MIT researchers create CMOS-compatible molybdenum ditelluride emitters and detectors for silicon photonics
Ultrathin films of the semiconductor emit and detect light when stacked on silicon wafers.
Creating optical systems in the form of silicon integrated circuits enables very compact, low-power, high-speed waveguide-based optoelectronic processing for communications and other uses. Ideally, all components in a silicon photonics chip should be CMOS-compatible, meaning that they can be fabricated using the same manufacturing processes used for silicon electronics. However, this is difficult to accomplish.
Now, researchers from the Massachusetts Institute of Technology (MIT; Cambridge, MA), Columbia University (New York, NY), the Barcelona Institute of Science and Technology (Barcelona, Spain), and the National Institute for Materials Science (Tsukuba, Japan) have developed molybdenum ditelluride (MoTe2)-based light-emitting diodes and photodetectors that can be integrated into silicon chips via the CMOS process.
The ultrathin MoTe2 belongs to an emerging group of materials known as two-dimensional transition-metal dichalcogenides. Unlike conventional semiconductors, the material can be stacked on top of silicon wafers, says Pablo Jarillo-Herrero, an associate professor of physics at MIT.
"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," Jarillo-Herrero says. "For example, gallium arsenide is very good for optics, but it cannot be grown on silicon very easily because the two semiconductors are incompatible."
In contrast, the 2-D molybdenum ditelluride can be mechanically attached to any material, Jarillo-Herrero says.
Another difficulty with integrating other semiconductors with silicon is that the materials typically emit light in the visible range, but light at these wavelengths is absorbed by silicon. Molybdenum ditelluride emits light in the infrared range, which is not absorbed by silicon, meaning it can be used for on-chip communications.
To use the material as a light emitter, the researchers first had to convert it into a P-N junction diode. In conventional semiconductors, this is typically done by introducing chemical impurities into the material. With the new class of 2D materials, however, it can be done by simply applying a voltage across metallic gate electrodes placed side-by-side on top of the material.
Once the diode is produced, the researchers run a current through the device, causing it to emit light. The device can also be switched to operate as a photodetector by reversing the polarity of the voltage applied to the device. This causes it to stop conducting electricity until a light shines on it.
In this way, the devices are able to both transmit and receive optical signals. 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 says.
The researchers are now investigating other materials that could be used for on-chip optical communication.
Most telecommunication systems, for example, operate using light with a wavelength of 1.3 or 1.5 µm, Jarillo-Herrero says. However, molybdenum ditelluride emits light at 1.1 µm. This makes it suitable for use in the silicon chips found in computers, but unsuitable for 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," he says. To this end, the researchers are exploring another ultrathin material, black phosphorus, which can be tuned to emit light at different wavelengths by altering the number of layers used. They hope to develop devices with the necessary number of layers to allow them to emit light at the two wavelengths while remaining compatible with silicon.
1. Ya-Qing Bie et al., Nature Nanotechnology (2017); doi: 10.1038/nnano.2017.209