal quantum dots that are integrable using complementary metal-oxide-semiconductor (CMOS) technology in a study that was published in Nature Photonics.

Shortwave infrared (SWIR) light, which is invisible to human vision, can provide hitherto unheard-of levels of performance, function, and reliability in high-volume computer vision-first applications in the consumer electronics, automotive, and service robots industries.

Smoke, haze, fog, and intense sunlight are only a few of the unfavorable environments in which image sensors with SWIR sensitivity can function dependably. Moreover, the SWIR spectrum offers sources of light that are safe for the eyes and makes it possible to use molecular imaging to determine material qualities.

The technological platform based on colloidal quantum dots (CQDs) presents a viable avenue to provide high-volume suitable image sensors in the near-infrared. Nanometric semiconductor crystals, or CQDs, are a kind of solution-processed material platform that enables access to the SWIR spectrum and can be combined with CMOS.

However, since SWIR-sensitive quantum dots (IV-VI Pb, Hg-chalcogenide semiconductors) frequently include heavy metals like lead or mercury, there is a fundamental barrier to turning these semiconductors into important supporting technologies for mass-market applications. The European legislation known as the Restriction of Hazardous Substances (RoHS) governs the use of certain compounds in consumer electronics that are sold commercially.

Researchers Yongjie Wang, Lucheng Peng, and Aditya Malla from ICFO, under the direction of ICREA Prof. at ICFO Gerasimos Konstantatos, have reported on the development of high-performance infrared photodetectors and a room-temperature shortwave infrared (SWIR) image sensor based on non-toxic colloidal quantum dots in a new study published in Nature Photonics. They worked in tandem with researchers Julien Schreier, Yu Bi, Andres Black, and Stijn Goossens from Qurv.

The research lays the groundwork for the commercialization of SWIR colloidal quantum dot technology in high-volume markets by describing a novel synthesis technique for size-tunable, phosphine-free silver telluride (Ag₂Te) quantum dots that maintains the beneficial characteristics of their conventional heavy-metal counterparts.

The researchers discovered silver telluride (Ag₂Te) as a by-product when they were figuring out how to synthesize silver bismuth telluride (AgBiTe₂) nanocrystals to increase the spectrum coverage of the AsBiS2 technology to improve the performance of photovoltaic systems. Similar to quantum dots, this material exhibited a potent and tunable quantum-confined absorption.

They turned their efforts to achieve and control a new process to synthesize phosphine-free versions of silver telluride quantum dots after realizing its potential for SWIR photodetectors and image sensors. Phosphine was found to have a negative effect on the optoelectronic properties of the quantum dots that are relevant to photodetection.

By employing several phosphine-free compounds, including tellurium and silver precursors, in their novel synthetic technique, the group was able to produce quantum dots with precisely regulated size distributions and excitonic peaks across an extensive range of the spectrum.

The newly synthesized quantum dots performed exceptionally well after manufacturing and characterization, showing clear excitonic peaks at wavelengths greater than 1500 nm. This is an unparalleled accomplishment in comparison to earlier phosphine-based methods for the synthesis of quantum dots.

The researchers then decided to use the produced phosphine-free quantum dots to build a straightforward lab-scale photodetector on a glass substrate covered in ITO (indium tin oxide) to evaluate the devices and gauge their characteristics.

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. 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.

Yongjie Wang, Study First Author and Postdoctoral Researcher, ICFO – The Institute of Photonic Sciences

Due to the photodiode’s poor performance in detecting SWIR light at first, a buffer layer was added throughout the designing process. After making this modification, the photodetector performance significantly improved.

The end product was a SWIR photodiode with a spectral range of 350 nm to 1600 nm, a linear dynamic range of more than 118 dB, a -3dB bandwidth greater than 110 kHz, and a room temperature detectivity of around 10¹² Jones.

To the best of our knowledge, the photodiodes reported here have for the first time realized solution-processed, non-toxic shortwave infrared photodiodes with figures of merit on par with other heavy-metal containing counterparts. 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.

Gerasimos Konstantatos, Study Lead Author and ICREA Professor, ICFO – The Institute of Photonic Sciences

After developing this heavy-metal-free quantum dot-based photodetector successfully, the researchers moved one step further and collaborated with Qurv, an ICFO spin-off, to showcase its potential through the case study of building a SWIR image sensor.

The researchers demonstrated a proof-of-concept, non-toxic, room temperature-operating SWIR quantum dot-based image sensor for the first time by integrating the novel photodiode with a CMOS-based read-out integrated circuit (ROIC) focal plane array (FPA).

By collecting numerous images of a target item, the study’s authors evaluated the imager to demonstrate its functionality in the short-wavelength infrared. Specifically, they were able to see the contents of opaque plastic bottles in the visible light spectrum and image the transmission of silicon wafers under the SWIR light.

“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 the automotive industry (cars) enabling vision and driving under adverse weather conditions. SWIR band around 1.35-1.40 µm, can provide an eye-safe window, free of background light under day/night condition, thus, further enabling long-range light detection and ranging (LiDAR), three-dimensional imaging for automotive, augmented reality and virtual reality applications,” Konstantatos further added.

The researchers now intend to improve the performance of photodiodes by redesigning the stack of layers that make up the photodetector device. They also intend to investigate different surface chemistries for Ag₂Te quantum dots to increase the material’s performance, thermal and environmental stability, and marketability.

Journal Reference:

Wang, Y., et. al. (2023) Silver telluride colloidal quantum dot infrared photodetectors and image sensors. Nature Photonics. doi:10.1038/s41566-023-01345-3.