An increase in computing performance has been achieved by squeezing ever more transistors into a tighter space on microchips. This downsizing has also meant packing the wiring within microprocessors ever more tightly together, leading to effects such as signal leakage between components, which can slow down communication between different parts of the chip. This delay, known as the “interconnect bottleneck,” is becoming an increasing problem in high-speed computing systems.
One way to solve the interconnect bottleneck problem is to use light rather than wires to communicate between different parts of a microchip; however, silicon — the material used to build chips — does not emit light easily.
A light emitter and detector were developed that can be integrated into silicon CMOS chips. The device is built from a semiconductor material called molybdenum ditelluride. This ultrathin semiconductor 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. In contrast to materials such as gallium arsenide that is used for optics, but cannot be grown easily on silicon, the 2D molybdenum ditelluride can be mechanically attached to any material.
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 simply 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 communication.
To use the material as a light emitter, it first had to be converted into a P-N junction diode, a device in which one side (the P side) is positively charged, while the other side (the N side) is negatively charged. In conventional semiconductors, this is typically done by introducing chemical impurities into the material. With the new class of 2D materials, 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, a current is run 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 light shines on it when the current restarts. In this way, the devices are able to both transmit and receive optical signals.
The researchers are 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; however, molybdenum ditelluride emits light at 1.1 micrometers. This makes it suitable for use in the silicon chips found in computers, but unsuitable for telecommunications systems. To this end, the researchers are exploring another ultrathin material called black phosphorus that 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.