Team successfully marries a CMOS IC with graphene, resulting in a camera able to image visible and infrared light simultaneously.
By Wisse Hettinga
Researchers at ICFO—the Institute of Photonic Sciences, located in Catalonia, Spain—have developed a broad-spectrum sensor by depositing graphene with colloidal quantum dots onto a standard, off-the-shelf read-out integrated circuit. It is the first-time scientists and engineers were able to integrate a CMOS circuit with graphene to create a camera capable of imaging visible and infrared light at the same time. Circuit Cellar visited ICFO
Stijn Goossens is a Research Engineer at ICFO- the Institute of Photonic Sciences.
and talked with Stijn Goossens, one of the lead researchers of the study.
HETTINGA: What is ICFO?
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GOOSSENS: ICFO is a research institute devoted to the science and technologies of light. We carry out frontier research in fundamental science in optics and photonics as well as applied research with the objective of developing products that can be brought to market. The institute is based in Castelldefels, in the metropolitan area of Barcelona (Catalonia region of Spain).
HETTINGA: Over the last 3 to 4 years, you did research on how to combine graphene and CMOS. What is the outcome?
GOOSSENS: We’ve been able to create a sensor that is capable of imaging both visible and infrared light at the same time. A sensor like this can be very useful for many applications—automotive solutions and food inspection, to name a few. Moreover, being able to image infrared light can enable night vision features in a smartphone.
HETTINGA: For your research, you are using a standard off-the-shelf CMOS read-out circuit correct?
GOOSSENS: Indeed. We’re using a standard CMOS circuit. These circuits have all the electronics available to read the charges induced in the graphene, the rows and columns selects and the drivers to make the signal available for further processing by a computer or smartphone. For us, it’s a very easy platform to work on as a starting point. We can deposit the graphene and quantum dot layer on top of the CMOS sensor (Photo 1).
PHOTO 1
The CMOS image sensor serves as the base for the graphene layer.
HETTINGA: What is the shortcoming of normal sensors that can be overcome by using graphene?
GOOSSENS: Normal CMOS imaging sensors only work with visible light. Our solution can image visible and infrared light. We use the CMOS circuit for reading the signal from the graphene and quantum dot sensors. Tt acts more like an ‘infrastructure’ solution. Graphene is a 2D material with very special specifications: it is strong, flexible, almost 100 percent transparent and is a very good conductor.
HETTINGA: How does the graphene sensor work?
GOOSSENS: There are different layers (Figure 1). There’s a layer of colloidal quantum dots. A quantum dot is a nano-sized semiconductor. Due to its small size, the optical and electronic properties differ from larger size particles. The quantum dots turn the photons they receive into an electric charge. This electric charge is then transferred to the graphene layer that acts like a highly sensitive charge sensor. With the CMOS circuit, we then read the change in resistance of the graphene and multiplex the signal from the different pixels on one output line.
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FIGURE 1
The graphene sensor is comprised of a layer of colloidal quantum dots, a graphene layer and a CMOS circuitry layer.
HETTINGA: What hurdles did you have to overcome in the development?
GOOSSENS: You always encounter difficulties during the course of a research study and sometimes you’re close to giving up. However, we knew it would work. And with the right team, the right technologies and the lab at ICFO we have shown it is indeed possible. The biggest problem was the mismatch we faced between the graphene layer and the CMOS layer. When there’s a mismatch, that means there’s a lack of an efficient resistance read-out of the graphene—but we were able to solve that problem.
HETTINGA: What is the next step in the research?
GOOSSENS: Together with the European Graphene Flagship project, we are developing a production machine that will allow us to start a more automated production process for these graphene sensors.
HETTINGA: Where will we see graphene-based cameras?
GOOSSENS: One of the most interesting applications will be related to self-driving cars. A self-driving car needs a clear vision to function efficiently. If you want to be able to drive a car through a foggy night or under extreme weather conditions, you’ll definitely need an infrared camera to see what’s ahead of you. Today’s infrared cameras are expensive. With our newly-developed image sensor, you will have a very effective, low-cost solution. Another application will be in the food inspection area. When fruit ripens, the infrared light absorption changes. With our camera, you can measure this change in absorption, which will allow you to identify which fruits to buy in the supermarket. We expect this technology to be integrated in smartphone cameras in the near future.
ICFO | www.icfo.eu
This article appeared in the September 326 issue of Circuit Cellar
