High-efficiency laser for silicon chips

Jülich, 17. March 2020Transistors in computer chips work electrically, but data can be transmitted faster with light. Researchers have therefore long been looking for a way to integrate a laser directly into silicon chips. Scientists at Forschungszentrum Jülich have now taken a step forward in that direction. Together with researchers from the Centre de Nanosciences et de Nanotechnologies (C2N) in Paris and the French company STMicroelectronics and CEA-LETI Grenoble, they have developed a compatible semiconductor laser made of germanium and tin that is already comparable in efficiency to conventional GaAs semiconductor lasers.
(Nature Photonics, DOI: 10.1038/s41566-020-0601-5)

Germanium-Tin Laser

Scanning electron micrograph of the germanium-tin laser (left). The germanium-tin layer, which is only a few micrometers wide, is deposited on a so-called stressor layer of silicon nitride and an aluminum base for better heat dissipation, and then coated with silicon nitride (right). The orientation of the germamium-tin compound to the further atomic spacings in the crystal lattice of the silicon nitride creates a stress in the embedded material, which ultimately results in optical amplification.
Forschungszentrum Jülich / Nils von den Driesch

Optical data transmission enables significantly higher data rates and ranges than common electronic methods, while requiring less energy. In computing and data centers, optical cables are therefore already standard from a length of about one meter. In the future, optical solutions will be required for ever shorter distances to transmit data from board to board or chip to chip due to the constantly increasing requirements. This is especially true for artificial intelligence (AI) systems, such as autonomous driving, where large amounts of data need to be transferred within a large network of sensors to train the chip and algorithms.

"What is missing as a priority is a low-cost laser, which is necessary to achieve very high data rates. An electrically pumped laser that is compatible with silicon-based CMOS technology would be ideal," explains Prof. Detlev Grützmacher, director at the Peter Grünberg Institute (PGI-9) at Forschungszentrum Jülich. "You could then simply form such a laser directly during chip fabrication, because all chip production is ultimately based on this technology."

The problem: Pure silicon is a so-called indirect semiconductor and unsuitable as a laser material. Other materials are therefore currently being used to build lasers. As a rule, so-called III-V compound semiconductors are used. "Their crystal lattice, however, has a completely different structure than that of silicon, which belongs to the fourth main group of the periodic table. Until now, laser components have therefore been produced externally and then have to be connected at great expense," explains Detlev Grützmacher.

The new laser, on the other hand, can be manufactured directly during the CMOS process. It is based on germanium, which, like silicon, comes from the fourth main group. In 2015, Jülich researchers had already demonstrated that laser light can be generated by adding tin. The key factor here is the high tin content; at that time it was over 8 percent, far above the solubility limit of 1 percent (see press release of Jan. 19, 2015).

"Pure germanium, like silicon, is by nature an indirect semiconductor. Only the high tin concentration ensures that it becomes a direct semiconductor for a laser source," explains Dr. Dan Buca, working group leader at the Peter Grünberg Institute (PGI-9).

Preparation of the germanium-tin compound

Preparation of the highly concentrated germanium-tin compound (chemical vapor deposition, CVD): Germanium and tin are introduced in the form of gaseous compounds such as G2H6 or SnCl4 and decomposed into reactive radicals, which react strongly exothermically on the heated substrate to release hydrochloric acid (HCl). The resulting heat of reaction contributes locally to the incorporation of germanium and tin into the crystal lattice. The process takes place below the actual crystallization temperature. If the temperature were higher, the tin would be "sweated out" again above the saturation point.
Forschungszentrum Jülich

The patented Jülich process is now used by several research groups around the world. By further increasing the tin concentration, it has already been possible to realize lasers that work not only at low temperatures but also at room temperature.

"However, a high tin content reduces the efficiency. The laser then requires quite a high pumping power. At 12 to 14 percent tin, 100 to 300 kW/cm2 are necessary," explains Nils von den Driesch. "We have therefore tried to reduce the tin concentration by additionally bracing the material, which allows the optical properties to be improved even more significantly."

For the new laser, the researchers have scaled down the tin content to around 5 percent - and reduced the required pump power to 0.8 kW/cm2. This produces so little waste heat that the laser is the first Group IV semiconductor laser that can be operated not only pulsed but also continuously in a so-called continuous wave mode.

"These values demonstrate that a germanium-tin laser is technologically feasible that matches the level of common III-V semiconductor lasers in terms of efficiency. This means that a laser for industrial applications that works at room temperature now seems within reach," explains institute director Detlev Grützmacher. This is because the function of the new laser is currently still limited to optical excitation and low temperatures in the range of - 200 to - 170 degrees Celsius.

Such a laser would be interesting not only for the optical transmission of data, but also for a wide range of other applications. For the corresponding wavelengths in the near infrared range between 2 and 4 micrometers, there are hardly any cost-effective alternatives so far. Potential applications range from infrared and night vision systems to gas sensors for infrared spectroscopy to monitor environmental and respiratory gases in climate research and medicine.

Here is how a laser works:

In a laser (light amplification by stimulated emission of radiation), energy is supplied to the laser medium by a pumping process. The pumping can be done optically by irradiation of light or electrically, whereby the necessary pumping power can vary greatly depending on the laser. The excited electrons are thus "pumped" to a higher metastable energy level. These states should last as long as possible, so that an "occupation inversion" can be built up in which a large number of the atoms or molecules are in the excited state.
As soon as one of the excited states falls back to its ground state, a photon is emitted. If this photon hits other excited states, they are also triggered to fall back to their ground state and emit an additional photon. This process is called "stimulated emission." This doubling of the stimulating photon causes the laser medium to act like a light amplifier. The "freshly created" second photon then in turn stimulates other excited atoms or molecules to emit. A chain reaction occurs in which a standing wave is formed between the two mirrors on the sides of the laser medium, with laser radiation escaping on one side, through the semi-transparent mirror.

Original publication:

Ultra-low threshold continuous-wave and pulsed lasing in tensile strained GeSn alloys

Anas Elbaz, Dan Buca, Nils von den Driesch, Konstantinos Pantzas, Gilles Patriarche, Nicolas Zerounian, Etienne Herth, Xavier Checoury, Sébastien Sauvage, Isabelle Sagnes, Antonino Foti, Razvigor Ossikovski, Jean-Michel Hartmann, Frédéric Boeuf, Zoran Ikonic, Philippe Boucaud, Detlev Grützmacher, Moustafa El Kurdi

Nature Photonics (published online 16 March 2020), DOI: 10.1038/s41566-020-0601-5

Further information:

Pressemitteilung des Centre de Nanosciences et de Nanotechnologies (C2N) in Paris vom 18. März 2020 (in Englisch)

Press release, 21. Februar 2017, “Zinn in der Photodiode” https://www.fz-juelich.de/SharedDocs/Pressemitteilungen/UK/DE/2017/2017-02-21-sigesn-diode.html?nn=692044

Press release, 19. Januar 2015 „Neuer Laser für Computerchips“

Peter Grünberg Institut, Halbleiter-Nanoelektronik (PGI-9)

The research was supported in part by the 'SiGeSn Laser for Silicon Photonics' project of the German Research Foundation (DFG).


Prof. Detlev Grützmacher, Director of the Peter Grünberg Institute, Semiconductor Nanoelectronics (PGI-9)
Tel. +49 2461 61-2340
E-Mail: d.gruetzmacher@fz-juelich.de

Dr. Dan Buca, Peter Grünberg Institute, Semiconductor Nanoelectronics (PGI-9)
Tel. +49 2461 61-3149
E-Mail: d.m.buca@fz-juelich.de

Dr. Nils von den Driesch, Peter Grünberg Institute, Semiconductor Nanoelectronics (PGI-9)
Tel. +49 2461 61-4505
E-Mail: n.von.den.driesch@fz-juelich.de

Press contact:

Dr. Regine Panknin
Public relations officer, Forschungszentrum Jülich
Tel.: 02461 61-9054
E-Mail: r.panknin@fz-juelich.de

Tobias Schlößer
Public relations officer, Forschungszentrum Jülich
Tel.: 02461 61-4771
E-Mail: t.schloesser@fz-juelich.de

Last Modified: 12.08.2022