Joint first authors Elham Fadaly (left) and Alain Dijkstra (right) operating an optical setup to measure the light emitted by the hexagonal silicon-germanium alloy. Photo: Sicco van Grieken, SURF.
Joint first authors Elham Fadaly (left) and Alain Dijkstra (right) operating an optical setup to measure the light emitted by the hexagonal silicon-germanium alloy. Photo: Sicco van Grieken, SURF.

Emitting light from silicon has been the 'Holy Grail' for the microelectronics industry for decades. Solving this puzzle would revolutionize computing, making chips faster than ever, and this is what researchers from Eindhoven University of Technology (TU/e) in the Netherlands have now done, by developing an alloy with silicon that can emit light. They report this advance in a paper in Nature, and will now start creating a silicon laser to be integrated into current chips.

Current computing technology, based on electronic chips, is reaching its ceiling. The limiting factor is heat, generated by the resistance electrons experience when traveling through the copper lines connecting the many transistors on a chip. To break through this ceiling will require a new computing technique that does not produce heat. One option is photonics, which uses photons (light particles), rather than electrons, to transfer data.

In contrast to electrons, photons do not experience resistance. As they have no mass or charge, photons scatter less within the material they travel through, meaning no heat is produced and energy consumption is therefore reduced. Moreover, by replacing electrical communication within a chip with optical communication, the speed of on-chip and chip-to-chip communication can be increased by a factor of a thousand.

Data centers would benefit most, with faster data transfer and less energy usage for their cooling systems. But photonic chips will also bring new applications within reach, such as laser-based radar for self-driving cars and chemical sensors for medical diagnosis or for measuring air and food quality.

Using light in chips requires a light source such as an integrated laser. The main semiconductor material that computer chips are made of is silicon. But bulk silicon is extremely inefficient at emitting light, and so was long thought not to be able to play a role in photonics. This caused scientists to turn to more complex semiconductors, such as gallium arsenide and indium phosphide. These are good at emitting light but are more expensive than silicon and are hard to integrate into existing silicon microchips.

To create a silicon-compatible laser, scientists needed to produce a form of silicon that could emit light, which is exactly what researchers from TU/e have now managed to do. Together with researchers from the Johannes Kepler University, Linz, in Austria and the universities of Jena and Munich, both in Germany, they combined silicon and germanium in a hexagonal structure that is able to emit light. A breakthrough after 50 years of work.

"The crux is in the nature of the so-called band gap of a semiconductor," explains lead researcher Erik Bakkers from TU/e. "If an electron 'drops' from the conduction band to the valence band, a semiconductor emits a photon: light." But if the conduction band and valence band are displaced with respect to each other, producing a so-called indirect band gap, no photons can be emitted – as is the case with silicon.

"A 50-year-old theory showed, however, that silicon alloyed with germanium shaped in a hexagonal structure does have a direct band gap, and therefore potentially could emit light," says Bakkers.

But shaping silicon into a hexagonal structure is not easy. As Bakkers and his team mastered the technique of growing nanowires, they found a way to create hexagonal silicon in 2015. This involved first growing nanowires made from another material into a hexagonal crystal structure, and then growing a silicon-germanium shell on this template.

"We were able to do this such that the silicon atoms are built on the hexagonal template, and by this forced the silicon atoms to grow in the hexagonal structure," said Elham Fadaly from TU/e, joint first author of the paper.

But the scientists were unable to make this hexagonal silicon-germanium alloy emit light, until now. They managed this through increasing the quality of the hexagonal silicon-germanium shells by reducing the number of impurities and crystal defects.

When exciting the nanowire with a laser, they could measure the efficiency of the new material. "Our experiments showed that the material has the right structure, and that it is free of defects. It emits light very efficiently," said Alain Dijkstra from TU/e, joint first author of the paper and responsible for measuring the light emission:

Creating a silicon laser is now just a matter of time, Bakkers thinks. "By now, we have realized optical properties which are almost comparable to indium phosphide and gallium arsenide, and the materials quality is steeply improving. If things run smoothly, we can create a silicon-based laser in 2020. This would enable a tight integration of optical functionality in the dominant electronics platform, which would break open prospects for on-chip optical communication and affordable chemical sensors based on spectroscopy."

In the meantime, his team is also investigating how to integrate the hexagonal silicon in cubic silicon microelectronics, which is an important prerequisite for this work.

This story is adapted from material from Eindhoven University of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.