Modern computers are, in many ways, limited by their energy consumption and cooling requirements. Some of that comes from the process of performing calculations. But often, the majority of energy use comes from simply getting data to the point where calculations are ready to be performed. Memory, storage, data transfer systems, and more all create power draws that, collectively, typically end up using more power than the processor itself.

Light-based communications offers the possibility of dropping power consumption while boosting the speed of connections. In most cases, designs have focused on situations where a single external laser supplies the light, which is divided and sent to the parts of the system that need it. But a new paper in Nature Nanotechnology suggests an alternate possibility: individual light sources on the chip itself. To demonstrate this possibility, the team put together an LED just two atoms thick and integrated it with a silicon chip. Better still, the same material can act as a photodetector, providing a way of building all the needed hardware using a single process.

Atomic

The work relied on two different atomically thin materials. These materials consist of a planar sheet of atoms chemically linked to each other. While their study was pioneered using graphene, a sheet of carbon atoms, they developed a variety of other materials with similar structures. The materials being used here are molybdenum ditelluride (MoTe 2 ), a semiconductor, and hexagonal boron nitride.

Normally, an LED requires a junction between p- and n-type semiconductors; light is emitted as electrons move from one to the other. For many materials, you can control whether they're p or n by doping small numbers of specific atoms into the semiconductor. But this won't work when your semiconductor is only one atom thick, like MoTe 2 . Doping won't work in this particular device, either, even though it has two layers of MoTe 2 (the authors say the second layer improves performance).

Instead, the researchers placed a layer of the boron nitride insulator on top of the MoTe 2 (which also protected the MoTe 2 from being oxidized). On top of that, they put a layer of current-carrying graphite, split into two electrodes. The presence of charge in these electrodes would electrostatically induce the equivalent of p- or n-type doping in the semiconductor.

This put all the pieces of an LED in place. To turn it into a useful device, the authors placed all this on top of silicon that had been prepared by drilling a carefully spaced series of holes in it. The spacing of these holes converted the silicon into a photonic crystal for infrared wavelengths, capable of directing light into or out of the MoTe 2 . The photonic crystal also bent the light so that, while the LED was located on top of it, the light traveled along the plane of the device. With a current of 2.3 microAmps, the LED operated as expected, producing light at roughly 1,175 nanometers, which places it in the near-infrared (the light was detected using a microscope).

It listens, too

The authors also reversed the device, turning it into a photodetector. If they directed a laser of the right wavelength into the device, they were able to induce a current to flow between the two electrodes on top of the device. A weak laser (20 microWatts, less than most laser pointers) was able to induce a current of 2.5 nanoAmps to flow across the device.

While all of this is impressive, it's not at the point where it solves the issue that the researchers say they want to address in their article's introduction: how to get optical communications to work on chip. As a photodetector, the external quantum efficiency is estimated to be only 0.5 percent, which would mean you'd have to send a substantial amount of light its way in order for it to register. And as an LED, this doesn't produce a ton of light.

The authors talk a bit about how putting a few of these LEDs in series could allow them to create an optically pumped laser. But at that point, you're talking about significantly increased complexity and power use. So, you can put this down as an impressive demonstration of how to integrate atomically thin materials with a standard silicon process. But it's going to need a fair bit of additional work before it becomes an option for computing.

Nature Nanotechnology, 2017. DOI: 10.1038/nnano.2017.203 (About DOIs).