Revolution: Light from silicon

Adapted from original text by Hilde de Laat, Science Information Officer at TU/e

It sounds like a Hollywood film: resolving an issue that has been the Holy Grail in some research field for over fifty years, directly at the start of your academic career. Still that is exactly what PhD researchers Elham Fadaly and Alain Dijkstra did. In a recent paper in Nature, they demonstrated that their hexagonal structure made out of silicon combined with germanium is able to efficiently emit light.

To use light on a chip, you need a light source; 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 to play no role in photonics. To create a silicon-compatible laser, scientists needed to produce a form of silicon that can emit light. That’s exactly what TU/e researchers together with colleagues from the universities of Jena, Linz and Munich did.

‘The crux is in the nature of the so-called bandgap of a semiconductor,’ says lead researcher Erik Bakkers. ‘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, which is called an indirect band gap, no photons can be emitted – as is the case in 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.

Here is where Elham Fadaly and Alain Dijkstra enter the stage. ‘When I started working on this project back in 2017, colleagues from our group Advanced Nanomaterials & Devices had already achieved good results in growing hexagonal silicon germanium. However, they had some serious issues with the quality and reproducibility of the material. They had already seen some first signs of light emission, but it was hard to say whether or not that light was coming from the pure material’, Fadaly says.

Producing hexagonal silicon is not easy, since the material originally has a strict cubic shape. The researchers used nanowires to do the trick. First, they grew nanowires made from another material, with a hexagonal crystal structure. Subsequently, they grew a silicon-germanium shell on this template to copy its structure. ‘To overcome the problems with the quality of the material we had, we decided to switch to another core template material. This material had unit cell dimensions that matched well with the desired SiGe alloys, which lead to a significant reduction of  the strain that hinders the optical properties,’ Fadaly explains. By carefully reducing the number of impurities and crystal defects, the team managed to increase the quality of the hexagonal silicon-germanium shells in such a way that the material now emits light very efficiently. ‘Eventually, we were also able to prove that the first signs of light my colleagues had seen before, were indeed also coming from the silicon alloy,’ Fadaly adds.

Though in hindsight this might sound like a straightforward road to success, there certainly were times when the PhD researcher was sure this was not going to work. ‘I started this project with high hopes. But when I came to the lab and saw all the challenges, I must admit I was hesitant about the possibilities to succeed.’ The real breakthrough feeling came upon her when the team started to see bright room temperature light emission comparable to that of other materials. ‘When we also proved that we can tune the emission by changing the percentage of silicon in the alloy, then I knew that we had something big’, she smiles. ‘And since the lifetime of the carriers is quite small, comparable to that of indium phosphide and gallium arsenide, this truly is the start of a very promising technology.’

Currently, the group is working hard to establish lasing in the material. Fadaly: ‘In a planar geometry, establishing a lasing effect is a lot easier than in the arrays of nanowires we are using. Tiny changes in the dimensions, faceting, or arrangement of the nanowires can lead to inhomogeneities in the light field, and that is devastating for lasing effects. We are now trying to determine the appropriate dimensions, control our nanowires’ morphology and further lower down the amount of impurities to find the sweet spot parameters for the gain medium.’

Group leader Bakkers is optimistic: ‘We have realized optical properties which are almost comparable to the known photonic materials indium phosphide and gallium arsenide, and the material’s quality is steeply improving. If things run smoothly, I expect that we can create a silicon-based laser sometime soon. That will revolutionize the microelectronics industry, by adding intra-chip and chip-to-chip optical communication, which requires much less energy and generates much less heat. This is important because it will help put a brake on the soaring energy consumption of microelectronics.’