TUE researchers squeeze light from silicon

The elusive silicon laser might be within reach now that researchers from Eindhoven University of Technology have demonstrated efficient light emission from silicon.

For all the magnificent things silicon is capable of, emitting light (or absorbing it, for that matter) is not one of them. Being an indirect bandgap semiconductor, excited electrons in silicon’s crystal lattice return to their ground state by releasing their energy as heat, not as light. By carefully manipulating the crystal structure, however, researchers from Eindhoven University of Technology and colleagues from Germany managed to make silicon shine. Creating a silicon laser is now just a matter of time, the scientists think.

The technological implications of that prospect are profound. Having access to a built-in light source means silicon’s electronic properties can be expanded upon with optoelectronic functionality, most importantly optical communication. Though electrons likely won’t become obsolete any time soon, being able to shuffle bits within a chip or between chips using speedy and energy-efficient optical communication would be a game-changer.

Paul van Gerven

Cubic

It’s not particularly hard to build a semiconductor laser. Direct bandgap semiconductors such as gallium arsenide and indium phosphide are great at emitting light when tickled electrically. Unfortunately, these and other compound semiconductors don’t combine easily with silicon, necessitating attachment of the laser to the silicon circuit in a separate and very delicate step. While progress is being made in that department, creating the laser with the same process technology that’s already being used for manufacturing chips would obviously be a superior alternative.

Research performed some fifty years ago predicts that silicon lasers are possible. The key lies within the crystal structure: whereas ‘regular’ silicon organizes in cubic shapes, theory predicts that silicon, when alloyed with its chemical relative germanium, becomes a direct bandgap semiconductor when organized in hexagonal patterns.

Matter of time

Shaping silicon hexagonally, however, isn’t easy. By gently growing silicon onto nanowires made from a material with a hexagonal crystal structure, the Eindhoven researchers led by Erik Bakkers figured they could force silicon into the desired pattern. But even after they succeeded, already in 2015, the exotic silicon remained dark.

It took another five years of careful tinkering with the growth process to reduce the numbers of impurities and crystal defects to obtain the quality required for light emission – for excellent light emission, in fact. “We’ve realized optical properties that are almost comparable to indium phosphide and gallium arsenide, and the material’s quality is steeply improving,” says Bakkers.

As a result, it appears to be a matter of time before a laser made from germanium-silicon alloys is developed that can be manufactured using conventional production processes. Bakkers: “If things run smoothly, we can create a silicon-based laser in 2020.” After that, the next step will be combining the hexagonal silicon with the cubic substrate used in microelectronics. The two forms are compatible in principle, but in this line of work, you expect the unexpected.

Credit: Eindhoven University of Technology/Nando Harmsen

The nanowires coated with a hexagonal, light-emitting silicon-germanium shell were grown using metal-organic vapor phase epitaxy (MOVPE).