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Laser-compatible germanium makes light work of on-chip communications

22 April 2013

A team of researchers has managed to make germanium suitable for lasers, enabling future microprocessor components to communicate using light.

Light emitting bridges of germanium can be used for communication between microprocessors (graphic: Hans Sigg, PSI)
Light emitting bridges of germanium can be used for communication between microprocessors (graphic: Hans Sigg, PSI)

Researchers from ETH Zurich, the Paul Scherrer Institute (PSI) and the Politecnico di Milano have jointly developed a manufacturing technique to render germanium laser-compatible through high tensile strain.

The scientists saye that they can use their method to alter the optical properties of germanium, which at present is unsuitable for lasers.

“With a strain of three per cent, the material emits around twenty-five times more photons than in a relaxed state,” explains Martin Süess, a doctoral student at the Laboratory for Nanometallurgy at ETH Zurich. “That’s enough to build lasers with,” adds his colleague Richard Geiger, a doctoral student at the Laboratory for Micro- and Nanotechnology at the PSI and the Institute for Quantum Electronics at ETH Zurich.

In order to bring the germanium into a laser-compatible, stretched form with the new method, the researchers use the slight tension generated in germanium when it evaporates on silicon, strengthening this prestrain with so-called microbridges.

They score exposed germanium strips, which remain attached to the silicon layer at both ends, in the middle on both sides. The two halves of the strip thus remain connected solely by an extremely narrow bridge, which is precisely where, for physical reasons, the strain of the germanium grows so intense that it becomes laser-compatible.

“The tensile strain exerted on the germanium is comparable to the force exerted on a pencil as two lorries pull upon it in opposite directions,” says PSI project manager Hans Sigg. The material properties change because the individual atoms move apart a little through the expansion of the material, which enables the electrons to reach energy levels that are favourable for the generation of photons.

This method could have a big impact on computer performance, as it opens up opportunities for new transmission paths that are not reliant on conventional conductors; in other words, light.

However, in order to achieve this, light sources must be small enough to fit on a chip and react well to silicon, the base material of all computer chips. Silicon itself is not suitable for the construction of laser light, hence the importance of this work on germanium laser-compatibility.

The team is currently in the process of constructing a germanium laser using their new methodology.

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