Because of their semiconducting properties, silicon and germanium are indispensable materials for microelectronics.

The semiconductors are easy to process and have excellent physical and chemical properties.

Unfortunately, their optical behavior is less favorable.

Despite many efforts, silicon can hardly be used as a light emitter.

In crystalline form silicon emits almost no radiation (with the exception of porous silicon).

For the production of light-emitting diodes and semiconductor lasers, other semiconducting materials such as gallium arsenide or indium phosphide are used, but these are difficult to integrate into silicon circuits.

A shortcoming that has preoccupied scientists for decades.

Manfred Lindinger

Editor in the “Nature and Science” section.

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But there is a silver lining on the semiconductor horizon. A German-Dutch research group has succeeded in efficiently stimulating an alloy of the two semiconductors silicon and germanium to glow. This creates an important prerequisite for a solid-state laser based on silicon and germanium, report Erik Bakkers and his colleague Elham Fadaly in Berlin.

For around fifty years, researchers have been trying to build light-emitting diodes and lasers out of silicon or germanium.

So far without much success.

The reason why silicon and germanium, unlike gallium arsenide, emit almost no light, is due to the cubic structure of their crystal lattices.

Electrons, excited for example by electric fields or light rays, have difficulty changing from the conduction band to the energetically lower valence band and sending out a photon.

The two bands are offset from one another in such a way that the process of generating light is difficult.

Luminous wires with a hexagonal crystal structure

"The crucial step was to produce germanium and silicon with a hexagonal crystal lattice," said Bakkers.

“With this crystal structure, the semiconductors have a direct band gap and can therefore generate light themselves.” The excited electrons can more easily change from the conduction band to the valence band and generate light quanta.

In 2015, Bakkers and his colleagues at the TU Eindhoven used a trick to produce hexagonal silicon and germanium.

To do this, they first grew nanowires from a material with a hexagonal crystal structure.

They then covered these wires with a layer of germanium and silicon.

Then something surprising happened: The underlying material forced the germanium-silicon alloy into a hexagonal structure.

But the semiconductors could not be stimulated to glow at first.

They contained too many impurities for that.

With the support of materials researchers at the Walter Schottky Institute of the Technical University of Munich, who repeatedly analyzed the optical properties of the samples, it was finally possible to grow thin wires made of hexagonal silicon and germanium with a high degree of purity.

When Bakkers and Fadaly irradiated the wires with laser light, they could almost not believe their eyes.

The breakthrough was made.

The silicon-germanium alloy emitted infrared photons in a wide spectral range.

Bakkers is convinced that the wavelengths could be used for optical communication.

“In the meantime we have achieved optical properties that are almost comparable to indium phosphide or gallium arsenide.” Building a laser from germanium and silicon seems only a matter of time.

Such a laser can be integrated directly into common computer chips and thus accelerate data processing.

Because silicon chips could then process electrical and optical signals at the same time.

The chips would not only be significantly faster as a result.

Bakkers is convinced that less heat would be generated with electro-optical data processing.