A new detector that can cover “two technically important ranges” could enhance camera chips, autonomous vehicles, and smartphones—and at a much easier-to-absorb price point.
Developed by a team led by Forschungszentrum Jülich (Jülich Research Center, part of the Helmholtz Association of German Research Centers; Jülich, Germany), the new detector/sensor can make two ranges of infrared (IR) radiation visible: near-IR (750–1400 nm) to shortwave-IR (SWIR; 1400–2500 nm). Traditional photodiodes and visible light have not been able to cover this, as they offer limited visibility and detail recognition.
According to the research, the technology is a two-terminal, dual-band detector that “provides a bias-switchable spectral response in [those] two distinct IR bands.”1 Specifically, the device’s photo response can be switched “by inverting the bias polarity” between the near-IR and SWIR bands.
Expanding IR capabilities
Dan Buca, a scientist at Forschungszentrum Jülich who was involved with the research, notes that “conventional camera chips made [solely] of silicon can only image the SWIR range to a very limited extent.” The use of materials that are difficult to combine with standard circuits and computer chips made of silicon “makes their integration in a chip complex and thus expensive.”
In their work, the researchers combined a thin layer of silicon with layers of compatible semiconductor materials that contain germanium (Ge) and germanium-tin (GeSn). These layers can be used to produce individual pixels, each of which can capture the same image in different IR ranges (see figure).
As examples, Giovanni Isella, a professor at Politecnico di Milano University (Italy) who participated in the research, cites security applications such as authenticating banknotes or even examining artwork.
“In the case of paintings,” he says, “we can use the Ge-GeSn pixel to look through layers of paint and see how the artist compose the painted or what is underneath.” And certain inks used as security features for banknotes seem to disappear when viewed under IR light. He adds that the new detector makes authenticating the banknotes easy.
“Our detector bridges a gap since it covers a range of the spectrum for which there have been no cost-effective sensors to date,” Buca says. “The smart combination of elements and alloys that are well compatible with silicon now enables us to use a straightforward manufacturing process with standard industry tools. Therefore, we are now able to construct very inexpensive camera chips that can be integrated in any smartphone just as in visible cameras currently in use.”
The development of the new detector has been prompted, in part, by increasing attention on applications relating to the Internet of Things (IoT) and Artificial Intelligence of Things (AIoT), as well as “a rising demand for high-spectral sensitivity” in imaging. The study notes that the near-IR and SWIR ranges “provide a wealth of information not accessible to cameras operating in the visible range (380–750 nm), which is extremely relevant for autonomous driving, security, medical, and environmental monitoring applications.”
1. E. T. Simola et al., ACS Photonics, 8, 7, 2166–2173 (2021); doi:10.1021/acsphotonics.1c00617.