Researchers in Spain have teamed up to produce a commercial infrared sensor that is compatible with CMOS chip making technology for low cost consumer applications.
Colloidal quantum dots (CQD) based image sensor technology offers a promising technology platform to enable high-volume short wavelength infrared (SWIR) image sensors.
The CQDs developed at the Insitut de Ciencies Fotoniques (ICFO) in Barcelona, Spain, use nanometric semiconductor crystals in a solution that are compatible with materials restrictions such as ROHS regulations and CMOS process technology, rather than using heavy metals such as lead or mercury (IV-VI Pb, Hg-chalcogenide semiconductors).
The researchers teamed up with Qurv, an ICFO spin-off, to demonstrate its potential by constructing a SWIR image sensor as a case study (shown above). The team integrated the new photodiode with a CMOS based read-out integrated circuit (ROIC) focal plane array (FPA) demonstrating for the first time a proof-of-concept, non-toxic, room temperature-operating SWIR quantum dot based image sensor.
The sensor was tested by taking several pictures of a target object. In particular, they were able to image the transmission of silicon wafers under the SWIR light as well as to visualize the content of plastic bottles that were opaque in the visible light range.
Currently available technologies in the SWIR range rely on costly epitaxial semiconductors that are not monolithically integrated with complementary metal–oxide–semiconductor electronics. Solution-processed quantum dots can address this challenge by enabling low-cost manufacturing and simple monolithic integration on silicon in a back-end-of-line process.
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The sensor stack is sensitive in the spectral range from 350 nm to 1,600 nm. The room-temperature detectivity is of the order of 1012 Jones, the 3 dB bandwidth is in excess of 0.1 MHz and the linear dynamic range is over 118 dB. The researchers also built a monolithically integrated SWIR imager based on solution-processed, toxic-heavy-metal-free materials, paving the way for this technology to the consumer electronics market.
The key is a new method for synthesizing size tuneable, phosphine-free silver telluride (Ag2Te) quantum dots while preserving the advantageous properties of traditional heavy-metal counterparts.
The researchers at ICFO were investigating how to synthetize silver bismuth telluride (AgBiTe2) nanocrystals to extent the spectral coverage of the AsBiS2 technology to enhance the performance of photovoltaic devices, and obtained silver telluride (Ag2Te) as a by-product.
This showed a strong and tuneable quantum confined absorption akin to quantum dots with potential for SWIR photodetectors and image sensors and pivoted their efforts to achieve and control a new process to synthesize phosphine-free versions of silver telluride quantum dots, as phosphine was found to have a detrimental impact on the optoelectronic properties of the quantum dots relevant to photodetection.
“For CMOS integrated CQD stacks, light comes from the top, whereas the bottom part of the device is taken by the CMOS electronics,” said Yongjie Wang, postdoc researcher at ICFO and first author of the study. “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 350nm to 1600nm.#
“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,” said Gerasimos Konstantatos, ICREA Prof. at ICFO. “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”.
“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 (cars) enabling vision and driving under adverse weather conditions,” says Konstantatos. “SWIR band around 1.35-1.40 µm, can provide an eye-safe window, free of background light under day/night condition, thus, further enable long-range light detection and ranging (LiDAR), three-dimensional imaging for automotive, augmented reality and virtual reality applications.”
Now 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.