Non-toxic quantum dots pave the way to new CMOS sensors

Although it is invisible to our eyes, light Shortwave infrared (or SWIR).for the English acronym) can enable unprecedented feasibility, functionality and performance of high-volume computer vision applications in robotics, automotive and consumer electronics.

Image sensors with sensitivity in the SWIR spectrum can be used in adverse conditions, such as very bright sunlight, fog, haze or even smoke. In addition, this area provides safe sources of illumination for the human eye and enables the detection of material properties through molecular imaging.

The technology, in turn, is based on colloidal quantum dots (CQD (for the acronym in English) offers a promising technological path for the development of SWIR image sensors. These dots are nanometer-sized semiconductor crystals, a platform that provides access to this short-wave infrared light spectrum and can be integrated into complementary metal oxide semiconductor (CMOS) technology commonly used in many computers and electronic devices.

However, turning SWIR-sensitive quantum dots into a valid technology for market-oriented applications faces obstacles because these CQDs often contain heavy metals such as lead or mercury (IV-VI chalcogenide semiconductors Pb, Hg). These materials are subject to the European Restriction of Hazardous Substances Directive (RoHS), a regulation that regulates their use in commercial consumer electronics applications.

In a new study published in the journal Nature photonicsICFO researchers Yongjie Wang, Lucheng Peng and Aditya Mallaled by Professor ICREA from ICFO Gerasimos Constantusin collaboration with researchers Julien Schreier, Yu Bi, Andres Black and Stijn Goosensfrom Qurv, a spin-off of ICFO, have developed high-performance infrared photodetectors and a short-wave infrared light (SWIR) image sensor that operate at room temperature and are based on the use of non-toxic colloidal quantum dots.

The work describes a new method for synthesizing these phosphine-free and size-modular quantum dots, which preserve the properties of dots containing heavy metals. This paves the way for the introduction of quantum dot-based technology that can operate in the SWIR range in high-volume markets.

Silver telluride (Ag) points₂Tea)

While they researched how to synthesize silver bismuth telluride (AgBiTe) nanocrystals₂) to increase the spectral coverage of bismuth sulfur arsenide (AsBiS).₂) To improve the behavior of photovoltaic devices, researchers obtained a byproduct: silver telluride (Ag₂Tea).

This material showed strong and tunable quantum confinement absorption, similar to that of quantum dots. Recognizing their potential for developing SWIR photodetectors and image sensors, the researchers then focused their efforts on developing a new method for synthesizing a phosphine-free version of silver telluride quantum dots, since phosphine has been shown to have a negative impact on the optoelectronic properties of quantum dots are relevant for photodetection.

As part of their new synthesis method, the research team used various phosphine-free compounds as precursors for silver and tellurium. They obtained quantum dots with a good size distribution and excitation peaks over a very wide range of the light spectrum. After their synthesis and characterization, the quantum dots showed remarkable performance with prominent excitonic peaks above 1500 nm, an unprecedented performance compared to previous phosphine-based quantum dot synthesis techniques.

The researchers then decided to implement these new quantum dots to fabricate a laboratory-scale photodetector (photodiode) on a glass substrate with a thin layer of ITO (indium tin oxide) to characterize the built device and measure its properties.

“These laboratory devices are operated by light coming in from below. In CMOS, which consists of stacked layers of quantum dots, the light is irradiated from above because the electronic part of the CMOS is at the bottom,” comments Yongjie Wang, postdoctoral researcher and lead author of the study, adding: “So the first challenge The challenge we had to overcome was to reverse the light input configuration of the photosensor, which seems simple in theory but turned out to be a complex task.”

Initially, the photodiode showed poor SWIR light detection performance. This forced the researchers to redesign, which involved incorporating an intermediate layer (buffer) at the heart of the device. This adjustment significantly improved the performance of the photodetector.

The resulting SWIR photodiode had a spectral range of 350 nm to 1600 nm, a linear dynamic range of over 118 dB, a bandwidth of -3 dB over 110 kHz, and a detection capacity at room temperature on the order of 1012 Jones.

“To the best of our knowledge, the photodiodes described in this work represent for the first time a processed and non-toxic solution of SWIR photodiodes that have similar performance factors to versions developed with heavy metal-containing quantum dots,” explains Professor Gerasimos Konstantatos ICREA of ICFO and Co -Author of the study.

“These results show that Ag quantum dots are present₂“The material we synthesized is a promising material,” he emphasizes, “that complies with the RoHS directive for developing high-performance and cost-effective SWIR light photodetection applications.”

With the successful development of this photodetector based on heavy metal-free quantum dots, the researchers took a further step in their work. They began a collaboration with the company Qurv, a spin-off of ICFO, to demonstrate its potential and began constructing a proof-of-concept SWIR image sensor made of non-toxic quantum dots that can operate at room temperature.

Yongjie Wang manipulates a sample of a quantum dot solution in the laboratory of the Institute of Photonic Sciences. / ICFO

The team integrated the photodiode built with a CMOS on a Digital Readout Integrated Circuit (ROIC) and a Focal Plane Assembly (FPA), creating for the first time the proof of concept of a SWIR image sensor based on non-toxic quantum dots and working at Room temperature.

The authors of the work tested the image sensor's ability to operate in the SWIR spectrum by taking various images. Specifically, they managed to obtain images of the contents of a plastic bottle that was opaque under visible light, as one of the possible examples carried out.

“Accessing SWIR with a cost-effective technology for consumer electronics means harnessing the potential of this spectrum for numerous applications, including improved automotive vision systems that enable driving in extreme weather conditions,” he explains. Gerasimos Constantus.

“The SWIR band between 1.35 µm and 1.40 µm provides a safety window for our eyes, free from background light day and night, and enables three-dimensional long-distance light detection and ranging (LiDAR) imagery for automotive applications “, augmented reality and virtual reality,” he emphasizes.

Now researchers want to increase the performance of photodiodes by designing and modifying the layers that make up the photodetector device. They also want to research new chemical surfaces for Ag quantum dots.₂They allow you to improve the performance as well as the thermal and environmental stability of the material on the way to market.