A combination light emitter and detector device that is compatible with silicon could help mitigate communications delays resulting from signal leakage between microchip components. The device is made from molybdenum ditelluride (MoTe2), an ultrathin semiconductor that belongs to an emerging group of materials known as 2D transition-metal dichalcogenides (TMDs). Researchers have designed a light-emitter and detector that can be integrated into silicon CMOS chips. This illustration shows a molybdenum ditelluride light source for silicon photonics. Courtesy of Sampson Wilcox. Unlike conventional semiconductors, MoTe2 can be stacked on top of silicon wafers. Additionally, MoTe2 emits light in the IR, a range that is not absorbed by silicon. Most semiconductor materials emit light in the visible range, and silicon absorbs light at these wavelengths. Researchers at Massachusetts Institute of Technology (MIT) developed the silicon waveguide-integrated light source and photodetector based on a p–n junction of bilayer MoTe2. Researchers converted the material into a P-N junction diode by applying a voltage across metallic gate electrodes placed side by side on top of the material — an approach that can be taken with the new class of 2D materials to which MoTe2 belongs. “That is a significant breakthrough, because it means we do not need to introduce chemical impurities into the material (to create the diode). We can do it electrically,” said professor Pablo Jarillo-Herrero. Once the diode was produced, the researchers ran a current through the device that caused the device to emit light. “So by using diodes made of molybdenum ditelluride, we were able to fabricate light-emitting diodes (LEDs) compatible with silicon chips,” Jarillo-Herrero said. Researchers found that the device could be switched to a photodetector mode by reversing the polarity of the voltage applied to it. The reversal in polarity caused the device to stop conducting electricity until a light was shined on it. In this way, the device demonstrated the ability to both transmit and receive optical signals. The research team believes that emerging 2D TMDs could offer a path for developing optical interconnect components that could be integrated with silicon photonics and complementary metal oxide semiconductors (CMOS) processing. “Researchers have been trying to find materials that are compatible with silicon in order to bring optoelectronics and optical communication on-chip, but so far this has proven very difficult,” said Jarillo-Herrero. “For example, gallium arsenide is very good for optics, but it cannot be grown on silicon very easily because the two semiconductors are incompatible.” The device is a proof of concept, and a great deal of work still needs to be done before the technology can be developed into a commercial product, Jarillo-Herrero said. The researchers are now investigating other materials that could be used for on-chip optical communication. Most telecommunication systems operate using light with a wavelength of 1.3 or 1.5 micrometers (μm), Jarillo-Herrero said. MoTe2 emits light at 1.1 μm, making it suitable for use in the silicon chips found in computers, but not for use in telecommunications systems. “It would be highly desirable if we could develop a similar material, which could emit and detect light at 1.3 or 1.5 μm in wavelength, where telecommunication through optical fiber operates,” Jarillo-Herrero said. To this end, the researchers are exploring another ultrathin material — black phosphorus — which can be tuned to emit light at different wavelengths according to the number of layers used. The team’s intent is to develop a device with the number of layers necessary to allow it to emit light at 1.3 and 1.5 μm in wavelength, while retaining its compatibility with silicon. This fabrication technology could provide opportunities for integrated optoelectronic systems. “The hope is that if we are able to communicate on-chip via optical signals instead of electronic signals, we will be able to do so more quickly, and while consuming less power,” Jarillo-Herrero said. The research was published in Nature Nanotechnology (doi:10.1038/nnano.2017.209).