An international team of researchers under the umbrella of the EU-funded Graphene Flagship have taken a significant step in thermal infrared (IR) photodetctors with the development of the most sensitive uncooled graphene-based thermal detector yet fabricated. These new photodetectors, known as bolometers, are so sensitive that they can register the presence of a scant few nanowatts of radiation. That level of radiation is about a thousandth of what would be given off by a hand waving in front of the detector.
In the research described in the journal Nature Communications, scientists from the University of Cambridge, UK; the Institute of Photonic Sciences (ICFO), Spain; the University of Ioannina, Greece; and from Nokia and Emberion found that the combination of graphene and pyroelectric materials—which generate a voltage when they are heated or cooled—yields a unique synergy that boosts the performance of thermal photodetectors.
The actual design of the device is fairly simple. The pyroelectric material acts as the substrate; a conductive channel made from single-layer graphene runs through it, and a floating gate electrode floats above it.
Changes in temperature create an electric field in the pyroelectric material and the floating gate focuses that field onto the graphene, which causes a change in the graphene’s electrical resistance. It is this change in resistance that is measured. What makes this latest thermal photodetector different from others is that it does not require built-in amplifiers to boost the electric field produced by the pyroelectric material.
“Pyroelectric materials produce charge, and that charge, rather than being directly measured by some external electronics, is used internally to “gate” the graphene and alter its conductivity,” explained Alan Colli, principal engineer at Emberion, said in an e-mail interview with IEEE Spectrum. (Emberion is a research group spun off from Nokia last summer.) “The resulting change in graphene current,” says Colli, “is much larger than the original pyroelectric current, hence it serves as an amplifier by definition.”
Graphene is able to perform this feat because it has nearly as much conductivity as a metal, but it responds to a field effect (the “gate”) like a semiconductor. But it’s not just any semiconductor; it’s one with an extremely high mobility, which translates into high gain and low noise.
Another benefit provided by the graphene, beyond it serving as an internal amplifier, is that it allows further integration with the external readout integrated circuit (ROIC) that collects all the photocurrent from each pixel in a photodetector and then transfers that signal into a readout. Because of graphene’s high conductivity and strong field effect, it is able to match the impedance of the ROIC, making the transmission from the pixels to ROIC as efficient as possible.
“To match the input impedance of the ROIC, you need something that is as conductive as possible,” said Colli in a press release. “The intrinsic conductivity of graphene helps the further integration with silicon.”
The international research team is bullish on the prospects of this technology, particularly for use in spectroscopy for security screening where high sensitivity at room temperature operation is required. Currently, IR detectors depend on background IR radiation being integrated into the system to generate a signal. With the graphene-based IR detector, it is possible to generate a high quality signal with less incident radiation, which in turn makes it possible to isolate parts of the IR spectrum.
“With a higher sensitivity detector, then you can restrict the band and still form an image just by using photons in a very narrow spectral range, and you can do multi-spectral IR imaging,” explained Colli. “For security screening, there are specific signatures that materials emit or absorb in narrow bands. So, you want a detector that's trained in that narrow band. This can be useful while looking for explosives, hazardous substances, or anything of the sort.”
While the European research team is very enthusiastic about the potential for the technology, there remain some key engineering issues that still need to be addressed.
“Full maturity of graphene processing and integration is the most general hurdle,” says Colli. “Specifically to this device, I believe it is necessary to increase the dynamic range and make the device operate over a wide range of ambient temperatures. At present, it is too vulnerable to large and sudden thermal shocks.”